Search

The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better understanding of microphysical deformation mechanisms. Here we use a combination of Scanning Electron Microscopy (SEM) and Broad Ion Beam (BIB) polishing to study the evolution of micro structure in samples of Callovo-Oxfordian Clay that were previously tested in the lab. Digital Image Correlation (DIC) enabled for the measurement of strain fields in the specimens, which were used as a guide to select regions in the sample for BIB-SEM analysis. Microstructures show evidence for dominantly cataclastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to nm- scale. At low strain, the dilatant fabric contains individually recognizable open fractures, while at high strain the reworked clay gouge contains broken non-clay grains, with a clear change towards smaller pores than the undeformed material and corresponding resealing of initial fracture porosity. This study might provide a first step towards a micro scale basis for constitutive models of deformation and fluid flow in cemented mudstones. [ABSTRACT FROM AUTHOR]

Lava dome eruptions subjected to high extrusion rates commonly evolve from endogenous to exogenous growth and limits to their structural stability hold catastrophic potential as explosive eruption triggers. In the conduit, strain localisation in magma, accompanied by seismogenic failure, marks the onset of brittle magma ascent dynamics. The rock record of exogenous dome structures preserves vestiges of cataclastic processes (Cashman et al., 2008; Kennedy and Russell, 2011) and of thermal anomalies (Kendrick et al., 2012), key to unravelling subsurface processes. Here, a combined structural, thermal and magnetic investigation of a shear band crosscutting a large block erupted in 2010 at Soufrière Hills volcano (SHV) reveals evidence of faulting and frictional melting within the magmatic column. The mineralogy of this pseudotachylyte vein offers confirmation of complete recrystallisation with an isothermal remanent magnetisation signature that typifies local electric currents in faults. The pseudotachylyte presents an impermeable barrier, which is thought to have influenced the degassing pathway. Such melting events may be linked to the step-wise extrusion of magma accompanied by repetitive long-period (LP) drumbeat seismicity at SHV (Neuberg et al., 2006). Frictional melting of SHV andesite in a high velocity rotary shear apparatus highlights the small slip distances (< 15 cm) required to bring 800 °C magma to melting point at upper conduit stress conditions (10 MPa). We conclude that frictional melting is an inevitable consequence of seismogenic, conduit-dwelling magma fracture during dome building eruptions and that it may have an important influence on magma ascent dynamics. [ABSTRACT FROM AUTHOR]

The tectonic units of Crete are divided in a Lower- and an Upper Nappe system that are separated by the so called Cretan Detachment. The Upper Nappe System was almost not affected by Alpine metamorphism but the Lower Nappe System experienced high-pressure/low-temperature metamorphism. Therefore, it has been suggested that the tectonic contact represents a low-angle top-to-the N extensional shear zone. However, a number of studies present conflicting and/or not very convincing kinematic indicators and came to the conclusion that the Cretan Detachment reflects bivergent extension with a strong component of coaxial deformation or a top-to-the S thrust. Recent data on calcite twinning, Raman spectroscopy of carbonaceous material and illite crystallinity from the Upper and Lower Nappe System also suggest that the Cretan Detachment might be a Miocene thrust, which was active at mid to upper crustal conditions..In this work, we investigate fault rocks from several locations in in Eastern Crete, where the Tripolitza Unit (Upper Nappe System) is juxtaposed against the Phyllite-Quartzite Unit s.l. (Lower Nappe System). The Tripolitza Unit in the hanging wall consists of Upper Triassic platform carbonates and the Phyllite-Quartzite Unit s.l. in the footwall is composed of violet-greenish slates and quartz-rich phyllites. Major displacement is localized in an almost horizontal detachment with up to several meter thick foliated ultracataclasites and fault gouges, which record numerous kinematic indicators like duplex structures, scaly fabrics, Riedel fractures, which consistently confirm top-to-N displacement. The cataclastic material is mainly derived from the violet and greenish slates in the footwall that are below the detachment zone strongly folded with subvertical limbs overprinted by a subhorizontal axial plane slaty cleavage. Asymmetric crenulation cleavage can be (and has been!) easily confused with SSC' fabrics, obscuring the clear picture of a top-to-the N sense of shear. The only slightly recrystallized carbonates of the overlying Tripolitza Unit are cut by parallel steeply S-dipping antithetic normal faults forming bookshelves, which are tilted towards the north. Numerous injection dykes, polished slicken sides and truncated grains at principal slip surfaces probably suggest seismic movement along the Cretan Detachment. New (U-Th)/He zircon ages are dispersed and as old as 130 Ma, but a significant population of the dates are ca. 14.5 Ma. These dates are much younger than the published zircon fission track dates for eastern Crete, and help constrain the temperature of deformation to 200 ± 20°C. Although we do not question the occurrence of an earlier thrusting component along this tectonic contact, our data from mainly cataclastic rocks clearly indicate Miocene top-to-N normal slip along the eastern part of the 'Cretan Detachment.' [ABSTRACT FROM AUTHOR]

We present the preliminary results of a multiscalar structural study of an outstanding exposure of the Belluno Thrust of the Italian Southern Alps (Monte Coppolo area, Feltre region). The Belluno Thrust is one of the main structures forming the orogen-scale WSW-ENE-trending thrust system that contributed to the building of the Southern Alps in the context of the Neogene Alpine evolution. Thrust tectonics, expressed by generally SSE-verging low-angle thrusts, started in the upper Oligocene and continues to present times, as documented by active faults considered to be the seismogenic sources affecting the eastern Southern Alps. In the Monte Coppolo area, the Belluno Thrust marks the tectonic juxtaposition of Early Jurassic oolitic and micritic limestones (the Calcari Grigi Group) on top of Late Jurassic-Early Cretaceous pelagic limestones (the Maiolica Fm). At the outcrop-scale, the thrust corresponds to an up to 2 m-thick complex deformation zone striking WSW-ENE and dipping NNW at less than 40°. Kinematic indicators confirm top-to-the-SSE tectonic transport. The fault consists of blandly anastomosed slip surfaces wrapping around lenses of massive cataclasites and less deformed, dm-to-half metre-thick strained lithons that sample both the hanging wall and the footwall blocks. At the top, the deformation zone is bounded by a sharp principal slip surface over which the Calcari Grigi Group is stacked. Microstructural investigations on samples that are representative of the identified fault facies denote that: (i) cataclasites consist of a fine-to-very fine- grained matrix (in the order of the clay-size fraction) hosting poorly-sorted and variably rounded fragments of limestone derived from both the hanging wall and footwall limestones as well as early cataclased fragments; (ii) the strained lithons display evidence of a long and complex deformation history documented by multiple arrays of shear and dilatant fractures filled by calcite, SC-like structures within foliated microdomains, and pervasive pressure-solution; (iii) the principal slip surface is a mm-thick surface that abruptly cuts at an angle < 10° an underlying foliated cataclasite; and (iv) the hanging wall Calcari Grigi Group limestone is deformed by a complex system of cross-cutting shear veins and diffuse evidence of pressure-solution. These preliminary observations constrain a thrusting faulting mechanism aided by major cataclastic flow and localised sliding within a seismic scenario, wherein strain localised into a progressively narrower fault core, likely assisted by fluid circulation and synkinematic clay authigenesis along individual slip surfaces. The principal slip surface likely represents the last increment of strain at the outcrop. These deformation processes are considered to be representative of the mechanical behaviour of the Belluno Thrust in response to the stress accumulation during its seismic cycle and provide further insights on strain localisation in carbonatic rocks. [ABSTRACT FROM AUTHOR]