2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 42
Presentation Time: 8:00 AM-12:00 PM


HILDEBRAND, Robert S., Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505, bob@roberthildebrand.com

A critical re-evaluation of the evolution of western North America suggests that it did not develop in a back-arc setting; but instead was a passive margin from the Lower Cambrian until it was partially subducted beneath an arc-bearing microcontinent named Rubia during the Cretaceous Sevier orogeny. Regionally-distributed gravels and conglomerates comprising clasts of eroded North American sedimentary rocks indicate that cratonic North America was arched upwards and passed over the outer trench swell during the Aptian at about 115 Ma. The leading edge of North America was then pulled down beneath Rubia and sedimentation of the Western Interior Basin commenced in the linear depression between the outer trench bulge and the collision zone. During the collision, rocks along the western edge of the North American passive margin were detached from their basement at about 98 Ma, fixed to the Rubian plate, and transported eastwardly a minimum of 200 kms to form the Sevier thrust-fold belt. Thrusting migrated eastward and continued until 65 Ma in the US and 58 Ma in Canada.

When the subducting plate failed and broke-off, thrusting in the upper plate thrust-belt stopped because the break-off produced a catastrophic stress inversion in both upper and lower plates. The buoyant North American craton was freed from its gravitational anchor and so rose rapidly. The uplift caused the hot arc crust sitting atop North America to extend and collapse gravitationally to form a belt of Paleocene metamorphic core complexes stretching from southern British Columbia to Death Valley.

Whether due to plate momentum or possibly to diachronous tearing, convergence didn't stop instantaneously at break-off. The combination of uplift plus convergence created intense coupling between the upper and lower plates, and the increased frictional drag caused stresses within the rapidly rising North American plate to change from extension to compression. It is this late-stage orogenic compression in the subducting North American plate that I suggest generated the Laramide thick-skinned thrusts and basement-involved folds starting at about 65 Ma.

Slab break-off was complete and an easterly-dipping subduction zone established beneath North America by 54 Ma, the age of magmatic arc rocks stretching from northwestern Wyoming to central British Columbia.