1,721,001 research outputs found
Transtensive faulting in carbonates at different crustal levels: examples from SW Helvetics and Central Apennines
No full textFault rocks at different crustal depths generate different fault structures and tectonites mostly depending on lithology, fluid flow and temperature. In this note we address the fault zone development and deformation mechanisms that occur in carbonate lithologies at different crustal levels. We compare faults exhumed near the brittle-ductile transition in the Helvetic Nappes in theAlps and similar faults from the upper crust in the Gran Sasso area in the Central Appenines. We observe the different role of deformation processes at different depth and lithology and assess the tendency for localization at the Brittle Ductile Transition
Transtensive faulting in carbonates at different crustal levels: Examples from SW Helvetics and Central Apennines
No full textTranstensive faulting in carbonates at different crustal levels: Examples from SW Helvetics and Central Apennines
No full textFault rocks at different crustal depths generate different fault structures and tectonites mostly depending on lithology, fluid flow and temperature. In this note we address the fault zone development and deformation mechanisms that occur in carbonate lithologies at different crustal levels. We compare faults exhumed near the brittleductile transition in the Helvetic Nappes in theAlps and similar faults from the upper crust in the Gran Sasso area in the Central Appenines. We observe the different role of deformation processes at different depth and lithology and assess the tendency for localization at the Brittle Ductile Transition
Gravity-driven deepwater fold-and-thrust belts as Critical Coulomb Wedges: Model limitations and the role of friction vs. fluid pressure
No full textGravity-driven Deepwater Fold-and-thrust Belts (GDFB) are the result of gravitational collapse of the sedimentary pile along the continental shelf. These systems can be described in the framework of the Critical Coulomb Wedge (CCW) mechanics to infer their mechanical properties and hydrological conditions. However, not all GDFB can be considered as critical wedges and their mechanical/hydrological parameters should be critically assessed. GDFB driven by gravity spreading and detached onto brittle overpressured shales are the most suitable systems to be modeled via the original CCW theory. In addition, the self-limiting mechanism of gravitational collapse implies that GDFB can reach the critical state only during limited time intervals (in some cases less than similar to 10 Myr), in the presence of large sedimentary loads and shortening rates higher than 1.5-2 mm/yr. We present a compilation of GDFB that most likely represent examples of critical tapers, almost exclusively located downdip of large deltas. We then analyze several analytical solutions to the CCW equation, constrained by laboratory and geophysical data, to discuss the role of material friction and fluid pressures in GDFB. Low friction and moderate fluid overpressure localized in the basal detachment are both essential to explain the observed shape of GDFB. Frictional and overpressure discontinuities between detachment and the overlying wedge are likely to be maintained in GDFB
Fault architecture and deformation mechanisms in exhumed analogues of seismogenic carbonate-bearing thrusts
No full textThe role of pre-existing anisotropies in fault mechanics: experimental insights from triaxial saw-cut experiments
No full textFault orientation and fault rock mechanical properties exert a major control on fault reactivation. Slip along faults that lie at high angle to the maximum compressive stress requires restrictive conditions, such as very low friction coefficient and/or near-lithostatic pore fluid pressure. However, the mechanism allowing a misoriented fault to initiate is still a matter of debate. Nevertheless, misoriented faults, such as the San Andreas fault, low-angle normal faults and sub-horizontal décollements, are recognized throughout the world. On a smaller scale, field observations show that mesoscale faults in mechanical multilayers often propagate within clay-rich layers at high angle to the maximum principal stress. These observations suggest that the presence of pre-existing anisotropies, not only resulting from faulting but also due to inherited foliation or sedimentary layering, plays a key role in determining fault geometry. To investigate this problem, we present a suite of triaxial experiments in which we deformed cylindrical samples of sandstone with pre-imposed saw-cuts with variable orientations respect to the axial stress. A layer of powdered marl or shale was placed within the saw-cut to simulate a pre-existing anisotropy. We performed: 1) conventional triaxial experiments to evaluate the failure envelope of the sandstone, and 2) biaxial experiments to evaluate the frictional strength of the marl and shale. Then, 3) we conducted triaxial experiments at constant confining pressure with saw-cuts oriented at different angles to the axial stress, from 30° (favourably oriented) up to 80° (severely misoriented). Microstructural observations on reactivated saw-cuts were conducted to investigate the deformation processes with increasing fault deformation. Finally, we compared the resulting deformation mode, i.e. saw- cut reactivation vs. new fracture development, with theoretical predictions based on sandstone strength and marl/shale friction. Our results show a complex mechanical behaviour that results from the interplay between the stress field orientation and the contrast in mechanical properties between the gouge and the surrounding. Reactivation occurs only in saw-cuts at angles to the axial stress that are lower than 60°-70°, consistently with theoretical predictions. Preliminary laboratory data show that under dry conditions the reactivation is a multi-stage process: after an initial compaction phase and a linear-elastic phase, yielding of the fault gouge occurs. Given the same boundary conditions, yielding occurs at the same differential stress independently of saw-cut orientation. After the yielding, a second linear-elastic phase precedes stick-slips followed by stable sliding. Integrating mechanical data and microstructural investigation we suggest that the first inelastic yielding occurs when the Mohr circle is tangent to the frictional reactivation criterion, obtained from friction experiments (2). Then, stick-slip and stable sliding are associated to localization of deformation along shear planes. We infer that they occur only when the stress path of the saw-cut intersects the frictional reactivation criterion
Frictional strength and deformation microstructures of mineralogically controlled Serpentinites
No full textMechanical heterogeneities in Umbria-Marche Apennines Thrusts: A field and experimental perspective
No full textPressure solution seams in carbonatic fault rocks: mineralogy, micro/nanostructures and deformation mechanism
No full textWe have investigated mineralogy and micro/nanostructures of pressure solution seams in four different carbonatic faults with kilometric displacement, cropping out in the Northern Apennines, Italy. Disregarding the different protoliths and deformation conditions, the stylolite-filling material has almost constant mineralogical characteristics, being invariably formed by an ultrafine
matrix that encloses relic insoluble grains, among which quartz, feldspars and detritic micas. The ultrafine matrix also hosts syn- and post-dynamic phases (e.g., foliationparallel goethite flakes and apatite euhedral nanocrystals in random orientation). The ultrafine matrix is formed by smectitic clays in nanosized (001) lamellae, showing pervasive interlayer fissuring, layer bending and preferred orientation parallel to the slipping surface. Stylolite mineralogy and micro/nanostructures may affect deformation
mechanisms and permeability properties of the fault rock.In particular, we propose that the extremely low friction coefficient of smectite would favour frictional sliding along the faults and that the fissured and oriented nanostructure of the smectite-dominated seams would enhance the sealing attitude of the structures in the fault-perpendicular direction
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