Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America (original) (raw)
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Journal of Geodynamics
Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes[ The inferred velocity models for the ChileÐPeruÐ Ecuador region reveal an increase of P velocity from 7[93 km:s at 39 km to 7[17 km:s at 149 km depth\ while the S velocity remains almost constant at 3[51 km:s from 39 to 109 km depth[ A velocity discontinuity "probably corresponding to the L discontinuity in the continental upper mantle# at 119Ð149 km depth for P and 199Ð119 km depth for S waves\ with a 2Ð3) velocity increase\ is inferred from the velocityÐdepth data[ Below this discontinuity\ P velocity increases from 7[43 km:s at 149 km to 7[51 km:s at 219 km depth and S velocity increases from 3[70 km:s at 109 km to 3[88 km:s at 189 km depth[ Travel time data from deep earthquakes at depths greater than 499 km in the BoliviaÐPeru region\ reveal P velocities of about 8[54 km:s from 499 to 469 km depth[ P velocityÐdepth data further reveal a velocity discontinuity\ either as a sharp boundary at 469 km depth with 7Ð09) velocity increase or as a broad transition zone with velocity rapidly increasing from 459 to 509 km depth[ P velocity increases to 09[64 km:s at 549 km depth[ A comparison with the latest global average depth estimates of the {559 km| discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region[ Further\ a velocity discontinuity at about 399 km depth with a 09) velocity increase seems to be consistent with travel time observations from deep earthquakes in this region[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Subducted Lithosphere Under South America From Multifrequency P Wave Tomography
Journal Of Geophysical Research: Solid Earth, 2021
has generated arc magmatism and produced the modern Andes. However, extrapolation of the modern east-dipping subduction scenario to before the Late Cretaceous does not readily explain the contrasting history of the ancestral Andes. Subducted former seafloor continues to exist in the mantle and remains visible to seismic tomography because P waves travel faster in it than in ambient mantle. We analyze such seismically fast domains, that is, "slabs" of interpreted paleo-seafloor, at depths of ∼300-1,800 km in our global tomography model DETOX-P1. Combining these observations with quantitative plate reconstructions and geological observations, we attempt to reconstruct the subduction history under South America. The slab that dips eastward down beneath the present-day Andes is relatively continuous to ∼900 km depth. Deeper down, slab geometries completely change. Voluminous slabs are unexpectedly imaged thousands of kilometers west of South America's reconstructed paleo-margin. We argue that in the simplest explanation all slabs sank essentially in place, and a trans-American, tectonic reconfiguration occurred the time equivalent to 900 km slab depth (∼80-90 Ma). Prior to this time, the "Andean" trench sat offshore (and the margin extended) but the subduction direction must have been oceanward, rather than eastward. MOHAMMADZAHERI ET AL.
Regional Variations in Upper Mantle Structure Beneath North America
1970
Several types of seismological data, including surface wave group and phase velocities, travel times from large explosions, and teleseismic travel time anomalies, have indicated that there are significant regional variations in the upper few hundred kilometers of the mantle beneath continental areas. Body wave travel times and amplitudes from large chemical and nuclear explosions are used in this study to delineate the details of these variations beneath North America. As a preliminary step in this study, theoretical P wave travel times, apparent velocities, and amplitudes have been calculated for a number of proposed upper mantle models, those of Gutenberg, Jeffreys, Lehman, and Lukk and Nersesov. These quantities have been calculated for both P and S waves for model CIT11GB, which is derived from surface wave dispersion data. First arrival times for all the models except that of Lukk and Nersesov are in close agreement, but the travel time curves for later arrivals are both qualit...
Journal of Geophysical Research, 2007
We combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the Americas. Using broadband waveform cross correlation, we measured 37,000 differential P and S traveltimes, 2000 PcP-P and ScS-S times along a wide corridor from Alaska to South America. We invert the data simultaneously to obtain P and S wave velocity models. We interpret slab structures and unravel subduction history by comparing our V S tomographic images with reconstructed plate motion from present-day up to 120 Myr. Convergence of the Pacific with respect to the Americas is computed using either (1) the Pacific and Indo-Atlantic hot spot reference frames or (2) the plate circuit passing through Antarctica. Around 800 km depth, four distinctive fast anomalies can be associated with subduction of the Nazca, Cocos, and Juan de Fuca plates beneath South, Central, and North America, respectively, and of the Pacific plate beneath the Aleutian island arc. The large fast anomalies in the lowermost mantle, which are most pronounced in the S wave models, can be associated with Late Cretaceous subduction of the Farallon plate beneath the Americas. Near 2000 km depth, the images record the post-80 Myr fragmentation of the proto-Farallon plate into the Kula plate in the north and the Farallon plate in the northeast. Near 1000 km depth, we infer separate fast anomalies interpreted as the Kula-Pacific, Juan de Fuca, and Farallon slabs. This interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction.
Journal of Geophysical Research, 2006
1] We present compressional (P) and shear (S) wave seismic velocity models for the upper mantle beneath southeastern Mexico derived from waveform inversion of triplicated seismic phases. The seismic waveform data produced by an earthquake located near the Mexico-Guatemala border were recorded by the La Ristra passive seismic array. The La Ristra seismic array consists of 54 broadband seismometers arranged linearly from west Texas to southeastern Utah. The orientation of the La Ristra array is nearly along the great circle from the event, and the distance (18.5°-26.5°) of the seismic array from southern Mexico is such that the data are ideal for investigating localized seismic structure of the upper mantle. Previous tomography and receiver function studies provide a priori knowledge of receiver-side crustal and upper mantle structure from which static adjustments were made to the seismic data. The waveforms were inverted for mantle velocity from 40 to 1000 km depth using a conjugate gradient algorithm. In the inversion, we evaluated a suite of starting models with different depths of the 410 km and 660 km discontinuities and varying velocity gradients. The best fitting models have velocity increases across the 410 km discontinuity of 6.2% and 7.3% for P and S wave velocities, respectively. The velocity jump across the 660 km discontinuity was found to be 3.3% for P waves and 6.3% for S waves. The size of the upper mantle discontinuities that we find are more in agreement with a pyrolite composition than standard reference models imply. A common feature of the best fitting models is a low-velocity zone above the 410 km discontinuity that is more prominent in the shear velocity model than the compressional velocity model. This feature may be due to partial melting induced by water release from the transition zone. The overall jump in velocity at 410 km is also larger than in previously published models with a lower gradient below. In addition, the P wave data require a small discontinuity at 490 km depth that is not resolved in the S data. Finally, the S wave data require an unusually high gradient beginning at about 600 km depth extending to the 660 km discontinuity. This feature may be due to a thermal and/or mineralogic anomaly due to a flat lying slab beneath eastern Mexico.
Mapping the lowermost mantle using core-reflected shear waves
Journal of Geophysical Research, 1994
A map of laterally varying D-double prime velocities is obtained for the region from 50 deg S to 50 deg N in latitude and 70 deg E to 190 deg E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS-S and 266 sScS-sS phase pairs. The long-period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D-double prime paths, and the remaining residuals are interpreted as the result of D-double prime heterogeneity. Ray-tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies. The resulting map reveals significant long-wavelength D-double prime structure including a 3% low-velocity region beneath northeastern Indonesia, surrounded by three identified high-velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3-5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low-velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D-double prime rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high-velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core-mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron-poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher-pressure polymorph.