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Shear Wave Splits, Plate Motions and the Mérida Andes, Western
Venezuela
Jeniffer Masy (Rice University), Fenglin Niu (Rice University), Alan Levander (Rice University), Michael Schmitz
(Fundacion Venezolana de Investigaciones Sismologicas-FUNVISIS)
We measured shear wave splitting from SKS data recorded by
the national seismic network of Venezuela and a linear broadband
PASSCAL/Rice seismic array across the Merida Andes installed as
part of the second phase of the BOLIVAR project. We modified the
stacking method of Wolfe and Silver [1998] and applied it to a total
of 22 stations. At each station, SKS waveforms from 2 to 36 earthquakes,
mostly from the Tonga subduction zone, were stacked to
yield a single measurement of the polarization direction and the
splitting time. The measured parameters indicate that region can be
divided into 3 zones, all congruent with surface motion observed
by GPS. The first zone is located north of part of the strike-slip plate
boundary prior to 7Ma. Here we observe the largest splitting times
(1.6-2.5s), and the fast S-wave directions are orientated roughly
in the EW direction, results consistent with previous studies and
also with observations made in eastern Venezuela along the active
strike slip plate boundary. An interpretation for these observations
was proposed by Russo and Silver [1994]. The rollback of the
Nazca plate induced a trench-parallel NS flow that passes around
the northwest corner of the South America continent and forms an
eastward flow beneath the Caribbean plate.
Zone two is located on the SA continent, east of the right lateral
Bocono fault, where the measured split times are the smallest (0.6-
1.0s) and have EW fastest direction. The observed fast direction and
splitting times are consistent with those observed at the Guarico
Basin, Maturin Basin and the Guayana shield in the east [Growdon
et al., 2009], and are probably related to the westward drift of the
South America continent.
Zone three is the Maracaibo block, which is bounded on the southeast by the Bocono fault, and on northwest by the Santa
Marta Fault. Split orientations along the Bocono fault are N45°E, suggesting that the observed seismic anisotropy is likely caused
by lithospheric deformation parallel to the fault. Split times in this zone range from 1s to 1.5s, requiring a vertically coherent
zone of deformation extending to at least 200 km, suggesting a deep origin for the Venezuelan Andes. A recent tomography
study [Bezada et al., 2010] suggests that beneath the Merida Andes the top of the flat slab is located at 200 km depth.
References
Measured splitting parameters are shown together with the topography
of the study region. The orientation of the black solid lines indicates
the orientation of the fastest direction. In general the study area can
be divided in 3 different zones. The first one is north of the Oca-Ancon
dextral strike slip fault, where splitting times are the largest (1.6 - 2.5 s)
with an east-west orientation. The second zone is inside Barinas Apure
Basin. The smallest split times are present in this region (0.6 - 1 s) also
with an east-west orientation. The third zone, southeast of Maracaibo
Block, shows a splitting direction N45E, parallel to Bocono Fault and
Mérida Andes.
Russo, R., and P. G. Silver (1994), Trench-Parallel Flow Beneath the Nazca Plate from Seismic Anisotropy, Science, 263(5150), 1105-1111.
Growdon, M. A., G. L. Pavlis, F. Niu, F. L. Vernon, and H. Rendon (2009), Constraints on mantle flow at the Caribbean–South American plate
boundary inferred from shear wave splitting, J. Geophys. Res., 114(B2), 1-7.
Wolfe, C., and P. Silver (1998), Seismic anisotropy of oceanic upper mantle: shear wave splitting methodologies and observations, J. Geophys.
Res.-Solid Earth, 103(B1), 749-771.
Bezada, M.J., A. Levander, and B. Schmandt, 2010, Subduction in the Southern Caribbean: Images from finite-frequency P-wave tomography,
submitted to J. Geophys. Res., May 2010.
Acknowledgements: We would like to acknowledge IRIS-PASSCAL and FUNVISIS for providing the data and the temporal stations used in
this study, and also for all the support they gave us during the deployment of the seismic stations. This research was supported by NSF grant
EAR-0607801.
II-184 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics