Crust-Lithosphere-Asthenosphere Dynamics in Mantle Plume Provinces with Emphasis on the Galápagos

Hotspot tracks, which most geoscientists attribute to the effects of mantle plumes on the overlying lithospheric plates, are characterized by distinct bathymetry, gravity signatures, structural geology, volcanology, petrology, and geochemistry; motivating us to try to understand the dynamics behind the space-time-histories of these systems. Making use of classical geodynamic paradigms, such as highly-viscous fluids (Stokes flow, for the mantle and/or lithosphere), elastic plate behavior, and heat flow, we develop conceptual frameworks to explain a number of distinct hotspot track features, and present quantitative models aimed at elucidating their origins./nThere is much diversity among the population of mantle plume hotspot tracks on the Earth's oceanic crust. For example, there are marked differences in the style of their bathymetry, as well as in their gravimetric signals, and also in the isotopic signature of extruded lavas. At the same time, important underlying differences are given by the age of the lithospheric plates under which the mantle plumes are impinging, lithospheric elastic thickness, the heat (or buoyancy) flux of individual mantle plumes, their melt production, crustal thickening, the proximity of spreading centers, etc. In the first chapter of this dissertation, making use of scaling theory, we show that for most oceanic hotspot tracks, the character of bathymetric expression (primarily rough vs. smooth topography) can be explained by three independent primary underlying factors – plate thickness, or equivalently plate age; plate speed; and plume buoyancy flux – combined into a single parameter, R, the ratio of plume heat flux to the effective thermal capacity of the moving plate overlying the plume./nThe Galápagos archipelago (off the west coast of equatorial South America), part of a >20 Ma old hotspot track formed by the underlying Galápagos mantle plume, currently exhibits a broad geographic distribution of volcanic centers of surprisingly variable age, unusual spatial patterns of geochemical enrichment, spectacular and enigmatic bathymetric features, important lithospheric and elastic thickness discontinuities, pronounced regional faults, the presence of the Nazca-Cocos spreading center nearby, and a Nazca plate – Galápagos hotspot relative velocity that varied through time. These factors have combined to create what is arguably the most complex mantle plume province on Earth, rivaled in this regard perhaps only by the Reunion and the Kerguelen-Ninetyeast hotspot provinces. The present-day Galápagos archipelago sits over a broad massive platform that has been formed primarily by intrusion and secondarily by extrusion, and that exhibits remarkable bathymetric gradients (comparable to those on the Hawaiian hotspot track). In the second chapter of this dissertation we show that some of these bathymetric features may be reasonably explained in the context of thermo-mechanical processes occurring on locally-weak crust/lithosphere, responding to internal gradients in lithostatic stresses, and that these processes of Quaternary history continue to the present day./nStudies in evolutionary biology, dating back to Darwin’s famous discoveries, have shown that many of the endemic Galápagos species (flora and fauna) must have evolved from species derived largely from South and Central America. Thus there is much scientific incentive to complement current phylogenetic knowledge regarding the origins of these endemic Galápagos species, with state-of-the-art geophysical models for the emergence and subsidence of the islands habitat on which these species must have evolved, most of which is no longer above sea level ! The third chapter of this dissertation represents a preliminary effort in this direction, combining multiple sources of dynamic topography during and following the formation of the Carnegie/Cocos ridges, which were constructed by the Galápagos mantle plume. We show that plate tectonic reconstructions, mantle plume dynamics, and crustal processes combined can reasonably account for changes in elevation along the Carnegie Ridge, that strongly support the idea that seamounts along most of this ridge were above sea level. In the regard, we note the irony that many Galápagos species, such as the famous marine iguanas, are older than the Galápagos Islands themselves, an observation explained perhaps by understanding the dynamic history of the Galápagos-Carnegie hotspot track through time, conforming to a spatio-temporal progression defined by the Nazca plate – Galápagos hotspot relative motion. These findings coming purely from geophysics, support a likely hypothesis/scenario of South American species migrating over now-submerged paleo-archipelagos along the Carnegie Ridge, finally giving rise to the present-day Galápagos flora and fauna.

Advisor: RICHARDS, MARK