Sedimentary Basin Development & Deformation Adjacent to the Mojave-Sonora Megashear, Sonora, Mexico: Evaluation of the Mid-Mesozoic Tectonics of the Southwestern Cordilleran Margin

New Mexico State University, Las Cruces NM

Investigators

Abstract

The identification of ancient continental transform margins has proven difficult for a variety of reasons. Transform faults are commonly reactivated subsequent to their active transcurrent displacement histories by younger tectonic processes. In addition, piercing points that permit assessment of offset are commonly equivocal and subject to multiple interpretations. However, analysis of sedimentary basins developed adjacent to transform faults, particularly faults that transect marginal continental lithosphere, provides a powerful tool for evaluation of ancient transform margins. Evaluation of the timing of deposition and deformation is especially critical, because the two processes should be partly coeval in strike-slip basins. In northern Sonora, the postulated Mesozoic type example of a continental transform, the Mojave-Sonora megashear, juxtaposes a continental crustal fragment, the Caborca block, against cratonal North America along what is now a complex structural zone. A narrow, deformed sedimentary basin of Late Jurassic-Early Cretaceous age lies along this structural boundary for a distance of more than 300 km. The subsidence history of the basin largely coincided with the posited displacement history of the megashear and so provides a unique opportunity to evaluate the megashear hypothesis and understand the processes that constructed the Late Jurassic-Early Cretaceous Cordilleran continental margin. A two-year field- and laboratory-based study proposed here will determine the depositional and structural evolution of the sedimentary basin adjacent to the megashear and the uplift history of the Caborca block, which forms the present southern edge of the basin. The objectives of the research fall into three major categories: 1. Basin origin and its Mesozoic proximity to the Caborca block. Lithostratigraphic, biostratigraphic, and geochronologic analysis will determine the lifespan and geometry of the basin, while provenance studies will determine if the Caborca block was a sediment source for the basin. Fossils in the strata and interbedded volcaniclastic rocks make this objective practicable. 2. Deformation kinematics and chronology of the basin. Strata in the basin are locally tightly folded, cleaved, and metamorphosed. However, deformation intensity is greater in older rocks than in younger ones, a variation which may be either temporal or related to burial depth of strata during deformation. Mesoscale and microscale structural analysis will evaluate kinematics and variability of deformation both stratigraphically and geographically with respect to the megashear. 40 Ar/ 39 Ar geochronology on metamorphosed pelites will determine the age of the deformation in the basin. 3. Time of uplift of Caborca block. The age of uplift of the basin margin will be determined from stratigraphic relations and 40 Ar/ 39 Ar thermochronology in order to establish time of uplift of the outboard block relative to deformation of the basin. The uplift analysis will also permit evaluation of provenance results. This integrated sedimentologic, structural and geochronologic study will evaluate the transform processes that are inferred to have created the southwestern Cordilleran margin beginning in the Late Jurassic. If the megashear hypothesis remains tenable, this study should serve as a template for other analyses of ancient continental transform margins. If the megashear hypothesis is refuted, the study will force re-evaluation of the Mesozoic tectonics of the southwestern Cordilleran margin and the kinematics of Pangean supercontinent break up in Mexico.