Qinling, Tibet): Evidence from the Hannan-Micang Crystalline Massifs and the Northern Sichuan Basin, Central China

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Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

Zhao Yang1,2,3,*, Lothar Ratschbacher1,*, Raymond Jonckheere1, Eva Enkelmann4, Yunpeng Dong2, Chuanbo Shen1,3, Maria Wiesinger1, and Qian Zhang2 1GEOLOGIE, TECHNISCHE UNIVERSITÄT BERGAKADEMIE FREIBERG, 09599 FREIBERG, GERMANY 2STATE KEY LABORATORY OF CONTINENTAL DYNAMICS, DEPARTMENT OF GEOLOGY, NORTHWEST UNIVERSITY, 710069 XI’AN, CHINA 3KEY LABORATORY OF TECTONICS AND PETROLEUM RESOURCES, CHINA UNIVERSITY OF GEOSCIENCES, MINISTRY OF EDUCATION, WUHAN 430074, CHINA 4DEPARTMENT OF GEOLOGY, UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO 45221, USA

ABSTRACT

This paper addresses the timing of fi nal foreland growth of China’s largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fi ssion-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fi ssion-track partial annealing zone (240 ± 30 °C) throughout the Paleozoic–Middle Triassic (481–246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2–2.5 °C/m.y. and 0.04–0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38–0.70 °C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4–5 °C/m.y.) since the Late Miocene (13–8 Ma), derived from apatite fi ssion-track temperature-time path models, signifi es involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4–2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.

LITHOSPHERE; v. 5; no. 4; p. 420–437 | Published online 10 May 2013 doi:10.1130/L260.1

INTRODUCTION deformation related to the exceptionally wide Tagh fault (e.g., Zheng et al., 2006; Palumbo et Pacifi c plate boundary (e.g., Ratschbacher et al., 2009). Along its eastern margin, the plateau The Triassic–Jurassic Qilian–Qinling– al., 2003; Li and Li, 2007). The Sichuan Basin, terminates abruptly against the Sichuan Basin, Dabie–Sulu (short Qinling) orogen of central fl oored by relatively rigid cratonic basement and forming spectacular topographic relief; there, the China involved subduction of cratonal crust characterized by fairly high Mesozoic subsid- plateau appears to be growing mostly vertically to and exhumation from mantle depths (e.g., ence rates (e.g., Guo et al., 1996; Meng et al., (e.g., Kirby et al., 2008; Z. Shen et al., 2009). Hacker et al., 2004). Dates are now available 2005) escaped strong shortening. Its northern It has been suggested that lateral plateau spread- for the onset of continental subduction (ca. 255 part, however, exposes in its bounding ranges ing diverges around the Sichuan Basin (Clark Ma; e.g., Cheng et al., 2011), the polyphase and marginal sedimentary rocks the leading and Royden, 2000), expanding southeastward ultrahigh-pressure metamorphism (ca. 245 Ma edge of the Qinling fold-and-thrust belt and thus toward southeastern Asia (e.g., Ouimet et al., and ca. 227 Ma; e.g., Hacker et al., 2006; D. Liu allows examination of the terminal stages of the 2010) and northeastward into the Qinling orogen et al., 2006; X. Liu et al., 2012), and the exhu- Qinling orogeny. (e.g., Enkelmann et al., 2006; Clark et al., 2010). mation of the ultrahigh-pressure rocks to the The Cenozoic Tibetan Plateau is Earth’s The northern Sichuan Basin and the Hannan- surface (≤190 Ma; Grimmer et al., 2003; Wang most spectacular evidence of upper-plate thick- Micang crystalline massifs occupy the junction et al., 2009). However, dating of foreland propa- ening in the aftermath of a continent-continent between the WNW-trending Qin Mountains gation, and in particular of the emplacement of collision (e.g., Argand, 1924). In the north, the (Qinling), part of the Qinling orogen, and the the thrust front of the Qinling orogen, has been plateau has overstepped the Tarim Basin, caus- NE-trending Longmen Shan, the northeast- hindered by the overprint of the foreland by ing the formation of the Tien Shan (e.g., Avouac ern margin of the Tibetan Plateau (Fig. 1; e.g., et al., 1993; Zubovich et al., 2010), and it is Burchfi el et al., 1995). Thus, the Proterozoic *E-mails: [email protected]; lothar@ currently growing northeastward into the Hexi crystalline basement core of the Hannan- geo.tu-freiberg.de. Corridor along the eastern end of the Altyn Micang crystalline massifs and the sedimentary

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Figure 1. Shuttle Radar Topography Mission (SRTM) digital elevation model (90 m horizontal resolution) with published low-temperature thermochronologic ages (in Ma and 1σ error) relevant to this paper. Red lines delineate likely late Cenozoic faults; data are compiled from Du et al. (1998), Enkelmann et al. (2006), Taylor and Yin (2009), and our ﬁ eld observations.

rocks of the northern Sichuan Basin provide rated into the eastward growth of the Tibetan massifs and the northern Sichuan Basin into the an excellent natural laboratory for studies of Plateau? We examine the late-stage cooling his- eastward-growing Tibetan Plateau. the foreland propagation of both the Qinling tory in the region east of the Longmen Shan and and the Tibetan Plateau orogens. The principal study whether the plateau is overstepping the GEOLOGIC FRAMEWORK questions this paper addresses are: (1) When Sichuan Basin. was the frontal part (the leading edge) of the Our analysis of the late-stage growth of the The Longmen Shan, Qinling, Daba Shan, Qinling fold-and-thrust belt emplaced? The Qinling and Tibetan Plateau orogens integrates and northern Sichuan Basin bound the Hannan- Qinling orogen terminates in the fold belt of the new and published zircon and apatite ﬁ ssion- Micang crystalline massifs in the west, north, northern Sichuan Basin with a blind thrust that track and (U-Th)/He thermochronology applied east, and south, respectively (Figs. 1 and 2A). dips N and NE underneath the Hannan-Micang to the Hannan-Micang crystalline massifs and The Longmen Shan separates the eastern Tibetan crystalline massifs and the Daba Shan (Fig. 2; Sichuan Basin rocks and the depositional his- Plateau from the Sichuan Basin; it formed dur- e.g., Shi et al., 2012); we date deformation at the tory of the northern Sichuan Basin. We argue ing the Late Triassic, Cretaceous, and Cenozoic deformation front by estimating cooling- and that enhanced cooling and exhumation from (Burchﬁ el et al., 1995; Liu et al., 1995; Arne exhumation-rate changes in the thrust hanging the late Middle Jurassic to the early Late Creta- et al., 1997; Kirby et al., 2000, 2002; Li et al., wall (the Hannan-Micang crystalline massifs) ceous signify propagation of the Qinling orogen 2012). The Qinling was assembled by collision and the fold belt (the northern Sichuan Basin). into its leading foreland, and that accelerated of the North and South China cratons and inter- (2) When were the Hannan-Micang crystalline cooling starting in the late Miocene marks the vening accretionary wedges and magmatic arcs; massifs and northern Sichuan Basin incorpo- incorporation of the Hannan-Micang crystalline collision occurred in the Triassic–Jurassic, and

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A E106° E107° E108° N34°

Q55 66± 3 80± 5 Qin 76± 5 Mountains 57± 3 59± 3 Foping 51± 2 Q65 74± 6 101± 8 48±2 62±3 52± 3 43± 6 Fig. 2b profile line 77± 4 73± 3 Q67 Q69 83.2± 5.5 D7611 77± 4 Hanzhong basin Hanzhong 184± 13() ZHe 62± 6 138.8± 10.7 92.8± 3.8 HN11 113.5± 4.5() AHe Xixiang N33° 114.4± 7.4 HN03103.8± 4.6 N33° 118.4± 2.3() AHe HN12 255± 22() ZFT Xixiang basin 90.3± 6.8 HN19 153± 3() ZHe sif 77±6D7613 123.5± 2.7() AHe 246± 19() ZFT 133.1± 6.0 Hannan mas F2 HN17 A 115.3± 3.2() AHe 113.7± 2.9() AHe 106.4± 5.8 70.1±4 .1 A' 81.0± 2.2() AHe 107.4± 3.8() AHe 176± 9() ZHe 73.2± 4.0() AHe F3 364±26(ZFT) 88.0± 3.7() AHe 73.2± 6.2() AHe 103.8± 5.1 MC25 Huijunba syncline 123.5± 6.0 110.7± 6.8 Daba Shan 87.6± 6.2() AHe 119.8± 5.9 Qingchuan fault (F1) 274± 27 95.8± 4.4MC01 103.7± 4.2 ()ZFT 79.0()± 2.2 AHe 110.1± 5.3 480± 34() ZFT Shan 93.9± 4.1 103± 3.7MC02 Micang massif 92.2± 1.8() AHe 270±25 () ZFT 82.9± 7.8 69.5± 1.2() AHe MC12 68.9± 3.5 50.1± 1.7() AHe 60.8± 5.7 51.5± 1.7() AHe Longmen 39.9± 2.5() AHe 73.8± 3.7 62.4± 5.2 MC14 31.9± 9.9() AHe 68.4± 3.4 66.8± 2.9 MC03 78.2± 5.8 MC04 B Nanjiang 64.1± 2.8 MC05 C 61.6± 3.0 MC11 N 70.2± 4.5MC08 B' Wangcang 60.3± 2.3 0 25 50() km C' 66.8± 2.5MC15 MC09 Northern Sichuan Basin E107° N32° E106° E108° N32° Early Late Earll y-Midd e Triassic Permian Pal eozoic Mianll ue comp ex Cenozoic Cretaceous Jurassic Jurassic F3 Guanba Fault Precambrian Proterozoic Paleozoic-Early Proterozoic A A' F1 Qingchuan Fault undivided mafic intrusion Mesozoic granites granite Stratigraphic section F2 Mujiaba Fault Xu et al., 2010 Thrust Normal Strike-sl ip Fault of unknown Blind fault(l eading thrust Sample location of this study Tian et al., 2012 fault fault fault kinematics of the Qinling orogen) Enkelmann et al., 2006 Chang et al., 2010

B N S m Hanzhong basin Hannan massif Huijunba syncline Micang massif Northern Sichuan basin 2000 Guanba fault Mujiaba Qingchuan fault fault 1000

0 unconformity

unconformity Neogene– Early Early–Middle 0 10 20 km Late Triassic Permian Quaternary Cretaceous Jurassic Jurassic

Early Late Early–Middle Proterozoic Proterozoic Thrust Normal Strike-slip fault (dextral) Paleozoic Proterozoic Proterozoic granite mafic intrusion fault fault

Figure 2. (A) Apatite and zircon fi ssion-track and (U-Th)/He ages (in Ma with 1σ error) plotted on geologic-tectonic map of the Hannan-Micang massif and northern Sichuan Basin; likely late Cenozoic faults are drawn as continuous lines. Ages in red denote samples recording late Cenozoic accelerated cooling in thermal history models. (B) Geologic cross section modifi ed from unpublished 1:200,000 geologic map of Hanzhong and Zhenba areas and our own fi eld work.

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reactivation by wrenching and rifting ensued eastern abutting Daba Shan, however, root in a ZFT partial annealing zone (ZPAZ) at 240 ± in the Cretaceous and Cenozoic (Meng and a basal detachment that underlies the Hannan- 30 °C (Zaun and Wagner, 1985; Hurford, 1986; Zhang, 1999; G.W. Zhang et al., 2001; Ratsch- Micang crystalline massifs and the entire Daba Brandon et al., 1998; Bernet, 2009), although bacher et al., 2003, 2006; Dong et al., 2011a). Shan fold-and-thrust belt and connects with the the variation of ZFT annealing with radiation The southwesterly convex Daba Shan orocline Qinling suture >100 km to the north (e.g., Shi et damage (Rahn et al., 2004) may cause consider- is part of the foreland fold-and-thrust belt of the al., 2012, and references therein). able shifts. The U content is commonly used to Qinling orogen; it formed in the Jurassic–Early Previous thermochronologic studies have assess the radiation damage in zircon; in gen- Cretaceous (Shi et al., 2012; Hu et al., 2012). covered the central and south Longmen Shan eral, it accounts for 80%–90% of the radiation The Sichuan Basin comprises a nearly complete (Arne et al., 1997; Kirby et al., 2002; Clark damage (e.g., Garver and Kamp 2002; Garver et Paleozoic–Cenozoic stratigraphic sequence that et al., 2005; Godard et al., 2009; Ouimet et al., 2005). The U content of our zircons ranges rests on Proterozoic Yangtze (South China) cra- al., 2010; Li et al., 2012), the western Qinling from 129 to 68 ppm, representing low to aver- ton basement that is comparable to the crystal- (Enkelmann et al., 2006; Zheng et al., 2006; J.H. age values (e.g., Garver and Kamp, 2002); this line rocks of the Hannan-Micang crystalline Liu et al., 2012), and the northeastern Tibetan range suggests that there is little difference in massifs (Guo et al., 1996; Richardson et al., Plateau and adjacent Sichuan Basin (Richard- radiation damage between our samples. There- 2008). The Hannan-Micang crystalline massifs son et al., 2008; Li et al., 2012); the northern fore, we interpret the broad ZFT age range to consist of the northern Hannan massif, central Longmen Shan is less studied (Li et al., 2012; refl ect slow cooling of the Hannan-Micang Huijunba syncline, and southern Micang massif Fig. 1 for data compilation). These studies have crystalline massif samples through the ZPAZ. (Fig. 2B); they expose Proterozoic metavolca- defi ned the age and mechanism of Tibetan Pla- Our new AFT ages span 114.4 ± 7.4–60.3 ± nosedimentary strata, voluminous Neoprotero- teau growth. U/Th-Pb zircon and 40Ar/39Ar and 2.3 Ma. All pooled ages are younger than the zoic granitoids, and, above an unconformity, Rb/Sr biotite dates for the Hannan-Micang intrusion or deposition ages of the sampled Sinian to Silurian clastic and carbonate cover crystalline massifs have documented rapid Late rocks. All bedrock samples except MC12 (P rocks (e.g., G.W. Zhang et al., 2001; Dong et al., Proterozoic postintrusive cooling (e.g., Z.Q. [χ2] = 3.1%) passed the χ2 test (P [χ2] > 5%), 2011a). In the northern Sichuan Basin, Upper Zhang et al., 2001). Three zircon (U-Th)/He but three Jurassic and Lower Cretaceous sedi- Triassic molasse unconformably covers Permian ages indicate involvement of the Hannan massif mentary rock samples failed the test. A plot of to Middle Triassic passive-margin limestones. into Qinling orogen foreland deformation since the single AFT ages against the host-rock strati- Throughout the Jurassic and Early Cretaceous, the Late Jurassic (Xu et al., 2010), and apatite graphic age indicates that some apatites of these the basin continued to serve as a clastic foreland fi ssion-track and (U-Th)/He ages trace terminal three samples were not fully reset (Fig. 3). C.B. depocenter to the Qinling orogen. The Huijunba southward propagation of the Qinling orogen by Shen et al.’s (2007) vitrinite data show that pale- syncline was part of the northern Sichuan Basin rapid pre–Late Cretaceous cooling (>100–90 otemperatures decreased from ~250 °C to 130 from the Late Neoproterozoic to the Early– Ma) in the northern Sichuan Basin (data com- °C from the Upper Triassic to Middle Jurassic Middle Jurassic; its sequence contains the Late pilation in Fig. 2A; Chang et al., 2010; Xu et strata (Fig. 3). Up-section projection of this tem- Triassic unconformity but lacks Upper Jurassic al., 2010; Tian et al., 2012). The growth of the perature gradient places the bottom of the AFT and Cretaceous strata (Fig. 2; S.B.G.M.R, 1991; Tibetan Plateau has involved the western Qin- partial annealing zone (APAZ; 110 ± 10 °C) in Guo et al., 1996; Meng et al., 2005). ling since 9–4 Ma (Enkelmann et al., 2006) and the Upper Jurassic–Lower Cretaceous strata; The major map-scale Cenozoic faults of the Hannan-Micang crystalline massifs since ca. this allows for partial preservation of detrital the Hannan-Micang crystalline massifs are the 16 Ma (Tian et al., 2012; Fig. 2A). grain ages in these strata. As only a few grain dextral transpressive Qingchuan fault (F1), and ages overlap the depositional ages, we used all the Mujiaba (F2) and Guanba (F3) faults, which NEW THERMOCHRONOLOGIC RESULTS single-grain ages to calculate the pooled ages have unknown kinematics (Fig. 2A). The late (also for those three sedimentary rocks samples) Cenozoic Hanzhong and Xixiang basins (Fig. We sampled granite, diorite, gabbro, and (Fig. 3; Table 1). 2A) appear to have been controlled by these Upper Triassic to Lower Cretaceous sandstones Mean AFT confi ned track lengths vary faults; together with the established late Ceno- from the Hannan-Micang crystalline massifs between 11.5 ± 0.1 μm and 13.8 ± 0.1 μm; the zoic stress fi eld of the West Qinling (compila- and the northern Sichuan Basin (Fig. 2A). We corresponding c-axis projected mean lengths are tion in Enkelmann et al., 2006), these basins selected 11 samples for apatite fi ssion-track 13.2 ± 0.1–14.9 ± 0.1 μm (Table 1). The track- allow speculations on the nature of the Mujiaba (AFT), six for zircon fi ssion-track (ZFT), and length distributions show negative skewness. and Guanba faults. Accordingly, the Hanzhong two samples for apatite (U-Th)/He (AHe) dat- Mean etch pit size parallel to the c-axis, Dpar basin may have formed along a releasing bend ing. Appendix A details the methodology used (Donelick, 1993), ranges from 1.9 ± 0.2 μm of the Qingchuan fault, and the Xixiang basin at the Freiberg track laboratory and the Tübin- to 2.4 ± 0.2 μm; sample HN11 has a 2.9 ± 0.3 may have formed along the eastern transten- gen (U-Th)/He laboratory. Tables 1–3 locate the μm value and also has the longest mean track sional horse tail of the Guanba fault (Fig. 2A). samples and present the AFT, ZFT, and AHe length of 13.81 ± 0.1 μm (14.9 ± 0.1 μm after The kinematics of the Cenozoic faults within data; Figure 2A shows the age distribution. c-axis projection). Most Dpar values are close to the Hannan-Micang crystalline massifs, the The bedrock ZFT ages cover 481 ± 34 Ma to those of Durango apatite (1.83 μm; Ketcham et Mujiaba and Guanba faults, would therefore 246 ± 19 Ma with no apparent correlation with al., 1999), and the small variation implies little also have been dextral transpressive. geographic and geologic position. All the ZFT chemical variability and similar annealing kine- Little has been published about the pre- samples passed the χ2 test, and thus all crystals matics among our samples. Cenozoic structures in the Hannan-Micang are concordant within the statistical uncertainty. Figure 4 displays track-length distribu- crystalline massifs and the northern Sichuan The Hannan (samples HN17, HN19, HN03) tions and presents temperature-time (T-t) path Basin (Du et al., 1998). Likely, the Cenozoic and Micang massif (MC01, MC02, MC25) ages models for 10 samples with 120–261 confi ned faults reactivate older structures. All faults of span 364 ± 26–246 ± 19 Ma and 481 ± 34–270 ± tracks (see Appendix A for our methodology to the Hannan-Micang crystalline massifs and the 25 Ma, respectively. Field-based studies suggest increase the number of etchable confi ned tracks

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/4/420/3039552/420.pdf by guest on 27 September 2021 YANG ET AL. (µm) Dpar (S.D.) ) C (µm) Skew (L ) C (µm) S.D. (L ) C Mean (L (µm ± S.E.) —track density in standard uranium glass; d ρ (µm) Skew (L) (µm) S.D. (L) -axis projected length; S.E.—standard error; S.D.—standard c )—mean C J1—Early Jurassic; J2—Middle J3—Late K1—Early (µm ± S.E.) calibration factor for IRMM-540R; nL Mean (L) ζ — ζ ) σ P-age (Ma ± 1 ) 2 χ ( (%) P ) –2 d cm ρ 6 (10

) 2 σ TABLE 1. APATITE FISSION-TRACK RESULTS APATITE 1. TABLE ± 1 ζ (a cm Lithology nG Ns Ni 481 Sandstone (J3) 16 770 630 261.8 ± 6.2 0.442 0 70.2 ± 4.5 505 Sandstone (J2) 36 4634 4503 261.8 ± 6.2 0.478 22.1 64.1 ± 2.1 261 ± 0.1 11.5 1.9 –0.25 ± 0.1 13.5 1.2 –0.23 978 Diorite (Pt3) 25 1747 960 261.8 ± 6.2 0.292 3.1 68.9 ± 3.5 515 Sandstone (T3) 39 2681 2310 261.8 ± 6.2 0.426 15.6 66.8 ± 2.9 126 ± 0.2 11.8 1.9 –0.27 ± 0.11 13.6 1.2 –0.25 550 Gabbro (Pt3) 28 1217 628 261.8 ± 6.2 0.278 99.9 70.1 ± 4.1 171 ± 0.1 11.5 1.5 –0.29 13.2 ± 0.1 1.1 –0.68 1.9(0.2) 844 Gabbro (Pt3) 20 2527 1682 ± 6.2 261.8 0.475 56.6 92.3 ± 3.8 240 13.8 ± 0.1 0.9 –0.13 14.9 ± 0.1 0.7 –0.27 2.9(0.3) 550 Sandstone (J1) 3 516 389 261.8 ± 6.2 0.453 25.6 78.2 ± 5.8 739 Granite (Pt3) 31 728 309 261.8 ± 6.2 0.295 100 90.3 ± 6.8 34 12.2 ± 0.3 1.5 –0.45 14.0 ± 0.2 1 –0.53 2.3(0.2) 624 Granite (Pt3) 18 821 439 261.8 ± 6.2 0.471 98.4 ± 7.4 114.4 195 ± 0.1 12.7 1.2 –0.52 14.1 ± 0.1 0.8 –0.57 2.0(0.2) 448 Sandstone (K1) 42 2643 2715 261.8 ± 6.2 0.470 0 60.3 ± 2.3 570 Granite (Pt3) 20 2058 1189 261.8 ± 6.2 0.462 50.9 103.8 ± 4.6 192 12.5 ± 0.1 1.2 –0.78 14.0 ± 0.1 0.7 –0.12 519 Granite (Pt3) 60 1493 752 251.7 ± 11.9 0.335 46.3 ± 5.5 83.2 497 Sandstone (J3) 33 1798 1145 261.8 ± 6.2 0.296 7.7 ± 3.0 61.6 122 12.2±0.2 1.6 -0.76 13.9±0.1 1.0 –0.08 2.3(0.2) 487 Diorite (Pt3) 15 823 448 251.7 ± 11.9 0.335 39.3 77.0 ± 6.0 500 Sandstone (J1) 11 385 236 261.8 ± 6.2 0.351 24 62.4 ± 5.2 461 Sandstone (K1) 65 4111 3082 253.4 ± 3.8 0.396 0 ± 2.5 66.8 175 12.2±0.2 1.9 -0.44 13.8±0.1 1.2 –0.19 2.0(0.4) (m) 1106 Granite (Pt3) 20 1732 1096 ± 6.2 261.8 0.467 53.9 95.8 ± 4.4 161 13.0 ± 0.1 1.2 –0.55 14.4 ± 0.1 0.8 –0.55 1073 Granite (Pt3) 191317 4149 2334 Granite (Pt3) ± 6.2 261.8 22 0.446 1106 538 29.4 261.8 ± 6.2 103.0 ± 3.7 138 0.448 12.9 ± 0.1 98.2 1.4 ± 6.8 110.7 168 –0.39 12.8 ± 0.1 ± 0.1 14.3 1.2 0.9 –0.26 ± 0.1 14.1 –0.35 1.0 –0.88 2.4(0.2) Elevation ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′

′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ and 32°21.12 32°53.64 32°26.84 32°48.49 32°56.18 33°02.61 32°17.94 32°21.65 32°14.87 106°58.5 107°10.11 107°10.69 106°38.62 107°08.38 107°07.45 106°51.88 107°39.59 107°10.74 107°10.62 107°46.46 106°55.39 106°54.19 107°23.10 107°26.08 107°26.56 107°09.54 107°36.18 107°10.29 Latitude (°N) Longitude (°E) Abbreviations: nG—number of counted grains; Ns—number spontaneous tracks; Ni—number induced )—chi-square probability; P-age—pooled age; nL—number of measured conﬁ ned tracks; Mean (L)—mean track length; (L )—chi-square probability; P-age—pooled age; nL—number of measured conﬁ 2 χ Note: ( HN11 33°03.74 HN17 HN19 HN12 HN03 MC08 32°13.10 MC12 MC01 32°41.91 MC25MC03 32°32.75 MC02 32°41.28 MC05 32°19.52 MC04 32°20.86 D7611 33°07.39 MC09 32°15.43 Cretaceous. MC11 D7613 32°57.76 P deviation; Skew—skewness of the track-length distribution; Dpar—mean etch pit diameter; Pt3—Late Proterozoic; T3—Late Triassic; T3—Late deviation; Skew—skewness of the track-length distribution; Dpar—mean etch pit diameter; Pt3—Late Proterozoic; MC14 MC15 Sample no.

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TABLE 2. ZIRCON FISSION-TRACK RESULTS

ζ σ ρ σ χ2 Sample Latitude (°N) Elevation (m) Lithology nG Ns Ni U (ppm) ±1 d ± 1 P ( ) P-age no. (Ma ± 1σ) Longitude (°E) (a cm2)(106 cm–2) (%) HN03 33°02.61′ 570 Granite (Pt3) 21 3795 185 70.9 85.3 ± 3.4 0.297 ± 0.008 90.3 255 ± 22 107°26.56′ HN19 32°48.49′ 739 Granite (Pt3) 28 5446 257 128.5 85.3 ± 3.4 0.278 ± 0.011 99.7 246 ± 19 106°55.39′ HN17 32°53.64′ 550 Gabbro (Pt3) 22 8400 282 73.1 85.3 ± 3.4 0.296 ± 0.007 76 364 ± 26 107°39.59′ MC01 32°41.91′ 1106 Granite (Pt3) 18 12,015 304 67.8 85.3 ± 3.4 0.297 ± 0.007 100 481 ± 34 107°07.45′ MC02 32°41.28′ 1073 Granite (Pt3) 11 3144 145 83.1 85.3 ± 3.4 0.298 ± 0.008 52.7 270 ± 25 107°08.38′ MC25 32°32.75′ 1317 Granite (Pt3) 8 3107 132 102.9 85.3 ± 3.4 0.298 ± 0.008 98.7 316 ± 32 106°51.88′ Note: Abbreviations: nG—number of counted grains; Ns—number of spontaneous tracks; Ni—number of induced tracks; U—concentration of uranium in zircons, ζ ζ ρ χ2 calculated according to Enkelmann et al. (2005); — calibration factor for IRMM-541; d—track density in standard uranium glass; P ( )—chi-square probability; P-age—pooled age.

TABLE 3. APATITE (U-Th)/He RESULTS Sample no. Elevation 4He 238U 235U 232Th 147Sm Length Width Ft Corrected age Mean age St. dev. (m) (mol) (mol) (mol) (mol) (mol) (µm) (µm) (Ma) (Ma) (Ma) MC01_1 1106 2.580E–14 2.492E–13 1.843E–15 6.100E–13 1.695E–13 141 98 0.72 71.5 MC01_2 2.754E–14 3.358E–13 2.484E–15 8.734E–13 2.531E–13 165 110 0.75 53.0 MC01_3 1.761E–14 1.211E–13 8.961E–16 3.491E–13 8.273E–14 132 88 0.68 98.5 MC01_4 2.264E–14 2.539E–13 1.878E–15 6.940E–13 2.437E–13 121 101 0.71 59.7 70.7 20.1 HN17_1 550 6.328E–15 1.744E–13 1.290E–15 4.441E–13 4.154E–13 133 70 0.63 28.2 HN17_2 6.595E–15 9.857E–14 7.290E–16 1.239E–13 2.926E–13 123 75 0.65 61.4 HN17_3 2.957E–15 5.522E–14 4.084E–16 1.184E–13 4.724E–13 105 88 0.67 40.5 43.4 16.8 Note: Ft—alpha-correction factor after Farley et al. (1996).

K1 MC15

K1 MC09 Figure 3. Single-grain apatite ﬁ s-

J3 sion-track ages (gray diamonds MC08 with 1σ error bars) compared with host-strata ages (black bars). Samples MC03, MC14, and MC05 J3 are younger than their host strata, MC11 indicating full reset, consistent with paleotemperature estimates J from vitrinite data (C.B. Shen et 2 maximum paleotemperature ~130°C al., 2007). Ages of samples MC11, MC05 MC08, MC09, and MC15 are close to or overlap the Upper Jurassic to Lower Cretaceous host-strata J1 maximum paleotemperature ~150°C ages, indicating the presence of MC14 partially reset grains and paleo- temperatures of ~110 °C. T MC03 3 maximum paleotemperature ~250°C

single grain age with 1σ error host strata age

0 50 100 150 200 250 300 350 Age (Ma)

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Track Length Distribution 0.50 0.45 Group 1 HN03 0.45 HN11 0.40 0 0.40 0.35 HN03 0.35 0.30 20 0.30 D:14.87± 0.66

c 0.25 D:14.02± 0.73 n 0.25 M:15.0± 0.80

40 u M:14.15± 0.84 q 0.20 GOF:0.37

e 0.20 r GOF:0.66

F N:240 0.15 N:192 0.15 60 ) 0.10

C 0.10

(

e r 80 PAZ 0.05 0.05

a r 0.00 0.00

p 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20

m 100

T 0.45 HN12 0.45 MC01 120 D:104± 5 0.40 0.40

M:104 0.35 0.35 140 GOF:0.93 0.30 D:14.11± 0.83 0.30

y D:14.37± 0.78

n 0.25 M:14.30± 0.85 0.25 e M:14.52± 0.83

160 u q GOF:0.97

160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 e 0.20 0.20 GOF:0.98

r F N:195 N:161 0 0.15 HN11 0.15 0.10 0.10 20 0.05 0.05

0.00 0.00 40 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0.40 0.40 MC02 MC25 60 0.35

)

C 0.35

(

e 0.30 r 0.30 u 80 t PAZ D:14.25± 0.93

r c 0.25 D:14.05± 1.04

e n 0.25 M:14.39± 1.00

u m GOF:0.96 M:14.21± 1.03

q e 100 0.20

e T 0.20 r N:138 GOF:0.99

F 0.15 0.15 N:168 120 D:92.4± 3. 8 M:92.7 0.10 0.10 GOF:0.93 140 0.05 0.05

0.00 0.00 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Length (µm) Length (µm) 0 0 HN12 MC01 20 20

40 40

60 60

)

(

u 80 u 80

t t PAZ

m m 100

e 100 PAZ e

120 D:114± 7 120 D:95.9± 4. 4 M:115 M:95.4 140 GOF:0.94 140 GOF:0.91

160 160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

0 0 MC02 MC25 20 20

40 40

60 60

)

(

r 80 r

u u 80

p PAZ

m 100 PAZ m

e e 100

120 120 D:103± 4 D:119± 7 140 M:103 140 M:120 GOF:0.94 GOF:0.99 160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Time (Ma)

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Group 2

0 0 MC05 MC03 20 20

40 40

60 60

)

(

r e 80 80

r a PAZ PAZ

100 m 100

120 120 D:64.1± 2.1 D:66.8± 2.9 140 M:63.9 140 M:69.0 GOF:0.95 GOF:0.45 160 160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Time (Ma) Track Length Distribution Track Length Distribution MC05 0.35 HN17 MC03 0.35 MC11 0.30 0.30 0.30 0.30 0.25 0.25 D:13.32± 1.21 0.25 D:13.2± 1.13 D:13.60± 1.24 0.25 M:13.39± 1.21 D:13.85± 1.02 0.20 M:13.52± 1.26 y 0.20 M:13.78± 1.19 c 0.20 M:14.06± 1.07

y 0.20 GOF:0.83 n

GOF:0.93 u GOF:0.67 GOF:0.99

e 0.15 N:171 q 0.15

u 0.15 e 0.15

r q N:261 N:126 N:122

F 0.10 0.10 0.10 0.10

0.05 0.05 0.05 0.05

0.00 0.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Length (µm) Length (µm) Length (µm) Length (µm) 0 0 HN17 MC11 20 20

D:70.1± 4.1 40 40 D:60.3± 2.9 M:70.1 M:60.3 GOF:1.00 60 GOF:0.99

) ) 60

(

u 80 u

t t 80

p PAZ p PAZ

e 100 e 100

120 120

140 140

160 160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Time (Ma) Figure 4 (on this and previous page). HeFTy-based 0 (Ketcham, 2005; Ketcham, et al., 2009) thermal history models: temperature-time paths and c-axis 20 projected confi ned fi ssion-track length distribu- overlap tions. Constraints imposed on the models are 40 given as boxes. Monotonic-variable paths were used to allow cooling and heating. Green paths: 60 acceptable fi ts (GOF [goodness of fi t] > 0.05); purple regions: good-fi ts (GOF > 0.5). D—determined PAZ apatite fi ssion-track age (in Ma with 1σ error) and 80 mean confi ned track length (in μm with standard

deviation). M—modeled apatite fi ssion-track age Temperature100 (°C) and mean confi ned track length. N—number of measured confi ned track lengths. Black circles 120 with 1σ error bars are apatite (U-Th)/He ages. Last onset of rapid cooling 13–8 Ma diagram (bottom center) overlaps all good-fi t tem- 140 perature-time path envelops of group 2 samples

(including those re-modeled from Enkelmann et 160 al., 2006; Fig. B1); rapid cooling started at 13–8 Ma. 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 PAZ—partial annealing zone. Time (Ma)

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and the modeling parameters). Two groups INTEGRATION: PUBLISHED DATA AND (Fig. 5A): Older ages are associated with higher are apparent: Group 1 samples (HN03, HN12, AGE-PARAMETER EVALUATION elevations. The calculated best-fi t linear trend to HN11, MC01, MC02, MC25) entered the APAZ the AFT age-elevation data in the Micang mas- between ca. 150 and 110 Ma, cooled through Here, we integrate our data with previously sif exhibits a break-in-slope at ~900 m, with the PAZ before ca. 90 Ma, and stayed for a published data and derive several independent exhumation rates declining from ~0.04 mm/yr prolonged time at low temperatures (≤60 °C). parameters pertinent to the cooling and exhu- prior to ca. 95 Ma to <0.02 mm/yr thereafter Group 2, with a broader track-length distribu- mation history as a foundation for a regional (Fig. 5A); both trends are poorly defi ned, and tion, includes three samples from the northern interpretation of the Qinling and Tibetan Pla- we will discuss the signifi cance of the break-in- Sichuan Basin strata (MC03, MC05, MC11) and teau forelands. Our and Tian et al.’s (2012) slope later herein. We interpret the higher-ele- one sample (HN17) from the Hannan-Micang AFT ages correlate with sample elevation in the vation trend as recording an exhumation event crystalline massifs. Group 2 samples entered Micang massif and the northern Sichuan Basin that brought the rocks from >110 °C through a the APAZ between ca. 110 and 80 Ma, resided for a prolonged time within the upper part of the APAZ (90–60 °C), and experienced accelerated cooling since 20–5 Ma. We re-modeled four of A Micang massif and Northern Sichuan Basin our previously published AFT data (Enkelmann apatite (U-Th)/He AHe AFT et al., 2006), located north of the Hanzhong 1800 Chang et al., 2010 Tian et al., 2012 basin (Q55, Q65, Q67, Q69; Fig. 2A; Appen- This study dix B, Fig. B1). Enkelmann et al. (2006) mod- 1600 apatite fission-track eled these samples with Laslett et al.’s (1987) Tian et al., 2012 annealing equation and without c-axis projec- 1400 This study MC25 tion. These samples belong to our group 2 data. Linear fit They entered the APAZ before 100 Ma, stayed 1200 Confidence 95% until 15–5 Ma in its upper part, and cooled to MC01 0.04mm/yr MC01 MC02 2 surface temperatures thereafter (Fig. B1). In the 1000 0.02mm/yr R =0.36 MC12 last diagram of Figure 4 (bottom center), we R2=0.88 overlapped all good-fi t (GOF > 0.5) T-t path 800 ~ 95Ma Elevation (m) Elevation envelops of group 2 samples (including those re-modeled from Enkelmann et al., 2006; Fig. 600 B1); these envelopes likely confi ne the extreme MC04 MC08 cooling-history range of each sample. Assum- 400 MC03 MC15 MC05 MC09 <0.02mm/yr ing that all group 2 models stem from rocks that MC14 R2=0.27 MC11 experienced the same cooling history, but that 200 the distinct cooling models were infl uenced by 20 40 60 80 100 120 140 various parameters (e.g., natural ones, such as Age(Ma) variable rock type, or laboratory induced ones, B Hannan massif such as counting effi ciency), causing their vari- 1800 apatite (U-Th)/He ability, the overlap defi ned by these envelopes Chang et al., 2010 This study AHe yields an “average” of all calculated T-t histories 1600 derived in this study. Taking conservative brack- apatite fission-track Xu et al., 2010 0.38mm/yr ets on this overlap (the “average”), we suggest 1400 This study 2 that rapid cooling started at 13–8 Ma. R =0.04

) Linear fit In total, seven apatite grains from two sam- 1200 Confidence 95% ples (MC01 and HN17) were used for (U-Th)/

He (AHe) dating (Table 3). The mean AHe ages (m ion 1000 are younger than the AFT ages of the same sam- vat HN11 ples and consistent with the AFT-derived T-t le 800 E HN19 paths (Fig. 4). However, the single-grain ages HN12 vary considerably; we speculate that among the 600 HN17 HN17 D7611 many reasons that may contribute to such an HN03 age variation, e.g., undetected U- and Th-rich 400 D7613 inclusions, disparate crystal size, zonation, slow cooling (e.g., Fitzgerald et al., 2006; Shuster et 200 al., 2006), the latter, i.e., the long residence of 20 40 60 80 100 120 140 our samples in the apatite He partial retention Age(Ma) zone (APRZ), is the main reason for the varia- Figure 5. (A) Apatite fi ssion-track and (U-Th)/He ages (1σ error bars) plotted against elevation for tion in the singe-crystal ages. This slow cooling the Micang massif and northern Sichuan Basin samples. The age-elevation plot is divided into an through the APRZ is also indicated by AHe and upper steep part and a lower fl at part; the break-in-slope is at ca. 95 Ma. The apatite (U-Th)/He AFT age-elevation trends (see following) and age-elevation plot defi nes an exhumation rate of ~0.02 m/yr since ca. 92 Ma. (B) As in A but for the the T-t path models (Fig. 4). Hannan massif samples. AFT—apatite fi ssion track.

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pre-Cretaceous APAZ; the lower elevation age- Ma), by far the youngest high-elevation sample μm), record more rapid exhumation. The lower- elevation trend may trace the slow passing of the from the Micang massif, are all located along elevation samples, with shorter tracks and larger samples through this APAZ, with ages repre- the Guanba fault (Fig. 2A); these three samples variation in mean track length (11.5 ± 0.1–12.5 senting a mixture between the higher-elevation will be discussed separately in the following. ± 0.2 μm), probably represent a pre-Cretaceous event and a younger one. In the Hannan massif, Plots of our and Tian et al.’s (2012) AFT upper APAZ. The Hannan massif track length– our and Xu et al.’s (2010) AFT ages range from mean track lengths against elevation reveal a elevation trend is as inconclusive as its age-ele- 70.1 ± 4.1 Ma to 133.1 ± 6.0 Ma, with most ages similar pattern as the age-elevation data for vation relationship (compare Figs. 5B and 6B): older than 90 Ma. These ages may also follow the Micang massif and northern Sichuan Basin Mean track lengths range from 11.5 ± 0.1 μm a weak age-elevation trend (Fig. 5B). However, (compare Figs. 5A and 6A): Higher-elevation to 13.8 ± 0.1 μm. However, the Hannan mas- the younger dates, HN17 (70.1 ± 4.1 Ma) and samples, with longer tracks and less mean sif track lengths are all longer than 14 μm after D7613 (77 ± 6 Ma), and also MC12 (68.9 ± 3.5 track-length variation (12.7 ± 0.2–13.2 ± 0.1 c-axis projection (Table 1), except HN17, which

A Micang massif and Northern Sichuan Basin B Hannan massif 1800 1800 Tian et al. (2012) 1600 1600 rapid exhumation this study 1400 MC25 1400

) 1200 ) M01C 1200

m m M02C

(

n 1000 n 1000

a HN11

v HN19 800 v 800

l MC11 MC03

E 600 E 600 HN17 HN12 HN03 M5C0 400 MC15 400 pPAZre- Cretaceous A 200 slow exhumation 200

0 0 10 11 12 13 14 15 10 11 12 13 14 15 Mean track length ( m) Mean track length ( m)

C Hannan and Micang massifs and D Northern Sichuan Basin 15 23. mixed

) 21.

group 1 n

m o 14 i

t undisturbed

(

19.

a i h basement

t v

g group 2 e

n 17.

13 e

l d

1.5 a

c d

n group2

t a 12

13. t

n S

a e 1.1

M 11 mainly from Micang massif and 09. northern margin of Sichuan Basin group 1 10 07. 40 50 60 70 80 90 100 110 120 130 140 10.0 11.0 12.0 13.0 14.0 15.0 Fission-track age (Ma) Mean track length ( m) Figure 6. (A) Mean confi ned track lengths (1σ error bars) vs. sample elevation for samples from the Micang massif and northern Sichuan Basin. Higher- elevation samples with longer tracks and less variation in mean track lengths indicate accelerated exhumation that brought the samples through an apatite partial annealing zone (APAZ); lower-elevation samples probably represent a pre-Cretaceous upper partial annealing zone. (B) Same plot as in A but for Hannan massif. (C) Plot of apatite fi ssion-track age (1σ error bars) against mean track length (1σ error bars), outlining an incomplete boomerang pattern (dashed line): Mean track lengths decrease with decreasing age. Samples with ages older than 90 Ma and mean track length of >12.5 μm indicate enhanced cooling; samples with ages younger than 90 Ma (mostly younger than 80 Ma) and mean track length <12.2 μm, distinct from the former samples, suggest longer residence in a partial annealing zone. (D) Mean track lengths related to their standard deviations; fi elds of “undisturbed basement” and “mixed” are defi ned by Gleadow et al. (1986).

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has the shortest value of 13.1 ± 0.1 μm. Apatite Micang massif and northern Sichuan Basin cline and the northern Sichuan Basin suggests from HN11, with the longest mean track length here, covering a comparable period (123.5 ± that the onset of accelerated exhumation did not of 13.8 ± 0.1 μm (14.9 ± 0.1 μm after c-axis 6–93.9 ± 4.1 Ma); enhanced exhumation would, start earlier than ca. 165 Ma, and we speculate projection) and the largest Dpar (2.9 ± 0.3 μm), however, be consistent with coeval enhanced that deposition terminated in the Middle Juras- appears more resistant to annealing. cooling, as derived from the Hannan massif’s sic in the Huijunba syncline due to migration of Plotting the AFT ages against mean track AFT T-t paths (Figs. 4 and 6B). Two of our AFT foreland deformation from the Hannan-Micang lengths for all Hannan-Micang crystalline ages (HN11, HN19) are younger than Chang crystalline massifs to the northern Sichuan massif samples reveals two groups (Fig. 6C): et al.’s (2010) AHe ages at the same elevation, Basin, where the deposition of thick deposits Group 1 apatites are older (123.5 ± 6.0–90.3 ± further casting doubt on the signifi cance of the and the onset of coarse clastic input suggest 6.8 Ma), have longer mean track lengths (>12.5 AHe age-elevation relationship. The 43.4 ± 16.8 onset of deformation during the late Middle– ± 0.1 μm; except sample HN19, with only 34 Ma AHe age of sample HN17 is much younger Late Jurassic (Fig. 7). confi ned tracks, and one sample from Tian et than other Hannan massif AHe ages; this sample Isopach maps of Mesozoic strata within al. [2012], which has a mean track length of also has the youngest AFT age (Fig. 5B; Table the Sichuan Basin (Guo et al., 1996; Li et al., 11.4 ± 0.3 μm), and have a narrower track- 1), and it is located along the Guanba fault (Fig. 2003; S.G. Liu et al., 2006) indicate migra- length distribution than group 2 (see follow- 2A; see discussion). tion of depocenters throughout the Late Trias- ing; Fig. 6D). All group 1 apatites fall into the sic and Jurassic; this has been interpreted as “undisturbed basement” fi eld (Fig. 6D), which STRATIGRAPHIC CORRELATION AND refl ecting foreland propagation of deforma- likely represents monotonic cooling from tem- EVOLUTION OF THE NORTHERN tion in the Qinling orogeny. The Late Triassic peratures where tracks fade to ambient tem- SICHUAN BASIN (Norian–Rhaetian) depocenter was in front of peratures (Gleadow et al., 1986); thus, group the Longmen Shan, with strata thickness ≥3 km 1 apatites may stem from above a fossil APAZ. The thermochronologic data analysis sug- (Li et al., 2003) and an average sediment accu- Group 2 apatites are younger (60.3 ± 2.3–82.9 gests more rapid exhumation and cooling dur- mulation rate of ~0.19 km/m.y. Lower Jurassic ± 7.8 Ma; mostly younger than 70 Ma), have ing the Early Cretaceous than during the Late sediment thickness is <400 m, which corre- shorter mean track lengths (11.5 ± 0.1–12.5 ± Cretaceous–Tertiary. The onset of this acceler- sponds to an abrupt decrease of the sediment 0.2 μm; except for one sample from Tian et al. ated exhumation/cooling period, however, can- accumulation rate to ~0.02 km/m.y.; sediment [2012] with a mean of 12.7 ± 0.2 μm), and have not be quantifi ed by AFT and AHe data due to thickness varied little throughout the Sichuan a wider track-length distribution than group 1 the inability of these thermochronometers to Basin, which suggests low erosion rates in the (Fig. 6C). Group 2 apatites occupy the “mixed” provide age information above temperatures adjacent drainages. In the late Middle Jurassic, fi eld and the transition zone between “mixed” of ~110–80 °C. Unfortunately, our ZFT ages a depocenter initiated in the northern Sichuan and “undisturbed basement” (Gleadow et al., (ca. 481–246 Ma) and Xu et al.’s (2010) zir- Basin, where the <2-km-thick (see Appendix 1986; Fig. 6D). Thus, group 2 samples, mainly con (U-Th)/He ages (ca. 184–153 Ma) vary C, section C–C′) Shaximiao Formation was from the Micang massif and the northern Sich- strongly and are possibly part of fossil partial deposited; the average accumulation rate was uan Basin, defi ne a pre-Cretaceous APAZ annealing/retention zones. Here, we approxi- ~0.2 km/m.y. Sediment accumulation rates based on both their ages and track lengths. In mate the age of onset of deformation around >0.1 km/m.y. persisted throughout the Late Figure 6C, longer lengths correlate with nar- the Hannan-Micang crystalline massifs, which Jurassic (<1830-m-thick strata, section C–C′) rower distributions for older ages, mean track we relate to the onset of enhanced exhumation/ and the Early Cretaceous. From the Late Cre- lengths vary less in samples older than 90 Ma, cooling, from a correlation of newly measured taceous onward, most of the northern Sichuan and mean track lengths decrease with decreas- Jurassic stratigraphic strata in the Huijunba Basin was an area of erosion. ing ages in samples younger than 90 Ma; this syncline (northern section A–A′; for previous pattern defi nes a “half” boomerang, and thus data, see S.B.G.M.R., 1989; Guo et al., 1996) DISCUSSION only the upper part of a fossil APAZ is cur- with data from the northern Sichuan Basin rently exposed in the Hannan-Micang crystal- (southern sections B–B′ and C–C′; Fig. 2A; see Published thermochronology (Z.Q. Zhang line massifs. also Meng et al., 2005; Burchfi el et al., 1995). et al., 2001; Zhou et al., 2002; Ling et al., Plotting our (sample MC01), Chang et al.’s We assume that disparities in the strata refl ect 2006; Zhao et al., 2006; Chang et al., 2010; (2010), and Tian et al.’s (2012) Micang massif north to south propagation of Qinling foreland Xu et al., 2010; Dong et al., 2011b; Tian et and northern Sichuan Basin AHe ages against deformation. al., 2012) and our new geothermochronology sample elevation defi nes a linear trend that sug- Figure 7 and Appendix C detail the evolu- defi ne the Neoproterozoic to Holocene ther- gests slow exhumation at ~0.02 mm/yr between tion of the Jurassic–Cretaceous strata; the Hui- mal evolution of the Hannan-Micang crystal- 92.2 ± 1.8 Ma and 31.9 ± 9.9 Ma; this rate is junba syncline and the northern Sichuan Basin line massif rocks (Fig. 8). This history consists similar to, albeit slightly higher than, the weakly sections are identical, at least up to the Middle of fi ve stages: (1) Neoproterozoic postmag- defi ned one calculated from the younger than 95 Jurassic (165 ± 5 Ma; ICS, 2010) basal section matic cooling; (2) thermal steady state within Ma part of the AFT age-elevation data (Fig. 5A). of the Shaximiao Formation. No younger Juras- a Paleozoic Yangtze block passive continental Chang et al.’s (2010) Hannan massif AHe age- sic strata are preserved in Huijunba syncline margin; (3) Middle Jurassic–Early Cretaceous elevation data suggest a poorly defi ned exhuma- north of the Micang massif. The absence of cooling due to foreland propagation of the Qin- tion rate of ~0.38 mm/yr (correlation coeffi cient strata younger than Middle Jurassic in the Hui- ling orogeny; (4) thermal steady state with Late R2 = 0.04); sample elevations vary from 1508 m junba syncline may be due to later (for example, Cretaceous formation and Tertiary persistence to 613 m within a narrow age spread (123.5 ± Cenozoic) erosion of potential Upper Jurassic– of a regional low-relief landscape; and (5) late 2.7–107.4 ± 3. 8 Ma; Fig. 5B). This rate would Cretaceous deposits, but it may also be due to a Cenozoic cooling due to eastward growth of be ~10× higher than the exhumation rate calcu- switch from deposition to erosion at ca. 165 Ma. the Tibetan Plateau. We discuss stages 2 to 5 lated from the AFT age-elevation trend of the The identical stratigraphy in the Huijunba syn- in the following.

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K limestone m 500 marlite 400 coal 300 mudstone 200 100 siltstone 0 sandstone

conglomerate Jp3 Penglaizhen Fm. Ca concretions plant fossil

Jp3

Jsn3 Jsn3 Suining Fm.

2 Js2

2 Js2 Shaximiao Fm.

1 Js2 1 Js2

Jq2 Jb Qianfoyan Fm.

Jq2

Jb1 Jb1 Ja Baitianba Fm.

Tx3 T2 BB-' CC-' AA-' sections located in Figure 2A. conformity erosional unconformity

Figure 7. Newly measured Jurassic lithostratigraphic sections (A–A′, B–B′, and C–C′) and their correlation from both sides of the Hannan-Micang massif. See Figure 2A for locations and Appendix C for details. The Jurassic sequence is identical up to the Early–Middle Jurassic; thereafter, Late Jurassic–Early Cretaceous deposition occurred only south of Micang massif.

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900 cooling rates of 1.2–1.6 °C/m.y. T-t paths show I Postemplacement cooling of granitoids that group 1 samples pass through the APAZ II Passive margin evolution in 20–30 m.y. before 120–90 Ma; the modeled 800 U-Pb Zircon III Qinling orogen intracontinental deformation cooling rates are 1.5–2.5 °C/m.y., and the exhumation rates are between 0.04 and 0.10 mm/yr, IV Persistence of a low-relief landscape 700 i.e., close to the results from the age-elevation V Late Cenozoic eastward growth of Tibet Plateau data. The inconclusive Hannan massif AFT age- elevation relationship (Fig. 5B) is probably due 600 ) to the relative low relief of the Hannan massif

)

(

C III 120 (Fig. 1), prohibiting sampling outside the APAZ.

° AFT and AHe constrained e

r I

(

thermal model Though both T-t paths and AFT age-eleva- 500 t

IV r

r 80

e tion data outline accelerated cooling and exhu-

t m mation during the Early Cretaceous, they do not

r 400 V T 40 e Rb-Sr Biotite reveal its onset. Our ZFT ages suggest that the

p Age (Ma) 0 Hannan-Micang crystalline massifs stayed in a

e 020406080100120 thermally stable passive continental margin set-

T 300 ting until the Middle Triassic (Fig. 8). After the ZFT II Ar-Ar Biotite ZHe Late Triassic hiatus in the Huijunba syncline 200 and deposition of coarse clastics in the northern probable onset of accelerated exhumation Sichuan Basin, which likely record the North AFT III (ca.165Ma ) China–South China collision (see previous), 100 AHe the stratigraphic sequence is identical up to the Early–Middle Jurassic on both sides of the IV 0 V Micang massif, i.e., in the Huijunba syncline 0 100 200 300 400 500 600 700 800 900 and the northern Sichuan Basin (Fig. 7); there- Age() Ma after, Late Jurassic–Early Cretaceous deposition Figure 8. Cooling history (gray envelope) of the Hannan-Micang massif rocks. The black dashed line likely occurred only south of the Micang massif. is the cooling path deduced from thermal history modeling of apatite fi ssion-track and (U-Th)/He The regional variation in sediment thickness and data: It is epitomized by the cooling curve of sample HN17 (see Fig. 4) in the inset. Postemplace- the depocenter migration in the Sichuan Basin ment quenching of Neoproterozoic granitoids (stage I) was followed by Paleozoic through Trias- indicate that the Hannan-Micang crystalline sic thermal steady state (stage II) during passive-margin evolution on the northern margin of the massifs were a source of detritus to the Sichuan South China craton. Enhanced cooling started at ca. 165 Ma, representing the onset of the intracontinental deformation stage III. Apatite fi ssion-track and (U-Th)/He data indicate slow cooling Basin and that orogenic propagation induced throughout Late Cretaceous–Tertiary (<95 Ma; stage IV); a low-relief landscape was established at basin subsidence in the northern Sichuan Basin the onset of this stage. Accelerated exhumation and cooling from 13 to 8 Ma onward (stage V) are beginning in the late Middle Jurassic. Thus, constrained by apatite fi ssion-track temperature-time models. Data sources: U/Pb zircon—Dong the sedimentologic evidence suggests that the et al. (2011b, and references therein); Rb/Sr and 40Ar/39Ar biotite—Z.Q. Zhang et al. (2001); zircon Qinling foreland deformation migrated into the fi ssion-track (ZFT)—this study; zircon (U-Th)/He (mean age)—Xu et al. (2010); apatite fi ssion-track Hannan-Micang crystalline massif area and the (AFT) and (U-Th)/He (AHe)—Chang et al. (2010), Tian et al. (2012), and this study. Closure temperatures are from Reiners and Brandon (2006) and Bernet (2009). northern Sichuan Basin during the late Middle Jurassic (−165 Ma); in the light of these inferences, Xu et al.’s (2010) Hannan massif 184– Paleozoic–Middle Triassic Passive from a passive margin into a southern foreland 153 Ma zircon (U-Th)/He dates can be inter- Continental Margin basin to the Qinling orogeny; clastic sediments preted to provide the thermochronologic record prevailed from the Late Triassic onward. In the for the incorporation of the Hannan-Micang Our ZFT ages (481 ± 34–246 ± 19 Ma) Huijunba syncline, foreland uplift is recorded crystalline massifs into the foreland growth of indicate that the Hannan-Micang crystalline by the Late Triassic hiatus. the Qinling orogen. massif rocks remained in the ZPAZ (240 ± 30 °C) for a prolonged period. Consistently, bio- Late Middle Jurassic (ca. 165 Ma) Early Late Cretaceous (ca. 95 Ma) to Late tite 40Ar/39Ar (closure temperature ~300 °C) to Early Late Cretaceous (ca. 95 Ma) Cenozoic (ca. 10 Ma) Thermal Stagnation and zircon (U-Th)/He ages (closure tempera- Intracontinental Foreland Growth ture ~180 °C) are at 796 ± 20 Ma (Z.Q. Zhang The AFT age-elevation data (Fig. 5A) indi- et al., 2001) and 184.3 ± 13.4–152.6 ± 2.9 Ma Several parameters, such as AFT age, AFT cate initiation of a prolonged phase of slow (Xu et al., 2010), respectively, and sedimentary track length–elevation relationships (Figs. 5A exhumation at ca. 95 Ma (<0.02 mm/yr; cool- strata in the Hannan-Micang crystalline mas- and 6A), and T-t paths (Fig. 4), indicate Early ing rates of <0.5–0.7 °C/m.y.). Consistently, sifs and northern Sichuan Basin record a Paleo- Cretaceous accelerated cooling and exhuma- the AHe age-elevation data (Fig. 5A) document zoic–Middle Triassic passive-margin evolution tion of the Hannan-Micang crystalline massif slow exhumation of ~0.02 mm/yr (cooling rate with Devonian and Carboniferous hiatuses rocks. The Micang massif and northern Sich- of 0.5–0.7 °C/m.y.) from at least 92.2 ± 1.8 Ma (Guo et al., 1996). The Triassic North China– uan Basin AFT age–elevation data constrain to 31.9 ± 9.9 Ma. Furthermore, group 1 T-t South China collision (Meng and Zhang, 1999; this event between ca. 125 and ca. 95 Ma, with paths (Fig. 4) indicate passage of most Hannan- Ratschbacher et al., 2003, 2006) transferred an exhumation rate of ~0.04 mm/yr; assuming Micang crystalline massif rocks through the the Hannan-Micang crystalline massif area a geothermal gradient of 30 ± 5 °C/km yields APAZ before ca. 90 Ma, subsequent to which,

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they stayed at temperatures <60 °C; the aver- Deposition withdrew into its southwestern cor- striking faults (Fig. 2A) and probably formed age cooling rate was ≤0.44 °C/m.y. Group 2 T-t ner in the Late Cretaceous–Eocene. Also other at releasing sites within an overall dextrally paths (Fig. 6) suggest prolonged residence of basins in central China show hiatuses in their transpressive late Cenozoic stress fi eld (Burch- other Hannan-Micang crystalline massif rocks sedimentation history; for example, the Ordos fi el et al., 1995; Enkelmann et al., 2006; Fan et within the upper part of the APAZ after ca. 90 Basin of northern Central China lacks sedimen- al., 2008; see previous sections). AFT samples Ma, followed by enhanced cooling since 13–8 tation in the Late Cretaceous–middle Miocene MC12, HN17, and D7613 were sampled along Ma. The cooling rate between ca. 90 and ca. 10 (S.B.G.M.R., 1989; C.Y. Liu et al., 2006). the Guanba fault (Fig. 2A). Sample MC12, Ma was very slow, ~0.38 °C/m.y. We infer that at an elevation of 978 m, has a 68.9 ± 3.5 Ma the waning phase of the enhanced cooling and Late Cenozoic (≤13–8 Ma) Eastward age, which is inconsistent with the Micang– exhumation stage (ca. 95 Ma; see previous) was Growth of the Tibetan Plateau northern Sichuan Basin age-elevation relation- synchronous with topographic decay and the ship (Fig. 5A). The T-t paths of sample HN17 establishment of a regional low-relief landscape Group 2 AFT T-t paths, based on the young- indicate accelerated cooling from 70 to 20 °C (or peneplain, which we regard as a featureless est ages, shortest confi ned track lengths, high- in the past ~10 m.y. (Fig. 4, group 2 samples). land surface of considerable area; e.g., Davis, est length deviations, and lowest elevation These samples may trace thermal advection in 1899; Hetzel et al., 2011). Previous thermochro- samples, suggest accelerated cooling from the the near-fi eld of the active Guanba fault. This nologic and geomorphologic research supports upper APAZ (70–60 °C) to surface temperatures possible thermal effect and the association of regional slow cooling and exhumation through- since 13–8 Ma (Fig. 4), with cooling rates of the Qingchuan and Guanba faults with the late out the Late Cretaceous–Tertiary and the pos- ≤5 °C/m.y.; assuming a geothermal gradient of Cenozoic Hanzhong and Xixiang basins also sible existence of a low-relief landscape in east- 30 ± 5 °C/km and a surface temperature at 20 suggest that the Hannan-Micang crystalline ern Tibet and central China. For example, based °C yields exhumation rates of 0.14–0.20 mm/ massifs were incorporated into the deformation on AFT T-t path models, Arne et al. (1997) yr, corresponding to 1.4–2.0 km of overburden along the eastern Tibetan Plateau margin in the recorded slow cooling from the Cretaceous to removal. Our modeling results correspond to late Miocene. ca. 20 Ma for the Longmen Shan. Enkelmann those of Tian et al. (2012) and the re-modeling Late Cenozoic enhanced cooling and exhu- et al. (2006) documented continuous slow cool- of Enkelmann et al.’s (2006) AFT data north mation have also been documented west and ing (~1.2 °C/m.y.) from 100–70 Ma to 9–4 Ma of the Hannan-Micang crystalline massifs. north of the Hannan-Micang crystalline massifs across the western Qinling. Kirby et al. (2002) Enhanced exhumation since 13–8 Ma is con- (see compilation in Fig. 9); consequently, these reported slow cooling (~1 °C/m.y.) along the sistent with sedimentation in the Hanzhong and areas have been interpreted as being incorpo- eastern margin of the Tibetan Plateau from the Xixiang basins (Fig. 2A; S.B.G.M.R., 1989): rated into the growth of the Tibetan Plateau. Jurassic until the late Miocene or early Pliocene, Pliocene mammal fossils in the middle to upper For example, Zheng et al. (2006) proposed ca. using 40Ar/39Ar K-feldspar and (U-Th)/He zir- section of the basin (Tang and Zong, 1987) sug- 8 Ma initiation of shortening along the Liupan con and apatite thermochronology. Sedimenta- gest an earlier, probably Upper Miocene basal Shan, forming the western margin to the Weihe tion in the Sichuan Basin also refl ects this stage: sequence. These basins are located along NE- graben (Figs. 1 and 9). To the north of the Han-

E102° E103° Li Haiyuan fault E104° E105° upan Shan E107° steep Tibetan Plateau E106° margin decreasing Ganshu fault E108° north of the Min Shan Weihe graben Wudu–Chengxian Basin N34° Taibai fault Kunlun fault zone Figure 9. Areas of late Cenozoic accelerated cooling and exhuma- Paleozoic-Triassic tion in the eastern Tibetan Pla- Hanzhong suture corridor Basin teau, the western Qinling, and Paleozoic-Triassic N33° the northern Sichuan Basin, as suture corridor outlined by previously published Min Shan area west of this boundary not Qingchuan fault Xixiang (Arne et al., 1997; Kirby et al., 2002; Xue Shan considered in this study Basin Clark et al., 2005; Enkelmann et al., Guanba fault fault 2006; Zheng et al., 2006; Godard Hannan- Micang et al., 2009; Ouimet et al., 2010; N32° Mujiaba massif E106° E107° E108° Li et al., 2012; J.H. Liu et al., 2012; E105° Tian et al., 2012) and our new low- area east of this boundary not temperature thermochronology. considered in this study Simpliﬁ ed tectonic boundary conditions are modiﬁ ed from Enkel- mann et al. (2006). N31° areas of Late Cenozoic accelerated cooling/exhumation Xianshuihe fault E104° Longmen Shan Sichuan Basin constrained by apatitefission- track and/ or (U–Th)/He ages

100 km constrained by apatite fission- track thermal history models

N30°

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nan-Micang crystalline massifs, Enkelmann et and ≤2 km of denudation, mark the eastward respectively. For each sample, 100.000 paths were calculated; we obtained at least several hundred (except sample HN11, al. (2006) documented accelerated exhumation growth of the Tibetan Plateau into the Hannan- which had <100) to up to 1800 “good” T-t paths. We did not since 9–4 Ma in the West Qinling. Two AFT Micang crystalline massifs and northern Sich- include the apatite (U-Th)/He ages as parameters in the mod- age-elevation profi les across the Qinling range uan Basin. Future work must identify whether els; instead, we plotted the (U-Th)/He ages in the T-t paths to test the validity of our models. adjacent to the Weihe graben documented ca. areas east of the Sichuan Basin, e.g., the Daba 10 Ma onset of accelerated exhumation (Fig. 1; Shan, are currently being involved in the over- Apatite (U-Th)/He Dating J.H. Liu et al., 2012). To the south and west of stepping of the Sichuan Basin by the growth of Clear and unbroken apatite grains without inclusions the Hannan-Micang crystalline massifs, Kirby the Tibetan Plateau. were selected using a binocular microscope. The grain et al. (2002) reported onset of rapid cooling at dimensions were measured for the calculation of the alpha- correction factor (Farley et al., 1996). After that, we packed ca. 12–5 Ma in the Longmen Shan and at 5–3 the single apatite grains in Nb-tubes for (U-Th)/He analysis. APPENDIX A Ma in the Min Shan (Fig. 1). The central Long- We analyzed 3–5 aliquots per sample. The samples were ana- men Shan shows ~0.65 mm/yr exhumation lyzed in the Patterson helium-extraction line at the University Zircon Fission-Track Dating of Tübingen, which is equipped with a diode laser to extract since ca. 11–8 Ma (Godard et al., 2009, 2010). Zircons were mounted in Tefl on, ground, and polished. the helium gas. Apatite grains were heated for 5 min at 11 Finally, AFT thermochronology traced regional The fossil tracks in the zircons were etched in a eutectic mix- Amps. Each grain was heated again and analyzed to make exhumation since ca. 10 Ma in both the north- ture of KOH and NaOH at 228 °C for 9–30 h. Zircon samples sure that the grain was degassed entirely in the fi rst step. were covered with 50-μm-thick, uranium-free muscovite The residues generally were <1% of the fi rst signal. After He ern to southern Longmen Shan (Li et al. 2012). external detectors and packed between three mounts of analysis, the grain packages were sent to the University of In this framework, we suggest that at 13–8 Ma, uranium glass (IRMM-541), inserted at the top, middle, and Arizona at Tucson for inductively coupled plasma–mass spec- bottom of the irradiation cans. The cans were irradiated in the trometry, U, Th, and Sm analysis. The analytical errors of the the Tibetan Plateau started to grow across the hydraulic channel of the FRM-II reactor, Munich, Germany. mass-spectrometer measurements were generally very low Sichuan Basin, progressively incorporating the The muscovite external detectors were etched in 40% HF for and did not exceed 2%; the reproducibility of the sample age northern Sichuan Basin and the Hannan-Micang 30 min at room temperature. All ages were determined using reveals a much higher external error. We therefore report the the zeta approach (Hurford and Green, 1982, 1983), employ- mean (U-Th)/He age and the standard deviation of the mea- crystalline massifs. ing the IRMM-541 monitor; the zircon zeta values were cali- sured aliquots as the sample error. brated by counting Fish Canyon tuff zircon age standards, which were irradiated together with samples (Table 2). We CONCLUSIONS APPENDIX B performed track counting on prismatic zircons with a Zeiss Axioplan microscope at ×1500 magnifi cation in transmitted The available data on cooling and exhuma- light; the corresponding muscovite external detectors were Re-Modeling Results of the West Qinling Apatite tion in the foreland of China’s two most promi- counted using an Autoscan (Autoscan Systems Pty. Ltd., Fission-Track Data of Enkelmann et al. (2006) Brighton, Australia) system. Re-modeling results for four of our previously published nent orogens, the Qinling and the Tibetan Pla- AFT data (Enkelmann et al., 2006) from north of the Hanzhong basin are presented in Figure B1. These samples were initially teau, constrain the mechanisms by which and Apatite Fission-Track Dating and Thermal History modeled with Laslett et al.’s (1987) fanning linear annealing the dates at which these orogens accommodated (T-t Path) Modeling equation and without c-axis correction. Here, we used Ket- We divided the apatites into two separates, one used their fi nal growth. The principal results from cham et al.’s (1999) annealing model, an l value of 15.9 μm, for age dating (age group) and the other used for confi ned 0 and c-axis projection (Donelick et al., 1999; Ketcham et al., the Hannan–Micang massifs and the northern track-length measurements (length group). Apatites were 2007; see Appendix A). The new parameters result in well- Sichuan Basin, where this late-stage orogenic mounted in epoxy, ground, and polished. The age-group defi ned cooling paths over the past 20 m.y., and thus the apatites were etched for 15 s in 23% HNO at 25 °C, and the growth can be studied in a combined approach, 3 re-modeling secured Enkelmann et al.’s (2006) inference of muscovite external detectors were etched in 40% HF for 30 a phase of late, rapid cooling (our group 2 samples) after a are as follows: min at room temperature. Ages were determined using the prolonged period of track accumulation at temperatures of (1) No or slow Paleozoic to Middle Triassic zeta approach, employing the IRMM-540R uranium glass; the accelerated track annealing. However, it did not yield a more apatite zeta values stem from independent calibrations of cooling in South China’s northern passive mar- precise estimate of the onset of this rapid terminal cooling, three persons by counting several Durango and Fish Canyon previously derived from a linear relationship between the gin was terminated in the Middle–Late Triassic tuff apatite age standards (Table 1). We performed the track fi ssion-track age and the number of tracks formed above and counting on prismatic apatite surfaces with a Zeiss Axioplan by the Qinling orogeny. below 60 °C; Enkelmann et al. (2006) suggested ca. 9–4 Ma, microscope at ×625 magnifi cation in transmitted light. The (2) The leading foreland edge, represented and the re-modeling yielded 15–5 Ma for the onset. muscovite external detectors were repositioned, trackside by the Hannan-Micang crystalline massifs and down, on the apatite mounts in the same position as dur- northern Sichuan Basin, was incorporated into ing irradiation; fossil tracks were counted by focusing on APPENDIX C the underside of the external detector without moving the the Qinling orogeny in the late Middle Jurassic microscope stage (Jonckheere et al., 2003). Where possible, (ca. 165 Ma); tectonic activity persisted up to we counted at least 20 crystals of each sample. Jurassic Strata in the Northern Sichuan Basin and the Early Cretaceous (ca. 95 Ma). This period The length-group apatites were etched for 20 s at 21 °C the Huijunba Syncline in 5.5 N HNO (Donelick et al., 1999). The track-length mea- shows cooling at 1.2–2.5 °C/m.y. and exhuma- 3 surements used ×1250 magnifi cation on a Zeiss microscope Baitianba Formation, Early Jurassic (J b) equipped with the Autoscan system. All suitable confi ned 1 tion at 0.04–0.10 mm/yr. In profi les B–B′ and C–C′, the Baitianba Formation con- tracks parallel to the prismatic surfaces were measured. (3) Negligible cooling and exhumation formably covers Upper Triassic rocks. It consists of basal con- Twelve samples were irradiated with heavy ions at GSI glomerate and fi ne-grained sandstone and mudstone inter- throughout the Late Cretaceous–Tertiary (Helmholtzzentrum fuer Schwerionenforschung GmbH) layered with coal beds in its middle to upper portions; it is Darmstadt to increase the number of etchable confi ned (ca. 95–10 Ma; 0.38–0.70 °C/m.y.; <0.2 mm/yr) ~380 m thick in B–B′ and ~300 m thick in C–C′. Conglomerate tracks (Jonckheere et al., 2007). T-t paths for each sample indicate the formation of a regional low-relief occurs together with coarse-grained sandstone and consists were derived through inverse Monte Carlo modeling using predominantly of well-rounded quartzite pebbles of 2–5 cm landscape in the waning stage of foreland the HeFTy software (version 1.6.7; Ketcham, 2005; Ketcham in B–B′ and 5–10 cm in C–C′. The basal Baitianba Formation in et al., 2009) and employing Ketcham et al.’s (1999) annealing growth (see item 2 above) and its regional per- A–A′ is ~330 m thick and pseudoconformably overlies Middle model. We used a fi xed l value of 15.9 μm instead of an initial sistence across the eastern Tibetan Plateau and 0 Triassic rocks. It consists of basal conglomerate but otherwise mean track length of 16.3 μm (Donelick et al., 1999), based is mainly composed of coarse-grained quartz sandstone; its much of central China. on personal calibrations of the induced confi ned track length middle and upper portions consist of grayish-green, fi ne- in Durango apatite. C-axis projection, using the method of (4) Overall, the Hannan-Micang crystalline grained sandstone and mudstone with coal layers. Donelick et al. (1999) and Ketcham et al. (2007), was applied massifs experienced <8 km of denudation since to account for the variation of track length with angle to the

the Paleozoic and no more than 4–5 km of denu- crystallographic c-axis. We used the monotonic-variable path Qianfoyan Formation, Middle Jurassic (J2q) dation from the early Mesozoic to the Cretaceous. setting to allow for both cooling and heating histories. The In the ~330- and ~523-m-thick sections B–B′ and C–C′, the Kolmogorov-Smirnov test was employed to assess the fi t Qianfoyan Formation consists of basal conglomerate (B–B′) (5) Enhanced cooling and exhumation, between modeled and measured track-length distributions, and medium- and coarse-grained sandstone (C–C′). The associated with fault activity since ca. 13–8 Ma with merit values of 0.5 and 0.05 for good and acceptable fi ts, upper section is composed of grayish-green, fi ne-grained

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0 0 Q55 Q65 20 20

40 40

60 60 ) C ° 80 PAZ 80 PAZ ature ( ature

100 er 100 p Temperature (°C) Temperature Tem 120 120 D:58.0±5 D:60.2±5 M:58.1 M:60.1 GOF:0.97 140 GOF:0.98 140

160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1501401301201101009080706050403020100 Time (Ma) Time (Ma) Track Length Distribution Track Length Distribution 0.40 0.35 Q55 Q65 Q67 Q69 0.30 0.35 0.30 0.30 D:12.46±1.51 0.25 D:13.31±1.25 0.30 0.25 0.25 D:13.11±1.17 D:13.51±1.07 M:12.65±1.39 M:13.36±1.12 M:13.48±1.23 0.25 GOF:0.68 0.20 M:13.69±0.99 0.20 0.20 GOF:0.82 GOF:0.98 GOF:0.96 N:187 N:112 N:185 0.20 0.15 N:173 0.15 0.15

Frequency 0.15

0.10 0.10 0.10 0.10

0.05 0.05 0.05 0.05

0.00 0.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Length (μm) Length (μm) 0 0 Q67 Q69 20 20

40 40

60 60 e (°C) ure (°C) ure

tur 80 80 PAZ a per emperat m T 100 100 PAZ Te

120 120 D:74.7±7 D:68.4±6 M:74.8 M:68.5 140 GOF:0.98 140 GOF:0.99

160 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Time (Ma) Figure B1. Remodeling results of the West Qinling apatite ﬁ ssion-track samples of Enkelmann et al. (2006).

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sandstone and greenish siltstone and mudstone. Section since the Cretaceous: Evidence from apatite (U-Th)/He ern Peru: The Journal of Geology, v. 113, p. 117–138, A–A′ (~738 m thick) shows similar composition but contains dating: Chinese Journal of Geophysics, v. 53, p. 912–920. doi:10.1086/427664. more siltstone and mudstone. Cheng, H., Zhang, C., Vervoort, J.D., Wu, Y., Zheng, Y., Zheng, Gleadow, A.J.W., Duddy, I.R., Green, P.F., and Lovering, S., and Zhou, Z., 2011, New Lu-Hf geochronology con- J.F., 1986, Confi ned fi ssion track lengths in apatite: A strains the onset of continental subduction in the Dabie diagnostic tool for thermal history analysis: Contribu- Shaximiao Formation, Middle Jurassic (J s) 2 orogen: Lithos, v. 121, p. 41–54, doi:10.1016/j.lithos.2010 tions to Mineralogy and Petrology, v. 94, p. 405–415, In sections B–B′ (~2020 m) and C–C′ (~1810 m), the Shaxi- .10.004. doi:10.1007/BF00376334. miao Formation conformably covers the Qianfoyan Forma- Clark, M.K., and Royden, L.H., 2000, Topographic ooze: Build- Godard, V., Pik, R., Lavé, J., Tibari, B., Cattin, R., de Sigoyer, J., tion; it is the thickest part of the Middle Jurassic succession, ing the eastern margin of the Tibet by lower crustal Pubellier, M., and Zhu, J., 2009, Late Cenozoic evolution and it is mainly composed of purple fi ne-grained sandstone, fl ow: Geology, v. 28, p. 703–706, doi:10.1130/0091-7613 of the central Longmen Shan (eastern Tibet): Insight siltstone, and mudstone with calc-concretions. Section B–B′ (2000)28<703:TOBTEM>2.0.CO;2. from (U-Th)/He thermochronometry: Tectonics, v. 28, has thick marls. In section A–A′, the upper portion of the Clark, M.K., House, M.A., Royden, L.H., Whipple, K.X., Burch- TC5009, doi:10.1029/2008TC002407. Jurassic is absent; there, the ~400-m-thick Shaximiao For- fi el, B.C., Zhang, X., and Tang, W., 2005, Late Cenozoic Godard, V., Lavé, J., Carcaillet, J., Cattin, R., Bourlés, D., mation consists of purple siltstones and mudstones that are uplift of southeastern Tibet: Geology, v. 33, p. 525–528, and Zhu, J., 2010, Spatial distribution of denudation in interbedded with medium- and coarse-grained sandstone; doi:10.1130/G21265.1. eastern Tibet and regressive erosion of plateau mar- the mudstones contain calc-concretions. Clark, M.K., Farley, K.A., Zheng, D., Wang, Z., and Duvall, A.R., gins: Tectonophysics, v. 491, p. 253–274, doi:10.1016/j 2010, Early Cenozoic faulting of the northern Tibetan .tecto.2009.10.026. Plateau margin from apatite (U-Th)/He ages: Earth and Grimmer, J.C., Ratschbacher, L., Franz, L., Gaitsch, I., Ticho- Suining Formation, Late Jurassic (J3sn) The Suining Formation (~430 m in C–C′ and ~500 m in Planetary Science Letters, v. 296, p. 78–88, doi:10.1016/j mirowa, M., McWilliams, M., Hacker, B.R., and Zhang, B–B′) conformably covers the Shaximiao Formation; it con- .epsl.2010.04.051. Y., 2003, When did the ultrahigh-pressure rocks reach tains reddish, coarse- and medium-grained sandstone inter- Davis, W.M., 1899, The geographical cycle: Geographical the surface?: A 207Pb/206Pb zircon, 40Ar/39Ar white mica, bedded with reddish siltstone and mudstone. Sandstone is Journal, London, v. 14, p. 481–504, doi:10.2307/1774538. Si-in-white mica, single-grain provenance study of mainly arkose and arkosic quartz arenite and has erosive Donelick, R.A., 1993, A Method of Fission-Track Analysis Dabie Shan synorogenic foreland sediments: Chemi- basal contacts with mudstone. Utilizing Bulk Chemical Etching of Apatite: Patent cal Geology, v. 197, p. 87–110, doi:10.1016/S0009-2541 No. 5.267.274, U.S.A. (02)00321-2. Donelick, R.A., Ketcham, R.A., and Carlson, W.D., 1999, Vari- Guo, Z.W., Deng, K.L., and Han, Y.H., 1996, Formation and

Penglaizhen Formation, Late Jurassic (J3 p) ability of apatite fi ssion-track annealing kinetics: II. Crys- Development of Sichuan Basin: Beijing, Geological Section B–B′ is incomplete. The Penglaizhen Formation tallographic orientation effects: The American Mineralo- Publishing House, 200 p. is ~1381 m thick in C–C′, consisting of basal conglomerate gist, v. 84, p. 1224–1234. Hacker, B.R., Ratschbacher, L., and Liou, J.G., 2004, Subduc- and coarse-grained sandstone, and sandstone, mudstone, Dong, Y.P., Zhang, G.W., Neubauer, F., Liu, X.M., Genser, J., tion, collision and exhumation in the Qinling-Dabie and siltstone in the upper portion. Pebbles are mostly well- and Hauzenberger, C., 2011a, Tectonic evolution of the orogen, in Malpas, J., Fletscher, C.J.N., Ali, J.R., and rounded quartzite. Gravelly sandstone and sandy conglom- Qinling orogen, China: Review and synthesis: Journal Aitchison J.C., eds., Aspects of the Tectonic Evolution erate facies are common and often display alternation of of Asian Earth Sciences, v. 41, p. 213–237, doi:10.1016/j of China: Geological Society of London Special Publica- planar-bedded conglomeratic and sandy layers. Fine-grained .jseaes.2011.03.002. tion 226, p. 157–175, doi:10.1144/GSL.SP.2001.226.01.09. sediments are more pronounced upward in the section. Dong, Y.P., Liu, X.M., Santosh, M., Zhang, X.N., Chen, Q., Hacker, B.R., Wallis, S.R., Ratschbacher, L., Grove, M., and Yang, C., and Yang, Z., 2011b, Neoproterozoic subduc- Gehrels, G., 2006, High-temperature geochronology ACKNOWLEDGMENTS tion tectonics of the northwestern Yangtze block in constraints on the tectonic history and architecture of South China: Constraints from zircon U-Pb geochronol- the ultrahigh-pressure Dabie-Sulu orogen: Tectonics, This work was supported by China Scholarship Council (CSC), ogy and geochemistry of mafi c intrusions in the Han- v. 25, TC5006, doi:10.1029/2005TC001937. Deutscher Akademischer Austausch Dienst (DAAD), Open nan Massif: Precambrian Research, v. 189, p. 66–90, Hetzel, R., Dunkl, I., Haider, V., Strobl, M., von Eynatten, H., Research Fund of Key Laboratory of Tectonics and Petro- doi:10.1016/j.precamres.2011.05.002. Ding, L., and Frei, D., 2011, Peneplain formation in leum Resources (China University of Geosciences Wuhan), Du, S.Q., Wei, X.G., Liu, Y.C., and Wu, D.C., 1998, The NE-SW southern Tibet predates the India-Asia collision and and Ministry of Education (TPR-2011-28,TPR-2012-25) grants nappe tectonics of the superposed E-W structure in plateau uplift: Geology, v. 39, p. 983–986, doi:10.1130 to Yang, Deutsche Forschungsgemeinschaft (DFG) grants Hannan-Micangshan area: Journal of Chengdu Univer- /G32069.1. Ra442/25 and 27 and Robert-Bosch Foundation grants to sity of Technology, v. 25, p. 369–374. Hu, J.M., Chen, H., Qu, H.J., Wu, G.L., Yang, J.X., and Zhang, Ratschbacher, an Austrian government Schrödinger Founda- Enkelmann, E., Jonckheere, R., and Ratschbacher, L., 2005, Z.Y., 2012, Mesozoic deformations of the Dabashan in tion scholarship to Wiesinger, and grants from the National Absolute measurements of the uranium concentration the southern Qinling orogen, central China: Journal Natural Science Foundation of China (grants 41190074, in thick samples using fi ssion-track detectors: Nuclear of Asian Earth Sciences, v. 47, p. 171–184, doi:10.1016/j 41225008, and 40902038) to Dong and Shen. Richard Gloag- Instruments & Methods in Physics Research: Section .jseaes.2011.12.015. uen is thanked for help with Figure 1. 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