Basin Research - Volume 17, Issue 2, 2005

Well‐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.

Neogene collision between Australia and the Banda Arc modified two adjacent depocentres within Australia's North‐West Shelf, the Browse and Bonaparte Basins. We identify two components of this modification: (1) continuous long‐wavelength amplification of Permo‐Carboniferous basement topography, and (2) flexure and normal faulting of Triassic–Recent sedimentary cover. Although this deformation was continuous across the Browse and Bonaparte Basins, the degree of basement architectural control, mechanisms of fault linkage and distribution of syntectonic accommodation space varied significantly between the two basins. These variations reflect fundamental differences in the structural relief, amplitude and depth of rifted basement on either side of a rupture‐barrier‐style accommodation zone, the Browse/Bonaparte Transition. This long‐lived architectural divide, of which there is no discrete structural expression, was amplified by Neogene collision. We examine tectonic rejuvenation of the Browse/Bonaparte Transition and describe a mechanism for actively sustaining long‐lived segmentation of the continental shelf.

Field, petrographic and fluid inclusion characteristics of sand injectites from five outcrop localities and from the subsurface of the Tertiary of the south Viking Graben are described. Although the case studies are from a wide variety of sedimentological, stratigraphic and tectonic settings, and hence their diagenetic evolutions differ significantly, it is possible and useful to assign diagenetic events to three distinct phases of fluid flow associated with sand injectites in sedimentary basins. Firstly, there is fluid flow associated with the injection of the fluid–sediment mix during shallow burial. Early diagenetic imprints in sand injectites reveal that basinal fluids, which may be released during movement along deeper‐seated faults, can be associated with this process and thus the injection process may reveal information on the timing of basin‐scale movement of fluids. Secondly, following the injection process, basinal fluids continue to migrate through uncemented injectites and mix with the ambient meteoric and/or marine pore fluids that invade injectites from the overlying and surrounding host sediments. Early, often pervasive, carbonate cementation is common within sand injectites and rapidly turns sand injectites into flow barriers during shallow (<1 km) burial. If early carbonate cementation is not pervasive, fluid inclusions in late quartz cement (∼>2 km of burial) reveal additional information on fluid flow associated with sand injectites during deeper burial. The latest phase of fluid flow occurs when sand injectites are reactivated as preferential fluid conduits during phases of deformation, when well‐cemented subvertical sand injectites become sites of focussed brittle deformation (fracturing). This study shows that sand injectites are a common and volumetrically important type of structural heterogeneity in sedimentary basins and that long‐lived fluid flow associated with sand injectites in very different settings can be assessed and compared systematically using a combination of petrography and fluid inclusion studies.

An inferred burial and exhumation history of Pennsylvanian strata in the central Appalachian foreland basin is constrained by integrating palaeothermometers, geochronometers and estimated palaeogeothermal gradients. Vitrinite reflectance data and fluid inclusion homogenization temperatures indicate that burial of Lower and Upper Pennsylvanian strata of the Appalachian Plateau in West Virginia exceeded ∼4.4 km during the late Permian and occurred at a rate of ∼100 m Myr⁻¹. Exhumation rates of ∼10 m Myr⁻¹ from the late Permian to the early Cretaceous are constrained using maximum burial conditions and published apatite fission track (AFT) ages. AFT and radiogenic helium ages indicate exhumation rates of ∼30–50 m Myr⁻¹ from the early to late Cretaceous. Radiogenic helium dates and present day sampling depths indicate that exhumation rates from the late Cretaceous to present were ∼25 m Myr⁻¹. Exhumation rates for Upper and Lower Pennsylvanian strata within the Appalachian Plateau are remarkably similar. Early slow exhumation was possibly driven primarily by isostatic rebound associated with Triassic rifting. The later, more rapid exhumation can be attributed to thermal expansion followed by lithospheric flexure related to sediment loading along the passive margin.

Three‐dimensional (3D) numerical modelling of fault displacement enables the building of geological models to represent the complex 3D geometry and geological properties of faulted sedimentary basins. Using these models, cross‐fault juxtaposition relationships are predicted in 3D space and through time, based on the geometries of strata that are cut by faults. Forward modelling of fault development allows a 3D prediction of fault‐zone argillaceous smear using a 3D application of the Shale Gouge Ratio. Numerical models of the Artemis Field, Southern North Sea, UK and the Moab Fault, Utah, USA are used to demonstrate the developed techniques and compare them to traditional one‐ and two‐dimensional solutions. These examples demonstrate that a 3D analysis leads to significant improvements in the prediction of fault seal, the analysis of the interaction of the sealing properties of multiple faults, and the interpretation of fault seal within the context of sedimentary basin geometry.

A sequential restoration based on combined backstripping and unfolding methods affords the opportunity to study the Cenozoic evolution of two low amplitude domes in the Mid‐Norwegian extensional margin, the Helland Hansen Arch and the Vema Dome. The integration of growth strata geometries observed in both flanks of the domes demonstrate that the structures grew by a variable combination of tectonics and differential compaction mechanisms. Sequential restoration shows that the Helland Hansen Arch grew between Early Oligocene and earliest Late Pliocene times (33–1.9 Ma). During the first phase of growth (33–9 Ma), the tectonic compression accounted for a minimum of 27% of the total dome amplitude. During Late Miocene to Pliocene times (9–1.9 Ma), differential compaction was the mechanism for dome growth. During Late Pliocene times, the Helland Hansen Arch grew with the highest rates coinciding with initial deposition of prograding wedges (3.6–1.9 Ma). In contrast, the Vema Dome started to develop in Early Eocene times and grew at a fairly constant rate up to Early Pliocene times at 3.6 Ma. The amplification of the Vema Dome took place through both differential compaction and tectonics between Early Eocene and Late Miocene times (54.8–7 Ma). The tectonic contribution accounted for a minimum of a 37% of the total dome amplitude. During Pleistocene times, the progradation of clastic wedges led to a decrease of the amplitudes of both the Helland Hansen Arch and the Vema Dome. The different timing of tectonic growth for analysed domes and arches suggest that a small and protracted phase of compression affected the Mid‐Norwegian Margin. This agrees with well‐known widespread contractional deformation affecting the Atlantic Margin of the European Plate during the Tertiary.

The well‐constrained seismic stratigraphy of the offshore Canterbury basin provides the opportunity to investigate long‐term changes in sediment supply related to the formation of a transpressive plate boundary (Alpine Fault). Reconstructions of the relative motion of the Australian and Pacific plates reveal divergence in the central Southern Alps prior to ∼20.1 Ma (chron 6o), followed by increasing average rates of convergence, with a marked increase after ∼6 Ma (late Miocene). A strike–slip component existed prior to 33.5 Ma (chron 13o) and perhaps as early as Eocene (45 Ma). However, rapid strike–slip motion (>30 mm yr⁻¹) began at ∼20.1 Ma (chron 6o). Since ∼20.1 Ma there has been no significant change in the strike–slip component of relative plate motion.

Sedimentation rates are calculated from individual sequence volumes that are then summed to represent sequence groups covering the same time periods as the tectonic reconstructions. Rates are relatively high (>22 mm yr⁻¹), from 15 to ∼11.5 Ma (sequence group 1). Rates decrease to a minimum (<15 mm yr⁻¹) during the ∼11.5–6 Ma interval (sequence group 2), followed by increased rates during the periods of ∼6–2.6 Ma (21 mm yr⁻¹; group 3) and 2.6–0 Ma (∼25 mm yr⁻¹; group 4). Good agreement between sedimentation and tectonic convergence rates in sequence groups 2–4 indicates that tectonism has been the dominant control on sediment supply to the Canterbury basin since ∼11.5 Ma. In particular, high sedimentation rates of 21 and ∼25 mm yr⁻¹ in groups 3 and 4, respectively, may reflect increased plate convergence and uplift at the Southern Alps at ∼6 Ma. The early‐middle Miocene (∼15–11.5 Ma) high sedimentation rate (22 mm yr⁻¹) correlates with low convergence rates (∼2 mm yr⁻¹) and is mainly a response to global climatic and eustatic forcing.