EXPLORING AN ALTERNATIVE EXPLANATION FOR THE LARAMIDE OROGENY

Much recent work presumes that early Tertiary basement-cored uplifts of the Rocky Mountain Foreland were produced by mechanical coupling of a flat-dipping Farallon Plate to the base of continental lithosphere in the western U.S. Although this hypothesis is physically appealling in its ability to relate a magmatic lull in the volcanic arc to inboard deformation and its physical mechanism for only minor deformation in the Colorado Plateau arcward from the foreland uplifts, several observations bring this model into question. Existence into the Miocene of Mesozoic lithosphere under the Sierra Nevada, the absence of a shift in the position of isotopic Precambrian province boundaries in the mantle, and some emerging difficulties with the timing of events in the arc and foreland are among the observations in need of explanation. Additionally, the cause(s) of flat-slab subduction across the entire western U.S. over more than 20 My remain a challenge.

We propose an alternative that only requires a limited portion of the slab to subduct shallowly, consistent with observations of the extent of the Pelona/Orocopia/Rand schists. Limited eastward arc migration in Late Cretaceous and Early Tertiary time across all of western North America reflected overall shallowing subduction; this was exacerbated in the western U.S. by the presence of thick Archean lithosphere under Wyoming, which disrupted asthenospheric counterflow, inducing extreme subsidence in the foreland while also further shallowing the slab segment descending under southern California. Stresses from the subsidence combined with increased end loads to drive contraction in the foreland. Oblique subduction combined with the disrupted counterflow ended arc magmatism over a broader region; minor magmatism along the Colorado Mineral Belt could be from adiabatic decompression associated with "Richter roll" convection aligned parallel to the Farallon-North America convergence vector. Post-Laramide volcanism could be a product of Laramide hydration of lithosphere with convective processes associated with the decay of the shallowly-dipping slab segment. Although much work is required to test this alternative, we hope by advancing this to highlight some of the difficulties remaining in understanding the geodyanmic processes of the Laramide Orogeny.