N. J. Kusznir

Cenozoic extension in northern Kenya: a quantitative model of rift basin development in the Turkana region

Recent geological and geophysical studies in the Turkana region of the northern Kenya rift have highlighted the presence of a number of major sedimentary depocentres (Morley et al., 1992) overlying anomalously thin crust (Mechie et al., 1994). The region overlies a putative structure linking the Mesozoic-Palaeogene rifts of south Sudan and eastern Kenya (Ibrahim et al., 1991; Bosworth, 1992). The flexural cantilever model of continental extension (Kusznir et al., 1991) has been used to describe the rift evolution of the Turkana region, constrained by seismic reflection data and surface geology. Sections showing crustal structure, basin geometry and β stretching factor profiles have been modelled for end Palaeogene (25 Ma), end Miocene (5 Ma) and end Pliocene (2 Ma). We also show that, whilst extension estimates giving a β stretching factor estimate of 1.55–1.65 are consistent with geophysical data showing that the Moho has been shallowed to 20 km by extension, a significant portion of that extension may pre-date the Neogene. We propose that across the Turkana region during the Palaeogene a series of basins formed, linking known loci of extension in southern Sudan with extension in the Anza/Kaisut system and South Kerio basins in northern and central Kenya. These basins have no surface outcrop expression, but are defined by gravity anomalies and a number of pieces of indirect evidence. Continued extension on the later East African rift system resulted in reactivation of some of these structures in Neogene-Recent times, and the abandonment of others. n nModelling of the rift basin geometry in the Turkana region using the flexural cantilever model gives a value of effective elastic thickness (Te) of 3.5 km. Flexural bending of the lithosphere associated with extensional fault block rotations is shown (following Buck, 1988) to generate large bending stresses which give rise to substantial upper lithosphere brittle failure and plastic deformation, so producing low values of lithosphere flexural strength and Te. The low values of Te estimated locally for the rift region by the flexural cantilever model differ from the much larger values of Te determined using gravity-topography coherence techniques (Ebinger et al., 1989) which sample regional lithosphere strength including that of strong continental shield outside the rift. n nThe maximum β stretching factor of 1.55–1.65 defined by the model is insufficient to generate the observed volcanics using published quantitative models of melt generation (McKenzie and Bickle, 1988). An anomalously hot mantle temperature is required to generate any melt at such low stretching factors. Whilst previous authors (Karson and Curtis, 1989; Latin et al., 1993) have proposed that large volumes of melt have been intruded into and ponded within the lithosphere, we show that crustal underplating and intrusion do not significantly alter estimates of crustal thickness and extension across the rift system.

The mechanics of continental extension and sedimentary basin formation : a simple-shear/pure-shear flexural cantilever model

Abstract Deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins. Major basement faults are generally planar and extend down to 10–15 km depth corresponding to the base of the seismogenic layer. Beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle. A mathematical model is described of the geometric, thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle. During faulting, upper crustal footwall and hanging-wall blocks behave as two mutually self supporting flexural cantilevers; their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse. For extension on multiple faults, interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics. The flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing, horizontal displacement and polarity. The effects of fault spacing, fault polarity and the amount of extension, as well as erosion of footwall uplift on crustal structure and basin geometry during both syn-rift and post-rift stages of basin evolution are explored. Models of “passive” rifting, involving lithosphere extension in response to the build-up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere, appear to conform to the main geological observations. The role of “active” rifting, driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain.

Stress concentration in the upper lithosphere caused by underlying visco-elastic creep

Abstract The implications of subdividing the lithosphere into upper elastic and lower viscoelastic layers are investigated by finite-element analysis. Application of uniform horizontal boundary stresses at the ends of a lithospheric plate leads to amplification of the stress in the elastic layer by a factor about equal to the ratio of lithospheric to elastic layer thicknesses, and the visco-elastic layer becomes nearly stress-free except near its ends. The time constant for approach to equilibrium is proportional to viscosity, being for our model 0.21 My for 10 23 N s m −2 , and there is some accompanying flexure of the lithosphere. Local variation in the thickness of the elastic layer causes inverse variation in the stress, in part explaining the stability of shield regions and the tectonic activity of hot plateau uplift regions. It is shown that stress amplification also occurs where the stresses arise from body forces such as differential loading and isostatic compensation across continental margins.

Quantitative analysis of Triassic extension in the northern Viking Graben

It has been recognized for over a decade that large-displacement, pre-Jurassic faults are present in the Northern Viking Graben, part of the North Sea rift. These faults define a series of major fault-blocks below the more obvious Jurassic rift basin. We attempt here to quantify the amount of extension associated with this older rift event, which is probably of early Triassic age. Quantitative modelling of the Triassic rift and the succeeding period of thermal subsidence has been undertaken, using a combination of flexural backstripping and flexural-cantilever forward modelling. These techniques suggest that Triassic extension across the Horda Platform (Norwegian sector) reached c. 40% (ß = 1.4). The consequences of this extension were deposition of a thick (>3 km) Triassic-Upper Jurassic syn-rift and post-rift sequence, prior to renewed, but minor, extension in the Late Jurassic-earliest Cretaceous. The thickness of the Viking Group reservoirs in the Troll area appears to have been almost entirely controlled by sediment loading during post-Triassic thermal subsidence. Jurassic extension on the Horda Platform was <5%, an order of magnitude less than the Triassic event. The Horda Platform is therefore principally an area of Triassic extension marginal to the main Jurassic rift further west. In the UK sector of the Viking Graben, Triassic structures are less obvious than those below the Horda Platform, because of Jurassic overprinting. They are, however, still present. Average Triassic extension across the East Shetland Basin was c. 15%, comparable with the magnitude of Jurassic extension in the same area. We believe that the Tern/Eider and Cormorant fault-blocks, with proven shallow basement, comprised a large eroded horst during the early Triassic, uplifted in the footwalls of major faults flanking the Magnus and Statfjord half-graben. During the Triassic, the Magnus half-graben was contiguous with the Unst Basin, now situated in the western footwall of the younger Jurassic basin. The presence of the Unst Basin suggests that Triassic extension occurred across the area that is now the northern Shetland Platform, continuing into the West Shetland area. Although the more obvious structures in the Viking Graben are Jurassic in age, the earlier Triassic event was equally as important in controlling the structural and stratigraphic history of the basin.

Mesozoic extension in the North Sea: constraints from flexural backstripping, forward modelling and fault populations

The magnitude and distribuion of late Jurassic extension in the Northern Viking Graben has been investigated by (i) syn-rift forward modelling using the flexural-cantilever model of continental rifting; (ii) post-rift flexural backstripping of a series of cross-sections; and (iii) the analysis of fault-population statistics. Application of these three techniques indicates that Jurassic extension on the tilted fault-block terrains of the East Shetland Basin, Tampen Spur and western Horda Platform is on average c. 15% ( β = 1.15). In the graben axis the regional value of β rises to c. 1.3, perhaps locally rising to 1.4. On the eastern Horda Platform Jurassic extension is low, β = c. 1.05. Flexural backstripping of the post-rift part of a cross-section through the Central Graben yields similar estimates of extension. At the flanks of the Jurassic basin β is estimated to be 1.2, rising to a likely maximum of c. 1.3 in the basin centre. These estimates of extension lie at, or towards, the low end of the previously published range of estimates. The principal reasons for this are (i) the incorporation of the thick sequence of Triassic and Lower Jurassic sediments in the backstripping; and (ii) the use of flexural isostasy (rather than Airy isostasy) in both the backstripping and forward modelling. The estimates of Jurassic extension obtained in this study do not account for the observed crustal thinning and, therefore, point to there having been a significant pre-Jurassic extensional event in the North Sea, of probable Triassic age. While a residual thermal anomaly from this event may have made a small contribution to post-Jurassic subsidence, compaction of the Triassic-Middle Jurassic sequence has been a significant contribution of the Triassic event to the thickness of the Cretaceous/Tertiary basin.

Continental lithosphere strength: the critical role of lower crustal deformation

Summary A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition. For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m−2 or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m−2. Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading. The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow q = 50–55 mW m−2) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling). Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.

The origin of tectonic stress in the lithosphere

Abstract The sources of lithospheric stress and their distinctive features are briefly reviewed. It is suggested that there are two main categories of lithospheric stress: renewable stress which persists despite continuing stress relaxation and non-renewable stress which can be dissipated by relief of the initial strain. The two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads. Non-renewable stress systems include bending stresses, membrane stresses and thermal stresses. An important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep. This applies to renewable stresses but not to the non-renewable type. It is suggested that only renewable stresses contribute significantly to tectonic activity. However, bending and thermal stresses are locally important in subducting lithosphere.

Forward and reverse modelling of rift basin formation

Abstract The McKenzie model of continental lithosphere extension describes the first-order lithosphere responses of crustal thinning and geothermal gradient increase following rifting, which lead to syn- and post-rift basin subsidence. At the sub-basin scale, seismic data show the fundamental importance of major basement faults in controlling the geometry and subsidence of rifted sedimentary basins. Reflection and earthquake seismology data show that these faults are generally planar and extend down 10–15 km to mid-crustal level. Below this depth deformation gives way to distributed ductile shear in the lower crust and mantle. Extensional faulting leads not only to hanging wall subsidence but also to footwall uplift. It can be successfully modelled using elastic dislocation, visco-elastic finite element and thin plate flexural-isostasy (flexural cantilever) theories. For extension on multiple faults, interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics. Mathematical forward models of rifting, incorporating upper crustal faulting, lower crust/upper mantle plastic stretching, lithosphere thermal perturbation and re-equilibration, sediment loading and flexural isostasy have been developed and applied to both syn- and post-rift stages of basin evolution. The models allow the effects on basin geometry and subsidence of fault spacing, fault polarity and extension magnitude, as well as sediment fill and footwall erosion, to be explored. Reverse post-rift modelling from present day sections may be used to constrain β stretching estimates and to predict palaeobathymetry and topography. Both forward and reverse models have been successfully applied to many rift basins worldwide, including the Viking Graben, North Sea and the East African Rift System, both of which are documented here. The forward model is also capable of simulating the first-order geometry of metamorphic core-complexes and so-called ‘detachment faults’ documented from the Basin-and-Range Province of the western USA.

2D flexural backstripping of extensional basins; the need for a sideways glance

Backstripping is a technique employed to analyse the subsidence history of extensional basins, and involves the progressive removal of sediment loads, incorporating the isostatic and sediment decompaction responses to this unloading. The results of backstripping calculations using 1D models employing local (Airy) isostasy and 2D models employing flexural isostasy are compared for three cross-sections of the North Sea rift basin. Backstripping is commonly used to estimate stretching factor (beta ) across extensional basins. At structural highs 1D Airy backstripping will overestimate beta by comparison with predictions from 2D flexural backstripping, because Airy isostasy fails to acknowledge the effects of lateral differential loading. Predictions of beta from 2D flexural backstripping are closer to those derived from forward modelling. 1D Airy backstripping also produces unrealistic internal deformation of individual fault-blocks and overestimates beta when the pre-rift sequence is not fully decompacted. The palaeobathymetric data required by 1D Airy backstripping are often inaccurate, which yields misleading results. 2D flexural backstripping has been formulated as reverse post-rift modelling, which is used to produce sequential (isostatically balanced) palinspastic post-rift cross-sections. These are calibrated using only high-quality palaeobathymetric data, allowing 2D flexural backstripping to be used to predict palaeobathymetry away from the calibration points.

Subsidence of the Vøring Basin and the influence of the Atlantic continental margin

The post-Cretaceous subsidence history of the Vøring Basin, part of the Atlantic passive margin offshore mid-Norway, has been investigated. Extension and β -factors related to rifting and continental break-up during the Palaeocene have been quantified using both forward and reverse basin-modelling techniques. In the preferred geological model it is assumed that rifting occurred in the Vøring Basin during the Palaeocene (prior to break-up), following an earlier rift event during the Late Jurassic. During Palaeocene rifting the basin may have been dynamically uplifted by the Iceland mantle plume. In the east of the basin there was no Palaeocene extension. Subsidence analysis shows that in the centre of the basin forward and reverse models converge to predict a modest Palaeocene stretching factor (β) of c. 1.15. In the west of the basin, closest to the Atlantic margin, forward models of upper-crustal faulting also predict a β of c. 1.15, but reverse (backstripped) models of subsidence predict a β of up to 1.75. We suggest that lower-crustal and mantle-lithosphere thinning close to the margin were greater than the extension accommodated by upper-crustal faulting and that some lower-crustal/mantle-lithosphere stretching associated with continental separation was partitioned below the Vøring Basin, up to 150 km landwards of the margin.