GRAVITATIONAL COLLAPSE OF A PALEOPROTEROZOIC OROGEN, SOUTHERN HUALAPAI MOUNTAINS, ARIZONA

Recent recognition of pervasively migmatized gneisses in the southern Hualapai Mts., AZ, raises questions about the origin and consequences of crustal anatexis in Proterozoic rocks of northwestern Arizona. This study uses structural and metamorphic data from the southern Hualapai Mts. to constrain the tectonic history of and elucidate partial-melting processes in the boundary zone between the Paleoproterozoic Mojave and Yavapai crustal provinces. Migmatitic gneisses and granites of the southern Hualapai Mts. contain a subhorizontal foliation (S₀/S₁) axial planar to abundant mesoscopic recumbent folds (F₁) and an E-W-trending, shallowly plunging lineation (L₁). N-S striking, conjugate, normal-sense shear bands both offset and sole into S₀/S₁ foliation. Kinematic indicators record subhorizontal, symmetric, E-W stretching.

Partial melting occurred by muscovite- and biotite-dehydration reactions under granulite facies conditions, but migmatites subsequently underwent considerable back reaction. The inferred peak assemblage was quartz + K-feldspar + plagioclase + biotite + sillimanite + garnet ± cordierite. Within leucosomes, quartz + K-feldspar + plagioclase locally reveal magmatic textures and are interpreted as crystallized partial melt. The highest P-T (GASP) determinations are 800+188°C and 9.2+3.0 kbar; the lowest, which we infer represents the terminus of the decompression path, yields T= 670 ± 144°C and P= 4.5 ± 2.0 kbar. Thus, the southern Hualapai rocks underwent at least 4 kbar of decompression at near-isothermal conditions.

Based on structural data and thermobarometry, we propose a multiphase history for the southern Hualapai Mts. Subhorizontal fabrics (S₀/S₁) formed initially during W-directed thrusting and crustal thickening associated with the regional D₁ of the Mojave and Yavapai provinces, which has been recognized elsewhere in northwestern Arizona at 1740 to <1715 Ma. Supracrustal sediments were buried to depths >25-30 km, where they underwent partial melting at temperatures >750°C. Large-volume melting destabilized the D₁ crust and promoted gravitational collapse. Rocks from the deepest parts of the D₁ orogen were tectonically exhumed to less than 12 km. Subhorizontal fabrics developed during D₁ thrusting were reactivated with normal-sense motion during exhumation.