Faults and Structures

A fault is a break in Earth’s crust where the blocks of rock overcome friction to move past each other in opposite directions. This movement can generate earthquakes if it occurs rapidly, or it can occur slowly as fault creep. The amount of movement is called the “fault displacement,” or the distance that rocks have moved on either side of the fault. Faults range in length from a few millimeters to hundreds of miles, and their corresponding fault displacements can also vary in size. Faults can be oriented vertically, horizontally, or some angle in between.

Because fault movement causes earthquakes, it is important to study the thousands of faults in Colorado to determine whether they have moved in the recent geologic past and whether they are capable of moving again in the near future. Paleoseismic studies help with this understanding. Faults have broken geologically young deposits (Quaternary period, 2.6 million to 11.7 thousand years ago) in many basins in Colorado. According to a recent inventory by CGS staff, there are at least 90 faults in Colorado that moved during the Quaternary Period and can be considered active.

Three faults in Colorado have received sufficient study to be included in the USGS National Seismic (Earthquake) Hazard Map, and are listed as being capable of generating earthquakes of 7.0 magnitude, or greater. There are many more faults in the state that could probably generate significant earthquakes, but have not received sufficient study, or documentation, to be included in the hazard map. With our current state of knowledge, it is not possible to predict when or where, the next large earthquake might occur in Colorado.

Types of Faults

Faults are classified based on the relative movement of each side of the fault with respect to the other side. Geologists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. “Dip-slip” faults are those where the rocks move vertically along the fault along the dip plane, with the rocks on “top/hanging wall” of the fault moving downward (“normal fault”) or upward (“thrust fault”) relative to the rocks on the “bottom/footwall” of the fault. “Strike-slip” faults are those where the motion is primarily horizontal along a fault, with rocks moving to the right (right-lateral) or left (left-lateral) with respect to rocks on the either side of the fault. A well-known example of a strike-slip fault is the San Andreas fault in California, which is a right-lateral fault.

In normal faults, the hanging wall moves down relative to the footwall. The Sangre de Cristo fault in the San Luis Valley is a normal fault with nearly four miles of displacement occurring over millions of years. In a reverse or thrust fault, the hanging wall moves up relative to the footwall. A thrust fault, such as the Williams Range fault bounding the west side of the Front Range, is a special case of a reverse fault where the dip of the fault plane is thirty degrees or less.

Studying Past Earthquakes

Because the interval between large earthquakes on any given fault is often in the neighborhood of hundreds or even thousands of years, it is necessary to study when large earthquakes have occurred on the faults in the past to better understand their potential future behavior. These studies of faults fall under the category of paleoseismicity, or the study of past earthquakes.

CGS scientists can often determine when a fault last moved, and estimate the magnitude of the earthquake that produced the last movement, by studying the geologic characteristics of faults. In some cases, it is possible to evaluate how frequently large earthquakes occurred on a specific fault during the recent geological past. This information is critical to understanding the earthquake hazard for each fault.

Trenching is a common technique for investigating an active fault with a surface rupture, carefully excavating a series of trenches across a fault zone to reveal the structure and magnitude of the fault displacement and determine the ages of past earthquakes on the fault.

Kirkham, Robert M., David C. Noe, Lauren Heerschap, James P. McCalpin, Shannon Mahan, and Matthew L. Morgan. “MI-100 Summary Report on the McQueary Gulch Trench, Williams Fork Mountains Fault, Grand County, Colorado.” Paleoseismic. Miscellaneous Investigation. Golden, CO: Colorado Geological Survey, June 2020.

Ostenaa, Dean A., and Mark S. Zellman. “MI-97 Paleoseismic Investigation of the Cheraw Fault at Haswell, Colorado.” Paleoseismic. Miscellaneous Investigations. Golden, CO: Colorado Geological Survey, November 2018.

Sackungen are a special category of faults that disrupt the ground surface in glaciated, alpine areas. They are not interpreted to extend very deep into the crust and are probably not capable of generating earthquakes. However, they can potentially yield information on past earthquakes on nearby faults.

• OF-98-08 Preliminary Quaternary Fault and Fold Map and Database of Colorado
• B-43 Earthquake Potential in Colorado, A Preliminary Evaluation
• B-46 Colorado Earthquake Data and Interpretation: 1867 to 1985
• B-52 Colorado Earthquake Information, 1867-1996
• and the precursor online map IS-60 Colorado Late Cenozoic Fault and Fold Database and Internet Map Server from 2002.

Mountain building episodes throughout Colorado’s history created geologic structures that are beautifully exposed across the state. Ancient structures that formed more than a billion years ago can be seen today and attest to the longevity of mountain building activity in Colorado. Young faults that offset 10,000-year-old glacial deposits and present-day earthquakes indicate the continuing rise of the mountains.

When rocks respond to the forces acting in Earth’s crust, they bend and/or break. When strata bend, a joint or fracture is formed. If the rocks move after they break, the fracture becomes a fault. In the faults, folds, and unconformities of Colorado we can look back to see how Earth continually makes and remakes itself. Colorado has every kind of fold and fault known to geologists.

Unconformity is the general term for missing pages of Earth history. There are three types of unconformities (below): angular unconformity, nonconformity, and disconformity.An angular unconformity (left) is an erosional surface separating steeply dipping rock layers below from gently dipping layers above. Nonconformity (middle) is an erosional surface separating igneous or metamorphic rocks below from sedimentary strata above. Disconformity (right) is an erosional surface separating horizontal strata below from horizontal strata above and where there is a gap in time.

Three types of geologic unconformities

500 million-year-old sandstones rest on 1.7 billion-year-old igneous and metamorphic rocks in Glenwood canyon

Exposed in the parking lot of Red Rocks Park & Amphitheater near Denver are red, 300-million-year-old sedimentary rocks resting on gray 1,700-million-year-old metamorphic rocks. About 1.4 billion years of history are missing in the geologic record. This gap is a nonconformity. Questions raised by the absence of strata at Red Rocks are at least partially answered by rocks in other ares of the Front Range. Studying these areas, geologists discovered that the missing strata were indeed deposited at Red Rocks and then later removed by erosion during an uplift associated with mountain building.

Similar nonconformities are also exposed in other places in Colorado. In Glenwood Canyon (pictured right), sandstone overlies metamorphic and igneous rocks with 1.2 billion years of history missing at the nonconformity. In the Black Canyon of the Gunnison and Colorado National Monuments, sedimentary rocks rest nonconformably on igneous and metamorphic rocks with a gap of 1.5 billion years.

In the angular unconformity pictured left, conglomerate overlies dipping shale in Little Snake Canyon. About 150 million years of Earth history occurred between the deposition of the rocks below and those above the unconformity.

A spectacular example of an angular unconformity in Box Canyon near Ouray is so striking that it often appears in geology texts. The steeply dipping Precambrian strata were originally deposited as horizontal layers and then buried. Later, the layers tilted and folded to a vertical position during a mountain building event and were eventually eroded and truncated. When the vertical Precambrian strata subsided below sea level, the Devonian marine sandstone was deposited on top of them.

The folds in Colorado’s rocks, are intriguing to geologists and lay viewers alike. For geologists who have studied Colorado rocks, the variety is surprising. The state has metamorphic folds, basement-cored folds, salt-cored folds, monoclines, syndepositional folds, anticlines, synclines, domes, basins, refolded folds, evaporite-flowage folds, collapse folds, disharmonic folds, and forced folds.

Anticline fold diagram

Anticline: folds with limbs that dip outward away from the hinge of the bend; convex upward

Syncline fold diagram

Syncline: folds with limbs that dip inward toward the hinge of the bend; concave upward

monocline on the northwest end of the Uncompahgre Plateau

Monoclines are folds that have only one limb with horizontal beds on either side of the steep limb. Monoclines range in size from rather small, north of Fort Collins, to huge monoclines associated with the Uncompahgre Plateau and White River Plateau basement blocks. Photo at right shows a well exposed monocline on the northwest end of the Uncompahgre Plateau.

Rabbit Mountain fold east of Lyons.

Basement-cored folds were formed during the Laramide mountain building event, when igneous and metamorphic rocks broke into large blocks throughout Colorado. These broken blocks pushed up into the overlying sedimentary rocks, forcing them to fold over the edges of the Precambrian blocks. The shape of the block determined the shape of the folds in the overlying sedimentary strata. Where the blocks moved up without any rotation, the overlying sedimentary rocks formed monoclines. Photo above-left shows an anticline four miles east of Lyons on Rabbit Mountain. The fold reflects the geometry of the basement block that moved up into the overlying sedimentary strata and created a fold in those strata.

Paradox Valley salt anticline diagram

Salt anticlines result when salt deposits flow upwards, folding the overlying sedimentary rocks into anticlines. Long, linear anticlines with cores of salt are found in the Eagle and Carbondale areas of central Colorado. Salt flowed upward into the cores of these anticlines for millions of years, creating angular unconformities in the sediments being deposited on the flanks of the growing folds. When the salt reached the surface and eroded, it dissolved much quicker than the overlying rocks and formed large, linear valleys over the crest of the anticlines. This rapid dissolution of the salt cores caused the rocks near them to collapse, creating the inward-dipping layers that define a syncline. Photo to right is a diagram of a the Paradox Valley salt anticline. The far end of the valley is the collapsed syncline.

When previously folded rocks were again subjected to heat and pressure, they were refolded and became refolded folds. Early geologists studying Precambrian structures found many clues indicating two periods of folding. Small, tight folds that were folded once during an earlier period of folding, then folded again and tightened by a second period of folding.

Anticline and synclines in metamorphic rocks near Blackhawk

Within these large folds are many small, tight folds that were formed during an earlier period of folding and were refolded by the second period

To the left, anticline and synclines in metamorphic rocks near Blackhawk. Within these large folds are many small, tight folds that were formed during an earlier period of folding and were refolded by the second period, shown in the photo to the right.