Rock magnetic evidence for the seismogenic setting of large earthquakes in the Longmen Shan fault zone

The seismogenic setting of earthquake fault zones is important for seismological research. To investigate the seismogenic environment of earthquakes in the Longmen Shan fault zone, we carried out microstructural and rock-magnetic analyses of pseudotachylyte, cataclasite and its wall rocks within cores from the Wenchuan earthquake Fault Scientific Drilling borehole 2 (WFSD-2). Pseudotachylyte, observed in a cataclasite zone, attests to that large earthquakes have occurred in the Longmen Shan fault zone. It has higher magnetic susceptibility values than cataclasite, while cataclasite has similar values as its wall rocks. The newly-formed magnetite contributes to the higher values of magnetic susceptibility of pseudotachylyte. In addition, a little newly-formed magnetite in the pseudotachylyte indicates that pseudotachylyte was formed at high temperature and a reducing environment. Frictional heating is the main factor which changes the rock magnetic properties of pseudotachylytes in the Longmen Shan fault zone, while fluids play a minor role. The cataclasite has never been subjected to heating over 300 degrees C in nature, which implies that the pseudotachylye was formed at depth. More than 20 layers of pseudotachylytes demonstrate that the Yingxiu-Beichuan fault zone is a long-term active fault, and ancient powerful earthquakes have occurred repeatedly in the Longmen Shan fault zone. These earthquakes most likely occurred in deep seismogenic environment, with strong reducing and weak fluid activity.