Problems of passive margins from the viewpoint of the geodynamics project: a review

Abstract

Passive margins form by continental splitting which may follow an early pre-split graben stage of development. Such margins are divisible into rifted and sheared types. After formation the margins develop by predominantly vertical tectonics as the intervening ocean widens by seafloor spreading. The plate splitting mechanism is not yet understood, but evidence from aseismic ridges and associated continental volcanism is relevant. Location of the original continent-ocean contact is also problematical in many regions, especially where quiet magnetic zones occur above crust of uncertain status. Rifted margins are notable for great subsidence resulting from early graben formation and later flexural subsidence of the shelf and slope regions. Graben formation is attributed to wedge subsidence of the upper crust in response to crustal stretching, possibly associated with doming before splitting. Four factors have been recognized as contributing to flexural subsidence: gravity loading, thermal subsidence of the adjacent oceanic lithosphere, possible thermal subsidence of the continental lithosphere following heating and erosion at the time of break-up, and thinning of the continental crust by seaward creep of lower crustal material. The relative importance of these four mechanisms needs clarification.