Strain localization and the onset of dynamic weakening in calcite fault gouge

To determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates, single-slide and slide–hold–slide experiments were conducted on 2–3-mm thick layers of calcite gouge at normal stresses up to 26 MPa and slip rates up to 1 m s−1. Microstructures were analyzed from short displacement (<35 cm) experiments stopped prior to and during the transition to dynamic weakening. In fresh calcite gouge layers, dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness. Strain is initially distributed across the full thickness of the gouge layer, but within a few millimeters displacement the strain becomes localized to a boundary-parallel, high-strain shear band c. 20 μm wide. During the strengthening phase, which lasts between 3 and 30 cm under the investigated conditions, the shear band broadens to become c. 100 μm wide at peak stress. The transition to dynamic weakening in calcite gouges is associated with the nucleation of micro-slip surfaces dispersed throughout the c. 100 μm wide shear band. Each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite, interpreted to result from grain welding driven by local frictional heating in the shear band. By the end of dynamic weakening strain is localized to a single 2–3-μm wide principal slip surface, flanked by layers of recrystallized gouge. Calcite gouge layers re-sheared following a hold period weaken nearly instantaneously, much like solid cylinders of calcite marble deformed under the same experimental conditions. This is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide, reducing the effective gouge layer thickness to a few microns. Our results suggest that formation of a high-strain shear band is a critical precursor to dynamic weakening in calcite gouges. Microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate. The delayed onset of dynamic weakening in fresh calcite gouge layers, particularly at low normal stresses, may inhibit large coseismic slip at shallow crustal levels in calcite-bearing fault zones

Catastrophic emplacement of giant landslides aided by thermal decomposition: Heart Mountain, Wyoming

The Heart Mountain landslide of northwest Wyoming is the largest known sub-aerial landslide on Earth. During its emplacement more than 2000 km3 of Paleozoic sedimentary and Eocene volcanic rocks slid >45 km on a basal detachment surface dipping 2°, leading to 100 yr of debate regarding the emplacement mechanisms. Recently, emplacement by catastrophic sliding has been favored, but experimental evidence in support of this is lacking. Here we show in friction experiments on carbonate rocks taken from the landslide that at slip velocities of several meters per second CO2 starts to degas due to thermal decomposition induced by flash heating after only a few hundred microns of slip. This is associated with the formation of vesicular degassing rims in dolomite clasts and a crystalline calcite cement that closely resemble microstructures in the basal slip zone of the natural landslide. Our experimental results are consistent with an emplacement mechanism whereby catastrophic slip was aided by carbonate decomposition and release of CO2, allowing the huge upper plate rock mass to slide over a ‘cushion’ of pressurized material

Interactions between low-angle normal faults and plutonism in the upper crust: Insights from the Island of Elba, Italy.

low-angle normal fault on the Island of Elba, Italy, was one of the principal structures active during extensional collapse of the Apennine fold-and-thrust belt. We investigate the relationships among the Zuccale fault, subsidiary footwall fault networks, and igneous bodies that were intruded into the immediate footwall of the Zuccale fault. Both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent WNW-ESE extension direction, suggesting that faulting and intrusion overlapped in time. Structure contour analysis indicates that the Zuccale fault has a regional domal morphology. The dimensions and spatial location of the dome correlate with the likely subsurface position of the Porto Azzurro pluton, originally intruded at ~6 km depth. We propose that doming of the Zuccale fault may have been related in part to emplacement of the Porto Azzurro pluton as a tabular intrusion, involving some component of vertical infl ation and roof uplift. The immediate footwall of the Zuccale fault is everywhere crosscut by a complex, linked network of high- and low-angle extensional faults with observed displacements of <10 m. Mutual crosscutting relationships suggest that low- and high-angle faults were active broadly contemporaneously. The fi - nal geometry of the footwall fault networks is adequately explained by their position with respect to the regional domal structure, and they suggest that certain sections of the Zuccale fault were back-rotated—during doming—out of an orientation capable of accommodating continued regional extension.Published329-346JCR Journalrestricte

Foreword to Special Issue: "Fault Zone Structure, Mechanics and Evolution in Nature and Experiment"

Editorial, numéro spécial sur "Fault zone structure, mechanics and evolution in nature and experiment", suite à la session sur le sujet à l'EGU 2011.International audienceTectonic faults in the Earth's upper crust are geometrically complex zones of localized deformation and rock damage that evolve over wide length- and time-scales. Fault zones influence a range of crustal processes including fluid flow, mechanical strength, basin evolution, and earthquake nucleation, propagation and arrest. Because of this, fault zones motivate considerable academic and commercial research, and have been the focus of a large body of work involving field observations, theory, numerical simulations, experiments and seismology. Despite recent progress, significant aspects of fault zone structure and evolution remain poorly understood. Uncertainties remain as to the main deformation mechanisms that are active during the seismic cycle. It is also largely unclear how deformation is partitioned between principal faults and adjacent regions of off-fault damage, and to what extent associated fluid flow is influenced by varying fracturemodes in different lithologies. A further problem concerns the evolution of fault roughness and fault rock granularity during slip and howthis impacts on fault mechanical behavior. Some of these issues are discussed in this Special Issue of the Journal of Structural Geology which arises from a session held at the EuropeanGeosciences Union (EGU) General Assembly in Vienna, April 2011. The contributions in this volume highlight some of the complexityofnatural fault systems. Common themes that linkmany of the contributions are the relationships between fault zone structure and fault dynamics, and the importance of fault zone structure for sub-surface fluid flow, particularly in fractured carbonates. The 11 papers here are divided in to two parts, although the subdivision is somewhat arbitrary given the multidisciplinary nature of many of the contributions. Papers in Part I, Fault zone structure and evolution, deal mainly with field characterization of fault zones at outcrop to regional scales, and associated observations of fault rock microstructures. Papers in Part II, Fault roughness and brittle rock damage, present field and experimental constraints on the geometry and mechanical properties of fault slip surfaces, pulverized and sheared fault rocks, and cataclastic deformation bands

The development of interconnected talc networks and weakening of continental low-angle normal faults

Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a significant mechanical problem. Here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite + abundant talc along a low-angle normal fault. Within the fault core, talc forms an interconnected foliated network that deforms by frictional sliding along 50-200-nm-thick talc lamellae. The low frictional strength of talc, combined with dissolution-precipitation creep, can explain slip on low-angle normal faults. In addition, the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures. The development of phyllosilicates such as talc by fluid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness. © 2009 Geological Society of America

Numerical analysis of fold curvature using data acquired by high-precision GPS

The maximum, minimum, Gaussian, and mean curvatures of a folded bedding surface are calculated from a high resolution data set. These data were collected using real-time kinematic (RTK) GPS and processed using commercially available software. The curvature parameters are calculated analytically from the first and second fundamental forms of the folded surface. Matrix algebra makes this method efficient for large data sets. Contoured maps of the curvature parameters can then be used as basemaps for the interpretation of other structural information such as fracture densities and orientations. This method provides precise analysis of folded surfaces in three dimensions and the data can be incorporated into larger-scale data sets obtained from seismic surveys

Fault structure, frictional properties and mixed-mode fault slip behavior

Recent high-resolution GPS and seismological data reveal that tectonic faults exhibit complex, multi-modeslipbehavior including earthquakes, creep events, slow and silent earthquakes, low-frequency events and earthquake afterslip. The physical processes responsible for this range of behavior and the mechanisms that dictate faultslip rate or rupture propagation velocity are poorly understood. One avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the Earth's surface with laboratory experiments on the frictionalproperties of the fault rocks. Here, we show that fault zone structure has an important influence on mixed-modefaultslipbehavior. Our field studies depict a complex fault zone structure where foliated horizons surround meter- to decameter-sized lenses of competent material. The foliated rocks are composed of weak mineral phases, possess low frictional strength, and exhibit inherently stable, velocity-strengthening frictionalbehavior. In contrast, the competent lenses are made of strong minerals, possess high frictional strength, and exhibit potentially unstable, velocity-weakening frictionalbehavior. Tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons. With continued deformation, fault creep will concentrate stress within and around the strong and potentially unstable competent lenses, which may lead to earthquake nucleation. Our studies provide field and mechanical constraints for complex, mixed-modefaultslipbehavior ranging from repeating earthquakes to transient slip, episodic slow-slip and creep event