Repeated Seismic Slips Recorded in Fluidized ultracataclastic Veins within Shallow Seismogenic Fault Zones

Abstract:

It is well known that direct evidence of earthquakes within fault zones is limited to the presence of pseudotachylyte. In addition to pseudotachylyte, previous studies have shown that the meso- and microstructural features of cataclastic veins that lack the primary cohesion of the host rocks, including crush-origin pseudotachylyte, fault breccia, may represent primary evidence of brittle deformation caused by recurrent seismic slip within shallow seismogenic fault zones (e.g., Lin, 1996, 2008).

In this presentation, we report on the structural mode of typical ultracataclastic veins including pseudotachylyte and fault gouge veins that formed repeatedly as simple veins and complex networks at shallow depth within two main active fault zones along the Arima-Takatsuki Tectonic Line (ATTL) and the Itoigawa-Shizuoka Tectonic Line (ISTL), Japan. Multistage veinlet cataclastic rocks, composed of aphanitic veins typical of pseudotachylyte and unconsolidated fault gouge, breccias, as complex networks along the main active fault zones. Early veins are generally fractured and overprinted by younger veins, indicating that vein-forming events occurred repeatedly within the same fault shear zone. Microstructurally, both the pseudotachylyte and fault gouge veins are characterized by a superfine- to fine-grained matrix and angular-subangular fragments ranging in size from submicron scale to several centimeters. Based on the meso- and microstructural features of veinlet ultracataclastic rocks and the results of powder X-ray diffraction analyses, we conclude that (i) the pseudotachylyte veins were generated mainly by crushing rather than melting at shallow fault zones, and (ii) multistage veinlet fault gouge and pseudotachylyte formed repeatedly within the fault-fracture zone via the rapid fluidization and injection of superfine- to fine-grained materials derived from the host rocks during large-magnitude earthquakes that occurred along the active fault zones within the ATTL and ISTL. Our results show that the fluidized ultracataclastic veins record paleoseismic faulting events that occurred at shallow seismogenic fault zones; consequently, these features are a type of earthquake fossil, as is melt-origin pseudotachylyte.