Rock record and magnetic response to large earthquakes within Wenchuan Earthquake Fault Scientific Drilling cores

Abstract

Fault-related pseudotachylytes are often considered to be produced by large seismic events. To investigate the rock record and magnetic response to large earthquakes within cores from the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD-2), we carried out microstructural, geochemical, and rock-magnetic analyses of representative cores. Based on microstructural observations and powder X-ray diffraction analyses, we found 21 layers of melt-origin pseudotachylytes from 579.62 to 599.31 m-depth in the cores. The presence of early-formed pseudotachylyte fragments in the new layer suggests that seismic faulting processes exploited the same fault strand more than once. Pseudotachylyte veins have higher values of magnetic susceptibility relative to wall rocks. Rock-magnetic results indicate that the magnetic minerals within the pseudotachylyte veins are magnetite with varying amounts of paramagnetic minerals. Magnetic hysteresis loops show that a reduction of the grain size of ferromagnetic minerals is not a plausible explanation for the higher magnetic susceptibility values in pseudotachylyte veins. Rock-magnetic analyses indicate that frictional heating (>500°C) occurred in the pseudotachylyte veins during large earthquakes. The resulting high temperatures induced thermal decomposition of paramagnetic minerals, forming magnetite and contributing to the higher magnetic susceptibility values. Different generations of pseudotachylytes and numerous high magnetic susceptibility zones together demonstrate that ancient powerful earthquakes may have occurred repeatedly in the Longmen Shan thrust belt.