GEOLOGY AND STRUCTURE OF LIMEROCK CANYON ASSEMBLAGE, SOUTHWESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

We present new geologic mapping and petrographic analysis of the Proterozoic (?) Limerock Canyon assemblage, studied previously by Miller (1934), Oakeshott (1958), and Powell (1993). Limerock Canyon is located in a recently burned area of the southwestern San Gabriel Mountains. We aim to provide a structural and petrographic characterization of marine metasedimentary strata that represent a key paleogeographic element situated at the Neoproterozoic rifted margin of Laurentia. These rocks are similar to strata on Ontario Ridge (Zylstra et al., 2017), and may provide constraints on right-lateral displacement across the south branch San Gabriel Fault. Limerock Canyon contains strongly foliated and folded metamorphic rock outcrops that were largely covered by chaparral brush until a December 2017 fire burned the area and provided access. Basement rocks are separated from Tertiary sandstone and conglomerate by the Lopez reverse fault on the southwest and southeast. The exposed map width of section is ~750 m, but folding suggests repetition of units. We recognize most of the rock units described by Powell (1993), and provide geological mapping and structural context. From most to least abundant, mapped metasedimentary units include: (1) sillimanite-garnet-bio±musc-quartzofeldspathic gneiss or schist with rare graphitic schist, (2) quartzite and feldspathic quarzite, and (3) white marble, and (4) grossular-diopside-calcsilicate gneiss. One target of future work is a distinctive coarse grained (Proterozoic?) biotite granite augen gneiss observed as stream boulders. This rock unit is probably exposed in upper Limerock Canyon, and may represent the basement of the metasedimentary package. Mesozoic(?) intrusive rocks include foliated diorite / amphibolite and unfoliated leucocratic biotite granite / aplite. Several NW-trending map-scale folds are identified. Foliations predominantly strike NNW with steep dips (55 to 85 degrees) to the southwest and northeast, except in hinge areas. The foliated diorite is not as deformed as the host gneisses, suggesting that it has experienced a younger metamorphic event. The granite sharply crosscuts all ductile fabrics. Ongoing detrital zircon analyses of quartzites will test correlations to the type Placerita Formation of Miller (1934), exposed 12 km northwest.