Thermo-Mechanical Properties of Earth’s Crust and Mantle: From Surface Deformation to Lower Mantle Structures in the Present Day and the Geological Past

This thesis comprises three distinct studies, each drawing on multiple, diverse datasets and modeling approaches to investigate the thermo-mechanical behavior of Earth's crust and mantle in different tectonic regimes. The first study addresses the longstanding debate regarding the stratified rheological structure of the Tibetan lithosphere using constraints from postseismic deformation and climate-driven hydrological loading at a range of timescales. By leveraging multiple geodetic datasets and geological records of time-dependent surface deformation, with a focus on the postseismic deformation following the 2021 Mw 7.4 Maduo earthquake, this study proposes a regional model that can explain observations across spatial and temporal scales more consistently. Our results reveal a low-viscosity zone in the middle crust of Tibet (20–40 km depth), consistent with many previous studies. However, the viscosity we infer for this layer is sufficiently high to imply strong mechanical coupling between the upper and lower crust (or the under-thrusting Indian lithosphere). The second study assesses the role of hydrous material in seismic detectability of slabs in Earth's lower mantle from a mineral physics perspective. It highlights the need to consider both temperature and composition when interpreting seismic observations for identifying structures, such as paleoslabs. Recent experiments show that hydrous phases (e.g., δ-Fe13 and (Al,Fe)-phase H) can coexist with basaltic assemblages under lower mantle conditions, and can thus alter the aggregate properties of slab material. Using mineral physics modeling, which incorporates new experimental data for these hydrous phases, this study examines the thermo-elastic and geophysical properties of lower mantle petrologies, including hydrous slabs, in the context of recent observations of seismic scatterers and reflectors, geodynamical constraints, and global tomography. Most prominently, it shows that hydrous phases can render cold subducted slabs seismically “invisible”, challenging the common assumption that slabs are distinctly faster than the ambient mantle at all mantle depths. The third study investigates the vertical motions of the Pacific seafloor in the Cretaceous using a newly developed Bayesian inversion method (InvoPlates) that constrains mantle thermal anomalies using lithosphere subsidence records over millions of years, present-day bathymetry, and paleo-carbonate compensation depths (CCD). The west-central Pacific’s anomalously shallow bathymetry (“Pacific Superswell”) and anomalous subsidence of atolls and guyots have been attributed to the "Darwin Rise", postulated to be a large superswell in the Cretaceous. This study constrains the spatio-temporal aspects of the Darwin Rise, which shows close correlation with the Ontong-Java Plateau formation, while de-tangling intrinsic plate cooling from lithosphere re-heating. It also challenges the traditional paleo-CCD reconstructions which link temporal CCD changes solely to changing ocean chemistry, key in models of long-term global carbon cycle, without considering time-varying plate-age independent vertical motions of the seafloor.