A new look at the Altaids: A superorogenic complex in northern and central Asia as a factory of continental crust. Part I: Geological data compilation (Exclusive of palaeomagnetic observations)

The Altaids are one of the largest superorogenic complexes in the world in which two genetically closely related orogenic complexes ended up generating much of northern Asia during the Palaeozoic and the early and medial Mesozoic. This immense superorogenic complex evolved as a consequence of the development of two large island arc systems called the Kipchak and the Tuva-Mongol arcs and that were similar in size to the present-day Southwest Pacific arc chains. They both have rifted from the then combined (or close) Siberian and Russian cratons during the latest Neoproterozoic/earliest Cambrian following the Baykalide/Preuralide orogeny. As a consequence of this rifting, the Khanty-Mansi Ocean opened behind them and they faced the Turkestan and the Khangai-Khantey Oceans, respectively. It is at the expense of these oceans that these two arc systems generated large subduction-accretion complexes. The Kipchak Arc was completely detached from the Siberian craton during the Neoproterozoic and it was reconnected with it along its trend by means of ensimatic arc systems that formed along its strike during the medial to late Cambrian. These ensimatic arcs also accumulated large volumes of subduction-accretion complexes in front of them during their migratory development throughout the Palaeozoic and, in Mongolia and in the Russian Far East, into the medial Mesozoic. As the accretionary complexes grew, magmatic fronts of their arcs migrated into them, turning them into arc massifs by magmatism and HT/LP metamorphism in arc cores. Especially near the Siberian Craton and in the Khangai-Khantey Ocean, the subduction-accretion complexes were fed by turbidites shed from old continental crustal pieces. Where arc magmatic axes migrated into such accretionary complexes, the material of which is of ancient continental provenance, they in places exhibit Proterozoic zircon ages and isotopic signatures inherited from their ancient source terrains leading to the mistaken conclusion of the presence of ancient continental crust under such arcs. It seems imperative to have proper field geological data together with the isotopic work to derive any reliable conclusions concerning crustal growth rates. We have compiled 1090 new, mostly zircon ages of magmatic and some metamorphic rocks from the literature for the whole of the Altaid supeororogenic complex. These ages show continuous arc activity from the Ediacaran into the early Cretaceous in the Altaids, although arc magmatism turned off already in the Triassic in the western Altaids. Much of the succeeding alkalic magma-tism in the western moiety of the superorogenic complex was related to strike-slip activity opening the West Siberian basins such as the Nurol and Nadym and the large pull-apart basins of Alakol, Junggar and Turfan. There are numerous other smaller areas of extension related to the late Altaid strike-slip activity and they too have alkalic magmatism associated with them. Some of the alkalic granites not related to the late strike-slip activity may have been related to slab fall-off after terminal collisions, although this is now difficult to document with any confidence. It is noteworthy that no Tibet-type collisional plateaux were ever produced as a consequence of Altaid collisions. We have been able to find no evidence anywhere in the Altaids for independent trans-oceanic migrations of numerous 'terranes' tied to individual subduction zones. Only two major subduction zones were responsible for the entire Altaid evolution from the beginning to the end and this is consistent not only with the present tectonics of the earth, where major subduction zones display great spatial continuity and temporal persistence, but also with the tomographic observations on well-imaged former subduction zones such as those associated with the Tethyan and the North American Cordilleran chains. The entire Altaid collage now occupies some 8,745,000 km². At least half of this area represents juvenile addition to the continental crust during the Ediacaran to the earliest Cretaceous interval. That is more than 10% of the entire land area of the Asian continent. Similar events are now going on in the Nipponides in eastern Asia, in the Oceanian arc systems in the southwestern Pacific Ocean and in places around the Caribbea and the southern Antilles. Altaids were one of the main factories-if not the main factory-for the generation of the continental crustal during the earlier half of the Phanerozoic on our earth. This was not because the growth rate of the crust was unusual, but because so much of it was produced in such a huge area and in an interval of some half a billion years.