The Jurassic to Paleogene detrital record of the Central Asian Orogenic Belt: results from the Kyrgyz Tien Shan
Résumé
The Central Asian Orogenic Belt (CAOB) represents one of the largest Phanerozoic accretionary orogens in the
world, pinched between North China - Tarim, Kazakhstan and the Siberian craton. It stretches from the Pamir
in the southwest, over the Tien Shan, Junggar, Altai and Sayan mountain ranges to the Baikal rift in the northeast.
The basement of the CAOB exists of various tectonic units that were assembled during several Paleozoic
collision-accretion events. By the Permian, the accretionary tectonics in the CAOB culminated as all major composing
units were joined. After final construction in the Late Paleozoic, the CAOB was subjected to several phases
of Mesozoic deformation and was again reactivated in the Late Cenozoic as distant effect of the India-Eurasia
collision. The Meso-Cenozoic reactivation episodes occurred in an intracontinental setting, related to tectonic farfield
effects originating at the Eurasian margins. Subsequently intracontinental orogens, such as Tien Shan, were
built superimposed on the inherited basement architecture and the erosion products of their exhuming mountain
ranges accumulated in sedimentary basins. New sedimentological and detrital zircon U-Pb (LA-ICP-MS) results
from several Jurassic to Paleogene sedimentary sections in Kyrgyzstan provide new insights in the Mesozoic - early
Cenozoic geodynamic evolution and related basin-range interactions of the Kyrgyz Tien Shan. Studied sedimentary
sections are located in the Fergana and Yarkand-Fergana basins to the west of the Talas-Fergana Fault (TFF) and
in the Issyk-Kul and Ming-Kush-Kökömeren basins to the east of the TFF. The U-Pb ages of the post-Precambrian
zircon grains found in 18 Jurassic to Paleogene sandstones, can be generally divided into four groups: Caledonian
(470-390 Ma), Hercynian (315-260 Ma), Triassic (250-210 Ma) and Jurassic (190-160 Ma) ages. The differences
in sedimentation pattern and detrital zircon U-Pb age components suggest that the TFF played an important role
in the distribution of the sedimentary deposits. During the humid Early-Middle Jurassic, the TFF probably acted
as a listric fault with strong subsidence in the Yarkand-Fergana basin. The provenance of the sediments can be explained
by both local sources from close to the TFF and more distal sources to the (south)west of the TFF. A strong
aridification event around the Jurassic-Cretaceous transition, together with a renewed compressive tectonic phase,
led to the deposition of large conglomerate packages to the west of the TFF. Within one of the conglomerates, a
mafic sill with an emplacement age of 144 8 Ma based on apatite U-Pb LA-ICP-MS dating indicates small-scale
volcanism during the Lower-Cretaceous. During the Late Cretaceous to early Paleogene, the Yarkand-Fergana
basin was likely connected to the Fergana basin and experienced some marine influence by the Paratethys Sea. In
contrast, to the east of the TFF, low-relief mainland was maintained during the Cretaceous and sedimentation -
without marine influence - resumed since the early Paleogene.