REFINING THE EARLY HISTORY OF THE MOJAVE-YAVAPAI COLLISION: PRE-COLLISION RIFTING AND POST-COLLISION GRAVITATIONAL COLLAPSE

Previous studies in the Paleoproterozoic orogen of N and NW Arizona outlined a history of protracted accretion and continental growth at the southern margin of Laurentia ca. 1.74-1.68 Ga, and provide a framework for more focused recent studies that probe the details of its earliest deformational history. These recent studies reveal evidence for 1) back-arc rifting prior to, and 2) gravitational collapse immediately following, initial collision of the continental Mojave block with Yavapai arcs during the regional D₁ event (1.74-1.71 Ga).

Mafic and ultramafic rocks are distributed widely throughout the isotopically mixed crust of the Mojave-Yavapai boundary zone as well as the isotopically evolved Mojave crust. In the Hualapai, Cerbat, and Peacock Mountains and the Cottonwood Cliffs, NW AZ, ultramafic and mafic rocks (including meta-pillow basalts) occur within bimodal sequences interlayered with compositionally immature siliciclastic metasedimentary rocks and locally, metacherts and calc-silicate rocks. We interpret this assemblage to have formed during rifting of Mojave continental crust, perhaps in a back-arc setting prior to D₁, a process that can also explain the mixed isotopic signature of the boundary zone.

Regional decompression following D₁ has generally been attributed to erosion-driven exhumation; however, structures in migmatites in the southern Hualapai Mts. (SHM), AZ, provide evidence for at least local gravitational collapse. Similar structures recognized elsewhere in NW AZ suggest that collapse following D₁ was likely regional in scale. Structures within migmatites in the SHM attributed to regional D₁ deformation comprise a foliation (S₀/S₁) that is axial planar to mesoscopic recumbent folds (F₁) and a gently E-W plunging lineation (L₁). N-striking, conjugate, extensional shear bands offset, sole into, and reactivate the S₀/S₁ foliation and record subhorizontal, coaxial, E-W extension. The highest P-T determinations are ca. 800°C and 0.9 GPa; the lowest, representing part of the decompression path, yield 660-670°C and 0.41-0.48 GPa, indicating >0.4 GPa of isothermal decompression. We suggest that conditions in the SHM preserved pre- and post-D₁ extensional fabrics that have been overprinted by D₂ deformation elsewhere in the SW US.