T72 Strain in the Footwalls of Detachment Faults: New Insights on the 40th Anniversary of the Seminal GSA Memoir “Cordilleran Metamorphic Core Complexes”

Tuesday, 27 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

STRUCTURAL, GEOCHRONOLOGIC, AND THERMOCHRONOLOGIC ANALYSIS OF THE CATALINA DETACHMENT FAULT AND ITS IMMEDIATELY UNDERLYING LOWER PLATE FAULT ROCKS AND STRUCTURES, RINCON MOUNTAINS, ARIZONA

Paper number: T72-2
Program: Technical Sessions
Time: 1:35 PM – 1:50 PM

Authors

Abstract

The mega-mullioned Catalina detachment fault of the Catalina-Rincon metamorphic core complex defines the upper surface of a 100–200-m-thick elastico-frictional shear zone. This shear zone contributed to ~35–40 km of top-to-SW displacement during exhumation of quasi-plastic lower-plate mylonites. The slip surface (and gouge smear) caps microbreccia and ultracataclasite (~5–10 m) derived from underlying chlorite breccia (up to ~80 m) that itself was derived from mylonite. A continuous though complicated strain localization zone, including discrete subdetachment faults, separates lower-plate mylonites (~2500-m-thick) from the elastico-frictional shear zone dominated by the cataclasite. Previous detailed mapping in 6 subareas and new mapping in the Hidden Spring subarea have established control for the character, geometry, and fabrics produced by shearing. Multispectral analysis has permitted the full trace length of the shear zone system to be delineated.

Brittle shear zone evolution included two coordinated deformation mechanisms, which evolved in a brittle/ductile transition probably shallower than 10 km. The strain localization at the base of the shear zone exploited ultramylonite and calc-mylonite tectonite within the lower plate as it was being exhumed. The detachment fault proper likely emerged in a manner proposed by Selverstone and Axen (2012): mini-detachments grew from c-surfaces in mylonite during strain-hardening accompanying transition from ductile to brittle conditions, with a progressively ‘smoothed’ detachment fault evolving through linkages of mini-detachments. Absolute timing for this sequencing is based on published and new geochronologic and thermochronologic results, achieved through U-Pb LA-ICPMS and Ar/Ar geochronology, and (U-Th)/He, 4He/3He, and apatite fission track thermochronology. Mylonitization is Oligocene (sheared 26.7 Ma granitic protolith), with no evidence in this area for older fabrics. Very rapid exhumation of lower-plate rocks proceeded from 26–21 Ma. Slip on the detachment fault ceased by 20 Ma. New 3D structural modeling illuminates the position and geometric variability of the elastic-frictional shear zone with respect to upper plate rocks and structures as well as geomorphic expressions within the overall core complex framework.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-354708

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