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Opposing Slabs under Northern South America
M.J. Bezada (Rice University, Earth Science Department), Alan Levander (Rice University, Earth Science Department),
B. Schmandt (University of Oregon, Department of Geological Sciences), Fenglin Niu (Rice University, Earth Science
Department), M. Schmitz (FUNVISIS, Caracas, Venezuela)
The eastern boundary of the Caribbean plate is marked by
subduction of the Atlantic under the Caribbean plate forming
the Lesser Antilles volcanic arc. The southeastern plate
boundary is a strike-slip margin while different configurations
of subduction of the southwestern Caribbean under
South America have been proposed. Using data from the
BOLIVAR/GEODINOS experiment [Levander, 2006] we
investigated the slab geometry in the upper mantle using
finite-frequency, teleseismic P-wave tomography. Waveforms
from P and PKIKP phases from 285 (Mb > 5.0) events occurring
at epicentral distances from 30o to 90o and greater than
150o were bandpass filtered and cross-correlated to obtain up
to three sets of delay times for each event. The delay times
were inverted using first Fresnel zone approximate finitefrequency
kernels. Our results show the subducting Atlantic
slab, as well as a second slab in the west of the study area that
we interpret as a subducting fragment of the Caribbean plate.
Both slabs have steep dips where imaged and can be traced
to depths greater than 600 km (Bottom figure). These results
are consistent with transition zone boundary topography as
determined by receiver function analysis (Juang et al, 2009.
Top and middle figures). The Atlantic slab extends continentward
south of the South America-Caribbean plate bounding
strike-slip margin. We interpret part of this extension south
of the faults as continental margin lithospheric mantle that
is detaching from beneath South America and subducting
along with the oceanic Atlantic slab. The steep subduction
of the Caribbean occurs ~600 km landward from the trench,
implying an initial stage of shallow subduction as far to the
east as the Merida Andes-Maracaibo region, as has been
inferred from intermediate depth seismicity. (Submitted to J.
Geophys. Res.)
References
J.P. Huang, E. Vanacore, F. Niu, and A. Levander, 2009, Mantle transition
zone beneath the Caribbean-South American plate boundary
and its tectonic implications, Earth Planet. Sci. Lett., 289,
105-111
Subducting plate structure under northern South America and the southern
Caribbean. ATL is the Atlantic (oceanic South American) plate, CAR is the
Caribbean plate. Top: Depth of the 410 discontinuity showing elevated boundary
where the slabs intersect the 410. Middle: Depth of the 660 discontinuity
showing depressed boundary where the slabs intersect the 660. Bottom: 3D
structure viewed from the northwest. The isosurface is the +1.5% anomaly. ATL
subducts westward beneath CAR and northern South America. CAR subducts
eastward beneath northern CA.
Levander, A., and 10 others, 2006, Evolution of the Southern Caribbean Plate Boundary, EOS, Trans. AGU, 87, Cover story, 97 and 100-101.
Acknowledgements: We would like to thank our colleagues at FUNVISIS, Caracas, Venezuela, the personnel at the PASSCAL Instrument
Center and at OBSIP, and Gary Pavlis and Frank Vernon for outstanding data collection. BOLIVAR was funded by NSF Continental
Dynamics Program grants EAR0003572 and EAR0607801, as well as by CONICIT and PDVSA. The 3D graphics were generated using
EarthVision¨ software from Dynamic Graphics, Inc.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-181