Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge?

Müller, Mark; Robinson, Christian; Minshull, T. A.; White, R. S.; Bickle, M. J.

doi:10.1016/s0012-821x(97)00030-7

Cited by 129 publications

(149 citation statements)

References 36 publications

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“…There is also a distinct lack of seismic studies at very highly segmented ridges. One recent seismic study of the SWIR near 57øE finds only 4 km of crust extending off axis to ages of at least 15 Ma [Muller, et al, 1996]. Further study may reveal additional ridges that produce thin crust.…”

Section: Discussionmentioning

confidence: 90%

Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges

Magde¹,

Sparks²

1997

J. Geophys. Res.

104

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Abstract. In contrast to the along-axis uniformity observed at the East Pacific Rise (EPR), crustal accretion at the Mid-Atlantic Ridge (MAR) appears to be a highly complex and heterogeneous process. Besides spreading rate, one of the first-order differences between the EPR and the MAR is the much higher degree of ridge segmentation observed in the Atlantic. Circular lows in the mantle Bouguer anomaly (MBA bull' s-eyes) are common at centers of spreading segments of the MAR, suggesting crustal thickness variations of up to 4 km along individual segments. We use a three-dimensional numerical model of mantle flow to examine the effect of ridge segmentation on mantle upwelling and the resulting overall crustal production and along-axis variations in crustal thickness. Mantle flow in our model is driven by both buoyant forces and segmented plate spreading. Various asthenospheric viscosity structures, plate spreading geometries, and mantle potential temperatures are explored. We find that a combination of buoyant mantle flow and three-dimensional melt migration can reproduce crustal thickness variations similar to those inferred from gravity. Buoyant flow gives rise to variations in upwelling velocity at along-axis wavelengths greater than 150 km but does not contribute to short-wavelength variations. However, three-dimensional melt migration may greatly enhance crustal thickness variations along all segments, independent of the wavelength of buoyant upwelling. We present an idealized model, in which melt first rises vertically and then flows along the base of the lithosphere toward the ridge axis, that easily produces crustal thickness variations greater than 4 km. The models also predict that the average crustal thickness should decrease with increasing amount of segmentation and decreasing spreading rate. Therefore the thinner, more heterogeneous crust observed at the MAR may result from the combined effects of slower spreading rate and more pervasive ridge segmentation.

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Section: Discussionmentioning

confidence: 90%

Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges

Magde¹,

Sparks²

1997

J. Geophys. Res.

104

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“…1508 m of material was recovered from Hole 735B, and fluid inclusion studies suggest that two kilometers of crust were removed from above the massif during detachment faulting [Vanko and Stakes, 1991]. Seismic profiles and sampling from a submersible indicate that the base of the gabbroic section lies 200 -600 m below the base of Hole 735B [Muller et al, 1997;Dick et al, 1999;see also Francis and Raitt, 1967;Brown and White, 1994]. Our study focuses on processes within the lower kilometer of Hole 735B, representing the middle of the lower crust (Figure 1).…”

Section: Southwest Indian Ridgementioning

confidence: 99%

Plagioclase preferred orientation in layered mylonites : evaluation of flow laws for the lower crust

Mehl¹

2008

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We evaluate the applicability of plagioclase and gabbro flow laws by comparing predicted and observed deformation mechanisms in gabbroic shear zones. Gabbros and layered gabbro mylonites were collected from the Southwest Indian Ridge (SWIR), ODP Hole 735B. Deformation temperatures are constrained by two-pyroxene thermometry, stress is estimated from grain size, and deformation mechanisms are analyzed by microstructure and the presence or absence of a lattice preferred orientation (LPO). Our analyses indicate that mylonite layers deformed at a strain rate in the range of 10 . Plagioclase in pure plagioclase mylonite layers exhibit strong LPOs indicating they deform by dislocation creep. Plagioclase grain size in mixed plagioclase-pyroxene mylonite layers is finer than in pure plagioclase layers, and depends on the size and proportion of pyroxenes. Progressive mixing of pyroxene and plagioclase within gabbro mylonite layers is accompanied by weakening of the LPO indicating that phase mixing promotes a transition to diffusion creep processes that involve grain boundary sliding. Our results indicate that experimental flow laws are accurate at geologic strain rates, although the strain rate for diffusion creep of fine-grained gabbro may be underestimated. At the conditions estimated for the SWIR crust, our calculations suggest that strain localization leads to a factor of two to four decrease in lower crustal viscosity. Even so, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle.

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“…Furthermore, the occurrence of a high-velocity lower layer is unlike that observed at very slow spreading rates [cf. Muller et al, 1997].…”

Section: Crustal Structure Of the Accommodation Zonementioning

confidence: 99%

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

et al. 2008

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The combined Greenland‐Senja Fracture Zones (GSFZ) represent a first‐order plate tectonic feature in the North Atlantic Ocean. The GSFZ defines an abrupt change in the character of magnetic anomalies with well‐defined seafloor spreading anomalies in the Greenland and Norwegian basins to the south but ambiguous and weak magnetic anomalies in the Boreas Basin to the north. Substantial uncertainty exists concerning the plate tectonic evolution of the latter area, including the role of the East Greenland Ridge, which is situated along the Greenland Fracture Zone. In 2002, a combined ocean‐bottom seismometer and multichannel seismic (MCS) survey acquired two intersecting wide‐angle reflection and coincident MCS profiles across and along the East Greenland Ridge. We present the results of integrated reflection seismic interpretation, first‐arrival tomography, 2D kinematic raytracing, full‐wave amplitude modeling, and gravity modeling of the intersecting profiles. The results show that (1) the Greenland Basin is characterized by a normal oceanic crustal velocity structure, (2) the velocity structure of the East Greenland Ridge is of overall continental type, and (3) a major faulted basin province above highly extended continental crust exists to the NE of the ridge. The results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the NW edge of the East Greenland Ridge and the transition to the NE Greenland margin.

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Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge?

Cited by 129 publications

References 36 publications

Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges

Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges

Plagioclase preferred orientation in layered mylonites : evaluation of flow laws for the lower crust

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

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