Collaborative Research: Formation of Basement-involved Foreland Arches: An Integrated EarthScope Experiment

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This EarthScope project, herein called the Bighorn Project, is an integrated geological and geophysical investigation of contractional basement-involved foreland arches. It addresses how these foreland arches form and how they are linked to plate tectonic processes. The research in the Rocky Mountain Bighorn Arch of northern Wyoming and southern Montana combines geological investigations of surface geometries and kinematic indicators with geophysical imaging of 3D crustal and upper mantle geometries from an active/passive seismic experiment. The resulting 4D (3D spatial and temporal), lithospheric-scale model of foreland arch deformation tests current hypotheses for basement-involved foreland thrust belts both in the Rockies and in active orogens of Asia and the Andes. These hypotheses include: 1) fault blocks defined by lithosphere-penetrating thrust faults, 2) subhorizontal detachment within the crust, 3) lithospheric buckling, and 4) pure shear lithospheric thickening. Our investigation to determine the mechanism driving basement-involved arch formation is advancing our understanding of continental lithospheric rheology. This three year (2009-12) collaborative project defines a lithospheric volume of 1.5 x106 km3 by integrating arch-scale upper crustal geometries derived from surface exposures and petroleum industry subsurface data (Eric Erslev, University of Wyoming; Christine Siddoway, Colorado College) with the results of a hybrid seismic experiment. The passive component of this experiment consists of a 1.25 year (2009-10) deployment of 27 broadband seismometers that densify the EarthScope transportable array (Megan Anderson, Colorado College), a 6.5 month deployment of 220 short period seismometers (Anne Sheehan, University of Colorado), and a 9 day deployment of 800 high frequency "Texan" seismometers (Kate Miller, University of Texas at El Paso). The active component consists of 9 shots (summer 2010) recorded by the above instruments and an additional 1600 "Texan" seismometers deployed for 5 days. These instruments are arrayed in a grid consisting of three SW-NE lines and two NW-SE lines with a total line length of 1000 km. Joint inversion of active and passive results defines crustal and upper mantle velocities and interface structures within the Bighorn Arch. These new seismic results are integrated (2010-12) within a GIS-based, 3D geospatial framework including data from exposures, geologic maps and industry subsurface data for the study area. Kinematic information from fracture transects (Erslev, Siddoway) and gravity modeling (Miller) is used to guide 3D, lithosphere-scale structural restorations to test the compatibility of different components in our 3D geometric model. The broader impact of the Bighorn Project partially lies in the development of fundamental methods for quantitative integration of geological and seismological research. In addition, the Bighorn Project involves a broad range of geoscientists, including one post-doctoral fellow, 4 graduate students, and 15 undergraduates, who will participate in IRIS and Keck Consortium sponsored research. The results have key implications for energy resources, allowing the prediction of open fractures that are critical to hydrocarbon production. The development of a dramatic animation showing the 4D structural development of the Bighorns Arch during the Laramide Orogeny provides important public outreach.