Understanding How Mountains Form Across Interior North America

  • View of the Bighorn Mountains from the east near Buffalo, Wyoming.

The Bighorn Mountains rise prominently above the flat, windswept plains of eastern Wyoming. With elevations exceeding 13,000 feet, the mountains' exposed crystalline rocks and the layered sedimentary rocks that cover them form a structural arch that is more than 100 miles long. The center of the arch is composed of 2.5 billion year old granite and gneiss crystalline rock that was thrust from the Earth's interior during a mountain building episode about 40-50 million years ago. These rocks are part of the North American craton, the original core of the continent. The margins of the arch are abrupt and are lined by clearly visible faults.

The uplift of such old rock so far inland from an active tectonic plate boundary has long been a geologic mystery. The The Bighorn Arch Seismic Experiment (BASE) examines the deep structure and composition of the Bighorn Mountains and the surrounding plains. Understanding the properties of these rocks and the orientations of their bounding faults will help determine the characteristics that led to uplift of the Bighorns.

The BASE project deployed 38 seismometers in the Bighorns Mountains sited between the seismic stations installed as part of the EarthScope Transportable Array. It also placed 172 sensors in five transects to obtain additional information about the structure beneath the Bighorns from seismic waves generated by distant earthquakes. Additionally, over 2200 geophones were deployed in two perpendicular lines for a seismic survey using controlled explosions. This combination of instruments and sources has produced an unprecedented level of detail across a wide area of the Bighorns and enabled cross-disciplinary analyses combining explosive and earthquake source seismology. Preliminary results from these studies show that the Moho discontinuity, the crust-mantle boundary, which underlies the Bighorn Arch, is much shallower than observed beneath most mountain ranges. This result suggests that the Bighorns are not supported by buoyant, thickened crust, but were likely formed by a deeper process, rooted in the mantle, that is still being investigated.

Figure 1A. Map of the Bighorn Mountains and the BASE study area, location and type of instruments deployed, and location of earthquakes and blasts.

The elevation profile across the east-west seismic line is shown above Figures 1B and 1C.

Figure 1B. A model across the Bighorn Arch derived from explosion and earthquake seismic waves. The model shows that seismic waves travel  relatively fast in the gneiss and granite beneath the Bighorns and that the Moho is relatively shallow.

Figure 1C. Unlike most mountain ranges, this image of processed seismic data provides independent confirmation that the Moho is unusually shallow beneath the Bighorns. This result suggests that the Bighorns are not supported by buoyant, thickened crust, but were likely formed by a deeper process, rooted in the mantle, that is still being investigated.
 

 

  • Boxes of seismic instruments are checked at the field headquarters before being placed in their field location.

  • A BASE field team poses next to a seismic station.

  • Geophones were used in a seismic survey to measure waves generated by controlled explosions.

Principal Investigators and Institutions:
Eric Erslev (University of Wyoming)
Kate Miller (Texas A&M University)
Anne Sheehan (University of Colorado-Boulder)
Christine Siddoway (Colorado College)

Field Dates:
June 2009 - November 2010

Funding Source:
National Science Foundation Earth Sciences - EarthScope: BASE Award