Skip to Main Content
Skip Nav Destination
GEOREF RECORD

Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California

Jonathan S. Miller, Allen F. Glazner, J. Douglas Walker and Mark W. Martin
Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California
Geological Society of America Bulletin (December 1995) 107 (12): 1441-1457

Abstract

The geologic history of the outer continental margin (eugeoclinal) rocks in the El Paso Mountains and northern Mojave Desert has long been important in models for the development of the active continental margin in the western Cordillera. Current interpretations call for either strike-slip or thrust juxtaposition of eugeoclinal rocks against miogeoclinal/cratonal (platformal) rocks, or some combination of both strike-slip faulting and thrusting. Two broad and interrelated aspects of the history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the eugeoclinal rocks? and (2) When were the eugeoclinal rocks thrust against platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures restore to a position structurally beneath the eugeoclinal rocks when Tertiary extension is restored, implying stacking by a thrust. New U-Pb zircon geochronology and whole rock geochemistry and Sr, Nd, and Pb isotopic data for plutons in the El Paso Mountains and northern Mojave Desert lend insight into the timing of this thrusting. The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and epsilon (sub Nd(t)) isotopic values from these plutons are distinct (< or =0.704 and > or =+2, respectively) from continental lithosphere isotopic signatures (> or =0.705 and < or =-2) of both Middle Jurassic plutons in the same area and Triassic plutons in the southern and eastern Mojave Desert. Feldspar common lead data for the Permian and Triassic plutons within the eugeoclinal outcrop belt also indicate limited crustal involvement and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Proterozoic basement is widespread. The observations and data reported here indicate that Late Permian-Early Triassic plutons in the northern Mojave Desert and El Paso Mountains were generated within or passed primarily through oceanic lithosphere, but later Jurassic plutons were derived from and/or interacted extensively with continental lithosphere. We hypothesize that the eugeoclinal rocks were deposited on oceanic crust that was thrust eastward over Precambrian cratonal basement and overlying strata between approximately 240 Ma and 175 Ma, a time of little documented tectonic activity in the Mojave Desert region. Postulated Permian or late Middle Jurassic east-directed thrusting is incompatible with our data and observations. The data also fill an important gap in palinspastic reconstructions of the early Mesozoic arc and indicate that this arc was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 107
Serial Issue: 12
Title: Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California
Affiliation: University of North Carolina at Chapel Hill, Department of Geology, Chapel Hill, NC, United States
Pages: 1441-1457
Published: 199512
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 90
Accession Number: 1996-005055
Categories: Geochemistry of rocks, soils, and sedimentsStructural geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. block diags., 3 tables, geol. sketch map
N34°19'60" - N35°34'60", W117°40'00" - W116°30'00"
Secondary Affiliation: University of Kansas, Lawrence, KS, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 199602
Close Modal

or Create an Account

Close Modal
Close Modal