The role of rift axis faulting in the final stages of magma-rich rifting: the Danakil Depression, Afar

Abstract

Traditionally interpretations assume that as magma-rich rift settings mature, the magmatism accommodates greater amounts of extension at the expense of mechanical deformation. However, the importance of faulting in the final stages of magma-rich rifting remains poorly constrained, with the data (e.g. structural geological mapping, seismic reflection and borehole data) from rifts near to break-up a rarity. The Danakil Depression (Northern Afar), is undergoing the final stages of continental break-up, thus providing the ideal natural laboratory to conduct high resolution, quantitative analysis on the architecture, extension and subsidence facilitated by faulting in an active rift setting before seafloor spreading initiates. >500 rift axis faults were identified using remote sensing data (satellite imagery, DEMs), with quantitative analysis showing an increase in fault density, length and connectivity away from magmatic segments. Kinematic and earthquake focal mechanism data demonstrate a transition from transtensional opening in the northern and central sub-regions of the rift to oblique opening in the southern Giulietti Plain and Tat-Ali sub-regions of the Danakil Depression. The oblique opening is attributed to the along-axis step between the Erta-Ale and Harak sub-regions. Integration of seismic reflection and borehole data with the mapped faults shows that extension is primarily accommodated by magmatism within the rift center, with faulting more significant towards the ends of the rift. ~30% of crustal extension is accommodated by axial faulting in areas of low magmatism, highlighting the importance of faulting even in the final stages of magma-rich rifting. Comparing our findings with spreading ridge morphology and structure, which is relevant due to the rift maturity and extensive magmatism present, we conclude that the Danakil Depression is in a transitional stage between continental rifting and seafloor spreading. Spatial changes in the importance of faulting and magmatism in accommodating extension, alongside rift morphology, resemble the relationships observed along spreading ridges. From our observations we have shown that axial faulting still plays a vital role in the final stages of break-up despite the increased importance of magmatism.