Abstract
We propose a two-stage tectonic evolution of SE Afar in Djibouti leading to the complex development of highly asymmetric conjugate margins. From c . 8.5 to c . 2 Ma, an early mafic crust developed, associated in the upper crust with synmagmatic growth faults dipping dominantly to the SW. After an erosional stage, a new detachment fault system developed from c . 2 Ma with an opposite sense of motion (i.e. to the NE), during an amagmatic extensional event. In the Asal area, break-up occurred after c . 0.8 Ma along the footwall of an active secondary detachment fault rooted at depth above the lithospheric necking zone. This evolution suggests that flip-flop detachment tectonics is developed during extension at passive margins, in connection with the dynamics of the melting mantle and the associated magma plumbing of the crust.
Highlights
Detachment faults are thought to play a significant role in accommodating extension at passive continental margins (e.g. Lavier & Manatschal 2006; Reston & McDermott 2011)
We propose a two-stage tectonic evolution of SE Afar in Djibouti leading to the complex development of highly asymmetric conjugate margins
Detachment faults accommodating the exhumation of mantle at ultraslow-spreading ridges have recently been shown to undergo a flip-flop tectonic evolution, with a possible link between discrete mantle melting events and changes in polarity (Sauter et al 2013)
Summary
The Afar depression (Fig. 1) is a key area for understanding the mechanisms of active plate break-up in a magma-rich context (e.g. Ayele et al 2009). The eastern Afar rift in Djibouti is an extensional faulted area extending from the Gobaad–Abhe discontinuity in the SW to the Danakil continental block in the NE (Fig. 1). The Stratoid floor is further dissected by a narrow and active en echelon tectonomagmatic axis, extending between the Ghoubbet–Asal and the Manda Inakir segments (referred to below as GAMI) (Fig. 1). Outside the currently active segments, seismic refraction and receiver function data recorded throughout eastern and central Afar suggest the existence of a 20–26 km thick crust of transitional character, comprising a continental lower crust intensively intruded by magma The predominantly basaltic floor of the SE Afar rift in Djibouti comprises the extensive Stratoid series and the
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