Propagation of rifting along the Arabia‐Somalia Plate Boundary: Into Afar

Abstract

Abstract. It is generally accepted that the Aden ridge has propagated westward from -58øE to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal-Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW-SE striking subrifts, all of which appear to have propagated northwestward. In Asal-Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia-Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, (•3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from -58øE to Asal-Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia-Somalia (or Danakil-Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N-S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en 6chelon subrifts, and a high-angle, smaller, nested, "subrifts within subrifts." The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia-Somalia and ArabiaNubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large-scale bookshelf faulting.

Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements

“…The direction of the axis of the smallest principal component of the strain rate tensor from the Tadjoura Gulf to the TG confirms the complexity of the deformation associated with the inland penetration of the divergent AR plate boundary west of longitude 042.5 • E and suggests that a large part of the extension is located west of the AG segment and not towards the Manda Inakir segment, as proposed by long-term models (Manighetti et al 1998;Acocella et al 2008). The velocities estimated south of the MI rift segment are significant (18.9 mm yr -1 for MAN2 and 21.2 mm yr -1 for SULA) and close to the value of the ASAB site (Danakil block along the Eritrean coast).…”

“…1). This branch, forming the northeastern edge of the AD along the Danakil block, propagates further north, along the 0.4-0.5-Myr-old Manda Inakir (MI) rift segment separated from the AG rift by a large zone of distributed deformation along the Mak'Arrassou NS-fault system, accommodating the transfer of the extensional deformation between both rifts (Tapponnier & Varet 1974;Manighetti et al 1998Manighetti et al , 2001; Fig. 1).…”

“…The Holocene and recent volcanism in Afar is mainly concentrated along the northward propagation of the East-African Rift through the Mid-Ethiopian Rift (MER) and two narrow branches corresponding to the inland propagation of the Aden and the RS oceanic spreading ridges (Barberi & Varet 1977;Manighetti et al 1998).…”

“…In northern Afar, the second branch of recent volcanic ranges corresponds to ∼NS-trending rift segments, including the Erta Ale and the Manda-Hararo segments (Barberi & Varet 1970, 1977Manighetti et al 1998). …”

“…Whereas the Loma and Manda-Gargori (MG) subrifts are structured by systems of antithetic faults, the Kurub and the Dama Ale-Gob'ad subrifts do not present large tectonic structures on surface, but are mainly defined by the presence of a large volcanic edifice. The mapping of small and large faults indicated in thin and bold lines respectively has been made using the SRTM DEM and modified from Tapponnier et al (1990), Hayward & Ebinger (1996), Manighetti et al (1998) and Jacques et al (2011). The axes and names of the active rift segments are indicated by dark purple lines and recent lava flows in light purple areas (modified from Barberi & Varet 1972).…”