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Kent, GM, Singh SC, Harding AJ, Sinha MC, Orcutt JA, Barton PJ, White RS, Bazin S, Hobbs RW, Tong CH, Pye JW.  2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities. Nature. 406:614-618.   10.1038/35020543   AbstractWebsite

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas(1) and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling(2,3). The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

Kent, GM, Harding AJ, Orcutt JA.  1993.  Distribution of Magma Beneat the East Pacific Rise Near the 9-Degrees-03' N Overlapping Spreading Center from Forward Modeling of Common Depth Point Data. Journal of Geophysical Research-Solid Earth. 98:13971-13995.   10.1029/93jb00706   AbstractWebsite

We have reprocessed six cross-axis and three along-axis common depth point (CDP) profiles near the 9-degrees-03'N overlapping spreading center (OSC) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Travel time modeling of the AMC reflector reveals an asymmetric distribution of melt across the 9-degrees-03'N OSC. The variation of modeled AMC width beneath either OSC limb is minimal, but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4.15 km near the 9-degrees-17'N ridge axis discontinuity. Additionally, melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis/neovolcanic zone but is displaced toward the western rise flank. Depth migration, based on a continuum velocity model consistent with postcritical reflections from the base of layer 2A, places the skewed AMC reflector beneath a nearly constant thickness sheeted dike section which dips approximately 10-degrees away from the rise axis. To confirm AMC continuity beneath the western rise flank, we use the Maslov synthetic seismogram method to show that amplitude enhancement of the AMC reflector is consistent with a continuous melt body underlying a thickening extrusive layer. Analysis of along-strike CDP profiles indicates an AMC which is neither overlapping nor discontinuous when projected onto the along-strike plane. Identifying intracrustal events on along-axis CDP lines. however, requires extreme caution; we have modeled out-of-plane scattering using a Kirchhoff formulation, and we show that a coherent event identified beneath the overlap basin results from diffraction off the AMC which lies nearly 3 km to the west of the profile. We attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust In this model, melt ascends upward (buoyancy forces) until deflected by the impermeable sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovolcanic zone where fissuring produces a temporary conduit for emplacement. Discrete jumps in modeled AMC width toward the overlap basin represent a further displacement/defocusing of melt supply (western AMC edge) relative to the neovolcanic zone (eastern AMC edge). The asymmetric pattern of melt therefore represents a gradual, en-echelon accommodation of melt supply across the 9 km of ridge axis offset at 9-degrees-03'N. Thus for asymmetric configurations, AMC width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone. Here we present a new model for OSC development which invokes a significant component of cross-axis melt migration. Moreover, abrupt changes in AMC width near ridge axis discontinuities (e.g., 9-degrees-17'N deviation in axial linearity) suggest that any along-axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models. The transition of melt supply beneath the overlap basin might favor a continuous low-velocity zone underlying this feature; if true, basin development may be related to the subsidence of a mechanically weak crustal lid. The proposed model for OSC development therefore views ridge axis discontinuities as the surficial response of misalignment and/or defocusing of melt supply in the uppermost mantle.

Kent, GM, Harding AJ, Orcutt JA, Detrick RS, Mutter JC, Buhl P.  1994.  Uniform Accretion of Oceanic-Crust South of the Garrett Transform at 14-Degrees-15' S on the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 99:9097-9116.   10.1029/93jb02872   AbstractWebsite

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (> 150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second-order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (< 1.0 km) melt lens which shows great along-strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9-degrees-30'N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14-degrees-27'S ridge axis discontinuity by a modest 250-300 m and appears to be continuous across this feature. Given the small aspect ratio (approximately 1.0 km by approximately 50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14-degrees-27'S offset is likely the result of a mixing boundary which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200-250 to 500-600 m) in thickness within 1-2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110-120 mm/yr) East Pacific Rise at 9-degrees-N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off-axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14-degrees-27'S discontinuity are caused by long-term reductions in magma supply. Second-order differences in the style of extrusive thickening may be related to structural differences within the low-velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Moho on cross-axis profiles may be traced to within approximately 1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Kent, GM, Babcock JM, Driscoll NW, Harding AJ, Dingler JA, Seitz GG, Gardner JV, Mayer LA, Goldman CR, Heyvaert AC, Richards RC, Karlin R, Morgan CW, Gayes PT, Owen LA.  2005.  60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe. Geology. 33:365-368.   10.1130/g21230.1   AbstractWebsite

Deformation across three major fault strands within the Lake Tahoe basin has been mapped by using a novel combination of high-resolution seismic chirp, airborne laser-and acoustic-multibeam-derived bathymetry, and deep- and shallow-water sediment cores. Submerged erosional terraces of late Pleistocene age (19.2 +/- 1.8 ka) record vertical deformation across fault strands that ranges between 10 and 15 m; offset of 10 m is observed across the southern part of the West Tahoe fault. Avalanche deposits from the catastrophic McKinney Bay slide (ca. 60 ka) are offset across the Stateline fault by at least 21-25 m. The submerged shoreline terraces and debris avalanche provide marker beds with which to constrain the extensional history of the region for the past 60 k.y. This history is then used to assess the future seismic hazard of the region. Data on deformation across these two important marker beds, combined with chronological control from C-14 and optically stimulated luminescence measurements, yield an estimate of extension across the Lake Tahoe basin that is 0.4-0.5 mm/yr. On the basis of these measurements, there exists the potential for a large, seiche wave-generating M7 earthquake every similar to 3 k.y. Late Pleistocene and Holocene vertical deformation rates within the Tahoe basin are characteristic of Basin and Range faulting and place the Tahoe basin within the western limits of the extensional Basin and Range province.

Kent, GM, Harding AJ, Orcutt JA.  1993.  Distribution of Magma Beneath the East Pacific Rise Between the Clipperton Transform and the 9-Degrees-17' N Deval from Forward Modeling of Common Depth Point Data. Journal of Geophysical Research-Solid Earth. 98:13945-13969.   10.1029/93jb00705   AbstractWebsite

We have reprocessed seven cross-axis common depth point (CDP) profiles between the Clipperton transform and the 9-degrees-17'N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross-axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9-degrees-53'N-9-degrees-35'N Devals, the 9-degrees-35'N-9-degrees-17'N Devals, and south of the 9-degrees-17'N Deval were < 0.7 km, 1.0-1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross-axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9-degrees-35'N and 9-degrees-17'N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity-depth functions. ESP velocity models were altered to compensate for off-axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two-dimensional velocity models constructed from modified ESP velocity-depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero-offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under-migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along-axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9-degrees-17'N Deval which suggests an AMC whose along-axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along-axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produce strong diffraction patterns as imaged in the CDP data.

Kent, GM, Harding AJ, Orcutt JA.  1990.  Evidence for a Smaller Magma Chamber Beneath the East Pacific Rise at 9-Degrees-30' N. Nature. 344:650-653.   10.1038/344650a0   AbstractWebsite

The size and shape of magma chambers beneath mid-ocean ridges are fundamental features that control the availability of melt, composition of magmas and formation of oceanic crust. Models derived from the study of ophiolites1, and from thermal considerations2, include a very large magma chamber, which can exceed 10–20 km in width. Cross-axis seismic reflection profiles from the East Pacific Rise, however, constrain the width of the axial magma chamber to be < 3–4 km (ref. 3). Even this may be an over-estimate, arising from the under-migration of diffracted energy generated at the edges of a smaller magma chamber. Here we show that forward modelling of these diffraction hyperbolae yields a distance between the best-fitting point diffractors, and by inference a magma chamber width, of only 800–1,200 m. Reflectivity modelling also suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt. A narrow and thin axial magma chamber would inhibit along-axis mixing4, and might thereby account for variations in magma composition along the East Pacific Rise5.

Kent, GM, Kim II, Harding AJ, Detrick RS, Orcutt JA.  1996.  Suppression of sea-floor scattered energy using a dip moveout approach - Application to the mid-ocean ridge environment. Geophysics. 61:821-834.   10.1190/1.1444007   AbstractWebsite

Multichannel seismic (MCS) images are often contaminated with in- and out-of-plane scattering from the sea floor. This problem is especially acute in the midocean ridge environment where sea-floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea-floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water-borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea-floor-scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea-floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common-midpoint (CMP) domain after DMO. Moreover, our DMO-based approach suppresses out-of-plane scattering, and therefore is not limited to removal of in-plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO-based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO-based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth.

Kennett, BLN, Harding AJ.  1984.  Guided Low-Frequency Noise from Airgun Sources. Geophysical Prospecting. 32:690-705.   10.1111/j.1365-2478.1984.tb01714.x   AbstractWebsite

The presence of the water layer in marine seismic prospecting provides an effective waveguide for acoustic energy trapped between the sea-bed and the sea-surface. This energy persists to large ranges and can be the dominant early feature on far-offset traces. On airgun records, there is commonly a lower frequency set of arrivals following the water-trapped waves. These arrivals are not as obvious with higher frequency watergun sources. By using a combination of intercept-time/slowness (τ—p) mapping on observational data and theoretical modelling, we are able to identify the origin of the events. If a very rapid increase in a seismic wavespeed occurs beneath the sea-bed sediments, a new waveguide is formed bounded by the sea surface and this transition zone. The low frequency waves are principally guided within this thicker waveguide. Numerical filtering in the τ—p domain followed by trace reconstruction is very effective in removing the low frequency noise.

Kennett, BLN, Harding AJ.  1985.  Is ray theory adequate for reflection seismic modelling? (a survey of modelling methods) First Break. 3:9-14.   10.3997/1365-2397.1985001   Abstract

The region of the Earth of interest far reflection seismology is characterised by rapid fluctuations in properties with depth superimposed on a smoother trend of general increase in seismic velocity with depth, together with significant horizontal variations. Since most hydrocarbon deposits are to be sought in traps formed by lateral contrasts it is most important that modelling techniques should give as accurate a representation of the seismic wave field as possible in a complex medium. In deep reflection seismology designed to look at the whole crust, the principal features of interest, such as thrusts, have often to be mapped by their expression in terms of horizontal changes in reflection character.

Kappus, ME, Harding AJ, Orcutt JA.  1995.  A Base-Line for Upper Crustal Velocity Variations Along the East Pacific Rise at 13-Degrees-N. Journal of Geophysical Research-Solid Earth. 100:6143-6161.   10.1029/94jb02474   AbstractWebsite

A wide aperture profile of the East Pacific Rise at 13 degrees N provides data necessary to make a high-resolution seismic velocity profile of the uppermost crust along a 52-km segment of ridge crest. Automated and objective processing steps, including tau - p analysis and waveform inversion, allow the construction of models in a consistent way so that comparisons are meaningful. A continuous profile is synthesized from 70 independent one-dimensional models spaced at 750-m intervals along the ridge. The resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability. The main features are a thin low-velocity layer 2A at the top with a steep gradient to layer 2B. The seafloor velocity is nearly constant at 2.45 km/s +/- 3% along the entire ridge. The velocity at the top of layer 2B is 5.0 km/s +/- 10%. The depth to the 4 km/s isovelocity contour within layer 2A is 130+/-20 m from 13 degrees to 13 degrees 20'N, north of which it increases to 180 m. The increase in thickness is coincident with a deviation from axial linearity (DEVAL) rioted by both a slight change in axis depth and orientation and in geochemistry. The waveform inversion, providing more details plus velocity gradient information, shows a layer 2A with about 80 m of constant-velocity material underlain by 150 m of high velocity gradient material, putting the base of layer 2A at approximately 230 m depth south of 13 degrees 20'N and about 50 m thicker north of the DEVAL. The overall lack of variability, combined with other recent measurements of layer 2A thickness along and near the axis, indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity, but the dynamics of emplacement. The homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10% against which other observed variations in similar regimes can be compared.

Kappus, ME, Harding AJ, Orcutt JA.  1990.  A Comparison of TAU-P Transform Methods. Geophysics. 55:1202-1215.   10.1190/1.1442936   AbstractWebsite

Many τ-p transform methods are available to seismic data analysts; selection of the appropriate method should depend on the nature of the source excitation, the intended use of the transformed data, limits imposed by sampling parameters, and computational cost. Using these criteria, we compare five methods on marine multichannel data and similar synthetic profiles. On fully sampled synthetic profiles, methods that handle the three‐dimensional (3-D) nature of the point source provide correct amplitude and phase information even at small slownesses, whereas 2-D and asymptotic approximate methods do not. When data from small ranges are not available, aliasing and truncation distort the amplitude of small slowness traces produced by all methods, but are most severe in the 3-D results. Transformation of data with increased input trace spacing or decreased depth to the reflector results in increased aliasing effects. When the intended use of the transformed data depends on correct arrival times only, and not on accurate amplitudes of small‐slowness traces, we recommend an asymptotic approximate method for its relative computational efficiency and comparative robustness with respect to aliasing and truncation. Uses that depend on correct amplitudes demand a τ-p transform method which honors the source geometry of the experiment.