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Hallenborg, E, Harding AJ, Kent GM, Wilson DS.  2003.  Seismic structure of 15 Ma oceanic crust formed at an ultrafast spreading East Pacific Rise: Evidence for kilometer-scale fracturing from dipping reflectors. Journal of Geophysical Research-Solid Earth. 108   10.1029/2003jb002400   AbstractWebsite

[1] A grid of seismic reflection data from 15 Ma Cocos Plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading (similar to 200 mm/yr, full rate) East Pacific Rise (EPR). The data contain an unexpected quantity of bright reflectors throughout the crust, generally < 5 km in length. Significant reflection dip angles ( &SIM; 10 - 30 &DEG;) are seen only on the isochron profiles, and several reflectors have a confirmed isochron dip orientation. Certain upper crustal reflectors (UCRs) appear to form a single network with isochron dip. The most prominent example ( apparent reflection coefficient &SIM; 0.2) projects toward a &SIM; 70 m disruption in the igneous seafloor, and waveform analysis demonstrates that the reflector is likely the product of a low-velocity zone on the order of tens of meters thick. Our observations suggest that the isochron-dipping UCRs represent small-slip, kilometer-scale faults/ fractures, and none are primarily related to the seismic 2/3 boundary or to the sheeted dike/gabbro transition. Lower crustal reflectors (LCRs) have no preferred orientation or regular pattern on the 30 km scale of this survey. Certain LCRs may be similar to the UCRs in origin, while others may originate from near-axis ductile flow, lithologic banding, off-axis magmatism, or some combination thereof. Overall, reflectors in this area are abundant and complex, consistent with previous off-axis studies, but at odds with observations of young crust (< 5 Ma) near the EPR. This dichotomy may be resolved by a delay in reflector formation, through some combination of postaccretionary tectonics, magmatism and hydrothermal alteration.

Han, L, Hole JA, Stock JM, Fuis GS, Kell A, Driscoll NW, Kent GM, Harding AJ, Rymer MJ, Gonzalez-Fernandez A, Lazaro-Mancilla O.  2016.  Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project. Journal of Geophysical Research-Solid Earth. 121:7469-7489.   10.1002/2016jb013139   AbstractWebsite

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from similar to 3 to similar to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below similar to 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at similar to 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Harding, AJ.  1985.  Slowness Time Mapping of Near Offset Seismic-Reflection Data. Geophysical Journal of the Royal Astronomical Society. 80:463-492.   10.1111/j.1365-246X.1985.tb05104.x   AbstractWebsite

The transformation of a set of seismograms to the delay time-slowness, τ—p, domain is presented as a sequence of Fourier and Bessel transforms, For a horizontally layered medium, this sequence gives an exact cylindrical wave decomposition of the response to a point source; correctly compensating for the phase shifting and geometrical spreading associated with transmission through the Earth. The resultant τ—p map or ‘slant stack’ contains true amplitude and phase information. The spatial aliasing properties of the transformation, when applied to a dataset, are greatly improved by the use of only outgoing waves in the Bessel transform. This is equivalent to using Hankel functions rather than Bessel functions, and is justified by the absence of incoming waves from most datasets. The WKBJ approximation to the medium response enables predictions to be made about the shape and amplitude variation with slowness of truncation effects. Theoretically the τ—p transformation is reversible, thus the τ—p domain is a suitable one in which to perform filtering operations before seismogram reconstruction.

Harding, AJ.  1985.  Inversion Methods for TAU-P Maps of Near Offset Data Linear Inversion. Geophysical Prospecting. 33:674-695.   10.1111/j.1365-2478.1985.tb00772.x   AbstractWebsite

Conventional velocity analysis, based on the ideas of rms velocity and hyperbolic reflection events in the x-t domain, is restricted in validity to near vertical incidence. Thus analysis of near-offset datasets usually requires the muting of wide-angle reflections from shallow interfaces before the rms velocities are determined. The ray-theoretical integral for the delay time τ, which depends on the slowness p and the velocity function, is valid for all angles. The wide-angle reflections can be used to improve the accuracy of the derived velocity function in the near surface region, if the recorded x-t data are mapped into the τ-p domain. By representing the velocity function between reflectors as a series of gradient zones, i.e. regions with a uniform increase in velocity with depth, the recovery of the velocities may be posed as a matrix linear inverse problem for the slopes of the gradient zones. In order to convert the problem to a linear one, the velocity discontinuities at the reflecting interfaces must be fixed in advance. Their positions are based on the behaviour of the τ-p map of the data. Finding a stable velocity model may require several iterations with the reflecting interfaces at different positions. An understanding of the workings of the inversion algorithm allied with an analysis of the causes of instability aids the search for a stable model.

Harding, AJ, Orcutt JA, Kappus ME, Vera EE, Mutter JC, Buhl P, Detrick RS, Brocher TM.  1989.  Structure of Young Oceanic-Crust at 13-Degrees-N on the East Pacific Rise from Expanding Spread Profiles. Journal of Geophysical Research-Solid Earth and Planets. 94:12163-12196.   10.1029/JB094iB09p12163   AbstractWebsite

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low-velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross-axis reflections from the magma chamber with a width of<5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.

Harding, AJ.  2001.  Seismic Structure. Encyclopedia of ocean sciences. ( Steele JH, Ed.).:6vol.(XLIX,3399p.)., San Diego: Academic Press Abstract
Harding, AJ, Hedlin MAH, Orcutt JA.  1998.  Migration of backscatter data from the Mid-Atlantic Ridge. Journal of the Acoustical Society of America. 103:1787-1803.   10.1121/1.421332   AbstractWebsite

In studies of low-frequency reverberation within the marine environment, a central concern is the relationship between reverberation events and morphological features of the seafloor. A time-domain migration algorithm for the reverberation intensity field is developed that produces scattering coefficient maps coregistered with a bathymetry database. The algorithm is tailored to broadband transient sources with good range resolution, and was developed to analyze an extensive set of reverberation records from a 200-255 Hz source collected on the flanks of the Mid-Atlantic ridge. The precise, sample-by-sample, tracking of wavefronts across elements of the bathymetry database that forms the foundation of the algorithms implementation results in reverberation maps that show a clear and detailed correlation between scattering and morphology with narrow scarp slopes consistently highlighted. Environmentally induced asymmetries in transmission loss and incidence angle are exploited to break the inherent left-right ambiguity of the receiver array. Iterative migration, assuming a dominant dependence of backscatter on grazing angle, produces images, even from individual records, that show good ambiguity resolution. Results from multiple records corroborate the effectiveness of the ambiguity resolution and demonstrate the stability of the scattering coefficient estimates and the acoustic system. (C) 1998 Acoustical Society of America.

Harding, AJ, Kent GM, Orcutt JA.  1993.  A Multichannel Seismic Investigation of Upper Crustal Structure at 9-Degrees-N on the East Pacific Rise - Implications for Crustal Accretion. Journal of Geophysical Research-Solid Earth. 98:13925-13944.   10.1029/93jb00886   AbstractWebsite

Reprocessed multichannel seismic profiles from the 9-degrees-N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide-angle reflections from the base of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3-0.45 s within 1-2 km of the axis, corresponding to an increase in layer thickness of 200-600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along-axis profile, apparent along-axis variability in the depth of the axial magnma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is constant.

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.

Holmes, RC, Tolstoy M, Harding AJ, Orcutt JA, Morgan JP.  2010.  Australian Antarctic Discordance as a simple mantle boundary. Geophysical Research Letters. 37   10.1029/2010gl042621   AbstractWebsite

Several complex models require unique mantle conditions to explain the Australian Antarctic Discordance (AAD), an unusually deep and rugged section of the Southeast Indian Ridge (SEIR) between similar to 120 degrees-128 degrees E. Seismic evidence suggests the AAD is instead the manifestation of two contrasting mantle domains converging along its eastern edge. Variations in axial morphology and flanking topographic relief along the SEIR arise as ridge segments to the west (Indian mantle) grade into a cooler melting regime while those to the east (Pacific mantle) are more magmatically robust. Seismic refraction data show crustal thickness decreases from the west into the AAD at a rate of 0.1 km/100 km, then rapidly increases from 4.8 +/- 0.4 km to 7.3 +/- 0.2 km across the eastern border. The AAD thus appears to be the terminal end of a long-wavelength reduction in melt supply at what may be the simplest global example of a mantle boundary. Citation: Holmes, R. C., M. Tolstoy, A. J. Harding, J. A. Orcutt, and J. P. Morgan (2010), Australian Antarctic Discordance as a simple mantle boundary, Geophys. Res. Lett., 37, L09309, doi: 10.1029/2010GL042621.

Hussenoeder, SA, Detrick RS, Kent GM, Schouten H, Harding AJ.  2002.  Fine-scale seismic structure of young upper crust at 17 degrees 20 ' S on the fast spreading East Pacific Rise. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb001688   AbstractWebsite

[1] The detailed upper crustal structure of the East Pacific Rise (EPR) at 17degrees20'S is examined by applying a genetic algorithm-based waveform inversion to five multichannel seismic lines: one on-axis and two to either side along 42- and 85-kyr-old crust. On-axis, a double-stepped velocity pattern is recorded beneath 70-100 m of low-velocity extrusives (2.1-2.4 km s(-1)). We define the upper velocity contrast as the base of seismic layer 2A due to its severity and continuity along and across axis. The more subdued and intermittent lower-velocity step is not observed off-axis. Material between the two high-gradient intervals is proposed to represent the pillow/dike transition, bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and/or deformation. Extrusive velocities increase quite rapidly in this area, with velocities similar to3 km s(-1) common in crust less than or equal to85 kyr old. This, plus a rapid (300-400 m) thickening of layer 2A observed within 1-4 km of the rise axis, indicates that this segment is undergoing focused melt delivery (<500-m-wide dike intrusion zone) and elevated hydrothermal activity. These findings demonstrate the ability of single-ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information.

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.