UNDERSTANDING THE CAUSAL MECHANISMS OF LARAMIDE BASEMENT-INVOLVED DEFORMATION: A PRIME TARGET FOR AN ENHANCED EARTHSCOPE PROGRAM

Connecting the passive seismic observations from the USArray program of EarthScope to crustal geology is a challenge because continuous resolution will start at a depth roughly equivalent to the 70 km spacing of the stations. Densified areas paralleling the proposed GeoSwath transect may bring continuous resolution to the lower crust, but geologic targets still need to be large and relatively simple to be resolved by this project.

The Laramide arches of the Rocky Mountains provide an excellent target for an enhanced EarthScope program. First, tectonic mechanism(s) for foreland basement-involved arch formation are highly contested, making the resolution of this question a potential EarthScope highlight. In addition, current Laramide hypotheses differ wildly in their predictions of lower crustal and upper mantle geometries. For instance, buckling models for the entire lithosphere suggest that the Moho is upwarped under arches whereas crustal detachment models predict that the Moho is relatively undeformed under arches. Moreover, structural modeling suggests relatively simple fault-bend fold geometries within the resolution of densified active and/or passive arrays. Finally, the relatively simple, pre-Laramide platformal structure of the northern Rocky Mountains allows interpreted deeper structures imaged by a densified array to be correlated to surface shortening.

The determination of Laramide tectonic mechanism(s) has importance on many levels. For the entire USArray project, whether the Rockies are detached or rooted to their underlying mantle will be critically important to every attempt to correlate mantle and crustal structures to the west. On a larger scale, this is an opportunity to explain one of the major remaining questions in plate tectonics and look at a critical indicator of continental rheology, which is another area of diverse hypotheses. From an applied perspective, a better understanding of the western USA may provide the framework for assessing and understanding the key variables controlling the west's hydrocarbon resources. And finally, the relative simplicity of Laramide arch deformation can be combined with the popularity of the Rocky Mountains themselves into an excellent teaching opportunity for the general public.