Rocky Mountains

Advanced Tectonics
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.

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, Colorado Plateau, southern Rocky Mountains, and adjacent High Plains. The central Colorado Plateau, for instance, experienced 1.5 to 2 km of vertical uplift and erosion during the past 5 million years at rates of ~250 to 700 m per million years (Murray et al. 2016). 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.
View over Elbow Lake in the Kananaskis Mountains, near Calgary, Alberta. Deformed sedimentary strata are visible in the cliffs.
View toward Parker Ridge with Icefields Parkway below, Jasper National Park, Alberta. Sedimentary rocks are deformed in folds and thrust masses.
Precambrian strata in Canadian Rocky Mountains front range at Waterton Lakes National Park, Alberta (U.S.-Canada border).
Mount Rushmore National Monument, Black Hills, South Dakota. The sculptures are carved in Precambrian granite that contains pegmatite veins.
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.
Vertical fissures in granite on the Needles Highway (state hwy. 87), Black Hills, east of Custer, South Dakota.
Pegmatite (light colored rocks at scene center) within a metamorphic sequence, US highway 16A, east of Custer, Black Hills, South Dakota.
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.
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.
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.
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.

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.
Zapata Falls on the western side of the Blanca Peak massif.
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).
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).
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.
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.
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.
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.
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).
Pegmatite outcrop within basement rocks uplifted along major thrust fault, Culebra Range, Sangre de Cristo Mountains.
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.
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.

Mt. Maxwell thrust fault features
Thrust fault in Sangre de Cristo Formation on side of Mt. Maxwell. Left: slickensides on fault surface. Right: chattermarks on fault surface. Such features indicate the direction of fault movement.
Thrust fault in Sangre de Cristo Formation on side of Mt. Maxwell. Left: highly fractured sandstone with quartz veins. Right: malachite veins in sandstone. Such veins are evidence for hydrothermal activity.

Reference

Rio Grande Rift system.

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Notice: Advanced tectonics 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. All text and imagery © J.S. Aber (2016).