CENOZOIC STRUCTURAL AND MAGMATIC EVOLUTION OF THE SONORAN BASIN-RANGE AND GULF OF CALIFORNIA RIFT SYSTEM

New geologic mapping and 40Ar/39Ar data from a transect across southern Sonora (Guaymas/San Carlos to the Sierra Madre Occidental) provide constraints on the distribution and timing of deformation and magmatism as this region evolved from an extensional arc to the modern oblique transform/rift system of the Gulf of California. Arc magmatism generally migrated from east to west across Sonora during the Oligo-Miocene, although magmatism was highly episodic both locally and regionally. Major pulses of volcanism occurred at ~37-33 Ma, 28-24 Ma (eastern Sonora), 22-20 Ma, 17-15 Ma, and 12-8 Ma (western Sonora). Subduction ceased at ~12 Ma at this latitude, yet voluminous intermediate to silicic magmatism (with strong arc-like geochemical signatures) continued for several millions afterwords.

Most of Sonora is highly extended, but the timing, magnitude, and character of extension varies. Intra-and back-arc extension generally mimicked but lagged behind arc magmatism, with major extensional deformation in eastern Sonora between 26-20 Ma, in central Sonora from 20-15 Ma, and in coastal Sonora between 12-9 Ma. Older (pre-12 Ma) extension in eastern and central Sonora was orthogonal to the coeval convergent boundary, whereas post-12 Ma deformation inboard of the lengthening transform boundary was largely restricted to coastal Sonora and was transtensional in character.

Tectonic models that invoke partitioning of late Miocene (12 to 6 Ma) Pacific-North American plate motion between dextral-slip outboard of Baja California and orthogonal extension inboard need to be re-evaluated, as there appears to be insufficient cumulative dextral slip outboard of Baja, and late Miocene deformation inboard of Baja includes a large amount of dextral shear. Evidence for this late Miocene transtensional deformation include multiple sets of NE- to EW-trending sinistral normal faults, NW-trending dextral faults, and large (~90°) clockwise block rotations.

Our work suggests that when subduction ceased due to passage of the triple junction, Baja was captured by the Pacific Plate and subsequent distributed transtensional deformation was localized inboard in the previous arc terrane. This highly oblique rift gradually narrowed and organized itself into the modern transforms and short spreading centers.