106 GSA Geophysics and Geodynamics Division 50th Anniversary Special Session and George Woollard Lecture II

Monday, 10 October 2022
1:30 PM - 5:30 PM
205

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

Paper number: 106-6
Program: Special Sessions
Time: 3:25 PM – 3:50 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-377323

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.