Pamir


CENOZOIC SEDIMENTS OF THE TAJIK BASIN: SEDIMENTOLOGY, PROVENANCE, AND THE TECTONIC RECORD OF THE PAMIR

is part of the DFG-founded Collaborative Research Bundle PAK 443: Tien Shan - Pamir Geodynamic Program (TIPAGE).

Collaborative Research Partners are:

Friedrich-Schiller-Universität Jena (Prof. Dr. Reinhard Gaupp, Dr. Thomas Voigt, Dipl. Geol. Martin Klocke)

Technische Universität Bergakademie Freiberg

GeoForschungsZentrum Potsdam

PROJECT SUMMARY

TIPAGE aims to exploit the Pamir, the western part of the Pamir-Tibet-Himalaya orogenic system, to address key questions in the geodynamics of continental collision, indentation, and orocline formation:

(1) The Pamir hosts Earth's most spectacular active intra-continental subduction zone beneath the

Pamir-Hindu-Kush. Does the current seismicity trace a single, partly overturned and distorted slab or two or more separate slabs? Are these slabs composed of continental lithosphere? What is the relationship between these slabs (mantle deformation) and deformation in the crust and at the surface? Why are intermediate-depth continental subduction zones present beneath the Pamir and not elsewhere in the collision zone, e.g. beneath Tibet and the Himalaya?

(2) The interior of the Pamir possibly hosted two Tertiary intra-continental subduction zones. Can these zones be unequivocally traced by accompanying belts of magmatism and deformation? Is middle and upper crustal deformation concentrated in the basement domes that occur in the Pamir or as distributed as in Tibet? Are these zones still active, when did they start/terminate to be active, and were they active coevally? How are they represented in the mantle, in the crust, and at the Moho? How do/did the active/Tertiary intra-continental subduction zones behave and what structures/strain develop(ed) in the mantle and crust when these zones were bent during orocline formation? Does lower crustal flow out of Tibet affect the Pamir?

(3) The Pamir formed at the tip of the western Indian promontory, whose northward path is well known, and is situated at the transition of the Tarim to the Tajik foreland basin. What determines the 3D strain field in the Pamir: the promontory or the tapering of the Tarim basin; how are development/ progradation of intra-continental subduction, crustal buckling/dome formation, oroclinal bending, lateral transpression/transtension zone formation connected with progressive India-Asia convergence, promontory indentation and rotation? What salient features of the Pamir can be explained by changes in India's motion: its transpressional (west) versus transtensional (east) boundaries, asymmetry in arrangement of crustal domes?

(4) The Pamir is externally drained and erosional products have accumulated mostly within a single basin system, the Tajik depression. Which effects does/did erosion and deposition have on the formation of the Pamir: how does/did the erosional network (rivers, drainage basins) and depositional system (Tajik basin) respond to the growth/decay of the orogen and the formation of the orocline; how was/is the initial orogen-parallel river system modified by oroclinal bending and growth of ranges oblique to the initial system; how does the foreland depositional system react?

These issues, concerning continental collision/subduction, indentation, and orocline formation in general, are enigmatic in older orogens but resolvable in young and active regions such as the Pamir-Tibet-Himalaya system. The Pamir offers distinct advantages over adjacent Tibet: a similar amount of Cenozoic shortening that is accommodated over ~50% less meridional orogenic width; the most active area of intermediate-depth intra-continental seismicity; the largest tract of high-temperature/intermediate pressure crystalline basement in the entire Pamir-Tibet-Himalaya orogen that traces thick-skinned, highstrain belts; distinct intra-continental magmatic belts; a complete record of orogen exhumation and foreland-basin deposition in the Tajik basin; an externally drained river
system that reveals neotectonics by drainage basin and river incision/capture/reversal evolution.

Klocke-1_working_area Klocke-2_geo_map

Assembly of the Pamir mountain system with working areas (modified after ROBINSON et al. 2004).

Geological map and cross section of the Mesozoic and Cenozoic filling of the Tajik depression with the working areas. Modified after THOMAS et al. (1994).

Sedimentology Group (Jena, Freiberg)

The project studies the response of the Pamir foreland depositional system (Tajik basin, Alai valley) to the erosional denudation in the Pamir and southern Tien Shan and thus will trace volume, source, and rates of erosion, and tectonic pulses in the orogenic hinterland. Principal hypotheses: Due to its areal extent, stratigraphic range, and the possibility to restore deformation of its fold-thrust belt, the Tajik basin allows a balance of orogenic erosion and syn-orogenic sediment deposition by permitting an estimate of the sediment volume eroded from the Pamir-southern Tien Shan orogen. As all major river systems of the southern Tien Shan and the Pamir drained into this basin, a complete record of the sediment-source regions and of source-region uplift and exhumation is to be expected. A combination of analyses, i.e. the study of depositional hiatuses, variations in facies patterns, changes in clast/grain compositions, and detrital geo/thermochronology, is able to trace hinterland tectonics.

The stratigraphy of the Tajik basin comprises shallow marine to non-marine sedimentary rocks deposited in a restricted, intra-continental basin (e.g. Vlasov et al. 1991). The Late Jurassic-Paleogene section includes mostly marine strata that are overlain by 350-400 m of Paleogene interbedded marine limestones and mudstones, with minor gypsiferous strata. These pre-orogenic strata depositionally overlie Paleozoic units of the Tien Shan; they appear to be part of an unbroken Paleozoic and Mesozoic stratigraphic sequence in the Darvaz range (Pavlis & Hamburger 1990). Thick syn-orogenic clastic rocks of Miocene to Pliocene age occur throughout the Tajik depression and consist largely of fluvial sandstone and conglomerate. They coarsen upward from lower Miocene siltstones/sandstones to upper Miocene and Pliocene conglomerates and thicken markedly toward the south from 2-3 km to >5 km. These typical foreland-basin deposits reflect the uplift and the approach of nappes from the south. Our work in the Darvaz range shows that the Cenozoic succession includes a spectacular record of climate, change and hinterland erosion. Depositional hiatuses, mostly marked by unconformities, variations in facies patterns, and changes in clast/grain compositions likely outline hinterland tectonics. The conglomerate-clast composition allows tracing the areas that were eroded: e.g. the base of the massive conglomerate section in the northern Darvaz range comprises mostly clasts of the Carboniferous-Permian carbonate platform of the northern Pamir, whereas the late Pliocene/Pleistocene rocks consist of granitoid and intermediate-acid volcanic rock pebbles likely derived from the Permian-Triassic arc of the northern Pamir and the Cretaceous-Cenozoic granitoids of the central Pamir.


Cooperation Partners

Tajik Group: Our cooperation partners (V.E. Minaev, S.K. Negmatullaev, N. Rasshabov, I. Oimahmadov, M. Gadoev, A.R. Faiziev and students) have 40-10 years of experience in field work concerning the near- and far-field of the Pamir and Tajik basin. They are organizing our field trips (cars, drivers, logistics, permits and the kitchen) and sample transport to Germany and they support us with their knowledge of the local and regional stratigraphy. Unfortunately, no real scientific work can be done in the Institute of Geology in Dushanbe which partly is in ruins and none of the labs is running.

Kyrgyz Group: Our cooperation partners are from International Research Center of the Russian Academy of Sciences (G.G. Schelochkov, A.K. Rybin) and the Central Asian Institute of Applied Geosciences (CAIAG; B. Moldobekov, S. Orunbaev) all located in Bishkek, Kyrgyzstan. CAIAG has been founded with support from the GFZ Potsdam and they manage sample transport.

US-Group: The US-PIs B.R. Hacker and J.M Mattinson submitted a funding request to the US-NSF for the project "How does the lower crust thicken and grow in continental-collision zones? A case study of the Pamir". This project focuses on testing the hypothesis of lower crustal flow from Tibet using quantitative petrology and high-precision geochronology using zircon and titanite within the Muskol, Sares, and Shakhdara domes of the central and southern Pamir.

Klocke-3_deposits
Klocke-4_unconformities

Deposits of the Miocene Baldshuan-Fm. with imbricated clast-supported conglomerates of a gravel bar above sand with isolated gravelly clasts. Lower part shows protruding bar-top boulders.

Unconformities between the syntectonic deposits of the Miocene Tavildara-Fm., the Karanak-Fm. and the Polizak-Formation (both Pliocene).