Field Trip to a Laramide Shallow Subduction Channel: Orocopia Schist in the Gavilan Hills, Southeasternmost California; with Incidental Localities for Three Blue Minerals

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Author
Haxel, Gordon B.
Jacobson, Carl E.
Epstein, Gabe S.
Affiliation
United States Geological Survey
Iowa State University
Northern Arizona University
Issue Date
2026-01
Keywords
Orocopia Schist
shallow subduction
Field trip guides
Cemetery Ridge
Plomosa Mountains
Chocolate Mountains

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Citation
Haxel, G.B., Jacobson, C.E., and Epstein, G.S., 2025, Field trip to a Laramide shallow subduction channel: Orocopia Schist in the Gavilan Hills, southeasternmost California; with incidental localities for three blue minerals: Arizona Geological Survey Contributed Report CR-25-A, version 2.0, 29 p., map scale 1:32 000.
Publisher
Arizona Geological Survey (Tucson, AZ)
Description
According to current tectonic models, much of southern California and southwest Arizona is underlain by the Late Cretaceous (Laramide) Pelona-Orocopia-Rand Schist (PORS) subduction complex, “the best-known archive of shallow subduction on the planet” (Chapman 2016). Subduction was shallow in both depth and inclination. One part of this complex, the Orocopia Schist subduction channel of southwest Arizona, has attracted particular attention over the past decade, for two reasons. First, it is exposed unusually far inland from the present and late Mesozoic Cordilleran continental margin. Second, Orocopia Schist in one area, Cemetery Ridge, uniquely includes well-preserved blocks of subducted oceanic-mantle peridotite. Most exposures of Orocopia Schist have become difficult to access, owing to enclosure within military reservations or Wilderness Areas, and deterioration of old exploration and mining roads. Moreover, in some areas Orocopia Schist has been degraded by hydrothermal alteration accompanying middle Cenozoic intrusions. Orocopia Schist at Cemetery Ridge, though accessible and largely unaltered, is atypical because of its high metamorphic grade. All three of these limitations can be circumvented by examining Orocopia Schist — readily accessible, minimally altered, and representative — in the Gavilan Hills (informal name), California; 40 km north-northwest of Yuma, Arizona. Orocopia Schist in the Gavilan Hills constitutes the core of an east-west–elongate dome, one of numerous culminations that define the 220 km–long Chocolate Mountains anticlinorium. The Gavilan Hills antiform is circumscribed on its east, north, and northwest sides by two exhumation faults, Chocolate Mountains and Sortan, overlying the Orocopia Schist. Two geologic traverses through the Orocopia Schist are described here. The longer of these traverses crosses the antiform, starting in Orocopia Schist on the south side and ending at the Chocolate Mountains fault on the north side. This route includes most major rock types of the schist, is ~ 4 km round trip, and requires 4–6 hours. The second, shorter traverse, ~ 1/2 km and 1 or 2 hours, examines actinolite pods within the Orocopia Schist near the start and end point of the first traverse. To facilitate reading in the field, most descriptions and interpretations of Orocopia Schist and PORS herein are brief and not individually attributed. Although blue rock-forming silicate minerals are relatively uncommon, the Gavilan Hills features three: kyanite, dumortierite, and riebeckite. Though unrelated to origin of the Orocopia Schist, these mineralogical curiosities may be of incidental interest. Section 5 describes a long-known locality for kyanite and dumortierite, and a recently recognized locality for riebeckite.
URI
http://hdl.handle.net/10150/679166
Additional Links
https://library.azgs.arizona.edu/
Language
en
Series/Report no.
Arizona Geological Survey Contributed Report
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
35.5
South Bounding Coordinate
32.5
West Bounding Coordinate
-119.5
East Bounding Coordinate
-111.5
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