Cenozoic clockwise rotation of the Tengchong block, southeastern Tibetan Plateau: A paleomagnetic and geochronologic study

    Cenozoic clockwise rotation of the southeastern Tibetan Plateau: a paleomagnetic study Daniela Kornfeld, Sabine Eckert, Erwin Appel, Lothar Ratschbacher, Benita-Lisette Sonntag, J¨org A. Pf¨ander, Lin Ding, DeliangLiu PII: DOI: Reference:

S0040-1951(14)00213-3 doi: 10.1016/j.tecto.2014.04.032 TECTO 126286

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

8 January 2014 11 April 2014 21 April 2014

Please cite this article as: Kornfeld, Daniela, Eckert, Sabine, Appel, Erwin, Ratschbacher, Lothar, Sonntag, Benita-Lisette, Pf¨ander, J¨ org A., Ding, Lin, DeliangLiu, Cenozoic clockwise rotation of the southeastern Tibetan Plateau: a paleomagnetic study, Tectonophysics (2014), doi: 10.1016/j.tecto.2014.04.032

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ACCEPTED MANUSCRIPT Cenozoic clockwise rotation of the southeastern Tibetan Plateau: a paleomagnetic study

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Daniela Kornfelda, Sabine Eckerta, Erwin Appela*, Lothar Ratschbacherb, Benita-Lisette

a

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Sonntagb, Jörg A. Pfänderb, Lin Dingc, DeliangLiuc

Department of Geosciences, University of Tübingen, Hölderlinstr.12, 72074 Tübingen,

Germany;

[email protected],

Geologie, Technische Universität Bergakademie Freiberg, Bernhard-von-Cotta-Str. 2, 09599

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b

erwin.appel@uni-

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tuebingen.de

[email protected],

Freiberg, Germany;

[email protected], [email protected], joerg.pfaender@tu-

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c

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freiberg.de

Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Lin Cui Lu 16 Hao

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Yuan, Beijing 100101, China;[email protected], [email protected]

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*Corresponding author Erwin Appel

[email protected] Phone no.: +49-(0)7071-2974132 Department of Geosciences, University of Tübingen, Hölderlinstr. 12, 72074 Tübingen, Germany

Abstract

Paleomagnetic data from ~50–35 Ma (likely ~40 Ma) mafic dykes in Yunnan, southeastern Tibetan Plateau, cutting ~115 Ma granitoids record the rotation of the Tengchong 1

ACCEPTED MANUSCRIPT (Lhasa)block

around

the

East

Himalayan

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(EHS).Ti-rich

titanomagnetiteandmagnetitecarry a primary magnetic component (Group1); a magnetic

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overprint resides in magnetite (Group2), likely induced by low-grade metamorphism between

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~30 and 10 Ma. The tilt-corrected overall mean directions are D/I = 89.8°/35.1° for Group1

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and D/I = 33.3°/41.4° for Group2. These data imply a clockwise rotation of ~87° (87.3 ± 12.5°) of the Tengchong block since remanence acquisition at ~40 Ma with respect to stable Eurasia. The average rotation rate of2.18 ± 0.31°/Myris at the upper limit of the present-day

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rotation rates around the EHS obtained from GPS velocities and Quaternary strain rates. The remagnetized Group2 indicates a—poorly defined—rotation of ~31° (31.2 ± 32.9°). Our key

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results―the detection of ~87° clockwise rotation, with high rotation rates following the India–Asia collision,and a decrease in rotation rates during the Miocene―suggest that first

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the Tengchong block rotated rapidly around the EHS,synchronous with eastward tectonic escape of lithospheric blocks. The later, slower, clockwise rotation—similar to those recorded

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geodetically—is probably related to viscous flow of Tibetan crust.

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Keywords: Paleomagnetism, geochronology, Tibetan Plateau, Eastern Himalayan Syntaxis, Tengchong block, crustal rotation

1 Introduction

Intra-continental convergence between India and Asia since ~50 Myr(e.g. Klootwijk et al., 1992; Leech et al., 2005; Najman et al., 2010)has caused significant deformation and rotation in Southeastern Asia;both—paleomagnetic (e.g. summarized in Otofuji et al., 2010) and geodetic data (e.g. Gan et al., 2007; Sol et al., 2007; Banerjee et al., 2008; Maurin et al., 2010)—record these crustal movements around the Eastern Himalayan Syntaxis (EHS). Two widely considered end-member models describe these crustal movements: The ‘tectonic 2

ACCEPTED MANUSCRIPT escape’ model implies lateral displacement of rigid blocks along lithosphere-scale shear zones(e.g. Molnar and Tapponnier, 1975; Tapponnier et al., 1982; Replumaz and Tapponnier,

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2003).The ‘crustal flow’ models describe material transfer from the orogenic interior to the

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exterior regions, channelized within a vertically decoupled lithosphere (e.g.Bird, 1991;

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Royden et al., 1997; Clark and Royden, 2000). These groups of models are called ‘escape’ and ‘crustal flow’ models in the following.

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Western Yunnan is a key area for tracing material transport around the EHS and for

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understanding its mechanism. Present-day surface velocities observed by GPS geodesy show a wide area of clockwise rotations around the EHS and S-ward crustal flow in this region (Fig. 1a; Gan et al., 2007; Sol et al., 2007; Banerjee et al., 2008; Maurin et al., 2010); the clockwise

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rotation resumes south of ~25°N withSW-ward flow into Myanmar and Thailand. Paleomagnetic results from the area east of the EHS, in the Shan-Thai and Lanping-Simao

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blocks, also record clockwise rotations (Fig. 1a; Huang and Opdyke, 1991, 1993; Funahara et al., 1992, 1993; Chen et al., 1995; Sato et al., 2001, 2007; Tanaka et al., 2008).Almost all

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results stem from Paleozoic–Mesozoic rocks; onlythe study of Chen et al. (1995) included data from Paleogene to Neogene strata. The availabledata thus record the rotation accumulated since pre-Cenozoic times and cannot resolve movements within specific time frames of the India–Asia collision and the Cenozoic development of the Tibetan Plateau.

In this paper, we present new paleomagnetic data from the Tengchong block, acquired from mafic dykes that intruded granitoids of the Gaoligong Mountains (Gaoligong Shan), a part of the Gangdese magmatic arc built on the Lhasa block (e.g. Xu et al., 2008, 2012; this study). The sampling area is located southeast of the town of Pianma (referred to as the Pianma– Nujiang section; Fig. 1b). Below, we argue that these dykes likely carry both a ~50–35 Ma

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ACCEPTED MANUSCRIPT (likely ~40 Ma) primary remanence and a ~30–10 Ma remagnetized component; thus, the results allow separating rotations accumulated over time.

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Figure 1 somewhere here

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2 Geological setting

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Several micro-continents were amalgamated to the southern margin of Asia prior to the India– Asia collision (e.g. Yin and Harrison, 2000; Metcalfe, 2002). The southernmost micro-

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continent is the Lhasa block, which is separated by the Bangong–Nujiang suture zone from the Qiangtang block in the north (Fig. 1a). The south-eastern continuation of the Lhasa block

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in Yunnan is the Tengchong block(Fig. 1a; Li et al., 2004; Xu et al., 2012). The latter is

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separated by the Gaoligong Shan shear zone (GSSZ) from the Baoshan block to the east. The Gaoligong Shan, the eastern segment of the Lhasa–Tengchong block (Fig. 1a), trendsN–S,

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and consists of possibly Precambrian metamorphic rocks, Late Paleozoic clastic rocks and carbonates, but,mostly of Mesozoic granitoids (e.g. Xu et al., 2012). Mafic dykes and

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stocks—~1 to ~50 m thick—cut the granitoids with sharp contacts (Fig. 2). The granitoids show I-type characteristics and positive εHf values similar to theGangdese batholith rocks (Xu et al., 2008, 2011).

In this study, we report paleomagnetic results from both the mafic dykesand the granitoids (Fig. 1b). The sampled dykes and granitesare undeformed or at most weakly deformed (Fig. 3) and all lie west of the Miocene dextral Gaoligong shear zone (Fig. 1b; Wang et al., 2006, 2008; Zhang et al., 2012; Eroğlu et al., 2013).

Figure 2 somewhere here Figure 3 somewhere here

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3 Paleomagnetic sampling and analytical methods

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We sampled eleven dykes and three granites along the road from the Nujiang valley to

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Pianma (Fig. 1b). More dykes may exist but dense vegetation and steep terrain preclude their exploration.At each site,we drilled eight to ten cores using a portable rock drill. The orientation of the cores was measured with a magnetic compass and an inclinometer.

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Whenever possible, we sampled across the full width of the dykes and measured the

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orientation of the dyke–granite contacts to be able to perform a tilt correction.

From each core, individual specimens―2.5 cm in diameter and ~2.3 cm in length―were cut.

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We then chose twin specimens from all sites for pilot analyses, one for alternating field demagnetization (AfD) and the other for thermal demagnetization (ThD). Remanence

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directions were measured with a 2G Enterprises He-free SQUID magnetometer 755–1.65 UC (sensitivity limit 10-7 Am-1 for a specimen volume of 10 cm3). For AfD, we used a 2G

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automatic degaussing system integrated in the SQUID magnetometer, applying steps of 2–20 mT with a maximum field of 100 mT. ThD was performed with ASC, MMTD-18, and Schoensted furnaces, progressing in 25–50°C steps up to 650°C. To determine the ferro(i)magnetic mineralogy,we subjected one specimen per site (after AfD) to stepwise acquisition of isothermal remanent magnetization (IRM), using a pulse magnetizer (MMPM9) for imparting an IRM and a spinner magnetometer (MINISPIN-Molspin) for measuring its intensity. The IRM acquisition curves were processed by cumulative log-Gaussian (CLG) distributions using the irmunmix2_2_1 and IRM_CLG1 software (Kruiver et al., 2001). After IRM acquisition, the saturation IRM (SIRM) was stepwise thermally demagnetized. High and low temperature thermomagnetic runs of magnetic susceptibility were performed for one crushed and powdered sample from each site using a Kappabridge KLY-3 (Agico) with a CS5

ACCEPTED MANUSCRIPT 3 temperature unit. Hysteresis loops were measured with a Princeton Measurements 2900 MicroMag alternating gradient force magnetometer (AGFM). For all dykes,we determined the

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anisotropy of magnetic susceptibility (AMS),applying15 directions on a MFK-1 Kappabridge

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(Agico) for 6 to 9 specimens per site. To determine the mineralogy of the magnetic

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remanence carrier, we investigated samples from three dykes by reflected and transmitted light microscopy,using a Leica DM 2500P microscope with a 20×/0.4 oil objective for reflected light microscopy. In addition, we analyzedtwo polished sections by an energy

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dispersive X-ray (EDX) system using an OXFORD INCA Energy 200 Premium Si (Li)

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SATW-Detector and a SEM LEO Model 1450 VP scanning electron microscope (SEM).

4 Magnetic mineralogy

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4.1 Mafic dykes

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In the samples fromsites LI03, LI05, and LI08, we identified (titano)magnetite byreflected light microscopy and SEM/EDX analysis(Fig. 4a, b). Crystalshapes and degree of alteration

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vary between the samples. The grain size of (titano)magnetite rangesfrom~15 to 200 µm. Ilmenite occurs together with (titano)magnetite (Fig. 4a).These intergrowths are typical for high-temperature oxidation (HTO) that occurs during initial cooling of the rock, resulting in intergrown spinel (near magnetite) and rhombohedral (near ilmenite) phases (Dunlop and Özdemir, 1997). Sample LI05 shows shrinkage cracks in (titano)magnetite (Fig. 4a, c),indicating low-temperatureoxidation (LTO) (Readman and O’Reilly, 1972)that causes lattice contraction. In sample LI08, ilmenite occurs together with hematite; ilmenite likely stems from exsolution of Ti-rich titanomagnetites, while hematite can be regarded as a final product of magnetite oxidation. EDX measurements ofthe Fe–Ti oxides revealed a broad range of Ti-contents from nearly pure magnetite to Ti-rich titanomagnetite, with compositions

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ACCEPTED MANUSCRIPT of TM60 to TM80. The latter are preserved in the particle interiors; alongthe rims,we

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observed phases close to ilmenite and rutile(examples in Fig. 4c, d).

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Figure 4somewhere here

High-temperature thermomagnetic runs of magnetic susceptibility (black and blue curves in Fig. 5a) indicate the presence of two ferro(i)magnetic phases. Magnetite was identified in all

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samples by a decrease in susceptibility at ~580°C. Four out of seven samples show a pronounced peak in the low-temperature curve at ~-150°C related to the isotropic point of

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magnetocrystalline anisotropy (Syono, 1965) (Fig. 5a) and the Verwey transition (Verwey, 1939) of magnetite. The presence of the Verwey transition demonstrates a composition close

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to pure magnetite (Moskowitz et al., 1998); the appearance of a pronounced isotropic point

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indicates the dominance of multi-domain (MD) particles (Moskowitz et al., 1998). Results

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from hysteresis loop measurements confirm domain states that range from pseudo-single domain (PSD) to MD (Fig. 6).PSD grains dominate and are suitableto carry a stable primary remanence (Dunlop and Özdemir, 1997). For some of the samples, a second decreaseappears

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in the heating curves of the thermomagnetic runs at intermediate temperatures (~250–450°C), likely representing a Ti-rich titanomagnetite phase which is destroyed during further heating (Fig. 5a). Incremental high-temperature thermomagnetic runs (Fig. 5b) show that the decay in the heating curves at intermediate temperatures is accompanied by destruction of a magnetic phase. Therefore, the decay at intermediate temperatures could represent an inversion of maghemite into hematite, instead of being indicative of the Curie temperature of Ti-rich titanomagnetite.The results from the thermomagnetic runs neither prove nor disprove the existence of Ti-rich titanomagnetite. As our EDX data demonstrate the presence of Ti-rich titanomagnetite,

we

infer

that

besides

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magnetite

the

dykes

contain

Ti-rich

ACCEPTED MANUSCRIPT titanomagnetite,which exsolved into phases with a composition close to magnetite and

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ilmenite during heating (Moskowitz, 1981).

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Figure 5 somewhere here

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Figure 6 somewhere here

IRM acquisition curves reveal saturation at fields≤300 mT (Fig. 5b). For seven sites, CLG-

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analysis (Fig. 5b) indicates the presence of two relatively soft ferrimagnetic phases. Both the lower coercive (B1/2=21–54 mT) and the higher coercive (B1/2=56–158 mT) fractions could be

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related to magnetite grains with different domain states. However, given the thermomagnetic results, the two components more likely represent a relativelysofter magnetite fraction and a

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relatively harder Ti-rich titanomagnetite fraction (magnetically harder magnetite may

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contribute to the latter one). The internal anisotropy of natural Ti-rich titanomagnetite is

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generally dominated by magnetostriction, and―in the common case of irregular internal stresses―this leads to relatively higher coercivities (Appel and Soffel, 1984; Appel, 1987). In samples from two sites, we detected only one component with B1/2 (41 and 51 mT) similar to

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the relativelylower coercive phase, indicating a dominance of magnetite. Three samples additionally contain small amounts of hematite (B1/2 =1389–1494 mT), but its contribution to the IRM is only 1–2%.

The ThD of the SIRM shows a decrease in magnetization with unblocking temperatures, corresponding to a combination of Ti-rich titanomagnetite and magnetite (dashed curve in the example of Fig. 5c), or tosolely magnetite (solid curve in the example of Fig. 5c). A broad range of unblocking temperatures between ~200 and 400°C reflects a variable but relatively high Ti-content and various degrees ofLTO.

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ACCEPTED MANUSCRIPT 4.2 Granites

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The IRM acquisition curves of our granite samples saturate at fields of ~300 mT (Fig. 5d)

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with B1/2= 28–53 mT; sample PM30 has a higher coercivity than the other samples. The ThD

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of the SIRM (decrease at ~580°C; Fig. 5d) revealsthe presence of magnetite.The thermomagnetic curves (e.g. red curve in Fig. 5a) are dominated by magnetite, identified by the decrease at ~580°C. A second decrease can be observed both in the heating and cooling

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curves; it is partly reversible and partly irreversible. The presence of Ti-rich titanomagnetite

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in granite can be excluded, because such compositionswillexsolve during slow cooling of the magma. Maghemite is also unlikely as the decay occurs at rather hightemperatures. A possible explanation is a grain-size effect, i.e. the presence of fine-grained magnetite particles that

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cross the single domain–superparamagnetic transition at intermediate temperatures.

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5 Geochronology of the dykes and granitoids

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5.1 Previous work

Xu et al. (2008) reported a ~42 Ma whole-rock 40Ar–39Ar emplacement age for a mafic dyke from the Pianma–Nujiang section (sample GL-24); a second dyke, from the southern Tengchong block,>100 km to the southwest of the Pianma–Nujiang section, yielded ~40 Ma. One granitoid from the western part of the Pianma–Nujiang section (sample GLS-8) yielded a 122 ± 2 Ma SHRIMP238U-206Pb zircon age (Xu et al., 2012). Cooling of the mylonites of the Gaoligong shear zone at the eastern end of the Pianma–Nujiang section through ≥300°C (closure of the

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Ar–39Ar system for biotite) occurred at ~17 Ma (17.9–16.7 Ma, samples

YNW-30A, YNW-29A, GM-2; Lin et al., 2009; Zhang et al., 2012) and through 120–80°C (partial annealing zone for the apatite fission-track system) at ~7 Ma (6.6 ± 1.2 Ma, sample Apx 10; Wang et al., 2008). 9

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To obtain insight into the age of regional magmatism and associated metamorphism, we

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compiled the Cretaceous and Cenozoic U–Th–Pb dates from the Mogok belt of east-central

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Myanmar and southwestern Yunnan, an equivalent to the Gaoligong Shan and Tengchong

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area, and the Chong Shan, a narrow belt of tectonites of magmatic and metamorphic origin just east of the Gaoligong Shan (Figs. 1a, 7a; Roger et al., 1999; Barley et al., 2003; Searle et al., 2007; Akzic et al., 2008; Song et al., 2010; Mitchell et al., 2012; Xu et al., 2012,

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unpublished own results); we excluded the Pleistocene to Recent volcanism of the Tengchong

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area. Cretaceous magmatism clusters at ~125 and 75–60 Ma; magmatism and related hightemperature metamorphism occurred throughout the Cenozoic, withgroups at ~50–30 Ma and ~20 Ma (Fig. 7a). Mica cooling ages obtained from

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Ar–39Ar thermochronology from the

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Gaoligong and Chong Shan,covering samples inside and outside the shear zones, cluster at

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~20–10 Ma (Fig. 7b; Lin et al., 2009; Zhang et al., 2012; Eroğlu et al., 2013).

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5.2 New geochronology

We analyzed six whole-rock mafic dyke samples by the

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Ar–39Armethod (Fig. 7c; Table 1;

Table DR1, auxiliary material in the electronic supplement) and three granitoids by the LAICPMS U–Pb zircon method (Fig. 7d; Table DR2, auxiliary material in the electronic supplement). Appendix 1 summarizes the geochronologic methods employed in this study.In thin section, themafic dyke samples showrelictic volcanic textures mostly traced by phenocrystic plagioclase; a greenschist-facies metamorphic mineral assemblage, comprising variable amounts of pale to green amphibole (actinolite, magnesio-hornblende), biotite, chlorite, epidote, albite, and locally rutile, overgrows the magmatic texture. The assemblage traces various stages of hydration with a likely peak temperature of~400°C. According to this overprint, the 40Ar–39Ar thermochronology of the dykes is complex (Fig. 7c; Table 1). 10

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Dyke sample YU9409Ayielded apparent ages between ~80–30 Ma; isochrons of selected data

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with approximately atmospheric 40Ar–36Ar intercepts may be interpreted as emplacement and

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overprint at ~70 and ~45 Ma, respectively. The apparent ages of sample YU9409G cover~35–

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25 Ma; selected data define isochrons that may be interpreted as dyke emplacement at ~30 Ma and hydration at ~25 Ma. Apparent ages of YU9419H1 comprise 55–15 Ma; selected data may suggest emplacement at ~41 Ma and overprint at ~28 Ma. Samples YU9409E1 and

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YU9409E2 comprise two parallel dykes within one outcrop. Most apparent ages of

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YU9409E1 cover~45–35 Ma; isochrons through selected data may indicate dyke emplacement or metamorphic overprint at ~38Ma. The apparent ages of YU9409E2 cover~40–20 Ma; an isochron through most of the data indicates either dyke emplacement or

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overprint at ~49 Ma. Sample YU9409Fapparent ages mostly range at ~45–40 Ma; isochrons

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fitted through various data subsets suggest emplacement at ~45 Ma.

Concordant zircon

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U–206Pb dates of granodiorite YU9408B2 cover ~130–94 Ma with

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inheritance at ~250 Ma and ~540 Ma (Fig. 7d); all zircons are magmatic, showing oscillatory growth zonation under cathodoluminescence and Th–U ratios between 0.13–0.90 (mean = 0.50). We interpret the main age cluster at 118 ± 2 Ma to date crystallization (Fig. 7d).The concordant

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U–206Pb zircon ages of granite YU9409G range from ~123 to ~109 Ma;

oscillatory growth zonation and Th–U ratios of 0.21–0.91 (mean = 0.46) suggest magmatic zircons. Inheritance is weak, with one concordant grain at ~191 Ma and two discordant grains. We interpret the Concordia age of the main clusterto date crystallization at 117 ± 2 Ma. The concordant

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U–206Pb zircon ages of granitic gneiss YU9409I cover ~125–101 Ma, with one

concordant grain at ~410 Ma and several discordant ones; all grains are magmatic (oscillatory growth zonation; Th–U = 0.16–0.89, mean = 0.56). The Concordia age of the main cluster likely dates crystallization at 112 ± 2 Ma. 11

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Given the occurrence of Cenozoic (~45–25 Ma; Fig. 7a) magmatism across western Yunnan 40

Ar–39Ar dyke dates at ~50–35

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and east-central Myanmar, we interpret the clustering of

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Ma(Fig. 7e) as recording their emplacement. The strong hydration precludes a more precise

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age assignment, leaving open whether dyke emplacement was a single, short-lived episode or a prolonged phase. We place the prominent hydration overprint at ~30–10 Ma, corresponding to the younger isochron dates obtained from the dykes and the

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Ar–39Ar mica agesof the

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Gaoligong and Chong Mountains (Fig. 7b). For our paleomagnetic study, we assume a ~115

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Maage for the Cretaceous magmatism (four granitoid ages at ~118–112 Ma) in the Pianma– Nujiang section, in accordance with the Early Cretaceous clusterin the dates of magmatism in western Yunnan–east-central Myanmar (Fig. 7a). We discuss the implications of these dates

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Figure 7somewhere here

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for remanence acquisition in section 7 below.

Table 1somewhere here.

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6 Paleomagnetic results

6.1 Analysis of remanence components

The natural remanent magnetization (NRM) values of the dykesrange between~0.01–10Am-1; those of the granitoidsbetween ~0.01–0.1 Am-1. Susceptibility (κ) values of the dyke samples at room temperature vary little (10-7–10-6 SI),thus indicating similar contents of ferro(i)magnetic minerals. The wide range—including relatively small values—of NRM intensities in the dykes may be due to oxidation of Ti-rich titanomagnetite.At 9 out of 12 dyke sites,two components with lower (LCC) and higher (HCC) coercivitywere separated betweenAfD intervals of 4–25 mT and 10–100 mT (Fig. 8a, c, e, g), respectively. According 12

ACCEPTED MANUSCRIPT to the ThD results (Fig. 8b, d, f, h), the magnetic remanence is carried by magnetite (full demagnetization achieved at 575°C). Theremanence directions separated from the ThD results

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show the same HCC directions than separated by the AfD. However, as the demagnetization

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paths for the AfDaresmoother, we used AfD for the remaining specimens, measuring 6–11

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specimens for each site. For determining remanence directions, we used principal component analysis (Kirschvink, 1980); this leads to the same results than using the great circle method

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Tables 2 and 3list the statistical results.

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(Fig. 8). Three sites (PM27, PM34, PM35) revealed only one component, likely the LCC.

Figure 8 somewhere here Table 2 somewhere here

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Table 3 somewhere here

The LCC in-situ site mean directions (Fig. 9a; overall mean D/I=10.2°/35.5°; k=23.7,

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α95=10.1°; n =10; outliers LI06 and PM35 excluded) plot close to the present-day Earth magnetic dipole field (D/I=0°/44.2°). Thus, the LCC likely represents a recent overprint and

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will notbe discussedfurther. The HCC directions show good grouping within sites (k>15 and mostly better; Table 2). In eight sites, they have a reverse polarity and in one site a normal polarity.The inclinations are relatively similar whereas the declinations are more scattered and deviate clockwise from north (Fig. 9b). Sites LI02, LI05 and LI06 (Group2) show clearly smaller declination values than sites LI03, LI04, LI07, LI08, PM29 and PM31 (Group1).The 95% confidence limit of all HCC in-situ site mean directions does not include the present Earth magnetic dipole field. In the granite samples, only one component―demagnetized at similar field intervals as for the LI02, LI05 and LI06 dyke samples―could be separated (Fig. 8g). In site PM 30, we identified a second, likely a HCC component, in two samples by AfD

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ACCEPTED MANUSCRIPT and in one sample by ThD (Fig. 8h). The remanence in the granites is carried by

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magnetite.We interpret these remanence directions as secondary, remagnetized.

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The overall in-situ mean direction of Group2 yields D/I = 34.0°/23.4° (k = 8.3, α95 = 28.2°; n

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= 6). The high scatter observed in the mean directions of this Group likely reflects its acquisition as amagnetic overprint.The overall in-situ mean direction of the Group1 dyke data is significantly different and better grouped (D/I = 84.5°/14.9°; k = 33.0, α95 = 11.8°; n =

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6).We note that Group2 directions show a significantly smaller (D=34.0°) clockwise deviation

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from north than Group1 directions (D=84.5°) (Fig. 9b).The main difference between these Groups is indicated by the demagnetization behavior. Group2 sites are almost completely demagnetized between ~30–50mT (Fig. 8c), whereas Group1 siteshave a higher coercivity

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and approach complete demagnetization only at higher fields of ~100 mT(Fig. 8a, e). The rock magnetic, light microscopy, and EDX results suggest the presence of Ti-rich

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titanomagnetite in the Group1 samples, while in theGroup2 sampleswe have only weak indications for it. Therefore, the Group1 remanence directions are likely of primary origin,

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carried by Ti-rich titanomagnetite and magnetically stable magnetite. Ti-rich titanomagnetites form during rapid magma cooling; they are metastable at room temperature and are sensitive to alteration (e.g. Özdemir, 1987). The magnetite is most likely derived from the HTO of Tirich titanomagnetite that occurs during initial cooling of the rock. The Group2 remanence directions are likely of secondary origin due to LTO of magnetite and alteration of Ti-rich titanomagnetite (exsolution into phases near magnetite and ilmenite). LTO and exsolution of titanomagnetite are alteration processes that are enhanced at elevated temperatures. Metamorphism is the most likely process inducing this alteration, accompanied by a partial magnetic overprint resulting in the remanence direction residing in the Group2 sites. Within such a scenario, Ti-rich titanomagnetite and primary magnetite formed during HTO are assumed to be better preserved in the Group1 sites. 14

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We did not sample the granites directly adjacent to the dykes and thus we are unable to

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present a contact test. However, as part of the dykes reveal the same directions as the granites

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(Group2), and part of them do not (Group1), a contact test would not help to further clarify

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the origin of the remanences.

How to explain the directions? There are two possibilities: The first is an apparent clockwise

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rotation caused by tilting around a horizontal axis, which could have deflected the remanence

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directions along a small circle (Waldhör and Appel, 2006). However, it is impossible to fit the site mean directions of both Groups to a sole small circle; thusa uniform tilt direction that would explain the results can be rejected (Fig. 9b). Moreover,we note that large tilt angles of

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~85° and ~100°are requiredto generate theoverall mean directions of the combined Group1+Group2 sites and of the Group1 sites, respectively(Fig. 9b). The second possibility is

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that Group1 directions areof primary origin, thus representing a large clockwise vertical-axis rotation, while Group2 directions record alater remagnetization,tracing only part of the same

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rotation. The second scenario seems much more likely because the tilt angles required by the first scenario are large;such tilt angles are likely not compatible with the sub-vertical dip of the dykes (Fig. 2c, 10c), whichwe assume to be close to the original dip. Another hint for vertical-axis rotation comes from the>45° difference in strike between ~53 Ma (one 40Ar–39Ar age), ~E-trending, unrotateddykes from the eastern Lhasa block west of the EHS (Liebke et al., 2010; our own orientation measurements) and the Yunnan dykes (Fig. 2c); assuming emplacement in a similarly oriented stress field around the onset of the India-Asia collision, their orientations indicate a clockwise rotation.Moreover, our dyke samples clearly record a thermal overprint (section 5).

Figure 9 somewhere here

15

ACCEPTED MANUSCRIPT

At each site we determined the dyke-granite contact planes (Fig.2c). They arewell grouped

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and relative rotations between the different dykescan be excluded. The slight scatter of the

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contact planes of the individual dykes probably arises from local heterogeneities, such as

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thickness changes and dyke bifurcation. The regional consistent, steep, mostly E-dip of the dykessuggests that theywere emplaced vertically. However, a slight tilting of the rocks in thePianma-Nujiang section is possible.He et al. (2007) and Chen et al. (2010) reported similar

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minor, differential tilting along the southern edge of the Lhasa terrane in southeastern Tibet

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west of the EHS between ~50–10 Ma.We therefore performed a tilt correction for both remanence Groups,using the mean orientation of the dyke-granite interfaces and assuming that the dykes were emplaced vertically(resulting in aback-tilting dip direction/dip of

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~238°/23°).As mentioned in the introduction the granites are undeformed or at most weakly deformed,thus substantial sub-horizontal rotation of the granites and dykes can be excluded.

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This allows to apply the fold test using the paleo-vertical orientation.The tilt corrected site mean directionisD/I = 89.8°/35.1° (k = 33.0, α95 = 11.8°; n = 6) for Group1. In

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comparisonwith the in situ direction (D/I = 84.5°/14.9°),tilt correction has only a minor influence on the declination values and thus the interpretation of block rotations.The same is true for the tilt corrected resultsof Group2 (D/I = 33.3°/41.4°). In the following discussion,we will refer to the tilt corrected result. We note thatGroup2 reflects a later time of remanence acquisition than Group1, andtherefore may only record part of the tilting. However, because of the minor difference of in situ and tilt corrected declination values this is not relevant for the interpretation of block rotations.

One important criterion for the reliability of the overall site mean directions is averaging on paleosecular variation (PSV). The

40

Ar–39Ar results (see interpretation below) and the

detection of normal and reverse polarity directions in the HCC of the dykes imply that the 16

ACCEPTED MANUSCRIPT time span of dyke emplacement was long enough to allow averaging on PSV; however, the number of sites may be too low to make this interpretation solid. Geomagnetic field models

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were used by Deenen et al. (2011) to determine A95 envelopes for VGP populations. If the A95

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value calculated for a mean VGP is between the A95min and A95max values predicted by the

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geomagnetic field models, then the scatter observed in the VGP population is consistent with PSV and well averages the latter. In sites LI05, LI07, PM31, A95 is below A95min (Deenen et al. 2011) implying that PSV is not perfectly sampled. Still the result could be sufficiently

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reliable for deriving geological implications. Using the method of McFadden (1980) to calculate the precision parameter k’ at the latitude of our sampling sites (~26°N) yields

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k’=28.5. Our k-values of most sites are above this value, but the site mean directions and the mean of all specimens are in the range of k’. According to these criterions, PSV is almost

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averaged out in the overall mean directions of our sampled dykes. Liebke et al. (2012) modeled the cooling time for mafic dykes with a thickness of 10 m intruding into a

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sedimentary rock. Their results revealed cooling times of maximum 500 years (until the dyke has reached a temperature of 100°C) that is clearly too short for averaging on PSV. Liebke et

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al. (2012) further discussed that multi-phase intrusions could improve the final result by partial averaging of PSV within sites.

Figure 10 somewhere here

6.2 Anisotropy of magnetic susceptibility

Measurements of the anisotropy of magnetic susceptibility (AMS) were performed for all 12 dyke sites (Fig. 10) to assess the possible influence of anisotropy on remanence directions;we used the statistics proposed by Jelinek (1978) for processing of the AMS results. Most sites show a good grouping of the principle axes, i.e. a triaxial susceptibility-ellipsoidshape, and in 17

ACCEPTED MANUSCRIPT part of the sites the kmax and kint axes occupy a great circle (Fig. 10). The kmin directions are close to the horizontal in most sites andcommonly are close to the poles of the dyke-granite

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interfaces (Fig. 10), thus supporting a sub-vertical dyke intrusion. The corrected degree of

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magnetic anisotropy is relatively high (P’ = 1.02–1.25). However, because the kmin directions

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spread within their sub-horizontal alignment, and also kmax and kint directions scatter clearly, we infer that anisotropy has a negligible influence on the overall mean directions.

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Figure 11somewhere here

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7 Tectonic implications

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As outlined above, block rotation due to crustal movement around the EHS is the most

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probable cause for the large clockwise deflections with respect to the expected declination of

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stable Eurasia. The magnitude of block rotation can be calculated by comparing the site mean directions of Group1 and Group2 to the expected Earth magnetic field directions at the time of remanence acquisition.Thus, what are the new whole-rock

40

Ar–39Ar dyke dates and the

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published geochronology telling us about the time of remanence acquisition? Group1 remanence directions are likely of primary origin, carried by Ti-rich titanomagnetite and magnetically stable magnetite; Group2 remanence directions are likely of secondary origin and due to low-temperature oxidation of magnetite and alteration of Ti-rich titanomagnetite (see section 6.1 above). As Ti-rich titanomagnetite is a stable igneous phase, preserved only in rapidly cooling igneous rocks, the fact that most of the dyke ‘ages’ and the Cenozoic regional magmatism cluster between ~50–35 Ma suggests that the Group1 remanence was acquired during dyke crystallization and does not represent a metamorphic overprint. The metamorphic alteration, clear from both petrographic and rock-magnetic evidence (≤ 400°C, see section 5.2), likely causes the scatter of the apparent ages and the K/Ca ratios. With the given scatter 18

ACCEPTED MANUSCRIPT in the apparent ages, the question of whether dyke emplacement was a single, short-lived episode or a prolonged phase is impossible to address. Given that the possible emplacement

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period spanned ~50–35 Ma, the magnetizations of individual sites could have been acquired

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over ~15 Myr, clearly affecting the tectonic interpretation of the declinations. We accounted

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for this effect in the following ways: First, we grouped the Group1 directions together and assigned to them a single remanence acquisition age at ~40 Ma. We think that this is justified for the derivation of general conclusions, given that no other Cenozoic remanence directions

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exist for this area, the complexity of the age assignment has been discussed on the base of

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detailed geochronologic–rock magnetic studies, and the tectonic interpretations reflect these uncertainties. Second, we calculated bulk rotations and rotation rates also for remanence

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acquisition ages at the likely upper and lower bounds of acquisition, i.e. ~50 and 35 Ma.

From the dyke geochronology itself, the timing of Group2 remanence is even worse than of

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Group1. However, the regional cooling form greenschist-facies metamorphism peaking at ~16 Ma (Fig. 7b) suggests that the metamorphic oxidation of magnetite and alteration of the Ti-

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rich titanomagnetites occurred in the Oligocene–Miocene. The time span over which the lowgrade metamorphism was attained probably explains the wide scatter in the directions from Group2 dykes and the granites. Again, for our tectonic interpretations, we use an age of Group2 remanence acquisition, with ~30 and 10 Ma as likely upper and lower bounds.

We use the new apparent polar wander path (APWP) for East Asia of Cogné et al. (2013) to determine the expected field directions (D/I = 2.5°/33.4° for ~40 Ma; 5.7°/35.1 and 2.4°/34.8° for ~50 Ma and ~35 Ma, respectively; D/I = 2.1–1.0°/36.2–41.1° for ~30–10 Ma) based on the present-day plate configuration. The overall tilt corrected mean direction of Group1 (D/I = 89.8°/35.1°) implies a 87.3 ± 13.2° clockwise rotation of the Tengchong block since remanence acquisition at ~40 Ma (84.1° ±16.1° and ~87.4° for ~50 M and ~35 Ma, 19

ACCEPTED MANUSCRIPT respectively; Fig. 11c). Group2 sites most likely recorded only part of this rotation due to the later magnetic overprint(31.2° ± 33.1° and 32.3° ± 33.1° for ~30 Ma and 10 Ma,

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respectively).Our upper bound for the Group2 remanence acquisition (~30 Ma) coincides with

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the likely age of the pervasive metamorphic overprint of basalt layerswithin Late Paleozoic

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strata from the Baoshan block east of the Pianma-Nujiang section (Kornfeld et al., in press).Thus, we combined the Group2 remanence directions of the Pianma-Nujiang dykes with the ~30 Ma remanence directions of the basaltic-layers from the Baoshan block (Fig.

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11b);this combined remanence direction results in an overall in-situ mean direction of D/I = 40.8°/38.6° (k = 12.6; α95 = 10.1°).The tilt corrected overall mean direction is D/I =

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41.5°/46.6° (k = 34.1; α95 = 7.2°), thus implying a 39.4 ±8.0° clockwise rotation since ~30 Ma. Confidence limits for the determined block rotation values were calculated from ΔDobs-

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D

and ΔDexp (95% confidence angles of the observed and expected declinations with ΔD=

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α95/cos I), using the error propagation law.

Large scale clockwise rotation around the EHS is indicated from several paleomagnetic

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studies. Chen et al. (1995) reported rotations of ~77° for the Lanping-Simao block since the Eocene–Oligocene. Sato et al. (2001) determined rotational magnitudes of >90° since the Eocene for the Lanping-Simao block, suggesting that the magnitude of clockwise rotation increases eastwarddue rotation accumulation across faults.For the Shan-Thai block, Tanaka et al. (2008) proposed a tectonic evolution with a first rigid-body clockwise rotation of ~20° during the initial stages of the India-Asia collision, and—after southward displacement of the block—localized clockwise rotations of >30°. They concluded that in the first stage the rotation was rapid, changing thereafter to slow and steady, with the rotational motions still ongoing today.

20

ACCEPTED MANUSCRIPT The present-day rotation rate around the EHS determined from Quaternary fault-slip rates and GPS velocities (Holt et al., 2000; Zhang et al., 2004; Shen et al., 2005; Gan et al., 2007;

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Allmendinger et al., 2007) isin the range of >2°/Myraround the EHS and is decreasing to

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<2°/Myr in the vicinityofthe syntaxis (generally 1.14–2.4°/Myr; Otofuji et al., 2010). From

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our paleomagnetic results, we can derive the Cenozoic ‘paleo’-rotation rates for the Tengchong block:therates since ~40 Ma, ~50 Ma, and ~35 Ma were 2.18 ± 0.33°/Myr, 1.68 ± 0.32°/Myr, and ~2.50°/Myr, respectively; the rates are at the upper limit of the present-day

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rotation rate. The rotation rates since ~30 Ma and ~10 Ma are1.31 ± 0.27°/Myr and 3.23 ±

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3.31°/Myr, respectively, thus—at least since ~30 Ma—at the lower limit of the present-day rotation rate.Overall, our results indicate a strong clockwise rotation of crustal material around the EHSearly in the India-Asia collision (e.g. 48.3 ± 15.4°, corresponding to a rotation

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D

rate of 4.83 ± 1.54°/Myrbetween ~40and ~30 Ma), whereas the later rotation was of smaller magnitude and consistent with present-day GPS rotation rates(e.g. 39.4± 11.1°, corresponding

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to a rotation rate of 1.31 ± 0.37°/Myr between ~30 Ma and today).

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Which tectonic framework of the evolution of the EHS region describes the large clockwise rotation of the Tengchong block best? The timing of the ‘tectonic escape’ scenario has mostly been developed from work along the Ailao Shan–Red River shear zone (Fig. 1a). Th–Pb dating of monazite inclusions in garnets, U–Pb zircon dating of granitoids that were interpreted to have formed pre- and syn-kinematic, and

40

Ar-39Ar dating of syn-kinematic

mica indicate that amphibolite-facies metamorphism, magmatism, and shearing occurred between >34 and ~17 Ma (e.g. Leloup et al., 2001; Gilley et al., 2003; Cao et al., 2011; Lu et al., 2012). Zones, conjugate to the Ailao Shan-Red River, that accommodated dextral strikeslip shear in the same time window have, however, not been detected so far. The dextral GSSZ was likely active between ~20-11 Ma, and thereafter shearing was accommodated along the Sagaing fault (e.g. Socquet and Pubellier 2005; Akzic et al., 2008; Lin et al., 21

ACCEPTED MANUSCRIPT 2009).The onset of ‘crustal flow’ is more difficult to determine: Normal slipalong low-angle, normal-shear, ductile detachments, bounding metamorphic core-complexes and associated

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conjugate strike-slip zones, and by the emplacement of dykes and tension veins date a change

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in the state of stress within the Tibetan Plateau, probably related to gravitational spreading

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(Dewey, 1988; England and Houseman, 1989; Ratschbacher et al., 1994; Copley, 2012), stronger coupling between Indian and Asian lithosphere (Copley et al., 2011) or lower crustal flow (Bird 1991; Royden et al., 1997). Low-temperature cooling ages from the Longmen

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Shan (e.g. Kirby et al., 2002; Godard et al., 2009) were used to infer the onset of shortening

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and uplift due to crustal flow along the eastern margin of the Plateau.Both extension within the Tibetan Plateau and shortening along its eastern margin likely started at ~17–15 Ma; the currently active ‘neotectonic’ deformation began at ~5 Ma, with acceleration of slip on pre-

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existing shear zones and the formation of the active ~NNE-trending graben systems in the southern and central Tibetan Plateau (e.g. Taylor et al., 2003; Dewane et al., 2006; Mahéo et

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al., 2007; Sanchez et al., 2010; Ratschbacher et al., 2011; Styron et al., 2013).

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Based on our results, we propose the following scenario for the EHS area of the Tibetan Plateau. After the initial India-Eurasia collision, the eastern part of the Gangdese belt already started to escape towards the EHS at ~50–40 Ma(Fig. 12a). This is consistent with significant E–W lengthening of the crust involved in the post-Eocene N–S shortening across the southern margin of the Lhasa Block and reflecting bulk orogen-parallel stretching facilitated by a weakly constrained lateral margin (e.g. Ratschbacher et al., 1992). We suggest that the Pianma-Nujiang section was located west of the EHS, i.e. north of the Indian plate margin, before the collision (Fig. 12a), moved to the east of the EHS at ~40 Ma, rotating clockwise around the EHS (Fig. 12b). ~Thus,theGroup1remanence records the total rotation imposed by the indentation of the EHS;a significantly higher total magnitude of rotation than the observed one (~87°) is unlikely. In the second stage, a slower rotation with the same magnitude of 22

ACCEPTED MANUSCRIPT present-day GPS velocities is indicated, probably related to crustal flow where the upper crust is decoupled from the upper mantle due to a weak lower crust (Royden, 1997; Clark and

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Royden, 2000).

Conclusions

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8

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Figure 12somewhere here

(1) We separated two groups of characteristic remanences in the mafic dykes and granitoids

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of the Pianma–Nujiang section of the Tengchong (Lhasa) block in western Yunnan. The Group2 lower coercive component, identified in most dykes and constituting the component

D

identified in the granites, iscarried by magnetite.This component likely is of secondary origin,

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acquired during a metamorphic overprint. The Group1 higher coercive component is carried

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by Ti-rich titanomagnetite and magnetically more stable magnetite, and likely represents a primary remanence acquired during emplacement of the dykes. (2) Based on five new and one published 40Ar–39Ar whole rock ages of the mafic dykes from

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the Pianma-Nujiang section that all show strong hydration due to greenschist-facies metamorphism, and a compilation of the ages of magmatism, metamorphism, and cooling in the Tengchong block, we suggest that the mafic dykes were emplaced at ~50-35 Ma, most likely at ~40 Ma, and that the hydration occurred between ~30–10 Ma. All granitoid host rocks of the mafic dykes in the Pianma–Nujiang section crystallized at ~115 Ma (one published and three new U–Pb zircon dates). (3) A clockwise rotation of ~87° (87.3 ± 13.2°)of the Tengchong block occurred since remanence acquisition in the dykes at ~40 Ma.The average rotation rate since ~40 Ma (2.18 ± 0.31°/Myr) is at the upper limit of the present-day GPS-derived rates around the EHS. The Group2 remanence directions, combined with remanence directions derived from 23

ACCEPTED MANUSCRIPT remagnetized basalt-layers in the Paleozoic strata of the Baoshan block east of the PianmaNujiang section,indicate a rotation of ~39° (39.4 ± 8.0°) since ~30 Ma. Consequently, our

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results imply a strong clockwise rotation of 48° (48.3 ± 15.4°) between ~40 and ~30 Ma,

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corresponding to a rotation rate of 4.83 ± 1.54°/Myr.These rates vary depending on the exact

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ages of the dyke emplacement (~50–35 Ma; Group1 remanence directions) and metamorphic overprint (~30–10 Ma; Group2 remanence directions) but the overall conclusion of a rapid early rotation during the first phase of the India–Asia collision and slower rotation later on,

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remains valid.

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(4) We explain the early rapid clockwise rotation of the Tengchong block by rigid block rotation synchronous with tectonic escape of lithospheric blocks during the northward push of the Indian plate. The slower clockwise rotation of the same magnitude as the present-day GPS

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velocities can be related to viscous flow of Tibetan crust around the EHS.

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Appendix 1 Geochronologic methods

Ar–39Ar Geochronology

40

Ar–39Ar geochronology was conducted at the Argon laboratory at TU Bergakademie

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40

Freiberg (ALF; Pfänder et al., 2010). The whole-rock samples were broken to <1 mm fragments and handpicked under a binocular microscope to select fresh material. The fragments were then ultrasonicated in alcohol and repeatedly cleaned in de-ionized water to remove the clay fraction, dried, and subsequently wrapped into Al foil. These sample packets were loaded in 5 × 5 mm wells on ~30 mm Al-discs for irradiation, which was done without Cd shielding at the LVR-15 research reactor in Rez, Czech Republic, at a thermal neutron fluence of ~4.8×1013 n/cm2s and a thermal to fast neutron ratio of ~2.1. Irradiated samples were unwrapped and loaded into small chutes of an autosampler system that allows sample 24

ACCEPTED MANUSCRIPT transfer to the high-temperature cell (HTC) without wrapping. Step heating was performed using a Createc® high-temperature cell as furnace; temperature control was managed by

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anEurotherm® 3504 controller. Ramp time was 3 minutes per step at a heating rate of

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100°C/min, heating time per step was 7 minutes. Gas purification was achieved within 10

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minutes using two AP10N getter pumps, one at room temperature and one at 400°C. Arisotope compositions were measured in static mode using a GV Instruments ARGUS noble gas mass spectrometer equipped with five Faraday cups and 1012 ohm resistors on mass

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positions 36–39 and a 1011 ohm resistor on mass position 40. Typical blank levels are

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<2.5×10-16mol40Ar and <8.1×10-18mol36Ar. Measurement time was 7.5 minutes per step acquiring 45 scans at 10 seconds integration time each. Mass bias was corrected assuming linear mass dependent fractionation and using an atmospheric

40

Ar–36Ar ratio of 295.5. For

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data handling and raw data reduction an in-house developed MATLAB®software packageassociated with a MySQL database system was used; isochron, inverse isochron, and

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plateau ages were calculated using ISOPLOT (Ludwig, 2008). All ages were calculated using Fish Canyon sanidine as a flux monitor (28.305 ± 0.036 Ma; Renne et al., 2010). Errors on

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ages are 1σ.Decay constants used are those given in Renne et al. (2010). Corrections for interfering Ar isotopes were done using (36Ar/37Ar)Ca= 0.000245, (39Ar/37Ar)Ca = 0.000932, (38Ar/39Ar)K = 0.01211, (40Ar/39Ar)K = 0.00183and applying 5% uncertainty.

U–Pb geochronology

We performed U–Pb zircon geochronology on grain mounts; multiple grains and several spots within some grains were analyzed in each sample. Grain liberation employed high-voltage pulse power fragmentation in the TU Bergakademie Freiberg SELFRAG ® facility (specifications see http://selfrag.com). Final separation was by magnetic, heavy liquid, and optical methods; grains were mounted in resin blocks and all grains were inspected by optical 25

ACCEPTED MANUSCRIPT microscopy and SEM-based cathodoluminescence before analysis. The U–Pb analyses were conducted

on

zircon

by

the

LA-ICPMS

method

at

the

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SenckenbergNaturhistorischeSammlungen Dresden, Germany.Zircons were analyzed for U,

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Th, and Pb isotopes by a Thermo-Scientific Element 2 XR sector field ICP-MS coupled to a

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New Wave UP-193 Excimer Laser System. Each analysis consisted of approximately 15 s background acquisition followed by 30 s data acquisition, using a laser spot-size of 20 to 35 µm. A common-Pb correction based on the interference- and background-corrected

204

Pb

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signal and a model Pb composition (Stacey and Kramers, 1975) was carried out if necessary. 207

Pb/206Pb lies outside of

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The necessity of the correction is judged on whether the corrected

the internal errors of the measured ratios. Raw data were corrected for background signal, common Pb, laser induced elemental fractionation, instrumental mass discrimination, and

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time-dependent elemental fractionation of Pb/Th and Pb/U using an Excel® spreadsheet program developed by Axel Gerdes (Geosciences, Johann Wolfgang Goethe-University,

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Frankfurt am Main, Germany). Reported uncertainties were propagated by quadratic addition of the external reproducibility obtained from the standard zircon GJ-1 (~0.6% and 0.5-1% for 207

Pb/206Pb and

206

Pb/238U, respectively) during individual analytical sessions and the

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the

within-run precision of each analysis. For further details on analytical protocol and data processing see Gerdes and Zeh (2006). Th/U ratios are obtained from the LA-ICP-MS measurements of investigated zircon grains. U and Pb content and Th/U ratio were calculated relative to the GJ-1 zircon standard and are accurate to approximately 10%.

Acknowledgments

Thisstudy was funded by the German Research Foundation (DFG) and is part of the Priority Program 1372 ‘Tibetan Plateau: Formation, Climate, Ecosystems (TiP)’. Thework was also supported by grants from the Chinese Academy of Sciences (XDB03010401) and the Chinese 26

ACCEPTED MANUSCRIPT Ministry of Science and Technology (2011CB403101). We thank Hartmut Schulz for support in the EDX/SEM analyses and Udo Neumann for support in reflected light microscopy. 40

Ar–39Ar geochronology, and Mandy Hofmann and Uwe

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BlankaSperner contributed to the

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Linnemann to U–Th–Pb analysis. The manuscript benefited from a pre-submission review by

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Peter Lippert.

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ACCEPTED MANUSCRIPT Figure 2. a) Simplified map of the Tibetan Plateau around the Eastern Himalayan Syntaxis. The study area is marked by the blue rectangle. Red arrows show present-day GPS velocities relative to Eurasia (after Gan et al., 2007); purple arrows indicate selected paleomagnetic declination data (after Otofuji

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the sampled granitoids (see Tables 1 and 2 for exact coordinates).

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et al. 2010). b) Simplified geologic map of the Pianma–Nujiang transect, showing the mafic dykes and

Figure 2. Field photographs of two of the sampled dykes are shown in a) LI05 and b) LI08. Stereoplot of dyke-granite interfaces from the Pianma-Nujiang section (left) and from dykes of the Lhasa block

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(right) in c).

Figure 3. Transmitted light microscopy photographs showing the matrix of dyke sites LI06 in a) and

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LI03 in b). In c) bright field and d) crossed polarizers, the undeformed matrix of one granite sample is shown.

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Figure 4. a) Reflected light microscopy photograph showing a grain with maghemitization and

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shrinkage cracks; b) a non-altered (titano-)magnetite; and c) a strongly altered particle (SEM picture).

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d) EDX results of a linescan of the particle in c); it reveals Ti-rich titanomagnetite in the particle’s interior while the rim has been altered to ilmenite and rutile. Figure 5. a) Magnetic susceptibility (κ) versus temperature curves of rock powders from sites LI05

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and LI08 (dykes) and PM26 (granite); low- and high-temperature curves at left and right, respectively. b) κ-T curves acquired by stepwise heating (300°C, 400°C and 600°C) of samples LI06 and LI07 (heating: full line, cooling: dashed line). c, d) IRM acquisition curves and thermal demagnetization of SIRM for representative specimens of the mafic dykes (c) and one granite (d); MMTD furnace inaccuracy causes the shift to higher temperatures in (c). Results of cumulative log-Gaussian analysis (Kruiver et al., 2001) of LI08 (c) and PM30 (d) (clow: lower coercivity component, chigh: higher coercivity component). Figure 6. a) Day-Plot (Day et al. 1977, Dunlop & Özdemir 1997) of one sample from each site. b) Hysteresis loops of two representative specimens (LI03-3-3, LI05-1-4); dashed line: no correction, solid line: corrected for paramagnetic fraction at 70% of Hmax. Mrs: saturation remanence, Ms: 48

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Figure 7. a) Compilation of the Cretaceous and Cenozoic U–Th–Pb dates from the Mogok belt of

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east-central Myanmar, an equivalent to the Gaoligong Shan and Tengchong area, and the Chong Shan,

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a narrow belt of tectonites of magmatic and metamorphic origin just east of the Gaoligong Shan; the Pleistocene–Recent volcanism of the Tengchong area is not included. Data are shown as histogram, Kernel density estimate (blue), and probability density plot; the data are plotted as circles below the

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diagram (calculated with the densityplotter, Vermeesch, 2012). Cretaceous magmatism clusters at ~125 and 75–60 Ma; magmatism and related high-temperature metamorphism occurred throughout the

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Cenozoic, with groups at ~50–30 Ma and ~20 Ma. b) Compilation of mica cooling ages obtain from 40

Ar–39Ar thermochronology from the Gaoligong and Chong Shan. c)

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results of mafic dykes of the Pianma–Nujiang section, displayed as plateau age spectra and

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corresponding inverse isochron diagrams. d) Zircon U–Pb Concordia diagrams of granitoid samples from the Pianma–Nujiang section of the Gaoligong Shan, Th/U ratios, and Concordia ages of selected

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data groups. e) Compilation of the new and published

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Gaoligong Shan and Tengchong area.

Figure 8. Intensity curves, Stereoplots and Zijderveld diagrams (in geographic coordinates) of alternating field demagnetization for representative specimens of a) LI03-5-3, c) LI05-2-3, e) LI08-5-2 (all dykes) and g) PM36-2-1 (granite); in b) LI03-1-3, d) LI05-1-2, f) LI08-8-2 (all dykes) and h) PM30-31 (granite) of thermal demagnetization.

Figure 9. Equal area projection in geographic coordinates of site mean directions of a) all LCC with outliers omitted. b) HCC showing the Group2 (remagnetized dykes, blue circles; granite, green circles) and Group1 (pink circles) site mean directions. Small circle paths of remanence directions for tilting in different directions (10° increments, tilting directions indicated at the margin) are shown. Overall site mean values (crosses) and the α95 confidence angles are displayed in different colors: all sites (black), Group2 (green), Group1 (pink). 49

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Figure 10. AMS principal axes of all dykes. k1=maximum, k2=intermediate and k3=minimum

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susceptibility directions. Solid great circle: Site specific dyke-granite contact interface.

Figure 11. a) Equal area projection showing the measured contact interfaces between dykes and the

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granite host (black for each site, red for the mean); the corresponding orientation for back-tilting of the overall mean direction is marked in blue. Equal area projection in geographic coordinates of site mean

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directions of b) HCC of Group2 dykes and granites and c) HCC of Group1 dykes. The horizontal small circles mark the α95 confidence range of the expected reference field (full lines, calculated by the

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APWP of Cogné et al. (2013)), for ~30 Ma (b) and at ~40 Ma (c), respectively. Overall mean directions (crosses) and the α95 confidence angles are shown in b) and c) for Group2 and Group1.

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Figure 12. Cartoon explaining the clockwise rotational movement around the EHS. (a) The Pianma-Nujiang section occupies an initial position to the west of the EHS, then (b) escaped to the east in the first stage of the India-Asia collision followed by a rapid rigid block rotation around the EHS between ~–30 Ma. The smaller, slower rotation thereafter is shown in c). The red arrows show the movement and rotation of the Pianma-Nujiang section rocks (black ellipse).

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Table 2. Summary of 40Ar/39Ar data.

YU9419H1 (~LI 07) YU9409E1 (~LI 05)

98.704 25.97 97 755 98.684 25.97 22 095

2668 2873

wrbasalt wrbasalt

45 44. 82

44. 82 48. 96

TFA (Ma) 55.97 ± 0.24 37.64 ± 0.09

21.23 ± 0.30 40.62 ± 0.07

WMA

n.a. 24.25 ± 0.28 30.11 ± 0.43 n.a.

36.19 ± 0.53 n.a.

98.684 25.97 22 095 98.697 25.97 9 847

2873 2949

wrbasalt wrbasalt

47. 7 49. 6

34.54 ± 0.17 42.88 ± 0.07

n.a.

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YU9409E2 (~PM29) YU9409F (~PM 27)

Isochro n (invers) 62.5 ± 2.3 25.3 ± 1.3 30.12 ± 0.16 27.66 ± 0.96 37.39 ± 0.53 38.6 ± 1.0 48.6 ± 2.0 45.11 ± 0.43

42.87 ± 0.58

MS WD 2.6

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0.24

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YU9409A (~PM34, PM35) YU9409G (~LI 07)

Longit Latitu Elevat Rock ude de ion (°E) (°N) (m) 98.677 25.98 wr2827 47 09 basalt 98.701 25.97 wr2784 08 697 basalt

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Sample

0.39 0.95 1.8 1.3 5.1 1.7

40/36 290 ± 1 292 ± 4 296 ± 3 291 ± 1 293 ± 1 432 ± 19 286 ± 2 280 ± 2

% Ar

Steps

42

3-13 (26)

29

8-14 (29)

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1-4, 6, 1619 (29)

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7-17 (29)

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8-15 (29)

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21-25 (29)

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8-22 (29)

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MSWD is the mean square weighted deviation, which expresses the goodness of fit of the isochron. Isochron and weighted mean ages (WMA) are based on fraction (%) of 39Ar and steps listed. N.a.: not applicable. Wr: whole rock. Preferred age interpretation: YU9409A basalt, emplacements age confined between ~60 and ~85 Ma, best 63 ± 10 Ma, overprint at ~45 Ma; YU0909G basalt, 25.0 ± 1.0 Ma and 30.1 ± 0.5 Ma; YU9419H1 basalt, mixing between atmosphere (Atm.) and a radiogenic component at ~40 Ma, and Atm. and a radiogenic component at 27.7 ± 1.5 Ma; YU9409E1 basalt, 38 ± 2 Ma; YU9409E2 basalt, 49 ± 3 Ma; YU9409F basalt, 45 ± 3 Ma.

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Table 2. Site mean values and statistical parameters of the paleomagnetic results. Site location In Situ

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Site

Lat. (N)

Long. (E)

NRM (mA/ m)

NRM/S usc. (A/m)

Dec. (°)

C

k

α95 (°)

4.3

28.4

92.5 35.7

8.0

Inc. (°)

N/N incl. (n/r)

IRM Comp. Com 1 Contri p. 2 B1/2 b. (%) B1/2 [mT] [mT]

Contri b. (%)

Dyke LCC

LI 03

25.950 98.750 64 31

116

LI 04

25.951 98.744 31 69

12

LI 05

25.973 98.678 56 96

697

24.1

LI 06

25.971 98.699 31 69

299

10.0

LI 07

25.977 98.704 56 75

625

32.6

LI 08

25.958 98.722 69 19

380

11.8 LCC

31.5

34.6

30.3

88

56.3

100

26.9 18.0 HCC 266.4 49.7 32.7

22.3

7/4 (4/0)

9.6

7/6 (0/6)

LCC

IP

HCC 226.4

11/9 (9/0) 11/5 (0/5)

17.2

37.8 16.6

23.2

HCC 271.7

50.2 23.5

13.1

8.6 24.2

HCC

36.4

LCC

42.9

HCC

45.9

LCC

5.4

10/4 (4/0) 10/4 (0/4)

157.6

12

35.5

99*

50.6

100

27.7

98*

30.9

66

103.9

34

55.4 51.3 275. 32.1 8

9.4

7/6 (6/0)

3.6

7/7 (7/0)

32.7 16.7 28.1 23.8

16.9

9/6 (6/0)

9.9

9/9 (0/9)

28.5

7.5 340. 6

23.7

8/7 (7/0)

3.0

8/8 (0/8)

10.6

LCC 360.0 16.7 8.2 HCC 271.7 -6.1 86.7

25.0 6.0

8/6 (6/0) 8/8 (0/8)

28.1

73

101.0

27

NU

LCC

T

16

HCC 256.0 -0.2

MA

LI 02

340.6 40.6

SC R

25.956 98.771 33 03

PM 27

25.975

98.695 7

693

15.8

LCC

43.5 33.4

9.0

10/9 (9/0)

25.2

94

90.5

6

PM 29

25.981 98.700 2 9

352

9.7

LCC 10.9 38.7 21.3 HCC 260.2 -5.5 30.9

14.9 9.4

9/6 (6/0) 9/9 (0/9)

26.3

44

81.3

56

PM 31

25.952 98.744 1 6

166

10/8 (8/0) 10/10 (0/10)

21.5

67

58.3

33

PM 34+

25.980 98.677 8 4

294

79.6

26

PM 30

18.6

39.2 20.5

12.5

HCC 261.9

90.8 20.8

5.1

LCC

11.3

20.1 55.4

10.4

6/5 (5/0)

41.4

100

LCC

303.9

15.0 42.4

20.5

9/5 (5/0?)

35.2

74

50

LCC

26.4

36.8 14.6

18.1

9/6 (6/0)

29.1

100

10

LCC HCC

0.5 57.4

28.4 32.8 3.5 29.5

16.3 23.1

9/4 (4/0) 7/3 (3/0)

52.6

100

8.9

CE P

25.978 98.697 6 7

D

TE

LCC

6248

AC

PM 25.980 98.677 35+ 8 4 Grani te PM 98.650 26.014 26 2

0.3

10.5

PM 25.965 98.773 71 LCC 14.8 27.0 37.8 12.6 6/5 (5/0) 28.3 100 36 5 8 + : sites from the same dyke, Lat.: latitude, Long.: longitude, NRM: mean natural remanent magnetization, Susc.: susceptibility, C: components (LCC low coercivity comp., HCC high coercivity comp.), Dec.: declination, Inc.: inclination, k: precision parameter, α95: 95% confidence angle, N/N incl (n/r).: number of specimens measured/ included in statistics (normal/reverse polarity); IRM: isothermal remanent magnetization (decomposed by cumulative log-Gaussian distributions after Kruiver et al., 2001), Comp.: component, Contrib.: contribution, *: samples may contain 1-2% of a higher coercitivity phase (B1/2>1T).

52

ACCEPTED MANUSCRIPT

Inc. (°)

k

α95 (°)

α95/cos I (°)

N incl.

Mean LCC (excl. outliers LI06, PM35): Mean HCC (* excl.Granites): Mean HCC:

9.2 71.8 62.8

34.9 15.9 18.7

25.3 8.6 7.0

9.3 18.6 17.6

11.3 19.3 18.6

11 9 12

Group1 HCC: Group2 HCC: Group1(overall mean direction tilt corrected): Group2 (overall mean direction tilt corrected):

84.5 38.5

14.9 20.0

33.0 8.4

11.8 24.5

12.2 30.7

6 6

89.8

35.1

33.0

11.8

14.4

33.3

41.4

8.4

24.5

32.7

SC R

IP

Dec. (°)

T

Table 3. Overall mean directions.

AC

CE P

TE

D

MA

NU

Dec.: declination, Inc.: inclination, k: precision parameter, α95: 95% confidence angle, α95/cos I is the 95% confidence limit of the declination, N incl.: number of specimens included in statistics, *sites excluded because of low k.

53

ACCEPTED MANUSCRIPT Highlights:

CE P

TE

D

MA

NU

SC R

IP

T

Rotation of Tibetan crust around the Eastern Himalayan Syntaxis. Obtaining time constraints for rotation of the Tengchong block. Strong clockwise rotation within a short time span. Rotation of the Tengchong block due to tectonic escape.

AC

   

54