The Song Ma Anticlinorium, northern Vietnam: the structure of an allochthonous terrane containing an early Palaeozoic island arc sequence

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Journal of Asian Earth Sciences, Vol. 15, No. 6, pp. 453±464, 1997 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1367-9120/98 $19.00 + 0.00 S0743-9547(97)00031-7

The Song Ma Anticlinorium, northern Vietnam: the structure of an allochthonous terrane containing an early Palaeozoic island arc sequence R. H. Findlay Department of Mineral Resources, Geological Survey Division, Private Mail Bag, Port Moresby, Papua New Guinea (Accepted in revised form 8 December 1997) AbstractÐThe Song Ma Anticlinorium displays evidence for four deformations, the ®rst two accompanied by high-grade to greenschist facies metamorphism, and the third by production of syn-low-grade metamorphic sheath folds. The fourth occurred during post-Cretaceous folding and thrusting which produced a thrust-fold complex across northern Vietnam. Whilst the Song Ma Anticlinorium may contain a metamorphosed island-arc volcano-sedimentary sequence, this Palaeozoic sequence cannot represent the active plate boundary in Triassic, Cenozoic or Recent times. The Song Ma Anticlinorium is not an Indosinian subduction zone, but is an allochthonous terrane which accreted to the South China Plate in Siluro-Devonian times. # 1998 Elsevier Science Ltd. All rights reserved

Introduction The Song Ma Anticlinorium has long been recognised as an anticlinorial metamorphic complex trending northwest across northern Vietnam (Tran Van Tri, 1973, 1979; Nguyen Ngoc Lien, 1980). It is juxtaposed (Fig. 1) against the Ordovician-Cretaceous Song Da region to the north by an extensive post-Cretaceous fault zone (Song Da Fault) and against the Truong Son Fold Belt (Tran Van Tri, 1979) to the south by the post-Triassic Song Ma Fault (Vietnamese 1:200 000 Son La and Dien Bien Phu geological maps) (Tran Dang Tuyet, 1978; Phan Son, 1978; see Figs 1 and 2). The Song Ma Anticlinorium is highly signi®cant for northern Vietnamese geology because it contains metamorphosed units thought characteristic of a subduction zone system (Tran Van Tri, 1979). The Anticlinorium has been interpreted as the possibly Indosinian boundary between the South China and Indochina plates e.g. Tran Van Tri (1979), Sengor et al. (1988), Bunopas and Vella (1978), Maranate and Vella (1986), Bunopas (1981) and Metcalfe (1993). However, until now no systematic structural geological studies have been made within the Anticlinorium even though the Vietnamese Geological Survey has produced 1:200 000 geological maps covering the region and have de®ned (Nguyen Ngoc Lien, 1980) the broad metamorphic zonation within the structure. Nor have any attempts been made to determine the kinematics of the Anticlinorium's boundary faults. Such studies are critical for determining the tectonic history and kinematics of the Song Ma Anticlinorium and hence for regional interpretations of northern Vietnamese geology. In order to assess regional geological interpretations of northern Vietnam the author worked in collaboration with the Director and the sta€ of the Petrology

Section of the Research Institute of Geology and Mineral Resources, Geological Survey of Vietnam, in and adjacent to the Song Ma Anticlinorium. This paper describes the structural evolution of the Song Ma Anticlinorium and is based on three detailed traverses (Thuan Chau to Co Ma; Son La to Chieng Khuong to Song Ma; and in the Nam Niem±Dien Lu region; Fig. 1) across the Anticlinorium. We conclude that the Song Ma Anticlinorium is part of a postCretaceous fold-thrust complex and represents an allochthonous terrane which docked with the South China plate in pre-Late Devonian times, and that it does not represent the post-Devonian boundary between the South China and Indochina plates.

Summary of regional geology The general geology of northern Vietnam is given in Fig. 1; Fig. 2 shows the geology of the Song Ma Anticlinorium. Broadly, south of the Song Hong Fault, northern Vietnam consists of ®ve geological entities (Fig. 1) which are, from north to south, the Con Voi region, the Tu Le region, the Song Da region, the Song Ma Anticlinorium and the Truong Son Fold Belt of Tran Van Tri (1979). The Con Voi region is dominated by a linear belt of highly strained high-grade schists assigned a Proterozoic age in the Vietnamese geological maps. Tapponnier et al. (1990) have obtained U/Pb data con®rming an Oligocene age for the major deformation and metamorphism in this schist belt, which lies between the Song Hong and Song Chay faults and which they interpret as the locus of a minimum of 500 km sinistral strike-slip along a ductile shear zone between the Indochina and South China plates. The northwestern part of the Con Voi region is occupied

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Fig. 1. Locality and geological region/terrane map. The Song Ma terrane has also been referred to as the Song Ma Anticlinorium (Tran Van Tri, 1979).

by the Phan Si Pan massif where the geology is dominated by migmatitic and granitic complexes o€set sinistrally by major faults. Slivers of Cambrian to Devonian sedimentary sequences also occur in the Con Voi region and trend more or less northwest. The Tu Le region was referred to as the Tu Le Rift Depression in Tran Van Tri (1979). It is a region dominated by Jurassic to Cretaceous calc-alkaline volcanic units and continental sedimentary rocks.

The Song Da region consists of Cambrian to Cretaceous sedimentary rocks ranging from marine carbonates to continental red beds, and includes a widespread series of Permian basalts and a PermoTriassic ``Gondwana'' sedimentary series. The Permian basaltic sequence includes pillow lavas, komatiitic lavas (Poliakov et al. 1991) and coarse, vesicular picrite lavas (Findlay pers. obs. 1993). The Song Ma terrane is an arched northwest-trending structure and has been referred to as the Song Ma

Fig. 2. Regional geology of study area, from the 1:1 000 000 geological map of northern Vietnam. ML-Muong Lam; NT-Nam Thi; CP-Co Phuong.

Song Ma Anticlinorium, northern Vietnam Anticlinorium (e.g. Tran Van Tri, 1979). It is dominated by low- to high-grade unfossiliferous schists intruded by Devonian and Triassic granitoids and is formed of pelitic schist, metagreywacke, greenschist and amphibolite, and marble. The southern part of the structure contains magnesitised and serpentinised ultrama®c bodies referred to as ophiolite by Vietnamese workers, and a gneissic plagiogranite called the Posen Complex in the Vietnamese 1:200 000 Dien Bien Phu and Son La geological maps (Tran Dang Tuyet, 1978; Phan Son, 1978). The Song Ma structure also contains non-schistose fossiliferous Middle Cambrian limestones at Dien Lu and is thought to include the possibly Permian Nui Nua ultrama®c massif west of Thanh Hoa. The Truong Son Fold Belt is a complex, faulted region dominated by Ordovician to Cretaceous sedimentary and subsidiary volcanic beds and contains possibly Cambrian but undated low- to high-grade metamorphic rocks at Phu Hoat (Tran Van Tri, 1973; Phan Cu Tien, 1989). The term Truong Son Fold Belt follows Tran Van Tri (1979). The Triassic to Cretaceous units in the northern part of the fold belt are correlates of those in the Song Da Zone (Tran Van Tri, 1973; Phan Cu Tien, 1989). The Truong Son Fold Belt is terminated near Da Nang by the Song Ca Fault (Fig. 1).

Song Ma Anticlinorium Stratigraphic and age relations The regional geology of the Anticlinorium is shown in Fig. 2. The core (Nam Co Formation) of the Song

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Ma Anticlinorium is ¯anked to the northwest by a sequence of metabasites, metagreywacke, and probable metatu€ (Song Ma Formation) with thin carbonate and siliciclastic beds (Pa Ham Formation). The southwest limb is dominated by similar rocks and includes also marble horizons, thin beds of chert, thin plagiogranite horizons (Posen Complex) and large pods of altered ultrama®c rock (Bo Xinh Complex) (stratigraphic nomenclature from Tran Van Tri, 1979). The core of the Anticlinorium is dominated by highly micaceous pelitic schist horizons called Nam Co Formation (Tran Van Tri, 1979). Kyanite, staurolite and sillimanite have been reported from these units (Nguyen Dinh Hop, pers. comm., 1993). Within the metamorphic rocks of the Anticlinorium, deformation and metamorphism are polyphasal, and the rocks are apparently unfossiliferous. The Song Ma Formation has been mapped as Cambrian and is held to overly the supposed Proterozoic Nam Co Formation with an unconformity in the northeast limb of the Anticlinorium (1:200 000 Dien Ben Phu and Son La Vietnamese geological maps). We saw no evidence for this unconformity as the Song Ma and Nam Co formations have the same polyphase structural and metamorphic history. Finally, although the Nam Co Formation has been subdivided according to age in the Vietnamese 1:200 000 geological maps, we know of no structural or geochronological data which support these divisions. In the southeast, the Anticlinorium strikes towards the inlier of Cambrian rocks exposed at Dien Lu (Fig. 3). The sequence here (Dien Lu Formation; Dinh Minh Mong, 1978) consists of non-metamorphic fossiliferous Lower to Middle Cambrian oolitic limestones overlying non-metamorphic beds of volcaniclastic

Fig. 3. Geological sketch map of the Dien Lu region, from the 1:200 000 Ninh Binh geological map, Vietnamese Geological Survey. Legend as for Fig. 2.

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lithicwacke which are inferred (see later) to be faulted against micaceous schist to the southwest. The Dien Lu Formation reappears southeast in the Thanh Hoa/ Sam Son region (Tran Van Tri, 1973), and is similar to Cambrian sequences (Phan Cu Tien, pers. comm., 1993) which, in the Ngai Tuyen region, rest on the South China Plate north of the Song Hong Fault (Fig. 1). The available geochronological data are limited and the palaeontologic data are reported in Vietnamese in the Vietnamese Geological Survey Explanatory Notes accompanying the 1:200 000 map series. Tran Van Tri (1979) cites Inouyia sp., Metanomocare sp., Utiidae and Damesellidae from the Song Ma Formation (see Phan Kim Ngan (1972) and Tong Bien Tap (1978) for source data, in Vietnamese). The only fossil localities shown in the 1:200 000 geological maps of the Song Ma Anticlinorium are in the Dien Lu Formation of the Dien Lu district, with an additional fossil locality lying in correlates of the Dien Lu Formation some 15±18 km to the northwest (Dinh Minh Mong, 1978). We note here that in the 1:1 000 000 geological map of northern Vietnam (Tran Van Tri, 1973) the distribution of Song Ma Formation and Dien Lu Formation di€ers from that in the 1:200 000 Ninh Binh geological map (Dinh Minh Mong, 1978). Our work in the Dien Lu inlier con®rms the rock distribution shown in Tran Van Tri (1973). This, and the distribution of fossil localities shown in the Dien Lu inlier by Dinh Minh Mong (1978), means that the schistose Song Ma Formation here is unfossiliferous. That is, in the entire Song Ma Anticlinorium, the only available palaenotological data are from the non-metamorphosed Dien Lu Formation. The only available geochronological data are three K/Ar ages of 455 Ma, 425 Ma and 360 Ma. These ages have been cited frequently in the Vietnamese literature but are rarely referenced; the source appears to be a thesis in Russian written by Phan Truong Thi (1978) and whose ages and abstract are cited by Nguyen Ngoc Lien (1980). The original ms is not available to the author. The 360 Ma age is from the thermal aureole of a granitoid intruding the schists of the Anticlinorium (Phan Cu Tien, 1989). The 455 Ma and 425 Ma ages may be taken as syn- to post-metamorphic cooling ages and may be interpreted as: (1) a metamorphic event with a minimum age of 455 Ma overprinted by a metamorphic event with a minimum age of 425 Ma; (2) a metamorphic minimum age of 455 Ma partially reset at 425 Ma by Late Devonian granitoid intrusion or delayed uplift; or (3) the metamorphic minimum age is 425 Ma, but there are local excess argon complications which would give older ages such as the 455 Ma age. Whatever the interpretation, the metamorphosed Nam Co and Song Ma Formations must be older than 425 Ma. The metamorphosed rocks assigned in the Song Ma Anticlinorium to the Pa Ham Formation must also be older than 425 Ma although their non-metamorphosed correlates are mapped as Ordovician-Devonian at Dien Lu (Dinh Minh Mong, 1978).

Summary of metamorphism The regional metamorphism has been ably summarised by Nguyen Ngoc Lien (1980) but has not yet been related to the detailed structural geology. Nguyen Ngoc Lien (1980) described a clockwise P-T-t path wherein the early phase of metamorphism ranged to kyanite-sillimanite zone conditions with staurolite and sillimanite growth in pelites at the maximum grade. The plagioclase content ranges from An10-28 (Nguyen Ngoc Lien, 1980) which may suggest partial closure of the peristerite gap as metamorphic conditions approached the kyanite zone. From the description by Nguyen Ngoc Lien (1980), the subsequent metamorphism was clearly thermal, was related to Devonian or younger granitoid intrusion, and produced andalusite and staurolite with albite and oligoclase; Nguyen Ngoc Lien (1980) notes that in the ®rst metamorphic phase the staurolite is concordant with the schistosity, whereas the subsequent metamorphic mineral, andalusite, cuts the schistosity. Tran Van Tri (1979) reported glaucophane from the Song Ma Anticlinorium, but cited no locality. Nguyen Ngoc Lien (pers. comm., 1993) investigated this report and his ®ve thin sections shown to me contain actinolite as the sole amphibole. This identi®cation has been con®rmed by X-ray studies (Nguyen Ngoc Lien, pers. comm., 1993). A verbal report of jadeite in the ultrama®c rocks was given to me during ®eld-work but according to subsequent discussion this report cannot be substantiated. Therefore, there appears to be no known evidence for blueschist facies metamorphism in the Song Ma Anticlinorium, although I believe that the metamorphic geology of the region may not have been either fully explored or fully reported in Englishlanguage publications. Our studies demonstrate three metamorphic events. The ®rst (M1) event produced a di€erentiation layering (quartz±mica in pelitic rocks; epidote±albite±actinolite and amphibole±plagioclase in metabasic rocks; gneissic layering in granitoids). The second event (M2) produced an extremely strong mica di€erentiation layering in the pelitic units leaving microfolded relics of the earlier fabric. This fabric was folded during M3 which produced a crenulation cleavage accompanied by growth of mica along the cleavage planes. Structural geology This section ®rst describes the structural features of the Song Ma Anticlinorium and then presents a section on the various local structural geometries in order to develop a regional structural geological interpretation. Our data are from: (1) the road traverse from Thuan Chau to Co Ma; (2) the road traverse from Son La to Chien Khuong via Mai Son and Ban Buon; (3) the area around Song Ma township; and (4) the Dien Lu-Nam Niem region in the Song Ma valley. Although the Song Ma Anticlinorium appears well mapped in the Vietnamese 1:200 000 geological maps, its structural history and fold geometries have not been described. Whilst our traverses do not allow domainal analysis they do yield the evolution of this complex structure. In sum, four folding deformations (F1, F2, F3, F4) occur within the Anticlinorium and

Song Ma Anticlinorium, northern Vietnam produced three metamorphic fabrics (S1, S2, S3) and associated intersection and crenulation lineations (L1, L2, L3). F4 is inferred from the change in geometry of the F3 structures, and its relation to faulting is discussed later. S-surfaces. The Anticlinorium's form surface is the regionally dominant, schistose, S2 fabric formed in the dominant pelitic units by parallel alignment of mica and ®ne lenses of quartz. S2, although parallel to gross lithological layering, is axial plane to very small (mm scale) hooks of quartz and thus clearly transposes an earlier fabric (S1). S2 is cut by two crenulation cleavages, one ®nely spaced and widespread, the other coarse and apparently domainal. In a very few localities only we could show that the ®ne crenulation cleavage is the earlier (S3), hence the coarsely spaced cleavage is referred to as S4. The major problem in distinguishing between the two cleavages is that they share a common steep dip and follow similar northwesterly trends. Lineations. No L1 was found, although within the Thuan Chau±Co Ma traverse (Fig. 2) we observed an early mineral lineation (L2) on S2. This L2 is formed from ®ne, weathered, possible Al2SiO5 (sillimanite, Nguyen Dinh Hop, pers. comm., 1993) marked now by generally rusty patches and less commonly by ®ne trains of biotite. Most outcrops of pelitic schist display one, and some two, younger crenulation lineations (L3 and L4).

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The orientations of the L3 crenulation lineations range between 0908 and 1608 (Fig. 6); this change in orientation occurs systematically across the Anticlinorium from Thuan Chau to (trend 0908) to Co Ma (trend 140±1608) and indicates that L3 must be folded by the Anticlinorium. L4 is a coarse crenulation trending northwest and was seen only in the Thuan Chau±Co Ma traverse. It appears to be developed domainly and is associated with a closely spaced cleavage also developed apparently domainally. The L4 trend follows the regional trend of the Song Ma Anticlinorium, which as noted is an F4 structure. Folds. The F2 folds in the Nam Co Formation form very small hooks of S1 quartz in the pelitic schist. Near Song Ma town, F1 is represented only be a metamorphic di€erentiation layering folded into F2 isoclinal folds (Fig. 4) with angular hinges and axial place schistosity (S2). The F3 folds form the dominant mesoscopic folds (scale 1 cm to 4 m, Fig. 4) seen by us principally in the Co Ma region and in the region around Song Ma town. The F3 fold geometries are complex as exempli®ed in the Co Phuong creek±Nam Thi creek region, at Muon Lam, and in the Posen Complex at Chieng Khuong. In general, the F3 folds generally are asymmetric, and have short rounded hinges and long limbs. The hinges are commonly thickened, whereas the limbs

Fig. 4. D3 folds: (A) and (B) in epidote-bearing greenschist 3 km southwest of Song Ma town; (C) in marble on S side of Nam Thi. Sketches from photographs taken down plunge of fold axes.

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are thinner. Interference of the F3 and F2 folds produces Type 3 (Ramsay 1967) interference structures. Northwest of Song Ma town the section between the Nam Thi and Muong Lam contains at Muong Lam southeast verging, angular F3 folds trending southeast and with a shallowly dipping axial surface (Fig. 5, locality 1). North of the Song Ma, the F3 folds at the Nam Thi are upright, plunge southeast, and have a sinistral vergence when viewed down-plunge (Fig. 5, locality 2). The relationship between these two sets of folds is best explained by refolding them by a southwest trending F4 syncline with a steep northeast dipping axial surface (Fig. 5, bottom, left). In contrast to the Nam Thi area in Co Phuong Creek, the upright F3 folds have dextral vergence (Fig. 5, locality 3, and bottom centre). This contrast speci®es an upright very tight F3 fold striking northwest between the lower part of Co Phuong Creek and the Nam Thi structures (Fig. 5, bottom, right). The full geometry is explained simply if the southwest-verging F3 folds at Muong Lam and southeast of Song Ma Town lay on the lower limb of this large, tight F3 structure (Fig. 5, bottom, right). Not only do the mesoscopic F3 folds suggest largescale F3 isoclinal folding (Fig. 5), but we also found evidence that F3 may have involved sheath folding. Near Muon Lam, hornblende±amphibolite horizons

are folded into tight, angular, southwest-verging F3 folds. The hornblendes are oriented parallel to the F3 fold axes and L3 crenulation lineation but rotate within a constant axial surface. Therefore, the F3 folds here have primarily curved axes and could be regarded as sheath folds (see below, Fig. 6 Net I) and the hornblende lineation may be a stretching lineation. Similarly, in the orthogneiss (Posen Complex) near the Chieng Khuong border-post some F3 fold axes showed a variable orientation in a uniform axial plane and again could be sheath folds. (Note that photography was not permitted in the vicinity of the Chieng Khuong border-post; the ®eld measurements here are presented below). Structural geometries. Figure 6 presents our structural measurements in the Song Ma Anticlinorium and is divided into two parts: stereonets A±D show principally the orientation of the planar features at di€erent localities and stereonets E±I illustrate the orientations of the linear features. Table 1 presents a discussion of the various geometries encountered. In respect of S-surface orientations, there are four main conclusions. First in the Co Ma±Thuan Chau traverse both S2 and S3 are folded about a 145±3258 (Net A) trend, which is the trend of the Song Ma Anticlinorium. This must therefore be an F4 structure. Second, the F3 axis, as de®ned by S2 measured around

Fig. 5. TopÐSketch geological map of the Song Ma district (from 1:200 000 Son La geological map) to show vergence changes of F3 folds in the southwest limb of the Song Ma Anticlinorium. BottomÐInterpretation of vergence geometries. LeftÐMuon Lam/Nam Thi region, F4 fold clearly de®ned; centreÐCo Phuong/Song Ma region, could de®ne F3 or F4 fold; rightÐaggregate demands F3 isoclinal fold refolded by F4.

Song Ma Anticlinorium, northern Vietnam

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Fig. 6. Structural geometries in Song Ma Anticlinorium. Equal area southern hemisphere projections. A±D are S-surfaces; E±I are linear features. (A) Thuan Chau-Co Ma traverse. Solid circlesÐdominant metamorphic foliation (S2); open circlesÐS3 crenulation cleavage, b to 1458/3258. (B) Thuan Chau-Chieng Khuong traverse. Solid circlesÐdominant metamorphic foliation (S2); open circlesÐS2 in plagiogranite (Posen Complex) at Chieng Khuong, b 20 to 2808. (C) Co Phuong, Nam Thi, Ban Muon. CrossesÐS2 at Co Phuong; open circlesÐS2, Ban Muon; open squaresÐS2, Nam Thi; solid circlesÐS3. The dominant b is between 108 to 3008 and 108 to 2908. (D) Dien Lu district. Solid trianglesÐL3 from schistose rocks; solid circlesÐS2 from schistose pelitic units near Dien Lu and along the Nam Niem traverse; open circlesÐS0 from the Middle Cambrian fossiliferous non-schistose rocks near Dien Luu. (E) Thuan Chau to Co Ma. CrossesÐmineral lineation; solid circlesÐL3 crenulation lineation; solid trianglesÐB3 fold axes. (F) Ban Buon to Chieng Khuong. Solid circlesÐL3; trianglesÐF3 axes; crossesÐmineral lineation. (G) Song Ma±Co Phuong-Nam Thi area. Solid circlesÐL3; solid triangles±F3 axes. Great circles are measured F3 axial surfaces. (H) Posen Complex plagiogneiss at Chieng Khuong. Solid trianglesÐF3 axes; open circlesÐfold axes of unknown age; solid circlesÐaxes of mesoscopic sheath folds; open trianglesÐmineral lineation; open squaresÐS2 around sheath folds for comparison with Net B; solid squaresÐS2 elsewhere in outcrop. (I) Muong Lam. Solid trianglesÐF3 axes; crossesÐhornblende lineation with F3 axial trace shown; open circlesÐmetamorphic foliation (S2).

F3 hinges in the Posen Complex (Net B), and along Co Phuong and Nam Thi creeks, Ban Muon and Muon Lam (Net C) trends 290±3008 and the folds have a double plunge (Net C). Third, the F3 deformation on the 2908 trend can be recognised readily in

the Dien Lu±Nam Niem district (Net D) and F3 folds both the Song Ma and Dien Lu formations. Fourth, the small circle distributions in Nets B, D and possibly A are indicators that the F3 structures are non-cylindrical which agrees with local evidence

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R. H. Findlay Table 1. Summary of structural geometries; refer to Fig. 7

NET A NET B NET C NET D NET E NET F NET G NET H NET I

Thuan Chau±Co Ma traverse. S2 folded about NW subhorizontal axis of Song Ma Anticlinorium. S3 lies on this great circle; Song Ma Anticlinorium is F4 structure. b 008 to 325/1458. Thuan Chau±Chieng Khuong traverse. S2 and gneissic foliation in Posen Complex folded about b 20 to 2808. Folds in Posen Complex are F3 folds, hence F3 axis is WNW as opposed to NW F4 axis. Note small circle distribution of some S2 measurements about NE±SW trend. Song Ma town±Nam Thi region. S2 folded about b 108 to 3008 to 108 to 2908 following F3 trend in Net B. S3 is steep. Dien Lu district. Both schistose and non-schistose rocks are folded about the WNW±ESE F3 trend marked by crenulation lineations in the schistose rocks. Note possible small circle distribution of S2 poles about NE±SW trend, as in Net B. Thuan Chau±Co Ma traverse. The F3 crenulation lineations de®ne a small circle about an axis trending ca 2958. The mineral lineations here are the ®ne rusty streaks thought to have been sillimanite grown during the F2 event. Ban Buong±Chieng Khuong. F3 crenulation lineations fold axes and mineral lineations track along a great circle dipping steeply SE but are folded around small circles about the NW Song Ma Anticlinorium. Co Phuong±Chieng Khuong. F3 fold axes and L3 trend WNW but some track along great circle dipping SW (see also NET F), whereas others track around small circles on a WNW trend indicating co-axial refolding by Song Ma Anticlinorium. Axes track along axial surface plots, possible sheath folding. Chien Kuong: the S2 around recognised F3 folds plots on an E-dipping great circle whose statistical axis coincides with the F3 fold axes and a mineral lineation. The steeply north-dipping girdles are measured F3 axial planes. The di€erence in orientation between the F3 trend in G and H could be due to mega-kinking. Muon Lam: poles to S2 de®ne NW±SE trending F3 structures. The F3 linear structures, the statistical F3 axis lie in a shallowly SE dipping great circle which is close to the measured axial surfaces shown by the partial great circle girdles for the measured F3 axial planes.

at Muon Lam and in the Posen Complex at Chieng Khuong. The linear structures measured are principally the F3 fold axis (B3) and L3 (nets, D, E, F, G, H). The geometries of these structures con®rm the west to northwest orientation of the F3 folds, which is close to that of the Song Ma Anticlinorium. However, there are excursions from this trend: in Net E the L3 lineations are rotated about a small circle across a northwest trend and this change in orientation takes place systematically from southwest to northeast across the Anticlinorium con®rming that F3 structures are refolded by the Anticlinorium. A similar feature is seen in Net F although the data are not numerous. The other aspect of Net F is that the L3 lineations track along a steeply southwest-dipping great circle. This could indicate sheath folding during F3, which is con®rmed by ®eld observations at Muon Lam and Chieng Khuong (see above). Similarly, in Net G, the F3 structures track along steeply northeast-dipping great circles which are the measured axial planes of the F3 folds; this is interpreted as indicative of sheath folding. Again in Net G and Net H there is also a small circle rotation of the F3 linear structures about a northwest trend suggesting refolding about the Song Ma Anticlinorium. The F3 axes and mineral lineations in Net I (Muon Lam) are plotted together with partial great circles showing the measured F3 axial surfaces. These F3 lineations show a scatter which includes the statistical fold axis along a south SE dipping great circle which is parallel to the F3 axial planes at Muon Lam. This is further con®rmation of F3 sheath folding. Finally, Nets F, G, H, and I show a rotation of the F3 axial planes which is tracked by the change in orientation of the statistical pole to these axial surfaces. This rotation clearly occurs about the NW trend of the F4 Song Ma Anticlinorium and follows the small circle rotation of the F3 lineations. Kinking. Post-S3 kink bands are common within the Anticlinorium. These structures are kinematically equivalent to faults and their geometry is indicated in Fig. 7. Brie¯y, reverse kink bands dominate and con-

®rm compression across the Anticlinorium; the orientation of these structures mimics that of the small-scale reverse faults and thrusts described later. Similarly, the two strike-slip kink bands labelled A and B in Fig. 7 con®rm compression across the Anticlinorium. A regional palaeostress ®eld may be determined from this array if all reverse kink bands are treated as contemporaneous. The poles to these kink bands de®ne a great circle, intimating a common s2, which lies moreor-less in the trend of the Song Ma Anticlinorium. Thus, s1 and s3 can be determined as the faults are reverse; s1 plunges about 25±0208. There is a close similarity between the geometries of the reverse kink bands and major faults in and adjacent the Song Ma Anticlinorium. There is also a close

Fig. 7. Kink band geometries across Song Ma Anticlinorium. Twin straight linesÐSong Ma Anticlinorium trend. Solid circlesÐpoles to kink bands; heavy broken great circles 1 and 2 show the two kink band systems. Great circle 1 is formed from conjugate kink bands illustrated by the dashed great circles. Great circle 2 is single kink bands with reverse fault geometries. Equal area lower hemisphere projection.

Song Ma Anticlinorium, northern Vietnam

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Fig. 8. Interpretation of pre-fault con®guration at Dien Lu.

similarity between the palaeostress ®elds derived from the kink bands, from considerations of the geometries of the minor structures in the major fault zones and the palaeostress directions derived from fault-striation data (Findlay and Phan Trong Trinh, 1997). These geometrical similarities are a sound argument that the kinking in the Song Ma Anticlinorium is related to the regional faulting. Faulting. The details of faulting in the study region are given in Findlay and Phan Trong Trinh (1997). The Vietnamese Geological Survey has mapped numerous northwest-trending faults crossing Vietnam between Da Nang and the Song Hong, and it is unequivocally clear from this mapping (Tran Van Tri, 1973; Phan Cu Tien, 1989) that these faults are younger than Cretaceous. Some of these faults cut Neogene strata, and both the Song Ma and Song Hong follow broad seismic corridors (Nguyen Dinh Xuyen, 1991; Nguyen Ngoc Thuy, 1991). Our results (Findlay and Phan Trong Trinh, 1997) con®rm thrusting and sinistral strike slip, and thrusts cut the northeast limb of the Anticlinorium a few kilometres southwest of Thuan Chau (approx. grid references 600,650±625,652 on the 1:200 000 Dien Bien Phu geological map). We also noted evidence in Co Phuong Creek for dextral slip; here ``ductile'' structures (C-S) fabric) show evidence for sinistral slip, whereas in a nearby ``brittle'' cataclastically deformed fault Riedel R shears suggest dextral slip. This contrast suggests early sinistral ``ductile'' shear preceded dextral ``brittle'' faulting. In the Dien Lu region (Fig. 3), the relations between topography, rock unit distribution and faults (Dinh Minh Mong, 1978) show that thrust faulting has pro-

duced a series of thrust fault duplexes. It is clear from the pre-fault geology in the Dien Lu inlier (Figs 3 and 8) that beds in the southern part of the inlier can be matched by reversal of thrusting. Although it is less easy to determine the pre-fault arrangement west of Dien Lu (Fig. 8), a fold style similar to that in the Dien Lu inlier may be obtained to the northwest of Dien Lu by treating the major faults as sinistral reverse faults (Fig. 8). Reconstruction of the pre-fault geology at Dien Lu (Fig. 8) aligns the schist now west of the Dien Lu inlier almost along the strike of the schist in the Dien Lu inlier. The Song Ma Fault forms the southern boundary of the Anticlinorium. According to Tran Dang Tuyet (1978), Phan Son (1978) and Dinh Minh Mong (1978), o€sets of Devonian and Triassic granitoids intruding the Anticlinorium could indicate 120 km of sinistral slip on this fault. As geophysical data (Nguyen Ngoc Thuy, 1991; Nguyen Dinh Xuyen, 1991; Le Than Hoi et al. 1991) indicate that the Song Ma Fault dips northeast at between 208 and 608, the Song Ma Fault must be a sinistral reverse fault.

Discussion Relationship between folding, faulting and kinking In general, the F3 axial surfaces dip both steeply northeast and also less steeply southwest and this suggests that they are folded. Using the vergences of the F3 folds in the Song Ma±Muon Lam region we are able to demonstrate here a southwest-facing F3 synform with a steep northeast dipping axial surface

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(Fig. 5). In this area also the F4 lineations are rotated along great circle plots which is consonant with simple shear along the F3 axial plance during F3 and also with the observation of small-scale sheath folds in the orthogneiss of the Posen Complex at Chieng Huong. The mapped trend of the Song Ma Anticlinorium is northwest, and this structure folds S2 and also S3. The rotation of the lineations in the Thuan Chau±Co Ma traverse (Fig. 6) and at Muon Lam is the result of their folding by the Song Ma Anticlinorium, a regional F4 fold. Local cross-folding, possibly by megakinking, is consistent with the small circle excursions seen in stereonets B and D, Fig. 6. It is clear that the F3 fold trend is subcoaxial to the mapped trend of the Song Ma Anticlinorium, and this is consistent with our observations that the patchily developed S4 fabric is similar in orientation to that of S3. Northern Vietnam has a marked northwest trending structural grain conferred by folds and faults. With the exception of the Song Hong shear zone (Tapponier et al., 1990), none of these structures were accompanied by regional metamorphism. The Nam Sap Syncline (Nguyen Xuan Bao and Phan Cu Tien 1978) northwest of Moc Chau is a structure important to understanding the timing of folding and faulting in northern Vietnam. This southwest verging structure deforms Cretaceous rocks, which are cut by what topographic relations demand (Findlay and Phan Trong Trinh, 1997) is a regionally extensive shallow thrust. It is clear from the data in Nguyen Xuan Bao and Phan Cu Tien (1978) that this region forms part of a post-Cretaceous fold-thrust belt. The Song Ma Anticlinorium also trends northwest. As F3 formed a metamorphic fabric it most likely occurred at or before the youngest K/Ar age of 425 Ma and cannot be related to the post-Cretaceous deformation. As the Nam Sap Syncline and the F4 Song Ma Anticlinorium post-date metamorphism and trend parallel to the regional folds in the F4 Song Da region, we conclude that this deformation most probably correlates with the regional post-Cretaceous folding in northern Vietnam and that the Song Ma Anticlinorium is not an Indosinian structure. It is logical to argue that the northwest trending folds and faults are related as both types of structure involve shortening in more-or-less the same direction. Thus, the regional northwest trending folds could be

an early ductile shortening strain which was followed by localisation of strain to reverse/thrust faults and to transpressive sinistral strike-slip faults. In this context and from the presence of thrust faults cutting both limbs of the Song Ma Anticlinorium, and from the structure of the Dien Lu area, it is reasonable to conclude that the Song Ma Anticlinorium consists of a large fault-bend fold/duplex system (Fig. 9). Kink folds are equivalent kinematically to faults and the geometry of kink faulting in the Song Ma Anticlinorium is consistent with reverse faulting. s1 derived from the kink bands fault system plunges at a shallow angle northeast, almost orthogonal to the Song Ma Anticlinorium, and is consistent with thrusting on the northwest trending faults. The mesoscopic evidence for faulting here also suggests a shallowly northeast plunging s1 (Findlay and Phan Trong Trinh, 1997). Tapponier et al. (1990) have identi®ed major sinistral slip during Oligocene times along the Song Hong Fault. Nguyen Trong Yem et al. (1991) and Phan Trong Trinh et al. (1994) present palaeostress maps for Miocene and Plio-Quaternary times in northern Vietnam and show a shallowly plunging, regional, east-directed s1 for Miocene times and a shallowly plunging north to northeast-directed s1 for PlioQuaternary times. Oligocene sinistral strike-slip across northern Vietnam would promote folding on a north to northnorthwest trend and as sinistral strike-slip continued the folds would rotate anticlockwise into a northwest trend. Thrust faulting on a north to northnorthwest trend would also occur, and again the faults would rotate to the northwest as sinistral slip continued. The Pliocene to Quaternary palaeostress vector (Phan Trong Trinh et al., 1994) would further amplify this rotation and would engender additional thrusting.

Status of Song Ma Anticlinorium as a plate boundary The Song Ma Anticlinorium was shown as a subduction zone between the South China and Indochina plates (Tran Van Tri, 1979) and has also been described by Bunopas and Vella (1978), Bunopas (1981), Maranate and Vella (1986), Sengor et al.

Fig. 9. Geological cross-section, Song Ma Anticlinorium.

Song Ma Anticlinorium, northern Vietnam (1988) and Metcalfe (1993) as the plate boundary. As the Anticlinorium contains fault-slices of metagreywacke, metabasite, marble, ultrama®cs and plagiogranite, it could be interpreted as the metamorphosed relic of a fore-arc and island arc complex. However, as the protolith of the Song Ma Anticlinorium had been metamorphosed by Silurian times it cannot have formed an island-arc system along an Indosinian (Permo-Triassic) subduction zone between the South China and Indochina plates. Second, the Song Ma Fault is only one of many major post-Triassic, probably Cenozoic, faults in the modern complex boundary region between the South China and North China plates and is neither the plate boundary fault now and nor can it have been in Permo-Triassic times. The relationships at Dien Lu are critical to the status of the Song Ma Anticlinorium. Here, the schists of the Anticlinorium are juxtaposed against the Lower to Middle Cambrian oolitic limestone beds and the nonmetamorphosed volcaniclastic lithicwacke of the Dien Lu Formation. The minimum age of the schists is 455±425 Ma (see earlier). The schists and the Dien Lu Formation are overlain unconformably by non-metamorphosed limestones of the Ordovician-Devonian Pa Ham Formation (Dinh Minh Mong, 1978). As the schists were metamorphosed in Middle OrdovicianSilurian times they cannot be overlain unconformably by the older non-metamorphosed Dien Lu Formation. Therefore, the two units must be juxtaposed by a hidden fault which predates the unconformably overlying and folded Pa Ham Formation but post-dates metamorphism. The Dien Lu Formation contains a Lower to Middle Cambrian fauna (Phan Kim Ngan, 1972; Tong Bien Tap, 1978; see also Tran Van Tri, 1979) which also occurs in similar rocks on the South China plate near Thai Nguyen (Phan Cu Tien, pers. comm., 1993) and which correlate with the Dien Lu Formation (Tran Van Tri, 1973). Therefore, the Dien Lu Formation must form part of the South China plate. The marked di€erence in protolith and metamorphic/ structural history between the juxtaposed Song Ma schists and the shelf sequence of the Dien Lu Formation suggests that the fault separating the two was a major structure; this fault could have been the South China plate boundary before deposition of the Pa Ham Formation. Therefore, the Song Ma Anticlinorium is most likely a relic of an allochthonous metamorphosed subduction-related sequence (Song Ma terrane) that docked with the South China plate after subduction and metamorphism (post-425 Ma) but before deposition of the non-metamorphosed rocks of the Pa Ham Formation. It is not possible to assess from our data the polarity of subduction and it may be an irrelevant exercise as the Song Ma Anticlinorium would seem to be an allochthonous terrane. These conclusions clarify the presence of Devonian ®sh at Ly Hoa, well south of the Song Ma Fault (Tong Dzuy Thanh et al., 1996). These ®sh occur in marginal marine Givetian-Lower Frasnian beds and were previously known only in the Lower Devonian beds of the South China plate. As the Song Ma terrane was a metamorphosed terrane which had accreted

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to and formed part of the South China plate before deposition of the Pa Ham Formation, then the ®shbearing rocks at Ly Hoa could well have been deposited on what was the South China plate in GivetianFrasnian times.

Conclusions There are four major deformations in the Song Ma Anticlinorium, the ®rst three of which involved metamorphic events. The ®rst three are of pre-Devonian age and may be Middle or Early Ordovician in age given the sparse K/Ar data available. The fourth deformation produced the anticlinorial structure which consists of contemporaneous post-Cretaceous folds and thrusts. This deformation has caused, and may still be causing, fragmentation of the southern part of the South China plate. The Song Ma Anticlinorium is bound by postTriassic and post-Cretaceous faults and the stratigraphic, structural and metamorphic history of the schistose rocks is unique. The anticlinorium therefore may be regarded as formed of a pre-Devonian terrane once separate from the rest of Vietnam. Present data favour accretion of this terrane to the South China plate after Cambrian times and before deposition of the Ordovician-Devonian Pa Ham Formation. Acknowledgements ÐThis paper, a contribution to IGCP 321, was sponsored by a grant from DITAC (Australia) to R.H. Findlay. Field-work was undertaken by invitation from Dr. Phan Cu Tien, Director, Research Institute for Geology and Mineral Resources, Vietnamese Geological Survey. The paper was prepared at the Geological Survey of Papua New Guinea by Dr. R.H. Findlay. Dr. D. Workman, University of Hong Kong, is thanked for his encouraging comments on the original drafts. Sponsorship from IGCP 321 to enable Dr. R.H. Findlay to present aspects of this work at the 4th and 5th IGCP Conferences is gratefully acknowledged. Assistance was also provided by BHP Minerals (Asia) Pty. Ltd. We thank our colleagues from RIGMR for collaboration during ®eld-work. These are: Phan (u Tien (Director), Nguyen Thu Gaio; Nguyen Din Hop; Pham Binh; Nguyen Ngoc Lien; Xuan Vo Dinh.

REFERENCES Bunopas, S. (1981) Palaeogeographic history of western Thailand and adjacent parts of Southeast AsiaÐa plate tectonics interpretation. Unpublished Ph.D. Thesis, Victoria University of Wellington, reprinted and published as Geological Survey Paper 2, 1982, Department of Mineral Resources, Bangkok. Bunopas, S. and Vella, P. (1978) Late Palaeozoic and Mesozoic structural evolution of northern Thailand, a plate tectonics model. In Proceedings of the 3rd Regional Conference on Geology and Mineral Resources of Southeast Asia, Bangkok, ed. P. Nutalaya, pp. 24±32. Dinh Minh Mong (1978) To Ninh Binh F-48 XXXIV, 1:200 000 Geological Map Series of S.R. Vietnam, Geological Survey of S.R. Vietnam, Hanoi. Findlay, R. H. and Phan Trong, Trinh (1997) The structural setting of the Song Ma Anticlinorium, Vietnam and the Indochina-South China plate boundary problem. Gondwana Research 1, 11±33. Le Than Hoi, Nguyen Tai Thinh, Chu Quoc Khanh, Nguyen Xuan Ngoan and Nguyen Huu Bac (1991) Geological structures of the Da and Ma Rivers area based on geophysical data. In Proceedings

464

R. H. Findlay

of the Second Conference on Geology on Indochina, pp. 84±87. Geological Survey of S.R. Vietnam. Maranate, S. and Vella, P. (1986) Palaeomagnetism of the Kohorat Group, Mesozoic, northeast Thailand. Journal of Southeast Asian Earth Sciences 1, 23±31. Metcalfe, I. (1993) Southeast Asian terranes: Gondwana origin and evolution. In Gondwana 8, eds R. H. Findlay, R. Unrug, M. R. Banks and J. J. Veevers, pp. 181±200. A.A. Balkema, Rotterdam. Nguyen Ngoc Lien (1980) Geology and metamorphism in the Song Ma zone of Vietnam, Geologiya i Geo®zika, 21, 61±71, (English translation 1980 by Allerton Press, UDC reference 552.16 (597.7), 46±54, (Note Anglicized spelling of Lien as Ljen). Nguyen Ngoc Thuy (1991) Geology-geophysical criteria of seismogenic zones in the territory of north Vietnam. In Proceedings of the 2nd Conference on Geology of Indochina, pp. 53±58. Geological Survey of S.R. Vietnam. Nguyen Dinh Xuyen (1991) Seismic zoning of the territory of Vietnam. In Proceedings of the 2nd Conference on Geology of Vietnam, pp. 39±44. Geological Survey of S.R. Vietnam. Nguyen Xuan Bao and Phan Cu Tien (1978) To Van Yen F-48XXVII, 1:200 000 Geological Map Series of S.R. Vietnam, Geological Survey of S.R. Vietnam, Hanoi. Nguyen Trong Yem, Phan Trong Trinh and Phung Van Phach (1991) Geological structure and stress ®eld of Hoa Binh region and its surrounding areas. In Proceedings of the 2nd Conference on Geology of Indochina, ed. Tran Van Tri, pp. 388±395. Geological Survey of S.R. Vietnam. Phan Kim Ngan (1972) New biostratigraphical data of the Lower Palaeozoic in the Than Hoa region, (in Vietnamese) (cited by Tran Van Tri, 1979). Geology Number 104, Vietnamese Geological Survey Publication. Phan Son (1978) To Son La F-48-XXVI, 1:200 000 Geological Map Series of S.R. Vietnam, Geological Survey of S.R. Vietnam, Hanoi. Phan Truong Thi (1978) Physicochemical conditions for the formation of regional metamorphic complexes of the S.R. Vietnam, Abstract of Dissertation for Candidate's Degree, Moscow, (in Russian; Russianised author spelling Fan Chyon Tkhi; cited by Nguyen Ngoc Lien, 1980). Phan Cu Tien, (1989) Geology of Kampuchea, Laos and Vietnam and accompanying 1:1 000 000 geological maps. Institute for

Information and Documentation of Mines and Geology, Hanoi, 149 pp. Phan Trong Trinh, Nguyen Trong Yem, Herre, L. P. and Tapponier, P. (1994) Late Cenozoic stress ®elds in north Vietnam from microtectonic measurements. In Proceedings of the International Workshop on Siesmotectonics and Siesmic Hazard in Southeast Asia, pp. 182±186. Geological Survey of S.R. Vietnam, Hanoi. Poliakov, G. V., Balykin, P., Glotov, A. I., Trong Quoc Hung, Ngo Thi Phuong, Hoang Huu Thanh and Bui An Nien (1991) HighMagnesian volcanites in Da River zone. In Proceedings of the 2nd Conference on Geology of Indochina. Geological Survey of Vietnam, ed. Tran Van Tri, pp. 247±261. Ramsay, J. G. (1967) Folding and Fracturing of Rocks. McGrawHill Book Co., Ltd., New York. pp. 568. Sengor, A. M. C., Altiner, D., Cin, A., Ustaomer, T. and Hsu, K. J. (1988) Origin and assembly of the Tethyside orogenic collage at the expense of Gondwanaland. In Gondwana and Tethys, ed. M. G. Audley-Charles and A. Hallam. Geological Society Special Publication 37, Oxford University Press, Oxford. Tapponnier, P., Lacassin, R., Lelaup, H., Scharer, U., Zhong Dalai, Wu Haiwei, Lui Ziaohan, Ji Shaocheng, Zhang Lianshang and Zong Jiayou (1990) The Ailo Shan/Red River metamorphic belt:Tertiary left-lateral shear between Indochina and South China. Nature 342, 431±437. Tong Bien Tap (1978) Dia chat to Ninh Binh F-48-XXXIV, (Explanatory notes in Vietnamese for the 1:200 000 Ninh Binh Geological Map). Geological Survey of S.R. Vietnam, Hanoi: 81 pp (in Vietnamese; English and Russian abstracts). Tong Dzuy, Thanh, Janvier, P. and Ta Hoa, Phuong (1996) Fish suggest continental connections between the Indochina and South China blocks in Middle Devonian time. Geology 24, 571±574. Tran Dang Tuyet (1978) To Dien Bien Phu F-48-XX, 1:200 000 Geological Map Series of S.R. Vietnam, Geological Survey of S.R. Vietnam, Hanoi. Tran Van Tri (1973) 1:1 000 000 Geological Map of Vietnam (the north part). The General Department of Geology of the Democratic Republic of Vietnam, Hanoi. Tran Van Tri. (1979) Geology of Vietnam (the North Part) Science and Technology Publishing House (1977 in Vietnamese, 1979 in English): pp. 78.