COMPETING HYPOTHESES TO EXPLAIN LARGE LATITUDINAL DISPLACEMENTS OF THE CABORCA BASEMENT BLOCK OF NORTHWESTERN MEXICO: WHAT ABOUT NOT HAVING DISPLACEMENTS AT ALL?

The concept of the Caborca basement block is part of one of the most influential hypotheses ever presented in Mexican Geology, the Mojave-Sonora megashear (MSM) of Anderson and Silver, postulated in the mid-sixties.

The MSM hypothesis suggests that a piece of Precambrian basement, and its Neoproterozoic and Early to Middle Mesozoic supracrustal rocks, was offset from western Nevada and displaced southeastward approximately 800 km into Sonora, Mexico, along a large left-lateral strike slip fault during the Late Jurassic time. The block to the southwest of the hypothetical fault is the Caborca block, whereas the basement to the northeast was named the North America block.

Dickinson and Lawton (2001) questioned the timing of truncation of the cordilleran passive margin and argued that the left-lateral strike-slip fault had to be active in Permo-Triassic time (281–232 Ma). This fault was named by these authors as the California-Coahuila transform (CCT). Poole et al. (2005) suggested that any slip of the Caborca block would have occurred prior to the formation of the intact passive margin sequence of SW Laurencia and proposed a Late Neoproterozoic fault that they named the Chihuahua-Sonora transform (CST). Whitmeyer and Karlstrom (2007) suggested a Late Mesoproterozoic-Grenville slip for the Caborca block. More recently, Lawton et al. (2017) reevaluated the timing of slip along the CCT, suggesting a new time frame for displacement between Late Pennsylvanian-to very Early Permian (~310–290 Ma). This readjustment was required to avoid conflict with the fact that subduction and formation of the cordilleran continental magmatic arc started ~284 Ma, and basically continued throughout the Permian and Triassic times (Arvizu et al., 2009; Arvizu and Iriondo, 2015).

These hypotheses have been questioned throughout the years with an increasing body of evidence suggesting the possibility that the Caborca block is in fact an autochthonous or para-autochthonous piece of basement. The only clear displacements are along Laramide thrusts that accounted for only a few tens of kilometers; these thrusts clearly affected the original Paleoproterozoic sutures proposed in Sonora. The passive margin sequence also appears to be basically unaffected by latitudinal displacements as suggested by Poole et al. (2005) and Lara-Peña et al., 2021.