A refinement of the chronology of rift-related faulting in the Broadly Rifted Zone, southern Ethiopia, through apatite fission-track analysis

Abstract

To reconstruct the timing of rift inception in the Broadly Rifted Zone in southern Ethiopia, we applied the fission-track method to basement rocks collected along the scarp of the main normal faults bounding (i) the Amaro Horst in the southern Main Ethiopian Rift and (ii) the Beto Basin in the Gofa Province. At the Amaro Horst, a vertical traverse along the major eastern scarp yielded pre-rift ages ranging between 121.4±15.3Ma and 69.5±7.2Ma, similarly to two other samples, one from the western scarp and one at the southern termination of the horst (103.4±24.5Ma and 65.5±4.2Ma, respectively). More interestingly, a second traverse at the Amaro northeastern terminus released rift-related ages spanning between 12.3±2.7 and 6.8±0.7Ma. In the Beto Basin, the ages determined along the base of the main (northwestern) fault scarp vary between 22.8±3.3Ma and 7.0±0.7Ma. We ascertain through thermal modeling that rift-related exhumation along the northwestern fault scarp of the Beto Basin started at 12±2Ma while in the eastern margin of the Amaro Horst faulting took place later than 10Ma, possibly at about 8Ma. These results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the Ethiopian Rift. Extensional basin formation initiated more or less contemporaneously in the Gofa Province (~12Ma) and Northern Main Ethiopian Rift (~10–12Ma) at the time of a major reorganization of the Nubia–Somalia plate boundary (i.e., 11±2Ma). Afterwards, rift-related faulting involved the Southern MER (Amaro Horst) at ~8Ma, and only later rifting seemingly affected the Central MER (after ~7Ma).