Abstract

The Vincent thrust of the San Gabriel Mountains, southern California, separates an upper plate that contains abundant granitic rocks from a lower plate composed of metagraywackes and metabasites (Pelona Schist). The Vincent thrust is part of a regional thrust system of Late Cretaceous-early Tertiary age (Vincent-Chocolate Mountain thrust). Several earlier workers have concluded that the upper plate was derived from the southwest, implying the former presence of continental rocks separated from North America by a marginal oceanic basin. Other workers consider that regional correlations indicate thrusting of the opposite sense.

The Pelona Schist of the San Gabriel Mountains has undergone a complex structural history. The earliest recognized structures are tight, synmetamorphic isoclinal folds (style 1 folds). These folds are present throughout the entire 4 km of presently exposed section and do not become more appressed or more abundant near the thrust. Isoclinally refolded folds and sheath folds were observed at several localities. Textural observations indicate that the present schistosity of the Pelona Schist formed by the extreme flattening of crenulations developed in an earlier schistosity. This is evidence that the isoclinally refolded folds, although only infrequently noticed in the field, are actually widespread.

The isoclinal folding was followed by the development of a macroscopic synform (Narrows Synform) and associated minor folds in the 700 m of Pelona Schist closest to the upper plate. These folds (style 2 folds) are more open than the style 1 folds and do not have a well-developed axial-planar schistosity. They are generally parallel in orientation to the style 1 folds. Both style 1 and style 2 folds are in turn parallel to tight folds in mylonitic rocks present at the base of the upper plate. It is believed that the three groups of folds are cogenetic and possibly were produced in a subduction zone.

The sheath folds and other evidence of stretching parallel to fold axes might indicate that fold axes were rotated into the direction of thrusting. On the other hand, the parallelism of the early, isoclinal folds and the later, relatively open style 2 folds is evidence against rotation. Thus, the orientation of fold axes relative to the thrusting direction is not clear.

Subsequent to metamorphism, the Pelona Schist and the mylonites were folded into a broad arch (style 3 folding). Uncertainty as to the orientation of the axes of style 1 and 2 folds relative to the thrusting direction, the presence of the style 3 folding, and recently described paleomagnetic evidence of large-scale rotation of crustal blocks in southern California make determination of thrusting direction impossible at the present time. Thus, no evidence was found either to support or to invalidate the marginal-basin model for the origin of the Pelona Schist.

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