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Authors: Smith, S. A. F.* 
Di Toro, G.* 
Kim, S.* 
Ree, J.-H.* 
Nielsen, S. B.* 
Billi, A.* 
Spiess, R.* 
Title: Coseismic recrystallization during shallow earthquake slip
Issue Date: Oct-2013
Series/Report no.: 1/41(2013)
DOI: 10.1130/G33588.1
Keywords: Rock mechanics
shallow earthquales
Subject Classification04. Solid Earth::04.01. Earth Interior::04.01.04. Mineral physics and properties of rocks 
04. Solid Earth::04.04. Geology::04.04.06. Rheology, friction, and structure of fault zones 
04. Solid Earth::04.04. Geology::04.04.09. Structural geology 
04. Solid Earth::04.07. Tectonophysics::04.07.07. Tectonics 
Abstract: Solidified frictional melts, or pseudotachylytes, remain the only unambiguous indicator of seismic slip in the geological record. However, pseudotachylytes form at >5 km depth, and there are many rock types in which they do not form at all. We performed low- to high-velocity rock friction experiments designed to impose realistic coseismic slip pulses on calcite fault gouges, and report that localized dynamic recrystallization may be an easy-to-recognize microstructural indicator of seismic slip in shallow, otherwise brittle fault zones. Calcite gouges with starting grain size <250 μm were confined up to 26 MPa normal stress using a purpose-built sample holder. Slip velocities were between 0.01 and 3.4 m s−1, and total displacements between 1 and 4 m. At coseismic slip velocities ≥0.1 m s−1, the gouges were cut by reflective principal slip surfaces lined by polygonal grains <1 μm in size. The principal slip surfaces were flanked by <300 μm thick layers of dynamically recrystallized calcite (grain size 1–10 μm) containing well-defined shape- and crystallographic-preferred orientations. Dynamic recrystallization was accompanied by fault weakening and thermal decomposition of calcite to CO2 + CaO. The recrystallized calcite aggregates resemble those found along the principal slip surface of the Garam thrust, South Korea, exhumed from <5 km depth. We suggest that intense frictional heating along the experimental and natural principal slip surfaces resulted in localized dynamic recrystallization, a microstructure that may be diagnostic of seismic slip in the shallow crust.
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