Fluid from the Earth's mantle as a lubricant
At the San Andreas Fault in California, the Pacific and North American plates slide along one another, causing crustal deformation across much of western North America. Recent studies suggest that high-pressure fluids at the plate boundary act as a lubricant. The chemical composition of the liquids indicates an origin from the earth's mantle. The San Andreas Fault is probably the most studied major fault zone on Earth. Despite the multitude of existing geological and geophysical data, some essential questions - such as the trigger and timing of the major earthquakes in this region - are still largely unanswered. In a work published by the U. S. Department of Energy's Office of Energy Research, geochemists Mack Kennedy, Yousif Kharaka and their colleagues have uncovered new insights into surprisingly intricate connections with the Earth's mantle deep below the Earth's surface.
The San Andreas Fault, a classic horizontal fault, marks the plate boundary between the Pacific and North American plates. The North American plate is moving south at a rate of about one centimeter per year, while the Pacific plate is drifting north. Both plates are also pressed against each other, so that the frictional resistance has to be overcome for the plate to move.
Interestingly, laboratory friction measurements on material from the fault zone revealed that significantly more shear stress than actually observed would be required to overcome the frictional blockage and cause the earth to move.
"The forces and motions should generate frictional heat," says Kennedy, a member of the Earth Sciences department at Ernest Orlando Lawrence Berkeley National Laboratory, "but paradoxically, no one has been able to determine the expected heat build-up in the vicinity of the fault. One possibility is that fluids under high pressure act as a kind of lubricant. Unusually high pressures have been measured in rock pores at moderate depths," says Kennedy, "but to fully understand what's going on at the fault, it's extremely important to find out what's happening at greater depths."
Geologists like Stanford University's Mark Zoback have proposed drilling a borehole right at the plate boundary: three kilometers deep or more. “It occurred to us that if fluids appear in a well, we should know where they are coming from. We did chemical analysis of the fluids in the trench system to measure helium isotope ratios,” says Kennedy.“We located as many springs, leachates and wells as we could to get clues about the composition of the fluids. We took samples and tested for carbon, hydrogen and noble gases. The fluid chemistry was in balance with the local geology as we expected. But in the course of this work we found helium-3 in all samples; we didn't expect that."
Kennedy determined helium ratios using a sophisticated gas separation system and mass spectrometer mounted on a truck trailer that can be deployed on site if necessary. He found different but comparatively high proportions of the rare helium-3 (helium with only one neutron in its nucleus) compared to the more common helium-4 (whose nucleus consists of two neutrons and two protons) in the San Andreas liquids. This provided revealing clues about their origin.
Two competing models have attempted to explain the formation of high-pressure fluids in fault zones.
The first is the so-called Byerlee-Sleep and Blanpied ("closed box" model); local crustal fluids, including groundwater, are believed to enter the fault zone in response to rift fissures and become trapped there by mineral reactions.
The Rice model explains high fluid pressure in a fault zone by the tip of a vertical "tongue" of high pressure fluids originating in the mantle at depths of 30 km and deeper.
The Earth's atmosphere contains less than one and a half atoms of helium-3 for every million atoms of helium-4. In crustal fluids, this ratio is even lower - only two-hundredths of the ratio in air. However, in mantle fluids, the ratio of helium-3 to helium-4 is about eight times higher than in air. In fluids from the region of the San Andreas Fault, Kennedy and his colleagues found helium-3 ratios ranging from over a tenth to four times the ratio in air-high ratios that are inconsistent with the fluid chemistry of the local rocks bring.
"Some of the liquids examined could only have come from the mantle," says Kennedy. "The Rice model is at least partially correct."
As liquids rise toward the Earth's surface, helium-3 from the mantle is increasingly diluted by helium-4 produced by the ongoing radioactive decay of various elements in the crust. The ratio at a given point gives an indication of how quickly the fluid will reach that point from the mantle. The distribution of Kennedy's results leaves open the possibility that mantle fluid flows into the San Andreas fault from a greater distance. As for the nature of the mantle fluid, says Kennedy, "we don't know the chemistry, but it's probably rich in carbon dioxide and maybe water that's under tremendous pressure" -- a mystery that even a deep well cannot satisfactorily explain -- " but we would like to obtain liquids directly from the ditch to better understand the situation. That's one of several good reasons to drill a deep well.”
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