31 Structural Geology, Tectonics, and Petrology

Friday, 23 April 2010
1:20 PM - 2:40 PM
Alpine Room

STRUCTURE OF THE NORTHERN GREAT PLAINS AND IMPLICATIONS FOR CONTINENTAL ASSEMBLY AND EVOLUTION

Paper number: 31-1
Program: Technical Sessions
Time: 1:20 PM – 1:40 PM

Authors

Abstract

Current understanding of the evolution of the North American plate in the northern Great Plains is constrained by basement outcrops on the Black Hills uplift combined with subsurface geologic, gravity, magnetic, heat flow, and very few seismic studies of the surrounding region. A Laramide arch, active ~64 – 40 Ma, extends from western Montana through the Black Hills uplift (structural relief ~ 2,000 m) and connects with the Chadron uplift of northwestern Nebraska. This trend crosses both the Wyoming/Trans-Hudson (WP/TH) and Trans-Hudson/Central Plains (TH-CP) province boundaries. A zone of small alkalic and calc-alkalic Laramide plutons (46-58 Ma) trend N.70oW across the Black Hills uplift possibly emplaced along an ancient basement fracture. Localization of the Laramide arch may lie along a transition from thin to thick lithosphere and reflect the surface manifestation of crustal shortening associated with deep-seated reverse faults. Understanding of the deep Proterozoic and Laramide structures requires remote sensing of the subsurface lithosphere beneath the Black Hills and surrounding regions

New seismic stations from the USArray’s transportable array and the proposed FlexArray will help us to better understand the origin and tectonic evolution of the North American continental lithosphere, as well as the role of lithospheric roots and asthenospheric mantle flow in driving North American plate motion. In order to image lithospheric structure, we have begun to analyze the initial passive source seismic data from the transportable array stations recently deployed in and around the Black Hills. Shear wave splitting data provide information on the mantle anisotropy in this region and receiver functions on the nature of the crust across the Trans-Hudson collisional boundary.

It is clear that a joint active and passive source seismic experiment and petrologic data designed to investigate inferred geologic structures that cross time boundaries at variable crustal and mantle depths are needed to understand the southern Trans-Hudson orogen and the later Laramide overprint. Such an experiment would allow us to image the transition from the primarily thin, actively-deforming lithosphere of the western U.S. to the stable, thick lithosphere beneath the Superior province.