Basement
Basement (according to the English term of art also Basement period) is the designation for the geologically old, at least one mountain formation (Orogenese) detected and thus always folded , partly highly metamorphic crust areas of continental blocks.
Etymology and word meanings in the narrower sense
The word component "mountains" goes back to the mining term for rock . Therefore, the word component "ground" is because the corresponding rocks below (therefore basement , engl. For, basement ') of the deposits of geologically young sedimentary basins that are not from a mountain formation were detected (Orogenese) and folded and as corresponding to the overburden designated will be encountered. But this is only one of several geological contexts in which we speak of basement today.
Another characteristic of a basement complex in the narrower sense is that the plate tectonic regime in which it was folded no longer exists - it is an old, inactive orogen . In this sense, there are rock complexes as a basement to be addressed
- beneath relatively thin sediment layers that were deposited on the tectonically stable areas of the continents over the past 500 million years. These areas are referred to as continental tables or continental platforms (e.g. the Eastern European platform ).
- on the earth's surface in the form of continental shields . These are also tectonically stable, very old basement complexes, but in fact they have never been overlaid by sediments since their formation (one speaks in this context of "consolidation") (e.g. Baltic or Canadian shield ).
- on the surface of the earth in the form of raised clods , the overburden of which was removed again as a result of uplift (e.g. parts of the Variscan basement , which today form numerous low mountain ranges in Central Europe, including the Vosges , Black Forest , Hunsrück , Taunus , Harz or Ore Mountains ). These broken clod mountains from old basement are also called basement upheavals. Because the fracture tectonic reactivation of the old crust takes place in a different plate tectonic regime than the previous folding of the basement units, the definition of inactive orogen remains fulfilled.
Word meanings in the broader sense
In the very young sedimentary basins, the formation of which is closely linked to the Alpine mountain formation , such as As the Vienna basin can, in principle, that geologically young, alpidisch folded rocks that underlie these basins as a basement or basement are called. In such constellations, older folded rocks are overlaid by younger unfolded layers, but the plate tectonic regime in which the Alps were formed still exists. The Alps and all other alpine mountain ranges (Pyrenees, Carpathians, etc.) are part of an active orrogenic belt . The openly emerging alpine folded rocks are therefore not to be referred to as basement in this context .
Older, demonstrably pre-deformed rock complexes within orogens are also regarded as bedrock or basement for the corresponding orogen . This is due to the fact that these units actually once formed the base of a sedimentary basin in which the younger rocks, for which no older folding is detectable, had been deposited. This sedimentary basin was subsequently folded together with its basement. In these cases the criterion of inactive orogen is again fully met because the new folding of the basement occurred in a different tectonic tension field than the previous one. In contrast to all other basement terms, however, the overburden is also folded here. In this sense, theoretically every basement complex (in the narrower sense) itself contains at least one generation of “older basement”. Examples of such “inner-orogenic” basement units include: a. the Cadomian pre-deformed units of the Variscan basement of Central Europe and the Variscan pre-deformed complexes in the Alps, e.g. B. the Mont Blanc massif .
Others
If the overburden is preserved, the direct contact between the basement and the overburden in the outcrop is often characterized by an angular discordance .
The oldest basement mountain complexes are the cores of today's continents, the so-called cratons . In the Canadian Shield , rocks with ages of 4,280 mya are known ( Nuvvuagittuq greenstone belt ) and in the Yilgarn craton of Western Australia a zircon grain that is even 4,400 mya was found. However, this is a so-called detritic zircon, i.e. In other words, the rock from which it was removed is a sedimentary rock in which it was embedded in what is known as a “secondary deposit” after the erosion of its actual igneous rock of origin . So it is only an indirect indication that the Yilgarn Kraton is one of the oldest basement complexes on earth.
Since they can contain indications of “petroleum traps” in the unfolded surface layers, the structure, inclination and depth of the bedrock of sedimentary basins are of particular interest in the exploration of hydrocarbon deposits . The exploration can be carried out using geoseismics and gravimetry , or geoelectrics if the depth is shallow .
Individual evidence
- ↑ Jonathan O'Neil, Richard W. Carlson, Don Francis, Ross K. Stevenson: Neodymium-142 Evidence for Hadean Mafic Crust. Science. Vol. 321, 2008, p. 1828, DOI: 10.1126 / science.1161925
- ↑ Simon A. Wilde, John W. Valley, William H. Peck, Colin M. Graham: Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature. Vol. 409, 2001, p. 175 ( PDF 197 kB)
literature
- Heinrich Bahlburg, Christoph Breitkreuz: Fundamentals of geology. 2nd Edition. Spectrum, Heidelberg 2003, ISBN 3-8274-1394-X
- Frank Press, Raymond Siever: General Geology. 3. Edition. Spectrum, Heidelberg 2003, ISBN 3-8274-0307-3