Crustal processes associated with the final stages of continental rifting in Northern Afar

Abstract

The transition from continental rifting to seafloor spreading is a key, yet enigmatic, stage of the plate tectonic Wilson Cycle. The Danakil region in the northern Afar depression marks the transition between late-stage continental rifting in the Main Ethiopian rift, and young seafloor spreading in the Red Sea, it is thus an ideal location to investigate the final stages of continental breakup. Here, the southern Red Sea rift steps on land and the active processes associated with continental breakup are exposed subaerially. Between February 2011 and February 2013, two seismic networks were installed in northern Ethiopia and Eritrea. In this thesis, continuous seismic data is used to detect and locate 4951 earthquakes and investigate how seismicity accommodates ex- tension and deformation as the region transitions to seafloor spreading. Results show that seismicity is focused at the rift axis and within a graben on the western rift margin. Seismicity at the rift axis accounts for ∼64% of the total seismic moment release and reveals a lower crustal magma complex beneath the Alu-Dalafilla volcanic system. This suggests that the majority of mechanical extension is focused at the rift axis, as is observed in the Main Ethiopian rift. However, seismic moment release rate shows that extension is predominantly accommodated though magmatism and this produces stacked sill magma plumbing systems, analogous to those seen at mid-ocean ridges. In contrast to the Main Ethiopian rift, high levels of seismicity are observed at the western rift margin. Earthquake focal mechanisms and seismic anisotropy show that extension in the marginal graben is oriented E-W, oblique to extension at the rift axis. The seismicity here predominantly occurs on an antithetic fault, extending to depths of ∼20 km. Large, antithetic faults have been observed in rifted margins worldwide, this research suggests such structures may form and remain active in the final stages of continental rifting. In addition, seismological data is combined with remote structural geology, geodetic observations and three-dimensional thermomechanical numerical modelling to show that an oceanic transform fault is initiating in the Danakil depression. This is the first direct observation of this process and demonstrates that these oceanic structures can initiate in the final stages of continental rifting, prior to seafloor spreading. The findings of this thesis reveal that many processes and structures which are associated with seafloor spreading can form during the final stages of continental breakup. Thus, the continent-ocean boundary must not be treated as a discrete point in space or time.

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Identifiers

ORCID for Finnigan McGowan Illsley-Kemp: orcid.org/0000-0002-7114-033X

ORCID for Derek Keir: orcid.org/0000-0001-8787-8446

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