Tectonophysics 617 (2014) 31–43 Contents lists available at ScienceDirect Tectonophysics j ourna l homepage: www.e lsev ie r .com/ locate / tectoPolyphase exhumation in the western Qinling Mountains, China: Rapid Early Cretaceous cooling along a lithospheric-scale tear fault and pulsed Cenozoic upliftBianca Heberer a,⁎, Thomas Anzenbacher a, Franz Neubauer a, Johann Genser a, Yunpeng Dong b, István Dunkl c a Dept. Geography and Geology, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria b State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi'an 710069, China c Geoscience Center, University of Göttingen, Goldschmidtstrasse 3, D-37077 Göttingen, Germany⁎ Corresponding author. http://dx.doi.org/10.1016/j.tecto.2014.01.011 0040-1951 © 2014 The Authors. Published by Elsevier B.Va b s t r a c ta r t i c l e i n f oArticle history: Received 16 June 2013 Received in revised form 14 November 2013 Accepted 12 January 2014 Available online 22 January 2014 Keywords: Qinling Mountain Age–elevation-relationship Thermochronology Yanshanian orogeny Tibetan Plateau upliftThe western sector of the Qinling–Dabie orogenic belt plays a key role in both Late Jurassic to Early Cretaceous “Yanshanian” intracontinental tectonics and Cenozoic lateral escape triggered by India–Asia collision. The Taibai granite in the northern Qinling Mountains is located at the westernmost tip of a Yanshanian granite belt. It con- sists of multiple intrusions, constrained by new Late Jurassic and Early Cretaceous U–Pb zircon ages (156± 3Ma and 124 ± 1 Ma). Applying various geochronometers (40Ar/39Ar on hornblende, biotite and K-feldspar, apatite fission-track, apatite [U–Th–Sm]/He) along a vertical profile of the TaibaiMountain refines the cooling and exhu- mation history. The new age constraints record the prolonged pre-Cenozoic intracontinental deformation aswell as the cooling historymostly related to India–Asia collision.We detected rapid cooling for the Taibai granite from ca. 800 to 100 °C during Early Cretaceous (ca. 123 to 100Ma) followedbya period of slow cooling from ca. 100Ma to ca. 25Ma, and pulsed exhumation of the low-relief Cretaceous peneplain during Cenozoic times. We interpret the Early Cretaceous rapid cooling and exhumation as a result from activity along the southern sinistral litho- spheric scale tear fault of the recently postulated intracontinental subduction of the Archean/Palaeoproterozoic North ChinaBlock beneath the Alashan Block. A Late Oligocene to EarlyMiocene cooling phasemight be triggered either by the lateral motion during India–Asia collision and/or the Pacific subduction zone. LateMiocene intensi- fied cooling is ascribed to uplift of the Tibetan Plateau. © 2014 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license. 1. Introduction The enigmatic Jurassic to Lower Cretaceous compressional “Yanshanian” orogenic belts (Dong et al., 2008; Enkelmann et al., 2006; Faure et al., 2012; Wong, 1927, 1929) of Central and Eastern China, which are rich in granites, are generally considered to have formed in response to geodynamic processes related to either (1) the subduction of the (Palaeo-)Pacific plate (e.g. Li and Li, 2007; Mao et al., 2010), (2) a stage of the closure of the Neotethys forming the Qiangtang orogenic belt on the Tibetan Plateau (e.g. Zhang et al., 2012), (3) the collision between Siberia and Mongolia after the closure of the Mongol–Okhotsk Ocean (e.g. Davis et al., 2001; Yin and Nie, 1993), (4) intracontinental W-directed subduction of the North China Block (NCB) underneath the Alashan Block (Faure et al., 2012) or a combina- tion of the effects of (1) to (3), i.e. multi-directional compressional deformation caused by convergence of multiple surrounding plates toward the NCB in the center (Dong et al., 2008). Generally, an earlier compressional stage (pre-136 Ma, Yanshanian tectonics sensu stricto according to Faure et al., 2012) and a later extensional stage are. Open access under CC BY-NC-ND licedistinguished. Particularly in Eastern China, Jurassic to Early Cretaceous tectonics are often related to thermal domes and metamorphic core complexes of extensional origin (Faure et al., 2012; Wu, 2005; Yang et al., 2006). Similar Jurassic to Early Cretaceous tectonic processes also affected the Qinling–Dabie orogen (Dong et al., 2011). Superimposed on these Cretaceous tectonic processes, the western part of the Qinling–Dabie orogen has also been affected by Cenozoic (since ca. 50 Ma) east-directed lateral extrusion away from the India–Asia colli- sion zone after closure of the Neotethys Ocean (Mercier et al., 2013 and references therein; Molnar and Tapponnier, 1977). However, the detailed evolutionary history of the Qinling orogen during Cretaceous to Cenozoic times is still unclear. In this study we present results from a multi-method thermo- chronological study (U–Pb on zircon, Ar–Ar on hornblende, biotite and K-feldspar, apatite fission-track (AFT), apatite (U–Th–Sm)/He (AHe) on an age–elevation profile of ca. 2500 m vertical distance of the Taibai granite, a crustal-scale footwall block in the Qinling. The aim was to extract more detailed information on both the Cretaceous and the Cenozoic cooling and denudation history by combining high- and low- T geochronometers, thus covering a wide range of crustal depths. Work focused along a vertical transect since such samples record thense. 32 B. Heberer et al. / Tectonophysics 617 (2014) 31–43passage through progressively lower temperatureswhile samples are ex- humed towards the surface. If exhumation could exhume samples from below the partial annealing/retention zones and closure depths of the re- spective geochronometer, then the onset of fast cooling and denudation can be directly identified by a distinct break in slope of plotted ages (Fitzgerald andGleadow, 1990; Fitzgerald et al., 1995; Stockli et al., 2000). Wefind that the Taibai granite represents thewesternmost pluton of a ca. 1000 km long Yanshanian granite belt extending to the Dabie Shan (Fig. 1). Results from geochronometric dating evidence rapid post- Yanshanian cooling, followed by a ca. 75 Ma long phase of tectonic qui- escence, and then pulsed cooling phases in theCenozoic.We expand the recently proposed model of Yanshanian tectonics (Faure et al., 2012) and explain the Taibai granite as formed on the westernmost tip of a lithospheric-scale tear fault, which was later reactivated by the North Qinling fault during Cenozoic extrusion away from the Tibet plateau as a result of the collision of India with Asia. 2. Geological setting The Qinling Mountains are part of the 2000 km long Central China orogen. They formed by the collisions between the North China, South Qinling and South China Blocks along the Shangdan suture to the north and theMianlue suture to the south in Paleozoic and Later Triassic times, respectively (e.g. Dong et al., 2011 and references therein; Li et al., 1989; Ratschbacher et al., 2006; Zhang et al., 1989). The area underwent Late Jurassic–Early Cretaceous (=Yanshanian) shortening, associated with the intrusion of A-type granites (Dong et al., 2011). Subsequently, the areawas reactivated by the Pacific back-arc extension (Engebretson et al., 1985). Located close to the northern margin of the Qinling Mountains, the Taibai granite (Fig. 2) was supposed to be a composite Indosinian granite with a U–Pb zircon age of 216±Ma (Xiao et al., 2000) overprinting a fo- liated quartz monzonite with a Rb–Sr age of 455 Ma (Zhou et al., 1994). To the north, the North Qinling Margin fault separates the CenozoicFig. 1. (a) Digital elevation model based on SRTM data of Eastern Asia showing the major tecto graben. Note the location and extent of the area covered by inset b indicated by shaded box. W Xi'an. White cross-hatched fields indicate zones with Yanshanian granites.Weihe graben and the Qinling Mountains. The Weihe graben is part of a Cenozoic graben system surrounding the Ordos Block, a major intracratonic basin (Fig. 1). Large and sharp contrasts in lithospheric (N200 km vs. 80 km) and crustal thickness (40–50 km in general and N37 km beneath rifts) exist between the Ordos Block and the adjacent rift areas (Chen et al., 2009). Such a thick lithospheric mantle root be- neath the Ordos attests that the block retained its rigidity and thickness over a long time period and exhibits features of a typical craton (Xu and Zhao, 2009). The Weihe graben exhibits a half-graben type structure along theNorthQinling fault. The basin isfilledwith ca. 6000mof lacus- trine and alluvial deposits covered by Quaternary loess (Mercier et al., 2013 and references therein). Locally, graben development and subsi- dence initiated in the Mid(?)–Late Eocene to Early Oligocene. Major in- creases in deposition rates are observed for the Late Oligocene–Early Miocene, LateMiocene and Quaternary (Liu et al., 2013). Latest Pliocene to Quaternary extension rates are estimated at 1.6 mm/yr (Zhang et al., 1998). During Quaternary times subsidence reached a maximum value of 1 mm/yr (Bellier et al., 1988b, 1991;Mercier et al., 2013; Zhang et al., 1998). Numerous destructive earthquakes occurred within the Weihe graben, e.g. the 1556 M8 Huaxian earthquake (Zhang et al., 1995). Ground fissures and ongoing faulting indicate that this region is still tectonically active today (Li et al., 2001). Between the Qinling Mts. and the Weihe basin, mainly strike-slip movements occurred along the WNW–ESE trending North Qinling fault zone (Fig. 2), a major duc- tile shear zone during Palaeozoic–Mesozoic times, followed by normal and strike-slip, brittle motions during Late Cretaceous and Cenozoic (Huang and Wu, 1992; Mattauer et al., 1985). Based on fault analyses, Mercier et al. (2013) recently proposed a complex, six-stage kinematic pattern for the North Qinling–Weihe region: Initial Late Cretaceous to Paleocene WNW–ESE extension (1) was followed by Early Eocene transpression (2) with NNE–SSW shortening, interpreted as a far-field effect of India–Asia collision. Dur- ing the Mid-Eocene to Early Oligocene NE–SW extension (3) prevailed, followed by a Late Oligocene–Early Miocene second transpressionalnic units. Y–H: Yinchuan–Hetao rift system; S–S: Shaanxi–Shanxi rift system; WG: Weihe hite asterisks denote the location of the Taibai Mtn. west of Xi'an and the HuaMtn. east of Fig. 2. Digital elevation model with superimposed geological map of the Taibai region, Qinling Mountains modified after Liu et al. (1996). Sample locations along the Taibai profile are indicated. New U–Pb data for the composite Taibai granite from this study are indicated in black, data from the literature are marked with a green (Zhou et al., 1994) and red (Xiao et al., 2000) asterisk. NQMF = North Qinling margin fault. 33B. Heberer et al. / Tectonophysics 617 (2014) 31–43event (4) with WNW–ESE shortening and surface uplift and exhuma- tion. NE–SW extension (5) was dominant during Late Miocene times. No structural data was found for the latest Miocene–Early Pliocene time (~9–3.5 Ma). A last tectonic phase was defined for Late Pliocene– Quaternary times, when NNW–SSE extension (6) occurred. Interesting- ly, alkaline volcanics extruded between 23 and 7 Ma in the western Qinling Mountains (Tang et al., 2012). At present, normal movement north of the Qinling Mountains in the NCB contrasts with eastward extrusion of the South China Block at a rate between 5 and 15 mm/a (Mercier et al., 2013; Wang et al., 2001; Zhang et al., 1998). A number of studies have addressed the cooling history of the Qinling Mts. using low-temperature thermochronology (Chen et al., 2001; Enkelmann et al., 2006; Hu et al., 2006; Yin et al., 2001). Recently, Liu et al. (2013) carried out AFT analyses along a similar vertical profile of the Taibai Mountain but located further to the west. They found an age gradient similar to our results, however, their AFT ages seem to be systematically younger than our AFT, and in parts even younger thanTable 1 Summary of locations, lithologies and results for samples from the Taibai transect. Lithology Altitude (m a.s.l.) Methods applied U–Pb age ± 1σ Ar–A (amp Granite 3375 U–Pb; Ar–Ar (Kf, bt), AFT 124.4 ± 1.3 Bt-amphibolite 3375 Ar–Ar (bt, amph) 122.2 Granite 3261 AHe Granite 2755 AHe; AFT Granite 2196 Ar–Ar (Kf, Bt); AFT; AHe Granite 1750 p–T; Ar–Ar (bt); AFT; AHe Granite 1163 U–Pb; Ar–Ar (Kf, Bt); AFT; AHe 155.5 ± 3.4 Granite 929 AFT; AHeour AHe data, a contradiction which cannot be readily dissolved at that stage. They find acceleration in cooling at ca. 10Ma along the Taibai and the Huashan transects (Fig. 1). A somewhat younger onset of rapid cooling (9–4Ma) in the southwestern QinlingMountainswas described by Enkelmann et al. (2006). A young (mostly Miocene) exhumation event following a long period of thermal stability is also a common fea- ture of cooling paths compiled for the eastern Tibetan Plateau (Roger et al., 2010, 2011). 3. Materials and methods We collected seven granite and one amphibolite samples of the Taibai granite along an elevation profile from altitudes 929 to 3375 m. Table 1 gives an overview of all samples, applied methods and results. On one sample (QL-49), hornblende barometry was carried out to reveal the depth of intrusion. Two representative granite samples (QL- 44 and QL-47A) were selected for U–Pb zircon dating (Table 2). Fromr age ± 1σ h) Ar–Ar age ± 1σ (bt) Ar–Ar age ± 1σ (Kf) AFT age ± 1σ AHe age ± 1σ 120.0 ± 0.6 117.0 ± 1.4 67.1 ± 2.7 ± 0.7 123.4 ± 0.3 44.9 ± 2.8 58.3 ± 3.3 35.5 ± 1.1 116.9 ± 1.0 113.0 ± 0.3 49.3 ± 2.5 26.7 ± 1.3 115.0 ± 0.4 40.3 ± 2.9 22.5 ± 3.6 119.9 ± 0.7 111.1 ± 1.9 34.5 ± 1.8 21.3 ± 1.9 26.6 ± 2.0 13.6 ± 0.9 Table 2 U–Pb analytical data of samples QL-44 and QL-47A of the Taibai granite. Sample spot # Isotope ratios Ages Concordia 207Pb/ 206Pb 1σ 207Pb/ 235U 1σ 206Pb/ 238U 1σ 208Pb/ 232Th 1σ 207Pb/ 206Pb 1σ 207Pb/ 235U 1σ 206Pb/ 238U 1σ 208Pb/ 232Th 1σ QL-44-01 0.0470 0.0015 0.1566 0.0033 0.0240 0.0004 0.0072 0.0001 49.8 75.1 147.7 2.9 152.8 2.3 144.5 2.0 96.7 QL-44-02 0.0457 0.0014 0.1594 0.0029 0.0252 0.0004 0.0071 0.0001 0.1 51.2 150.2 2.6 160.1 2.4 142.1 2.0 93.8 QL-44-03 0.0460 0.0016 0.1564 0.0039 0.0245 0.0004 0.0070 0.0001 0.1 77.6 147.6 3.4 156.2 2.4 141.7 2.9 94.5 QL-44-04 0.0473 0.0016 0.1717 0.0037 0.0262 0.0004 0.0079 0.0001 65.0 76.8 160.8 3.2 166.4 2.5 159.5 2.5 96.6 QL-44-05 0.0474 0.0015 0.1572 0.0033 0.0239 0.0004 0.0063 0.0001 66.6 75.3 148.3 2.9 152.5 2.3 126.6 1.8 97.2 QL-44-06 0.0508 0.0016 0.1711 0.0035 0.0243 0.0004 0.0073 0.0001 233.0 72.0 160.3 3.0 154.6 2.3 146.6 2.3 103.7 QL-44-07 0.0539 0.0020 0.1987 0.0053 0.0266 0.0004 0.0074 0.0001 367.9 80.4 184 4.5 169.1 2.6 147.9 2.8 108.8 QL-44-08 0.0480 0.0015 0.1525 0.0027 0.0230 0.0003 0.0070 0.0001 95.9 71.8 144.1 2.4 146.3 2.2 140.9 2.0 98.5 QL-44-09 0.0476 0.0015 0.1599 0.0033 0.0243 0.0004 0.0083 0.0001 77.6 75.5 150.6 2.9 154.5 2.3 166.7 2.8 97.5 QL-44-10 0.0476 0.0015 0.1637 0.0033 0.0248 0.0004 0.0082 0.0001 79.1 75.4 153.9 2.9 158 2.4 164.7 2.2 97.4 QL-44-11 0.0488 0.0016 0.1679 0.0036 0.0249 0.0004 0.0077 0.0001 137.7 76.4 157.6 3.2 158.4 2.4 155.3 2.5 99.5 QL-44-12 0.0482 0.0016 0.1661 0.0035 0.0249 0.0004 0.0083 0.0001 107.9 75.9 156 3.0 158.7 2.4 166.7 2.9 98.3 QL-44-13 0.0500 0.0017 0.1692 0.0040 0.0245 0.0004 0.0081 0.0001 195.6 79.0 158.7 3.5 155.7 2.4 162.9 2.9 101.9 QL-44-14 0.0529 0.0017 0.1776 0.0035 0.0243 0.0004 0.0073 0.0001 324.4 71.6 166 3.0 154.6 2.3 147.2 2.2 107.4 QL-44-15 0.0486 0.0044 0.1149 0.0098 0.0171 0.0003 0.0097 0.0003 126.9 198.7 110.4 8.9 109.4 2.2 194.2 6.9 100.9 QL-44-16 0.0528 0.0018 0.1735 0.0037 0.0238 0.0004 0.0087 0.0002 318.4 73.9 162.5 3.2 151.6 2.3 175.1 3.1 107.2 QL-44-17 0.0528 0.0017 0.2316 0.0044 0.0317 0.0005 0.0119 0.0002 321.7 71.2 211.5 3.6 201.3 3.0 239 3.9 105.1 QL-44-18 0.0794 0.0026 0.8925 0.0185 0.0813 0.0013 0.0617 0.0012 1182.6 64.2 647.7 9.9 504.1 7.6 1209.9 22.6 128.5 QL-44-19 0.0499 0.0020 0.1620 0.0049 0.0235 0.0004 0.0064 0.0002 191.4 90.4 152.5 4.3 149.7 2.4 127.9 3.2 101.9 QL-44-20 0.0618 0.0026 0.2419 0.0081 0.0283 0.0005 0.0131 0.0003 668.7 88.8 219.9 6.6 180 2.9 263 6.7 122.2 QL-44-21 0.0535 0.0019 0.1833 0.0041 0.0248 0.0004 0.0098 0.0002 350.4 76.1 170.9 3.5 158.1 2.4 197.3 3.4 108.1 QL-44-22 0.1286 0.0040 5.1089 0.0830 0.2880 0.0044 0.1054 0.0018 2078.7 53.4 1837.6 13.8 1631.4 22.0 2024.8 33.7 112.6 QL-44-23 0.0541 0.0019 0.2054 0.0046 0.0275 0.0004 0.0099 0.0002 374.4 76.2 189.7 3.9 175.1 2.7 199.2 3.5 108.3 QL-44-24 0.0554 0.0019 0.1897 0.0041 0.0249 0.0004 0.0096 0.0002 426.6 74.7 176.4 3.5 158.2 2.4 192.2 3.5 111.5 QL-44-25 0.0500 0.0019 0.1657 0.0043 0.0240 0.0004 0.0104 0.0003 194.9 84.7 155.7 3.8 153.1 2.4 209.1 6.5 101.7 QL-44-26 0.0533 0.0019 0.1843 0.0041 0.0251 0.0004 0.0098 0.0002 340.8 76.7 171.8 3.5 159.8 2.4 196.9 3.6 107.5 QL-44-27 0.0556 0.0018 0.2733 0.0052 0.0357 0.0006 0.0128 0.0003 435.4 71.7 245.3 4.2 225.9 3.4 257.1 5.0 108.6 QL-44-28 0.0521 0.0020 0.1870 0.0052 0.0261 0.0004 0.0099 0.0002 288.2 86.3 174 4.4 165.8 2.6 198.1 4.5 104.9 QL-44-29 0.0544 0.0018 0.1898 0.0037 0.0253 0.0004 0.0110 0.0002 389.2 72.5 176.4 3.2 161.1 2.4 220.9 4.3 109.5 QL-44-30 0.0535 0.0019 0.1806 0.0040 0.0245 0.0004 0.0107 0.0002 351.0 77.0 168.6 3.4 156 2.4 215.2 4.1 108.1 QL-47A-01 0.0476 0.0014 0.1319 0.0020 0.0201 0.0003 0.0065 0.0001 80.7 69.0 125.8 1.8 128 1.9 130.8 1.8 98.3 QL-47A-02 0.0508 0.0015 0.1400 0.0021 0.0200 0.0003 0.0064 0.0001 232.1 66.6 133 1.9 127.5 1.9 129.1 1.8 104.3 QL-47A-03 0.0599 0.0018 0.1531 0.0023 0.0185 0.0003 0.0068 0.0001 600.1 62.3 144.6 2.0 118.3 1.8 137.5 1.9 122.2 QL-47A-04 0.0615 0.0018 0.1633 0.0026 0.0192 0.0003 0.0075 0.0001 658.1 62.6 153.6 2.2 122.8 1.8 151.9 2.1 125.1 QL-47A-05 0.0538 0.0016 0.1438 0.0022 0.0194 0.0003 0.0069 0.0001 360.3 65.6 136.4 2.0 123.8 1.8 138.8 1.9 110.2 QL-47A-06 0.0558 0.0017 0.4818 0.0076 0.0625 0.0009 0.0184 0.0003 445.8 65.0 399.3 5.2 391 5.7 369.3 5.2 102.1 QL-47A-07 0.0498 0.0015 0.1303 0.0020 0.0190 0.0003 0.0066 0.0001 184.7 67.9 124.4 1.8 121.2 1.8 133.8 1.9 102.6 QL-47A-08 0.0577 0.0017 0.1314 0.0021 0.0165 0.0003 0.0076 0.0001 518.3 64.6 125.3 1.9 105.5 1.6 152.9 2.2 118.8 QL-47A-09 0.0488 0.0015 0.1332 0.0021 0.0198 0.0003 0.0066 0.0001 139.8 68.7 127 1.9 126.3 1.9 132.9 1.9 100.6 QL-47A-10 0.0511 0.0015 0.1339 0.0020 0.0190 0.0003 0.0064 0.0001 243.6 66.8 127.6 1.8 121.4 1.8 129.6 1.8 105.1 QL-47A-11 0.0617 0.0019 0.1668 0.0027 0.0196 0.0003 0.0075 0.0001 662.6 63.2 156.6 2.3 125.2 1.9 150.9 2.2 125.1 QL-47A-12 0.0497 0.0015 0.1380 0.0022 0.0201 0.0003 0.0068 0.0001 182.6 68.7 131.2 2.0 128.4 1.9 136.5 2.0 102.2 QL-47A-13 0.0489 0.0015 0.1329 0.0022 0.0197 0.0003 0.0070 0.0001 142.6 70.0 126.7 2.0 125.8 1.9 140.5 2.1 100.7 QL-47A-14 0.0703 0.0021 0.1914 0.0030 0.0198 0.0003 0.0077 0.0001 936.5 60.5 177.8 2.6 126 1.9 155.8 2.2 141.1 QL-47A-15 0.0539 0.0016 0.1395 0.0023 0.0188 0.0003 0.0061 0.0001 367.5 66.9 132.6 2.0 119.8 1.8 122 1.8 110.7 QL-47A-16 0.0513 0.0016 0.1347 0.0022 0.0190 0.0003 0.0068 0.0001 253.9 68.1 128.3 1.9 121.6 1.8 137.2 2.0 105.5 QL-47A-17 0.0494 0.0016 0.1343 0.0024 0.0197 0.0003 0.0076 0.0001 167.3 71.6 128 2.2 125.8 1.9 153.4 2.4 101.7 QL-47A-18 0.0851 0.0026 0.2057 0.0035 0.0175 0.0003 0.0096 0.0002 1317.7 58.9 189.9 3.0 112 1.7 193.2 2.9 169.6 QL-47A-19 0.0491 0.0015 0.1362 0.0024 0.0201 0.0003 0.0079 0.0001 152.5 71.4 129.7 2.1 128.4 2.0 159.9 2.4 101.0 QL-47A-20 0.0520 0.0016 0.1398 0.0023 0.0195 0.0003 0.0068 0.0001 283.3 68.3 132.9 2.1 124.6 1.9 136.3 2.0 106.7 QL-47A-21 0.0527 0.0016 0.1396 0.0022 0.0192 0.0003 0.0067 0.0001 315.0 67.3 132.7 2.0 122.7 1.9 134 2.0 108.1 QL-47A-22 0.0531 0.0016 0.1388 0.0023 0.0190 0.0003 0.0074 0.0001 331.2 68.9 132 2.1 121.1 1.9 148.5 2.2 109.0 QL-47A-23 0.0495 0.0015 0.1306 0.0021 0.0191 0.0003 0.0061 0.0001 169.7 70.0 124.6 1.9 122.2 1.9 121.9 1.8 102.0 QL-47A-24 0.0699 0.0022 0.1629 0.0027 0.0169 0.0003 0.0073 0.0001 924.3 62.2 153.2 2.4 108.1 1.7 146.1 2.2 141.7 QL-47A-25 0.0519 0.0016 0.1392 0.0022 0.0195 0.0003 0.0067 0.0001 280.0 67.9 132.3 2.0 124.2 1.9 135.7 2.0 106.5 QL-47A-26 0.0519 0.0016 0.1390 0.0023 0.0194 0.0003 0.0062 0.0001 280.0 68.7 132.1 2.0 124 1.9 125.2 1.9 106.5 QL-47A-27 0.0518 0.0016 0.1421 0.0023 0.0199 0.0003 0.0067 0.0001 275.7 68.6 134.9 2.1 127 1.9 133.9 2.0 106.2 QL-47A-28 0.0528 0.0016 0.1375 0.0022 0.0189 0.0003 0.0065 0.0001 318.8 68.0 130.8 2.0 120.7 1.9 131.2 2.0 108.4 QL-47A-29 0.0880 0.0027 0.2757 0.0045 0.0227 0.0004 0.0126 0.0002 1383.0 57.7 247.3 3.6 144.8 2.2 252.4 3.8 170.8 34 B. Heberer et al. / Tectonophysics 617 (2014) 31–43ten samples mineral concentrates of amphibole, biotite and K-feldspar have been dated with 40Ar/39Ar in order to reveal the cooling history after intrusion. AFT and AHe analyses served to document the low-temperature cooling history of the Taibai massif. We adopted the following closure temperatures (Tc): amphibole 550 ± 25 °C (Harrison, 1981), biotite 300 ± 25 °C (Harrison et al., 1985); K- feldspar 200 ± 25 °C (Lovera et al., 1989), AFT partial annealing zone (APAZ) 60–120 °C (Fitzgerald et al., 1991) and AHe partial retention zone (PAZ) 40–80 °C (House et al., 1999). 40Ar/39Ar ages are given at the 1σ, U–Pb, (U–Th–Sm)/He and AFT ages at the 2σ level.4. Results In the following we give an overview of all results. More informa- tion on analytical procedures and detailed results are given in the e-component. 4.1. P–T estimate Applying the hornblende barometry (Schmidt (1992) in themodifi- cation of Anderson and Smith (1995)) for sample QL-49 yields a 35B. Heberer et al. / Tectonophysics 617 (2014) 31–43pressure of ca. 4.2 ± 0.2 kbar (at T = 775 °C) corresponding to a depth of ca. 12–13 km (Fig. 3). For the temperature estimate, the geother- mometer of Holland and Blundy (1994) was applied. The PET software (Dachs, 2004) was used for calculations.4.2. LA-ICP-MS zircon U–Pb dating Results of LA-ICP-MS zircon U–Pb dating are given in Table 2 and concordia diagrams are shown in Fig. 4. We use 206Pb/238U ages, since they are generally more precise for an age b1000 Ma. Only those ages are used, which are within a concordancy of 90 and 110%. Zircon grains of sample QL-44 from the lower region of the Taibai Mountain yield a crystallization age of 155.5±3.4Ma. The sample also contains inherited zircon grainswith ages of 201.3±3.0 and 225.9±3.4Ma interpreted to represent Indosinian ages. The uppermost sample QL-47A gives an age of 124.4 ± 1.3 Ma and contains three inherited grains with ages be- tween 148.6 ± 2.2 Ma and 159.9 ± 2.4 Ma likely inherited from the older Taibai granite.4.3. 40Ar/39Ar dating An overview of all 40Ar/39Ar mineral ages is included in Table 1 and Ar release patterns and isochron plots are shown in Fig. 5. Detailed analyses for all minerals are listed in the e-component. Amphiboles of sample QL-47B yield an age of 122.2 ± 0.7 Ma. Biotite ages range between 115.0 ± 0.4 Ma (QL-49) and 123.4 ± 0.3 Ma (QL-47B). Four biotite concentrates yield staircase 40Ar/39Ar release patterns in low- temperature steps. These are a result of Ar loss caused by chloritization, as shown by optical microscopy. The exact age of argon loss is insignif- icant because of low proportions of radiogenic 40Ar*. K-feldspar ages range from 111.1 ± 1.9 Ma (QL-44) to 117.0 ± 1.4 Ma (QL-47A). The K-feldspar release patterns do not show a similar argon loss as indicated by thebiotite concentrates from the same samples. Consequently, theAr loss as observed in the biotite release patterns does not indicate a stage2 4 6 780 820740 860 QL-49 Temperature [°C] Pressure [bar] PT-diagram (Holland & Blundy, 1994) P = 4.2 ± 0.2 k n= 18 Pr es su re [k ba r] Es tim at es 1 3800 4000 4200 4400 4600 2 3 4 5 6 a) b) Fig. 3. (a) P–T Estimates for sample QL-49 based on the plagioclase-amphibole thermometer o estimates.of reheating but rather variable access of hydrothermalwater,which led to chloritization of biotite (See Fig. 5.) 4.4. Apatite fission-track and (U–Th–Sm)/He analyses Results of AFT and AHe analyses are given in Tables 3 and 4. Ages of both AFT and AHe thermochronometers vary significantly with eleva- tion and AHe ages are consistently younger than AFT ages (Fig. 6a). As a result of track annealing AFT ages decrease systematically with increasing palaeo-depth. Ages range from 67.1 ± 2.7 at 3375 m to 26.6 ± 2.0 at 929 m. All ages pass the chi2-test (Galbraith and Laslett, 1993). The number of track lengths was generally very low and only the uppermost oldest sample (QL-47) yielded sufficient lengths for further interpretation (13.48 ± 1.41 μm; n = 80). Apparent AHe ages decrease as a result of the diffusive loss of 4He and range from 51.9 ± 7.3 Ma at 3261 m (QL-46) to 13.6 ± 0.9 Ma at 929 m (QL-50). 5. Interpretation of AFT and AHe data and the derivation of the Taibai cooling history AFT age–elevation profiles with a distinctive age pattern have be- come a common tool to infer the low-temperature exhumation history (e.g. Fitzgerald et al., 1995; Wagner and Reimer, 1972). Samples from el- evation transects record thepassage throughprogressively lower temper- atures as the rock column is exhumed towards the surface (Fitzgerald et al., 1995), assuming a vertical particle pathway (e.g. Parrish, 1983) and a steady closure isotherm at constant depth. However, complica- tions may arise from advection of isotherms due to rapid denudation and the deflection of near-surface isotherm due to topography with compressed isotherms beneath valleys and wider-spaced isotherms beneath ridges (Mancktelow and Grasemann, 1997; Stuwe et al., 1994). Threshold values for perturbation of the 110 °C isotherm are ex- humation rates≥0.5 km/Ma, topographic wavelength of≥10 km and a relief of ≥2 km (Braun, 2002; Mancktelow and Grasemann, 1997;5 6 7 4 650 700 750 800 QL-49 Temperature [°C] PT-diagram (Anderson & Smith, 1995) bar Pr es su re [k ba r] 4800 f Holland and Blundy (1994) and Anderson and Smith (1995). (b) Histogram of pressure 114 118 122 126 130 134 Mean= 124.4±1.3 [1.0%] 95% conf. Wtd by data-pt errs only, 0 of 19 rej. MSWD = 2.0, probability = 0.006 (error bars are 2σ) box heights are 2σ 138 142 146 150 154 158 162 166 170 174 Mean=155.5±3.4 [2.2%] 95% conf. Wtd by data-pt errs only, 0 of 13 rej. MSWD = 5.6, probability = 0.000 (error bars are 2σ) 118 122 126 130 134 0.0175 0.0185 0.0195 0.0205 0.0215 QL47A 206Pb 238U data-point error ellipses are 2σ 140 150 160 170 0.021 0.023 0.025 0.027 0.12 0.13 0.14 0.15 0.13 0.15 0.17 0.19 0.21 QL44 206Pb 238U 207Pb/235U 207Pb/235U data-point error ellipses are 2σ Fig. 4. Concordia diagrams of U–Pb analyses on samples QL-47A and QL-49 with LA-ICP-MS. 36 B. Heberer et al. / Tectonophysics 617 (2014) 31–43Stuwe and Hintermuller, 2000; Stuwe et al., 1994). No topography- induced perturbation of isothermswas found for similar boundary con- ditions in an AFT study along a tunnel transect (Glotzbach et al., 2009). The AFT age pattern from the Taibai Mountain shows no discernible deviation from a linear trend, taking the magnitudes of uncertainties into account. However, the simplest possible interpretation of uni- form/steady exhumation at an average rate of 0.06 mm/a would yield a much more narrow age range for both thermochronometers and may not account for the observed large age variation. Both the AFT and the AHe data do require a more complex cooling history including a considerable residence time within the APRZ and APAZ, respectively. Thus, we interpret the age versus elevation/depth profile (Fig. 6a) as uplifted partial annealing and partial retention zones. Fig. 6b shows a concept of expected AFT and AHe ages versus depth profiles within a thermotectonically stable setting: Close to the surface AFT and AHe ages are expected to be constant, indicating an older exhumation event. When entering the PRZ, AHe ages decrease, whereas AFT ages still remain constant. Within the next deeper layer, the apatite partial retention zone (APRZ) and apatite partial annealing zone (APAZ) over- lap, and ages of both thermochronometers decrease.When reaching the upper temperature limit of the APRZ, AHe ages will remain constant, whereas AFT ages further decrease until the high-temperature limit of the APAZ is reached. For our data this schematic pattern allows us to de- rive an estimate of the pre-exhumation palaeo-geothermal gradient (Stockli et al., 2000): Under the assumption that the PRZ ranges from 40 to 80 °C and the PAZ from 60 to 120 °C (dpar values indicate Ca-rich F-apatites, Table 3), the zone of a contemporaneous decrease in ages spans only 20 °C. Following the concept of exhumed AFT-PAZs and AHe-PRZs (Fitzgerald and Gleadow, 1990; Fitzgerald et al., 1995) we in- terpret themarked inflection point within the AHe profile at ca. 2000m as the base of theHePRZ and suggest that intensified cooling initiated at around 22–25 Ma. A second inflection point is observed at 1000 m. Anapatite PAZ is not well defined, since our AFT data fit well to a linear trend. However, parallel trends of both systems over a vertical distance of more than 1000 m would reduce the geothermal gradient to below 20 °C/km. We therefore propose that the uppermost AFT sample, which shows a slight deviation from a linear trend, was located above the upper (low temperature) limit of the fossil PAZ. By placing the upper PAZ limit at ca. 3000 m and the lower PRZ limit at ca. 2000 m, a palaeo-geothermal gradient in line with a long-term stable situation is derived (see below for a more detailed discussion of the gradient). In order to derive more robust cooling histories in linewith our data from age–elevation-relationships all samples were forward modeled with HeFTy (Ketcham, 2005). Time–temperature historieswere derived and the resulting AFT and AHe data were compared with the measured parameters (Fig. 7). As indicated by the goodness-of-fit values (GOF), which range between 0.5 and 1 (Fig. 7), the proposedmodel successful- ly integrates our AFT and AHe data sets and provides a coherent inter- pretation of the measured ages. Based on the Ar–Ar ages, the AFT and AHe age–elevation profile, and HeFTy modeling, we suggest the following five-step cooling history (Fig. 7): (1) An early Cretaceous exhumation event brought most of our samples into the partial annealing/retention zones. This phase of rapid cooling (ca. 30 °C/Myr) following emplacement of the granite body is constrained byAr–Ar dating. (2) Itwas followedby a longperiod of very slow, continuous cooling (b1 °C/Myr) between ca. 100 and 25 Ma. (3) Intensified cooling (ca. 5 °C/Myr) started at ca. 22–25 Ma (break-in-slope of AHe data). Preceded by (4) a phase of again slow cooling, (5) a last pulse of exhumation and topographybuilding brought the rocks to the surface. By modeling and by comparisons with other low-T studies (see below for discussion) the onset of this youngest phase of minor cooling is bracketed between ca. 7 and 5 Ma. The final cooling of the lowermost sample differs from the other samples, as indicated by the second break in slope at ca. 1000 m 36Ar 40Ar no isochrone plot available . 001 . 002 . 003 . 004 . 005 39Ar/40Ar 39Ar/40Ar 36Ar 40Ar 0 . 001 . 002 . 003 . 004 . 005 . 04 . 06 . 08 . 10 . 12 . 14 . 02 36Ar 40Ar . 001 . 0 . 003 . 004 . 005 36Ar 40Ar . 001 . 002 . 003 . 004 . 005 QL-44 (2) Bt 36Ar 40Ar . 001 . 002 . 003 . 004 . 005 36Ar 40Ar . 001 . 002 . 003 . 004 . 005 QL-44 (1)Bt no isochrone plot available Age = 118.7 ± 1.5 Ma Initial 40Ar/36Ar =231 ± 22 MSWD = 15 QL-4 Bt Age = 121.2 ± 4.4 Ma Initial 40Ar/36Ar =178 ± 30 MSWD = 6.1 Age = 118.7 ± 1.5 Ma Initial 40Ar/36Ar =231 ± 22 MSWD = 1502 9 Bt Age = 1 5.16 0.72 Ma 78 7 .5 QL-4 Bt Age = 118.7 ± 1.5 Ma Initial 40Ar/36Ar =231 ± 22 MSWD = 15 8 Bt QL-4 Bt Age = 118.7 ± 1.5 Ma Initial 40Ar/36Ar =231 ± 22 MSWD = 15 7A Bt 20 3 7 15 10 4 QL-4 Bt Age = 118.7 ± 1.5 Ma Initial 40Ar/36Ar =231 ± 22 MSWD = 15 7B Bt 21 0 3 95 16 67 3 8 66 13 24 cum. %39Ar rel. 119.97 ± 0.63 Ma 0 20 40 60 80 100 123.37 ± 0.34 Ma 0 50 100 150 200 250 119.42 ± 0.54 Ma 0 40 80 120 160 ap pa re nt a ge [M a] ap pa re nt a ge [M a] 0 40 80 120 160 118.37 ± 0.66 Ma 0 40 80 120 160 ap pa re nt a ge [M a] ap pa re nt a ge [M a] 0 40 80 120 160 ap pa re nt a ge [M a] ap pa re nt a ge [M a] ap pa re nt a ge [M a] 0 40 80 120 160 0 40 80 120 160 ap pa re nt a ge [M a] 115.04 ± 0.20 Ma 116.86 ± 0.99 Ma 0 50 100 150 200 250 39Ar/40Ar . 04 . 06 . 08 . 10 . 12 . 14 . 02 QL-44 (2) Bt 250-355µm QL-48 Bt 250-355µm QL-47A Bt 250-355µm QL-47B Bt 250-355µm QL-45A (2) Bt 250-355µm QL-45A (1) Bt 250-355µm QL-49 Bt 250-355µm QL-44 (1) Bt 250-355µm 36Ar 40Ar 0 . 001 . 002 . 003 . 004 . 005 . 04 . 06 . 08 . 10 . 12 . 14 . 02 cum. %39Ar rel. 0 20 40 60 80 100 ap pa re nt a ge [M a] 0.0 5.0 10.0 15.0 ap pa re nt 37 Ar /3 9A r 100 120 140 160 180 200 Age = 121 ± 11 Ma Initial 40Ar/36Ar =323 ± 39 MSWD = 2.7 122.19 ± 0.73 Ma 119.05 ± 0.63 Ma QL-47B Amph QL-47B Amph 200-250µm QL-47B Amph 36Ar 40Ar .001 . 002 . 003 . 004 36Ar 40Ar 36Ar 40Ar ap pa re nt a ge [M a] 100 120 140 160 180 117.0 ± 1.4 Ma 113.00 ± 0.32 Ma 111.10 ± 1.90 Ma ap pa re nt a ge [M a] 100 120 140 160 180 ap pa re nt a ge [M a] 100 120 140 160 180 cum. %39Ar rel. 0 20 40 60 80 100 Age = 107.4 ± 5.7 Ma Initial 40Ar/36Ar =939 ± 260 MSWD = 66 .001 . 002 . 003 . 004 Age = 115.7 ± 6.1 Ma Initial 40Ar/36Ar =444 ± 98 MSWD = 5.3 . 001 . 002 .003 .004 Age = 113.9 ± 3.3 Ma Initial 40Ar/36Ar =638 ± 82 MSWD = 101 39Ar/40Ar . 04 . 06 . 08 . 10 . 12 . 14 . 020 QL-47A Kfsp QL-48 Kfsp QL-44 Kfsp QL-47A Kfsp 250-355µm QL-48 Kfsp 250-355µm QL-44 Kfsp 250-355µm psfKtB Amph Fig. 5. 40Ar/39Ar release patterns of amphibole, biotite and K-feldspar from the samples of the Taibai granite. 37B. Heberer et al. / Tectonophysics 617 (2014) 31–43(Fig. 6a). For this sample, a reheating event, probably due to hydrother- mal fluxes, must have occurred. Such a scenario seems likely, consider- ing, that the sample is situated closest to the range-bounding normal fault. Indeed, numerous hot springs occur along the North Qinling mar- gin fault (e.g. Huaqing). Alteration fabrics visible in thin section, e.g. the transformation of feldspar and biotite, also support this interpretation. 6. Discussion Two main aspects of the results shall be discussed further: (1) the Late Jurassic to Cretaceous intrusion and high-temperature cooling ages and its implications for Early Cretaceous Yanshanian tectonic models of Central China, and (2) the subsequent, ca. 75Myr long history of slow cooling followed by episodic cooling and its relation to the India–Asia collision and other major tectonic events. 6.1. Yanshanian magmatism and cooling along a lithospheric-scale tear fault The Taibai granite experienced multiple intrusion episodes: For its northern unit, Zhou et al. (1994) reported a whole-rock Rb–Sr age of 455 Ma, and Xiao et al. (2000) an age of 216 ± 14 Ma. Our analyses yield two different ages of 155.5 ± 3.4 Ma and 124.4 ± 1.3 Ma, which represent the youngest (known) stages of intrusion activity of this com- posite pluton (Fig. 2). Since all new 40Ar/39Ar mineral ages postdate the younger U–Pb zircon age (124.4 ± 1.3 Ma), samples must have resided at high temperatures or were thermally affected by the youngest intru- sion. Upper Jurassic to Cretaceous intrusions are widespread in the North Qinling–Dabie Mts. further east and thus granitoids from the Taibai vertical profile form the westernmost tip of a ca. 1200 km long granitic belt with similar ages (Fig. 1) (Dong et al., 2011; Mao et al.,2005; Ye et al., 2006; Zhu, 1995; Zhu et al., 2008). Several 100s of km to the WNW of Taibai, Li et al. (2013) recently reported alkaline basalt and basaltic andesite with an age of ca. 106–103 Ma which would imply a further extension of the magmatic belt. The linear granite belt includes a wide range of compositions including I-, S- and A-type gran- ites, alkaline rocks and widespread Mo mineralization. Since most bear a calcalkaline signature, they were traditionally interpreted to record southward intracontinental subduction of the North China Block (NCB), accompanied by abundant gold and molybdenite mineraliza- tions (Du et al., 1994; Huang et al., 1994; Li et al., 2003; Stein et al., 1997; Wang et al., 2012). However, up to now a corresponding mantle slab has not been detected, although a number of geophysical transects cross this boundary (Chen et al., 2009; van der Meer et al., 2010). Also, large-scale overthrusts of the Qinling onto the NCB have not been re- ported. Mao et al. (2010) presented new SHRIMP U–Pb ages from the Eastern Qinling region and differentiated between a Late Jurassic– Early Cretaceous (158 ± 3 to 136 ± 2 Ma) and an Early Cretaceous (134±1 to 108 ± 2Ma) pulse. They relate both episodes to subduction of the Izanagi plate (ancient Pacific plate) beneath Eastern China, which occurred at a shallow angle during the earlier phase (Li and Li, 2007). However, since this subduction is far away and the orientation of the granite belt is ca. perpendicular to the Pacific coast, it may not explain the entire Late Jurassic–Early Cretaceous-aged granite belt. Based on the mixed nature and the asthenospheric contribution to the melt, we suggest magma uprise along a lithospheric-scale tear fault as a conse- quence of lithospheric thinning, asthenospheric upwelling and partial melting (Mao et al., 2008, 2011). This would fit into a model recently proposed by Faure et al. (2012), within which west-directed intra- continental subduction of the NCB underneath the Helanshan fold- and-thrust belt plays a key role (Fig. 8a). TheN–S trendof theHelanshan contrasts with the predominantly E–W striking tectonic framework of Ta bl e 3 Re su lt s fr om ap at it e fi ss io n- tr ac k an al ys is .n is th e nu m be ro fg ra in s co un te d. ρ s ,ρ i,ρ d an d N s, N i,N d ar e th e nu m be ra nd de ns it y of sp on ta ne ou s, in du ce d an d do si m et er tr ac ks re sp ec ti ve ly ;χ 2 is th e ch i- sq ua re d pr ob ab ili ty .A ze ta fa ct or of 36 2. 8 ± 3. 27 (f or H eb er er ) w as ba se d on st an da rd ca lib ra ti on s of D ur an go an d Fi sh Ca ny on Tu ff us in g th e ex te rn al de te ct or m et ho d an d CN 5 as th e do si m et er gl as s. A ll re po rt ed ag es ar e ce nt ra la ge s. Th e et ch ed pi tp ar am et er D pa r( D on el ic k, 19 93 ) w as m ea su re d to co ns tr ai n an ne al in g ki ne ti cs (4 dp ar s fo r ea ch gr ai n) . Sa m pl e A lt it ud e (m ) n ρ s ,× 10 5 cm − 2 N s ρ i ,× 10 6 cm − 2 N i P (χ 2 ) ρ d ,× 10 6 cm − 2 N d A ge ,M a ± 1σ U (p pm ) M ea n D pa r (μ m ) SD (μ m ) M ea n le ng th an d D pa r (μ m ,1 σ ) N um be r of le ng th s Q L- 50 -0 6 92 9 20 1. 61 1 20 0 1. 72 7 21 44 67 1. 57 6 49 60 26 .6 2. 0 13 .7 1. 45 0. 17 – – Q L- 44 -0 6 11 63 20 3. 47 7 46 4 2. 91 1 21 20 53 1. 59 5 50 20 34 .5 1. 8 22 .8 1. 65 0. 15 13 .6 7 ± 1. 71 1. 66 ± 0. 4 11 Q L- 49 -0 6 17 50 19 1. 58 2 23 7 1. 12 2 16 81 96 1. 58 49 72 40 .3 2. 9 8. 9 1. 7 0. 16 – – Q L- 48 -0 6 21 96 17 4. 78 9 47 6 2. 77 8 27 61 81 1. 58 4 49 84 49 .3 2. 5 21 .9 1. 65 0. 15 13 .0 3 ± 1. 91 1. 74 ± 0. 2 14 Q L- 45 A -0 6 27 55 15 4. 78 7 40 5 2. 36 1 19 97 79 1. 59 1 50 08 58 .3 3. 3 18 .5 1. 7 0. 16 12 .9 7 ± 2. 0 1. 93 ± 0. 13 22 Q L- 47 A -0 6 33 75 19 7. 34 8 99 9 3. 11 6 42 36 33 1. 58 7 49 96 67 .1 2. 7 24 .5 1. 96 0. 16 13 .4 8 ± 1. 41 1. 89 ± 0. 12 80 38 B. Heberer et al. / Tectonophysics 617 (2014) 31–43China and eastern Asia (Figs. 1 and 8). Crustal-scale shortening within the Helanshan fold-and-thrust belt between the continental NCB in the east and the Alashan Block in the west (Fig. 1) and eastward vergency within the Helanshan and Zhuozi fold-and-thrust belt have been outlined by Darby and Ritts (2002) and corroborate this model. The presence of a foreland basin in front and significant shortening within the Helanshan indicate significant intracontinental convergence between the NCB and the Alashan Block during Late Jurassic and Early Cretaceous (Darby and Ritts, 2002). As proposed by Faure et al. (2012) the NCB is therefore an isolated indentation-type block. The Yinshan–Yanshan belt represents its north- ern lateral boundary and the North Qinling–Dabie belt its sinistral transpressive southern transform boundary transferring and connecting westward motion of the NCB against the South China Block (Fig. 8). The driving mechanism could be flat-slab subduction, which leads to in- creased interplate coupling and transmission of stress and deformation far into the upper plate (e.g. English et al., 2003; Gutscher et al., 2000). Such Late Jurassic to Early Cretaceous flat-slab subduction (Fig. 8b) pre- ceded by slab rollback (Fig. 8c)was recently proposed by Kiminami and Imaoka (2013). The Cretaceous-aged, ca. E- to ESE-trending strike-slip faults and related basins within the Qinling Mountains (Zhang et al., 2001) represent part of a tear fault connecting shortening at the south- ern tip of the Helanshanwith an unknown tip point east of present-day Dabie Shan. Since reliable markers are missing the amount of indentation of the NCB is unclear. One speculative marker might be the correlation of the Palaeozoic Shangdan suture in the Qinling Mts. with the Palaeozoic North Quaidam suture zone in the southern Qilian Mts. This would yield anoffset of 200 to 400km,which is cumulative for both Cretaceous and Cenozoic times. Depending on the complex activity along the lithospheric-scale tear fault, some of the granitoids underwent rapid cooling subsequent to in- trusion, whereas others resided for a longer time at depth. 40Ar/39Ar mineral ages record intense cooling from ca. 550° (amphibole) to 200 °C (K-feldspar) between ca. 123 Ma and 111 Ma (Fig. 7). A phase of strong surface uplift and denudation is corroborated by the presence of ca. E-trending ductile shear zones throughout the QinlingMts., which yield similar Late Jurassic to Early Cretaceous mineral ages for shearing, andwhich are connectedwith narrow, fault-bounded Lower Cretaceous sedimentary basins filled with conglomerates and sandstones. Along the southern margin of the Qinling Mts., Lower Jurassic strata were overthrusted, and the foreland fold–thrust belt also implies the growth of topography (Ratschbacher et al., 2003; Zhang et al., 2001). Evidence for Late Jurassic to Cretaceous rapid exhumation of some sectors of the Qinling orogen and adjacent areas was also found in other thermo- chronological studies (Chang et al., 2010; Hu et al., 2006; Ratschbacher et al., 2006; Shen et al., 2007; Zheng et al., 2004). Deformation andmagmatic activity concentrated along the 1200 km long E–Wbelts might further be a consequence of large and sharp steps in lithospheric thickness, defining long-lasting zones of weakness be- tween the Ordos Block and the adjacent Qinling–Dabie and Yinshan– Yanshan orogens (Chen et al., 2009). Upwelling magma was possibly deflected around the deep lithospheric root of the Ordos Block, causing widespread lateMesozoic igneous rocks around butmuchweakermag- matic activity within the Ordos basin (e.g. Yuan et al., 2007). In our model the Helanshan fold–thrust belt represents an intra- continental suture. Based on global mantle tomography van der Meer et al. (2010) outline a remnant of a subducted oceanic plate beneath Central China (Central China slab). Slab ages are given as 260/250 to 121–84 Ma, bracketing the onset and end of subduction. Van der Meer et al. (2010) interpret the Central China slab as the remnant of eastward subducted Palaeotethys lithosphere below Cathaysia and as the south- ern continuation of the Mongol–Okhotsk slab. We suggest that the intracontinental subduction proposed by Faure et al. (2012) is linked to a larger, segmented, possibly mixed oceanic/continental subduction zone, traced by the today's N–S trending Central China slab deeply Ta bl e 4 Re su lts fr om ap at ite (U – Th – Sm )/ H e an al ys is .A m ou nt of he liu m is gi ve n in na no -c ub ic -c m in st an da rd te m pe ra tu re an d pr es su re .A m ou nt of ra di oa ct iv e el em en ts ar e gi ve n in na no gr am s. Ej ec tio n co rr ec t. (F t) :c or re ct io n fa ct or fo ra lp ha -e je ct io n (a cc or di ng to Fa rl ey et al .,1 99 6; H ou ri ga n et al ., 20 05 ). U nc er ta in tie s of he liu m an d th e ra di oa ct iv e el em en tc on te nt s ar e gi ve n as 1 si gm a, in re la tiv e er ro r% .U nc er ta in ty of th e sa m pl e av er ag e ag e is 1 st an da rd er ro r, as (S D ) / (n )1 /2 ;w he re SD = st an da rd de vi at io n of th e ag e re pl ic at es an d n = nu m be r of ag e de te rm in at io ns .D iv is io n of st an da rd de vi at io n is ap pl ie d on ly in ca se s of n N 2. A liq uo t H e 23 8 U 2 32 Th Th /U ra ti o Sm Ej ec ti on U nc or r. H e- ag e Ft -c or r. H e- ag e 1σ Sa m pl e un w ei gh te d av er .& s. e. V ol . s. e. M as s s. e. Co nc . M as s s. e. Co nc . M as s s. e. Co nc . co rr ec t. [n cc ] [n cc ] [n g] [n g] [p pm ] [n g] [n g] [p pm ] [n g] [n g] [p pm ] (F t) [M a] [M a] [M a] Q L- 44 a2 0. 35 0 0. 00 7 0. 11 1 0. 00 2 12 .1 0. 09 6 0. 00 2 10 .5 0. 87 1. 38 8 0. 04 8 15 0 0. 80 19 .9 24 .9 0. 6 Q L- 44 a3 0. 16 6 0. 00 3 0. 08 1 0. 00 2 16 .2 0. 03 7 0. 00 1 7. 3 0. 45 0. 66 6 0. 02 4 13 2 0. 71 14 .4 20 .3 0. 5 Q L- 44 a4 0. 21 5 0. 00 4 0. 11 0 0. 00 2 16 .3 0. 05 2 0. 00 1 7. 7 0. 47 0. 89 8 0. 03 1 13 3 0. 74 13 .7 18 .6 0. 5 21 .3 1. 9 Q L- 50 a1 0. 03 6 0. 00 1 0. 02 0 0. 00 1 3. 6 0. 01 6 0. 00 0 2. 9 0. 81 0. 37 8 0. 01 3 70 0. 77 11 .1 14 .5 0. 6 Q L- 50 a2 0. 09 8 0. 00 2 0. 05 8 0. 00 1 6. 0 0. 03 6 0. 00 1 3. 8 0. 63 0. 82 8 0. 03 8 86 0. 87 11 .1 12 .7 0. 4 13 .6 0. 9 Q L- 46 a1 0. 69 6 0. 01 2 0. 14 5 0. 00 3 18 .4 0. 02 3 0. 00 1 2. 9 0. 16 1. 72 8 0. 06 0 21 8 0. 72 34 .9 48 .4 1. 1 Q L- 46 a2 0. 59 5 0. 01 1 0. 13 1 0. 00 2 12 .6 0. 02 5 0. 00 1 2. 4 0. 19 1. 82 5 0. 06 3 17 5 0. 78 32 .3 41 .5 1. 0 44 .9 2. 8 Q L- 48 a1 0. 17 0 0. 00 4 0. 05 5 0. 00 1 8. 9 0. 03 6 0. 00 1 5. 8 0. 65 0. 74 7 0. 02 6 12 2 0. 76 20 .3 26 .7 0. 7 Q L- 48 a2 0. 11 1 0. 00 3 0. 04 4 0. 00 1 9. 8 0. 02 2 0. 00 1 4. 8 0. 49 0. 53 7 0. 02 0 12 0 0. 71 17 .2 24 .4 0. 7 Q L- 48 a3 0. 43 8 0. 00 8 0. 12 7 0. 00 2 9. 6 0. 08 9 0. 00 2 6. 7 0. 70 1. 39 8 0. 04 9 10 5 0. 79 22 .7 28 .9 0. 7 26 .7 1. 3 Q L- 45 a1 0. 10 7 0. 00 2 0. 02 9 0. 00 1 10 .5 0. 01 8 0. 00 1 6. 7 0. 64 0. 25 1 0. 00 9 91 0. 70 24 .9 35 .5 1. 1 Q L- 49 a1 0. 05 3 0. 00 2 0. 01 9 0. 00 1 7. 0 0. 01 1 0. 00 0 3. 8 0. 55 0. 36 7 0. 01 4 13 2 0. 69 17 .7 25 .6 1. 0 Q L- 49 a2 0. 02 2 0. 00 1 0. 01 4 0. 00 1 6. 0 0. 00 7 0. 00 0 3. 2 0. 53 0. 31 1 0. 01 1 13 6 0. 65 10 .0 15 .3 0. 8 Q L- 49 a3 0. 06 1 0. 00 2 0. 01 9 0. 00 1 5. 4 0. 01 1 0. 00 0 3. 2 0. 60 0. 46 4 0. 01 7 13 1 0. 74 19 .8 26 .6 1. 0 22 .5 3. 6 39B. Heberer et al. / Tectonophysics 617 (2014) 31–43buried in the mantle (van der Meer et al., 2010). In such a case, the ori- entation of the slab is now different from the mostly ca. W–E trending subduction zones proposed for Eastern Asia (Yin and Harrison, 2000). The Banda arc may represent an appropriate model for the interaction of oceanic and continental lithosphere and rotation of subducted slabs (Spakman and Hall, 2010). Our model shows similarities to the classical STEP fault model of Govers andWortel (2005), in which a STEP fault is defined as the lateral boundary of a retreating subduction zone. However, in our case the fault is propagating away from the intracontinental subduction zone as a lateral transform boundary of an indenting continental block (Fig. 8a). 6.2. The Cenozoic cooling history of the Taibai granite After the initial stage of rapid cooling (cooling phase 1 in Fig. 7), a stable thermotectonic environment developed and prevailed for ca. 75Myr (cooling phase 2). During this phase of insignificant exhumation and peneplanation the age patterns and the distinctly shaped APAZ and APRZ evolved (Fig. 6a). Based on these patterns we derived a geother- mal gradient of ca. 20 °C/km. Such a pre-exhumation thermal structure seems reasonable for a thickened, stable crustwithin a plate interior set- ting. Widespread Mesozoic lithospheric extension (decratonization) with high heat flow and voluminous magmatism was restricted to the eastern part of the NCB (Chen et al., 2009; Zhang, 2013), and even beneath the younger rift systems surrounding the Ordos basin the crust is relatively thick (N37 km) (Li et al., 2006; Zheng et al., 2006). An identical gradient of 20 °C/km was suggested for the Mesozoic for the Sichuan basin by Richardson et al. (2008). Based on our AHe profile we find a further pronounced cooling phase (cooling phase 3) for the latest Oligocene. However, this cooling phase did not bring our samples to the surface, and our data clearly argue for pulsed exhumation since the latest Oligocene (see below for discussion of latest cooling stage). Intensified cooling during ca. 22–25 Ma is corroborated by palaeostress analyses from Mercier et al. (2013) for the Weihe graben. They describe the Late Oligocene–Early Miocene as a period of major transpression, uplift and exhumation. Sediment accumulation rates in the graben show a marked increase at ca. 25 Ma (inset Fig. 7). Thus the sedimentological record, stored in the adjacent basin supports that denudation and surface uplift of the range accompanied rapid cooling. A slightly younger (~21 Ma) onset of rapid cooling and denudation were found for the Ordos Basin, based on a compilation of FT ages and the sedimentary history of the basin and the surrounding grabens (Yuan et al., 2007). Intensified thermo- tectonism was widespread during this phase, e.g. major fault reactiva- tion occurred in the Himalayas (e.g. Le Fort, 1975), as well as major compression in the Northern Tibetan Plateau (Jolivet et al., 2001). Whether major plate boundary reorganizations in the Pacific were the underlying cause for intensified thermotectonism or if these observa- tions are at all related to the same process remains a matter of debate (for more details see Mercier et al., 2013). So far, the long-term cooling history derived from the Taibai vertical transect is rather similar to cooling paths from awide area ranging from eastern Tibet to the Dabie Shan: Roger et al. (2011) compiled t–T-paths from the eastern part of the Tibetan Plateau and find a long period of at least 100 Myr without major tectonic events, erosion or sedimentation following initial rapid cooling for the South Songpan–Garzê, Kunlun and Yidun Blocks. This phase of tectonic quiescence was superseded by exhumation linked to the uplift of the Tibetan Plateau starting at ca. 30 Ma. Based on low-T thermochronology from samples from the Sichuan basin Richardson et al. (2008) found evidence for widespread erosion of 1–4 km that began sometime after 40 Ma. They correlate this event with the reorganization of the drainage pattern and the tran- sition from internal to external drainage,which, according to Roger et al. (2011)might also indicate the onset of topography building in NE Tibet. In a recent contribution, Shi et al. (2012) derive a strikingly similar long- term cooling pattern for samples from the Dabashan at the interface of 40 B. Heberer et al. / Tectonophysics 617 (2014) 31–43the Sichuan basin and the Qinling–Dabie orogenic belt: AFTA and thermal track length modeling indicate rapid cooling before 130– 90 Ma and after 25 Ma and slow cooling in-between. All these thermochronological studies have in common that no major signal of rapid cooling is preserved for the time of the collision of India with Asia at ca. 50 Ma (e.g. Searle et al., 1987; Tapponnier and Molnar, 1977). However, younger (mostly Miocene) exhumation/ cooling events have often been interpreted as far-field effects of this collision (e.g. Enkelmann et al., 2006; Liu et al., 2013). Alternatively, Pacific plate subduction or a combination of both geodynamic mechanisms (e.g. Ma et al., 1982) have been suggested to be the trigger for Late Cenozoic tectonism (Ma et al., 1982, 1984; Yin, 2010). Enkelmann et al. (2006) outlined a corridor of rapid cooling and strong exhumation at the very end of the thermal history for samples from the Southwestern Qinling Mountains. Our study, located just outside this zone of late Cenozoic uplift, strongly corroborates their finding of an eastward decrease of latest-stage exhumation. Whereas these authors find final exhumation of 2–3 km for the SW Qinling, our data clearly suggest less than 1 km for the latest step (step 5 of our cooling model). As mentioned above, our samples must have resided above the PRZ prior to final cooling. Thus they do not define an exact onset of final cooling. Modeling yields similarly good results for 7–5 Ma. A Late Miocene intensified cooling and tectonism is corroborated by sev- eral studies: One of the strongest arguments comes from the change of course of the Yellow River. Its current bent-shaped course around the Ordos basin (Fig. 1) initiated during Late Miocene to Early PlioceneAPAZ Age AFT AHe age AFT age constant constant constant constant decreasing constant decreasing decreasing constant decreasing 20°C/km 1000 m 1000 m 2000 m 2000 m t1 t2 40° 60° 80° 120° AHe APRZ 0 1000 1500 2000 2500 3000 3500 10 20 30 40 50 60 70 80 Age (AHe ) Al tit ud e base of exhumed APRZ (~80 °C) top of exhumed APAZ ? (~60 °C) a) b) ? and AFT Fig. 6. a) Age-elevation correlation for apatite fission-track and apatite (U–Th–Sm)/He samples from the Taibai mountain. The break-in-slope within the AHe data at ca. 2000 m marks the onset of rapid cooling at ca. 22 to 25 Ma. A second inflection point at ca. 1000 m is interpreted as a later reheating event, probably as a result of hydrothermal fluxes. b) Concept of expected AFT and AHe ages versus depth profiles within a thermotectonically stable setting. Ages of both thermochronometers decrease simulta- neouslywith depth where the APRZ and the APAZ overlap. T2 marks the onset of an exhu- mation event, which uplifted the APRZ and APAZ; t1 marks an older exhumation event.times due to the development of a topographic divide, which blocked the river from flowing eastward (Lin et al., 2001). Sedimentological ev- idence from theWeihe graben east of Xi'an strongly points to a marked change in palaeoenvironmental and climatic conditions in late Miocene times (Kaakinen and Lunkka, 2003). A lack of major changes in the sur- face gradient within the Bahe Formation (deposited between 11 and 7.3 Ma) indicates that no significant surface uplift of the adjacent QinlingMts. occurred in this time period. However, the onset of aeolian Red Clay deposition immediately above the Bahe sediments is linked to a major uplift phase of the Tibetan Plateau and the development of the monsoon system in China (Zhisheng et al., 2001). Enkelmann et al. (2006) andWang et al. (2012) bracketed the onset of this final exhuma- tion event between ca. 9 and 4 Ma and both the lower crustal flow model (Clark and Royden, 2000) and the South China Craton acting as a backstop (Tapponnier et al., 2001) equally well describe diminishing exhumation towards the east along a rheologically weak crustal corri- dor beneath the QinlingMts. In sum, all these lines of evidence strongly support a causal relationship of our last cooling step initiated at ca. 7–5 Ma and Tibetan Plateau uplift. 7. Conclusions 1. The Taibai granite body is formed by multiple intrusions. New U–Pb zircon data yield Late Jurassic (156 Ma) and Early Cretaceous ages (124 Ma) at a pressure of 4.2 ± 0.2 kbar (at 775 °C) corresponding to a depth of ca. 12–13 km. The Taibai granite forms thewesternmost0 20 40 60 80 100 120 140 160 180 200 (Ar-Ar Kf) 300 (Ar-Ar Bt) 550 (Ar-Ar Hbl) 700 (U-Pb Zr) 140160 120 100 80 60 40 40 20 20 0 0 Time (My) Te m pe ra tu re (° C) XXXXX X X X XX X mid-Miocene reheating of lowermost sample ? 1 2 3 4 51 cooling step Goodness-of-fit values from HeFTy modeling 400 200 0 Sedimtentation History Weihe basin compacted sed. accum. D ep os itio n ra te (m m/ yr) 1.0; 0.7 AHe; AFT age; AFT length 0.8; 0.7 0.7; 0.9 0.5; 0.9 0.9; 0.6 0.6; 0.7 ;0.9 Fig. 7. Cooling history of all samples from the Taibai profile. The cooling paths are constrained by U–Pb ages, Ar–Ar ages, and forward modeling with HeFTy (Ketcham, 2005). Inset shows the sedimentation history of the Weihe graben taken from Liu et al. (2013), modified after Bellier et al. (1988a) and Xing et al. (2005). Note that the Late Oligocene–Miocene onset of intensified cooling is paralleled by an increase in sediment accumulation. Late Miocene cooling also roughly coincides with higher accumulation rates. ca . 10 0 km Granite ascend along lithospheric-scale tear fault foreland basin SOUTH CHINA BLOCK NORTH CHINA BLOCK ALASHAN BLOCK Helanshan mk 002 . ac WE a) b) c) Stage 1 at ca. 150 Ma: NORTH CHINA BLOCK PACIFIC PLATE HelanshanNo magmatism W-ward motion E ALASHAN BLOCK W Asthenospheric upwelling Rollback Migration of magmatic front Stage 2 at ca. 130 Ma: NORTH CHINA BLOCK PACIFIC PLATE Helanshan E ALASHAN BLOCK W Fig. 8. a) Tectonicmodel of Yanshanian tectonics according to Faure et al. (2012) and results from this study. b) Schematic cross-sections from the (Palaeo-)Pacific subduction zone in the W to the Alashan Block in the E modified after Kiminami and Imaoka (2013). Flat-slab subduction increases interplate coupling and induces a W-ward motion of the North China Block. c) Slab rollback of the (Palaeo-)Pacific plate prior to flat-slab subduction. 41B. Heberer et al. / Tectonophysics 617 (2014) 31–43tip of ca. 1200 km longNorthQinling–Dabie granite belt at the south- ern margin of the North China Block. 2. The exhumation history of the Taibai granite is divided into five dis- crete periods: (1) After emplacement of the youngest intrusions the Taibai samples cooled rapidly at rates of ca. 30 °C/Myr. (2) At ca. 100 Ma cooling rates decreased drastically to less than 1 °C/Myr, and tec- tonic quiescence prevailed for ca. 75Ma. (3) A second phase of rapid exhumation was initiated at 22–25 Ma (ca. 5 °C/Myr). (4) Cooling rates decreased at ca. 15Ma and (5) accelerated again to rates similar to phase 3 during the last ca. 7 Myr. 3. Changes in cooling rate are interpreted as reflecting pulses of tec- tonic activity. Rapid cooling after emplacement occurred along a large tear fault lateral to the front of a zone of intracontinental convergence. Linking the latest Oligocene–Miocene pulse of de- formation and uplift of the QinlingMountains with the underlying geodynamic causes, e.g. India–Asia collision, Pacific subduction, opening of the Japan Sea is difficult and awaits further detailed studies. Minor late-stage cooling is related to the growth of the Tibetan Plateau and confirms diminishing rates of exhumation eastward. 4. Our study also highlights the usefulness of combining various geochronometric tools to detect the long-term cooling history. In particular, AFT and AHe, which both are most sensitive at shallow crustal levels, applied in tandem along a vertical profile, may delin- eate episodes of mountain building for areas, where exhumation has not been sufficiently efficient to exhume samples from below the PAZ but from below the PRZ. 5. Finally, the long-term cooling history derived on the Taibai vertical profile underlines, that preexisting weak corridors in the lithosphere are susceptible to deformation and magmatic activity. Located between the rigid Sichuan and Ordos Blocks the Qinling Mts. have been influenced by successive tectonic events, concentrated along major tectonic structures.Acknowledgments We acknowledge support of the fieldwork by the Eurasia–Pacific Uninet. We particularly acknowledge support by grant P22110 of the Austrian Science Fund FWF and grants 41190074 and 41225008 of NSFC.Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.tecto.2014.01.011.References Anderson, J.L., Smith, D.R., 1995. The effects of temperature and fO2 on the Al-in- hornblende barometer. Am. Mineral. 80, 549–559. Bellier, O., Mercier, J.L., Vergely, P., Long, C.X., Ning, C.Z., 1988a. 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