Brushlines in fault pseudotachylytes: A new criterion for coseismic slip direction

Striations along fault planes, such as slickensides, generally indicate slip direction. These linear structures typically form through friction between two solids. Here we describe brushlines, a new class of fault striation in pseudotachylytes, formed during coseismic slip at the interface between host rock and frictional melt. Unlike slickensides, brushlines primarily lack longitudinal step-like asymmetry and display a hemicylindrical morphology. This new class of striation forms while the frictional melt has sufficiently low viscosity to be brushed along the direction of coseismic displacement by small asperities on the host-rock block. In the Hoping River pseudotachylytes of eastern Taiwan, the long axis of brushlines is parallel to the earthquake slip direction determined using the offset of piercing points across the fault. This study shows that the pseudotachylyte-host rock interface may host previously overlooked, invaluable coseismic kinematic information.