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Fluid-mediated transition from dynamic rupturing to aseismic slip at the base of the seismogenic continental crust
No full textISSN:0012-821XISSN:1385-013XISSN:1385-013
Early-Oligocene low-angle normal faulting in the Eastern Alps
No full textDuring Early-Oligocenic cooling of the Rieserferner pluton (RFP, Eastern Alps) through amphibolite-greenschist facies temperature conditions, a set of shallowly (15-25 ◦ ) E-dipping joints, epidote and quartz veins developed. These joint and veins were exploited as top-to-the-East normal ductile shear zones, then overprinted by a set of steeper brittle-ductile shear zones with the same orientation and kinematics. Strong fluid-rock interaction, feldspar destabilization and chlorite+white mica+calcite veins characterize these brittle-ductile mylonites. As a whole, the orientation, kinematics and meso-microstructural evolution of these shear zones are similar to that described for the Katschberg normal fault (KNF, Genser and Neubauer, 1989), a regional scale low-angle normal fault (LANF). Therefore, some genetic relationship between the two might be outlined.
The characteristics, timing and the comparison of this set of structures with the KNF allow us to investigate both regional geology topics and mechanical processes related to LANF, in particular:
1) The KNF is miocenic in age (23 Ma). Comparing field, microstructural and published thermo-chronological data, we infer that structures in the RFP developed between 30 and 26 Ma, defining, therefore, the first occurrence of exhumation tectonics in the future Tauern Window region. In addition, diffuse extensional structures might be responsible for the “regional E-down tilting” inferred from thermochronological data (Steenken et al., 2002).
2) Given the limited amount of accommodated strain, these structures might represent the incipient stages of deformation and nucleation (on brittle precursors) of a regional-scale LANF. Nucleation probably occurred at very low-angle, given the small amount of supposed “regional E-down tilting” (5 ◦ , Steenken et al., 2002) and the unlikely occurrence of rolling-hinge model rotations. The transition from shallow- to high-angle dips probably represents the evolution in geometry of fault system during footwall uplift and decreasing confining pressure. Veins and fluid-rock interaction suggest the occurrence of high pore-fluid pressure.
3) Different weakening mechanisms contributed to slip along misoriented planes: (i) reaction weakening processes in mylonites exploiting epidote veins; (ii) inherently weak quartz with respect to the host granitoid during quartz vein shearing; (iii) feldspar-to-mica reaction during later stage brittle-ductile mylonite development. The results of rheological models about shear zone deformation will be discussed focussing to the strength evolution and to the LANF-slip/seismicity conundrum
Structural Evolution of Periadriatic Plutons and its implications on solid-state deformation of granitoid rocks
Full text linkExhumed granitoid plutons are an ideal natural research target for studying the processes of nucleation and evolution of ductile and brittle deformation structures. Granitoid plutons, unaffected by later tectono-metamorphic cycles, preserve pristine deformation structures developed during cooling from magmatic to host rock ambient temperatures, that can be assumed as representatives of structures that at different structural levels of the continental crust. The main focus of this Ph.D. project is the analysis of deformation structures of the Rieserferner pluton – one of the major Periadriatic intrusions. The aim of the study is two-fold: (i) reconstruct the tectonic framework during the different stages of the pluton structural evolution, and (ii) determine the processes controlling localized ductile strain at different scales and the environmental conditions at which they occurred.
The structural evolution during pluton cooling consists of 5 main deformation stages, that have been bracketed in time and thermal conditions according to microstructural and textural analysis, literature and field data: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins and associated ductile shear zones; (ii) shallowly dipping joints with associated epidote and quartz veins and ductile shear zones; (iii) steeply dipping mafic dykes and calcite-white mica-bearing brittle-ductile faults; (iv) steeply dipping pseudotachylyte-bearing cataclastic faults; and (v) zeolite-bearing faults. Integrating new field, microstructural and geothermo-chronological data with published data we have related the deformation sequence to the Tertiary tectonics of the Eastern Alps. (i) three main ductile deformation stages developed during Oligocene, followed by two brittle deformation stages during Miocene; (ii) paleostress inversion from kinematic analyses suggest a complex stress field variation during the structural evolution mainly due to switch in relative magnitudes of principal stress components; (iii) the described paleostress evolution reflect the sequence of tectonic processes occurred during Oligocene and Miocene at the scale of the Eastern Alps, from slab break-off to indentation and lateral escape tectonics.
Microstructural investigations were mainly focused on the analysis of ductile shear zones exploiting epidote- and quartz-rich veins. Softening and localization in quartz veins was mainly controlled by grain size reduction by recrystallization. EBSD mapping and image analyses have shown that different crystallographic orientations of quartz vein crystals controlled the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains ( ≈ 10). Recrystallization by Subgrain Rotation (SGR) lead to the development of fine-grained ultramylonitic quartz veins, in which, the observed CPO banding have been inherited from the original crystallographic orientation of the vein crystals. Localization of ductile strain within heterogeneous shear zones exploiting epidote veins was mainly obtained through myrmekite development and following shearing. EBSD investigations suggest that myrmekite induced a switch in the dominant deformation mechanism, from dynamic recrystallization by SGR to diffusion-assisted grain boundary sliding (GBS) during shearing of plagioclase + quartz aggregates.
Thermodynamic modelling was aimed to define the temperature-pressure-fluid conditions under which deformation these processes occurred. Pseudosections computed for the chemical systems NaCaKFMASHO and MnNaCaKFMASHO suggest that: (i) the epidote-veining event in the RFP likely occurred at temperatures between 520°C and 490°C at water-saturated conditions; (ii) the main deformation phase likely occurred at 460±40°C and 0.35 ± 0.05 GPa, lasting probably during pluton cooling down to 350°C at slightly under-saturated water-conditions
Structural evolution of the Rieserferner pluton in the framework of the Oligo-Miocene tectonics of the Eastern Alps
No full textThe structural evolution of the Oligocene Rieserferner pluton (Eastern Alps), developed during pluton cooling and exhumation, includes localized deformation structures consisting of: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins, and related ductile shear zones (formed at ∼600 °C); (ii) shallowly dipping joints with associated epidote and quartz veins, and related ductile shear zones (∼450 °C); (iii) steeply dipping mafic dykes and calcite + white mica-bearing brittle-ductile faults (∼300 °C); (iv) steeply dipping pseudotachylyte-bearing faults (< 250 °C); and (v) zeolite-bearing faults (< 200 °C). The structures (i-iii) and (iv-v) can be dated to the Oligocene and Early Miocene, respectively. The kinematic analysis suggests a change in local shortening direction from N290°to N350°, interpreted to reflect a change from transtension in the Early Oligocene, associated with a N060°-directed plate divergence, to transpression in the Miocene, associated with a (N)NW-directed plate convergence. This evolution reflects the sequence of transient and long-term geodynamic processes occurring in the Eastern Alps during the Tertiary, including slab break-off of the subducted European slab, the indentation of the Dolomites promontory against the Alpine stack of nappes, and the exhumation and lateral escape tectonics of the Alpine belt
In–situ quantification of mechanical and permeability properties on outcrop analogues of offshore fractured and weathered crystalline basement: Examples from the Rolvsnes granodiorite, Bømlo, Norway
Full text linkFractured and weathered crystalline basement units below erosional unconformities potentially represent unconventional
reservoirs for georesources (oil, mineral and water). The reservoir properties and characteristics
strongly depend on secondary processes connected to the local structural and alteration/weathering history.
Here we present the results of in–situ field quantification of mechanical (uniaxial compressive strength) and
petrophysical (permeability) properties of a fractured and weathered crystalline basement at selected outcrops
on the island of Bømlo (western Norway). The Bømlo outcrops are believed to represent an onshore analogue of
the unconventional oil reservoir hosted in the offshore Utsira High granodioritic fractured basement (northern
North Sea). The off– and onshore crystalline basements have both undergone surficial weathering during the
Mesozoic, as shown by the occurrence of a dated, variably thick saprolitic profile on top of fresh fractured
basement blocks. The Bømlo crystalline basement is characterized by a complex and highly permeable fracture
network. Fault rocks within its fault zones are characterised by an anisotropic mechanical strength and by an
average permeability that is two orders of magnitude larger than that of the host rock. The matrix permeability
and mechanical strength are significantly affected by alteration/weathering products. Analysis of the textural
and mineralogical characteristics of the weathered outcrops allowed us to constrain the variation of permeability
and mechanical strength as a function of increasing alteration and to infer their distribution in the, now eroded,
top–basement weathering profile on Bømlo. Weathering enhances permeability and drastically decreases the
mechanical strength. Nevertheless, evolved saprolitic horizons may act as low–permeability top–seal units to the
fractured and weathered crystalline basement reservoir. The obtained permeability and mechanical data are
finally used to better constrain the potential reservoir rocks, the fluid migration pathways, and to discuss their
role in the geomechanics of a conceptualised fractured and weathered crystalline basement unconventional
reservoir
Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation
Full text linkAt low-temperature and high-stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low-temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary-ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high-stress conditions. Pre-indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low-temperature plastic strains. Our results have implications for fault strength at the frictional-viscous transition and during transient deformation by LTP, such as seismogenic loading and post-seismic creep
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