Mantle flow and the geological record, dynamical mechanisms for continental epeirogeny

grantor: University of TorontoThe geologic record shows that continents have periodically experienced episodes of large-scale subsidence and uplift which result in widespread marine transgressions. The focus of my thesis is to consider the dynamic topography caused by fluid convection in the mantle as a mechanism for these enigmatic epeirogenic motions. The first part of the thesis examines subsidence at continental margins and the role of subduction-induced flow in producing the surface deflections. Numerical simulations of mantle convection in a Cartesian geometry are adopted to model the flow dynamics associated with stiff subducting slabs. An analysis of the stratigraphy and paleotectonics of the Russian Platform during the Paleozoic reveals a correlation between subsidence of the platform and subduction events at the continental margin. Numerical models of mantle flow and dynamic topography are computed and the predictions are able to reconcile the observed sedimentation history. The Late Paleozoic subsidence of the Karoo Basin is also considered in this context. Numerical simulations indicate that dynamic tilting caused by subduction-induced mantle flow works in tandem with flexural loading of the lithosphere to explain the subsidence and uplift history of the basin. The latter half of the thesis focuses on the interaction of various classes of mantle flow with the endothermic phase change at 660 km depth and investigates the associated surface manifestations. A model of fluid convection in axisymmetric spherical geometry is introduced to simulate flow regimes in a multi-phase mantle. My numerical simulations show that 'mantle avalanche' events are capable of causing long-wavelength lithospheric subsidence with vertical amplitudes of ~2500 m over timescales of 150-200 m.y. Based on these results, it is speculated that an avalanche event may provide a plausible explanation for the Devonian evolution of the Laurussian supercontinent. I also consider the dynamic topography associated with more 'generic' mantle flow regimes, namely 'stiff' descending plumes and slabs. The plume/slab flow models are found to induce wide (~1000 km) topographic deflections of amplitude ~1 km which persist over timescales of 100-150 m.y. Simple models of sedimentation in these dynamic depressions suggest that these classes of mantle flow may be a plausible mechanism for the enigmatic development of transient intracratonic basins.Ph.D

Transient Injection of Flow: How Torn and Bent Slabs Induce Unusual Mantle Circulation Patterns Near a Flat Slab

Abstract Torn and bent slabs are usually associated with flat‐slab subduction where the descending plate develops a horizontal geometry beneath the overlying continent. How such slab dynamics modify the surrounding mantle flow and the overriding plate remains enigmatic. Here, we conduct three‐dimensional subduction numerical experiments to investigate the flat slab to steep‐angle slab transition region and examine the impact of slab geometry changes on mantle flows. The results show that the along‐strike change due to flattening a segment of slab induces oblique flow toward the mantle corner at the transition region where flat slab bends to steep‐subducting slab. Slab tears can occur due to the buoyancy contrast between an oceanic ridge and the surrounding dense oceanic crust and/or presence of weak zones. A vertical tear at the side edge of a flat slab causes toroidal flow around the steep‐angle slab where sub‐slab mantle floods rapidly through the tear. Flow through the tear spreads to all directions including upwelling toward the continental base that may trigger slab and partial melting and thus affect arc magmatism. A horizontal tear that occurs ahead of the flat portion where slab resumes its steep‐angle results in enhanced plate‐motion parallel flow. Notably, the tear‐induced flow behaves as a rapid injection through the slab breach with the peak velocity up to an order of magnitude higher than plate motion, lasting for 1–2 million years after initially tearing. This rapid pulse flow might be recorded in surface tectonics as distinct transient events of topography or metamorphism

Postcollisional lithospheric evolution of the Southeast Carpathians: Comparison of geodynamical models and observations

Seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the Southeast Carpathians region. A series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface-crustal response to lithosphere delamination and slab break-off. The results show that a delamination-type removal (“plate-like” migrating instability) causes a characteristic pattern of surface uplift/subsidence and crustal extension/shortening to occur due to the lithospheric deformation and dynamic/thermal forcing of the sublithospheric mantle. These features migrate with the progressive removal of the underlying lithosphere. Model results for delamination are comparable with observables related to the geodynamic evolution of the Southeast Carpathians since 10 Ma: the mantle structure inferred by seismic tomography, migrating patterns of uplift (>1.5 km) and subsidence (>2 km) in the region, crustal thinning in the Carpathian hinterland and thickening at the Focsani depression, and regional extension in the Carpathian corner (e.g., opening of Brasov basin) correlating with volcanism (e.g., Harghita and Persani volcanics) in the last 3 Myr

Lasting mantle scars lead to perennial plate tectonics

Mid-ocean ridges, transform faults, subduction and continental collisions form the conventional theory of plate tectonics to explain non-rigid behaviour at plate boundaries. However, the theory does not explain directly the processes involved in intraplate deformation and seismicity. Recently, damage structures in the lithosphere have been linked to the origin of plate tectonics. Despite seismological imaging suggesting that inherited mantle lithosphere heterogeneities are ubiquitous, their plate tectonic role is rarely considered. Here we show that deep lithospheric anomalies can dominate shallow geological features in activating tectonics in plate interiors. In numerical experiments, we found that structures frozen into the mantle lithosphere through plate tectonic processes can behave as quasi-plate boundaries reactivated under far-field compressional forcing. Intraplate locations where proto-lithospheric plates have been scarred by earlier suturing could be regions where latent plate boundaries remain, and where plate tectonics processes are expressed as a ‘perennial’ phenomenon

Exploring the theory of plate tectonics: the role of mantle lithosphere structure

This review of the role of the mantle lithosphere in plate tectonic processes collates a wide range of recent studies from seismology and numerical modelling. A continually growing catalogue of deep geophysical imaging has illuminated the mantle lithosphere and generated new interpretations of how the lithosphere evolves. We review current ideas about the role of continental mantle lithosphere in plate tectonic processes. Evidence seems to be growing that scarring in the continental mantle lithosphere is ubiquitous, which implies a reassessment of the widely held view that it is the inheritance of crustal structure only (rather than the lithosphere as a whole) that is most important in the conventional theory of plate tectonics (e.g. the Wilson cycle). Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures and, as such, linked to the Wilson cycle and inheritance. We consider the current fundamental questions in the role of the mantle lithosphere in causing tectonic deformation, reviewing recent results and highlighting the potential of the deep lithosphere in infiltrating every aspect of plate tectonics processes