Structural Inheritance Controls Strain Distribution During Early Continental Rifting, Rukwa Rift

Abstract

Little is known about rift kinematics and strain distribution during the earliest phase of extension due to the deep burial of the pre-rift and earliest rift structures beneath younger, rift-related deposits. Yet, this exact phase of basin development ultimately sets the stage for the location of continental plate divergence and breakup. Here, we investigate the structure and strain distribution in the multiphase Mesozoic-Cenozoic magma-poor Rukwa Rift, East Africa during the earliest phase of extension. We utilize aeromagnetic data that image the Precambrian Chisi Suture Zone (CSZ) and bounding terranes, and interpretations of 2-D seismic reflection data to show that, during the earliest rift phase (Permo-Triassic Karoo): (1) the rift was defined by the Lupa Fault, which exploited colinear basement terrane boundaries, and a prominent intra-basinal fault cluster (329° ±9.6) that trends parallel to and whose location was controlled by the CSZ (326°); (2) extensional strain in the NW section of the rift was accommodated by both the intra-basinal fault cluster and the border fault, where the intra-basinal faulting account for up to 60% of extension; in the SE where the CSZ is absent, strain is primarily focused on the Lupa Fault. The early-rift strain in the Rukwa Rift is thus, not accommodated by distributed faulting as suggested by classic rift models; instead, strain focuses relatively quickly on large faults and intra-basinal fault clusters following pre-existing intra-basement structures; (3) two styles of early-phase strain localization are evident, in which strain is localized onto a narrow discrete zone of basement weakness in the form of a large rift fault (Style-1 localization), and onto a broader discrete zone of basement weakness in the form of a fault cluster (Style-2 localization). We argue that the CSZ and adjacent terrane boundaries represent zones of basement mechanical weakness that controlled the first-order strain distribution and rift development during the earliest phase of extension. The established early-rift structure, modulated by structural inheritance, then persisted through the subsequent phases of rifting. The results of our study, in a young, and relatively well-exposed and data-rich rift, are applicable to understanding the structural evolution of deeper, buried ancient rifts.

Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: the Malawi Seismogenic Source Model (MSSM)

“…Lower, intermediate, and upper α bf values of 0.5, 0.7, and 0.9 are applied in the MSSM. These values reflect observations of the relative contribution to rift opening between intrarift and border faults in Malawi (Shillington et al, 2020;Wedmore et al, 2020a), elsewhere along the EAR (Kolawole et al, 2021b;Muirhead et al, 2016Muirhead et al, , 2019Wright et al, 2020), and in analogue and numerical models (Agostini et al, 2011;Gupta et al, 1998). The South Basin is bound onshore to the east by the Metangula fault, which exhibits a 500-700 m high escarpment (Laõ-Dávila et al, 2015).…”

“…However, we do not consider this a realistic scenario, since other factors (e.g. structural inheritance) will cause intrarift faults to accommodate regional rift extension prior to significant flexural extension (Kolawole et al, 2021b;Wedmore et al, 2020a), and so c hwf is truncated at values >5.…”

Evolution of Rift Architecture and Fault Linkage During Continental Rifting: Investigating the Effects of Tectonics and Surface Processes Using Lithosphere‐Scale 3D Coupled Numerical Models

“…We observe differing sediment thickness in the basins surrounding the high‐elevation plateau in our simulations as well (Figure 4). However, the Rukwa Basin experienced multiple rifting phases: the oldest deposits are of Permo‐Triassic age and Neogene deposits are commonly 2–3 km thick with a maximum thickness of Cenozoic deposits of >5 km (Morley et al., 1992; Morley et al., 1999; Macgregor, 2015; Kolawole et al., 2021).…”

Geodynamics of continental rift initiation and evolution

“…Hence, favourably-oriented pre-existing planes of strength contrast will fail first and localise deformation prior to the nucleation of discontinuities in the pristine portions of the rock 88 . In a normal faulting stress regime, inherited fabrics are ubiquitously exploited [G] by normal or oblique-normal rift faults [99][100][101] . The exploitation behaviour can however be very complex, as faults form as a result of 3D anisotropies interacting with the 3D stress field.…”