TheLaramide orogeny was a time period ofmountain building in westernNorth America, which started in theLate Cretaceous, 80 to 70 million years ago, and ended 55 to 35 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by thisorogeny was deep-seated,thick-skinned deformation, with evidence of this orogeny found fromCanada to northernMexico, with the easternmost extent of the mountain-building represented by theBlack Hills ofSouth Dakota. The phenomenon is named for theLaramie Mountains of easternWyoming. The Laramide orogeny is sometimes confused with theSevier orogeny, which partially overlapped in time and space.[1]
The orogeny is commonly attributed to events off the west coast of North America, where theKula andFarallon Plates were sliding under theNorth American Plate. Most hypotheses propose that oceanic crust was undergoingflat-slab subduction, that is,subduction at a shallow angle. As a consequence, nomagmatism occurred in the central west of the continent, and the underlying oceaniclithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.
Magmatism associated with subduction occurred not near the plate edges (as in thevolcanic arc of theAndes, for example), but far to the east, along theColorado Mineral Belt.[2] Geologists call such a lack of volcanic activity near asubduction zone amagmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotterasthenosphere.[3] One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of theRocky Mountains. Part of the proto-Rocky Mountains would be later modified by extension to become theBasin and Range Province.
The Laramide orogeny produced intermontanestructural basins and adjacentmountain blocks by means of deformation. This style of deformation is typical ofcontinental plates adjacent toconvergent margins of long duration that have not sustained continent/continent collisions. Thistectonic setting produces a pattern of compressive uplifts and basins, with most of the deformation confined to block edges. Twelve kilometers of structural relief between basins and adjacent uplifts is not uncommon. The basins contain several thousand meters ofPaleozoic andMesozoicsedimentary rocks that predate the Laramide orogeny. As much as 5,000 meters (16,000 ft) ofCretaceous andCenozoic sediments filled these orogenically defined basins. DeformedPaleocene andEocene deposits record continuing orogenic activity.[4]
During the Laramide orogeny, basin floors and mountain summits were much closer to sea level than today. After the seas retreated from the Rocky Mountain region,floodplains,swamps, and vast lakes developed in the basins. Drainage systems imposed at that time persist today. Since theOligocene, episodicepeirogenic uplift gradually raised the entire region, including the Great Plains, to present elevations. Most of the modern topography is the result ofPliocene andPleistocene events, including additional uplift, glaciation of the high country, and denudation and dissection of older Cenozoic surfaces in the basin by fluvial processes.[4]
In the United States, these distinctive intermontane basins occur principally in the central Rocky Mountains fromColorado andUtah (Uinta Basin) toMontana and are best developed inWyoming, with theBighorn,Powder River, andWind River being the largest. Topographically, the basin floors resemble the surface of the western Great Plains, except for vistas of surrounding mountains.[4]
At most boundaries, Paleozoic throughPaleogene units dip steeply into the basins off uplifted blocks cored byPrecambrian rocks. The eroded steeply dipping units formhogbacks andflatirons. Many of the boundaries arethrust orreverse faults. Although other boundaries appear to bemonoclinal flexures, faulting is suspected at depth. Most bounding faults show evidence of at least two episodes of Laramide (Late Cretaceous andEocene) movement, suggesting both thrust andstrike-slip types of displacement.[4]
According to paleontologist Thomas M. Lehman, the Laramide orogeny triggered "the most dramatic event that affected Late Cretaceous dinosaur communities in North America prior to their extinction."[5] This turnover event saw the replacement of specialized and highly ornamentedcentrosaurine andlambeosaurines by morebasal upland dinosaurs in the south, while northernbiomes became dominated byTriceratops with a greatly reducedhadrosaur community.[6]
This article incorporatespublic domain material fromHegde, M.Wyoming Intermontane Basins.National Aeronautics and Space Administration. Archived fromthe original on 2011-06-17.
