EXTENSIONAL TECTONICS, A SLAB WINDOW AND THE ARRIVAL OF THE GULF OF CALIFORNIA ABOUT 17MA, WHILE THE COYOTE MOUNTAINS OF SOUTHERN CALIFORNIA, USA UNDERGO 90° CLOCKWISE ROTATION IN SONORA, MEXICO

Abstract

Prior to 5.5(?) Ma, Winker and Kidwell (1996) place the Coyote Mountains (CMs) of southern California, in Sonora, Mexico. In Sonora, the CMs were deformed by Basin and Range Extension and related regional subsidence that started about 25 Ma (Nourse et al., 1994). The Basin and Range Extension was the first of three episodes of extensional tectonics recorded in the CMs (Morgan and Morgan, 2017). This first episode of extension left the CMs with the normal Ocotillo Canyon Fault and a related graben containing non-marine syn-extensional sediments (lower part of the Split Mountain Group). Sometime after the first and before the start of the second episode of extensional tectonics, the CMs start to rotate clockwise about 90°. We interpret the rotation as the product of right-lateral faulting (possible transtensional). During the rotation, 17.1 Ma (Morgan et al., 2012) volcanics of the Alverson Formation (upper part of the Split Mountain Group) are deposited in the CMs as a result of the formation of Dickinson’s Slap Window (Dickinson, 1997). The formation of the Slab Window also coincides with the start of the second episode of extensional tectonics and continued regional subsidence. The second episode of extension and continuing subsidence facilitate the arrival of the Gulf of California during the deposition of the volcanics of the Alverson Formation in the CMs (Woodring, 1931 and Morgan and Morgan 2015). The marine-nonmarine, second episode, syn-extensional sediments of the Viejo Formation (Morgan and Morgan, 2015) of the Imperial Group, represents the Gulf’s arrival in the CMs, while the CMs are still in Sonora. The Viejo Formation is represented by several transgressional-regressional sequences, all of which were deposited near sea- level. A combination of the following: 1) a weak, thin, warn crust; 2) right-lateral faulting (possible transtensional); 3) continuing extensional tectonics and subsidence, is inferred from the CMs. From these inferences it is reasonable to expect pull-a-part basins to be a part of the young Gulf.