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

Abstract

The Cenozoic East African Rift System (EARS) is an exceptional example of active continental extension, providing opportunities for furthering our understanding of hydrocarbon plays within rifts. It is divided into structurally distinct western and eastern branches. The western branch comprises deep rift basins separated by transfer zones, commonly localised onto pre-existing structures, offering good regional scale hydrocarbon traps. At a basin-scale, local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution. Here, we consider the evolution of the Central basin of the Malawi Rift, in particular the influence of pre-existing structural fabrics.Integrating basin-scale multichannel 2D, and high resolution seismic datasets we constrain the border, Mlowe-Nkhata, fault system (MNF) to the west of the basin and smaller Mbamba fault (MF) to the east and document their evolution. Intra basin structures define a series of horsts, which initiated as convergent transfers, along the basin axis. The horsts are offset along a NE-SW striking transfer fault parallel to and along strike of the onshore Karoo 2 (Permo-Triassic) Ruhuhu graben. Discrete pre-existing structures probably determined its location and, oriented obliquely to the extension orientation it accommodated predominantly strike-slip deformation, with more slowly accrued dip-slip.To the north of this transfer fault, the overall basin architecture is asymmetric, thickening to the west throughout; while to the south, an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western MNF. The presence of the axial horst increasingly focussed sediment supply to the west. As the transfer fault increased its displacement, so this axial supply was interrupted, effectively starving the south-east while ponding sediments between the western horst margin and the transfer fault. This asymmetric bathymetry and partitioned sedimentation continues to the present-day, overprinting the early basin symmetry and configuration. Sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small (10-100 m) scale. The observed influence of basin-scale transfer faults on sediment dispersal and fault compartmentalisation due to pre-existing structures oblique to the extension orientation is relevant to analogous exploration settings.

Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

“…Additionally, preliminary observations from the SEGMeNT activesource refraction study report a significant increase in the crossover distance between sedimentary and crustal refractions in the Central Basin (Accardo et al 2016), and preliminary P-wave velocity models contain a several kilometres thick layer with P-wave velocities between 3 and 5 km s −1 , which may be consistent with older, more indurated Karoo sediments (Shillington et al 2015). We suggest that the low-velocity anomaly within the Central Basin represents an increased sediment package resulting from both greater sediment deposition related to Neogene-recent rifting (Mortimer et al 2016) as well as the presence of several kilometres of Karoo-age sediment.…”

“…If the 3 per cent reduction in velocity in the Central Basin relative to the North Basin is due to syn-rift sediment, then this would require an increase in the thickness of basin sediments, and thus could imply an increase in total extension, from north to south. Alternatively, the Central Basin is also thought to contain several kilometres of Karoo super-group sediments (Ebinger et al 1987;Mortimer et al 2016), which are exposed onshore in the Ruhuhu basin (Kreuser et al 1990) and could contribute significantly to the strong short-period velocity anomaly. Several lines of evidence point to the presence of Karoo sediments in the Central Basin including a transition in the character of seismic reflectors between the North and Central basins (Ebinger et al 1987) and the location/orientation of horsts within the Central Basin which are thought to be controlled by pre-existing structures associated with Karoo rifting (Mortimer et al 2016).…”

“…Alternatively, the Central Basin is also thought to contain several kilometres of Karoo super-group sediments (Ebinger et al 1987;Mortimer et al 2016), which are exposed onshore in the Ruhuhu basin (Kreuser et al 1990) and could contribute significantly to the strong short-period velocity anomaly. Several lines of evidence point to the presence of Karoo sediments in the Central Basin including a transition in the character of seismic reflectors between the North and Central basins (Ebinger et al 1987) and the location/orientation of horsts within the Central Basin which are thought to be controlled by pre-existing structures associated with Karoo rifting (Mortimer et al 2016). Additionally, preliminary observations from the SEGMeNT activesource refraction study report a significant increase in the crossover distance between sedimentary and crustal refractions in the Central Basin (Accardo et al 2016), and preliminary P-wave velocity models contain a several kilometres thick layer with P-wave velocities between 3 and 5 km s −1 , which may be consistent with older, more indurated Karoo sediments (Shillington et al 2015).…”

Terrane Boundary Reactivation, Barriers to Lateral Fault Propagation and Reactivated Fabrics: Rifting Across the Median Batholith Zone, Great South Basin, New Zealand

“…The distribution of different crustal rheologies and their associated strength variations, such as between distinct crustal terranes or between igneous batholiths and adjacent terranes, may inhibit fault nucleation and propagation in some areas while promoting it in others (Critchley, 1984;Howell et al, 2019;Koopmann et al, 2014;Magee et al, 2014;Peace et al, 2017). Preexisting structures in the crust may also localize deformation and control the geometry and evolution of fault and rift systems (e.g., Daly et al, 1989;Dawson et al, 2018;Fazlikhani et al, 2017;Fossen et al, 2016;Morley, 2017;Mortimer et al, 2016;Peace et al, 2017;Phillips et al, 2016;Rotevatn, Kristensen, et al, 2018;Vasconcelos et al, 2019). In addition, preexisting structures and fabrics at outcrop scale may be exploited by and control the geometry of later faults and fractures (Chattopadhyay & Chakra, 2013;De Paola et al, 2005;Dichiarante et al, 2016;Duffy et al, 2015;Kirkpatrick et al, 2013;Morley, 2010;Morley et al, 2004;Mortimer et al, 2016;Paton & Underhill, 2004;Phillips et al, 2017).…”

“…Preexisting structures in the crust may also localize deformation and control the geometry and evolution of fault and rift systems (e.g., Daly et al, 1989;Dawson et al, 2018;Fazlikhani et al, 2017;Fossen et al, 2016;Morley, 2017;Mortimer et al, 2016;Peace et al, 2017;Phillips et al, 2016;Rotevatn, Kristensen, et al, 2018;Vasconcelos et al, 2019). In addition, preexisting structures and fabrics at outcrop scale may be exploited by and control the geometry of later faults and fractures (Chattopadhyay & Chakra, 2013;De Paola et al, 2005;Dichiarante et al, 2016;Duffy et al, 2015;Kirkpatrick et al, 2013;Morley, 2010;Morley et al, 2004;Mortimer et al, 2016;Paton & Underhill, 2004;Phillips et al, 2017). Equally, preexisting structures do not always influence rift physiography; some may remain passive during subsequent tectonic events, while certain structures may only be selectively reactivated (e.g., Reeve et al, 2013;Roberts & Holdsworth, 1999).…”