| dc.contributor.author | Osler, John Collamer. | en_US |
| dc.date.accessioned | 2014-10-21T12:37:42Z | |
| dc.date.available | 1993 | |
| dc.date.issued | 1993 | en_US |
| dc.identifier.other | AAINN87487 | en_US |
| dc.identifier.uri | http://hdl.handle.net/10222/55363 | |
| dc.description | Two dimensional dynamic models for mantle circulation beneath mid-ocean ridges are characterized by two end members. Models with passive upwelling induced by the separation of the lithospheric plates predict a broad region of partial melting, lateral migration of melt to the accretionary axis, and a dependence of crustal thickness upon spreading rate. Models with flow dominated by buoyancy forces localized beneath the spreading centre predict that upwelling occurs rapidly in a narrow zone and crustal thickness is independent of spreading rate. The Labrador Sea contains a rare example of an abandoned mid-ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates. As its thermal regime and spreading rate must have changed as spreading stopped, studying the crustal structure provides a means by which the predictions of thermal and petrological models for the processes and structure of active spreading centres may be assessed. | en_US |
| dc.description | Seismic refraction data were collected along two refraction lines in the Labrador Sea, R1 along strike of the extinct ridge and R2 crossing it orthogonally. One and two dimensional analyses of the refraction data using travel time and synthetic seismogram techniques reveal major variations in crustal thickness and velocity. In the extinct spreading centre, a crustal thickness of approximately 4 km is determined, compared with 5.5 km for the flanks. Substantial lateral variations in P-wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading centre. Low velocities are also observed in the uppermost mantle underlying the extinct spreading centre and are interpreted as being the result of hydrothermal alteration. | en_US |
| dc.description | The anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction. The observations are consistent with thermal models which suggest a spreading rate dependence on crustal generation at slow spreading rates, though the thinning is not as appreciable as predicted. The seismic structure is also used to develop two dimensional gravity models along line R2. These models support the seismic observations of crustal thinning and demonstrate that the gravity field does not require a deep low density gabbroic root zone to underlie the extinct spreading centre. | en_US |
| dc.description | Thesis (Ph.D.)--Dalhousie University (Canada), 1993. | en_US |
| dc.language | eng | en_US |
| dc.publisher | Dalhousie University | en_US |
| dc.publisher | | en_US |
| dc.subject | Geophysics. | en_US |
| dc.subject | Physical Oceanography. | en_US |
| dc.title | Crustal structure of the extinct spreading centre in the Labrador Sea: Implications for dynamic models of flow beneath mid-ocean ridges. | en_US |
| dc.type | text | en_US |
| dc.contributor.degree | Ph.D. | en_US |
