Inversion tectonics during continental rifting: The Turkana Cenozoic rifted zone, northern Kenya

Abstract

Remote sensing data and revised seismic reflection profiles provide new insights about the origin of inverted deformation within Miocene‐Recent basins of the Turkana rift (northern Kenya) in the eastern branch of the East African rift system. Contractional structures are dominated by weakly inverted sets of fault blocks within <3.7 Myr old synrift series. Most of reverse extensional faults involve components of oblique‐slip, whereas associated hanging wall folds are characterized by large wavelength upright folding. The area of basin inversion is restricted to a 40 × 100 km elongated zone overlying a first‐order N140°E trending fault zone in the basement, referred to as the N'Doto transverse fault zone (NTFZ). In the proposed kinematic model, inversion tectonics is assigned to permutation of principal stress axes (σ1/σ2) in addition to the clockwise rotation of extension (from nearly N90°E to N130°E) during Pliocene. The transition from pure extension (Miocene) to a wrench faulting regime (Pliocene) first results in the development of T‐type fault networks within a dextrally reactivated shear zone (NTFZ). Inversion tectonics occurred later (<3.7 Ma) in response to a still rotated (∼20°) shortening axis (σ1) oriented N40°E that caused the oblique compression of earlier (NS to N20°E) extensional structures within the NTFZ. The origin of basin inversion and strain concentration in the Turkana rift is thus directly linked to a crustal weakness zone, transverse to the rift axis, and involving steep prerift anisotropies.