2042 Economos et al. | Late Cretaceous magmatic-tectonic evolution of the central Mojave Desert GEOSPHERE | Volume 17 | Number 6 Research Paper Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc Rita C. Economos 1 , Andrew P. Barth 2 , Joseph L. Wooden 3 , Scott R. Paterson 4 , Brody Friesenhahn 1 , Bettina A. Wiegand 5 , J. Lawford Anderson 6 , Jennifer L. Roell 2 , Emerson F. Palmer 2 , Adam J. Ianno 7 , and Keith A. Howard 8 1 Roy M. Huffington Department of Earth Sciences, Southern Methodist University, P.O. Box 750395, Dallas, Texas 75275-0395, USA 2 Department of Earth Sciences, Indiana University/Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, Indiana 46202, USA 3 U.S. Geological Survey (Retired), Marietta, Georgia 30064, USA 4 Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA 5 Applied Geoscience, University of Göttingen, Goldschmidtstrasse 3, Göttingen 37077, Germany 6 Department of Earth and Environment, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA 7 Geology Department, Juniata College, 1700 Moore Street, Huntingdon, Pennsylvania 16652, USA 8 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA ABSTRACT The Mojave Desert region is in a critical position for assessing models of Laramide orogenesis, which is hypothesized to have initiated as one or more seamounts subducted beneath the Cretaceous continental margin. Geo- chronological and geochemical characteristics of Late Cretaceous magmatic products provide the opportunity to test the validity of Laramide orogenic models. Laramide-aged plutons are exposed along a transect across the Cor- dilleran Mesozoic magmatic system from Joshua Tree National Park in the Eastern Transverse Ranges eastward into the central Mojave Desert. A transect at latitude ~33.5°N to 34.5°N includes: (1) the large upper-crustal Late Creta- ceous Cadiz Valley batholith, (2) a thick section of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late Cretaceous midcrustal sheeted complex emplaced at ~20 km depth that transitions into a migmatite complex truncated along the San Andreas fault. This magmatic section is structurally correlative with the Big Bear Lake intrusive suite in the San Bernardino Mountains and similar sheeted rocks recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb geochronology of 12 samples via secondary ionization mass spectrometry (SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass Deep Scientific Drillhole) indicates that all Cretaceous igneous units investigated were intruded between 83 and 74 Ma, and Cajon Pass sam- ples include a Jurassic age component. A compilation of new and published SIMS geochronological data demonstrates that voluminous magmatism in the Eastern Transverse Ranges and central Mojave Desert was continuous throughout the period suggested for the intersection and flat-slab subduction of the Shatsky Rise conjugate deep into the interior of western North America. Whole-rock major-element, trace-element, and isotope geochemistry data from samples from a suite of 106 igneous rocks represent the breadth of Late Cretaceous units in the transect. Geochemistry indicates an origin in a subduc- tion environment and intrusion into a crust thick enough to generate residual garnet. The lack of significant deflections of compositional characteristics and isotopic ratios in igneous products through space and time argues against a delamination event prior to 74 Ma. We argue that Late Cretaceous plutonism from the Eastern Transverse Ranges to the central Mojave Desert represents subduction zone arc mag- matism that persisted until ca. 74 Ma. This interpretation is inconsistent with the proposed timing of the docking of the Shatsky Rise conjugate with the margin of western North America, particularly models in which the leading edge of the Shatsky Rise was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of plutonism precedes the timing of the passage of the Hess Rise conjugate beneath western North America at ca. 70–65 Ma. The presence, geochemical composition, and age of arc products in the Eastern Transverse Ranges and central Mojave Desert region must be accounted for in any tectonic model of the transition from Sevier to Laramide orogenesis. ■ INTRODUCTION The transition from Sevier to Laramide orogenesis in the western United States was marked by major changes in tectonic style, faulting, and paleo- elevation (Dickinson et al., 1978). A common feature of models for the tectonic evolution of this period is the migration of the locus of magmatism away from the continental margin (Coney and Reynolds, 1977; Copeland et al., 2017; Chapman et al., 2018), in some cases calling for near-horizontal (e.g., “flat”) slab geometry due to the docking of the conjugates of the Shatsky and Hess GEOSPHERE, v. 17, no. 6 https://doi.org/10.1130/GES02225.1 11 figures; 1 set of supplemental files CORRESPONDENCE: reconomos@smu.edu CITATION: Economos, R.C., Barth, A.P., Wooden, J.L., Paterson, S.R., Friesenhahn, B., Wiegand, B.A., Anderson, J.L., Roell, J.L., Palmer, E.F., Ianno, A.J., and Howard, K.A., 2021, Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc: Geosphere, v. 17, no. 6, p. 2042–2061, https://doi.org/10.1130/GES02225.1. Science Editor: Shanaka de Silva Associate Editor: G. Lang Farmer Published online 5 November 2021 Received 20 December 2019 Revision received 19 May 2021 Accepted 20 July 2021 © 2021 The Authors This paper is published under the terms of the CC-BY-NC license. R.C. Economos https://orcid.org/0000-0002-8484-7190 GEOSPHERE Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/6/2042/5476143/2042.pdf by guest on 21 May 2023