Basin Research - Volume 3, Issue 4, 1991

The flexural rigidity of the oceanic lithosphere is strongly dependent on its temperature structure at the time of loading. It is commonly assumed that the depth to the 450°C isotherm defines the effective elastic thickness Te of the lithosphere. However, recent gravity studies across the Baltimore Canyon and Nova Scotian margins suggest that temperature may play a more complicated role in controlling the mechanical strength of extended continental lithosphere. For example, the flexural strength of the Baltimore Canyon margin (with sediment thicknesses of ≅ 15 km) appears to be controlled by the depth to the 150°C isotherm whereas the strength of the Nova Scotian margin (with sediment thicknesses cf ≅ 10 km) is controlled by the depth to the 250°C isotherm. The apparent correlation between sediment thickness and controlling isotherm suggests that sediment blanketing may play a role in modifying the flexural strength of extended continental lithosphere. This hypothesis was investigated by simulating the sedimentation history of a margin as a Gaussian function in which sedimentation peak and rate are determined by the mean and standard deviation of the function. The temperature structure of the lithosphere is continually modified as sediments are deposited on, and incorporated into the temperature structure of, the underlying lithosphere. Given a ‘starting’ value of Te defined by the degree of extension of the lithosphere, the modification of Te appears to be directly proportional to the sedimentation rate and cumulative sediment thickness, and inversely proportional to the time at which the sedimentation rate is a maximum. The first‐order consequence of sediment blanketing is to reduce the cooling rate of the lithosphere relative to cooling in the absence of sediments. At thermal equilibrium, the initial value of Te is reduced by the cumulative sediment thickness. Local isostatic conditions (i. e. Te≅ 0) can only be approached when the sedimentation rate is unrealistically high (> 1000 m/Myr) during the rift or early post‐rift phase of basin development. However, while these early loads may be locally compensated, any subsequent loads will be regionally compensated. Thus, it is unlikely that the low present‐day flexural strengths interpreted from the Baltimore Canyon and Nova Scotian passive continental margins are a consequence of sediment blanketing.

Fission track analyses of detrital components in the Permo‐Triassic Karoo Basin (South Africa), highlight the potency of tectono‐magmatically driven fluids to penetrate wide and far in foreland basins. The data, together with the data published on Karoo tectonics and magmatism, support a model which requires that fluids were driven north out of the Cape‐Karoo orogen during the Cape Orogeny (270–200 Ma). Later fluids were redistributed and aquifers rejuvenated during (and after) the final break‐up of Gondwana (<200 Ma). The fission track data indicate that thermal annealing of fission tracks in zircon occurs non‐uniformly between individual zircon grains. This model is in agreement with recent models applied to deformed foreland basins and implicates tectonic fluids in U metallogenesis.

Detailed seismic reflection data combined with regional magnetic, gravity and geological data indicate that the Drummond Basin originated as a backare extensional basin associated with Late Devonian and Early Carboniferous active margin tectonism in the northern New England Fold Belt. Seismic reflection data have been used to generate a two‐way time map of seismic basement, providing a clear view of the basinal geometry and structural development. Broadscale structural asymmetry of the basin implies that simple shear along a deep, upper‐crustal detachment provided the extensional mechanism and generated an inter‐related set of listric normal faults and associated transfer faults, as well as steeply‐dipping planat normal faults. The orientation of normal faults near the basin margins appears to have been controlled by regional basement structural trends. Transfer‐fault trends were approximarely orthogonal to the line of plate convergence as assessed from the orientation of coeval are, forcare and subduction complex stratorectonic elements. Three distinct phases of infill are represented in the basinal stratigraphic succession. The first consists largely of volcanics and volcaniclastics, indicating that effusive magmatism and extension were closely associated in space and time. The second is quartzose and of basement derivation, but was not derived from footwall blocks at the faulted basinal margins to the east and north. Uplifted hanging‐wall crust beyond the western basinal margin, a product of west‐directed simple shear detachment, was the likely source terrain. The final infill phase consisted of volcaniclastics considered to have been derived from a coeval volcanic are to the east. Major faults at the basin margins provided conduits for magmatism during extensional basin development, and long after the basinal history was complete. During the Late Carboniferous and mid‐Triassic, the basin was affected by two discrete episodes of compressional deformation. This led to inversion with the development of folds, and reverse and wrench faults now seen at the surface.

Excellent exposure, well‐controlled palaeobathymetry, and tightly‐spaced, high‐precision radiometric age control in the El Gallo Fm. permit rigorous quantitative analysis. Backstripping of these proximal nonmarine, forearc basin deposits reveals that, during the Late Cretaceous, the Rosario embayment of the Peninsular Ranges forearc was undergoing an episode of rapid tectonic subsidence. This subsidence had several marked effects on the sedimentology of the Rosario embayment: formation of a broad alluvial plain consisting of coarse‐grained clastics; rapid (∼ 600 m Myr^‐1) aggradation of sediments; and a retrogradational succession of facies, capped by a marine transgression, as deposition failed to keep pace with eustatic rise and subsidence.

Long‐term sedimentation is driven by some combination of two allocyclic mechanisms: tectonic subsidence and eustatic sea‐level rise. In order to evaluate which force predominated during deposition of the El Gallo Fm., the processes of sedimentation, compaction, and isostasy are evaluated through the interval in question. A sensitivity analysis is performed, in which the maximum tectonic and maximum eustatic contributions are estimated, along with the best‐fit model. These results are qualitatively the same: tectonic subsidence was the major driving force of sedimentation in the Rosario embayment in late Campanian time. Regional sedimentological similarities suggest that this tectonic subsidence may have characterized the Peninsular Ranges forearc margin at this time, reflecting an episode of active down‐faulting during the Late Cretaceous.

The Western Irish Namurian Basin developed in Early Carboniferous times as a result of extension across the Shannon Lineament which probably coincides with the lapetus Suture. During the late Dinantian, axial areas of the NE‐SW elongate trough became deep, whilst shallow‐water limestones were deposited on the flanks. This bathymetry persisted into the Namurian when carbonate deposition ceased. In axial areas, a relatively thick mudstone succession spans earliest Namurian to Chokierian whilst on the northwestern marginal shelf, a thin, condensed Namurian mudstone sequence, in which pre‐Chokierian sediments are apparently absent, rests unconformably on the Dinantian. From late Chokierian to early Kinderscoutian, the basin was filled by sand‐dominated clastic sediments. Sand deposition began in the axial area with deposition of a thick turbidite sequence, the Ross Formation, which is largely equivalent to the condensed mudstone succession on the flanks. Turbidity currents flowed mainly axially towards the north‐east and deposited a sequence lacking well‐defined patterns of vertical bed‐thickness change. Channels and slide sheets occur towards the top of the formation. The turbidite system seems to have lacked well‐defined lobes and stable distributary channels.

Overlying the Ross Formation, the Gull Island Formation shows a decreasing incidence of turbidite sandstones at the expense of increasing siltstones. This formation is characterized by major slides and slumps interbedded with undisturbed strata. In the flanking areas of the basin, the formation is thinner, has only a few turbidites in the sequence above the condensed mudstones and contains only one slide sheet. Overall the formation is interpreted as the deposit of a major prograding slope, the lower part representing a ramp upon which turbidites were deposited, the upper part a highly unstable muddy slope lacking any conspicuous feeder channels through which sand might have been transferred to deeper water. Progradation of the slope appears to have been increasingly from the northwestern flank of the trough which is similar to the direction deduced for the overlying deltaic Tullig cyclothem which completes the initial basin fill.

Whilst several features of the succession can be explained by envisaging the whole sequence as the product of one linked depositional system, the shifting directions of palaeocurrents and palaeoslope raise problems. The switch from axial to lateral supply casts doubt on the strict application of Walther's Law to the total sequence and seems to demand large avulsive shifts of the delta system on the shelf area to the west.

Book reviewed in this article:

Allochthonous Terranes J. F. Dewey, I. G. Gass, G. B. Curry, N. B. W. Harris and A. M. C. Sengör.

Interior Cratonic Basins M. W. Leighton, D. R. Kolata, D. F. Oltz and, J. J. Eidel (eds)

Active Margin Basins K. T. Biddle (ed.)