Structural constraints on the timing of left-lateral shear along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China Michael P. Searle Department of Earth 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 lithologies tation of India into Asia (Molnar and Tap- left-lateral Red River shear zone (RRSZ) and at high temperature (~500–550 °C) for the ponnier, 1975; Tapponnier et al., 1986, 1990, metamorphic belt and the Pliocene—active ALS and lower temperatures (~250–150 °C) 2001; Peltzer and Tapponnier, 1988; Leloup et right-lateral Red River fault (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 gneisses. 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- feldspar 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 mylonites 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 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/4/316/3338386/316.pdf by guest on 27 September 2021 Left-lateral shear along the Red River shear zone 90TIBET 92 94 96 98 100 Gangdese batholith LHASA Namche Barwa Gangdese batholith (105- c.50Ma) BLOCK 30 Syntaxis e Yarlung Tsangpo suture 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 r Lesser Himalaya ive Xuelong Shillong plateau PreCambrian R 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 au l f t - 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 Sagaing fault Dien Ben Phu fault Yebokson Mengxing fault A La 20 r 20 a Mai Sariang fault Luang Prabang k a Chiang Rai Ramree n Is. – Y o m MMB D BAY a Chiang Mai OF 18 BENGAL 18 T Vientiane n- Uttaradit suture Langsang Na gneiss basin Pathein Mae Ping fault Quaternary calc-alkaline i- Lao volcanoes Rangoon Moulmein Naga-Chin-Arakan Yoma fold belt Ranges 16 e I Sukhota n rra lta Mt.Victoria- Kawlun belt MVK o waddy de z Three Pagodas fault n Jade Mines belt HP ophiolites JM o i t BURMA PLATE BURMA c Katha- Gangaw ranges KGR u d ANDAMAN b Bangkok Tagaung-Myktyina belt TMB u s SEA 14 n Mogok metamorphic belt MMB a m Chiang Mai- Lincang belt a Chonburi d n Andaman Gaoligong- Salween A Islands Mergui gneisses GS Gulf of Red River-Ailao Shan 9092 94 96 98 100 Thailand 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 Mineral 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). Geosphere, August 2010 317 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/4/316/3338386/316.pdf by guest on 27 September 2021 Searle et al. 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 mountain 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.