Oceanic crust

: A study was performed of the seismic velocity structure of the South China Shelf from the westernmost parts of the Beibu Gulf, adjacent to the Gulf of Tonkin, south of Hainan Island and offshore southern mainland China, as far east as 118 deg east. Stacking velocities from petroleum industry multichannel seismic surveys were compiled into a database from which the depth or two way travel time to any given velocity interval could be calculated. The data was filtered to remove major decreases in velocity with depth, as these normally reflected decreasing data quality below -3 km. Data quality is typically good to 3 km, the approximate level of basement, and shows a good correlation with pre-existing seismic velocity data from sonobuoys. Seismically the shelf sediments are usually flat bedded, with generally well defined reflective character. The slope does not appear to be strongly affected by slumping, although locally such disturbances ar,e noted. The seismic data show that the se...

Located South of the High Atlas, in Morocco, The Anti-Atlas is a 700 km-long chain trending NE-SW. In the Central Anti-Atlas region, between Warzazat and Taznakht, the Proterozoic Pan-African basement (X1 to X2-3) crops out in isolated areas (boutonnières), where it is overlaid by late to post Pan-African Upper Proterozoic and Palaeozoic rocks. Late to post Pan-African Upper Proterozoic rocks (X3) have been classically divided into three units (X3i, X3m and X3s) which include volcanic rocks — mainly rhyolites— and continental siliciclastic rocks, the older units intruded by late granites (Choubert, 1952 and Choubert et al., 1970). Rocks belonging to the upper unit of post Pan-African Upper Proterozoic rocks (X3s) were deposited in basins bounded by faults with a dominant dip-slip normal motion; as a result, this unit have a variable thickness, being locally absent in the uplifted blocks. Uppermost Proterozoic (Adoudounian) and Palaeozoic rocks deposited unconformable on the older ro...

The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, ...

... and include different allochthonous units such as the Kocali Complex, the Karadut Complex, the Cilo Limestone, and the Hatay Ophiolites (Senel, 2002). ... Geochemical characteristics of the volcanic rock units will be the subject of another paper (Varol et al., in preparation). ...

Early development of marine electromagnetic methods, dating back about 80 years, was driven largely by defense/military applications, and use for these purposes continues to this day. Deepwater, frequency-domain, electric dipole-dipole, controlled-source electromagnetic (CSEM) methods arose from academic studies of the oceanic lithosphere in the 1980s, and although the hydrocarbon exploration industry was aware of this work, the shallow-water environments being explored at that time were not ideally suited for its use. Low oil prices and increasingly successful results from 3D seismic methods further discouraged investment in costly alternative geophysical data streams. These circumstances changed in the late 1990s, when both Statoil and ExxonMobil began modeling studies and fieldtrials of CSEM surveying in deep water (around [Formula: see text] or deeper), specifically for characterizing the resistivity of previously identified drilling targets. Trials offshore Angola in 2000–2002 ...

The Snowball Earth theory initially proposed by Kirschvink (Kirschvink, 1992) to explain the Neoproterozoic glacial episodes, suggested that the Earth was fully ice-covered at 720 My (Sturtian episode) and 640My (Marinoan episode). In the absence of continental weathering due to the spreading of large ice sheets (Hyde et al 2000, Donnadieu 2003), the main processes changing the seawater composition are

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

Upper Triassic volcanic and sedimentary rocks of the Mamonia Complex in southwestern Cyprus are exposed in erosional windows through the post-Cretaceous cover, where the Mamonia Complex is tectonically imbricated with the Troodos and Akamas ophiolitic suites. Most of these Upper Triassic volcanic rocks have been considered to represent remnants of Triassic oceanic crust and its associated seamounts. New Nd and Pb isotopic data show that the whole Mamonia volcanic suite exhibits features of oceanic island basalts (OIB). Four rock types have been distinguished on the basis of the petrology and chemistry of the rocks. Volcanism began with the eruption of depleted olivine tholeiites (Type 1) and oceanic island tholeiites (Type 2) associated with deep basin siliceous and/or calcareous sediments. The tholeiites were followed by highly phyric alkali basalts (Type 3) interbedded with pelagic Halobia-bearing limestones or white reefal limestones. Strongly LREE-enriched trachytes (Type 4) wer...

The global mid-ocean ridge (MOR) is one of Earth’s most fundamental geologic structures.Active volcanic, tectonic, hydrothermal, and biological processes occurring at the MOR affect nearly every attribute of the world’s oceans and oceanic crust. For the
past quarter century the overarching goal of the U.S. National Science Foundation
(NSF)–funded multidisciplinary Ridge 2000 program (http:// www .ridge2000
.org/) and its predecessor, Ridge Interdisciplinary Global Experiments (RIDGE), here collectively called “R2K,” has been to use observations,experiments, and models to answer fundamental questions about oceanic spreading center processes.

We present the results of a field, geochemical and geochronological study of a ∼5000 km2 area of the Sirwa Window of the Anti-Atlas Orogen of Morocco. The region includes the northern edge of the Palaeoproterozoic (Eburnean) West African Craton (Zenaga Complex) and the southern margin of the Neoproterozoic (Pan-African) Anti-Atlas Orogen. The Zenaga Complex comprises medium grade supracrustal schists and intrusive granitoid orthogneisses, three of which gave within-error U–Pb SHRIMP zircon dates of ∼2035 Ma. The Anti-Atlas Orogen contains a vast thickness of volcano-sedimentary rocks known collectively as the Anti-Atlas Supergroup. The oldest of these comprises three, probably coeval, sequences collectively known as the Bleı̈da Group. The Bleı̈da Group includes tectonic inliers of schists and orthogneisses which gave a SHRIMP date of 743±14 Ma; medium-grade ophiolitic rocks in the central part of the area (Khzama and Nqob fragments) and a low-grade clastic-chemical volcano-sedimenta...

In the northwestern part of Anatolia along the Izmir-Ankara Suture Zone, the Kazdağ and Uludağ metamorphic massifs form an E-W trending belt between the Sakarya Continent in the north and the Menderes Massif in the south. Internal succession of these two massifs have been described as metamorphic complexes consisting of various kinds of micaschists, quartz mica schist, gneisses, amphibolites and marbles. In the Kazdağ metamorphics, metaophiolites have been described additionally (Okay et al., 1991; Yaltırak and Okay, 1994; Okay et al., 1996; Duru et al., 2004). These metamorphic complexes were considered to form the basement of the Sakarya Continent tectonically overlain by the Early Permian (Topuz et al., 2004) to Late Triassic (Okay and Monie, 1997; Okay et al., 2002) Karakaya Complex. This old basement and the Karakaya Complex were suggested to be unconformably overlain by Liassic and younger platform limestones and detritals (Altıner et al., 1991). In the literature, it has also...