33 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

This session explores key structural processes that govern the temporal, mechanical, and rheological evolution of crustal/lithospheric extensional and strike-slip systems with a focus on novel field observations, microstructural and analytical methodologies, and numerical modeling.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Syn-extensional magmatism and constraints on timing of ductile deformation within the Catalina Rincon Metamorphic Core Complex

Paper number: 33-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Catalina-Rincon Mountains adjacent to Tucson, Arizona, are the type locality for North American metamorphic core complexes. Classically, three phases of ductile deformation are recognized that correlate to plutonic events at ca. 70Ma, 50Ma, and 25Ma based on geochronology and cross-cutting relationships between undeformed and mylonitic plutons. However, it remains debated whether most ductile deformation occurred during Eocene or Oligo-Miocene time. We address this controversy by first establishing consistent field criteria to identify undeformed intrusions (a lack of internal mylonitization and ductile folding) and then constraining timing using geochronology of cross-cutting intrusions. We conducted detailed geologic mapping (using the StraboField app) along a NE-SW-oriented ~10 km traverse through Tanque Verde Wash between the Catalina and Rincon mountains to document rock types, cross-cutting relationships, and melt-present versus solid-state ductile deformation fabrics. Mylonitic gneisses exhibit consistent top-to-SW and top-to-NE sense-of-shear in the southwestern and northeastern part of the transect, respectively.  We documented a new mappable rock unit consisting of syn-magmatically mixed/mingled mafic and felsic bodies. Syn-magmatic deformation fabrics are overprinted by solid-state ductile and brittle deformation.  Preservation and alignment of schlieren and feldspar grains with stretching lineations in the augen gneiss suggest a transition from melt-present to solid-state deformation and mixing of mafic and felsic bodies within an extensional strain regime. Selected geochronological analysis on 21 samples that include SW- and NE-sheared augen gneiss, leucogranite, and mafic and felsic mingling/mixing samples from the traverse will constrain the timing of ductile deformation. U-Th-Pb geochronology on zircons from a top-to-SW-sheared augen gneiss yields a well-defined lead-loss discordia line with an upper intercept of ~1.4Ga (corresponding to the age of Proterozoic granites in the region) and a lower intercept of ~31Ma (corresponding to the earliest onset of extension in the region). Given unambiguous evidence that the gneiss protolith was emplaced coeval with top-to-SW extension, we interpret the protolith to have been derived from melting of ~1.4Ga granite at ~31Ma, during which inherited ~1.4Ga zircon grains experienced lead loss.  Additional geochronology targeting zircon rims and igneous garnet will test our hypothesis that synextensional magmatism is more prevalent than previously thought (in this and likely other Cordilleran metamorphic core complexes) because of the underappreciation of melt-present synextensional deformation and the role of zircon inheritance.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412713

© Copyright 2025 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.