The tectonic history of the southwestern UNited States and Sonora, Mexico, during the past 100 M.Y.

Abstract

The area between the Colorado Plateau on the north, New Mexico and Chihuahua to the east, and southern and Baja California to the southwest has undergone a complicated thermal and tectonic history climaxing in medial Cretaceous time and manifested in a sequence of events which continue to the present time, independent of plate boundary transform faulting. This sustained sequence of events was initiated by subduction‐related magmatic emplacement, largely between 120 and 50 Ma, beginning in the west and sweeping eastward across the region. The emplacement of these rocks generated a large isostatic welt which began to elevate in late medial Cretaceous time in the west central portion of the area and spread outward until in Oligocene time the west coast of southern California was emergent almost to the continental escarpment. Both erosional and tectonic unloading accompanied elevation, with the axis of the welt achieving its maximum elevation in earliest Cenozoic time when north central Sonora may have had elevations of the order of 5000 m. Body forces caused the welt to relax laterally, producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east. To the east these structures are included in what is called the Larimide Orogeny; to the west they include such features as the Santa Rosa mylonite belt and related structures. As with the magmatic emplacement, the gravitational spreading began in the west. During the Eocene, additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt, and subhorizontal detachment began along the brittle‐ductile boundary. During Oligocene and early Miocene time, renewed isostatic uplift resulted from rising magma, and continued erosional and tectonic denudation spawned both brittle and ductile extension, hydrothermal alteration, and mineral deposition. From mid‐Miocene to the present time the central portion of the area has subsided, while the marginal mountain ranges have elevated.

Tectonics of the Colorado Plateau and Its Margins

“…The presence of high topography along this margin is geologically recorded by northeast-flowing paleorivers (Young & Hartman 2014). Gastil et al (1992) suggested a highland on the order of 5 km high based on remnants of these paleorivers (Rim Gravels) on the northern flank of the highland and unconformities in southern California. J.B. utilized whole-rock La/Yb ratios from continental arc rocks in southern Arizona and northern Sonora to estimate crustal paleothickness of 62-48 km from 76 to 61 Ma.…”

Colorado River flow and biological productivity in the Northern Gulf of California, Mexico

“…Cartographers generally recognize the region, from north to south, as the Coachella, Imperial, and Mexicali Valleys, as well as the floodplain of the Colorado River that abuts the Upper Gulf of California. Sediments in the Salton Trough have accumulated atop a Paleozoic basement of limestone, sandstone, conglomerate, and metamorphic rocks (Gastil et al 1992, Delgado-Granados et al 1994, Nations and Gauna 1998, Fletcher and Munguía 2000, Bialas and Buck 2009. The sediment-basement interface is irregular and occurs at depths from 1.4 to 5.6 km (Anderson et al 2003, Lovely et al 2006, Crowell et al 2013, Pacheco et al 2006.…”

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

“…The top of the subducting plate was at 35 km depth, some 150-180 km inland from the trench, and led to the underplating of trench sediment beneath Cordilleran arc-related rocks. This architecture has been documented in southern California (Burchfiel and Davis 1975;Gastil et al 1992), but no better, more coherent exposures of the deep roots of the system exist than those in the central California Coast Range.…”