Geologic map and sections, western end of the San Gabriel Mountains, Sylmar Quadrangle, California: Supplement 1 from "Areal geology and contact relations of the basement complex and later sediments, west end of the San Gabriel Mountains, California" (Thesis)

Description

The west end of the San Gabriel Mountains is a relatively small block of pre-Cretaceous metamorphic complex faulted from the main mass of the San Gabriel Mountains by the San Gabriel fault. Their core of metamorphic rocks is flanked on three sides by Tertiary sediments represented by Eocene, Pliocene, and Pleistocene formations. The Eocene and Pliocene are characteristic off-shore and littoral marine deposits. The Pleistocene is principally of terrestrial origin. The area is one of structural complexity. This can be accounted for, in part, by assuming that this end of the mountain block acted as a centre of rotation for north-south compressional forces that were active to the west. The San Gabriel Range is believed to be a fault block, raised to its present elevation principally by movements along faults which parallel the north and south margins. The faulting is not restricted to the extreme margins, but often is located within the range itself. The west slope of the range is characterized by depositional relations between the sediments and the underlying mountain mass. If any faulting has occurred, it is thought to be farther west than the contact between the sediments and the basement complex and to be concealed beneath the younger sediments.

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