Search

Abstract: Submicron Raman spectroscopy mapping is able to unambiguously distinguish the main serpentine minerals within their in situ microstructural context. The high spatial resolution (~370 nm), large‐area coverage (up to hundreds of micrometres in each dimension), and ability to map directly on polished thin sections allows novel interpretations to be made regarding the nature and evolution of serpentinite fault rock textures. The potential of this method is illustrated by examining submicron‐scale textures of scaly serpentinites (e.g., dissolution seams, mineral growth in pressure shadows, distribution, and intergrowth of serpentine minerals) from a lithospheric‐scale shear zone in New Zealand and a subduction‐related serpentinite body in California, USA. [ABSTRACT FROM AUTHOR]

Earthquakes occur along faults in response to plate tectonic movements, but paradoxically, are not widely recognized in the geological record, severely limiting our knowledge of earthquake physics and hampering accurate assessments of seismic hazard. Light-reflective (so-called mirror like) fault surfaces are widely observed geological features, especially in carbonate-bearing rocks of the shallow crust. Here we report on the occurrence of mirror-like fault surfaces cutting dolostone gouges in the Italian Alps. Using friction experiments, we demonstrate that the mirror-like surfaces develop only at seismic slip rates (~1 m/s) and for applied normal stresses and sliding displacements consistent with those estimated on the natural faults. Under these experimental conditions, the frictional power density dissipated in the samples is comparable to that estimated for natural earthquakes (1-10 MW/m²). Our results indicate that mirror-like surfaces in dolostone gouges are a signature of seismic faulting, and can be used to estimate power dissipation during ancient earthquake ruptures. [ABSTRACT FROM AUTHOR]

The authors respond to the comments made on their article about the interactions between low-angle normal faults and plutonism in the upper crust in the island of Elba, Italy. They appreciate the discussers for the additional evidence that they provided regarding the presence of the Zuccale low-angle normal fault on the southside of the Calamita Peninsula in Elba, Italy. They also retain that this fault has a broad dormal geometry on a regional scale.

The Longmenshan fault that ruptured during the 2008 Mw 7.9 Wenchuan (China) earthquake was drilled to a depth of 1200 m, and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575-595 m. We report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths. Results indicate that the Longmenshan fault at this locality is extremely weak at seismic slip rates. In situ synchrotron X-ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products, similar to the occurrence of graphite in the natural principal slip zone of the 2008 Wenchuan rupture. We surmise that graphitization occurred due to frictional heating of carbonaceous minerals. Because graphitization was associated with strong dynamic weakening in the experiments, we further infer that the Longmenshan fault was extremely weak at borehole depths during the 2008 Wenchuan earthquake, and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]