“SALMON TECTONICS” AS A POSSIBLE EXPLANATION FOR LARAMIDE SEDIMENTATION AND UNDERPLATING OF SCHIST IN SOUTHERN CALIFORNIA AND SOUTHWESTERN ARIZONA

The Late Cretaceous-early Tertiary evolution of the U.S. Southwest is marked by underplating of eugeoclinal sediment beneath the Cretaceous magmatic arc (Pelona-Orocopia-Rand Schist). Previous U-Pb analysis of detrital zircons within the schist has revealed systematic provenance and depositional age patterns. Northern schists were derived from a protolith of Campanian age containing predominantly Early to early Late Cretaceous zircons suggestive of a Sierra Nevada arc source (“Sierran” end member). Sediment accreted to the south was deposited later (Maastrichtian to Paleocene) and has a mix of Proterozoic, Triassic, Jurassic, latest Cretaceous, and early Tertiary zircons evocative of overlying basement of the Mojave Desert, Transverse Ranges, and southwest Arizona (“Mojave/Arizona” end member). We have recently confirmed that sediment of appropriate age to represent the schist protolith exhibits similar trends. Maastrichtian to lower Eocene strata from proximal parts of the forearc basin in the La Panza and Transverse Ranges have zircon populations similar to the Mojave/Arizona end-member of the schists, consistent with their southerly location. In contrast, Sierran zircon suites are characteristic of similar aged sediments comprising the distal forearc basin and trench-slope basins in Salinia and the Sur-Obispo terrane (Santa Cruz, Point San Pedro, Pigeon Point, Monterey, Big Sur, Cambria, and Atascadero areas). An intriguing, although non-unique, interpretation of our data from both schists and sediments is based upon eastward migration of the topographically high magmatic front with flattening of the Farallon slab. Because the locus of underplating tracks the source area of schist detritus (i.e., both move eastward in tandem), we refer to the process as “salmon tectonics.” Specifically, older sediment derived from the western Sierran source was accreted first and closest to the margin. Younger sediment derived from more easterly, craton-dominated crust was subsequently accreted farther inboard due to shallowing of subduction and progressive erosion of the base of the crust with time.