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Abstract

The detachment (“simple-shear”) model of continental extension predicts that opposing passive- margin pairs are asymmetrical. The asymmetry is reflected in the morphology, structure, uplift/ subsidence history and/or thermal evolution of the passive margins. This results in two broad classes of passive margins—upper-plate margins in the hanging wall of the detachment, and lower-plate margins in the footwall. The detachment model applied here incorporates pure-shear extension above and below the master detachment, and ramp-flat geometry for the detachment.

The detachment model is applied to the United States Atlantic-northwest Africa and the southern Australia-Antarctica margin pairs in some detail, and to the southern and northern Atlantic Oceans, the Tasman Sea margins and the Provencal basin in lesser detail. Asymmetry of various characteristics has been recognized in all cases, and upper- versus lower-plate settings have been determined. In the case of the United States Atlantic-northwest Africa margins, a switch in asymmetry along the margin pair is interpreted across a major transfer fault. On the United States Atlantic margin, the Blake Plateau is considered to be an upper-plate margin, whereas the Carolina-Baltimore Canyon Trough segments exhibit lower-plate characteristics. An attempt is made to palinspastically reconstruct the southern Australia-Antarctica margins, including the continental extension of more than 200 km.

Application of the detachment model to these passive margins has resulted in a number of outcomes of general interest. Most importantly, attempts to make a simple distinction between “pure-shear” and “simple-shear” models of extension should be abandoned in favor of determination of the three- dimensional finite deformation field across the whole extended region. The model supports the view that at least some of the magnetic quiet zones (MQZ) that occur oceanward of many passive margins consist largely of highly extended, metamorphosed and underplated (intruded) continental crust rather than oceanic crust. Subsequent subduction of this thin continental crust will potentially lead to arc magmas with distinct continental signatures. Predrift reconstruction of passive margins can be substantially refined by (1) careful determination of the lithospheric strain field (including detachment geometry) during continental extension, and (2) recognition of major transfer faults on opposing margins.

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