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Pulsed Miocene Range Growth in Northeastern Tibet: Insights from Xunhua Basin Magnetostratigraphy and Provenance

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Pulsed Miocene Range Growth in Northeastern Tibet: Insights from Xunhua Basin Magnetostratigraphy and Provenance Downloaded from gsabulletin.gsapubs.org on July 5, 2012 Pulsed Miocene range growth in northeastern Tibet: Insights from Xunhua Basin magnetostratigraphy and provenance Richard O. Lease1,†, Douglas W. Burbank1, Brian Hough2, Zhicai Wang3, and Daoyang Yuan3 1Department of Earth Science, University of California, Santa Barbara, California 93106, USA 2Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA 3State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China ABSTRACT the north at ca. 22 Ma. Subsequently, growth and deposition initiating in the late Miocene on of the north-trending Jishi Shan occurred both the northern and eastern plateau margins. Sedimentary rocks in Tibetan Plateau at ca. 13 Ma and is highlighted by an accel- Some of the most compelling evidence derives basins archive the spatiotemporal patterns eration in Xunhua Basin accumulation rates from low-temperature thermochronological his- of deformation, erosion, and associated cli- between 12 and 9 Ma, as well as by a signifi - tories in uplifted mountain ranges where age- mate change that resulted from Cenozoic cant change in detrital zircon provenance of depth or time-temperature relationships exhibit a continental collision. Despite growing under- nearby Linxia Basin deposits by 11.5 Ma. Ini- clear transition to rapid cooling. Accelerated late standing of basin development in northeast- tial growth of the WNW-trending Laji Shan Miocene–Pliocene development of the eastern ern Tibet during initial India-Asia collision, in the early Miocene and subsequent growth plateau margin is revealed by detailed thermo- as well as in the late Miocene–Holocene, sur- of the north-trending Jishi Shan ~10 m.y. chronological records from the Longmen Shan prisingly little is known about the interven- later support interpretations of a middle (Godard et al., 2009), Min Shan (Kirby et al., ing period: a time when the plateau may have Mio cene kinematic reorganization in north- 2002), and Gonga Shan areas (Clark et al., 2005; undergone fundamental tectonic changes. To eastern Tibet. Ouimet et al., 2010), as well as fi ssion-track- fi ll this gap, we present new magnetostratig- length modeling from a portion of the Qinling raphy from a >2300-m-thick fl uviolacustrine INTRODUCTION (Enkelmann et al., 2006). In northeastern Tibet, succession that spans ca. 30–9.3 Ma. An inte- thermochronologic age-elevation relationships grated analysis of sedimentology, subsidence, Over the past decade, growing evidence in- also clearly demarcate a late Miocene onset of and provenance from this section reveals the dicates that contractional deformation near the accelerated cooling in the Liupan Shan (Zheng sequential, pulsed erosion of multiple ranges present northern margin of the Tibetan Plateau et al., 2006) and north Qilian Shan (Zheng et al., bordering the Xunhua Basin. Emergence of commenced a few million years after initial 2010), which is corroborated by sedimentologi- the WNW-trending Laji Shan is highlighted India-Asia continental collision (Yin et al., cal and magnetostratigraphic archives adjacent by a doubling of sediment accumulation 2002, 2008a; Yin and Harrison, 2000), despite to these ranges (Fig. 1A; Bovet et al., 2009; rates between 24 and 21 Ma and a transition the >3000 km distance separating northeast- Wang et al., 2011a). Finally, magnetostrati- to coarse allu vial facies at 20.3 Ma. Detrital ern Tibet from the plate boundary at ca. 50 Ma graphic records adjacent to the Riyue Shan zircon U/Pb age spectra show that these (Molnar and Stock, 2009). Along the northeast- (Fang et al., 2005; Lease et al., 2007), Qinghai coarse sediments came from basement ter- ern plateau margin, a regional 25° clockwise ro- Nan Shan (Zhang et al., 2011), and Gonghe Nan ranes within the Laji Shan. Together these tation by ca. 41 Ma (Dupont-Nivet et al., 2008) Shan (Craddock et al., 2011) reveal the late Mio- observations suggest accelerated growth and the onset of localized thrust-induced cool- cene emergence of these ranges. of the Laji Shan and its coupled foreland ing (Clark et al., 2010) and fault gouge (Duvall Despite increasing knowledge of plateau- basin at ca. 22 Ma. The most rapid accumu- et al., 2011) at ca. 45–50 Ma directly support margin deformation during both the Eocene lation rates in Xunhua Basin occur within an Eocene onset of contractional tectonism in onset of continental collision as well as during the fi nest-grained strata and suggest an un- the Xining-Lanzhou region (Fig. 1A). To the the late Miocene, geological knowledge of the derfi lled basin during the fastest interval of west, along the northern plateau margin, initia- interval between these two episodes remains Laji Shan deformation. Growth of the Laji tion of a suite of contractional structures in the sparse. For example, the large body of work re- Shan occurred northward of the contempo- northern Qaidam Basin and adjacent ranges at vealing extensive late Miocene–Pliocene range raneous plateau margin, which had been de- ca. 65–50 Ma (Yin et al., 2008a) corroborates growth in northeastern Tibet (Fig. 1A) stands in fi ned since ca. 45–50 Ma by the West Qinling, this early deformation. Furthermore, the oldest contrast to only meager evidence for Oligocene– lying ~60 km farther south. Hence, following strata within continuous Cenozoic stratigraphic middle Miocene range growth. Proxy data sets, ~20–25 m.y. of apparent stability, the defor- successions in both the Xining (Dai et al., 2006) such as middle Miocene vertical-axis rotations mation front in this region jumped ~60 km to and Qaidam Basins (summarized in Yin et al., (Yan et al., 2006) or decreases in detrital ther- 2008b) are nearly coeval with the initial phase mochronology lag times (Zheng et al., 2003), †Present address: Institut für Geowissenschaft, Uni- of India-Asia continental collision. are often unable to delimit proximate causes versität Tübingen, 72074 Tübingen, Germany; e-mail: On the other hand, several studies document (Zhang et al., 2011) or source areas. In addition, [email protected] an episode of accelerated deformation, erosion, recent thermochronological results that suggest GSA Bulletin; May/June 2012; v. 124; no. 5/6; p. 657–677; doi: 10.1130/B30524.1; 15 fi gures; 2 tables; Data Repository item 2012078. For permission to copy, contact [email protected] 657 © 2012 Geological Society of America Downloaded from gsabulletin.gsapubs.org on July 5, 2012 Lease et al. Riyue Xiejia Magnetostratigraphic section A B Xining Huangshuihe R. Shan Detrital zircon sample Basin 100°E 105°E 7 Fault Thermochron QS Quaternary 6 9–11 L Late Cenozoic 20 km 8 a j i Fig.13 S h Early Cenozoic QNS LJS a Cretaceous (-Jurassic) 5–7 21 16 n KS H LPS Triassic (-Carboniferous) –24 Ashi- YellowYel R. 7–10 Ganjia 5 Triassic-Permian plutons ~35 RS: 8–10 gong l J Z ow R. Pr2 Silurian (-Devonian) a JS: 12–14 Hualong i m Cambrian (-Ordovician) S GNS 36°N Guide h a s Early Paleozoic plutons 35°N WQ a z Basin a Xunhua Basin h K1 7–10 45–50 n r Pr3 Proterozoic N i i K Q: 4–9 Xunhua 15 W S Fig.14 oww R. e 4x YellYello s h MS: 3–5 t Maogou a 4 14 K2 Q LMS: 8–11 i n Wangjiashan n l Linxia 30°N i Basin GS: 9–13 N n QNS Mountain range g 5–7 Age of growth (Ma) 102°E 103°E Figure 1. (A) Northeastern margin of the Tibetan Plateau showing ages of onset of accelerated Cenozoic range growth derived from bedrock thermochronology and sedimentary records. Contractional deformation across WNW-trending faults commenced shortly after initial India- Asia continental collision (yellow circles), with a later phase of incremental expansion and accelerated exhumation in the late Miocene (blue circles) occurring toward both the east and north. In this paper, we discuss new evidence for early–middle Miocene range growth (white circles). Data from west to east: the Kunlun Shan (KS) ca. 35 Ma (Clark et al., 2010), Qilian Shan (QS) 9–11 Ma (Bovet et al., 2009; Zheng et al., 2010), Qinghai Nan Shan (QNS) 5–7 Ma (Zhang et al., 2011), Gonghe Nan Shan (GNS) 7–10 Ma (Craddock et al., 2011), Riyue Shan (RS) 8–10 Ma (Fang et al., 2005; Lease et al., 2007), Gonga Shan area (GS) 9–13 Ma (Clark et al., 2005; Ouimet et al., 2010), Laji Shan (LJS) 21–24 Ma and Jishi Shan (JS) 12–14 Ma (Lease et al., 2011; this paper), West Qinling (WQ) 45–50 Ma (Clark et al., 2010; Duvall et al., 2011), Longmen Shan (LMS) 8–11 Ma (Godard et al., 2009), Min Shan (MS) 3–5 Ma (Kirby et al., 2002), Qinling (Q) 4–9 Ma (Enkelmann et al., 2006), and Liupan Shan (LPS) 7–10 Ma (Wang et al., 2011a; Zheng et al., 2006). K—Kunlun fault and H—Haiyuan fault. (B) Geological map of Xunhua Basin study area with magnetostratigraphic section locations (stars) (this paper; Dai et al., 2006; Fang et al., 2003, 2005; Hough et al., 2011), modern detrital zircon samples (numbered circles) (this paper; Lease et al., 2007), and major thermochronology transects (hexa- gons) (Lease et al., 2011; Clark et al., 2010) shown. Geology is modifi ed after Qinghai Bureau of Geology and Mineral Resources (QBGMR) (1991). For each detrital zircon sample, the associated catchment is outlined. sequential, early and middle Miocene growth of Factors that remain unknown are whether For clear answers to these questions, we need the Laji-Jishi Shan complex (Lease et al., 2011) signifi cant basin development and contractional to defi ne the onset, pace, nature, and extent of have yet to be substantiated by stratigraphic ar- deformation took place in the Xining-Lanzhou basin development and range growth.
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