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

• Main Author: Hosford, Allegra
• Other Authors: WOODS HOLE OCEANOGRAPHIC INSTITUTION MA
• Format: Text
• Language: English
• Published: 2001
• Subjects: Geography; *EVOLUTION(GENERAL); *EARTH CRUST; VELOCITY; TIME INTERVALS; THICKNESS; OPENING(PROCESS); AGING(MATERIALS); CRACKS; RATES; THESES; EXTRUSION; MELTS; SEISMIC DATA; GRAVITY; REFRACTION; SUPPLIES; ACCUMULATION; SEALING COMPOUNDS; VALLEYS; OCEANIC CRUST; SEAFLOOR SPREADING; ANTARCTIC REGIONS; RIDGES; Antarctic; The Antarctic; Indian; Mid-Atlantic Ridge; Antarc*
• Online Access: http://www.dtic.mil/docs/citations/ADA397383; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA397383

Half of the ocean crust is formed at spreading centers with total opening rates less than 40 km/Myr. 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 km/Myr) and the ultra-slow spreading Southwest Indian Ridge (SWIR) (full rate, 14 km/Myr). 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.