GEOLOGIC RECONNAISSANCE AND GEOCHRONOLOGY OF PROTO-GULF OF CALIFORNIA EXTENSION, WESTERN SONORA

Plate-circuit prediction of relative Pacific—North America motion requires hundreds of kilometers of ‘proto-Gulf of California’ extension and dextral shear from 14 to 6 Ma that remains unresolved with land-based geologic studies. Geologic data that defines the degree and obliquity proto-Gulf extension are especially desirable because, unlike dilation of continental crust elsewhere in North America, extension in northwest Mexico has progressed to continental rupture and formation of new oceanic crust. We present new geologic reconnaissance mapping and ⁴⁰Ar/³⁹Ar geochronology of proto-Gulf of California extension of western Sonora, Mexico. Extension in the Caborca region, located ~100 km from the Gulf of California, exhumed greenschist-grade metasedimentary and metavolcanic rocks as a metamorphic core complex in the Sierra La Gloria, Sierra El Batamote, and Cerro Carnero. A previously published K-Ar cooling age on biotite from mylonitized granodiorite at Cerro Carnero indicates exhumation of the footwall from 6 to 10 km depth since 14.8 ± 0.5 Ma. We dated tilted volcanic rocks from the lower part of the related supradetachment basin at 24.57 ± 0.10 Ma. Near Hermosillo, also located ~100 km from the Gulf, outcrops of tilted, welded tuff were dated at 12.37 ± 0.10 Ma. This new age, together with other lithologic, paleomagenetic, and geochemical data, are consistent with correlation of these outcrops to the Tuff of San Felipe known from coastal Sonora and northeastern Baja California. Moderately tilted conglomerates in the Sierra Picu, located ~40 km from the Gulf, contain 12.44 ± 0.05 Ma tuff clasts. Adjacent to the coastline, steeply-tilted volcaniclastic rocks at Puerto Lobos are 15.37 ± 0.61 Ma and steeply-tilted ca. 12.6 Ma Tuff of San Felipe near Bahia Kino is unconformably overlain by moderately-tilted 11.47 ± 0.05 Ma basalt. Taken together, these results indicate that proto-Gulf extension is widespread from central to western Sonora. Zones of large-magnitude proto-Gulf extension are recognized at Caborca and adjacent to the Gulf of California coastline.