Large-scale Oroclines Formed During the Permian-Mesozoic Amalgamation of Central Asia: Are They Good Analog Models for Hercynian Europe?
Subduction-related volcanics of Devonian age form an inverted U-shaped belt in Kazakhstan and yielded paleomagnetic inclinations indicating coherent northern hemisphere paleolatitudinal positions between 15 and 40 degrees North during the Mid-Late Devonian. The directions, on the other hand, show a large but systematic variation, in which declination correlates with the general trend of the NE, NW and SW branches of the horseshoe-shaped belt. This, combined with Permian, Silurian, and Ordovician paleomagnetic data indicates large-scale oroclinal bending during the late Paleozoic from an originally straight Andean margin situation at the eastern side of a Kazakhstan microcontinent. The mechanism, thought responsible for the oroclinal rotations, was a convergence of Siberia and Baltica, which involved a relative rotation between these two blocks of about 125 degrees causing buckling, bending and rotations of the Kazakhstan arc, as it was squeezed in between. During the Jurassic - Early Cretaceous a similar rotational convergence took place between Siberia and North China, which closed the Mongol-Okhotsk Ocean (MOO) and created a strongly curved belt wrapped around the suture of MOO. Paleomagnetic determinations of rotations do not yet have full coverage to call this belt an orocline, but the kinematic evidence seems to favor this. A third strongly curved belt exists in Hercynian Europe, forming the Ibero-Armorican Arc (IAA). Oroclinal bending has been documented by one-on-one correspondence between paleomagnetic declinations and strikes of the Cantabrian horseshoe- shaped belt at the core of the IAA. The suggestion is created that the Ibero-Armorican belt, wrapped around this Cantabrian core, is also an orocline, but the paleomagnetic data are too scarce and not of sufficient reliability to establish this as fact. All three belts have continent-scale dimensions and can reasonably be expected to have involved the crust as well as possibly upper mantle lithosphere in the oroclinal bending, in strong support of some of Carey's original proposals in the 1950's.
The drift Hystory of Iran from the Ordovician to the Triassic
New Late Ordovician, Permian, and Triassic paleomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician-Triassic. A robust agreement of paleomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician-earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian-early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere as the result of the opening of the Neotethys Ocean along the eastern margin of Gondwana during the Permian. Since possibly the late Early Triassic, Iran was located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Paleotethys in ~35 Myr at an average plate speed of ~7-8 cm/yr. As a novel conclusion, we find that timing, rates, and geometry of Cimmerian tectonics are broadly compatible with the transformation of Pangea from an Irvingian B to a Wegenerian A-type configuration with Neo-Tethyan opening taking place contemporaneously essentially in the Permian.
Lithospheric-mantle thinning beneath the Alpine-Himalayan Belt. Influence of mantle dynamics on tectonic evolution
The Alpine-Himalayan belt stretches from the Iberian Peninsula to Southeast Asia and is the result of closure of the Tethys Ocean and continental collision between the Eurasian plate and African, Arabian and Indian plates. Some anomalous chains located along this belt have been proposed as thinned lithospheric mantle (e.g. Atlas and Zagros Mountains, Tibetan Plateau). The slight crustal thickening and high topography, the presence of Cenozoic alkaline volcanism and the interpretation of geopotential fields support the hypothesis of a remarkable lithospheric thinning below the Atlas Mountains. Beneath the Zagros Belt a lithospheric mantle thinning is needed in order to fit elevation, geoid, gravity and thermal data. Uplift late in the tectonic evolution of the Tibetan Plateau, the widespread extension, and the associated magmatism have been attributed to removal of the lower part of the lithospheric mantle and its replacement by hotter and lighter asthenosphere, a two-dimensional lithospheric thermal and density model of the present day structure and numerical modeling of the evolution of the Plateau supports this interpretation. Despite all the studies carried out in these orogens, there is still much debate on the respective chronologies of uplift, the mechanisms responsible, and the present orogenic structure. This presentation will show the influence of lithospheric mantle thinning beneath a compressional mountain belt on topography and surface deformation and, in particular, on block rotations.