Gwenn Peron-Pinvidic

Continental Rifted Margins 1


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main theoretical categories of rifting: active and passive. Active rifting is controlled by the warm asthenospheric mantle that rises beneath the base of the continental lithosphere, causing thinning of the plate and uplift at the rift zone. Passive rifting is controlled by distant extensional stresses, causing stretching and lithospheric thinning and passive rise of the asthenosphere below the rift zone

       1.2.2. Plate tectonic setting classification

      We review the main rift classifications and their characteristics. In order to construct a clear list of the various types of rifts, we follow Ruppel’s (1995) early approach and propose that we first classify rifts based on their regional tectonic setting: intracontinental, convergent and divergent. This results in an overarching basic classification that has the advantage of embracing the majority of rift basin types, although some hybrid and alternative cases are always possible. The influencing parameters will be summarized in the next chapter.

       1.2.2.1. Intracontinental rifts

      Intracontinental rifts develop in the interior of a continental plate, far from any plate boundary, thus with no apparent direct tectonic connection to the regional stress fields produced by plate tectonics. However, continental plates can deform internally in response to far-off changes in stress conditions, such as changes in the plate boundary configuration (e.g. changes in the dip/velocity/direction of the subduction slab). Intracontinental rifts are classified according to their geometry (narrow vs. wide) or their structural setting (aulacogens and intracratonic rifts).

      Narrow intracontinental rifts

Schematic illustration of the narrow rift mode.

       Case example: The East African Rift

      The geological reason and exact mechanisms responsible for the formation of the EAR are debated. Dating volcanic rocks from the Main Ethiopian Rift indicates that the rifting process probably began ~30 Ma ago with the eruption of voluminous flood basalts, which resulted in the formation of the Ethiopian and Somalian plateaus (Corti 2009; Rooney 2017). The basins’ initiations are diachronous along the EAR with distinct episodes of extension followed by periods of relative tectonic quiescence. The fault systems responsible for the formation of the separate rift basins are understood to date from Oligocene and Mio-Pliocene times (Chorowicz 2005).

      The observation and mapping of thick volcanic sequences apparently emplaced before the rift-faulting events favored the development of plume theories. Since the observations are not straightforward to interpret and the models multiple, several plume hypotheses have been developed. These invoke the presence of several distinct localized plumes (Kenya, Ethiopia, Arabia) (Montelli et al. 2006), a super-plume (Ebinger and Sleep 1998; Bastow et al. 2008) or a composite scenario including a deep plume originating at the core–mantle boundary and feeding multiple plume-stems in the upper mantle beneath the EAR (Hansen et al. 2012). Tectonic theories focus instead on the regional tectonic setting; the development of extensional stresses is due to plate reorganization from a ridge jump in the Indian Ocean (Burke 1996). Mantle convection cells activated due to lithospheric thinning (producing thermal gradients) and/or similar, cratonic-edge-driven cells can then explain melt genesis and volcanic intrusions and extrusions (King and Ritsema 2000). Ongoing discussions and debates may today favor a composite origin with both plate tectonics and plume magmatism contributing to the extensional geometries, with feedback effects on one another (Rooney 2017, 2019, 2020a, 2020b, 2020c).

       The aulacogen case