Seismogenic structure of a crystalline thrust fault: fabric anisotropy and coeval pseudotachylyte–mylonitic pseudotachylyte in the Grizzly Creek Shear Zone, Colorado

Abstract Field and microstructural observations from the Proterozoic Grizzly Creek Shear Zone suggest that crustal-scale fabric anisotropy exerted a significant control on earthquake rupture propagation during deformation at mid-crustal depths. The shear zone developed in amphibolite-facies supracrustal gneisses and granitoids, and consists of a 0.4–0.7 km-wide zone of high-strain rocks with foliation transposed to 256°/51°NW and top-to-the-south kinematics. The shear zone is overprinted by hundreds of veins of pseudotachylyte, mylonitic pseudotachylyte and ultramylonite. Field observations and whole-rock geochemical data suggest that pseudotachylyte fault veins formed as a result of first-generation rupture through intact rock. Pseudotachylytes are preferentially localized in as many as nine decametre-scale rupture zones dispersed across the width of the shear zone, concordant to foliation. We present a conceptual model for the asymmetric development of anisotropic fabric in a thrust-related fault zone in crystalline metamorphic rocks. Progressive tectonic exhumation of hanging wall rocks during thrusting results in the development of a crustal-scale anisotropic fabric that provides a preferentially weakened zone that could accommodate the propagation of earthquake ruptures from the seismogenic zone into the middle crust.