Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2001
Description:
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-l segment (35°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. A small-offset non-transform discontinuity
between two ridge segments is just as stable as two neighboring transform
discontinuities, although a single mantle Bouguer gravity anomaly centered over the non-transform
offset indicates that this boundary does not significantly perturb underlying
mantle flow. Off-axis magnetic anomalies are recorded with high fidelity despite the
very low spreading rates and the absence of a basaltic upper crust in one area. The lower
crust may be the dominant off-axis carrier of the magnetic signal, contrary to traditional
models of crustal magnetic structure. Morphological and gravity data show evidence of
asymmetric crustal accretion across the SWIR ridge axis, with slightly warmer mantle
temperatures beneath the slower-spreading African (north) plate.
Description:
Funding was provided by the National Science Foundation through Contract No.
OCE-9300450 and by the Joint Oceanographic Institutions through Subcontract No.
JSC1-00.
Keywords:
Earth
;
Crust
;
Geophysics
;
Maurice Ewing (Ship) Cruise EW96-08
;
Yokosuka (Ship) Cruise
;
Kairei (Ship) Cruise
;
Conrad (Ship) Cruise RC2709
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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