Seismic Cycle in Bituminous Dolostones (Monte Camicia Thrust Zone, Central Apennines, Italy)

Abstract

Seismic ruptures often propagate along fault zones cutting km-thick sequences of carbonates (e.g., Wenchuan Mw 7.8, 2008, China; L’Aquila Mw 6.1, 2009, Italy). As a consequence, fault rock assemblages may record the seismic cycle under a wide range of loading conditions, temperatures (>1,000°C during co-seismic slip), and fluid-rock interactions. The Monte Camicia Thrust Zone in the Italian Central Apennines is exhumed from ∼3 km depth. We studied the seismic cycle recorded by a network of faults cutting bituminous dolostones in the footwall of the Monte Camicia Thrust. These faults accommodate up to several meters of displacement. Slip zones are mm- to cm-thick and bounded by ultra-polished (“mirror-like”) surfaces independently of their displacement. At the microscale, deformation is accommodated by cataclasis and pressure-solution in carbonates, and viscous flow in the foliated bitumen. Some of the faults with displacements >0.10 m have multiple slip zones, separated by “mirror-like” surfaces, and include clasts of foliated bitumen and fragments of older slip zones sealed by calcite precipitation. We conclude that (a) slip zones record post- to inter-seismic (foliated bitumen) and co-seismic (fragments of bitumen) deformation in a fluid-rich environment (calcite precipitation) and (b) mirror-like surfaces formed during all phases of the seismic cycle.