No description available.

No description available.

No description available.

Mantle seismic tomography has historically relied on radially symmetric ID velocity models to trace ray paths through the mantle. The resulting travel time residuals are used to invert for seismic velocity perturbations around this 1D model. However, we know the Earth deviates from such ID velocity models; for example there are global variations in crustal thickness; in the age of oceanic lithosphere and presence of subducting oceanic lithosphere. In light of this, an a priori model which incorporated the three types of surface observable heterogeneity outlined above was constructed as part of this thesis. Tracing ray paths through this more heterogeneous starting model resulted in new travel time residuals which were subsequently employed in a simultaneous tomographic inversion solving for earthquake relocation parameters and slowness perturbations. This inversion method allows us to investigate whether tomography using a priori models results in improved images of mantle velocity perturbations and systematic earthquake relocations. A graphical earthquake browser was specifically written to establish, in a consistent manner, the shape of subducting oceanic lithosphere for all the major subduction zones. The resulting population of earthquakes, which best represent the shape of Wadati-Benioff zones, were subsequently interpolated into profiles following the path of oceanic lithosphere as it subducts. The temperature field in and around each profile was generated using a new analytic solution of the heat equation for subducting lithosphere, adapted to incorporate slab shape. The upper mantle a priori model was constructed on an equal area tomographic grid by combining the thermal models of the subducting lithosphere, plate cooling models of oceanic lithosphere and variations in crustal thickness away from that prescribed in a ID velocity model. Efficient 20 ray tracing through the a priori model was achieved via the adaptation of a ID ray tracer by perturbing the reference ID model, iasp91, using the a priori velocities in the cells connecting the event to the recording station for each ray. A new travel time residual was calculated and subsequently used in the simultaneous solution for slowness perturbation and earthquake relocation. So as not to bias the earthquake relocation procedure, phases were selected so as to maximise the azimuth and epicentral distance coverage, while minimising the number of duplicated ray paths which would be redundant in the inversion. The data selection resulted in some 3,450 events emitting 785,000 teleseismic P phases (bottoming in the lower mantle). The cell based SIRT inversion procedure, used to solve the standard system of linear tomographic equations, was augmented by explicit damping and smoothing matrices so as to control both poorly resolved cells and the relative importance between earthquake relocation parameters and slowness perturbations. For comparison, the ray population was also traced through the 3SMAC upper mantle model before undertaking a similar inversion. The 5° x 5° equal area, 100 km thick, cell inversions resulted in systematic earthquake relocations with an average relocation distance of= 5 km. In the upper mantle, the inversion procedure adjusts the a priori subducting slab velocity contrast, revealing images of subducting oceanic lithosphere. In the lower mantle, there is little difference between inversions produced in this thesis and those available digitally. Some of the main features are the pronounced lineations interpreted as the Farallon slab (beneath North and South America) and the Tethys (beneath Eurasia) clearly imaged between 1200 and 1500 km depth. All inversions undertaken in this thesis image hotspots throughout the upper mantle, and in places these pronounced slow features are observed passing through the upperllower mantle transition. A section through the South Pacific superswell images slow material as a continuous body, to at least 1300 km. Synthetic recovery tests indicate these hotspot features are well resolved.

No description available.

No description available.

Geological, geophysical, and geochemical evidence indicates that a significant portion of the continental mantle lithosphere may be absent in a number of regions near plate boundaries or plate interiors. Delamination and viscous Rayleigh-Taylor instability (“dripping”) are widely cited to account for the missing lithosphere, however these removal processes are poorly constrained. This thesis examines the dynamics of delaminating and dripping mantle lithosphere, in particular focusing on the response of the crust to underlying lithospheric removal. Using forward computational models, I explore whether certain (surface) geological observables may be diagnostic of either removal mechanism. Surface topography associated with delamination has a broad zone of uplift above the lithospheric gap and a mobile zone of subsidence at the delamination hinge, whereas with dripping lithosphere, the topographic expression is symmetric and fixed above the mantle lithosphere downwelling. The pattern of crustal deformation is also distinctly asymmetric with delamination compared to dripping lithosphere. Expanding on these results, I investigate whether present day geological geophysical observables in Eastern Anatolia are consistent with delamination of the mantle lithosphere. Experimental results demonstrate that well-developed plateau uplift, syn-convergent extension, and crustal thinning in the central part of the Anatolian plateau are consistent with a topographic profile at longitude 42ºE and a geologically interpreted zone of syn-convergent extension in eastern Anatolia. With three-dimensional physical scaled analogue modeling experiments, I consider the process of oceanic plate subduction evolving into continental delamination. Model results show that slower plate convergence with retreating ocean lithosphere subduction can develop into delamination,whereas for the experiments with higher plate convergence, the crust above the consumed mantle lithosphere becomes accreted on the retro-plate similar to flake tectonics. The results suggest that delamination is a process analogous to subduction retreat; however, delamination involves decoupling of the retreating mantle lithosphere slab from the buoyant continental crust.

Geological, geophysical, and geochemical evidence indicates that a significant portion of the continental mantle lithosphere may be absent in a number of regions near plate boundaries or plate interiors. Delamination and viscous Rayleigh-Taylor instability (“dripping”) are widely cited to account for the missing lithosphere, however these removal processes are poorly constrained. This thesis examines the dynamics of delaminating and dripping mantle lithosphere, in particular focusing on the response of the crust to underlying lithospheric removal. Using forward computational models, I explore whether certain (surface) geological observables may be diagnostic of either removal mechanism. Surface topography associated with delamination has a broad zone of uplift above the lithospheric gap and a mobile zone of subsidence at the delamination hinge, whereas with dripping lithosphere, the topographic expression is symmetric and fixed above the mantle lithosphere downwelling. The pattern of crustal deformation is also distinctly asymmetric with delamination compared to dripping lithosphere. Expanding on these results, I investigate whether present day geological geophysical observables in Eastern Anatolia are consistent with delamination of the mantle lithosphere. Experimental results demonstrate that well-developed plateau uplift, syn-convergent extension, and crustal thinning in the central part of the Anatolian plateau are consistent with a topographic profile at longitude 42ºE and a geologically interpreted zone of syn-convergent extension in eastern Anatolia. With three-dimensional physical scaled analogue modeling experiments, I consider the process of oceanic plate subduction evolving into continental delamination. Model results show that slower plate convergence with retreating ocean lithosphere subduction can develop into delamination,whereas for the experiments with higher plate convergence, the crust above the consumed mantle lithosphere becomes accreted on the retro-plate similar to flake tectonics. The results suggest that delamination is a process analogous to subduction retreat; however, delamination involves decoupling of the retreating mantle lithosphere slab from the buoyant continental crust.

Seismic data from 31 seismic stations, consisting of 16 SPREE (Superior Province Rifting Earthscope Experiment) and 15 TA (Transportable Array) instruments located from 80 - 97W and 41 – 55N were used to measure the lateral variation in the lithospheric fabric beneath the Superior Province and the Mid-Continent Rift (MCR). I made shear wave splitting measurements of SK(K)S phases by using the eigenvector minimization approach of Silver and Chan (1991). Error surfaces for multiple events were stacked in back-azimuthal swaths to examine directional variability. A single anisotropic layer model is sufficient to explain my data. My results show a high split time in the western Superior Province (WSP), very weak splits in the Nipigon Embayment and a moderate split in the eastern Superior. I observed low split times in the Penokean, Yavapai and Matzazal Provinces. A region of very low split is newly detected by this study immediately to the east of Lake Superior. The MCR shows moderate to low split times. There are subtle variations in the direction of the fast shear wave across the study region. The fast directions align with the direction of the absolute plate motion and the direction of tectonic boundaries in most regions. Lateral variation of anisotropy and lithospheric fabric is observed across the study area. The strong fabric observed in the western Superior is truncated to its east and to its south. I interpret southward truncation to be due to the Mid-Continent Rift. My result shows that lithospheric fabric in the Nipigon Embayment (NE) located just east of the WSP has been lost or seriously modified. The NE is interpreted to be an hotspot feature, which may have initiated the MCR. Moreover, the result of this study suggests that the lithosphere in the MCR may have been thinned or modified though not as much as the lithosphere of the NE. The newly discovered localized low split zone northeast of the MCR is similar in split time and extent to the feature in the NE. The relatively weak split in the eastern Superior Province may possibly be attributed to partial loss or modification of preexisting fabric resulting from the Great Meteor hotspot track.

As part of the southern Ontario POLARIS project, this thesis uses magnetotelluric methods to investigate the lithospheric architecture of the Proterozoic Grenville Province and its margin with the Archean Superior Province. The first multi-dimensional crustal and lithospheric resistivity images for this region are presented. The resistivity structure of the Phanerozoic sedimentary rocks in the lower Great Lakes region was determined using 1-D methods. The responses are strongly affected by a 20-23 S conductive layer within the sedimentary rocks, interpreted to be associated with Upper Ordovician shale units. This layer excludes resolution of resistivity structure of underlying crust. The resistivity structure of the Precambrian crust and lithosphere was determined using 2-D methods. Different strike azimuths were determined for the crust, the upper lithospheric mantle and the deeper mantle layer. The crustal resistivity model for a profile from 50oN79oW to 43oN76oW images resistive Laurentian margin rocks dipping southeast to the base of the crust, bounded by the Grenville Front and the Central Metasedimentary Belt Boundary Zone. In a 2-D model of the mantle lithosphere for the same profile, a conductor at 70-150 km depth, located along-strike from the Mesozoic Kirkland Lake and Cobalt kimberlite fields, is interpreted to be due to mantle re-fertilization. Results from multiple MT profiles indicate conductive (<10 Ω.m) lithospheric mantle beneath the Central Metasedimentary Belt and show that the northwestern Grenville Province is characterized by large-scale, resistive lithosphere (>10,000 Ω.m) extending for about 300 km beneath the Grenville Province and 800 km along strike. Lithospheric thickness is interpreted to be 280 km; local decreases in this depth are attributed to refertilization of the lower mantle lithosphere by fluids associated with Cretaceous kimberlite magmatism. Anisotropic 2-D modeling reveals minimal electrical anisotropy (<10%) at mantle depths in contrast to the factor of 15 anisotropy determined in earlier 1-D studies. This result suggests that observed MT response anisotropy is caused by large-scale structures. Strike direction in the upper lithospheric mantle is interpreted to be related to the Archean fabric of the Superior craton and in the deeper, conductive, mantle it is interpreted to have been established in the Cretaceous.