Paleozoic events in the Altaids

The Altaids constitute a large and complex tectonic collage that accreted around the Angaran platform from late in the Proterozoic to early in the Mesozoic Era. Its oldest part, the Baikalides, formed between about 850 and 570 million years ago along the southern periphery of the Angaran platform. A number of island arcs and microcontinents were accreted onto Angara along a suture containing ophiolitic remnants of old ocean floor.

After the Baikalian collisions, rifting outboard of the accreted fragments opened a new oceanic area, the floor of which had begun subducting under the enlarged continental nucleus in early Paleozoic time—perhaps during the Ordovician Period (about 485 to 444 million years ago). That subduction accumulated a large accretionary prism (wedge of deformed and partially metamorphosed sediments and rocks scraped from the ocean floor as it subducted) consisting of deep-sea muds (now slates), sandstones (deposited by large submarine turbidity currents), and siliceous sedimentary rocks (cherts) that were all structurally mixed with ophiolites (fragments of oceanic crust). Those rocks now form the basement of much of the Altai Mountains. Much subduction-related magmatism was associated with the growth of the Altai accretionary prism. Another accretionary prism was growing at the same time in the ocean, far from the Altai, and that material now forms the basement of much of Kazakhstan. It was consolidated and made into a small continent by repeated deformation and magmatism throughout the early Paleozoic.

The later Paleozoic development of the Altaid tectonic collage included the convergence and final collision of the Kazakhstan continental block with the enlarged Angaran nucleus during the middle of the Carboniferous Period (about 320 million years ago). The collision occurred along the southwestern Altai suture, the northerly continuation of which is now buried under the younger Mesozoic deposits of the West Siberian Plain. To the east it continues into Mongolia and there unites with the circum-Altaid suture zone coming from the west—i.e., from the Tien Shan. Another Carboniferous collision in the Tien Shan welded the North Tarim fragment to the Altaid collage. Shortly afterward, in the early Permian Period (about 290 million years ago), north-plunging subduction along the present-day Kunlun Mountains—which originally lay flush to the south of the North Tarim fragment—rifted open the Junggar (Dzungarian) and Tarim basins. Those are analogous in their tectonic setting to the present-day Sea of Japan (East Sea).

The Altaid evolution came to an end in the west when the Russian platform collided with Asia along the Ural Mountains between the Arctic Ocean and the Aral Sea. That collision occurred during the Carboniferous Period (about 359 to 299 million years ago) in the south but later—during the Permian Period (299 to 252 million years ago)—in the north, creating the supercontinent of Laurasia. Later collisions in the south and southeast terminated the Altaid evolution.

Paleozoic events in the Tethysides

Along the northern margin of the Tethysides, there was a continuous transition from the Altaid evolution into the Tethyside or, more strictly speaking, into the Cimmeride evolution. In northern Tibet the Kunlun Mountains (a part of the Cimmerides) may also be considered the southernmost representatives of the Altaid collage that was described above. They are made up of a huge subduction-accretion complex and of arc-related magmatic rocks—such as granites, granodiorites, and andesites, the ages of which range from the early Cambrian to Late Triassic (i.e., from about 540 to 200 million years ago)—that had begun accumulating along the southern margin of the North Tarim fragment, from which that subduction-accretion complex was later separated by the opening of the Tarim Basin during the Permian. The accretionary complex continues westward into the Pamir and Hindu Kush ranges in Tajikistan and northern Afghanistan and finally constitutes almost the entire pre-Triassic basement of Turkmenistan. The North China block became a part of Asia during the late Paleozoic, although a small westerly vanishing, wedge-shaped ocean between it and the rest of nuclear Asia remained open along a line roughly following the present course of the Shilka River in southern Siberia.

Orogenic deformation, magmatism, and metamorphism during the Carboniferous and Permian periods have become known in parts of Asia that then either belonged to Gondwanaland or had just separated from it as a result of the rifting of the Paleo-Tethys Ocean behind the separating Cimmerian continent. In northern and eastern Turkey, southwestern Iran, and Oman, folding and thrust faulting were in places accompanied by granitic and andesitic magmatism and high-temperature, low-pressure metamorphism, all collectively suggesting the activity of a subduction zone dipping under Gondwanaland. The same subduction zone may have been responsible for the rifting of the Neo-Tethys in the middle Permian as a back-arc basin similar to the present-day Sea of Japan.

Late Permian andesitic volcanics in the Hoh Xil Mountains in northern Tibet and late Paleozoic granites in western and peninsular Thailand, accompanied by compressional deformation and metamorphism, also suggest that a subduction zone existed along the northern margin of the Cimmerian continent. In those parts of Asia, the separation of parts of the Cimmerian continent from northern Gondwanaland may have already been under way during the Carboniferous, as shown by the deposition of the Phuket Group—a formation of glacially modified clastic sedimentary rocks in western Thailand some 3,600 feet (1,100 metres) thick—and of correlative rocks in adjacent Myanmar, Malaysia, and the Indonesian island of Sumatra.

The Yangtze paraplatform and the Kontum block are believed to have been parts of Gondwanaland during the early Paleozoic, but they rifted away from it sometime in the Devonian Period (about 419 to 359 million years ago). Two other fragments in southeastern China, the Huan’an and Dongnanya, have basements that had been consolidated mainly in the late Proterozoic and that also may have rifted from Gondwanaland sometime during the middle Paleozoic.

At least two island arcs collided with the Kontum block along its northeastern margin during the Paleozoic to enlarge it to what is called the Annamia block. The earlier island arc docked along a suture that now coincides with the Annamese Cordillera in northern Vietnam in the Devonian or slightly earlier. The later one collided along a suture zone farther to the north, along the present-day Ma River, during the early Carboniferous and caused a major south-directed deformation that included considerable thrusting.

Subduction during Carboniferous to Permian times was active along the present-day western margin of the Annamia block, giving rise to much arc-related magmatism and mineralization. That same magmatic zone extended down into the eastern half of the Malay Peninsula. Subduction was probably also active along the present western margins of the Huan’an and Dongnanya blocks, although late Paleozoic magmatism there was much sparser than in Southeast Asia.

Paleozoic events in the continental nuclei

Only three major nuclei underwent Paleozoic tectonic events not obviously related to their flanking orogenic belts. The Arabian platform underwent a major extensional tectonic event from the late Proterozoic to the middle Cambrian that created large north-south (“Arabian-trend”) and northwest-southeast (“Najd-trend”) rift basins in which clastics and evaporites (Jubaylah and Hormuz) were deposited. Those extensional basins were reactivated repeatedly until the early Carboniferous and then again in the late Permian. Active normal faulting in central Saudi Arabia late in the Ordovician (between about 460 and 444 million years ago) was coeval with sediment deposition caused by the Saharan glaciation (the Raʾan shales with striated sandstone boulders). A major marine invasion from the east in the late Permian covered more than half of the Arabian platform. The submergence of the platform coincided with the opening of the Neo-Tethys along its eastern margin and with a global rise in sea level following the late Carboniferous–early Permian glaciation in Gondwanaland. Striated pavements and glacial sedimentary deposits in the southern part of the Arabian platform (e.g., the Al-Khlata Formation in Oman) provide evidence of that glaciation.

A prolonged period of emergence up to the late Carboniferous characterized the Paleozoic history of the Indian platform, except for its northern margin, which was involved in the later Himalayan deformation. Late in the Carboniferous, glacially influenced terrestrial sedimentation commenced with the Talcher tillite formation in erosional bedrock depressions. In the early Permian a number of rift valleys oriented east-west and northwest-southeast originated, possibly related to extensions that farther north led to the opening of the Neo-Tethys. Terrestrial deposition continued in those rifts and in the surrounding areas, with local interruptions, until early in the Cretaceous Period (about 145 million years ago), forming the Gondwanan deposits. Farther north in what later became the Himalayas, there was continuous marine sedimentation, with only local interruptions related to global changes in sea level and gentle oscillations of the platform.

After the early Cambrian deposition of evaporites in extensional basins, the Angaran platform remained geologically calm, and shallow marine clastic and carbonate rocks were deposited on it. In the late Devonian (385 to 359 million years ago), however, the platform’s present northeastern margin was rifted; in addition to creating a major ocean, that activity produced two large rift valleys that now extend into the Angaran platform (the Vilyuy, or Viliui, and Chatanga rifts). Extensive basaltic volcanism accompanied that rifting event, followed by a period of heavy sedimentation along a northeast-facing continental margin.