Browsing by Author "Masy, Jeniffer"

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    Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela
    (Elsevier, 2015) Masy, Jeniffer; Niu, Fenglin; Levander, Alan; Schmitz, Michael
    We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere–asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01–0.05 Hz (20–100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5° × 0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50–60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.
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    Seismic imaging of the Upper Mantle structure and dynamics beneath the Southern Caribbean plate boundary and Venezuela
    (2013-11-01) Masy, Jeniffer; Niu, Fenglin; Levander, Alan R.; Alexander, David; Morgan, Julia K.
    The Caribbean-South America plate boundary has a complicated tectonic history that has been matter of debate and the focus of many studies for decades, yet many questions remain unanswered. The aim of this work, developed within the framework of the BOLIVAR (USA) and GEODINOS (Venezuela) projects, is to use different seismological techniques to study the lithospheric structure under the southern Caribbean and Venezuela, in order to understand some aspects of the present structure and its tectonic evolution. A shear wave splitting analysis in northwestern Venezuela revealed three areas with different deformation mechanisms: (1) Islands and coastal regions have large splitting times (~2-3 s) and a fast polarization direction parallel to the direction of the relative plate motion of the Caribbean plate respect to South America, which can be explained by a strong eastward flow confined at the CAR-SA plate boundary. (2) The stable South America plate showed weak seismic anisotropy with an origin likely in the asthenosphere. (3) Large splitting times and a ~NE-SW fast direction are observed at stations deployed along the Mérida Andes range, suggesting that the subcontinental mantle is also deformed beneath the range. It is likely the lithospheric mantle played a major, if not dominant, role in the formation of the Mérida Andes. The upper mantle structure of the area was obtained by combining three types of seismic data: Ps and Sp receiver functions and Rayleigh wave tomography. Results reveal the presence of the Moho of the subducting Caribbean Plate beneath the northwestern part of the Maracaibo Block. Tomographic images indicate that the subducting Atlantic slab appears to be attached to the continental South American lithosphere, pulling it down and removing the continental lithospheric mantle beneath the Serrania del Interior. A lithospheric thickness map was also obtained. The lithosphere asthenosphere boundary shows significant variations and seems to correlate well with major tectonic provinces in the region. Finally ambient noise cross-correlations between station pairs yields to Empirical Greens Function as waveform data input for the adjoint tomography based on spectral element methods. The adjoint tomography utilizes a more accurate full wave finite-frequency theory compared to the previous ray theory, and will iteratively refine the initial smooth 3D model to achieve more detailed high-resolution images of the upper most mantle structure of eastern Venezuela. Low velocity anomalies correspond to the major sedimentary basins and high velocity anomalies correspond to the stable craton.