Crustal structure of the Mid-Atlantic ridge crest at 37  N

Abstract

A structural model of the Mid-Atlantic Ridge at 37" N is proposed on the basis of travel-time data and synthetic seismograms. At the ridge axis the crust is only 3 km thick and overlies material with an anomalously low 'upper mantle' velocity of 7.2 km s-'. Crustal thickening and the formation of layer 3 and a layer with velocity 7.2-7.3 km s-l takes place within a few kilometres of the axis, producing a 6-7 km thick crust by less than 10 km from the axis. A normal upper mantle velocity of 8.1 km s-l exists within 10 km of the axis. Shear waves propagate across the axis, thus precluding the existence of any sizeable magma chamber at shallow depth.

Igneous Petrogenesis

“…For example, geophysical studies of segments of the East Pacific Rise have revealed the existence of a low-velocity zone beneath the ridge axis (Orcutt et al 1975, McClain et al 1985, which could represent a magma reservoir. In contrast, similar studies of the much slower spreading FAMOUS area of the Mid-Atlantic Ridge have failed to reveal any substantial axial magma chamber (Fowler 1976).…”

The Oblique Seismic Experiment on Deep Sea Drilling Project Leg 52

“…Comparison between the observed signals on the geophone channels at short ranges and the theoretical explosive source function (Fowler, 1976a) shows that the theoretical source is much shorter than the apparent direct arrival (Figure 10). Also, the first bubble pulse wavelet is greater than the initial wavelet.…”

“…The causes of intrinsic attenuation are reviewed by Jackson and Anderson (1970) and Gordon and Nelson (1966). Residual attenuation is a worthwhile quantity in its own right in the interpretation of crustal structure using amplitude analysis (Kennett, 1975;Fowler, 1976b). It may also give a measure of inhomogeneity in the crust.…”

The tectonics and three-dimensional structure of spreading centers : microearthquake studies and tomographic inversions

“…The dashed line shows the plane-layered structure assumed for calculating microcarthquake locations. The velocity structure determined by Fowler (1976] for the median valley of the Mid-Atlantic Ridge near 37'N is also shown. See text for a discussion of the effects of varying the assumed velocity model on hypocenter determinations.…”

“…The locations of the 18 events with good depth control were also estimated using the crustal structure proposed by Fowler [1976] for the median valley of the Mid-Atlantic Ridge near 37*N. This P wave velocity structure, shown in Figure 6, differs significantly from the model used above, including somewhat higher velocities through much of the crust but a lowervelocity upper mantle. A V,/V, ratio of 1.75 was assumed to obtain the corresponding S wave velocity structure.…”