PRE-MIOCENE AGED SAPROLITE ZONE IN THE COYOTE MOUNTAINS, SALTON TOUGH, SOUTHERN CALIFORNIA

A 12+ meter thick paleo-saprolite zone (PSZ), in the Coyote Mountains (CM), has been mapped under Miocene aged subaerial sediments of the Split Mountain Group. PSZ and sediments are on a horst, in a graben associated with the first two of three episodes of extension linked to the southern Basin and Range Province. Sediments are under the volcanics of the 17 Ma aged Alverson Formation. PSZ is developed on a foliated Jurassic granitoids and Paleozoic (?) to Triassic metasediments. The presence of this PSZ implies a period of tectonic stability that is difficult to associated with the tectonically active CM. West of the CM, Gastil (1961) mapped an “Old Erosional Surface” (OES) on the Peninsular Ranges Batholith. The PSZs developed on this OES represents a ”stable terrain”. Gastil’s OES is equivalent to Axen et al.’s (2000) “Well-Preserved Paleogene Erosional Surfaces”. Axen et al. have dated this Surface at 65 - 45 Ma and interpret the Surface as representing “tectonic quiescence”. The removal of the upper part of the Batholith facilitated the formation of the OES. Axen et al. dated the removal of 12 + kilometers of the upper part of the Batholith using cooling ages from biotite, potassium feldspar and apatite crystallization ages. Starting in the west (104 to 101 Ma) the cooling ages become progressively younger to the east (87 to 74 Ma). Axen et al. have a cooling age in the CM of 74 Ma that matches, to some extent, the cooling ages of other basement exposures in the Salton Trough and the eastern side of the Batholith. The comparison of the cooling dates, PSZ and OES between the Salton Trough, CM and the east side of the Batholith indicates a continuation of the basement terrain, PSZ, and “OES” to the east (to Sanora?). Incised into the “OES”, above the West Salton Detachment, is an Eocene aged river channel (Gastil, 1961; Minch, 1979; Abbott and Smith, 1989) containing cobbles that have been uniquely source to Sonora, Mexico.

Abbott, P. L., and Smith, T. E., 1989, Sonora. Mexico source for the Eocene Poway conglomerate of southern Calif.; Geology, v. 17, p, 329-332.

Axen, G. J., Grove, M., Stock, D., Lovera, O. M., Rothstein, D, A., Fletcher, J. M., Furley, K., Abbott, P. L., 2000, Thermal evolution of Monte Blanco dome: Low-angle normal faulting during Gulf of Calif. rifting and late Eocene denudation of the eastern Peninsular Ranges: Tectonics 19, 197-212.