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Tolstoy, M, Harding AJ, Orcutt JA, Detrick RS, Kent GM, Mutter JC, Buhl P.  1997.  Deepening of the axial magma chamber on the southern East Pacific Rise toward the Garrett Fracture Zone. Journal of Geophysical Research-Solid Earth. 102:3097-3108.   10.1029/96jb03226   AbstractWebsite

A wide-aperture profile along the ridge axis from 14 degrees 29'S to 13 degrees 39'S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 m thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13 degrees N on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is rear rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as: a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along-axis flow.

Tolstoy, M, Harding AJ, Orcutt JA.  1993.  Crustal Thickness on the Mid-Atlantic Ridge - Bulls-Eye Gravity-Anomalies and Focused Accretion. Science. 262:726-729.   10.1126/science.262.5134.726   AbstractWebsite

Spreading segments of the Mid-Atlantic Ridge show negative bull's-eye anomalies in the mantle Bouguer gravity field. Seismic refraction results from 33-degrees-S indicate that these anomalies can be accounted for by variations in crustal thickness along a segment. The crust is thicker in the center and thinner at the end of the spreading segment, and these changes are attributable to variations in the thickness of layer 3. The results show that accretion is focused at a slow-spreading ridge, that axial valley depth reflects the thickness of the underlying crust, and that along-axis density variations should be considered in the interpretation of gravity data.

Tong, CH, Lana C, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2005.  Subsurface tectonic structure between overlapping mid-ocean ridge segments. Geology. 33:409-412.   10.1130/g21245.1   AbstractWebsite

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The overlap region between mid-ocean ridge segments at 9 degrees N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 9 degrees 03'N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related.

Tong, CH, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2004.  Effects of tectonism and magmatism on crack structure in oceanic crust; a seismic anisotropy study. Geology. 32:25-28.   10.1130/g19962.1   AbstractWebsite

We analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9 degrees N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20 degrees -30 degrees ) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest.

Tong, CH, Pye JW, Barton PJ, White RS, Sinha MC, Singh SC, Hobbs RW, Bazin S, Harding AJ, Kent GM, Orcutt JA.  2002.  Asymmetric melt sills and upper crustal construction beneath overlapping ridge segments: Implications for the development of melt sills and ridge crests. Geology. 30:83-86.   10.1130/0091-7613(2002)030<0083:amsauc>2.0.co;2   AbstractWebsite

A new three-dimensional tomographic velocity model and depth-converted reflection images of the melt sills beneath the 9degrees03'N overlapping spreading center on the East Pacific Rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs. Despite the similarly curved ridge crests, the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs. The overlap basin is closely related to the propagating eastern limb in terms of its seismic structure. By contrast, the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins. As the overlapping spreading center is migrating southward, the differential development of melt sills and ridge crests may be inferred from the results of this study. Ridge propagation appears to involve two major processes: the advancement of the melt sill at the ridge tip and the development of ridge-crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb. The latter process may result in the abandonment of the current neovolcanic axis, leading to the self-decapitation of the propagating limb. By contrast, the self-decapitation of the western limb is related to the receding melt sill, which lags behind the anticlockwise rotational motion of the ridge crest.

Tong, CH, Barton PJ, White RS, Sinha MC, Singh SC, Pye JW, Hobbs RW, Bazin S, Harding AJ, Kent GM, Orcutt JA.  2003.  Influence of enhanced melt supply on upper crustal structure at a mid-ocean ridge discontinuity: A three-dimensional seismic tomographic study of 9 degrees N East Pacific Rise. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb002163   AbstractWebsite

[1] We present a three-dimensional upper crustal model of the 9degrees03'N overlapping spreading center (OSC) on the East Pacific Rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels. Our P wave velocity model obtained from tomographic inversion of similar to 70,000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs. Extrusive volcanic rocks above the western melt sill are inferred to be thin ( similar to 250 m). More extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill. The extrusive layer inferred above the eastern melt sill thickens from similar to 350 ( at the neovolcanic axis) to 550 m ( to the west of the melt sill). Volcanic construction is likely to be significant in the formation of ridge crest morphology at the OSC, particularly at the tip of the eastern limb. On the basis of our interpretation of the velocity model, we propose that enhanced magma supply at crustal levels at the OSC may provide an effective mechanism for the migration of ridge discontinuities. This "dynamic magma supply model'' may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs.