STRUCTURAL GEOLOGY OF THE PLACERITA FORMATION, WESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

We describe structural relationships from the Placerita Formation of Miller (1934) and Oakeshott (1958), exposed at Placerita-Los Pinetos Canyons; also Limerock Canyon,12 km to the southeast. This formation is composed of a metamorphosed Neoproterozoic continental margin assemblage (pelitic gneiss, quartzite, marble, calcilicate) intruded by Jurassic and Cretaceous plutons and dikes. The Placerita-Los Pinetos exposures are exhumed in the footwall of the Late Miocene Placerita normal fault; Limerock Canyon exposures are imbricated between Quaternary strands of the Sierra Madre frontal thrust system. Both areas are broken by NE-striking left-lateral faults that are common in the San Gabriel Mountains. 1.74 Ga granite basement is identified only in structural culminations in Limerock Canyon.

Rocks in both study areas preserve a penetrative prograde upper amphibolite facies fabric characterized by steeply dipping, NW-striking foliation and ubiquitous tight folding along NW-trending hinges. The age of deformation at Limerock Canyon is bracketed by sills of fine-grained metadiorite (181±1 Ma) that share the fabric and NE-striking granite dikes (151±1 Ma) that crosscut all fabric. In Los Pinetos Canyon, Late Jurassic deformation is also suggested by syntectonic granite sills (148±4 Ma) and a crosscutting granite pluton (147±3 Ma). Structural elements at Limerock Canyon (N25W/90 mean foliation; S26E/16 best-fit fold axes) are rotated ~ 30^o clockwise from those at Placerita Canyon (N60W/75 NE mean foliation; N55W/40 best-fit fold axes). Most axial planes at Placerita Canyon are steeply inclined to the NNE, suggesting SW vergence. Greenschist grade Laramide(?) ductile fabric is locally developed in the 78±1 Ma Los Pinetos quartz diorite, but not in the 84±1 Ma Limerock Peak granodiorite.

The Placerita Formation records an important Late Jurassic tectonic event previously unrecognized in the San Gabriel Mountains. Laramide fabrics of the Vincent thrust in the eastern San Gabriel Mountains and at Mill Canyon window to the northeast (Barth et al., 2019) are only weakly developed in Placerita Canyon. Comparison with other likely exposures of Placerita Formation 75-100 km farther southeast at Potato Mountain, Ontario Ridge, and Lytle Creek suggests continuity of a significant Late Jurassic deformational belt.