Structure, Tectonics, and Sediment Flow
Into the Lesser Antilles Subduction Zone
Principal Investigators: Nathan Bangs, Gail Christeson,Yosio Nakamura, and
Tom Shipley.
Funded by: National Science Foundation
Check out data from the cruise in 1998.
Subduction zones are extremely dynamic regions of the earth's surface. They are the
collision sites between earth's tectonic plates, where a subducting plate is carried deep
into the earth's mantle beneath an overriding plate. Because of the collision, subduction
zones are also extremely dynamic geologic setting with numerous tectonic, deformational
and metamorphic processes occur. Most notably, seafloor sediments that accumulate on the
subducting plate collide with the overriding plate at subduction zones and are thickened
into accretionary wedges thousands of meters thick, squeezed, baked, and uplifted, in
major mountain building events that transform soft sediments into hard rock. Some seafloor
sediment can also bypass the collision by being carried underneath the plate-boundary
fault down to great depths within the earth's mantle. These sediments are melted and
generate magma, which buoys up to earth's surface and erupts explosively at major island
arc volcanic systems. Mount Saint Helens, which erupted in 1980, is a recent example of
such an explosive eruption caused by the magma that is generated from subduction of
sediments at a subduction zone. Also, earth's largest and most frequent earthquakes, such
as many from around the Pacific rim, are generated along plate-boundary faults within
subduction zones. How sediments flow through the subduction zone, and the degree of
sediment metamorphosis from squeezing and baking are believed to significantly affect
stress accumulation along the plates as they slide past each other and therefor affect
large earthquake genesis at subduction zones.
At UTIG, understanding subduction zone processes has been a long standing
research goal. The margins along Central America, the Caribbean, Japan, Alaska, and
Antarctica have been the subject of several subduction zone studies during the past 20
years. This new research project, "Structure, Tectonics, and Sediment Flow into the
Lesser Antilles Subduction Zone," proposes to use the Lesser
Antilles subduction zone in the Caribbean as a case study for examining sediment
deformation and flow through the subduction zone.
Scientific Objectives
The new research effort is directed at examining the structure of the crustal plates
where they meet. We believe the crustal geometry of these two plates may affect the
behavior of sediments caught up in the subduction zone. The geometry of the rigid portions
of the overriding plate where they first overlap, may have a big impact on the
partitioning of sediment either onto the overriding plate, or down with the subducting
plate. We are interested in how the plate geometry controls the flow of sediment
through the subduction zone. Secondly, the geometry of rigid crust that makes up the
overriding plate, will have a big impact on how stress is transmitted into the
sediments that are caught up in the subduction zone. By imaging the geometry of the
overriding forearc crust, and the deformational structures of the accretionary wedge of
sediment that accumulates in front of the overriding crust, we hope to learn how crustal
geometry affects the transmission of stress into the accretionary wedge and how it impacts
this mountain building event.
The upcoming experiment will be conducted using both seismic reflection and refraction
techniques to probe deep within the subduction zone. A 17-day
cruise is planned on the R/V Maurice Ewing east of the volcanic island of
Guadeloupe. During the cruise we will acquire 1,600 km of multichannel seismic reflection
data. These data will be acquired both along strike and across strike of the subduction
zone and are designed to image crust and sediment structures from the seabottom, which is
5 km below the sea surface, to 10 km subseafloor. Thirty two ocean bottom seismometers
will also be deployed to acquire seismic refraction data. The refraction data will allow
us to measure the seismic velocity within the deep structure to determine the boundary
between the higher-velocity rock of the crust, from lower-velocity sediments. These data
will also help delineate the degree of sediment metamorphosis, and determine at what
depths sediments are of sufficient strength to build up large amounts of stress energy
that could be released in a large earthquake.
Education and Training
The project is a three-year project that will include both graduate and undergraduate
participation. Students will participate in all phases of the project from data
acquisition at sea, to data reduction, processing, and analysis. Projects such as this one
that acquire seismic data are expensive and are relatively rare in academia. However,
these project provide invaluable training to students who are interested in continuing
with careers in the oil industry and service industries, both in acquisition at sea and
with data processing and interpretation. Specific to this project, forearc basins, such as
the basin to be examined as part of this study, are also of current interest by the oil
industry for hydrocarbon exploration. This project will acquire both "standard"
seismic reflection profiling data as well as seismic refraction data, which will provide a
range of educational opportunities for students. This project will also directly provide
support for student stipends, travel, acquisition of state-of-the-art computer
workstations and software that will be used by students for data processing,
interpretation, course work and thesis projects.
Significance
The results from this project will place new constraints on the geometry of the
crust and the flow of sediments past the crustal plates as they enter the Lesser Antilles
subduction zone. These results will help to constrain the controlling influences of
crustal geometry on sediment deformation in the accretionary complex which attaches to the
overriding plate. Furthermore, the results will reveal how crustal geometry controls the
partitioning of sediment as either accreted to the overriding plate or subducted into the
mantle to promote magma genesis. How and where sediments are partitioned within the
subduction zone may also have an impact on where and if significant earthquakes can be
generated. The controlling effects of the plate geometries on sediment flow through the
subduction zone will bear significantly on models of earthquake generation.
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