Basin Research - Volume 14, Issue 2, 2002

ABSTRACT Foreland basins form by lithospheric flexure under orogenic loading and are filled by surface transport of sediment. This work readdresses the interplay between these processes by integrating in a 3D numerical model: the mechanisms of thrust stacking, elastic flexural subsidence and sediment transport along the drainage network. The experiments show that both crustal tectonic deformation and vertical movements related to lithospheric flexure control and organise the basin‐scale drainage pattern, competing with the nonlinear, unpredictable intrinsic nature of river network evolution. Drainage pattern characteristics are predicted that match those observed in many foreland basins, such as the axial drainage, the distal location of the main river within the basin, and the formation of large, long‐lasting lacustrine systems. In areas where the river network is not well developed before the formation of the basin, these lithospheric flexural effects on drainage patterns may be enhanced by the role of the forebulge uplift as drainage divide. Inversely, fluvial transport modifies the flexural vertical movements differently than simpler transport models (e.g. diffusion): Rivers can drive erosion products far from a filled basin, amplifying the erosional rebound of both orogen and basin. The evolution of the sediment budget between orogen and basin is strongly dependent on this coupling between flexure and fluvial transport: Maximum sediment accumulations on the foreland are predicted for a narrow range of lithospheric elastic thickness between 15 and 40 km, coinciding with the Te values most commonly reported for foreland basins.

We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change (SLR) show that response depends on the relative rates of rock uplift (U) and wave‐base erosion (ɛw). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall (U‐SLR) and ɛw, and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ɛw. Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (ɛf). In the presence of a rapid offshore decrease in rock‐uplift rate (U), short (∼5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.

ABSTRACT Sedimentary rocks rich in organic matter, such as coal and carbonaceous shales, are characterized by remarkably low thermal conductivities in the range of 0.2–1.0 W m⁻¹ °C⁻¹, lower by a factor of 2 or more than other common rock types. As a result of this natural insulating effect, temperature gradients in organic rich, fine‐grained sediments may become elevated even with a typical continental basal heat flow of 60 mW m⁻². Underlying rocks will attain higher temperatures and higher thermal maturities than would otherwise occur. A two‐dimensional finite element model of fluid flow and heat transport has been used to study the insulating effect of low thermal conductivity carbonaceous sediments in an uplifted foreland basin. Topography‐driven recharge is assumed to be the major driving force for regional groundwater flow. Our model section cuts through the Arkoma Basin to Ozark Plateau and terminates near the Missouri River, west of St. Louis. Fluid inclusions, organic maturation, and fission track evidence show that large areas of upper Cambrian rocks in southern Missouri have experienced high temperatures (100–140 °C) at shallow depths (< 1.5 km). Low thermal conductivity sediments, such as coal and organic rich mudstone were deposited over the Arkoma Basin and Ozark Plateau, as well as most of the mid‐continent of North America, during the Late Palaeozoic. Much of these Late Palaeozoic sediments were subsequently removed by erosion. Our model results are consistent with high temperatures (100–130 °C) in the groundwater discharge region at shallow depths (< 1.5 km) even with a typical continental basal heat flow of 60 mW m⁻². Higher heat energy retention in basin sediments and underlying basement rocks prior to basin‐scale fluid flow and higher rates of advective heat transport along basal aquifers owing to lower fluid viscosity (more efficient heat transport) contribute to higher temperatures in the discharge region. Thermal insulation by organic rich sediments which traps heat transported by upward fluid advection is the dominant mechanism for elevated temperatures in the discharge region. This suggests localized formation of ore deposits within a basin‐scale fluid flow system may be caused by the juxtaposition of upward fluid discharge with overlying areas of insulating organic rich sediments. The additional temperature increment contributed to underlying rocks by this insulating effect may help to explain anomalous thermal maturity of the Arkoma Basin and Ozark Plateau, reducing the need to call upon excessive burial or high basal heat flow (80–100 mW m⁻²) in the past. After subsequent uplift and erosion remove the insulating carbonaceous layer, the model slowly returns to a normal geothermal gradient of about 30 °C km⁻¹.

We report on new stratigraphic, palaeomagnetic and anisotropy of magnetic susceptibility (AMS) results from the Amantea basin, located on‐shore along the Tyrrhenian coast of the Calabrian Arc (Italy). The Miocene Amantea Basin formed on the top of a brittlely extended upper plate, separated from a blueschist lower plate by a low‐angle top‐to‐the‐west extensional detachment fault. The stratigraphic architecture of the basin is mainly controlled by the geometry of the detachment fault and is organized in several depositional sequences, separated by major unconformities. The first sequence (DS1) directly overlaps the basement units, and is constituted by Serravallian coarse‐grained conglomerates and sandstones. The upper boundary of this sequence is a major angular unconformity locally marked by a thick palaeosol (type 1 sequence boundary). The second depositional sequence DS2 (middle Tortonian‐early Messinian) is mainly formed by conglomerates, passing upwards to calcarenites, sandstones, claystones and diatomites. Finally, Messinian limestones and evaporites form the third depositional sequence (DS3). Our new biostratigraphic data on the Neogene deposits of the Amantea basin indicate a hiatus of 3 Ma separating sequences DS1 and DS2.

The structural architecture of the basin is characterized by faulted homoclines, generally westward dipping, dissected by eastward dipping normal faults. Strike‐slip faults are also present along the margins of the intrabasinal structural highs. Several episodes of syn‐depositional tectonic activity are marked by well‐exposed progressive unconformities, folds and capped normal faults. Three main stages of extensional tectonics affected the area during Neogene‐Quaternary times: (1) Serravallian low‐angle normal faulting; (2) middle Tortonian high‐angle syn‐sedimentary normal faulting; (3) Messinian‐Quaternary high‐angle normal faulting.

Extensional tectonics controlled the exhumation of high‐P/low‐T metamorphic rocks and later the foundering of the Amantea basin, with a constant WNW‐ESE stretching direction (present‐day coordinates), defined by means of structural analyses and by AMS data. Palaeomagnetic analyses performed mainly on the claystone deposits of DS1 show a post‐Serravallian clockwise rotation of the Amantea basin.

The data presented in this paper constrain better the overall timing, structure and kinematics of the early stages of extensional tectonics of the southern Tyrrhenian Sea. In particular, extensional basins in the southern Tyrrhenian Sea opened during Serravallian and evolved during late Miocene. These data confirm that, at that time, the Amantea basin represented the conjugate extensional margin of the Sardinian border, and that it later drifted south‐eastward and rotated clockwise as a part of the Calabria‐Peloritani terrane.

The c. 2.97–2.71 Ga Witwatersrand Basin located in the Kaapvaal craton of South Africa represents a remnant of a large Late Archaean sedimentary basin that hosts the world's premier gold deposit within a series of conglomerate horizons. Evidence of postdepositional gold mobility within these conglomerates associated with hydrothermal–metamorphic activity has led to speculation about the Late Archaean to Palaeoproterozoic geothermal gradients in the basin. We use surface heat flow and heat production data from rocks in the basin and its environs in order to calculate detailed temperature profiles for the central Kaapvaal craton that show that the steady state crustal geotherm during the Late Archaean and Palaeoproterozoic was relatively cool at 15–20 K km⁻¹. The geotherm in the upper crustal strata is also largely unaffected by substantial increases in the heat flow into the base of the crust. Consequently, regional greenschist facies metamorphism of the basin sediments could only have been achieved during a transient thermal event that advected heat into the upper crust. The most likely candidate for this is the Bushveld magmatic event at 2.06 Ga.

ABSTRACT The regional thermal history of the north‐eastern Sverdrup Basin, Canadian Arctic Archipelago, has been assessed using apatite fission‐track thermochronology and vitrinite reflectance data. Fission‐track data for 27 samples from six wells through the Mesozoic section on Axel Heiberg and Ellesmere Islands reveal significant Palaeocene cooling associated with basin inversion during the Eurekan Orogeny. Fission‐track data for 29 outcrop samples, ranging in stratigraphic age from Cambrian to Tertiary, also reveal significant Palaeocene cooling. Vitrinite reflectance data from carbonaceous shales and coal seams in well and outcrop samples are consistent with these conclusions. The degree of Palaeocene cooling observed is greatest for well and outcrop samples in the cores of anticlines or the hanging walls of thrust faults, such as the Fosheim anticline, and faults, such as the Lake Hazen fault system, and the East Cape and Vesle Fiord thrust faults. Palaeocene cooling is largely attributed to the denudation of structures during the Eurekan Orogeny.

At one locality on north‐western Ellesmere Island, which is on the northern flank of the Sverdrup Basin, the underlying Franklinian basement rocks yield Early Cretaceous fission track ages with relatively long mean track lengths. This indicates that this part of the basin was uplifted at this time and that subsequent sedimentation and subsidence in the Cretaceous and early Tertiary were modest. This locality thus appears to be on the rift shoulder, which developed along the flank of the Amerasia Basin in the Lower Cretaceous.

At a locality on western Axel Heiberg Island, which is downflank from the rift shoulder, the Upper Jurassic Awingak sandstone has a Late Cretaceous fission track age. This is best explained by heating above the total annealing temperature for fission‐tracks in apatite by extensive Lower Cretaceous intrusions and subsequent heat dissipation and cooling in the Late Cretaceous followed by further substantial cooling due to Tertiary denudation.

These results indicate that maximum burial temperatures occurred in the presently exposed Mesozoic section prior to basin inversion during the Eurekan Orogeny. It can therefore be inferred that peak hydrocarbon generation and primary migration predated the formation of structural traps during the Tertiary at shallow depths within the northern Sverdrup Basin.

Neoproterozoic sedimentary basins cover a large area of central Australia. They rest upon rigid continental crust that varies from c. 40–50 km in thickness. Whilst the crust was in part formed during the Archaean and early Palaeoproterozoic, its final assembly occurred at approximately 1.1 Ga as the Neoproterozoic supercontinent, Rodinia, came into being. The assembly process left an indelible imprint on the region producing a strong crustal fabric in the form of a series of north dipping thrusts that pervade much of the thick craton and extend almost to the Moho. Following a period of stability (1.1–0.8 Ga), a large area of central Australia, in excess of 2.5 × 10⁶ km², began to subside in synchroneity. This major event was due to mantle instability resulting from the insulating effect of Rodinia. Initially, beginning c. 900 Ma, a rising superplume uplifted much of central Australia leading to peneplanation of the uplifted region and the generation of large volumes of sand‐sized clastic materials. Ultimately, the decline of the superplume led to thermal recovery and the development of a sag basin (beginning at c. 800 Ma), which in turn resulted in the redistribution of the clastic sediments and the development of a vast sand sheet at the base of the Neoproterozoic succession.

The superbasin generated by the thermal recovery was short lived (c. 20 M.y.) but, in conjunction with the crustal fabric developed during supercontinent assembly, it set the stage for further long‐term basin development that extended for half a billion years well into the Late Palaeozoic. Following the sag phase at least five major tectonic episodes influenced the central Australian region. Compressional tectonics reactivated earlier thrust faults that had remained dormant within the crust, disrupting the superbasin, causing uplift of basement blocks and breaking the superbasin into the four basins now identified within the central Australian Neoproterozoic succession (Officer, Amadeus, Ngalia and Georgina Basin). These subsequent tectonic events produced the distinctive foreland architecture associated with the basins and were perhaps the trigger for the Neoproterozoic ice ages. The reactivated basins became asymmetric with major thrust faults along one margin paralleled by deep narrow troughs that formed the main depocentres for the remaining life of the basins. The final major tectonic event to influence the central Australian basins, the Alice Springs Orogeny, effectively terminated sedimentation in the region in the Late Palaeozoic (c. 290 Ma). Of the six tectonic episodes recorded in the basinal succession only one provides evidence of extension, suggesting the breakup of east Gondwana at the end of the Rodinian supercontinent cycle may have occurred at close to the time of the Precambrian–Cambrian boundary. The central Australian basins are thus the products of events surrounding the assembly and dispersal of Rodinia.

The outer Adriatic zones of the central Apennines (Italy) provide good conditions for analysing geometry and kinematics of the earliest normal faults, superposed onto the thrust belt. During the latest stages of thrusting onto the Adriatic foreland (late Pliocene–early Pleistocene), the outermost imbricates of the thrust belt were subjected to normal faulting, coeval with differential uplift. Crosscutting normal faults get younger towards the foreland, thus the easternmost normal faults record the latest stages of fault propagation and growth. The Caramanico fault, on the western flank of Mt. Maiella, is the largest outcropping normal fault of the outer zones. This high‐angle fault (dip > 70°) has cumulative offsets ≤ €4.2 km, and propagated with slip rates of 2.6 mm/year in a short time interval (≤ 1.6 Ma), concomitant with intense uplift of Mt. Maiella. In contrast with normal faults in a more internal position, the Caramanico fault maintains a high‐angle planar geometry, and does not reach the major basal detachment of the thrust belt. Thus the fault did not cause large extensional displacements; its major role was rather to accommodate ongoing components of vertical uplift of the overthickened thrust wedge. Downfaulting of the thrust belt on the western flank of Mt. Maiella represents the youngest end member of the same processes that have operated since 11 Ma in the Tyrrhenian hinterland, where large extensional strains and crustal thinning of the orogenic belt were achieved by long‐lasting activity of listric normal faults detached at lower crustal depths.