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A
Arnulf, AF, Harding AJ, Kent GM, Wilcock WSD.  2018.  Structure, seismicity, and accretionary processes at the hot spot-influenced axial seamount on the Juan de Fuca Ridge. Journal of Geophysical Research-Solid Earth. 123:4618-4646.   10.1029/2017jb015131   AbstractWebsite

Axial Seamount is the most volcanically active site of the northeast Pacific, and it has been monitored with a growing set of observations and sensors during the last two decades. Accurate imaging of the internal structure of volcanic systems is critical to better understand magma storage processes and to quantify mass and energy transport mechanisms in the crust. To improve the three-dimensional velocity structure of Axial Seamount, we combined 469,891 new traveltime arrivals, from 12 downward extrapolated seismic profiles, with 3,962 existing ocean-bottom-seismometers traveltime arrivals, into a joint tomographic inversion. Our approach reveals two elongated magma reservoirs, with melt fraction up to 65%, representing an unusually large volume of melt (26-60km(3)), which is likely the result of enhanced magma supply from the juxtaposition of the Cobb hot spot plume (0.26-0.53m(3)/s) and the Axial spreading segment (0.79-1.06m(3)/s). The tomographic model also resolves a subsided caldera floor that provides an effective trap for ponding lava flows, via a trapdoor mechanism. Our model also shows that Axial's extrusive section is thinnest beneath the elevated volcano, where anomalously thick (11km) oceanic crust is present. We therefore suggest that focused and enhanced melt supply predominantly thickens the crust beneath Axial Seamount through diking accretion and gabbro crystallization. Lastly, we demonstrate that our three-dimensional velocity model provides a more realistic starting point for relocating the local seismicity, better resolving a network of conjugate outward and inward dipping faults beneath the caldera walls.(c) 2018. The Authors.

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.

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.

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.

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, 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.

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Babcock, JM, Harding AJ, Kent GM, Orcutt JA.  1998.  An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13 degrees 30 ' N and 12 degrees 20 ' N. Journal of Geophysical Research-Solid Earth. 103:30451-30467.   10.1029/98jb01979   AbstractWebsite

We investigate the along-axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13 degrees 30'N to 12 degrees 20'N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of similar to 500 m as determined from migrated cross-axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13 degrees N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13 degrees 05'N and 13 degrees 01'N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17 degrees 26'S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9 degrees N, 14 degrees S, and 17 degrees S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along-axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (similar to 165 m) at 13 degrees N than at the faster spreading, more robust 9 degrees N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13 degrees 20'N and 12 degrees 46'N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Singh SC, Tong CH, Pye JW, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2003.  A three-dimensional study of a crustal low velocity region beneath the 9 degrees 03 ' N overlapping spreading center. Geophysical Research Letters. 30   10.1029/2002gl015137   AbstractWebsite

[1] Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9degrees03'N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

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.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Tong CH, Pye JW, Singh SC, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2001.  Three-dimensional shallow crustal emplacement at the 9 degrees 03 ' N overlapping spreading center on the East Pacific Rise: Correlations between magnetization and tomographic images. Journal of Geophysical Research-Solid Earth. 106:16101-16117.   10.1029/2001jb000371   AbstractWebsite

We report a three-dimensional (3-D) seismic reflection and tomographic survey conducted at the 9 degrees 03'N overlapping spreading center (OSC) on the East Pacific Rise to understand crustal accretion at this feature. Inversions of travel time data from 19 ocean bottom hydrophones provide a 3-D image of the shallow velocity structure beneath the nontransform offset and associated discordance zone. Seismic analysis indicates that layer 2A thickness varies between 100 and 900 in and averages 430 in throughout the study area. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East Pacific Rise. The crust affected by the OSC migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks.

Begnaud, ML, McClain JS, Barth GA, Orcutt JA, Harding AJ.  1997.  Velocity structure from forward modeling of the eastern ridge-transform intersection area of the Clipperton Fracture Zone, East Pacific Rise. Journal of Geophysical Research-Solid Earth. 102:7803-7820.   10.1029/96jb03393   AbstractWebsite

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to ''fill in'' the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal-attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

Blackman, DK, Slagle A, Guerin G, Harding A.  2014.  Geophysical signatures of past and present hydration within a young oceanic core complex. Geophysical Research Letters. 41:1179-1186.   10.1002/2013gl058111   AbstractWebsite

Borehole logging at the Atlantis Massif oceanic core complex provides new information on the relationship between the physical properties and the lithospheric hydration of a slow-spread intrusive crustal section. Integrated Ocean Drilling Program Hole U1309D penetrates 1.4km into the footwall to an exposed detachment fault on the 1.2Ma flank of the mid-Atlantic Ridge, 30 degrees N. Downhole variations in seismic velocity and resistivity show a strong correspondence to the degree of alteration, a recorder of past seawater circulation. Average velocity and resistivity are lower, and alteration is more pervasive above a fault around 750m. Deeper, these properties have higher values except in heavily altered ultramafic zones that are several tens of meters thick. Present circulation inferred from temperature mimics this pattern: advective cooling persists above 750m, but below, conductive cooling dominates except for small excursions within the ultramafic zones. These alteration-related physical property signatures are probably a characteristic of gabbroic cores at oceanic core complexes. Key Points Borehole T indicates shallow present circulation, conductive regime > 750 mbsf Narrow fault zones have seismic, T, resistivity signal indicating localized flow Hydration of gabbroic oceanic core complexes is limited below fault damage zone

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.

Brothers, DS, Kent GM, Driscoll NW, Smith SB, Karlin R, Dingler JA, Harding AJ, Seitz GG, Babcock JM.  2009.  New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California. Bulletin of the Seismological Society of America. 99:499-519.   10.1785/0120080135   AbstractWebsite

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe-Dollar Point fault (WTDPF), the western-most normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPF. Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is similar to 3.7 m. Coregistered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y. B.P., with a preferred age of 4.1-4.5 k.y. B. P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred between similar to 3-10 k.y. B.P., which is consistent with the MRE age in FLL. Offset of Tiogaaged glacial deposits provides a long-term record of vertical deformation on the WTDPF since similar to 13-14 k.y. B.P., yielding a slip rate of 0.4-0.8 m/yr. In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin.

Brothers, DS, Driscoll NW, Kent GM, Harding AJ, Babcock JM, Baskin RL.  2009.  Tectonic evolution of the Salton Sea inferred from seismic reflection data. Nature Geoscience. 2:581-584.   10.1038/ngeo590   AbstractWebsite

Oblique extension across strike-slip faults causes subsidence and leads to the formation of pull-apart basins such as the Salton Sea in southern California. The formation of these basins has generally been studied using laboratory experiments or numerical models(1-4). Here we combine seismic reflection data and geological observations from the Salton Sea to understand the evolution of this nascent pull-apart basin. Our data reveal the presence of a northeast-trending hinge zone that separates the sea into northern and southern sub-basins. Differential subsidence (>10 mm yr(-1)) in the southern sub-basin suggests the existence of northwest-dipping basin-bounding faults near the southern shoreline, which may control the spatial distribution of young volcanism. Rotated and truncated strata north of the hinge zone suggest that the onset of extension associated with this pull-apart basin began after similar to 0.5 million years ago. We suggest that slip is partitioned spatially and temporally into vertical and horizontal domains in the Salton Sea. In contrast to previous models based on historical seismicity patterns(5), the rapid subsidence and fault architecture that we document in the southern part of the sea are consistent with experimental models for pull-apart basins(1).

Brothers, D, Harding A, Gonzalez-Fernandez A, Holbrook WS, Kent G, Driscoll N, Fletcher J, Lizarralde D, Umhoefer P, Axen G.  2012.  Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California. Geophysical Research Letters. 39   10.1029/2011gl050828   AbstractWebsite

Subduction of the Farallon plate beneath northwestern Mexico stalled by similar to 12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (similar to 24.5 degrees N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached similar to 40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting similar to 8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes. Citation: Brothers, D., A. Harding, A. Gonzalez-Fernandez, W. S. Holbrook, G. Kent, N. Driscoll, J. Fletcher, D. Lizarralde, P. Umhoefer, and G. Axen (2012), Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California, Geophys. Res. Lett., 39, L09307, doi:10.1029/2011GL050828.

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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, 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.

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.

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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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).