Abstract

ERTS-1 and Skylab images show that transverse shear is pervasive in a variety of geologic and physiographic provinces in Southwestern U.S.A. and Mexico. Most of the transverse structures trend east-west to west-north-west oblique to the regional tectonic grain.

ERTS imagery of Mexico suggests that the Parras lineament corresponds to a zone of transverse sinistral shear. Profound deformation is observed in Mesozoic and older rocks whereas the late Tertiary volcanics of the Sierra Madre Occidental and late Tertiary sedimentary rocks of the Gulf of Mexico are only slightly affected.

Farther north structural linears corresponding to the Texas Lineament have been observed. Like the Parras shear, maximum deformation involves rocks of pre-late Cenozoic age. The present disposition of the Paleozoic miogeosynclinal Millard Belt in Nevada and its displaced extension in Sonora is consistent with a left-lateral Texas shear. A tectonic model reconciling the apparent conflict with the San Andreas fault system is presented.

In the Coast Ranges and Peninsular Ranges of California, short segments of old transverse shear faults are observed within blocks sliced and displaced by the younger San Andreas fault system. These faults probably represent remnants of shearing deformation within the Mesozoic subduction and intrusive belt.

Transverse faults mark the northern and southern boundaries of the Sierra Nevada Block. In the Basin and Range Province, en echelon transverse faults cut across north-trending rift structures. The relation between the Death Valley and the Furnace Creek fault will be discussed as a typical example.

It appears that regmatic transverse shear developed during both compressional and tensional diastrophic cycles of the western Cordillera. An analysis based on the Plate Tectonic theory is presented.