T287 Best Student Geologic Map Competition (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

DETAILED GEOLOGIC MAPPING OF MYLONITIC UNITS IN THE FOOTWALL OF THE PLOMOSA DETACHMENT FAULT, WEST-CENTRAL ARIZONA

Paper number: T287-7
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

New 1-10,000-scale geologic mapping reveals that the footwall of the Plomosa detachment fault is dominated by early Miocene mylonitic fabrics associated with detachment faulting, demonstrating that the northern Plomosa Mountains is a metamorphic core complex similar to adjacent core complexes in the region. Mylonitic fabrics have systematic NE-SW-trending lineations (average T/P = [220/09]), and record a top-NE sense of shear. The Plomosa detachment fault dips ~10–30° ENE along most of the range, but dips shallowly N along its northern extent, defining a broad NE-trending corrugation parallel to its slip direction. NW-striking normal faults, joints, and dikes throughout the footwall record NE-SW extension consistent with that of the mylonitic fabrics and detachment fault.

Three mylonitic units dominate the footwall of the Plomosa Mountains core complex: 1) Orocopia Schist, 2) an early Miocene Intrusive Complex, and 3) layered gneiss. The quartzofeldspathic Orocopia Schist encompasses 10.5 km2 of the northern Plomosa Mountains, and locally contains graphitic plagioclase porphyroblasts. We mapped 24 occurrences of course-grained green actinolite pods up to 1.5 m-wide within the Schist. These pods are high in Mg, Cr, and Ni, suggesting derivation from an ultramafic protolith. A laterally continuous unit of hornblende amphibolite (~3.5 km long, 10–150 m thick), interpreted as Orocopia metabasalt, is localized along a moderately SE-dipping contact between the Schist and the gneiss, and contains metachert layers 3–30 cm thick. L>S mylonitic fabrics are common throughout the Schist and gneiss, and a zone of L-tectonites is localized along their contact. The Intrusive Complex, which is dominated by biotite granodiorite and hornblende diorite, parallels the detachment fault along its eastern margin.

Late-stage dextral- and sinistral-normal faults strike N and ENE, respectively, and associated barite and carbonate veins strike NE. These faults locally cut moderately-consolidated colluvium deposits, and veins cut NW-striking joints and faults associated with detachment faulting. Together these structures record a minor phase of NW-SE extension, which we attribute to regional post-middle Miocene dextral faulting, with the northern Plomosa Mountains having occupied a transtensional releasing bend.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-300989

© Copyright 2017 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.