• journal_title：Geology
• Contributor：S.A.F. Smith ; G. Di Toro ; S. Kim ; J.-H. Ree ; S. Nielsen ; A. Billi ; R. Spiess
• Publisher：Geological Society of America
• Date：2013-01-01
• Format：text/html
• Language：en
• Identifier：10.1130/G33588.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：1
• firstpage：63
• section：Articles

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.