Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins

Abstract

Traditionally active rifts are thought to evolve in response to thermal upwelling of the asthenosphere, whereas passive rifts develop in response to lithospheric extension driven by far-field stresses. Depending on the volumes of extension-related magmatism two end-member passive margin types, either volcanic or magma-poor are defined. Volcanic rifted margins evolve by extension accompanied by extensive extrusive magmatism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor rifted margins in contrast show wide domains of extended crust with wide-ranging extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be systematically delayed to post-breakup.In this study three unique rifted margins are presented: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the South China Sea that may represent an intermediary form of continental extension between the end member extremes, and the southernmost South Atlantic with well expressed conjugate volcanic rifted margins, which are traditionally interpreted as result of a mantle plume, the Tristan da Cunha hot-spot.The accurate timing of the events when continental rifting initiated and stopped, and when subsequent sea-floor spreading began is crucial to refine models of margin development. Therefore, a detailed description of rift-onset and breakup unconformities is presented for the three continental margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively. The investigation reveals that a major controlling mode of deep, hot-spot related mantle processes on the rift evolution and rift-related magmatism cannot be observed. Instead, it is suggested that passive margin evolution is controlled by several lithosphere-scale processes and parameters, including the mode of rift propagation and propagation barriers.