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http://hdl.handle.net/2122/9312
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 | Journal: | Geology (Geological Society of America) | Series/Report no.: | 1/41(2013) | Publisher: | Geological Society of America | Issue Date: | Oct-2013 | DOI: | 10.1130/G33588.1 | Keywords: | Rock mechanics shallow earthquales |
Subject Classification: | 04. 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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