Planar Normal Faults Research Articles

Study on relationship between deep and shallow structures along north boundary fault of Yanqing-Fanshan basin

Based on the results of surface geology, shallow and deep seismic survey, features of micro-earthquake activity along the north boundary fault of Yanqing-Fanshan sub-basin and their relationship with the surface active faults and the deep-seated crustal structure are analyzed using the recordings from the high-resolution digital seismic network. The focal mechanism solutions of micro-earthquakes, whose locations are precisely determined by the seismic network, have confirmed the structural characteristics to be the rotational planar normal fault and demonstrated the surface traces of the north boundary fault of Yanqing-Fanshan sub-basin. By using the digital recordings of earthquakes with the high resolutions and analyzing the mechanism solutions, our study has revealed the relationship between the geological phenomena in the shallow and deep structures in Yanqing-Huailai basin and the transition features from the brittle to ductile deformation with the crustal depth.

Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

The Wasatch fault is a ∼370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low‐temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two‐dimensional (2‐D) thermal‐kinematic models coupled with cooling‐rate‐dependent kinetic models which predict exhumed apatite fission track (AFT) and (U‐Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr−1, footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45° and 60°, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat‐producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U‐Th)/He ages. Predicted and observed ages are compared using a reduced chi‐square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45° or 60° dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr−1 at around 5 Ma. The model also suggests an exhumation duration of ∼12 Myr ± 2 Myr).

Neotectonics of the Vajont dam site

The disastrous Vajont landslide (NE Italy) of 9 October 1963 is generally thought to have occurred on an existing failure surface. Reassessment of the morphological and structural evidence suggests that movement was on a normal fault plane which had juxtaposed Cretaceous limestone and highly fractured rock debris, thus rendering the dam site unusually susceptible to massive sliding. The proposed fault is consistent in strike with the regional lineament pattern. Although movement was triggered by the combined effects of heavy rainfall and changes in reservoir level, there is circumstantial evidence that seismicity played a contributory part in mobilising the slide by increasing pore pressure at the base of the slide as well as by any associated shaking.

Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles

Subduction of Atlantic seafloor under the Caribbean plate causes shallow earthquakes within the Lesser Antilles volcanic arc. Such earthquakes, above the subduction interface, show strike‐slip or normal fault plane solutions, the latter with ∼E‐W striking nodal planes. To better assess seismic hazard and the coupling between volcanism and tectonics, we investigated faulting related to overriding‐plate deformation in the Guadeloupe archipelago. Using aerial photographs, satellite SPOT images, and topographic maps (1/25000 scale), we mapped active and middle to late Pleistocene fissures and normal fault systems that cut the uplifted coral platforms Grande‐Terre and Marie‐Galante and the volcanic rocks of Basse‐Terre. The available marine geophysical data show that the faults extend offshore to bound submarine rifts. The E‐W striking, 1500 m deep, V‐shaped Marie‐Galante rift separates the two islands of Marie‐Galante and Grande‐Terre. Normal faults in the north of Grande‐Terre appear to mark the similarly V‐shaped, western termination of the 5000 m deep, N°50E to N130°E striking Desirade graben. Three shallow, M ∼ 5.5 earthquakes (6 May 1851, 29 April 1897, 3 August 1992) appear to have ruptured segments of the Marie‐Galante rift boundary faults. The young “La Grande Découverte” volcanic complex of Basse‐Terre, including the 1440 A.D. Soufrière dome, lies within the western termination of the Marie‐Galante rift. The ancient volcanic shoulders of the rift buttress the active dome to the north and south, which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the Caribbean Sea, or southeastward into the Atlantic Ocean. The Marie‐Galante rift is typical of other troughs transverse to the northeastern edge of the Caribbean plate. We interpret such troughs, which are roughly orthogonal to the arc, to result from slip‐partitioning and extension perpendicular to plate convergence. That they disappear southward implies that they result from interaction between the Caribbean and North American plates.

Geometric and morphologic evolution of normal fault planes and traces from 2D to 4D data

The detailed 3D geometry of normal fault planes is described and analysed using datasets from outcrop studies (2D), seismic surveys (3D) and analogue models (4D). Different geometric configurations of simple isolated normal faults are studied by reference to processes of normal fault propagation. When a normal fault propagates without interacting with other fault zones, the entire border of the principal plane displays characteristic connected secondary structures. These secondary structures cause bifurcations of the principal fault terminations. The along-strike terminations of the principal plane display typical bifurcation configurations (‘ear geometry‘). The orientation of the bifurcations depends on the vertical direction of propagation (downwards and/or upwards). The along-dip terminations display en échelon secondary fault planes linked to the principal plane and are described as ‘lobate geometry’. A 3D genetic model of isolated normal fault geometry is proposed with a new general terminology for the secondary structures. When two isolated normal faults propagate towards each other and overlap, the two principal planes connect up via a relay fault. The resulting geometry is a longer fault exhibiting a characteristic undulation with two inactive branches.

Propagation, interaction and linkage in normal fault systems

Recent advances are reviewed in the understanding of the geometry and development of steep, planar normal faults with up to hundreds of meters throw in multilayered sedimentary rocks. This essential class of structures is of particular importance in basin formation and hydrocarbon development. The role of interaction and linkage between fault segments is emphasized. Such normal faults usually consist of complex zones of overstepping and linked segments, within which relay ramps are significant structures. Displacement is transferred between normal faults that overstep in map view and that dip in the same direction by tilting of beds to form a relay ramp and by the development of minor faults. Oversteps and bends also occur along normal faults in cross-section, these commonly being controlled by lithological variations. Damage zones are zones of fractures developed around faults as they initiate, propagate, link and build up displacement. Various models have been developed to account for the accumulation of slip and the finite displacement on faults. Recent models emphasize the importance of fault interaction and other mechanical effects in causing variations in the displacement characteristics of faults. A population of normal faults typically displays a linear length–displacement ratio and obeys a power–law-scaling relationship for displacement. It is likely, however, that complex networks of faults and reverse-reactivated normal faults do not obey simple scaling relationships, with the pattern and history of deformation changing with scale.

Analogue modelling of fault reactivation: tectonic inversion and oblique remobilisation of grabens

Analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation. Models were first subjected to extension, which lead to the formation of two linear grabens. Each model was then subjected to a second phase deformation, either parallel or oblique to the previous initial extension, and either extensional or contractional. The influence of sedimentation has been evaluated using experiments with and without sedimentation. In cases of oblique secondary deformation, all newly formed faults were parallel to the older grabens, thus they were oblique to the direction of the principal stress directions during the second deformation phase. In experiments without sedimentation, all older faults were reactivated, whereas in experiments with sedimentation, some of the older faults were not reactivated. In the case of an oblique compressional secondary deformation phase with post rift sedimentation, strain partitioning occurred between reactivated older normal faults and new reverse faults. σ2 was vertical on reactivated normal fault planes, whereas σ3 was vertical on reverse fault planes. In the case of oblique compressional secondary deformation phase without post rift sedimentation, no strain partitioning was observed. In this model, σ3 was vertical on every fault plane. It is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning.

Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic

The Falkland Islands lie in a critical location for our understanding of Gondwana supercontinent break-up. Subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the North Falkland Basin's Late Jurassic syn-rift fill history. Results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults, which divide the basin into a series of north-south trending half-grabens and minor grabens. Well data (in the form of exploration wireline logs, cores, petrographic thin sections, major and trace element analyses) indicate that their syn-rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows, hyperconcentrated flows and pyroclastic ash-fall. The geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread, Middle Jurassic to Early Cretaceous, silicic Chon Aike Large Igneous Province, in southern South America and Antarctica. Diagenesis has obscured provenance information and severely reduced porosity within the syn-rift sediments, causing them to have poor reservoir potential. The results enable a radically new interpretation of this key period in the North Falkland Basin's evolution, which helps modify and unify previously disparate views. It is now suggested that basin development was the direct result of the Chon Aike back-arc stretching, influenced in part by the underlying Palaeozoic basement trends. This was followed by Cretaceous-Recent post-rift thermal subsidence of the basin, which was only arrested by local structural inversion related to Atlantic opening. Consequently, it is thought that the extension that created the North Falkland Basin was independent of both Early–Middle Jurassic rotation of the Falkland Islands microplate and the evolution of the South Atlantic Ocean in the Cretaceous and Cenozoic.

Scaling model experiments on propagation of electromagnetic waves using salt solution to simulate the Earth's crust: Loma Prieta and Kobe earthquakes

Propagation of electromagnetic (EM) waves from an earthquake focus in the conductive Earth has been investigated using 1/1,000,000 scaling models taking earth-ionosphere and ocean-Moho plane parallel-plate waveguides into account. Microwaves at a frequency, ω m, a million times higher than that of seismic EM signal (SEMS), ω, were generated at the model focus. They are propagated in a salt solution modeling the earth's crust and reflected by ocean, fault planes, ionosphere and Moho plane all made by aluminum. Distribution of EM power was mapped by scanning a detector antenna over the model Earth's surface. The skin depth, δ, calculated by the exact skin depth equation, 1/ δ= ω( με/2) 1/2 [(1+(1/ ωερ) 2) 1/2 −1] 1/2 where dielectric constant, ε and permeability, μ are the same but resistivity, ρ, 10 −6 times smaller than that of Earth, gave 10 −6 times small skin depth validating the model scaling index. Images for evanescent and wave-ripple standing waves disturbed by normal, strike-slip and dip-slip conductive fault planes have been obtained using an aluminum plate. The co-circular contour map above the epicenter due to evanescence was pushed to the north east direction from the epicenter by the presence of ocean for the Loma Prieta earthquake, while to north direction for the Kobe earthquake. The intensity of EM ULF emissions for the Loma Prieta earthquake is discussed quantitatively.

New seismotectonic data from an intraplate region: focal mechanisms in the Armorican Massif (northwestern France)

Focal mechanism solutions are determined for 11 small intraplate earthquakes that occurred between 1990 and 1998 in Normandy and the Channel Islands. These mechanisms are obtained from the P-wave first-motion polarities recorded by stations from local and regional seismic networks. The accuracy of each hypocentral location is closely examined and the quality of each fault plane solution is discussed by considering the influence of the velocity structure. The predominant feature of the computed focal mechanisms is the relatively widespread near-horizontal NE–SW T-axis orientation. Horizontal P-axes strike roughly NW–SE. Mechanism solutions for the earthquakes in the Avranches region show left-lateral strike slip on a NNW–SSE fault zone. For the overall region, it seems that nodal planes of normal faulting solutions trend NW–SE or WNW–ESE, whereas those of thrust faulting solutions trend NE–SW. This is in agreement with the general regional stress pattern. The NE–SW normal fault plane solution of the 1990 Jersey event is unique because it is not consistent with the regional style of faulting.

Is lithostatic loading important for the slip behavior and evolution of normal faults in the Earth's crust?

Normal faults growing in the Earth's crust are subject to the effects of an increasing frictional resistance to slip caused by the increasing lithostatic load with depth. We use three‐dimensional (3‐D) boundary element method numerical models to evaluate these effects on planar normal faults with variable elliptical tip line shapes in an elastic solid. As a result of increasing friction with depth, normal fault slip maxima for a single slip event are skewed away from the fault center toward the upper fault tip. There is a correspondingly greater propagation tendency at the upper tip. However, the tall faults that would result from such a propagation tendency are generally not observed in nature. We show how mechanical interaction between laterally stepping fault segments significantly competes with the lithostatic loading effect in the evolution of a normal fault system, promoting lateral propagation and possibly segment linkage. Resultant composite faults are wider than they are tall, resembling both 3‐D seismic data interpretations and previously documented characteristics of normal fault systems. However, this effect may be greatly complemented by the influence of a heterogeneous stratigraphy, which can control fault nucleation depth and inhibit fault propagation across the mechanical layering. Our models demonstrate that although lithostatic loading may be an important control on fault evolution in relatively homogeneous rocks, the contribution of lithologic influences and mechanical interaction between closely spaced, laterally stepping faults may predominate in determining the slip behavior and propagation tendency of normal faults in the Earth's crust.

Matrix strains along normal fault planes in the Campanian White Chalk of Belgium: structural consequences

Significant matrix strains were associated with normal faulting in the Campanian chalk of the Mons basin. These transformations, mainly due to pressure solution, located against the fault planes, brought about systematic changes in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the material. The measurements of other physical properties (elastic waves velocity, capillary rise, permeability) of the chalk provide good evidence for estimating the width of the transformed zone: it ranges between 100 and 150 mm. These transformations cannot be detected when the medium is observed at the grain scale: the most visible evidences of transformation are limited in a 50 mm thick fringe. The transformation may have occurred either in a closed or in an open system. If we follow the first assumption we can estimate the change in volume, and therefore the change in horizontal dimension of the faulted blocks: it is small compared to the horizontal extension. The second assumption is supported by a few evidences: tilted and striated rostra of belemnites along fault planes. In that case the geometrical change can be estimated too and reveals that the horizontal shortening is in the same range as the horizontal extension. It means that the extensional deformation of the Mons basin might have been partially hindered by chemically induced matrix strains.

History and tectonic implications of low‐angle detachment faults and orogen parallel extension, Picentini Mountains, Southern Apennines fold and thrust belt, Italy

Late Miocene to Pliocene movement on low‐angle extensional faults within the internal Southern Apennines orogenic belt was superposed on an earlier, Miocene imbricate thrust stack. The low‐angle faults formed within the interior of the belt during orogen parallel extension as thrust imbrication continued in the foreland. Extreme tectonic thinning defines discrete structural domains of hyperextension which are linked by a complex system of extensional and transcurrent faults. Some of the best examples of hyperextension structures in the Southern Apennines are exposed in the Picentini Mountains. In this area, detailed mapping, fault‐kinematic analysis, and excellent stratigraphie control contributed to the construction of restorable cross sections and forward models of deformation. With these constraints, it is possible to document extensional displacement on shallowly dipping supercrustal faults whose orientation is a primary feature and is not due to later tilting. During movement, upper plate rocks were disarticulated by listric and planar normal faults that soled into a ramp‐flat detachment system. The depth of the basal detachment increased in the direction of upper‐plate motion and ranged from 5 to 10 km. Displacement on the low‐angle detachment was accompanied by block tilting in the upper plate assemblage, incisement and excisement of the upper and lower plate rocks as fault trajectories changed through time, and the progressive cataclasis of hanging wall and footwall assemblages. Preexisting thrusts were only locally reactivated during extension, and faults emanating from the underlying decollement systems cross the imbricated thrust sheets at moderate to high angles. Longitudinal extension resulted in thinning of the thrust stack to less than half the original thickness and had a cumulative magnitude of between 200 and 250% (beta>2).

Recent tectonic evolution and present stress in the Northern Apennines (Italy)

The present‐day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6–7 km, 125 fault plane solutions of crustal earthquakes with M<5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M≤6) reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by Shmin direction mainly trending NE–SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shmin is oriented NW–SE, and focal solutions are thrust or strike‐slip, with maximum compression around NE–SW. Conversely, south of 43°N, breakouts evidence an orthogonal direction of horizontal compression (NW–SE), following the Southern Apennine trend, where a widespread NE–SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution inferred from structural geology, we argue that the extension‐compression pair, characteristic of the post‐Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension‐compression pair has been controlled by progressive retreat of the slab.

Frontier Exploration in the South Atlantic: Structural Prospectivity in the North Falkland Basin

Seismic stratigraphic analysis of the continental shelf north of the Falkland Islands confirms that it is dissected by a significant extensional fault system that broadly defines the North Falkland Basin. The basin comprises a series of extensional subbasins that developed during two distinct rift episodes. Analysis of structural styles shows that while extension occurred predominantly on a series of planar normal faults with changes in rift polarity in northern areas, the southern portion of the basin has listric faults that sole out along preexisting thrusts. After rifting, the basin largely underwent postrift thermal subsidence with the passive infill of remnant topography; however, evidence exists for at least two uplift events affecting the postrift sediments. The earlier period of uplift tilted the basin to the south, resulting in the formation of a southerly prograding forced regressive wedge; the later episode of uplift tilted the basin northward, resulting in truncation of earlier strata and minor compressional reactivation and inversion in southernmost parts of the basin. The later tilting allows for the possibility that the original rift system once extended across the Falkland Islands. Although the lack of well control does not permit a comprehensive assessment of hydrocarbon plays, the seismic data do at least demonstrate the trapping potential of the basin. Interpreted structural and depositional styles found within the prerift, synrift, and postrift sequences are similar to those found in proven hydrocarbon provinces such as the North Sea and the Gulf of Suez. The success or failure of the frontier hydrocarbon province thus is likely to depend upon factors other than structural ones.

Tectonics of the Red Sea region reassessed

The brittle upper level of the continental crust had been rifted with or without ocean opening many times in many places during the geological past and the process is still happening. Since the advent of plate tectonic theory in the early 1960s, the formation of such rifts has been viewed in the context of plate tectonic processes that caused the repeated dispersal of supercontinents. Several researchers focused on the mechanisms of formation of continental rifts because some rifts, like the Red Sea and Gulf of Aden, are precursors to ocean basins and many hydrocarbons yet to be located which are either directly or indirectly related to rift structures. The East African Rift System and the Red Sea-Gulf of Aden young oceans have been considered as prime examples of the early stage of continental separation that has long been a testing ground for classical hypotheses of continental drift. The Red Sea separates the once contiguous Neoproterozoic Arabian–Nubian Shields and started opening about 25 Ma ago. Geophysics and geochronology of dredged basaltic rocks indicate that sea-floor spreading began at only about 4–5 Ma. Numerous multidisciplinary investigations have been carried out in this region. However, several questions remain unresolved. Examples pertain to the nature of the crust that underlies the shelves, the extent of the ocean floor, the interplay between sea-floor spreading, crustal extension and plutonic activity and mechanisms of rifting. Several mechanisms of rifting have been proposed for the formation of the Red Sea. Examples include extension by prolonged steep normal faulting (horst–graben terrain), early diffuse ductile extension followed by brittle deformation, low-angle lithospheric simple shear, low-angle shear and magmatic expansion, lithospheric thinning by faulting and dike injection, northeastward migration of asymmetric rifting over a fixed mantle plume and the formation of pull-apart basin(s) by transtension. The major differences between the various models center on the relative timing of updoming, rifting and magmatism and whether the rifting was active and driven by a mantle plume or passive and due to lateral extension of the lithosphere leading to reactive effects in the mantle. New geological field data from the western margin of the Southern Red Sea in Eritrea reveal two main stages of NE–SW extension history. The first semi-brittle stage (⩾30 Ma) was dominantly characterized by top-to-east low-angle detachments. The second brittle stage of extension (since ∼22 Ma) occurred on a new system of dominantly down-to-southwest planar normal faults and dikes with NW–SE strikes. The earlier semi-brittle stage of extension corresponds to the predicted low-angle simple shear zone through the lithosphere and the later gives some support to the models that invoke graben–horst formation along steep normal faults that ultimately soled out to detachments at intermediate crustal level or merge with the Moho.

Shale tectonics and deformation associated with active diapirism: the Jerudong Anticline, Brunei Darussalam

The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N–S-trending anticline overlies a high-angled basement fault zone that was episodically active, moving predominantly sinistrally in response to transpressive stresses. The earliest stage of deformation in the study area was mid-Miocene E–W to NE–SW-trending growth faulting and shale-diapir growth. Early normal faults exerted an important influence on the large- and small-scale bedding geometries and facies changes of syn-tectonic shallow-marine, shoreface and tidal strata. In particular there are laterally rapid facies changes and differential rotation of strata across growth faults. Diapir activity is indicated by the presence of shale dykes which commonly intrude normal fault planes, and concordant shale intrusions. Shale intrusions, associated minor thrust faults and normal faults were rotated during a late Miocene–early Pliocene phase of folding related to transpression. Few shale dykes were intruded during this phase, probably because the horizontal principal stresses were relatively large compared with the pore-fluid pressure. At the end of the transpressive phase, shale dykes were intruded into steeply dipping beds as a result of stress relaxation. Continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and Holocene-age mud volcanoes were developed.

The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow

Abstract Pressure solution plays a major role in fault initiation and development in carbonate rocks. Strike-slip faults in the Triassic and Jurassic limestones of Somerset, UK, initiated as en echelon extension fractures, which became linked by pressure solution seams. Shear occurred along the pressure solution seams as the bridges between the veins rotated. The linked vein-pressure solution seam systems developed into pull-aparts and eventually into through-going faults. Normal fault planes in the Cretaceous Chalk at Flamborough Head, Yorkshire, UK, commonly have the pitted appearance of slickolites. Phyllosilicates are often concentrated along faults in Chalk, the thickness of the phyllosilicate gouge being proportional to fault displacement. The enrichment of phyllosilicates along the faults is due to pressure solution rather than simply to phyllosilicate smear. Pressure solution can be concentrated at the contractional quadrants of faults, particularly where there is a contractional overstep onto an adjacent fault. Metre-scale oversteps between strike-slip faults in Somerset often have pressure solution seams, while contractional oversteps and bends at Flamborough Head are accommodated by compaction of beds, with pressure solution apparently being important. The concentration of phyllosilicates by pressure solution can hinder fluid flow along and across faults in carbonate rocks. A high density of pressure solution seams between overstepping faults can effectively link these faults, increasing both the effective length of the barrier to fluid flow and the fault seal potential.

Mid-Tertiary rifting and magmatism in the Traill Ø region, East Greenland

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of >350 km and over an across-strike width of >100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area. 40 Ar– 39 Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of c. 1 km, occurred at c. 54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at c. 36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.

Numerical modelling of the effects of fault slip on fluid flow around extensional faults

Fluid flow and deformation in regions of fractured rock around extensional faults have been modelled using distinct element methods (UDEC code). The basic methodology is described in terms of a simple model of a planar normal fault zone, at the Earth's surface. The model is then modified to simulate deformation at greater depths and to investigate irregularities in fault shape (including dilational and anti-dilational fault jogs). The results obtained show that the deformation of a faulted region resulted in significant variation in fracture dilation (porosity), stress distribution, fluid pressure and fluid flow. The geometry of models and the applied boundary conditions had important effects on deformation and fluid flow. At shallow depth, dilation and fluid flow occurred both in the fault zone and the hangingwall, with little change in the footwall. At greater depth, the higher compressive stresses tended to close all fractures, except within the fault zone where the shear displacements caused local dilation. The presence of anti-dilational bends reduced the dilation and fluid flow in the fault zone, but promoted greater deformation in parts of the hangingwall. The dynamic response of fracture aperture, pore pressure and shear displacement to fault slip was also studied. The modelled results are in reasonable correspondence with observed natural examples and have practical significance for evaluating fluid flow and deformation in regions which exhibit normal faulting.