Rice Geologists Find Evidence Fault Cuts Down to Mantle

CONTACT: Lia Unrau

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RICE GEOLOGISTS FIND EVIDENCE FAULT CUTS DOWN TO MANTLE

The northern San Andreas fault extends through the
Earth’s lower crust and through the Moho, the boundary layer between the crust
and the mantle of the Earth, two Rice University researchers and a colleague
from Virginia Tech found.

The geologists recently studied the northernmost part of the San Andreas
fault system in northern California.

It is the first time scientists have evidence that what is happening at the
surface in this area&emdash;strike-slip faulting&emdash;is also taking
place to depths as great as the upper mantle. This strike-slip fault system is
where the Pacific and North American plates slide by one another.

The findings show that a layer in the lower crust is offset vertically at its
top and its base in the vicinity of two faults in the region, suggesting that
the surface strike-slip deformation extends down into the upper mantle, said
Alan Levander, Rice professor of geology and geophysics. “This is one important
observation in a number of findings resulting from a large-scale
multi-institutional study of the Triple Junction region,” he said.

Timothy Henstock, research scientist at Rice University, said, “We are the
first to look in detail at the structure in this region. Our results give a good
framework within which to understand the earthquakes which are occurring.”

The findings have been published in a paper titled, “Deformation in the Lower
Crust of the San Andreas Fault System in Northern California,” which appears in
the Oct. 24 issue of Science. The paper was co-authored by Henstock, Levander
and John Hole, assistant professor of geological sciences at Virginia
Polytechnic Institute.

In the future, the findings may also help scientists understand links between
what kind of earthquakes happen and the structures below the surface, Henstock
said.

The San Andreas fault system, which at the northern latitude is made up of
three faults, is expanding northward parallel to the California coast at a
location 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 the
evolution of the San Andreas Fault system,” Henstock said. “We’re much closer to
where the initial active process is taking place, and this helps us deduce how
activity in the crust and in the mantle relate to one another.”

Levander said, “It appears that processes in the mantle are controlling the
deformation process directly above in the upper crust.”

The San Andreas fault started in southern California about 27 million years
ago, but at this location, about 100 miles south of the Mendocino Triple
Junction, it is relatively young — only two million years old. Learning about
the evolution of the fault system and how it is moving northward may help
scientists understand how the fault evolved in the San Francisco area. The
finding provides evidence against the suggestion that the three major faults
which make up the San Andreas fault system are connected at shallow depths when
the fault system is young.

The study made use of marine, off-shore and land seismic reflection and
refraction surveys of the deep crust, 15-19 miles down, for the full width of
the fault system from east to west — about 120 miles. Looking at data collected
using three independent methods, the researchers came to the same conclusion in
each instance.

Levander and his colleagues think that the Gorda plate, which is fragmenting,
is being pushed east, breaking up underneath the North American plate and
sticking to the underside. Some fragments are as large as a room; others are the
size of a city like Houston. The Rice researchers believe that this is the most
probable cause of the offset crust.

The research was funded by the National Science Foundation’s Continent
Dynamics Program, and included researchers from Oregon State University, Lehigh
University, Stanford University, and the United States Geological Society, as
well as Rice and Virginia Tech.

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