Crustal Accretion and Evolution at Slow and Ultra-Slow Spreading Mid-Ocean Ridges
WOODS HOLE OCEANOGRAPHIC INSTITUTION MA
Pagination or Media Count:
Half of the ocean crust is formed at spreading centers with total opening rates less than 40 kmMyr. The objective of this Thesis is to investigate temporal variations in active ridge processes and crustal aging at slow-spreading centers by comparing axial crustal structure with that on conjugate flanks of the slow-spreading Mid-Atlantic Ridge MAR full rate, 20 kmMyr and the ultra-slow spreading Southwest Indian Ridge SWIR full rate, 14 kmMyr. Seismic refraction data collected along the rift valley and flanking rift mountains of the OH-1 segment 35 deg N at the MAR show that the entire crustal section is constructed within a zone that is less than 5 km wide. Shallow-level hydrothermal circulation within the axial valley is suggested by the rift mountain seismic profiles, which show that the upper crust is 20 thinner and 16 faster along strike than zero-age crust. These effects probably result from fissure sealing within the extrusive crust. Deeper crustal velocities remain relatively constant at the segment midpoint within the first 2 Myr, but are reduced near the segment offsets presumably by faulting and fracturing associated with uplift out of the rift valley. A temporal variation in axial melt supply is suggested by a 15 difference in along-strike crustal thickness between the rift valley and rift mountains, with relatively less melt supplied today than 2 Ma. Crustal accretion at the SWIR appears to occur in a similar manner as at the MAR, although gravity and seismic data indicate that the average crustal thickness is 2-4 km less at the ultra-slow spreading SWIR. A 25 Myr record on both flanks of the ridge shows that seafloor spreading has been highly asymmetric through time, with 35 faster crustal accretion on the Antarctic south plate.