CONTROLS ON BASEMENT-INVOLVED FORELAND DEFORMATION: A VIEW FROM THE LARAMIDE ROCKY MOUNTAINS, USA

What controls basement-involved foreland deformation? While Laramide structures are inactive, excellent exposures combined with abundant subsurface data provide important constraints on this problem. On the scale of individual structures, earlier hypotheses invoking dominantly vertical or strike-slip tectonics have been falsified by extensive evidence for NE-SW to E-W shortening by slightly transpressive thrust tectonics. Laramide structural geometries can be replicated by rotational fault-bend folding of crystalline basement blocks whose bounding thrust faults are linked to flexural slip folding of the upper-most sedimentary strata through intermediary triangular shear (trishear) zones. Pre-existing weaknesses appear to guide local deformation geometries without controlling regional patterns.

Proposed Laramide connections to plate processes have included shear and hydration from below during low-angle subduction as well as collisional plate coupling to the west. Laramide deformation was largely amagmatic, correlating with a Cordilleran magmatic gap. In the northern Rockies, the regular spacing and asymmetrical geometries of many Laramide arches combined with limited geophysical evidence for an unfolded Moho suggest buckling of the upper crust above a subhorizontal detachment with minimal involvement of the lower lithosphere. The temporal overlap of thin-skinned Sevier and thick-skinned Laramide thrusting also suggests that the motive force(s) driving crustal shortening originated from the main Cordilleran orogen to the west, not from below the Rockies. The southern Rocky Mountains differ in three important ways: 1) the minimally-deformed Colorado Plateau separates the Laramide arches from the main Cordilleran thrust belt, 2) post-Laramide magmatism and extension may have concealed Laramide deformation of the Moho, and 3) the enigmatic, early-Laramide Colorado Mineral Belt formed a northeast-trending belt of magmatic centers across both the Colorado Plateau and the southern Rockies. While subduction-related shear, erosion and/or hydration of the North American mantle lithosphere beneath the Rockies may have contributed to Laramide deformation, similar basement-involved deformation in more purely collisional orogens suggest that plate coupling to the west drove foreland shortening during the Laramide orogeny.