Abstract

A dike swarm in the Spangler Hills, N. of the Garlock fault, is believed to be the offset extension of a similar swarm in the Granite Mountains, S. of the fault. The projected intersections of the swarms with the fault are about 40 mi. apart. The sense is left lateral. The swarm in the Spangler Hills can be traced N. from the fault for about 145 mi.; the swarm in the Granite Mountains can be traced S. of it for at least 7 mi. Both swarms are about 12 mi. wide. The dikes in the 2 swarms have similar widths; their trends differ by about 25 degrees 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.

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