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No description available.

Whether by accretion, magmatic addition, or refinement of more mafic lithologies, continental arcs are likely zones for the creation of "average" continental crust with intermediate silica content. This dissertation contains the results of broadband seismic studies carried out in two field areas, an active subduction zone and the remnants of an extinct arc, with the aim of understanding lithospheric evolution at convergent margins. The analytical techniques of receiver function calculation and surface wave tomography are applied to data sets collected above the Andean subduction zone in Chile and western Argentina and in the Coast Mountains Batholith of central British Columbia. We present the first in-depth comparison of receiver functions calculated using the high frequencies available in records of intermediate-depth local earthquakes with those calculated from the lower frequency data in records of larger teleseismic events. The comparison reveals that the lower crust beneath the Western Sierras Pampeanas contains a gradational velocity increase over ~20km above a small velocity step at the Moho. Surface wave tomography confirms the existence of an unusually high velocity anomaly in the mantle above the slab and yields estimates of slab thickness on the order of 50 km. To the south of the flat slab region, we see evidence of active mantle wedge convection above the steep slab, but no evidence of the lithosphere-asthenosphere boundary beneath the subducting Nazca plate. In the Coast Mountains Batholith (CMB), receiver functions image a bright, continuous Moho throughout the study region. Combined with petrologic modeling, the receiver function data point toward convective removal of any ultramafic root that formed beneath the CMB. Low absolute shear wave velocities in the upper mantle resolved via surface wave analysis strengthen the case for root removal beneath the eastern section of the CMB. On the far western edge of the CMB, we find evidence of a partially reformed lithosphere outboard of a major tectonic boundary. These observations shed light on the distillation of felsic to intermediate continental crust from more mafic primary magmas in active subduction zones and the eventual return of the complementary ultramafic residuals to the convecting mantle.

The Cape Verde Rise and Islands are considered to be the result of 'Hot Spot' activity. The results of reheating can be observed by changes in the physical properties of the lithosphere. The purpose of this work is to study the relationship of the islands to the rise and relate any changes in the physical properties of the lithosphere to its thermal structure. A geophysical investigation of the lithosphere of the Cape Verde Rise has been carried out using measurements of total magnetic field, seismic reflection profiles, sonobuoy wide angle reflection/refraction experiments, free air gravity and bathymetry from surface ships. Geoid height data from the GE0S3 and SEASAT satellite missions have also been used. Total magnetic field anomalies are attributed to sea floor spreading during the Mesozoic and used to show that 4 fracture zone traces exist in the vicinity of the archipelago. The history of spreading is comparable to corresponding studies in the Western North Atlantic. Seismic stratigraphy from continuous reflection profiles and velocities from wide angle reflection/refraction studies are consistent with uplift during the Early Miocene followed by a period of island building volcanism. Depths to oceanic layer ?, after correction for sediment loading. show that the rise is.;2 km shallower than expected for Mesozoic crust. The present depth is equivalent to only 25 Ma oceanic crust. A 1-dimensional examination of the relationship between free air gravity anomalies and bathymetry using linear transfer function techniques indicates that the lithosphere of the Cape Verde Rise has an effective elastic thickness (EET) of only 15_t3 km when compared with a thin plate flexure model. This value of EET is less than expected for Mesozoic crust from a global compilation of EFT against age of crust at the time of loading. Using geoid height data the above analysis has been extended to 2-dimensions and a value of 13 km has been observed for the EET. The same thin plate flexure model was used for the geoid response as was used for the gravity. The Cape Verde Rise is associated with both a reduction in EFT and a considerable depth anomaly. This is considered to be evidence for thermal rejuvenation. Hawaii, however, is only associated with a depth anomaly and no substantial reduction in EET is observed. This difference between the effect of 'Hot Spot' activity on Hawaii and on the Cape Verde Rise is considered to be due to the motion of the oceanic plate over the mantle heat source.

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The Drybones Bay kimberlite has offered an opportunity to compare geophysical observations of the mantle lithosphere beneath the southwest Slave craton with geochemical and petrological interpretations through mantle xenoliths and xenocrysts. Coarse-textured garnet and spinel-garnet peridotite xenoliths, and garnet and garnetclinopyroxene xenocrysts were analyzed for a suite of trace elements. The Ni-in-garnet geothermometer (Canil, 1999) yielded equilibration temperatures between 850-1200 DC, and corresponding depths of 90-160 km, assuming a 41m Wm-2 paleogeotherm. This method estimates the thickness of the southwest Slave craton in the Ordovician at approximately 160 km, within the stability field of diamond. Trace element analysis revealed a distinct geochemical anomaly at a depth of approximately 120 km, which marks the transition from wehrlite to metasomatized wehrlite and lherzolite. This anomalous depth has been observed in geophysical studies, and has been interpreted as a structural discontinuity. / Graduate / 0372

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No description available.