Abstract
Abstract During earthquakes, melt produced by frictional heating can accumulate on slip surfaces and dramatically weaken faults by melt lubrication. Once seismic slip slows and arrests, the melt cools and solidifies to form pseudotachylytes, the presence of which is commonly used by geologists to infer earthquake slip on exhumed ancient faults. Field evidence suggests that solidified melts may weld seismic faults, resulting in subsequent seismic ruptures propagating on neighboring pseudotachylyte-free faults or joints and thus leading to long-term fault slip delocalization for successive ruptures. We performed triaxial deformation experiments on natural pseudotachylyte-bearing rocks, and show that cooled frictional melt effectively welds fault surfaces together and gives faults cohesive strength comparable to that of an intact rock. Consistent with the field-based speculations, further shear is not favored on the same slip surface, but subsequent failure is accommodated on a new subparallel fault forming on an off-fault preexisting heterogeneity. A simple model of the temperature distribution in and around a pseudotachylyte following slip cessation indicates that frictional melts cool to below their solidus in tens of seconds, implying strength recovery over a similar time scale.
Highlights
During earthquake slip on a plane or within a narrow fault zone, heat (Q) is produced in proportion to the shear stress required to cause slip (t) and the total displacement (d)
Once faults are dynamically weakened by melt lubrication, frictional resistance is insufficient to generate the heat to drive further melt production, but the melt cools and solidifies to pseudotachylyte (PST); solidification likely leads to strength recovery and may play a role in cessation of slip
Some field studies have speculated that solidified melts may weld seismic faults (Di Toro and Pennacchioni, 2005), hindering either further seismic slip or incremental repeated slip along the same fault, causing slip to migrate to a new rupture surface in the host rock (e.g., Chester and Chester, 1998)
Summary
During earthquake slip on a plane or within a narrow fault zone, heat (Q) is produced in proportion to the shear stress required to cause slip (t) and the total displacement (d). The foliated, micaceous Alpine fault zone mylonites are structurally heterogeneous on the millimeter scale (Fig. DR2 in the GSA Data Repository1). Mylonites we observe numerous subparallel PST layers in close proximity (Fig. 1B), and the bulk of melt-generating slip occurred on planes parallel to their foliation (fault veins; Sibson, 1975), which may indicate low and/or heterogeneous strength (Toy et al, 2011).
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