Terran
A terran (from the Latin terra "earth") is a crustal block of regional extent that has accumulated due to large tectonic shifts to another continent , to which it has a different geological history.
The concept arose from observations made by David Lawrence Jones of the US Geological Survey in the 1970s in the western Cordillera of North America, with the first observations being made in Alaska (1972). The name came from William Porter Irwin , and in 1977 Jones named one of the most famous Terrans Wrangellia . By the early 1980s, the image of the North American Cordilleras solidified as a collection of terranos on the Laurentia land mass.
properties
The crustal area of a terran is characterized by uniform petrography and an enclosing line structure of mostly inactive geological faults . A terran has a self-contained geological overall picture, which is determined by stratigraphy, fauna / flora, structures, age and type of metamorphosis , igneous rocks , metallogeny , paleomagnetics and geophysical properties. It is bounded on all sides by faults or thrusts that may contain rocks from a subduction trench such as Mélangen or Ophiolite . It can consist solely of a ceiling unit without crust roots.
Emergence
Terranes are mainly found within a mountain belt ( orogen ) of active continental margins ( subduction zones ) and differ from adjacent areas due to their differing lithology or mineral paragenesis . It is therefore assumed that terranos are allochthonous (non-local) fragments of other continents ( microplates ), island arches or deep sea mountains welded on by the plate tectonic movement . The crustal parts are not subduced, but sheared off from their base and pushed onto the continent ( autopsied ). In such cases, at the edge of the terran facing the sea, there is often a narrow border of autopsied, i.e. postponed seabed . A terran can travel a great distance before accretion . The North American Cordilleras consist to a large extent of such exotic crust blocks, in the Canadian Cordilleras and Alaska, terranes were first identified and scientifically investigated.
Types of Terrans
A distinction is made between growing terranes ( accretion terranes ) and group-forming superterranes (English: composite terranes ), the surrounding areas of which are characterized by different tectonic development.
Further differentiation options exist in the geological history of terrans:
- Exotic Terran: Drifted through an oceanic basin and contains an overall geological picture different from the region to which the terran was accreted. These differences cannot be explained by changing facies. Examples are micro-continents and island arcs.
- Proximal Terran: Detached from a continental margin and moved along the same continental margin for 100 km. The limiting lateral shift zone can be changed by the subsequent collision (e.g. covered by ceiling complexes; rotates during the collision; the original direction of movement is inverted; the lateral shift has partially changed into an upward or overtravel). A proximal terran can show an overall geological picture similar to the region it was relocated to.
A particular problem lies in the detection of proximal terrans, for which a minimum displacement of 100 km is specified, the extent of the displacement being difficult to grasp.
Occurrence
Well-known examples are the Canadian Cordillera , the East Siberian Primorsky region and smaller microplates in Western and Central Europe, such as the Hebridean Terran . Large parts of Western Europe consist of terranos that were displaced over distances of 1000 to 4000 km during the Upper Devonian and Lower Carboniferous on the northern edge of Gondwana . These terranes were welded to Laurussia during the collision of Gondwana with Laurussia in the Carboniferous, when the supercontinent Pangea was formed. They are therefore proximal in relation to Gondwana (e.g. Northwest Africa) and exotic in relation to Laurussia (e.g. British Isles - Northern Germany - Northern and Eastern Poland).
literature
- JD Keppie: Northern Appalachian terranes and their accretionary history. Geological Society of America, Special Paper, Vol 230 1989, pp 159-192
- MS Oczlon: Terrane Map of Europe. Gaea Heidelbergensis, Vol. 15, 2006
- David G. Howell: Terranes , Scientific American, Nov. 1985
- David G. Howell: Tectonics of suspect terranes , Chapman and Hall 1989
Web links
Individual evidence
- ^ Rolf Meissner, The Little Book of Planet Earth, Copernicus Books 2002, p. 199
- ↑ Biography of David Jones, Berkeley