Nanometric flow and earthquake instability

Fault zones accommodate relative motion between tectonic blocks and control earthquake nucleation. Nanocrystalline fault rocks are ubiquitous in “principal slip zones” indicating that these materials are determining fault stability. However, the rheology of nanocrystalline fault rocks remains poorly constrained. Here, we show that such fault rocks are an order of magnitude weaker than their microcrystalline counterparts when deformed at identical experimental conditions. Weakening of the fault rocks is hence intrinsic, it occurs once nanocrystalline layers form. However, it is difficult to produce “rate weakening” behavior due to the low measured stress exponent, n, of 1.3 ± 0.4 and the low activation energy, Q, of 16,000 ± 14,000 J/mol implying that the material will be strongly “rate strengthening” with a weak temperature sensitivity. Failure of the fault zone nevertheless occurs once these weak layers coalesce in a kinematically favored network. This type of instability is distinct from the frictional instability used to describe crustal earthquakes.

A natural example of brittle-to-viscous strain localization under constant-stress conditions: a case study of the Kellyland fault zone, Maine, USA

This article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a ∼50 m wide band of pulverized foliated granite, (3) a ∼2.8 m wide breccia zone hosting coeval shear zones, and (4) a >100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96–104 MPa at temperatures of 400–450 °C and is marked by coeval pulverization and formation of breccia. Interseismic viscous creep at similar flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted shear zones, brittle fractures and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the >100 m wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive creep at 112–124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle–viscous transition and uppermost viscous zone are the strongest part of the crust.

Carbonate clumped isotope thermometry of fault rocks and its possibilities: tectonic implications from calcites within Himalayan Frontal Fold-Thrust Belt

Disentangling the temperature and depth of formation of fault rocks is critical for understanding their rheology, exhumation, and the evolution of fault zones. Estimation of fault rock temperatures mostly relies on conventional geothermometers of metamorphic minerals and element partitioning analysis, which are largely inapplicable in shallow crustal fault rocks. Here, we demonstrate the applicability of the carbonate clumped isotope thermometer in low-grade carbonate-bearing fault rocks from the Himalayan frontal wedge (northwest India). Coalescing carbonate clumped isotope thermometry and calcite e-twin morphology allows us to constrain the temperature and depth of formation of the two main thrusts of the Himalayan frontal wedge, the Nahan thrust (170 ± 10 °C; 6–7 km depth), and the Main Boundary thrust (262 ± 30 °C; 10–11 km depth). The integration of the adopted analytical techniques can promote the application of calcite-based clumped isotope thermometry to the fault zone processes and refinement of shallow crustal fault zone models.

The Quaternary Active El Arrayan Fault, Santiago, Chile

Understanding the location and nature of Quaternary active crustal faults is critical to the reduction of both fault rupture and strong ground motions hazards in the built environment. Recent work along the San Ramon Fault in Santiago, Chile demonstrates that crustal seismic sources are important hazards. We present the results of a second likely Quaternary active fault (the El Arrayan Fault, EAF) that runs through the City of Santiago. The EAF was discovered at an outcrop in El Arrayan (Lo Barnechea) with up to the North reverse motion and sinistral (left-lateral) motion clearly visible and coincident with fault rocks (gouge, cataclasite, and breccia) and higher topography (i.e. uplift) in the hanging wall. The EAF is at least 12 km long, strikes North-Northwest to South-Southeast, and is steeply dipping (mean dip 77º NE). Clear geomorphic expression with sinistral displaced streams (up to ~210 m) suggest that this fault is Quaternary active and an important local source of fault rupture and crustal strong ground motions. Because no fault zone avoidance criteria in Chile, there is need for enhanced fault mapping, legislation, implementation of active fault rupture avoidance areas in Chile to reduce the risk posed by active crustal structures.

Evidence for 40–41 km of dextral slip on the southern Death Valley fault: Implications for the Eastern California shear zone and extensional tectonics

Recognition of a pair of pre-Neogene markers together with analysis of published data indicate ~40 km of dextral slip across the southern Death Valley fault zone, California, USA. Stratigraphic overlaps on fault rocks indicate much of the dextral slip predates the late Miocene, placing a significant fraction of the dextral slip in the same time window as regional extension and challenging interpretations that the modern strike-slip system became active post–6–3 Ma. However, these results are consistent with regional evidence that dextral transtension began by ca. 12 Ma.