Science Opportunities - Basic Research Opportunities in Earth Science - page 58

Lower Continental Crust
The lower continental crust forms a weak, ductile layer that reduces the
coupling between the brittle deformation of the earthquake-producing upper crust
and the aseismic motions in the mantle. Its properties, which are highly variable
from place to place, play an important role in determining the width of active
fault zones, postseismic response and interseismic strains, the heights of mountain
belts, and the subsidence rates of sedimentary basins and passive continental
margins. Surprisingly, however, some of the most basic parameters of the lower
crust, such as its average composition, are still poorly constrained, and the role of
key processes, such as magmatic underplating, remains largely speculative.
Better knowledge of the lower crust can be obtained from geochemical and rock
mechanical studies of tectonically exhumed sections, as well as from samples
brought to the surface in volcanic eruptions and from geophysical research on
active and ancient deformations. Substantial geological field studies in key areas
will be needed to set up the tectonic framework for such investigations, and the
integration of disciplinary data will require the construction of quantitative
models that can account for the geomechanical behavior of the continents from
the surface to the mantle.
Continental Deep Structure
Substantial knowledge has been gained about the structure of the continental
lithosphere, but fundamental questions concerning its composition and formation
remain unanswered. They pertain, in particular, to the anomalously thick
lithosphere that underlies the Archaean cratons, which comprise the oldest parts
of the crust and have acted as nuclei for continental growth. Substantial
implications for Earth evolution may be drawn from the compositional and
thermal contrasts among these continental keels, younger lithosphere, and the
underlying convecting mantle. The keels are buoyant and not easily disrupted or
subducted, and they act to organize the largescale patterns of mantle dynamics,
perhaps affecting convective processes as deep as the core-mantle boundary.
EarthScope—A Major Initiative
Building on the success of EAR's CD program, the geoscience community
is working with NSF, the U.S. Geological Survey, the National Aeronautics and
Space Administration, and other agencies to initiate the