Foreland basin

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Foreland basins are sedimentary basins that are in front of orrogens . They are tectonically related and occur in the foothills by sinking a continental crustal section in front of the ballast of advancing orogenic ceiling front .

history

Foreland basin, engl. foreland basins , have been known for more than 110 years and were also referred to as such in classical geosyncline theory . Kay still calls them exogeosynclinals in his work from 1951 .

In plate tectonics , the term foreland basin was taken up again in 1974 by WR Dickinson in the scientific literature and later elaborated by C. Beaumont. He should, however, from the concept of pre-depth ( Franz Eduard Suess , 1909), engl. foredeep , as it had been used by Miall and Bally in 1981, must be separated because the foredeep is only a section of the foreland basin.

definition

The Persian Gulf, a peripheral foreland basin

Foreland basins are tectonically caused lithospheric depressions on or relatively close to the continental margin. They are located on continental crust, which is subducted in the course of a collision or driven over or pushed back by an approaching orogen front.

description

Foreland basins are asymmetrical in cross-section; they have their deepest point in the immediate vicinity of the deferred belt of folds or the thrust front. Their spatial location does not remain fixed, however, but rather moves visibly towards the interior of the continent over time.

Foreland basins are created by a flexure in the continental lithosphere , which is due to the loading of the advancing orogen. A direct consequence of the lithospheric flexure is the bulging of the continental crust behind its break point. A so-called forebulge is created , which can emerge to the surface and then divide the foreland basin. The so-called backbulge , a rather shallow sedimentary basin, lies towards the craton of the forebulge . The foredeep follows in the direction of orogen - the deepest part of the foreland basin - and the somewhat shallower wedge-top , which is already located above the accretion wedge.

The temporal development of foreland basins is thus directly linked to the advance of the orogenic front of the emerging mountains . The structure of some basins can be severely disturbed by this or even disappear completely under tectonic blankets. Conversely, the sedimentary sequence in the foreland basin provides valuable information on the development of the orogen. For example, on the basis of the foreland basin sediments (by means of crevice traces and closure temperatures of minerals), time-temperature-pressure paths (tTp paths) for the orogen can be obtained.

Sedimentation in foreland basins

The sediment input into the foreland basins comes from two very different erosion areas (continental shelf sediments on the one hand and orogenic erosion debris on the other). This has a direct effect on the arrangement of the individual facies spaces within the pelvis, which are generally aligned parallel to the longitudinal axis of the pelvis. However, transversal faults and lineaments in the underlying basement can change this somewhat simplistic scheme and break up the foreland basin into several lower basins. In some foreland basins, sediment can also be discharged sideways (example: two-stream lowland - Persian Gulf).

The onset of sedimentation is often preceded by a hiatus .

In terms of time, the sediment input can be roughly divided into two stages: an initial underfilled basin stage and a final overfilled basin stage .

During the underfilled stage, turbidites , hemipelagic claystones and carbonates of the shelf area are mostly deposited, while continental molasse sediments clearly predominate during the overfilled stage . The foreland basin can ultimately silt up completely and be filled with alluvial fan sediments .

Pool types

From a genetic point of view, there are two very different types of foreland basins:

  • Peripheral foreland basins.
  • Foreland basins located behind intrusive arches.

Peripheral foreland basins arise in the course of a continental collision directly on the continental margin, which descends directly in front of the orogen front in an A subduction zone . In retro-arc foreland basins located behind intrusive arches , the thrust front lies behind an arch of igneous intrusive rock that is located much further in the interior of the continent (cordillera type). This type is coupled to a B subduction zone , i. H. a collision with oceanic crust that leads to the formation of an igneous island arc . The thrust front, which already runs quite far inside the craton, can be compared to a back thrust . The retro-arc type can have two very different foreland basins, on the one hand a very flat, thin-skinned basin, on the other hand a broken basin, which is interspersed with towering basement blocks. The thin-skinned basins are coupled at a steep subduction angle, the fractured basins at a relatively shallow angle.

Examples

The Ganges foreland basin with the Himalayas pushing over from the north

The most impressive example of a peripheral foreland basin is the Indus - Ganges - Brahmaputra lowlands in the south of the Himalayan shift. As a result of the continental collision between the Eurasian Plate and the Indian subcontinent, the Himalayas have lifted themselves out of several thrust systems ( Main Boundary Thrust , Main Central Thrust etc.) by 18 to 20 kilometers since the Middle Miocene , which corresponds to a rate of 70 centimeters per thousand years . The Siwaliks , which consist of more than 5 kilometers of sediment, arose in its foreland basin to the south . The Siwaliks are characterized by their constant migration to the south with internal thrusts and constant sediment shifting.

Further examples of peripheral foreland basins are Mesopotamia and the Persian Gulf ( Zagros thrust ), the Molasse basin north of the Alps , the Po - Adria basin, the East Carpathian basin , the Aquitaine basin and the Ebro basin of the Pyrenees .

Examples of the Cordilleras are the basins on the eastern edge of the Rocky Mountains , which later gave way to the Western Interior Seaway , as well as the basins on the Andean eastern beach ( e.g. the Sierras Pampeanas are a broken basin).

In Paleozoic foreland basin of the Lower came Old Red Sandstone of southwestern Britain , the Silurian - Carboniferous the central Appalachians , the Upper Carboniferous of the Central European Rhenoherzynikums and possibly the Upper Carboniferous of the Ouachitas for deposition.

Individual evidence

  1. ↑ M. Kay: North American geosynclines . In: Mem. Geol. Soc. At the. tape 48 , 1951, pp. 143 .
  2. ^ WR Dickinson: Plate tectonics and sedimentation . In: Tectonics and Sedimentation. Spec. Publ. Soc. Econ. Paleont. Miner. tape 22 , 1974, p. 1-27 .
  3. ^ C. Beaumont: Foreland basins . In: Geophys. JR Astr. Soc. tape 65 , 1981, pp. 291-329 .
  4. ↑ AD Miall: alluvial sedimentary basins: tectonic setting and basin architecture. Sedimentation and Tectonics in Alluvial Basins . In: Spec. Pap. Geol. Ass. Can. tape 23 , 1981, pp. 1-33 .
  5. ↑ AW Bally: Thoughts on the tectonics of folded belts. Thrust and Nappe Tectonics . In: Spec. Publ. Geol. Soc. Lond. tape 9 , 1981, pp. 13-32 .
  6. ^ AB Hayward,: Sedimentation and basin formation related to ophiolite nappe emplacement. Miocene SW Turkey . In: Sedimentary Geology . tape 40 , 1984, pp. 105-129 .
  7. ^ RJ Weimer: Relation of unconformities, tectonics and sea level changes, Cretaceous of the Denver basin and adjacent areas. Mesozoic Paleogeography of the west-central United States . In: Spec. Publ. Soc. Econ. Paleont. Miner. Rocky Mtn. Section . 1983, p. 359-376 .

literature

  • M. Leeder: Sedimentology and Sedimentary Basins. From Turbulence to Tectonics . Blackwell Science, 1999, ISBN 0-632-04976-6 .
  • HG Reading: Sedimentary Environments and Facies . Blackwell Scientific Publications, 1978, ISBN 0-632-01223-4 .