THE RETREAT OF THE PARATETHYS FROM THE TARIM BASIN (WEST CHINA)
LINKED TO THE EOCENE-OLIGOCENE CLIMATE TRANSITION AND THEINDO-ASIA COLLISION
Bosboom, R.E.
Utrecht University, Faculty of Geosciences, 02-03-2008
ABSTRACT
The Paleogene sediments of the southwest Tarim Basin along the West Kunlun Shan in westernmost China include the latest remnants of the easternmost extent of the Paratethys Sea, an epicontinental sea that extended across the Eurasian continent during the Eocene. In this study magneto- and biostratigraphic results from two sections recording the marine to continental transition between the Bashibulake Formation and Kezilouyi Formation are presented, which provide a chronological framework to reconstruct the role of both the Paratethys and the Tibetan uplift with respect to the Eocene-Oligocene transition (EOT) of the global climate and the associated aridification of the Asian interior.
Curie balance processing of representative specimens showed that the marine and continental sediments yield different rock magnetic behaviors. Upon thermal demagnetization, the characteristic remanent magnetization (ChRM) of the marine type specimens was isolated between 100 and 450°C, suggesting that iron-sulfides such as greigite are the most likely carriers of the ChRM. The ChRM of the specimens of the continental type consists of a low-temperature and a high-temperature component, which were unblocked from 350 to 600°C and from 635 to 675°C respectively, suggesting magnetite and hematite are the dominant ferromagnetic carriers. As the specimens of the marine type almost exclusively yielded normal polarity directions and did not pass the reversal and fold tests, secondary magnetization in the present-day field is not excluded. However, for the specimens of the continental type both reversed and normal polarity directions were isolated and positive reversal and fold test results were acquired. Hence, the recognition of a remarkably long reversed polarity zone in the continental red beds, together with early Oligocene biostratigraphic age identification of sampled microfossils recovered from the marine sediments and published accumulation rates allowed for a convincing correlation to polarity chron C12r.
Accordingly, the ultimate retreat of the Paratethys from the Tarim Basin is dated at approximately 33.3 Ma. As the field sections are located in the basin depocentre, the regression of the Paratethys is considered a diachronous process which was caused by the eustatic sea-level fall of the EOT starting at 33.9 Ma. Accordingly, the ultimate of the Paratethys Sea is indeed linked to the EOT and played a key role in enhancing the aridification of the Asian interior. This outcome is in agreement with those climate models suggesting the redistribution of the thermal contrast between land and sea had a profound effect on local climate by changing the regional atmospheric pressure system and by intensifying the seasonal contrast.
The acquired ChRM directions of the continental specimens from both sections showed that a significant clockwise vertical-axis rotation of approximately 15° has occurred along the West Kunlun Shan since 33 Ma. If comparing these results to other published rotations for the West Kunlun, it appears the rotation must have occurred between 33 and 30 Ma. This early Oligocene tectonic activity is further supported by the observed increase in accumulation rates in the upper parts of the sections and by the continuous progradation of the river-dominated delta system seen at both localities. The observations are likely related to the suggested northward indentation of the Pamir within the Indo-Asia collision system and the associated dextral slip along the Karakorum fault, which probably enhanced the ultimate retreat of the Paratethys Sea from the Tarim Basin by disconnecting it from adjacent basins to the west. This early northward indentation of the Pamir may very well be related to early uplift of the northern margin of the Tibetan Plateau and hence its contribution to the Asian aridification by changing the regional atmospheric circulation patterns is not excluded.
INTRODUCTION
The Eocene-Oligocene transition (EOT) is considered the most dramatic change in Cenozoic climate. This shift from greenhouse to icehouse conditions occurred 34 million years ago as the first ice cap formed on the Antarctic continent (Coxall et al., 2005). As this cooling event had a major impact on both hemispheres, one of its most probable causes seems a global reduction of atmospheric carbon dioxide concentrations, likely related to an increased uptake of atmospheric CO2 by weathering in the Indo-Asia collision zone (DeConto et al., 2003).
Major consequences involved a eustatic fall in sea-level, widespread disruption of the global thermohaline circulation and significant floral and faunal turnovers on the continents, of which the European ‘Grande Coupure’ is the best documented example (Ivany et al., 2000).
Recently the Faculty of Geosciences at the Utrecht University, in cooperation with the University of Lanzhou in central China, has shown, by applying magneto- and cyclostratigraphy, that the EOT coincides with the intense aridification of the Asian continental interior (Dupont-Nivet et al., 2007). Other recently published papers also report significant consequences, including the faunal turnover known as the ‘Mongolian Remodeling’ (Meng & McKenna, 1998), change in the regional paleoenvironment of North-America (Zanazzi et al, 2007) and an increase in the intensity of the Asian monsoons (Ramstein et al., 1997; Zhang et al., 2007).
Although the age correlation of changes in Asian climate to the EOT is well-established, the exact forcing mechanisms and processes responsible for the aridification of the central part of Asia remain matter of strong debate. Previous studies have related the observed aridification either to the early uplift of the Tibetan Plateau or to the westward retreat of the Paratethys Sea, an epicontinental sea that extended across the Eurasian continent during the Eocene. An increasing number of studies suggests the northern Tibetan Plateau may have already formed during the late Eocene (Mock et al., 1999; Jolivet et al., 2002; Yin & Harrison, 2000; Yin et al., 2002; Graham et al., 2005; Dai et al., 2006). These conclusions are primarily inferred from mountain range exhumations and the depositional histories of surrounding basins, which are based upon age constraints provided by biostratigraphy, magnetostratigraphy, thermochronology and fission track analyses. However, general circulation models suggest the intensification of the Asian monsoon system and the aridification of central Asia was not only caused by Tibetan uplift (Ramstein et al., 1997; Zhang et al., 2007). These models show the retreat of the Paratethys and the associated redistribution of the thermal contrast between land and sea caused a significant change in the regional pressure system and sharpened the seasonal contrast. Hence, many studies regard the change from marine to continental conditions a far more likely forcing mechanism of the observed change in Asian climate, particularly when considering the simultaneous drop in global sea-level. However, significant proof for this role of the Paratethys has only been illustrated by modeling and not by actual field evidence.
Accordingly, the objective of this project is to precisely determine the timing and in turn the cause and the effects of the retreat of the Paratethys by the application of high-resolution magneto- and biostratigraphy to key sedimentary successions which have recorded this event. The Paleogene sediments of the southwest Tarim Basin along the West Kunlun Shan (Shan means range) in westernmost China are characterized by such a marine to continental transition and comprise the latest remnants of the easternmost extent of the Paratethys Sea before its subsequent westward retreat (Sobel, 1999; Yin et al., 2002; Jin et al., 2003). The transition is often believed to be related to the initial uplift of the Tibetan Plateau by northward indentation of the West Kunlun Shan within the Indo-Asia collision zone, which resulted in a disconnection from the adjacent basins to the west (Burtman &
Molnar, 1993). On the other hand, its correlation to the global eustatic ~60 m sea-level fall of the EOT is also a plausible hypothesis. Hence, this fields study wishes to build an accurate stratigraphic framework, well-established in time, of the marine-continental successions observed in southwest Tarim, which may constrain the cause of the ultimate retreat of the Paratethys and its effect on the Asian aridification.
GEOLOGICAL SETTING
Background
The Paleogene marine deposits of interest are, as mentioned, located along the southwestern margin of the Tarim Basin in the foothills of the West Kunlun Shan, which is along the northwestern rim of the Tibetan Plateau (Fig. 1). The area is characterized by an uninviting arid climate and a landscape of endless sands along the southern margin of the Taklamakan Desert. It is part of the Xinjiang Uyghur Autonomous Region, which is both politically and economically important because of its many neighboring countries and major oil occurrences. As the region is rather remote and isolated, the availability of geologic data is still relatively limited and allows for fundamental research to be conducted. Accordingly, this paper presents the results of the stratigraphic study of that field region, where careful and detailed magneto- and biostratigraphic sampling and logging has been carried out.
The Tarim Basin is by far the largest intracontinental basin of China and one of the largest closed drainage basins in the world, extending over 1500 km in east-west direction and occupying an area of over 500.000 km². The basin is part of a relatively resistant and undeformed crustal block within the Indo-Asia collision system. The successive accretion of continental terranes on the southern Eurasian margin started during the late Triassic and ended by the collision and subsequent progressive movement of the Indian plate towards Eurasia, still active today. As shortening continued after the collision of the two continents approximately 70-50 Ma, the Tarim Basin became marginally overthrusted by the Tian Shan in the north, the Pamir in the west and the Kunlun Shan in the south (Burtman & Molnar, 1993; Yin & Harrison, 2000; Jolivet, 2001; Ratschbacher, 2001). The Kunlun Shan fold-and-thrust belt is part of the field area and extends over 3000 km with an average elevation of 5000 m. It is divided into the West and East Kunlun by the major sinistral Altyn Tagh fault system, which terminates on the dextral Karakorum fault (Fig. 1).
Stratigraphy
The stratigraphy of the Tarim Basin rests on a basement of crystalline metamorphic gneiss of Archean and Proterozoic age, covered by strongly deformed sediments of the Paleozoic and Mesozoic, which are in turn unconformably overlain by less deformed Cenozoic sediments. Marine conditions prevailed till the beginning of the Permian after which continental deposition dominated (Jolivet, 2001; Zheng, 2006). The shallow marine and continental formations of the Cenozoic succession are discussed in detail here.
It seems marine deposition initiated in the late Cretaceous as an eastward transgression resulted in the appearance of a shallow epicontinental sea in the southwest of the Tarim Basin, which was connected to the adjacent Tadjik-Ferghana Basin and
Alai Basin in the west (Burtman & Molnar, 1993; Sobel, 1999; Ratschbacher, 2001; Popov, 2004). Two major transgressive cycles are distinguished during which the interconnected depressions became a unified basin in which abundant grey and green sandstones, marls and limestones were deposited, each characterized by distinct species of bivalves, ostracods and foraminifera. The regressive intervals in between consist of massive evaporites, gypsiferous shales and red marls (Mao & Norris, 1988). The overall thickness of these marine deposits reaches a maximum of over 1500 m in the area near Yarkand (Fig. 1) in the centre of the West Kunlun (Hu, 1992). The ultimate retreat of the Paratethys from the Tarim Basin
resulted in a shift from marine to terrestrial conditions, which divides the Tertiary sequence in two distinct parts (Fig. 2). The upper part accordingly consists of a series of red-beds of fluvial and lacustrine origin, which includes conglomerates, sandstones and shales, locally interbedded with gypsum (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, 1993; Zheng, 2006). The total Tertiary sequence has an approximate thickness of 4 km (Graham, 2005) and its exposure is limited to the marginal areas of the Tarim basin, as Quaternary loess and sand dunes cover the larger part of the basin-fill.
The designation of regional stratigraphic units of Paleogene age is not uniform. In this study the nomenclature of Yin et al. (2002) is used for clarity. This implies the lower part comprises the shallow marine and lagoonal deposits of the Kashi Group, which are overlain by the fluvial and lacustrine deposits of the Wuqia Group. Age constraints are limited, but based upon fossil assemblages the marine deposits are believed to have been deposited from latest Cretaceous until late Oligocene. The marine stratigraphic units of the Kashi Group are the Aertashi, Qimugen, Kalatar, Wulagen and Bashibulake Formations, which primarily consist of grey limestones rich in bivalves and ostracods, massive gypsum beds, grey dolomites, grayish-green mudstones and brownish-red gypsiferous mudstones.
As this study focuses on the transition from marine to continental conditions, the marine Bashibulake Formation is of interest here. In general five stratigraphic members are recognized within this formation, which have been assigned conflicting ages. The lower three members are considered to be of early to middle Eocene age based upon bivalves, whereas age estimates for the fourth and fifth member range from late Eocene based on calcareous nanno-fossils, early Oligocene based on bivalves and late Oligocene based on benthic foraminifera. The fourth member is considered the last unit deposited under marine conditions (Sobel & Dumitru, 1997; Yin et al., 2002). Preliminary inspection of ostracods and foraminifera that were recovered from this member indicates exclusively Early Oligocene (Rupelian) faunas (Dr. Marius Stoica, personal communication). The Bashibulake Formation ranges in thickness from 210 to 350 m, but can locally be completely absent. It consists of grayish-green mudstones, siltstones and shell beds in its lower part and purplish-red mudstones, siltstones and sandstones form its upper part. Characteristic fossils include oysters, other bivalves, gastropods, echinoderms, ostracods and foraminifera (Mao & Norris, 1988). The overlying stratigraphic unit is the continental Kezilouyi Formation of the Wuqia Group which is a 200 to 500 m-thick sequence of red-brown mudstones, siltstones and sandstones with numerous gypsum interbeds (Scharer et al., 2004).
PALEOMAGNETIC SAMPLING
In the summer of 2007 two sections in the field area have been studied in detail during two full weeks to establish the desired stratigraphic framework. These sections are referred to as the Aertashi and Kezi sections, after the villages located nearby. The Aertashi section has already been studied earlier by Yin et al. (2002) and Sobel & Dumitru (1997). The first group has built a stratigraphic framework for the Neogene based upon magnetostratigraphy and fission track dating, whereas the latter primarily focused on fission track dating of the Jurassic to Neogene succession. The Aertashi section (37°58'N, 76°33'E) is well-exposed along the Yarkand River and along the road from Kochum to Aertashi (Fig. 2). The Kezi section (38°26'N, 76°24'E) is exposed along the Kezi river and its tributary streams. The strata at both localities are exposed continuously with homoclinal ~30° dip along broad folds. The structural orientation of the bedding is rather consistent within both localities and shows little variation. At each section, the poles of the measured bedding orientations fall along a great circle, showing that both sections are part of a simple cylindrical fold system. Best-fit fold axes have a negligible plunge of less than 7° and their orientations are N170° and N100° respectively for the Aertashi and Kezi section. As can be seen on the geologic map (Fig. 3) the sections intersect the same two stratigraphic units of Paleogene age. As the two sections are separated laterally by over 50 km, their stratigraphic correlation is not straightforward and is primarily based upon the recognition of five lithostratigraphic units within the overall transition from green marine to red continental beds. Of these the lower four units have been sampled in the Kezi section (Fig. 4 and 5).
The two sections were measured to decimetric precision with measuring tape and magnetic compass. The lowest unit (-90 to -40 meter level for Aertashi; -125 to -40 meter level for Kezi) comprises massive calcareous green sandstones and limestones with shell and oyster beds at their top which are interbedded with green marls. The second unit (-40 to -25 meter level for Aertashi; -40 to -20 meter level for Kezi) is characterized by green marls with scarce fossil evidence. A thick interval of red laminated mud interbedded with limestones and irregular bedded green calcareous (bioturbated) siltstones and fine sandstones rich in fossils characterizes the third unit (-25 to 5 meter level for Aertashi; -20 to 10 meter level for Kezi). The boundary between the third and the fourth unit marks the transition from a marine to a continental depositional environment and is easily recognized in the field by a change in color of the deposits from green to red (Fig. 2). The fourth unit (5 to 185 meter level for Aertashi; 10 meter level to top for Kezi) itself consists of red laminated mud interbedded with increasingly dominant presence of siltstones and fine sandstones with local ripple marks, minor incised channels
and nodular gypsum. The fifth unit (185 meter level to top for Aertashj) strongly coarsens upward and comprises an alternation of red muds and dominant thick fine to medium sandstone beds with planar laminations, trough cross-bedding and major channels. In this unit local gypsum interbeds become more numerous and locally the effects of slumping are observed. The entire lithostratigraphy is interpreted as the progradation of a river-dominated delta system within an overall regressive sequence. The sedimentary facies change from a low-energy shallow marine delta front affected by mass-movements, to a low-energy delta plain affected by minor
transgressions and high-energetic storm events, to a high-energy deltaic-fluvial floodplain. Note that evidence for unconformities has not been found and as such the entire succession appears to be continuous at both localities.
The paleomagnetic samples were collected with a standard electric drill powered by a portable gasoline generator and subsequently orientated with a magnetic compass. The orientations were later corrected for the local declination of 3°. For each horizon one or two oriented cores were obtained. The average sampling interval for the Aertashi section is 1.3 m, as samples were collected from 307 horizons with a total thickness of 390.5 m. Of these oriented samples 15 were collected by hand sampling. The Kezi section has a composite thickness of 224.8 m and yielded core samples from 207 horizons, which results in an average sampling interval of only 1.1 m. The mentioned sampling distances are generally larger in poorly exposed and fragile strata, whereas smaller in well-developed and easily drillable strata. As derived sedimentation rates for continental deposits of the field area are estimated in between 0.1 and 1.0 m.kyr-1 (Yin et al. 2002), the average sampling interval corresponds to a maximum of approximately 13 kyr. This time span is acceptable for accurate magnetostratigraphic dating of
the Paleogene, of which the polarity chrons have a relatively long average duration of close to 1 Myr (Butler, 2002).
PALEOMAGNETIC ANALYSES
Rock magnetism & ChRM directions
Paleomagnetic analyses were carried out in the shielded paleomagnetic laboratory of the Faculty of Geosciences at the Utrecht University. The collected field samples were cut into core specimens of approximately 2 cm in length, whereas the hand samples were cut into cubic-shaped specimens of roughly 2 × 2 × 2 cm. Susceptibility measurements at room temperature were performed on a KLY-2 Kappa bridge for each specimen. Before the actual thermal demagnetization ten representative specimens of characteristic lithologies were powdered and analyzed for their thermomagnetic behavior, using a Curie balance and a CS-3 device coupled to a KLY-3 Kappa bridge, in order to assess the Curie temperature of potential magnetic carriers and to devise the best procedure for subsequent demagnetization of the natural remanent magnetization (NRM).
Based upon the derived rock magnetic properties, upon Curie balance processing of the samples, two behavioral types are distinguished (Fig. 6a and b). This transition in magnetic behavior is observed at level 12.2 ± 0.2 meter level in the Aertashi section (Fig. 4) and at 12.8 ± 0.4 meter level in the Kezi section (Fig. 5), which is within a few meters from the boundary between the third and fourth unit. The first type, in the lower part of the stratigraphy, is referred to as the ‘green’ or ‘marine’ type and includes the marine limestones and green clastics of the first and second unit and the red clastics of the third unit. In general, these lithologies show very weak and irreversible Curie balance behavior (magnetic moment in the order of 1 to 10 mA.m-2.kg-1) with the magnetic intensity peaking around 500°C, probably because iron-oxides are oxidized to magnetite which is subsequently converted to hematite at even higher temperatures. Reduction in intensity at around temperatures of 580 and 670°C confirms the presence of respectively magnetite and hematite. Few samples show a decrease in intensity at lower temperatures, suggesting that iron-sulfides such as greigite function as ferromagnets. The second behavioral type is referred to as the ‘red’ or ‘continental’ type and includes the continental red clastics of the fourth and fifth unit, which express considerably stronger irreversible thermomagnetic behavior (magnetic moment in the order of 10 to 100 mA.m2.kg-1) and which frequently show a decrease in magnetization near the Curie temperature of magnetite and more notably of hematite.
Single specimens of most horizons were thermally demagnetized in a shielded oven and their magnetization at each temperature step was measured by a three-axis 2G Enterprises cryogenic magnetometer. To optimize the demagnetization procedure, a first set of representative samples of both the marine (30 samples) and the continental interval (30 samples) were thermally demagnetized in detail using in between 12 to 20 steps from room temperature up to the respective unblocking temperatures. For the limestones, green clastics and red clastics within the marine to continental transitional interval, the best thermal demagnetization results are acquired by using in between 8 to 12 temperature steps of 50 °C from 100 to 600 °C and an additional step at 380°C. Above the marine to continental transition, the red beds show different magnetic behavior. These samples were demagnetized by using 8 to 18 temperature steps of 50°C from 350 up to 600°C, after which the steps were reduced to 10 and finally 5°C after 620°C as the unblocking temperatures of the NRM carriers were reached.
Upon thermal demagnetization, the characteristic remanent magnetization (ChRM) was defined using visualization on orthogonal (Zijderveld) plots and equal-area plots. In general, ChRM components are recognized after progressive removal of an overprint (possibly viscous and recent) at temperatures of 100 to 350°C and confirm the rock magnetic properties identified from the Curie balance results (Fig. 6).
The ChRM of the marine type is generally observed from 100 to 450°C, suggesting that indeed iron-sulfides such as greigite are the most likely carriers of the ChRM (Fig. 6c, d and e). Infrequently both a low temperature component (LTC) ranging from 100 to 300°C and a high temperature component (HTC) from 300 to 450°C can be distinguished. Nonetheless, as the number of temperature steps is limited, the ChRM of the green samples is interpreted as a single component which generally decays towards the origin. As the magnetization is very weak and ranges in between 20 and 4000 µA.m-1, the demagnetization trajectories are often rather unstable, in particular for those samples rich in fossils and calciumcarbonate. At temperatures above 300°C unstable behavior becomes more apparent and above 500°C a strong increase in the magnetization results most likely from the mentioned conversion of iron-oxides to magnetite.
After stepwise removal of the secondary remanent magnetization of the red samples, it is shown that generally a LTC and a HTC can be discerned (Fig. 6f, g and h). The LTC is on average removed by thermal demagnetization from 350 to 600°C, whereas the HTC is generally unblocked from 635 to 675°C. This supports magnetite is the dominant carrier of the LTC and hematite acts as the principal ferromagnet carrying the HTC. From 600 to 640°C, in between the LTC and HTC, little decay is observed, though occasionally a third temperature component can be recognized. The LTC and in particular the HTC tend to decay towards the origin.
The magnetic intensity generally ranges in between 200 and 14000 µA.m-1. The continental type commonly shows stable thermomagnetic behavior up to temperatures of 600°C, except for those samples with weak magnetization due to, for example, high gypsum content. Unstable behavior at higher temperatures is attributed, at least partly, to the distortion of the ChRM by a weak magnetic field generated in the oven. Those samples with a reversed polarity typically show an obvious increase in the magnetic intensity at a temperature of 350°C, which is most likely due to a relatively strong normal overprint in the present-day magnetic field. For some samples only the HTC shows an unambiguous demagnetization trajectory towards the origin, which implies secondary magnetization is occasionally persistent up to temperatures of 600°C.
To ensure correct interpretation of the datasets, the ChRM directions of the different temperature components were calculated from the orthogonal plots by application of principal component analysis
(Kirschvink, 1980), both for in-situ and bedding tilt-corrected data. The line-fits required a minimum of four temperature steps and were forced through the origin only if directions clustered in a distinct normal or reversed position, albeit did not decay linearly. Maximum angular deviations (MAD) up to 30° were accepted if the polarity could be clearly discerned. Midpoints in between temperature steps and mean component orientations for each polarity were obtained from the ChRM directions by Fisher statistics (Fisher, 1953).
Following these statistical procedures, 112 out of the 177 specimens of the Aertashi section yielded reliable ChRM directions, which results in an average magnetostratigraphic interval of 3.5 m. The Kezi section yielded reliable ChRM directions for 102 out of the 129 demagnetized specimens, resulting in an average magnetostratigraphic interval of 2.2 m. After rejection of the most scattered directions by the Vandamme method (Vandamme, 1994), overall 142 and 121 in-situ and 140 and 125 tilt-corrected ChRM directions were acquired for the Aertashi and Kezi section respectively (Table 1 and Fig. 7a-h). The paleomagnetic data are separated by section (Aertashi or Kezi) and behavioral type (‘green’ or ‘red’). The equal-area plots of ChRM directions corrected for bedding tilt reveal that samples of the marine type generally show no rotation of the declination with respect to North and exhibit normal polarity. As the number of reversed directions of the marine type is very limited, the corresponding reversed mean directions are not taken into account into the discussions below. The majority of the tilt-corrected ChRM directions of the continental type show ~30° rotation of the declinations with respect to North and reversed polarity, except for the red beds at the top of the Aertashi section which display normal polarity directions. Comparison between the LTC and HTC directions of the red beds demonstrates that their Fisher means are rather similar and that the LTC is more scattered, which is ascribed to the observed increase in unstable behavior of the red samples above temperatures of 600°C (Fig. 7i-l).
TABLE 1. SUMMARY OF THE FISHER MEAN DIRECTIONS.
Section Type Polarity In-situ (IS) or
tilt-corrected (TC) N D I κ α95 IS 17 355.7 42.0 12.9 10.3 N TC 14 23.2 16.5 14.3 10.9 IS 5 160.0 -30.9 3.6 47.0 Green (marine) R TC 3 171.9 -38.0 14.9 33.2 IS 37 1.1 44.8 11.8 7.2 N TC 37 27.8 22.0 14.8 6.3 IS 83 174.7 -60.6 8.5 5.7 Aertashi Red (continental) R TC 86 222.6 -29.9 10.1 5.0 IS 56 0.1 47.2 15.3 5.0 N TC 56 3.8 34.8 15.9 4.9 IS 6 189.0 -37.4 3.9 39.1 Green (marine) R TC 5 180.2 -24.9 4.1 43.0 IS - - - - - N TC - - - - - IS 59 208.0 -49.5 9.8 6.2 Kezi Red (continental) R TC 64 206.6 -36.3 8.9 6.3
Note: N is normal; R is reversed; IS is in-situ; TC is tilt-corrected; N is the total number of directions; D is the declination; I is the inclination; κ is the precision parameter; α95 is the confidence limit.
Reliability of ChRM directions
The reversals test was applied to determine if the acquired ChRM components are of a primary origin. Note that a fold test was not applicable at each section due to the very homogeneous structural dip.
At both sections, the reversals test (McFadden & McElhinny, 1990) fails when applied to all directions, which is in essence related to the declination difference between normal polarity directions (mostly from samples of marine origin) and reversed polarity directions (mostly from continental red beds). Based upon the unsatisfactory test results the primary origin of the observed ChRM directions needs to be assessed more profoundly.
The approximate present-day field in the field area has a declination of 3.0° and an inclination of 57.2°, which plots very close to the confidence limit of the in-situ directions of all normal datasets. This does not however necessarily favor secondary magnetization. An independent fold test at each section was not applicable, as mentioned, but generally the overlap in the α95confidence limit of the Fisher mean directions does increase
significantly after tilt correction for both the two sections and the two behavioral types, except for the samples of the marine type of the Kezi section (Fig. 7m and n). This inconsistency suggests that the marine samples of at least the Kezi section have indeed been subjected to an overprint, whereas the continental samples of both sections are likely to reflect primary magnetization.
If considering only the ChRM directions from the red samples, the reversals test is not applicable to the Kezi section as this locality solely yielded samples of reversed polarity. For the samples of the continental type of the Aertashi section, however, both normal and reversed polarities are observed. Those directions do not pass the reversals test either, but if only the ChRM directions of the best quality are taken, which means the directions show explicit polarity and unambiguous linear decay towards the origin, the samples of the continental type of the Aertashi section do pass the reversals test with a classification C (McFadden & McElhinny, 1990). The angular difference is 17.2° between the tilt-corrected mean directions of normal and reversed polarity, which is less than the critical angle of 17.9°. The C-classification, rather than B or A, is attributed to the significant scatter
within the dataset and the variation in declination and inclination between the mean directions of normal and reversed polarity. If taking into account this positive outcome of the reversals test for the samples of the continental type at Aertashi, this indicates the ChRM directions of the red beds appear to be obtained shortly after their deposition. Note however that the LTC might still include some overprint in the present-day field, as the LTC directions in the equal-area plots are slightly more rotated towards North compared to the HTC directions (Fig. 7i-l). This principally concerns the Aertashi section, as the confidence limits of the LTC and HTC means at Kezi show nearly full overlap and are hence not significantly different. Yet, the difference is negligible and the effect of secondary magnetization should be considered minor. The origin of the ChRM of the marine type sediments is however more questionable, particularly for the Kezi section.
The reversals test is not applicable to the samples of the marine type as these have almost exclusively yielded normal directions. As mentioned, after tilt-correction the mean normal polarity of the green samples at Aertashi shows full overlap in confidence limit with the mean of the normal tilt-corrected red samples, whereas the same samples at Kezi show no overlap at all in confidence after tilt-correction. This outcome suggests remagnetization of the marine beds of the Kezi section is quite likely. More importantly, at Kezi the change in polarity from normal to reversed occurs in the stratigraphic interval from 5.7 to 12.4 meter level, which includes the observed change in rock magnetic behavior in between 12.2 and 13.2 meter level. The fact that the change in polarity coincides with the change in behavioral type further supports the hypothesis that the marine sediments of the Kezi section have acquired a secondary magnetization. On the other hand, at Aertashi the change in polarity occurs in the stratigraphic interval from -21.5 to 2.9 meter level, which is considerably lower than the change in behavioral type found between 12.0 and 12.4 meter level. Therefore, the results from the lower part of the reversed interval at Aertashi which have a marine behavior must be primary. This suggests that not all marine sediments in the lower normal interval are remagnetized such that the reversal of interest may be recorded. More work need to be performed to confirm this assertion.
PALEOMAGNETIC RESULTS
Tectonic rotation analysis
The positive reversals test for the red samples of the Aertashi section and the grouping of the mean directions of the red samples of both sections, allow for calculation of the magnitude of the vertical-axis rotation and flattening (Table 2). The green samples are not taken into account here as their secondary nature has not been refuted by a positive reversal or fold test. Based upon the mean directions and the reference pole for Eurasia during the Eocene (Besse & Courtillot, 2002), both localities indicate consistent clockwise rotations suggesting the field area in the southwest of the Tarim Basin has undergone a clockwise rotation of roughly 15° since the early Oligocene deposition of the sediments. The degree of flattening is around 25° for both sections.
TABLE 2. SUMMARY OF TECTONIC ROTATION AND FLATTENING.
Section Type Location
Observed
direction Reference pole p Expected direction Rotation Flattening
Lat. Long. D I α95 Lat. Long. α95 D ± ∆D I ± ∆I R ± ∆R F ± ∆F
Aertashi Red 37.97 76.55 28.8 31 6.2 81.3 162.4 3.3 51.9 11.1 ± 4.2 57.5 ± 3.1 17.7 ± 6.7 26.5 ± 5.5 Kezi Red 38.43 76.4 25.9 34.3 8.8 81.3 162.4 3.8 51.5 11,1 ± 4.9 57.9 ± 3.5 14.8 ± 9.4 23.6 ± 4.8 Note: D is declination; I is inclination; α95 is the confidence limit; p is the co-latitude; ∆ is the 95% deviation; R and
F are the vertical-axis rotation and flattening based upon the difference between the observed and expected directions. The reference pole is for Eurasia during the period from 45 to 35 Ma and based upon data from Besse & Courtillot (2002).
Correlation to the GPTS
Here the correlation of the magnetostratigraphic sections with the geomagnetic polarity time scale (GPTS) is presented, constrained by biostratigraphic age estimates and published sedimentation rates. The recognized polarity zones are in essence based upon the virtual geomagnetic pole (VGP) latitudes calculated from the ChRM directions. Reversal boundaries have been chosen as the midpoints in between two adjacent sample levels with opposing polarity. Polarity zones based upon a single direction within a considerable interval of continuous and opposing polarity are generally neglected.
From the magnetostratigraphic analyses it appears that only a limited number of polarity zones has been sampled in both sections. The Aertashi section comprises two normal polarity zones at its bottom and top, which are separated by a relatively long and continuous reversed interval and of which the top normal polarity zone possibly includes a small reversed interval (Fig. 4). The Kezi section solely incorporates one polarity reversal from normal at its bottom to reversed at its top (Fig. 5). The limited number of polarity zones does not allow for pattern correlation to the GPTS, particularly if considering that the normal polarity zones at the base of both
sections are as mentioned likely to reflect to some extent secondary magnetization by the present-day field. However, the presence of the remarkably long and continuous reversed zone at Aertashi in the stratigraphic interval from 0 to 215 m strongly limits the number of potential correlations to the GPTS.
Yin et al. (2002) estimated the sedimentation rate to be 0.1 m.kyr-1 at the base of their Aertashi section, which should more or less coincide with the top of the succession sampled in this study. This value yields a duration of 2.15 Myr for the long reversed polarity zone in Aertashi. If taking into account the mentioned biostratigraphic age estimates from late Eocene to late Oligocene for the last marine member of the Bashibulake Formation, the substantial reversed zone should either correlate to chron 20r of middle Eocene age or to chron 12r of early Oligocene age. Hence, based upon these correlations to the GPTS (Ogg & Smith, 2004) the marine to continental transition is located near the boundary between C21n and C20r or between C13n and C12r, dating the ultimate retreat of the Paratethys respectively at approximately 45.4 Ma or 33.3 Ma. The latter age is supported by the mentioned preliminary Rupelian biostratigraphic age identification (Dr. Marius Stoica, personal communication) from ostracods and foraminifera sampled in the marine parts of the sections. This correlation implies that the long reversed polarity zone identified in the Aertashi section is indeed C12r with a reported duration of 2.15 Myr. This duration yields a sediment accumulation rate of 0.1 m.kyr-1 for the 215-meter length of the polarity zone, in excellent agreement with the reported rate of Yin et al. (2002). According to this correlation, the overlying normal polarity zone would correspond to C12n with a duration of 0.50 Myr. The normal interval at the top of the Aertashi section has a minimum thickness of 95 m, if neglecting the presence of the small reversed zone in between stratigraphic levels 270 and 280 m, which requires a minimum accumulation rate of 0.19 m.kyr-1. This suggests the accumulation rates have increased to double in the upper part of the Aertashi section.
In the overall successions a distinct cyclicity of sedimentary facies is observed. This cyclicity is recognizable by a repetitive pattern of less resistant, slope forming, fine-grained and thinly bedded intervals alternated by prominent, ledge forming, coarse-grained and thickly bedded intervals. These cycles can also be identified in the susceptibility measurements (Fig. 4 and 5). At Aertashi the cycle thickness steadily increases from around level 190 m upwards, doubling in thickness towards the top of the section. This suggests a gradual increase in accumulation rate that could explain the larger thickness of the normal interval and the doubling of the accumulation rate at the top of the section. The average duration of these cycles can be computed to test whether they can be associated to typical periods of astronomical forcing (eccentricity at 413 kyr and 100 kyr, obliquity at 41 kyr or precession at 23 kyr). In the Aertashi section 33 distinct cycles are observed between stratigraphic level 44 and 189 m, of which the computed average duration is 43.9 kyr per cycle based upon the rate of 0.1 m.kyr-1. This average duration is close to the period of obliquity, which is however quite controversial as the effects of astronomical forcing by obliquity are generally only evident at high latitudes. Between level 189 and 293 m 8 of the very same cycles are recognized, of which the maximum average duration is 68.4 kyr per cycle based on the minimum rate of 0.19 m.kyr-1, which could again correspond to obliquity if considering this is a maximum value. In between levels 195 and 236 m 12 thinner cycles are recognized which have a maximum average duration of 18.0 kyr per cycle based upon the minimum rate of 0.19 m.kyr-1, which could correspond to the period of precession. Accordingly, the effect of astronomical forcing is not fully discarded but requires further research to constrain the accumulation rate by for example performing spectral analyses on the susceptibility measurements.
DISCUSSION
Tectonic implications
The observed rotations are consistent in sense, time and magnitude at both localities suggesting the rotations are related to the very same tectonic mechanism which has affected the area along the West Kunlun Shan since the early Oligocene. The magnitude is however in disagreement with paleomagnetic data by Rumelhart et al. (1999). Their structural study suggests that the region has experienced an insignificant clockwise rotation of 6.2 ± 7.9° since the late Eocene (see correction in Rumelhart et al. (2000)). However, Rumelhart et al. (1999) based their rotation for the West Kunlun on three different sections, being the Aertashi (discussed here), Yinjisha and Puska sections. At Aertashi they calculated a minor clockwise rotation for sediments which were younger than 30 Ma based upon the magnetostratigraphic correlation by Yin et al. (2002). The Yingjisha section, also showing no significant rotation, corresponds to the Uytak section of Chen et al. (1992), of which the sampled sediments have been reinterpreted by Gilder et al. (2001) to be younger than 21 Ma. From the Puska section a minor clockwise rotation was acquired from sediments younger than 50 Ma, however, as Puska is located over 200 km to the east of the Aertashi section where the Kunlun Shan changes to an east-west trend (Fig. 1), the Puska section has probably experienced a different tectonic regime. This means the observed clockwise rotation of 15° is indeed consistent along the West Kunlun and occurred as a major tectonic event after 33 Ma and probably before ~30 Ma west of Puska.
The occurrence of a major clockwise vertical-axis rotation along the West Kunlun during the early Oligocene is in agreement with the current views on the role of the Cenozoic northward indentation of the Pamir in the Indo-Asia collision zone (Yin & Harrison, 2000). This early northward movement of the Pamir is based on major vertical-axis rotations acquired from sediments of Paleocene to Miocene age along the northern margin of the Pamir, ranging from clockwise in the east to counterclockwise in the west (Thomas, 2002). The observed geometry of rotations is believed to be related either to oroclinal bending and radial thrusting of the Pamir arc (Bazhenov, 1994; Robinson et al., 2004) or to induced opposite shear along the eastern and western margins of the Pamir, sinistral and dextral respectively (Searle, 1996). The latter implies the 15° clockwise rotation is directly related to a mechanism of dextral slip along the Karakorum fault. The occurrence of such a major tectonic event during early Oligocene moreover agrees with the maximum exhumation age of 30.0 Ma near Aertashi reported by Sobel & Dumitru (1997), which they assign to an early Oligocene exhumation event along the Karakorum fault. Accordingly, the early Oligocene rotation would support the original view by Burtman & Molnar (1993) that the northward indentation of the Pamir and West Kunlun Shan caused the disconnection of the Tarim Basin from adjacent basins to the west, which enhanced the ultimate retreat of the Paratethys Sea.
Note that the probable increase in sedimentation rate at the top of the Aertashi section is either an indication of climate change, basinal subsidence, uplift of the hinterland or base-level lowering. The former seems to be an unlikely candidate in an overall trend of regional aridification, as an increase in sedimentation rate is likely to be related to an increase in water supply and hence erosion rate. The lowering of base-level due to the observed regression would lead to the development of unconformities like incised valleys or at least to the formation of paleosoils typical of an arid climate, like caliche horizons (Reading, 2006). However, as no unconformities or paleosoils are present in the sedimentary record in the field, the enhanced sedimentation rate and also the overall progradation of the river-dominated delta system seem to be primarily tectonically driven (Reading, 2006). This subsidence or uplift of the field region may indicate that uplift of the northern Tibetan Plateau already occurred at middle Paleogene times, which would support those studies indicating an early formation of the northern Tibetan Plateau (Mock et al., 1999; Jolivet et al., 2002; Yin & Harrison, 2000; Yin et al., 2002; Graham et al., 2005;Dai et al., 2006) and which suggests Tibetan uplift might have indeed enhanced the Asian aridification by changing the regional atmospheric circulation system. Early Oligocene tectonic activity along the West Kunlun Shan may be related to the computed clockwise rotation, which would further support the indentation of the West Kunlun Shan as the cause of the closing of the Tarim Basin and in turn the regression of the Paratethys Sea.
The 25° flattening of inclination is quite substantial and according to ongoing discussions either reflects inclination shallowing, tectonic displacement of Eurasia or variations in the non-dipole components of the geomagnetic field (Dupont-Nivet et al., 2002). Further research of the former may be directed at constraining the role of depositional and compactional processes on the rock magnetic properties by measuring the anisotropy of magnetic susceptibility (AMS).
Climatic implications
The suggested age of the ultimate retreat of the Paratethys Sea of 33.3 Ma means the event occurred within a period of 0.6 Myr after the EOT at 33.9 Ma. This suggests a link between the EOT and the regression of the Paratethys, though the last marine occurrence does not precisely correspond to the EOT. However, isopach maps displaying the thickness of the marine sediments of the Paratethys realm strongly suggest that the westward retreat of the Paratethys is a diachronous process (Hu, 1992; Meng et al., 2001). As the field area of this study is located in the most central and deepest part of the Paratethys Sea near Yarkand, the suggested age is indeed a good indication of the ultimate retreat of the sea from the Tarim Basin. However, the initiation of the retreat may have started earlier along the margins of the basin. This diachroneity remains to be dated in sections along the eastern and northern margins of the Paratethys where the marine successions have a limited thickness. The ages of these sections should be somewhat older, which would emphasize the link between the EOT and the retreat more precisely. The timing of the geological event nonetheless suggests the redistribution of land and sea, and in turn the thermal contrast, was caused by the global sea-level fall during and shortly after the EOT and indeed had a key role in enhancing the Asian aridification by changing the regional pressure system and sharpening the seasonal contrast as suggested by certain climate models (Ramstein et al., 1997; Zhang et al., 2007).
CONCLUSIONS
The preceding magnetostratigraphic analyses of the Paleogene sediments recording the marine to continental transition in the southwest of the Tarim Basin provide new insight on the role of the Paratethys Sea and the Tibetan Plateau during the EOT and the associated Asian aridification. The retreat of the Paratethys from the Tarim Basin is considered a diachronous process which was initially caused by the global sea-level fall related to the EOT at 33.9 Ma and lead to its ultimate retreat at 33.3 Ma in the basin depocentre. Hence, the retreat of the
Paratethys is indeed linked to the EOT and had a key role in enhancing the Asian aridification. Early Oligocene tectonic activity is verified by a major 15° clockwise vertical-axis rotation along the West Kunlun Shan occurring between 33 and 30 Ma, continuous progradation of the overall river-dominated delta system and an increase in sedimentation rate. These observations are likely related to the early northward indentation of the Pamir within the Indo-Asia collision system, which in all probability enhanced the ultimate retreat of the Paratethys by disconnecting the Tarim Basin from adjacent basins and which suggests early Oligocene uplift of the northern margin of the Tibetan Plateau might have also played a role in the aridification of the Asian interior.
ACKNOWLEDGEMENTS
This study has been done as a thesis for the Master Geology at the Utrecht University. I am most grateful to Dr. Guillaume Dupont-Nivet of the Paleomagnetic laboratory, who has supervised me during the whole duration of this project. This thesis is part of his current research on the tectonic and climatic implications of the Tibetan uplift during the Paleogene. He has provided essential guidance through every component of academic research and his careful review of this paper has significantly improved its content. I also would like to thank student Li Chuanxin from Peking University, Dr. Wout Krijgsman and Prof. Cor Langereis for their tremendous input during the two weeks in the field. Tom Mullender and Dr. Mark Dekkers need to be thanked for their laboratory assistance and Prof. Cor Langereis for valuable discussions considering thermomagnetic behavior. Dr. Marius Stoica and Dr. Ghe Popescu from the Bucarest University need to be mentioned here for providing the essential biostratigraphic age constraints from microfossils. Gratitude is finally shown to the Molengraaff Fund and Netherlands Science Organisation (NWO) as their contribution made this project financially possible in the very first place.
REFERENCES
Bazhenov, M.L., Perroud, H., Chauvin, A., Burtman, V.S., and Thomas, J.C., 1994. Paleomagnetism of Cretaceous red beds from Tadzhikistan and Cenozoic deformation due to India-Eurasia collision. Earth
and Planetary Science Letters 124, 1–18.
Besse, J. and Courtillot, V., 2002. Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr. Journal of Geophysical Research, 107, 1-31.
Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, 1993. Regional Geology of Xinjiang Uygur Autonomous Region, Geological Publishing House, Beijing, 1841 p.
Burtman V.S., and Molnar, P., 1993. Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir. Geological Society of America Special Paper 281, 76 p.
Butler, R.F., 1992. Paleomagnetism: Magnetic Domains to Geologic Terranes. Blackwell Scientific Publications, 238 p.
Chen, Y., Cogné, J.P. and Courtillot, V., 1992. New Cretaceous paleomagnetic poles from the Tarim Basin, Northwestern China. Earth and Planetary Science Letters 114, 17-38.
Coxall, H. K., Wilson, P. A., Palike, H., Lear, C. H. & Backman, J., 2005. Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean. Nature 433, 53-57.
Dai, S., Fang, X., Dupont-Nivet, G., Song, C., Gao, J., Krijgsman, W., Langereis, C., and Zhang, W., 2006. Magnetostratigraphy of Cenozoic sediments from the Xining Basin: Tectonic implications for the northeastern Tibetan Plateau. Journal of Geophysical Research, 111.
DeConto, R.M., and Pollard, D., 2003. Rapid Cenozoic glaciation of Antartica induced by declining atmospheric CO2. Nature 421, 245-249.
Dupont-Nivet, G., Guo, Z., Butler, R. F. & Jia, C., 2002. Discordant paleomagnetic direction in Miocene rocks from the central Tarim Basin: Evidence for local deformation and inclination shallowing. Earth
Planetary Science Letters 199, 473-482.
Dupont-Nivet, G., Krijgsman, W., Langeris, C.G., Abels, H.A., Dai, S. & Fang. X., 2007. Tibetan Plateau aridification linked to global cooling at the Eocene-Oligocene transition. Nature 445, 635-638.
Fisher, R.A., 1953. Dispersion on a sphere, Proc. Royal Society 217 (Series. A), 295–305.
Graham, S.A., Chamberlain, C.P., Yue, Y., Ritts, B.D., Hanson, A.D., Horton, T.W., Waldbauer, J.R., Poage, M.A. and Feng, X., 2005. Stable isotope records of Cenozoic climate and topography, Tibetan Plateau and Tarim Basin. American Journal of Science 305, 101-118.
Gilder, S., Chen, Y., and Sen, S., 2001. Oligo–Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia. Journal of Geophysical Research 106, 30505–30521.
Hu, B., 1992. Petroleum geology and prospects of the Tarim (Talimu) Basin, China. In: Halbouty, M.T., (Ed.), Giant Oil and Gas Fields of the Decade 1978–1988, AAPG Memoir, 54, 493–510.
Ivany, L.C., Patterson, W.P., and Kyger, C.L., 2000. Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary. Nature 407, 887-890.
Jin, X., Wang, J., Chen, B. & Ren, L., 2003. Cenozoic depositional sequences in the piedmont of the west Kunlun and their paleogeographic and tectonic implications. Journal of Asian Earth Sciences 21, 755-765.
Jolivet, M., Brunel, M., Seward, D., Xuc, Z., Yang, J., Roger, F., Tapponnier, P., Malavieille, J., Arnaud, N. and Wu, C., 2001. Mesozoic and Cenozoic tectonics of the northern edge of the Tibetan plateau: fission-track constraints. Tectonophysics 343, 111– 134.
Kirschvink, J.L., 1980. The least squares line and plane and the analysis of paleomagnetic data. Geophysical
Journal of the Royal Astronomical Society 62, 699–718.
Mao, S. and Norris, G., 1988. Late Cretaceous–early Tertiary dinoflagellates and acritarchs from the Kashi area, Tarim Basin, Xinjiang Province, China. Life Science Contributions, Royal Ontario Museum 150, 93 p. McFadden, P.L. and McElhinny, M.W., 1990. Classification of the reversals test in paleomagnetism.
Geophysical Journal International 103, 725–729.
Meng, J. & McKenna, M. C., 1998. Faunal turnovers of Palaeogene mammals from the Mongolian Plateau.
Nature 394, 364-367.
Meng, Q., Hu, J. and Yang, F., 2001. Timing and magnitude of displacement on the Altyn Tagh fault: constraints from stratigraphic correlation of adjoining Tarim and Qaidam basins, NW China. Terra Nova 13 (2), 86-91.
Mock, C., Arnaud, N. and Cantagrel, J.M., 1999. An early unroofing in northeastern Tibet? Constraints from 40Ar/39Ar thermochronology on granitoids from the eastern Kunlun range (Qinghai, NW China). Earth
Planetary Science Letters, 171, 107–122.
Ogg, J.G., and Smith, A.G., 2004. The geomagnetic polarity time scale. In: Gradstein, F.M., Ogg, J.G., and Smith, A.G., (Eds.), A geologic time scale 2004, Cambridge, Cambridge University Press, 63-86. Popov, S., Rögl, F., Rozanov, A.Y., Steininger, F.F., Shcherba I.G., and Kovac, M., 2004.
Lithological-Paleogeographic maps of Paratethys 10 Maps Late Eocene to Pliocene. Courier Forschungsinstitut
Senckenberg 250, 1-42.
Ramstein, G., Fluteau, F., Besse, J. & Joussaume, S., 1997. Effect of orogeny, plate motion and land–sea distribution on Eurasian climate change over the past 30 million years. Nature 386, 788-795.
Ratschbacher, L., 2001. A short story about the geological history of the Pamir. Open report, University of Mining and Technology Freiberg, Institute of Geology, Department of Tectonophysics.
Reading, H.G., (Ed.), 2006. Sedimentary Environments: Processes, Facies and Stratigraphy, Blackwell Publishing Inc., Oxford, 688 p.
Robinson, A.C., Yin, A., Manning, C.E., Harrison, T.M., Zhang, S.-H., and Wang, X.-F., 2004. Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China. Geological Society of America Bulletin, 953-973.
Rumelhart, P.E., Yin, A., Cowgill, E., Butler, R.F., Zhang, Q. and Wang, X.F., 1999. Cenozoic vertical-axis rotation of the Altyn Tagh fault system, Geology 27(9), 819-822.
Rumelhart, P.E., Yin, A., Cowgill, E., Butler, R.F., Zhang, Q. and Wang, X.F., 2000, Correction to: Cenozoic vertical-axis rotation of the Altyn Tagh fault system, Geology 28(5), 480-480.
Scharer, K.M., Burbank, D.W., Chen, J., Weldon, R.J., Rubin, C., Zhao, R. and Shen, J., 2004. Detachment folding in the Southwestern Tian Shan – Tarim foreland, China: shortening estimates and rates. Journal
of Structural Geology 26, 2119-2137.
Searle, M.P., 1996. Geological evidence against large-scale pre-Holocene offsets along the Karakoram fault: implications for the limited extrusion of the Tibetan plateau. Tectonics 15, 171– 186.
Sobel, E.R., 1999. Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim Basin, northwest China.
Geological Society of America Bulletin 111, 709–724.
Sobel, E.R., Dumitru, T.A., 1997. Thrusting and exhumation around the margins of the Western Tarim Basin during the India–Asia collision. Journal of Geophysical Research 102, 5043–5063.
Thomas, J.C., Lanza, R., Kazansky, A., Zykin, V., Semakov, N., Mitrokhin, D. and Delvaux, D., 2002. Paleomagnetic study of Cenozoic sediments from the Zaisan basin (SE Kazakhstan) and the Chuya depression (Siberian Altai): tectonic implications for central Asia. Tectonophysics 351, 119-137. Vandamme, D., 1994. A new method to determine paleosecular variation. Physics of the Earth and Planetary
Interiors, 85, 131-142.
Yin, A. and Harrsion, T.M., 2000. Geologic evolution of the Himalayan-Tibetan Orogen. Annual Review Earth
and Planetary Sciences 28, 211–280.
Yin, A., Rumelhart, P.E., Butler, R., Cowgill, E., Harrison, T.M., Foster, D.A., Ingersoll, R.V., Zhang, Q., Zhou, X.Q., Wang, X.F., Hanson, A. and Raza, A., 2002. Tectonic history of the Altyn Tagh fault system in
northern Tibet inferred from Cenozoic sedimentation. Geological Society of America 114 (10), 1257-1295.
Zanazzi, A., Kohn, M.J., MacFadden, B.J. & Terry Jr., D.O., 2007. Large temperature drop across the Eocene-Oligocene transition in central North America. Nature 445, 639-641.
Zhang, Z., Wang, H., Guo, Z. & Jiang, D., 2007. What triggers the transition of palaeoenvironmental patterns in China, the Tibetan Plateau uplift or the Paratethys Sea retreat? Palaeogeography, Palaeoclimatology,
Palaeoecology 245, 317-331.
Zheng, H., Huang, X. and Butcher, K., 2006. Lithostratigraphy, petrography and facies analysis of the Late Cenozoic sediments in the foreland basin of the West Kunlun. Palaeogeography, Palaeoclimatology,
