Abstract
Laboratory experiments on serpentinite suggest that extreme dynamic weakening at earthquake slip rates is accompanied by amorphisation, dehydration and possible melting. However, hypotheses arising from experiments remain untested in nature, because earthquake ruptures have not previously been recognised in serpentinite shear zones. Here we document the progressive formation of high-temperature reaction products that formed by coseismic amorphisation and dehydration in a plate boundary-scale serpentinite shear zone. The highest-temperature products are aggregates of nanocrystalline olivine and enstatite, indicating minimum peak coseismic temperatures of ca. 925 ± 60 °C. Modelling suggests that frictional heating during earthquakes of magnitude 2.7–4 can satisfy the petrological constraints on the coseismic temperature profile, assuming that coseismic fluid storage capacity and permeability are increased by the development of reaction-enhanced porosity. Our results indicate that earthquake ruptures can propagate through serpentinite shear zones, and that the signatures of transient frictional heating can be preserved in the fault rock record.
Highlights
Laboratory experiments on serpentinite suggest that extreme dynamic weakening at earthquake slip rates is accompanied by amorphisation, dehydration and possible melting
Ancient earthquake ruptures have not previously been recognised in exhumed serpentinite shear zones[33] and hypotheses developed from deformation experiments and numerical modelling concerning possible dynamic weakening mechanisms remain untested in nature
We analysed the content of these serpentinite inclusions by transmission electron microscopy (TEM) and document below the appearance of distinct textures and mineral assemblages that can be related to the progressive amorphisation and dehydration of serpentinite within the inclusions, which we interpret to have occurred during propagation of a coseismic thermal pulse
Summary
Laboratory experiments on serpentinite suggest that extreme dynamic weakening at earthquake slip rates is accompanied by amorphisation, dehydration and possible melting. Our results indicate that earthquake ruptures can propagate through serpentinite shear zones, and that the signatures of transient frictional heating can be preserved in the fault rock record. In the case of serpentinite, it has been proposed that this transition in fault stability is enhanced at high slip rates by dynamic weakening mechanisms that involve flash heating of asperity contacts[25,26,27], thermal pressurisation of pore fluids[28,29,30] and mineral dehydration reactions that release structurally-bound fluid into the coseismic slip zone[31,32]. Ancient earthquake ruptures have not previously been recognised in exhumed serpentinite shear zones[33] and hypotheses developed from deformation experiments and numerical modelling concerning possible dynamic weakening mechanisms remain untested in nature. Whether creeping serpentinite shear zones can transiently host dynamic earthquake ruptures remains an open question
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