Basin Research - Volume 6, Issue 2‐3, 1994

Estimates of dynamic motion of Australia since the end of the Jurassic have been made by modeling marine flooding and comparing it with palaeogeographical reconstructions of marine inundation. First, sediment isopachs were back stripped from present‐day topography. Dynamic motion was determined by the displacement needed to approximate observed flooding when allowance is made for changes in eustatic sea‐level. The reconstructed inundation patterns suggest that during the Cretaceous, Australia remained a relatively stable platform, and flooding in the eastern interior during the Early Cretaceous was primarily the result of the regional tectonic motion. Vertical motion during the Cretaceous was much smaller than the movement since the end of the Cretaceous. Subsidence and marine flooding in the Eromanga and Surat Basins, and the subsequent 500 m of uplift of the eastern portion of the basin, may have been driven by changes in plate dynamics during the Mesozoic. Convergence along the north‐east edge of Australia between 200 and 100 Ma coincides with platform sedimentation and subsidence within the Eromanga and Surat Basins. A major shift in the position of subduction at 140 Ma was coeval with the marine incursion into the Eromanga. When subduction ended at 95 Ma, marine inundation of the Eromanga also ended. Subsidence and uplift of the eastern interior is consistent with dynamic models of subduction in which subsidence is generated when the dip angle of the slab decreases and uplift is generated when subduction terminates (i.e. the dynamic load vanishes). Since the end of the Cretaceous, Australia has uniformly subsided by about 250 m with little apparent tilting. This vertical subsidence may have resulted from the northward migration of the continent from a dynamic topography high and geoid low toward lower dynamic topography and a higher geoid.

Backstripping and apatite fission track analysis are used to constrain the Mesozoic vertical motion of the eastern Australian basins (Eromanga, Surat and Clarence‐Moreton). The backstripping results show that subsidence was linear during the Jurassic, and the rate of subsidence shows an overall increase (by a factor of about 2) towards the eastern margin. The Cretaceous section is well preserved only in the Eromanga Basin, and the backstripping results show that the apparent subsidence rate increased by a factor of 5–10 during the Early Cretaceous. The sediments show a lithological cyclicity which is the result of a variable influx of volcanogenic detritus from the convergent eastern margin. The rapid Cretaceous subsidence corresponds to a large influx of this volcanogenic material, resulting in progressively non‐marine deposition at a time when global sea‐level was rising.

The apatite fission track data suggest that the Cretaceous section was probably deposited over the Surat and Clarence‐Moreton Basins but has since been eroded off. The exhumation‐induced cooling may have commenced earlier in the eastern region (Late Cretaceous to Early Tertiary) and slightly later to the west (Early to Middle Tertiary). Furthermore, the inferred total amount of removed section is greater (˜2.5 km) in the east than in the west (<1 km). The present‐day thermal regime in the Eromanga Basin is considered to be a relatively recent (<10Ma) phenomenon, as non‐zero fission track ages are maintained in sediments currently at temperatures of ˜120°C.

Overall, the regional backstripping and apatite fission track results support a model of platform tilting, This is related to the inferred subduction along a convergent margin on eastern Australia during the Jurassic to Early Cretaceous. The cessation of subduction, and subsequent opening of the Tasman Sea in the Late Cretaceous, was accompanied by uplift on the eastern margin and the termination of widespread deposition on the platform.

The history of the African continent since the beginning of fragmentation of Pangaea is one of widespread rifting in the Late Jurassic to Early Cretaceous, extensive inundation under elevated Cretaceous sea‐levels, widespread epeirogenic uplift beginning in the Early Tertiary and off‐rift domal uplift associated with alkaline igneous activity in the Tertiary and especially in the Neogene. The cratonic basins have developed against this post break‐up background,

The cratonic basins of Africa are located between a relatively dense array of highspots and hotspots. Although there is a clear underlying structural fabric in the form of the Central African and West African rift systems, the present location of these basins cannot always be unequivocally linked to the presence of underlying rifts. The Congo Basin stands out as overlying a rigid rather than stretched lithosphere, and no reasonable density contrasts in the crust can explain the isostatic anomalies over the basin. Either there is a cold, dense region situated at depths typical of the upper mantle beneath the basin, or a downward‐acting dynamic force on the base of the lithosphere is necessary to explain the gravity field. Together, this points to the likelihood of a convective downwelling beneath the African plate under the Congo region.

The African cold‐spot model may be applicable to the Early Palaeozoic cratonic basins of North America. Their location and timing of development with respect to the adjacent plate margin, size and shape may be analogous to the African examples.

Two long‐standing discrepancies between observations and predictions from geophysical models for the evolution of intracratonic basins are: (1) thermal indicators, such as organic maturity, document higher basin temperatures than predicted by thermal conduction; and (2) periods of rapid/slow subsidence which deviate from the exponentially decreasing subsidence rate consistent with thermal contraction. A possible explanation for these problems is free thermal convection in the continental crust beneath the basin. Using a finite difference model for coupled fluid flow and heat transport, the Keweenawan rift beneath the Michigan basin is simulated as a plug of fractured igneous rock 10 km thick and 45 km wide. Overlying the igneous body and adjacent impermeable basement rocks in the model is 4 km of Proterozoic and Palaeozoic sediments of intermediate permeability. Model results indicate that during short‐lived (a few million years) periods of free thermal convection in the igneous body, temperature gradients in the overlying sediments can nearly double and rapid heat loss causes additional subsidence at the centre of the basin. Locally, additional tectonic subsidence can be more than 25 m. After fractures are sealed and free thermal convection slows or stops, basin subsidence is anomalously slow as the basement reheats back to conductive equilibrium.

Episodic tectonic subsidence can be seen from all backstripped wells from the Michigan Basin. These episodes are observed as periods of relatively rapid subsidence, contrasting with the relatively linear subsidence. Steady tectonic subsidence curves have been explained as being due to a decaying thermal anomaly in the underlying uplifted asthenosphere. Rapid episodes of subsidence require another explanation and prompt us to examine the tectonic history of surrounding areas for correlative events.

The Taconic orogeny has been inferred to have influenced the development of the Michigian Basin. To analyse the extent of coupling between the Appalachian foredeep and the Michigan Basin we have used Ordovician formation tops correlated among 172 wells distributed over the whole of Michigan's Lower Peninsula. The spatial variation of tectonic subsidence over this interval supports this suggestion.

Contour maps of decompacted sediment thickness and subsidence rate show substantial changes in form and magnitude during the development of the Tippecanoe I sequence. Early basin‐centred subsidence, during deposition of the Shakopee, St. Peter and Glenwood formations, was followed by tilting toward the Appalachian orogen concurrent with the early stages of the Taconic orogeny. This tilting continued, declining gradually, during deposition of the Black River, Trenton and Utica formations. Deposition during Tippecanoe I in the Michigan Basin may indicate modulation of continuing thermally driven subsidence by flexure of the lithosphere due to Taconic thrust loading, but the distinctive spatial distribution of subsidence over each interval may indicate that the important mechanisms making space available in the basin operated independently, not concurrently, invalidating the flexural model for the coupling and suggesting other regional, tectonic possibilities for models of basin initiation, evolution and filling.

The stress from the initial coalescence of Pangaea during the mid‐Carboniferous (320 Ma) collision of Gondwanaland and Laurussia in the Variscan (Sudetic) magmatic fold belt was transmitted through Pangaea to generate the nappes and thrusts that dismembered the intracratonic Centralian Superbasin during the Alice Springs Orogeny and the megakinks that terminally deformed the Lachlan fold belt along the subducted Andean‐type margin of eastern Australia. Definitive collision was followed by a lacuna on the Pangaean platform that reflects uplift. The first release of heat at ≅ 300 Ma from the self‐induced Pangaean heat anomaly weakened the hypersensitive neocratonic crust of the inactive but still hot European and eastern Australian magmatic fold belts to become stress guides for right‐lateral transtension during anticlockwise rotation of Pangaea. Deep transtensional fractures provided a way into and through the neocratonic crust for magma ranging from S‐type granite to rhyodacitic ignimbrite to basalt, with alkaline undersaturated rocks in some rifts. Lagging behind the magma, the platform subsided at ≅290 Ma in basins by differential weakening of the crust during the release of Pangaean heat. The Gondwana facies accumulated in the Gondwanaland province and the Stephanian‐Rotliegend succession in Europe. The basins of Europe and eastern Australia continued to grow by transtension followed by mid‐Permian (270–265 Ma) thermal sagging and rifting. Their histories then diverged. Europe maintained its post‐orogenic course except in the Alpine region, where rifting and sagging continued to accommodate the western Tethys. The eastern fringe of Australia entered a new, Innamincka, orogenic cycle that developed in embryo at 265 Ma to a fully developed magmatic arc and yoked foreland basin at 258 Ma.

Due to the frequently observed disparities between stretching amounts obtained from faults, crustal thickness and tectonic subsidence, the development of intracratonic basins cannot always be explained by a simple model of lithospheric extension. Basin evolution may then be regarded as the result of superimposed and successive processes.

The Paris Basin is chosen as a type example for the discussion of European intracratonic basins. The tectonic subsidence, reconstructed using a standard method, is small (maximum of 1600 m, mean rate of 8.5 mm/ka). The long‐term linear or concave curves are interrupted by periods of short‐term acceleration and deceleration. Thus, tectonic subsidence is clearly discontinuous and five phases, with duration of 20–60 Myr, constitute the long‐term Meso‐Cenozoic Subsidence. The boundaries and pattern of each phase are identical all over the basin. Subsidence is interpreted as the result of several modifications of the lithospheric structure. The uniform stretching model can explain the first Triassic to early 1, iassic phase, the only one with a concave‐upward trend (deceleration). The Jurassic to Cretaceous subsidence could be explained by superposing (1) a long‐term component caused by lower crustal flow and/or underplating and (2) several short‐term accelerations (convex‐upward trend) related to compressive or transpressive forces. Geophysical control is insufficient to test the first postulate accurately and the generation of sufficiently high compressional stresses during the Jurassic‐Cretaceous is questionable for the second.

Other Northwest European basins are compared with the Paris Basin. Although similar features can be observed, the overall image is not uniform: ‘Paris Basin’ (intracratonic) as well as ‘Celtic Sea Basin’ (passive margin) signatures, variable long‐term trends, lack of synchronism of subsidence phases. This picture necessitates different driving mechanisms for subsidence across Northwest Europe. The variable subsidence patterns and processes result from the ‘remote’ geodynamics of the Atlantic and Tethyan realms, combined with mainly ‘active’ processes similar to those proposed for the Paris Basin.