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B.Sc. (Honours) Thesis

COOLING HISTORY OF A RIFTED PASSIVE MARGIN – NEW INSIGHTS FROM (U-TH)/HE THERMOCHRONOLOGY ALONG THE LABRADOR PASSIVE MARGIN

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The Labrador Sea is the result of rifting and breakup between Labrador and Greenland. The asymmetry of magmatism, bathymetric/topography, sediment distribution and crustal structure along both margins suggests a lithospheric scale simple shear model of rifting. The initiation of rifting has been attributed to the Late Triassic to Jurassic (223–150 Ma) based on phlogopite Rb-Sr dating and U-Pb dating of perovskite found in dikes emplaced in southwest Greenland. Less magmatic activity from rifting has been found on the Labrador side with the oldest ages determined through fossil evidence of a diatreme yielding Early Jurassic–Early Cretaceous ages (197-145 Ma). There is still ongoing debate on the initiation of seafloor spreading, which represents the end of rifting. The oldest undisputed evidence of oceanic crust formation has been dated to 63.0 ± 0.7 Ma correlating to magnetic anomaly polarity Chron C27. Older ages have been attributed to 72.1 Ma (Chron C32) for Southern Labrador and 66.0 Ma (Chron C28) for northern Labrador. Cessation of spreading has been determined stratigraphically to the late Eocene to early Oligocene. Low temperature thermochronometry includes a range of methods used to retrieve the thermal history of the uppermost crust, allowing to date and identify tectonic, magmatic and/or surface processes that have contributed to this thermal history. In this study, five bedrock samples were collected along a 200 km transect along the Labrador passive margin between Nain (56.5417°N, 61.6969°W) and Hopedale (55.4580°N, 60.2115°W). Samples were dated using apatite and zircon (U-Th)/He thermochronometry, with closure temperatures of 70°C and 170°C, respectively. A numerical modelling software called HeFTy was used to quantify the crustal cooling of the margin to help produce time – temperature paths denoting exhumation. Ages obtained from our samples showed slow cooling followed by rapid exhumation initiating during the Jurassic – Early Cretaceous contemporaneous to initiation of rifting.Time-Temperature cooling paths showed a northward decreasing age of initiation of cooling which was interpreted to be due to diachronous rifting from south to north yielding progressively younger ages in the North.

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Pages: 84
Supervisor: Isabelle Coutand