Using Ambient Noise to Image the Northern East African Rift

Abstract

The northern East African Rift (EAR) is a unique location where we observe continental rifting in the Main Ethiopian Rift (MER) transitioning to incipient seafloor spreading in Afar. Here we present a 3‐D absolute shear wave velocity model of the crust and uppermost mantle of the northern EAR generated from ambient noise tomography. We generate 4,820 station pair correlation functions, from 170 stations (present over 12 years), which were inverted for phase velocity from 8–33 s period and finally for 3‐D absolute shear velocity structure to 60‐km depth. Everywhere in the uppermost mantle, shear velocity is slower than expected for a mantle peridotite composition (<4.1 km/s). This suggests the presence of pervasive partial melt, with focused upwelling and melt storage beneath the MER, where the slowest velocities (3.20 km/s ± 0.03) are observed. Average crustal shear velocity is faster beneath Afar (3.83 km/s ± 0.04) than the MER (3.60 km/s ± 0.04), albeit Afar has localized slow velocities beneath active volcanic centers. We interpret these slow‐velocity regions (including the MER) as magmatic intrusions and heating of the crust. Beneath the northwestern plateau, crustal velocities are laterally heterogeneous (3.3–3.65 ± 0.05 km/s at 10 km), suggesting a complex geological history and inhomogeneous magma distribution during rift development. Comparison between the MER and Afar allows us to draw conclusions between different stages of rifting. In particular, the MER has the slowest crustal velocities, consistent with longer magma residence times in the crust, early during the breakup process.

Mechanism for Deep Crustal Seismicity: Insight From Modeling of Deformation Processes at the Main Ethiopian Rift

“…Several lines of evidences suggest that deformation style and seismicity in the MER vary along (e.g., Déprez et al., 2013; Muluneh et al., 2017) and across the rift (Keranen et al., 2009; Kogan et al., 2012). Future studies should address the role of contrasting rheologies and thermal properties between the plateau and the rift in controlling the depth distribution of seismicity (e.g., Chambers et al, 2019).…”

Accommodation of East African Rifting Across the Turkana Depression

“…Landscape features in the profiles are labeled, including Ta = Lake Tana; A = Addis Ababa; LA = Lake Abaya; O = Omo River; CB = Chew Bahir; T = Teltele Plateau; LT = Lake Turkana; K = Kino Sogo Fault Belt; H = North Horr; HH = Hurri Hills; LN = Lake Navisha; N = Nairobi. Major faults indicated with blue lines and volcanic centers with dashed red lines (Ebinger et al, 2000;Vetel et al, 2005;Brune, 2016;Corti et al, 2018). Note the Kenya Profiles has slightly longer axes then the other four profiles to better view the data.…”

“…The relay from hybrid extension in the MER to higher strain rate and localized extension in the Turkana Depression also precludes distributed stretching related to preexisting crustal weaknesses or obliquity of the Depression, as proposed in some models of the study area (Agostini et al, 2009;Brune, 2016;Corti, 2012). In particular, the Anza Graben has been cited as an inherited structure that could influence strain partitioning in the Turkana Depression (Figure 1) (Brune, 2016), but the Anza Graben appears to be aseismic, and lacks morphological evidence for young faults (e.g., Vetel & Le Gall, 2006). The length scale over which strain is accommodated in the Turkana Depression is only tens of kilometers ( Figure 4) and finite strain rates do not extend into or toward the Anza Graben.…”

Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography

“…A major barrier to understanding magmatic processes subsurface has been that the majority of passive seismic stations were deployed near the rift valley only, preventing comparison to off rift structures. In addition, despite the numerous seismic images created, absolute seismic velocities for the crust and upper mantle are unconstrained beneath a broad region including multiple sectors of the rift and to significant distances off rift (Chambers et al, 2019;Keranen et al, 2009). Seismic tomography provides constraints on the elastic properties of the Earth, which are used to understand tectonic and magmatic processes.…”

“…The rift has been extensively imaged, creating multiple velocity models for smaller disparate sections of the region, which are not necessarily directly comparable due to variations in methodology, resolution, and scale. For example, ambient noise tomography (ANT) has been used to produce 3 models of the northern EAR that are not directly comparable due to variations in the type of velocity (phase, group and shear velocity) and regions covered (Afar, central Main Ethiopian Rift and broader rifted regions) (Chambers et al, 2019;Kim et al, 2012;Korostelev et al, 2015). Comparisons between models are important for determining the relationship between magmatic and tectonic processes including variations in plate structure, melt generation and melt migration processes.…”