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86 Cretaceous plutons that intrude the Central and Southern Pamirs record a long-lasting magmatic history. Their zircons and those from Late Miocene xenoliths show that the most distinct tectonic events were Cambro-Ordovician (~575-410 Ma), Triassic (~250- 200 Ma, due to subduction along the Jinsha suture and/or Triassic rifting (e.g. in the Rushan Pshart/Bangong-Nujiang area and between the SW and SE Pamirs)), Middle Jurassic (~200-150 Ma, due to subduction along the Rushan Pshart suture), and mainly Cretaceous. The Cretaceous activity may have been bimodal, with peaks at ~120 Ma and ~80 Ma. The Mid-Cretaceous event may reflect arc activity in Asia prior to the accretion of the Karakorum block, probably along the Tirich Mir fault. The Late Cretaceous magmatism may be a sign of flat-slab subduction along the Shyok suture north of the Kohistan-Ladakh arc. Before India and Asia collided, the Pamir region from the south Tien Shan to the Indus-Yarlung was an Andean-style plate margin. From the Kunlun arc in the north to the southern Qiangtang block in the south, the Pamirs and Tibet seem to have dominantly (meta)sedimentary crust, characterised by the Karakul- Mazar/Sonpan-Ganze accretionary wedge rocks. The crust south of the southern Qiangtang block is of likely granodioritic composition, reflecting long-lived subduction, arc formation and Cretaceous/Tertiary underthrusting of arc segments beneath the Qiangtang block. The accretion history was accomplished by the final collision of India with Eurasia at ~55 Ma. The long lasting history as an active continental margin ended and the Pamirs came into an intracontinental position. Crustal stacking in the Pamirs region initiated shortly after the India-Asia collision and led to much more thickened crust than previously thought (70-80 km). Due to crust thickening and successive melt generation, several Tertiary intrusions penetrated the Pamirs. More widespread are leucogranites in the South Pamirs, whereas localised migmatisation and melt generation led to subalkaline and alkaline intrusions into Central Pamiran basement domes in the Tertiary. The Tertiary magmatism was probably episodic with subalkaline intrusions in Late Eocene/Oligocene and aplitic and basalt dyke emplacement around ~20-11 Ma. Medium to high-grade metamorphism was about 20-15 Ma. Crustal shortening was most extreme in the Pamiran region and strain is mostly concentrated along rheologically weakened zone along the margins of the accreted crustal blocks. One exception is the Central Pamirs with its basement anticlinoriums. Apatite and zircon fission track ages support the above outlined accretion history. Tertiary sedimentary rock samples and their determined grain ages highlight the exhumation and cooling of the rising Kunlun after Early to Mid Palaeozoic arc accretion and subduction related magmatism (peak ages in the Kunlun ~ 370 Ma). The Tien Shan range was subject of two collisions in the Palaeozoic: an Early to Mid Palaeozoic collision was only weakly detected in Tertiary Tien Shan sedimentary basins, but Late Palaeozoic/Triassic cooling and exhumation after collision together with coeval basin formation catching the sediments of the rising Tien Shan and Kunlun mountains is well detectable in the Tien Shan (~266-245 Ma). Triassic ages around ~242 Ma are also recorded in Tertiary northern Qiangtang intramontane basins and can be related to subduction of the Jinsha oceanic crust beneath the Qiangtang block and/or be related to increased heat flow due to basin formation in several areas surrounding the Qiangtang, Lhasa, and Kunlun regions. Mid-Jurassic peak ages, coupled with Jurassic
compressive tectonics and the shedding of a huge amount of Tertiary sediments into the adjacent basins, are most likely sourced by Mid-Jurassic collision of the SE Pamirs and Lhasa blocks along the Rushan Pshart and Bangong-Nujiang zones. Few Late Cretaceous fission track ages of sedimentary rocks and hardrocks imply a thermal event affecting the Qiangtang block and the Tien Shan. At least for the Qiangtang, a thermal influence is suggested from the Late Cretaceous collision of the Kohistan Ladakh arc along the Shyok suture. All other fission track ages are related to India-Asia post collisional tectonics. In the southermost Tien Shan, apatite fission track ages of granitoids point to Late Miocene exhumation (~11 Ma). Exhumation was probably induced in response to crustal stacking along major thrust faults and dextral transpressional strike-slip faults. Fission track ages of the Karakul lake batholith gave ages from 56 to 18 Ma with a younging towards the north. Either Tertiary tectonics (e.g. piggy-back thrust teconics in which footwall burial and hangingwall exhumation propagated towards north) or cooling after a Late Cretaceous reheating event, induced by underthrusting of the Karakul-Mazar belt beneath the northern Qiangtang, may have caused this age distribution. The most penetrative exhumation event is localised in the Muzkol and Sares domes of the Central Pamirs and occurred between ~25-15 Ma. Between 22 to 15 Ma the cooling rates increased to >40°C/Ma. Tertiary intramontane basins along the margins of the dome contain coarse grained conglomerates composed of the Mesozoic cover sequence of the dome. Reset zircons of sediments from a basin south of the dome yielded ages of 20 to 19 Ma. Basalts cutting through the section are preliminary interpreted as within plate origin. They yielded ~20 Ma Ar/Ar whole rock ages. K/Ar sericite ages range from 33 to 14 Ma and increase in age away from the dome (from north to south). The older K/Ar sericite ages (>25 Ma) are interpreted to be partly reset. Dome exhumation between 25 to 15 Ma seem to have heated the Palaeogene sedimentary basin. Cretaceous granitoids intruding Late Palaeozoic to Jurassic rocks in blocks rimmed by Tertiary dextral transpressional faults. At least since Miocene the area was strongly deformed along the right lateral splays of the Karakoram fault, which seem to be responsible for exhumation at 11-10 Ma. From Late Miocene to Recent, final cooling of the samples was at rates of 5-16°C/Ma. Obviously, deformation did not propagate continuously from south to north. Instead, it might have been concentrated into rheologically weak zones like the Central Pamirs where probably intracontinental subduction along the Jinsha suture facilitated deformation and melt generation. Coeval with the fast Early Miocene dome exhumation, the northern margin of the Karakul-Mazar belt was exhumed along the right-lateral transpressional Markansu fault. After the Early Miocene exhumation, deformation seems to have shifted to locations dominated by large strike slip faults responsible for lateral rock transportation and compensating for continuous N-S compression between India and Asia; these are e.g. the Karakoram fault, dextral transpressional faults along the southern margin of the Tien Shan and along the Pamiran frontal range. These are active at least since 11-10 Ma. 87
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Institut für Geowissenschaften (IF
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Key words: Tien Shan, Pamirs, Tibet
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* Nr. 22 MESCHEDE, M. (1994): Tecto
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* Nr. 55 REINECKER, J. (2000): Stre
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The amalgamation of the Pamirs and
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Wer aber je auf Asiens Erdboden sch
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viii (1971). Black lines with arrow
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x sample locations and age distribu
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xii Swedish expeditions followed. I
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2 postulated that the Safed Khers a
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4 Zusammenfassung Die paläozoische
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6 (d) Oberkreide Spaltspurenalter w
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8 deformation propagate from the pl
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10 source terrains, changes in the
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12 Geochemisches Zentrallabor, Univ
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14 mean UO+/U+ and Pb+/U+ value. Si
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16 apatites and zircons were counte
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18 number of simulation runs and th
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Two Palaeozoic sutures divide the C
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igneous and sedimentary rock sequen
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Triassic and the formation of an is
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27 Lake Karakul The Lake Karakul ar
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3.3.2 Major and trace elements Majo
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32 of 26-40 for samples P2, P5, and
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34 some of the zircons suffered Pb
Page 60 and 61: 36 fractions are situated close to
Page 65: 3.4.2 Rb/Sr dating The Rb/Sr minera
Page 69 and 70: 45 The Cretaceous plutons show mode
Page 72 and 73: 48 Thompson et al. (1984), who see
Page 74 and 75: 50 granites and have Sm-Nd model ag
Page 76 and 77: 52 associations in Afghanistan is f
Page 78 and 79: 54 Kara Kunlun or Mazar accretionar
Page 80 and 81: 56 Early Jurassic low-angle normal
Page 82 and 83: 58 Coward et al. (1988) pointed out
Page 84 and 85: 60 oceanic crust for the consumptio
Page 86 and 87: 62 Palaeogene volcanic rocks of cen
Page 88: 64 3.7 Conclusions Based on new own
Page 91 and 92: 4 Cenozoic exhumation history of th
Page 93 and 94: Southern Tien Shan One Permian gran
Page 97 and 98: Thermal modelling The results of th
Page 99 and 100: amphibole, white mica, and biotite
Page 101 and 102: the zircon PAZ. The apparent apatit
Page 103 and 104: elt likely operated as an out-of-se
Page 105: ages from gneissic rocks within the
Page 108 and 109: 84 south to 18 Ma in the north; the
Page 112 and 113: 88 6 References Allègre C.J., Cout
Page 114 and 115: 90 House, scale 1:2 000 000. Chen F
Page 116 and 117: 92 Karakorum (Sinkiang, China); the
Page 118 and 119: 94 Hurford A.J. and Hammerschmidt K
Page 120 and 121: 96 without double spiking. Geochimi
Page 122 and 123: 98 Geological Society of America, 2
Page 124 and 125: 100 Zamoruyev A., 1995a. Neotectoni
Page 126 and 127: 102 Ann. Rev. of Earth and Planetar
Page 128: 104 this work, especially all membe
Page 132 and 133: II Appendix A Tab. A1 Sample descri
Page 134 and 135: IV Tab. A2 continued Pamir frontal
Page 136 and 137: VI Tab. A2 continued Tertiary exten
Page 138 and 139: VIII Tab. A3 continued Sample Miner
Page 140 and 141: X Tab. A6 Rb/Sr and Sm/Nd isotopic
Page 142 and 143: XII Tab. A7 continued Apparent ages
Page 144 and 145: XIV Tab. A9 U/Pb isotopic ratios fr
Page 146 and 147: XVI Tab. A9 continued 207 Pb/ 206 P
Page 148 and 149: XVIII Tab. A9 continued 207 Pb/ 206
Page 150 and 151: XX Appendix B Plate B1 Cathodolumin
Page 152 and 153: XXII Plate B3 Cathodoluminescence i
Page 154 and 155: XXIV Plate B5 Cathodoluminescence i
Page 156 and 157: XXVI Tab. C2 Determination of the
Page 158 and 159: XXVIII
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XXX
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XXXII
Page 164 and 165:
XXXIV
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XXXVI Tab. C5 continued Sample grai
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XXXVIII Tab. C5 continued Sample gr
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XL Tab. C5 continued Sample grain-n
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XLII Tab. C5 continued Sample grain
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XLIV
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XLVI Tab. C6 Zircon fission track a
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XLVIII
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LII
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LIV Tab. C7 Apatite and zircon fiss
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