Rifting at the Boundary of Contrasting Terrane Fabrics, Luama Rift, DR Congo

Here, we investigate the structure of the Permo-Triassic (Karoo) Luama Rift basin, DR Congo, In plan-view the basin displays a well-developed yet puzzling, NW-trending, 55 km-wide graben that branches out into a network of nine narrower (5 - 27 km-wide) troughs at its NW tip. We trace tectonic structures with the aid of 30 m-resolution satellite radar digital elevation models (DEM), and identify the spatial extents of rift-related sedimentary deposits and crystalline basement exposures using published geologic maps. Our results show that: (1) the Luama Rift developed along the boundary between the Paleoproterozoic Ruzizian-Ubendian Orogenic Belt (RUOB) to its NE, and the Mesoproterozoic Kibaran-Burundian Orogenic Belt (KBOB) to its SW; (2) the splays at the rift tip exhibit an hierarchical structure of higher frequency rift branching (4 branches) proximal to the main graben, and lowest frequency of branching (1 or 2 branches) at the distal parts of the rift tip; (3) although the main graben and its faults strike 151°± 3, the splays at the rift tip are sub-parallel (NW-SE, 120° - 150°) and sub-orthogonal (NE-SW, ~060°) to the main graben; (4) RUOB terrane fabrics dominantly trend NW-SE (148°), whereas the KBOB fabrics dominantly trend NE-SW (053°± 4.2); and (5) RUOB fabrics are parallel to the main graben faults, and are sub-orthogonal (θ = 15 - 28°) to the Karoo extension direction (SHmin), whereas the KBOB fabrics are sub-orthogonal to the main graben and highly-oblique/parallel (θ = > 45°) to the Karoo SHmin. We suggest that although the fabrics of the RUOB were preferentially selected for reactivation, the main graben localized at the boundary of the two terranes in response to the crustal-scale rheological contrast across the boundary. Further, we suggest that the systematic order of rift branching at the rift tip indicates that during the Karoo extension, the rift was propagating northwestwards. However, we speculate that local stress rotations at the propagating rift tip may have facilitated the reactivation of both the RUOB and KBOB fabrics which controlled the geometries of the rift branches. Our results provide a compelling example of how the interplay between inherited structures and extension direction influence strain localization and rift propagation within the continental crust.