Controls on Asymmetric Rift Dynamics: Numerical Modeling of Strain Localization and Fault Evolution in the Kenya Rift

Abstract

Asymmetric rifting, which is characterized by a dominant single border fault, is known to have played a major role in the evolution of many past rift systems (e.g., Schlische et al., 2003;Withjack et al., 2013). It can also be observed in many presently active extensional tectonic settings (e.g., Gawthorpe & Leeder, 2008;Ebinger & Scholz, 2011), where it governs basin geometry, topography evolution, erosion, and sedimentation patterns. One such setting is the largest continental rift system in existence today: the East African Rift System (EARS). It exhibits a wide array of developmental stages from youthful extension with incipient faulting to final continental breakup (Corti, 2009;Ebinger & Scholz, 2011;Ring, 2014) and is thus an ideal location for studying the stages of early rifting that involve asymmetric normal fault activity followed by hanging-wall segmentation. In this study, we focus on the southern and central sectors of the Kenya Rift, which are in an early phase of active continental rifting (Ebinger et al., 2017) characterized by the transition from waning border fault activity to enhanced intra-basinal faulting and subsidence (Muirhead et al., 2016).The first-order tectonic characteristics of continental rifts are known to be influenced by a large range of structural, petrological, and thermal parameters, which have been investigated in previous numerical modeling studies (e.g.,

Mid‐Pleistocene to Recent Crustal Extension in the Inner Graben of the Northern Kenya Rift

“…Our data shows that most of the current extension is taking place within the inner graben and that this process has been going since the mid‐Pleistocene. While amagmatic extension processes can explain the surface‐deformation similar to the faults observed (e.g., Richter et al., 2021), it is likely that the surface manifestations are at least partly influenced by magmatic activity and the shallow intrusion of dikes into the crust (e.g., Pollard et al., 1983). In Kenya, volume estimates of plutonic rocks associated with the widespread extrusive units suggest a total volume of 111.2 × 10 3 km 3 of added gabbroic rocks in the crust of the Kenya Rift between 0° and 2°N (Karson & Curtis, 1989); as this estimate is only based on units that are preserved at the surface, the actual intrusive volume might be even higher (e.g., Guth, 2015; Prodehl et al., 1997).…”

“…Currently, the overall subsidence and extension in the Northern Kenya Rift reflects deformation processes within the inner graben; this area corresponds to the youngest manifestation of extensional faulting that transitioned from a pronounced asymmetry with major faults associated with an east-dipping master fault in the west and subsequently established antithetic, west-dipping normal faults to the formation of the magmatically and tectonically inner graben (Bosworth & Maurin, 1993;Dunkley et al, 1993;Saneyoshi et al, 2006). This transition to a more symmetric rift and localized subsidence and extension can be observed in all sectors of the Kenya Rift (e.g., Baker et al, 1988;Bosworth, 1987;Hautot et al, 2000;Rosendahl, 1987;Strecker et al, 1990), a sequence of events that is also predicted by numerical models of strain localization (e.g., Richter et al, 2021). Geological mapping of the major east-dipping normal faults in the Northern Kenya Rift (i.e., the Sattima and Kaimo faults) documents that despite their major offsets during the Tertiary, these structures are no longer tectonically active (e.g., Carney, 1972;Lippard, 1972;Mugisha et al, 1997).…”

Evolution of Rift Systems and Their Fault Networks in Response to Surface Processes

“…It is also similar to the first phase in two‐phase rifting (Agostini et al., 2009; Corti, 2012), where large faults border a central graben. Many rift segments in East Africa such as the Malawi and the Central Kenya rifts constitute examples of this phase, with active border faults surrounding the central graben (Ebinger & Scholz, 2012; Richter et al., 2021; Williams et al., 2019).…”

Evolution of rift systems and their fault networks in response to surface processes

“…It is also similar to the first phase in two-phase rifting (Agostini et al, 2009;Corti, 2012), where large faults border a central graben. Many rift segments in East Africa such as the Malawi and the Central Kenya rifts constitute examples of this phase, with active border faults surrounding the central graben (Ebinger and Scholz, 2012;Williams et al, 2019;Richter et al, 2021).…”