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Arnulf, AF, Harding AJ, Kent GM, Carbotte SM, Canales JP, Nedimovic MR.  2014.  Anatomy of an active submarine volcano. Geology. 42:655-658.   10.1130/g35629.1   AbstractWebsite

Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot (northwestern USA). Our results reveal a complex melt body, which is similar to 14 km long, 3 km wide, and up to 1 km thick, beneath the summit caldera. The estimated volume of the reservoir is 18-30 km(3), more than two orders of magnitude greater than the erupted magma volumes of either the A. D. 1998 or 2011 eruption. Our images show a network of sub-horizontal to shallow-dipping (<30 degrees) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites.

Arnulf, AF, Harding AJ, Kent GM, Singh SC, Crawford WC.  2014.  Constraints on the shallow velocity structure of the Lucky Strike Volcano, Mid-Atlantic Ridge, from downward continued multichannel streamer data. Journal of Geophysical Research-Solid Earth. 119:1119-1144.   10.1002/2013jb010500   AbstractWebsite

The shallow velocity structure of the Lucky Strike segment of the Mid-Atlantic Ridge is investigated using seismic refraction and reflection techniques applied to downward continued multichannel streamer data. We present a three-dimensional velocity model beneath the Lucky Strike Volcano with unprecedented spatial resolutions of a few hundred meters. These new constraints reveal large lateral variations in P wave velocity structure beneath this feature. Throughout the study area, uppermost crustal velocities are significantly lower than those inferred from lower resolution ocean bottom seismometer studies, with the lowest values (1.8-2.2km/s) found beneath the three central volcanic cones. Within the central volcano, distinct shallow units are mapped that likely represent a systematic process such as burial of older altered surfaces. We infer that the entire upper part of the central volcano is young relative to the underlying median valley floor and that there has been little increase in the layer 2A velocities since emplacement. Layer 2A thins significantly across the axial valley bounding faults likely as the result of footwall uplift. The upper crustal velocities increase with age, on average, at a rate of similar to 0.875km/s/Myr, similar to previous measurements at fast-spreading ridges, suggesting hydrothermal sealing of small-scale porosity is progressing at normal to enhanced rates.

Bazin, S, van Avendonk H, Harding AJ, Orcutt JA, Canales JP, Detrick RS, Grp M.  1998.  Crustal structure of the flanks of the East Pacific Rise: Implications for overlapping spreading centers. Geophysical Research Letters. 25:2213-2216.   10.1029/98gl51590   AbstractWebsite

Tomographic inversion of seismic refraction data from the flanks of the East Pacific Rise (EPR), 17 degrees 15'S, shows that the thickness of layer 2 varies by as much as 500 meters off axis. A thick layer 2 is found in crust affected by migration paths of overlapping spreading centers (OSC). However, no significant variation in crustal thickness is detected throughout the study area. The crustal structure differences documented in this paper are primarily related to this paleo-tectonic setting rather than the east-west asymmetries characteristic of this region of the southern EPR.

Henig, AS, Blackman DK, Harding AJ, Canales JP, Kent GM.  2012.  Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core complex, 30 degrees N, Mid-Atlantic Ridge. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004059   AbstractWebsite

Detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the Atlantis Massif oceanic core complex (OCC), contrasting notably with its conjugate ridge flank. Multichannel seismic (MCS) data are downward continued using the Synthetic On Bottom Experiment (SOBE) method, providing unprecedented detail in tomographic models of the P-wave velocity structure to subseafloor depths of up to 1.5 km. Velocities can vary up to 3 km/s over several hundred meters and unusually high velocities (similar to 5 km/s) are found immediately beneath the seafloor in key regions. Correlation with in situ and dredged rock samples, video and records from submersible dives, and a 1.415 km drill core, allow us to infer dominant lithologies. A high velocity body(ies) found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the OCC, indicating a propagating magmatic source as the origin for this pluton(s). The western two-thirds of the Southern Ridge is capped in serpentinite that may extend nearly to the base of our ray coverage. The distribution of inferred serpentinite indicates that the gabbroic pluton(s) was emplaced into a dominantly peridotitic host rock. Presumably the mantle host rock was later altered via seawater penetration along the detachment zone, which controlled development of the OCC. The asymmetric distribution of seismic velocities and morphology of Atlantis Massif are consistent with a detachment fault with a component of dip to the southeast. The lowest velocities observed atop the eastern Central Dome and conjugate crust are most likely volcanics. Here, an updated model of the magmatic and extensional faulting processes at Atlantis Massif is deduced from the seismic results, contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow-rate spreading centers.

Arnulf, AF, Harding AJ, Singh SC, Kent GM, Crawford W.  2012.  Fine-scale velocity structure of upper oceanic crust from full waveform inversion of downward continued seismic reflection data at the Lucky Strike Volcano, Mid-Atlantic Ridge. Geophysical Research Letters. 39   10.1029/2012gl051064   AbstractWebsite

We present a fine-scale 2D velocity structure beneath the Lucky Strike Volcano on the Mid-Atlantic Ridge (MAR) using an elastic full waveform inversion (FWI) method. The FWI is a data driven procedure that allows simultaneous exploitation of both reflections and refractions energy in multi-channel seismic data to create a single self-consistent, high-resolution velocity image of the upper crust that can be used for geologic interpretation. The long-wavelength background P-wave velocity model required by the local optimization approach was created using a combination of downward continuation and 3D first-arrival travel-time tomography. The elastic waveform inversion was applied to carefully windowed downward continued data, where wide-angle reflections and refractions arrive in front of the water-wave and are thus isolated from the high-amplitude seafloor scattering energy that is particularly acute in areas of rough igneous seafloor. Waveform inversion reduces the misfit of the initial model by 76% after 19 iterations and strongly reduced the size of the residuals relative to the signal size. The final model shows fine scale structure beneath the northern part of the Lucky Strike volcano on a resolution of tens of meters. Evidence for successive lava sequences testifies to the constructional origin of the upper section of layer 2A. Normal faults are revealed within the shallow crust and are strongly correlated with seafloor observations. Citation: Arnulf, A. F., A. J. Harding, S. C. Singh, G. M. Kent, and W. Crawford (2012), Fine-scale velocity structure of upper oceanic crust from full waveform inversion of downward continued seismic reflection data at the Lucky Strike Volcano, Mid-Atlantic Ridge, Geophys. Res. Lett., 39, L08303, doi: 10.1029/2012GL051064.

Nedimovic, MR, Carbotte SM, Harding AJ, Detrick RS, Canales JP, Diebold JB, Kent GM, Tischer M, Babcock JM.  2005.  Frozen magma lenses below the oceanic crust. Nature. 436:1149-1152.   10.1038/nature03944   AbstractWebsite

The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust(1-3). Thermal modelling(4-6), tomography(7), compliance(8) and wide-angle seismic studies(9), supported by geological evidence(3,10-18), suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of similar to 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate(19) Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

Blackman, DK, Canales JP, Harding A.  2009.  Geophysical signatures of oceanic core complexes. Geophysical Journal International. 178:593-613.   10.1111/j.1365-246X.2009.04184.x   AbstractWebsite

P>Oceanic core complexes (OCCs) provide access to intrusive and ultramafic sections of young lithosphere and their structure and evolution contain clues about how the balance between magmatism and faulting controls the style of rifting that may dominate in a portion of a spreading centre for Myr timescales. Initial models of the development of OCCs depended strongly on insights available from continental core complexes and from seafloor mapping. While these frameworks have been useful in guiding a broader scope of studies and determining the extent of OCC formation along slow spreading ridges, as we summarize herein, results from the past decade highlight the need to reassess the hypothesis that reduced magma supply is a driver of long-lived detachment faulting. The aim of this paper is to review the available geophysical constraints on OCC structure and to look at what aspects of current models are constrained or required by the data. We consider sonar data (morphology and backscatter), gravity, magnetics, borehole geophysics and seismic reflection. Additional emphasis is placed on seismic velocity results (refraction) since this is where deviations from normal crustal accretion should be most readily quantified. However, as with gravity and magnetic studies at OCCs, ambiguities are inherent in seismic interpretation, including within some processing/analysis steps. We briefly discuss some of these issues for each data type. Progress in understanding the shallow structure of OCCs (within similar to 1 km of the seafloor) is considerable. Firm constraints on deeper structure, particularly characterization of the transition from dominantly mafic rock (and/or altered ultramafic rock) to dominantly fresh mantle peridotite, are not currently in hand. There is limited information on the structure and composition of the conjugate lithosphere accreted to the opposite plate while an OCC forms, commonly on the inside corner of a ridge-offset intersection. These gaps preclude full testing of current models. However, with the data in hand there are systematic patterns in OCC structure, such as the 1-2 Myr duration of this rifting style within a given ridge segment, the height of the domal cores with respect to surrounding seafloor, the correspondence of gravity highs with OCCs, and the persistence of corrugations that mark relative (palaeo) slip along the exposed detachment capping the domal cores. This compilation of geophysical results at OCCs should be useful to investigators new to the topic but we also target advanced researchers in our presentation and synthesis of findings to date.

Forsyth, DW, Scheirer DS, Webb SC, Dorman LM, Orcutt JA, Harding AJ, Blackman DK, Morgan JP, Detrick RS, Shen Y, Wolfe CJ, Canales JP, Toomey DR, Sheehan AF, Solomon SC, Wilcock WSD, Team MS.  1998.  Imaging the deep seismic structure beneath a mid-ocean ridge: The MELT experiment. Science. 280:1215-1218.   10.1126/science.280.5367.1215   AbstractWebsite

The Mantle Electromagnetic and Tomography (MELT) Experiment was designed to distinguish between competing models of magma generation beneath mid-ocean ridges. Seismological observations demonstrate that basaltic melt is present beneath the East Pacific Rise spreading center in a broad region several hundred kilometers across and extending to depths greater than 100 kilometers, not just in a narrow region of high melt concentration beneath the spreading center, as predicted by some models. The structure of the ridge system is strongly asymmetric: mantle densities and seismic velocities are lower and seismic anisotropy is stronger to the west of the rise axis.

Singh, SC, Kent GM, Collier JS, Harding AJ, Orcutt JA.  1998.  Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise. Nature. 394:874-878.   10.1038/29740   AbstractWebsite

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images(1-4) have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties(5-7). Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14 degrees 00' S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80-90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

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

Not available

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   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.

Arnulf, AF, Harding AJ, Singh SC, Kent GM, Crawford WC.  2014.  Nature of upper crust beneath the Lucky Strike volcano using elastic full waveform inversion of streamer data. Geophysical Journal International. 196:1471-1491.   10.1093/gji/ggt461   AbstractWebsite

Seismic full waveform is an emerging technique for determining the fine-scale velocity structure of the subsurface. Here, we present results of elastic full waveform inversion (FWI) along three multichannel seismic lines at the Lucky Strike volcano on the Mid-Atlantic ridge that provides a velocity image of the upper oceanic crust with unprecedented resolution (50-100 m). We have used a two-step process combining downward continuation with a time-domain, elastic FWI. The downward continuation procedure enhances the refracted arrivals and wide-angle reflections, and reduces the scattering noise due to rough seafloor. Since both sources and receivers are downward continued to the seafloor, the computational cost of FWI is reduced, as one does not need to model the thick water layer. Our results clearly demarcate two layers within seismic Layer 2A; a low-velocity, highly heterogeneous layer likely reflecting the complexity of accretion that is underlain by a more homogeneous high-velocity gradient layer. The base of Layer 2A is defined as a lithological boundary that can be offset by faulting. Thick (> 400 m) units of anomalously low-velocity material (< 2.5 km s(-1)) beneath different summital edifices on the central volcano indicate that a thick pile of high-porosity extrusive rocks can be supported without collapsing, suggesting that while in general there is pore closure with depth this is not the cause of high velocities we observe. Hydrothermal deposition sealing of small-scale porosity is shown to be a secondary process, which likely explains the upper crustal velocity increase with age, but is not responsible for the high-velocity gradient Layer 2A. Finally, the rapid thinning of the entire Layer 2A in the vicinity of the main normal faults suggests the tectonic thinning of a geologically defined layer, further confirming the lithological origin of the high-velocity gradient zone at the base of seismic Layer 2A.

Phipps Morgan, J, Harding AJ, Kent GM, Orcutt JA, Chen YJ.  1994.  An observational and theoretical synthesis of magma chamber geometry and crustal genesis along a mid-ocean ridge spreading center. Magmatic Systems. ( Ryan M, Holton J, Dmowska R, Eds.).:139-178., Burlington: Elsevier Abstract
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).

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, 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
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.

Hussenoeder, SA, Collins JA, Kent GM, Detrick RS, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1996.  Seismic analysis of the axial magma chamber reflector along the southern East Pacific Rise from conventional reflection profiling. Journal of Geophysical Research-Solid Earth. 101:22087-22105.   10.1029/96jb01907   AbstractWebsite

The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through a combination of waveform modeling the near-vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source-receiver offset (or slowness). Our results show that the AMC reflector observed along the southern EPR is best modeled by a thin (< 100 m thick) sill of partial melt (V-s not equal 0 km/s) sandwiched between higher-velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities. The comparatively high P wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content. This may have important implications for eruption mechanisms and along-axis mixing of magma at the EPR. There is no simple relationship between morphologic indicators of magma supply (e.g., axial depth or volume) and sill thickness, depth, or velocity. Magma sill properties may be closely tied to the eruption and replenishment cycle of the AMC and thus may vary on a much shorter spatial and temporal scale than axial morphology, which reflects longer-term variations in magma supply to the ridge.

Singh, SC, Collier JS, Harding AJ, Kent GM, Orcutt JA.  1999.  Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise. Geology. 27:219-222.   10.1130/0091-7613(1999)027<0219:sefahl>;2   AbstractWebsite

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50-60-m-thick roof is overlain by a 150-200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

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.

Mutter, JC, Carbotte SM, Su WS, Xu LQ, Buhl P, Detrick RS, Kent GM, Orcutt JA, Harding AJ.  1995.  Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17-Degrees-05' and 17-Degrees-35'S. Science. 268:391-395.   10.1126/science.268.5209.391   AbstractWebsite

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'5 image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions. These observations indicate that it is where the width and depth of the magma chamber differ from regional trends, indicating an enhanced magmatic budget, that is diagnostic of current magmatism.

Singh, SC, Harding AJ, Kent GM, Sinha MC, Combier V, Bazin S, Tong CH, Pye JW, Barton PJ, Hobbs RW, White RS, Orcutt JA.  2006.  Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise. Nature. 442:287-290.   10.1038/nature04939   AbstractWebsite

The determination of melt distribution in the crust and the nature of the crust - mantle boundary ( the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres(1-4) but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Van Ark, EM, Detrick RS, Canales JP, Carbotte SM, Harding AJ, Kent GM, Nedimovic MR, Wilcock WSD, Diebold JB, Babcock JM.  2007.  Seismic structure of the Endeavour Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity. Journal of Geophysical Research-Solid Earth. 112   10.1029/2005jb004210   AbstractWebsite

[ 1] 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 ( similar to 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 degrees 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.

Arnulf, AF, Singh SC, Harding AJ, Kent GM, Crawford W.  2011.  Strong seismic heterogeneity in layer 2A near hydrothermal vents at the Mid-Atlantic Ridge. Geophysical Research Letters. 38   10.1029/2011gl047753   AbstractWebsite

We present a high-resolution 3D seismic image beneath the Lucky Strike volcano on the Mid-Atlantic Ridge using streamer tomography. To obtain a high-resolution ray coverage in layer 2A, we first downward continue the multichannel seismic (MCS) data close to the seafloor generating a synthetic ocean bottom experiment (SOBE) and then apply 3D travel-time tomography. We find that the upper crust is laterally heterogeneous on 2-3 km scale, with unusually low velocities (1.8-2.2 km. s(-1)) in the upper few hundred meters beneath the Lucky Strike volcanic edifices, but normal layer 2A velocities (2.2-3.0 km. s(-1)) beneath the lava lake. The low velocities could be due to extremely high porosity (25-50%) in recently erupted, highly fractured pillow lavas. The hydrothermal vent fields seem to lie at the boundary between the high-porosity edifices and the lower porosity lava lake. We have also imaged a reflector at the base of the volcanic edifices that is distinct from the deeper high-velocity gradient transition zone from layer 2A to 2B imaged so far. The new technique provides an image of the oceanic crust with resolutions comparable to that of seafloor geology, leading to new insight about volcanic and hydrothermal processes. Citation: Arnulf, A. F., S. C. Singh, A. J. Harding, G. M. Kent, and W. Crawford (2011), Strong seismic heterogeneity in layer 2A near hydrothermal vents at the Mid-Atlantic Ridge, Geophys. Res. Lett., 38, L13320, doi:10.1029/2011GL047753.

Nedimovic, MR, Carbotte SM, Diebold JB, Harding AJ, Canales JP, Kent GM.  2008.  Upper crustal evolution across the Juan de Fuca ridge flanks. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc002085   AbstractWebsite

Recent P wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2A doubles or reaches similar to 4.3 km/s found in mature crust in < 10 Ma after crustal formation. This velocity change is commonly attributed to precipitation of low-temperature alteration minerals within the extrusive rocks associated with ridge-flank hydrothermal circulation. Sediment blanketing, acting as a thermal insulator, has been proposed to further accelerate layer 2A evolution by enhancing mineral precipitation. We carried out 1-D traveltime modeling on common midpoint supergathers from our 2002 Juan de Fuca ridge multichannel seismic data to determine upper crustal structure at similar to 3 km intervals along 300 km long transects crossing the Endeavor, Northern Symmetric, and Cleft ridge segments. Our results show a regional correlation between upper crustal velocity and crustal age. The measured velocity increase with crustal age is not uniform across the investigated ridge flanks. For the ridge flanks blanketed with a sealing sedimentary cover, the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks (similar to 60% increase versus similar to 28%) over the same crustal age range of 5-9 Ma. Extrapolation of velocity-age gradients indicates that layer 2A velocity reaches 4.3 km/s by similar to 8 Ma on the sediment blanketed flanks compared to similar to 16 Ma on the flanks with thin and discontinuous sediment cover. The computed thickness gradients show that layer 2A does not thin and disappear in the Juan de Fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust. However, layer 2A on the fully sedimented ridge-flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks (330 +/- 80 versus 430 +/- 80 m). The change in thickness occurs over a 10-20 km distance coincident with the onset of sediment burial. Our results also suggest that propagator wakes can have atypical layer 2A thickness and velocity. Impact of propagator wakes is evident in the chemical signature of the fluids sampled by ODP drill holes along the east Endeavor transect, providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration.