The Development of Late‐Stage Continental Breakup: Seismic Reflection and Borehole Evidence from the Danakil Depression, Ethiopia

Abstract

During continental breakup, the locus of strain shifts from a broad region of border faulting and ductile plate stretching to a narrow zone of magma intrusion in a young ocean basin. Recent studies of volcanic rifts and margins worldwide suggest this shift occurs subaerially, before the onset of seafloor spreading. We test this hypothesis using recently acquired seismic reflection and borehole data from the Danakil Depression, Ethiopia, a unique region of transition between continental rifting and seafloor spreading. Our data, located near Dallol, ~30 km northwest of the Erta'Ale Volcanic Segment, reveal a remarkably thick (>1‐km) sequence of young (~100‐ka) evaporites in a basin bound by a major (≤400‐m‐throw), east‐dipping normal fault. To generate such a large amount of subsidence in such a relatively short time, we propose that upper‐crustal extension in Danakil is currently dominated by faulting, not magmatic intrusion. Given the region's markedly thinned crust (~15‐km‐thick), relative to elsewhere in Afar where magma‐assisted rifting dominates and maintains crustal thickness at ~25 km, mechanical extension in Danakil is likely coupled with ductile extension of the lower‐crust and mantle lithosphere. Despite proximity to the voluminous lavas of the active Erta'Ale Volcanic Segment, evidence for igneous material in the upper ~2 km of the 6‐ to 10‐km‐wide basin is limited. Late‐stage stretching was likely aided by thermal/strain‐induced lithospheric weakening following protracted magma‐assisted rifting. Basin formation immediately prior to the onset of seafloor spreading may also explain the accumulation of thick marine‐seepage‐fed evaporite sequences akin to those observed, for example, along the South Atlantic rifted margins.

Kinematics and deformation of the southern Red Sea region from GPS observations

“…6 and 7). Similarities with other areas in northern Afar (e.g., Bastow et al 2018), suggest that mechanical extension is dominant over magma assisted deformation around the gulf. Tectonic structures in the area (e.g., Drury et al 1994;Ghebreab & Talbot 2000;Sani et al 2017, Fig.…”

“…7). In northern Afar, such broad regions of distributed deformation have been found to characterize areas where mechanical extension is dominant over magma assisted extension (e.g., Bastow et al 2018), with faulting in the upper crust and ductile flow in the lower crust being the main deformation drivers (Bastow & Keir 2011;Ebinger et al 2017). The modeling provides first-order constraints on the distribution of active deformation in the Gulf of Zula area without accounting for possible local complexities within the deformation field (e.g., Ghebreab & Talbot 2000;Sani et al 2017).…”

Origin and Evolution of the Halo-Volcanic Complex of Dallol: Proto-Volcanism in Northern Afar (Ethiopia)

“…Two main thick halite sequences can be identified: the lower rock salt formation (LRS) and the upper rock salt formation (URS) ( Figure 1C) separated by a potash unit, the Houston Formation, which has been the target of mining exploration since the beginning of the last century (Holwerda and Hutchinson, 1968). Although the age of the complete salt series is unknown, K/Ar dating at different depths in the URS suggested ages of 76 ka at 73 m depth and 88 ka at 137 m depth (Barberi et al, 1972) confirming a Pleistocene origin in agreement with the age of the last marine incursion (Jaramillo-Vogel et al, 2018) and with the ∼100 ka age estimated for the onset of the last evaporative sequence (Bastow et al, 2018). The LRS extends from the Pleistocene (upper sequence part) to likely the Miocene (bottom sequence), probably being a lateral marine facies equivalent to the Danakil Formation.…”

“…The closure of the seawater passage led to the formation of a classical evaporitic drawdown facies belt from the depression margins to the center (Holwerda and Hutchinson, 1968;Warren, 2015b). The open marine reef facies transitioned to marineseepage gypsum pavements (Hutchinson and Engels, 1970;Bastow et al, 2018) that can be seen below the reef rim (no. 8 in Figures 1B, 2).…”

“…The Danakil depression is a N-S basin ca. 200 km long, 50-150 km wide and down to 120 m below sea level (bsl), lying at the triple junction between the Nubian, Somalian, and Arabian plates and the Danakil microplate (Hutchinson and Engels, 1972;Collet et al, 2000;Eagles et al, 2002;Tesfaye et al, 2003;Bonatti et al, 2015;Corti et al, 2015;Bastow et al, 2018; Figure 1A). Two noticeable escarpments limit the depression.…”