Evidence from the Hannan-Micang crystalline ... - GeoScienceWorld

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 1

GEOLOGIE, TECHNISCHE UNIVERSITÄT BERGAKADEMIE FREIBERG, 09599 FREIBERG, GERMANY STATE KEY LABORATORY OF CONTINENTAL DYNAMICS, DEPARTMENT OF GEOLOGY, NORTHWEST UNIVERSITY, 710069 XI’AN, CHINA KEY LABORATORY OF TECTONICS AND PETROLEUM RESOURCES, CHINA UNIVERSITY OF GEOSCIENCES, MINISTRY OF EDUCATION, WUHAN 430074, CHINA 4 DEPARTMENT OF GEOLOGY, UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO 45221, USA 2 3

ABSTRACT This paper addresses the timing of final 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 fission-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 fission-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 100 km to the north (e.g., Shi et al., 2012, and references therein). Previous thermochronologic studies have covered the central and south Longmen Shan (Arne et al., 1997; Kirby et al., 2002; Clark et al., 2005; Godard et al., 2009; Ouimet et al., 2010; Li et al., 2012), the western Qinling (Enkelmann et al., 2006; Zheng et al., 2006; J.H. Liu et al., 2012), and the northeastern Tibetan Plateau and adjacent Sichuan Basin (Richardson et al., 2008; Li et al., 2012); the northern Longmen Shan is less studied (Li et al., 2012; Fig. 1 for data compilation). These studies have defined the age and mechanism of Tibetan Plateau growth. U/Th-Pb zircon and 40Ar/39Ar and Rb/Sr biotite dates for the Hannan-Micang crystalline massifs have documented rapid Late Proterozoic postintrusive cooling (e.g., Z.Q. Zhang et al., 2001). Three zircon (U-Th)/He ages indicate involvement of the Hannan massif into Qinling orogen foreland deformation since the Late Jurassic (Xu et al., 2010), and apatite fission-track and (U-Th)/He ages trace terminal southward propagation of the Qinling orogen by rapid pre–Late Cretaceous cooling (>100–90 Ma) in the northern Sichuan Basin (data compilation in Fig. 2A; Chang et al., 2010; Xu et al., 2010; Tian et al., 2012). The growth of the Tibetan Plateau has involved the western Qinling since 9–4 Ma (Enkelmann et al., 2006) and the Hannan-Micang crystalline massifs since ca. 16 Ma (Tian et al., 2012; Fig. 2A). NEW THERMOCHRONOLOGIC RESULTS

We sampled granite, diorite, gabbro, and Upper Triassic to Lower Cretaceous sandstones from the Hannan-Micang crystalline massifs and the northern Sichuan Basin (Fig. 2A). We selected 11 samples for apatite fission-track (AFT), six for zircon fission-track (ZFT), and two samples for apatite (U-Th)/He (AHe) dating. Appendix A details the methodology used at the Freiberg track laboratory and the Tübingen (U-Th)/He laboratory. Tables 1–3 locate the samples and present the AFT, ZFT, and AHe data; Figure 2A shows the age distribution. The bedrock ZFT ages cover 481 ± 34 Ma to 246 ± 19 Ma with no apparent correlation with geographic and geologic position. All the ZFT samples passed the χ2 test, and thus all crystals are concordant within the statistical uncertainty. The Hannan (samples HN17, HN19, HN03) and Micang massif (MC01, MC02, MC25) ages span 364 ± 26–246 ± 19 Ma and 481 ± 34–270 ± 25 Ma, respectively. Field-based studies suggest

LITHOSPHERE | Volume 5 | Number 4 | www.gsapubs.org

| RESEARCH

a ZFT partial annealing zone (ZPAZ) at 240 ± 30 °C (Zaun and Wagner, 1985; Hurford, 1986; Brandon et al., 1998; Bernet, 2009), although the variation of ZFT annealing with radiation damage (Rahn et al., 2004) may cause considerable shifts. The U content is commonly used to assess the radiation damage in zircon; in general, it accounts for 80%–90% of the radiation damage (e.g., Garver and Kamp 2002; Garver et al., 2005). The U content of our zircons ranges from 129 to 68 ppm, representing low to average values (e.g., Garver and Kamp, 2002); this range suggests that there is little difference in radiation damage between our samples. Therefore, we interpret the broad ZFT age range to reflect slow cooling of the Hannan-Micang crystalline massif samples through the ZPAZ. Our new AFT ages span 114.4 ± 7.4–60.3 ± 2.3 Ma. All pooled ages are younger than the intrusion or deposition ages of the sampled rocks. All bedrock samples except MC12 (P [χ2] = 3.1%) passed the χ2 test (P [χ2] > 5%), but three Jurassic and Lower Cretaceous sedimentary rock samples failed the test. A plot of the single AFT ages against the host-rock stratigraphic age indicates that some apatites of these three samples were not fully reset (Fig. 3). C.B. Shen et al.’s (2007) vitrinite data show that paleotemperatures decreased from ~250 °C to 130 °C from the Upper Triassic to Middle Jurassic strata (Fig. 3). Up-section projection of this temperature gradient places the bottom of the AFT partial annealing zone (APAZ; 110 ± 10 °C) in the Upper Jurassic–Lower Cretaceous strata; this allows for partial preservation of detrital grain ages in these strata. As only a few grain ages overlap the depositional ages, we used all single-grain ages to calculate the pooled ages (also for those three sedimentary rocks samples) (Fig. 3; Table 1). Mean AFT confined track lengths vary between 11.5 ± 0.1 μm and 13.8 ± 0.1 μm; the corresponding c-axis projected mean lengths are 13.2 ± 0.1–14.9 ± 0.1 μm (Table 1). The tracklength distributions show negative skewness. Mean etch pit size parallel to the c-axis, Dpar (Donelick, 1993), ranges from 1.9 ± 0.2 μm to 2.4 ± 0.2 μm; sample HN11 has a 2.9 ± 0.3 μm value and also has the longest mean track length of 13.81 ± 0.1 μm (14.9 ± 0.1 μm after c-axis projection). Most Dpar values are close to those of Durango apatite (1.83 μm; Ketcham et al., 1999), and the small variation implies little chemical variability and similar annealing kinematics among our samples. Figure 4 displays track-length distributions and presents temperature-time (T-t) path models for 10 samples with 120–261 confined tracks (see Appendix A for our methodology to increase the number of etchable confined tracks

423

424

33°07.39′ 107°23.10′ 32°57.76′ 107°36.18′ 33°02.61′ 107°26.56′ 32°56.18′ 106°54.19′ 32°48.49′ 106°55.39′ 33°03.74′ 107°46.46′ 32°53.64′ 107°39.59′ 32°41.91′ 107°07.45′ 32°41.28′ 107°08.38′ 32°26.84′ 106°38.62′ 32°32.75′ 106°51.88′ 32°21.12′ 107°10.11′ 32°20.86′ 107°10.62′ 32°19.52′ 107°10.74′ 32°13.10′ 107°10.69′ 32°15.43′ 107°26.08′ 32°17.94′ 107°09.54′ 32°21.65′ 107°10.29′ 32°14.87′ 106°58.5′

461

500

Sandstone (K1)

Sandstone (J1)

Sandstone (J3)

Sandstone (K1)

448

497

Sandstone (J3)

Sandstone (J2)

Sandstone (J1)

481

505

550

Sandstone (T3)

Granite (Pt3)

1317

515

Diorite (Pt3)

Granite (Pt3)

Granite (Pt3)

978

1073

1106

Gabbro (Pt3)

Gabbro (Pt3)

844

550

Granite (Pt3)

Granite (Pt3)

739

624

Granite (Pt3)

Diorite (Pt3)

487

570

Granite (Pt3)

Lithology

519

Latitude (°N) Elevation and (m) Longitude (°E)

65

11

33

42

16

36

3

39

22

25

19

20

28

20

31

18

20

15

60

nG

4111

385

1798

2643

770

4634

516

2681

1106

1747

4149

1732

1217

2527

728

821

2058

823

1493

Ns

3082

236

1145

2715

630

4503

389

2310

538

960

2334

1096

628

1682

309

439

1189

448

752

Ni

253.4 ± 3.8

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

261.8 ± 6.2

251.7 ± 11.9

251.7 ± 11.9

0.396

0.351

0.296

0.470

0.442

0.478

0.453

0.426

0.448

0.292

0.446

0.467

0.278

0.475

0.295

0.471

0.462

0.335

0.335

ρd (106 cm–2)

0

24

7.7

0

0

22.1

25.6

15.6

98.2

3.1

29.4

53.9

99.9

56.6

100

98.4

50.9

39.3

46.3

P (χ2) (%)

66.8 ± 2.5

62.4 ± 5.2

61.6 ± 3.0

60.3 ± 2.3

70.2 ± 4.5

64.1 ± 2.1

78.2 ± 5.8

66.8 ± 2.9

110.7 ± 6.8

68.9 ± 3.5

103.0 ± 3.7

95.8 ± 4.4

70.1 ± 4.1

92.3 ± 3.8

90.3 ± 6.8

114.4 ± 7.4

103.8 ± 4.6

77.0 ± 6.0

83.2 ± 5.5

P-age (Ma ± 1σ)

175

122

261

126

168

138

161

171

240

34

195

192

nL

TABLE 1. APATITE FISSION-TRACK RESULTS ζ ± 1σ (a cm2)

12.2±0.2

12.2±0.2

11.5 ± 0.1

11.8 ± 0.2

12.8 ± 0.1

12.9 ± 0.1

13.0 ± 0.1

11.5 ± 0.1

13.8 ± 0.1

12.2 ± 0.3

12.7 ± 0.1

12.5 ± 0.1

1.9

1.6

1.9

1.9

1.2

1.4

1.2

1.5

0.9

1.5

1.2

1.2

-0.44

-0.76

–0.25

–0.27

–0.26

–0.39

–0.55

–0.29

–0.13

–0.45

–0.52

–0.78

Mean (L) S.D. (L) Skew (L) (µm ± S.E.) (µm) (µm)

13.8±0.1

13.9±0.1

13.5 ± 0.1

13.6 ± 0.11

14.1 ± 0.1

14.3 ± 0.1

14.4 ± 0.1

13.2 ± 0.1

14.9 ± 0.1

14.0 ± 0.2

14.1 ± 0.1

14.0 ± 0.1

Mean (LC) (µm ± S.E.)

1.2

1.0

1.2

1.2

1.0

0.9

0.8

1.1

0.7

1

0.8

0.7

S.D. (LC) (µm)

–0.19

–0.08

–0.23

–0.25

–0.88

–0.35

–0.55

–0.68

–0.27

–0.53

–0.57

–0.12

Skew (LC) (µm)

2.0(0.4)

2.3(0.2)

2.4(0.2)

1.9(0.2)

2.9(0.3)

2.3(0.2)

2.0(0.2)

Dpar (S.D.) (µm)

Note: Abbreviations: nG—number of counted grains; Ns—number of spontaneous tracks; Ni—number of induced tracks; ζ—ζ calibration factor for IRMM-540R; ρd—track density in standard uranium glass; P (χ2)—chi-square probability; P-age—pooled age; nL—number of measured confined tracks; Mean (L)—mean track length; Mean (LC)—mean c-axis projected length; S.E.—standard error; S.D.—standard deviation; Skew—skewness of the track-length distribution; Dpar—mean etch pit diameter; Pt3—Late Proterozoic; T3—Late Triassic; J1—Early Jurassic; J2—Middle Jurassic; J3—Late Jurassic; K1—Early Cretaceous.

MC15

MC14

MC11

MC09

MC08

MC05

MC04

MC03

MC25

MC12

MC02

MC01

HN17

HN11

HN19

HN12

HN03

D7613

D7611

Sample no.

YANG ET AL.

www.gsapubs.org | Volume 5 | Number 4 | LITHOSPHERE

Late-stage foreland growth of China’s largest orogens

| RESEARCH

TABLE 2. ZIRCON FISSION-TRACK RESULTS Sample no. HN03 HN19 HN17 MC01 MC02 MC25

Latitude (°N)

Elevation (m)

Lithology

nG

Ns

Ni

ζ ±1σ

U (ppm)

ρd ± 1σ

2

Longitude (°E) 33°02.61′ 107°26.56′ 32°48.49′ 106°55.39′ 32°53.64′ 107°39.59′ 32°41.91′ 107°07.45′ 32°41.28′ 107°08.38′ 32°32.75′ 106°51.88′

6

P (χ2)

–2

(a cm )

(10 cm )

(%)

P-age (Ma ± 1σ)

570

Granite (Pt3)

21

3795

185

70.9

85.3 ± 3.4

0.297 ± 0.008

90.3

255 ± 22

739

Granite (Pt3)

28

5446

257

128.5

85.3 ± 3.4

0.278 ± 0.011

99.7

246 ± 19

550

Gabbro (Pt3)

22

8400

282

73.1

85.3 ± 3.4

0.296 ± 0.007

76

364 ± 26

1106

Granite (Pt3)

18

12,015

304

67.8

85.3 ± 3.4

0.297 ± 0.007

100

481 ± 34

1073

Granite (Pt3)

11

3144

145

83.1

85.3 ± 3.4

0.298 ± 0.008

52.7

270 ± 25

1317

Granite (Pt3)

8

3107

132

102.9

85.3 ± 3.4

0.298 ± 0.008

98.7

316 ± 32

Note: Abbreviations: nG—number of counted grains; Ns—number of spontaneous tracks; Ni—number of induced tracks; U—concentration of uranium in zircons, calculated according to Enkelmann et al. (2005); ζ—ζ calibration factor for IRMM-541; ρd—track density in standard uranium glass; P (χ2)—chi-square probability; P-age—pooled age.

TABLE 3. APATITE (U-Th)/He RESULTS Sample no. MC01_1 MC01_2 MC01_3 MC01_4 HN17_1 HN17_2 HN17_3

Elevation (m)

He (mol)

U (mol)

U (mol)

232 Th (mol)

1106

2.580E–14 2.754E–14 1.761E–14 2.264E–14 6.328E–15 6.595E–15 2.957E–15

2.492E–13 3.358E–13 1.211E–13 2.539E–13 1.744E–13 9.857E–14 5.522E–14

1.843E–15 2.484E–15 8.961E–16 1.878E–15 1.290E–15 7.290E–16 4.084E–16

6.100E–13 8.734E–13 3.491E–13 6.940E–13 4.441E–13 1.239E–13 1.184E–13

550

4

238

235

147

Sm (mol)

Length (µm)

1.695E–13 2.531E–13 8.273E–14 2.437E–13 4.154E–13 2.926E–13 4.724E–13

141 165 132 121 133 123 105

Width (µm) 98 110 88 101 70 75 88

Ft

Corrected age (Ma)

Mean age (Ma)

St. dev. (Ma)

0.72 0.75 0.68 0.71 0.63 0.65 0.67

71.5 53.0 98.5 59.7 28.2 61.4 40.5

70.7

20.1

43.4

16.8

Note: Ft—alpha-correction factor after Farley et al. (1996).

K1

MC15

K1 MC09 Figure 3. Single-grain apatite fission-track ages (gray diamonds with 1σ error bars) compared with host-strata ages (black bars). Samples MC03, MC14, and MC05 are younger than their host strata, indicating full reset, consistent with paleotemperature estimates from vitrinite data (C.B. Shen et al., 2007). Ages of samples MC11, MC08, MC09, and MC15 are close to or overlap the Upper Jurassic to Lower Cretaceous host-strata ages, indicating the presence of partially reset grains and paleotemperatures of ~110 °C.

J3 MC08

J3

MC11

J2

MC05

maximum paleotemperature ~130°C

J1

MC14


T3

MC03


single grain age with 1σ error 0

50

100

150

200

host strata age 250

300

350

Age (Ma)


425

YANG ET AL.

Track Length Distribution

Group 1

0.50

HN03

0.45

HN03

0.40

0.35

0.35

0.30

Frequency

20

40

0.20 0.15

0.25 0.20 0.15

0.10

0.10

0.05

0.05

0.00 0

2

4

6

8

0.00 10 12 14 16 18 20 0

HN12

0.45

140

160 160 150 140 130 120 110 100

90

80

70

60

50

40

30

20

10

0

0.25 0.20

0.30

10 12 14 16 18 20

0.20

0.15

0.15

0.10

0.10

0.05

0.05

0.00 40

0.00 0

2

4

6

8

0.30

Frequency

D:92.4 ± 3 . 8 M:92.7 GOF:0.93

120

140 160 150 140 130 120 110 100

90

80

70

60

50

40

30

20

10

0.20

0.20 0.15

0.10

0.10

0.05

0.05

10 12 14 16 18 20

0

2

4

6

0.00 8 10 12 14 16 18 20 0 Length (µm)

2

4

6

8 10 12 14 16 18 20 Length (µm)

0

0

MC01

HN12 20

40

40

60

60

Te m p e r a t u r e ( ° C )

20

80

PAZ

100

D:114 ± 7 M:115 GOF:0.94

120

140

160 160 150 140 130 120 110 100

90

80

70

60

50

40

30

80

PAZ

100

D:95.9 ± 4 . 4 M:95.4 GOF:0.91

120

140

20

10

160 160 150 140 130 120 110 100

0

0

90

80

70

60

50

40

30

20

10

0

20

10

0

0

MC02

MC25 20

40

40

60

60


20

80

100

8

D:14.05 ± 1.04 M:14.21 ± 1.03 GOF:0.99 N:168

0.25

0.15

0

6

0.30

D:14.25 ± 0.93 M:14.39 ± 1.00 GOF:0.96 N:138

0.25

0.00

4

0.35

0.35

100

2

MC25

0.40

60

PAZ

0

10 12 14 16 18 20

MC02

0.40

80

D:14.37 ± 0.78 M:14.52 ± 0.83 GOF:0.98 N:161

0.25

20


8

0.35

D:14.11 ± 0.83 M:14.30 ± 0.85 GOF:0.97 N:195

0.30

HN11


6

0.40

0.35

0


4

MC01

0.45

0.40

D:104 ± 5 M:104 GOF:0.93

120

PAZ

120

160 160 150 140 130 120 110 100

80

PAZ 100

120

D:103 ± 4 M:103 GOF:0.94

140

90

80

Time (Ma)

426

2

100

Frequency


PAZ

D:14.87 ± 0.66 M:15.0 ± 0.80 GOF:0.37 N:240

0.30

D:14.02 ± 0.73 M:14.15 ± 0.84 GOF:0.66 N:192

0.25

60

80

HN11

0.45

0.40

0

70

60

50

40

30

D:119 ± 7 M:120 GOF:0.99

140

20

10

0

160 160 150 140 130 120 110 100

90

80 70 Time (Ma)

60

50

40

30



| RESEARCH

Group 2 0

0

MC03

20

20

40

40

60

60 Te m p e r a t u r e ( ° C )


MC05

80

PAZ 100

120

PAZ 100

120

D:64.1 ± 2.1

D:66.8 ± 2.9

M:63.9

140

80

M:69.0

140

GOF:0.95 160 160 150 140 130 120

GOF:0.45 110

100

90

80 70 Time (Ma)

60

50

40

30

20

10

160 160 150 140 130 120

0

110

0.35

0.30

90

80 70 Time (Ma)

0.35

MC03

HN17 0.30

Frequency

0.15

0.25 0.20 0.15

0.25

D:13.2 ± 1.13 M:13.39 ± 1.21 GOF:0.83 N:171

0.15 0.10

0.10

0.05

0.05

0.05

0.05

0

2

4

6

0.00 2

4

6

8 10 12 14 16 18 20 Length (µm)

10

0

2

4

6

8 10 12 14 16 18 20 Length (µm)

0

2

4

6

8 10 12 14 16 18 20 Length (µm)

0

MC11

HN17 20

20

D:70.1 ± 4.1 M:70.1 GOF:1.00

60

80

PAZ

60

80

PAZ 100

120

120

140

140

100

90

80 70 Time (Ma)

60

50

40

Figure 4 (on this and previous page). HeFTy-based (Ketcham, 2005; Ketcham, et al., 2009) thermal history models: temperature-time paths and c-axis projected confined fission-track length distributions. Constraints imposed on the models are given as boxes. Monotonic-variable paths were used to allow cooling and heating. Green paths: acceptable fits (GOF [goodness of fit] > 0.05); purple regions: good-fits (GOF > 0.5). D—determined apatite fission-track age (in Ma with 1σ error) and mean confined track length (in μm with standard deviation). M—modeled apatite fission-track age and mean confined track length. N—number of measured confined track lengths. Black circles with 1σ error bars are apatite (U-Th)/He ages. Last diagram (bottom center) overlaps all good-fit temperature-time path envelops of group 2 samples (including those re-modeled from Enkelmann et al., 2006; Fig. B1); rapid cooling started at 13–8 Ma. PAZ—partial annealing zone.


30

20

10

0

160 160 150 140

130 120 110

100

90 80 70 Time (Ma)

60

50

40

30

20

10

0

0

20

overlap 40 Temperature (°C)

160 160 150 140 130 120 110

D:60.3 ± 2.9 M:60.3 GOF:0.99

40


40

100

20

0.00 0

0


0.15

0.10

0.00 8 10 12 14 16 18 20 0 Length (µm)

30

D:13.85 ± 1.02 M:14.06 ± 1.07 GOF:0.99 N:122

0.20

0.10

0.00

40

MC11

0.25

D:13.60 ± 1.24 M:13.78 ± 1.19 GOF:0.67 N:126

0.20

Frequency

D:13.32 ± 1.21 M:13.52 ± 1.26 GOF:0.93 N:261

0.20

50

0.30

0.30

0.25

60



MC05

100

60

PAZ 80

100

120 140

160 160 150 140 130 120 110 100 90 80 70 Time (Ma)

onset of rapid cooling 13–8 Ma

60

50

40

30

20

10

0

427

YANG ET AL.

428

(Fig. 5A): Older ages are associated with higher elevations. The calculated best-fit linear trend to the AFT age-elevation data in the Micang massif exhibits a break-in-slope at ~900 m, with exhumation rates declining from ~0.04 mm/yr prior to ca. 95 Ma to 110 °C through a

INTEGRATION: PUBLISHED DATA AND AGE-PARAMETER EVALUATION

Here, we integrate our data with previously published data and derive several independent parameters pertinent to the cooling and exhumation history as a foundation for a regional interpretation of the Qinling and Tibetan Plateau forelands. Our and Tian et al.’s (2012) AFT ages correlate with sample elevation in the Micang massif and the northern Sichuan Basin

A

Micang massif and Northern Sichuan Basin apatite (U-Th)/He Chang et al., 2010 Tian et al., 2012 This study

1800

Elevation (m)

1600 1400

apatite fission-track Tian et al., 2012 This study

1200

Linear fit Confidence 95%

AFT

AHe

MC25

MC01

MC01

1000

0.02mm/yr R 2= 0 . 88

MC02

0.04mm/yr R 2= 0 . 36

MC12 ~ 95Ma

800 600

MC04 MC03 MC05 MC14 MC11

400 200 20

40

MC09

MC08 MC15 0.5) T-t path envelops of group 2 samples (including those re-modeled from Enkelmann et al., 2006; Fig. B1); these envelopes likely confine the extreme cooling-history range of each sample. Assuming that all group 2 models stem from rocks that experienced the same cooling history, but that the distinct cooling models were influenced by various parameters (e.g., natural ones, such as variable rock type, or laboratory induced ones, such as counting efficiency), causing their variability, the overlap defined by these envelopes yields an “average” of all calculated T-t histories derived in this study. Taking conservative brackets on this overlap (the “average”), we suggest that rapid cooling started at 13–8 Ma. In total, seven apatite grains from two samples (MC01 and HN17) were used for (U-Th)/ He (AHe) dating (Table 3). The mean AHe ages are younger than the AFT ages of the same samples and consistent with the AFT-derived T-t paths (Fig. 4). However, the single-grain ages vary considerably; we speculate that among the many reasons that may contribute to such an age variation, e.g., undetected U- and Th-rich inclusions, disparate crystal size, zonation, slow cooling (e.g., Fitzgerald et al., 2006; Shuster et al., 2006), the latter, i.e., the long residence of our samples in the apatite He partial retention zone (APRZ), is the main reason for the variation in the singe-crystal ages. This slow cooling through the APRZ is also indicated by AHe and AFT age-elevation trends (see following) and the T-t path models (Fig. 4).

0.38mm/yr R 2= 0 . 04

Linear fit Confidence 95%

1200 1000

HN11

800

HN19 HN12

600

HN17

HN17

D7611 HN03

D7613

400 200 20

40

60

80

100

120

140

Age(Ma) Figure 5. (A) Apatite fission-track and (U-Th)/He ages (1σ error bars) plotted against elevation for the Micang massif and northern Sichuan Basin samples. The age-elevation plot is divided into an upper steep part and a lower flat part; the break-in-slope is at ca. 95 Ma. The apatite (U-Th)/He age-elevation plot defines an exhumation rate of ~0.02 m/yr since ca. 92 Ma. (B) As in A but for the Hannan massif samples. AFT—apatite fission track.



pre-Cretaceous APAZ; the lower elevation ageelevation trend may trace the slow passing of the samples through this APAZ, with ages representing a mixture between the higher-elevation event and a younger one. In the Hannan massif, our and Xu et al.’s (2010) AFT ages range from 70.1 ± 4.1 Ma to 133.1 ± 6.0 Ma, with most ages older than 90 Ma. These ages may also follow a weak age-elevation trend (Fig. 5B). However, the younger dates, HN17 (70.1 ± 4.1 Ma) and D7613 (77 ± 6 Ma), and also MC12 (68.9 ± 3.5

A

Ma), by far the youngest high-elevation sample from the Micang massif, are all located along the Guanba fault (Fig. 2A); these three samples will be discussed separately in the following. Plots of our and Tian et al.’s (2012) AFT mean track lengths against elevation reveal a similar pattern as the age-elevation data for the Micang massif and northern Sichuan Basin (compare Figs. 5A and 6A): Higher-elevation samples, with longer tracks and less mean track-length variation (12.7 ± 0.2–13.2 ± 0.1

B

Micang massif and Northern Sichuan Basin

1800

| RESEARCH

μm), record more rapid exhumation. The lowerelevation samples, with shorter tracks and larger variation in mean track length (11.5 ± 0.1–12.5 ± 0.2 μm), probably represent a pre-Cretaceous upper APAZ. The Hannan massif track length– elevation trend is as inconclusive as its age-elevation relationship (compare Figs. 5B and 6B): Mean track lengths range from 11.5 ± 0.1 μm to 13.8 ± 0.1 μm. However, the Hannan massif track lengths are all longer than 14 μm after c-axis projection (Table 1), except HN17, which

Hannan massif

1800

Tian et al. (2012)

1600

1600

rapid exhumation MC25

Elevation (m)

1200 1000 800 600

1400 1200

M C 01

M C 02

Elevation (m)

1400

this study

1000 800 MC11

MC03

HN17

600

HN12 HN03

M C0 5

400

HN11

HN19

400

MC15

p re - Cretaceous A PAZ slow exhumation

200

200

0

0 10

11

13

12

15

14

10

11

C

12

2.3 2.1

group 1

1.9

group 2

1.7 1.5 1.3

Standard deviation

13

15

mixed

Mean track length ( m)

14

14

D

Hannan and Micang massifs and Northern Sichuan Basin

15

13

12



undisturbed basement

group2

1.1

11

mainly from Micang massif and northern margin of Sichuan Basin 10 40

50

60

70

80

90

0.9

group 1 100

110

Fission-track age (Ma)

120

130

140

0.7 10.0

11.0

12.0

13.0

14.0

15.0


Figure 6. (A) Mean confined track lengths (1σ error bars) vs. sample elevation for samples from the Micang massif and northern Sichuan Basin. Higherelevation 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 fission-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 12.5 ± 0.1 μm; except sample HN19, with only 34 confined tracks, and one sample from Tian et al. [2012], which has a mean track length of 11.4 ± 0.3 μm), and have a narrower tracklength distribution than group 2 (see following; Fig. 6D). All group 1 apatites fall into the “undisturbed basement” field (Fig. 6D), which likely represents monotonic cooling from temperatures where tracks fade to ambient temperatures (Gleadow et al., 1986); thus, group 1 apatites may stem from above a fossil APAZ. Group 2 apatites are younger (60.3 ± 2.3–82.9 ± 7.8 Ma; mostly younger than 70 Ma), have shorter mean track lengths (11.5 ± 0.1–12.5 ± 0.2 μm; except for one sample from Tian et al. [2012] with a mean of 12.7 ± 0.2 μm), and have a wider track-length distribution than group 1 (Fig. 6C). Group 2 apatites occupy the “mixed” field and the transition zone between “mixed” and “undisturbed basement” (Gleadow et al., 1986; Fig. 6D). Thus, group 2 samples, mainly from the Micang massif and the northern Sichuan Basin, define a pre-Cretaceous APAZ based on both their ages and track lengths. In Figure 6C, longer lengths correlate with narrower distributions for older ages, mean track lengths vary less in samples older than 90 Ma, and mean track lengths decrease with decreasing ages in samples younger than 90 Ma; this pattern defines a “half” boomerang, and thus only the upper part of a fossil APAZ is currently exposed in the Hannan-Micang crystalline massifs. Plotting our (sample MC01), Chang et al.’s (2010), and Tian et al.’s (2012) Micang massif and northern Sichuan Basin AHe ages against sample elevation defines a linear trend that suggests slow exhumation at ~0.02 mm/yr between 92.2 ± 1.8 Ma and 31.9 ± 9.9 Ma; this rate is similar to, albeit slightly higher than, the weakly defined one calculated from the younger than 95 Ma part of the AFT age-elevation data (Fig. 5A). Chang et al.’s (2010) Hannan massif AHe ageelevation data suggest a poorly defined exhumation rate of ~0.38 mm/yr (correlation coefficient R2 = 0.04); sample elevations vary from 1508 m to 613 m within a narrow age spread (123.5 ± 2.7–107.4 ± 3. 8 Ma; Fig. 5B). This rate would be ~10× higher than the exhumation rate calculated from the AFT age-elevation trend of the

430

Micang massif and northern Sichuan Basin here, covering a comparable period (123.5 ± 6–93.9 ± 4.1 Ma); enhanced exhumation would, however, be consistent with coeval enhanced cooling, as derived from the Hannan massif’s AFT T-t paths (Figs. 4 and 6B). Two of our AFT ages (HN11, HN19) are younger than Chang et al.’s (2010) AHe ages at the same elevation, further casting doubt on the significance of the AHe age-elevation relationship. The 43.4 ± 16.8 Ma AHe age of sample HN17 is much younger than other Hannan massif AHe ages; this sample also has the youngest AFT age (Fig. 5B; Table 1), and it is located along the Guanba fault (Fig. 2A; see discussion). STRATIGRAPHIC CORRELATION AND EVOLUTION OF THE NORTHERN SICHUAN BASIN

The thermochronologic data analysis suggests more rapid exhumation and cooling during the Early Cretaceous than during the Late Cretaceous–Tertiary. The onset of this accelerated exhumation/cooling period, however, cannot be quantified by AFT and AHe data due to the inability of these thermochronometers to provide age information above temperatures of ~110–80 °C. Unfortunately, our ZFT ages (ca. 481–246 Ma) and Xu et al.’s (2010) zircon (U-Th)/He ages (ca. 184–153 Ma) vary strongly and are possibly part of fossil partial annealing/retention zones. Here, we approximate the age of onset of deformation around the Hannan-Micang crystalline massifs, which we relate to the onset of enhanced exhumation/ cooling, from a correlation of newly measured Jurassic stratigraphic strata in the Huijunba syncline (northern section A–A′; for previous data, see S.B.G.M.R., 1989; Guo et al., 1996) with data from the northern Sichuan Basin (southern sections B–B′ and C–C′; Fig. 2A; see also Meng et al., 2005; Burchfiel et al., 1995). We assume that disparities in the strata reflect north to south propagation of Qinling foreland deformation. Figure 7 and Appendix C detail the evolution of the Jurassic–Cretaceous strata; the Huijunba syncline and the northern Sichuan Basin sections are identical, at least up to the Middle Jurassic (165 ± 5 Ma; ICS, 2010) basal section of the Shaximiao Formation. No younger Jurassic strata are preserved in Huijunba syncline north of the Micang massif. The absence of strata younger than Middle Jurassic in the Huijunba syncline may be due to later (for example, Cenozoic) erosion of potential Upper Jurassic– Cretaceous deposits, but it may also be due to a switch from deposition to erosion at ca. 165 Ma. The identical stratigraphy in the Huijunba syn-

cline and the northern Sichuan Basin suggests that the onset of accelerated exhumation did not start earlier than ca. 165 Ma, and we speculate that deposition terminated in the Middle Jurassic in the Huijunba syncline due to migration of foreland deformation from the Hannan-Micang crystalline massifs to the northern Sichuan Basin, where the deposition of thick deposits and the onset of coarse clastic input suggest onset of deformation during the late Middle– Late Jurassic (Fig. 7). Isopach maps of Mesozoic strata within the Sichuan Basin (Guo et al., 1996; Li et al., 2003; S.G. Liu et al., 2006) indicate migration of depocenters throughout the Late Triassic and Jurassic; this has been interpreted as reflecting foreland propagation of deformation in the Qinling orogeny. The Late Triassic (Norian–Rhaetian) depocenter was in front of the Longmen Shan, with strata thickness ≥3 km (Li et al., 2003) and an average sediment accumulation rate of ~0.19 km/m.y. Lower Jurassic sediment thickness is