Rift Transfer Zones and the Stages of Rift Linkage in Active Segmented Continental Rift Systems

Abstract

Although much is known about the interaction of faulting and sedimentation within the basins of active segmented continental rift systems, little is known about these processes within the transfer zones of varying geometries that separate the young interacting segments. We address this problem in the humid, magma-poor juvenile western branch of the East African Rift System (WB-EARS). First, we present a broader classification of rift transfer zone geometries that accommodate both the plan-view geometries and border fault polarity patterns of the interacting rift segments. Within the framework of the transfer zone geometries, we explore the large-scale cross-over relief profiles, and the relationships with the spatiotemporal development of rift-linking faults (breaching faults) and axial stream patterns. Our results show that: 1.) distinct long-wavelength 2-D cross-over topographic relief shapes, directionality of axial stream flow (sediment routing patterns), and breaching fault patterns characterize rift transfer zones at the various stages of the linkage of interacting rift basins, 2.) these stages include unbreached, partially-breached, recentlybreached, and breached transfer zones, 3.) deforming transfer zones exhibit different styles of directionality of breaching, including a unidirectional (distinct propagator and receiver segments) and bi-directional propagation (both segments act as propagators and receivers) which may also modulate the cross-over relief shape, 4.) transfer zone breaching is facilitated by rift-flank deformation, and/or rift tip propagation structures in the form of rift splaying, border fault rotation, and fault cluster networks, 5.) the lateral propagation of breaching faults at the rift tips and flanks may be modulated by the extension direction and inherited basement structures. Our findings offer a broader insight into the geometries and structural evolution of rift transfer zones, and provide first-order predictions of large-scale sedimentation patterns of humid early-stage continental rift environments. Our models provide testable hypotheses for linking rift architecture and patterns of early-stage sedimentation applicable to ancient rift basins.

Knickpoint morphotectonics of the Middle Shire River basin: Implications for the evolution of rift interaction zones

“…We note that the estimated ages of linkage is a lower bound considering that knickpoint formation must have occurred after basin linkage and that knickpoint migration rate may have been lower during well-documented Late Pleistocene (135-75 ka) megadrought period in the region (Scholz et al, 2007). Nevertheless, the results provide a minimum quantitative constraint on the timing of rift linkage between the southern Malawi Rift and Shire Rift Zone across the Middle Shire basin, which was only speculated on in previous studies of the Middle Shire RIZ (Dulanya, 2017;Kolawole et al, 2021).…”

“…We also acknowledge that there could have been some complexity in the spatiotemporal sequence of opening of the Shire basin (i.e., north south the Shire River network), such as an initial opening of the Middle Shire to the Upper Shire prior to its linkage with the Lower Shire section, thus introducing some uncertainties into our age estimates. However, our interpretation of events is supported by syn-rift paleo-lake sediments at the southern end of the Zomba Graben including the Matope Beds and yet-to-be buried bedrock along the rift floor south of the Matope beds, suggesting a previously dammed southern-end of the Upper Shire segment (Bloomfield & Garson, 1965;Dulanya, 2017;Kolawole et al, 2021). Furthermore, an overall southward propagation of longterm basin development at the scale of the Malawi Rift is consistent with previous studies (Scholz et al, 2020).…”

“…Along this section, the Shire River flows over unconsolidated alluvium and colluvium (Shela, 2010) and is characterized by meanders with a high sinuosity index (ca. 1.17; Kolawole et al, 2021), reflecting the low topographic gradient (15 m) of the 130 km-distance between Mangochi and Matope. This lower gradient is also reflected by records of the Upper Shire silting up in the early 20th century, and causing the upper 65 km section of the river to temporally flow back into Lake Malawi (Dixey, 1924).…”

“…The Middle Shire River is characterized by erosion and narrow steep‐sided river sections reflecting recent incision (Bloomfield & Garson, 1965a; Kolawole et al, 2021; Lister, 1967). A number of prominent rivers, some of which are tributaries of the Shire River, originate in the Middle Shire section, including the Lisungwe and Wamkurumadzi Rivers.…”

“…Rift‐scale earthquake focal mechanism stress inversion also indicates a regional ENE–WSW‐trending minimum compressive stress, although, at the scale of individual faults, local stress rotations are possible (Williams et al, 2019). The tectonic style of the Middle Shire is dominated by a set of curvilinear rift faults striking NW to N that extend northwards from the Lower Shire Graben Rift Zone to the Southern Malawi Rift's Zomba Graben (Figure 3; Kolawole et al, 2021). These are intersected by an orthogonal set of fractures with a NE‐strike.…”