The Cameroon Volcanic Line (CVL) is an 1800-km-long line of Cenozoic volcanoes that does not show a chronological progression consistent with hotspot-related volcanism. We investigate seismic anisotropy to determine the upper-mantle lattice preferred orientation and constrain the mantle flow pattern using a temporary array of 32 broad-band seismographs deployed throughout Cameroon between 2005 and 2007 along with two additional permanent seismographs in adjacent countries. We determine the fast direction and lag time beneath each station by stacking SKS and SKKS splitting measurements from multiple events. The results indicate four regions with different splitting parameters. The Congo Craton in southern Cameroon and the Garoua rift region in northeast Cameroon have northeast—southwest-oriented fast directions and split times of about 1 s. Between the Congo Craton and the CVL, in central Cameroon, the fast directions are variable and have small splitting times of 0.3 s or less. Along the CVL, where previous studies show a strong slow velocity anomaly in the mantle, the fast direction is oriented approximately north—south, with splitting times of about 0.7 s. We interpret measurements from southern Cameroon and northeast Cameroon as indications of lattice-preferred orientation frozen into the Congo Craton and subcontinental lithosphere related to relict plate motion and deformation. The distinct pattern of splitting along the CVL suggests the existence of small-scale convection in the asthenosphere related to the formation of the CVL, perhaps driven by the adjacent cold edge of the Congo Craton.
The origin of the Cameroon Volcanic Line (CVL), a 1600 km long linear volcanic chain without age progression that crosses the ocean‐continent boundary in west‐central Africa, is investigated using body wave tomography. Relative arrival times from teleseismic P and S waves recorded on 32 temporary seismic stations over a 2‐year period were obtained using a multichannel cross‐correlation technique and then inverted for mantle velocity perturbations. The P and S wave models show a tabular low‐velocity anomaly directly beneath the CVL extending to at least 300 km depth, with perturbations of −1.0 to −2.0% for P and −2.0 to −3.0% for S. The S wave velocity variation can be attributed to a 280 K or possibly higher thermal perturbation, if composition and other effects on seismic velocity are negligible. The near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the CVL that invoke plumes or decompression melting under reactivated shear zones, but are possibly consistent with a model invoking edge‐flow convection along the northern boundary of the Congo Craton lithosphere. If edge‐flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the CVL, then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness.
Abstract Seismic anisotropy provides essential information for characterizing the orientation of deformation and flow in the crust and mantle. The isotropic structure of the Antarctic crust and upper mantle has been determined by previous studies, but the azimuthal anisotropy structure has only been constrained by mantle core phase (SKS) splitting observations. This study determines the azimuthal anisotropic structure of the crust and mantle beneath the central and West Antarctica based on 8—55 s Rayleigh wave phase velocities from ambient noise cross‐correlation. An anisotropic Rayleigh wave phase velocity map was created using a ray—based tomography method. These data are inverted using a Bayesian Monte Carlo method to obtain an azimuthal anisotropy model with uncertainties. The azimuthal anisotropy structure in most of the study region can be fit by a two‐layer structure, with one layer at depths of 0–15 km in the shallow crust and the other layer in the uppermost mantle. The azimuthal anisotropic layer in the shallow crust of West Antarctica, where it coincides with strong positive radial anisotropy quantified by the previous study, shows a fast direction that is subparallel to the inferred extension direction of the West Antarctic Rift System. Fast directions of upper mantle azimuthal anisotropy generally align with teleseismic shear wave splitting fast directions, suggesting a thin lithosphere or similar lithosphere‐asthenosphere deformation. However, inconsistencies in this exist in Marie Byrd Land, indicating differing ancient deformation patterns in the shallow mantle lithosphere sampled by the surface waves and deformation in the deeper mantle and asthenosphere sampled more strongly by splitting measurements.
The lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar. We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H–κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31–36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson's ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s–1), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s–1). However, markedly slow shear-wave velocities (4.2–4.3 km s–1) are observed beneath the northern tip, central part and southwestern region of the island where the major Cenozoic volcanic provinces are located, implying the lithosphere has been significantly modified in these places.
The Ethiopia/Afar hotspot has been frequently explained as an upper mantle continuation of the African superplume, with anomalous material in the lower mantle under southern Africa, rising through the transition zone beneath eastern Africa. However, the significantly larger amplitude low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared to the anomalies beneath neighboring regions, has led to questions about whether or not along-strike differences in the seismic structure beneath eastern Africa and western Arabia are consistent with the superplume interpretation. Here we present a new P-wave model of the hotspot's deep structure and use it to evaluate the superplume model. At shallow (< ∼400 km) depths, the slowest velocities are centered beneath the Main Ethiopian Rift, and we attribute these low velocities to decompression melting beneath young, thin lithosphere. At deeper depths, the low velocity structure trends to the northeast, and the locus of the low velocity anomaly is found beneath Afar. The northeast-trending structure with depth is best modeled by northeastward flow of warm superplume material beneath eastern Africa. The combined effects of shallow decompression melting and northeastward flow of superplume material explain why upper mantle velocities beneath Ethiopia/Afar are significantly slower than those beneath neighboring East Africa and western Arabia. The superplume interpretation can thus explain the deep seismic structure of the hotspot if the effects of both decompression melting and mantle flow are considered.
Research Article| December 26, 2017 Locations and Source Parameters for Calibration Events in Turkey, Saudi Arabia, Ethiopia, and Tanzania J. P. O'Donnell; J. P. O'Donnell aDepartment of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, j.p.odonnell@leeds.ac.ukbNow at School of Earth and Environment, The University of Leeds, Leeds LS2 9JT, United Kingdom. Search for other works by this author on: GSW Google Scholar S. Shamsalsadati; S. Shamsalsadati aDepartment of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, j.p.odonnell@leeds.ac.uk Search for other works by this author on: GSW Google Scholar R. A. Brazier; R. A. Brazier aDepartment of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, j.p.odonnell@leeds.ac.uk Search for other works by this author on: GSW Google Scholar A. A. Nyblade A. A. Nyblade aDepartment of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, j.p.odonnell@leeds.ac.uk Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2018) 108 (1): 145–154. https://doi.org/10.1785/0120170180 Article history first online: 26 Dec 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. P. O'Donnell, S. Shamsalsadati, R. A. Brazier, A. A. Nyblade; Locations and Source Parameters for Calibration Events in Turkey, Saudi Arabia, Ethiopia, and Tanzania. Bulletin of the Seismological Society of America 2017;; 108 (1): 145–154. doi: https://doi.org/10.1785/0120170180 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The identification of GT595% seismic events is of fundamental importance to ground‐based explosion monitoring. However, the lack of GT0 event (i.e., explosion) data needed to develop new regionally tailored GT595% event‐identification criteria hampers the identification of new events. As a means of circumventing this problem, we show that existing regionally tailored GT595% criteria are potentially transferrable to tectonic analogs. We invoke this principle to identify new calibration events in Turkey, Tanzania, and Saudi Arabia. Six events are located in the east Anatolian plateau, meeting existing GT595% criteria developed for the tectonically analogous Tibetan plateau. Nine events are located in the Tanzania craton and 15 in the Arabian shield, meeting GT595% criteria developed for the tectonically analogous Kaapvaal craton. We additionally expand the existing database of GT595% events in Ethiopia by identifying six events in the Afar region, fulfilling GT595% criteria previously developed for the Ethiopian Rift. Source properties including the moment tensor, moment magnitude, radiated energy, corner frequency, and static stress drop are determined for the larger‐magnitude events identified in each region. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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