The Alai Valley of Kyrgyzstan is an east-west-oriented intramontane basin along the northwestern perimeter of the India-Eurasia collision zone. It is the vestige of a once continuous sedimentary basin linking the Tadjik and Tarim depressions to the west and east, respectively. The valley is bounded to the south by the active Main Pamir Thrust (MPT) of the Trans Alai, and by the Alai Range of the southern Tien Shan to the north. West of 073°E, the MPT is divided into two segments, linked by a NW-oriented transfer zone with dextral transfer faults and thrust faults. We present a series of N-S structural cross-sections based on field observations and seismic reflection data in order to better understand space and time relationships of collision-related progressive basin closure during the Cenozoic. The Paleozoic basement of the Alai Valley and surrounding regions consists of Silurian terrigenous deposits, Devonian limestones and marbles, and Carboniferous metasedimentary rocks that were largely deformed during the Hercynian cycle. The basement is unconformably overlain by Jurassic conglomerates, sandstones, and claystones interbedded with gypsum and coal. In the Tien Shan, Cretaceous rocks unconformably overlie Paleozoic basement or Jurassic rocks. Early Cretaceous strata were deposited in a continental setting and mainly include coarse-grained red sandstones and mudstones, 200-300 m thick. In the late Cenomanian, marine condition prevail indicated by massive carbonates, marls, claystones, and gypsum, up to 900 m thick in the eastern part of the valley. These units are overlain by early Paleogene marine and lagoonal deposits, up to 750 m thick, including limestones, dolomite, marls and mudstones intercalated with gypsum. Oligocene to early (?) Miocene strata (Massaget Complex) are the oldest continental Cenozoic deposits in the area. They are characterized by massive conglomerates and sandstone layers, alternating with claystones. According to seismic reflection and borehole data, they conformably lie on Paleogene strata and are 900 m thick. Late (?) Miocene to early (?) Pliocene strata (Baktry Formation) are the basal unit of a large asymmetric orogenic wedge, composed of conglomerates and siltstones up to 1500 m thick, which is separated from lower Neogene deposits by an unconformity. However, in the southwest, along the Trans Alai mountain front, an approximately 3 km-thick sequence of chronologically poorly constrained Neogene cross-bedded red sandstones and mudstones is conformable with Paleogene strata. Upsection, the sequence is overlain by 1000 m-thick conglomerates (Late (?) Pliocene to Pleistocene Sokh Formation) in front of the Pamir thrust systems. Dip sections of the seismic reflection profiles show that the Alai Valley is an asymmetric full-ramp basin bounded by S- and N-vergent thrust systems with the basin deepening toward the south. Based on these observations, shortening in the Tien Shan must have started in early Neogene time. This is clearly documented by the shift from marine to continental sedimentation in the late Eocene-early Oligocene, and by the marked angular and erosional unconformities separating Paleogene and Neogene strata, where the Paleozoic basement rocks were thrusted over the Meso-/Cenozoic sedimentary cover. During the early Neogene, shortening was distributed across the present Alai valley by a series of northward verging thrust faults. In the late Miocene, however, the thrust front retreated southward; since then, shortening restricted to the MPT generated the large asymmetric late Neogene orogenic wedge. The external part of the Trans Alai range is defined by a stack of steeply southward dipping thrust sheets involving Cretaceous and Paleogene units, which are detached along Paleogene gypsum layers. These rocks overthrust folded Neogene syntectonic conglomerates. The youngest event related to basin closure is the northward stepping-out of a thrust, which involves the Neogene gravels in the hanging wall. For example, west of the Minjar drainage at about 072°E, a tight asymmetric E-W trending anticline is truncated at the range front by the active MPT, and interpreted as a fault-propagation fold; as the thrust stepped northward and cut through Neogene deposits, it also displaced Quaternary gravels. Due to sustained tectonic activity, it is also responsible for displacing several generations of climate-controlled terraces that abruptly terminate at the mountain front. This attests to the recency of deformation along this range sector, which contrasts sharply with the inactive southern Tien Shan. In addition, these observations show that the closure of the Alai Valley is currently accomplished by progressive N-directed thrusting.
|Original language||English (US)|
|Number of pages||2|
|Journal||Journal of Asian Earth Sciences|
|State||Published - May 12 2001|
ASJC Scopus subject areas
- Earth-Surface Processes