40Ar/39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone,Cape Breton Island,Canadian Appalachians |
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| Institution: | 1. ARC Center of Excellence for Core to Crust Fluid Systems (CCFS), Curtin University, Perth 6845, Australia;2. The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, Perth 6845, Australia;3. Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;4. Institute of Earth Science, China University of Geosciences, Beijing 100083, China;5. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan;2. Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa;3. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China;4. Centre for Tectonics Resources and Exploration, Department of Earth Sciences, University of Adelaide, SA 5005, Australia;1. Institute for Study of the Earth''s Interior, Okayama University, Misasa, Tottori 682-0193, Japan;2. Sobolev Institute of Geology and Mineralogy, Siberian Branch Russian Academy of Sciences, Novosibirsk 630090, Russia;3. Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, 2109, Australia;4. Novosibirsk State University, Novosibirsk 630090, Russia |
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| Abstract: | The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ～5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ～1 km wide that formed at a more shallow level. Hornblende 40Ar/39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ～565 and ～545 Ma; in the younger zone (within 4.5 km of the centre), they are between ～425 and ～415 Ma; and in the transitional zone in between, they decrease abruptly from ～545 to ～425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ～28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (～500°C) until differential uplift occurred. The 40Ar/39Ar ages in the zone (～425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ～565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones. |
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