Plate interactions control middle late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range

Middle-late Miocene (proto-Gulf; ∼12-6 Ma) extension around the Gulf of California (Gulf Extensional Province) is commonly interpreted as resulting from partitioning of oblique Pacific-North American plate motion into strike-slip displacement along the margin and east-northeast extension perpendicular to the margin within the North American plate. We propose that this mechanism also applies to kinematically similar, predominantly east-northeast extension that occurred at the same time throughout the southern Basin and Range province, from southern Arizona and New Mexico to the Trans-Mexican Volcanic Belt. New field and ⁴⁰Ar/ ³⁹Ar data in Sinaloa and Durango confirm that this episode of extension occurred on the mainland side of the Gulf and in the Basin and Range east of the Sierra Madre Occidental, which is generally considered the eastern margin of the Gulf Extensional Province. Published data indicate the middle-late Miocene episode also occurred across the northern and southern ends of the Sierra Madre where the Gulf Extensional Province connects with the Basin and Range: (1) from central Sonora into southern Arizona and New Mexico, and (2) from Nayarit into central Mexico north of the Trans-Mexican Volcanic Belt. This episode appears to have affected an area that continues to the eastern edge of the Basin and Range province in Texas and San Luis Potosi. Recognition that this episode of extension affected the entire southern Basin and Range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the Gulf Extensional Province alone. Published plate reconstructions require 160 to 110 km of east-northeast extension between ∼12 and 6 Ma. If taken up solely within the Gulf Extensional Province, this would have generated 66 to 78% extension, which is much greater than observed. Spread across the entire southern Basin and Range it requires only ∼20% total extension, which is more consistent with observations of cumulative extension between 12 and 6 Ma. Extension was partitioned into the Gulf Extensional Province because (1) it lies between two stable batholith belts (Mesozoic Peninsular Ranges on the west and mid-Tertiary Sierra Madre Occidental on the east) that resisted extension and (2) the Gulf was thermally weakened by immediately preceding arc magmatism. Extension in the main Basin and Range province in part probably avoided the relatively strong, batholithic crust of the Sierra Madre Occidental.