Van Ark Emily M.

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Van Ark
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Emily M.
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  • Article
    Upper crustal structure and axial topography at intermediate spreading ridges : seismic constraints from the southern Juan de Fuca Ridge
    (American Geophysical Union, 2005-12-14) Canales, J. Pablo ; Detrick, Robert S. ; Carbotte, Suzanne M. ; Kent, Graham M. ; Diebold, John B. ; Harding, Alistair J. ; Babcock, Jeffrey M. ; Nedimovic, Mladen R. ; Van Ark, Emily M.
    We use multichannel seismic reflection data to image the upper crustal structure of 0-620 ka crust along the southern Juan de Fuca Ridge (JdFR). The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow (Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment, to 2.3 km at the northern end near the site of the 1980’s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 km to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250-300 m thick at the ridge axis along the Cleft segment, and 300-350 m thick along the axis of the Vance segment. However off-axis layer 2A is similar in both segments (500-600 m), indicating ~90% and ~60% off-axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3-4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.
  • Article
    Seismic structure of the Endeavour Segment, Juan de Fuca Ridge : correlations with seismicity and hydrothermal activity
    (American Geophysical Union, 2007-02-03) Van Ark, Emily M. ; Detrick, Robert S. ; Canales, J. Pablo ; Carbotte, Suzanne M. ; Harding, Alistair J. ; Kent, Graham M. ; Nedimovic, Mladen R. ; Wilcock, William S. D. ; Diebold, John B. ; Babcock, Jeffrey M.
    Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9–1.4 s (∼2.1–3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4–1.2 km wide and appears to dip 8–36° to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 ± 0.09 s (380 ± 120 m) below the seafloor on the along-axis line and 0.38 ± 0.09 s (500 ± 110 m) on the cross-axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9–2.4 s below the seafloor. By projecting hypocenters of well-located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal-related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.
  • Article
    Time variation in igneous volume flux of the Hawaii-Emperor hot spot seamount chain
    (American Geophysical Union, 2004-11-09) Van Ark, Emily M. ; Lin, Jian
    Satellite gravity, ship track bathymetry, sediment thickness, and crustal magnetic age data were combined to calculate the residual bathymetry and residual mantle Bouguer gravity anomaly (RMBA) for the northwest Pacific Ocean. The Hawaii-Emperor hot spot track appears on the RMBA map as a chain of negative anomalies, implying thickened crust or less dense mantle. The hot spot swell is clearly visible in a broad band of half-width ∼500 km for about 2000 km downstream from the current hot spot location, corresponding to hot spot ages of 0–25 Ma. A much narrower expression of the hot spot is visible for the rest of the chain at hot spot ages of 25–80 Ma. Comparison of the observed RMBA with various compensation models reveals that the relatively narrow features of the Hawaii-Emperor seamounts are best explained as being supported by plate flexure, while the Shatsky Rise, Hess Rise, and Mid-Pacific Mountains oceanic plateaus are best fit by Airy isostasy with a thickened crustal root. Amplitude comparisons between the RMBA predictions of various compensation models and the observed RMBA for the Hawaiian swell are ambiguous. However, on the basis of the shape of the predicted anomalies, we favor a model of flexure in response to a buried load at 120 km depth. We further calculate igneous (i.e., crustal) volume flux along the axis of the Hawaii-Emperor hot spot by integrating cross-sectional areas of gravity-derived excess crustal thickness and seafloor elevation, respectively, with respect to the normal oceanic crust. The highest values of the calculated igneous volume flux along the Hawaiian and Emperor ridges (∼8 m3/s) occur at present and at about 20 Ma. The flux was reduced to only 50% of this maximum (∼4 m3/s) at 10 Ma. The calculated igneous volume flux is systematically smaller (maximum values of ∼4 m3/s) along the Emperor ridge. Overall, the Hawaiian and Emperor ridges appear to have experienced quasi-periodic variations in fluxes on timescales of 6–30 Ma. Furthermore, during the low-flux periods at 25–48, 57, and 75 Ma the height and size of individual hot spot seamounts appear to be noticeably less than those of the high-flux periods. We hypothesize that the variations in the fluxes of the Hawaiian ridge might be controlled by the thickness of the overlying lithosphere at the time of hot spot emplacement, while the variations along the Emperor ridge may be influenced by the dynamics of the slow absolute motion of the hot spot at the time.
  • Thesis
    Seismic and gravitational studies of melting in the mantle's thermal boundary layers
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2007-06) Van Ark, Emily M.
    This thesis presents three studies which apply geophysical tools to the task of better understanding mantle melting phenomena at the upper and lower boundaries of the mantle. The first study uses seafloor bathymetry and small variations in the gravitational acceleration over the Hawaii-Emperor seamount chain to constrain the changes in the igneous production of the hot spot melting in the mantle which has created these structures over the past 80 My. The second study uses multichannel seismic reflection data to constrain the location and depth of axial magma chambers at the Endeavour Segment of the Juan de Fuca spreading ridge, and then correlates these magma chamber locations with features of the hydrothermal heat extraction system in the upper crust such as microseismicity caused by thermal cracking and high temperature hydrothermal vent systems observed on the seafloor. The third study uses two-dimensional global pseudospectral seismic wave propagation modeling to characterize the sensitivity of the SPdKS seismic phase to two-dimensional, finite-width ultra-low velocity zones (ULVZs) at the core-mantle boundary. Together these three studies highlight the dynamic complexities of melting in the mantle while offering new tools to understand that complexity.