DEVELOPMENT OF CRUSTAL-SCALE FABRIC ANISOTROPY AS A MECHANISM FOR DOWNWARD PROPAGATION OF EARTHQUAKE RUPTURES INTO THE MIDDLE CRUST

Numerical modeling and limited observational evidence from seismology suggests that large earthquake ruptures can propagate downwards from the seismogenic zone into the middle crust, where crystal-plastic deformation mechanisms operate. Geological evidence of this phenomenon is potentially preserved by the presence of mylonitic pseudotachylyte (mpt) in exhumed fault zones, since this fault rock is widely interpreted to represent a record of seismicity in the middle and lower crust. In the Sibson-Scholz fault zone model, ruptures propagating downward from the base of the seismogenic zone encounter increasing crustal strength through the upper part of the brittle-plastic transition, which can slow ruptures and serve as a barrier to deeper propagation (e.g., Das and Scholz, 1983). It is proposed here that fault-zone mechanisms that weaken this part of the brittle-plastic transition may accommodate deeper rupture propagation. One proposed mechanism is the development of a crustal-scale anisotropic fabric in crystalline rocks. Progressive deformation in a mid-crustal, thrust-related shear zone results in the preferential development of fabric anisotropy as hanging wall rocks are translated updip into the seismogenic zone and foliation is transposed into parallelism with the evolving thrust. The resulting structure provides an anisotropically weakened connection between the middle crust and the seismogenic zone that can favor downward propagation of earthquake ruptures from the base of the seismogenic zone into the middle crust. This model is supported by field and microstructural observations from the Proterozoic Grizzly Creek shear zone (GCSZ) at Glenwood Canyon, Colorado. The GCSZ is a top-to-south thrust that developed in amphibolite facies supracrustal gneisses and granitoids, and consists of a 0.5-km-wide zone of high-strain rocks with foliation transposed to 256/51˚NW. The GCSZ is overprinted by hundreds of veins of pseudotachylyte (pst) and mpt localized into as many as nine decameter-scale rupture zones dispersed across the width of the shear zone. The zones are concordant with foliation suggesting that fabric controlled localization. Since pst and mpt are not preferentially localized in mylonites, they are not likely to be a result of mid-crustal plastic instabilities.