Partitioning of intermontane basins by thrust - related folding , Tien Shan , Kyrgyzstan

Well-preserved, actively deforming folds in the Tien Shan of Kyrgyzstan provide a natural laboratory for the study of the evolution of thrust-related folds. The uplifted limbs of these folds comprise weakly indurated Cenozoic strata that mantle well-lithified Palaeozoic bedrock. Their contact is a regionally extensive unconformity that provides a persistent and readily traceable marker horizon. Based on the deformation of this marker, preserved fold geometries support simple geometric models for along-strike gradients in fold amplitude and displacement along the underlying faults, linkage among multiple structures, transfer of displacement among folds and evolution of the folds as geomorphic entities. Subsequent to initial uplift and warping of the unconformity surface, steeply dipping reverse faults cut the forelimbs of many of these folds. Wind gaps, water gaps, recent faulting and progressive stripping of the more readily eroded Cenozoic strata indicate the ongoing lateral propagation and vertical growth of fault-related folds. The defeat of formerly antecedent rivers coincides in several places with marked increases in erosional resistance where their incising channels first encountered Palaeozoic bedrock. Persistent dip angles on the backlimbs of folds indicate strikingly uniform geometries of the underlying faults as they propagate both laterally and vertically through the crust. Deformation switches irregularly forward and backward in both time and space among multiple active faults and folds with no systematic pattern to the migration of deformation. This distributed deformation appears characteristic of the entire Kyrgyz Tien Shan. shortening rates of #13 mm yr−1 within the Kyrgyz INTRODUCTION Tien Shan (Abdrakhmatov et al., 1996) and suggest that, when the shortening along the northern margin of the Intracontinental mountain building in convergent settings usually occurs as a result of the nucleation and growth Tarim Basin is included, the total shortening is likely to be #20 mm yr−1 (Michel et al., 1997): an amount equal of thrust faults. As thrust belts emerge as topographic entities, they define intermontane basins in which the to #40% of the modern Indo-Asian convergence rate. In western Kyrgyzstan, the Tien Shan comprises five detritus eroded from the bounding ranges accumulates. As convergence continues, at least three patterns of thrust-bounded ranges which are aligned approximately east–west and are separated from each other by intermondeformation can accommodate the ongoing shortening: the early formed thrusts can continue to move and tane basins typically 15–50 km wide (Chediya, 1986; Ghose et al., 1996) (Fig. 1). Whereas shortening is accumulate more and more displacement; the zone of deformation can widen as new thrusts emerge along the presently occurring along existing range-bounding thrusts and the overall zone of deformation appears to be widenflanks of the mountain belt; or new thrusts can develop within and between the existing mountains during ing to the north (the first two options above), it is clear that the present intermontane basins are also being densification of the thrust system. In this last scenario, the new thrusts will often encroach upon or subdivide partitioned by the development of new thrust faults. In the southern Naryn Basin and directly to the south the pre-existing intermontane basins. The Tien Shan of Kyrgyzstan represent a zone of of the Naryn River, an array of thrusts and fault-related folds have emerged which define the current basin margin intracontinental convergence in which shortening occurs in response to the ongoing convergence between India (Fig. 2) and which separate the Naryn Basin from the At Bashi basin to the south (Omuraliev & Korzhenkov, and Siberia. Recent geodetic studies indicate present © 1999 Blackwell Science Ltd 75 D. W. Burbank et al. Fig. 1. Schematic map of the major ranges and basins of the Tien Shan in the Kyrgyz Republic. Study area encompasses the ranges which define the present-day southern margin of the Naryn Basin and northern margin of the At Bashi Basin. Modified after Geological map of Kirghiz SSR (1980). 1995). These structures range from nascent folds to southern Naryn Basin comprise Palaeozoic miogeoclinal strongly emergent bedrock folds which are cut by steeply strata, primarily Carboniferous carbonate and siliciclastic dipping reverse faults and which exhibit as much as 7 km strata. Following Palaeozoic and early to mid Mesozoic of structural relief. The resultant ranges provide an deformation, the bedrock throughout the study area was excellent opportunity to examine the growth of faultlargely bevelled by erosion prior to the onset of extensive related folds, displacement transfer between multiple Cenozoic deposition (Makarov, 1977; Sadybakasov, 1990). structures, spatial and temporal displacement variations, The profound, commonly angular, unconformity and the geomorphic response to rock uplift within these across the pre-Cenozoic rocks provides a remarkable active folds and thrusts. In this paper, we present simple structural marker that can be used to define the magnimodels which relate the growth of individual and multude of folding and rock uplift within many parts of the tiple structures with geomorphic processes. Based on Tien Shan (Chediya, 1986; Omuraliev & Korzhenkov, reconnaissance mapping, field observations, and 1995). During the early Cenozoic in the study area, the interpretation of satellite images and aerial photography, tilt and topographic relief on this unconformity appear we examine the ranges bounding the southern Naryn to have been minimal. Prominently bedded Cenozoic Basin in the context of the models. Finally, we discuss strata overlying the Palaeozoic bedrock can be traced what the structures and geomorphology suggest in terms extensively along the flanks of folds in the study area. of the sequence of deformation and the way in which the Because the stratigraphic height of nearly any traceable upper crust responds to continued contractional Cenozoic bed above the unconformity varies by less than stresses. a few metres across the several kilometres of these exposures, it is clear that the unconformity can be BACKGROUND GEOLOGY OF THE considered to be a nearly horizontal datum at the time TIEN SHAN of initial Cenozoic deposition. Thus, subsequent tilting, uplift, folding and faulting of this surface provide key The Tien Shan of Kyrgyzstan are bounded by the Tarim controls on the magnitude of Cenozoic deformation. Basin to the south and the relatively stable Kazakh Palaeontological studies indicate that Cenozoic strata platform to the north. Comprising primarily Palaeozoic in the Naryn and At Bashi basins span from Oligocene rocks and lesser amounts of lower Mesozoic strata to Quaternary times (Turbin et al., 1972). These strata (USSR, ANKSaMoG, 1980), the Tien Shan were deare nonmarine and comprise facies associations interformed by two Palaeozoic orogenies (Burtman, 1975). preted to represent alluvial fan, fluvial, floodplain and The approximately east–west-trending ‘Nikolai Line’ in shallow lacustrine depositional settings. In the study area, the central Tien Shan (Fig. 1) defines a tectonic boundary the Cenozoic strata prior to deformation were generally in central Kyrgyzstan between extensively intruded rocks 4–6 km thick (Makarov, 1977; Sadybakasov, 1990). These to the north that were deformed in early Palaeozoic times strata are weakly to moderately cemented and are far and unintruded strata to the south that were deformed more readily eroded than the underlying Palaeozoic during late Palaeozoic times (Burtman, 1975). Lying south of the Nikolai Line, the bedrock ranges of the rocks. © 1999 Blackwell Science Ltd, Basin Research, 11, 75–92 76 Partitioning of intermontane basins