FORMATION OF COMPOUND STRIKE-SLIP-FAULT ZONES, MOUNT ABBOT QUADRANGLE, CALIFORNIA

Numerous strike-slip fault zones in granitic rocks of the Mount Abbot quadrangle, California, developed from steeply-dipping, subparallel joints. These joints generally were less than 50 m long and were spaced several centimeters to several meters apart. Some joints subsequently slipped and became small faults. Simple fault zones formed as oblique dilatant fractures (splay fractures) linked non-coplanar faults side-to-side and end-to-end. These simple fault zones are as much as 1 km long and accommodated displacements as great as 10 m. Compound fault zones formed as splay fractures linked small faults and simple fault zones. They are as much as several kilometers long and accommodated displacements as great as 100 m. These zones are distinctly different from 'Riedel shear zones' and the-mechanics of their formation are unlikely to be described well by Mohr-Coulomb mechanisms. Simple and compound fault zones are composed of non-coplanar segments that join at steps or bends; splay fracture length determines step widths. The longest splay fractures occur along the longest fault zones, allowing step widths to increase as the length and displacement across the zones increase. These findings are consistent with the structure of some active seismogenic faults, and they provide a mechanically consistent, field-based conceptual model for fault zones that grow in basement rocks from a preexisting set of joints. © 1990.