Science Opportunities - Basic Research Opportunities in Earth Science - page 57

geodesy. 9 Particular opportunities exist for high-resolution geodetic
measurements near active faults and other regions of concentrated deformation,
for more detailed paleoseismic studies to gain a better history of particular fault
ruptures, and for more extensive geological investigations of the way that older
fault systems evolved through time. Direct measurement of fluid pressures and in
situ sampling by deep drilling will be needed to understand the role of water,
which is a key constituent in deformation processes.
Fluids in the Crust
Fluids play a major role in crustal processes at all depths, even in small
abundances. Laboratory experiments have shown that trace amounts of water can
have substantial effects on the ductile and frictional properties of rocks,
underlining the importance of this ubiquitous fluid in deformation processes. The
effects of fluid mobilization on chemical and thermal processes are also well
established by direct observations in shallow environments, and they can be
reliably inferred from high-resolution geophysical measurements and
observations on metamorphic rocks uplifted from depth. The microscopic fluid-
mineral interface, often modulated by biological processes (at least in the near-
surface environment), governs the way in which fluids permeate and react with
soils and other crustal materials. Many problems related to water and other fluids
in the near-surface Critical Zone have been discussed in earlier sections of this
report. Little is currently known about the dynamics of fluid conditions in the
deeper parts of the crust. For example, the interior zones of well-developed
strike-slip faults such as the San Andreas are thought to be much weaker than
typical crustal rocks, and increased fluid pressures in the fault zone, perhaps
dynamically maintained by the earthquake cycle itself, have been implicated as a
possible cause of this weakness. Surface observations have been inadequate to
test this hypothesis, and deep drilling will probably be required to resolve the
issue. Other problems include the influence of water on the mechanics of
detachment surfaces and décollements, the nature of deep circulation systems in
hydrothermal areas and sedimentary basins, the fluid content of the lower crust,
and the fluid flux from the mantle.
9 See also Support for Research in Tectonics at NSF, White Paper from the Division of
Structural Geology and Tectonics, Geological Society of America, July 24, 1998.