Beschreibung: The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust.

Beschreibung: Highlights • Analysis of crystallographic preferred orientations of high-pressure polymineralic rocks by time-of-flight neutron diffraction • Elastic properties of a complete set of subduction channel rocks calculated from their crystallographic preferred orientation • Vp/Vs ratio and P-wave anisotropy of eclogites and metasediments • Influence of eclogite retrogression during exhumation on their elastic properties • Evaluation of the seismic signature of both clastic and carbonate sediments in subduction channels Abstract Crystallographic preferred orientations (CPO) of rocks from an exhumed subduction channel of the Alpine orogen were determined using time-of-flight neutron diffraction. This method allows the investigation of large polymineralic samples and, more importantly, the application of full pattern fit methods to constrain CPOs of mineralogically complex rocks. Samples studied include intensely deformed fresh and retrogressed eclogites, as well as metasediments, which are interleaved with the eclogites in the subduction channel. From the CPO, seismic properties of the samples were calculated. P- wave anisotropies of the eclogite samples are fairly low, with an average of about 1.5%, and mainly constrained by pronounced omphacite CPO. Growth and deformation of retrograde amphibole in the eclogites also led to a pronounced CPO, which has a large impact on seismic anisotropies by raising them to up to 3.7% and changing the orientations of velocity maxima. Elastic anisotropies of the subducted metasediments are higher (up to 7.4%) and constrained by quartz and mica CPO in clastics and by calcite CPO in marble. VP/VS ratios may help to distinguish fresh eclogites from retrogressed ones, and both rock types from mantle peridotites of downgoing lithospheric slabs in seismic imaging. Our data also indicate that subducted terrigenous sediments are not only strongly anisotropic, but also have low VP/VS ratios. This way there may be potential to image them by seismic tomography at depth in active subduction channels.

Beschreibung: The Giudicarie fault system represents the central, approximately NE–SW-trending segment of the Periadriatic fault system. Based on new fission track data the near surface evolution of the units bordering the Giudicarie fault system (GFS) is analysed. New and published zircon fission track (ZFT) data are compiled in an age-contour map of the area, and depict some first order features of cooling and exhumation pattern. Out of the three main Permian plutons located in the footwall of the GFS, the northern (Brixen pluton) and southernmost (Kreuzberg pluton) yielded ZFT ages of about 100 Ma, while the intermediate Ifinger pluton cooled through the zircon partial annealing zone only in the Miocene. The Ifinger granodiorite overthrusts the Southalpine basement and the Permian rocks of the Athesian Volcanic District along the NW dipping brittle Naif fault. Across this thrust an important age jump from Miocene to Permian ZFT ages is observed, suggesting that the deformation has jumped towards the southeast, thus deactivating the related segment of the GFS. The most eye-catching feature of the age-contour map is the corridor of young, Miocene ZFT ages from small tonalitic intrusions along the Northern Giudicarie fault. This corridor connects Early Miocene (17–23 Ma) ZFT ages of the NE-Adamello with the Middle–Late Miocene (19–9 Ma) ZFT ages of the Meran–Mauls basement and the western Tauern window and provides evidence for a polyphase deformation along the GFS. A three-step evolution model is proposed for the GFS: (a) shearing of the northern rim of the Adamello batholith along the dextral strike slip Periadriatic fault system; (b) bending of the central part of the fault to a NE direction together with the adjacent intrusive rocks; (c) Dissection of the bend part (Meran–Mauls fault) by the sinistral transpressive Northern Giudicarie fault and exhumation of small tonalitic bodies along this structure. Highlights ► Different exhumation history for Permian intrusions along the Giudicarie fault system. ► Very consistent ZFT ages of ~ 15 Ma for the entire Meran–Mauls basement. ► Corridor of Miocene zircon fission track ages along the Giudicarie fault system. ► Evidence for a polyphase deformation along the Giudicarie fault system.

Beschreibung: The petrophysical properties of fine-grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations (CPOs). In this contribution we show that Rietveld-based synchrotron texture analysis is a new and valuable tool to quantify textures of water-saturated fine-grained phyllosilicate-rich sediments, and assess the effects of compaction and tectonic deformation. We studied the CPO of compositionally almost homogeneous silty clay drillcore samples from the Nankai Accretionary Prism slope and the incoming Philippine Sea plate, offshore SW Japan. Basal planes of phyllosilicates show bedding-parallel alignment increasing with drillhole depth, thus reflecting progressive burial and compaction. In some samples calcite and albite display a CPO due to crystallographically controlled non-isometric grain shapes, or nannofossil tests. Consolidated-undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the CPOs of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis. There is at least a qualitative relation between CPO intensity and strain magnitude. Scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening. This indicates that the microfabrics are sensitive strain gauges, and contribute to anisotropic physical properties along with the CPO.

Beschreibung: Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from ∼250 to ∼700 °C was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional–viscous transition) at 280±30 °C. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between ∼280 and ∼400 °C, subgrain rotation recrystallization (SGR) in the ∼400–500 °C interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at ∼500 °C. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630±30 °C, the textures also include submaxima, which are indicative of combined basal 〈a〉- and prism [c] slip.

Beschreibung: The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300–380 °C, 0.25 GPa confining pressure, a flow stress range of ~ 0.1–0.2 GPa, and a strain rate of ~ 10− 13 s− 1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 °C, 1.2–1.5 GPa confining pressure, a flow stress of ~ 0.3–0.4 GPa, a strain rate of ~ 10− 6 s− 1, and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphiné twins within porphyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding.