Evolution of a thick-skinned thrust system and associated basins, Northern Tien Shan, Kzakhstan
Final Report Abstract
The Eocene to Quaternary Ili basin occupies a large segment of the northern Tien Shan foreland in Kazakhstan. The eastern part of the Ili basin, studied here, is located between the overlapping thrust fronts of the western and eastern branches of the modern Tien Shan. Displacement is transferred between both fronts across the Ili Basin along a NNE-trending band of sinistral strike-slip deformation. Throughout the history of the Ili Basin, exclusively continental sediments were deposited in alluvial fan, mudflat, fluvial and lacustrine environments. All interpreted environments have equivalents in the recent Ili system: a trunk stream fed by several large braided rivers transports sand and clay to a closed-drainage lake basin. The tributary streams form fluvial fans with only few active channels. Some of these rivers disappear before reaching the main stream. Short alluvial fans prograde from the uplifting mountain ranges to near the trunk stream and interfinger with extended mud flats. The basinwide interplay of lake and river deposition together with typical stacking patterns of lithostratigraphic units points to base-level control which occurred simultaneously in the entire Ili basin, suggesting a coupling of the Ili Basin to global sea-level fluctuations via the Paratethys. The transfer zone is expressed as a swath of ranges of varying size and orientation rising from the basin. Deformation is almost exclusively thick-skinned, involving the Permo-Carboniferous and older basement. The main basement thrusts and reverse faults strike E to NE; a secondary set strikes NW. The Ili Basin has not evolved as a typical foreland basin. Part of the basin evolution pre-dates the main phase of localized basement thrusting and folding. Eocene to Oligocene sandstones indicate distal sources. However, coarse-grained units with compositions indicating proximal sediment sources are present throughout the basin history. This is consistent with long-lived trends of decreasing thickness and increasing stratigraphic condensation over the present-day basement uplifts. Structural evidence shows that early deposition was accompanied by E-W-directed extension that lasted until the Late Miocene. Whereas the axis of the Ili basin is subsiding and receiving sediment to the present, the evolution on its margins proceeds from deposition over peneplanation to incision, suggesting uplift encroaching on the basin from the south and north. Most folds and faults in the basin are older than the formation of the peneplain which comprises pediments and alluvial surfaces a few hundred thousand to a few ten thousand years old. Active deformation affecting the Quaternary piedmont alluvial surfaces is evidenced by subtle folding as indicated by warped terraces, and by a few localized flexures and thrust faults. 10Be dating of an alluvial surface offset by a major thrust fault indicates a local shortening rate of about 0,25 mm/a, substantially less than the bulk GPS shortening rate for the area and suggesting distributed deformation in keeping with geological observations. Large-scale active uplift of the regional transfer zone is suggested by the deflection of the Charyn River into a NNE trend for ca. 100 km of channel length before it joins the westward flowing Ili trunk stream. GPS data indicate a present-day eastward increase in northward motion of some 2 mm/y across the transfer zone, corroborating our interpretation of distributed sinistral strike-slip deformation. GPS data furthermore show that the western and eastern branches of the Tien Shan rotate counterclockwise and clockwise, respectively. Their relative motion is also partly resolved in the transfer zone. Localized E-W extension similar to the early stages of the Ili Basin is a consequence of the present-day GPS displacement field. Inherited structures modulate but do not dominate the Cenozoic deformation in our study area. Many Cenozoic structures truncate Paleozoic ones. The shortening direction from geological data is NNW, roughly perpendicular to the main set of folds and thrust faults, consistent with earthquake focal mechanisms. Reactivation of Paleozoic structures can be demonstrated for one regional fault and for frequent smaller, steeply dipping NW-trending faults of probably Permo-Carboniferous age.
Publications
- (2009). Rating of seismicity and reconstruction of the fault geometries in northern Tien Shan within the project "Seismic Hazard Assessment for Almaty". Journal of Geodynamics 48, 269-278
Torizin, J., Jentzsch, G., Malischewsky, P., Kley, J., Abakanov, N., Kurskeev, A.
- (2011) Some Cenozoic tectonics of Northern Tien Shan. 5th International Symposium “Modern Problems of Geodynamics and Geoecology of Intracontinental Orogens”. Bishkek. ISBN 978-9967-26-317-8
Seib, N., Kley, J., Voigt, T., Kober, М., Buvtyshkin, V.
- (2011) Thick-Skinned Tectonics In The Northern Tien Shan Foreland, Kazakhstan. Poster T43F-2439, AGU Fall Meeting 2011
Kober, M., Kley, J, Seib, N., Voigt, T.
- (2011) Thick-skinned thrusting in the northern Tien Shan foreland, Kazakhstan: Structural inheritance and polyphase deformation. Fragile Earth. Joint Meeting GSA-GV-DGG, Munich, A18
Kober, M, Seib, N., Kley, J., Voigt, T.
- (2011): Tectonic and sedimentary evolution of the Ili Basin (northern Tien Shan, Kazakhstan). EGU General assembly 2011, Vienna
Voigt, T., Kober, M., Seib, N., Kley, J.
- (2013): Continental climate dynamics prior to uplift of the Tien Shan: The Oligocene-Miocene succession of the Ili Basin, SE Kazakhstan. EGU General assembly 2013, Vienna
Weber, Y., Voigt, S., Pross, J., Voigt, T., Kley, J.
- (2013): Thick-skinned thrusting in the northern Tien Shan foreland, Kazakhstan: Structural inheritance and polyphase deformation. In: M. Nemcok, A. Mora, J.W. Cosgrove (eds.): Thick-Skin-Dominated Orogens: From Initial Inversion to Full Accretion. Geological Society of London, Special Publications 377
Kober, M., Seib, N., Kley, J., Voigt, T.
(See online at https://doi.org/10.1144/SP377.7)
