EXTENSIONAL AND TRANSTENSIONAL CONTINENTAL ARC BASINS OF SOUTHWEST U.S. AND MEXICO
1. Early-stage low-lying continental arc basins formed in early Mesozoic time, by sinking and rollback of very old, cold subducting slab(s). These basins are floored by supracrustal rocks, showing that uplift did not precede magmatism, and form near or below sea level. In the extensional setting (orthogonal subduction), abundant, widespread large-volume silicic calderas bury fault scarps and horst blocks with the “instant sediment” of explosive volcanic eruptions, resulting in a paucity of epiclastic basin fill. High sedimentation rates and uniform, fast subsidence result in a lack of unconformities in the basins. In the transtensional setting (oblique subduction), in contrast, local restraining bends produce reverse faults, with pop-ups that shed slide blocks and epiclastic debris into basins, locally cannibalizing basin fills to produce deep unconformities. Silicic giant continental calderas also form in this setting, but they are restricted to symmetrical basins at releasing stopovers, while releasing bends produce fields of small monogenetic volcanoes.
2. Late-stage high-standing continental arc basins formed in Cenozoic time, in response to slab rollback beneath continental crust thickened by low-angle subduction during late Mesozoic time. Late-stage extensional continental arc basins are wholly nonmarine and are floored by deeply eroded crust. Voluminous eruptive products that fill and escape intra-arc grabens are funneled far away from the arc, through canyons carved during the preceding phase of crustal shortening. Silicic caldera fields form, but are concentrated in areas of thickest crust. Late-stage transtensional continental arc basins in the U.S. and Mexico formed in response to exploitation of thermally-weakened arc crust, corresponding to a change from more westerly motion to more northerly motion of the Pacific plate at ~12 Ma. In this setting, the biggest arc volcanic centers are located on pull-apart structures; these include the active Lassen center, as well as Pliocene and Miocene centers.