Himalaya tectonostratigraphic zones for Wiki

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, tectonostratigraphy is

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

that refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets, or nappes, in areas of

thrust tectonics Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the Tectonics, tectonic processes associated with, the shortening and thickening of the Crust (geology), crust or lithosphere. It is one of the three main ...

or to the effects of tectonics on lithostratigraphy.

Tectonically formed stratigraphy

One example of such a tectonostratigraphy is the Scandinavian Caledonides. Within the entire exposed 1800 km length of this orogenic belt the following sequence is recognised from the base upwards: * Autochthon : undisturbed foreland of the Baltic plate * Parautochthon : thrust sheets that have moved only a short distance (up to tens of kilometres) from their original position * Lower

allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a window. A klippe is a solitary outcrop of the nappe in the middle of autochthonous material. An allochthon, or an alloc ...

: far travelled thrust sheets derived from the Baltic plate passive margin, mainly sediments associated with the break-up of

Rodinia Rodinia (from the Russian родина, ''rodina'', meaning "motherland, birthplace") was a Mesoproterozoic and Neoproterozoic supercontinent that assembled 1.26–0.90 billion years ago (Ga) and broke up 750–633 million years ago (Ma). wer ...

* Middle allochthon : also derived from the margin of the Baltic plate,

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

basement and its psammitic cover * Upper allochthon : thrust sheets including

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

and ophiolitic sequences * Uppermost allochthon : thrust sheets containing sediments with fossil assemblages indicating an origin on the margin of the Laurentian plate This vertically stacked sequence thus represents the passive margins of Baltica and Laurentia and intervening island arcs and

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

s telescoped together and emplaced on top of the

Baltic Shield The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's crust belonging to the East European craton, East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed ...

, involving hundreds of km of shortening. Within this overall stratigraphy the individual layers have their own tectonostratigraphy of stacked thrust sheets.

Effects of active tectonics on lithostratigraphy

Tectonic events are typically recorded in sediments being deposited at the same time. In the case of a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

, for instance, the sedimentary sequence is normally broken down into three parts: * The ''pre-rift'' includes a sequence deposited before the onset of rifting, recognised by the lack of thickness and sedimentary facies changes across the rift faults. * The ''syn-rift'' includes a sequence deposited during active rifting, typically showing facies and thickness changes across the active faults,

unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

on the fault footwalls may pass laterally into continuous conformable sequences in the hanging walls. * The ''post-rift'' includes a sequence deposited after the rifting has finished, it may still show thickness and facies changes around the rift faults due to the effects of differential compaction and remnant rift topography, particularly in the earliest part of the sequence. This relatively straightforward nomenclature may become difficult to use, however, in the case of multiphase rifting with the post-rift from one event being the pre-rift to a later event.

References

{{reflist Stratigraphy Tectonics

Tectonically formed stratigraphy

Effects of active tectonics on lithostratigraphy

See also

References