Frictional melting and thermal fracturing recorded in the pseudotachylyte derived from pelagic sedimentary rocks

Pseudotachylytes produced by coseismic slip provide an important information on earthquake faulting. We conducted microstructural and numerical analyses of the pseudotachylyte found in the Jurassic accretionary complex, central Japan. The pseudotachylyte-bearing thrust develops as an out-of-sequence thrust branched from the subduction plate boundary. The pseudotachylyte is ~2 mm-thick and sharply separates fractured grey chert above from cataclasite below. The pelagic black carbonaceous mudstone layer is distributed at the stratigraphic equivalent interval of the pseudotachylyte. The microstructures of the pseudotachylyte are characterized by fault and injection veins, fragments of grey chert in the dark matrix of illite composition, and local embayed pseudotachylyte boundaries. The gray chert adjacent to the pseudotachylyte and fragments in the pseudotachylyte are intensely cracked, showing a jigsaw puzzle structure. These microstructural features suggest preferential melting of illite, thermal fracturing, and thermal erosion during earthquake faulting. Frictional heat, estimated from temperature rise, volume fraction of fragments, and pseudotachylyte thickness, is 7.1 MJ m^-2. The comparison of fragment volume fraction between the pseudotachylyte and the pelagic black carbonaceous mudstone layer suggests that half amount of fragments in the pseudotachylyte were derived from wall rocks. The numerical modeling, considering temperature rise, heating duration, and thermal properties of gray chert, indicates that the temperatures of wall rocks within a few millimeters from the pseudotachylyte boundaries exceeds the temperature of alpha-beta transition in quartz (573 ℃) during earthquake faulting. The crack density in wall rocks is rapidly increased in a few millimeters from the pseudotachylyte boundaries. The fractal dimension of the particle size distributions of fragmented chert in and adjacent pseudotachylyte ranges from 1.77 to 3.20, while that of the cataclasite varies from 2.84 to 3.67. There features suggest that intensely cracked wall rocks and fragments in the pseudotachylyte represent thermal fracturing associated with thermal expansion of quartz grains in gray chert, which could contribute to unstable fault slip by decreased elastic stiffness of surrounding rocks.