Tectonothermal Evolution of the Broadly Rifted Zone, Ethiopian Rift

Abstract

The Broadly Rifted Zone (BRZ) of southern Ethiopia is a long-lived and structurally complex segment of the East African Rift System. However, due to poor surface exposure of early synrift strata and a dearth of subsurface data, the evolution of the BRZ remains poorly understood. We present new apatite (U-Th-Sm)/He and augmented apatite fission track low-temperature thermochronology data from the Beto and Galana basin boundary fault systems to constrain the tectonothermal evolution of the western and eastern BRZ, respectively. Time-temperature reconstructions suggest that East African Rift System-related extension began concurrently across the BRZ in the early Miocene (20-17 Ma), at least 6 Myr prior to faulting in the Main Ethiopian Rift further north. Increased time-temperature resolution provided by multithermochronometer analyses reveals contrasting along-strike spatiotemporal variations in Beto and Galana margin cooling histories, which appear to mirror the disparate structural geometries of their basin-bounding normal fault arrays. Longitudinal contrasts in basin architecture and rift-related cooling histories across the BRZ may reflect the region's heterogeneous distribution of preexisting basement fabrics, namely, the presence of a previously reported N-NNE trending Neoproterozoic suture zone beneath the eastern BRZ. Its influence may explain both the development of long, curvilinear faults and the gradual basinward migration of strain exhibited by the easternmost BRZ, absent further west. The anomalous evolution of the BRZ compared to the greater Ethiopian Rift, both in its earlier onset and its wider deformation zone, likely results from its inheritance of preattenuated lithosphere, thermomechanically modified by earlier Cretaceous-Paleogene Anza-South Sudan rifting and/or Eocene plume impingement.

Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA

“…The formation of the active volcanic centers of the Jemez lineament over this LAB step suggests that inherited structure from ancient features continues to play a role in controlling deformation in the region (Baldridge et al, ). Similar to the RGR, changes in rift deformation style occur in the East African rift system and are coincident with “steps” in crustal thickness attributed to deep‐seated Neoproterozoic sutures and other inherited weaknesses (e.g., Boone et al, ; Corti et al, ), suggesting that such controls are common in continental rift systems.…”

“…Fault segment growth and linkage can be observed and quantified with low‐temperature thermochronometry analyses in vertical transects collected on exhuming normal fault blocks (Abbey & Niemi, ; Boone et al, ; Curry et al, ; Stockli et al, ; Stockli et al, ). Low‐temperature thermochronometers are powerful tools for understanding near‐surface (1–7 km) thermal histories of the crust and thus are sensitive to processes that affect the upper crust, including erosion and faulting.…”

Interactions between propagating rifts and linear weaknesses in the lower crust

“…14A). Recent work (Balestrieri et al, 2016;Boone et al, 2019) has illustrated a complex scenario of rift initiation in the MER; deformation propagated northward in the southern MER, while the northern MER propagated southward, into the central MER. The boundary conditions in model F (α = 90°) are therefore analogous to the tectonic setting of the northern MER (Fig.…”

Development of the Nyika Plateau, Malawi: A Long Lived Paleo‐Surface or a Contemporary Feature of the East African Rift?

“…In some instances, median ages are provided in the text and in figures along with the inter quartile range (IQR) to help with broad trend interpretation and summary. The IQR is a robust statistical measure in intrasample age dispersion and, thus, a proxy for cooling rate through the PRZ (rapidly cooled samples yield lower IQRs, Boone et al., 2019). Only single grain ages were used for data interpretation and thermal history modeling.…”