Heterogeneous Strain Distribution & the Effects of High Frequency Climate Change on the Evolution of Early-Stage Rift Systems: Case Studies from Lakes Tanganyika and Malawi (Nyasa), East Africa
Date of Award
Doctor of Philosophy (PhD)
The break-up of the earth’s crust through the formation of continental rift basins is a fundamental aspect of plate-tectonics and the first step that ultimately leads to plate-spreading, new continent formation, and the creation of ocean basins. Within continental rifts, processes associated with extensional faulting drive uplift and subsidence of the landscapes forming rift valleys, and climatically influenced processes act upon these transient landscapes, erode them, modify relief, and produce sediments that infill the rift valleys creating the stratigraphic record. Continental rifts are highly sensitive to the interplay between extensional faulting, sedimentary processes, and climate change, and variations in the evolution of the system can be related to both tectonic and climatic processes.The far-field forces, and/or a summation of the forces available to split a continent are considered insufficient to rupture the strong continental crust. This paradox may be overcome through a combination of the mechanical and thermal effects of magmatism, and inherited anisotropies within the earth’s crust such as pre-existing structures. Such inhomogeneities from earlier rift or orogenic events can assist in weakening and stretching the earth’s crust. While extensional faulting drives continental rifting, changes in the relief of the landscape, erosion of different lithologies, and the location of sedimentation within continental rifts, rapid changes in climate, including temperature, precipitation, and vegetation cover, affect rates of erosion and the flux of sediment into the rift valley. Thus, any spatio-temporal variation in sediment production affects the architecture of the sediments that fill the rift, as well as potential feedbacks in the structural evolution of the rift system. Previous studies have mainly focused on the influence of pre-existing structures and rapid climate change on rift kinematics and physiography at scales <100 km, with few studies assessing the rift-regional (>100’s km) scale. In addition, there is limited information on how these processes influence rift physiography along strike and how these control the location and geometry of rift segments throughout rifting. Within ancient continental rift basins, the record of these processes is typically deeply buried and inaccessible; high resolution data on ages and rates are lacking, and the resulting sedimentary fill may be deeply buried or overprinted by latter deformation events. This lack of high-resolution constraints makes it challenging to evaluate and isolate the tectonic and climatic contributions, and thus our ability to constrain the spatio-temporal variations in basin paleo-environment, sedimentation patterns, or the role of climate and tectonics on moderating these processes is restricted. The late-Cenozoic East African Rift System provides a natural laboratory in which to study active continental rifting processes, and the intersection of climate and tectonics at a young extensional plate boundary. In chapter 2 the influence of pre-existing structures on active rifting is investigated through balancing and restoring a series of 12 regional cross sections across the Lake Tanganyika Rift, using the structural restoration tool Lithotect, constructed from newly reprocessed seismic reflection data in the depth domain. This rift transects a series of different terrains containing both pre-existing structures and relatively homogeneous cratonic blocks. A spatio-temporal integration of the cross sections, and comparison with the various basement terranes the rift transects, reveals that extension in cratonic blocks is more widely distributed compared to areas with pre-existing structures where strain rapidly localizes onto border faults. These results reveal how pre-existing structures control extensional faulting, ultimately impacting the structural geometry and physiography of rift systems; however, characterizing the recent spatio-temporal evolution of individual faults remains elusive due to a lack of age control. Characterizing individual rift faults is crucial for understanding the fundamental controls on fault evolution, their contributions to rift opening, and for assessing their seismic hazards. In chapter 3, fault offset is measured on both late-Quaternary and Basement surfaces in the Lake Malawi (Nyasa) Rift to elucidate spatio-temporal patterns in fault evolution. Almost all faults offset the late-Quaternary surface and show sawtooth profiles of displacement along strike, indicating recent activity and potential segmentation. Observed extension on intra-rift faults, that is those within the rift valley, is approximately twice that predicted from the bending of the earth’s crust in response to such faulting; accordingly, these faults contribute to active continental breakup, rather than acting as a passive response to such phenomena. The observed distribution of extension is inferred to be influenced by variations in lithospheric structure and crustal heterogeneities that are documented along the rift axis. Despite crustal inheritance and tectonics exerting a fundamental control on the larger scale development of rift systems, these rates are not expected to fluctuate cyclically on the timescales of c.a., 100 kyr or less as climate is observed to fluctuate. In chapter 4, the effects of high-frequency climatic and tectonic interactions on landscape evolution, basin physiography, and sediment flux into the Lake Malawi Rift over the past 140 kyr are investigated using the landscape evolution model pyBadlands. This work provides a unique look into the overall functioning of lacustrine source-to-sink systems in juvenile continental rift basins. The onset of arid climate conditions causes extreme reorganization of river systems and the formation of mega-catchments that flow axially into a shallow restricted paleo-lake. Sedimentation rates within the much smaller, restricted lake are two times higher because of increased sediment focusing via these river systems. Wetter climates cause rapid expansion of lake area and destruction of these axial mega-systems. The redistribution of mass into focused depocenters during arid intervals, caused by the changes in basin physiography and sediment flux, has the potential to promote narrow rifting modes, prolong fault lifespan before abandonment, and to induce basement-detached deformation through sediment mobilization. This dissertation provides a more complete understanding of how pre-existing structures interact with tectonic and climate processes to shape the evolution of juvenile continental rift systems. While pre-existing structures and crustal heterogeneities determine fault evolution and initial basin geometry, high frequency climate change acts upon this landscape causing sediment focusing, drainage reorganization, and changes in erosion rates. Moreover, this dissertation provides a framework to test the feedbacks between high frequency climate forcings and tectonic evolution of juvenile rift systems.
Wright, Lachlan, "Heterogeneous Strain Distribution & the Effects of High Frequency Climate Change on the Evolution of Early-Stage Rift Systems: Case Studies from Lakes Tanganyika and Malawi (Nyasa), East Africa" (2022). Dissertations - ALL. 1592.