Earthquake rupture in shallow portion of subduction zone: An ancient record from the Ishido thrust (out-of-sequence thrust) in Boso Peninsula, central Japa

Abstract:

Although earthquake ruptures in shallow portion of plate boundary have recently been identified (e.g. Tohoku, Nankai, etc.), their mechanisms why the shallow portion of plate boundary composed mainly of clay minerals can accumulate strain and make seismic slip are under controversial. In this presentation, we show an ancient record of earthquake rupture in shallow portion of subduction zone, characterized by frictional melting of smectite.

Two young and non-metamorphosed accretionary complexes developed in the Miura and Boso peninsulas, central Japan: 1) the Early to Middle Miocene and 2) the Late Miocene to Early Pliocene accretionary complex. Based on the vitrinite reflectance, the maximum burial depth were estimated as 2-3 km (65-90˚C) and less than 1 km (about 20 ˚C), respectively. The former complex thrust up above the latter one, leading to formation of the Ishido thrust. The difference of maximum paleotemperature and low angle of the thrust dipping (<30˚) implies the total net slip should be over a few km. The fault core is composed of black-colored thin (<1 mm) pseudotachylite and fault gouge. Under SEM-BSE observation, the pseudotachylite is characterized by homogeneous glassy matrix including fragments of quartz/feldspar, submicron-sized Fe-rich spherules, and vesicles. Based on the mineralogy of the host rock, spot analyses and elemental mapping, origin of the pseudotachylite was apparently frictional melting of smectite containing Fe. Fe-rich spherules formed by rapid cooling of pseudotachylite. On the other hand, occurrences of the fault gouge: no grain-preferred orientations, folding with pseudotachylite, intrusion into pseudotachylite and vice versa, suggest that the gouge corresponds to the fluidized zone associated with high-speed shear. This fluidized zone contains large number of pseudotachylite fragments, and some pseudotachylite cut the fluidized zone, indicative of plural earthquake ruptures occurred along this thrust.