passive margins

passive margins Continental margins, the linear zones that mark the position of the edge of continental crust, are classified as passive or active according to whether they are attached to adjacent oceanic crust, as part of the same plate, or detached from it across a plate boundary. Passive margins may be further subdivided according to their maturity, and whether or not they are protected by offshore island arcs. Extensive mature passive margins fringe the Atlantic Ocean, East Africa, India, Antarctica, and the western seaboard of Australia. The best example of an immature passive margin is the Red Sea, only 10 Ma old, an embryonic ocean. As well as providing the setting for much of the world's oil and gas, most of the developed and developing world is bordered by passive margins. Consequently, oil companies and national governments alike have invested considerably in understanding their development.

Mature passive margins develop during oceanic widening as part of a continuous cycle of ocean opening and widening, subsequent shrinking, continental collision (orogenesis), and continental splitting (rifting). Consequently, they display characteristic syn-rift and post-rift sedimentary successions related, respectively, to the transition from rifting and isostatic subsidence to the cessation of lithospheric extension and the onset of ‘passive’ thermal subsidence that accompanies the production of new oceanic crust.

Continental rifting commonly continues until the crust has been thinned to practically zero thickness, such that mantle rocks erupt at the surface to form new oceanic crust. Studies of present and former continental rift basins show that once the crust has been thinned to half of its normal 35 km thickness liberation of basaltic partial melts results in extrusive volcanism. Volcanic rocks of Middle Jurassic age mark the point in time in which the North Sea crust was thinned beyond this critical limit. However, further rifting ceased soon after this episode of volcanism, and the basin did not develop into a fully fledged ocean basin.

The onset of ocean opening is signified by two major changes: the cessation of extensional faulting, and the thermal relaxation of thinned, and therefore hot, lithosphere. These changes mark the onset of post-rift deposition. The base of the post-rift sequence is usually defined by a marked, end-rift unconformity, recording the depositional hiatus accompanying the thermal subsidence of a formerly uplifted region of net erosion and non-deposition. Interpretation of boreholes drilled into passive margins reveals a distinct transition from generally land-based syn-rift sedimentation to deepening marine environments of deposition during the post-rift phase. Thermally subsiding basins are always considerably larger than the area of their precedent rift basin. Marine flooding of the end rift unconformity, and the deepening of the post-rift marine environment, reflects the areal extent and prolonged nature of thermal subsidence (Fig. 1). Using borehole data, particularly from the eastern United States shelf, modelling of the subsidence history of passive margins has revealed that initial thermal subsidence occurs typically at a rate of 30 m per million years, subsequently decaying exponentially until it stabilizes some 150 Ma later.

Jonathan P. Turner