4 Late Cretaceous–Paleogene Tectonism from the Southwestern Cordillera to the Rocky Mountains

Wednesday, 12 May 2021
8:30 AM - 11:10 AM
Mackay School of Earth Sciences & Engineering Room

MULTI-MINERAL U-PB GEO- AND THERMOCHRONOMETRIC CONSTRAINTS ON LARAMIDE-AGE DEFORMATION IN THE MARIA FOLD-AND-THRUST BELT (SW USA)

Paper number: 4-5
Program: Theme Sessions
Time: 9:50 AM – 10:10 AM

Authors

Abstract

The Maria fold-and-thrust belt (MFTB) is an anomalously S- to SW-vergent vestige of the Late Cretaceous fold-thrust system of the North American Cordilleran orogen and preserves contractional and extensional fabrics that are variably overprinted by the Miocene Colorado River extensional corridor of southeastern California and west-central Arizona. While MFTB contractional kinematics are fairly well studied, the exact timing of MFTB thrust structures and the timing and kinematics of regional, Late Cretaceous-Paleogene extension are poorly constrained. To constrain cross-cutting relationships, we applied zircon and apatite U-Pb geochronometry to date syn- and post-kinematic intrusions. Top-SW contractional structures like the Big Maria syncline in the Big and Little Maria Mountains and the Tyson thrust in the Dome Rock Mountains are cut by ~80-64 Ma post-kinematic dikes and plutons. The apatite U-Pb system has a closure temperature ~450°C, making it an ideal thermochronometer to constrain shear zone deformation within amphibolite- to upper greenschist-facies mylonitic fabrics. Apatite U-Pb dates from rocks in extensional top-NE amphibolite- to upper greenschist-facies mylonitic shear bands within Jurassic and Mesoproterozoic plutons are ~105 to 55 Ma in the Big Maria and Riverside Mountains, suggesting that the western MFTB cooled below ~450°C during Late Cretaceous to Paleocene extension. To the east, the ~87 Ma Tyson Wash Granite in the Dome Rock and Moon Mountains cuts SW-vergent contractional structures, but is involved in top-NE extensional shear. Top-NE extensional reactivation of the Tyson-Valenzuela thrust system was coeval with pegmatite dike emplacement ~70 Ma. Apatite U-Pb analyses from top-NE extensional fabrics and in-situ titanite U-Pb analyses from calc-silicate rocks along the Tyson-Valenzuela thrust system in the Dome Rock and eastern Moon Mountains are the target of future analyses.

Geological Society of America Abstracts with Programs. Vol. 53, No. 4

doi: 10.1130/abs/2021CD-362516

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