Source-to-Sink Analysis of Rift Basin Tectonics and Sedimentation

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Abstract

This thesis investigates the development of regional drainage patterns within intra-continental rift basins using remote sensing data, field studies and numerical models. By examining modern-day extensional settings, such as the Basin and Range, USA, and the East African Rift System (EARS), this study elucidates the controls upon regional source-to-sink systems and assesses the findings in relation to existing, conceptual tectono-stratigraphic and drainage models. Rift basins are generally well studied and facies models well established. However, there is a tendency to overlook the regional perspective. Many drainage evolution studies and tectono-stratigraphic models focus upon the development of individual basin-bounding faults and half-grabens, often overlooking the influence of regional-scale drainage evolution upon landscape and stratigraphic development. On a regional scale, extensional basins are segmented into numerous sub-basins, which: (i) exist at different elevations; (ii) subside at different rates; (iii) vary in their degree of fluvial connectivity; and, (iv) may experience significant shifts between erosional and depositional regimes as drainage networks evolve. Through observations of the Basin and Range, and three-dimensional numerical modelling, it is shown how sub-basins with identical tectonic and climatic boundary conditions can exhibit vastly different stratigraphic fills depending on the degree of fluvial connectivity and their relationship to adjacent sub-basins. In addition, drainage integration is recognised as an overlooked, yet important process in the source-to-sink evolution of rift basins. Drainage integration between sub-basins of varying elevations is shown to cause widespread erosion and sediment bypass in the upstream basin, while contemporaneously increasing sediment supply to the downstream basin. A case study of the Tecopa Basin shows drainage integration as a powerful driver of base level fall and landscape transience in the absence of significant tectonic or eustatic controls. The differential gradients created by base level fall cause further upstream drainage rearrangements.To understand the evolution of regional hinterland drainages, two rift segments of varying maturity are compared. The Okavango Rift Zone represents a rift initiation phase segment while the Albertine Rift represents a rift climax phase segment. Through analysis of the contributing drainage patterns a model is proposed where early rift drainage is dominated by antecedent directions, with large, low relief, low slope catchments. As rifts mature catchments increase in relief and slope but reduce in asymmetry as the influence of antecedent drainage direction is reduced via tectonic tilting, drainage reversals and ponding.The thesis concludes that current tectono-stratigraphic models of sub-aerial rift settings commonly overlook the role of inter-basin erosion, under-appreciate the influence of antecedent drainage direction in hinterland input, and over-emphasise the role of axial rivers. Future tectono-stratigraphic models should acknowledge distinctions based upon the degree of fluvial connectivity (isolated or integrated sub-basins) and the regional position relative to adjacent sub-basins (upstream and terminal and sub-basins).

Keyword(s)

drainage integration; rift basin; tectono-stratigraphic

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