Seismic Study of the Crust and Mantle Lithosphere in Cameroon,West Africa.pdf

1、Therefore,in two of the three studies that form this dissertation,I focused on developing high-resolution3-D models of radial and azimuthal anisotropic from both ambient seismic noise and earthquake data.My goal is to un

2、derstand 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 st

3、udy area.The3-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 obs

4、erved 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 aniso

5、tropy 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

6、 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 anisotro

7、py varies systematically from predominantly negative in the middle crust to positive in the lower crust.The3-D model of S-wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and

8、earthquake surface wave dispersion revealed slow phase velocities along the CVL in contrast with the neighboring Congo Craton,in agreenment with previous studies.Apart from the Congo Craton and the Oubanguides Belt,the u

9、ppermost 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

10、 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

11、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

12、 of deformation and revealed that the mantle lithosphere has been deformed.
　　The third study included in this thesis is remarkable in two aspects.It involved the use of multiple seismic datasets and improved inversio

13、n methodologies to refine previously existing1-D shear wave velocity models and estimate new crustal parameters.The new library of1-D Vs models shows significant improvement over the previous1-D models,places tighter con

14、straints 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 dispers

15、ion data at several grid nodes,I am able to image the precise geometry and constrain the locations of mafic bodies(Vs of3.7-4kms-1)that intruded into the upper crust(2-8km)during the formation of Gondwana.Investigating t

16、he 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,ther

17、e are no clear variations trend in the southem part of the Cameroon Volcanic Line,the Congo Craton and the Oubanguides Belt.The inverted Vp/Vs ratio shows variation that ranged from1.67-1.85with uncertainties that are ge