Thesis defense
Student: Jamison Faustino Gomes de Assunção
Program: Geophysics
Title: “Sensitivity Analysis of Lithospheric Convergence Velocity in Numerical Simulations of Self-Sustained Andean-Type Subduction”
Advisor: Prof. Dr. Victor Sacek - IAG/USP
Judging Committee:
- Prof. Dr. Victor Sacek – Presidente e Orientador
- Prof. Dr. Claudio Alejandro Salazar Mora - IGc/USP
- Dr. Andrea Piccolo - University of Leeds
- Dr. Nicolas Riel - Johannes Gutenberg-Universität Mainz
- Dr. Rafael Monteiro da Silva - IAG/USP (Pós-Doc)
Abstract
In an Andean-type subduction system, where an oceanic lithosphere dips beneath a continental lithosphere, the convergence speed is predominantly governed by ridge push and slab pull forces. However, numerical models have shown that the subduction style is significant sensitive to the geometry and rheology of the mantle. This thesis aims to shed light on how subduction dynamics is affected by changes in both geometry and rheology, and explore an approach for simulating subduction that by incorporating an effective partial melt zone (PMZ). For this, the geodynamic finite difference numerical code LaMEM was used, which allows the simulation of thermo-mechanical evolution of the crust and mantle in the geological time scale. Several sets of simulations were performed with different oceanic plate lengths at the surface (OPLS), ranging from 1000 to 5000 km. The overall subduction pattern displayed a relatively rapid acceleration of the subducting plate, followed by a deceleration of approximately the same duration until the plate reached the lower mantle. This was then followed by a stable phase with minimal variation in the convergence velocity between the subducting lithosphere and the overriding continental lithosphere. A linear relationship was found between the OPLS and the average convergence speed after the plates reach the 660-km discontinuity. The average convergence velocity decreased by a rate of approximately 1.3 cm/yr per each additional 1000 km of OPLS. Furthermore, it was observed that the convergence speeds remained consistently stable over time, supporting the hypothesis of sustained convergence rates for tens of millions of years. The sensitivity of the average convergence velocity to variations in lower mantle viscosity was also tested. A decrease in the viscosity from 1.5×10^22 to 0.5×10^22 Pa·s increased the convergence speed of the subducting plate–after reaching the 660-km discontinuity–by up to a factor of three, hindering convergence stability. The effect of a PMZ on convergence speedand subduction kinematics was also tested. The results indicate that introducing a PMZ, modeled via asthenospheric rheology scaling, does not significantly alter the convergence speed stability or affect the kinematics. Instead, it primarily mimics the behavior of a scenario with a smaller oceanic plate. Lastly, the simulations reproduced an Andean-type subduction system, capturing the onset uplift of a ∼3 km-high cordillera, regional subsidence in the interior of the continent, and reproduced the amplitude of the mid-ocean ridges consistent with observed elevation patterns in normal and flat-slab segments.
Keywords: subudction, numerical model, finite difference method, Andean-type subduction, low viscosity zone