New insights on the origin of the Rocky Mountains
(PhysOrg.com) -- The formation of the Rocky Mountains in Colorado has always puzzled scientists. Some 600 miles inland and far removed from the nearest tectonic plate, the only comparable inland mountain range is the Himalaya, which scientists deduced were formed by the collision of the Indian plate with the Eurasian plate.
Rocky Mountains - Wikimedia
"But there really was no India slamming into North America," said Craig Jones, a research fellow of the Cooperative Institute for Research in Environmental Sciences (CIRES) and a professor of geological sciences at the University of Colorado at Boulder. "Just how the Rockies have formed is an enigma."
But now scientists have further insight into the solution of this mystery. Jones and his team of researchers have proposed a new model of the mountains' creation and published their results in the February edition of the journal Geosphere. Not only could their research explain the origin of the Rockies, it could also elucidate other geological phenomena: why a swath of gold, silver and other precious metal deposits stretches across Colorado, and why a marine basin deepened in the states of Colorado and Wyoming just before the Rockies rose. The sediments of this marine basin are the Pierre Shale, a layer of dark-gray shale lying along the Front Range of Colorado.
"Pierre Shale has this nasty tendency to bow up people's basements," Jones said. "Why more than a mile of this stuff was dumped into this area has been puzzling."
Previously scientists believed that the oceanic plate subducting - moving under - North America rose to rub against the continent's bottom all the way from the ocean to Colorado. The theory was this action pushed the landmass into mountains much like a rug piles up underfoot, said Jones. But the hypothesis just doesn't explain the facts, he said. "That model predicted removal of material that is still found to lie underneath California and Arizona," he said. "That in and of itself was unsatisfying."
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Hydrodynamic mechanism for the Laramide orogeny
Craig H. Jones1, G. Lang Farmer1, Brad Sageman2 and Shijie Zhong3
-Author Affiliations
1Department of Geological Sciences and CIRES (Cooperative Institute for Research in Environmental Science), University of Colorado at Boulder, 2200 Colorado Avenue, Boulder, Colorado 80309-0390, USA
2Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, Illinois 60208-2150, USA
3Department of Physics, University of Colorado at Boulder, 2000 Colorado Avenue, Boulder, Colorado 80309-0390, USA
Abstract
The widespread presumption that the Farallon plate subducted along the base of North American lithosphere under most of the western United States and ∼1000 km inboard from the trench has dominated tectonic studies of this region, but a number of variations of this concept exist due to differences in interpretation of some aspects of this orogeny. We contend that five main characteristics are central to the Laramide orogeny and must be explained by any successful hypothesis: thick-skinned tectonism, shutdown and/or landward migration of arc magmatism, localized deep foreland subsidence, deformation landward of the relatively undeformed Colorado Plateau, and spatially limited syntectonic magmatism. We detail how the first two elements can be well explained by a broad flat slab, the others less so. We introduce an alternative hypothesis composed of five particular processes: (1) a more limited segment of shallowly subducting slab is created by viscous coupling between the slab and the Archean continental keel of the Wyoming craton, leaving some asthenosphere above most of the slab; (2) dynamic pressures from this coupling localize subsidence at the edge of the Archean Wyoming craton; (3) foreland shortening occurs after the subsidence of the region decreases gravitational potential energy, increasing deviatoric stresses in lithosphere beneath the basin with no change to boundary stresses near the subduction zone or changes to basal shear stress; (4) shear between the slab and overriding continent induces a secondary convective system aligned parallel to relative plate motion, producing the Colorado Mineral Belt above upwelling aligned along the convection cell; (5) the development of this convective system interrupts the flow of fresh asthenosphere into the arc region farther west, cutting off magmatism even in segments of the arc not over the shallowly dipping slab.
Received 31 December 2009.
Revision received 18 June 2010.
Accepted 7 July 2010.
© 2011 Geological Society of America
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