A
three-dimensional tomographic view of a ridge-transform intersection along the
East Pacific Rise
Michael Begnaud, Ph.D.
Monterey Bay Aquarium Research Institute
Friday, March 20, 1998
12:00 Noon —Pacific Forum
I performed a three-dimensional (3-D) tomographic analysis for data from the second
deployment of the Clipperton Area Seismic Study to Investigate Compensation (CLASSIC)
experiment. The CLASSIC experiment took place during the Spring of 1994 along the
Clipperton Transform of the East Pacific Rise. Using a tomography method modified from
Toomey et al. [1994] and 20,668 shot-receiver pairs, I have obtained 3-D structure for an
area centered at the eastern ridge-transform intersection (RTI). The northern ridge
segment has been characterized as "magma-starved" due to its narrow
cross-section, depth, and the lack of an axial magma chamber (AMC) reflector. Tomographic
analysis indicates the presence of an erratic axial low-velocity zone (LVZ) near the RTI
at ~1.5-2 km deep merging into a relatively deep (2.5-3 km) axial LVZ to the north. The
width of the LVZ varies from ~5-10 km with velocities reduced by ~0.5-1.0 km/s.
The characterization of the northern ridge segment as "magma-starved" is
overstated; the term "magma-reduced" may be more appropriate. The presence of
the LVZ suggests an ample supply of magma, while the lack of an AMC reflector indicates a
low percentage of melt. Upper crustal velocities at the RTI are lower (~1-2 km/s) than
adjacent areas, indicating that RTI topography results from increased volcanism. Directly
north of the transform, velocities are also lower (~1.0-1.5 km/s) suggesting an increase
in porosity for the region. Reduced velocities along the transform occur at depth, perhaps
related to compressive faulting along the transform. The lower velocities parallel the
transform trace, again suggesting faulting as the mechanism for velocity reduction.