Structure and mechanical properties of seismogenic fault zones in carbonates

In many seismically active areas (e.g. Italy, Greece) earthquakes, sometimes destructive, nucleate within (aftershocks surely do) and propagate through carbonates in the upper crust (e.g. L’Aquila earthquake, 2009, Mw 6.1). Seismology, geophysics and geodesy furnish key parameters related to the earthquake source (e.g. seismic moment, static stress drop, radiated energy) but lack sufficient resolution to constrain detailed three-dimensional fault zone geometry and coseismic on- and off-fault deformation processes at scales relevant to earthquake physics. In this thesis it is proposed to study the internal structure and mechanics of fault zones hosted in carbonate rocks using a multidisciplinary approach, complementary to the seismological-based one. This includes detailed structural survey to quantify the architecture of exhumed fault zones in carbonates both by field and remote sensed methods (e.g. use of a drone to get high-resolution aerial images), rock deformation experiments under conditions relevant to the seismic cycle (e.g. use of rotary shear apparatus, uniaxial press, Split Hopkinson Pressure Bar), microstructural-mineralogical characterization (optical and scanning electron microscopy, electron microprobe analyses, X-ray powder diffraction, cathodoluminescence, X-ray microtomography, white light interferometry, image analysis) of natural and experimental fault rocks to infer the physico-chemical processes occurring during earthquakes. Two fault zones cutting dolostones exhumed from < 3 km depth in the Italian Southern Alps were described: the Borcola Pass Fault Zone (BPFZ) and the Foiana Fault Zone (FFZ). In both cases the internal structure of the two fault zones was strongly influenced by the reactivation of preexisting anisotropies such as regional-scale joint sets and bedding surfaces. The BPFZ is a secondary strike-slip branch of the regional Schio-Vicenza Line that developed in a fluid-rich upper crustal environment. The microstructural characteristics of the principal and secondary slip zones of the BPFZ, including detailed analysis of the clast size distribution of injected cataclasites, suggested coseismic fluidization processes during faulting, most likely related to the propagation of ancient seismic ruptures in to the shallow crust. The FFZ is a major sinistral transpressive fault zone that developed in a fluid-poor upper crustal setting. Systematic along-strike and down-dip changes in the structure of the FFZ were recognized, allowing a comparison to be made between field observations and the predictions of three-dimensional earthquake rupture simulations. A noteworthy characteristic of the FFZ is the presence of thick belts (hundreds of meters) of in-situ shattered dolostones cut by discrete mirror-like fault surfaces. The origin of mirror-like fault surfaces and in-situ shattered dolostones in the FFZ was investigated using, respectively, low- to high-velocity (0.0001-1 m/s) rotary shear friction experiments on dolostone gouges and low- to high-strain rate (quasi-static 10-3 s-1, dynamic > 50 s-1) uniaxial compression tests on dolostone rock cylinders. At applied normal stresses and displacements consistent with those estimated for the FFZ, experimental mirror-like fault surfaces comparable to the natural examples (e.g. clast truncation along fault surfaces, similar surface roughness) were formed in rotary-shear experiments only at seismic slip rates (v ≥ 0.1 m/s). I suggest therefore that small-displacement mirror-like fault surfaces developed in dolostone gouge layers represent markers of seismic slip. In-situ shattered dolostones similar to those found within the FFZ (i.e. rock fragments up to a few millimeters in size elongated in the stress wave loading direction, incipient zones of microfracturing down to the micrometer scale) were formed during uniaxial compression tests only above strain rates of ~ 200 s-1. The association of in-situ shattered dolostones cut by discrete mirror-like fault surfaces is interpreted to record the propagation of multiple earthquake ruptures within the shallow crustal portions of the FFZ. Lastly, the structural complexity of the studied fault zones in terms of three-dimensional geometry of the fault-fracture network, fault rock spatial distribution, fault orientation and kinematics, compares favorably to the predicted damage distribution in three-dimensional earthquake rupture simulations, as well as the structure of active seismic sources hosted in carbonate rocks as illuminated by seismological technique

"Coseismic foliations" in gouge and cataclasite: experimental observations and consequences for interpreting the fault rock record

Foliated gouges and cataclasites are commonly interpreted as the product of distributed (aseismic) fault creep. However, foliated fault rocks are often associated with localized slip surfaces, the latter indicating potentially unstable (seismic) behavior. One possibility is that such fault zones preserve the effects of both seismic slip and slower aseismic creep. An alternative possibility explored here is that some foliated fault rocks and localized slip surfaces develop contemporaneously during seismic slip. We studied the microstructural evolution of calcite- dolomite gouges deformed experimentally at slip velocities <1.13 m/s and for total displacements of 0.03 - 1 m, in the range expected for the average coseismic slip during earthquakes of Mw 3-7. As strain progressively localized in the gouge layers at the onset of high-velocity shearing, an initial mixed assemblage of calcite and dolomite grains evolved quickly to an organized, foliated fabric. The foliation was defined mainly by compositional layering and grain size variations that formed by cataclasis and shearing of individual foliation domains. Quantitative image analysis (e.g. grain size, strain) showed that the most significant microstructural changes in the bulk gouge occurred before and during dynamic weakening (<0.08 m displacement). Strain was localized to a bounding slip surface by the end of dynamic weakening and thus microstructural evolution in the bulk gouge ceased. Our experiments suggest that certain types of foliated gouge and cataclasite can form by distributed brittle \u201cflow\u201d as strain localizes to a bounding slip surface during coseismic shearing. We will also present preliminary observations of natural calcite-dolomite foliated cataclasites from the Campo Imperatore normal fault, central Italy, which bear striking resemblance to our well-characterized experimental examples

Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)

How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5–7 km depth and ≤ 300°C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125–118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth

Static versus dynamic fracturing in shallow carbonate fault zones

International audienc

Frictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile)

Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km-long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-, epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s−1) resulted in the production of vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust