MULTIPLE GENERATIONS OF PSEUDOTACHYLYTES FROM BRITTLE TO DUCTILE REGIMES DURING EXHUMATION OF SEISMOGENIC FAULT ZONE

The process of earthquake generation and rupture propagation within a seismogenic zone is inherently complex at all scales, but a lack of adequate geologic data inhibits any detailed investigations into the actual process of rupture and slip within the seismic fault zone. Fault-related pseudotachylyte is the only material that recorded the seismic nature of ancient faults from brittle to semi-ductile and ductile regimes. Cataclasite-related pseudotachylytes (C-Pt) form in brittle regimes and have been described from more than a dozen countries. In contrast, pseudotachylytes associated with mylonites (M-Pt) form in semi-brittle to ductile shear zones and have only been described from several localities. Although the formation mechanisms of M-Pt are still in dispute, it is generally thought that these pseudotachylytes form during episodes of seismic faulting as the case for C-Pt. Fault-related pseudotachylytes therefore provide insight into the earthquake generation process within the seismogenic fault zone. Two types of pseudotachylyte veins are documented in this presentaion: cataclasite-related pseudotachylyte (C-Pt) and mylonite-related pseudotachylyte (M-Pt), which were developed in two thrusting related fault zones: the Dahezhen shear zone related to rapid exhumation of the ultrahigh-pressure (UHP) complex in the Qinling-Dabie Shan collisional orogenic belt, central China, and the Woodroffe shear zone, Musgrave Mountains, central Australia. M-Pt is associated with mylonite-development and is overprinted by C-Pt. The foliations of M-Pt overprinted by mylonite are generally parallel to that of the country mylonite zone. The lineations within the M-Pt veins are generally oriented parallel to that of the country mylonite. The C-Pt veins cross-cut the foliations of mylonite and M-Pt, whereas M-Pt veins are overprinted by the mylonite. These facts show that the M-Pt formed cyclically in the ductile region due to propagation of seismic fracturing in the brittle regime down to the greater depth than the base of seismogenic zone. The coexistence of C-Pt, cataclasite, M-Pt and mylonite in the same shear zones suggests that repeated seismic slips occurred in both the brittle and ductile portions of the crust during the exhumation of seismogenic fault zones.