Intraplate Basin Research Articles

Pre-Cretaceous Intraplate Basins of NE Africa

Pre-Cretaceous Intraplate Basins of NE Africa

Iberia Versus Europe--Effects of Continental Break-Up and Round-Up on Hydrocarbon Habitat: ABSTRACT

Based on the continuity of foldbelts and the positions of intermountain continental nuclei and transcontinental megashears, a close Pangea fit is proposed for the central and north Atlantic borderlands. The Variscan arch segment missing between Brittany and Galicia in the Gulf of Gascony (Biscaye) can tentatively be identified with the Flemish Cap block off Newfoundland. At the same time the northwest African-Gondwana border (central Morocco) was located some 800 km farther to the west-northwest, as compared to its present position in southwestern Europe (Iberia). During the opening of the central and northern segments of the Atlantic Ocean (Jurassic and Cretaceous) and during the closure of the western Mediterranean basin, i.e., the thrust of Africa toward southern Europe (Tertiary), the European continental mass underwent deformation in the transtensive and transpressive modes, which reactivated parts of its inherited structural network. The trailing south European continental margin was partially dismembered into loosely bound continental blocks, to be assembled again during the subsequent Alpine orogenic cycle. These events can be compared with processes known in the northernmost and western segments of the North American continent. Mechanisms are proposed for the formation and deformation of inter- and intraplate basins by way of moderate shifts (wrenching)more » and slight rotations, the direction of which changed during the Mesozoic-Tertiary according to the global stress field. The above evolution and mechanisms had multiple and decisive effects on hydrocarbon generation, habitat, and accumulation.« less

Dependence of the flexural rigidity of the continental lithosphere on rheology and temperature

While the first-order mechanical properties of oceanic and continental lithosphere are well defined and explain the observed increase in lithospheric flexural strength with thermal age, the temporal variation of continental flexural rigidity following loading, and its dependence on lithospheric composition and temperature structure, is only just beginning to be appreciated. We describe here the results of a quantitative Theological model in which laboratory-determined rock deformation data and transienttemperature distributions, resulting from intra-plate basin formation within continental interiors, have been integrated. Continental flexural rigidity is shown to be controlled by crustal thickness, lithospheric thickness through the temperature structure, and the interaction of these factors during basin formation. For thermally young lithosphere, the flexural rigidity is dominated by the quartzo-feldspathic theology of the crust, while for the thermally older lithosphere, it is dominated by the olivine rheology of the mantle. Flexural data, through thermo-mechanical models of lithospheric deformation, provide a powerful constraint on both the transient and steady-state temperature structure of the lithosphere.

A thermo-mechanical model of continental lithosphere

An understanding of the mechanical response of the continental lithosphere to vertical surface loading is crucial when considering geodynamic processes such as intraplate basin formation, postglacial rebound or mountain building. Previous models of the mechanical behaviour of the lithosphere, which are more successful in oceanic than in continental regions, have used specific isotherms to delineate an effectively elastic zone of the lithosphere and result in mathematically convenient descriptions of mechanical behaviour. Heat flow and lithosphere flexure data for continents, however, suggest that this relationship is more complex, which can be demonstrated by numerical modelling of the loading of a heterogeneous visco-elastic lithosphere with temperature-dependent viscosity. Viscous stress relaxation in the lower and hotter section of the lithosphere results in a region of stress concentration dependent on the thermal state of the lithosphere, its structure and the time since loading. Here we present initial numerical models, based on a relatively simple steady-state thermal model constrained by present-day heat flow, which explain observations of continental flexure except in regions which have experienced a long or complex thermal history since loading.

The North China Basin: An example of a Cenozoic rifted intraplate basin

The North China basin was a stable continental region that has undergone several distinct phases of rifting and subsidence during the Mesozoic and Cenozoic eras. The last major phase, featuring block faulting, rapid subsidence and widespread calc‐alkaline basaltic volcanism, began during early Tertiary time. This intraplate rifting appears to be the result of an approximately 30% extension and thinning of the lithosphere beneath the northeastern China. By the late Tertiary period, active rifting had slowed and postrift thermal subsidence had begun. This regional extension and subsidence resulted in the present day North China basin, a large saucer‐shaped, oil producing basin with thinner crust and lower upper mantle P and S wave velocities than the surrounding regions. During the Quaternary period, however, the subsidence rate increased in the North China basin. The frequent occurrence of destructive earthquakes and observations of relatively high heat flow suggest that the Quaternary tectonic activity differs from the late Tertiary simple thermal subsidence pattern and may indicate a new phase of rifting.

Variations in fluvial style in the lower Cenozoic synorogenic sediments of the Canadian Arctic Islands

The Eureka Sound Formation (Maastrichtian to Eocene or Oligocene) occupies seven intraplate basins in the Arctic Islands and equivalent beds form part of the continental-margin wedges flanking the Arctic Ocean and Baffin Bay. Shallow-marine, deltaic, fluvial and lacustrine facies are present. Fluvial assemblages can be subdivided into: (1) assemblage F: cyclic, generally fine-grained deposits with point bar crossbedding, deposited by high-sinuosity rivers; (2) assemblage G 1: noncyclic sandy sequences of Platte braided type; (3) assemblage G 2: cyclic sandy and conglomeratic assemblages of Donjek type; (4) assemblage H: laminated sandstone with minor crossbedding—ephemeral Bijou Creek or Malbaie type; and (5) assemblage J: cobble and boulder conglomerate deposited on alluvial fans—Scott type. Variations in fluvial style within and between basins are related to local tectonic and, possibly, climatic effects. Adjacent to active faults bounding two of the basins the deposits form coarsening-upward megasequences, showing transitions from assemblages E (deltaic) or F to G and H, and culminating in assemblage J. These sequences document the progradation of a braidplain and alluvial-fan complex in front of rising fault-bounded uplifts. Only at Strathcona Fiord in central Ellesmere Island does coal comprise a significant part of the formation. There the sediments also contain a rich vertebrate fauna. This and other broad facies variations may reflect local climatic differences generated by the rugged topography.

Geologic Evolution of Petroliferous Basins on Continental Shelf of China

The coastline of southeastern China is about 18,000 km (11,200 mi) in length, and its aggregate continental shelf area within 200-m (660-ft) water depth is well over 1 million km2 (390,000 mi2). Recent geophysical exploration and petroleum drilling records aid in understanding the geologic evolution of these petroliferous basins. Two types of tectonic basins are present on the continental shelf areas: (1) Bohai Gulf, South Yellow Sea, and Beibu Gulf are intraplate polyphase rift-depression basins, and (2) East China Sea, mouth of the Pearl River, and the Yingge Sea are epicontinental rift-depressions basins. Both types are believed to be of extensional origin. The severe convergence of the Indian plate with the Eurasia plate produced east-northeast-s reading of the South China Sea basin, which resulted in two triple junctions on its northern margins. The Pacific plate was subducted by downthrust beneath the Eurasia continental crust. The extension mechanism could be the rising of an upper mantle plume to produce two weak north-northeast-trending fracture zones. A series of intraplate and epicontinental rift-depression basins was formed. The depositional models and sea level variations of these basins have been interpreted from drilling records and seismic profiles. They can be explained by the tectonoeustatic changes in sea level and Cenozoic climatic changes in China.

Closing of the Protocadic Ocean and intraplate basins

THE presence of several long-lived, deeply subsiding basins, autogeosynclines1, such as those in Michigan, Illinois, North Kansas and Williston suggests that the North American craton or plate remained in a constant position relative to the underlying mantle during the Palaeozoic.