How the Rockies Rose from the Sea

April 5 (Tuesday), 6 p.m., Teton Co. Library Auditorium –Open to Public. Presentation: ” How the Rockies rose from the sea: crustal-scale seismic data and structural modeling from the NSF-funded Bighorn Project”. Presented by Eric Erslev, Univ.s of WY & CO State.

How did the mountains in Wyoming, throught the Rocky Mountains form?

The formation of the Rocky Mountains during the Laramide Orogeny (70-50 million years ago) has been a long-standing puzzle due to their highly-variable structures and long distances to plate tectonic boundaries. Recent studies have shown that the Rockies are a thrust belt formed by lateral shortening, not by piston-like vertical tectonics as proposed in the 1970s. But uncertainty about their deeper geometries has resulted in a multitude of hypotheses invoking either a push from the west or shear from below.  These hypotheses were tested by the NSF/EarthScope-funded Bighorn Project which studied a 60,000 km2 area encompassing the Bighorn Mountains and parts of the Bighorn and Powder River basins.

As part of the project, Anne Sheehan (University of Colorado), Kate Miller (Texas A&M), Megan Anderson (Colorado College) and their students applied multiple seismic methods to image the crust in northern Wyoming. This Bighorn Arch Seismic Experiment (BASE) showed that the crust-mantle contact (Moho) lacks the fold and fault shortening that uplifted individual mountain ranges. Instead, the Moho preserves Precambrian crustal geometries that formed billions of years ago. This discordance between the upper and lower crust requires a sub-horizontal crustal detachment at ~30 km depth. Thus, Laramide Rocky Mountain arches like the Bighorns formed like folds in a table cloth that develop in front of your hand when you push a table cloth away from yourself. Slippage between the table cloth and the table is analogous to the detachment fault slip that formed the Rockies.

Computer models show how the Rocky Mountains rose from the sea as they were pushed ENE during the Laramide Orogeny. These models explain how our enigmatic Rockies were rooted in the mountains and plate boundaries to the west, and predict fracture orientations that determine fluid flow in the Rockies and seismic hazards in analogous active mountain belts.