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Canales, JP, Singh SC, Detrick RS, Carbotte SM, Harding A, Kent GM, Diebold JB, Babcock J, Nedimovic MR.  2006.  Seismic evidence for variations in axial magma chamber properties along the southern Juan de Fuco Ridge. Earth and Planetary Science Letters. 246:353-366.   10.1016/j.epsl.2006.04.032   AbstractWebsite

Multichannel seismic data collected along the Cleft segment on the southern Juan de Fuca Ridge show that this intermediate-spreading center is underlain by a mid-crustal reflector interpreted as the top of an axial magma chamber (AMC). The AMC reflection is present along most of the segment, and deepens gently from 2.0 km near the southern end of the segment beneath the RIDGE Cleft Observatory Site, to 2.3 km at the northern end beneath the site of the mid-1980s submarine eruption. We analyzed the one-dimensional seismic structure of the AMC at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields. At the northern site, waveform modeling in the time intercept-slowness (tau-p) domain indicates that the AMC is similar to 100 m thick and it is characterized by a decrease in P-wave velocity from 6 km/s to 3.7 km/s. In contrast, the P-wave velocity within the shallower, similar to 100-m-thick AMC at the southern site is higher (5.0 km/s). The decrease in seismic velocity within the AMC indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate-spreading segments. The data show a coherent seismic phase interpreted as the P- to S-wave conversion at the AMC (PAMCS). Stacking of this event shows that the PAMCS is only present along the northern part of the segment. Our results thus suggest along-axis variations in the crystallinity of the AMC. The AMC along Cleft varies from a high crystal content (< 30% melt) sill at the southern end of Cleft, to a largely melt (60-75%) sill at the source of the 1980s eruption at the northern end. The variations in magma chamber properties inferred from our seismic data correlate with changes in lava chemistry and with the location of hydrothermal plumes, and they all suggest that focused, high-temperature hydrothermal venting along intermediate-spreading ridges is closely linked to the physical state of the underlying magma chamber. (c) 2006 Elsevier B.V. All rights reserved.

Canales, JP, Detrick RS, Bazin S, Harding AJ, Orcutt JA.  1998.  Off-axis crustal thickness across and along the East Pacific Rise within the MELT area. Science. 280:1218-1221.   10.1126/science.280.5367.1218   AbstractWebsite

Wide-angle seismic data along the Mantle Electromagnetic and Tomography (MELT) arrays show that the thickness of 0.5- to 1.5-million-year-old crust of the Nazca Plate is not resolvably different from that of the Pacific Plate, despite an asymmetry in depth and gravity across this portion of the East Pacific Rise. Crustal thickness on similarly aged crust on the Nazca plate near a magmatically robust part of the East Pacific Rise at 17 degrees 15'S is slightly thinner (5.1 to 5.7 kilometers) than at the 15 degrees 55'S overlapping spreading center (5.8 to 6.3 kilometers). This small north-south off-axis crustal thickness difference may reflect along-axis temporal variations in magma supply, whereas the across-axis asymmetry in depth and gravity must be caused by density variations in the underlying mantle.

Canales, JP, Detrick RS, Carbotte SM, Kent GM, Diebold JB, Harding A, Babcock J, Nedimovic MR, van Ark E.  2005.  Upper crustal structure and axial topography at intermediate spreading ridges: Seismic constraints from the southern Juan de Fuca Ridge. Journal of Geophysical Research-Solid Earth. 110   10.1029/2005jb003630   AbstractWebsite

[1] We use multichannel seismic reflection data to image the upper crustal structure of 0 - 620 ka crust along the southern Juan de Fuca Ridge. 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 1980s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 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 similar to 90% and similar to 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.

Canales, JP, Carton H, Mutter JC, Harding A, Carbotte SM, Nedimovic MR.  2012.  Recent Advances in Multichannel Seismic Imaging for Academic Research in Deep Oceanic Environments. Oceanography. 25:113-115. AbstractWebsite
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Carbotte, SM, Nedimovic MR, Canales JP, Kent GM, Harding AJ, Marjanovic M.  2008.  Variable crustal structure along the Juan de Fuca Ridge: Influence of on-axis hot spots and absolute plate motions. Geochemistry Geophysics Geosystems. 9   10.1029/2007gc001922   AbstractWebsite

Multichannel seismic and bathymetric data from the Juan de Fuca Ridge (JDFR) provide constraints on axial and ridge flank structure for the past 4-8 Ma within three spreading corridors crossing Cleft, Northern Symmetric, and Endeavour segments. Along-axis data reveal south-to-north gradients in seafloor relief and presence and depth of the crustal magma lens, which indicate a warmer axial regime to the south, both on a regional scale and within individual segments. For young crust, cross-axis lines reveal differences between segments in Moho two-way traveltimes of 200-300 ms which indicate 0.5-1 km thicker crust at Endeavour and Cleft compared to Northern Symmetric. Moho traveltime anomalies extend beyond the 5-15 km wide axial high and coincide with distinct plateaus, 32 and 40 km wide and 200-400 m high, found at both segments. On older crust, Moho traveltimes are similar for all three segments (similar to 2100 +/- 100 ms), indicating little difference in average crustal production prior to similar to 0.6 and 0.7 Ma. The presence of broad axis-centered bathymetric plateau with thickened crust at Cleft and Endeavour segments is attributed to recent initiation of ridge axis-centered melt anomalies associated with the Cobb hot spot and the Heckle melt anomaly. Increased melt supply at Cleft segment upon initiation of Axial Volcano and southward propagation of Endeavour segment during the Brunhes point to rapid southward directed along-axis channeling of melt anomalies linked to these hot spots. Preferential southward flow of the Cobb and Heckle melt anomalies and the regional-scale south-to-north gradients in ridge structure along the JDFR may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution.

Carbotte, SM, Detrick RS, Harding A, Canales JP, Babcock J, Kent G, van Ark E, Nedimovic M, Diebold J.  2006.  Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge. Geology. 34:209-212.   10.1130/g21969.1   AbstractWebsite

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1-2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is similar to 50-200 m deep and 1-8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheollogy of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Chereskin, TK, Harding AJ.  1993.  Modeling the Performance of an Acoustic Doppler Current Profiler. Journal of Atmospheric and Oceanic Technology. 10:41-63.   10.1175/1520-0426(1993)010<0041:mtpoaa>2.0.co;2   AbstractWebsite

A systematic examination of measurement error in acoustic Doppler current profiler (ADCP) velocity estimates, in the limit of large signal-to-noise ratio, is made using a system model and sonar signal simulations coupled into an ADCP. The model is extremely successful in predicting ADCP performance. The signal simulations provide model validation. Three main sources of error are examined: frequency tracking, measurement variance (inherent variance of pulse-to-pulse incoherent volume reverberation), and measurement bias. A theoretical lower bound on measurement variance is directly tested by coupling simulated signals into an ADCP. The observed measurement variance is approximately twice the theoretical value and varies as the inverse of the product of the pulse and averaging period (bin). Model predictions of velocity errors for back-to-back beam pairs measuring a sequence of increasing velocity-shear profiles in a medium of randomly distributed scatterers are in excellent agreement with errors measured from simulated signals coupled into an ADCP. Trade-offs between velocity error, vertical and temporal resolution, and maximum range are discussed, with specific focus on optimizing parameters available to users of commercial instruments. For reasonable parameter choices in low velocity-shear ocean conditions, the predicted error in horizontal velocity from effects considered in this study is 1-2 cm s-1. In large-shear conditions, the predicted error using the same parameters as in low shear is much worse, approximately 10 cm s-1. Optimal parameter choices, however, can reduce the error in large-shear conditions to 1-4 cm s-1.

Chereskin, TK, Levine MD, Harding AJ, Regier LA.  1989.  Observations of Near-Inertial Waves in Acoustical Doppler Current Profiler Measurements Made During the Mixed Layer Dynamics Experiment. Journal of Geophysical Research-Oceans. 94:8135-8145.   10.1029/JC094iC06p08135   AbstractWebsite

Measurements of upper ocean shear made during the Mixed Layer Dynamics Experiment (MILDEX) provide evidence of large horizontal scale motion at near-inertial frequency. The measurements consist of shipboard acoustic Doppler current profiles. Four large-scale spatial surveys of 2–4 days duration were made by the R/V Wecoma as a set of boxes approximately 60 km per side around a drifting current meter buoy. Velocity time series from the drifting buoy and from sonar measurements made from FLIP also indicated the presence of motions at near-inertial frequency. Horizontal length and time scales of the motion are estimated from the phase of the shear vector measured during the spatial surveys. Estimates of the length scale of the waves range from 500 to 1000 km, and the frequency is approximately 1.1f. The behavior of the phase is found to be consistent with a model of narrow-band inertial waves with vertical structure such that there is a zero crossing in velocity at the base of the mixed layer (40–60 m).

Chereskin, TK, Harding AJ.  1992.  A Model Approach To Predicting Errors In Acoustic Dopler Current Profiles. OCEANS '92. 'Mastering the Oceans Through Technology'. Proceedings.. 2:602-606.   10.1109/oceans.1992.607650   Abstract

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