The CAFE Experiment : a joint seismic and MT investigation of the Cascadia Subduction System

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013In this thesis we present results from inversion of data using dense arrays of collocated seismic and magnetotelluric stations located in the Cascadia subduction zone region of central Washington. In the migrated seismic section, we clearly image the top of the slab and oceanic Moho, as well as a velocity increase corresponding to the eclogitization of the hydrated upper crust. A deeper velocity increase is interpreted as the eclogitization of metastable gabbros, assisted by fluids released from the dehydration of upper mantle chlorite. A low velocity feature interpreted as a fluid/melt phase is present above this transition. The serpentinized wedge and continental Moho are also imaged. The magnetotelluric image further constrains the fluid/melt features, showing a rising conductive feature that forms a column up to a conductor indicative of a magma chamber feeding Mt. Rainier. This feature also explains the disruption of the continental Moho found in the migrated image. Exploration of the assumption of smoothness implicit in the standard MT inversion provides tools that enable us to generate a more accurate MT model. This final MT model clearly demonstrates the link between slab derived fluids/melting and the Mt. Rainier magma chamber.Funding for this work was made possible by the American Society for Engineering education through a National Defense Science and Engineering Fellowship, and by the National Science Foundation through two grants for the CAFE and CAFE MT projects

Joint seismic and MT investigation of the Cascadia subduction system

Thesis (Ph. D. in Geophysics)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013.Page 176 blank. Cataloged from PDF version of thesis.Includes bibliographical references.In this thesis we present results from inversion of data using dense arrays of collocated seismic and magnetotelluric stations located in the Cascadia subduction zone region of central Washington. In the migrated seismic section, we clearly image the top of the slab and oceanic Moho, as well as a velocity increase corresponding to the eclogitization of the hydrated upper crust. A deeper velocity increase is interpreted as the eclogitization of metastable gabbros, assisted by fluids released from the dehydration of upper mantle chlorite. A low velocity feature interpreted as a fluid/melt phase is present above this transition. The serpentinized wedge and continental Moho are also imaged. The magnetotelluric image further constrains the fluid/melt features, showing a rising conductive feature that forms a column up to a conductor indicative of a magma chamber feeding Mt. Rainier. This feature also explains the disruption of the continental Moho found in the migrated image. Exploration of the assumption of smoothness implicit in the standard MT inversion provides tools that enable us to generate a more accurate MT model. This final MT model clearly demonstrates the link between slab derived fluids/melting and the Mt. Rainier magma chamber.by R. Shane McGary.Ph.D.in Geophysic

Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4230–4253, doi:10.1002/2014GC005509.Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.Phil Wannamaker and Virginie Maris gratefully acknowledge funding by the U.S. National Science Foundation under grants EAR08–43725 and EAR08–38043 through the Earthscope and Geophysics programs. The 2D inversion capability received development support under U.S. Department of Energy contract DE-PS36-04GO94001. Rob Evans was supported through Earthscope grant EAR08–44041 and Shane McGary through a National Defense Science and Engineering Graduate (NDSEG) fellowship. Fieldwork in Canada was made possible by an NSERC Discovery Grant and a Canadian Foundation for Innovation award to Martyn Unsworth.2015-05-1

Investigating Subsurface Void Spaces and Groundwater in Cave Hill Karst Using Resistivity

Grand Caverns Natural National Landmark lies in the southeastern Shenandoah Valley and is home to the oldest show cave in the United States. The park and adjacent private lands include a complex of five known caves: Grand Caverns, Madison Cave, Steger’s Fissure, Jefferson Cave, and Fountain Cave, all encompassed within the northern section of Cave Hill. The cave complex lies below a series of sinkholes that run in two approximately north to south parallel lines. One sinkhole, the site of a U.S. Geological Survey study, was selected as a location of interest due to the possible existence of a perched aquifer between it and Grand Caverns below. The southern section of Cave Hill lacks sinkholes and large known caverns, although a few smaller caves exist in the area. The southern section does contain a large karstic swale feature, which we selected as a second location of interest to look for unknown void spaces associated with groundwater. Understanding groundwater distribution can give insight into the relationship between the geomorphologic karst features at the surface and the caves below. We conducted an electrical resistivity survey to investigate the spatial relationship between groundwater and karstic features at both locations. Electrical resistivity is useful in karst because we can generate a model of resistivity distribution in the subsurface that allows us to identify groundwater and void spaces based on the conductive nature of water and the high resistance of air filled voids. A total of ten resistivity lines, consisting of 14, 28, and 56 -electrodes spaced 6.25 m apart, were deployed in and/or around the features at both locations. Both dipole-dipole and Schlumberger arrays were collected during each deployment; these arrays were then merged and inverted using AGI EarthImager 2D-Inversion Software. Inverted resistivity sections were correlated to geologic cross-sections, high resolution airborne-derived LiDAR digital elevation models, known surface features, and cave depth survey data. By using the different data types, we were able to correlate many of the features present in the resistivity sections. The results indicate that bedding geometry and rock type are the dominate factors that define groundwater distribution and type of karstic features found within Cave Hill. In the northern section, the images showed several perched aquifers, two of which were dry, situated above the Grand Caverns and correlated with individual sinkholes. We also imaged the water table approximately 61 meters below the surface. The images correlated to the digital elevation model and cross-sections suggests that the sinkholes formed along a calcareous arenite confining ridge to the east which blocks the northwest flow of surface water off Cave Hill. In the southern section, the resistivity sections suggest that groundwater flows along bedding planes with trend of a antiform fold axis which is situated approximately parallel to and beneath the swale feature. The images also show that long and narrow void spaces exist beneath the areas of groundwater saturation on the flanks and within the swale. These void spaces appear larger in areas where water enters beneath the swale and in areas where the swale diverges course