Crustal Accretion and Evolution at Slow and Ultra-slow Spreading Mid-Ocean Ridges

Allegra Hosford Scheirer, Ph.D., 2001
Jian Lin and Maurice Tivey, Advisors

This thesis examines variations in ridge processes and crustal aging at slow-spreading centers. Seismic refraction data collected on 0 and 2 Myr-old crust on the Mid-Atlantic Ridge show that the crustal section is constructed within a narrow zone (<5 km). Hydrothermal circulation within the axial valley is indicated by a thinner and faster upper crust off axis relative to on-axis crust. Off axis, deeper crustal velocities near the segment offsets are reduced from the on-axis values, presumably by fracturing during uplift out of the rift valley. A 15% smaller crustal thickness beneath the rift valley than beneath the rift mountains suggests a smaller axial melt supply today than 2 Ma. Seafloor spreading has been highly asymmetric at the Southwest Indian Ridge since at least 25 Ma, with a 35% larger half-rate on the Antarctic plate. A non-transform discontinuity is just as stable as neighboring transform discontinuities, although gravity data suggest that this boundary does not significantly perturb mantle flow. Magnetic data indicates that the lower crust may be the dominant off-axis source of magnetic anomalies. Morphological and gravity data show evidence of asymmetric crustal accretion across the ridge axis, with slightly warmer mantle temperatures beneath the slower spreading African plate.