BERKELEY, CA --
For years geologists have tried to understand why the San
Andreas Fault is so weak. In work supported by the U.S. Department of Energy's
Office of Energy Research, geochemists Mack Kennedy, Yousif Kharaka, and their
colleagues have found that part of the answer lies in the fault's surprisingly
intricate connections with the Earth's mantle, deep underground.
The San Andreas, a classic strike-slip fault, marks the collisional
boundary where the Pacific and North American plates meet. The forces at the
boundary are compressive, yet fault failure is by shear, as the Pacific plate
slides steadily if intermittently northward. Friction measurements in the
laboratory on fault zone materials suggest that considerably more shear stress
than is actually observed should be required for the fault to fail and the
earth to move.
Mack Kennedy pictured with the mass spectrometer, an
instrument which helped determine that fluids welling up from the earth's mantle
can be found within the fault zone
"The forces and movement of the fault should produce frictional heating,"
says Kennedy, a member of the Earth Sciences division of the Ernest Orlando
Lawrence Berkeley National Laboratory, "but paradoxically, nobody's seen the
expected heating in the vicinity of the fault. One possibility is that
high-pressure fluids are acting as a sort of lubricant." Abnormally high
pressures have been measured in rock pores at shallow depths, Kennedy says,
"but to fully understand how the fault works it is extremely important to find
out exactly what's down there."
Geologists including Mark Zoback of Stanford University have proposed
drilling a deep hole right through the fault, three kilometers deep or more.
"It occurred to us that if a bore hole encountered fluids, we would need to
know where they came from. We set out to do chemical studies of fluids
associated with the fault system, as well as measuring the ratios of helium
isotopes," says Kennedy. "We located all the springs, seeps, and wells we could
that showed evidence of deep-circulating fluids. We sampled them for carbon
dioxide, hydrogen, noble gases, and so on. The fluid chemistry was in
equilibrium with the local geology, as we'd expected, but in the course of this
work, we found a helium-three signature in all the samples, which we did not
Kennedy determines helium ratios using a a sophisticated gas-separation
system and mass spectrometer, mounted in a truck trailer that can go on
location when necessary. He found variable but comparatively high ratios of
rare helium three (helium with only one neutron in its nucleus) to more common
helium four (whose nucleus consists of two neutrons and two protons) in the San
Andreas fluids, which proved to be telling clues to their origin.
Two competing models have sought to explain the origin of high-pressure
fluids in fault zones. One, the Byerlee-Sleep and Blanpied model, or "closed
box" model, suggests that local crustal fluids, including groundwater, are
drawn into the fault zone in response to fault rupture and become trapped by
mineral reactions; when the sealed fault zone compacts, the high fluid
pressures required to weaken it are reestablished.
In the Rice model, by contrast, high fluid pressure in the fault is only
the tip of a vertical "tongue" of high-pressure fluids originating in the
mantle, 30 kilometers deep and deeper, that are focused into the fault zone by
a root zone through the ductile base of the crust.
The Earth's atmosphere contains fewer than one and a half helium-three
atoms for every million atoms of helium four. In crustal fluids, the ratio is
even less -- only two hundredths of the ratio in air. But in mantle fluids, the
ratio of helium three to helium four is about eight times greater than in the
air. In fluids from the San Andreas Fault region, Kennedy and his colleagues
found helium-three ratios that varied from over a tenth to as much as four
times the ratio in air -- high ratios that were unrelated to the fluid
chemistry in the local rocks.
"Some of this fluid could have come only from the mantle," says Kennedy.
"The Rice model is at least partially correct."
The degree to which high-pressure mantle fluids contribute to the weakness
of the San Andreas Fault, while large, remains indefinite, because Kennedy and
his colleagues can't be sure if their sample fluids were tapped directly from
the fault zone or from the adjacent crust. Meanwhile, the discoveries have
raised interesting questions about the structure of the fault itself.
As fluids flow upward, helium three from the mantle is increasingly
diluted by helium four produced from the steady radioactive decay of various
elements in the crust. The ratio at a given site yields an estimate of how
quickly the fluid reached that site from the mantle. The distribution of
Kennedy's results leaves open the possibility that mantle fluid is flowing into
the San Andreas Fault from great distances away.
"There may be a regional decollement that extends as far east as the
Sierra Nevada -- maybe even under the Sierra," says Kennedy, noting the
presence of soda springs near the crest of the Sierra which contain carbon
dioxide that may have come from the mantle.
As for the nature of the mantle fluid, Kennedy says, "We don't know the
chemistry, but it's likely to be rich in carbon dioxide and perhaps water under
tremendous pressure" -- a mystery even a deep well won't answer in a
straightforward way -- "but we'd really like to get fluids directly from the
fault, to help us understand what makes the fault move the way it does. That's
one of several good reasons to bore a deep well."
Kennedy is a member of Berkeley Lab's
Center for Isotope Geochemistry. He and his colleagues presented their results in an article in Science, 14 November 1997.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.