Authors / CoAuthors
Czarnota, K. | White, N.
Abstract
Whether rift basins form as a consequence of pure shear or simple shear stretching of the lithosphere or a hybrid of these two end members has long been the focus of debate (McKenzie, 1978; Wernicke, 1985; Rosenbaum et al., 2008). It is generally accepted that under low strain pure shear dominates yet the debate rages with respect to highly extended continental margins. The key dataset to resolve this debate is the spatial distribution of syn-rift and post-rift basin subsidence resulting from mechanical thinning of the lithosphere and subsequent thermal re-thickening of the lithospheric mantle to its pre-rift thickness. An often-overlooked element of this debate is what lithospheric template is being stretched (Crosby et al., 2010). Most geodynamic models simply assume a standard lithospheric thickness of 100120 km, yet in the last decade teleseismic tomography has revealed that much of the Earth's continental land mass is underlain by lithosphere over double this thickness (Priestley and McKenzie, 2013). Here, we kinematically model the subsidence history of the Canning basin following Crosby et al. (2010). This intracratonic rift basin putatively overlies lithosphere 180 km thick, imaged using shear wave tomography (Kennett et al., 2013). The entire subsidence history of the, < 300 km wide and < 6 km thick, western Canning Basin is adequately explained by Ordovician rifting of pre-existing 100120 km thick lithosphere followed by post-rift thermal subsidence as described by the established pure shear model. In contrast, the < 150 km wide and 15 km thick Fitzroy Trough of the eastern Canning Basin reveals an almost continuous phase of normal faulting between Ordovician and Carboniferous Periods followed by negligible post-rift thermal subsidence. This pattern cannot be accounted for by a simple shear model (c.f. Drummond et al., 1991), as there is no record of excess post-rift subsidence in the basin, nor does the data fit the standard pure shear model. We attribute this difference in subsidence to a sharp change in mantle lithospheric thickness between the west and eastern Canning Basin. The presence of ~20 Ma diamond bearing lamproites intruded into the basin depocentre indicate that the present lithospheric thickness exceeds ~180 km (Evans et al., 2012). In order to account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted by 5070 kg m3. The actual depletion of the lowermost lithospheric mantle was assessed by modeling REE concentrations of the ~20 Ma lamproites along with other ultrapotassic rocks from the Kimberley, Yilgarn and Pilbara blocks following the method of Tainton and McKenzie (1994) which reveal a depletion of 4070 kg m3. This result suggests that thermal re-thickening of the lithospheric mantle did not occur following rifting, as it is unlikely that such a strongly depleted mantle source was available in the Phanerozoic to be frozen into the lowermost lithospheric mantle. Therefore, we conclude that thinning of thick lithosphere to thicknesses > 120 km is thermally stable and is not accompanied by post-rift thermal subsidence driven by thermal re-thickening of the lithospheric mantle. The discrepancy between estimates of lithospheric thickness derived from subsidence data in the Western Canning and that derived from shear wave tomography suggests that the latter technique cannot resolve lithospheric thickness variations on < 300 km half wavelengths.
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nonGeographicDataset
eCat Id
87793
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
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Australia
Keywords
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- External Publication
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- WA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2015-01-01T00:00:00
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geoscientificInformation
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[-24.0, -16.0, 119.0, 129.0]
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Spatial Resolution
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