Structural constraints on the timing of left-lateral along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China

Michael P. Searle Department of Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Meng-Wan Yeh Center for General Education, National Taiwan Normal University, Taipei, Taiwan Te-Hsien Lin Sun-Lin Chung Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

ABSTRACT before a signifi cant phase of tight to isoclinal rotation and continental extrusion of Indochina– folding. Ductile, left-lateral strike-slip shear SE Asia as a result of the collision and inden- The >1000-km-long Oligocene—Miocene fabrics were superimposed on all tation of India into Asia (Molnar and Tap- left-lateral Red River shear zone (RRSZ) and at high (~500–550 °C) for the ponnier, 1975; Tapponnier et al., 1986, 1990, metamorphic belt and the Pliocene—active ALS and lower (~250–150 °C) 2001; Peltzer and Tapponnier, 1988; Leloup et right-lateral Red River (RRF), stretch- after peak metamorphism and after granite al., 1993, 1995, 2001). The active right-lateral ing from SE Tibet to the South China Sea, intrusion. A few very small biotite (±Grt ± RRF has been mapped along the northeastern has been cited as one of the primary exam- Tur) leucogranite veins and dykes crosscut margin of the Ailao Shan (in places also called ples of a lithospheric scale strike-slip fault the ductile strike-slip shear fabrics at Yuan- the “Mid-Valley fault”) or the Yuanjiang Fault that has resulted in syn-kinematic metamor- jiang, in the Ailao Shan. Low-angle normal (Leloup et al., 1995) and shows a reversal of the phism and partial melting and accommo- faulting along the margins of the metamor- earlier left-lateral fabrics within the Ailao Shan dated several hundred to a thousand kilo- phic massif accommodated fi nal exhumation massif (Allen et al., 1984; Leloup et al., 1995; meters of horizontal motion as a result of the of the Red River . Using published Replumaz et al., 2001; Schoenbohm et al., 2005, indentation of India into Asia. Alternatively U-Th-Pb and 40Ar/39Ar ages of granites along 2009). However, both the active Mid-Valley and we interpret the metamorphic complexes the shear zone, the age of left-lateral ductile Yuangjiang Faults occur in the heavily veg- along the RRSZ as exhumed metamorphic shearing along the RRSZ can be constrained etated Red River valley, and Schoenbohm et al. core complexes of older rocks, subsequently as between the earlier deformed leucogran- (2009) could fi nd no evidence for the existence affected by Oligocene–Early Miocene left- ites (31.9–24.2 Ma) and the later crosscutting of the Yuangjiang Fault. lateral shear and localized partial melting dykes (21.7 Ma) with exhumation-related The Red River fault extends from the upper (leucogranite dykes), Miocene low-angle cooling continuing until ~17 Ma. reaches of the Mekong River north of the East- normal faulting along margins (Range Front ern Himalayan syntaxis, southeastward through faults), and Pliocene active dextral strike-slip INTRODUCTION the Three Gorges regions into Yunnan (Fig. 1). faulting (RRF). Along the Ailao Shan (ALS) Some authors (Tapponnier et al. 1982, 1986, and Diancang Shan (DCS) ranges in Yunnan, The Ailao Shan–Red River (ASRR) meta- 1990; Leloup et al. 1993, 1995, 2001) proposed SW China, early amphibolite facies meta- morphic belt forms a series of NW-SE aligned that the Red River fault includes all the different morphic rocks were intruded by K- metamorphic complexes (Xuelong Shan, Dian- strands along its trace, whereas others (Wang orthogneisses of Triassic age (Indosinian). cang Shan, Ailao Shan in Yunnan, and the and Burchfi el, 1997; E. Wang et al., 1998; LA-ICP-MS U-Pb zircon dating reveals a DayNuiConVoi [DNCV] complex in North Burchfi el et al. 2008) proposed that there is no complex history with zircon cores show- Vietnam) showing Oligocene–Early Miocene link between the southern segment, south of the ing evidence of Indosinian (~239–243 Ma) metamorphism and left-lateral shearing aligned “Midu Gap” (Ailao Shan and SE into North to Neoproterozoic magmatism. Zircon rims along the Late Pliocene–active, right-lateral Vietnam), and the northern segment (Diancang show an Oligocene (~26 Ma) magmatic or Red River Fault (RRF). Both the earlier left- Shan and to the NW). Burchfi el et al. (2008) metamorphic overprint. Biotite granodio- lateral shear zone and the active right-lateral mapped three separate belts of mylonitic rocks rites and syenites of mantle origin intruded fault have been interpreted as a 1000-km-long, in southern Yunnan and Vietnam: the Ailao Shan, the gneisses during the Oligocene (~35 Ma). lithospheric-scale strike-slip fault that resulted a middle belt and the DNCV sepa- Later biotite leucogranites intruded the in syn-kinematic metamorphism and leuco- rated by zones of weakly to un-metamorphosed orthogneisses and migmatite host rocks granite melting, and accommodated clockwise Palaeozoic-Triassic sedimentary rocks. E. Wang

Geosphere; August 2010; v. 6; no. 4; p. 316–338; doi: 10.1130/GES00580.1; 14 fi gures; 1 table.

316 For permission to copy, contact [email protected] © 2010 Geological Society of America

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90TIBET 92 94 96 98 100 Gangdese batholith LHASA Namche Barwa Gangdese batholith (105- c.50Ma) BLOCK 30 Syntaxis e Yarlung Tsangpo zone Indus Tsa sutur Mishmi hills ngpo NB Jiale fault Mesozoic shelf margin sediments Puqu fault Lohit thrust (+ upper Pz)

Hkakabo High Himalaya metamorphic rocks Bhutan STD Razi Lesser Himalaya Arunachal 5881 28 MCT HIMALAYA Bhramaputra basin Putao Tertiary molasse sediments INDIAN PLATE Lesser Himalaya ver Shillong plateau PreCambrian Ri Xuelong tra Gaoligong Shan Shan MBT apu Naga hills am Mount Guwahati Bhr Assam Jade Mines Loimue D ia Dali nc 26 an 26 Shillong g Sh Myitkyina an Fig.2

MVK Taung B’DESH Manipur Thonlong Bhamo GS s lt A e u ila g KGR fa o 24 n 24 g R S a n e h r i a MMB d d n n Ganges a basin TMB a R YANGTSE Chindwin W Simao i m Mogok v Mizoram r e delta Li r u basin fa B lt u fau lt - ting Nan Chittagong o Fig.5 d Lincang fault

n 22 I 22 Mandalay Mt. BURMA WEST SHAN - Coxs Victoria THAI Bazar Mt. Minbu Popa basin Meiktila

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d n Andaman Gaoligong- Salween A Islands Mergui gneisses GS Gulf of Red River-Ailao Shan 9092 94 96 98 100 metamorphic complexes

Figure 1. Geological map of SE Asia showing the Red River Shear Zone from Tibet to the South China Sea, after Bureau of Geology and Resources of Yunnan (1983), Leloup et al. (1995), and Searle et al. (2007). Boxes show location of the maps in Figures 2 (Diancang Shan) and 5 (Ailao Shan).

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et al. (1998) and Burchfi el et al. (2008) pro- tion with earlier published U-Th-Pb age data Shan”) suture zone. In the northeast part of the posed that the name Red River fault should be (Schärer et al., 1990, 1994; Zhang and Schärer, Diancang Shan, Devonian limestones have been restricted to the young and active faults that 1999; Gilley et al., 2003; Sassier et al., 2009) to mapped unconformably overlying the meta- can be traced from the Ailao Shan south into discuss the differing models for the evolution of morphic rocks (Bureau of Geology and Mineral Vietnam. Following E. Wang et al. (1998) in the RRSZ. Resources of Yunnan, 1983). this paper, we distinguish between the older New quarry exposures in the southern part exhumed high-grade gneisses and mylonites GEOLOGY OF THE DIANCANG SHAN of the DCS (GPS: N 25.57198°; E 100.18116°; along the Red River shear zone (RRSZ) from elevation 1940 m) show excellent structural the Pliocene–active Red River fault (RRF). We The Diancang Shan (DCS) range is a NW-SE relationships between the host gneisses and also recognize a low-angle normal fault (Range aligned range ~80 km long and intruding sets of granodioritic and leucogra- Front fault of Leloup et al., 1995) that bounds 10–15 km wide (Fig. 2) and exposes a series nitic melts. These exposures clearly show that the margins of the metamorphic massifs. of high-grade metamorphic rocks bounded by the host gneisses and migmatites have been Recent discussions concerning the evolu- Mesozoic or Tertiary continental clastic red- intruded by an early series of K-feldspar ortho- tion of the RRSZ and RRF have concerned beds on either side. We distinguish here between gneisses (Fig. 3a) and a later set of biotite leu- four major aspects: (a) the timing of motion (a) the Red River ductile shear zone mylonites in cogranites, sometimes also containing along these faults, (b) the depth that such faults the margins of the massif, (b) the low-angle duc- and tourmaline. Schistosity and folds within the extend (upper crust, whole crust, or into the tile to brittle normal fault that bounds the mar- host gneisses are truncated by the dyke margins mantle), (c) the amounts of geological offset, gin of the massif west of Dali, and (c) the active (Fig. 3b). A prominent phase of tight to isocli- and (d) the degree of metamorphism and melt- Red River fault, of which we found no trace, at nal folding affects all these rocks. Stretching ing produced during strike-slip motion (e.g., least in the southern DCS region. E. Wang et al. lineations associated with left-lateral shearing Searle, 2006, 2007; Leloup et al., 2007). All (1998) considered that the NNW trending Tong- cut across all lithologies, are sub-horizontal, the metamorphic and igneous rocks were pre- dian fault, thought to be the continuation of the and were clearly superimposed after metamor- viously interpreted as having formed as a result RRF (Leloup et al., 1995), was not aligned with phism, folding, and granite intrusion (Fig. 3c). A of shear heating during Tertiary left-lateral the DCS and actually swings around east-west late set of brittle fractures cuts the host gneisses strike-slip shearing along the Ailao Shan–Red to cut off the Diancang Shan metamorphic rocks and the granite dykes (Fig. 3d). Normal faults River shear zone (Tapponnier et al., 1982, west of Dali (Fig. 2), also implying that the DCS bound both fl anks of the DCS massif and dip 1986, 1990; Leloup et al., 1993; 1995, 2001; massif no longer has active faults. Burchfi el et at angles between 45 and 25° along the north- Leloup and Kienast, 1993). An alternative al. (2007) show active outward-dipping nor- east and up to ~75° along the southwest margin, hypothesis correlates the metamorphism and mal faults bounding the DCS gneisses with no away from the massif. mantle-derived granites to earlier metamorphic strike-slip motion. Between the Diancang Shan The DCS is bounded along both NE and events, unrelated to the mylonites of RRSZ. in the NW and the Ailao Shan to the SE, the SW margins by outward-dipping normal faults In this scenario, fabrics formed by 80 km long “Midu Gap” shows no metamor- suggesting that the width of the DCS gneisses left-lateral strike-slip shearing occurred after phic rocks. Leloup et al. (1993) interpreted this increases with depth. The along the metamorphism and granite intrusion (Chung gap as a very large scale oblique extensional C′ SW margin of the DCS dips SW at moderate et al., 1997, 2008; Jolivet et al., 2001; Searle, shear plane; there is, however, no evidence of angles (~45°), whereas along the NE margin 2006, 2007; Anchiewicz et al., 2007; Yeh et al., this linking fault at the surface. dips are steeper toward the NE (60–80°) with 2008). Jolivet et al. (2001) fi rst suggested that The mylonite fabric cuts through several steep fold axial planes in the center of the massif the RRSZ was purely an upper crustal fault, different earlier metamorphic rocks, including (Fig. 4). Earlier pre-strike-slip D1 upright and above a major mid-crust detachment. K-feldspar augen gneisses, amphibolites, bio- D2 recumbent folds are cut and transposed into Many of the critical interpretations center tite gneisses and migmatites, with rare bands of parallelism with the later, steep left-lateral shear around whether the metamorphism is pre- calc-silicate and quartzite. Leloup et al. (1993) fabrics (D3) that run along the NE margin of kinematic or syn-kinematic with respect to left- mapped all the Diancang Shan gneisses as a the DCS. Stretching lineations are mainly hori- lateral shearing and whether the leucogranite 10–15-km-wide Tertiary shear zone and showed zontal, and strain markers are consistently left- sills-dykes are pre- syn- or post-kinematic with that left-lateral shear fabrics occurred irrespec- lateral. The Range Front normal faults along respect to left-lateral shearing. During our fi eld- tive of the dip direction of the schistosity. These both SW and NE margins of the DCS cut all ear- work in 2007, we visited the Diancang Shan authors and Harrison et al. (1996) used Ar-Ar lier fabrics (D4). In the southern DCS foliations and Ailao Shan in order to assess the regional dating to constrain the age of left-lateral shear- are aligned east-west and are truncated by the structural setting of leucogranites and gneisses ing as occurring between ~35 Ma and 17 Ma low-angle normal faults that bound the southern along the RRSZ. In this paper, we fi rst discuss with a ductile to brittle phase of normal faulting margin of the DCS gneisses. the major differences in interpretation of the that began at 4.7 Ma. leucogranite ages with respect to the strike-slip Metamorphic rocks are, however, not GEOLOGY OF THE AILAO SHAN fabrics. Further to our previous studies of the restricted to the RRSZ (Bureau of Geology and DNCV complex along the RRSZ in Vietnam Mineral Resources of Yunnan, 1983). “Older” The Ailao Shan metamorphic massif (Fig. 5) (Searle, 2006, 2007; Chung et al., 2008; Yeh metamorphic rocks have also been mapped is up to 20 km wide and more than 300 km long et al., 2008), this paper contributes more to the away from the RRSZ, and these include the (Bureau of Geology and Mineral Resources of debate by presenting some important fi eld con- multi-colored Dali marbles that show extreme Yunnan, 1983), extending SE into Vietnam into straints from the Diancang Shan and Ailao Shan ductile deformation. The age of metamorphism the Sapa metamorphic belt NE of the metamorphic rocks in Yunnan. We also report is as yet unknown, although we suspect that alkali syenite massif and SW of the DayNui- some new zircon LA-ICP-MS and SHRIMP they could be related to the Triassic Indosin- ConVoi (DNCV) metamorphic complex ages. Finally, we use these data in conjunc- ian collision event along the Jingsha (“–Ailao (Leloup et al., 1995, 2001). Whereas the Sapa

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Figure 2. Geological map of Diancang Shan after Geological Map of Yunnan, scale 1:500,000, overlain onto a Digital Eleva- tion Model, showing sample locations. Equal area stereonet plots show fi eld measurements of poles to matrix foliation (solid squares) and mineral stretching lineations (open triangles). General NW-SE striking, NE-dipping strike-slip related foliations with NW-SE double plunging sub-horizontal lineations were measured from the Jade Belt trail section, west of Dali. NE-SW striking foliations with SE-plunging sub-horizontal lineations were measured for the southern XiQuen old highway section.

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regional metamorphic rocks show little evi- orthogneisses and K-feldspar augen gneisses rarely contain a stretching lineation. Although dence of strike-slip shearing, the DNCV massif with minor amounts of calc-silicate marble, Leloup et al. (1995) describe both faults as does expose mylonites along two steep faults quartzite, and rare pelites. P-T conditions of active, we could fi nd no trace of active that bound the margins of the massif (Leloup metamorphism have been estimated at between along the NE-dipping low-angle Range Front et al., 2001; Searle, 2006; Anczkiewicz et al., 3 and 7 kbars and 550–780°C (Leloup et al., fault, and no outcrop indication of an active 2007; Yeh et al., 2008). The active RRF sup- 1993; Leloup and Kienast, 1993). Granodio- Mid-Valley fault along the heavily forested posedly follows the northeastern margin of the rites, biotite granites and late biotite ± garnet ± Hong-He (Red River) valley in the central Ailao Ailao Shan (Leloup et al., 1995) but cuts across tourmaline leucogranites intrude the gneisses. Shan. The SW margin of the low-grade the earlier RRSZ to follow the southwest- All these rocks have been affected by strong shows a discontinuous belt of ultramafi c rocks, ern margin of the DNCV (Song Hong fault). deformation, and late mylonite fabrics related mainly serpentinized harzburgites that have Another active strike-slip fault bounds the to left-lateral strike-slip shearing along the fault presumably been related to the Song Ma suture northeastern margin of the DNCV (Song Chai have been superimposed on all lithologies. zone (Leloup et al., 1995). Vertical shear bands fault). Both margins of the DNCV show steep The NE margin of the Ailao Shan metamor- cut through the serpentinised harzburgites and mylonite zones with left-lateral kinematic indi- phic complex is a NE-dipping normal fault low-grade schists indicating that metamorphism cators and brittle normal faulting that cut earlier (Range Front fault of Leloup et al., 1995) jux- preceded strike-slip shearing along the RRSZ. fl at-lying and gently folded schistosities within taposing Triassic to Tertiary red-beds directly the DNCV (Jolivet et al., 2001; Searle, 2006, against high-grade gneisses. The SW margin of Yuanjiang Area Anczkiewicz et al., 2007; Viola and Anczkie- the high-grade gneisses is a steep NE-dipping wicz, 2008; Yeh et al., 2008). or vertical fault (Ailao Shan fault of Leloup et Although outcrop is not particularly good in In the Ailao Shan, high-grade gneisses al., 1995) juxtaposing the gneisses against low- the forested Ailao Shan , we found form a long linear belt between 0–10 km wide grade schists to the SW. These low-grade schists two small areas along scoured river courses (Fig. 5). These rocks are dominantly biotite are not as highly deformed as the gneisses and to the west of Yuanjiang where full outcrop

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Figure 3. Photos of Diancang Shan gneisses showing relative chronology of metamorphism, granite intrusion and strike-slip shear- ing. (A) K-feldspar augen gneiss from Diancang Shan showing strong shear fabrics. (B) Tight folds and schistosity in Diancang Shan gneiss cut by a biotite granite dyke intruded after metamorphism and folding in host gneiss. (C) Granite dyke intruding Diancang Shan gneiss with strike-slip fabrics (dashed lines, hammer handle) cutting both host gneiss and granite. (D) Late brittle fractures cross-cutting granite dykes and host gneisses.

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Figure 4. (A) Block diagram showing fabrics in the central southernmost Diancang Shan from thin sections; left fi gure plane- polarized light, middle fi gure crossed polars, right fi gure interpretation. This region shows two superimposed folding events: pre-strike-slip D1 upright folds with NW-SE striking fold axial planes (S1 green) and the later D2 NW plunging recumbent folds with NE-SW striking fold axial planes (S2 red). D1 and D2 folds are cut by later D3 left-lateral strike-slip fabrics along the NE margin of the DCS (not shown at this locality). (B) Block diagram showing fabrics along the southeastern margin of the Diancang Shan from thin sections; left fi gure plane-polarized light, middle fi gure crossed polars, right fi gure interpretation. This section shows strike-slip related D3 fabrics (blue) cutting earlier D2 fabrics (red). The strike-slip fabrics are only present along the northeastern and southeastern parts of the DCS.

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Figure 5. Geological map of the Ailao Shan, modifi ed from Geological Map of Yunnan, scale 1:500,000, super- imposed on a Digital Elevation Model showing sample locations. Equal area stereonet plots show fi eld measure- ments of poles to matrix foliation (solid square) and mineral stretching lineation (open triangle). Steep NW-SE striking matrix foliations with NW-SE double plunging sub-horizontal lineations were measured from the Ailao Shan region.

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permitted detailed structural mapping of the leucogranites, sometimes also containing garnet yses of granite break out and intrude across the RRSZ mylonites. The fi rst locality (Fig. 6: GPS: and tourmaline, have intruded the gneisses par- fabric of the host gneiss, including cutting ear- N 23.55663°; E 101.91552°; elevation: 712 m) allel to the foliation, which has then been subse- lier boudinaged set 1 dykes and layer-parallel is a small tributary of the Yuanjiang River. Host quently folded (Fig. 7a). There is good evidence set 2 sills (Fig. 7c). NW-SE aligned stretching rock gneisses include garnet amphibolite, bio- for pre-kinematic intrusion of granites into the lineations parallel to the strike-slip shear direc- tite orthogneiss and K-feldspar augen gneiss gneisses (Fig. 7a) and also a later set of post- tion are common in both gneisses and granite striking between 122 and 132° with very steep kinematic dykes that cut folds in the gneisses (Fig. 7d), although fabrics are less prominent dips (between 90 and 75°NE). Biotite bearing (Fig. 7b). In some places, small dykes or apoph- in some of the late cross-cutting apophyses. Numerous kinematic indicators (C-S, C′-S mylonite fabrics, rolled K-feldspar porphyro- blasts, asymmetric boudins, etc.) consistently show left-lateral shear. Some outcrops show a mixture of fabrics with both early leucogranites infolded with the host gneisses, and later leuco- granites cross-cutting the fabrics (Fig. 7e). Some of these later leucogranites have broken up the host gneisses into detached xenoliths (Fig. 7f) clearly indicating that the gneisses formed fi rst and the leucogranite formed later. U-Pb zircon and titanite ages from monzo- nites, monzo-syenites and from else- where along the Ailao Shan range from 31.9 ± 0.3 Ma to 26.3 – 23.0 ± 0.3 Ma (Schärer et al., 1990; Zhang and Schärer, 1999), ages that date the timing of crystallization of the granite. These authors and Leloup et al. (1995, p. 55) stated that “since the melts are deformed, the ages imply that left-lateral shear was in prog- ress in both massifs [ALS and DCS] at least before 26 Ma until after 22 Ma.” In contrast, Searle (2006, 2007) and Chung et al. (2008) interpreted most of the granites along the DNCV metamorphic belt along the RRSZ in North Vietnam, as well as the alkaline FanSiPan syenite, SW of the RRSZ, (U-Pb titanite age of 35.2 ± 0.4 Ma; Zhang and Schärer, [1999]) as pre-kinematic. Left-lateral strike-slip fabrics were superimposed onto the granites at tem- peratures (ca 500–550°C) high enough to plas- tically deform , but still below melting temperatures as recorded by U-Th-Pb zircon– monazite ages. Detailed mapping of outcrops in Figure 6, however, shows ambiguity, with evidence that could be interpreted as pre-kinematic, syn- kinematic, or post-kinematic granite intrusion. Pre-kinematic granites are strongly affected by deformation and boudinage and are infolded with the gneisses, as also seen in the Diancang Shan. A few instances also show late veins and apophyses of post-kinematic granite intrusion where very small veins cut across the mylonite fabric. These late veins originate from the cores of parallel granite sheets and are the fi nal melt Figure 6. Sketch map of fi eld outcrop of the RRSZ mylonites, along a stream section west of phase in the RRSZ mylonite belt. Yuanjiang in the Ailao Shan (GPS: N 23° 55663; E 101°91552; elevation: 712 m). Leucogran- A second outcrop was also mapped in detail ites are mainly parallel to left-lateral strike-slip shear fabrics in the host garnet amphibolite, near a small hydro-electric power station west biotite orthogneiss and K-feldspar augen gneiss. In some places, small dykes or apophyses of Yuanjiang in the Ailao Shan (Fig. 8: GPS: N break out and intrude across the fabric of the host gneiss including cutting tight to isoclinal 23. 55447°; E 101. 91688°; elevation: 701 m). folds, and the earlier granite. This outcrop is the same locality as shown in

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Figure 7. (A) Photos of Ailao Shan gneisses from the RRSZ. (a) Pre-kinematic granites illustrated by tightly folded granite and host gneiss. (B) Post-kinematic granite dyke intruding folded calc-silicate metamorphic rocks. (C) Three sets of granite intrusion with early boudinaged granites (1), later layer-parallel sills with internal fabric (2) and latest cross-cutting dyke (3). (D) Strike-slip shear fabric superimposed onto earlier granite intrusive contact with host gneiss. (E) Isoclinally folded pre-kinematic granites and later granite dyke cross-cutting gneissic fabric. (F) Post-kinematic granite intruding and breaking up host gneisses into xenoliths and fl oating blocks.

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Zhang and Schärer (1999, Fig. 2c), Leloup et al. ite sheets are all aligned parallel to the fabric (Fig. 9b). We interpret these granites as having (1995, fi gure 13, p. 34), and Sassier et al. (2009, in the host gneiss, and also contain the strike- intruded syn- to post-strike-slip shearing. A few fi gures 4, 5). It shows very strong mylonite slip shear fabric (Fig. 9a, b; set 1 sills), and are thin (<10–20 cm) late-stage leucogranites (con- fabrics striking 120–125° and clearly shows therefore interpreted here as having intruded taining Bt, Grt, and Tur) cross-cut the shearing at least three generations of leucogranites pre-strike-slip shearing. Set 2 dykes clearly fabrics and are mainly undeformed (Fig. 9a, b, (Fig. 9a, b). Early biotite granites (set 1) intrude cross-cut metamorphic fabrics and set 1 dykes set 3 dykes). These small dykes are interpreted K-feldspar augen gneiss, biotite orthogneiss, (Fig. 9a), but elsewhere in the same outcrop here as intruding during, and mainly after duc- and uncommon calc-silicate rocks. These gran- appear to be parallel to the metamorphic fabric tile strike-slip shearing. The set 3 dykes are apparently the same set as the one dated by Th-Pb monazite dating by Gilley et al. (2003) at 21.7 ± 0.2 Ma and by Sassier et al. (2009) at AL07-26a 22.55 ± 0.25 Ma. Based on these ages, we now N interpret the timing of ductile shearing along this segment of the RRSZ to have occurred between ~29 – 22 Ma. The microstructural evolution of the Yuan- jiang locality shows the dominant left-lateral beach large S-C fabrics related to shearing along the RRSZ approx 1m boulder (Fig. 10). These fabrics clearly cut earlier- formed , amphiboles and biotite, sug- 1m gesting that metamorphic mineral growth pre- ceded ductile strike-slip shearing.

GPS: ZIRCON U-PB GEOCHRONOLOGY OF N 23°55447’ THE RRSZ GNEISSES E 101°91688’ 170 Four gneiss samples from the Diancang Shan elevation: 701m and Ailao Shan areas were subjected to zircon separation and U-Pb dating (Fig. 11; Table 1). 130 Zircons were separated from ~3 kg samples using conventional heavy-liquid and magnetic separation techniques. Cathode-luminescence AL07-26c (CL) images of individual zircon grains were taken at the Institute of Earth Sciences, Aca-

River demia Sinica, Taipei, for examining the inter- 142 nal structures and selecting suitable positions for U-Pb isotope determinations. Zircon U-Pb isotopic analyses were performed using a New Wave UP213 laser ablation system combined 130 with an Agilent 7500s quadrupole ICP-MS AL07-26b (inductively coupled plasma mass spectrom- eter) housed at the National Taiwan University, Taipei. The LA-ICPMS operating conditions and analytical procedures were same as those 130 reported in Chiu et al. (2009). Given that in a/g LA-ICP-MS zircon U-Pb isotopic analysis, 78 Bt Tur ± Ms Leucogranite precise age measurements using 207Pb/235U and 95 Bt Granite gneiss 207Pb/206Pb ratios are feasible usually only for 100° DAM Kfs augen gneiss + Bt zircons older than ~800 Ma, essentially due to the fact that 235U comprises less than 1% of natu- ral U and thus relatively little 207Pb can be pro- duced in the case. The weighted mean of pooled 206Pb/238U ages are used to represent the ages of the young zircons dated. The analytical results Figure 8. Map of fi eld outcrop of RRSZ mylonites near a small hydro-electric power sta- are summarized in Table 1. tion west of Yuanjiang in the Ailao Shan (GPS: N23°55447; E 101°91688; elevation: 701 m). Early biotite granites intrude K-feldspar augen gneiss and uncommon calc-silicate rocks Diancang Shan DS07–16 (Fig. 11 A-B) and contain a fabric that is parallel to the left-lateral shear fabrics formed during slip along the RRSZ. A few thin late-stage leucogranites (containing Bt, Grt and Tur) cross-cut the Sample DS07–16 is a muscovite-biotite- shearing fabrics and are mainly undeformed. garnet from the southern margin of the

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A B PosPost-kinematicosost-kt--kkinenenematmam t c Sett 3 dykdykesyykes

2

3 PrePre-kinematicree-kik nememe at c SiSSilll ssetete 2

3 1

2 1

Figure 9. (A, B) Three sets of granite sills-dykes in the Yuanjiang dam outcrop. Early set 1 sills are parallel to the RRSZ mylonites. Syn- to post-kinematic set 2 dykes were intruded during left-lateral strike-slip shear. Later set 3 leucogranite dykes dated by Th-Pb monazite at 22.55 ± 0.25 Ma by Sassier et al. (2009) cross-cut all fabric and the two earlier sets of dykes.

Figure 10. Block diagram of thin sections of garnet amphibolite from the Yuanjiang region of the Ailao Shan; left fi gure plane- polarized light, middle fi gure crossed polars, right fi gure interpretation. This region shows typical NW-SE aligned, SW-dipping, left-lateral S (red)–C (blue) fabrics related to shear along the Red River ductile shear zone. Note that earlier-formed garnets are truncated by later strike-slip fabrics.

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DCS (GPS: N 25.57624°; E 100.20002°; eleva- Diancang Shan DS07–19A (Fig. 11 C-D) loss. The two high-U zircon rims exhibit very tion 1970 m) This sample contains short pris- low Th/U ratios (0.006–0.010), similar to the matic zircons between 150–300 µm in length. Sample DS07–19A is a biotite orthogneiss zircons crystallizing at high metamorphic grade CL images show growth zoning and resorption that forms the host rock to several granitic (Williams et al., 1996), and yield 206Pb/238U ages features, typically observed in magmatic and intrusions and was collected from the quarry at ca. 24 Ma, which we interpret as indicating metamorphic zircons, respectively (Hoskin site in the southern DCS (GPS: N 25.57198°; the timing of metamorphic overgrowth. and Schaltegger, 2003). The magmatic cores E 100.18116°; elevation 1940 m). Zircons from have highly various U concentrations (150– this sample are stubby to elongate in shape, Ailao Shan AL07–24A (Fig. 11 E-F) 3281 ppm), whereas the rim areas are more showing rounded to subrounded terminations. uniform in U (256–590 ppm). We performed 35 Most zircons that show highly luminescent cores Sample AL07–24A is a mylonitic biotite–K- spots of analyses on both core and rim, if avail- in CL images have low to medium U concentra- feldspar orthogneiss collected from the Yuanji- able, of the zircon separates. Among these, 22 tion ranging from 62 to 554 ppm. In two zircon ang site in the Ailao Shan (Fig. 6: GPS: N 23. analyses (Table 1) form a concordant cluster grains, rims with dark CL images and high U 55663°; E 101. 91552°; elevation: 712 m). This yielding a mean 206Pb/238U age of 243.0 ± 1.7 Ma (2089–4492 ppm) are recognized. Twenty-fi ve sample represents the host orthogneiss into (2σ; MSWD = 0.6), which is interpreted to rep- spots on 24 grains were analyzed (Table 1). 24 which the two later sets of leucogranitic dykes resent the protolith age of the gneiss. Some analyses yielded a concordant cluster with a have been intruded. The sample contains mostly older spots that range from ~300–1000 Ma are mean 206Pb/238U age of 234.9 ± 1.8 Ma (MSWD = euhedral, transparent, colorless to pale brown discordant and considered to represent inherited 0.3), which is interpreted to represent the forma- zircons, which have average crystal lengths of zircons that may have experienced Pb lost when tion age of the magmatic protolith. Few analyses ~150–300 µm and length-to-width ratios from the protolith was metamorphosed during the that plot slightly below the Concordia line show 2:1–3:1. CL images show that most zircons have Triassic Indosinian time. younger ages, probably due to radiogenic Pb complex internal structures. The cores show

Figure 11 (Continued on following page). Selected CL images and Concordia plots of zircons from the ASRR gneisses.

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magmatic growth zoning with variable U from E 101. 91688°; elevation: 701 m). This rock is an that the sample was derived from an Indosinian 54 to 923 ppm and Th/U ratios from 0.22 to orthogneiss and is the host rock into which the magmatic event that later underwent a metamor- 1.12. Twelve zircon grains gave older 206Pb/238U three sets of dykes have been intruded (Fig. 9). phic overprint during the Oligocene. ages from 239 to 818 Ma, suggesting multiple Zircons from this sample are dominated by clear magmatic sources of the protolith. Most zircons to brownish, rounded to short prismatic, partly LEUCOGRANITE AGES AND STRIKE- contain rims that are characterized by higher irregular grains, ranging in size between ~100 SLIP SHEAR FABRICS and more variable U concentrations ranging and 300 µm. Most zircons reveal sector zoning from 280 to 4913 ppm, with Th/U from 0.11 to in CL images. Rim structures that are occasion- In the Ailao Shan, U-Pb dating of zircons, 1.01. Twenty-two analyses on the rims yielded a ally observed have an outer zone of high lumi- monazites, and xenotimes from granite veins weighted mean 206Pb/238U age of 26.2 ± 0.3 Ma nescence, indicative of low U concentration. parallel to the foliation and granite veins (MSWD = 2.0), implying a magmatic or meta- Thirty-eight spots were analyzed on 31 zircons affected by left-lateral shearing gave ages rang- morphic origin. These data indicate a complex from this sample. A cluster of 26 analyses gave ing from 26.3 ± 0.2–23 ± 0.2 Ma (Schärer et history of sample AL07–24A, which contains a weighted mean 206Pb/238U age of 239.4 ± al., 1990, 1994). Two “layered granitic intru- inherited zircons of Indosinian to Neoprotero- 1.9 Ma (MSWD = 1.3), interpreted to refl ect sions (YS-53 and YS-13) within the gneiss mas- zoic magmatism. The rock was overprinted by the emplacement age of the magmatic protolith. sif” gave monazite ages of 31.9 ± 0.3 Ma and an Oligocene magmatism or metamorphism Several older grains, ranging from early Palaeo- 25.8 ± 0.2 Ma (Zhang and Schärer, 1999, p. (ca. 26 Ma) during which zircon rims overgrew. zoic and Neoproterozoic, indicate the presence 70). In the Diancang Shan, ages from a “cross- of earlier magmatic events in the region. Four cutting aplitic layer (MD-1)” are between 24.4 Ailao Shan AL07–26A (Fig. 11 G-H) young ages from 29.0 to 34.0 Ma were obtained ± 0.2 Ma and 22.3 ± 0.3 Ma, suggesting that from the rims (Table 1), which have medium the fabric must have been pre-24.4 Ma (Zhang Sample AL07–26A was collected from the to high U concentrations (429–3785 ppm) and and Schärer, 1999, p. 69). Schärer et al. (1990, Yuanjiang dam site (Fig. 8: GPS: N 23. 55447°; low Th/U ratios (0.02–0.05). These data suggest 1994) and Zhang and Schärer (1999, p. 67)

Figure 11 (Continued). Selected CL images and Concordia plots of zircons from the ASRR gneisses.

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TABLE 1 . ZIRCON U-PB AGE DATA FROM THE AILAO SHAN AND DIANCANG SHAN SHEAR ZONE Spot U Th/U 207Pb/206Pb ± 1s 206Pb/238U ± 1s 207Pb/235U ± 1s error 206Pb/238U age 207Pb/206Pb age corr. (Ma ± 1s) (Ma ± 1s) AL07-24A wt. mean = 26.2 ± 0.3 Ma (2s) AL07-24A-01 2581 0.358 0.05074 0.00064 0.00391 0.00007 0.02737 0.00071 0.690 25.2 0.4 AL07-24A-03 1828 0.617 0.04660 0.00092 0.00408 0.00008 0.02620 0.00093 0.552 26.2 0.5 AL07-24A-04 2358 0.498 0.04680 0.00079 0.00401 0.00008 0.02586 0.00084 0.614 25.8 0.5 AL07-24A-05 551 0.752 0.04986 0.00268 0.00405 0.00009 0.02784 0.00203 0.305 26.1 0.6 AL07-24A-07 3398 0.344 0.04634 0.00102 0.00401 0.00007 0.02561 0.00090 0.497 25.8 0.4 AL07-24A-08 1212 0.202 0.04773 0.00155 0.00409 0.00008 0.02689 0.00129 0.408 26.3 0.5 AL07-24A-09 1952 0.289 0.04693 0.00070 0.00435 0.00008 0.02816 0.00082 0.632 28.0 0.5 AL07-24A-10 1578 0.595 0.05028 0.00108 0.00404 0.00008 0.02798 0.00105 0.528 26.0 0.5 AL07-24A-11 1323 0.588 0.04610 0.00089 0.00398 0.00007 0.02528 0.00082 0.542 25.6 0.5 AL07-24A-12 795 0.763 0.05218 0.00208 0.00408 0.00009 0.02935 0.00172 0.376 26.2 0.6 AL07-24A-13 415 0.826 0.04641 0.00459 0.00403 0.00009 0.02579 0.00302 0.191 25.9 0.6 AL07-24A-14 992 0.719 0.04766 0.00321 0.00405 0.00009 0.02662 0.00228 0.259 26.1 0.6 AL07-24A-15 1360 0.578 0.04611 0.00062 0.00408 0.00007 0.02592 0.00071 0.626 26.2 0.5 AL07-24A-16 4913 0.236 0.04611 0.00030 0.00399 0.00007 0.02538 0.00051 0.873 25.7 0.4 AL07-24A-17 1855 0.261 0.04618 0.00073 0.00405 0.00008 0.02576 0.00080 0.636 26.1 0.5 AL07-24A-18 1500 0.301 0.05034 0.00106 0.00411 0.00008 0.02851 0.00105 0.529 26.4 0.5 AL07-24A-19 1087 0.641 0.04618 0.00210 0.00403 0.00008 0.02564 0.00152 0.335 25.9 0.5 AL07-24A-20 979 0.680 0.04729 0.00226 0.00440 0.00009 0.02869 0.00186 0.316 28.3 0.6 AL07-24A-21 1423 0.186 0.04847 0.00130 0.00409 0.00008 0.02736 0.00118 0.454 26.3 0.5 AL07-24A-22 385 0.223 0.05368 0.00092 0.03785 0.00066 0.28013 0.00876 0.558 239.0 4.0 AL07-24A-23 54 0.641 0.06498 0.00084 0.10747 0.00200 0.96271 0.02582 0.694 658.0 12.0 AL07-24A-24 571 0.709 0.06602 0.00053 0.11795 0.00209 1.07363 0.01945 0.978 719.0 12.0 AL07-24A-25 119 1.124 0.06870 0.00064 0.09921 0.00180 0.93964 0.01999 0.853 610.0 11.0 AL07-24A-26 179 0.649 0.07057 0.00060 0.11053 0.00198 1.07551 0.02107 0.914 676.0 11.0 AL07-24A-27 267 0.426 0.06432 0.00075 0.04028 0.00075 0.35720 0.00898 0.741 255.0 5.0 AL07-24A-28 430 0.364 0.06720 0.00054 0.13572 0.00241 1.25730 0.02293 0.974 820.0 14.0 AL07-24A-29 1536 0.110 0.04958 0.00138 0.00428 0.00009 0.02924 0.00132 0.466 27.5 0.6 c 923 0.535 0.06635 0.00053 0.11888 0.00211 1.08743 0.01974 0.978 724.0 12.0 AL07-24A-30 315 1.010 0.05443 0.00380 0.00413 0.00010 0.03095 0.00281 0.267 26.6 0.6 AL07-24A-31 365 0.840 0.06821 0.00056 0.12407 0.00221 1.16680 0.02157 0.964 754.0 13.0 AL07-24A-32 280 0.862 0.04609 0.00240 0.00402 0.00009 0.02556 0.00174 0.329 25.9 0.6 AL07-24A-33 102 1.020 0.06921 0.00081 0.08353 0.00156 0.79695 0.02004 0.743 517.0 9.0 AL07-24A-34 209 1.099 0.06581 0.00057 0.13534 0.00244 1.22796 0.02420 0.915 818.0 14.0 AL07-24A-35 300 0.952 0.06893 0.00058 0.12599 0.00227 1.19730 0.02287 0.943 765.0 13.0 AL07-26A wt. mean = 239.4 ± 1.9 Ma (2s) AL07-26A-01 483 0.178 0.05102 0.00046 0.03919 0.00071 0.27569 0.00573 0.872 248.0 4.0 c 527 0.439 0.05252 0.00049 0.03914 0.00073 0.28342 0.00603 0.877 248.0 5.0 AL07-26A-02 611 0.149 0.05142 0.00046 0.03781 0.00068 0.26802 0.00554 0.870 239.0 4.0 c 318 0.248 0.07900 0.00067 0.19915 0.00368 2.16919 0.04149 0.966 1171.0 20.0 1172.0 17.0 AL07-26A-03 1201 0.093 0.05165 0.00043 0.03740 0.00067 0.26633 0.00509 0.937 237.0 4.0 c 389 0.222 0.05523 0.00058 0.03825 0.00072 0.29126 0.00686 0.799 242.0 4.0 AL07-26A-04 1075 0.115 0.05110 0.00043 0.03742 0.00067 0.26360 0.00507 0.931 237.0 4.0 AL07-26A-05 725 0.134 0.05100 0.00044 0.03763 0.00068 0.26459 0.00525 0.911 238.0 4.0 AL07-26A-06 886 0.113 0.05146 0.00044 0.03788 0.00069 0.26873 0.00523 0.936 240.0 4.0 AL07-26A-07 889 0.221 0.05083 0.00043 0.03765 0.00068 0.26383 0.00515 0.925 238.0 4.0 AL07-26A-08 710 0.153 0.05161 0.00045 0.03914 0.00071 0.27854 0.00551 0.917 248.0 4.0 AL07-26A-10 1051 0.113 0.05137 0.00044 0.03631 0.00066 0.25717 0.00500 0.935 230.0 4.0 AL07-26A-11 1010 0.158 0.05091 0.00044 0.03791 0.00070 0.26608 0.00528 0.931 240.0 4.0 AL07-26A-12 870 0.129 0.05197 0.00045 0.03803 0.00070 0.27247 0.00541 0.927 241.0 4.0 AL07-26A-13 656 0.150 0.05097 0.00046 0.03797 0.00070 0.26684 0.00552 0.891 240.0 4.0 AL07-26A-14 526 0.216 0.05144 0.00047 0.03790 0.00070 0.26877 0.00566 0.877 240.0 4.0 AL07-26A-15 792 0.101 0.05144 0.00046 0.03756 0.00069 0.26640 0.00542 0.903 238.0 4.0 AL07-26A-16 594 0.134 0.05145 0.00047 0.03808 0.00070 0.27011 0.00569 0.873 241.0 4.0 AL07-26A-17 772 0.140 0.05074 0.00046 0.03757 0.00069 0.26283 0.00542 0.891 238.0 4.0 (continued)

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TABLE 1 . ZIRCON U-PB AGE DATA FROM THE AILAO SHAN AND DIANCANG SHAN SHEAR ZONE (continued) Spot U Th/U 207Pb/206Pb ± 1s 206Pb/238U ± 1s 207Pb/235U ± 1s error 206Pb/238U age 207Pb/206Pb age corr. (Ma ± 1s) (Ma ± 1s) AL07-26A-18 1704 0.061 0.05141 0.00044 0.03657 0.00067 0.25915 0.00501 0.948 232.0 4.0 c 1706 0.309 0.06056 0.00052 0.09184 0.00174 0.76683 0.01477 0.984 566.0 10.0 AL07-26A-20 922 0.259 0.05124 0.00046 0.03787 0.00070 0.26750 0.00546 0.906 240.0 4.0 AL07-26A-21 3785 0.021 0.04810 0.00059 0.00451 0.00008 0.02994 0.00075 0.708 29.0 0.5 c 328 0.163 0.05088 0.00054 0.03840 0.00073 0.26935 0.00639 0.801 243.0 5.0 AL07-26A-22 3091 0.031 0.04942 0.00064 0.00458 0.00008 0.03119 0.00082 0.664 29.4 0.5 c 375 0.238 0.05385 0.00054 0.03832 0.00073 0.28453 0.00648 0.836 242.0 5.0 AL07-26A-23 682 0.084 0.05132 0.00047 0.03845 0.00072 0.27207 0.00570 0.894 243.0 4.0 AL07-26A-24 360 0.095 0.06915 0.00060 0.14148 0.00266 1.34884 0.02663 0.952 853.0 15.0 AL07-26A-25 429 0.045 0.05114 0.00545 0.00526 0.00019 0.03706 0.00511 0.262 34.0 1.0 c 615 0.680 0.05162 0.00049 0.03618 0.00069 0.25752 0.00555 0.885 229.0 4.0 AL07-26A-26 819 0.398 0.05210 0.00126 0.03770 0.00069 0.27079 0.01061 0.467 239.0 4.0 AL07-26A-27 1529 0.144 0.05189 0.00046 0.03773 0.00072 0.26995 0.00540 0.954 239.0 4.0 AL07-26A-28 913 0.242 0.06686 0.00058 0.08399 0.00159 0.77424 0.01511 0.970 520.0 9.0 AL07-26A-29 3194 0.020 0.04609 0.00087 0.00452 0.00009 0.02871 0.00094 0.608 29.1 0.6 AL07-26A-30 1057 0.299 0.07700 0.00067 0.17222 0.00328 1.82824 0.03599 0.967 1024.0 18.0 1121.0 17.0 AL07-26A-31 1612 0.061 0.05910 0.00051 0.08466 0.00161 0.68980 0.01340 0.979 524.0 10.0 AL07-26A-32 1498 0.321 0.06368 0.00054 0.11403 0.00216 1.00119 0.01920 0.988 696.0 12.0 AL07-26A-33 763 0.369 0.07520 0.00064 0.17588 0.00334 1.82346 0.03507 0.987 1044.0 18.0 1074.0 17.0 DS07-16 wt. mean = 243.0 ± 1.7 Ma (2s) DS07-16-01 294 0.265 0.05213 0.00057 0.04024 0.00073 0.28923 0.00694 0.756 254.0 5.0 DS07-16-02 473 0.112 0.05120 0.00049 0.03854 0.00069 0.27204 0.00589 0.827 244.0 4.0 DS07-16-03 335 0.308 0.05186 0.00056 0.03814 0.00069 0.27273 0.00646 0.764 241.0 4.0 DS07-16-04 283 0.270 0.05367 0.00060 0.03935 0.00071 0.29122 0.00705 0.745 249.0 4.0 DS07-16-05 298 0.258 0.05101 0.00058 0.03816 0.00069 0.26836 0.00660 0.735 241.0 4.0 DS07-16-06 292 0.341 0.05403 0.00061 0.03804 0.00069 0.28338 0.00693 0.742 241.0 4.0 DS07-16-07 590 0.289 0.05221 0.00047 0.03830 0.00068 0.27569 0.00571 0.857 242.0 4.0 DS07-16-08 387 0.241 0.05149 0.00053 0.03797 0.00069 0.26952 0.00618 0.793 240.0 4.0 DS07-16-09 310 0.310 0.05211 0.00067 0.03811 0.00071 0.27379 0.00733 0.696 241.0 4.0 DS07-16-10 371 0.217 0.05033 0.00059 0.03811 0.00070 0.26442 0.00660 0.736 241.0 4.0 DS07-16-11 475 0.222 0.05215 0.00048 0.03838 0.00069 0.27592 0.00581 0.854 243.0 4.0 DS07-16-12 326 0.218 0.05382 0.00055 0.03856 0.00070 0.28610 0.00652 0.797 244.0 4.0 DS07-16-13 288 0.294 0.05007 0.00055 0.03777 0.00069 0.26073 0.00629 0.757 239.0 4.0 DS07-16-14 256 0.259 0.05121 0.00109 0.03834 0.00067 0.27070 0.00948 0.499 243.0 4.0 DS07-16-15 304 0.270 0.05222 0.00055 0.03848 0.00070 0.27701 0.00647 0.779 243.0 4.0 DS07-16-16 395 0.198 0.05130 0.00050 0.03797 0.00069 0.26856 0.00595 0.820 240.0 4.0 DS07-16-17 265 0.413 0.05719 0.00061 0.03869 0.00071 0.30509 0.00717 0.781 245.0 4.0 DS07-16-18 435 0.219 0.05034 0.00089 0.03824 0.00067 0.26542 0.00827 0.562 242.0 4.0 DS07-16-19 278 0.201 0.05341 0.00053 0.04946 0.00090 0.36423 0.00811 0.817 311.0 6.0 DS07-16-20 282 0.287 0.05173 0.00055 0.03855 0.00071 0.27494 0.00651 0.778 244.0 4.0 DS07-16-21 150 0.508 0.07220 0.00066 0.16609 0.00305 1.65328 0.03465 0.876 991.0 17.0 DS07-16-22 762 0.730 0.05795 0.00048 0.08378 0.00151 0.66943 0.01251 0.964 519.0 9.0 DS07-16-23 577 0.113 0.10118 0.00083 0.26314 0.00495 3.67102 0.07673 0.900 1506.0 25.0 1646.0 14.0 DS07-16-24 1359 0.329 0.05811 0.00047 0.08655 0.00155 0.69337 0.01266 0.981 535.0 9.0 DS07-16-25 1647 0.388 0.05709 0.00046 0.08136 0.00146 0.64037 0.01167 0.985 504.0 9.0 DS07-16-26 343 0.258 0.05256 0.00059 0.03875 0.00072 0.28082 0.00691 0.755 245.0 4.0 DS07-16-27 462 0.274 0.06535 0.00062 0.08556 0.00158 0.77088 0.01673 0.851 529.0 9.0 DS07-16-28 1417 0.273 0.07305 0.00059 0.13985 0.00251 1.40853 0.02541 0.995 844.0 14.0 DS07-16-29 940 0.450 0.10913 0.00088 0.28530 0.00514 4.29267 0.07770 0.995 1618.0 26.0 1785.0 14.0 DS07-16-30 586 0.195 0.07050 0.00058 0.16035 0.00290 1.55854 0.02915 0.967 959.0 16.0 DS07-16-31 328 0.290 0.05292 0.00058 0.03837 0.00071 0.27995 0.00672 0.771 243.0 4.0 DS07-16-32 294 0.559 0.05649 0.00066 0.05095 0.00096 0.39675 0.01000 0.748 320.0 6.0 DS07-16-33 317 0.285 0.05384 0.00066 0.03906 0.00074 0.28995 0.00760 0.723 247.0 5.0 DS07-16-34 3281 0.054 0.05654 0.00046 0.06649 0.00120 0.51828 0.00953 0.982 415.0 7.0 DS07-16-35 815 0.463 0.07132 0.00058 0.15855 0.00287 1.55901 0.02866 0.985 949.0 16.0 (continued)

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TABLE 1 . ZIRCON U-PB AGE DATA FROM THE AILAO SHAN AND DIANCANG SHAN SHEAR ZONE (continued) Spot U Th/U 207Pb/206Pb ± 1s 206Pb/238U ± 1s 207Pb/235U ± 1s error 206Pb/238U age 207Pb/206Pb age corr. (Ma ± 1s) (Ma ± 1s) DS07-19A wt. mean = 234.9 ± 1.8 Ma (2s) DS07-19A-02 85 0.578 0.05138 0.00249 0.03669 0.00070 0.25996 0.01673 0.296 232.0 4.0 DS07-19A-03 121 0.870 0.05113 0.00075 0.03718 0.00071 0.26208 0.00768 0.652 235.0 4.0 DS07-19A-05 88 0.575 0.05368 0.00094 0.03732 0.00072 0.27621 0.00904 0.589 236.0 4.0 DS07-19A-06 63 0.613 0.05084 0.00286 0.03699 0.00077 0.25931 0.01920 0.281 234.0 5.0 DS07-19A-07 65 0.610 0.05527 0.00124 0.03694 0.00074 0.28146 0.01088 0.518 234.0 5.0 DS07-19A-08 107 0.633 0.05243 0.00081 0.03743 0.00072 0.27060 0.00819 0.636 237.0 4.0 DS07-19A-09 66 0.658 0.05236 0.00120 0.03751 0.00074 0.27078 0.01055 0.506 237.0 5.0 DS07-19A-10 87 0.629 0.05254 0.00254 0.03708 0.00074 0.26859 0.01756 0.305 235.0 5.0 DS07-19A-11 78 0.559 0.05632 0.00112 0.03636 0.00071 0.28232 0.01006 0.548 230.0 4.0 DS07-19A-13 69 0.654 0.05017 0.00117 0.03763 0.00074 0.26031 0.01025 0.499 238.0 5.0 DS07-19A-14 69 0.645 0.05465 0.00113 0.03784 0.00075 0.28512 0.01047 0.540 239.0 5.0 DS07-19A-15 105 0.645 0.05200 0.00093 0.03742 0.00073 0.26826 0.00894 0.585 237.0 5.0 DS07-19A-16 67 0.641 0.05136 0.00120 0.03734 0.00074 0.26436 0.01044 0.502 236.0 5.0 DS07-19A-17 62 0.613 0.05177 0.00277 0.03753 0.00077 0.26791 0.01910 0.288 238.0 5.0 DS07-19A-18 93 0.667 0.05174 0.00093 0.03722 0.00072 0.26549 0.00883 0.582 236.0 4.0 DS07-19A-19 90 0.704 0.05256 0.00270 0.03641 0.00074 0.26387 0.01816 0.295 231.0 5.0 DS07-19A-20 64 0.592 0.05197 0.00139 0.03711 0.00075 0.26594 0.01158 0.464 235.0 5.0 DS07-19A-21 80 0.676 0.05265 0.00119 0.03716 0.00075 0.26973 0.01050 0.518 235.0 5.0 DS07-19A-23 86 0.592 0.05022 0.00244 0.03664 0.00074 0.25366 0.01671 0.307 232.0 5.0 DS07-19A-26 246 1.053 0.05140 0.00066 0.03759 0.00071 0.26639 0.00715 0.704 238.0 4.0 DS07-19A-27 126 0.855 0.05264 0.00283 0.03718 0.00098 0.26984 0.02059 0.345 235.0 6.0 DS07-19A-28 111 0.676 0.05077 0.00124 0.03757 0.00074 0.26294 0.01068 0.485 238.0 5.0 DS07-19A-29 554 1.220 0.05358 0.00054 0.03663 0.00068 0.27058 0.00611 0.822 232.0 4.0 DS07-19A-30 4492 0.010 0.04739 0.00061 0.00369 0.00007 0.02413 0.00067 0.683 23.8 0.5 DS07-19A-31 2089 0.006 0.05054 0.00154 0.00379 0.00008 0.02643 0.00127 0.439 24.4 0.5 c 90 0.763 0.05096 0.00226 0.03635 0.00082 0.25538 0.01624 0.355 230.0 5.0 Note: c = core. The bold data were used for the weighted mean age calculations.

correctly state that such ages date the crystal- still high temperature (~ 450–550 °C), were all been published by Sassier et al. (2009); these lization of the granite melt. However, they also formed after granite crystallization, so the U-Pb new data now require a revision of timing con- concluded that “magmatism and left-lateral granite ages will only give a maximum age of straints of ductile shear along the RRSZ. slip movements occurred coevally during latest strike-slip initiation (Searle, 2006, 2007). Ancz- Only a few dykes cross-cut the prominent Eocene – Oligocene times from 33 to 22 Ma” kiewicz et al. (2007) showed that microstruc- regional left-lateral shear fabrics, notably the even though some of their samples (e.g., MD-1 tures from the RRSZ mylonites in the DNCV in later set of cross-cutting dykes at Yuanjiang in Diancang Shan) cross-cut fabrics. Leloup et al. North Vietnam were compatible with maximum the Ailao Shan (Zhang and Schärer, 1999, fi g. (1995) further stated that “since the melts are deformation temperatures around 500–550 °C, 2c; Leloup et al., 1995, fi g. 13a; Sassier et al., deformed, the ages imply that left-lateral shear temperatures that are less than those required for 2009, fi gs. 4, 5). The latter authors published was in progress in both massifs from at least granite melting. Complete dynamic recrystalli- 232Th-208Pb high-resolution ion microprobe before 26 Ma until after 22 Ma”. zation of calcite occurs at ~300 °C (Passchier ages that accurately constrain the emplacement The possible exhumation path through time and Trouw, 1996). Above this, at temperatures age of the three sets of leucogranites. Ages of of a typical rock from the RRSZ is illustrated less than ca. 300 °C, brittle faulting produces the three groups were ca. 22.5 Ma for the later in Figure 12. U-Pb ages are interpreted as dat- in the upper seismogenic layer. cross-cutting dykes, 26 Ma and 30 Ma for the ing the timing of crystallization of the granite at Searle (2006, 2007) suggested that the earlier more deformed dykes parallel to the temperatures probably >750°C (Parrish, 2001). majority of the leucogranites along the RRSZ matrix fabric. The late cross-cutting dykes at Following crystallization, transpressional exhu- were pre-kinematic and deformed at high Yuanjiang are apparently the same as the sam- mation up to mid-crust levels resulted in the rock temperatures within the solid-state ductile ple (YU-4a-00) dated by Gilley et al. (2003, cooling through 40Ar/39Ar closure isotherms for regime after crystallization of the granite. p. 14–10) at 21.7 ± 0.2 Ma. This suggests that hornblende (~ 550°C), K-feldspar (500 °C), Photographs of the deformed leucogranites the ductile shear fabric that is dominant along and biotite (~ 300 °C) (Parrish, 2001). High- in Leloup et al. (1995) also suggest that they the Ailao Shan must have been older than 21.7 ± temperature ductile mylonite fabrics (Hanmer have been deformed subsequent to crystalli- 0.2 Ma. We subsequently visited this site and and Passchier, 1991) were superimposed at tem- zation and emplacement in the host gneisses, others around Yuanjiang in 2007 and confi rm the peratures high enough for feldspars to deform with extensive boudinaged and stretched pods. structural succession as published by Sassier et plastically (~ 450–500 °C) and to deform Subsequently, new Th-Pb ages from a few al. (2009). During our fi eldwork we also found plastically (~ 300 °C). These fabrics, although minor cross-cutting leucogranite veins have numerous examples of folded and boudinaged

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fabrics in leucogranites within gneisses. The provide compelling evidence of a Triassic Indo- formed (D2). Th-Pb ages from matrix monazites

critical observation that both amphibolite layers sinian metamorphic event in the protolith (D1). In from the Ailao Shan range from 74 to 20 Ma, and and leucogranite layers show similar boudinage Vietnam, there is geochronological evidence of from the DNCV in North Vietnam from ca. 220– textures suggest that high-temperature ductile high-temperature granulite facies events during 44 Ma (Gilley et al., 2003). The wide range of strain was imposed in the solid state after both the Ordovician and the Permo- Triassic (Roger Th-Pb monazite ages makes interpretation very metamorphism and leucogranite intrusion. The et al., 2007). A widespread regional Permo- diffi cult. Some old ages could be recording a only exception seems to be the youngest set of Triassic Indosinian metamorphism was caused by mixture of incompletely reset detrital monazites; cross-cutting leucogranite dykes in the Yuanji- the collision of the Sibumasu- Qiangtang some of the oldest could actually be recording ang section (Figs. 8, 9). with the South China terrane (Carter et al., 2001; an Indosinian metamorphic event. Clearly these Maluski et al., 2001). High-temperature and ultra- ages cannot be related to either shear heating CHRONOLOGY OF METAMORPHISM, high temperature granulites were formed at 250– along the RRSZ (Leloup and Kienast, 1993) or MELTING, AND SHEARING ALONG 240 Ma (U-Pb ages, Roger et al., 2007; Sm-Nd to strike-slip shearing along the fault (Leloup THE RRSZ ages, Nakano et al., 2007) in the Kontum massif, et al., 1995, 2001, 2007). Gilley et al. (2003) central Vietnam, along with both garnet granites showed by time-dependent conductive thermal Field structural mapping can be used to con- and orthopyroxene–bearing granites (Owada et modeling that the heat supply through strike-slip struct a relative chronology of metamorphism, al., 2007). shearing would not have been suffi cient to raise granite intrusion and deformation along the RRSZ In the RRSZ, K-feldspar augen gneisses temperatures up to those required for high-grade in the DCS and ALS. The age of regional meta- were intruded into the host orthogneiss and metamorphism or melting. morphism has not been constrained yet by U-Th- both underwent regional metamorphism during The metamorphic fabric, both in the DCS Pb dating in this area, but our U-Pb zircon data which a regional metamorphic schistosity was and the ALS was clearly present prior to the

Strike- slip fault profile Closure + temperatures isotopic systems Rock Fault Mechanism Rheology Seismicity

Clay gouge U-Th-He apatite FT apatite Brittle faulting Brittle 200 FT zircon faulting Cataclasites Layer

Ar biotite Onset Quartz Seismogenic Ar muscovite plasticity

400 Mylonites Onset Feldspar max. rupture plasticity depth of Ar hornblende ? Ductile Migmatites shear flow

600 Plastic zone Muscovite dehydration Temperature (°C) Temperature melting

U-Pb monazite Aseismic viscous 750°C, 5kb U-Pb zircon 800 850°C, 5kb Biotite dehydration Granulite Amphibolite Greenschist melting

1000

Figure 12. Cross-section depth profi le through the RRSZ together with signifi cant isochrones derived from closure temperature of zircons and monazites in the U-Th-Pb system, closure temperatures of hornblende, biotite and muscovite in the Ar/Ar system and apatite and zircon in the Fission track system (Parrish, 2001). Also shown are the depths of the brittle-ductile transition, the onset of feldspar and quartz plasticity.

332 Geosphere, August 2010

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intrusion of biotite granodiorites and later set tion on cooling histories of exhumed metamor- that the basin started to open ca. 45 Ma and 1 biotite ± garnet ± tourmaline leucogranite phic and magmatic rocks. 40Ar/39Ar ages only more rapidly at ca. 35 M, but began to invert dykes. Host gneisses and the granite intrusive record a point on, or a small part of the cooling diachronously between 21 and 14 Ma (Clift rocks were then folded together by tight to curve; they do not give any information on tim- and Sun, 2006). The timing of the rapid basin

isoclinal folding (D1-D2). All these rocks were ing of peak metamorphism, melting or strike- deepening corresponds roughly to the timing of then subjected to high-strain during left-lateral slip shearing. 40Ar/39Ar ages from the DayNui- initiation of ocean fl oor spreading in the South

strike slip motion along the RRSZ (D3). Sub- ConVoi gneisses along the RRSZ in Vietnam China Sea (32–17 Ma; Briais et al., 1993, or horizontal stretching lineations parallel to the record a rapid cooling event at 27–21 Ma (P-L. 30–16 Ma; Cande and Kent, 1992). Ocean fl oor fault (120–140°) were superimposed on all Wang et al., 1998, 2000) immediately following spreading accelerated to 7.3 cm/yr at 25 Ma, and rocks within the shear zone. The fi nal magmatic leucogranite melting ages (Schärer et al., 1990, ended at 20.5 Ma (Zhu et al., 2009). The timing phase was a set of very small, thin leucogran- 1994; Zhang and Schärer, 1999). The RRSZ of the basin (21–14 Ma) corresponds ite dykes and veins that crosscut the mylonitic mylonites in Vietnam have fi ssion track ages to our proposed timing of cessation of ductile fabric (set 3 dykes in Figure 9a, b). Whereas all from 30 ± 4–20 ± 3 Ma (Viola and Anczkiewicz, left-lateral shearing and exhumation along the previous granites are all pre-kinematic, these 2008). 40Ar/39Ar ages from the Ailao Shan range RRSZ but before the more recent brittle right- dykes and veins are syn-to post-kinematic with from ~22–19 Ma, and ages from the Diancang lateral strike-slip faulting (Burchfi el et al. 2008). respect to the ductile strike-slip shear fabrics. Shan gneisses range from ~23–7 Ma (Leloup The fi nal deformation involved brittle strike- et al., 1993, 1995; Harrison et al., 1996). These MODEL FOR RRSZ IN THE DIANCANG slip faults at high structural levels and the authors reported a transition of 40Ar/39Ar cooling SHAN AND AILAO SHAN

range-bounding normal faults (D4) that accom- ages along the RRSZ from 17 Ma in the NW modated the fi nal few km of exhumation of the part of the Ailao Shan to 25 Ma in Vietnam and Our model for the RRSZ in Yunnan is based DCS and AS metamorphic rocks. proposed that the RRSZ propagated from SE to on fi eld structural evidence and timing con- Searle (2006, Fig. 5) reviewed all the geo- NW at a rate of 4.5 cm/a (“zipper tectonics”). straints as described above. The geometry of the chronological data from rocks along the RRSZ However, if the RRSZ was a result of the inden- RRSZ gneisses in the Diancang Shan (Fig. 14a) in Yunnan and North Vietnam. He concluded tation of the Indian plate into Asia as required and the Ailao Shan (Fig. 14b) do not conform to that strike-slip shearing occurred after 21 Ma, by the block extrusion model (Tapponnier et al., models of classical strike-slip fault zones. The the U-Pb age of the youngest leucogranite show- 1982. 1986, 1990; Leloup et al., 1993, 1995, DCS is bounded along both NE and SW mar- ing post-magmatic, high-temperature (~550 °C) 2001) the fi rst affects, and hence the oldest ages, gins by outward-dipping normal faults suggest- strike-slip shear fabrics. Subsequently Leloup et should be recorded nearer to the Indian indenter, ing that the width of the DCS gneisses increases al. (2007) and Sassier et al. (2009) reported new not farthest away from it. The “zipper” model is with depth. Foliation planes within the internal U-Th-Pb ages from the Yuangjiang site (Fig. 8). in direct contradiction to the 40Ar/39Ar and fi s- and northern part of the DCS and the DNCV in The earlier, more deformed dykes (set 1) are sion track data. Vietnam are sub-horizontal and cut by steeper 29.9 ± 0.5 Ma, the intermediate dykes (set 2) There is abundant evidence that major topog- left-lateral shear zones along the margins. are 26.8 ± 0.7 - 24.2 ± 0.4 Ma, and the late set raphy existed in the Tibetan Plateau region Stretching lineations are mainly horizontal and 3 cross-cutting leucogranite dykes are 22.5 ± during the late Oligocene–early Miocene, if strain markers are consistently left-lateral. Met- 0.2 Ma (Sassier et al., 2009). Using all the not even older. Apatite and zircon fi ssion track amorphic rocks are present outside and adja- U-Th-Pb age data (Schärer et al., 1990, 1994; thermochronology from sands along the Red cent to the DCS massif (Dali marbles) and the Zhang and Schärer, 1999; Gilley et al., 2003), River show accelerated cooling in the source DNCV (Sapa marbles to the SW and Yen Bai as well as the new Sassier et al. (2009) ages, we regions since 25 Ma (Clift et al., 2006). Major marbles to the NE) are unrelated to strike-slip interpret the timing of ductile shearing along the Miocene incision in the upper Red River drain- shearing and are possibly of Indosinian age. RRSZ in the Ailao Shan to be after 31.9 (age age basin was probably 13–9 Ma in Yunnan The RRSZ in the Ailao Shan (Fig. 14b) also of early set 1 deformed granites) and before (Clark et al., 2004). A later, important Pliocene has a Range Front fault along the NE margin of 21.7 ± 0.2 Ma, the age of the set 3 cross-cutting surface uplift is also recorded by (U-Th)-He the Ailao Shan that dips NE, away from the mas- dykes at this locality. The revised time chart for low-temperature thermochronology (Schoen- sif. The precise angle of dip is diffi cult to deter- the RRSZ (Fig. 13) now shows that the ductile bohm et al., 2006). Clift et al. (2006) and Clift mine because the fault is poorly exposed along left-lateral shearing along the RRSZ in the Ailao and Sun (2006) showed that the volume of sedi- the forested slopes of the Ailao Shan and the Red Shan occurred within a very narrow span during ment in the Yinggehai-Song Hong basin off- River, but the true dips are likely to be between the latest Oligocene – earliest Miocene, and was shore Vietnam is much greater than the volume 40 and 70°NE. The Ailao Shan fault along the followed by brittle faulting. The timing of end- of rock removed from the modern river basin, center of the massif (Leloup et al., 1995) sepa- ing of brittle faulting is diffi cult to constrain in supporting evidence that the ancient Red River rates low-grade mica schists to the SW from the the DCS and AS, but in the Gulf of Tonkin, off- may have included the upper Mekong and upper high-grade amphibolites, ortho gneisses, augen shore North Vietnam, seismic profi les show that Yangtze rivers, before their capture ca 24 Ma. gneisses, and mylonites of the RRSZ to the NE. Red River related strike-slip faults are truncated The Yinggehai-Song Hong basin in the Gulf of Foliations in the low-grade schists have dips by a prominent fl at-lying unconformity dated at Tonkin was part of the rifted South China Sea less than 45° and strike-slip lineations are much 5.5 Ma (Rangin et al., 1995). margin offshore North Vietnam, but records less common, so we do not include these rocks renewed rifting and subsidence as a result of in the RRSZ sensu stricto. Uncommon serpen- CHRONOLOGY OF COOLING AND transtension along the Red River fault. This tinized harzburgites occur close to the shear EXTENSION ALONG THE RRSZ basin contains 17 km of sediment eroded from zone in the NW part of the Ailao Shan, but to the southeastern margin of the Tibetan Plateau the SE around Mojiang, they are actually within 40Ar/39Ar dating of micas and fi ssion track and the Red River hinterland. Multichannel low-grade schists 5–15 km SW from the Red dating from apatite and zircon provide informa- seismic refl ection data and oil well data show River mylonite zone. The fault does not f ollow

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Ma EPOCH Vietnam RRSZ Gulf of Tonkin Gulf of SE Asia South China Sea DayNuiConVoi Ailao Shan Diancang Shan away from away 0 Xuelong Shan PLEISTOCENE passive PLIOCENE sedimentation 5 5.5 Ma end of left- lateral strike- slip RRSZ 10 trans- pression

15 15.5 Ma MIOCENE RRSZ- RRF brittle strike- slip - normal 20 3 trans-

2 25 Thai granite Thai granite Doi Inthanon granite ductile

? SPREADING SOUTH CHINA SEA SOUTH CHINA ? ductile shear 30 30 Ma Ar/ Ar cooling

OLIGOCENE 1 DNCV Bu Kang

35 m s SW- NE extension FanSiPan granite UPPER UPPER UPPER High- Alkali magmatism 40 MOGOK metamorphism, Burma

45 MIDDLE MIDDLE EOCENE

50 matrix monazites monazite inclusions U- Pb Sassier

LOWERU- Pb Scharer LOWER LOWER Ar- Ar 55 Th- Pb Gilley ages m monzonite PALEOCENE s syenite to 74 Ma

Figure 13. Chronology of metamorphism, magmatism, and deformation in the RRSZ using fi eld evidence and geochro- nology from the Diancang Shan and Ailao Shan in Yunnan and the DNCV in North Vietnam. U-Th-Pb age data are from Schärer et al. (1990, 1994), Zhang and Schärer (1999), Gilley et al. (2003), and Sassier et al. (2009). 1,2,3 relate to the three sets of leucogranite dykes at Yuanjiang, dated by Sassier et al. (2009). Our interpreted age of ductile left-lateral shearing along the RRSZ lies in between 2 and 3. Timing of Mogok metamorphism in Burma is from Searle et al. (2007), and alkaline magmatism from Chung et al. (1997, 2005). Timing of sea-fl oor spreading in the South China Sea is from Briais et al. (1993), and unconformities are related to interpretation of seismic data from Rangin et al. (1995).

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0 Neogene Quaternary- Neogene ? Mesozoic ? Mesozoic km Palaeozoic - ? Indo- Sinian marbles

10 Indo- Sinian ? basement ? ?

20 granites

AILAO SHAN B Ailao Shan Range Front fault fault

Serpentinized AILAO harzburgites SHAN Red River

SW NE

0 Neogene redbeds Redbeds ? Jurassic km ? ?

10 Indo- Sinian Low- grade basement mica schist

High- grade amphibolite, 20 augen gneiss mylonites

? mid-crustal ductile shear zone

Figure 14. Schematic structural cross sections showing geometries of the Red River shear zone (A) in the Diancang Shan and (B) in the Ailao Shan.

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an old suture zone, but rather cuts obliquely cene extensional faulting along the fl anks of els. They do not form classic strike-slip “fl ower across different geological zones. Since we pro- the metamorphic massifs (Range Front fault). structures,” with outward verging thrusts; they pose that the mantle-sourced granodiorites and If 40Ar/39Ar mica ages are used as a rough proxy are more like an exhumed deep-crust SW- early granites were intruded prior to strike-slip for timing of cooling through the ductile-brittle verging slab placing high-grade gneisses over ductile shearing, there is now no evidence that transition (~350°C), then this transition and low-grade schists, subsequently overprinted the RRSZ tapped down into the mantle as pro- rapid cooling occurred between ca. 25–22 Ma and cut by NW-SE strike-slip shearing fabrics posed by Tapponnier et al. (1990) and Leloup et in the Ailao Shan (Leloup et al., 2001). relating to left-lateral shear along the RRSZ. al. (1995). It is also clear that other Oligocene Finally, although there are numerous active The overall structure resembles an elongate alkali granite–syenite intrusions, both in Yun- faults in the Yunnan region (e.g.: E. Wang et al., gneiss similar to those in metamorphic nan and North Vietnam (e.g., FanSiPan granite), 1998; Burchfi el and Wang, 2002; Burchfi el et core complexes, although the origin of the DCS were intruded prior to strike-slip shear (Chung al., 2007, 2008; Schoenbohm et al., 2009), there and ALS is not the same as Cordilleran-type et al., 1997, 2008; Searle, 2006) and not dur- is no apparent active faulting linking the three metamorphic core complexes. ing strike-slip shearing (Leloup et al., 1995; metamorphic segments in the DCS, ALS, and Early granodioritic melts were mantle- Zhang and Schärer, 1999; Liang et al., 2007). DNCV complex, Vietnam. Although numerous derived, whereas later leucogranitic melts Undeformed syenites–alkali granites on Mount authors cite evidence for Quaternary or Recent containing Bt ± Grt ± Tur were derived from FanSiPan 10 km south of the RRSZ in North motion along the RRF with some displaced crustal melting. All these granites (except for Vietnam have been cut through by a few discrete stream channels crossing the fault, it is diffi cult a few very small set three cross-cutting veins) left-lateral shears with minor offsets, but the to see how it could be active today, given the were formed prior to mylonite fabric formation granite and the metamorphic rocks around Sapa seismic quiescence and the fact that the GPS and shearing along the RRSZ. U-Th-Pb ages of are not formed by shear heating along the RRSZ velocity fi eld cuts across the strike of the fault at K-feldspar augen gneisses, granodiorites, and (Jolivet et al., 2001; Searle, 2006, 2007). The right angles (Shen et al., 2005). many leucogranites will only give a maximum K-rich magmas are not restricted to the RRSZ, age of initiation of strike-slip shearing along the and were part of a regional high-K magmatic CONCLUSIONS RRSZ. U-Th-Pb ages from the few, very small event that lasted from 40 to 30 Ma, and possibly leucogranite dykes that cross-cut the left-lateral up to 20 Ma (Chung et al., 1997, 2005, 2008; We suggest that the metamorphism along strike-slip shear fabrics will give a minimum age Guo et al., 2004). both the DCS and AS was not formed during of left-lateral ductile shearing along the RRSZ. We suggest that the RRSZ gneisses were the Tertiary by shear heating along the RRSZ, Ductile shearing along the RRSZ occurred after not formed from shear heating during strike- but occurred mainly prior to strike-slip shear- Oligocene metamorphic zircon rim growth and slip faulting, but are uplifted basement rocks, ing, similar to the DNCV complex in Viet- post-early folded leucogranite intrusions (31.9 – possibly of Indosinian age, with an Oligocene– nam (Jolivet et al., 2001; Searle, 2006, 2007; 24.2 Ma), but prior to the later cross-cutting Early Miocene high-temperature overprint Anchiewicz et al., 2007; Yeh et al., 2008). U-Pb small dykes and veins (21.7 Ma). Brittle fault- event that resulted in localized partial melting zircon geochronology suggests that the RRSZ ing, mainly transtensional normal faulting along to form leucogranite dykes. In Yunnan, the Dali gneisses were metamorphosed during the Tri- the fl anks of the AS and DCS metamorphic marbles outside of the RRSZ show that meta- assic Indosinian event but have an Oligocene massifs, occurred after this time. morphism is not restricted to the shear zone. thermal overprint prior to intrusion of the Late U-Pb ages of the alkali granites must be pre- In Vietnam, high-grade marbles around Sapa to Oligocene–Early Miocene leucogranite dykes. strike slip shearing, although the 40Ar/39Ar ages the SW of the RRSZ and around Luc Yen to the Metamorphic rocks have been exhumed along may well relate to transpressional exhumation NE of the RRSZ also show regional metamor- the RRSZ but are not restricted to the shear zone; during the Miocene. Leloup et al. (1995, 2001) phism outside the RRSZ. Miocene leucogranite similar metamorphic rocks occur away from the interpreted the high temperature shear as coeval dykes also occur outside of the RRSZ (Ancz- shear zone in Yunnan (e.g., Dali marbles) and in with felsic magmatism that lasted from 33 to kiewicz et al., 2007), suggesting that their Vietnam (e.g., Song Chai dome, Sapa, and Luc 22 Ma (Schärer et al. 1990, 1994; Zhang and origin was not related to the strike-slip fault. Yen marbles, etc.). Metamorphism may have Schärer, 1999). However, if the shear fabrics Ubiquitous left-lateral strike-slip shear fab- reached hornblende melting temperatures (ca were superimposed onto earlier granite as we rics in RRSZ gneisses are post- metamorphic, 900 °C) resulting in in situ tonalitic partial melts suggest, then the timing of fault initiation must post- sillimanite growth (Jolivet et al., 2001; in orthogneiss migmatites. Maximum P-T con- be post-25–24 Ma. Searle, 2006; Anczkiewicz et al., 2007; Yeh ditions were reached prior to strike-slip shear- The precise age of metamorphism is not yet et al., 2008). However, transpressional motion ing, and the source of heat was not from shear known for certain, and RRSZ gneisses may well along the Red River fault was related to their heating along the fault. Transpression during show multiple periods of metamorphism. Trias- uplift and exhumation. Along the NE fl ank of early Miocene left-lateral strike-slip shearing sic metamorphism is known from North Viet- the Ailao Shan and Diancang Shan, the meta- exhumed deep crustal metamorphic rocks and nam (244 ± 7 Ma; Carter et al., 2001) as well morphic fabrics are truncated by the later steep superimposed strike-slip ductile mylonite fab- as Early Miocene melting (26.0–23.7 ± 1.7 Ma; NE-dipping range-bounding brittle normal rics onto earlier metamorphic fabrics. Nagy et al., 2000) in the Bu Kang dome, NE of fault. Harrison et al. (1992, 1996) proposed Strong, steeply inclined, left-lateral mylonite the RRSZ. Gilley et al. (2003) dated monazites that exhumation was related to transtension fabrics with very strong NW-SE aligned stretch- from the Xuelong Shan, Diancang Shan and

and propagated from SE to NW with time. ing lineations (D5) were superimposed on all Ailao Shan by in situ Th-Pb ion microprobe dat-

However, extension and transtension will not earlier metamorphic fabrics (D1-D4) by large- ing, with ages ranging from 34 – 21 Ma. Matrix exhume any rocks without a push from below. scale, left-lateral strike-slip shearing along the monazite ages from the DNCV complex are far It is far more likely that early transpression RRSZ. The ALS-DCS metamorphic massifs more complicated with ages ranging from 208– caused the upward motion of the RRSZ rocks form NE-dipping (45°-vertical) ductile shear 21 Ma. Monazite inclusion ages from DNCV and this was synchronous, or followed by, Mio- zones, thickening toward deeper structural lev- gneisses at Bao Yen in North Vietnam were 117 ±

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Chu, M.-F., 2009, Zircon U-Pb and Hf isotopic con- petrological and thermal evolution of a Tertiary ductile 2 Ma and 85.1 ± 1.1 Ma (Gilley et al., 2003), straints form eastern transhimalayan batholiths on strike-slip shear zone, Diancang Shan, Yunnan: Jour- clearly long before initiation of the RRSZ. the precollisional magmatic and tectonic evolution in nal of Geophysical Research, v. 98, p. 6715–6743, doi: These authors, however, interpreted the old ages southern Tibet: Tectonophysics, v. 477, p. 3–19, doi: 10.1029/92JB02791. 10.1016/j.tecto.2009.02.034. Leloup, P.H., Lacassin, R., Tapponnier, P., Schärer, U., as representing inheritance. Clearly much more Chung, S.-L., Lee, T., Lo, C., Wang, P., Chen, C., Yem, N., Dalai, Z., Xiaohan, L., Liangshang, Z., Shaocheng, high-precision U-Th-Pb dating of metamorphic Hoa, T., and Genyao, W., 1997, Intraplate extension J., and Trinh, P.T., 1995, The Ailao Shan- Red River prior to continental extrusion along the Ailao Shan- shear zone (Yunnan, China), Tertiary transform bound- and magmatic rocks that are structurally well Red River shear zone: Geology, v. 25, p. 311–314, ary of Indochina: Tectonophysics, v. 251, p. 3–84, doi: constrained is now required along the RRSZ. doi: 10.1130/0091-7613(1997)025<0311:IEPTCE> 10.1016/0040-1951(95)00070-4. 2.3.CO;2. Leloup, P.H., Arnaud, N., Lacassin, R., Kienast, J.R., Har- ACKNOWLEDGMENTS Chung, S.-L., Chu, M., Zhang, Y., Xie, Y., Lo, C., Lee, T., rison, T.M., Trong, T.P., Replumaz, A., and Tappon- Lan, C., Li, X., Zhang, Q., and Wang, Y., 2005, Tibetan nier, P., 2001, New constraints on the structure, ther- tectonic evolution inferred from spatial and tempo- mochronology, and timing of the Ailao Shan-Red River MPS thanks NERC grant NER/K/S/2000/951 for ral variations in post-collisional magmatism: Earth- shear zone, SE Asia: Journal of Geophysical Research, funding fi eldwork in Vietnam, and the Royal Soci- Science Reviews, v. 68, p. 173–196, doi: 10.1016/j v. 106, p. 6683–6732 doi: 10.1029/2000JB900322. ety for funding fi eldwork in Yunnan. M.-W. Yeh and .earscirev.2004.05.001. Leloup, P.H., Tapponnier, P., and Lacassin, R., 2007, Discus- T.-H.L. and S.-L.C. thank the National Science Coun- Chung, S.-L., Searle, M.P., and Yeh, M.-W., 2008, The age sion on the role of the Red River shear zone, Yunnan cil, Taiwan, for fi nancial support for fi eldwork. We also of the potassic alkaline igneous rocks along the Ailao and Vietnam, in the continental extrusion of SE Asia: thank Y. Iizuka at IES and H.-Y. Chiu at NTU for their Shan–Red River shear zone: Implications for the onset Journal of the Geological Society, v. 164, p. 1253– age of left-lateral shearing: A Discussion: Journal of 1260, doi: 10.1144/0016-76492007-065. help with CL-imaging and LA-ICPMS experimenta- Geology, v. 116. Liang, H.-Y., Campbell, I.H., Allen, C.M., Sun, W.-D., Yu, tion. We are grateful to B.C. Burchfi el for discussions, Clark, M.K., Schoenbohm, L.M., Royden, L.H., Whipple, H.-X., Xie, Y.-W., and Zhang, Y.-Q., 2007, The age P.H. Leloup for extremely detailed comments on an K.X, Burchfi el, B.C., Zhang, X., Tang, W., Wang, of the potassic alkaline igneous rocks along the Ailao earlier version of the manuscript, and P. Kapp and an E., and Chen, L., 2004, Surface uplift, tectonics Shan–Red River shear zone: Implications for the onset anonymous person for insightful reviews. and erosion of eastern Tibet from large-scale drain- age of left-lateral shearing: The Journal of Geology, age patterns: Tectonics, v. 23, p. TC1006, doi: v. 115, p. 231–242, doi: 10.1086/510801. REFERENCES CITED 10.1029/2002TC001402. 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Wang, E., Burchfi el, B.C., Royden, L.H., Liangzhong, C., MANUSCRIPT RECEIVED 25 NOVEMBER 2009 Searle, M.P., Noble, S.R., Cottle, J.M., Waters, D.J., Mitch- Jishen, C., Wenxin, L., and Zhiliang, C., 1998, Late REVISED MANUSCRIPT RECEIVED 31 MARCH 2010 ell, A.H.G., Hlaing, T., and Horstwood, M.S.A., 2007, Cenozoic Xianshuihe-Xiaojiang, Red River, and Dali MANUSCRIPT ACCEPTED 1 APRIL 2009

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