IMA2014 Abstract Volume Hossain 400

400 PC3 – IMPACT CRATERING, HIGH PRESSURE SHOCK METAMORPHISM AND LUMINESCENCE STUDIES __________________________________________________________________________________________________________________________________________ Characterization of impact-induced brittle deformation: Ries meteorite crater, Germany Hossain M*, Kruhl J Technical University Munich, Germany, *sakawat.hossain@tum.de Shock waves of the Ries (Figure 1) meteorite impact caused fragmentation of the target rock far beyond the range and style typical for regional brittle deformation. In horizontal and vertical directions fracture patterns vary systematically with increasing distance from the impact centre. Quantification of fracture patterns with different fractal-geometry methods shows that impact fragmentation largely follows power laws, but to variable extent on different scales and at different locations. Box counting of limestone fracture patterns in a ca. 114 m long and 9 m high vertical section at the crater margin shows two different power-law relationships. They are interpreted as resulting from two different pattern-forming processes: pre-impact compaction of the sediment and impact-induced deformation. The strong pattern anisotropy and its spatial variation are quantified by the Mapping of Rock Fabric Anisotropy (MORFA) method [1]. The pattern's local systematic variation is interpreted as resulting from shatter-cone style fractures on the dm- to m-scale [2]. The regular variation of complexity and orientation of the fracture patterns seems to be typical for impact-related fragmentation [3]. Fragment size distributions (FSD) and fracture patterns of boulders collected inside the crater show power-law relationships over one order of magnitude, most likely resulting from material excavation through non-ballistic ejection during the impact. The two different power-law relationships for the FSD and fracture patterns of the Ries drill cores, however, demonstrate two subsequent pattern-forming processes. Probably, the first one is related to shock-wave fragmentation, the second one to elastic rebound of the transient crater floor from a depth of ca. 4.5-5 km. Whereas the box-counting dimension (D) of the fragmentation patterns do not vary with depth, the D-values of the two different power-law relationships for FSD of the drill cores decrease. Only at greatest depth of ca. 1200 m the higher D-value is clearly increased. This can be related to stress localization with enhanced comminution [4]. In general, fractal geometry - specifically when based on automated procedures - proves a powerful tool for quantifying and analysing complex rock structures. Genesis of Vredefort pseudotachylitic breccias Mohr-Westheide T1*, Reimold WU1,2, Tagle R3, Mader D4, Koeberl C4,5 1 - Museum für Naturkunde Berlin, Germany, *tanja.mohr-westheide@mfn-berlin.de 2 - Humboldt University Berlin, Germany 3 - Bruker Nano GmbH, Berlin, Germany 4 University of Vienna, Austria 5 - Natural History Museum Vienna, Austria Generation of pseudotachylitic breccias (PTB) in impact structures has been controversial [1], with debate of genesis by friction melting, shock compression melting, decompression melting, combination of these processes, and - as of late intrusion of allochthonous impact melt. Resolving this requires detailed multidisciplinary analysis to characterize the nature of these breccias. PTB are the most prominent impact-induced deformation phenomenon in the central uplift of the Vredefort Impact Structure - the Vredefort Dome [2, 3]. We present chemical data (INAA, XRF and EMPA) for mm to several m wide PTB from granitic and mafic (dioritic) host rocks and compare with compositions of the respective host rocks. μ-XRF spectrometry (M6 Jetstream, Bruker) was used for comparison of compositions of PTB and granitic host rock in a 71 x 52 cm slab from a PTB occurrence on Leeukop Hill. EMP analysis of PTB groundmass in comparison to XRF bulk chemical analysis of PTB and their host rocks revealed that PTB generally display close chemical relationships to the adjacent host rock. For example, comparing the chondrite-normalized REE abundance patterns shows that pairs of PTB and respective host rock samples plot generally together. This is in agreement with the general PTB/host rock XRF data systematics from this and earlier studies. In granitic environments, the refractory behavior of quartz seems to be the main reason for the slight chemical differences between PTB and host rock. The μ-XRF derived element distribution maps for the slab from Leeukop Hill show very similar chemical compositions for the granitic host rock and the melt breccia. This confirms that melt was formed from material of the same composition and that mm to dm wide breccia veinlets are of local origin. In larger occurrences, admixture of a small component of precursor material from a somewhat wider (say, 50-100 m) source volume is possible. Chemical investigations of PTB in mafic host rocks revealed that the elements associated with plagioclase and/or hydrous ferromagnesian minerals are enriched in PTB veins. PTB seemingly occur preferentially in amphibole-rich host rock portions of intermediate gneisses, which confirms the macroscopic observations of [4, 5]. In addition, the veins selected for analysis do not provide textural evidence for shearing/faulting. PTB of up to 1 m width all contain clast populations that represent local lithologies only, with distinct differences between clast population and host rock mineral abundances likely the result of different mechanical behavior and different melting temperatures of the various minerals. [1] Mohr-Westheide T. and Reimold W.U. (2011). Meteorit. Planet. Sci., 46, 543-555. [2] Dressler B.O. and Reimold W.U. (2004). Earth-Science Rev., 67, 1-60. [3] Reimold W.U. and Gibson R.L. (2006). Geol. Soc. Amer. Spec. Pap. 405, pp. 233-253. [4] Reimold W.U. and Colliston W.P. (1994), Geol. Soc. Amer. Spec. Pap. 293, 177-196. [5] Reimold W.U. (1991). N. Jhrb. Min., 161, 151-184. Figure 1: Ries impact area with crater outline, crater centre, location of Ries research borehole RRB, boulder sample location (Unterwilflingen) and position of Eireiner quarry. Boundary of the quarry: dashed line. Total length of section-1 (quarry wall): 114.03 m; general height: 9 m. N, M and S: north, middle, and south part, as used for quantification [3]. [1] Peternell M. et al. (2011). Journal of Structural Geology, 33, 609-623. [2] Osinski G.R. and Pierazzo E. (2013). Impact Cratering, pp. 45-64. [3] Hossain M.S. and Kruhl J.H. (2014), Pure and Applied Geophysics, submitted. [4] Buhl, E. et al., 2013, Journal of Structural Geology, 56, 20-33. View publication stats