Geologic analysis of ASTP photographs of parts of Southern California

Abstract

On July 16, 1975, the crew of the Apollo-Soyuz Test Project obtained a series of remarkably clear color stereoscopic photographs of the central Transverse Ranges and the Los Angeles Basin in southern California. These photographs are used to identify and interpret known geologic features of the Transverse Ranges, and to search for previously unrecognized features. High-altitude aerial photographs provide useful supplements to the orbital photographs. Major well-known high-angle faults of the region (the San Andreas, San Gabriel, and San Jacinto faults) are clearly visible; low-angle faults, even if active, are less apparent. The sharpness of the visual definition of a fault zone, therefore, should not be used as an exclusive criterion for recency of activity. Conversely, pronounced topographic expression may be developed by preferential erosion along inactive fault zones. Vegetation can produce severe masking of originally strong lithologic contrasts. Within and bounding the San Gabriel Mountains, current and recent fault breakage indicate a complex interplay of strike-slip and thrust fault movement. A previously unrecognized pattern of east-northeast-trending lineaments and the lobate character of the mountain front offer new perspectives to possible tectonic interpretations of this mountain range. They suggest the possibility that fault movement may represent a series of shingled slabs moving either in independent or in grouped increments, in response to compressive components across the Pacific/North America plate boundary.

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