Abstract

The east-central part of the left-lateral Garlock fault, in southeast California, is associated with three broad folds that trend and plunge northeast. The folds, which lie north, south, and astride the fault, postdate deposition of alluvial gravels that at one time formed a continuous northwest-sloping fan; the gravels rest conformably on lake beds that contain the Lava Creek B ash bed (0.62 Ma). The anticline, named after Christmas Canyon, has vertical relief of as much as 100 m. Topographic profiles suggest that the gravels exposed along the synclinal troughs to its southeast (Pilot Knob Valley syncline) and northwest (Teagle Wash syncline) lie at about the same elevations as they did when part of the originally active fan, implying that the synclines are passive results of the anticline development. If so, the original slope of the middle Pleistocene alluvial sediment sheet was about 0.6° northwest. The age and dimensional controls allow calculation of the rates of tilting and horizontal shortening. Corrected tilting rates vary from about 2.5° to 4.7°/m.y.; the total shortening of about 7.5 m developed at a minimum rate of 12.5 m/m.y.

The orientations of these folds imply northwest-southeast compression, which is anomalous when viewed in the context of regional tectonics. The left-lateral Garlock fault and the north-northwest-trending Searles Valley syncline and Slate Range anticline imply northeast-southwest compression, which geophysical criteria confirm. Geologic evidence of northwest-southeast compressional deformation, dating to pre-Miocene (Paleocene?) time, however, is known from several other localities along the Garlock fault. These anomalous features are interpreted as evidence that episodes of north-west-southeast compression along the Garlock fault do occur, and although oriented about 90° from the average of conventional—and presently observed—compression directions, they have occurred during much (most?) of the Cenozoic Era, including the past 0.6 m.y.

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