Earth and Planetary Science Letters 210 (2003) 545^560 www.elsevier.com/locate/epsl Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China Xiaomin Fang a;b, Carmala Garzione c;Ã, Rob Van der Voo d, Jijun Li a, Majie Fan a a MOE National Laboratory of Western China’s Environmental Systems and College of Resources and Environment, Lanzhou University, Lanzhou, Gansu 730000, PR China b State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xi’an, Shanxi 710054, China c Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA d Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA Received 24 October 2002; received in revised form 24 February 2003; accepted 28 February 2003 Abstract This study provides a detailed magnetostratigraphic record of subsidence in the Linxia Basin, documenting a 27 Myr long sedimentary record from the northeastern edge of the Tibetan Plateau. Deposition in the Linxia Basin began at V29 Ma and continued nearly uninterruptedly until V1.7 Ma. Increasing rates of subsidence between 29 and 6 Ma in the Linxia Basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to Late Oligocene^Late Miocene time. By Late Miocene^Early Pliocene time, a decrease in subsidence rates in the Linxia Basin associated with thrust faulting and a V10‡ clockwise rotation in the basin indicates that the deformation front of the Tibetan plateau had propagated into the currently deforming region northeast of the plateau. ß 2003 Elsevier Science B.V. All rights reserved. Keywords: Asia; magnetostratigraphy; £exural basin; fold and thrust belt; Tibetan Plateau 1. Introduction teau. Along the northeastern edge of the Tibetan Plateau, fold-thrust belts, such as the Nan Shan, The timing and mechanisms of growth of the Qilian Shan, and Liupan Shan, are presently Tibetan Plateau are still poorly understood be- undergoing shortening as they are incorporated cause of a lack of widespread and precise con- into the plateau [1^5] (Fig. 1A). Today, strike- straints on the timing of deformation in the pla- slip deformation dominates the northern part of the Tibetan Plateau [1]. It has been suggested that left slip at the eastern termination of the Altyn Tagh fault is absorbed by northeast-directed * Corresponding author. Tel.: +1-716-273-4572; Fax: +1-716-244-5689. shortening, accommodating growth of the north- E-mail address: [email protected] eastern part of the plateau [2,3,6], whereas trans- (C. Garzione). pressional deformation is occurring in the Liupan 0012-821X/ 03 / $ ^ see front matter ß 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0012-821X(03)00142-0 EPSL 6620 19-5-03 546 X. Fang et al. / Earth and Planetary Science Letters 210 (2003) 545^560 Fig. 1. (A) Location of the study area (rectangle) relative to major tectonic elements of the Tibetan Plateau (simpli¢ed from [4]). The gray region shows the extent of the Tibetan Plateau. The darker gray region shows the extent of the Songpan^Ganzi accre- tionary wedge. (B) Simpli¢ed geologic map of the Linxia Basin on the northeastern margin of the Tibetan Plateau [19], showing localities of studied sections and fossil mammals in the Linxia Basin. Thick lines are faults. Teeth are shown in the hanging wall of thrust faults. Black dots show locations of measured sections: Wangjiashan (W), Maogou (M), Dongxiang (D), and Dong- shanding (DS); and faunal assemblages: Longguan fauna (1), Longdan fauna (2), Sigou fauna (3), and Shanzhuang fauna (4). EPSL 6620 19-5-03 X. Fang et al. / Earth and Planetary Science Letters 210 (2003) 545^560 547 Shan at a southward bend in the east^southeast- and V33 Ma for the Nan Shan fold-thrust belt, striking Haiyuan fault [3,4,6] (Fig. 1A). GPS mea- northeast of Qaidam [14]. Yin et al. [14] interpret surements across northeastern Tibet show that these thrust systems as forming in relation to the this region is moving NE^NNE at velocities of Altyn Tagh fault, which suggests initiation of 19^15 mm/yr relative to the ¢xed South China strike-slip deformation by 49 Ma. However, Ter- Block [7]. However, localities in the foreland, tiary piercing points from the central and eastern north of the Altyn Tagh fault and Qilian Shan, portion of the Altyn Tagh fault have been inter- display much lower and more northerly velocities preted to suggest a younger age of initial strike- of 4^10 mm/yr relative to South China [7], indi- slip motion of Late Oligocene^Early Miocene cating that the northeastern part of the plateau is [15]. Stratigraphic patterns and paleocurrent indi- moving eastward relative to the northern fore- cators also suggest that Tarim and Qaidam basins land. were separated by the Altyn Tagh fault by Mio- Widespread deformation and associated sedi- cene time [16]. This strike-slip deformation led to mentary basin deposition across the Tibetan Pla- an increase in exhumation in the Qilian Shan, as teau provide a record of upper crustal shortening well as the Kunlun Shan and Altun Shan, in Mio- in the plateau. Based on the age of magmatism, cene^Pliocene time, recorded by apatite ¢ssion deformation, and the seismic structure of the lith- track ages [11,17]. Increasing rates of exhumation osphere at signi¢cant tectonic boundaries in the in the Qilian Shan in Pliocene^Quaternary time plateau, it has been suggested that deformation have been interpreted based on an increase in took place from south to north, causing stepwise sediment accumulation rates at V6 Ma and the uplift of the Tibetan Plateau [8]. Age constraints deposition of a thick gravel succession of upper from sedimentary ¢ll in a £exural basin in the Pliocene^Pleistocene age [5,18]. The growing eastern Qiangtang Block document Cretaceous^ number of studies of the northern margin of the Paleogene deformation associated with thrust Tibetan Plateau continues to support the notion faulting, whereas strike-slip faults in this region that deformation associated with the Altyn Tagh postdate basin development [9]. Volcanic rocks fault propagated towards the northeast through unconformably overlying and intruding these de- time. Linxia Basin deposits provide a unique op- posits constrain the youngest sedimentation to portunity to place spatial and temporal con- Middle^Late Eocene age [9]. The Eocene age for straints on the deformation history of the north- shortening in this region is corroborated by a eastern margin of the Tibetan Plateau. In this belt of V50^40 Ma magmatism in the Tanggula study, we report on the magnetostratigraphy of mountains, which has been interpreted as result- the Linxia Basin, thereby documenting both the ing from the southward subduction of Asian lith- timing and mechanisms of subsidence in this re- osphere along the Jinsha suture along the Eocene gion. This is the longest magnetostratigraphic rec- edge of the high Tibetan Plateau as it existed at ord, in terms of time, documented from the ter- that time [8,10]. restrial Cenozoic. Along the northern margin of the plateau, ini- tial deformation in Eocene^Oligocene time has been inferred from apatite ¢ssion track analyses 2. Geologic background from rocks along the Altyn Tagh and Kunlun faults surrounding the Qaidam Basin [11,12]. The Linxia Basin is a sub-basin of the Long- These data are supported by seismic data from zhong Basin, located just northeast of the topo- northern Qaidam, which show growth strata graphic front on the northeastern edge of the Ti- that indicate deformation began by at least Oli- betan Plateau and south of the Haiyuan fault, a gocene time [13]. Magnetostratigraphic ages from large-scale, left-lateral, strike-slip fault [4,6], and the southern Tarim Basin corroborate these data, the Liupan Shan to the northeast (Fig. 1A). The indicating an initial age of crustal thickening of lateral extent of Linxia Basin is marked by struc- V49 Ma on the margins of the Qaidam Basin tural boundaries on the northern, western, and EPSL 6620 19-5-03 548 X. Fang et al. / Earth and Planetary Science Letters 210 (2003) 545^560 southern edges of the basin, but is poorly de¢ned inated by mudstone and sandstone of £uvial and towards the east. To the west and south, the ma- lacustrine origin. The stratigraphy has been sub- jor basin-bounding faults within the Tibetan Pla- divided into eight formations based on lithofacies, teau are the Laijishan fault and North Qinling contacts and paleontology (Fig. 2), and can be fault, respectively (Fig. 1B). Linxia Basin sedi- traced across the basin. Three stratigraphic sec- mentary rocks onlap Cretaceous sedimentary tions were measured along a pro¢le perpendicular rocks in the Maxian Shan to the north (Fig. to the northeastern margin of the Tibetan Plateau 1B). Throughout the central part of the Linxia (Fig. 2). These sections provide a proximal to dis- Basin, the oldest deposits were laid down on gran- tal pro¢le across the basin from the Tibetan Pla- ite of inferred Paleozoic age [19]. Southwest of the teau margin towards the Maxian Shan. Both the Linxia Basin, the Tibetan Plateau consists of De- most proximal Wangjiashan (WJS) and central vonian^Permian terrestrial and marine deposits of Maogou sections contain lithologies that can be the Kunlun^Qaidam terrane and Triassic subma- correlated to one another. The more distal Dong- rine fan deposits of the Songpan^Ganzi accretion- xiang section, which is dominantly mudstone, ary wedge, which were shed by the east^southeast cannot be easily correlated based on lithology striking Qinling mountain belt to the east of the and therefore only provides information on the plateau (Fig.