Magma-Assisted Continental Rifting: The Broadly Rifted Zone in SW Ethiopia, East Africa

Abstract

The Gofa Province and Chew Bahir Basin in the Broadly Rifted Zone (BRZ) between the southern Main Ethiopian Rift (sMER) and the northern Kenya Rift (nKR) record early volcanism and associated faulting in East Africa; however, the spatiotemporal relationships between volcanism and faulting remain poorly constrained. We applied apatite (U-Th)/He (AHe) and zircon (U-Th)/He (ZHe) thermochronometry to Neoproterozoic basement rocks from exhumed footwall blocks of the extensional Gofa Province and Chew Bahir Basin, and analyzed our result in the context of well-dated regional volcanic units in the BRZ to unravel the interplay between tectonic exhumation, faulting and volcanism. Single-grain AHe ages ranging from 1.0 to 136.8 Ma were recorded in 32 samples, and single-grain ZHe ages from three samples range between 142.2 and 335.6 Ma. The youngest AHe ages were obtained from the Chew Bahir Basin and the narrow deformation zone in the Gofa Province. Our thermal modeling results reflect little or no significant regional crustal cooling prior to extensive volcanism, which started at about 45 Ma. Conversely, new and previously published thermal history models suggest that widespread crustal cooling related to regional extension occurred between ∼27 and 20 Ma. Thermal modeling results from subsets of samples indicate that following this initial diffuse extensional deformation, renewed exhumation occurred along a narrow zone within the Gofa Province and the Chew Bahir Basin during the middle to late Miocene (15-6 Ma) and Pliocene (<5 Ma), respectively. The crustal cooling phases follow a regional trend in volcanic episodes. For example, initial cooling between 27 and 20 Ma corresponds with the end of widespread flood-basalt volcanism (45–28 Ma), suggesting that spatially diffuse normal faulting may have initiated shortly after the emplacement of voluminous and areally extensive flood basalts. The Miocene and Pliocene shifts in deformation along the Mali-Dancha and Bala-Kela basins in the Gofa Province and the Chew Bahir Basin, respectively, may indicate strain localization during the late stage of rifting and ongoing tectonic interaction between the sMER and the nKR. Our results support the notion of crustal weakening by massive volcanism as a precursor to widespread extensional faulting, and thus offer further insights into magma-assisted deformation processes in the East African Rift System.