S. Roller

Exhumation of high- and ultrahigh-pressure metamorphic rocks by slab extraction

Exhumation of high- and ultrahigh-pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks, erosion of their overburden, or extensional thinning of the overburden. Some high-pressure terranes, such as the Adula nappe in the Central Alps, fit none of these scenarios. We propose an additional way in which part of the overburden may be removed: it may sink off into the deeper mantle (slab extraction). Structural and metamorphic relationships in and around the Adula nappe indicate that the emplacement of this Alpine high- to ultrahigh-pressure nappe (to 3.2 GPa) in a pile of lower-pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins, ultimately leading to the extraction of the intervening slab. In terms of mechanics, the cause of the exhumation is, in this case, not the buoyancy of the high-pressure rocks, but the negative buoyancy of the extracted slab.

Isotopic evidence (B, C, O) of deep fluid processes in fault rocks from the active Woodlark Basin detachment zone

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 (δ13CPDB down to −17‰; δ18OPDB down to −22‰) formed by both dehydration and decarbonation. Boron contents are low (<7 ppm), indicating high-grade metamorphic source rock for the fluids. Some of the δ11B signatures (17–35‰; 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.

Deformation fabrics of faulted rocks, and some syntectonic stress estimates from the active Woodlark Basin detachment zone

Abstract Ocean Drilling Program (ODP) Leg 180 investigated, in the western Woodlark Basin off Papua New Guinea, the nature and evolution of continental extension, eventually leading to crustal break-up and sea-floor spreading. At Moresby Seamount, the rift-related extension is localized at a recently active low-angle (30°) detachment fault, partly buried beneath a Pliocene-Pleistocene sedimentary synrift sequence. Data from three drillsites sample the detachment fault itself, secondary faults in its hanging wall and a steep normal fault cutting the footwall. The fault plane itself is manifested as a strongly altered fault gouge. Deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms, and accompanied by intensive veining and pervasive diagenetic cementation. The metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth. Many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization. Syn-mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust. Two palaeo-piezometers were applied to calcite-filled veins that show evidence of plastic deformation. Differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall.

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 tertiaren alpinen Metamorphose maximal erreichten Drucke. Wahrend 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 Polaritat des Drucksprunges fuhrte dazu, das die Deckengrenze zumeist als gewaltige sudostvergente Abschiebung interpretiert wurde (z.B. Ballevre & Merle 1993, Reddy et al. 1999). Strukturgeologische Gelandebeobachtungen ergeben jedoch sowohl fur das Hangende als auch das Liegende der Combinstorung die folgende kinematische Entwicklung: