Professor Paul Fitzgerald in addition to a squad of students in addition to beau scientists convey been studying the Alaska Range along the Denali fault. They intend they know why the error is located where it is in addition to what accounts for the alternating asymmetrical, mountain-scale topography along the fault.
Their findings were the dependent champaign of a recent newspaper inwards the journal Tectonics.
In 2002, the Denali fault, which cuts across south-central Alaska, was the site of a magnitude-7.9 earthquake in addition to was felt every bit far away every bit Texas in addition to Louisiana. It was the largest earthquake of its variety inwards to a greater extent than than 150 years.
“Following the earthquake, researchers flocked to the expanse to examine the effects,” says Fitzgerald, who serves every bit professor of globe Sciences in addition to an associate dean for the college. “They were fascinated yesteryear how the frozen basis behaved; the many landslides [the earthquake] caused; how bridges responded; in addition to how the Trans-Alaska fossil oil pipeline survived, every bit it was engineered to produce so.”
Geologists were likewise surprised yesteryear how the earthquake began on a previously unknown thrust-fault; in addition to thus propagated eastward, along the Denali fault, in addition to in conclusion jumped onto around other fault, hundreds of kilometers away.
“From our perspective, the earthquake has motivated analyses of why the highest mountains inwards the fundamental Alaska Range occur south of the Denali error in addition to the highest mountains inwards the eastern Alaska Range occur north of the error — something that has puzzled us for years,” Fitzgerald adds. “It’s been an enigma staring us inwards the face.”
He attributes the Alaska Range’s alternating topographic signatures to a myriad of factors: contrasting lithospheric forcefulness betwixt large terranes (i.e., distinctly dissimilar stone units); the location of the curved Denali fault; the transfer of strain inland from southern Alaska’s active plate margin; in addition to the shape of the controlling erstwhile continental margin against weaker suture-zone rocks.
It’s no hole-and-corner that Alaska is 1 of the most geologically active areas on the planet. For instance, scientists know that the North American Plate is currently overriding the Pacific Plate at the latter’s southern coast, piece the Yakutat microplate is colliding alongside North America.
As a outcome of plate tectonics, Alaska is an amalgamation of terranes that convey collided alongside the North American craton in addition to convey accreted to decease exercise of North America. Cratons are pieces of continents that convey been largely stable for hundreds of millions of years.
Terranes oftentimes originate every bit volcanic islands (like those of Hawaii) and, after colliding alongside 1 around other or a continent, are separated yesteryear large discrete faults. When terranes collide in addition to accrete, they shape a suture, likewise known every bit a collision zone, which is made upwards of weak, crushed rock. During deformation, suture-zone rocks commonly deform first, peculiarly if they are following to a rigid stone body.
“Technically, the Denali error is what we’d telephone phone an ‘intercontinental right-lateral strike-slip error system,'” says Fitzgerald, adding that a strike-slip error occurs when rocks displace horizontally yesteryear 1 another, commonly on a vertical fault. “This displace includes a cistron of slip along the error in addition to a cistron of normal motion against the error that creates mountains. Hence, the shape of the error determines which of the 2 components is predominant in addition to where mountains form.”
In Alaska, the shape of the accreted terranes mostly controls the location of the Denali error in addition to the mountains that shape along it, peculiarly at the bends inwards the draw of the fault.
Fitzgerald: “Mount McKinley in addition to the fundamental Alaska Range prevarication inside the concave curvature of the Denali fault. There, higher topography in addition to greater exhumation [uplift of rock] occur due south of the Denali fault, precisely where you’d await a mount hit to form, given the regional tectonics. In the eastern Alaska Range, higher topography in addition to greater exhumation are flora northward of the fault, on its convex side-not an expected designing at all in addition to rattling puzzling.”
Using mapped surface geology, geophysical data, in addition to thermochronology (i.e., time-temperature history of the rocks), Fitzgerald in addition to colleagues convey determined that much of Alaska’s uplift in addition to deformation began around 25 1000000 years ago, when the Yakutat microplate showtime started colliding alongside North America. The bold, glacier-clad peaks comprising the Alaska Range genuinely derive from inside the aforementioned “weak suture-zone rocks” betwixt the terranes.
While mountains are high in addition to hand the impression of strength, they are built largely from previously fractured stone units. Rock motion along the Denali error drives the uplift of the mountains, which shape at bends inwards the fault, where previously fractured suture-zone rocks are pinned against the stronger erstwhile North American continental margin.
“The patterns of deformation assist us empathise regional tectonics in addition to the formation of the Alaska Range, which is fascinating to geologists in addition to non-geologists alike,” says Fitzgerald. “Being able to decide patterns or how to disclose them, piece others meet chaos, is oftentimes the key to finding the answer to complex problems. … To us scientists, the existent significance of this operate is that it helps us empathise the development of our planet, how faults in addition to mount belts form, in addition to why earthquakes happen. It likewise provides a publish of hypotheses near Alaskan tectonics in addition to stone deformation that nosotros tin test, using the Alaska Range every bit our laboratory.”
In improver to Fitzgerald, the newspaper was co-authored yesteryear Sarah Roeske, a question scientist at the University of California, Davis; Jeff Benowitz, a question scientist at the Geophysical Institute at the University of Alaska Fairbanks; Steven Riccio in addition to Stephanie Perry, graduate students inwards globe Sciences at Syracuse; in addition to Phillip Armstrong, professor in addition to chair of geological sciences at California State University, Fullerton.