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Orocopia Schist in the northern Plomosa Mountains, west-central Arizona; a Laramide subduction complex exhumed in a Miocene metamorphic core complex

E. D. Strickland, John S. Singleton and G. B. Haxel
Orocopia Schist in the northern Plomosa Mountains, west-central Arizona; a Laramide subduction complex exhumed in a Miocene metamorphic core complex
Lithosphere (December 2018) 10 (6): 723-742

Abstract

We document field relationships, petrography, and geochemistry of a newly identified exposure of Orocopia Schist, a Laramide subduction complex, in the northern Plomosa Mountains metamorphic core complex of west-central Arizona (USA). This core complex is characterized by pervasive mylonitic fabrics associated with early Miocene intrusions. The quartzofeldspathic Orocopia Schist records top-to-the-NE mylonitization throughout its entire approximately 2-3 km structural thickness and 10 km (super 2) of exposure in the footwall of the top-to-the-NE Plomosa detachment fault. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarse-grained actinolitite pods, both of which are characteristic of the Orocopia and related schists. Actinolitite pods are high in Mg, Ni, and Cr, and are interpreted as metasomatized peridotite-an association observed in Orocopia Schist at nearby Cemetery Ridge. A 3.5-km-long unit of amphibolite with minor interlayered ferromanganiferous quartzite is localized along a SE-dipping contact between the Orocopia Schist and gneiss. Based on their lithologic and geochemical characteristics, we interpret the amphibolite and quartzite as metabasalt and metachert, respectively. The top of the Orocopia Schist is only approximately 3-4 km below a ca. 21 Ma tuff in the footwall of the Plomosa detachment fault, suggesting that a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted in the latest Cretaceous but before most Miocene core complex exhumation. The Orocopia Schist in the northern Plomosa Mountains is located near the center of the Maria fold-and-thrust belt, which likely represented a crustal welt in the Late Cretaceous. The keel of this crustal welt may have been sheared off by the shallowly dipping Farallon slab prior to underplating of rheologically weak Orocopia Schist. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation recorded in Orocopia Schist in the Gavilan Hills of southeasternmost California, which shortly postdated schist underplating, suggesting that subduction of schist may have triggered Paleogene extension in the region.


ISSN: 1941-8264
EISSN: 1947-4253
Serial Title: Lithosphere
Serial Volume: 10
Serial Issue: 6
Title: Orocopia Schist in the northern Plomosa Mountains, west-central Arizona; a Laramide subduction complex exhumed in a Miocene metamorphic core complex
Affiliation: Colorado State University, Department of Geosciences, Fort Collins, CO, United States
Pages: 723-742
Published: 201812
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 79
Accession Number: 2019-002271
Categories: Structural geologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N33°47'60" - N34°00'00", W114°10'00" - W114°00'00"
Secondary Affiliation: U. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2019
Program Name: USGSOPNon-USGS publications with USGS authors
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