Geology of The Death Valley Area - Extension Creates The Basin and Range

Extension Creates The Basin and Range

Starting around 16 Ma in Miocene time and continuing into the present, a large part of the North American Plate in the region has been under extension by literally being pulled apart. Debate still surrounds the cause of this crustal stretching, but an increasingly popular idea among geologists called the slab gap hypothesis states that the spreading zone of the subducted Farallon Plate is pushing the continent apart. Whatever the cause, the result has been the creation of a large and still-growing region of relatively thin crust; the region grew an average of 1 inch (2.5 cm) per year initially and then slowed to 0.3 inches (0.76 cm) per year in the last 5 million years. Geologists call this region the Basin and Range Province.

Extensional forces causes rock at depth to stretch like silly putty and rock closer to the surface to break along normal faults into downfallen basins called grabens; small mountain ranges known as horsts run parallel to each other on either side of the graben. Normally the number of horsts and grabens is limited, but in the Basin and Range region there are dozens of horst/graben structures, each roughly north-south trending. A succession of these extend from immediately east of the Sierra Nevada, through almost all of Nevada, and into western Utah and southern Idaho. The crust in the Death Valley region between Lake Mead and the southern Sierra Nevada has been extended by as much as 150 miles (240 km).

The Furnace Creek Fault system, located in what is now the northern part of Death Valley, started to move about 14 Ma and the Southern Death Valley Fault system likely began to move by 12 million years ago. Both fault systems move with a right-lateral offset along strike-slip faults; these type of faults rub past each other so that a theoretical observer standing on one side facing the other side sees it move right. Both fault systems run parallel to and at the base of the ranges. Very often the same faults move laterally and vertically, simultaneously making them strike-slip and normal (i.e. oblique-slip). These two systems are also offset from each other; the area between the offset is thus put under enormous oblique tension, which intensifies subsidence there; Furnace Creek Basin opened in this area and the rest of Death Valley followed in stages. One of the last stages was the formation of Badwater Basin, which occurred by about 4 Ma. Data from gravimeters show that Death Valley's bedrock floor tilts down toward the east and is deepest under Badwater Basin; there is 9,000 feet (2,700 m) of fill under Badwater. By about 2 Ma Death Valley, Panamint Valley and their associated ranges were formed.

Much of the extra local stretching in Death Valley that is responsible for its lower depth and wider valley floor is caused by left lateral strike-slip movement along the Garlock Fault south of the park (the Garlock Fault separates the Sierra Nevada range from the Mojave Desert). This particular fault is pulling the Panamint Range westward, causing the Death Valley graben to slip downward along the Furnace Creek Fault system at the foot of the Black Mountains. The rocks that would become the Panamint Range may have been stacked on top of the rocks that would become the Black Mountains and the Cottonwood Mountains. Under this interpretation, as the Black Mountains began to rise, the Panamint/Cottonwood Mountains slid westward off of them along low-angle normal faults, and starting around 6 Ma, the Cottonwood Mountains slid northwest off the top of the Panamint Range. There is also some evidence that the Grapevine Mountains may have slid off the Funeral Mountains. Another interpretation of the evidence is that the Black and Panamint Mountains were once side-by-side and were pulled apart along normal faults. These normal faults, in this view, are steep near the surface but become low angle at depth; the mountain blocks rotated as they slid to create the tilted mountains seen today.

Total movement of the Pamamint block between the Garlock and Furnace Creek Faults is 50 miles (80 km) to the northwest, creating Death Valley in the process. A few of the 20 to 25 degree-sloped surfaces along which this mass of 20,000 to 30,000 feet (6,100 to 9,100 m) of rock slipped, are exposed in Death Valley. These features are called "turtlebacks" due to their turtle shell-like appearance.