EARLY PALEOZOIC GEODYNAMICS OF THE URALS AND PAI-HOI: EVIDENCE FOR RAPID SUBSIDENCE IN THE ABSENCE OF CRUSTAL EXTENSION

Abstract

Subsidence of continental crust in fold belts is commonly interpreted as related to extension or elastic flexure of the lithosphere near convergent plate boundaries. In the Late Cambrian - Early Ordovician, relative extension in the most of the Uralian rift zone was as small as a few percent in the south and within 10% in the north. The average thickness of synrift sediments that filled the rift basins during slow subsidence rarely exceeded 1 km. In the Pai-Hoi, no lithospheric extension was evident. Most of the Early Paleozoic crustal subsidence in the Urals and Pai-Hoi occurred in several short episodes in the Early Ordovician and earliest Silurian and produced up to 2-4 km deep basins on the shallow-water shelf. This subsidence was not accompanied by any notable extension typical of rifts, as evidenced by the absence of deformation in the upper crust. Nearly all episodes of rapid subsidence were in the time when no plate collision occurred in the region of the Urals and Pai-Hoi, except for one event that took place at the Early-Middle Ordovician boundary in the Northern and Polar Urals concurrently with a collision east of the formed basin. However, the basin shows a westward rather than eastward deepening, which rules out subsidence related to elastic bending of the lithosphere toward the convergent plate boundary. Rapid subsidence in the absence of lithospheric extension in the basins or plate collision in their surroundings required contraction of rocks in the lithosphere. Rapid contraction could have been provided solely by metamorphism in the basic lower crust catalized by infiltration of volatiles from the asthenosphere. Formation of deep-water basins without lithospheric extension and prior to intense compression occurred widely in the Alps and other orogens. Therefore, precollisional crustal subsidence in fold belts was controlled by deep-level processes rather than by plate motions.