Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

Scott M White

Abstract

The morphological and structural characteristics of the global spreading ridge system are systematically related to spreading rate and magma supply (e.g. Macdonald, 1982). Because intermediate spreading-rate ridges (ISRs) spreading between 60 and 80 mm/yr have characteristics of both fast and slow spreading ridges, they are ideal environments to study the differences controlling upper crustal accretion. We examine the distribution of seamounts along spreading ridges to understand the relationship between seamount volcanism and axial morphology along four ISRs: the Juan de Fuca Ridge (JdFR), the Galápagos Spreading Centers (GSC), the Southeast Indian Ridge (SEIR), and the Eastern Lau Spreading Centers (ELSC). Before determining the distribution of volcanoes on the seafloor, the Modified Basal Outlining Algorithm (MBOA) developed by Bohnenstiehl et al. (2012) is assessed for its accuracy in picking volcanic edifices relative to a traditional field-based geologic mapping approach at the Springerville Volcanic Field, Arizona. There, MBOA is able to closely match the geologic map, under-predicting height and area by 4% and over-predicting volume by 13%. Applying MBOA to determine the seamount abundance, volume and spatial distribution along the ISRs indicate that (1) seamounts do not form at the ends of segments along the JdFR, GSC or ELSC, but do form at the ends of segments along the SEIR, (2) a systematic relationship exists between axial morphology and ridge migration in the hotspot reference frame along the SEIR, and (3) segments offset in the direction of ridge migration have axial highs along the leading segment end and axial valleys along the trailing segment end. Given that ridge migration oblique to spreading direction causes enhanced melting beneath the leading segment end (Katz et al., 2004), we attempt to understand the formation of faults when the near-field spreading direction is oblique to the far-field stretching direction (i.e., the direction plates are pulled apart). Three dimensional finite-element modeling of an idealized MOR indicates that faults slip in a direction halfway between the spreading and stretching directions outside of the axial graben. Inside the axial graben, faults slip in a direction approaching the stretching direction. This indicates simply pulling the lithosphere causes the orientation of volcanic and tectonic structures to change without changing the underlying structure of the upwelling asthenosphere.

Rights

© 2013, Julia Kathryn Howell

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