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

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The identification of ancient continental transform margins has proven difficultfor a variety of reasons. Transform faults are commonly reactivated subsequent to theiractive transcurrent displacement histories by younger tectonic processes. In addition,piercing points that permit assessment of offset are commonly equivocal and subject tomultiple interpretations. However, analysis of sedimentary basins developed adjacent totransform faults, particularly faults that transect marginal continental lithosphere,provides a powerful tool for evaluation of ancient transform margins. Evaluation of thetiming of deposition and deformation is especially critical, because the two processesshould be partly coeval in strike-slip basins. In northern Sonora, the postulated Mesozoictype example of a continental transform, the Mojave-Sonora megashear, juxtaposes acontinental crustal fragment, the Caborca block, against cratonal North America alongwhat is now a complex structural zone. A narrow, deformed sedimentary basin of LateJurassic-Early Cretaceous age lies along this structural boundary for a distance of morethan 300 km. The subsidence history of the basin largely coincided with the positeddisplacement history of the megashear and so provides a unique opportunity to evaluatethe megashear hypothesis and understand the processes that constructed the LateJurassic-Early Cretaceous Cordilleran continental margin.A two-year field- and laboratory-based study proposed here will determine thedepositional and structural evolution of the sedimentary basin adjacent to the megashearand the uplift history of the Caborca block, which forms the present southern edge of thebasin. 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 thelifespan and geometry of the basin, while provenance studies will determine if theCaborca block was a sediment source for the basin. Fossils in the strata and interbeddedvolcaniclastic rocks make this objective practicable.2. Deformation kinematics and chronology of the basin. Strata in the basin arelocally tightly folded, cleaved, and metamorphosed. However, deformation intensity isgreater in older rocks than in younger ones, a variation which may be either temporal orrelated to burial depth of strata during deformation. Mesoscale and microscale structuralanalysis will evaluate kinematics and variability of deformation both stratigraphically andgeographically with respect to the megashear. 40 Ar/ 39 Ar geochronology onmetamorphosed 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 bedetermined from stratigraphic relations and 40 Ar/ 39 Ar thermochronology in order toestablish time of uplift of the outboard block relative to deformation of the basin. Theuplift analysis will also permit evaluation of provenance results.This integrated sedimentologic, structural and geochronologic study will evaluatethe transform processes that are inferred to have created the southwestern Cordilleranmargin beginning in the Late Jurassic. If the megashear hypothesis remains tenable, thisstudy should serve as a template for other analyses of ancient continental transformmargins. If the megashear hypothesis is refuted, the study will force re-evaluation of theMesozoic tectonics of the southwestern Cordilleran margin and the kinematics ofPangean supercontinent break up in Mexico.