Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift

Abstract

[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 strikeslip 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 N110 E. 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.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…10 km) beneath the central rift valley (Birt et al 1997). Recorded seismicity occurs throughout the region, with relatively deep lower crustal earthquakes beneath the Manyara basin (Albaric et al 2009;Mulibo & Nyblade 2009;Craig et al 2011;Albaric et al 2014), and beneath the Natron basin (Lee et al 2016,Weinstein et al, submitted).…”

“…This method assumes purely horizontal interfaces, isotropic medium and an average crustal P-wave velocity (Zhu & Kanamori 2000). In our case, we extracted the mean crustal P-wave velocity for each station from Albaric et al (2014) velocity model (Table 1). We defined H and k to initially range between [20-50 km] and [1.6-2.0], respectively.…”

“…Moho depth and Vp/Vs ratio (k) results from the H-k stacking method for all seismic stations. The mean crustal P-wave velocity used for each station is taken from Albaric et al (2014). We have an average of 67 events processed per station (46 P-RF and 21 PP-RF), with a mean Moho depth of 35.9 km and an average Vp/Vs of 1.77.…”

“…Our RF analysis can hardly be conclusive for the presence of azimuthal anisotropy within the lithosphere, and particularly in this LVZ. Recent SKS analyses emphasize the presence of mantle anisotropy (Albaric et al 2014;Ebinger et al 2016), but it barely explains 9 per cent of velocity reduction within the mantle in all directions. We thus plead for the contribution of another main factor.…”

“…Imaging lithospheric structures is one major step to understand the interactions between mantle rheology, magmatism, tectonics and inherited structures in this rifted area. Previous studies focused in this specific region were either local tomography restricted to the crustal structures (Albaric et al 2014), or teleseismic analyses on a too large scale to reach a sufficient lateral resolution for detailed lithospheric discontinuities (Ritsema et al 1998;Priestley & McKenzie 2006).…”