PALEOPROTEROZOIC CRUSTAL-SCALE GRAVITATIONAL COLLAPSE NORTHWESTERN ARIZONA: NEW INSIGHTS INTO AN ANCIENT OROGENY

Migmatites in the southern Hualapai Mts. (SHM), AZ, provide evidence for the first documented episode of gravitational collapse in the SW US Paleoproterozoic orogen. Gneisses in the SHM contain a subhorizontal foliation (S₀/S₁) axial planar to abundant mesoscopic recumbent folds (F₁) and a gently E-W plunging lineation (L₁). These fabrics are similar to others in AZ that have been attributed to the regional, ca. 1.74-1.71 Ga D₁ deformational event, and we interpret them in that context. N-striking, conjugate, extensional shear bands both offset and reactivate the S₀/S₁ foliation. Kinematic indicators record subhorizontal, symmetric, E-W stretching.

The migmatites were produced by partial melting via biotite-dehydration reactions under granulite facies conditions, but they subsequently underwent considerable back reaction. The current peak assemblage is quartz + K-feldspar + plagioclase + biotite + sillimanite + garnet ± cordierite ± andalusite. Leucosomal quartz + K-feldspar + plagioclase locally reveal magmatic textures and are interpreted as crystallized partial melt. The highest P-T determinations are 800±188°C and 9.2±3.0 kbar; the lowest, which we infer represents part of the decompression path, yields 660-670°C and 4.1-4.8 kbar. Thus, the SHM underwent at least 4 kbar of decompression at near-isothermal conditions.

We propose a multiphase history for the SHM. Subhorizontal fabrics (S₀/S₁) formed 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 1.74-1.71 Ga. Supracrustal sediments were buried to depths >25-30 km, where they underwent partial melting at temperatures at >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 <15 km. The subhorizontal thrust-related fabrics (D₁) were reactivated with normal-sense motion during exhumation.

The SHM show little evidence for the ca. 1.7 Ga regional D₂ event (Yavapai orogeny) that produced a penetrative NE-striking, subvertical fabric throughout the SW US. We suggest that extensive overprinting of earlier fabrics by D₂ has hindered recognition of post-D₁ extensional fabrics elsewhere in the SW.