S44A-01
Upper Mantle Structure of South America from Joint Inversion of Waveforms and Fundamental-mode Group Velocities of Rayleigh Waves
A new tomographic S-wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S & Rayleigh waveforms and fundamental-mode Rayleigh-wave group velocities, to better constrain upper mantle S-velocity structure and Moho depth. We used about 5700 Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent. The joint inversion of this data set provided a new 3D upper mantle S-velocity model and a Moho depth model for South America, which fits both the group velocity and regional waveform data sets well. New features of the final 3D S-velocity and Moho depth model correlate well with known geotectonic units on a regional scale. The Moho depth ranges from 30 km in the central Chaco basin to 42 km beneath the Amazonian craton and 45-70 km beneath the orogenic Andean belt. The imaged S-velocity indicates an average lithosphere thickness of around 160 km for the Amazonian craton. High velocities are imaged beneath the Amazon and São Francisco cratons and part of the Paraná and Parnaíba basins down to about 150 km. Low to very low velocities are imaged beneath the central Andes and the Chaco, Pantanal and northwestern Paraná basins.
S44A-02 INVITED
The Effective Elastic Thickness of South America and its Implications for Intracontinental Deformation.
The flexural rigidity or effective elastic thickness of the lithosphere, Te, primarily depends on its thermal gradient and composition. Consequently, maps of the lateral variability of Te in continents reflect their lithospheric structure. We present here a new Te map of South America generated using a compilation of satellite-derived (GRACE and CHAMP missions) and terrestrial gravity data (including EGM96 and SAGP), and a multitaper Bouguer coherence technique. Our Te maps correlate remarkably well with other proxies for lithospheric structure: areas with high Te have, in general, high lithospheric mantle shear wave velocity and low heat flow and vice versa. In this paper we focus on the Te of the stable platform. We find that old cratonic nuclei (mainly Archean and Early/Middle Proterozoic) have, in general, high Te (> 70 km), while the younger Patagonian Phanerozoic terrane has much lower Te (20-30 km), suggesting that Te is related to terrane age as has already been noted in Europe. Within cratonic South America, Te variations are observed at regional scale: relatively lower Te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures, rift zones and sites of hotspot magmatism. Today, these low Te areas are surrounded by large cratonic nuclei. They concentrate most of the intracontinental seismicity, exhibit relatively high surface heat flow and low seismic velocity at 100 km depth. This implies that intra-continental deformation focuses within relatively thin, hot and hence weak lithosphere, that cratonic interiors are strong enough to inhibit tectonism, and that the differences in lithospheric rigidity, structure and composition between stable cratons and sites of intra-continental deformation are not transient, and may have been maintained, in some cases, for at least 500 m.y.
S44A-03 INVITED
The crust and lithosphere thicknesses in South America: trying to find the lithosphere- asthenosphere boundary
During the past years, a series of seismological investigations have been carried out to study the crustal and mantle structures all over the world. In South America, this investigation has not been an easy task as there are different regions where the geodynamics involves the subduction of an oceanic plate, the building of a mountain range as the Andes, the interaction with older lithosphere as the Brazilian Shield and the presence of active deformation fronts between the last two regions. In order to investigate the thickness of the lithosphere in such a complex context we have performed S-wave receiver function analysis (Vinnik and Farra, 2000; Li et al., 2004). The S receiver function technique looks for the S-to-P converted waves at seismic discontinuities beneath a station in the same way as the conventional P receiver function method that deals with P-to-S conversions. The S receiver function technique have proved to be useful to map the Moho and the LAB in many regions where other methods (i.e. surface waves) failed to provide reliable information (e.g. Li et al., 2004; Kumar et al., 2004a, 2004b; Sodoudi et al., 2006). We present here the results of S receiver function technique that has been applied to all the available temporary seismic experiments (e.g. BANJO, SEDA, REFUCA, BLSP) and the permanent stations from the IRIS network. We have been able to investigate the upper mantle discontinuities at all the depths beneath the stations and obtained coherent Moho depths along the entire Andes and in other South American continental regions. The LAB has been clearly detected below some stations, particularly those that are located far away from the subduction zone. By comparing our results with those from the P receiver functions, we have been able to further constrain the thicknesses of the crust and LAB in different regions including shields, mobile belts, basins and mountain ranges. At many stations we have also been able to map the upper mantle discontinuities of the 410 and 660 in South America. The topography of these discontinuities is governed by the temperature in the mantle transition zone and reflects the thermal status of the upper mantle
S44A-04
New Moho map for South America from receiver functions and surface waves
We present new constraints on crustal structure and thickness that we obtained from receiver functions (RF) from 18 temporary seismic stations in Brazil. These stations, of the Brazilian Lithosphere Seismic Project 2002 (BLSP02, Feng et al., 2004), were deployed in seismically inactive regions, previously very sparsely covered with seismic stations. Crustal ages there are predominantly Proterozoic and Archean. We determine crustal thickness and Poisson's ratio from two grid search methods: the commonly used H-k method by Zhu and Kanamori (2000), and the waveform misfit stacking method of Van der Meijde et al. (2003). Combined with approximately 40 receiver functions from other parts of eastern South America our data show no significant correlation between crustal thickness and age. However, our data do suggest that the average Poisson's ratio of the crust increases with decreasing age. The average difference between our results and crustal thickness of Crust2.0 is about 4 km, with some stations differing by as much as 11 km. In order to obtain a continuous sampling, we interpolate crustal thickness estimates from RF analysis between the stations. For this interpolation, we jointly invert the RF point constraints and the surface wave group velocities. We use both the Love and Rayleigh wave group velocities from Feng et al., 2004, which consist of about 6000 Rayleigh-- and 3500 Love wave dispersion curves of good quality.
S44A-05 INVITED
Subduction of the Chile Ridge Imaged by Teleseismic Travel-time Inversion
Making use of teleseismic data recorded at 46 seismic stations deployed in southern Chile from December 2004
to February 2007 (see the Chile Ridge Subduction Project; http:seismology.geology.ufl.edu/chile), we present a
preliminary inversion for the mantle P-wave velocity structure beneath the region. The area covered by the network
(42-46 degrees South and 72-78 degrees West) lies above the projected position of the subducted Chile ridge,
which separates the Nazca and Antarctic oceanic plates. Because the Nazca plate subducts nearly 5 cm/yr faster
than the Antarctic plate, the trailing edge of the last Nazca lithosphere formed before the ridge subducts has been
inferred to separate steadily from the leading edge of the Antarctic lithosphere, forming progressively larger slab
windows with depth. We use the teleseismic inversion to test models for such slab windows. The P-wave relative
delay times are obtained via a multi-channel cross correlation of band-passed waveforms for each teleseismic
event. These data are then inverted using an iterative, robust, non-linear scheme which parameterizes the 3-D
velocity variations as splines under tension constrained at over 30,000 nodes beneath the region.
http:seismology.geology.ufl.edu/chile
S44A-06
Shear Wave Splitting and Seismic Anisotropy in the Chile Ridge Subduction Region
We present new shear wave splitting measurements of SK(K)S and PKS phases recorded at 39 broadband
seismic stations in the Chile triple junction region. The network, deployed December 2004-February 2005 and
operated jointly by the University of Florida and the Universidad de Chile (Santiago), spans the region where the
Chile Ridge subducts beneath South America, from the Pacific coast of the Taitao Peninsula to the Argentine
border, and extending 250 km north and south of the actual triple junction. Given increasing temperature with
depth, it has long been hypothesized that ridge subduction should result in creation of slab windows
asthenosphere-filled gaps between continually separating edges of oceanic lithosphere formed at the Earth's
surface. The Chile Ridge Subduction Project (seismology.geology.ufl.edu/chile) was formulated in part to test this
notion. The network was demobilized during January-February 2007. In conjunction with teleseismic travel time
inversions and studies of seismic attenuation in the Chile Ridge subduction region, shear wave splitting as
recorded at the Project network will likely provide an excellent snapshot of upper mantle flow in the region: The
tomography and attenuation studies will allow us to define the distribution of lithosphere and asthenosphere to
determine if slab windows exist in the area. The shear wave splitting, which may be caused by mineral alignment
during upper mantle flow or possibly by aligned pockets of partial melt, will then define the upper mantle flow
pattern and/or partial melt distribution in the region of ridge subduction, and possibly also mineral alignments in
the subducted Nazca and Antarctic plates. We will present preliminary results at the meeting.
http:seismology.geology.ufl.edu/chile
S44A-07
Wide-angle velocity modeling and receiver functions imaging a lithospheric shear tear in the southeast Caribbean-South America plate boundary
Two models for slab detachment have been proposed for the lithospheric structure of the northern South America plate boundary with the southeast Caribbean, where westward subduction of oceanic South America transitions to east-west transform between continental South America and the Caribbean plate. In the tensile tear model, oblique convergence causes northwest-dipping subduction, and tension on the subducting slab results in detachment orthogonal to the motion vectors. Conversely, the shear tear model predicts detachment parallel to the motion vectors along a vertical plane, with shear stress focused on the edge of the propagating transform boundary. We present new active-source onshore-offshore wide-angle tomography, integrated with new passive-source receiver function analysis, from profile 64W of the BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) project. Profile 64W is a 460 km-long, north-south, onshore-offshore reflection/refraction/teleseismic transect located approximately at 64 deg W longitude that extends from the southeastern Caribbean across the Serrania del Interior and into the Maturin basin. The various datasets image deep crustal and upper mantle structure across the entire diffuse plate boundary zone. The active-source components of profile 64W include 33 OBSs and 344 land seismic stations which recorded 7500 offshore airgun shots and 2 chemical explosive land shots. Receiver functions along 64W were picked from hundreds of events at 18 temporary and permanent broadband stations. Close agreement exists between the wide-angle inversion of first arrivals, PmP, and Pn, and receiver function analysis for the Moho conversion, indicating that the Moho deepens northward from 35 km beneath the Guiana shield craton (from receiver functions only) to 45 km beneath the Serrania del Interior; to the north, Moho abruptly shoals to a depth of 25 km. We interpret this step change in Moho depth to be the lower crustal plate boundary between the Caribbean and South America, and coincident with a shear tear through the South American lithosphere. North of the step, Moho depth increases slightly to 28 km beneath Margarita Island, then gradually shallows to 20 km and flattens north of La Blanquilla Island. Thrust faults beneath La Blanquilla and the Serrania del Interior bound the 250 km-wide diffuse plate boundary, the former marking Caribbean underthrusting beneath the remnant arc, the latter marking the frontal thrust of the fold and thrust belt. We hypothesize that isostatic rebound following slab detachment may contribute significantly to the uplift of the Serrania del Interior, which reaches 2500 m elevation, and infer a role in the exhumation of HP/LT metamorphic rocks found along the strike- slip system. We conclude that the Serrania del Interior and Maturin basin do not represent a collisional orogeny and foreland basin, respectively, but are instead resultant from the geodynamic response to the propagating lithospheric shear tear.
S44A-08
Evidence from seismic tomography for a possible geodynamic link between the active and passive margins of South and North America
South and North America each have a seismically active western margin and a seismically passive eastern margin bordering a seismically stable Precambrian Craton. Regional seismic waveform imaging of the upper mantle beneath these two continents relies on this seismic activity, as recorded at seismic stations across the continents. Imaged upper-mantle structure beneath the active margins of both continents is remarkably similar and characterized by very low S-wave velocities. The low S-velocity regions are adjacent to or above high S-velocity regions. In South America the high-velocity region likely represents subducting lithosphere of the Nazca Plate, dipping under the low-velocity zone. In North America the high-velocity region likely represents subducted lithosphere from the Farallon Plate, sunk to the deep upper mantle. East of these striking subduction-related structures, both the South American and North American upper mantles display a strong, thick, high-velocity lithosphere beneath the Precambrian Cratons. However, in South America this lithosphere is significantly thinner and weaker than that in North America. Furthermore, sub-lithospheric structure appears different as well. In North America, low velocities are imaged in a large depth range beneath the eastern passive margin. These low velocities might represent an upwelling of a somewhat hydrous mantle. This hydrous mantle might be the result of Farallon subduction beneath the western margin, which could have carried small quantities of water into the top of the lower mantle and transition zone. This hydrous, upwelling mantle might act to weaken the passive- margin lithosphere and allow subduction of Atlantic lithosphere in the near geologic future.