1040 Science in China Ser. D Earth Sciences 2005 Vol.48 No.7 1040—1051 Magnetostratigraphy of the late Cenozoic Laojunmiao anticline in the northern Qilian Mountains and its implications for the northern Tibetan Plateau uplift FANG Xiaomin1,2, ZHAO Zhijun3,2, LI Jijun2, YAN Maodu2, PAN Baotian2, SONG Chunhui2 & DAI Shuang2 1. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China; 2. Key Laboratory of Western China's Environmental Systems, Ministry of Education & College of Resources and Environment, Lanzhou University, Lanzhou 730000, China; 3. College of Geography, Nanjing Normal University, Nanjing 210097, China Correspondence should be addressed to Fang Xiaomin (email: [email protected]) Received May 22, 2003 Abstract Cenozoic sediments in the foreland basin——Jiuquan Basin in west Hexi Corridor recorded tectonic uplift information of the Qilian Mountains. High resolution paleomagnetic dating of the Laojunmiao (LJM) section across the central LJM anticline in the southern Jiuquan Basin reveals ages of the Getanggou Member, Niugetao Member in the Shulehe Formation, the Yumen Conglomerate, Jiuquan Conglomerate and Gobi Formation at >13―8.3 Ma, 8.3―4.9 Ma, 3.66― 0.93 Ma, 0.84―0.14 Ma and 0.14―0 Ma, respectively. Sedimentary evolution study suggests that the Qilian Mountains should begin to rise gradually since ~8―6.6 Ma, accompanied by sedimentary environments changing from lacustrine mudstones-sandstones to alluvial conglom- erates. Rapid uplift of the Qilian Mountains began at ~3.66 Ma, followed by a series of stepwise or intermittent intensive uplifts at about <1.8―1.23 Ma, 0.93―0.84 Ma and 0.14 Ma, which finally resulted in the present high Qilian Mountains. Keywords: Qilian Mountains, Qinghai-Tibetan Plateau, Jiuquan Basin, magnetostratigraphy. DOI: 10.1360/03yd0188 The uplift process of the Qinghai-Tibetan Plateau Any surface uplift of the Plateau will enlarge relief, holds the key to understand the dynamic mechanisms which should be recorded directly by sediments in of continental crust shortening and mountain-building related basins. Only tectonic-caused surface uplift is of and to test the relationship between the Tibetan uplift our interest here. By dating the absolute ages of the and tectonic-climatic coupling and environmental im- Cenozoic foreland stratigraphy of the Siwalik Group, pacts[1―4]. However, there are still many debates in the and deciphering signals of tectono-sedimentary envi- process and mechanism of how the Tibetan Plateau ronmental changes, the uplift history of the Himalayas uplifted to the present configuration. Among various has been reconstructed[1,2]. However, less work has approaches to solve these key questions, dating of the been done in the northern Tibetan Plateau. Two excep- Cenozoic stratigraphy in foreland basins around the tional cases come from the works in the northeastern margin of the Tibetan Plateau is of great significance. and northwestern Tibetan Plateau[5—8]. The former has Copyright by Science in China Press 2005 Magnetostratigraphy of the late Cenozoic stratigraphy in northern Qilian Mountains 1041 reconstructed the evolution of the northeastern Tibetan from a northeastern accretion of a large scale upper Plateau since 29 Ma, based on detailed chronology and crust decollement decoupled from the lower crust and environmental studies supported by the National Ti- mantle, which is controlled by a co-action of the betan Project in the Chinese 8th Five-Year Plan in Kunlun strike slip and the Altyn Tagh strike slip Science, which demonstrates that the northeastern faults[11,12]. Here we make very detailed magneto- [6,7] Plateau began to uplift rapidly at ~3.6 Ma . The lat- stratigraphic study in the most representative basin — ter measured a long section at Yecheng in the southern the Jiuquan Basin in the northern Tibetan Plateau (Fig. Tarim Basin and showed that the West Kunlun Moun- 1), with an attempt to focus on the well-known Yumen tains underwent a rapid uplift also at ~3.6 Ma[8]. conglomerates and other related tectonic events. However, evidence only from the two localities are too 1 Geological setting few to allow the reconstruction of process of uplift of the entire Tibetan Plateau, more locations are needed. The Hexi Corridor is a foreland basin, defined by The Cenozoic stratigraphy in the foreland Hexi Corri- the NWW northern Qilian Mountains fault to the south dor is a key locality, where it is right situated at the and the Longshou Shan fault to the north (Fig. 1). It is central part of the principal stress of the N-S compres- further dismembered into several sub-basins by a sion——a place still tectonic active in present day[9―11]. group of active NNW-NW dextral transpressional Nevertheless, one of popular models believes that the sub-faults and their controlled upheavals. For example, uplift process of the NE Tibetan Plateau might differ the Wenshu Shan upheaval and the Jiayuguan fault from that of the main Plateau south of the Kunlun divide the Jiuquan Basin into the Jiuxi Basin and Jiu- Mountains. The NE Tibetan Plateau may be formed dong Basin, the Yumu Shan upheaval and the Yumu Fig. 1. Tectonic location and N-S cross section of the Jiuxi Basin (a) and the Laojunmiao anticline (b). MF1 and MF2, Miaobei No.1 and No. 2 faults; BF, Bainan fault; XF, Xinminbao fault; QF, Northern Qilian Mountains fault; KF, Kuantan Shan fault; S, Silurian; C, Carboniferous; P-T, Per- mian to Triassic; K1x, Lower Cretaceous; E3h, Huoshaogou Fm; E3b, Baiyanghe Fm.; N, Shulehe Fm; Q1, Yumen Conglomerate; I-I′, the sampling profile. 1042 Science in China Ser. D Earth Sciences Shan fault separates the Jiudong Basin and the Mingle glomerate according to its cementation and thought Basin, and the Dahuang Shan borders the Mingle Ba- that its age should be Neogene and the age of the Jiu- sin and the Wuwei Basin. quan Gravel Bed should be also older[15]. In 1958, Song Zhichen pointed out that the Shulehe Formation The Jiuxi Basin is a sedimentary basin located at should be the Pliocene in age and the Baiyanghe For- the westest Hexi Corridor. It is delineated by the Qil- mation may be the Miocene or Pliocene in age ac- ian Mountains to the south, the Chijin Gorge—Kuan- cording to fossil pollen spores in the Jiuquan Basin[16]. tan Shan—Hei Shan to the north, the Xiaohongliu Since 1993, preliminary paleomagnetic works were Gorge to the west, and the Jiayuguan—Wenshu Shan made for the Cenozoic stratigraphy in the basin[17—19], to the east. Tectonically it is directly controlled by the yielding rough ages generally agreeing with those northern Qilian Mountains thrust fault and Altyn Tagh suggested by fossil mammals. Because all these are strike slip fault (Fig. 1). The sedimentary sequences largely based on estimations from stratigraphic rela- completely recorded the tectonic uplift process of the tionship and also the obtained paleomagnetic ages northern Qilian Mountains. The basin can be tectoni- have very big uncertainties due to very large sampling cally divided in three zones: the southern upheaval, the intervals and incomplete observed magnetic chrons, it central syncline depression consisting of Cenozoic is necessary to make a detailed systematic chrono- stratigraphy, and the northern monoclinal slope of Eo- logical study of the Cenozoic stratigraphy in the Hexi gene base rocks (Fig. 1). The southern upheaval zone Corridor. Therefore, we chose the most representative is of great interest. It consists of a series of NW anti- late Cenozoic stratigraphic LJM anticline section in clines involving mostly the late Cenozoic stratigraphy the Jiuquan Basin as our study area. and the Yumen conglomerates (Fig. 1). In geomor-- phology, the Jiuxi Basin is contrast to the Qilian The LJM anticline, where the Chinese first petro- Mountains by large different elevations. The basin- leum borehole was drilled, is located at Yumen City surface is at altitudes of 2000―2300 m, while the Qi- (97°32′E, 39°47′N), belongs to one of the series of lian Mountains stands up to over 4500 m a.s.l. (Fig. 1). anticlines in the southern upheaval of the Jiuquan Ba- sin (Fig. 1). It has a gentle southern side, but a very 2 Section and stratigraphy steep or even reversed northern one. The Shiyou River Although stratigraphy of the Hexi Corridor has cuts northwards across the central anticline. From the been long studied, it is still poorly understood because core of the anticline to northward, there were exposed of lacking accurate age constraints. In 1893, Russian sequences of the Gongxingshan, Getanggou and Niugetao Members of the Shulehe Fm, the Yumen geologist Obuluchev investigated Gansu and named Conglomerate, the Jiuquan Gravel Bed and the Gobi the Tertiary stratigraphy “Hanhai System”[13]. In 1942, Gravel Bed. The thickness of the section is 1960 m Sun Jianchu divided the Tertiary Gansu System into after tilt-calibrated. Six unconformities were observed the Miocene Baiyanghe System and the Pliocene in the section, numbered downwards as U1 to U6 lo- Shulehe System and thought that the huge thick grav- cated at depths of 20, 247, 310, 552, 754 and 1160 m, els are Quaternary in age. He named the lower and respectively. U1 separates the Gobi Gravel Bed with upper gravels the Early Pleistocene Yumen Gravel Bed the Jiuquan Gravel Bed, U2 the Jiuquan Gravel Bed and the middle Pleistocene Jiuquan Gravel Bed. The with the Yumen Conglomerate, U3 the upper with mid- Baiyanghe System was further subdivided upperwards dle Yumen Conglomerate, U4 the middle with lower into the Jianquanzi Member, Ganyouquan Member Yumen Conglomerate, U5 the Yumen Conglomerate and Shiyougou Member, and the Shulehe System up- with the Shulehe Fm, and U6 the upper with middle perwards into the Gongxingshan Member, Getanggou Shulehe Fm (Fig.