Post–12 Ma deformation in the lower Colorado River corridor, southwestern USA: Implications for diffuse transtension and the Bouse Formation

Abstract

Structural evidence presented here documents that deformation was ongoing within the lower Colorado River corridor (southwestern USA) during and after the latest Miocene Epoch, postdating large-magnitude extension and metamorphic core complex formation. Geometric and kinematic data collected on faults in key geologic units constrain the timing of deformation in relation to the age of the Bouse Formation, a unit that records the first arrival and integration of the Colorado River. North-south–striking extensional, NW-SE–striking oblique dextral, NE-SW–striking oblique sinistral, and east-west–striking contractional faults and related structures are observed to deform pre– (>6 Ma), syn– (6–4.8 Ma), and post–Bouse Formation (<4.8 Ma) strata. Fault displacements are typically at the centimeter to meter scale, and locally exhibit 10-m-scale displacements. Bouse Formation basalt carbonate locally exhibits outcrop-scale (tens of meters) syndepositional dips of 30°–90°, draped over and encrusted upon paleotopography, and has a basin-wide vertical distribution of as much as 500 m. We argue that part of this vertical distribution of Bouse Formation deposits represents syn- and post-Bouse deformation that enhanced north-south–trending depocenters due to combined tectonic and isostatic subsidence in a regional fault kinematic framework of east-west diffuse extension within an overall strain field of dextral transtension. Here we (1) characterize post-detachment tectonism within the corridor, (2) show that diffuse tectonism is cumulatively significant and likely modified original elevations of Bouse Formation outcrops, and (3) demonstrate that this tectonism may have played a role in the integration history of the lower Colorado River. We suggest a model whereby intracontinental transtension took place in a several hundred kilometers-wide area inboard of the San Andreas fault within a diffuse Pacific–North America plate margin since the latest Miocene.

Geodetic Extension Across the Southern Basin and Range and Colorado Plateau

“…Singleton (2015) found evidence for post ∼11-Ma, NW-striking dextral and oblique dextral-normal faults at the Buckskin-Rawhide metamorphic core complex near longitude W113.75° and latitude N34°. Thacker et al (2020) analyzed structural data of the lower Colorado River corridor located inboard of the SAFS along California-Arizona border (Figure 1) and characterize post-12 Ma deformation occurring before, during, and after the deposition of the ca. 6-4.8 Ma Bouse Formation as a broad, diffuse zone of low strain dominated by E-W extension with a portion of the E-W extension taken up on reactivation of NW-SE striking faults resulting in oblique dextral-normal and dextral faults.…”