Contrasting Styles of Salt-Tectonic Processes in the Ionian Zone (NW Greece and S Albania)

Posted on 23 Jun 2023 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/essoar.168748485.58030882/v1 — This a preprint and has not been peer reviewed. Data may be preliminary. Contrasting Styles of Salt-Tectonic Processes in the Ionian Zone (NW Greece and S Albania) Juan I Soto1 , MARKOS DAMIANOS TRANOS2 , Zamir Bega3 , Tim Dooley4 , Pablo Hernández5 , Michael R. Hudec1 , Takis Konstantopoulos6 , Ervin Lula7 , Konstantinos Nikolaou6 , Rubén Pérez5 , Juan Pablo Pita5 , Juan Antonio Titos7 , Constantinos Tzimeas8 , and Adela Herra Sánchez de Movellán5 1 University of Texas at Austin KING FAHD UNIVERSITY OF PETROLEUM AND MINERALS 3 Independent Consultant 4 Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin 5 REPSOL Exploración S.A. 6 ENERGEAN 7 Granada University 8 Independent Exploration Geoscience Consultant 2 June 23, 2023 1 1 Contrasting Styles of Salt-Tectonic Processes 2 in the Ionian Zone (NW Greece and S Albania) 3 4 5 J. I. Soto1,2*, M. D. Tranos3, Z. Bega4, T. Dooley1, P. Hernández5, M. R. Hudec1, P. A. 6 Konstantopoulos6, E. Lula7,8, K. Nikolaou6, R. Pérez5, J. P. Pita5, J. A. Titos7, C. Tzimeas9, 7 and A. Herra Sánchez de Movellán5 8 9 10 1 11 12 2 13 14 3 15 4 Independent consultant, Tirana, Albania. 16 5 REPSOL Exploración S.A., Madrid, Spain. 17 6 Energean Oil & Gas, Athens, Greece. 18 7 Departamento de Geodinámica, Granada University, Granada, Spain. 19 8 San Leon Energy Plc, Tirana, Albania. 20 9 Independent Exploration Geoscience Consultant, Athens, Greece. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, USA. On leave of absence from: Departamento de Geodinámica, Granada University, Avenida de Fuente Nueva s/n, 18071 Granada, Spain. Department of Structural, Historical & Applied Geology, University of Thessaloniki, Thessaloniki, Greece. 21 22 * Corresponding author: Juan I. Soto (juan.soto@beg.utexas.edu) 23 24 25 Key points • 26 27 and the interpretation of new seismic profiles • 28 29 30 The Ionian Zone in Greece and Albania is studied with surface geology, well information, Triassic salt pillows, isolated thick diapirs, and elongated salt walls condition the style of this Alpine fold and thrust belt • Salt tectonics provides new clues about Alpine orogenic evolution, opening new opportunities for geologic storage in the region 1 31 Abstract 32 The Ionian Zone (IZ) is one of the key elements of the external zones of the Albanian Hellenides 33 orogen and contains large outcrops of Triassic evaporites. The IZ consists of various thrust sheets 34 with a general westward vergence, stacking over the Apulian and Pre-Apulian zones, and 35 repeating a thick carbonate sequence of Late Triassic or Early Jurassic to Eocene age. Thrusting 36 becomes younger toward the west with a piggyback sequence, starting during the latest 37 Oligocene Epoch in the internal Ionian and ending in the Pliocene in the external Ionian. We have 38 studied the IZ in southern Albania and northwestern Greece using field observations and borehole 39 data and by fully interpreting a recently acquired 2D seismic data set. Our objectives are to 40 establish the geometry and nature of the contacts associated with the major Triassic outcrops, to 41 unravel precursor salt diapirs, and to assess their role during the Alpine contraction. Salt 42 structures include gentle salt pillows, isolated salt plugs and diapirs, thrust welds, and salt walls. 43 We show how these structures control the geometry and kinematics of the Alpine thrusts or the 44 location of recent strike-slip faults. Salt minibasins have also been identified, demonstrating salt 45 mobility conditioned Mesozoic sedimentation in the Ionian Basin. The use of salt-tectonics 46 principles to evaluate the structural style and evolution of the IZ fold and thrust belt also opens 47 new directions for interpreting the subsurface and could help to better define structures that can 48 be used as opportunities for geologic storage in the region. 49 50 Keywords: 51 Fold and thrust belts, salt tectonics, diapirs, seismic interpretation, Hellenides, Albanides. 52 53 1. Introduction 54 Within the Alpine belts developed in the eastern Mediterranean Sea, one of them is the fold and 55 thrust belt (FTB) that extends from the Alps and the Carpathian arc along the eastern Adriatic 56 coast to Greece and Crete, including the Dinarides, Albanides, and Hellenides. This orogenic belt 57 encircles the Aegean extensional basin and faces the Apennines, with a structural vergence 58 opposite that of the Apennines (Figure 1). The general structure of the Dinarides–Albanides– 2 59 Hellenides (DAH) consists of the imbrication of several terrains linked to the Adriatic plate, 60 which collided with several elements of the Eurasian plate during the Alpine orogeny, closing 61 and obducting the oceanic domains that separated these plates (Aubouin, 1959a; Aubouin et al., 62 1976; Aubouin et al., 1962; Bernoulli, 2001; Bernoulli & Laubscher, 1972; Cavazza et al., 2004; 63 Channell et al., 1979; Channell et al., 2022; Degnan & Robertson, 1998; Dercourt et al., 1993; 64 Handy et al., 2019; Kilias et al., 2016; Kilias et al., 2001; Mountrakis, 2006; Nieuwland et al., 65 2001; Papanikolaou, 2013; Papanikolaou, 2021; Papanikolaou, 1996-1997; Robertson & Shallo, 66 2000; Schmid et al., 2008; Schmid et al., 2020). 67 Specifically, in the present-day region of Albania and Greece, the configuration of the FTB is 68 conditioned by several unique features. First, the external domains of the Albanides-Hellenides 69 (AH) involved thick Mesozoic carbonate successions. Next, although the AH belt contains 70 obducted elements from several preorogenic oceans, its present-day orogenic front lies on a 71 domain of strongly-thinned continental crust, in the present-day Adriatic Sea (Hieke et al., 2003; 72 Kokinou et al., 2003; Makris et al., 2013). And finally, the curvature of the AH, south of Crete, is 73 produced by the superposition of active northward subduction of the African plate and 74 simultaneous high-magnitude extension of the AH forming the Aegean Sea (Brun et al., 2016; 75 Jolivet & Brun, 2010; Jolivet & Faccenna, 2000; Kilias et al., 2002; Mountrakis, 2006; van 76 Hinsbergen et al., 2006)). 77 Our study region is located precisely in the outer domains of the AH, between southern Albania 78 and northwestern Greece (in the Epirus region; Figure 2). In this particular sector, the zone we 79 have studied covers the entirety of one of the main elements of the outer zones of the orogen: the 80 Ionian Zone (IZ; (Philippson, 1898) or “adriatisch-ionische Zone” (Renz, 1925, 1955)). For our 81 purposes, the structural organization of this FTB consists of thin-skinned thrust systems directed 82 westward, whose configuration and nomenclature differ between those proposed in Albania and 83 those realized in Greece. 84 Another value in the study region is that there is an active exploration of the resources in these 85 countries for the discovery and extraction of hydrocarbons (included in Figure 2) (Arvanitis et al., 86 2020; Bega, 2013; Bega & Soto, 2017; Curi, 1993; David et al., 2014; Kamberis et al., 2022; 87 Maravelis et al., 2012; Mavromatidis, 2009; Moorkens & Döhler, 1994; Rigakis et al., 2007; 88 Sadekaj et al., 1992; Sejdini et al., 1994; Shehu & Johnston, 1991; Stournaras, 1985; Zelilidis et 3 89 al., 2015; Zelilidis et al., 2003; Zelilidis et al., 2016). In addition, we believe that a precise study 90 on the deep and shallow nature of the salt structures and the associated salt-tectonic processes 91 will be valuable for the new challenges of geologic storage (e.g., CO2 and Hydrogen) in these 92 countries (e.g., (Arvanitis et al., 2020; Hatziyannis et al., 2009; Stournaras, 1985)). 93 In the Epirus area, several previous studies have interpreted the existence of diapiric salt bodies, 94 nucleated in the evaporitic successions of the Late Triassic Epoch. This has been suggested in 95 some specific structures both in Albania and Greece (Aliaj, 1974; Berberi et al., 1990; Jardin et 96 al., 2011; Jenkins, 1972; Kamberis et al., 1996; Kamberis et al., 2013; Karakitsios, 1995; 97 Karakitsios & Rigakis, 2007; Nikolaou, 1986; Plaku & Murataj, 1974; Prifti et al., 2013; 98 Underhill, 1988; Velaj, 2001, 2015; Velaj et al., 1999; Zelilidis et al., 2016). However, we 99 believe that major questions remain to be answered completely in the area. For example, what 100 kind of observations can we make in such a FTB to confirm that diapiric structures previously 101 existed or, on the contrary, that the structural evolution of particular structures can be explained 102 uniquely by shortening (e.g, (Kamberis et al., 2022))? What is the internal architecture of the IZ? 103 Can we use a single frame of reference to explain the whole IZ in Albania and Greece? What role 104 did the Triassic evaporites play in the IZ during the Alpine shortening? What is the structure of 105 the evaporite bodies at surface and at depth? What preorogenic salt structures can we still 106 differentiate in this FTB and when did they form? 107 With these general tectonic objectives in mind, our work has consisted of the analysis of three 108 different types of data: (1) an extensive reconnaissance of the Triassic structures of the IZ in the 109 field; (2) seismic interpretation of a new set of unpublished two-dimensional (2D) seismic 110 profiles obtained in the NW onshore region of Greece, and; (3) a review of previous geophysical 111 data in the IZ, such as gravity, previous deep-seismic reflection profiles, and a comprehensive 112 borehole data set. Our main objective has been to characterize in detail the geometries of the FTB 113 and the role played by the Triassic evaporites, using the principles of salt tectonics (e.g., (Jackson 114 & Hudec, 2017)). For the overall interpretation of these observations, we will also use a set of 115 experimental models with various types of diapiric structures deformed under compression, and 116 data from previous studies in the area, such as the distribution of paleomagnetic rotations in the 117 IZ. 4 118 2. Tectonic Setting 119 The AH FTB consists of a set of terrains linked to the Adria plate, forming various metamorphic 120 units and two obducted nappes of ophiolites (Western and Eastern Vardar ophiolites), which 121 constitute the inner zones of the orogen (e.g., Pelagonian Massif in Greece and Albania) (Figure 122 2) (Kilias et al., 2016; Kilias et al., 2001; Meço et al., 2000; Papa, 1970; Papanikolaou, 2009; 123 Papanikolaou, 1996-1997; Plougarlis et al., 2021; Robertson & Shallo, 2000; Schmid et al., 2008; 124 Schmid et al., 2020; Smith & Moores, 1974). In the study area, during the Alpine orogeny, these 125 terrains were thrusted westward over a deepwater basin infilled by Paleogene–early Neogene 126 flysch series (Aubouin, 1959b; Papanikolaou, 2021; Robertson & Shallo, 2000; Skourlis & 127 Doutsos, 2003; Thiébault, 1982). These tectonic elements constitute the external zones of the AH 128 FTB and extend under different names from Greece (Pindos and Gavrovo–Tripolitza zones), 129 Albania (Krasta–Cukali and Kruja zones), to Montenegro, Croatia, and Bosnia (e.g., Dalmatian, 130 Bubdva, Beotian, Pre-Karst, and Bosnian Flysch zones) (for a recent and comprehensive revision 131 see (Schmid et al., 2008; Schmid et al., 2020)). 132 Underlying these flysch sequences of Oligocene to lower Miocene age is the IZ, formed by a 133 Mesozoic (to Eocene) carbonate sequence and large outcrops of Triassic evaporites (Figure 2). 134 The IZ is organized in three large thrust sheets, detached along (and possibly within) the Triassic 135 evaporites (Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Jenkins, 1972; Karakitsios & 136 Rigakis, 2007; Lykakis et al., 2021; Roure et al., 2004; Sadekaj et al., 1992; Sejdini et al., 1994; 137 Sotiropoulos et al., 2022; The_B.P._Co_Ltd., 1971; Underhill, 1988, 1989; Velaj, 2001; Velaj et 138 al., 1999; Zelilidis et al., 2015). 139 The orogenic front of the IZ consists of a thrust system with various salients and reentrants, and 140 some oblique structures that are locally known as the Vlora–Dibër (or Vlora–Elbasan; VDL), 141 Corfu–Picari (CPL), and Kefallinia–Lefkas–Lixourion (or Cephalonia fault; KLL) lineaments 142 (from north to south) (Bega & Soto, 2017; Kokinou et al., 2005; Kokinou et al., 2006; Mehillka 143 & Canaj, 1996; Picha, 2002; van Hinsbergen et al., 2006). Beneath this front, detached elements 144 of the Mesozoic Apulian carbonate platform are found, forming what is known locally as the Pre- 145 Apulian or Sazani–Paxi zones (Aliaj, 1987; Aubouin et al., 1962; Papa & Kondo, 1968; Prifti & 146 Uţă, 2012; Underhill, 1988, 1989). To the west, beneath much of the Adriatic Sea, the present- 147 day marine basin consists of a Miocene to present-day sedimentary section overlying the thick 5 148 Mesozoic carbonate sequence of the Apulian and the pre-Apulian platforms (Cazzini et al., 2015; 149 Fantoni & Franciosi, 2010; Fournillon et al., 2017; Kamberis et al., 1996; Sotiropoulos et al., 150 2022). Existing data show that the AH FTB orogenic front in the offshore region is formed by a 151 thrust stack with the IZ over the Pre-Apulian zone (Kamberis et al., 1996), and both lie above the 152 Apulian platform that dips eastward by flexure due to the load of the AH orogenic edifice 153 (Scrocca et al., 2022). Due to this process, a thick marine foreland basin originates, in which 154 post-Messinian sediments can reach thicknesses of more than 4 km in the northern part of the IZ, 155 in a domain known as the Albanian foredeep (Figure 2) (Aliaj, 2006; Nieuwland et al., 2001). 156 The tectonic situation of orogenic convergence extends to the present day. GPS data in the area 157 reflects this, showing that the continental region of the AH FTB is now moving south in northern 158 Greece and North Macedonia and WSW in western Greece and southern Albania, with velocities 159 of up to 26–32 and 4–5 mm/yr, respectively (see vectors in Figure 2) (Caporali et al., 2020; 160 Cocard et al., 1999; D’Agostino et al., 2008; D’Agostino et al., 2020; Pérouse et al., 2016; van 161 Hinsbergen et al., 2006). This curved motion of the AH FTB appears to accelerate south of the 162 KLL dextral transform (transpressional) fault zone, because further south the orogen is associated 163 with an active subduction of the oceanic (or strongly thinned continental) crust of the Ionian 164 Basin. Kinematic data from the Apulian and southern Italian domains, on the contrary, show a 165 movement against present-day Eurasia, directed NNE (4.2 mm/yr; (D’Agostino et al., 2008; 166 D’Agostino et al., 2020; Pérouse et al., 2016)). 167 The Alpine tectonic activity, which shaped the AH FTB, therefore extends to the present day, 168 with a complex orogenic front in which the northward movement of Apulia and the overthrusting 169 of the IZ over both an oceanic crust (Ionian Basin) and elements of the Mesozoic Apulian 170 carbonate platform must play a different role. For example, we also suggest that the position of 171 the Pre-Apulian Zone, with a thick accumulation of Mesozoic carbonates (e.g., (Fournillon et al., 172 2017; Kokinou et al., 2005; Kokinou et al., 2003; Papa & Kondo, 1968; Sotiropoulos et al., 173 2022)), acted as a tectonic buttress against the westward advance of the IZ (Bega, 2020; Bega & 174 Soto, 2017; Underhill, 1989). The role that the geometry of the eastern escarpment of the Apulian 175 platform may also have on the dynamics and configuration of the IZ at the FTB front is not the 176 subject of our study, but it is worth noting because of the development of complex salients and 177 embayments developed by this orogenic front (Argnani, 2013; Underhill, 1989). 6 178 3. General Structure of the Orogenic Front 179 The general configuration of the external zones of the AH FTB is illustrated in Figure 3 with two 180 crustal-scale cross sections. The cross section in Albania (Figure 3a) is located just north of the 181 surface termination of the IZ, also north of the Dumrë diapir, across the central Albanian 182 foredeep. This cross section merges surface geological observations from several authors together 183 with data from the offshore region (Bega, 2013; Cazzini et al., 2015; de Alteriis, 1995; Fantoni & 184 Franciosi, 2010; Kamberis et al., 2022; Moisiu & Gurabardhi, 2004; Scrocca et al., 2022; Xhomo 185 et al., 2002). The deep structure, showing the internal crustal structure and characteristics, has 186 been drawn by reinterpreting gravity models from previous studies (Frashëri et al., 2009). We 187 have also included the density and compressional-wave velocity (Vp) values established there for 188 the Triassic evaporites (Velaj, 2002; Velaj et al., 1999) (Table 1). Note, that we propose the 189 existence of a thin synrift to postrift succession of Triassic evaporites beneath the Mesozoic 190 carbonate succession of the Apulian platform. 191 The configuration of the AH FTB in the central region of the IZ in Greece is schematized in the 192 cross section of Figure 3b. In this section we have modified previous geological interpretations 193 from the Corfu region and the structure from the Pindos Zone to the Mesohellenic trough 194 (Doutsos et al., 2006; Ferrière et al., 2004; Monopolis & Bruneton, 1982; Waters, 1994), 195 incorporating our interpretations. The deep structure here consists of a moderate subduction of 196 the oceanic crust beneath the Apulian shelf, under a thinned continental crust in which a lower 197 crust is still preserved (with Vp= 6.9×103 m s-1) (Makris, 1985; Makris et al., 2013). The crustal 198 structure, its thickness, and the existence of two large Mesozoic carbonate units (Sazani–Paxi or 199 pre-Apuliaz zone versus IZ) separated by a thick evaporite sheet (with Vp= 4.2–4.3×103 m s-1) 200 have been taken according to seismic tomography data carried out by several authors recently 201 (Makris & Papoulia, 2018; Makris & Papoulia, 2019). Although it has not been sufficiently 202 documented, the thick Apulian Mesozoic carbonate platform must have a thin evaporite sequence 203 below it, as it has been suggested by some authors (Cazzini et al., 2015; Fantoni & Franciosi, 204 2010; Kamberis et al., 1996; Kokinou et al., 2005; Kokinou et al., 2003). Perhaps the evaporites 205 are only composed of anhydrite, as documented by some wells and subsurface data in the 206 Gargano area and surroundings in offshore Italy (e.g., Puglia–1 and Gargano–1D wells; 7 207 (Agostinetti et al., 2007; Bosellini & Morsilli, 2001; de'Dominicis & Mazzoldi, 1987; Improta et 208 al., 2000; Patacca et al., 2008; Santantonio et al., 2013). 209 Two aspects already known and of special interest for our study are also documented in both 210 sections. First, the important role played by the Triassic evaporites as a detachment level for thin- 211 skinned thrust systems in the IZ. This has been extensively documented in previous studies 212 (Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Jenkins, 1972; Kamberis et al., 2022; 213 Karakitsios & Rigakis, 2007; Lykakis et al., 2021; Maravelis et al., 2012; Roure et al., 2004; 214 Sadekaj et al., 1992; Sejdini et al., 1994; Sotiropoulos et al., 2022; The_B.P._Co_Ltd., 1971; 215 Underhill, 1988, 1989; Velaj, 2002; Velaj et al., 1999; Waters, 1994; Zelilidis et al., 2015). 216 Second, the existence of a thick body of evaporites in the central sector of the IZ (also 217 documented by well data; e.g., Delvinaki–1 and Parakalamos–1 wells in Figure 3b), which may 218 have deeper thrust sheets of the Mesozoic carbonate succession, and whose geometry, origin, and 219 influence on the orogeny will be explored in this study. 220 3.1. Main phases of deformation 221 In the external zones of the AH FTB, the age of shortening is well known according to the dating 222 of synorogenic sediments, which are flysch sediments filling large synclinal basins, but also 223 considering the deformations affecting the offshore foredeep (Figure 3). In Albania and Greece, 224 the age of flysch successions, syntectonic sediments affected by IZ thrusts, and age 225 determinations in foredeep wells describe a thrust system whose age is progressively younger 226 toward the front (to the west) and down the thrust stack. The age succession describes a 227 piggyback thrust system, with ages varying progressively from late Oligocene and Aquitanian– 228 Burdigalian in its innermost (eastern) part (Institut_Francais_du_Pétrole_Mission_Grèce, 1966; 229 Jenkins, 1972; Kamberis et al., 2000; Makrodimitras et al., 2010; Pieri, 1990; Roure et al., 2004; 230 Savoyat, 1977; Sejdini et al., 1994; Sotiropoulos et al., 2003; Speranza et al., 1992) to post- 231 Messinian (affecting evaporites of this age; see (Aliaj, 2006; Kokkalas et al., 2013; Kosmidou et 232 al., 2018; Lacombe et al., 2009; Papanikolaou, 2021; Schröder, 1986)) and Pliocene in the 233 outermost (western) thrusts (e.g., (Kamberis et al., 1996; Karakitsios et al., 2017; Kokkalas et al., 234 2013; Nieuwland et al., 2001; Speranza et al., 1995; Underhill, 1988, 1989)). 235 The geometry and age of the foredeep infill also describe that the frontal thrusts are upper 236 Miocene to Pliocene in age, in some cases affecting the Messinian evaporites and unconformity 8 237 in the offshore region (Aliaj, 2006; Bega, 2020; Gjika et al., 2001; Karakitsios et al., 2017). Some 238 studies suggest the occurrence of three punctuated shortening episodes; one in the lower to 239 middle Miocene (Tranos et al., 2020), a second in the Pliocene, and the latest during the 240 Pleistocene; (Kamberis et al., 2022; Sorel et al., 1992)), although we do not have the data to 241 better refine this suggestion. 242 Deformation in the frontal domain of the AH FTB extends to the present day, as evidenced by 243 paleomagnetic rotations measured in Pliocene-to-Recent sediments, GPS-derived kinematic 244 vectors (Figure 2), and the seismic activity linked to some of the frontal thrusts (in the vicinity of 245 the coastal region from Albania to Greece; e.g., (Caporali et al., 2020; D’Agostino et al., 2020; 246 Kokkalas et al., 2013; Louvari et al., 2001; Matraku et al., 2023; Schmitz et al., 2020; Valkaniotis 247 et al., 2022)) and to the strike-slip faults affecting the orogenic front (Figure 2). The two main 248 seismically active strike-slip faults in this region are the Kefallinia–Lefkas–Lixourion dextral 249 transpressional fault zone in Greece (Bonatis et al., 2021; Kokinou et al., 2005; Kokinou et al., 250 2006; van Hinsbergen et al., 2005) and in Albania, the dextral strike-slip fault zone associated 251 with the Shkodra–Tropoja (or Shkodra–Peje) Lineament and its offshore continuation with the 252 Lezha (or Lezhe) fault (Aliaj, 1999; Aliaj et al., 2004; Caporali et al., 2020; Mehillka & Canaj, 253 1996). 254 4. Stratigraphy of the Ionian Zone 255 The stratigraphic column of the IZ was established since the first surveys on this domain of the 256 AH FTB. The first detailed biostratigraphic and lithostratigraphic study was carried out in the 257 region of Greece by (Renz, 1913, 1955), to which we must add the studies of (Nowack, 1924, 258 1926) in Albania. We have synthesized in Figure 4 our observations in the study region in 259 Albania and Greece, also using information from multiple authors (Aubouin, 1958; Aubouin et 260 al., 1977; Avramidis et al., 2002; Bellas, 1997; Bourli et al., 2019; Dalipi et al., 1972; 261 Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Jenkins, 1972; Kamberis et al., 2022; 262 Karakitsios, 1995, 2013; Karakitsios & Rigakis, 2007; Meço et al., 2000; Papanikolaou, 2021; 263 Renz, 1913, 1925, 1955; Savoyat, 1977; The_B.P._Co_Ltd., 1971; Triantaphyllou, 2013; 264 Zelilidis et al., 2015; Zelilidis et al., 2003). Along with the synthetic column, lithological 265 variations, thicknesses and chronostratigraphic information, we have also included other useful 266 data, such as the position and characteristics (porosity and total organic [TOC] content) of the 9 267 different elements of the petroleum system of the IZ as well as the position of the structural 268 detachment horizons. 269 Three major lithostratigraphic sequences are present. First, the Upper Triassic evaporites, 270 possibly Carnian–Norian in age (Renz, 1925). Then, a thick carbonate succession that extends 271 from the uppermost Triassic (Norian–Rhaetian) Series to the upper Eocene (Priabonian) Series. 272 And finally, separated by a regional discontinuity, a flysch sequence of upper Eocene and 273 Oligocene to lower Miocene age (Aquitanian and Burdigalian). Several continental sediments 274 from the Burdigalian to the present erosively cover the previous sequences. 275 The Mesozoic carbonate sequence consists of the following lithostratigraphic formations (for 276 which we will use the terminology established by multiple authors in northern Greece). A thick 277 package of dolomites (“Obertriadischer Dolomit” or “Hauptdolomit” of Upper Triassic, Norian- 278 Rhaetian to Hettangian age; (Renz, 1955)), is followed by the dolomitic limestones of the 279 Pantokrator Formation (“Pantokratorkalk”; Sinemurian) and the massive limestone series of the 280 Siniais Formation (Pliensbachian; (Flügel, 1983)). In the lower part of the Pantokrator Limestone 281 (or Formation), thin levels of organic-rich black shales (up to 39% TOC content) have also been 282 described (Renz, 1955). The Middle Jurassic succession (Toarcian to Kimmeridgian) shows 283 important variations in thickness and sedimentary facies (Aubouin, 1958; Bernoulli & Renz, 284 1970; Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Karakitsios, 1995; Nikolaou, 1988; 285 Renz, 1955; Savoyat, 1977). The following members can be distinguished: an “ammonitico 286 rosso” marly limestone member and the marly limestones of the Lower Posidonia mudstones 287 (Toarcian); limestones with filaments (“Calcaires à filaments;” Bajocian) and another package of 288 marls and marly limestones of the Upper Posidonia mudstones (Kimmeridgian). Both marl 289 intervals have high organic contents (up to 3.5% TOC) and constitute regionally the main source 290 rocks of the IZ petroleum system. Overlying these discontinuous successions and with variable 291 thickness and stratigraphic facies is a limestone and white marly limestone package of variable 292 thickness (up to ~800 m) that is the Vigla Limestone (Tithonian to Cenomanian; (Danelian et al., 293 2004)). Above this formation are two limestone members of approximately uniform thickness, 294 which are the Senonian (~150 m) and Eocene (~800–900 m) limestones, whose ages are 295 Coniacian to Maastrichtian and probable Paleocene to upper Eocene (Priabonian), respectively. 296 Some phosphate- and chert-rich intervals have been found in south Albania and north Greece in 10 297 the transition from the Vigla to the Senonian limestones (Serjani, 1991). Studies conducted by 298 this team and previously by (Karakitsios, 2013; Karakitsios & Rigakis, 2007; Makri et al., 2023), 299 supported by basin thermal models and subsurface data, show that the middle to upper part of the 300 Eocene limestone, where the marly intercalations are more abundant, constitute a regional 301 reservoir sequence in the IZ. 302 The carbonate succession will be abbreviated as Mesozoic in age, although it extends from the 303 Lower Jurassic (Hettangian) to the upper Eocene (Priabonian). Within this formation, it should be 304 noted that all the marls and marly limestone sequences of Middle Jurassic age, constitute a 305 horizon of special interest for our study, both for its lithological and sedimentary facies variations 306 and for constituting an efficient structural detachment horizon. 307 Above this sequence, and separated by a regional discontinuity, are the detritic successions of the 308 flysch units (Aubouin et al., 1977). In this group, three stratigraphic formations can be 309 distinguished. The lower one is the thin level, not always observable, of the “base flysch” 310 formation (upper Eocene in age; Priabonian). Above this level, two flysch series are easily 311 recognized in the field due to their lithological differences, sedimentary facies, and colors, such 312 as the Ayii Pantes (Oligocene) and Upper Flysch formations (late Oligocene to Aquitanian and 313 Burdigalian). 314 Of special interest for our study are the Triassic evaporites. We dedicate a separate section to 315 them, differentiating the observations that can be made at the surface from those derived from 316 subsurface data. We believe that we present for the first time, a novel and extensive set of well 317 data, gathering information from multiple wells in Albania and Greece, bringing together 318 information that is generally not easily accessible to the scientific community. 319 4.1. Characteristics of Triassic evaporites 320 Several authors have analyzed the age of the evaporite sequence in the IZ, and we can assign it to 321 the Upper Triassic (Carnian–Norian) Epoch (Dragastan et al., 1985; Jenkins, 1972; Muhameti & 322 Pejo, 1974; Pomoni-Papaioannou & Tsaila-Monopolis, 1983; Velaj, 2001)), which is in 323 agreement with the age of the Keuper evaporites in much of the Mediterranean Alpine belts (see 324 summary in (Soto et al., 2017)). However, some authors attribute a pre-Carnian (Bornovas, 325 1960), a Lower–Middle Triassic (Pomoni-Papaioannou & Tsaila-Monopolis, 1983) or a 11 326 Permian(?)–Triassic lato sensu age to the evaporites (e.g., (Meço et al., 2000; Moorkens & 327 Döhler, 1994; Papanikolaou, 2021)). 328 Triassic evaporites at the surface are usually covered by a thick layer of dolomitic breccias 329 (Figure 5b), being poorly exposed due to weathering and the occurrence of soil and vegetation 330 cover (“obertriadischer Dolomit” or “Hauptdolomit”; (Renz, 1955)). It is interpreted that these 331 breccias were formed immediately after deposition, in an arid evaporitic environment, also 332 having early diagenetic dissolution processes (Getsos et al., 2004; Karakitsios & Pomoni- 333 Papaioannou, 1998). However, it is possible to find some continuous sections in which a thick 334 section of reddish to green or grayish marls and clays grades upward to a gypsum-rich series 335 below the breccias (Figure 5a). Levels of black oleaginous clays (which may have up to 17% 336 TOC) may also be encountered toward the upper part of the evaporite sequence and within the 337 lower part of the dolomites (Bornovas, 1960; Karakitsios & Rigakis, 1996; Renz, 1955). The red 338 clay and marl sequences usually have abundant intercalations of alabastrine gypsum and thin 339 levels of black dolomites (Pomoni-Papaioannou & Tsaila-Monopolis, 1983), with local lenses of 340 sub-volcanic rocks (Dalipi et al., 1972). Some authors have interpreted that these gypsum layers, 341 with massive and bedded micro-fabrics, are transformed and recrystallized during burial to 342 anhydrite (Bornovas, 1960; Pomoni-Papaioannou & Tsaila-Monopolis, 1983). 343 In the outcropping evaporites, especially toward the base, stratified packages of black limestones 344 with Foustapidima (Alexandridis et al., 2022; Bourli et al., 2022; Renz, 1955) and with Cardita 345 guembeli (Dragastan et al., 1985), known regionally as “Cardita limestone” (“karnischen 346 Fustapidimakalk” and “Carditakalke”; (Jacobshagen, 1986; Renz, 1955)) and which have been 347 dated as Upper Triassic (Carnian–Norian) in age, are also occasionally found. 348 Recognizable structures in the evaporite successions comprise cemented polymictic breccias with 349 abundant centimeter and millimeter clasts of variable lithologies, such as gray limestones, 350 gypsum, dolomites, and marls (inset in Figure 5a). In the evaporite levels it is common to observe 351 structures showing ductile flow, such as asymmetric and sheath folds (Figure 5c–d) or ductile 352 shear zones with gypsum and anhydrite porphyroclasts (Figure 5g–h). When the limestone and 353 dolomitic layers are intercalated within the gypsum-rich sequences, they usually develop boudins 354 and discontinuous clasts, evidencing their fracturing due to layer-parallel extension, while the 355 evaporites show ductile behavior. 12 356 4.2. Borehole information on Triassic evaporites 357 Information of the Triassic evaporites is more complete if borehole data are used (Figure 6). In 358 this figure we have assembled a set of wells that especially document the lithologic and thickness 359 variation of Triassic successions in the IZ in Albania (Figure 6a) and Greece (Figure 6b–c). The 360 thicker evaporite sequences occur in the Filiates–1 (~3.6 km) and Butrinti–1 (>3.9 km) wells. The 361 successions in the more complete wells generally have two sections: an upper one formed by 362 anhydrite and gypsum with various levels of dolomites and clays or marls, and a lower one rich 363 in halite with centimetric to metric intercalations (rarely more than 150–200 m) of anhydrite and 364 gypsum. According to well data, the two sequences have different average densities, varying 365 between 2400 and 2900 kg×m-3 between the gypsum and anhydrite-rich and the halite-rich 366 sequences, respectively (Table 1). Between the two sections (anhydrite- and gypsum-rich above 367 and halite-rich below; also reported by (Pomoni-Papaioannou et al., 2004)) there is always a 368 gradational boundary, possibly due to internal shearing within the evaporites (Velaj et al., 1999). 369 Some wells also show thin intervals of other evaporites, like potassic salts, probably rich in 370 sylvite and carnallite (e.g., Astakos–1 well). 371 As we will document later, the large thickness variations (kilometers) in the evaporite 372 successions are not only stratigraphic but are also due to the existence of precursor diapiric 373 structures. 374 Taking the wells along a W–E transect in northern Greece (from Butrinti–1 and Filiates–1 to 375 Parakalamos–1 and Demetra–1; Figure 6a–b), it can be deduced that halite is more abundant 376 toward the west, whereas the evaporite sequences are richer in anhydrite and gypsum toward the 377 east. This agrees with the estimated average densities in these wells, decreasing westward (richer 378 in halite) from Parakalamos–1 to Delvinaki–1 and Filiates–1 (2950 to 2400 kg×m-3, respectively; 379 Table 1). Likewise, and using information from wells in Albania, a N–S variation in the Triassic 380 evaporites, being progressively richer in anhydrite toward the south (with average values 381 increasing toward the south from 2490 to 2590 kg×m-3; Table 1). 382 In our interpretation, these variations probably reflect a lithological variation of the evaporitic 383 sequences in the Upper Triassic IZ basin, being richer in halite toward the West and toward the 384 center, in a region that lies between Albania and Greece. This idea was also pointed out in the 385 region by some previous authors (Pieri, 1990; Savoyat, 1977; Veizaj & Polo, 1999). 13 386 5. Data and Methods 387 Our study has employed three types of data and methods. On the one hand, we have carried out 388 field observations, reinterpreting the salt structures according to modern concepts of salt tectonics 389 (Jackson & Hudec, 2017). In this way we have revised the cartographic information in the area, 390 including new observations and proceeding to unify with our own criteria the geological maps of 391 Greece and Albania, since they use different chronostratigraphic criteria for the IZ. 392 We have also used all the seismic lines available in the region, from those coming from previous 393 campaigns, which have been converted into digital format, to the new seismic lines acquired in 394 2018–2019 by Repsol and Energean in the continental region of northern Greece. The 395 distribution of these seismic lines is shown in Figure 7. The 2D seismic lines have been depth 396 processed by both companies and we have had access to the prestack depth migration (PSDM) 397 profiles in digital format. The depth conversion has been carried out using the average seismic 398 velocities (Vp) of the main lithostratigraphic successions of the IZ (Table 1). 399 The average density and Vp values have been obtained by averaging the sonic data measured in 400 several of the wells in the region (Ágios Georgios–3, Demetra–1, Filiates–1, Lavdani–1 and 401 Parakalamos–1, see situation in Figure 7), according to the study of (Olaya et al., 2020). The 402 average values obtained in the different stratigraphic successions of the IZ are shown in Table 1. 403 The seismic velocities of the Triassic evaporites that we have thus established range from 5.0 to 404 5.8×103 m s-1 (Table 1). These average velocities are higher than those of halite or rock salt (4.2– 405 4.75×103 m s-1), and also turn out to be higher than those estimated for the Triassic evaporites by 406 seismic tomography in the region of Greece (4.2–4.3×103 m s-1) or those deduced from boreholes 407 in Albania (4.5–5.0×103 m s-1) (Makris & Papoulia, 2014, 2018; Makris & Papoulia, 2019; Velaj, 408 2002; Velaj et al., 1999). Our interpretation is that the estimated mean Vp demonstrates that the 409 evaporite succession has in addition to halite, other higher velocity lithologies such as dolomites, 410 gypsum, and anhydrite (Jones & Davison, 2014). For example, amounts around 50% of gypsum 411 and anhydrite in a saline evaporite can cause the average velocity to be between 4.9 and 5.5×103 412 m s-1 (e.g., (Cornelius & Castagna, 2018)). The abundance of dolomitic layers (mean Vp of 413 6.3×103 m s-1; (Jones & Davison, 2014)) would further elevate the mean velocity of Triassic 414 evaporites in the study area. 14 415 416 417 Table 1. Characteristics of the different seismic units of the IZ. Average densities (ρ) and compressionalwave velocities (Vp) used for seismic processing are according to this study and (Olaya et al., 2020). Detailed information of the different stratigraphic units is shown in Figure 4. 418 Unit Lithology Vp (m s-1) a ρ (kg×m-3) a Lower Miocene flysch mudstones, sandstones 2,500–2,800 – Oligocene flysch mudstones, sandstones 2,500–4,500 2,500–2,600f Eocene limestone Cretaceous limestones Jurassic sequences Triassic evaporites limestones b 4,700 limestones, cherty limestones 5,300–6,000 limestonesc with mudstone intervalsd gypsum, halite, anhydrite, and brecciated dolostones at the top 5,000–6,700 5,000–5,800(5) 2,600–2,700g 2,400–2,900h Notes: a. Average values estimated according to well-log properties. b. Vigla and Senonian limestones. c. Pantokrator and Siniais limestones. d. Lower and Upper Posidonia mudstones. e. Seismic refraction studies in Greece suggest variable sonic velocities for the evaporites, ranging from 4,200– 4,300 m s-1 (Makris & Papoulia, 2014, 2018; Makris & Papoulia, 2019) to 5,000-5,500 m s-1 (Kokinou et al., 2003), while in Albania average sonic velocities of 4,500–5,000 m s-1 are commonly used (Velaj, 2002; Velaj et al., 1999). f. In Albania, a progressive increase in the density of the flysch sequences from 2,470 km×m-3 at the surface and up to 2,650 kg×m-3 at 3 km depth has been measured in some wells (Veizaj & Polo, 1999). g. The average density estimated with wells in Albania is quite uniform and does not show variations with depth. The mean value is 2,670 kg×m-3 (Veizaj & Polo, 1999). Gravity modeling studies in Greece also suggest an average density of 2,600 kg×m-3 for the carbonate sequence (Makris et al., 2013). h. Densities are quite variable in the Triassic evaporites, ranging between 2,400 and 2,900–3,000 kg×m-3 in gypsum- or anhydrite-rich sequences, respectively. Other studies in Albania document variations between 2,490 to 2,590 kg×m-3, suggesting an average value of 2,550 kg×m-3 (Veizaj & Polo, 1999). 419 420 The deep seismic processing has followed a workflow that started with near-surface modeling 421 (NSM) using first-break arrival times including long offsets (detailed information on the depth 422 migration is in (Olaya et al., 2020)). Diving-wave tomography provided a model that was 423 constrained according to surface geology and available well velocities. Flooding models were 424 later created based on the deepest NSM velocities as permitted by the maximum depth 425 penetration. This model enabled us to image a deeper top of evaporites and carbonates sections. 426 Images corresponding to the flood models were used to provide interpretation for guiding deeper 427 model building in an iterative workflow for KDP–RTM migration, which incorporated the 428 measured dips at the surface and the geological models for the sections. 429 The iterative process allowed us to improve the seismic image, finding that the Triassic evaporite 430 bodies show a strong lateral variation of the sonic velocities (Vp ranging from 5.0 to 5.8–6.0×103 431 m s-1). Tops and bases of evaporites are usually difficult to image, especially because they do not 432 exhibit a strong variation of acoustic impedance with respect to the Jurassic or Cretaceous 433 carbonates (which exhibit Vp from 5.0–6.0×103 m s-1 and locally 6.7×103 m s-1). Another 15 434 problem intrinsic to our seismic images comes from the high-dip areas in some structures and, 435 possibly, the complex internal structure of the evaporite bodies. In the thicker bodies of 436 evaporites, we have identified discontinuous strong reflections with variable dip, which might 437 correspond to intrasalt layering due to other lithologies within the Triassic evaporites. On the 438 contrary, when evaporites are in contact with the underlying flysch units, there is always a strong 439 reflection associated with the boundary, which can be either caused by a strong acoustic 440 impedance between evaporites and the flysch sequences or the occurrence of highly strained 441 fabric associated with thrust surfaces. 442 The seismic interpretation has been carried out based on the information provided by the wells 443 (detailed in Figure 7), of which we have presented a selection in Figure 4. Due to confidentiality 444 issues, we cannot show the seismic image so we have chosen to present line drawings of the 445 seismic profiles. The seismic interpretations have been correlated among the 2D seismic lines, 446 making the structures coherent and compatible at all line intersections. 447 Finally, a series of physical models of shortened salt structures are used to compare with 448 structures we see in the IZ, and postulate on their origins. A detailed description of the physical 449 modeling, the materials used, the experimental setup, and how the data was acquired is presented 450 in the Appendix. 451 6. Observations and Results: Salt Structures in the Ionian Zone 452 6.1. General structure of the Ionian Zone 453 The general structure of the IZ, and its relations with other domains in the external zones of the 454 AH FTB (Kruja and Gavrovo–Tripolitza, Krasta–Cukali and Pindos, and Parnassos–Giona zones) 455 are shown in the tectonic map in Figure 7. This map integrates our observations in a detailed 456 framework that brings together multiple cartographic sources, unifying the different geological 457 criteria used in Albania and Greece (Bornovas & Rondogianni-Tsiambaou, 1983; 458 Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Kamberis et al., 2000; Kamberis et al., 2013; 459 Onuzi & Ymeri, 2020; Sejdini et al., 1994; Velaj, 2001; Xhomo et al., 2002; Zelilidis et al., 460 2015). This map has greater detail in the onshore region, but we have also integrated the available 461 information in the marine region (Bell et al., 2008; Brooks & Ferentinos, 1984; Cushing, 1985; 462 Fournillon et al., 2017; Kokinou et al., 2005; Kokinou et al., 2006) to have a complete overview 16 463 of the AH FTB orogenic front and of the relationships between the IZ with the Sazani–Paxi (Pre- 464 Apulian) zone, the Apulian platform and the Ionian oceanic basin (Argnani et al., 1993; Argnani 465 et al., 2009; Fantoni & Franciosi, 2010; Finetti & Del Ben, 2005). The structure of the western 466 Hellenic accretionary wedge (or the Mediterranean Ridge) is simplified according to (Polonia et 467 al., 2011). The main active and recent faults are taken from various sources (Brooks et al., 1988; 468 Kassaras et al., 2020; Lekkas et al., 1997; Ntokos, 2018; Roberts & Jackson, 1991; Tranos et al., 469 2020). 470 In this tectonic map (Figure 7), we have highlighted the Triassic evaporite bodies and the 471 surrounding structures, as well as the distribution of the oil and gas shows (and fields) (Bega, 472 2013; Bega & Soto, 2017; Curi, 1993; David et al., 2014; Mavromatidis, 2009; Moorkens & 473 Döhler, 1994; Rigakis et al., 2007; Sejdini et al., 1994; Zelilidis et al., 2015; Zelilidis et al., 474 2003). Finally, the map also includes the positions of the seismic lines and wells we have used in 475 our study. 476 Thrusting within the IZ produces kilometric tectonic repetitions of the evaporites (Figures 3, 7–8) 477 (Kamberis et al., 2022; Sadekaj et al., 1992; Sotiropoulos et al., 2022; Underhill, 1989; Velaj, 478 2001, 2015) and their ductile expulsion and flow feeding thick frontal plugs, like for example in 479 the Dumrë diapir (Figure 7) (Bega & Soto, 2017; Velaj et al., 1999). Our interpretation of the 480 general structure of the IZ is given in the two regional cross sections in Figure 8 and is based on 481 historical (from the 60’s to the 90’s) and recent borehole information and the interpretation of 482 two large seismic line transects oriented parallel to the main shortening direction (NE–SW). The 483 seismic transects are built up by joining some of the recently acquired 2D seismic profiles in 484 northern Greece and available seismic profiles in southern Albania. The two sections are 485 constrained by surface observations, such as dip measurements included along the topographic 486 profile. 487 The names given in Greece and Albania to these thrust sheets vary, but we propose the following 488 correlation from innermost (east and higher) to outermost (west and deeper): Berati or internal 489 Ionian, Kurveleshi or middle Ionian, and Ҫika or external Ionian 490 (Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Karakitsios & Rigakis, 2007; Papanikolaou, 491 2021; The_B.P._Co_Ltd., 1971; Zelilidis et al., 2015; Zelilidis et al., 2003). In Albania, another 492 thrust sheet, the Sqepuri thrust sheet (Figure 8a), has been locally differentiated (Bare et al., 17 493 2001; Meço et al., 2000; Roure et al., 2004; Sadekaj et al., 1992; Sejdini et al., 1994; Velaj, 2001; 494 Velaj et al., 1999; Xhomo et al., 2002). Due to its position and deep structure, we interpret the 495 Sqepuri as an imbricate of the Kurveleshi (middle Ionian) sheet. Stratigraphic variations in the 496 Mesozoic succession between these three thrust sheets have been described in previous studies of 497 the IZ (Aliaj, 1974; Aubouin, 1958, 1959b; Bega & Soto, 2017; Bernoulli & Renz, 1970; 498 Institut_Francais_du_Pétrole_Mission_Grèce, 1966; Karakitsios, 1995; Nikolaou, 1988; Plaku & 499 Murataj, 1974; Prifti et al., 2013; Renz, 1955; The_B.P._Co_Ltd., 1971; Velaj, 2002; Velaj et al., 500 1999). However, based on our field observations, we believe that these variations, rather than 501 sedimentary facies, are thickness variations toward previous diapiric structures. However, more- 502 detailed field work is needed to resolve this question. 503 Larger accumulations of evaporites also are present. We interpret these to be caused by precursor 504 diapirs where the wells show kilometer thicknesses of the evaporite succession (especially the 505 wells Filiates–1 and Butrinti–1; Figure 6). The shapes of these bodies, their structures, and the 506 relationships with the nearby sequences allow us to differentiate several major types of salt 507 diapiric structures in the IZ. The general structure of some of these is documented by more- 508 detailed mapping in Figures 9 through 11 and in selected field observations (Figure 12). 509 The four types of salt diapiric structures in the IZ are as follows: 510 1. Subcircular- to elliptical-plan diapirs, which appear to be shortened and tectonically 511 translated salt plugs (SP), such as the Dumrë (DSP) (Bega & Soto, 2017; Jardin et al., 512 2011; Velaj et al., 1999) and Filiates (FSP) (Figure 10) structures. 513 2. Salt walls (SW) of pluri-kilometer length and of preferential orientation subperpendicular 514 to shortening, such as those of Butrinti–Xarra (BSW) (Figures 10 and 11a–b), Delvinaki– 515 Parakalamos (DSW) (Figure 9), and Fyteies–Trífos (FSW) (Figure 11). 516 517 518 3. Gentle unrooted salt pillows (P), which are detached and displaced by thrusts, such as the Delvinë pillow (DP) (Bega & Soto, 2017). 4. Pinched-off diapirs, which we also found. These exhibit reverse faults cutting the thrust- 519 related anticline and the secondary thrust welds. Anticlines of this type with associated 520 thrust welds (WA) have been identified in Vasiliko (VWA) (Figures 9 and 12c–d), 521 Navarica (NWA) (Figures 9 and 12i–j), the Kassidhiáris Mountains, near the 18 522 Parakalamos–1 well (KWA) (Figures 9 and 12 g–h) and the shortcut thrust seen in Parga– 523 Sarakiniko (PWA) (Figure 10), 524 Our description of these structures will bring together field and cartographic observations 525 (Figures 9–11 and 12, respectively), with the interpretation of selected seismic profiles (Figures 526 13 and 14), in whose interpretation we have integrated a revision of the chronostratigraphic 527 information provided by the wells (Figure 6). 528 6.2. The Butrinti–Xarra salt wall and other salt structures in South Albania 529 On the border between Albania and Greece, an elongate and narrow (20×3–5 km) salt structure 530 extends from the Greek coast near Sayiadha to Lake Butrint, just north of Vrina (or Vrinë) in 531 Albania. The Butrinti–1 well shows that this salt structure is at least 4 km thick and that the 532 evaporites lack a Mesozoic or Neogene overburden (Figure 6a). Our review of the previous 533 mapping of the area is shown in Figures 9 and 10, and we interpret this structure to correspond 534 with an elongate NW–SE salt wall, which we have termed the Butrinti–Xarra salt wall (BSW). 535 Several observations support this interpretation: 536 1. The eastern flank of the wall is formed by an anticline parallel to the salt wall and the 537 Mesozoic successions (the Vigla and the Senonian limestones; Upper Jurassic to Upper 538 Cretaceous) dip westward against the salt structure. 539 540 541 2. A similar fold is found on the western flank of the salt wall and local thrusts are developed over the evaporites (Figure 12a–b). 3. Successions are progressively younger to the SE on the eastern flank of the wall, with 542 obliquity relations to the salt structure (with the Eocene limestones coming to rest on the 543 salt wall between Konispol and Sayiadha). 544 4. To the north, the wall culmination deepens northward and north–south folds affect only 545 the Upper Triassic dolomitic succession and the Pantokrator Formation (Hettangian to 546 Sinemurian). 547 With these observations, we interpret that the salt wall is shortened in the NE–SW direction, 548 allowing the approach of the two flanks that are deformed with anticlines against the wall, while 549 the salt wall is shortened and probably upbuilds, preventing the deposition of Paleogene or 550 Neogene sediments on top. In addition, according to observations around Sayiadha, we propose 19 551 the wall is resolved toward the SE by a subvertical weld that is partially cut by a thrust sheet to 552 the SW (Figure 10). The connections with the Filiates structure are not visible at the surface, 553 because the whole area is covered by very recent sediments (the Kalamas River delta plain; 554 (Chabrol et al., 2012)). We believe that the BSW is connected to the Filiates structure through a 555 narrow area, where the evaporites are squeezed, forming a weld zone that is also cut by a SW- 556 directed thrust (near Smertos) (Figure 10). 557 According to our seismic interpretation (Figures 13g, 14g), the BSW is displaced to the SW by a 558 basal thrust that lies at about 5.5 km depth beneath the Butrinti–1 well [identified at 2 s two-way 559 travel time (TWT) and using a mean Vp of 5.5×103 m s-1], with evaporites probably detached on 560 the lower Miocene flysch. Beneath the BSW, we identify another IZ sheet, which may have 561 deeper evaporites in a para-autochthonous position (at about 11–12 km depth). The western flank 562 of the BSW corresponds with a listric thrust, detached above the evaporites, in which a strong 563 succession of Upper Triassic dolomites and the Pantokrator Formation, describes a hanging wall 564 ramp of a NE-directed backthrust over the salt wall. 565 Two other salt structures have been studied in southern Albania. In a previous work, we 566 described the Delvinë salt structure (Bega & Soto, 2017) (Figure 9). Because of its geometry, as a 567 gentle culmination of the Triassic evaporites with progressive onlaps of the Upper Triassic 568 dolomites and Pantokrator formations on the two flanks, we consider that this structure 569 corresponds with a salt pillow (the DSP). According to these observations, we infer that the salt 570 pillow emerged in the Mesozoic basin of the IZ during Upper Triassic to Middle Jurassic time. 571 The other structure we have studied in Albania is the Navarica (or Navaricë) structure in the 572 Livinë Mountains (Figures 9 and 12i–j). Here, Triassic evaporites outcrop as gypsum-rich black 573 clays and dolomites at the core of a thrust-cut anticline. Above the evaporites, we find folded 574 onlaps in the Upper Triassic successions and in those of the Pantokrator Formation. We interpret 575 the general geometry as corresponding to a decapitated diapir, with the central conduit still open 576 but not completely welded. It would therefore be an example of a diapiric anticline cut by a thrust 577 weld. 578 6.3. The Delvinaki–Parakalamos salt wall and the Vasiliko pinched-off anticline 20 579 The first major structure we describe in Greece includes two large Triassic outcrops in Delvinaki 580 and Parakalamos (DSW). The overall structure of the salt walls has been reinterpreted, including 581 our observations as shown in Figure 9. Both bodies are linked by a narrow corridor of evaporites 582 just south of Delvinaki. The Delvinaki structure elongates NW–SE until it disappears 583 progressively to the N, where the Vigla Formation is directly in contact with the evaporites. The 584 entire south contact is a low-angle thrust where evaporites overthrust the Aquitanian–Burdigalian 585 flysch. Discontinuous and thin lenses of Cretaceous–Eocene and even Oligocene flysch 586 formations can be found along this contact, almost always in an inverted position and with a thin 587 thickness. On the NE flank of the Delvina structure, oblique W–E folds and progressive onlap 588 relationships to the NW are identified. The Delvinaki–1 well also shows that the evaporites 589 overthrust the Oligocene flysch succession, probably in normal polarity (Figure 6b). 590 The other evaporite body, around Parakalamos, is bounded to the north by W–E-oriented, left- 591 lateral strike-slip faults, while on its eastern edge there are NW–SE folds that run obliquely to the 592 evaporites. The western edge corresponds with a thrust toward the east, which progressively 593 becomes to the south into a subvertical fault zone (or weld), where Cretaceous successions (Vigla 594 and Senonian limestones) collide with opposing thrusts. Continental sediments of Messinian age 595 seal the structure here. The Parakalamos–1 well did not reach the base of the evaporites but 596 shows that this succession is more than 1.5 km thick (Figure 6b). 597 We interpret both outcrops to correspond with a complex salt wall, the DSW, perhaps composed 598 of two close and joined diapirs in plan view and finally thrusting SW over the lower Miocene 599 flysch. Note that between the two diapirs, when the orientation of the salt wall is west–east, and 600 forms a small angle with the shortening direction (NE–SW), a major strike-slip fault system is 601 formed along the salt wall. 602 In this same sector, just west of the Parakalamos wall, in the Kassidhiáris Mountains, we interpret 603 the structure to describe a tight anticline with subvertical flanks, cut by double-vergent, high- 604 angle thrusts (Figures 9 and 12g,h). Surface observations, together with information from wells in 605 the area (Lavdani wells 1 through 3 and Parakalamos–1), allow us to interpret this structure as 606 corresponding to a tight box-like anticline [a detached lift-off fold; (Mitra, 2003)], whose core 607 has a vertical weld, connected with the source layer of evaporites, which do not outcrop (see inset 608 in Figure 12h). 21 609 The deep structure of the DSW is illustrated by two profiles perpendicular to the structure 610 (Figures 13e,f and 14e,f). We interpret this salt wall as part of an allochthonous sheet that moves 611 SW over a tight syncline affecting only the flysch successions so that the basal thrust of the 612 evaporites eventually cuts a thin reverse flank, as attested by data from the Delvinaki–1 well. Our 613 review of the stratigraphy of this well suggests that levels that would be lower Miocene are 614 already beneath the Oligocene flysch. On the other hand, our seismic interpretation suggests that 615 the DSW is compressed and that outside this structure, the evaporites become a constant layer 616 with local highs formed by squeezed anticlines. Below this thrust sheet, which corresponds to the 617 inner IZ (or Berati), we interpret that there is another thrust sheet detached over the Triassic 618 evaporites, with internal imbrications and some backthrust structures (Figure 14f). This lower 619 sheet would therefore be the middle IZ (or Kurveleshi). 620 The Vasiliko salt anticline (VWA) allows a detailed observation of a subvertical secondary weld 621 (Figure 12c–d). In this domain the evaporites are strongly deformed by subvertical brittle shear 622 zones, bounding lenticular domains of gypsum and clays in which an associated cataclastic 623 foliation is found. This weld is the core of a tight anticline deforming successions of the Vigla 624 Formation. Field observations and the development of inverted flanks near the salt weld suggest 625 that the VWA must have a non-outcropping upper bulge (or teardrop), disconnected from the 626 pedestal at the root. This is the geometry we include in seismic interpretations of the area 627 (Figures 13e, 14e). 628 6.4. The Filiates diapir and the Parga–Sarakiniko salt thrust 629 The Filiates structure is shown in the mapping in Figure 10. The plan-view shape of this structure 630 is complex, resembling a rhombus whose boundaries outcrop only to the north, east, and south. 631 Inside, the evaporites generally dip to the north, defined by some thick gypsum package (Figure 632 5a). Given information from the Filiates–1 well, the evaporite thicknesses are about 3.75 km 633 (Figure 6b). We interpret that this structure corresponds to a diapir (or plug) whose present-day 634 contacts are strongly modified by Alpine shortening: the Filiates salt plug (FSP). For example, in 635 the north (near Filiates), Cretaceous successions (especially the Vigla and the Senonian 636 limestones) dip southward against the diapir, and above the diapir, these same successions are 637 condensed and deformed by west–east folds (isolated outcrops between Smertos and Filiates). 638 We interpret the northern contact of the FSP to currently be a high-dipping reverse fault that 22 639 laterally transitions to a WNW–ESE, dextral strike-slip fault. On that flank, we have identified a 640 set of NW–SE folds that affect the entire Jurassic and Cretaceous succession, which is locally 641 thicker here and is deformed by overturned and asymmetric, NE-vergent folds that may be locally 642 cut by folded thrusts (Figure 12e,f and inset). On the eastern edge and to the south, the FSP roof 643 thrusts westward (with sheets involving mainly the Vigla and the Senonian limestones). Some of 644 these thrusts imbricate the diapir evaporites themselves (NE of Igoumenitsa, in the Varathi 645 Mountains). The FSP is terminated to the SE by a narrow shear zone that we interpret as a salt 646 weld, in which the evaporites are characterized by pervasive ductile shearing (Figure 5g,h). This 647 weld is cut by a SE-directed thrust, which in its footwall develops a S–verging anticline with a 648 kilometer-long reverse flank affecting the Vigla Formation (observed between the cape of 649 Vrachonisída Ágios Dionýsios and Plataria; Figure 10). 650 The deep structure of the FSP is shown in four seismic profiles and their interpretation; thus, we 651 have studied two sections subparallel to the shortening direction (NE–SW; Figures 13a,b and 652 14a,b) and two perpendicular (NW–SE; Figures 13c,d and 14c,d). The FSP is a thick salt body 653 with continuous internal reflections (probably from other evaporites) that are folded (Figure 14a), 654 while the base is a gently dipping thrust advancing SW over the Oligocene and lower Miocene 655 flysches (as is confirmed by the well Filiates–1; Figure 6b). The FSP is part of a thrust sheet 656 (external IZ or Ҫika) that is detached over a para-autochthonous succession that may have minor 657 internal imbrications (Figure 14b). Its extension outside the study area, already in the marine 658 region, could contain another salt wall (trending NW–SE) that is the one outcropping on Corfu 659 Island (Figures 3b, 7) (e.g., (Tserolas et al., 2019; Waters, 1994)). 660 The FSP has two flanks with different geometries in the suprasalt series. The SW flank is formed 661 by a backthrust (to the NE) involving a thick Middle Jurassic to Lower Cretaceous limestone 662 succession (including the Vigla Formation). The eastern flank, on the contrary, contains a thick 663 succession of the same sequences but also exhibits deformed disharmonic folds against the 664 parallel succession of Triassic dolomites and the Pantokrator Formation, which might represent a 665 folded megaflap (Figure 12e,f and inset). We interpret this flank to correspond to a minibasin that 666 developed from the Middle Jurassic to the Lower Cretaceous on the NE flank of the FSP and then 667 shortened along with the diapir, developing backthrusts that may incorporate the evaporites of the 668 source layer (Figure 14a–c). 23 669 In this northern sector of Greece, we have also studied the area between Parga, Sarakiniko, and 670 Perdika (Figure 10). Here we interpret that a tight NW–SE anticline develops, with an incomplete 671 secondary weld that is finally displaced to the SW by an out-of-sequence thrust that superimposes 672 the salt anticline (PWA) on the Oligocene flysch (the tectonic window between Parga and 673 Perdika) and finally, at the front (to the SW), on Messinian evaporites, as observed in the cliffs of 674 the Sarakiniko and Arillas beaches. 675 6.5. The Fyteies–Trífos salt wall 676 The other sector studied in Greece contains a large elongated NNW–SSE structure between the 677 gulfs of Arta and Preveza, outcropping from the locality of Paliampela to that of Katochi (Figure 678 11). By its shape and geometrical relationships, this structure corresponds to a relatively narrow 679 (60×6–7 km) salt wall (the Fyteies–Trífos salt wall, FSW), on which multiple doline fields are 680 developed and in which the salt wall is covered by subhorizontal packages of brecciated massive 681 dolomites and small outcrops of the Vigla Formation. Structures within these later outcrops 682 describe small synclinal basins, with progressive unconformities, wedging, and debris flow levels 683 (Figure 12k,l). We interpret these domains as local minibasins, with condensed sequences of 684 Cretaceous sediments whose sedimentation was conditioned by the local movement (uplift and 685 subsidence) of the Triassic evaporites. 686 The FSW exhibits three major structural features. First, the entire eastern contact is a narrow, 687 subvertical, left-lateral strike-slip fault zone, which was cut by the Trifos South–1 well (Figure 688 6c). The deformation associated with the fault is largely produced by brittle faulting, where 689 lenses of hundreds of meters of Mesozoic limestones (Pantokrator, Vigla, and Senonian 690 limestones) are found. No traces of deformation are associated with this fault zone within the 691 FSW, except in rare NW–SE dextral faults that occasionally develop only in dolomitic packages 692 overlying evaporites. This fault zone (the Aitoloakarnania, left-lateral strike-slip fault) has 693 associated seismic activity and constitutes the active western edge for the Kalivia–Lisimakia or 694 Trichonis basin (Tranos et al., 2020; Underhill, 1989). 695 Second, to the north, two windows are developed under the basal salt thrust of the FSW, 696 outcropping the Oligocene flysch in its hanging wall (around the Trifos–2 well). In this same 697 sector, we interpret that the salt wall is losing height and the Neogene sediments of the Drimos 24 698 region come to rest directly on the subsalt sequences, suggesting an almost complete welding of 699 the allochthonous evaporite layer. 700 Finally, the western edge of the FSW presents two types of singular structures. In the north, from 701 Paliampela to Kompoti, the salt wall overthrusts westward, an inverted succession (through a 702 north–south synclinal fold) of the Oligocene flysch, with condensed layers at the top 703 corresponding to the Vigla and the Senonian limestones. In contrast, south of Aetos, the entire 704 western edge of the FSW corresponds with backthrusts to the east, developing two large thrust 705 sheets (one each at Vasilopoulos and Bampini) and associated folds that superimpose a thick 706 Mesozoic limestone succession (from the Pantokrator to the Senonian limestones) on the 707 evaporites. Although none of these thrusts imbricate the Triassic evaporites, one of these folds is 708 nucleated in them (Astakos–1 well; Figure 6c), so the Triassic succession, although being a 709 detachment level for these backthrusts on the western flank of the FSW, must also flow into the 710 anticlinal cores during Alpine shortening. 711 7. Experimental Models of Fold and Thrust Belts with Precursor Diapirs 712 Numerous publications have recently analyzed the role of preexisting salt structures in a thin- 713 skinned FTB (Callot et al., 2007; Callot et al., 2012; Dooley et al., 2009b; Dooley et al., 2015; 714 Duffy et al., 2018; Granado et al., 2019; Rowan et al., 2022; Rowan & Vendeville, 2006; 715 Santolaria et al., 2021; Santolaria et al., 2022). Our goal has not been to perform new experiments 716 that analyze this well-known situation, but rather to gather published and unpublished 717 information from existing experiments that explore some of the main problems we have 718 identified in the IZ. For example, what structural differences can we find between a shortened 719 diapir and a shortened wall? What structural style develops during shortening of variably oriented 720 salt walls? What structural style do shortened salt structures develop in section? And how does 721 deformation vary spatially and temporally? 722 Note that some tectonic processes that are operating in the IZ e have not been analyzed by 723 experimental models, such as the inversion of synrift salt structures (e.g., (Dooley & Hudec, 724 2022; Hudec & Jackson, 2007; Jackson & Hudec, 2017; Letouzey et al., 1995)), the role of salt- 725 layer (and basement) tilting on subsequent shortening (e.g., (Dooley et al., 2007; Santolaria et al., 726 2022)), and how the deformation evolves having varying shortening directions over time. 25 727 The plan-view evolution in of a system of isolated diapirs and variably oriented salt walls during 728 shortening is illustrated in Figures 15 and 16, respectively. These figures show how the structures 729 on the surface of the models vary under increasing shortening strains. 730 In the case of isolated diapirs (Figure 15), shortening migrates progressively toward the foreland 731 (left in all models), generating initial soft links between neighboring diapirs (Figure 15a), then 732 doubly vergent thrusts in the hinterland (Figure 15b,c), and finally frontal thrusts that can verge 733 toward the hinterland (Figure 15d) (see also (Callot et al., 2007; Callot et al., 2012; Dooley et al., 734 2015; Santolaria et al., 2022)). The foreland migration of the deformation is caused by the fact 735 that the entire suprasalt succession is detached above the salt layer. Other results of this model 736 indicate: (1) that pop-ups are generated by the linkage of neighboring diapirs; (2) that these 737 structural ridges (pop-up anticlines that have a broad and flat culmination) become narrower 738 around the diapirs and migrate toward the foreland, progressively moving away from their zone 739 of origin and end up displaced with respect to the position of the original diapir, and (3) that the 740 geometries of the thrusts are conditioned by the diapirs. For this last point, all the models 741 preserve an early structural feature, such as the thrust reentrants toward the position of the diapirs 742 (see also (Dooley et al., 2009a; Duffy et al., 2018; Rowan et al., 2022; Santolaria et al., 2021)). 743 In the case of shortened salt walls, the structural evolution is strongly conditioned by their 744 position, but also by their orientation with respect to the shortening direction (Figure 16) (see 745 further details in (Dooley et al., 2009b; Duffy et al., 2018; Jahani et al., 2009)). Some major 746 features are shared with the previous model, such as the advance of the deformation toward the 747 foreland, the nucleation of thrusts and associated folds in the salt structures, and the displacement 748 of the shortening structures as the deformation advances. The novel features are as follows: 749 1. Walls oriented at a high angle to shortening nucleate long thrusts and anticlines (mostly 750 foreland-vergent) associated with the salt high (e.g., walls e and f in Figure 16); 751 2. Walls oblique to shortening develop systems of curved, long faults and folds, and; 752 3. “Relay or stepover” structures form between nearby walls, and also strike-slip faults with 753 variable kinematics, but dominantly transpressional along these walls (wall d in Figure 754 16c–d). 755 Walls oriented parallel to the shortening evolve into a large dome by ductile salt flow, eventually 756 creating a local major diapir with a subcircular shape (wall a in Figure 16c,d). The figure also 26 757 illustrates how the thrusts originate in the salt walls, but as shortening increases, deformation 758 propagates in the salt roofs and the salt walls are squeezed and welded, developing grabens with 759 normal faults along their crests due to outer arc extension above the rising diapiric salt (e.g., wall 760 e in Figure 16d). 761 The final geometry of the structures around salt walls with different orientation is shown with 762 several sections in Figure 17 (using the end result of the model in Figure 16). The walls oriented 763 parallel to shortening (number 1 in Figure 17) become shorter and narrower as the salt flows 764 upwards to form allochthonous salt sheets (Figure 17a). In this case, the salt wall remained open 765 due to its length relative to the amount of shortening it absorbed, and may develop salt wings 766 along the roof bedding. The walls perpendicular to shortening (number 4 in Figure 17), on the 767 contrary, are closed more rapidly, developing subvertical secondary welds that link upwards to 768 allochthonous salt sheets or salt bulbs, and finally end up being cut and displaced by foreland- 769 directed thrusts (Figure 17a,b). Oblique walls (numbers 2, 3, and 5 in Figure 17), after 770 considerable shortening, develop both vertical secondary welds with double-vergent thrusts 771 (number 3 in Figure 16c), as well as secondary welds cut by thrust welds (numbers 2, 3, and 5 in 772 Figure 16b,c). 773 The temporal evolution of a shortened salt wall from an arched roof to upright weld, to finally a 774 displaced secondary weld is shown in Figure 18. Shortening begins by focusing on the weak 775 diapir, resulting in its gradual closure displacing diapiric salt both upwards and downwards 776 (Figure 18a). Once the wall closes, a vertical secondary weld develops (Figure 18). Finally, if the 777 shortening continues, a foreland-vergent thrust develops, cutting and displacing the original 778 vertical weld (Figure 18c), similar to those seen in Figure 17. Other noteworthy processes are the 779 development of pop-ups above the wall crest as salt is forced to upwards during initial 780 shortening, and the existence of two opposing salt flows. Initially, salt flows from the wall into 781 the source layer, inflating it to fill the space created by the rising hangingwall (number 2 in 782 Figure 18a), followed by the development of an upward salt flow from the diapir due to the 783 impingement by approaching wallrocks, which initially feeds the salt bulb at shallow levels and 784 then a salt nappe at the surface after the roof was breached (numbers 3 and 4 in Figure 18b). Both 785 types of flow were characterized by (Dooley et al., 2009a), who called them outward and inward 786 plumes, respectively. 27 787 Finally, we have gathered experiments to explore the geometry of two types of structures. Firstly, 788 the structural configuration that develops in relation to the so-called “Q-tips” (Rowan & 789 Vendeville, 2006) (Figure 19a). And also, the structures that can develop in the vicinity of 790 welded walls (Figure 19d). The models we have chosen to study these structures come from salt 791 walls oriented at a high angle to the shortening direction (80º). These models differ in the 792 thickness of the original source layer, and thus height of the diapir that could be generated. 793 “Q tips” structures are characterized in plan-view by a secondary weld with the orientation of the 794 original salt wall, connecting two isolated salt bodies at their lateral terminations. The structural 795 style can be very variable depending on the orientation of the cross section. In those parallel to 796 shortening, the central weld develops in the source layer, while an upper diapir upwarp the 797 overburden that is also deformed by double-vergent thrusts (Figure 19b). Sections perpendicular 798 to the shortening, however, show a diapir with two large lateral wings, again bounded by 799 opposing thrusts (which in this case would correspond to lateral ramps) (Figure 19c). 800 In our model with higher salt walls and thus thicker diapir-bounding sequence, a secondary weld 801 develops as seen previously, but also two strike-slip faults with opposite displacements that end 802 against the lateral terminations of the wall. These “tear faults” (number 7 in Figure 19d) bound a 803 suprasalt “indentor” that into the wall, displacing diapiric salt and allowing the development of 804 the vertical secondary weld. After advanced stages of shortening this secondary weld is thrusted 805 and displaced (number 5 in Figure 19e). 806 We suggest that many of these structures developed in the experimental models explain cases that 807 we have observed in the IZ, as we will discuss in the next section. 808 8. Discussion 809 8.1. Influence of preexisting diapirs on the orogenic evolution of the Ionian Zone 810 Our study has demonstrated several types of salt structures in the IZ. We have characterized their 811 geometry and relationships with the salt roofs, assessing the role they have played in the 812 shortening of the IZ during the Alpine orogeny. In addition to detailed observations and 813 geological cross sections generated with structural criteria and supported on seismic 814 interpretations, our overview of the diapiric structures and the structural organization of the IZ is 815 summarized in Figure 20. This is a simplified tectonic map of the entire IZ, bringing together our 28 816 own observations and previous mapping information in Albania and Greece. The main tectonic 817 features have been simplified from Figure 7, highlighting the different types of salt diapirs 818 identified. 819 Four types of structures nucleated in the Triassic evaporites exist in the IZ (Figure 20): 820 1. Salt pillows such as the Delvinë (or Delvina) (DSP) and Picari–Kardhiq (PK). 821 2. Diapirs rising from a point source (sensu (Jackson & Talbot, 1991)), and which are often 822 deformed and unrooted/displaced, currently forming part of high-allochthonous thrust 823 sheets, such as the Dumrë and Filiates diapirs (or plugs; DSP and FSP, respectively). 824 3. Salt walls rising from line sources (sensu (Jackson & Talbot, 1991)), such as the Corfu 825 (CSW) and those studied here, the Butrinti–Xarra (BSW) and Fyteies–Trífos (FSW). 826 4. Welded pinched-off anticlines that are finally cut by out-of-sequence thrusts that displace 827 the secondary weld, such as the structures in the Kassidhiáris Mountains (KWA), 828 Navarica (NWA), Vasiliko (VWA), and the shortcut thrust seen in Parga–Sarakiniko 829 (PWA). 830 The Delvina–Parakalamos structure (DSW) can be considered a mixed case, because although 831 globally it is an allochthonous salt wall, it is internally composed of two diapirs connected by a 832 narrow corridor where the suprasalt succession reaches greater thickness (cf. just south of the 833 Delvinaki locality in Figure 9). This type of geometry has been found in other salt walls, for 834 example in the North Sea (e.g., (Tvedt et al., 2016)), so we are inclined to interpret it as a salt 835 wall with two local bulges or diapirs. 836 In light of our observations and experimental models described in the previous section, it is clear 837 that the relief, shape, and orientation of salt structures condition the structural style of the IZ 838 FTB. However, there is a fundamental difference between the geology under study and our 839 models due to the nature of the Triassic evaporites that are formed by many lithologies, and to a 840 lesser extent by halite, which gives them a higher viscosity and heterogeneity than the 841 experimental models. The existence of thin salt levels in an evaporite sequence with abundant 842 competent lithologies that will tend to deform brittlely gives the Keuper evaporites rheological 843 characteristics that have not yet been studied in other Alpine FTBs. For example, it may be that 844 the higher viscosity of the Triassic evaporites here causes the deformation of the salt diapirs to be 29 845 resolved by salt-involved thrusts and with only partial diapir welding as the more mobile units 846 are expelled and the less mobile ones are left behind. 847 The following major features observed in the IZ also appear in the models: 848 849 1. Thrust curvature of thrusts toward the position of diapirs, forming reentrants; e.g., in DSW and FSP (as in Figure 15b,c). 850 2. Secondary welds and thrust welds, with out-of-sequence thrusts cutting salt bulges (e.g., 851 KWA, NWA, PWA, and VWA), with double-vergent thrusts also developing from such 852 salt structures (as in Figures 17c, 18). 853 3. Variable structural style depending on the orientation of the salt wall versus the direction 854 of shortening, as may occur in the oblique walls of BSW, DSW, and FSW, as opposed to 855 the perpendicular salt wall of Corfu (as shown in Figures 16, 17). So oblique salt walls 856 develop strike-slip faults only along their edges (e.g., Aitoloakarnania fault in FSW; 857 (Tranos et al., 2020; Underhill, 1989)); in turn, their kinematics depend on the initial 858 orientation of the wall with respect to shortening direction (as in Figure 16). 859 4. Pinching at the centers of the salt walls, creating two salt bodies at their ends (“Q-tips”) 860 joined by a narrow conduit (or even welded in a secondary weld), as found between the 861 Delvinaki and Parakalamos culminations in DSW or the thrust welds found between BSW 862 and FSW (as in Figure 19a). 863 5. Almost complete welding of the salt walls perpendicular to shortening, developing two 864 strike-slip faults with opposite kinematics at their ends, bounding an indentor in the 865 suprasalt succession (as in Figure 19b). This geometry is probably the case for the Corfu 866 wall (with the right-lateral, South Salento strike slip fault in the north of this wall; 867 (Tserolas et al., 2019)), outside the studied area, which also describes a salient of the IZ 868 orogenic front against the Pre-Apulian zone (Figure 20). 869 6. Finally, on the scale of the entire AH orogen, the thrusts of the IZ have a unique shape. 870 The thrusts arc toward the foreland, drawing several salients, such as the one located at 871 the front (and west) of the island of Corfu. The thrust system converges in two lateral 872 branches where the IZ ends, such as in the Zákynthos region, to the south, and in the NE– 873 SW alignment of Vlora–Dibër–Elbasan, around the Dumrë diapir (Figures 7, 20). South 874 of Zákynthos and west of Peloponnesus, in the offshore area, Triassic evaporites are only 30 875 represented by a thin layer (Karakitsios et al., 2017; Kokinou et al., 2005; Kokinou et al., 876 2003; Kosmidou et al., 2018; Sotiropoulos et al., 2022). To the north of the Vlora–Dibër– 877 Elbasan Lineament, there is no trace of these evaporites in the subsurface (Bega, 2013, 878 2020). The existence of salt walls also conditions the shape of the front, but the lateral 879 terminations and the shape of the IZ FTB may be due to the original distribution of the 880 Triassic evaporites and the evaporite facies distribution (being richer in halite toward the 881 center and West of the basin). So, as described by some experimental models (Cotton & 882 Koyi, 2000; Letouzey et al., 1995), the lateral limits of a salt layer determine that the 883 thrust system develops large lateral ramps as is seen in both regions. 884 We have analyzed the pattern of rotations deduced from paleomagnetic studies. To compare this 885 type of information with our interpretation of salt tectonics in the IZ, we have chosen only those 886 rotations established for the carbonate section (Jurassic to Paleocene), presenting the results in 887 Figure 20 (revising data from (Birch, 1990; Broadley et al., 2006; Horner & Freeman, 1983; 888 Márton et al., 1990; Waters, 1994)). All studies of this type have shown two main features: 889 1. The entire IZ has experienced a variable clockwise rotation that began in an early 890 preorogenic epoch, probably since the Late Cretaceous Epoch (Broadley et al., 2006; 891 Kissel et al., 1985; Speranza et al., 1995; Speranza et al., 1992), and 892 2. Strike-slip faults such as the Kefallinia–Lefkas–Lixourion transpressional fault and the 893 Vlora–Dibër–Elbasan Lineament, have been active during the orogeny and until at least 894 the Pliocene Epoch, producing major local rotations (Duermeijer et al., 1999; Duermeijer 895 et al., 2000; Mauritsch et al., 1995; Speranza et al., 1995; van Hinsbergen et al., 2005). 896 Some authors have also proposed that the rotations vary progressively in the IZ, 897 decreasing toward the foreland (westward) and that there are differential rotations 898 between some thrust sheets and others (e.g., (Broadley et al., 2006; Kondopoulou, 2000; 899 Mauritsch et al., 1995)). However, the distribution of rotations, when compared to diapirs, 900 shows a feature not sufficiently well appreciated previously: many of the salt structures 901 show variable rotations around them (see, for example the data around FSP and DSW). 902 Given the available data, we suggest that the diapiric highs thus conditioned the internal 903 rotation within the same thrust sheet, which would agree with what is shown in the 904 experimental data, where it can be seen not only that the thrusts rotate toward the diapirs, 31 905 but that the suprasalt successions also rotate differentially on either side of these 906 structures (Figure 15). This suggestion is useful for paleomagnetic studies in other FTBs 907 with salt structures. 908 8.2. How to differentiate preexisting diapirs in a fold and thrust belt 909 The case of the IZ differs from some other cases of salt-involved FTBs, because of several 910 peculiarities that should be highlighted. 911 1. The Triassic evaporites (Keuper) are an effective detachment level during shortening, but 912 they have a great lithological heterogeneity, with anhydrite, gypsum, and dolostones 913 (occasionally also weaker potassic salts) predominating many times over halite, and being 914 richer in halite toward the center of the basin and basinward. This heterogeneity confers a 915 greater viscosity to the salt layer undergoing deformations and gives them a complex 916 rheology in which levels of different thicknesses coexist with brittle behavior, as opposed 917 to others that behave in a ductile manner. This is not a singularity of the IZ, as it is a 918 constant in the Keuper evaporites in the Alpine belts of the Mediterranean (see (Soto et 919 al., 2017)) and in other FTBs such as those of the Appalachians (Mount, 2014), the Sub- 920 Andean FTB (Baby et al., 2018), and the Zagros FTB (Najafi et al., 2014; Sherkati & 921 Letouzey, 2004). Although it differs from other FTBs in where the evaporites are 922 dominated by halite (e.g., in Central Sivas Basin; (Kergaravat et al., 2017)). 923 2. In our case, we have not identified structures such as allochthonous salt sheets (e.g., 924 (Flinch & Soto, 2022)), nor salt wings, nor thick sedimentary sequences filling secondary 925 minibasins that can completely weld the source layer (e.g., (Kergaravat et al., 2017)). This 926 is probably due to a lower thickness of the IZ evaporites, as well as to their 927 aforementioned higher viscosity and complex rheology. 928 3. Another problem that is often discussed in the interpretation of FTBs with evaporites is 929 whether the observed structures can be explained only by thrust and fold tectonics without 930 the need for preexisting diapirs (e.g., (Kamberis et al., 2022; Liesa et al., 2023; Lykakis et 931 al., 2021)). Our interpretation has demonstrated the existence of preorogenic diapirs of 932 different shapes and orientations, conditioning the final style of the FTB. Figure 21 933 schematizes the observations we made to illustrate the criteria we propose to resolve this 934 uncertainty. We compare an idealized preorogenic situation (Figure 21a) and how the salt 32 935 geometries would eventually look in the FTB (Figure 21b). The diagrams idealize a salt 936 basin sloping toward the foreland (left), in which there could be a carbonate platform as 937 represented by the Apulian Zone. In this simple configuration, we ignore the role of the 938 basement (either with normal faults of imbricated by thrusting; e.g., (Kamberis et al., 939 2022; Kamberis et al., 1996; Kokinou et al., 2005; Kokinou et al., 2003; Kokkalas et al., 940 2013)) and the existence of local thickness variations in synrift to postrift evaporites. 941 4. The increased thickness of the salt layer toward the foreland could be the cause for the 942 source layer to progressively develop gentle pillows (like those of Delvinë [DP] and 943 Picari–Kardhiq [PK]), punctual-source diapirs or salt plugs (like Filiates [FSP]), and 944 finally salt walls (like those of Butrinti–Xarra [BSW], Corfu [CSW], and Fyteies–Trífos 945 [FSW]). In suprasalt successions such as those represented here by the carbonate 946 sequence (Jurassic to Eocene), preorogenic structures such as those observed (Figure 20) 947 may develop with progressive onlaps over the pillows (as in DP), truncations and 948 important thickness variations around the diapirs (as in the NE edge of FSP), and 949 condensed (or absent) successions over the higher-relief salt walls (as in BSW, FP, and 950 FSW). Diapirs growth occurred during the Middle Jurassic to Lower Cretaceous (i.e., 951 Lower Posidonia mudstones to the Vigla Formation). 952 5. As for the tectonic inversion by shortening of the salt province (in age that would vary 953 progressively toward the foreland from latest Oligocene to Pliocene), structures such as 954 those described above would develop (Figures 20 and 21): unrooted pillows displaced by 955 thrusts located at their base (as in DP and PK); tight anticlines with vertical welds that 956 may be sheared forming thrust welds (as in Kassidhiáris [KWA], Navarica [NWA], and 957 Vasiliko [VSW]); Q-tips diapirs joined by secondary welds that may become cut and 958 displaced by thrusts (as in between Butrinti and Filiates); opposite-vergence faults on 959 either side of former tight diapirs (as at Filiates [FSP]); strike-slip faults at the lateral 960 terminations of salt walls perpendicular to the shortening direction, similar to the tear 961 faults limiting indentors (as at Corfu [CSW]); and strike-slip (transpressional) faults along 962 the edges of walls oblique to shortening (such as the Aitoloakarnania fault on the eastern 963 edge of FSW; (Tranos et al., 2020; Underhill, 1989)). 964 Our study presents a novel, detailed, and accurate analysis of the shallow and deep structure of a 965 large part of the IZ in Albania and Greece and characterizes the roles of the different types of salt 33 966 diapirs in the orogen. It also provides knowledge that can be useful for the search for resources in 967 these countries as well as for the assessment of what role these structures can play in objectives 968 linked to geologic storage (like CO2 and Hydrogen) in salt diapirs (Arvanitis et al., 2020; 969 Hatziyannis et al., 2009; Stournaras, 1985). 970 9. Conclusions 971 Using field observations, reviews of existing mapping and other tectonic data (such as 972 paleomagnetic rotations), interpretation of PSDM seismic profiles (including those from a recent 973 acquisition campaign), and information from borehole data, we present a tectonic synthesis of the 974 architecture of the Ionian Zone (IZ) in Albania and Greece. 975 We characterize several bodies of salt diapirs nucleated in Upper Triassic (Keuper) evaporites, 976 whose compositions prove to be highly heterogeneous, from both a rheological point and 977 compositional point of view, as they have an upper-level rich in anhydrite, gypsum, and 978 dolomites and a lower, halite-rich sequence. During the Alpine shortening, deformation within 979 the Triassic evaporites is accommodated by simultaneous brittle failure in the more competent 980 layers (e.g., dolostone and anhydrite) and ductile flow in the less viscous lithologies (e.g., halite). 981 The salt bodies identified vary from low-relief salt pillows (e.g., Delvinë and Picari–Kardhiq 982 structures), isolated salt plugs (e.g., at Filiates), and salt walls of varying orientations to the 983 shortening, such as those perpendicular to shortening (e.g., Corfu) and others of variable 984 obliquity (e.g., Butrinti–Xarra and Fyteies–Trífos). 985 We have characterized the structural style around these salt structures, showing that their 986 presence, geometry, relief, and orientation conditioned the shape and kinematics of the thrusts 987 and associated folds and faults. Combining surface observations, borehole data, and seismic 988 interpretations, along with comparison to physical modeling results, we have reconstructed for 989 the first time in the region the deep structure of the different thrust sheets as well as the internal 990 structure and geometry of the main salt diapirs. We interpret that diapirs to have an early growth 991 stage, before the orogeny, most probably from the Middle Jurassic to the Lower Cretaceous 992 Epoch, and then to have been unevenly shortened during the Alpine shortening (from the end of 993 the Oligocene to the upper Miocene–Pliocene Epoch), according to a piggyback sequence of 994 thrusting. 34 995 This global study of the IZ can be used to better target exploration efforts in the region, but we 996 are also confident that it will open new directions of study in the search for geologic storage 997 linked to the energy transition in Albania and Greece (e.g., carbon and hydrogen storage in salt 998 diapirs). 999 1000 Acknowledgments 1001 We thank Nancy Cottington and Pablo Ruiz for initial figure drafting. Repsol and Energean Oil 1002 & Gas are specially acknowledged for their invitation and financial support to conduct the 1003 fieldwork for this study and for giving us access to the seismic data set they recently acquired in 1004 onshore northern Greece. The project was also funded by the Applied Geodynamics Laboratory 1005 (AGL) Industrial Associates program, comprising the following companies: BP, Chevron, 1006 Condor Petroleum Inc., Eni, ExxonMobil, Fairfield Geotechnologies, Hess, Murphy Oil 1007 Corporation, Oxy, Petrobras, Petronas, PGS, Repsol, China National Petroleum Company 1008 Development (RIPED), Rockfield, Shell, Talos Energy, TGS, and Woodside Energy 1009 (https://www.beg.utexas.edu/agl/sponsors). JIS also thanks Spyros Bellas (now at the Technical 1010 University of Crete) and Yannis Bassias from Hellenic Hydrocarbon Resources Management 1011 S.A. for supporting this research in Greece. Publication authorized by the Director, Bureau of 1012 Economic Geology, The University of Texas at Austin. 1013 Data Availability Statement: The original seismic data in digital format used for the study of the 1014 subsurface structure are not included in this article due to restrictions by the Government of 1015 Greece. Readers can request a copy of this data and ask for permission by sending a request to 1016 Repsol and Energean. The 2023 version of Adobe Illustrator used to draw figures was available 1017 through a license agreement between Adobe and the Bureau of Economic Geology (UT Austin). 35 1018 10. References 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 Agostinetti, N. P., Giacomuzzi, G., & Chiarabba, C. (2007). 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This polymer has a 1648 density of 970 kg m-3 at room temperature and a dynamic shear viscosity of 2.5×104 Pa s at a 1649 laboratory strain rate of 3×10-1 s-1 (e.g., (Vendeville & Jackson, 1992; Weijermars et al., 1993)). 1650 The salt roof consists of differently colored mixtures of silica sand with a bulk density of ~1,700 1651 kg m-3, grain size of 300–600 µm, internal friction coefficient (µ) of 0.55–0.65, and hollow 1652 ceramic microspheres (“glass beads”) having a bulk density of 650 kg m-3, an average grain size 1653 of 90–150 µm, and a typical µ of 0.45 (e.g., (Dooley et al., 2009a; Rossi & Storti, 2003)). 1654 These mixtures moderated the density of the overburden surrounding the diapirs, and the 1655 suprasalt roof, such that the effects of density-driven loading were minimized and diapir rise, 1656 after downbuilding, was solely a function of the imposed shortening (see (Dooley et al., 2007; 1657 Dooley et al., 2009a), for further details). In all the experiments, there is a realistic density 1658 contrast of 1:1.1 between the salt and the surrounding strata, respectively. 1659 Diapirs (stocks and walls) in the experimental models illustrated in Figures 15–17 and 19 were 1660 grown by downbuilding. A layer of dense granular material was deposited uniformly above our 1661 model salt layer and, with the use of templates, this granular material was removed from 1662 predisposed locations by means of a vacuum. The differential load between covered and 1663 uncovered model salt drove our model salt into areas where the cover was removed. As the 1664 diapirs grew upwards additional layers of granular materials were deposited just cresting the tops 1665 of the growing diapirs, further driving diapir rise. After reaching a sufficient height, primarily 1666 governed by the source-layer thickness, the diapirs were covered with a uniform roof and 1667 shortening was initiated. The diapir in Figure 18 was built using the same methodology as 1668 described in (Dooley et al., 2009a), whereby polymer layers were stacked atop each other and 1669 surrounded by sediments. Again, once a certain height was achieved, the diapir was eventually 1670 covered with a uniform-thickness roof and shortening commenced. 1671 Computer-controlled cameras photographed the obliquely lit upper surface of the models at set 1672 time intervals between 3 and 10 minutes depending on the length of time of the experiment. 1673 Surface height-change maps were generated by means of a spherical laser scanner. This high48 1674 resolution laser scanning mapped topography during each experiment, allowing us to track relief 1675 changes on a millimeter scale. Each scan produces more than 8×106 points, from which CAD 1676 applications and other software to render high-resolution, interactive, 3-D surfaces. In all the 1677 experiments, regional shortening direction occurs westward (toward the left in all the 1678 illustrations). Dip sections are the sliced and photographed cross sections, whereas depth slices 1679 are virtual sections constructed from the voxel model. 49