Pamir Thrust Research Articles

Regional Differences in Geological Conditions Related to Reservoir-forming in the Foreland Fold and Thrust Belt of Southwest Tarim Basin

The foreland fold and thrust belt of southwest Tarim basin can be divided into four structural zones, and different tectonic sectors have quite different geological conditions related to reservoir-forming. Most favorable locations are Pusha–Keliyang zone and Sugaite–Qimugen zone, not only with many fault-propagation folds, triangle-zone structures, and three sets of excellent source rocks including Lower–Middle Cambrian carbonate, Lower Carboniferous–Lower Permian, and Jurassic dark mudstone but also with a series of good petroleum reservoir and seal combinations such as Lower-Cretaceous sandstone reservoirs associated with Paleogene cap rocks, Paleocene carbonate reservoirs, and Miocene sandstone reservoirs associated with their corresponding mudstone cap rocks, respectively. The second favorable location is Hotan thrust nappe zone with some good tectonic traps including traps of par-autochthonous system and in-situ system, and Lower-Permian mudstone as a set of high quality regional cap rocks, but their source and reservoir are relatively bad, that is, Lower-Permian carbonate source rocks just meet the criteria of gas source rocks, and Lower-Permian sandstone or carbonate reservoirs and Lower-Ordovician carbonate reservoirs are all low-porous and low-permeable in spite of Lower-Permian Pusige mudstone as excellent regional cap rocks. The last, Pamir thrust zone might not be favorable for reservoir-forming due to its large-scale nappes and distinctly-exposed thrust faults which result in the lack of good tectonic traps although there are some advantageous petroleum source-reservoir-seal combinations.

Shear waveQstructure and its lateral variation in the crust of China and surrounding regions

SUMMARY We have obtained three-layer crustal models of shear wave Q (Qμ) for several surface wave paths in China and peripheral regions using a single-station, multimode method in which amplitude spectra of fundamental-mode and higher-mode Rayleigh waves, computed for known source depths and mechanisms, are compared to measured spectra. The three layers, totaling 60 km in thickness, roughly comprise the entire crust in the Tibetan plateau and include both the crust and part of the uppermost mantle in other regions where the crust is thinner. Qμ in the shallowest layer, 10 km thick, is lowest (about 40) in the western portion of the Tibetan plateau and highest (about 250) in southeastern China. In the middle layer, at 10–30 km depth, Qμ is lowest (60–80) beneath Tibet and the Pamir thrust system and highest (120–140) in central China and parts of the Sino-Korean platform. Uncertainties of Qμ in the deepest layer, at 30–60 km depth, are much greater than in the upper two layers but available results suggest that Qμ is lowest (about 80) under the Pamir thrust system and highest (about 180) under southern Mongolia. The densities of event and station coverage, although wanting in some regions, allow us to develop the first regionalized maps of crustal Qμ variation for continents. The maps, for depth ranges 0–10 km and 10–30 km, generally show good agreement with Q results obtained earlier with other phases and good correlation with the tectonics of this active region.

Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China

The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium- to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230–200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (~650 °C, 8 kbar), which structurally overlies a lowgrade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125–110 Ma, followed by emplacement over the low-grade metagraywacke along a north-northeast–directed thrust prior to ca. 100 Ma. Together these results indicate signifi cant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (~650–700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7–8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (~29 km) and the present dip of the Kongur Shan normal fault (~40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases signifi cantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.

Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim basin, northwest China

The Jurassic through Cretaceous southwest Tarim basin, northwest China, contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with Eurasia of the Changtang block during Late Triassic–Early Jurassic time and with the mega-Lhasa block during Late Jurassic–Early Cretaceous time. This tectonism is chronologically linked with the creation of a narrow lower Middle Jurassic transtensional basin with thick sedimentary infill, succeeded by a broader Upper Jurassic–Lower Cretaceous compressional(?) basin with thinner sedimentary infill. The older basin formed between a north-northwest–striking dextral fault on the eastern side of the southwest Tarim basin in the Tian Shan and a postulated strike-slip or normal fault on the western margin of the basin along the Kunlun Shan. The former fault is now the Talas-Ferghana fault; the latter may be a predecessor to the Main Pamir thrust. Subsidence analysis of the thickest sedimentary section suggests thermal subsidence, interpreted as the result of transtension between the two basin-bounding faults. The younger basin extends farther east and west and does not preserve evidence of activity along the Talas-Ferghana fault. The change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin-bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes.

Seismotectonic range-front segmentation and mountain-belt growth in the Pamir-Alai region, Kyrgyzstan (India-Eurasia collision zone)

The Trans Alai Range along the northern perimeter of the Pamir region of Kyrgyzstan is at the northern edge of the India-Eurasia collision zone. The range defines a section of Main Pamir thrust, which is divided into eastern, central, and western segments that record differential absorption of plate convergence. The 50-km-long, east-west–striking, central fault segment of the Main Pamir thrust has dip-slip thrust fault offsets (with a minimum Holocene dip-slip rate of ∼6 mm/yr) and is linked to the other segments via northwest-striking dextral transfer. The western transfer faults are well characterized geomorphically, and the westernmost records the predominant transition to north-vergent thrusting along the western segment via north-vergent, low-angle thrust faults rooted in a steeply south-dipping dextral shear zone. In contrast, the eastern segment deformation is widely distributed and geomorphically less evident, but the transfer also takes place in a structurally complex zone. Asymmetric offset of a regionally correlated terrace, geomorphic zonation, and the distribution of historic earthquakes suggest that the central segment is mechanically linked with the western segment, but not with the eastern segment. The progressive closure of the Alai Valley by the northward advance of thrusting exemplifies the annihilation of an intramontane basin. If the high surface uplift rates implied by these geologic observations have been maintained for the last few million years, they attest to the high level of tectonic activity and the recency of construction of the Trans Alai Range.

Thrusting and exhumation around the margins of the western Tarim basin during the India‐Asia collision

The Cenozoic collision between India and Asia has deformed a large part of central Asia. To the north of Tibet around the margins of the western Tarim basin, major basin‐vergent thrusting has uplifted and exhumed thick Jurassic to Neogene sedimentary sections; this presumably reflects the propagation of collision‐induced deformation into the basin. Apatite fission track data from five sections involved in this thrusting record strong late Oligocene to middle Miocene exhumation and cooling. On the northwest margin of the basin on the piedmont of the Tian Shan, a section exhumed by thrusting yields an exhumation age of 13.6±2.2 Ma (±1σ). Four Miocene sandstones from a second section 40 km to the east yield detrital source area cooling ages which decrease upsection from 25.0±3.9 to 13.1±2.2 Ma. Landsat imagery suggests that probable sediment source areas were dominated by Neogene thrusting, so these ages likely record progressive unroofing in Tian Shan thrust systems. Deformed Miocene to Pleistocene strata indicate that thrusting has continued and propagated basinward up until the present. Previously published apatite data from the Junggar basin on the northern flank of the Tian Shan yield a similar age of 24.7±3.9 Ma. On the southwest margin of Tarim on the piedmont of the western Kunlun Shan, three sections yield cooling ages of 19.8±0.9 Ma, 20.0±3.1 Ma, and roughly 20 Ma. Farther south at Kudi, previous work has yielded apatite cooling ages of 17±2 Ma and a zircon cooling age of 22±2 Ma. These similar cooling ages over a ≈250 km long belt in the western Kunlun Shan are associated with the transpressional Kumtag fault and the Main Pamir Thrust (MPT). Geologic relations within the western Kunlun Shan suggest that the MPT‐Kumtag fault system offsets the originally linear trend of the Paleozoic‐early Mesozoic Kunlun arc system by 200–300 km, accommodating much of the Neogene northward indentation of the Pamir block. We propose that the ≈20 Ma ages slightly postdate the initiation of this indentation and consequent crustal thickening. Taken together, the Tian Shan and Kunlun Shan results indicate that crustal thickening, in part accommodated by strike‐slip faulting, became the dominant mode of deformation by ≈25–20 Ma in a large region extending from the Pamir and west Kunlun Shan north to the Tian Shan.

Active tectonics of the Pamirs and Karakorum

The source parameters of moderate‐ to large‐sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three‐dimensional configuration of the subducted lithosphere in the Pamir‐Karakorum intracontinental convergence zone. The continental collision has resulted in intense deformation in the upper crust. The deformation is distributed over a broad area and is absorbed by thrust and strike‐slip faulting. Along the northern margin of the Pamirs, where the deformation due to continental collision is most intense, the Northern Pamir Thrust and several major fault zones mark the present boundary. Most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region. Along the Pamir Front, thrust faulting dominates, while on the western and eastern edges of the Pamirs the style of deformation is characterized by oblique thrusting with a component of strike‐slip motion. Thrust‐type events beneath the Tadjik Depression indicate that both the sedimentary rocks and the basement are involved in shortening. Right‐lateral strike‐slip motion is observed on the eastern edge of the Pamirs where they border the Tarim Basin and in the Talas‐Fergana‐Kun Lun fault zone. A southward dipping seismic zone beneath the Pamirs and Karakorum indicates that the Asian lithosphere has been subducted along the Pamir Front to a depth of at least 200 km. This interpretation is consistent with published geological and geophysical data. Focal mechanism solutions of some intermediate‐depth events beneath the Pamirs show strike‐slip faulting with approximately N‐S horizontally oriented P axes, indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation. A 90‐km‐deep event beneath the Karakorum is interpreted as occurring in the subducted Asian lithosphere; the fact that the P axis of this event is oriented parallel to the descending plate suggests downdip compression within the Asian lithosphere. The overall tectonics of the region is interpreted as a consequence of the underthrusting Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere.