Fault Array Evolution Research Articles

4D fault evolution revealed by footwall exhumation modelling: A natural experiment in the Malawi rift

The evolution of normal fault arrays during rift extension reflects paleo-plate boundary conditions and lithospheric rheology, while controlling seismic hazard and the distribution of basin-hosted resources. Yet, constraining their spatiotemporal development is challenging, particularly where geophysical and subsurface data are absent. Here, we test footwall exhumation modelling using thermochronology as a means of elucidating 4D normal fault array evolution, using the Miocene Central Basin of the Malawi Rift as a natural laboratory. Along-strike trends in exhumational cooling recorded by vertical transects of apatite fission-track and (U–Th)/He data from the basin-bounding Usisya fault scarp reveal a diachronous footwall uplift history that closely reflects 4D trends in hangingwall subsidence recorded by previously published seismic and well data. Initially, pronounced footwall exhumation is restricted to the centres of a series of four isolated normal faults, mirroring the distribution of early syn-rift depocentres. The later onset of footwall exhumation in the intervening areas marks subsequent fault segment propagation and linkage as they formed the through-going Usisya fault system. Elsewhere, cumulative exhumation recorded in the Usisya footwall remains low, coinciding with more significant intra-basinal faulting. This study shows that footwall exhumation modelling constrained by thermochronologic data can reveal the spatiotemporal evolution and strain partitioning within normal fault arrays.

Fault Throw and Regional Uplift Histories From Drainage Analysis: Evolution of Southern Italy

AbstractLandscapes can record elevation changes caused by multiple tectonic processes. Here, we show how coeval histories of spatially coincident normal faulting and regional uplift can be deconvolved from river networks. We focus on Calabria, a tectonically active region incised by rivers containing knickpoints and knickzones. Marine fauna indicate that Calabria has been uplifted by >1 km since ∼0.8–1.2 Ma, which we used to calibrate parameters in a stream power erosional model. To deconvolve the local and regional uplift contributions to topography, we performed a spatiotemporal inversion of 994 fluvial longitudinal profiles. Uplift rates from fluvial inversion replicate the spatial trend of rates derived from dated Mid‐Late Pleistocene marine terraces, and the magnitude of predicted uplift rates matches the majority of marine terrace uplift rates. We used the predicted uplift history to analyze long‐term fault throw, and combined throw estimates with ratios of footwall uplift to hanging wall subsidence to isolate the nonfault related contribution to uplift. Increases in fault throw rate—which may suggest fault linkage and growth—have been identified on two major faults from fluvial inverse modeling, and total fault throw is consistent with independent estimates. The temporal evolution of nonfault related regional uplift is similar at three locations. Our results may be consistent with toroidal mantle flow generating uplift, perhaps if faulting reduces the strength of the overriding plate. In conclusion, fluvial inverse modeling can be an effective technique to quantify fault array evolution and can deconvolve different sources of uplift that are superimposed in space and time.

Tipping the balance: Shifts in sediment production in an active rift setting

Landscapes in actively developing rifts respond to tectonic forcing over a similar time scale to that of fault array evolution (i.e., 105–106 yr). Consequently transient landscapes (i.e., not in topographic steady state) predominate, characterized by focused incision along extensional fault scarps and regional tectonic tilting of surface slopes across strike. Using a field-calibrated numerical model to explore the controls on landscape evolution across the Corinth rift, central Greece, we demonstrate that this tilting, although subtle, leads to a shift in dominant source area as well as a shift toward sediment-starved conditions within the basin. We show, by comparing model runs with and without imposing tectonic forcing, that the impact of active faulting on relief development along the most active Corinth rift margin locally increases erosion rates and footwall incision. However, the overall sediment flux from this margin is reduced because back-tilting lowers erosion rates in catchment headwaters. Conversely, the hanging-wall side of the rift, as it is downwarped, supplies relatively more sediment as rift-directed channel slopes increase even though the relief is decreasing. In summary, we show that tilting plays a key role in controlling the syn-rift sediment flux and, in a counterintuitive way, modifies the relationship between topographic relief and catchment-averaged erosion rates. Our results provide a new perspective on the origin and timing of sediment starvation relative to structural development in rifts.

Normal fault array evolution above a reactivated rift fabric; a subsurface example from the northern Horda Platform, Norwegian North Sea

AbstractThe impact of a pre‐existing rift fabric on normal fault array evolution during a subsequent phase of lithospheric extension is investigated using 2‐D and 3‐D seismic reflection, and borehole data from the northern Horda Platform, Norwegian North Sea. Two fault populations are developed: (i) a population comprising relatively tall (>2 km), N‐S‐striking faults, which have >1.5 km of throw. These faults are up to 60 km long, penetrate down into crystalline basement and bound the eastern margins of 6–15 km wide half‐graben, which contain >3 km of pre‐Jurassic, likely Permo–Triassic, but possibly Devonian syn‐rift strata; and (ii) a population comprising vertically restricted (<1 km), NW‐SE‐striking faults, which are more closely spaced (0.5–5 km), have lower displacements (30–100 m) and not as long (2–10 km) as those in the N–S‐striking population. The NW‐SE‐striking population typically occurs between the N‐S‐striking population, and may terminate against or cross‐cut the larger structures. NW–SE‐striking faults do not bound pre‐Jurassic half‐graben and are largely restricted to the Jurassic‐to‐Cretaceous succession. Seismic‐stratigraphic observations, and the stratigraphic position of the fault tips in both fault populations, allow us to reconstruct the Late Jurassic‐to‐Early Cretaceous growth history of the northern Horda Platform fault array. We suggest the large, N‐S‐striking population was active during the Permo–Triassic and possibly earlier (Devonian?), before becoming inactive and buried during the Early and Middle Jurassic. After a period of relative tectonic quiescence, the N‐S‐striking, pre‐Jurassic fault population propagated through the Early‐Middle Jurassic cover and individual fault systems rapidly (within <10 Ma) established their maximum length in response to Late Jurassic extension. These fault systems became the dominant structures in the newly formed fault array and defined the locations of the main, Late Jurassic‐to‐Early Cretaceous, syn‐rift depocentres. Late Jurassic extension was also accommodated by broadly synchronous growth of the NW‐SE‐striking fault population; the eventual death of this population occurred in response to the localization of strain onto the N–S‐striking fault population. Our study demonstrates that the inheritance of a pre‐existing rift fabric can influence the geometry and growth of individual fault systems and the fault array as a whole. On the basis of observations made in this study, we present a conceptual model that highlights the influence of a pre‐existing rift fabric on fault array evolution in polyphase rifts.

Fault array evolution in extensional basins: insights from statistical analysis of gravel deposits in the Cecina River (Tuscany, Italy)

Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany, Italy) have been combined to unravel changes in the palaeo-drainage system. Data from 16 outcrops were collected and 6400 clasts described. Facies analysis, micro-palaeontology and macro-palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units. Clast lithology is the main discriminator among these units. Cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the Lower Pleistocene deposits and constrain the re-routing of the lower palaeo-Cecina River from a supposedly south-east to north-west direction to the present east to west direction. Short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the Lower Pleistocene, when the re-routing of the Cecina River caused the capture of these streams. This evolution suggests a change in the tectonic regime of the area. The fan deltas developed on the hanging wall of normal faults sub-parallel to the coast; a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull-apart basin, which is now exploited by the Cecina River. This study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area.

Temporal and spatial development of a gravity-driven normal fault array: Middle–Upper Jurassic, South Viking Graben, northern North Sea

Three-dimensional seismic and well data from the South Viking Graben, northern North Sea Basin, is used to investigate the temporal and spatial development of a gravity-driven normal fault array above an evaporite-rich detachment. Two moderate throw (500–900 m), Middle to Upper Jurassic normal faults (the Gudrun and Brynhild Faults) are developed within the study area. Both faults die-out laterally and tip-out upwards at different structural levels within the syn-rift succession. Both faults terminate downwards into Late Permian evaporites (Zechstein Group) and do not offset pre-evaporite basement units. This thin-skinned fault array developed in response to westwards tilting of the hangingwall of the South Viking Graben during Late Jurassic rifting, and consequent westward gliding and extensional break-up of units above the mechanically-weak evaporite horizon. Isochron mapping and well-based correlation of Middle to Upper Jurassic syn-rift units allow constraints to be placed on the temporal evolution of the fault array. Several stages of structural development are observed which document; (i) a period of relatively minor, early (i.e. pre-rift) halokinesis; (ii) variable spatial activity on individual faults within the array; and (iii) the progressive upslope migration of active faulting within the array as a whole. The progressive upslope migration of fault activity is interpreted to reflect progressive “unbuttressing” and extensional faulting of upslope, post-evaporite units. The overall structural style and kinematic evolution identified here shares many characteristics with both ‘ rift–raft tectonics’ documented in other rifts developed above an evaporitic sub-stratum and ‘ raft tectonics’ described from passive margin basins containing thick mobile salt or shale intervals. This style of fault array evolution differs from that observed in rifts lacking mobile layers at-depth and highlights the importance of these units in the structural development of rifts.

Relay ramp evolution and mass flow deposition (Upper Kimmeridgian–Lower Volgian) in the Tail End Graben, Danish North Sea

ABSTRACTThe Central Graben in the Danish North Sea sector consists of a series of N–S to NW–SE trending, eastward‐tilted half‐grabens, bound to the east by the Coffee Soil Fault zone. This fault zone has a complex Jurassic history that encompasses at least two fault populations; N–S to NNW–SSE striking faults active in the Late Aalenian–Early Oxfordian, and NNW–SSE to WNW–ESE striking faults forming in Late Kimmeridgian time (sensu gallico), following a short period of tectonic quiescence. Sediment transport across the Coffee Soil Fault zone was controlled by fault array evolution, and in particular the development of relay ramps that formed potential entry points for antecedent drainage systems from the Ringkøbing–Fyn High east of the rift. Fault and isochore trends of the Upper Kimmeridgian–Lower Volgian succession in the northeast Danish Central Graben show that accommodation space was initially generated close to several minor, isolated or overlapping faults. Subsidence became focused along a few master faults in the Early Volgian through progressive linkage of selected faults. Seismic time isochore geometries, seismic facies, amplitude trends and well ties indicate the presence of coarse clastic lithologies locally along the fault zone. The deposits probably represent submarine mass flow deposits supplied from footwall degradation and possibly also from the graben hinterland via a relay ramp. The latter source appears to have been cut off as the relay ramp was breached and the footwall block are uplifted. Fault growth and linkage processes thus controlled the spatial and temporal trends of accommodation space generation and sediment supply to the rift basin.

The role of evaporite mobility in modifying subsidence patterns during normal fault growth and linkage, Halten Terrace, Mid‐Norway

AbstractWell‐calibrated seismic interpretation in the Halten Terrace of Mid‐Norway demonstrates the important role that structural feedback between normal fault growth and evaporite mobility has for depocentre development during syn‐rift deposition of the Jurassic–Early Cretaceous Viking and Fangst Groups. While the main rift phase reactivated pre‐existing structural trends, and initiated new extensional structures, a Triassic evaporite interval decouples the supra‐salt cover strata from the underlying basement, causing the development of two separate fault populations, one in the cover and the other confined to the pre‐salt basement.Detailed displacement–length analyses of both cover and basement fault arrays, combined with mapping of the component parts of the syn‐rift interval, have been used to reveal the spatial and temporal evolution of normal fault segments and sediment depocentres within the Halten Terrace area. Significantly, the results highlight important differences with traditional models of normal fault‐controlled subsidence, including those from parts of the North Sea where salt is absent. It can now be shown that evaporite mobility is intimately linked to the along‐strike displacement variations of these cover and basement faults. The evaporites passively move beneath the cover in response to the extension, such that the evaporite thickness becomes greatest adjacent to regions of high fault displacement. The consequent evaporite swells can become large enough to have pronounced palaeobathymetric relief in hangingwall locations, associated with fault displacement maxima– the exact opposite situation to that predicted by traditional models of normal fault growth. Evaporite movement from previous extension also affects the displacement–length relationships of subsequently nucleated or reactivated faults. Evaporite withdrawal, on the other hand, tends to be a later‐stage feature associated with the high stress regions around the propagating tips of normal faults or their coeval hangingwall release faults. The results indicate the important effect of, and structural feedback caused by, syn‐rift evaporite mobility in heavily modifying subsidence patterns produced by normal fault array evolution. Despite their departure from published models, the results provide a new, generic framework within which to interpret extensional fault and depocentre development and evolution in areas in which mobile evaporites exist.

Implications of fault array evolution for synrift depocentre development: insights from a numerical fault growth model

Implications of fault array evolution for synrift depocentre development: insights from a numerical fault growth model

Implications of fault array evolution for synrift depocentre development: insights from a numerical fault growth model

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.

Abstract: Fault array evolution as a control on syn-rift stratigraphy and play development: examples from the Gulf of Suez and North Sea

Abstract: Fault array evolution as a control on syn-rift stratigraphy and play development: examples from the Gulf of Suez and North Sea

Role of fault interactions in controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift, Sinai, Egypt

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse‐grained delta system to evolution of a fault array in a Miocene half‐graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall‐sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin‐bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault‐related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point‐sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along‐strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.

Three-dimensional geometry and evolution of a salt-related growth-fault array: Eugene Island 330 field, offshore Louisiana, Gulf of Mexico

The geometry and evolution of a growth-fault array bounding the Eugene Island 330 Field were studied using 3-D seismic interpretation and analysis of displacement patterns. The 3-D geometry of the kinematically coherent fault array is complex and characterized by both lateral and vertical branching and linkage of individual fault segments, so that fault patterns and interactions very considerably between different structural levels. Three types of faults are distinguished: growth faults with throw increasing downward; compensation faults with closed tip lines; and composite faults, which combine elements of both growth and compensation faults. Reconstruction of the fault array's evolution using displacement backstripping illustrates the growth and linkage of some isolated faults and the upward separation of others into distinct segments. The results may have important implications for understanding the history of hydrocarbon migration and entrapment in the EI-330 field and other fault-bounded minibasins in the Gulf of Mexico.

A mechanism to explain rift-basin subsidence and stratigraphic patterns through fault-array evolution

Rift-basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate. The physical basis for this transition is not well understood, although an increase in extension rate is commonly implied. Here, a numerical fault-growth model is used to investigate the influence of segment linkage on fault-displacement-rate patterns along an evolving normal fault array. The linkage process we describe is controlled by a stress feedback mechanism, which leads to enhanced growth of optimally positioned faults. Model results indicate that, even with constant extension rates, slow displacement rates prevail during an initial phase of distributed extension, followed by an increase in displacement rates as strain becomes localized on linked fault arrays. This is due to the dynamics of fault interactions rather than mechanical weakening. Comparison of model simulations with rift-basin subsidence and stratigraphic patterns in the Gulf of Suez and North Sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault-zone evolution, without invoking a change in regional extension rates.

Simulating polyphase faulting with a tensorial 3D model of fault growth

Abstract Fault growth in brittle media has previously been extensively studied via a numerical approach using scalar representation of the stress and strain fields. More realistic simulations and further investigations of fault array evolution demand a fully tensorial three dimensional (3D) representation of these fields. In this paper we present a tensorial model of the spontaneous birth and growth of faults in a 3D medium. The medium is elastic and attenuating up to a stress threshold, determined by the Mohr-Coulomb criterion, where brittle failure is modelled by a partial shear stress drop. Elastic radiations generated by the rupture are explicitly extrapolated in time by a finite-difference scheme of the equation of dynamics until the static state is reached. We do not consider the dynamic process explicitly; further ruptures can only be triggered by the resulting static stress field, or by the imposed straining of the medium. Preliminary simulations of 3D straining of a 2D plate show how a pre-existing fault set (appearing as a perturbed stress field)can influence the development of a second fault set. We believe that our model provides a valuable tool for the study of fault development and in particular for the assessment of the effects of anisotropic stresses around faults on strain accumulation and the spatial organization of crustal deformation.