Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N

Toomey, D. R.; Solomon, Sean C.; Purdy, G. M.

doi:10.1029/94jb01942

Cited by 220 publications

(255 citation statements)

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“…The magnitude of upper crustal heterogeneity found around Hole 504B is significantly less than that reported by Barclay et al [1998] within the Mid-Atlantic Ridge rift valley but comparable to the smaller amounts of upper crustal velocity heterogeneity observed off-axis at the fast spreading East Pacific Rise [Toomey et al , 1994] and at the coaxial segment on the Juan de Fuca Ridge [Sohn et al, 1997]. This apparent spreading rate dependence to the magnitude of upper crustal heterogeneity almost certainly reflects a greater temporal and spatial complexity of magmatic and tectonic processes at slower spreading ridges.…”

Section: Implications For Nature Of Seismic Layer 2/3 Boundarymentioning

confidence: 60%

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

Detrick¹,

Toomey²,

Collins³

1998

J. Geophys. Res.

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Abstract. We have used over 16,000 compressional wave travel times recorded on 11 ocean bottom seismometers to determine the magnitude of P-wave heterogeneity and anisotropy in the upper 2 km of the oceanic crust in an -• 150 km 2 area around Hole 504B, the deepest hole drilled into the oceanic crust. Our best fitting one-dimensional, isotropic upper crustal velocity model for the 150 km 2 area around the drill site is similar to velocity models at Hole 504B determined using downhole sonic logs, vertical seismic profiles, oblique seismic experiments and conventional refraction profiling. There is no evidence seismic layer 2 is anomalously thin at Hole 504B, suggesting that the occurrence of the seismic layer 2/3 boundary within the upper sheeted dike section, which has been documented in Hole 504B, is a general feature of this 5.9 Ma crust. On the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in upper crustal P wave velocity around Hole 504B is comparatively small. Within the 571-m thick extrusive section maximum P wave velocity differences are -300-400 rn s '• (a 6%-8% velocity anomaly). These velocity differences decrease to <100-200 rn s -• (<2%-3% of the average velocity) within the sheeted dike section > 1 km into the crust. The highest upper crustal velocities are found beneath basement ridges west and south of Hole 504B, while the lowest upper crustal velocities occur beneath the flanking basement troughs. There is an azimuthal dependence to the travel time residuals (slow direction N-S) which is consistent with 2%-4% crack-induced anisotropy in the upper crust. The magnitude of upper crustal heterogeneity observed near Hole 504B is significantly less than that found in "zero-age" crust at the Mid-Atlantic Ridge but comparable to that observed off-axis in crust formed at faster spreading ridges.

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Section: Implications For Nature Of Seismic Layer 2/3 Boundarymentioning

confidence: 60%

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

Detrick¹,

Toomey²,

Collins³

1998

J. Geophys. Res.

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“…The graph method can be applied to a sheared grid in marine refraction tomography [Toomey et al, 1994], because them is no limitation on the complexity of the geometry of the grid, provided the topology remains simple. We trace rays from each instrument to the shot points using reciprocity in the wave field.…”

Section: Ray Tracingmentioning

confidence: 99%

“…The seismic velocity structure is represented by a grid composed of columns of grid points that are sheared vertically to include bathymetric relief [Toomey et al, 1994]. In two dimensions, parallellogram shaped cells fill the space between the sheared grid points (Figure 2).…”

Section: Model Parameterizationmentioning

confidence: 99%

A two‐dimensional tomographic study of the Clipperton transform fault

Avendonk¹,

et al. 1998

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Abstract. From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to Investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray-tracing problem is a hybrid of the graph (or shortest path) method and a ray-bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6-5.9 km under the transform fault to produce the best fit of the Prop phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.

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“…In the inversion procedure the slowness (i.e., velocity) model is defined on a 160-by-15-km regular grid with a nodal spacing of 0.2 km. The perturbational grid had a spacing of 5 km and 0.36 km in the horizontal and vertical directions, respectively, and the smoothing parameter is 800 in both directions [Toomey et al, 1994].…”

Section: Seismic Modelingmentioning

confidence: 99%

Crustal and upper mantle seismic structure beneath the rift mountains and across a nontransform offset at the Mid‐Atlantic Ridge (35°N)

Canales¹,

Detrick²,

Lin³

et al. 2000

J. Geophys. Res.

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Abstract. We present new results on the crustal and upper mantle structure beneath the rift mountains along two segments of the Mid-Atlantic Ridge and across a nontransform offset (NTO). Our results were obtained from a combination of forward modeling and twodimensional tomographic inversion of wide-angle seismic refraction data and gravity modeling. The study area includes two segments: OH-1 between the Oceanographer fracture zone and the NTO-1 at 34ø35'N and OH-2 between NTO-1 and the NTO at 34ø10'N. The center of OH-1 is characterized by anomalously thick crust (-8 km) with a thick Moho transition zone with Vp=7.2-7.6 km/s. This transition zone, coincident with a gravity low, is probably composed of gabbro sills alternating with dunites, as observed in some ophiolites. OH-1 has larger alongaxis crustal thickness variations than OH-2, but average crustal thicknesses are similar (6.0+1.2 km at OH-l, 6.1+0.7 at OH-2). Thus we do not find significant differences in magma supply between these segments, in contrast to what has been inferred from morphological and gravity studies. At both segments the shoaling of the Moho is more rapid at the inside than at the outside corners, consistent with models in which the inside-corner crust is tectonically modified. The structural differences between inside-and outside-corner crust are more apparent at OH-2, suggesting that the extrusive layer is thinner at the inside corner of OH-2 than at the inside corner of OH-1, probably due to differences in axial morphology and along-axis magma transport. NTO-1 is characterized by a nearly constant velocity gradient within the upper 5 km and low upper mantle velocities (7.4-7.8 km/s). The anomalous structure beneath NTO-1 is interpreted as fractured mafic crust. The P wave velocities and densities required to match the gravity data suggest that serpentinites are common beneath the NTO-1 and possibly beneath the inside corners. Serpentinization could be as much as 40% at-3.8 km below seafloor and probably does not occur at subseafloor depths greater than -6.2 km at the NTO-1. Our results indicate that in a slow spreading environment where magmatism and tectonism are equally important, the seismic Moho cannot be correlated with an unique geological structure. At the center of a segment the seismic Moho may represent the lower boundary of an interlayered grabbro-dunite transition zone, while beneath the inside corner and NTO where the crust is thinner, it may correspond to an alteration front.

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Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N

Cited by 220 publications

References 50 publications

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

A two‐dimensional tomographic study of the Clipperton transform fault

Crustal and upper mantle seismic structure beneath the rift mountains and across a nontransform offset at the Mid‐Atlantic Ridge (35°N)

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