Rohr Kristin Marie Michener

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Rohr
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Kristin Marie Michener
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  • Article
    Focused fluid flow along the Nootka fault zone and continental slope, explorer-Juan de Fuca Plate Boundary
    (American Geophysical Union, 2020-08-07) Riedel, Michael ; Rohr, Kristin Marie Michener ; Spence, George D. ; Kelley, Deborah S. ; Delaney, John R. ; Lapham, Laura L. ; Pohlman, John W. ; Hyndman, Roy D. ; Willoughby, Ele C.
    Geophysical and geochemical data indicate there is abundant fluid expulsion in the Nootka fault zone (NFZ) between the Juan de Fuca and Explorer plates and the Nootka continental slope. Here we combine observations from >20 years of investigations to demonstrate the nature of fluid‐flow along the NFZ, which is the seismically most active region off Vancouver Island. Seismicity reaching down to the upper mantle is linked to near‐seafloor manifestation of fluid flow through a network of faults. Along the two main fault traces, seismic reflection data imaged bright spots 100–300 m below seafloor that lie above changes in basement topography. The bright spots are conformable to sediment layering, show opposite‐to‐seafloor reflection polarity, and are associated with frequency reduction and velocity push‐down indicating the presence of gas in the sediments. Two seafloor mounds ~15 km seaward of the Nootka slope are underlain by deep, nonconformable high‐amplitude reflective zones. Measurements in the water column above one mound revealed a plume of warm water, and bottom‐video observations imaged hydrothermal vent system biota. Pore fluids from a core at this mound contain predominately microbial methane (C1) with a high proportion of ethane (C2) yielding C1/C2 ratios <500 indicating a possible slight contribution from a deep source. We infer the reflective zones beneath the two mounds are basaltic intrusions that create hydrothermal circulation within the overlying sediments. Across the Nootka continental slope, gas hydrate‐related bottom‐simulating reflectors are widespread and occur at depths indicating heat flow values of 80–90 mW/m2.
  • Technical Report
    A geophysical survey within the Mesozoic magnitic anomaly sequence south of Bermuda
    (Woods Hole Oceanographic Institution, 1979-12) Purdy, G. Michael ; Rohr, Kristin Marie Michener
    This geophysical survey of an approximate 1° square covers Mesozoic magnetic anomalies M0, M2, and M4 south of Bermuda. Bathymetry, magnetics, seismic reflection profiling, and seismic refraction data are presented. The isochron trend within the survey area at magnetic anomaly M4 time is 025°. Two left lateral fracture zones exist: the southern fracture zone has an offset of <10 km at M4 time and 33 km at M0 time. The northern fracture zone has an offset of 37 km at M4 time and 26 km at M0 time. These changes in offset are accounted for by asymmetric spreading, an 11° change in trend of anomaly M0 relative to M4, and by M0 time, growth of a small right lateral fracture zone. Seismic refraction data provide poor control on the shallow crustal structure but suggest the presence of significant lateral inhomogeneities within layer 2.
  • Thesis
    A study of the seismic structure of upper oceanic crust using wide-angle reflections
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1983-01) Rohr, Kristin Marie Michener
    The lateral homogeneity of oceanic crust on the scale of a seismic experiment is a condition that most methods of seismic interpretation depend on. Whether this condition is in fact true is largely unknown and only recently have efforts been made to test this hypothesis. This thesis is part of that effort and is focussed on determining with as much resolution as possible the seismic structure of upper oceanic crust, i.e. Layer 1 and the uppermost part of Layer 2. This portion of the crust is of interest, because of the effect of the sediment-basement interface on the transmission and conversion of seismic energy, also because of the possibility of detecting lateral heterogeneities in upper Layer 2 caused by faulting, hydrothermal circulation etc. The data employed are a set of wide-angle reflections from oceanic crust 130 m.y. old in the western North Atlantic Ocean southwest of Bermuda. First, the sedimentary structure is determined by stacking the data along hyperbolae and interpreting the stacking velocities and two-way normal incidence travel-times for interval velocities. This method has not been applied to deep sea marine data before; it gives a more detailed velocity structure of the sediments than does a traditional study of the basement reflections' travel-times. Second, the same data are mapped into tau-p space in order to measure the velocity gradient in oceanic basement; unfortunately the scatter in the tau-p picks caused by the topography of the basement reflector combine with the properties of the tau-sum inversion to make such a measurement impossible. Third, the amplitudes of the basement reflections observed on three seismic lines are modelled by synthetic seismograms; each can be matched by velocity-depth models which contain a transition zone between the sediments and the basement. The different thicknesses of this transition zone near the three receivers is an indication that the top few hundred meters of Layer 2 are laterally heterogeneous on a scale of 3 to 8 km.