Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

Finnigan Illsley‐Kemp,James O S Hammond,Jonathan M Bull,Derek Keir,Cynthia Ebinger,J.‐Michael Kendall,Berhe Goitom,Atalay Ayele,Manahloh Belachew,Thomas M Gernon

doi:10.1002/2017jb014902

Abstract

AbstractContinental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late‐stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ∼64% of the seismic moment release and exhibits a swarm‐like behavior. In contrast, seismicity at the marginal graben is characterized by high‐frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low‐frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ∼12 km beneath Alu‐Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E‐W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long‐term extension.

Highlights

The breakup of continents to form ocean basins is a fundamental process in plate tectonic Wilson Cycles, the processes governing the transition from continental rifting to seafloor spreading have largely remained enigmatic
The earthquake locations can be broadly categorized into two groups: seismicity at the rift axis and at the western marginal graben system (Figure 3)
We use a new catalogue of over 4951 local earthquakes in the Danakil region of Northern Afar to show that seismicity is focused at both the rift axis, and the western marginal graben (Figure 3)

Summary

Introduction

The breakup of continents to form ocean basins is a fundamental process in plate tectonic Wilson Cycles, the processes governing the transition from continental rifting to seafloor spreading have largely remained enigmatic. It is unclear how crustal extension is accommodated and whether it predominantly occurs through mechanical faulting of the crust or magmatic activity. Numerical models demonstrate that magmatic activity plays an important role in strain localization from rift onset to plate rupture [e.g., Buck, 2004; Bialas et al, 2010; Allken et al, 2012]. The role that mechanical faulting and seismicity play in the final stages of continental rifting remain poorly understood, in large part because the process is occurring in only a few remote areas worldwide [e.g., Bosworth et al, 2005; Bastow and Keir, 2011; Persaud et al, 2016]

Results

Discussion

Conclusion