Basin Research - Volume 12, Issue 3‐4, 2000

We present conceptual models for the tectono‐sedimentary evolution of rift basins. Basin architecture depends upon a complex interaction between the three‐dimensional evolution of basin linkage through fault propagation, the evolution of drainage and drainage catchments and the effects of changes in climate and sea/lake level. In particular, the processes of fault propagation, growth, linkage and death are major tectonic controls on basin architecture. Current theoretical and experimental models of fault linkage and the direction of fault growth can be tested using observational evidence from the earliest stages of rift development. Basin linkage by burial or breaching of crossover basement ridges is the dominant process whereby hydrologically closed rifts evolve into open ones. Nontectonic effects arising from climate, sea or lake level change are responsible for major changes in basin‐scale sedimentation patterns. Major gaps in our understanding of rift basins remain because of current inadequacies in sediment, fault and landscape dating.

A two‐dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is presented and applied. This model is a generalization of a one‐dimensional algorithm which minimizes the misfit between predicted and observed patterns of basin subsidence. Our calculations include the effects of two‐dimensional conduction and advection of heat as well as flexural rigidity. More importantly, we make no prior assumptions about the duration, number or intensity of rifting periods. Instead, the distribution of strain rate is permitted to vary smoothly through space and time until the subsidence misfit has been minimized. We have applied this inversion algorithm to extensional sedimentary basins in a variety of geological settings. Basin stratigraphy can be accurately fitted and the resultant spatiotemporal distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes. Perhaps surprisingly, the smallest misfits are achieved with flexural rigidities close to zero. Spatiotemporal strain rate distributions will help to constrain the dynamical evolution of thinning continental lithosphere. The strain rate pattern governs the heat‐flow history and so two‐dimensional inversion can be used to construct accurate maturation models. Finally, our inversion algorithm is a stepping stone towards a generalized three‐dimensional implementation.

A reconnaissance slip‐rate‐estimation technique is needed for hundreds of uncharacterized normal faults in the Great Basin. We use the occurrence and/or absence of primary tectonic geomorphology, alluvial fault scarps and fault facets, to distinguished between categories of faults that have differences of orders of magnitudes in slip rate. Normal faults that lack alluvial fault scarps and fault facets (Type 3) have vertical slip rates on the order of 0.001 m kyr⁻¹. Faults with alluvial fault scarps but that lack active fault facets (Type 2) have vertical slip rates on the order of 0.01 m kyr⁻¹. Faults with active fault facets with a minimum height of 30 m (Type 1) are the most important faults in the Great Basin with respect to seismic hazard, tectonic deformation, and basin development and deposition. These faults generally have vertical slip rates of 0.1 m kyr⁻¹ and faster. For better precision we have developed a relationship between vertical slip rate and maximum basal facet height: Log10  sv=0.00248h−0.938 where Sv is vertical slip rate in m kyr⁻¹ and H is maximum basal facet height in metres. The standard deviation of this relationship is 0.239, which corresponds to plus or minus a multiplicative factor of 1.7 in vertical slip rate.

These criteria were used to estimate vertical slip rates for major normal faults in Nevada, USA. Based on the rates and orientations of these faults, seven Quaternary tectonic subprovinces are distinguished. The fastest faults (≥0.5 m kyr⁻¹) are generally located in the Walker Lane belt, perhaps because of higher extension rates created by the addition of strike‐slip tectonics. Earthquake occurrence rates based on the faults in Nevada and their estimated slip rates are about a factor of two lower than historical earthquake rates.

We investigate the effects of interaction between growing normal faults on the creation of accommodation in extensional half grabens. Fault evolution is simulated using a numerical model in which we calculate both the stress field around each fault and the changes in stress level on neighbouring faults caused by individual slip events (earthquakes). These stress changes govern the interaction and determine both the direction and rate of lateral fault propagation and the accumulation of displacement. The spatial distribution of subsidence resulting from fault growth is examined parallel and transverse to strike to document the three‐dimensional evolution of accommodation creation. The numerical experiment permits analysis of the geometry, inception and growth history of individual fault‐controlled depocentres during development of a linked normal fault array. Model results indicate that the pattern of fault‐controlled subsidence, and hence hangingwall accommodation creation, is spatially and temporally variable as the timing and rates of fault movement vary considerably. In general, there is a progression from many relatively small and isolated subbasins initially, to the development of a larger laterally continuous half graben in the hangingwall to a major through‐going linked fault system. Many smaller faults initially active in footwall and hangingwall areas become inactive as the extension localizes onto the major structure. This switching‐off of some faults, combined with focusing of the extensional deformation, leads to an increase in the rate of displacement accumulation on the remaining active fault. As dip‐slip displacement is a proxy for hangingwall subsidence, we interpret this prominent rate change in terms of the ‘rift‐initiation’ to ‘rift‐climax’ transition, previously recognized in synrift stratigraphy. A general picture of depocentre development relative to timing of linkage emerges from the simulated fault evolution that provides some simple conceptual models to be tested. However, the important result of this paper is that it shows the degree to which fault activity can vary in space and time both along the same fault zone and also across strike. We briefly discuss the implications of model results for stratigraphic architecture in rift basins. Our conclusion is that some of the stratigraphic complexity of rifts previously ascribed to other controls (e.g. sediment supply, eustasy, etc.) may be tectonically controlled and that with improved three‐dimensional imaging of rift basins these effects may be recognized.

Through examination of the scaling relations of faults and the use of seismic stratigraphic techniques, we demonstrate how the temporal and spatial evolution of the fault population in a half‐graben basin can be accurately reconstructed. The basin bounded by the ≫62‐km‐long Strathspey–Brent–Statfjord fault array is located on the western flank of the Late Jurassic northern North Sea rift basin. Along‐strike displacement variations, transverse fault‐displacement folds and palaeo‐fault tips abandoned in the hangingwall all provide evidence that the fault system comprises a hierarchy of linked palaeo‐segments. The displacement variations developed while the fault was in a prelinkage, multisegment stage of its growth have not been equilibrated following fault linkage. Using the stratal architecture of synrift sediments, we date the main phase of segment linkage as latest Callovian – middle Oxfordian (10–14 Myr after rift initiation). A dense subpopulation of faults is mapped in the hangingwall to the Strathspey–Brent–Statfjord fault array. The majority of these faults are short, of low displacement and became inactive within 3–4 Myr of the beginning of the extensional event. Subsequently, only the segments of the proto‐Strathspey–Brent–Statfjord fault and a conjugate array of antithetic faults located 3.5 km basinward continued to grow to define a graben‐like basin geometry. Faults of the antithetic array became inactive ∼11.5 Myr into the rift event, concentrating strain on the linked Strathspey–Brent–Statfjord fault; hence, the basin evolved into a half‐graben. As the rift event progressed, strain was localized on a smaller number of active structures with increased rates of displacement. The results of this study suggest that a simple model for the linkage of 2–3 fault segments may not be applicable to a complex multisegment array.

Field data from onshore exposures of the Oligo‐Miocene Gulf of Suez Rift in the Sinai document the passive rotation of early formed mesoscale synthetic and antithetic faults and associated half‐graben due to long‐lived activity on large displacement (2–5 km) block‐bounding faults. Early formed small‐displacement (<350 m) mesoscale antithetic faults and half‐graben within regional‐scale fault blocks underwent progressive steepening due to footwall uplift, rotational faulting and footwall flexing on large‐displacement, block‐bounding faults. In contrast, mesoscale synthetic faults were progressively rotated to shallower angles. Analysis of palaeohorizontal surfaces within synrift sediments deposited in half‐graben adjacent to the mesoscale faults indicate passive rotations of up to 25° about horizontal axes since deposition. Passive burial and in‐filling of early formed mesoscale faults and half‐graben by synrift sediments is consistent with extension being transferred from numerous mesoscale faults to few block‐bounding macroscale faults as extension preceded. Furthermore, this transfer of extension appears to be associated with a marked change in basin configuration, synrift sediment dispersal patterns and facies development. Identification of early formed, passively rotated normal faults and half‐graben is important for correctly reconstructing the early stages of basin palaeogeography and sediment dispersal, and for addressing models of rift basin evolution.

Normal faults in middle Miocene sedimentary rocks of the Mae Moh mine have their geometries and displacement patterns well constrained by outcrop and subsurface data. Seventeen of the largest faults are described and analysed. The 17 faults have very different displacement profiles, with differences between the profiles being explicable in terms of the linkage of initially separate faults. Lateral tip gradients show a large range from 0.035 to 0.6. Fault displacement–length (D–L) relationships plot with considerable scatter. Following previous studies, data points with relatively high D–L ratios are attributed to displacement transfer between overlapping faults; faults with relatively low D–L ratios display linkage of two or more faults. In transfer zones conservation of displacement between the faults ranges from high (> 70%) to low (10–40%). This appears to depend upon whether the faults propagated relatively early (high displacement transfer) or late (low displacement transfer) into overlapping configurations. Where displacement transfer is low, extension appears to be conserved in a broader zone on adjacent mappable faults, with little increase in ductile deformation.

A number of recent papers have stressed the importance of lateral and vertical fault propagation on sediment geometries in active rift settings. However, the majority of these studies have been based on outcrop data. This contribution addresses the evolution of a single, major normal fault and its interaction with adjacent active faults using high‐resolution 3D seismic data from the Smørbukk and Smørbukk South hydrocarbon fields, Halten Terrace, Mid‐Norway. The major fault dividing the two fields, the Trestakk–Smørbukk fault, evolves from a southern segment with a well‐defined set of rift wedges in its hangingwall to a northern segment where the fault tip is buried and a fault‐tip fold is developed. Isochore maps of three Jurassic intervals illustrate a south to north evolution where, initially, Early Jurassic fault activity is limited to the southern part of the study area. Middle to Upper Jurassic intervals display a northwards migration in activity and linkage with two other major faults in the study area. This northwards migration had a profound effect on sediment geometries and depocentres in an area where previously only Late Jurassic rift activity has been recognized.

This study addresses the complex relationship between an evolving fault population and patterns of synrift sedimentation during the earliest stages of extension. We have used 3D seismic and well data to examine the early synrift Tarbert Formation from the Middle–Late Jurassic northern North Sea rift basin. The Tarbert Formation is of variable thickness across the study area, and thickness variations define a number of 1‐ to 5‐km‐wide depocentres bounded by normal faults. Seismic reflections diverge towards the bounding faults indicating that the faults were active contemporaneous with the deposition of the formation. Many of these faults became inactive during later Heather Formation times. The preservation of the Tarbert Formation in both footwall and hangingwall locations demonstrates that, during the earliest synrift, the rate of deposition balanced the rate of tectonic subsidence. Local space generated by hangingwall subsidence was superimposed upon accommodation generated due to a regional rise in relative sea‐level. In basal Tarbert Formation times, transgression across the prerift coastal plain produced lagoons and bays, which became increasingly marine. During continued transgression, barrier islands moved landward across the drowned bays. In the southern part of our study area, shallow marine sediments are erosionally truncated by fluvial deposition. These fluvial systems were constrained by fault growth monoclines, and flowed parallel to the main faults. We illustrate that stratal architecture and facies distribution of early sedimentation is strongly influenced by the active short‐lived faults. Local depocentres adjacent to fault displacement maxima focused channel stacking and allowed the aggradation of thick shoreface successions. These depocentres formed early in the rift phase are not necessarily related to Late Jurassic – Early Cretaceous depocentres developed along the major linked normal fault systems.

The stratigraphy of rift basins is a direct result of sediment liberation and transport through catchment–fan systems whose dynamics are controlled by both external and internal factors. We investigate the response of catchment–fan systems established across an active normal fault to variations in both tectonic and climatic boundary conditions. Numerical experiments show that the ratio of fan area to catchment area provides a sensitive indicator of tectonic activity. A step decrease in fault slip rate results in a delayed response by the catchment–fan systems; the response time is ∼50 kyr for a variety of parameter values. Decreased slip rate also gives rise to an abrupt but transient pulse in sediment discharge from the fans due to a drop in the hangingwall subsidence rate. In contrast, variations in climatic activity, using precipitation rate as a proxy, produce extremely rapid responses throughout the catchment–fan system. Thus, high‐frequency climatic changes will overprint lower frequency tectonic variations in the stratigraphic record of fan deposits. Finally, we map out possible combinations of fault geometry, fault slip rate and precipitation rate that allow fan progradation and high rates of sediment discharge from the system.

Fundamental to interpreting the stratigraphic architecture within a basin is understanding the relationship between a basin’s external conditions and its stratigraphic response. Here we present a model of fluvial basin filling that is based on two field observations. Firstly, natural fluvial systems commonly have an upstream region dominated by gravel transport and a downstream region dominated by sand transport with the transition between the two being relatively abrupt. Second, gravel bed and sand bed systems operate at nearly constant but different mean Shields stresses. Our model is based on a single, linear diffusion equation but is unique in that we use distinct transport (diffusion) coefficients for the two dominant fluvial regimes: a proximal gravel region and a distal sand region. This problem is complicated by the existence of two moving boundaries: the position of the distal fluvial toe and the position of the gravel–sand transition. We present a rigorous numerical treatment of both of these moving boundaries and verify our numerical formulation by comparing the model results to a semi‐analytical solution technique.

We use the model to examine the stratigraphic response to perturbations in four external boundary conditions: sediment supply, water supply, rate of subsidence and gravel fraction. The response is analysed in terms of the phase relation between forcing and the position of the gravel front, the position of the fluvial toe, proximal accumulation rate and distal accumulation rate. The model supports the results of earlier single‐diffusion models suggesting that the form of the response is dependent on the period of the perturbation relative to the intrinsic basin response time. For forcing periods less than the intrinsic basin response time, basin response is nearly constant and independent of the forcing period, suggesting that the transport system controls basin response. For forcing periods greater than the intrinsic response time of the basin the response time of the basin increases directly with the forcing period, suggesting that the transport system plays no role in limiting basin response. For gravel–sand systems we show that the intrinsic response time is a function of the ratio of gravel to sand entering the basin. Forcing of the above external boundaries, both slowly and rapidly relative to the basin response time, produces both distal and proximal unconformities. We present a nondimensional ‘unconformity number’ that constrains the amplitude and period of forcing necessary to generate proximal unconformities.

Alluvial slopes are piedmonts characterized by parallel steam channels rather than alluvial fans. They are common landforms in extensional basins of the south‐western United States but have received little attention from geomorphologists or sedimentologists. Persistence of channellized flow across piedmonts, as opposed to sheetflooding due to loss of flow confinement on alluvial‐fan surfaces, distinguishes alluvial‐slope and alluvial‐fan facies. Miocene strata of the Tesuque Formation (Española basin, New Mexico) and Pliocene strata of the St. David Formation (San Pedro Valley, Arizona) provide examples of extensional basin–piedmont successions constructed by discrete gravel and sand bedload channels and aggrading interfluve floodplains and aeolian sand sheets. Distinction of alluvial‐fan and alluvial‐slope piedmont deposits has several important implications. The contrasting facies geometries associated with the two landforms produce distinctly different aquifer and reservoir properties. It is hypothesized that alluvial slopes are more likely to form than alluvial fans where mountain fronts lack abrupt structural and topographic definition. This circumstance will most likely be met (a) along tectonically inactive and embayed mountain fronts and (b) on the hangingwall ramp side of half grabens.

This paper reviews and synthesizes several Holocene field examples of river response to lateral ground tilting. Key aspects of alluvial architecture modelling in extensional basins are addressed, including the nature of gradual lateral migration, the spatial and temporal history of avulsive sequences, and the underlying controls that determine whether a river responds to lateral tilting through gradual migration or avulsion. A new conceptual model for gradual lateral migration is proposed that unifies previously disparate models. Tilt‐induced avulsion in several field examples is associated with sequences that move towards and away from the locus of subsidence during active and quiescent tectonic periods, respectively. These avulsion sequences closely correspond to those produced by several 2D and 3D alluvial architecture models. The rate of lateral tilt appears to control the style of channel movement, with gradual migration occurring at low tilt rates, and avulsion at higher rates. This apparent dependence on tilt rate suggests the mode of channel movement, and also the avulsion frequency, may in part be a function of the imposed tectonic regime.

The inorganic geochemistry and mineralogy of synrift lacustrine shales from the Early Cretaceous Congo rift basin provide insight into source terrane, palaeoclimate and evolution of rift topography. The basin formed in a Late Proterozoic metamorphic belt as South America and Africa began to separate. Within the synrift section, a late rift sequence (Marnes Noires and Argilles Vertes Formations) can be distinguished from an underlying active rift sequence (Vandji, Sialivakou and Djeno Formations), based on diminished faulting and more uniform subsidence.

Provenance did not vary significantly until deposition of the youngest part of the rift sequence. Al2O3/TiO2 and K2O/Na2O ratios, generally constant, rise sharply in upper Marnes Noires and Argilles Vertes, demonstrating decreased contribution from Proterozoic volcanic sources. Parameters including (quartz+feldspar)/total clay and SiO2/Al2O3 suggest that depositional systems reorganized during lower Djeno deposition, possibly due to rejuvenation of faulting. Grain size decreases in the late rift section; however, the parameter SiO2/Al2O3 increases. This is attributed to chemical or biogenic deposition of silica.

The proportion of chemical sedimentation increases upward in the synrift section, peaking in the Marnes Noires Formation, where the concentrations of organic carbon and carbonate and the proportion of SiO2 to siliciclastic‐associated elements reach a maximum. This is interpreted as resulting from input of high dissolved chemical load in the late rift stage and is attributed to increased chemical weathering in the basin as rift topography diminishes. The slower flow of ground and surface water to the rift lakes enhanced weathering and dissolution of minerals in rocks and sediments surrounding the rift lake. The carbon isotopic composition of carbonate decreases from +6 at the base of the rift section, associated with carbon from Late Proterozoic carbonates, to 0, indicating increasing contribution of light carbon; the source of light carbon is interpreted as vegetation, which increases as rift topography degrades.