Concurrent Cretaceous-Paleogene Deformation, Basin Development, Magmatism, and Shallow Angle Subduction in the Laramide Province of the SW US and Northern Mexico

The Laramide province of southern Arizona, New Mexico, and northern Mexico differs from the classic Rocky Mountain Laramide region in two ways: (1) It consists dominantly of NW-trending basement uplifts and basins; and (2) syndeformational magmatism took place from ca. 80 to 40 Ma. In the shallow-angle subduction model, association of magmatism with Laramide deformation is problematic because published numerical thermal models of shallow-angle subduction predict that melting occurs only in narrow bands of space and time. These models fail to accommodate the broad areas (AZ, NM, Mexico) of protracted magmatism.

We propose that the structural and magmatic style of the Laramide event in the SW US and northern Mexico resulted from shallow-angle subduction of the Farallon plate into shallow asthenosphere underlying recently-rifted continental lithosphere. The subducted plate penetrated subhorizontally into hot convecting mantle, producing arc-like magmas as the plate dehydrated and triggered mantle melting. Reactivation of Jurassic normal faults, themselves possible reactivated Precambrian structures, controlled Laramide structural trends. The Late Jurassic rifting event, or Border rift, is defined by accumulations of alluvial and oceanic strata intercalated with rhyolitic ash-flow tuffs and asthenosphere-derived basalts. Border rift mafic rocks are present in Upper Jurassic-Lower Cretaceous strata of the McCoy Mountains Formation of SE CA/SW AZ, Kimmeridgian/Tithonian strata in the Chiricahua Mountains of SE AZ, Upper Jurassic strata in the Little Hatchet Mountains of SW NM, and as allochthonous blocks in diapiric Upper Jurassic evaporites in La Popa basin of NE Mexico. These rift sequences indicate that extension ended ca. 140 Ma, some 60 million years prior to Farallon shallow-angle subduction. Thermal diffusivity calculations indicate that asthenosphere elevated during Jurassic extension would have remained hot for more than 60 million years. Small-scale mantle convection associated with onset of Farallon subduction likely increased the longevity of hot, shallow asthenosphere.