116 research outputs found
Physical properties of seismogenic Triassic evaporites in the northern Appennines (Central Italy)
see Abstract Volum
Deformation processes, textural evolution and weakening in retrograde serpentinites
Serpentinites play a key role in controlling fault rheology in a wide range of geodynamic settings, from oceanic and continental rift zones to subduction zones. In this paper, we provide a summary of the most common deformation mechanisms and frictional strengths of serpentine minerals and serpentinites. We focus on deformation mechanisms in retrograde serpentinites, which show a progressive evolution from undeformed mesh and bastite pseudomorphic textures to foliated, ribbon-like textures formed by lizardite with strong crystallographic and shape preferred orientations. We also discuss the possible mechanical significance of anastomosing slickenfibre veins containing ultraweak fibrous serpentines or relatively strong splintery antigorite. Our review and new observations indicate that pressure solution and frictional sliding are the most important deformation mechanisms in retrograde serpentinite, and that they are frictionally weak (Ό~0.3). The mineralogical and microstructural evolution of retrograde serpentinites during shearing suggests that a further reduction of the friction coefficient to Ό of 0.15 or less may occur during deformation, resulting in a sort of continuous feedback weakening mechanism
Stabilization of fault slip by fluid injection in the laboratory and in situ
Faults can slip seismically or aseismically depending on their hydromechanical properties, which can be measured in the laboratory. Here, we demonstrate that fault slip induced by fluid injection in a natural fault at the decametric scale is quantitatively consistent with fault slip and frictional properties measured in the laboratory. The increase in fluid pressure first induces accelerating aseismic creep and fault opening. As the fluid pressure increases further, friction becomes mainly rate strengthening, favoring aseismic slip. Our study reveals how coupling between fault slip and fluid flow promotes stable fault creep during fluid injection. Seismicity is most probably triggered indirectly by the fluid injection due to loading of nonpressurized fault patches by aseismic creep
Fault structure and slip localization in carbonate-bearing normal faults: An example from the Northern Apennines of Italy
Carbonate-bearing normal faults are important structures for controlling fluid flow and seismogenesis
within the brittle upper crust. Numerous studies have tried to characterize fault zone structure and
earthquake slip processes along carbonate-bearing faults. However, due to the different scales of
investigation, these studies are not often integrated to provide a comprehensive fault image. Here we
present a multi-scale investigation of a normal fault exhumed from seismogenic depths. The fault extends
for a length of 10 km with a maximum width of about 1.5 km and consists of 5 sub-parallel and
interacting segments. The maximum displacement (370e650 m) of each fault segment is partitioned
along sub-parallel slipping zones extending for a total width of about 50 m. Each slipping zone is
characterized by slipping surfaces exhibiting different slip plane phenomena. Fault rock development is
controlled by the protolith lithology. In massive limestone, moving away from the slip surface, we
observe a thin layer (<2 cm) of ultracataclasite, cataclasite (2e10 cm) and fault breccia. In marly limestone,
the fault rock consists of a cataclasite with hydrofractures and smectite-rich pressure solution
seams. At the micro-nanoscale, the slip surface consists of a continuous and thin (<300 mm) layer
composed of coarse calcite grains (~5e20 mm in size) associated with sub-micrometer grains showing
fading grain boundaries, voids and/or vesicles, and suggesting thermal decomposition processes.
Micrometer-sized calcite crystals show nanoscale polysynthetic twinning affected by the occurrence of
subgrain boundaries and polygonalized nanostructures. Investigations at the kilometres-tens of meter
scale provide fault images that can be directly compared with high-resolution seismological data and
when combined can be used to develop a comprehensive characterization of seismically active fault
structures in carbonate lithologies. Micro and nanoscale investigations along the principal slipping zone
suggest that different deformation processes, including plastic deformation and thermal decomposition,
were active during seismic slip
ï»żFault weakening due to CO2 degassing in the Northern Apennines: ï»żshort- and long-term processes
ï»żThe influx of fluids into fault zones can trigger two main types of weakening
processes that operate over different timescales and facilitate fault movement and
earthquake nucleation.
Short-term and long-term weakening mechanisms along faults require a
continuous fluid supply near the base of the brittle crust, a condition satisfied in the
extended/extending area of the Northern Apennines of Italy. Here carbon mass
balance calculations, coupling aquifer geochemistry to isotopic and hydrological data, define the presence of a large flux (âŒ12,160 t d-1) of deep-seated CO2 centred in the
extended sector of the area. In the currently active extending area, CO2 fluid
overpressures at âŒ85% of the lithostatic load have been documented in two deep (4-5
km) boreholes.
In the long-term, field studies on an exhumed regional low-angle normal fault
show that during the entire fault history, fluids reacted with fine-grained cataclasites
in the fault core to produce aggregates of weak, phyllosilicate-rich fault rocks that
deform by fluid assisted frictional-viscous creep at sub-Byerlee friction values (Ό <
0.3). In the short-term, fluids can be stored in structural traps, such as beneath mature
faults, and stratigraphical traps such as Triassic evaporites. Both examples preserve
evidence for multiple episodes of hydrofracturing induced by short-term cycles of
fluid pressure build-up and release.
Geochemical data on the regional-scale CO2 degassing process can therefore be
related to field observations on fluid rock interactions to provide new insights into the
deformation processes responsible for active seismicity in the Northern Apennines
Activity-based cost analysis of hepatic tumor ablation using CT-guided high-dose rate brachytherapy or CT-guided radiofrequency ablation in hepatocellular carcinoma
Connecting seismically active normal faults with Quaternary geological structures in a complex extensional environment: The Colfiorito 1997 case history (northern Apennines, Italy)
Thermal weakening friction during seismic slip experiments and models with heat sources and sinks
Experiments that systematically explore rock friction under crustal earthquake conditions reveal that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surfaces have been invoked to explain pronounced velocity weakening. Both contact asperity temperature Ta and background temperature T of the slip zone evolve significantly during high-velocity slip due to heat sources (frictional work), heat sinks (e.g., latent heat of decomposition processes), and diffusion. Using carefully calibrated High-Velocity Rotary Friction experiments, we test the compatibility of thermal weakening models: (1) a model of friction based only on T in an extremely simplified, Arrhenius-like thermal dependence; (2) a flash heating model which accounts for the evolution of both V and T; (3) same but including heat sinks in the thermal balance; and (4) same but including the thermal dependence of diffusivity and heat capacity. All models reflect the experimental results but model (1) results in unrealistically low temperatures and model (2) reproduces the restrengthening phase only by modifying the parameters for each experimental condition. The presence of dissipative heat sinks in stage (3) significantly affects T and reflects on the friction, allowing a better joint fit of the initial weakening and final strength recovery across a range of experiments. Temperature is significantly altered by thermal dependence of (4). However, similar results can be obtained by (3) and (4) by adjusting the energy sinks. To compute temperature in this type of problem, we compare the efficiency of three different numerical approximations (finite difference, wavenumber summation, and discrete integral)
Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions
The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient ” of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits ” = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter aâb and the critical slip distance D c , combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment M o = ~4 Ă 1010 for an M w = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3â7 km
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