LARAMIDE CRUSTAL DETACHMENT IN THE ROCKIES: CORDILLERAN SHORTENING OF A FLUID-WEAKENED CRATON

While the association of basement-involved foreland orogens with low-angle subduction is well documented, how and why these cordilleran orogens shorten adjoining, previously-stable cratons remain controversial. The NSF/EarthScope Bighorn Project addressed these questions by imaging the Moho in northern Wyoming and showing that its miss-matches with the overlying Bighorn Arch requires ENE-directed Laramide detachment at ~30 km depth. Detachment of this thin, allochthonous wedge created the anastomosing network of Laramide basement arches by combining ENE-directed detachment with secondary NNW-SSE shortening due to wedging.

The contrast between Laramide shortening (~N65E) and plate convergence (~N25E) directions argues against Laramide deformation being driven by direct coupling with the Farallon Plate as it passed beneath the Rockies. Instead, similar slip directions in the Laramide and Sevier thrust belts indicate a shared origin from cordilleran end-loading. Slip partitioning between hinderland strike-slip and foreland thrust faults can explain the different plate and Laramide slip directions.

Detachment of a thin sheet of cratonic crust requires an extremely weak detachment, something that can’t be ascribed to heat since plate-on-plate contact during low-angle subduction should cool, and thus strengthen, overlying cratons. Instead, suppression of fluid-consuming melting and corner flow processes during low-angle subduction virtually necessitates fluid escape into overlying cratons. Similar conditions of retrograde metamorphism and/or increased fluid pressure occur in the lower crust of subduction forearcs, where they are revealed by slow-slip earthquake processes on similarly weak faults. Thus, Laramide basement-involved foreland shortening was driven by Cordilleran end-loading and aided by subduction dewatering that unglued the craton and allowed lower-crustal detachment.

This hypothesis predicts that major shortening in active analog orogens (e.g., the Sierras Pampeanas of Argentina) will involve a period of slow-slip earthquakes on a crustal detachment.