Large Lateral Displacement on Garlock Fault, Califonia, as Measured from Offset Dike Swarm

Abstract

A dike swarm in the Spangler Hills, north of the Garlock fault, is believed to be the offset extension of a similar swarm in the Granite Mountains, south of the fault. The projected intersections of the swarms with the fault are about 40 miles apart. The sense is left lateral. The swarm in the Spangler Hills can be traced north from the fault for about 145 miles; the swarm in the Granite Mountains can be traced south of it for at least 7 miles. Both swarms are about 12 miles wide. The dikes in the two swarms have similar widths; their trends differ by about 25, however, and this is attributed to rotation of the southern fault block. Fine-grained diorite porphyry forms most of the dikes in both swarms, although microcrystalline granite porphyry forms dikes near their eastern edges. The host rocks are mostly quartz monzonite or granodiorite. No other comparable swarms crop out along the fault. The dikes were probably emplaced during very late Mesozoic time. As the Garlock fault was probably formed in late Mesozoic or early Tertiary time, the offset of the vertical swarm approximates the total horizontal offset along the fault.