T75 Cryptic Uplift of the Interior of the U.S. Cordilleran Orogen

Sunday, 31 October 2010
1:30 PM - 5:30 PM
Room 405

POSSIBLE CLASTS OF SHINUMO QUARTZITE (EASTERN GRAND CANYON) IN LOWER MIOCENE CONGLOMERATES OF THE SESPE FORMATION (COASTAL SOUTHERN CALIFORNIA), AND IMPLICATIONS FOR THE UPLIFT AND EROSION HISTORY OF THE SOUTHWESTERN US

Paper number: T75-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

(U-Th)/He ages from the eastern Grand Canyon region of the SW Colorado Plateau suggest a pulse of ~1500 m of exhumation at ca. 20 Ma, with very little subsequent erosion (Flowers et al., 2008 GSAB; Wernicke, 2010, GSAB). Throughout E Grand Canyon, the supermature Shinumo Quartzite (SQ) is widely exposed. Samples from the densest portions of the SQ exhibit 4 characteristics, each either unknown or uncommon in other orthoquartzites (Oqt) in the SW US: (1) they contain no feldspar, having only quartz and quartzolithic clasts; (2) they have a bimodal clast-size distribution with peaks at ca. 0.4 and 0.7 mm, with grains coarser than 1.0 mm rare or absent; (3) clasts are uniformly well rounded; and (4) magnetization is moderately inclined downward. Other dense OQ in the SW US generally contain feldspar, and fractions of clasts that are either coarser than 1 mm, subangular to subrounded, or both. Although some supermature compositions have been reported from Ediacaran-Cambrian OQs in the Death Valley region, they are not a dominant component of their host formations, leaving SQ by far the primary “reservoir” of exposed supermature Oqt in the SW US. Further, any supermature Ediacaran-Cambrian Oqts are distinguishable from the SQ by their shallow upward or equatorial magnetizations.

We collected 53 clasts of OQ from early Miocene Sespe conglomerates in the Santa Monica (ca. 21-17 Ma) and Santa Ana Mts. (known and presumed early Miocene strata) in the Los Angeles area. Of this suite, 3 samples (1 from the Santa Monicas, 2 from the Santa Anas) clearly exhibit characteristics (1)-(3). All but one of a subset of the other samples examined petrographically (n=27) contain feldspar, and a significant fraction of clasts are either too poorly sorted, too angular, or both. Preliminary paleomagnetic analyses indicate that the 3 supermature clasts exhibit a moderate paleomagnetic inclination, while two additional clasts chosen at random from the other samples yield the expected shallow inclinations. If the supermature samples are indeed SQ, then the erosion surface in E Grand Canyon was unroofed to at least the level of SQ exposures no later than 17 Ma. This in turn supports limited post-17 Ma erosion (a few 100 m), and requires a hydrological connection between the SW Colorado Plateau and the Pacific coast in early Miocene time.