[1] We analyze geophysical data that extend from 0 to 25-Myr-old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a $400 km wide corridor between large-offset fracture zones of the SWIR. In contrast to the Mid-Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present-day rift valleys. This pattern appears to be primarily a function of along-axis variations in crustal thickness, rather than magnetic mineralogy. Off-axis, magnetization amplitudes at paleo-segment ends are more positive than at paleo-segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo-segment midpoints reduces magnetization amplitudes by 70-80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.
Abstract. We present the crustal and mantle velocity structure along the strike of the eastern rift mountains at 35øN on the Mid-Atlantic Ridge. These results were obtained by an inversion of -1800 Pg/Pn and-•450 PmP travel times and by gravity modeling. As commonly observed at slow spreading mid-ocean ridges, thicker crust (9 km) occurs at the segment midpoint, while thinner crust (7 km) is found toward the segment ends. This along strike variation occurs primarily in the lower crust, which is 7 km thick at the segment center and 4-6 km thick at the segment ends. In contrast, the thickness of the upper crust is relatively constant along strike. At the segment ends, relatively low velocities extend for 10-15 km along strike and from the seafloor to 4 km depth. These low velocities may indicate an attenuated melt supply and/or fracturing and alteration within the shallow to mid-crust. Directly beneath a cluster of three seamounts at the segment center is a region of relatively high velocity (+0.5 km/s) in the mid-crust. This feature may correspond to a frozen magma chamber that fed the overlying volcanoes. A synthesis of these results with those from two companion experiments along the rift valley and the conjugate flank provide a detailed record of crustal accretion and evolution at this segment. Specifically, the crustal velocity structures of each flank are nearly identical, and they exhibit a thinner and 16% faster upper crust than is observed on axis. The lower crust is remarkably similar in all three settings, except for a low-velocity body on axis, which is interpreted as a partially molten zone. The maximum crustal thickness is also similar in all three profiles, but north of the segment center, zero-age crust is nearly 4 km thinner than beneath the eastern flank and 2 km thinner than beneath the western flank. These differences may indicate that segment-centered mantle upwelling varies on a timescale of-2 m.y.
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