CURRENT PROBLEMS IN BASEMENT-INVOLVED LARAMIDE FORELAND DEFORMATION, ROCKY MOUNTAINS, USA: 3D STRUCTURAL EVOLUTION AND CONNECTIONS TO PLATE PROCESSES

How and why did the basement-involved Laramide structures of the Rocky Mountains form? Proposed answers to these questions remain highly diverse and contradictory because essential geometric and kinematic elements of Laramide deformation are just now being resolved. Overall, however, a broad consensus favoring lateral shortening and horizontal compression continues to build.

On a small scale, the diverse geometries of individual structures, which are well imaged by excellent exposures and extensive subsurface information, clearly require 3D kinematic models with realistic fault-related folding mechanisms integrating regional information on structural setting. The classification of second-order anticlines as backlimb tightening, backthrust tip and subthrust splay structures is just the beginning of a rational framework for Laramide deformation. Important three-dimensional elements like crestal faulting on doubly-plunging anticlines and transpressive slip partitioning remain largely unexplored. On a more regional scale, the geometries of Laramide arches at depth, where they must connect to larger tectonic processes, remain uncertain. For instance, current hypotheses linking Laramide deformation to plate processes predict diverse Moho geometries under individual Laramide arches. Lithospheric buckling models predict upward folding of the Moho under arches, crustal detachment models predict a relatively undisturbed Moho, and lithospheric “squashing“ models predict downward folding of the Moho under individual arches. A better delineation of Laramide deformation in the lower crust and mantle lithosphere is needed before the dynamics of Laramide deformation and its connection to plate processes can be determined. In addition, there is substantial, heated disagreement as to the proportion of strike-slip to thrust deformation over the Laramide province.

The effect of pre-existing weaknesses also remains uncertain. Precambrian weaknesses are followed by some Laramide structures yet are ignored by others. In addition, post-Laramide reactivation of Laramide faults also had uncertain controls on subsequent mid-Tertiary and Rio Grande rift deformation, both of which may have had substantial transtensional components.