Interactions between a flat slab and overriding plate: Controlling factors of subduction dynamics and continental deformation
Creators: Liu, Xiaowen
Publisher: name: University of Alberta Library
Year: 2022

<p>Subduction zones are active plate margins that have an important role in mantle and lithosphere processes. Subduction is driven by the negative buoyancy of the downgoing oceanic plate, resulting in a steep descent angle at most subduction zones. However, in approximately 10% of modern subduction zones, shallow to flat-slab subduction occurs. In these regions, the oceanic plate unbends to become subhorizontal, underplating the continental plate for several hundred kilometers inboard of the plate margin at depths of 45-150 km. Development of a flat slab is generally characterized by migration and termination of arc magmatism, reduced continental surface heat flow, and thick-skinned deformation in the continental interior. The mechanisms that lead to flat-slab subduction include anomalous buoyancy of the oceanic plate, high trenchward velocity of the continental plate, and increased suction force in the mantle wedge. Recent studies have made significant progress in mapping flat slabs and understanding how a flat slab develops. However, there are many fundamental problems that are still not well understood, such as why the depth of the flat slab varies between different regions, and how flat slabs affect the thermal structure and deformation of the continental upper plate.<br>
This thesis uses 2D thermal-mechanical models to investigate flat-slab dynamics and the interactions between a flat slab and the overlying continent. Models focus on regions of flat-slab subduction in western North and South America, where slab flattening is driven by a combination of trenchward continental motion and subduction of a buoyant aseismic oceanic ridge. The models show that flat-slab depth is primarily determined by the initial thickness of the overriding plate, such that the slab is deeper below an initially thick (cool) continent. The depth is also controlled by the strength of the continental mantle lithosphere, whereby a weak (hydrated) lithosphere is able to be displaced by the flat slab. Through reasonable variations in these parameters, the depth of the flat slab varies by more than 100 km, in good agreement with observed depths. The models also indicate that the observed low surface heat flow in flat-slab regions does not solely result from conductive cooling by the cold slab. The continental lithosphere cools over 10’s of millions of years through conductive heat transfer after flat-slab emplacement. The timescale of cooling is mainly controlled by flat-slab depth, with an earlier onset of cooling and a greater amount of cooling for a shallower slab. The magnitude of cooling is also enhanced by longer slab emplacement times, faster convergence rate, and an older oceanic plate. The presence of a flat slab can also trigger thick-skinned deformation of the continent 100’s of km from the plate margin. Deformation is the result of both high compressional stresses through end-loading at the distal plate margin and the presence of pre-existing weak structures in the continent. Deformation is modulated by dynamic topography associated with the flat slab, where dynamic uplift inhibits deformation. The model results are in good agreement with geological and geophysical observations from flat-slab regions. This thesis provides new insights into the flat-slab regions in both North and South America and deepens our understanding of flat-slab subduction dynamics and the origin of thermal anomalies and thick-skinned deformation of the Laramide Orogeny in North America and the Sierras Pampeanas in South America.</p>