Multi-observable probabilistic inversion for the thermochemical structure of the lithosphere

The present-day thermal, physical and compositional structure of the Earth's crust and upper mantle is of fundamental importance in geosciences. It does not only provide insights into the forces that drive tectonic activity in the Earth but it can also assist on the discovery of deep mineral resources. In this thesis, I further develop a recent multi-observable probabilistic inversion method [1{3] and focus on its application to produce realistic and well-constrained estimates of the thermophysical and chemical structure of the crust and uppermost mantle in the following three regions: i) the western U.S, ii) the Western Australian Craton (WAC) and iii) the North China Craton (NCC). Geophysical observables used in this study (when available) include Rayleigh and Love dispersion curves, P-wave receiver functions, geoid height, absolute elevation and surface heat ow. These data sets are jointly inverted through a thermodynamically- and internally-consistent approach within a probabilistic framework. 

The application of this inversion approach to the Arizona Transition Zone (ATZ) of the western U.S and its immediate adjacent areas (e.g. southern Colorado Plateau) confirm a significant crustal thickening from ~ 28 km in the SW of the Arizona Transition Zone and southern Basin and Range to ~ 48 km beneath the southern Colorado Plateau. Inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere-asthenosphere boundary is not deeper than ~ 70 km in most of the region. We find that major pre-Cambrian surface lineaments and/or shear zones (e.g. Bright Angel Fault system, Holbrook lineament) separate crustal domains with distinct bulk properties, suggesting that the juxtaposed crustal blocks still retain, at least in part, their original characteristics. However, widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths, locally overprinting their original compositions and creating highly heterogeneous crustal sections. A dominant and large-scale internal crustal pattern of SW dipping planes/structures is evident in our models. This can reconcile the current inconsistency in predicted deep fault orientations from earthquake focal mechanisms and from large surface lineaments.

In a much larger area in the western U.S, encompassing the Central and Southern Rocky Mountains, Great Plains, the Rio Grande Rift, Colorado Plateau and some parts of the Basin and Range, our 3D thermophysical crustal structure reveals an excellent correlation between the locations of Cenozoic volcanism and areas that are characterized by low crustal densities (or velocities), high temperatures and higher topography (e.g., southern Rockies, boundaries of the CP, etc.). The crustal model exhibit large lateral variability at short wavelengths, with some clear indications of intra-crustal magmatic intrusions and the presence of partial melt. Our density model negatively correlates with topography in the Southern Rocky Mountains, suggesting that crustal buoyancy plays a major role in supporting the high topography. In the Colorado Plateau area, a mantle contribution to elevation is more prominent. 

In Western Australia, where previous estimates of the seismic LAB and temperature structure are highly variable, our inversion shows a relatively thin crust (~ 35.6 km) and a sharp Moho interface, compatible with an undisturbed mid-Archean crust. Thermal and compositional structure reveals a thick (~ 278 km), signi_cantly cold and highly depleted lithosphere (Mg # 92). The obtained radial anisotropy structure casts some doubt on previous definitions of the Lehmann discontinuity and shows horizontal rock fabric in the lower crust and lithospheric mantle (Vsh > Vsv).  

In the NCC, an integrated thermochemical model is presented. Consistent with previous results, we find widespread lithospheric modification/erosion in the eastern NCC. In this part of the NCC, the lithosphere is thin (_ 100 km) and chemically fertile (refertilized), in accordance with independent xenolith evidence. A high temperature anomaly in the sub-lithospheric mantle is imaged beneath and around the cratonic keel of the Western NCC and its distribution correlates well with the location of recent volcanism in the region. This anomaly seems to be associated with upwellings of sub-lithospheric material driven by the large-scale circulation of the Pacific slab. The shallowest parts of the sub-lithospheric upwelling create forced downwellings and erosion of the basal parts of the lithosphere, visible in our models.