Mantle influence on crustal discontinuity revealed by He isotopes and resistivity in the East African Rift System

Abstract

As one of the most renowned continental rifts on Earth, the East African Rift System (EARS) is characterized by intense rifting and active volcanic activity, creating a favorable geological setting for the development of crustal discontinuity and ground fissures. In this study, crustal discontinuities are defined as regions of weakness resulting from fault activity and crust-mantle interaction. This study investigates the crustal discontinuity in the Central Kenyan Rift (CKR) through a multidisciplinary approach that includes mapping, trenching, gas geochemical testing, magnetotellurics, surface deformation analysis, correlation analysis and geodynamic modelling. A total of 83 ground fissures were identified, mostly distributed along the eastern side of the rift zone and are primarily characterized by opening and uplifting of the flank. Helium isotope values (R_C/R_A) of gases escaping the fissures were generally greater than 0.2. Compared to the volcanic gases, those from the fissure is more indicative of mantle origin. Fissured areas are located above two crustal discontinuities identified by low-resistivity zones. These discontinuities (weakness) are located at depths of approximately 1 km and 5–20 km, with thicknesses of 200–600 m and 2–14 km, respectively. In the areas with deep discontinuities, the thicknesses of the anomaly zone increase with the R_C/R_A values, while the opposite is observed in the areas with shallow discontinuities. The spatial distribution of fissures suggests the westward-dipping mantle plume in the EARS is the main cause of the crustal discontinuity. This research not only enhances the understanding of the crust-mantle interaction in EARS but also aids in evaluating the fracture network in the crust.