Crustal structure and magmato‐tectonic processes in an active rift (Asal‐Ghoubbet, Afar, East Africa): 2. Insights from the 23‐year recording of seismicity since the last rifting event

Abstract

[1] The Asal-Ghoubbet (AG) Rift has sustained a major volcano-tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys. It is therefore an ideal place to study the transient magmato-tectonic processes that operate after a rifting episode. We examine the space-time evolution of $2500 Md 2.8 earthquakes recorded in the rift from 1979 to 2001. We focus on the relationships between this seismic activity and both the three-dimensional structure of the rift and its postrifting behavior depicted from geodesy. The results highlight the major role of the central magmatic system (Fieale-Shark Bay) on the structure, seismic activity, and overall behavior of the rift. From 1978 to 1986, the rift opens at a fast rate, yet mainly aseismically; the opening is magmatically driven and accommodated. Since 1986, when the opening rate abruptly decreased, the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system. These pulses result from that magmatic zone undergoing alternating stretching and inflating episodes. Thus, while the plate-driven induced stresses have been rebuilding in the rift since 1987, the rift opening is still essentially accommodated in the axial magmatic zone. The AG Rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode. That transient opening has essentially occurred aseismically, and most tectonic faults remain relaxed and locked.

Flip-flop detachment tectonics at nascent passive margins in SE Afar

“…From c. 0.8 Ma onwards, the GAMI axis represents the earliest stage of oceanization, with the development of an active oceanic-type volcanotectonic segment bounded by conjugate high-angle faults (e.g. De Chabalier & Avouac 1994; Doubre et al 2007). Fig.…”

Lithospheric strength and strain localization in continental extension from observations of the East African Rift

“…Xenolith suites from the MER and extruded lavas in the Afar provide evidence of ongoing destruction of the lithospheric mantle as the rifting style approaches oceanic‐type extension [ Rooney et al , 2005; Ayalew et al , 2006]. More detailed studies of the magmatic segments in the Afar [e.g., Doubre et al , 2007a, 2007b] suggest crust is further thinned by magmatism locally.…”

“…In the Afar, the influence of magma on rifting is even more pronounced. Active magma emplacement occurs within the crust [ Cattin et al , 2005; Wright et al , 2006; Ayele et al , 2007; Vigny et al , 2007; Grandin et al , 2009; Keir et al , 2009], mafic crustal replacement is pervasive [ Barberi and Varet , 1975], and considerable extension may occur aseismically via magmatic accretion [ Doubre et al , 2007a, 2007b]. …”

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

“…[3] The East African Rift System is a classic example of continental breakup, where the deformation of the continental crust is located along magmatic segments [e.g., Manighetti et al, 1998;Manighetti et al, 2001;Doubre et al, 2007aDoubre et al, , 2007bEbinger et al, 2008;Ferguson et al, 2013]. The central Main Ethiopian Rift marks the transition between rifting of thick continental crust in the southern and central East African Rift System and incipient seafloor spreading in the northern Afar, into the Afar Depression [e.g., Hayward and Ebinger, 1996;Rooney et al, 2007].…”

“…While the crustal thickness outside of the Asal Rift is about 20-25 km [Hammond et al 2011], deep seismic sounding [Ruegg, 1975] and others geophysical approaches give crustal thickness beneath the rift of $5 km [Tarantola et al, 1980;L epine and Hirn, 1992;De Chabalier and Avouac, 1994;Doubre et al, 2007aDoubre et al, , 2007b, the first 500 m near the rift axis corresponding to the basalts flooded since $1 Myr [Demange and Puvilland, 1993].…”