Abstract
AbstractDuring 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.
Highlights
During continental breakup, the locus of extension is expected to shift from broad (~80-km-wide) regions dominated by long (>60-km), large-offset (>1-km) border faults to narrow (~20-km-wide), elongate (~60km) zones of focused dike intrusion (e.g., Ebinger et al, 2017; Ebinger & Casey, 2001; Hayward & Ebinger, 1996)
Seismic and borehole data indicate that the large-throw, east-dipping normal faults demarcate the boundary of a thick evaporite depocenter near Dallol in the northern Danakil Depression (Figure 5)
The occurrence of ductile plate stretching and upper crustal mechanical faulting immediately prior to the onset of seafloor spreading in Danakil suggests that the resulting basin subsidence could accommodate deposition of thick evaporite sequences, like those observed along the South Atlantic rifted margins (e.g., Pickup et al, 1996; Ranero & Pérez-Gussinyé, 2010)
Summary
The locus of extension is expected to shift from broad (~80-km-wide) regions dominated by long (>60-km), large-offset (>1-km) border faults to narrow (~20-km-wide), elongate (~60km) zones of focused dike intrusion (e.g., Ebinger et al, 2017; Ebinger & Casey, 2001; Hayward & Ebinger, 1996). The presence of markedly thinned Danakil crust, revealed by wide-angle seismic data, coupled with subsidence and deposition of Pliocene-to-Recent age evaporitic sedimentary rocks (Atnafu et al, 2015; Bonatti et al, 1971; Holwerda & Hutchinson, 1968; Lalou et al, 1970), suggests that the final stages of continental breakup may be characterized by an abrupt, late phase of ductile plate stretching and associated upper crustal mechanical extension (Bastow & Keir, 2011; Keir et al, 2013) Such late-stage stretching, which would enhance decompression melting, may explain why there is an increase in the volume of young basalt flows in the Erta’Ale Volcanic Segment (EAVS). These new data offer a unique insight into the anatomy of a young sedimentary basin, forming subaerially during the final stages of continental rifting
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