Basin Research - Volume 1, Issue 1, 1988

Abstract Expressions are obtained for temperature as a function of depth, and for surface elevation and surface heat flow using a simplified model to represent lithosphere extending on a low‐angle detachment surface. The geometry of the resulting basin is determined by the dip of the detachment surface φ and the original thickness of the crust, h. For small extension the width of the basin is h/tan φ and with increasing extension the width of the basin cannot exceed 2h/tan φ before sea‐floor spreading begins. The asymmetry of heat flow and subsidence profiles across the basin is described and the predictions of the model are compared with those of the model for uniform extension by pure shear. The amplitude of thermal subsidence for the detachment‐zone model is typically half as great as for the pure‐shear model with the same extension factor. As the total subsidence is the same for each model the initial subsidence is correspondingly greater for the detachment‐zone model. The time‐integrated anomalous heat flow in the detachment‐zone model is also approximately half that in the pure‐shear model.

Abstract Recent improvements in biostratigraphic and magnetostratigraphic control in the Eocene sediments of the Hampshire Basin prompted direct comparison of depositional sequences in outcrop with those predicted by the latest and most detailed Exxon coastal onlap chart. This study focused on the upper two cycles of the London Clay Formation, the Bracklesham Group and the Barton Formation, comprising nine depositional sequences, each a few 10s of metres thick. The sediments were divided into three basic facies associations: marine, estuarine and alluvial. Depositional sequences invariably rest on a regional erosion surface cut during sea‐level lowstand. The lower part of each sequence consists typically of ‘estuarine’ sediments (including tidal channel, lagoon, tidal flat and marsh deposits), laid down under brackish conditions during the early stages of sea‐level rise. Estuarine deposits are typically erosively overlain by marine shoreface or shelf deposits; the eroded, pebble‐strewn contact marks the passage of the marine shoreface. Marine deposits may be erosively overlain by alluvial sediments that record coastal progradation in response to stable or slowly falling sea level. Magnetostratigraphy, in the form of truncated or absent magnetozones provides supporting evidence for significant erosion during periods of lowstand. Every sequence can be matched to the Exxon coastal onlap chart, with one exception, which, on biostratigraphic and magnetostratigraphic evidence has been shown to be absent from the Hampshire Basin. The Exxon chart suggests that in this exceptional instance, coastal onlap was insufficient to effect marine deposition in the Hampshire Basin.

Abstract Reconstructions of the relative positions of the Indian, African, and Antarctic plates and their uncertainties are given for the times of selected magnetic anomalies that could be identified on adjacent pairs of these plates. Among the most certain reconstructions are those for the Antarctic and African plates, which can be determined directly from recently published magnetic anomalies from both sides of the Southwest Indian Ridge. As Patriat and his colleagues reported, there was an important change in direction and a decrease in rate of separation between Africa and Antarctica between the times of anomalies 33 and 20. India moved rapidly away from both Africa and Antarctica in the Late Cretaceous and early Tertiary periods, but slowed markedly near the time of anomaly 20 (≅ 45 Myr). The positions of the Indian plate with respect to the others are poorly constrained between the times of anomaly 5 (≅ 10 Myr) and anomaly 23 (≅ 54 Myr), but using the reconstructions of the African and Antarctic plates, the uncertainties can be reduced. Despite the relatively large uncertainties, the positions of anomalies 5, 6, and 13 on the Antarctic and Indian plates apparently cannot be described by the same parameters that describe the history of separation of Australia and Antarctica. Therefore, Stein and Okal's contention that Australia and India lie on separate plates appears to be valid not only for the present, but for the last 35 Myr.

Abstract A distinction is drawn between sea‐floor canyons, which are incised into bedrock, and fan valleys and deep‐sea channels, which are cut in unconsolidated sediment. The formation of continental margin canyons/fans and deep‐sea channels is an inevitable consequence of continental margin rifting and sea‐floor subsidence. Such submarine sediment transport systems are amongst the longest‐lived physiographic features on earth, with the Bounty Channel system being more than 50 Myr old. Many deep‐sea channels form the distal part of ocean‐margin sediment transport systems, being incised 100–350 m into ocean‐floor sediments, traversing great distances over the ocean‐basin floor, and generally terminating on an abyssal plain. The course of each deep‐sea channel is, however, unique. Channel locations are controlled primarily by inherited basement relief, and, during their evolution, by rates and patterns of lithospheric subsidence and sedimentation. In the early stages of ocean‐basin formation, deep‐sea channels may issue from the axial parts of marginal rifts, or directly from slope canyon‐fan systems. As an ocean basin widens, margin‐connected channels may become trapped within the strip of oldest (and therefore deepest) oceanic crust at the continent/ocean interface, and will therefore be margin‐parallel features. In some cases, as for the Cascadia Channel, channels may escape from the ocean‐margin deep, bypassing the spreading ridge via a fracture zone. Deep‐sea channels and their associated sediments are influenced also by global sea‐level change, by rate of turbidity current generation from the headward continental margin, by rates of pelagic sediment supply, by differential levee development consequent upon the Coriolis effect, and by the operation of deep‐sea current systems with their associated sediment drifts. The survival of deep‐sea channels as long‐lived features necessitates that rates of long‐term subsidence at the channel terminus exceed sediment accumulation.

Abstract Analogue models are a powerful tool for investigating progressive deformation in extensional fault systems. This paper presents exciting new insights into the progressive evolution of hanging wall structures in listric extensional terranes. Analogue models, scaled to simulate deformation in a sedimentary sequence, were constructed for simple listric and ramp/flat listric extensional detachments. For each detachment geometry homogeneous sand, sand/mica and sand/clay models were used to simulate respectively, deformation of isotropic sediments, of anisotropic sediments and of sedimentary sequences with competency contrasts.

Roll‐over anticlines with geometrically necessary crestal collapse graben structures are characteristic of the steepening‐upwards segments of listric extensional fault systems in all of our models. With progressive deformation, crestal collapse grabens show hanging wall nucleation of new faults. Variations in graben size, amount of fault rotation and throw, are dependent on detachment curvature and amount of extension. Individual faults and associated fault blocks may significantly change shape during extension. Complex and apparently conjugate fault arrays are the result of superposition of successive crestal collapse grabens.

Ramp/flat listric extensional fault systems are characterized by a roll‐over anticline and a crestal collapse graben system associated with each steepening‐upwards segment of the detachment and a ramp zone consisting of a hanging wall syncline and a complex deformation zone with local reverse faults. The roll‐over anticlines and crestal collapse graben are similar in geometry to those formed in simple listric extensional systems.

The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of hanging wall structures. Analysis of particle displacement paths for these experiments provides new insights into the mechanical development of roll‐over anticlines. Two general models for deformation above simple listric and ramp/flat listric extensional detachments have been erected.