Kinematik des Deckenkontaktes zwischen der Combinzone und der Zermatt-Saas-Zone (Penninische Decken, Westalpen) und deren Bedeutung für die Exhumierung der Zermatt-Saas-Zone

Die Grenze zwischen zwei ophiolithischen Decken der penninischen Alpen, der Zermatt-Saas-Zone (unten) und der Combinzone (oben), markiert zugleich einen bedeutenden Sprung der bei der tertiären alpinen Metamorphose maximal erreichten Drücke. Während die Zermatt-Saas-Zone Ultrahochdruckmetamorphose (25–30 kbar/550–600°C, Bucher et al. 2005) erfuhr, erreichte die Combinzone lediglich blauschieferfazielle Bedingungen (13–18 kbar/380– 550°C, Bousquet et al. 2004). Vor allem die Polarität des Drucksprunges führte dazu, daß die Deckengrenze zumeist als gewaltige südostvergente Abschiebung interpretiert wurde (z.B. Ballèvre & Merle 1993, Reddy et al. 1999). Strukturgeologische Geländebeobachtungen ergeben jedoch sowohl für das Hangende als auch das Liegende der Combinstörung die folgende kinematische Entwicklung: i) Nordwestvergente, überschiebende Scherung (D1), ii) (Süd)westvergente Scherung (D2),iii) Südostvergente, abschiebende Scherung (D3). Alle drei Deformationsphasen fanden in beiden Einheiten unter grünschieferfaziellen Bedingungen statt... Die Rekonstruktion ergibt, daß die Combinstörung hauptsächlich als Überschiebung aktiv war. Die Exhumierung der Gesteinseinheiten im Liegenden wurde nicht durch Extension, sondern durch vertikale Ausdünnung der Kruste während horizontaler Kontraktion bewirkt.conferenc

(Table 1) Petrological description and consolidation properties of ODP Site 186-1150 and Hole 186-1151A

In order to determine the shear parameters of the forearc sedimentary strata drilled during Ocean Drilling Program Leg 186, West Pacific Seismic Network, Japan Trench, eight whole-round samples were selected from different depths in the drilled sections of Sites 1150 and 1151. Whereas Site 1150 lays above the seismically active part of the subduction zone, Site 1151 is situated in an aseismic zone. The aim of the triaxial tests was, apart from determination of the static stress strain behavior of the sediments, to test the hypothesis that the static stress strain parameter could differ for each sites. In order to simulate undrained deformation conditions according to the high clay mineral content of the strata, consolidated undrained shear tests were performed in a triaxial testing setup. Measurements of water content, grain density, organic content, and microtextural investigations under the scanning electron microscope (SEM) accompanied the compression experiments. After the saturation and consolidation stages were completed, failure occurred in the compression stage of the experiments at peak strengths of 280-7278 kPa. The stiffness moduli calculated for each sample from differential stress vs. strain curves show a linear relationship with depth and range between 181 and 5827 kPa. Under the SEM, the artificial fault planes of the tested specimen only show partial alignment of clay minerals because of the high content of microfossils

Carbon, boron and oxygen isotope ratios of sediments from the Moresby Seamount

We report results from boron, carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during ODP Leg 180 in the western Woodlark Basin, off Papua New Guinea. In this area of active continental extension, crustal break-up and incipient seafloor spreading, a shallow-dipping, seismically active detachment fault accommodates strain, defining a zone of mylonites and cataclasites, vein formation and fluid infiltration. Syntectonic microstructures and vein-fill mineralogy suggest frictional heating during slip during extension and exhumation of Moresby Seamount. Low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids (delta13C PDB down to -17 per mil ; delta18O PDB down to -22 per mil ) formed by both dehydration and decarbonation. Boron contents are low (<7 ppm), indicating high-grade metamorphic source rock for the fluids. Some of the delta11B signatures (17-35 per mil ; parent solutions to calcite vein fills) are low when compared to deep-seated waters in other tectonic environments, likely reflecting preferential loss of 11B during low-grade metamorphism at depth. Pervasive devolatilization and flux of CO2-rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip. Multiple rupture-and-healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and, hence, seismogenic fault movement