EASTERN GRAND CANYON PROVENANCE FOR ORTHOQUARTZITE CLASTS IN LOWER MIOCENE CONGLOMERATES OF THE SESPE FORMATION NEAR MALIBU, CA

We measured characteristic paleomagnetic magnetizations in 44 densely cemented orthoquartzite clasts from the Sespe Formation in the Santa Monica and Santa Ana mountains (lower Miocene Piuma Member and related strata, 35 samples), and in Simi Valley (middle Eocene, 9 samples). Only clasts with well-defined internal bedding were selected for analysis, in order to determine their pre-Sespe paleomagnetic inclinations. Miocene Sespe clast inclinations define a bimodal population with peaks near 15° and 45°, with 60% in the steeper population. Of eight detrital zircon (DZ) analyses from the steeper Miocene mode, one clast is Mesozoic and probably derived from the Upper Jurassic Morrison Formation. All seven remaining analyses include DZ modes at 1.1, 1.4, and 1.7 Ga. DZ analyses on shallowly inclined Miocene clasts yielded one distribution similar to those from the steeper mode and another with a unimodal peak at 1.7 Ga. Two Eocene clasts with moderate inclination also yielded distributions with unimodal peaks at 1.7 Ga. These clasts were probably derived from Paleoproterozoic orthoquartzites (Mazatzal, Del Rio, and others) that characterize the central Arizona highlands. An inventory of characteristic magnetizations from several hundred individual sample cores from orthoquartzite source regions in the Cordilleran interior define a low-inclination population of Neoproterozoic-Paleozoic age in the Mojave Desert-Death Valley region, and a moderate- to high-inclination population in the 1.1 Ga Shinumo Formation, the latter source exposed only within a few hundred meters elevation of the bottom of eastern Grand Canyon. Because all Miocene Sespe samples with steeper inclinations have Grenville-age DZ peaks, a source region in the central Arizona highlands is ruled out, leaving the Shinumo Formation as the only plausible, presently exposed source. If it is, then eastern Grand Canyon had been eroded to within a few hundred meters of its current depth by early Miocene time (c. 20 Ma), which is confirmed independently by (U-Th)/He ages from eastern Grand Canyon and environs that document a pulse of late Oligocene-early Miocene unroofing. Collectively, these data define a mid-Tertiary, SW-flowing drainage system (Arizona River) between the Grand Canyon region and coastal California.