Seismic Study Of The Crust And Mantle Lithosphere In Cameroon,West Africa

One of the main goals of seismology is to use seismic waves emanating from active and passive sources to make inferences about the properties and structure of the Earth and substantial successes have been recorded in recent decades.Some of these can be attributed to the rapid advancement in computing technology and the ever increasing deployment of dense seismic networks at different locations on the earth.The latter has led to the availability of quality seismic data and allowed a great variety of seismic methods to be applied.Among these methods,seismic tomography is known to be very powerful for imaging the earth's structure and resolving volumetric heterogeneities.Using this method,a handful of studies have been performed using data recorded by the Cameroon Broadband Seismic Experiment(CBSE)with the goal of investigating the origin of the enigmatic Cameroon Volcanic Line.These studies have resulted into the development of several geodynamic models involving plume and non-plume models to explain the origin of the Cameroon Volcanic Line(CVL)and other tectonic features in Cameroon.However,questions remain as to which of the model best explains the origin of the hotspot intra-plate volcanism and has led to a series of scientific debate in literature without reaching a consensus.Numerous laboratory experiments and petrophysical studies have shown that most of the rocks forming the earth's crust and mantle are anisotropic in nature and their preferred orientation of minerals;small scale compositional layering,oriented system of cracks,micro-cracks and fractures can give a clue about their formation and geodynamic evolution.However,detailed study revealing the depth variation of seismic anisotropy(both radial and azimuthal anisotropy)in the crust and mantle lithosphere in Cameroon has not been performed.Therefore,in two of the three studies that form this dissertation,I focused on developing high-resolution 3-D models of radial and azimuthal anisotropic from both ambient seismic noise and earthquake data.My goal is to understand the structure and deformation of the Cameroon lithosphere in sufficient details to resolve the competing geodynamic models for the origin of the CVL and advance understanding of other tectonic features in the study area.The 3-D radial anisotropic model reveals the spatial variation of strong to weak positive(Vsh>Vsv)and negative(Vsv>Vsh)radial anisotropy in the Cameroon crust.In the upper crust,the negative radial anisotropy observed at the location of a mantle plume around Mt.Oku is attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material.Likewise,a strong positive radial anisotropy is observed on the crustal suture located between the Congo Craton and the Oubanguides Belt indicates the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone.The middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow-induced alignment of anisotropic minerals that crystallized during magma intrusion.The magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust.The 3-D model of S-wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion revealed slow phase velocities along the CVL in contrast with the neighboring Congo Craton,in agreement with previous studies.Apart from the Congo Craton and the Oubanguides Belt,the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere.The direction of fast axis in the upper crust is mostly NE-SW,but trending approximately N-S around Mt.Oku and the southern CVL.The observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities.A widespread zone of weak-to-zero azimuthal anisotropy in the mid-lower crust shows evidence for vertical mantle flow or isotropic mid-lower crust.In the uppermost mantle,the fast axis direction changed from NE-SW to NW-SE around Mt.Oku and northern Cameroon.This suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed.NE-SW fast azimuths are observed beneath the Congo Craton and are consistent with the absolute motion of the African plate,suggesting a mantle origin for the observed azimuthal anisotropy.Our tomographically derived fast directions are consistent with the local SKS splitting results in some locations and depths,enabling us to constrain the origin of the observed splitting.The different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in Cameroon.The third study included in this thesis is remarkable in two aspects.It involved the use of multiple seismic datasets and improved inversion methodologies to refine previously existing 1-D shear wave velocity models and estimate new crustal parameters.The new library of 1-D Vs models shows significant improvement over the previous 1-D models,places tighter constraints on the uppermost crustal structure and reveals a better spatial correlation with known tectonic features.By interpolating the ellipticity measurements and performing joint inversions with group and phase dispersion data at several grid nodes,I am able to image the precise geometry and constrain the locations of mafic bodies(Vs of 3.7-4 kms-1)that intruded into the upper crust(2-8 km)during the formation of Gondwana.Investigating the relationship between crustal parameters,I found a positive correlation between the inverted Moho depth and surface topography within the Adamawa Plateau and the Garoua Rift in agreement with Airy isostasy.However,there are no clear variations trend in the southern part of the Cameroon Volcanic Line,the Congo Craton and the Oubanguides Belt.The inverted Vp/Vs ratio shows variation that ranged from 1.67-1.85 with uncertainties that are generally less than 0.05 at crustal depths.This may suggest a felsic crustal bulk composition and high upper-crustal temperatures in Cameroon compared to the average continental crust.The combination of the results in this thesis with previous studies reveals an increasing evidence for the existence of a small scale upper mantle convection that is possibly driven by the variable lithospheric thickness between the southern Congo craton and the neighboring regions.However,future studies involving the deployment of both ocean bottom seismometer and land seismic stations will make it possible to completely image both the continental and oceanic sectors of the CVL and test the proposed hypothesis before making a final conclusion and bringing the long time scientific debate to an end.