Abstract

The structure elements of metamorphic and granitic rocks along the Sierra Nevada front between Mt. Lyell and Mt. Whitney combine into a regional pattern dominated by northwest, northeast nearly north-south, and west-northwest directions, all reflected in the much younger plan of Tertiary-Pleistocene border faults. The elements of basement structure indicate strong upward motion, accompanied or followed by horizontal shift and local overthrusting. The motion on northwest and west-northwest structures was like the shift that has been observed on the San Andreas fault. On northeast structures, the movement seems to have been like that which caused the horizontal displacement reported by Hulin on the Garlock fault.

This shear pattern suggests north-south compression. Into such a system, the nearly north-south structures fit as tensional features.

The intersections of the northwest and northeast shift zones were loci of especially intense deformation, and in some cases they were especially favored by intrusions. Pleistocene volcanic outlets were grouped on similar intersections in the fault pattern.

The internal structures of the intrusions and their contact relations with the metamorphic rocks seem to preclude any great amount of downward stoping, and seem to indicate forceful intrusion. Time relations between intrusion and wall-rock deformation in general, however, reduce forceful intrusion to a comparatively minor factor.

The data suggest that space for the intrusions was provided mainly by buckling, as a result of north-south compression, of the already tightly folded metamorphic rocks. The importance of the derivation of the granite from metamorphic rocks essentially in situ cannot yet be estimated.

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