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.
The primary goal of the Bighorns Project was to understand how contractional, basement-involved foreland arches, such as those that comprise the Rocky Mountains of the western United States, are formed and how arch formation is linked to plate tectonic processes. This is one of the fundamental, unsolved problems in continental tectonics, in part because this type of mountain belt forms at great distances from subducting plate boundaries. The Bighorns Project involved a collaboration of scientists and students at five universities and included acquisition of 3D seismic recordings of earthquakes and controlled blasts as well as other geologic data. Through integration of 2D and 3D seismic images and 4D structural studies of an archetypical basement-cored foreland arch, the Bighorn Mountains of north Central Wyoming, the group tested hypotheses for arch formation in order to enhance our understandings of the lithosphere in continental interiors. The main responsibility of the investigators for this award centered on producing geophysical models along two transects, BASE01 and BASE02 (Figure 1). A geologic cartoon (Figure 2) that results from a joint interpretation of the velocity, gravity, and magnetic models along the BASE01 transect illustrates results. At the scale of the crust, the cartoon illustrates three features that are new and/or unexpected in the region. First, an up warp of the Moho (crust-mantle boundary) from ca. 50 km to 37 km depth occurs beneath the Bighorn Arch. Second, a 20-km thick high-density "7.x" layer, long thought to be a pervasive feature of the lower crust of the Wyoming, is severely attenuated or entirely missing in the region associated with up warped Moho. Third, the boundary between two Archean-age (> 2.3 billion years old) geologic features, the Bighorn batholith and the Bighorn Gneiss terrane is shown to be a crustal-scale feature characterized by a steeply west-dipping boundary. There are a number of possible implications of these findings for the tectonics of the region. A regional map of Moho derived from receiver function analysis done as part of the Bighorns project, suggests that the up warp likely has a Precambrian origin because it has a trend that differs substantially from that of the Bighorn Arch. Uplift of the Bighorn Arch may have occurred along a deeply-buried crustal fault that nucleated as a result of the upwarp. The occurrence of a crustal-scale discontinuity between the Bighorn batholith and the Bighorn Gneiss terrane, suggests that it may also have played a role in nucleating the fault. Possible explanations for the absence of the high-density "7.x" layer in the lower crust and Moho up warp include the possibility that pieces of the crust fell away (delamination) during late stages of the formation of the original continental crust or that the lower crust and Moho of the region were reworked during Mesoproterozoic (ca. 1.5 billion years ago) rifting.
