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Crustal Structure and Upper Mantle Anisotropy of the Afar Triple Junction U Kumar et al. Earth, Planets and Space (2021) 73:166 https://doi.org/10.1186/s40623-021-01495-0 FULL PAPER Open Access Crustal structure and upper mantle anisotropy of the Afar triple junction U. Kumar1, C. P. Legendre1,2* and B. S. Huang1 Abstract The Afar region is a tectonically distinct area useful for studying continental break-up and rifting. Various conficting models have been suggested to explain the lateral variations of the anisotropy in this region. To address this issue, we investigated the tectonics of the Afar region using receiver function and shear-wave splitting measurements based on broadband seismic data from 227 stations in the region. Further, the receiver function results were inverted to obtain the crustal thickness and Vp/Vs ratio of the region. Our results reveal a thick African crust (thicker than 40 km) with typical Vp/Vs values for the continental crust, elongated down to 21 km along the rift system with very high Vp/Vs values near the fractured zones, suggesting crustal thinning near the fractured zones. Our shear-wave splitting measurements indicate a general fast axis orientation of N030E. However, substantial disparities in the fast anisotropy direction exist in the triple junction region, with some stations displaying a direction of N120E, which is perpendicular to the fast directions measured at the surrounding stations. In addition, many stations located close to the rifts and within the Arabian Plate provide mostly null measurements, indicating the presence of fuids or isotropic media. This study uses several methodologies to unravel the structure and evolution of the Afar region, providing valuable insight into the Afar, a tectonically distinct region, which will be useful for elucidating the mechanisms and characteristics of a continental break-up and the rifting process. Keywords: Afar, Continental break-up, Continental rifting, Crustal structure, Receiver function, Shear-wave splitting Introduction called East African Rift System), an active continental rift Geological settings zone in East Africa, is a divergent tectonic plate bound- Te Afar triple junction (Fig. 1) is located in East Africa ary (Kusky et al. 2010) dividing the African Plate into connecting three branches of a complex rift system. Tis two tectonic units: the Nubian and Somalian plates (Gar- area is an example of the last stage of continental rifting funkel and Beyth 2006). Te Afar triple junction accom- and the early stage of seafoor spreading, leading to the modates the divergent motions between the Arabian, creation of oceanic crust (Mohr 1970; Tesfaye et al. 2003; Nubian, and Somalian plates along the Red Sea, Gulf of Rychert et al. 2012). Te Afar hotspot is located at the tri- Aden, and East African rifts. Te kinematics of the Afar ple junction between the Red Sea rift, the Gulf of Aden, triple junctions are generally studied using long-term and the Ethiopian rift zone (Mohr 1970; Legendre 2013). deformation and geodetic observations (McClusky et al. Te Red Sea rift is considered as the boundary between 2010) as well as geodynamical modeling (Koptev et al. the Arabian and African (Nubian) plates, whereas the 2018). Gulf of Aden separates the Arabian and African (Soma- Te opening of the Red Sea and the Gulf of Aden lian) plates (Bird 2003). Te Ethiopian rift zone (also started in the Late Eocene–Early Oligocene (Ghebreab 1998) and lead to the split between Africa and Arabia in the Early Miocene (Jofe and Garfunkel 1987). Te *Correspondence: [email protected] early stage of the collision between the Mediterranean 1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan Full list of author information is available at the end of the article and Biltis segments (McQuarrie et al. 2003) occurred © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Kumar et al. Earth, Planets and Space (2021) 73:166 Page 2 of 17 Fig. 1 Tectonic setting of the Afar triple junction. The red triangles are active Holocene volcanoes (Venzke 2013). The magenta line represents the plate boundary (Bird 2003). The black circles represent the locations of the epicenters of Earthquakes (Mw > 4 ). Most of the Earthquakes in the region have shallow depths. The black text framed in magenta provides the names of the diferent plates (Bird 2003): AFn, African (Nubian) plate; AFs, African (Somalian) plate; Ar, Arabian plate. The green text framed in blue provides the names of the three rifts: ER, Eden rift; GA, Gulf of Aden; R,- Red Sea rift. The blue text framed in brown (Af) indicates the location of the Afar triple junction synchronously with the massive eruption of food basalt rotation with respect to the Nubian Plate (with a rotation associated with the Afar plume at around 30 Ma (Hof- pole located in the central Red Sea). Doubre et al. (2017), mann et al. 1997). based on a combination of GPS and Interferometric Syn- thetic Aperture Radar (InSAR) measurements, suggested Previous geophysical studies that the northern part of the Somalia plate evolves inde- Several geophysical tools have been employed in this pendently from the entire African (Somalian) plate. region to investigate its tectonics. In recent decades, Previous seismological models from body-wave tomog- the Global Positioning System (GPS) has been widely raphy (Benoit et al. 2006; Koulakov 2007) and surface- deployed in this region to investigate the relative motions wave tomography (Sebai et al. 2006; Guidarelli et al. 2011; of the diferent tectonic units (Walpersdorf et al. 1999; Legendre 2013) corroborated the presence of negative McClusky et al. 2010; Doubre et al. 2017). velocity anomalies beneath the triple rift systems. Also, Te main results of geodetic measurements along the they have suggested that the Afar hotspot and other vol- Afar triple junction have suggested counterclockwise canic provinces in the region (Wignall 2001; Mège and Kumar et al. Earth, Planets and Space (2021) 73:166 Page 3 of 17 Korme 2004) are originating from the African super- rigid blocks in the region. Further geophysical modeling plume (Ni et al. 2002; Simmons et al. 2007), upwelling (Simmons et al. 2007; Reilinger and McClusky 2011) has from the lower mantle. enabled the kinematics and dynamics of the region to Te crustal thickness in the region is of great interest as be refned. However, conficting models have been sug- it is crucial to unraveling the mechanisms of the tectonic gested to explain the lateral variations of the anisotropy evolution of the Afar triple junction. Te Moho depth in the Afar triple junction region. Gao et al. (2010) sug- has been widely investigated using the receiver function gested that the optimal source of anisotropy is located technique (Dugda et al. 2005; Dugda and Nyblade 2006; at approximately 300 km depth (in the asthenosphere), Hammond et al. 2011; Reed et al. 2014; Tompson et al. whereas Keir et al. (2011) found that this anisotropy orig- 2015) and the crustal thickness ranges from ~ 15 km inates mostly from deformation-related structures in the beneath the spreading centers up to ~ 40–45 km outside crust and lithosphere. the continental break-up area (Hammond et al. 2011). In this study, we performed a systematic investigation Previous anisotropy observations have mainly been of the crustal structure and mantle anisotropy in the Afar achieved by the shear-wave splitting of teleseismic region by means of receiver function and shear-wave (Gashawbeza et al. 2004; Walker et al. 2005) or local splitting analyzes. Knowledge about the crustal thick- Earthquakes (Keir et al. 2011). Several other shear-wave ness as well as crustal and lithospheric anisotropies (Leg- splitting measurements performed in the region and sur- endre et al. 2017, 2020) can provide helpful insight into roundings have been compiled into international shear- the crustal and mantle fow afecting the regional dynam- wave splitting databases (Wüstefeld et al. 2009; IRIS ics (Legendre et al. 2016; Fan et al. 2020). Te main con- 2012). Deformation-related structures generally explain tribution of this study is a total of 431 new shear-wave the anisotropic pattern in the Afar region. Te general splitting measurements in the region as well as 34 meas- orientation of the anisotropies is associated with mantle urements of crustal thickness and ratio of seismic com- fow; in contrast, subtle changes in the fast direction are pressional and shear-wave velocities (Vp/Vs). In the frst related to the presence of fuids, isotropic media, or verti- step, we will describe the data and methodologies used in cal mantle fow. Dyke-induced faulting and alignment of this study (“Data and methods”). “Results” presents our melt pockets near volcanic centers also have very distinct new fndings based on crustal thickness, velocity ratio anisotropic patterns in the region (Gashawbeza et al. between compressional waves and shear waves as well as 2004). shear-wave splitting of the SKS phase. “Discussion” dis- Vp cusses the implications of our computed crustal depth, Vs Tectonics of the Afar triple junction ratio, and anisotropy in terms of the structures, composi- Te tectonics of the Afar triple junction is dominated by tion, and evolution of local and regional features.
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