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Deep Crustal Earthquakes in North Tanzania, East Africa

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Deep Crustal Earthquakes in North Tanzania, East Africa Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift Julie Albaric, Jacques Déverchère, Julie Perrot, Andrey Jakovlev, Anne Deschamps To cite this version: Julie Albaric, Jacques Déverchère, Julie Perrot, Andrey Jakovlev, Anne Deschamps. Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2014, 15 (2), pp.374-394. 10.1002/2013GC005027. insu-00985250 HAL Id: insu-00985250 https://hal-insu.archives-ouvertes.fr/insu-00985250 Submitted on 29 Apr 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Article Volume 15, Number 2 6 February 2014 doi: 10.1002/2013GC005027 ISSN: 1525-2027 Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift Julie Albaric Laboratory Domaines Oceaniques, Universite de Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Plouzane, France Now at NORSAR, Gunnar Randers vei 15, 2007 Kjeller, Norway ([email protected]) Jacques Deverche`re and Julie Perrot Laboratory Domaines Oceaniques, Universite de Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Plouzane, France Andrey Jakovlev Laboratory of Forward and Inverse Seismic Modeling, Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia Anne Deschamps Laboratory Geoazur, Universite Nice Sophia-Antipolis, Observatoire de la Cote^ d’Azur, Geoazur, Sophia-Antipolis, France [1] In this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike- slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110E. Crustal seismic tomography reveals low-velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge. Components: 12,526 words, 8 figures, 3 tables. Keywords: lower crustal earthquakes; seismic swarm; East African Rift System; aseismic deformation; seismic anisot- ropy; seismic tomography. Index Terms: 7270 Tomography: Seismology; 7218 Lithosphere: Seismology; 8109 Continental tectonics: extensional: Tectonophysics; 8159 Rheology: crust and lithosphere: Tectonophysics; 8180 Tomography: Tectonophysics; 8178 Tecton- ics and magmatism: Tectonophysics. © 2013. American Geophysical Union. All Rights Reserved. 374 ALBARIC ET AL.: DEEP CRUSTAL EARTHQUAKES IN TANZANIA 10.1002/2013GC005027 Received 4 September 2013; Revised 16 December 2013; Accepted 17 December 2013; Published 6 February 2014. Albaric, J., J. Deverche`re, J. Perrot, A. Jakovlev, and A. Deschamps (2014), Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift, Geochem. Geophys. Geosyst., 15, 374– 394, doi:10.1002/2013GC005027. 1. Introduction [Shudovsky et al., 1987; Nyblade and Langston, 1995; Albaric et al., 2009], but which factors con- [2] The East African Rift System (EARS) offers the trol this stress release remains unclear. Do they rare opportunity to study the different stages of conti- primarily result from large zones of weakness (as nental extension, from its initiation in the North Tan- suture zones, cratonic border faults or other zones zanian Divergence (NTD; Figure 1) to oceanic of lower shear strength than surrounding rocks) accretion in Afar. The dynamics of the EARS has within strong crust [e.g., Lyakhovsky et al., 2012; been linked to the effect of one or several mantle Fagereng, 2013]? Or are they triggered by fluids plumes [Burke, 1996; Ebinger and Sleep, 1998; settled within critically loaded shear zones [e.g., Nyblade et al., 2000; Pik et al., 2006; Hansen et al., Reyners et al., 2007; Lindenfeld et al., 2012] or by 2012; Mulibo and Nyblade, 2013a, 2013b]. In magmatic processes below/within the crust [e.g., mature stages of rifting, continental breakup docu- Keir et al., 2009; Thybo and Nielsen, 2009; mented in Afar and Ethiopian rift regions is mainly Schmeling and Wallner, 2012]? controlled by magmatic processes, with the occur- [4] From a seismic monitoring of the NTD con- rence of large dike-opening events [e.g., Wright ducted during 6 months in 2007, we have observed et al., 2006]. In contrast, it is unclear how strain that a significant part of the seismicity occurs in accommodation is partitioned between faulting and the lower crust and clusters below Lake Manyara magmatism in immature stages like in the NTD or in [Albaric et al., 2010]. In this paper, we perform other tips of the EARS. On the one hand, the rela- several analyses based on seismological data in tively small amount of surface volcanism observed order to investigate (1) the triggering mechanisms in the NTD region [Dawson, 1992] and the reported of this deeply rooted and long-duration seismicity seismic activity of some large border faults [Ebinger and (2) the respective roles of magmatism and et al., 1997, 1999; Mulibo and Nyblade, 2009] sug- crustal fabrics for rift initiation in the NTD. Thanks gest fault-dominated deformation mechanisms such to our dense temporary seismological network, we as those described in rifting models where faulting characterize the seismogenic deformation in great plays a major role until the lithosphere has been detail (focal mechanisms, stress inversion) and thinned enough [Corti, 2009, and references therein]. explore how it is related to crustal velocity anoma- On the other hand, recent seismic and geodetic sur- lies (seismic tomography) and to deeper processes veys performed ca. 100 km north of the NTD, in the occurring in the lithosphere (seismic anisotropy). Lake Natron area (Figure 1), indicate that significant surface deformations are caused by magmatic injec- tions [Calais et al., 2008]. This observation suggests 2. Tectono-Volcanic Setting that similar seismo-magmatic events could occur in the NTD, in particular at large depths where their [5] The EARS corresponds to the divergent detectability is declining. The lithosphere under the boundary between two main tectonic plates, the NTD is indeed relatively thick and rigid because of Nubian and Somalian plates, and several micro- the presence of the Tanzanian craton [Le Gall et al., plates [Calais et al., 2006]. The rift continues 2008; Perez-Gussinyeetal., 2009]. more than 4000 km from the Afar triple junction in the north to Mozambique in the South. The [3] A common related question is the meaning of EARS has started developing in the north, as deep crustal earthquakes often reported in modern attested by the age of the volcanic samples col- continental rifts [Bungum and Nnko, 1984; Camel- lected in the Ethiopian rift (40–45 Ma) [George beeck and Iranga, 1996; Deverche`re et al., 2001; et al., 1998]. The most recent volcano-tectonic Reyners et al., 2007; Keir et al., 2009; Albaric activity in the EARS is found in the NTD (8–5 et al., 2010; Lindenfeld et al., 2012]: their occur- Ma) [after Dawson, 1992] (Figure 1), with a clear rence requires a highly resistant mafic lower crust shift of magmatic activity eastward between 5 and 375 ALBARIC ET AL.: DEEP CRUSTAL EARTHQUAKES IN TANZANIA 10.1002/2013GC005027 30° 40° 50° Earthquake depth (km) 5101520 25 30 35 40 45 Afar Lake Magadi 10° Magnitude M Lake L Tanzanian Natron 234 Craton 0° NTD G. 10° ? L. Chyulu Hills 500 km Mozambique Ki. Ng. Me. MF A EF Lake Eyasi Lake KENYA Manyara T ANZANIA MBULU A' Domain Pangani Range BF Kw. Masai Block H. ? km Mbulu Domain Masai EF MF Block AA'Kw ? ? Depth (km) Distance (km) Shear zones Major sedimentary areas Rift-involved basement Surface Tanzanian craton limit Main volcanoes (0-8 Ma.) Proterozoic belt Buried Tanzanian craton limit Pliocene-recent volcanics Achaean Tanzanian craton Normal faults Proterozoic Mozambiquian
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