Triassic
The Triassic is a geologic period and a stratigraphic system that spans 50.5 million years from the end of the Permian Period 251.902 Ma to the beginning of the Jurassic Period 201.4 Ma. The Triassic Period is the first and shortest geologic period of the Mesozoic Era, and the seventh period of the Phanerozoic Eon. The start and the end of the Triassic Period featured major extinction events.
Chronologically, the Triassic Period is divided into three epochs: the Early Triassic, the Middle Triassic, and the Late Triassic. The Triassic Period began after the Permian–Triassic extinction event that much reduced the biosphere of planet Earth. The fossil record of the Triassic Period presents three categories of organisms: animals that survived the Permian–Triassic extinction event, new animals that briefly flourished in the Triassic biosphere, and new animals that evolved and dominated the Mesozoic Era. Reptiles, especially archosaurs, were the chief terrestrial vertebrates during this time. A specialized group of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period. Archosaurs that became dominant in this period were primarily pseudosuchians, relatives and ancestors of modern crocodilians, while some archosaurs specialized in flight, the first time among vertebrates, becoming the pterosaurs. Therapsids, the dominant vertebrates of the preceding Permian period, saw a brief surge in diversification in the Triassic, with dicynodonts and cynodonts quickly becoming dominant, but they declined throughout the period with the majority becoming extinct by the end. However, the first stem-group mammals, themselves a specialized subgroup of cynodonts, appeared during the Triassic and would survive the extinction event, allowing them to radiate during the Jurassic. Amphibians were primarily represented by the temnospondyls, giant aquatic predators that had survived the end-Permian extinction and saw a new burst of diversification in the Triassic, before going extinct by the end; however, early crown-group lissamphibians also became more common during the Triassic and survived the extinction event. The earliest known neopterygian fish, including early holosteans and teleosts, appeared near the beginning of the Triassic, and quickly diversified to become among the dominant groups of fish in both freshwater and marine habitats.
The vast supercontinent of Pangaea dominated the globe during the Triassic, but in the latest Triassic and Early Jurassic it began to gradually rift into two separate landmasses: Laurasia to the north and Gondwana to the south. The global climate during the Triassic was mostly hot and dry, with deserts spanning much of Pangaea's interior. However, the climate shifted and became more humid as Pangaea began to drift apart. The end of the period was marked by yet another major mass extinction, the Triassic–Jurassic extinction event, that wiped out many groups, including most pseudosuchians, and allowed dinosaurs to assume dominance in the Jurassic.
Etymology
The Triassic was named in 1834 by Friedrich August von Alberti, after a succession of three distinct rock layers that are widespread in southern Germany: the lower Buntsandstein '', the middle Muschelkalk and the upper Keuper.Dating and subdivisions
On the geologic time scale, the Triassic is usually divided into Early, Middle, and Late Triassic Epochs, and the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are:Paleogeography
At the beginning of the Triassic, all the major continents were amalgamated into the supercontinent of Pangea. Centred on the equator, this stretched in an arc from the north to south polar regions with Laurussia in the north and Gondwana in the south. The Paleo- and Neo-Tethys oceans lay within the arc of the supercontinent with the vast Panthalassa Ocean beyond. North China and Amuria, and South China were separated from Pangea by the Paleoasian Ocean, but this closed by the Late Triassic.Pangea was surrounded by subduction zones that dipped beneath the supercontinent. The great mountain ranges that marked the Late Paleozoic continental collisions were largely eroded and were being replaced by regions of thinned crust that lay along the lines of the future Atlantic, Indian and Southern oceans.
The supercontinent changed motion from drifting westward to rotating counterclockwise during late Permian. This continued until the Carnian, after which it resumed the westward motion. These changes in motion were triggered by the opening of the Neo-Tethys, and closing of the Paleo-Tethys respectively, and affected tectonic regimes particularly along the southern and western margins.
The narrow Cimmerian terranes that had rifted from the northern margin of Gondwana in the Permian continued to drift northwards; the Paleo-Tethys Ocean closing in front of them and the Neo-Tethys opening behind.
Eruptions of the Siberian Traps Large Igneous Province persisted into the Early Triassic and the Central Atlantic Magmatic Province were active by the Late Triassic as a prelude to seafloor spreading in the Central Atlantic at the boundary of the Triassic and Jurassic.
Northern Pangea (Laurussia)
In northern-eastern Pangea, the Siberian Traps LIP continued to erupt into the Middle Triassic. Lower Triassic major deltaic systems advanced across the shallow Arctic Ocean. Their catchment areas the high ground of the Urals, Fennoscandinavia, the Canada-Greenland Shield and the Lomonosov High. During the Late Triassic, in response to the opening of the Central Atlantic to the south, tectonic movements between North America and Baltica led to convergence in the High Arctic with uplift, folding and thrusting in the Barents Sea and North Siberian margin.Major extensional tectonic movements, that began in the late Permian, continued across Laurussia from North/Central Europe in the east to eastern Canada in the west, with north-south trending graben systems developing across Europe, including further subsidence in the Northern and Southern Permian basins. Lower Triassic sediments deposited in these basins are continental to brackish marine in nature. They are overlain by shallow marine carbonates and mudstones and evaporites of the Zechstein Sea.
Further south, during the Norian, the opening of the Central Atlantic led to the formation of narrow, deep water basins in the Eastern Mediterranean area. Corsica, Sardinia, Calabria, and the Balearic terranes were attached to Europe, whilst Apulia, Adria, and the terranes of southern Turkey remained attached to the African plate.
Along the western margin of Laurussia, a continental magmatic arc extended from the southwestern United States to the Arctic with an associated continental-scale foreland basin system. Localised back-arc basins developed within this, in regions of extension. Along the northern section of the margin, the Permian to Early Triassic counterclockwise rotation of Pangea resulted in the closure of the narrow Slide Mountain Ocean and the Sonoma orogeny. In south, it led to a reduction in the dip of the subducting slab and thickening of the continental crust.
Southern Pangea (Gondwana)
Much of Africa was stable and above sea level, with only a few Triassic-aged lake sediments known, although along the northern coast marine sediments were deposited during periods of higher sea levels.Northeast-southwest trending rifting along the eastern edge of Africa between Madagascar and eastern Africa continued from the Late Carboniferous into the Triassic, with substantial mountains rising along the edge of the rift and the formation of a series of pull-apart basins. Gradual marine incursions from the Neo-Tethys resulted in the deposition of Lower and Middle Triassic marine sediments in these basins. Upper Triassic sediments are continental in nature and this rift system ultimately failed and a new north-south trending rift system developed in the Jurassic.
The opening of the Neo-Tethys created passive margins along the Arabian and Indian margins. Rifting in the Neo-Tethys extended westwards between the Pontides and Taurides terranes of Turkey during the Late Triassic. The Tethyan Himalayan block remained attached to India but was separated by the thinned crust of Greater India, the northern margin of which supplied sediments to the passive margin.
The counterclockwise rotation of Pangea precipitated dextral transpression across the NNE-SSW trending, west-dipping subduction zone, along the eastern Australian margin, which culminated in the Hunter-Bowen orogeny. Following this, the magmatic arc rotated to north-south and compression gave way to extension. This was accompanied by subduction rollback and back-arc basin formation.
Along the southwestern margin of South America, low plate convergence and subduction rates, triggered by the assembly and rotation of Pangea, resulted in subduction rollback and extension across the back-arc region. This generated large amounts of felsic magmatism. These extensional forces stretched across the continent with the formation of large northwest-trending basins with thick sedimentary deposits and the extension related magmatism.
Closure of the Paleo-Tethys
The Paleo-Tethys ocean formed as the continents surrounding it assembled to form Pangea in the Late Palaeozoic. The Eurasian sector of Pangea lay along its north and northwestern margin. To the northeast, the narrow Paleoasian Ocean lay between Eurasia, and North China and Tarim, and to the east, South China and Annamia. To the south were the Cimmerian terranes, Lhasa, and Sibumasu. These terranes had rifted from northeastern Gondwana during the Permian. As they drifted northwards through the Triassic, the Paleo-Tethys closed in front of them, and the Neotethys opened behind.The Paleo-Tethys was being consumed by subduction zones along the southern margin of North China, much of the Eurasian margin, and along the northern margin of the Qiangtang-Annamia and Lhasa-Sibumasu blocks.
Collisions between Annamia and South China ; between Sibumasu and South China–Annamia ; and, between Qiangtang and Lhasa resulted in the Indosinian orogeny and the formation of a single large Eastern Asian continent. At about the same time, the final closure of the Paleoasian Ocean led to the collision of Tarim and North China with the Kazakhstan and Siberian regions of Pangea, to form the Central Asian orogenic belt. South China collided with North China, forming the Central China orogenic belt. The segment of the Paleo-Tethys between North China and Qiangtang may never have fully closed, but was filled with Permo-Triassic turbidites preserved in the West Kunlun and Bayanhar belts of the Central China orogenic belt.
The amalgamation of these East Asian blocks with Pangea in the Late Triassic maximised the land area of the supercontinent. It coincided with a period of dramatic climate change and the development of the megamonsoon, although the relationship between these is the subject of ongoing research.
The western Paleo-Tethys remained open until about 205 Ma, when the Iranian blocks collided with the Turan platform, on the southern margin of Eurasia, resulting in the Cimmerian orogeny. This extended from the Anatolian Plateau in the northwest, and merged with the Indosinian orogenic belt in the east. Late Triassic deformation across the Eastern Mediterranean area and much of the Middle East was complex, with regional scale strike-slip faulting and continued subduction below the Iranian margin.