PALEOTECTONICS OF THE WESTERN US AND NORTHERN MEXICO DURING 90-48 MA, INCLUDING ~400-KM LEFT-SLIP ON A FAULT EXTENDING FROM CALIFORNIA TO COAHUILA

Bird & Ingersoll (2022, Geosphere) documented how kinematic F-E program Restore computes paleo-velocity fields and integrates them through time to create paleogeologic maps. Velocity is fit by weighted-least-squares to 4 datasets of: fault traces and offsets, section length changes, paleostress directions, and paleomagnetic inclinations and declinations. Here we report further reconstructions prior to 48 Ma.

During the Laramide orogeny (75-39 Ma), crustal velocities in the Rocky Mountain foreland averaged less than 2~4 mm/a to the NE, consistent with the hypothesis of drag by horizontal subduction of the Farallon plate. Horizontal subduction may have been caused by a buoyant oceanic plateau that subducted beneath southern California, but later foundered under South Dakota when the basalt/eclogite phase change reversed its buoyancy.

The paleogeologic map at 60 Ma shows the North American belt of Cretaceous granodiorite (Kg) plutons, formed as a continuous magmatic arc above the Franciscan subduction zone, to be offset 370-480 km in southern California by sinistral faulting along SE trends. We describe a proposed Nacimiento-Sonoyta-Fronteras-San Marcos fault trace defined by offsets of the Kg belt and restored fault traces, which was active during 75-60 Ma. This trace can explain: (1) a terrane boundary in pre-Tertiary rocks along the Arizona/Sonora border; (2) offset of both margins of the Kg belt; (3) N termination of the Sierra Madre Occidental; (4) limitation of Cu-bearing mid-Laramide intrusions to the N side of the trace; and (5) evidence for an extensional interlude separating two phases of the Laramide/Hidalgoan orogeny in southern Arizona-New Mexico-Texas.

Sinistral slip on an alternative Nacimiento-Caborca-Durango-Zacatecas fault trace gives an alternative reconstruction, which we present at another talk in this meeting. Both models result in problematic reconstructed shapes for the Franciscan trench, which may imply/require additional submarine faults in the continental margin.

Maps at 90-87 Ma display a late stage in the Sevier orogeny, when shortening rates across the central part of the orogen (34°-42°N) were 2 mm/a. Our model is consistent with a slow intraplate orogeny in which topographic stresses under the slope to the east of the “Nevadaplano” controlled the locus of thrusting.