Rocky Mountains

GO 568 Structural Geology
James S. Aber


Tectonic overview

The modern Rocky Mountains extend from New Mexico northward into western Canada. The Rockies are not a continuous mountain uplift, but rather consist of several dissimilar segments with intervening sedimentary basins. Parts of the Rocky Mountains are found in New Mexico, Colorado, Utah, Wyoming, South Dakota, Montana, Idaho, Alberta and British Columbia. The Rockies are an "interior" mountain system that developed within North America far from the subduction zone along the Pacific margin. Two major episodes of orogeny have taken place during the Phanerozoic in the Rocky Mountain region.

  1. Ancestral Rocky Mountains -- Deformation and uplift along faults brought crystalline basement rocks to the surface in a series of mountain ranges during the Pennsylvanian and Permian. These uplifts were in approximately the same positions as the modern Rocky Mountains in Colorado and Utah. Erosion of uplifts generated great quantitites of sediments that were deposited in adjacent basins.

  2. Larimide Orogengy -- Late Cretaceous and Paleocene faulting and thrusting of crustal slices once again brought crystalline basement rocks to the surface in mountain ridges and anticlinal uplifts. This deformation culminated in the Eocene with massive sediment accumulation in adjacent basins. Later, lesser uplifts led to pulses of sedimentation during the Oligocene and Miocene.

Neither the ancestral nor modern Rocky Mountains were accompanied by substantial igneous or metamorphic activity. A tectonic explanation for the Larimide Orogeny remains controversial. A popular idea is that rapid subduction along the west coast during late Cretaceous led to a flattening of the subduction zone. Such a low-angle subduction zone could have extended as far east as Colorado without generating much volcanism. Widespread volcanism in the Rockies reflects later tectonic developments--Yellowstone hot spot and Rio Grande Rift system. The present elevation of the Rocky Mountains is a result of wholesale crustal uplift throughout the southwestern United States, including the Grand Canyon region. Regional uplift may be due in part to hot-spot doming (Yellowstone) and overrunning the East Pacific Rise.

Northern Rocky Mountains

View downstream, Kananaskis River in the Kananaskis Mountains, near Calgary, Alberta. Mountains are comprised entirely of deformed sedimentary strata, namely Mesozoic carbonates. Photo date 8/85, © J.S. Aber.
View over Elbow Lake in the Kananaskis Mountains, near Calgary, Alberta. Deformed sedimentary strata are visible in the cliffs Photo date 8/84, © J.S. Aber.
View toward Parker Ridge with Icefields Parkway below, Jasper National Park, Alberta. Sedimentary rocks are deformed in folds and thrust masses. Photo date 8/84, © J.S. Aber.
Precambrian strata in Canadian Rocky Mountains front range at Waterton Lakes National Park, Alberta (U.S.-Canada border). Photo date 8/84, © J.S. Aber.
Mount Rushmore National Monument, Black Hills, South Dakota. The sculptures are carved in Precambrian granite that contains pegmatite veins. Photo date 6/97, © J.S. Aber.
Head of Crazy Horse sculpture in progress, Black Hills, north of Custer, South Dakota. Notice the mafic inclusions above the nose and eye. The construction site is open for tourists only one day per year. Photo date 6/97, © J.S. Aber.
Vertical fissures in granite on the Needles Highway (state hwy. 87), Black Hills, east of Custer, South Dakota. Photo date 6/97, © J.S. Aber.
Pegmatite (light colored rocks at scene center) within a metamorphic sequence, US highway 16A, east of Custer, Black Hills, South Dakota. Photo date 9/79, © J.S. Aber.
Outcrop of pegmatite, Wind Cave National Park, Black Hills, South Dakota. Precambrian crystalline rocks are exposed in the core of the anticlinal uplift of the Black Hills. Photo date 9/79, © J.S. Aber.
Outcrop of Pahasapa Limestone (Mississippian) in the plateau of the southern Black Hills, South Dakota. Uplifted, thick limestone hosts Wind Cave and many other cave systems in the Black Hills. Photo date 9/79, © J.S. Aber.
Minnelusa Formation (Pennsylvanian) overlies the Pahasapa Limestone in the southern Black Hills, South Dakota. The Minnelusa consists of clastic strata, namely sandstone and shale, derived from the ancestral Rocky Mountains. Photo date 9/79, © J.S. Aber.
The so-called "Red Valley" north of Hot Springs, South Dakota. Underlain by Triassic redbeds, which are widespread in the western United States. These soft and easily eroded red strata are situated in the outer portion of the Black Hills uplift. Photo date 9/79, © J.S. Aber.

Southern Rocky Mountains

Landsat MSS false-color image, southern Colorado and northern New Mexico (Sept. 1977). A - Spanish Peaks in the Raton Basin, B - Sangre de Cristo Mountains, C - San Luis Valley, D - Great Sand Dunes, and E - Raton Mesas. Image obtained from the EROS Data Center.
Blanca Peak as seen from the south. Blanca Peak is supported by uplifted crystalline basement rocks; its summit is 14,345 feet (4372 m), one of the highest peaks in Colorado. Photo date 7/99, © J.S. Aber.
Zapata Falls on the western side of the Blanca Peak massif. Photo date 6/00, © J.S. Aber.
View west toward the Culebra Range of the Sangre de Cristo Mountains. The highest summit on left is Trinchera Peak, 13,517 feet (4120 m). Photo date 7/00, © J.S. Aber.
Trinchera Peak on the crest of the Culebra Range. The peak is supported by vertical quartzose sandstone at the base of the Sangre de Cristo Formation (Pennsylvanian-Permian). Photo date 6/99, © J.S. Aber.
Exposure of Sangre de Cristo Formation near Cuchara, Colorado. This formation was deposited as alluvial sediments washed out of the ancestral Rocky Mountains during their uplift in the late Pennsylvanian and Permian. The formation is at least 4 km (3 miles) in thickness and was deformed during the Larimide Orogeny. Photo date 6/99, © J.S. Aber.
Shelf Road south of Cripple Creek, Colorado. Gently tilted Harding Sandstone (upper, right) over Manitou Limestone (lower, right) on the flank of the Colorado Front Range. Photo date 6/00, © J.S. Aber.
Sandstone hogback of the Dakota and Purgatorie formations (lower Cretaceous) creates a "stone wall" along the front range of the southern Rocky Mountains. Seen here near Cuchara, Colorado. Photo date 6/00, © J.S. Aber.
Major thrust fault in Culebra Range, Sangre de Cristo Mountains. The reddish color rocks at top of the near ridge are crystalline basement, part of the thrust sheet. Next picture shows a closeup view of the thrust. Photo date 8/03, © J.S. Aber.
Major thrust fault in Culebra Range, Sangre de Cristo Mountains. Seen here are Pennsylvanian limestone and sandstone. Strata stand in vertical position in the footwall (left center); the hanging wall has gently dipping strata (right). Photo date 8/03, © J.S. Aber.
Pegmatite outcrop within basement rocks uplifted along major thrust fault, Culebra Range, Sangre de Cristo Mountains. Photo date 8/03, © J.S. Aber.
Larimide uplift culminated in the Eocene, and the mountains underwent substantial weathering and erosion in a tropical or subtropical climate. One result was deposition of kaolinite-rich sediment in adjacent basins. Here the Dawson Arkose outcrops near Calhan, in the Denver Basin of east-central Colorado. Photo date 8/03, © J.S. Aber.
Bridge over Royal Gorge of the Arkansas River near Canon City, Colorado. Deep erosion of the canyon, like the Grand Canyon in Arizona, demonstrates substantial post-Larimide crustal uplift in the southern Rocky Mountains and Colorado Plateau during the Neogene. Photo date 8/03, © J.S. Aber.

Rio Grande Rift system.

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Notice: Structural Geology is presented for the use and benefit of students enrolled at Emporia State University. Others are welcome to view the course webpages. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. GO 568 © J.S. Aber (2003).