The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

Derek Keir,Francesco Mazzarini,Ian D Bastow,Giacomo Corti,Tyrone O Rooney

doi:10.1002/2014tc003698

Abstract

The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

Highlights

Continental rifts display significant along-strike variations in volumes of magmatism that causes a heterogeneous igneous record along ancient rifted continental margins
The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north
The Miocene-Recent Main Ethiopian Rift (MER) accommodates extension between the Nubian and Somalian Plates, constituting the northern part of the East African rift system, and forms the youngest arm of the rift-rift-rift triple junction currently positioned in central Afar (Figure 1) [e.g., Tesfaye et al, 2003; Wolfenden et al, 2004; Ayele et al, 2007]

Summary

Introduction

Continental rifts display significant along-strike variations in volumes of magmatism that causes a heterogeneous igneous record along ancient rifted continental margins. Increased volumes of magma intrusion have previously been attributed to enhanced melting of the mantle caused by elevated potential temperature [e.g., White and McKenzie, 1989; White et al, 2008], anomalous volatile content in the asthenosphere [e.g., Lizarralde et al, 2007; Shillington et al, 2009], or higher extension rate [Bown and White, 1995]. Rifting kinematics has been suggested to influence the temporal development of melting and locus of intrusion, with oblique extension causing accelerated localization of deformation to a narrow axial zone and facilitating more localized plate thinning [e.g., Corti et al, 2003]. Ongoing seismic and tectonic deformation in the MER [e.g., Biggs et al, 2011; Keir et al, 2009; Pagli et al, 2014] and KEIR ET AL

Results

Discussion

Conclusion