Rice Geologists Find Evidence Fault Cuts Down to Mantle

first_imgAddThis Share CONTACT: Lia Unrau PHONE: (713) 831-4793 E-MAIL:[email protected] GEOLOGISTS FIND EVIDENCE FAULT CUTS DOWN TO MANTLEThe northern San Andreas fault extends through theEarth’s lower crust and through the Moho, the boundary layer between the crustand the mantle of the Earth, two Rice University researchers and a colleaguefrom Virginia Tech found.The geologists recently studied the northernmost part of the San Andreasfault system in northern California.It is the first time scientists have evidence that what is happening at thesurface in this area&emdash;strike-slip faulting&emdash;is also takingplace to depths as great as the upper mantle. This strike-slip fault system iswhere the Pacific and North American plates slide by one another.The findings show that a layer in the lower crust is offset vertically at itstop and its base in the vicinity of two faults in the region, suggesting thatthe surface strike-slip deformation extends down into the upper mantle, saidAlan Levander, Rice professor of geology and geophysics. “This is one importantobservation in a number of findings resulting from a large-scalemulti-institutional study of the Triple Junction region,” he said.Timothy Henstock, research scientist at Rice University, said, “We are thefirst to look in detail at the structure in this region. Our results give a goodframework within which to understand the earthquakes which are occurring.”The findings have been published in a paper titled, “Deformation in the LowerCrust of the San Andreas Fault System in Northern California,” which appears inthe Oct. 24 issue of Science. The paper was co-authored by Henstock, Levanderand John Hole, assistant professor of geological sciences at VirginiaPolytechnic Institute.In the future, the findings may also help scientists understand links betweenwhat kind of earthquakes happen and the structures below the surface, Henstocksaid.The San Andreas fault system, which at the northern latitude is made up ofthree faults, is expanding northward parallel to the California coast at alocation called the Mendocino Triple Junction — where the edges of the Pacific,North American and Gorda plates rub together.“This finding is important because we’re looking at a very early stage in theevolution of the San Andreas Fault system,” Henstock said. “We’re much closer towhere the initial active process is taking place, and this helps us deduce howactivity in the crust and in the mantle relate to one another.”Levander said, “It appears that processes in the mantle are controlling thedeformation process directly above in the upper crust.”The San Andreas fault started in southern California about 27 million yearsago, but at this location, about 100 miles south of the Mendocino TripleJunction, it is relatively young — only two million years old. Learning aboutthe evolution of the fault system and how it is moving northward may helpscientists understand how the fault evolved in the San Francisco area. Thefinding provides evidence against the suggestion that the three major faultswhich make up the San Andreas fault system are connected at shallow depths whenthe fault system is young.The study made use of marine, off-shore and land seismic reflection andrefraction surveys of the deep crust, 15-19 miles down, for the full width ofthe fault system from east to west — about 120 miles. Looking at data collectedusing three independent methods, the researchers came to the same conclusion ineach instance.Levander and his colleagues think that the Gorda plate, which is fragmenting,is being pushed east, breaking up underneath the North American plate andsticking to the underside. Some fragments are as large as a room; others are thesize of a city like Houston. The Rice researchers believe that this is the mostprobable cause of the offset crust.The research was funded by the National Science Foundation’s ContinentDynamics Program, and included researchers from Oregon State University, LehighUniversity, Stanford University, and the United States Geological Society, aswell as Rice and Virginia Tech.###last_img