TETHYS

one of the eight large, regular satellites of Saturn. Essentially composed of pure water ice, Tethys has a diameter of 1,060 km (657 miles). It orbits Saturn at a distance of 294,660 km (182,689 miles) and is involved in an orbital resonance with Mimas such that it completes precisely one orbit for every two of Mimas' orbits. Tethys possesses two noteworthy features. The first of these is a long crack extending along three-quarters of the satellite's circumference and forming 5 to 10 percent of its surface. It is theorized that the crack was produced by freezing and expansion of the water that composes the satellite's interior. The second notable feature is a crater that measures 400 km (250 miles) in diameter and has a large central peak. former equatorial ocean that is believed to have separated the former supercontinent of Laurasia in the north from Gondwana, or Gondwanaland, in the south during much of the Mesozoic Era (245 to 66.4 million years ago). Laurasia consisted of North America and Eurasia north of the Alpine-Himalayan mountain ranges, while Gondwana consisted of South America, Africa, peninsular India, Australia, Antarctica, and those Eurasian regions south of the Alpine-Himalayan chain. The continental convergences and collisions that eventually eliminated Tethys created the Alpine-Himalayan mountain belt. Tethys was named in 1893 by the Austrian geologist Eduard Suess after the sister and consort of Oceanus, the ancient Greek god of the ocean. The presence of a former Mesozoic marine realm in the place of the present Alpine-Himalayan chain of mountain systems was recognized late in the 19th century by the German geologist Melchior Neumayr on the basis of the distribution of large thicknesses of marine (and in places fully oceanic) sedimentary rocks and common faunal affinities of their fossils from the Alps and Carpathians through Turkey and Iran all the way to the Himalayas and Burma (Myanmar). Suess interpreted this marine realm as a former ocean in 1893. Later, in the first half of the 20th century, this marine realm was mistakenly interpreted as a long-lived geosynclinei.e., a narrow, elongate, subsident trough in the Earth's crustthat had become subject to orogenic (mountain-building) phases since the middle Mesozoic and the remnants of which now form the Mediterranean Sea. With the rise of the theory of plate tectonics and its application to historical geology, it was recognized that the concept of the geosyncline was incorrect, that Tethys must have been an ocean rather than a trough, and that it can only have existed since the formation of Pangaea (by the collision of Laurasia and Gondwana) during Permian-Triassic times. Studies in the late 20th century revealed that at least two Tethyan oceans may have successively occupied the area between Laurasia and Gondwana during the Mesozoic Era. The earlier, called the Paleo (Old) Tethys Sea, came into existence along with Pangaea late in the Paleozoic Era (about 320 million years ago). During the Permian and Triassic periods (286 to 208 million years ago), Paleo Tethys formed an enormous, eastward-opening, oceanic embayment of Pangaea. This ocean was eliminated by the detachment (from northern Gondwana) and northward rotation of a strip of continental material called the Cimmerian continent that eventually collided with the southern margin of Laurasia in the Early Jurassic Period (208 to 187 million years ago). The vestiges of the Paleo Tethys Sea are now preserved in the mountain ranges of northern Turkey, Transcaucasia (the Caucasus and the Pamirs), northern Iran and Afghanistan, and northern Tibet (Kunlun Mountains) and reach into China and Indochina. The Neo (Younger) Tethys Sea, or simply the Tethys Sea, began forming in the wake of the rotating Cimmerian continent in the earliest part of the Mesozoic Era. The breakup of Pangaea into Laurasia (north) and Gondwana (south) by continental drift and the concurrent growth of the Atlantic and Indian oceans resulted in the gradual elimination of Tethys by the convergence and eventual collision of the fragments of former Gondwana with Eurasia. Tethys was finally closed in the Cenozoic Era (66.4 million years ago to the present) when India, Arabia, and Apulia (consisting of parts of Italy, the Balkan states, Greece, and Turkey) finally collided with the rest of Eurasia to erect the modern Alpine-Himalayan ranges, which extend from Spain (the Pyrenees) and northwest Africa (the Atlas) along the northern margin of the Mediterranean Sea (the Alps, Carpathians) into southern Asia (the Himalayas) and reach Indonesia. The eastern part of the Mediterranean Sea is a remnant of the Tethys Sea. The effects of the closure of the Tethys Sea had so severely defaced those of earlier closures that the prior existence of the Paleo Tethys Sea was not recognized by geologists until the 1980s. Another important effect of the evolution of the Tethys Sea was the formation of the giant hydrocarbon-bearing basins of North Africa and the Middle East.