Basin Research - Volume 11, Issue 4, 1999

We compare the present‐day sediment discharge (solid phase) of some of the largest rivers in Asia to the average discharge deduced from the mass accumulated in several sedimentary basins during the Quaternary. There is a very good correlation, especially for the largest rivers: the Ganges–Brahmaputra, the Changjiang, the Huanghe and, to a lesser extent, the Indus and the Zhujiang. This suggests that present‐day average discharge at the outlet has remained constant throughout the Quaternary at least for very large rivers (drainage area of the order of 10⁵–10⁶ km²). This, in turn, suggests either that continental denudation of large Asian catchments has remained on average constant, implying a strong tectonic control on erosion during the Quaternary, or that the river network has the ability to buffer changes in hillslope erosion or in sea‐level in order to conserve the total discharge at the outlet. We show how this buffering capacity relies on the characteristic reaction time‐scale of Asian alluvial plains (of the order of 10^5–6 years), that is, much higher than the time‐scales of the Quaternary climate oscillations (of the order of 10⁴ years). A short‐term perturbation originating in hillslopes will be diluted by the floodplain. At the outlet the signal should have a longer time span and a smaller amplitude. In the same manner, an alluvial plain should not instantaneously react to a 10⁴‐year sea‐level drop because of its inertia. Along with long‐term tectonic control we infer this buffering to be the main cause for the average constancy of sediment yield of large Asian rivers during the Quaternary.

Dynamic topography formed over subducting oceanic lithosphere has been proposed as a mechanism to explain certain otherwise anomalous long‐wavelength patterns of subsidence inferred from ancient strata. Forward modelling of mantle flow in response to a subducting slab predicts amplitudes and distributions of dynamic topography that may occur due to various subducting slab geometries and histories. Plotting calculated dynamic topographies at a point against time produces tectonic subsidence curves. These subsidence curves show features such as evolution from convex to concave shape, amplitudes up to ~2000 m, subsidence rates up to ~220 m Myr⁻¹, and a general decrease in subsidence amplitude away from the subduction zone, over a distance of ~2000 km. On many convergent continental margins, dynamic topography is likely to be superimposed on other subsidence mechanisms. In back‐arc basins, subsidence due to dynamic topography should be distinguishable from that due to extensional tectonics based simply on the temporal subsidence evolution expressed in the subsidence curve shapes. In a foreland basin setting, comparing dynamic topography models with forward models of flexural loading suggest the two processes can generate similar temporal subsidence patterns, but that dynamic topography causes subsidence over significantly greater wavelengths. Matches between calculated subsidence due to dynamic topography and backstripped subsidence patterns from Upper Cretaceous strata of the Western Interior Basin, USA, support the hypothesis that a long‐wavelength ‘background subsidence’ was caused by dynamic topography.

Tertiary foreland sedimentation in SE France occurred along the western sidewall of the Alpine orogen during collision of the Apulian indentor with the European passive margin. A detailed reappraisal of the stratigraphy and structure of the Southern Subalpine Chains (SSC) in SE France shows that Tertiary depocentres of differing character developed progressively toward the foreland during ongoing SW‐directed shortening. The geodynamic controls on each of four stages of basin development are evaluated using a flexural isostatic modelling package of thrust sheet emplacement and foreland basin formation. (1) The initial stage (mid to late Eocene) can be explained as a flexural basin that migrated toward the NW, closing off to the SW against the uplifting Maures–Esterel block. This broad, shallow basin can be reproduced in forward modelling by loading a lower lithospheric plate with an effective elastic thickness of 20 km. (2) The end of detectable flexural subsidence in the early Oligocene coincides with the emplacement of the internally derived Embrunais–Ubaye (E‐U) nappes, which caused 11 km of SW‐directed shortening in the underlying SSC. The lack of Oligocene flexural subsidence dictates that the E‐U units were emplaced as gravitational nappes. Within the SSC, Oligocene sedimentation was restricted to small thrust‐sheet‐top basins recording mainly continental conditions and ongoing folding. Further west, Oligocene to Aquitanian NNW–SSE extension generated the Manosque half‐graben as part of the European graben system that affected an area from the Gulf of Lion to the Rhine graben. (3) Following the Burdigalian breakup of the Gulf of Lion rift, a marine transgression migrated northward along the European graben system. Subsequent thermal subsidence allowed 1 km of marine sediments to be deposited across the Valensole and Manosque blocks, west of the active SSC thrust belt. (4) Mio‐Pliocene conglomeratic deposits (2 km thick) were trapped within the Valensole basin by the uplifting Vaucluse block to the west and the advancing Alpine thrust sheets to the east. Late Pliocene thrusting of the SSC across the Valensole basin (approx. 10.5 km) can be linked along a Triassic detachment to the hinterland uplift of the Argentera basement massif.

Seismic and stratigraphic data of the inland Volterra Basin and of the Tuscan Shelf (Northern Tyrrhenian Sea) have been analysed to determine the tectono‐sedimentary evolution of this part of the Northern Apennines from the early Miocene (about 20 Ma) to the present. The area is a good example for better understanding the evolution of postcollisional related basins.

The study area is characterized by a series of sedimentary basins separated by tectonic ridges. Similar environmental conditions existed both onshore and offshore as indicated by the occurrence of similar seismic units. The units are separated by major unconformities. The cross‐sectional geometries of the deposits of these basins, as defined through seismic reflection profiles, change in a quasi‐regular manner through time and space.

Early stages (late Burdigalian to early Tortonian) of evolution of the basins are marked by either flat‐lying deposits, quasi‐uniform in thickness, probably remnants of originally wider and shallow settings, or, in places, by relatively small bowl‐shaped basins. The latter may have been strongly affected by the pre‐existing topography and tectonics, as they developed at or near the leading edges of pre‐Neogene substrate thrusts. These early deposits represent sedimentation during a transitional period from the end of compressional tectonics to the start of an extensional phase and represent a pre‐narrow rift stage of evolution of the region. The subsequent stage of tectonic evolution (late Tortonian to early Messinian), where preserved, is recorded by fault‐bounded triangular‐shaped basins interpreted as half‐grabens. This is one of the periods of major development of narrow rifts in the area. The following stage (late Messinian to Early Pliocene) is marked by variable types of basins, showing wide and deep bowl‐shaped geometries persistent in the offshore, whereas inshore (Volterra Basin) they alternate with half‐graben, synrift deposits. This period thus represents a transitional stage where part of the system is still affected by synrift sedimentation and part is developing into incipient post‐rift conditions. This stage was followed in early to middle Pliocene times by wide bowl‐shaped to blanket‐type deposits both in offshore and in inshore areas, indicating regional post‐rifting conditions. The pre‐, syn‐ and post‐rift stages have followed each other through time and space, starting first in the westernmost offshore area and shifting later toward the east, inshore.

The Valencia Trough is a rift formed during the late Oligocene – early Miocene opening of the western Mediterranean Sea. In this paper, we focus on the crustal structure and on the deep structure of the basin which is hard to delineate because of the widespread volcanism that conceals part of the basement. This work is the result of the study of a dense network of seismic profiling surveys and exploratory wells made in the region. The structure of the deep basement reveals the importance of transfer fracture zones which represent steps in the deepening of the basin. The thinning of the crust follows the basement deepening and we find the same partitioning of structural blocks at the crustal level. Transfer faults also represent limits in the thinning of the crust and each compartment thus delineated has a different thinning and different extensional ratios. Such a discrepancy between the thinning of the upper crust and the thinning of the lower crust may be common in many other rift zones, but is seldom as well imaged as in this study of the Valencia Trough. The transfer zones are related to extensional processes but a simple shear opening is envisaged to explain the discrepancies between thinning and extension and the asymmetry of the margins. The more efficient thinning in the lower crust can be explained by a thermal anomaly in accordance with the recent evolution of the trough. The steady thinning of the margins is discussed in terms of a marginal basin in a compressional context.