Matching simulated fracture data with field measurements using joint intensity

Abstract

Survivor grains are rounded, isolated clasts in fault gouge that escaped the grain fracturing so evident in fault breccia. Surrounded by a matrix of clay or comminuted parent rock, they are free to rotate without interference from other clasts. Using an optical microscope, electron microprobe images, and image analysis software, I collected data on the long axis orientations of elongate (a/b>1.4) survivors in 15 samples of fault gouge and breccia from Death Valley, California. Survivor grains display a profound shape preferred orientation (SPO) which can be used to infer kinematic parameters of gouge deformation. The SPO depends on the gouge type. In clay gouge the SPO is inclined to the shear plane, similar to the P-foliation usually defined by a phyllosilicate fabric. In flow-banded granular gouge the SPO is parallel to the shear plane and banding. To explore the implications of the SPO data, kinematic models of passive markers in simple shear (March model), rigid ellipsoids in simple shear (Jeffery model), and rigid ellipsoids in general shear (Ghosh and Ramberg model) were numerically implemented. Comparison of SPO data and model results suggests two explanations for the inclined SPO of clay gouge: either the simple shear strain experienced by clay gouge is low (γ<4) or the shear strain is unlimited but gouge deformation included apparent thickening of the shear zone. In the accompanying paper the latter option is assumed and a kinematic model of brittle shear zones which includes another commonly observed microstructure of fault gouge, Riedel shears, is developed to explain the inclined SPO in clay gouge.