Chapter 2. Tectonics, climate and sedimentation in the Albertine Rift

General Aspects of Rifting.- Evolution of Continental Rift Systems in the Light of Plate Tectonics.- Evolution of the Earth's Crust.- The Origin and Evolution of Rifts and Rift Valley Structures: A Mechanistic Interpretation.- Experimental Model Studies of Rift-Valley Systems (Abstract).- Rates of Sedimentation and Vertical Subsidence in Neorifts and Paleorifts.- Active Continental Rifts.- Deep Structure and Evolution of the Baikal Rift Zone.- Two Stages Rhinegraben Rifting.- The Rio Grande Rift and A Diapiric Mechanism for Continental Rifting.- Owens Valley - A Major Rift between the Sierra Nevada Batholith and Basin and Range Province, USA.- Paleorifts.- Rift Systems in the Western United States.- The Late Precambrian Central North American Rift System - A Survey of Recent Geological and Geophysical Investigations.- An Early Paleozoic Continental Rift System in Galicia (NW Spain).- The Midland Valley as a Rift, Seen in Connection with the Late Palaeozoic European Rift System.- Main Geologic Features of the Oslo Graben.- The Tectonic History of the Oslo Region.- The Oslo Region during the Early Palaeozoic.- Continental Margins.- The Margin South of Australia - A Continental Margin Paleorift.- Passive Continental Margins and Intra-Cratonic Rifts, A Comparison.- Observations on the Margin off Norway (66-70 N) and the History of Early Cenozoic Rifting.- Pelagic Sedimentation in Immature Ocean Basins.- North Sea Rift and Basin Development.- Geophysical Studies.- A Comparison of Magnetic Anomalies in the Red Sea and the Gulf of Aden.- Palaeomagnetism in the Oslo Rift Zone.- Seismic Mapping of the Fennoscandian Lithosphere and Asthenosphere with Special Reference to the Oslo Graben Region.- The Ideal-Body Concept in Interpretation of the Oslo Rift Gravity Data and their Correlation with Seismic Observations.- Comparative Studies.- Comparison of the East African Rift System and the Permian Oslo Rift.- A Comparison between the Older (Karroo) Rifts and the Younger (Cenozoic) Rifts of Eastern Africa (Abstract).- Main Features of Evolution and Magmatism of Continental Rift Zones in the Cenozoic.- The St. Lawrence Paleo-Rift System: A Comparative Study.- Some Problems of Rifting Development in the Earth's History.- Rift Valleys on Earth, Mars and Venus.- Paleorifts - Concluding Remarks.

Whilst Albertine Rift marginal lacustrine and deltaic depositional environments each produce characteristic lithofacies which together can form a complex stratal architecture, ≥80% of the onshore area of these early continental rift basins is actually dominated by fluvial systems and their associated rift valley terrestrial environments. Albertine Rift rivers today can be classified as flank fan drainage, flank drainage rivers or long-axial systems. Dependent upon slope gradient and sinuosity, and reflecting the bed or suspended load transported, alluvial channels can cycle through four main stages before entering a delta distributary system at the lake shoreline. Higher energy flows closer to rift margins are dominated by gravel bed loads, transitioning to mixed and then suspended loads as gradients decrease out across the rift valley floor. Typical fluvial geomorphological features such as riffle–pool sequences, channel and side bars, meanders, point bars and oxbow lakes are accompanied by development of characteristic ichnofabrics and rhizofabrics in riparian sands and interfluve silts and clays. The four different fluvial stages can be recognized in onshore Pleistocene–Holocene sedimentary successions of both Lake Edward and Lake Albert, with well-sorted fluvial sandbodies, containing termite nests, correlating with petroleum reservoir intervals in the subsurface.

Continental rifting is a fundamental geological process that has been responsible for breaking supercontinents apart and opening new ocean basins throughout Earth's history and the East African Rift System (EARS) is the best active example of this happening in the world today. The Albertine Rift lies at the northern end of the Western Arm of the EARS, consisting primarily of the Lake Albert and Lake Edward basins as they pass along the border between Uganda and the Democratic Republic of Congo. This Memoir describes the key geomorphological features found in the Albertine Rift Valley at the present day, identifying the sedimentary dynamic elements that dovetail together to characterize the dominant depositional environments in these early continental rift basins. These are then used to interpret the Pliocene–Holocene sedimentary sequences, documented during geological field surveys over an eight-year period, exposed around the onshore parts of the Lake Albert and Lake Edward rift valleys. Ultimately, the sedimentary dynamics of the Albertine Rift, and the stacking of its 3D stratigraphic architecture, is shown to beat to the rhythm of oscillating global glacial–interglacial climatic cyclicity and how this affects the Equatorial Tropics of Africa.

The genus Myosorex has a classic relict distribution within sub-Saharan Africa. Montane populations in eastern and western equatorial Africa are separated by ca. 2900 km. Until this study, the closest known populations in southern Africa were separated by nearly 2000 km from the closest populations in the Albertine Rift Valley. Here we document previously unknown populations of Myosorex, representing two new endemic taxa from montane forests adjacent to the Albertine Rift. In conjunction with additional data from Malawi, we fill in major gaps in our knowledge of the biodiversity and distribution of this genus in the areas of the Albertine and Malawi Rift Valleys. We demonstrate that this gap is an artefact of survey effort and collecting serendipity. The two new species described herein, as well as other species of Myosorex from north of the Zambezi River, exhibit limited distributions and are confined to montane habitats, typically above 1000 m. Our new species of Myosorex from Kahuzi-Biega NP (...

Rift-related igneous activity and metallogenesis in SW Bergslagen, Sweden

A persistent problem that has hampered a general understanding of Albertine Rift early continental rift basins has been the lack of a high-resolution stratigraphic framework within which to correlate sediments across their onshore outcrop. Crucial to resolving this problem is resolving how the 3D stacking architecture of sediments is controlled by a combination of evolving rift basin structure and the glacial climatic cyclicity experienced at the Equatorial Tropics of East Africa during deposition. Rift basin structural styles and subsidence rates are modelled and their merits discussed in the light of the field evidence gathered from around the Albertine Rift. After establishing the mechanism of structural development in the basins, the impact of glacially driven climatic oscillations on sedimentary lithofacies is also incorporated to produce a combined glacial climatic cyclicity model for Albertine Rift basin evolution. This superimposes lake level transgressive–regressive cyclicity upon a tectonic regime of fault propagation and depocentre retreat away from rift margins. The model predicts the resulting stratigraphic architecture that should develop in outcrop areas, and is supported by field evidence of not only lithofacies characteristic of arid (glacial) and wet (interglacial) climatic conditions but also cycles of 3D terrace development, channel incision and sediment backfilling.

From Permian to Jurassic times the northern Viking Graben and adjacent platform areas experienced multiple rifting, the Permian-early Triassic and middle-late Jurassic rift episodes, separated by an intervening middle Triassic-middle Jurassic inter-rift period dominated by relative tectonic quiescence. The associated syn- and inter-rift strata show large variations in sedimentary architecture as a result of temporal and spatial variations in tectonic deformation and subsidence, sediment supply, climate and accommodation creation. The Permian-early Triassic syn-rift succession is believed to consist predominantly of non-marine, arid to semiarid, aeolian, sabkha, alluvial and lacustrine strata, probably interbedded with marine strata on the Horda Platform and in the Viking Graben. The middle Triassic-middle Jurassic experienced several subsidence stages which, together with climatic variations, exerted a major control on the periodic outbuilding and retreat of rift marginal, alluvial and shallow marine clastic wedges. Evidence for fault block rotation suggests that the subsidence was caused partly by minor extensional stages. As such, the middle Triassic-middle Jurassic does not fit the type of development assumed to be typical for either post- or pre-rift basins. Hence, the notation inter-rift is assigned to this period and the associated succession. The middle-late Jurassic rift episode was characterized by multiple rift phases separated by intervening stages of relative tectonic quiescence. The syn-rift infill is mixed non-marine and marine and consists of fluvial through shallow marine and shelfal deposits to deeper marine sediment gravity flow and (hemi-)pelagic strata. At the larger scale, related to the entire middle-late Jurassic rift episode, the syn-rift infill in general shows a two-fold sandstone-mudstone lithology motif, typical of underfilled rift basins. At the intermediate scale, related to single rift phases, threefold sandstone-mudstone-sandstone, twofold sandstone-mudstone and single mudstone lithology motifs are present, typical of sediment overfilled/sediment balanced, sediment underfilled and sediment starved rift basins, respectively. The spatial and temporal variations in the syn-rift infill reflect relative distance to the rift basin hinterland areas (which had a large sediment yield potential) and overall increased tectonic subsidence and enhanced rift topography as the rift basin evolved. This suggest that the tectonostratigraphic evolution of the northern North Sea rift basin can be viewed at several scales: at the largest scale the rift basin evolved through multiple rift episodes, which commonly had a duration of several tens of Ma. The rift episodes are separated by inter-rift periods. Rift episodes are subdivided into intervals representing distinct rift phases. These rift phases were separated by tectonic relatively quieter intervals, here referred to as tectonic quiescence stages. Inter-rift periods are subdivided into prolonged tectonic quiescence intervals separated by short-lived rift stages or minor rift phases. Distinct rift phases and inter-rift tectonic quiescence intervals commonly represent periods of a few to 10+ Ma, and correspond to second-order sequences or ‘megasequences’. At the smaller scale, syn-rift successions can be subdivided into packages related to distinct rotational tilt event or faulting events (deformation spans), representing hundreds of ka to few Ma and corresponding to third-order sequences. Solitary, large-magnitude faulting events (deformation clines) are likely to exert a major control on high frequency base- or sea-level fluctuations and thus on the development of higher-order sequences. However, such a control is difficult to prove and can probably only be recognized in sub-basins with abundant wells and a dense well spacing.

Tectonic Models for the Evolution of Sedimentary Basins

Rifts are one of the most spectacular features of global morphology. Within oceans they separate plates as new oceanic crust is created. Within continents they may form deep valleys such as the Rhine graben, within which runs one of the world's busiest waterways. They are often floored by deep lakes, such as Lake Baikal, the deepest lake in the world today whose floor lies 1700 m below the surface of the lake. Ancient rifts are the sites of petroleum accumulations beneath the margins of the Atlantic, in the North Sea and in China, of coals and oil shales and of minerals, including phosphates, barite, Cu-Pb-Zn sulphides and uranium (Robbins 1983). Of all the world's rifts none can match in scale and diversity the Great Rift System, which runs for over 7000 km and includes the East African Rift System and its extension through the Red Sea into the Dead Sea, to form a unique feature of global geology. Its study is important not only for what it can tell about the nature and origin of present-day rifts, the thermal, magmatic and tectonic processes which gave rise to them, climatic changes, and sedimentary, particularly lacustrine and volcaniclastic, processes. It is also essential for the understanding of the processes which formed passive continental margins, all of which are underlain by rifted sedimentary basins, and failed rifts whether they formed within continents or at the junction of continents and oceans. Early workers on rifts were impressed by the wide valleys, 40-50 km across, filled by young alluvial and lacustrine sediments and bordered by steep escarpments rising in some cases a few hundred metres, in others up to 3-4000 m above the valley floor. Many rifts are the sites of volcanic activity, sometimes of rather unusual composition. These unusual volcanic source rocks may in turn be the cause of peculiar lake water chemistries, many rift lakes containing unique suites of evaporitic minerals. Although at one time some people considered rifts to have been formed by compression (rampvalley hypothesis), discussion today revolves principally around the extent to which rift valleys are extensional features or are the result of oblique-slip movements between two laterally moving blocks. There is no doubt now that some rift valleys are the result of strike-slip movement. The Dead Sea/ Gulf of Elat rift lies along a lineament that separates two lithospheric plates and, as Arabia moves northwards relative to the Palestinian (Mediterranean) block and faults curve or sidestep each other, extensional pull-apart basins are formed, such as the Dead Sea whose surface lies further below sea level than any other place in the world. A feature of these strike-slip rift valleys is that the extensional valleys pass, along the rift, into mountainous zones of compression and uplift. Rapid erosion from these mountains leads to substantial sediment supply for the basins from within the fault system itself. Ancient strike-slip rift basins are therefore characterized by very thick, rapidly accumulated basin fills with areas of contemporaneous compressional tectonics and unconformities not far away. Rift valley basins that are extensional features on a regional scale tend to sink rather more slowly, have less sediment entering them, and show no evidence of synchronous compressional tectonics in their neighbourhood. Cross-sections through them show normal faulting, probably listric in form. The main controversy with regard to extensional rift valleys is whether magmatic activity in rifts reflects an underlying heat source such as a convection plume in the mantle or hot spot, the rifting then being the consequence of expansion and uplift brought about by thermal activity, or whether the magmatic activity is merely the result of regional or local extensional stresses which have passively allowed magma to come to the surface. Some rifts that penetrate continents are clearly failed rifts or aulacogens that began as one arm of a triple junction. As ocean floor spreading developed along two arms, one arm was left as a 'failed rift' in the sense that new oceanic crust was not created. Perhaps the best known is the Benuc trough within which the Niger delta developed, However, the northern (Viking) graben of the North Sea has been claimed to be one (Whiteman et al. 1975), and so has that part of the East African Rift known as the Afar triangle where thc Red Sea and the Gulf of Aden Rifts successfully drifted, whilst the Afar triangle 'failed'. Molnar & Tapponnier (1975) have argued that other rifts are the result of continental collision, in particular that the impact of India with Asia caused not only major lateral displacement along

The Albertine Rift is dominated by Lakes Albert and Edward, which together represent two of the great rift lakes of East Africa. Rift basin lakes form one of the most obvious geomorphological features in an early continental rift basin, as crustal extension proceeds, causing the rift valley floor to subside and diverting the regional drainage pattern into the depocentre. Modern-day open lacustrine sedimentation in Albertine Rift lakes is dominated by deposition of organic-rich clays and diatomites, with fan deltas and turbidites delivering clastics from the flanks and long axes of the rift basins, respectively. Oxidation colours in the clays can be used to indicate bottom-water anoxia and seasonal mixing. Field evidence from Plio-Pleistocene rift-fill sediments demonstrates that diatomite horizons mark lacustrine transgressions, with deeper clay intervals recording elevated uranium (U) and thorium (Th) concentrations. Alkaline lake marly limestones, with algal mats, were discovered in two areas of onshore Lake Edward and preserve exceptionally high U and Th signatures. Similar values were identified in lacustrine clays of northern Lake Albert and suggest the development of restricted alkaline lake conditions within the Albertine Rift during its geological evolution were more common than previously acknowledged.

The primary objective of International Ocean Discovery Program Expedition 381 was to retrieve a record of early continental rifting and basin evolution from the Corinth rift, central Greece. Continental rifting is fundamental for the formation of ocean basins, and active rift zones are dynamic regions of high geohazard potential. However, the detailed spatial and temporal evolution of a complete rift system needed to understand rift development from the fault to plate scale is poorly resolved. In the active Corinth rift, deformation rates are high, the recent synrift succession is preserved and complete offshore, earlier rift phases are preserved onshore, and a dense seismic database provides high-resolution imaging of the fault network and of seismic stratigraphy around the basin. As the basin has subsided, its depositional environment has been affected by fluctuating global sea level and its absolute position relative to sea level, and the basin sediments record this changing environment through time. In Corinth, we can therefore achieve an unprecedented precision of timing and spatial complexity of rift-fault system development, rift-controlled drainage system evolution, and basin fill in the first few million years of rift history. The following are the expedition themes: High-resolution fault slip and rift evolution history, Surface processes in active rifts, High-resolution late Quaternary Eastern Mediterranean paleoclimate and paleoenvironment of a developing rift basin, and Geohazard assessment in an active rift. These objectives were and will be accomplished as a result of successful drilling, coring, and logging at three sites in the Gulf of Corinth, which collectively yielded 1645 m of recovered core over a 1905 m cored interval. Cores recovered at these sites together provide (1) a longer rift history (Sites M0078 and M0080), (2) a high-resolution record of the most recent phase of rifting (Site M0079), and (3) the spatial variation of rift evolution (comparison of sites in the central and eastern rift). The sediments contain a rich and complex record of changing sedimentation, sediment and pore water geochemistry, and environmental conditions from micropaleontological assemblages. The preliminary chronology developed by shipboard analyses will be refined and improved during postexpedition research, providing a high-resolution chronostratigraphy down to the orbital timescale for a range of tectonic, sedimentological, and paleoenvironmental studies. This chronology will provide absolute timing of key rift events, rates of fault movement, rift extension and subsidence, and the spatial variations of these parameters. The core data will also allow us to investigate the relative roles of and feedbacks between tectonics, climate, and eustasy in sediment flux and basin evolution. Finally, the Corinth rift boreholes will provide the first long Quaternary record of Mediterranean-type climate in the region. The potential range of scientific applications for this unique data set is very large, encompassing tectonics, sedimentary processes, paleoenvironment, paleoclimate, paleoecology, geochemistry, and geohazards.

Post-cratonization rifting has emerged as a prominent research focus in structural geology due to its association with significant hydrocarbon accumulations. Such rift systems are extensively developed within and along the margins of cratonic regions during the Mesoproterozoic to Neoproterozoic, notably in areas such as the Siberian Craton, Australian Craton, and North American Craton. The genesis of these rift systems is typically attributed to extensional tectonic regimes that evolved during the post-orogenic reconfiguration of cratonic lithosphere. These systems represent critical tectono-sedimentary processes that influence crustal thinning, fault block development, and the formation of accommodation space, playing a key role in hydrocarbon source rock maturation, reservoir development, and trap formation. Recent advancements in natural gas exploration within the Ediacaran strata of the Sichuan Basin have revealed the substantial hydrocarbon resource potential of the Neoproterozoic sequences in the Upper Yangtze Craton. These exploration successes are intimately associated with the development of deep-seated extensional rift systems in the Yangtze Craton, which are interpreted as the result of rapid lithospheric extension following cratonization during the early Neoproterozoic. Despite these breakthroughs, a comprehensive understanding of the structural geometry, kinematic evolution, and petroleum systems of these rift systems remains limited, highlighting the need for further systematic investigation. This study integrates two-dimensional and three-dimensional seismic data with magnetotelluric data, deep borehole records, and field outcrop observations to construct, for the first time, a three-dimensional structural model of the Neoproterozoic rift systems in southwestern Sichuan Basin. The results reveal two distinct rifting phases during the Early to Middle Neoproterozoic, with rift dimensions ranging from 3-8 km in width and 7-23 km in length. The rift systems and associated fault networks predominantly display NE and NNE trends, with faults generally dipping northwestward. These faults governed the development of half-grabens during both rift phases, each accompanied by sedimentary deposits reaching thicknesses of 2–3 km. The stratigraphic sequences within the rifts exhibit strong correlations with the Neoproterozoic strata exposed along the western margin of the Yangtze Craton. Chronological evidence indicates that the first rift phase (800–720 Ma) was characterized by independently developed sub-rift basins. The second rift phase (720–635 Ma) inherited and expanded upon the earlier rifting, culminating in the development of a unified, large-scale half-graben that overlies the sub-rifts of the first phase. During the late syn-rift stage, significant compressional uplift along the western Yangtze Craton margin induced structural inversion of several pre-existing rift normal faults in southwestern Sichuan and the formation of pre-Ediacaran reverse faults. This compressional event eroded over 3 km of rift-related sequences. The Neoproterozoic rifting and subsequent compressional deformation along the western Yangtze Craton margin are closely tied to subduction and rollback dynamics of the Pan-Oceanic plate. This study emphasizes the excellent conditions for hydrocarbon source rock and reservoir formation in the Neoproterozoic of southwestern Sichuan, highlighting its vast potential as a target for future hydrocarbon exploration.

The East African rift system in the light of KRISP 90

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Summary Lithospheric stretching can successfully account for the overall evolution of many sedimentary basins, and detached normal faulting the detailed geometry of the upper crust. In an instantaneously stretched, isostatically compensated basin, the equations which describe these two processes can be combined to define a ‘notional depth to decollement’. Only at this level can the sole to the normal fault system maintain a constant depth below sea-level during extension. The notional depth to decollement depends primarily on the mean density of the basin fill and for a constant-density basin fill (e.g. sea water) coincides with the level of no vertical motion during stretching. In a sediment-filled basin, the notional depth to decollement will increase with the stretching factor β as early-deposited sediments are compacted and the mean density of the sediment column increases. In general, a physical sole fault will not lie at the notional depth to decollement, and must move vertically to maintain isostatic equilibrium: such movement precludes the use of balanced cross-section techniques to determine the physical depth to decollement. In typical crustal situations, uplift of the sole fault will be more common than subsidence and will in turn cause uplift of any residual, unfaulted basement blocks which rest upon it. Footwall uplift can also be modelled using area-balance constraints, referred to the notional depth to decollement rather than to the physical sole fault. The amount of uplift depends primarily on the initial fault spacing. Three fields can be distinguished: one in which footwalls subside at an increasing rate as β increases, one in which they are uplifted above sea-level then subside below sea-level, and one in which they are uplifted then subside, but always remain above sea-level. Similar relationships exist in an uncompensated basin, where the depth to the physical sole fault replaces the notional depth to decollement. Curves showing uplift and subsidence versus β have been constructed in dimensionless form (referred to depth to decollement) and for a model basin with an exponential sediment compaction relationship. They agree closely with uplift/subsidence histories inferred from seismic and well data for the North Sea, and with published descriptions of other areas (the Armorican margin, the Aegean Sea).