An exploration of the effect of anisotropy on seismic tomography in different geological settings

Abstract

Seismic tomography is used in wide ranges to understand the inner structure of Earth based on the dependence of seismic velocity to temperature and other factors such as composition, volatile content, and seismic anisotropy. Despite it is well known that the mantle is seismically anisotropic, the isotropic mantle is often assumed in seismic tomography. In this dissertation, we explore the upper mantle velocity structure and seismic anisotropy in three distinct tectonic settings: 1) relic subduction zone in the westernmost Mediterranean, 2) isostatically-unbalanced the Moroccan Atlas Mountains in the intraplate of Africa, and 3) geologically active Central Appalachian Mountains in the passive margin of North America. Taking a benefit of the dense seismic coverage and extraordinary shear-wave splitting (SWS) observation (i.e., large splitting time and an arcuate pattern of FPD) along the tight Gibraltar Arc in the western Mediterranean, we show that unaccounted-for seismic anisotropy may have been mapped as an artificial low-velocity structure below the subducted Alboran slab. Also, considering seismic anisotropy is predominantly from the alignment of the seismically fast axis of olivine in mantle parallel to the mantle deformation, it is suggested that the mantle is less entrained below the slab and perhaps toroidal mantle flow is more dominant in the region. This leads us to explore the low-velocity structure below the Moroccan Atlas of Northern Africa, where is known to be isostatically unbalanced. From incorporating a seismic anisotropy model inspired by SWS observation, we find that the low-velocity structure is consistent with Canary Hotspot origin and supporting the high topography of the mountains instead of its crustal root. As the volume of the low-velocity structure decreases significantly in the depth > 90 km, we find that seismic anisotropy, which is possibly aligned with the flow direction of the anomalous mantle, may have contributed to the low-velocity structure is in the tomography. Meanwhile, we explore the velocity structures of the Central Appalachian Mountains and vicinity. We find the low-velocity structure below the Central Appalachian Mountains and it is spatially matching with the steep increase of attenuation. In addition, considering the anisotropy, we are convinced that the asthenospheric upwelling is imaged as the low-velocity structure in our detailed tomography and it may have contributed to the change in measured anisotropy in the eastern region of the mountain. The exploration of velocity structure and seismic anisotropy in the tomography across the three regions helps us to understand the mantle dynamics of different tectonic settings.