Observational characteristics of non-Atlantic extensional systems (onshore)
2007. "Observational characteristics of non-Atlantic extensional systems (onshore)", Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup, G. D. Karner, G. Manatschal, L. M. Pinheiro
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The Red Sea is an ideal locale for testing differing models and hypotheses for rift evolution and the initiation of sea-floor spreading. The Red Sea is an active rift system that formed by the rifting of Precambrian continental lithosphere beginning in the late Oligocene, leading to breakup and sea-floor spreading by approximately 5 Ma in the southern Red Sea to the south of about 19°30′N. In the northern Red Sea, north of approximately 23°30′N, organized sea-floor spreading is not observed, although individual volcanoes are located within discrete ‘deeps’ spaced along the axial depression. These have been interpreted as marking the beginning of a transition from amagmatic, mechanical rifting to magmatic, oceanic spreading. Based on seismic reflection and refraction, gravity, magnetic and heat flow data in the northern Red Sea, it has been suggested that rift development occurs via the rotation of large crustal fault blocks that sole into a zone of plastic creep in the lower crust, resulting in a flat Moho and high upper crustal relief. Melt formed within individual rift segments is focused to form small axial volcanoes. That is, the northern Red Sea is on the verge of replacing horizontal translation with focused mantle upwelling and organized sea-floor spreading. In marked contrast, many passive margins (e.g. West Africa, Brazil, and NW and NE Australia) are characterized by the stacking of regional synrift sag packages, the thickness and distribution of which are inconsistent with the minor amounts of brittle deformation mapped from seismic sections. A fundamental implication of this observation is that rifts characterized by large offset fault systems, (i.e. faults that generate synrift accommodation, such as in the Basin and Range province and East Africa) will not proceed to breakup. The challenge is to understand why different portions of the Red Sea show different stages in the development of a spreading centre during continental rifting. Two hypotheses exist: (1) the structural framework deduced in the north simply continues to the south where sea-floor spreading exists and that the two regions are registering a difference in total extension. Thus, the northern Red Sea has experienced insufficient extension to breach the continental lithosphere but, in time, should develop into a spreading centre; or (2) the lithosphere of the northern Red Sea region is rheologically stronger compared with the lithosphere of the southern Red Sea, perhaps as a consequence of the thermal effects of the Afar plume, and the northern Red Sea will never evolve to sea-floor spreading. The existence of large rotated fault blocks, as implied from the inversion of gravity and magnetic anomaly data, favours the latter.
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This book summarizes our present understanding of the formation of passive continental margins and their ocean–continent transitions. It outlines the geological, geophysical and petrological observations that characterize extensional systems, and how such observations can guide and constrain dynamic and kinematic models of continental lithosphere extension, breakup and the inception of organized sea-floor spreading. The book focuses on imaging, mapping and modelling lithospheric extensional systems, at both the regional scale using dynamic models to the local scale of individual basins using kinematic models, with an emphasis on capturing the extensional history of the Iberia and Newfoundland margins. The results from a number of other extensional regimes are presented to provide comparisons with the North Atlantic studies; these range from the Tethyan realm and the northern Red Sea to the western and southern Australian margins, the Basin and Range Province, and the Woodlark basin of Papua New Guinea. All of these field studies, combined with lessons learnt from the modelling, are used to address fundamental questions about the extreme deformation of continental lithosphere.