Orthogonal to oblique rifting: effect of rift basin orientation in the evolution of the North basin, Malawi Rift, East Africa

Abstract

The East African Rift system has long been considered the best modern example of the initial stages of continental rifting. The Malawi Rift is characteristic of the western branch of the East African Rift system, composed of half‐grabens of opposing asymmetry along its length. There are striking similarities between basins within the Malawi Rift, and others along the western branch. Each exhibits similar bathymetry, border‐fault length, rift zone width and fault segment length. The North Basin of the Malawi Rift differs from others in the rift only in its orientation: trending NW–SE as opposed to N–S. Although there is general agreement as to the geometry of the Malawi Rift; debate as to the amount of strike–slip vs. dip–slip deformation and the influence of underlying Pan‐African foliation remains. This study presents new data from a closely spaced shallow [2 s two‐way travel time (TWT)] seismic reflection data set integrated with basin‐scale deeper (6 s TWT) seismic reflection data that document the structural evolution of the border and intra‐basin faults. These data reveal that the different trend of the North Basin, most likely to have been influenced by the underlying Pan‐African foliation, has played an extremely important role in the structural style of basin evolution. The border‐fault and intra‐basin structures nucleated during extension that was initially orthogonal (ENE). During this time (>8.6 to ∼0.5–0.4 Ma) intra‐basin faults synthetic to the west‐dipping border‐fault nucleated, whereas strain was localised on the segmented border‐fault early on. A later rotation of extension orientation (to NW) led to these established faults orienting oblique to rifting. This generated an overall dextral strike–slip setting that led to the development of transfer faults adjacent to the border‐fault, and the generation of flower structures and folds over the greater displacement intra‐basin faults.

Implications of structural inheritance in oblique rift zones for basin compartmentalization: Nkhata Basin, Malawi Rift (EARS)

“…Thus existing pre-Cenozic structures have controlled the location of intra-basin transfer zones that become increasingly important as the amount of dip-slip across them accrues. This contrasts strongly with the asymmetric Karonga basin to the north, which does not experience the influence of underlying Karoo fabrics to the same effect, nor contains transfer faults within the basin (Mortimer et al, 2007).…”

“…The border fault system, unlike those to the north (Karonga basin, Mortimer et al, 2007) is not superimposed upon an underlying Ubendian fabric. The border fault nucleation and growth through fault tip propagation and linkage through breaching relay structures between overlapping segments (Trudgill and Cartwright, 1995) occurred in a manner anticipated for normal fault systems (Dawers and Anders, 1995;Trudgill and Cartwright, 1995;Gupta et al, 1998;Cowie et al, 2000).…”

“…Variation in sediment thickness, generated by subtracting interpolated surfaces, has been used to locate the depocentre for each successive sequence along structures. Depocentre distribution and evolution provide an approximation for the displacement pattern, and thus evolution, of faults (e.g., Schlische and Anders, 1996;McLeod et al, 2000, Contreras et al, 2000Paton, 2006;Mortimer et al, 2007).…”

“…The north, Karonga Basin, has undergone oblique extension with a significant component of dextral slip (Chorowicz and Mukonki, 1980;Wheeler and Karson, 1989;Wheeler and Rosendahl, 1994) with a temporal variation in rift obliquity that occurred during the Pleistocene. While initially the region experienced ENE-WSW oriented extension, the changeover to the neotectonic kinematic regime has subjected this region to NW-SE extension Scott et al, 1992;Mortimer et al, 2007). This change in stress orientation occurred c. 500 Ka based upon dated tuffs and the correlation with 100 kyr duration packages in seismic reflection data (Mortimer et al, 2007).…”

“…While initially the region experienced ENE-WSW oriented extension, the changeover to the neotectonic kinematic regime has subjected this region to NW-SE extension Scott et al, 1992;Mortimer et al, 2007). This change in stress orientation occurred c. 500 Ka based upon dated tuffs and the correlation with 100 kyr duration packages in seismic reflection data (Mortimer et al, 2007). This change in orientation is not, however, documented to the west of the basin, where Karonga earthquake focal mechanisms indicate pure extension, and a W-E extension orientation (Biggs et al, 2012).…”