Formation of basement-involved foreland arches: Integrated structural and seismological research in the Bighorn Mountains, Wyoming

What: Field-based research in structural geology and geophysics in the Bighorn Mountains of Wyoming

When: July 11 and August 14 (5 weeks; participant stipend will be augmented due to the 5-week duration of the project)

Who: Project will involve 3 faculty and 9 Keck students, plus NSF Earthscope research collaborators

  • Director: Christine Siddoway, Colorado College
  • Faculty: Megan Anderson, Colorado College
  • Eric Erslev, University of Wyoming

The Keck project will coincide with an NSF EarthScope project being carried out in the Bighorns, so there are collaborators who will be involved in the project. These include PhD students Karen Aydinian (U. Wyoming), Will Yeck (U. Colorado); seismologists Anne Sheehan (U. Colorado) and Kate Miller (Texas A&M); and more!

Project Description and Goals

Quintessential geological structures in the Rocky Mountains of North America, now recognized to exist in the interior of many continents, are basement-involved foreland arches. They consist of faulted Precambrian crystalline rock forming a quasi-barrel-shaped topographic and structural culmination (the “arch”), covered by sedimentary rocks that are deformed into exceptionally lovely monoclines and anticlines at the mountain-range scale. The apparent folding of the Great Unconformity that separates crystalline basement from cover rocks presents an intriguing mechanical problem, in that the fold arch occurs at a scale that requires deformation of the entire lithosphere (not simply the crust).

The Bighorns Keck project will use structural geology and seismology to examine four competing models for the lithospheric-scale structure of the Bighorns arch (see diagram, next page) and to determine the process of formation of basement arches. The work will bring fundamental new insights about the processes of deformation of strong, old lithosphere within continent interiors, distant from active plate boundaries. Students will have access to new data from an innovative passive and active source hybrid experiment designed to image the lithosphere, that will be conducted by EarthScope project seismologists during the time period of the Keck project.

The goals of the project are:

  1. to conduct detailed structural studies of the upper crust along range-parallel and range-perpendicular transects, that aim to gather robust sets of measurements on every rock formation in order to document contrasting rheological responses and/or strain histories between units;
  2. to compile structural data within an accurate geospatial framework using GIS, to be tied to the lithospheric scale structure determined from seismology;
  3. to involve undergraduates in a regional-scale seismological survey that images arch-scale structures on the Moho and faults within the mid-crust; and
  4. to merge the seismic and structural datasets into structural profiles that tie it structures exposed at the surface to structures within the Rocky Mountains lithosphere underpinning the Bighorn arch.

Structural geology projects will center upon individual study regions situated along the study transects. Geophysics projects will involve seismology data interpretation or subsurface exploration of faults using gravity or magnetics in the field. All students will participate in seismology data collection using instrument sensors called ‘Texans’, which collect seismic signals that to be used to image the crust-mantle boundary (Moho) and faults within the crust. The seismic experiment will involve “active sources,” meaning that seismic energy will be generated from detonation of explosives. All of the participants’ results will be brought together within balanced two-dimensional models along the transects that will be used to evaluate the competing hypotheses for the lithospheric scale structure of the Bighorn range.

Geological Background

The mechanism for the formation of mountain ranges within continental interiors, far from tectonic boundaries where differential motion between plates causes focused crustal deformation, is in many respects just as mystifying today in the 21st century as when Plate Tectonic Theory became the accepted unifying model for geological processes on Earth in the 1960s (Oreskes and LeGrand, 2005). In many respects, the foundations of geological knowledge of foreland mountain ranges comes from the Rocky Mountains of the western U.S.A. (e.g. Coney, 1976), and particularly the ranges of the state of Wyoming where the term “Laramide Orogeny” originated. The structural geology of the Laramide mountain ranges is world-renowned because of the aesthetic folded form of sedimentary cover rocks upon faulted Precambrian crystalline basement (Figure 1) which poses interesting mechanical problems. Comparable features that are distant from plate boundaries are now recognized in central Asia, far to the north of the Himalayan orogen (e.g. Scharer et al., 2004); in central Europe outside of the Alps (Marotta and Sabadini, 2003); and also in the well known “sister” system to the Laramide, the Sierras Pampeanas of Argentina (Jordan and Allmendinger, 1986; Cristallini et al., 2004, and refs cited therein).

Geological Setting of the Bighorn Arch

The Laramide Orogeny in the Rocky Mountains produced regional-scale anticlines, or arches, (Figs. 3,4) that are cored by Precambrian plutonic and/or metamorphic rock. The Phanerozoic platform sedimentary sequence of carbonate and clastic units defines the fold structures, showing that the arches may be asymmetrical and thrust-bounded (e.g. Fig. 4; east-central Bighorn arch, Stone, 2003) or symmetrical, with smaller outward- and inward-directed thrusts on both sides of the basement block (e.g., north and south ends of the Bighorn Arch; e.g. Hallberg et al., 1999). The differing structural geometries may be used to determine the nature of concealed fault/fold relationships at depth, for example whether deformation occurred upon rotational fault-bend folds vs. detachment folds (Erslev, 1986; Stanton and Erslev, 2004). Even with excellent surface and near-surface structural and industry data, the crustal structure of the Bighorn arch is known to depths of just 10 km (Fig. 4).

Student projects

6 student projects / “Crustal Structure Group” coordinated by Siddoway and Erslev:

Three pairs of two students will conduct ground traverses across the Bighorn range to acquire systematic fault and fracture data for the lithological units at all stratigraphic levels. One student in each team will have primary responsibility for the west side of the structural arch, and one, the east side; and each will alternate having a “lead” or an “assistant” role. The contrasting geometries of the opposing flanks will ensure that each member of a pair will attain a ‘different’ solution from the other. The contrasting exposures and structural segmentation between fault blocks (north, central, south) will mean emphasis of the work is on field structural data measurement, followed by computer-based data compilation and stereographic analysis for kinematic interpretation. The fracture kinematic data will be used for detailed structural balancing of the fold and fault structures in the cover rocks. The task of balancing will help students determine whether deformation of successive stratigraphic units and in different regions of the Bighorn range occurred in response to a) regional shortening (expected to yield sub-parallel strain axes throughout, as is the assumption for 2D profiles such as the one by Stone, 2003 in Fig. 4), b) localized strike-slip accommodation (expressed as strike-slip faults extending throught the axis of the arch), c) gravitational sliding (sliding radial to the arch) or ‘relaxation’ in a direction opposite to Laramide shortening). By sampling pre-, syn-, and post-orogenic strata, fracture timing will be established, resolving an active debate as to the origins of fractures in the region (e.g. Hallberg et al., 1999 vs. Stanton and Erslev, 2004).

The transects will cross the southern, central and northern range crests, making use of US Highways 14 and 16 and forest roads for access (Fig. 3). Standard geometrical and kinematic methods (e.g. Marrett and Allmendinger, 1990) will be used to distinguish components of regional shortening, expected to be uniform in trend, from gravitational components expected to be radial to the arch, and from and strike-slip components along zones of structural transfer. The work will produce slip trajectories for range blocks be used to restore the arch in 3D. The projects will involve sampling for thin section analysis of brittle microstructures for determination of deformation mechanisms, qualitative depth estimates, and possible thermochronology of fault materials (using adularia; illite; apatite).

Students involved in structure transects will also participate in the installation of instruments for the contemporaneous seismic experiments, and each student pair will work with one member of the “lithosphere group” (see below) who has primary responsibility for seismic analysis for the region surrounding their transect. In this way, members of the “crustal structure group” will gain experience with seismological methods.

Questions that may be investigated by the Crustal Structure Group are:

  • What is the pattern of Laramide slip across the Bighorn Arch that will need to be restored in EarthScope earth models?
  • What are the age and mechanisms of the extensional fractures which are so critical to regional petroleum exploration and development?
  • What is the role of gravitational spreading and arch-axis-parallel extension during the formation of basement-core foreland arches?
  • What is the role of basement weaknesses in the localization of subsequent Laramide deformation?
  • What field evidence exists for rotations about vertical versus horizontal axes, and what contemporary perspective can we gain from examining the historical development of thought on the process of arch formation?

3 student projects / “Lithosphere Group” coordinated by Megan Anderson:

Three geophysics-oriented projects will involve analysis of data from the broadband passive source seismic experiment begun in 2009. The aim is to determine the depth and spatial extent of the crust-mantle boundary (Moho) beneath the Bighorns and/or major mid-crustal faults. Data from 2007-8 USArray stations (http://www.iris.edu/gmap?xmlref=http://www.iris.edu/earthscope/usarray/_US-TA-OpStationList.xml) will be used for lower resolution coverage of the surrounding region.

In the field, students will take part in installation and data collection for instruments used in the hybrid active-passive seismic experiment. During the project, as many as 1800 seismometers at a time will record active source shots as well as background seismic sources such as teleseisms, local earthquakes, mine blasts, and ambient noise. The “people power” to be contributed by the Keck party of 12 people is needed! The instruments will be installed at intervals along three parallel SW-NE trending lines and two cross-cutting NW-SE trending lines. A portable laboratory of laptop computers will accompany the project group to the field area, and the “Lithosphere Group” will begin work with seismic data sets that have been collected prior to the group’s arrival in the Bighorns, under the instruction of Megan Anderson (Keck project faculty member) and graduate students who are working with her. Once their initial training is complete, the Keck students will also be involved in preliminary analysis of active source lines of seismic data as the data is collected during the project. Thus, students in this group will gain practical experience with instrumentation and deployment, even as their computer analysis of new data sets proceeds throughout the month of work in the field area. In addition, students doing seismology projects will participate in field study and measurement of selected faults and fracture arrays in the field, in order to broaden their own skill set and to gain close familiarity with the work of the Crustal Structure group.

Questions to be investigated by the Lithosphere Group are:

  • What is the depth of the Moho underneath and flanking the Bighorns Mountain range?
  • How do faults located by structural studies extend into the subsurface as constrained by seismic reflectors in the active-source data?
  • Are there any strong mid-lower crustal structual boundaries that may be connected to faults located by structural and active source studies?
  • What is the deformational mode (e.g. pure shear, decollement) of the mantle as it is shortened along with the crust?

Alternative possible projects for student applicants who have other, complementary interests:

  • 1 project: conduct a gravity survey across selected faults in the Bighorn range to assess upper crustal properties and the character and geometry of brittle faults in the upper crust. Recommended for a student with mathematics and physics background who wishes to conduct geophysical studies at the crustal scale (as could be relevant for a student who anticipates future specialization in mineral exploration or geohydrology).
  • 1 or 2 projects: Compile surface measurements within a GIS of a part of the Bighorn range, and incorporate existing geological data from wells and industry sources. Successive layers of lithological units will be constructed as “sheets” at a deepest to highest levels contribute to the range-scale 3D crustal model of the Bighorns. Suitable for students with computer science and graphic representation experience.

Field Conditions

The project will begin and end at Colorado College. The CC Geology Department will provide vehicles and field equipment (including geophysical instrumentation) to support the work. The project will conclude with three days of sample preparation and seismic data analysis, during which time students will write a progress report and research plan for continuing their studies at their home campus.

While in the field, accommodations will be in bunkhouses at the Iowa State University field station in Shell, Wyoming, and rental houses in Buffalo or Sheridan, Wyoming. Meal preparation will be undertaken by group members, on a rotating basis, using civilized kitchen facilities in the accommodations. There will be opportunities for backpacking/camping in the backcountry while doing traverses of 3 to 4 days duration (in the company of a faculty member). Students need to be equipped with their own tents, sleeping bags, and suitable clothing for a range of weather conditions. Students should be prepared for alpine terrain and weather conditions at elevations of 9000 to >12,000 feet (ever with the possibility of snow), as well as arid, hot environments of the Bighorn Basin. Potential hazards are precipitous cliffs, enraged moose or bear (if startled), rattlesnakes, mosquitoes, and recreational ATVs (on the 4th of July).

Recommended course background

Structural geology, Tectonics, Introductory Physics, and Geology field camp. Introductory Geophysics recommended for students who wish to undertake seismology, gravity, or magnetics research.

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