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Shaping a Continent: Version 1.0

"I'm always fascinated when I stand on a mountain or in a valley and look out around the area," says Princeton University geophysicist Nadine McQuarrie. "I wonder: How was this landscape created?" McQuarrie wonders most often about a particularly spectacular landscapethe sunken desert valleys flanked by mountain ranges in the southwestern United States: Death Valley, the Mojave Desert, the Sonoran Desert, and more.

mtndesert

A typical mountain-flanked valley in the Basin and Range province.

AMNH


Geologists call this landscape the Basin and Range Province. It begins in eastern California and extends to lower Idaho, central Utah, and the western panhandle of Texas. The valleys and mountains run roughly north-south, each parallel and separated by about 30 kilometers: a kind of continental corduroy. How did the Basin and Range get this way? Scientists like McQuarrie are now marrying traditional fieldwork with cutting-edge computer modeling to produce the first animated, theoretical picture of the region's geological evolution.

The Wild West

The continental crust in the Basin and Range won't sit still. It slowly stretches and cracks, rises and sinks, suffering millions of little earthquakes in the process. At faultliterallyis plate tectonics: the motion of the giant rafts of Earth's lithosphere (the crust and upper mantle) that float on the asthenosphere, the sea of partially molten rock beneath. The western border of the Basin and Range is the San Andreas Fault, which divides Baja California and Los Angeles from the rest of the continent. That's where the Pacific Plate grinds northwestward against the North American Plate at a rate of about 5 centimeters per year.

Yet deformation on the San Andreas Fault takes up only about 3 centimeters per year of the plate motion. The remaining deformation is distributed among the web of hundreds of faults in the Basin and Range Province. Think of the array of fault blocks in this region as a plate of broken glass (the upper layer of crust) lying atop a sheet of Silly Putty (the underlayer of crust). The northwestward motion of the Pacific Plate essentially pulls the continental crust apart, stretching the ductile "Silly Putty" in a westward direction. This process, called extension, has been occurring in the Basin and Range for 36 million years. The "glass shards" on top, being more brittle, shift up, down, side to side, or away from each other. These movements create mountains and valleys.

Geologists understand the broad strokes of continental deformation. The details, however, still need piecing together. "The theory of plate tectonics tells us with great accuracy how the relatively few plates that exist on the Earth's surface are moving with respect to one another," says Brian Wernicke, a geologist at the California Institute of Technology and a collaborator on the Basin and Range model. "What it doesn't predict is how individual pieces should be moving in order to get the job done, if you will, of having one plate move past another."

Making Models
nadine

Princeton geologist Nadine McQuarrie with her team's computer model of the history of geological deformation in the American southwest.

AMNH


Direct tracking of the speed and direction of individual fault blocks has been possible only in the last 10 years with the advent of the Global Positioning System (GPS). An array of GPS devices has been installed throughout the Basin and Range for this purpose. To reconstruct the movement of the fault blocks for the 35,999,990 years before that, scientists must turn to traditional fieldwork.

"There have been geologists crawling over the landscape of western North America for the last 150 years," says McQuarrie. "So we have a very good understanding about where the faults are, how much motion has been on those faults, what's the direction of that motion, and over what window of time those faults have moved." In fact, very few other geological provinces have yielded such a wealth of data.

McQuarrie, Wernicke, and their team enter as much published physical information as they can find from both the GPS network and the field studies into the model. This computer analog of the deformation of the Basin and Range can be run as an animation, compressing 36 million years into 20 seconds. It shows where the fault blocks were (theoretically) located at specific intervals of time, shaping the southwestern United States from an unrecognizable form to the one we see on maps today.

Chasing Lost Marbles

The Basin and Range computer model is a work in progress. When data are not available for an area, the model interpolates to get the fault blocks from point A to point B. This "educated guesswork" can cause the model to predict unusual thingstwo mountain ranges on top of one another, for example, or a fault shift that hasn't been observed in the field.

Settling differences between scientific observations and the model's predictions requires a constant exchange. An example is the Bristol-Granite Mountains Fault in the California region of the Mojave Desert. Richard Lease, a former student of McQuarrie's at Princeton, has returned to this fault again and again to reconcile a gap in the model. According to the model, the western side of this fault, like the San Andreas Fault, is moving northwestward relative to the eastern side. The model predicted a displacement of 20 to 30 kilometers over the last 12 million years, the length of time that the southern portion of the Basin and Range has been extending. Together, the lateral shift and the extension created a wide valley flanked by two mountain ranges. The valley is there in plain sight, but the 20 to 30 kilometers of displacement hadn't yet been observed in the geological record.

rock_lookin

Geologist Richard Lease inspects a river-weathered rock—a key clue to the deformation history of California's Bristol-Granite Mountains Fault zone.

AMNH


Lease looked for evidence of this displacement by following an ancient "trail of breadcrumbs" sprinkled across the fault: rocky debris geologists called clasts, or, to Lease, the "Lost Marbles." Lease first discovered them atop the Marble Mountains, which edge the valley. The rocks were unusually roundedsome as big as baseballs, others like bowling ballswhich indicated they had been carried and weathered by a river. Then Lease looked for an identical deposit on the mountain range opposite the fault. "The cool thing was that we only found the Lost Marbles at one place on the other side of the fault," says Lease. "We went up and down [the fault line], but we only saw them to the north." Analysis of the mineral content and size distribution of the clasts in the two deposits confirmed that they were compositionally identical.

The conclusion: the two deposits were left by the same ancestral river, which flowed across the fault when the "seam" of the valley was zippered shut 12 million years ago. Additional evidence of a later flow of volcanic ash allowed Lease and his team to calculate a lateral shift of 21 kilometers, which is within the range the model had predicted.

Beyond testing the model's accuracy, field studies like Lease's produce new measurements to fold into a "2.0 version" of the model. But that, concedes McQuarrie, may take another decade to perfect. "I don't know if we'll ever know exactly if the model is correct," she says. Still, for these geologists, the lure of the unknown simply fuels more questionsand answersabout the Southwest's ever-changing landscape.

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