IMPLICATIONS OF PERMIAN MAGMATISM IN SOUTHWEST LAURENTIA FOR TECTONIC RECONSTRUCTIONS

The truncation of the western Laurentian margin in late Paleozoic time was closely followed by a transition from passive-margin sedimentation to subduction and magmatism, traditionally considered to have begun at approximately 250 Ma. The transcurrent margin incorporated sinistral translation of crustal blocks such as the Caborca terrane, which is commonly inferred to have been translated ~800 km southeastward from eastern California beginning in Middle-Late Pennsylvanian time (~305 Ma). Geochemistry and zircon dating of the Los Tanques pluton in northwest Sonora, Mexico, and volcanic clasts in the Permian-Triassic Antimonio Formation west of Caborca, Sonora, provide insights into the changing tectonic regimes of southwestern Laurentia.

The Los Tanques pluton and Antimonio Formation clasts range from ~280 Ma to ~265 Ma, and are weakly peraluminous, high-K calc-alkaline, and are enriched in heavy rare earth elements. ΕNd values (assuming a crystallization age of 280 Ma) are -3.2 – -8.3, and Sr_ivalues range from 0.7071 to 0.70978; these values are similar to some Permo-Triassic plutons in the Mojave Desert that record earliest magmatism in that region.

We use these data to infer that early magmatic products were emplaced into continental crust, and that the Caborca terrane arrived wholly or in large part to its present configuration by ~280 Ma. Considering a likely travel distance of ~800 km over ~25 m.y., a offset rate of 35-40 km/Myr is calculated, similar to rates on the Holocene San Andreas fault. Induced subduction that initiated along the fault continued irregularly northwestward over the ensuing 50 Myr. Subduction initiated during collapse of the fault or a strongly transpressional period, as a west-facing system. Published data suggest that magmatism remained active in the Caborca region until ca. 250 Ma and was followed by a lull until ca. 225 Ma. The lull was accompanied by erosion of volcanoes and uplift and erosion of plutons; detrital zircons were redistributed over time into early Mesozoic units across southwestern North America.