Date of Award

12-19-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Scholz, Christopher

Keywords

East Africa Rift, Lake Overflow, Rift Structure, Seismic Facies, Shear zones, Tectonostratigraphy

Subject Categories

Geology | Geomorphology

Abstract

Active continental rifts are ideal sites for understanding the break-up of continents, and long-lived rift lake environments are known as important reservoirs for endemic communities and biodiversity. The sedimentary fill of extensional continental rifts within the East Africa Rift System (EARS) records a long history of continental extension and variable tropical climate that is unparalleled in its duration and fidelity. Continental extensional basins are sensitive to variations caused by the interplay between tectonics, sedimentary processes, and climate change. However, to discern the sedimentary fill history and its related tectonostratigraphy, as well as for reconstructing past climate changes, high-fidelity data such as basin-scale high-resolution seismic reflection data and continuous drilling information from stratigraphic boreholes or exploratory wells are required. Constraining the geometry of geological structures in extensional basins such as those within the EARS is important for studies of sedimentation, limnology, and hazards. Although previous studies have mapped geological structures of the Lake Tanganyika Rift in East Africa, many aspects of the geometry of the rift structures are still poorly known. Previous studies either used data with limited spatial coverage, with low resolution, or both. Much earlier rift basins of the Karoo Permo-Triassic system below the late-Cenozoic- Lake Tanganyika Rift sediments have been proposed to have acted as weak zones during the Cenozoic rifting, however this hypothesis has not before been explicitly tested. Understanding the evolution of tectonic rift lakes is critical for studying continental breakup, recovering terrestrial climate histories, assessing hydrocarbon resources and biological evolution. Therefore, it is important to study how rift lakes evolve, interact and link in space and time, as they are highly sensitive to tectonic processes and climate variability. For instance, recent studies speculate that in the past, Lake Rukwa flowed into Lake Tanganyika, East Africa during its high stands. Spectacular oversized deltaic prograding clinoforms, which may be linked to the outflow of Lake Rukwa, are observed in seismic reflection data within the Lake Tanganyika Rift on the Karema platform. In the absence of borehole data in this area, a numerical modelling approach may be required to test these lakes’ connectivity. Chapter 2 of this study uses recently acquired, state-of-the-art 2D seismic reflection data, together with reprocessed legacy data, to evaluate the evolution and distribution of sedimentary facies over the Lake Tanganyika Rift. Using seismic stratigraphic analysis, I reconstruct past depositional environments and the paleogeography of the lake and assess how tectonic-driven subsidence and hydroclimate variability modifies lacustrine basins. I identify six syn-rift seismic units overlying the acoustic basement and identify sedimentary units beneath the syn-rift sequence that suggest episodes of pre-rift sedimentation. This analysis suggests that the earliest -stage rift system is of low-relief that is usually dominated by alluvial, fluvial, and shallow lacustrine conditions. As the basin continued to evolve in space and time, the lacustrine environment increased in water depth, and catchment relief and accommodation increased, consistent with a more mature rift. Under these conditions, active extensional continental rifts exhibit extensive deltaic deposits and deep-water fans, and locally, canyons, channels, channel-levee complexes, turbidites, slumps and other mass flow deposits. In the latter part of continental rift lake history, erosional surfaces and abundant lowstand delta facies are observed, indicating periods of dramatic hydroclimate cycles. I assess the relative timing of key features of the rift, including the emergence of major structures and rift segment boundaries, and development of major drainages and linkages to upstream rift lakes. This study illustrates a shallow to deep progression of rift valley environments and then restriction of littoral habitats that might have influenced the evolution of its unique endemic organisms. Rift structures such as rift segments and accommodation zones, border faults, rift-parallel fault blocks, and transfer faults exert a first-order control on the spatial and temporal distribution of sedimentary facies and speciation of endemic species in continental rifts. The Western Branch of the EARS provides a natural laboratory to investigate how basement anisotropies assist in rupturing thick, strong continental crust in the magma-poor setting. Using the Lake Tanganyika Rift as a case study, chapter 3 of this dissertation integrates newly acquired, aeromagnetic and Full Tensor Gradiometry (FTG) data with 2-D seismic reflection data to assess the deep basin and underlying basement structure. I consider the evolution of the Kavala Island Ridge (KIR), a major rift segment boundary and assess the relationship of major structures around the KIR to inherited crustal anisotropies. Derivative-filtered aeromagnetic and gravity grids show a dominance of NW-trending structural fabrics at long wavelengths (>5km) corresponding to the deeper basement depths, and dominant NW-SE with a secondary NNE-SSW fabrics at shorter wavelengths (<3 km) representative of the shallower structures and intra-basinal structures. Long wavelength anomalies indicate that deep structures have a general NW-SE strike parallel to most of the regional basement structures. Short wavelength anomalies reveal that shallow subsurface structures are dominated by a NW-SE strike with a secondary NNE-SSW strike. The NW-SE striking structures are attributed to the inheritance of pre-existing basement structures. Seismically-constrained 2-dimensional forward modeling of the aeromagnetic and gravity data reveals: 1) an anomalously high-density (2.35-2.45 g/cc) deep-seated, fault-bounded wedge-shaped sedimentary unit that directly overlies the pre-rift basement, likely of Mesozoic rift (Karoo) origin; 2) a ~4 km-wide sub-vertical low-density (2.71 g/cc) structure within the 3.2 g/cc basement, interpreted to be an inherited basement shear zone, 3) a large intra-basinal early-rift fault co-located with the modeled shear zone margins, defining a persistent intra-basin ‘high’, and 4) a shallow intra-sedimentary V-shaped zone of comparatively dense material (~2.2 g/cc), interpreted to be a younger axial channel complex confined between the intra-basin ‘high’ and border fault. The results provide insights on early-rift Tanganyika Rift architecture which was modulated by basement structure, and its influence on the subsequent rift structure. This study provides useful information for future research, especially on the evolution of juvenile rifts and how they transition into passive margins, as well as providing useful context for hydrocarbon exploration. Analysis of the stratigraphy on and around the KIR indicates that relief on the ridge was established early in rift history, and that it has been a persistent geographic boundary for much of the lifespan of Lake Tanganyika. To test how juvenile active continental rifts and rift-lakes interact and link in space and time as they respond to slow tectonic processes and high-frequency climatic change, I employ a numerical modeling approach. In chapter 4, I integrate a FastScape algorithm that utilizes the stream power law to predict landscape evolution with the Linear Upslope Model for orographic precipitation to analyze and document the topographic evolution and hydrological connectivity of Lakes Tanganyika and Rukwa in the vicinity of the Rungwe volcano. The analysis suggests that as climate fluctuates, two active rift lakes that are in proximity and structurally linked may interact and connect, depending on the magnitude of precipitation. The models predict that Lake Rukwa overflowed into Lake Tanganyika causing the deposition of deltaic sediments along the Karema Platform. This implies that in the geologic past Lake Rukwa was a much larger lake than present; the small-sized underfilled condition of modern Lake Rukwa may be due to diminished precipitation and a regional rain shadow, produced by the growth of the Rungwe volcanic edifice. This study provides background paleogeographic context for the LTR and Rukwa rifts and how tectonics modulates surface processes on the scale of large continental rifts, informative for understanding the origins of endemic species, water column exchanges and geo-resource exploration. However, understanding the details of changing hydrologic and water column chemistries over geologic time ultimately requires testing through scientific drilling. To better constrain the timing nature of the identified seismic units as well as proving the hydrological connectivity between Lakes Tanganyika and Rukwa, deep scientific drilling intersecting the crystalline basement is required. Also, to prove the connectivity between Lake Rukwa to Lake Tanganyika over the past several hundred thousand years, core sampling of material from the Karema depression is required. Furthermore, to obtain a full picture of Cenozoic juvenile rifting, additional ‘infill’ multi-channel basin-scale seismic reflection or refraction data will be needed.

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