Late Cenozoic magnetochronology and ... - Wiley Online Library

6 downloads 2 Views 940KB Size Report
Apr 27, 2007 - 24 to 7 Ma [e.g., Hendrix et al., 1994; Burtman et al., 1996;. Métivier ..... Wang, 1983; Wu and Sun, 1987; Sun and Wu, 1988; Ma et al.,. 2004].

Click Here

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B04107, doi:10.1029/2006JB004653, 2007

for

Full Article

Late Cenozoic magnetochronology and paleoenvironmental changes in the northern foreland basin of the Tian Shan Mountains Jimin Sun,1 Qinghai Xu,2 and Baochun Huang1 Received 24 July 2006; revised 2 November 2006; accepted 8 December 2006; published 27 April 2007.

[1] The folded Neogene deposits in the northern foreland basin of the Tian Shan

Mountains provide important information about tectonic history and environmental changes of northwestern China. On the basis of lithostratigraphic and magnetostratigraphic evidence, we develop a new chronology for the previous studied Kuitun He section. Our palynological study on the late Miocene–Pliocene sediment of the Dushanzi section yields new information about vegetation and climate change during the period of 8.7–2.58 Ma. The results indicate that steppe taxa (Artemisia and Chenopodiaceae) were generally dominant in the studied areas between 8.7 and 2.58 Ma, implying that a dry climate has occurred in the inland basins of northwestern China at least since 8.7 Ma ago. Although the general climate pattern indicated by our palynological results displays to some extent of drought, a warm and humid phase occurred at 5.8–3.9 Ma ago. This climatic optimum is comparable with the other records from the Chinese Loess Plateau, central Japan, the Sub-Himalayan Zone, and the marine eustatic sea level rise, implying this seems to be, at least, a regional climatic optimum. Citation: Sun, J., Q. Xu, and B. Huang (2007), Late Cenozoic magnetochronology and paleoenvironmental changes in the northern foreland basin of the Tian Shan Mountains, J. Geophys. Res., 112, B04107, doi:10.1029/2006JB004653.

1. Introduction [2] The Neogene deposits in the foreland basins of high Asia provide opportunities not only for investigating the uplift history related to the India-Asia collision but also for studying the paleoenvironmental changes in relation to changing geographical patterns. The uplift of high Asia during the Cenozoic has resulted in slope instability [Scheidegger, 1999], intense physical weathering, high runoff, and quick accumulation of sediments in the foreland basins. The late Neogene deposits in the northern foreland basins of the Tian Shan Mountains, the focus of our research, constitute the late Cenozoic record of crustal shortening and paleoenvironmental changes in an active tectonic region. [3] The Cenozoic deposits in the northern piedmont of the Tian Shan Mountains are considered as the standard Neogene sequences in northwestern China [Editing Committee of the Stratigraphy in China, 1999]. Many studies have been carried out dealing with stratigraphy [Editing Committee of the Stratigraphy in Xinjiang, 1981; Chen et al., 1994], mammalian fossils [Peng, 1975], Ostracoda [Editing Committee of the Stratigraphy in China, 1999], and the tectonic deformation of the Neogene strata [Avouac and Tapponnier, 1993; Avouac et al., 1993; Hendrix et al., 1

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2 College of Resources and Environment, Hebei Normal University, Shijiazhuang, China. Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JB004653$09.00

1994; Molnar et al., 1994; Burchfiel et al., 1999; Fu et al., 2003]. Recently, detailed magnetostratigraphy of the strata has been established [Sun et al., 2004], and the comparison with the standard magnetic polarity data of Cande and Kent [1995] provides a chronology for the thick Neogene deposits. Another recent magnetostratigraphy study gives different lithostratigraphy and age control, especially for the famous Xiyu Formation [Charreau et al., 2005]. Such debates will cause confusion; therefore it is necessary to revise both the lithostratigraphy and the magnetostratigraphy in the region studied. [4] As the late Cenozoic is a time of major vegetation changes, partly related to tectonic events in high Asia [Molnar and England, 1990], detailed study of the Neogene palynostratigraphy in the foreland basins of the Tian Shan Mountains is anticipated to reveal the interplay between mountain building and climatic changes. The aims of this study are (1) to provide a more detailed lithostratigraphy in the region studied, (2) to produce a new highresolution age constraint on these Neogene deposits, and (3) to reconstruct the long-term climatic changes based on palynological studies of the late Miocene-Pleistocene terrestrial sequences.

2. Geological Settings and Stratigraphy 2.1. Geological Settings [5] To be one of the most active mountain belts in the world [DeMets et al., 1990], the Tian Shan range is the longest and highest mountain belt in central Asia. It consists of several parallel ranges that extend east-west for more than 2100 km (Figure 1a), with peaks generally higher than

B04107

1 of 14

B04107

SUN ET AL.: MAGNETOCHRONOLOGY AND PALEOENVIRONMENT

B04107

Figure 1. (a) Active tectonic map of central Asia (modified from Avouac and Tapponnier [1993]). ALTF, Altyn Tagh Fault; HYF, Haiyuan Fault; KLF, Kunlun Fault; MFT, Main Frontal Thrust. (b) Geological map showing the three thrust faulting and folding zones (I, II, III) on the northern flank of the Tian Shan range. 3000 m above sea level (asl) and maximum elevations exceed 7000 m. Lying north of the Tarim basin and the Pamir (Figure 1a), the crustal shortening of the Tian Shan range reflects the intracontinental deformation in response to the ongoing convergence between India and Siberia [DeMets et al., 1990], indicating a close link with the India-Asia collision system [Avouac et al., 1993]. The Tian Shan range was originally formed during the Paleozoic accretion of

paleo-Asia [Windley et al., 1990; Deng et al., 2000]. There has been general agreement that most of the relief in the Tian Shan range had been smoothed out by the end of the Mesozoic and that the Cretaceous and early Tertiary were particularly quiet tectonic periods [Burtman, 1975; Deng et al., 2000]. Its relief was reactivated during the late Cenozoic [Tapponnier and Molnar, 1979; Yin et al., 1998; Allen et al., 1999; Burchfiel et al., 1999; Fu et al., 2003], and this region

2 of 14

B04107

SUN ET AL.: MAGNETOCHRONOLOGY AND PALEOENVIRONMENT

B04107

Figure 2. Present vegetation belts from the Tian Shan range to its northern foreland basin. has been undergoing crustal shortening since the early Miocene for a total of about 200 ± 50 km [Avouac et al., 1993; Abdrakhmatov et al., 1996]. The deformation within the Tian Shan range is mostly related to India-Eurasia convergence [Burchfiel et al., 1999], but the time of initiation of deformation in this region remains unclear ranging from 24 to 7 Ma [e.g., Hendrix et al., 1994; Burtman et al., 1996; Me´tivier and Gaudemer, 1997; Yin et al., 1998; Sun et al., 2004; Huang et al., 2006a]. To constrain the dynamics of the Tian Shan range, it is necessary to have detailed lithostratigraphy and convincible chronology of the thick Cenozoic deposits in its foreland basin. [6] The main range of the Tian Shan Mountains consists of Paleozoic rocks, and it is flanked by three low-elevation mountains of deformed Mesozoic and Cenozoic rocks, which form three parallel ridges of anticlines (Figure 1b). All the three anticlines (I to III) are characterized by an elongate shape and west-east striking axes, indicating a north-south contraction of the range. The Dazimiao anticline (I) consists of Mesozoic and Cenozoic strata, the Huoerguosi anticline (II) mainly consists of Paleogene to Pleistocene strata, whereas the Dushanzi anticline and the neighboring Anjihai anticline (III) are composed of Neogene to Pleistocene deposits (Figure 1b). [7] In this paper, we focus on studying NeogenePleistocene deposits of the third thrusting and folding zone (III) (Figure 1b). The region studied has an elevation of about 1000 m asl, and it is located in a semiarid region with a mean annual temperature of 7.3°C and a mean annual precipitation of 183 mm. Vegetations on the northern slopes of the Tian Shan is extremely diverse, due to a wide range in altitude and climate, changing from the desert steppe (elevation less than 800 m asl), dry steppe (800 – 1700 m asl), montane needleleafed forest (1700 – 2800 m asl), to subalpine meadow (2800– 3300 m asl) (Figure 2). 2.2. Stratigraphy [8] The third thrusting and folding zone (III) consists of several discontinuous segments cut by the Kuitun He and

Anjihai rivers (Figure 3a). The late Cenozoic deposits exceed 3000 m thick. The best outcrops occur at Kuitun He, Dushanzi, and Anjihai (Figure 3a), the strata among the three sites can be well correlated (Figures 3b– 3d). [9] The oldest strata of the Dushanzi anticline is the Taxihe Formation (Figures 3b and 3c), it is mainly composed of fine reddish to brownish mudstone, and becomes intercalated with green sandstone and/or grey conglomerates in its uppermost part. Its thickness varies from several hundreds of meters to 1200 m, and its age is interpreted to be Miocene based on the presence of Ostracoda fossils [Editing Committee of the Stratigraphy in China, 1999] and the paleomagnetic results [Sun et al., 2004; Charreau et al., 2005]. [10] Resting conformably upon the Miocene Taxihe Formation, the middle part of the strata is the Dushanzi Formation [Editing Committee of the Stratigraphy in Xinjiang, 1981] with a thickness of 400 –540 m. It consists of alternatively deposited brownish mudstone and grey conglomerates (Figures 3b – 3d). The gravel beds vary between several to tens of meters thick. The age of the Dushanzi Formation is interpreted to be Pliocene based on biostratigraphic control [Editing Committee of the Stratigraphy in China, 1999] and the paleomagnetic results [Sun et al., 2004]. [11] The uppermost part is typical massive molasse deposits, consisting of dark grey pebble to boulder conglomerates (Figures 3b– 3d). It is the most prominent strata and named as Xiyu Formation by Huang et al. [1947]. It rests conformably upon the Pliocene Dushanzi formation. The best outcrops of the Xiyu Formation occur at Dushanzi and Kuitun He (both the west and east banks, see Figure 4a for location) with a thickness of about 1700 m (Figures 3b and 3c), and there is a sharp boundary between the Xiyu Formation and the underlying Dushanzi Formation (Figures 4b– 4d). All these demonstrate that the lithology of the Xiyu Formation is quite different from the underlying Dushanzi Formation. Therefore it is not a fact that ‘‘it is difficult to clearly delimit the Neogene Dushanzi Formation

3 of 14

B04107

SUN ET AL.: MAGNETOCHRONOLOGY AND PALEOENVIRONMENT

B04107

Figure 3. (a) Landsat TM image showing the Dushanzi and Anjihai anticlines. (b– d) Stratigraphy of the strata at Kuitun He (west bank), Dushanzi, and Anjihai, respectively. N1, N2, Q1, Q2, Q3 represent the Miocene, Pliocene, early Pleistocene, middle Pleistocene, and late Pleistocene, respectively. Early Pleistocene vertebrate fossil of Equus was found at the base of the Xiyu Formation at Anjihai by Peng [1975]. The bold dashed lines indicate our field paleomagnetic sampling. from the overlying Xiyu Formation’’ as argued by Charreau et al. [2005]. Fossils of Equus sanmeniensis were found at the base of the Xiyu Formation in the Anjihai section [Peng, 1975] (see Figure 3d), suggesting the age of the Xiyu Formation is younger than 2.6 Ma. However, the recent study attributed a basal age of 3.8 Ma to the Xiyu formation [Charreau et al., 2005], because they included the upper

part of the Pliocene Dushanzi Formation to the early Pleistocene Xiyu Formation. This age assignment of the Xiyu Formation is obviously controversial with other previous studies on the Cenozoic deposits of the northern Tian Shan range [Avouac and Tapponnier, 1993; Deng et al., 2000; Sun et al., 2004]. The following lines of evidence support our view.

4 of 14

B04107

SUN ET AL.: MAGNETOCHRONOLOGY AND PALEOENVIRONMENT

B04107

Figure 4. (a) Landsat TM image showing the western part of the Dushanzi anticline as well as the locations of the three mentioned sites. (b –d) Photos showing the stratigraphic boundary between the Xiyu Formation and the Pliocene Dushanzi Formation at Dushanzi, and west and east banks of the Kuitun He, respectively. Lithostratigraphic boundary between the two formations is distinct. [12] 1. The section studied by Charreau et al. [2005] is located on the west bank of the Kuitun He River. The sharp boundary between the Xiyu Formation and the underlying Pliocene Dushanzi Formation is distinct (Figure 4c). This boundary can be more easily observed in both the Dushanzi section (Figure 4b) and on the east bank of the Kuitun He section (Figure 4d), because field observations indicate that the conglomerates of the Xiyu Formation make a morphologically resistant horizon that often forms the jagged crests (Figures 4b and 4d). [13] 2. Equus became widespread in the North American faunas after about 3.3 Ma, but conditions for dispersal to Asia were apparently not favorable until about 2.6 Ma after the formation of the northern Hemisphere ice sheet [Opdyke et al., 1979; Lindsay et al., 1980]. Therefore the occurrence of the Mammalian fossil of Equus sanmeniensis near the base of the Xiyu Formation in the Anjihai section suggests an early Pleistocene age of the Xiyu Formation. [14] 3. Although we did not find Equus at the studied sites of Dushanzi and Kuitun He, which are about 50 km west of the Anjihai section, all the three sites belong to the same folding and thrusting zone, paralleling the Tian Shan range. Regional geological investigations indicate that deformation in the region studied began after the deposition of the lower part of the Xiyu conglomerates as indicated by

syntectonic deposition [Burchfiel et al., 1999], in this sense, a diachronous boundary near the beginning of the early Quaternary Xiyu Formation within the studied Dushanzi anticline could not exist. [15] 4. The onset of large ice sheets in the high northern latitudes has been demonstrated to be around 2.58 Ma [Shackleton et al., 1984; Jansen et al., 1988; Ruddiman and Raymo, 1988], and abundant glaciers after this time would have been responsible for rock erosion and clastic material production. The accumulation of the thick Xiyu conglomerates, which contain cobbles and boulders of large size, reflects the onset of the ice age in the nearby mountains [Avouac et al., 1993]. In other words, climatic cooling rather than tectonics has played a dominant role in producing the thick Xiyu conglomerates [Avouac et al., 1993; Sun et al., 2004]. Accepting this interpretation, that base of the Xiyu conglomerate should be the same age in the region studied and would be around 2.58 Ma.

3. Material and Methods [16] The recent magnetostratigraphic study of Charreau et al. [2005] gives different lithostratigraphy and chronology for the upper part of the Neogene strata of the Kuitun He section (west bank); it is largely due to the too few

5 of 14

B04107

SUN ET AL.: MAGNETOCHRONOLOGY AND PALEOENVIRONMENT

B04107

Figure 5. Photo showing the upper part of the Kuitun He section (west bank). Our sampling is from the thin siltstone intercalations near the base of the Xiyu Formation to the Pliocene Dushanzi Formation. samples (less than 30) for a thickness of more than 400 m. Therefore it is necessary to have more detailed magnetostratigraphic studies on the upper part of this section. [17] Field observations at the Kuitun He section (west bank) indicate that the Xiyu Formation is dominated by massive conglomerates, which are impossible for paleomagnetic sampling. However, near the base of this formation, there are several thin (10 to 20 cm thick) siltstone intercalations. Therefore our sampling began from the interbedded thin siltstone layers near the base of the Xiyu Formation down to the Pliocene Dushanzi Formation (Figure 5), with a sampling interval of about 5 m. In this study, 176 oriented specimens were taken from the upper part of the Kuitun He section (west bank), and at every sampling position, we collected two samples. All the samples were cored with a portable gasolinepowered drill. Orientations were measured by using an inclinometer for determing inclination (dip) of the core axis and magnetic compass (Suunto MC-2) for determining azimuth of core axis. The accuracy of orientation by such methods is about ±2°. [18] All samples were subjected to stepwise (averaging 18 steps) thermal demagnetization, using the following stepwise heating routine: (1) 50°C steps up to 500°C, (2) 25°C steps up to 610°C, and (3) 10°C or 5°C steps up to 670°C or 680°C. Magnetic remanence was measured with a 2G, three-axis, cryogenic magnetometer housed in field-free space (

Suggest Documents