Tectono-Thermal Evolution of the Mule Mountains and Little Maria Fold and Thrust Belt Shear Zones in Southeastern California

Abstract

The Maria fold and thrust belt (MFTB) in southeastern California and northwestern Arizona is characterized by south-verging folds and top-to-the-south thrusts that place Proterozoic basement on deformed Paleozoic–Mesozoic sedimentary and volcanic strata. Three major phases of deformation have been proposed to describe the evolution of the Maria fold and thrust belt related to Sevier and Laramide events, metamorphic core complex extension, and Basin and Range events. The Maria fold and thrust belt crops out in the Little Maria and Mule Mountains which makes these locations essential to understanding the nature and timing of deformation. Three models are considered for the development of the Little Maria and Mule Mountain structures: (1) A folded thrust fault that connects shear zones in the Mule Mountains and Little Maria Mountains; (2) structures are developed as a result of synconvergent extension; and (3) structures record Sevier and Laramide tectonics and are overprinted by Oligo-Miocene extension. This study applies structural geology, geochronology, and low-temperature thermochronology to samples collected from the hanging wall, footwall, and shear zones of faults in the Little Maria Mountains and the Mule Mountains to determine the timing of cooling associated with deformation and exhumation as well as timing of crystallization. Our results from the Mule Mountains suggest a shortening event ca. 162 - 174 Ma coeval with arc magmatism in the Sierra Nevada and Peninsular Range Batholith. The timing of exhumation in the Mule Mountains is consistent with end of Sevier, beginning of Laramide tectonic events ca. 50 Ma. The Little Maria Mountains exhibit extension with methods and timing of exhumation related to core complex metamorphism and Basin Range extension in two pulses: (1) ca. 50–60 Ma and (2) ca. 40–25 Ma.