Little is known about rift kinematics and strain distribution during the earliest phase of extension 24 due to the deep burial of the pre-rift and earliest rift structures beneath younger, rift-related 25 deposits. Yet, this exact phase of basin development ultimately sets the stage for the location of 26 continental plate divergence and breakup. Here, we investigate the structure and strain distribution 27 in the multiphase Mesozoic-Cenozoic magma-poor Rukwa Rift, East Africa during the earliest 28 phase of extension. We utilize aeromagnetic data that image the Precambrian Chisi Suture Zone 29 (CSZ) and bounding terranes, and interpretations of 2-D seismic reflection data to show that, 30 during the earliest rift phase (Permo-Triassic Karoo):(1) the rift was defined by the Lupa Fault, 31 which exploited colinear basement terrane boundaries, and a prominent intra-basinal fault cluster 32 (329±9.6) that trends parallel to and whose location was controlled by the CSZ (326);(2) 33 extensional strain in the NW section of the rift was accommodated by both the intra-basinal fault 34 cluster and the border fault, where the intra-basinal faulting account for up to 60% of extension; 35 in the SE where the CSZ is absent, strain is primarily focused on the Lupa Fault. The early-rift 36 strain in the Rukwa Rift is thus, not accommodated by distributed faulting as suggested by classic 37