Stratigraphy and Structure of the Pelona Schist in the North Fork Lytle Creek Drainage of the San Gabriel Mountains, California — Implications for Palinspastic Reconstruction

ABSTRACT

The basement rocks of the eastern San Gabriel Mountains have been the subject of many studies in the last several decades. The Pelona Schist in this region is disrupted by both NE-striking and NW-striking fault systems that intersect one another. The nature of intersections of such faults in the eastern San Gabriel Mountains is enigmatic. Does one set of faults turn and blend into the other, or does one set of faults truncate and post-date the other? Detailed geologic mapping was conducted at one such intersection in the North Fork of Lytle Creek where three faults are in proximity to the Pelona Schist and Tertiary granite in the hopes of constraining motion along the faults. Structure and stratigraphy of the Pelona Schist and the intrusive contact of Tertiary granite were used to palinspastically reconstruct a crude, Late Miocene paleogeology with respect to these mapped rock units. Since 5-2 Ma motion along the San Antonio Canyon fault was constrained to 2.9 km based on alignment of a metabasalt layer and two post-metamorphic arches within the Pelona Schist, as well as alignment of the San Gabriel and Icehouse Canyon faults. The NE-striking, left-lateral motion of San Antonio Canyon fault is hypothesized to predate movement on the NW-striking, right-lateral Scotland, San Jacinto, and Glen Helen faults, where Quaternary slip was constrained to 1.9 km, 5.9-8.1 km, and 4.4 km, respectively. Testing of the hypotheses embodied in the reconstruction is possible by further field mapping and investigation.