Downloaded from geology.gsapubs.org on September 13, 2011 Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift Ralf Hetzel1, István Dunkl2, Vicky Haider2, Marcus Strobl1, Hilmar von Eynatten2, Lin Ding3, and Dirk Frei4 1Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany 2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Sedimentologie/ Umweltgeologie, Goldschmidtstraße 3, 37077 Göttingen, Germany 3Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 4Department of Earth Sciences, Stellenbosch University, Private Bag X1, 7602 Matieland, South Africa ABSTRACT STUDY AREA The uplift history of Tibet is crucial for understanding the geodynamic and paleoclima- The investigated bedrock peneplain is located tologic evolution of Asia; however, it remains controversial whether Tibet attained its high in the northern Lhasa block (Fig. 1A, inset) and elevation before or after India collided with Asia ~50 m.y. ago. Here we use thermochronologic was carved into Cretaceous granitoids and very and cosmogenic nuclide data from a large bedrock peneplain in southern Tibet to shed light on low grade metamorphic sediments of Jurassic the timing of the uplift. The studied peneplain, which was carved into Cretaceous granitoids age. Field investigations and the analysis of and Jurassic metasediments, is located in the northern Lhasa block at an altitude of ~5300 m. digital elevation models show that originally Thermal modeling based on (U-Th)/He ages of apatite and zircon, and apatite fi ssion track the peneplain extended for at least ~150 km data, indicate cooling and exhumation of the granitoids between ca. 70 and ca. 55 Ma, followed east-west and ~75 km north-south. Streams that by a rapid decline in exhumation rate from ~300 m/m.y. to ~10 m/m.y. between ca. 55 and incised the original erosion surface have gen- ca. 48 Ma. Since then, the peneplain has been a rather stable geomorphic feature, as indicated erated a local relief of as much as a few hun- by low local and catchment-wide erosion rates of 6–11 m/m.y. and 11–16 m/m.y., respectively, dred meters and divide the peneplain into dif- which were derived from cosmogenic 10Be concentrations in bedrock, grus, and stream sedi- ferent well-preserved parts that are at similar ment. The prolonged phase of erosion and planation that ended ca. 50 Ma removed 3–6 km of elevations of ~5200 m to ~5400 m (Strobl et al., rock from the peneplain region, likely accomplished by laterally migrating rivers. The lack of 2010). The best preserved portion of the original equivalent sediments in the northern Lhasa block and the presence of a regional unconformity planation surface occurs near the town of Ban- in the southern Lhasa block indicate that the rivers delivered this material to the ocean. This goin, where it was eroded into granitoids that implies that erosion and peneplanation proceeded at low elevation until India’s collision with intruded Early Cretaceous sediments (Fig. 1; Asia induced crustal thickening, surface uplift, and long-term preservation of the peneplain. see the GSA Data Repository1 for a descrip- tion of geomorphology and fi eld photographs). INTRODUCTION siderable north-south shortening in the Lhasa Locally, the granitoids underneath the peneplain The growth of the Tibetan Plateau, the high- block and the Qiangtang terrane at long 85°E are overlain by continental red beds of Eocene est plateau on Earth, with a mean elevation of are crosscut by undeformed granitoids dated at age (Qu et al., 2003) along a gently dipping 5 km above sea level (Fielding et al., 1994), has ca. 99 Ma, ca. 113 Ma, and ca. 153 Ma (Mur- unconformity (Fig. 1). These red beds contain long been attributed to India’s collision with phy et al., 1997). Likewise, shortening at 87°E abundant granitic detritus, indicating that the Asia (Argand, 1924; Dewey et al., 1988; Tap- occurred before ca. 118 Ma and there is no evi- granitoids had been exhumed to the surface by ponnier et al., 2001), which started ca. 50 Ma dence for deformation between the Cenomanian Eocene time. Field observations show that the (Patriat and Achache, 1984; Rowley, 1996; Naj- (ca. 95 Ma) and the early Tertiary (Kapp et al., peneplain exposes bedrock or is covered by man et al., 2010). However, the preceding accre- 2007). Hence, the thickened crust was subject to block fi elds generated by frost weathering of the tion of continental terranes to Asia (e.g., Dewey erosion for tens of millions of years before the granitoids (Fig. DR2A in the Data Repository). et al., 1988) raises the possibility that crustal collision of India, which may have reduced the Where present, the soil between the blocks is thickening, and hence surface uplift, occurred crustal thickness substantially before the India- thin (<30 cm) and contains large amounts of much earlier. It has been argued that the colli- Asia collision started. The detritus derived from granite grus. sion between the Lhasa block and the Qiantang the erosion of the Early Cretaceous orogen is terrane (Fig. 1A, inset) resulted in crustal short- partly preserved in the mid-Cretaceous Takena METHODS AND RESULTS ening, which may have raised southern Tibet to Formation of the Lhasa block (Dewey et al., We dated the emplacement age and the cool- an elevation of 3–4 km during the Cretaceous 1988; Leeder et al., 1988), but was also trans- ing history of the granitoids in the Bangoin (Murphy et al., 1997; Kapp et al., 2005, 2007). ported farther south and deposited in the Xigaze region with U/Pb geochronology and low-tem- However, the following observations suggest forearc basin (Dürr, 1996) located just north of perature thermochronological methods (Table 1; that crustal shortening in several regions of the the Indus-Yarlung suture (Fig. 1A, inset). Tables DR1–DR5). Five U/Pb ages reveal that Lhasa block and the Qiangtang terrane does Here we apply an independent approach the granitoids intruded their sedimentary host not necessarily imply that southern Tibet as a to constrain the early uplift of southern Tibet, rock between ca. 120 and ca. 110 Ma. The sub- whole reached a high elevation and remained which is based on quantifying the age and geo- sequent cooling history is constrained by seven high until the onset of the India-Asia collision. morphic evolution of a large bedrock peneplain First, marine limestones document that many using low-temperature thermochronology and 1GSA Data Repository item 2011287, geomorphic regions of southern Tibet remained close to sea cosmogenic nuclides. We use the term peneplain description of peneplain, details of geochronologic level until the Albian (ca. 100 Ma) or Cenoma- to denote a nearly featureless, gently undulating and cosmogenic radionuclide samples, and analy- ses, is available online at www.geosociety.org/pubs/ nian (ca. 95 Ma) (Marcoux et al., 1987; Leeder land surface of considerable area, which has ft2011.htm, or on request from editing@geosociety et al., 1988; Yin et al., 1988). Second, thrust been produced by erosion almost to base level .org or Documents Secretary, GSA, P.O. Box 9140, fault systems interpreted to have caused con- (cf. Jackson, 1997). Boulder, CO 80301, USA. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, October October 2011; 2011 v. 39; no. 10; p. 983–986; doi:10.1130/G32069.1; 2 fi gures; 2 tables; Data Repository item 2011287. 983 Downloaded from geology.gsapubs.org on September 13, 2011 89°50'E 90° 90°10' 89°50'E 90° A B Elevation (m) 5628 16.29 ± 0.50 31°30'N 31°30'N China 5350 H-30 5075 6.97 ± 0.22 H-24 H-31 12.61 ± 0.39 4800 India 7.90 ± 0.25 4524 DC-31 6.91 ± 0.22 90°10' H-29 Bangoin 10.66 ± 0.33 H-23 ult Qaidam Bangoin gh fa Ta basin tyn Al 11.09 ± 0.34 DC-33 Jinsha suture 31°20' 31°20' 14.47 ± 0.45 Qiangtang terrane 12.30 ± 0.38 Ba 6.76 ± 0.21 ngo ng suture study N Lhasa block area I 10.54 ± 0.33 ndus-Yarlung suture 6.58 ± 0.21 H i 6.44 ± 0.20 m a l 05 a y a km Grus Early Cretaceous granitoids Eocene red beds Local and catchment-wide Sample Bedrock Early Cretaceous sediments Alluvial sediments erosion rates (m/m.y.) 6.89 ± 0.22 type Stream sediment Figure 1. A: Geologic map of peneplain region in northern Lhasa block near town of Bangoin and sample locations. U/Pb dating and thermo- chronology performed on granitoid samples revealed their intrusion ages and their subsequent cooling history. Inset maps show continental terranes of Tibetan Plateau, bounded by suture zones, and depict their location in Central Asia. B: Digital elevation model (30 m resolution) of study area with local and catchment-wide erosion rates (m/m.y.) quantifi ed from concentrations of cosmogenic 10Be in quartz. Peneplain is in brown. TABLE 1. U/Pb AND THERMOCHRONOLOGICAL AGE DATA hence erosion rates, were presumably different Sample U/Pb age*† (U-Th)/He zircon age§ Apatite fi ssion track age§ (U-Th)/He apatite age§ from those today. On the fl at peneplain, where number (Ma) (Ma) (Ma) (Ma) a thin veneer of soil is present between bedrock H-23 117.0 ± 2.8 77.1 ± 7.9 59.4 ± 2.3 56.2 ± 0.9 blocks in most areas, a warmer and more stable H-24 – 80.2 ± 5.0 58.5 ± 3.3 56.3 ± 0.7 climate in the Tertiary may have caused soils to H-29 111.7 ± 1.6 75.1 ± 6.6 56.8 ± 2.8 53.6 ± 1.2 be thicker than today.