Ev. Artyushkov et al., Early Paleozoic geodynamics of the Urals and Pai-Hoi: Evidence for rapid subsidence in the absence of crustal extension, GEOL GEOFIZ, 41(12), 2000, pp. 1670-1689
Subsidence of continental crust in fold belts is commonly interpreted as re
lated to extension or elastic flexure of the lithosphere near convergent pl
ate boundaries. In the Late Cambrian-Early Ordovician, relative extension i
n the most of the Uralian rift zone was as small as a few percent in the so
uth and within 10% in the north. The average thickness of synrift sediments
that filled the rift basins during slow subsidence rarely exceeded 1 km. I
n the Pai-Hoi, no lithospheric extension was evident.
Most of the Early Paleozoic crustal subsidence in the Urals and Pai-Hoi occ
urred in several short episodes in the Early Ordovician and earliest Siluri
an and produced up to 2-4 km deep basins on the shallow-water shelf. This s
ubsidence was not accompanied by any notable extension typical of rifts, as
evidenced by the absence of deformation in the upper crust. Nearly all epi
sodes 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 pla
ce 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 roc
ks in the lithosphere.
Rapid contraction could have been provided solely by metamorphism in the ba
sic lower crust catalized by infiltration of volatiles from the asthenosphe
re. Formation of deep-water basins without lithospheric extension and prior
to intense compression occurred widely in the Alps and other orogens. Ther
efore, precollisional crustal subsidence in fold belts was controlled by de
ep-level processes rather than by plate motions.