The Structural Setting of the Song Ma Region, Vietnam and the Indochina-South China Plate Boundary Problem

Gondwana Research, V l , No. 1. pp. 11-33. 0 1997 International Association for Gondwana Research, Japan ISSN: 1342-937X

The Structural Setting of the Song Ma Region, Vietnam and the Indochina-South China Plate Boundary Problem R. H. Findlay' and Phan Trong Trinh2 'Geological Survey of Papua New Guinea, Private Bag, Port Moresby, Papua New Guinea National Institute of Scientific Research and Technology,Nghia DO, Tu Liern, Hanoi, Vietnam (Manuscript accepted June 16, 1997)

Abstract The Song Ma-Song Da region of northern Vietnam contains the Song Ma Anticlinorium, a polydeformed polymetamorphosed, early Palaeozoic island arc/ forearc terrane accreted to the South China plate in Siluro-Devonian times. The Song Ma Anticlinorium is not an Indosinian subduction zone and nor is the postTriassic, post-Cretaceous Song Ma Fault. The Song Ma Fault is one of many northwest-trending, post-Cretaceous, high-angle reverse oblique-slip faults and thrusts responsible for shortening and strike-slip transposition of northern Vietnam. Folds penecontemporaneous with this faulting include the Song Ma Anticlinorium, which is formed probably of thrust-ramp folds. The faulting and penecontemporaneous folding developed during Oligocene sinistral strike-slip on the Song Hong Fault and was reactivated during Pliocene-Quaternary times. We conclude that the present plate boundary is a broad deformation zone between Da Nang and the Song Hong and that parts of the South China plate extend well to the south of the Song Hong Fault.

Key words: Vietnam, faulting, tectonics, plate boundary, palaeostress.

Introduction It is well-known that the seismically active region of northern Vietnam lies across the boundary between the South China and Indochina plates. This geologically complex region is cut by numerous northwest-trending faults (Tran Van Tri, 1973; Phan Cu Tien, 1989), two of which have been nominated in particular as forming the plate boundary. Tapponnier et al. (1990) argued strongly that the Song Hong fault formed a major sinistral strike-slip boundary in Oligocene times and is still active as a dextral fault, whereas others (eg Bunopas and Vella, 1978; Tran Van Tri, 1979; Bunopas, 1981; Maranate andVella, 1985; Sengor et al., 1988; Metcalfe, 1993) have cited or figured the song Ma-Song Da region as a geosuture, regarded commonly as Indosinian age, between the South China and Indochina plates. Tran Van Tri, (1979) in particular identified the Song Ma Fault as a palaeosubduction zone. The assessment of the Song Ma-Song Da zone (Fig. 1) as the Indochina-South China plate boundary hinges on two aspects of the region's structural geology: the nature of the regional faulting in the Song Da-Song Ma zone and the structural evolution of the Song Ma Anticlinorium. In order to carry out this assessment, we document the previously undescribed history and kinematics of folding and faulting in the Song Ma-Song Da region and discuss the significance of

our results in their regional tectonic setting. In this assessment we make full use of the Vietnamese Geological Survey's 1:200000 geological maps of northern Vietnam; as with all national geological maps, these maps provide a valuable dataset for regional geological and tectonic interpretations. Our conclusions have profound implications for palaeogeographic interpretations of the northern Vietnam-Laos-South China region. The field-data supporting the paper were gathered from three traverses across the Song Ma Anticlinorium and from work along strike of the major faults along the boundary between the Song Ma Anticlinorium and the Song Da Zone. Excursions were made to Phu Yen and to the Hoa Binh area to view important contacts in these two regions. All localities are indicated in Figs. 1 and 2; all stereographic projections are equal area, southern hemisphere projections.

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 five geological entitles (Fig. 1) which are, from north to south, the Con Voi region, the Tu Le region, the Song

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Fig. I .

R. H. FINDLAY AND PHAN TRONG TRINH

Locality and geological regionlterrane map. The Song Ma terrane (see conclusions) is referred to also as the Song Ma Anticlinorium (Tran Van Tri, 1979).

Da region, the Song Ma Anticlinorium, and the Truong Son Fold Belt of Tran Van Tri (1 979). 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 confirming 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 strikeslip along a ductile shear zone between the Indochina and South China plates. The northwestern part of the Con Voi region is occupied by the Fan Si Pan massif where the geology is dominated by migmatitic and granitic complexes offset 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 Permo-Triassic “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); Cretaceous rocks of this type have been reported from a dispersed palaeoseamount/oceanic plateau in the Caribbean and Colombian regions (Kerr et al., 1996a, b, c). The Song Ma Anticlinorium is an arched northwest -trending structure and has been referred to as the Song MaAnticlinorium (eg 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 serpentinisedultramafic bodies referred to as ophiolite by Vietnamese workers, and a gneissic plagiogranite called the Posen Complex in the Vietnamese 1:200000 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% Dien Lu and is thought to include the possibly Permian Nui Nua ultramafic 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 Gondwana Research, V l , No. 1,1997

STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

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 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).

Structural Geology of Song Ma-Song Da Region Song Ma Anticlinoriiiriz We traversed the Song Ma Anticlinorium from: Dien Lu along the Nam Niem; Ban Buon to Song Ma town and in the Song Ma valley (Co Phuong and Nam Thi streams); and Thuan Chau to Co Ma (Fig. 2). The data from these traverses are includcd with discussion of the data available from the 1:200000 Dien Bien Phu, Son La, Van Yen and Ninh Binh Vietnamcse Geological Survey maps which form, as do all national geological survey maps, a commonly uncited but invaluable data-base. Mer~,iizorplzisniarid Structure Nguyen Ngoc Lien (1 980) described the metamorphism in the Song Ma Anticlinorium. Metamorphism follows a clockwise P-T-t path; early ? kyanite-sillimanite zone conditions produced staurolite and sillimanite at the metamorphic maximum. Plagioclases are described as An

Fig. 2.

13

suggesting partial closure of the peristerite gap and onset of kyanite-zone conditions. Subsequent metamorphism was thermal, was related to probably Devonian granitoid intrusion, and produced andalusite and staurolite with albite and oligoclase.The andalusites cut the schistosity whereas the early stauroliteis concordant with the schistosity.Nguyen Ngoc Lien (1980) also noted an inversion in the metamorphic grade, which we take as evidence for thrusting. Tran Van Tri (1979) reported glaucophane from the Song Ma Anticlinorium but cited no locality or author. Nguyen Ngoc Lien (pers. Comm., 1993) investigated this locality and his samples contain solely actinolite. Thus the suggestion of highpressure low-temperaturemetamorphism related to subduction (Tran Van Tri, 1979) cannot be substantiated though it must not be ignored as detailed studies of the metamorphism here appear to be in their infancy. We found evidence for three metamorphic events. The first (M,) event produced a metamorphic differentiation layering, S, ( quartz-mica in pelitic rocks; epidote-albite-actinolite and amphibole-plagioclase in metabasic rocks; gneissic layering in granitoids). The second event (M,) produced an extremely strong mica differentiation layering (S, ) in the pelitic units and which formed axial planar to microfolded relics of the S,fabric; S,is commonly parallel to S,. Both fabrics were folded during M, which produced an axial planar crenulation cleavage (S,)accompanied by growth of mica along the cleavage planes. We found unequivocal evidence for multiple deformation, reported in detail elsewhere (Findlay, in press). The first

Geological and locality map of the Song Ma region to show general study area (excluding Nui Nua). Note that in Fig. 2 the Pa Ham and Nam Pia formations at Dien Lu have been simplified as Devonian; Dinh Minh Mong (1978) has mapped the Pa Ham Formation as ? Ordovician-Devonian.

Gondwcino Research, K l , N o . I , I997

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R. H. FINDLAY AND PHAN TRONG TRINH

deformation (F,) is represented by transposed fine quartz hooks and thin lenses of quartz (1-2mm wide) some of which contain the early metamorphic fabric (S,) formed during M,. These structures represent the first metamorphic fabric which was intensely transposed by the second fabric (S,). S, is a strong schistosity and is axial planar to the fine quartz hoots described above and to isoclinal folds (F,) folding compositional banding in the metabasite/pelite layers and marble beds in the Song Ma township area and at Co Ma. The S, fabric was folded during M, which produced a steeply dipping, fine crenulation cleavage and mesoscopic folds with rounded hinges and relatively long limbs. The F, folds in the marble beds tend to have more angular hinges. The geometry of the F, folds, as seen in the orthogneiss of the Posen Complex at Chieng Huong (Fig. 2), indicates syn-F, sheath folding, which is confirmed by stretching parallel to the F, axes and by the rotation of mesoscopic fold axes and fold axial lineations in a uniform F, axial surface (see Findlay in press). The general trend of the F, structures is broadly. In the Thuan Chau-Co Ma traverse, rotation by the F, deformation has produced a range of F, trends from 90" to about 160". The F, axial surface generally dips steeply northeast. The progressive changes in the geometry of the F, structures across the Anticlinorium and refolding relationships seen at Muon Lam (Findlay in press) confirm that the Song Ma Anticlinorium formed during F,. This was accompanied by an apparently domainal development of a coarsely-spaced crenulation cleavage (S4). Post-S, kink-bands are common throughout the Anticlinorium were some are spatially associated with the thrust faults which form part of fault system along the northern boundary of the Anticlinorium. The kinematics of the kink bands are discussed more fully later. Stratigraphy The Song Ma Anticlinorium has been described by Tran Van Tri (1979) as bound to the south by a palaeosubduction zone (Song Ma Fault). Tran Van Tri (1979) also describes the stratigraphy which suggests that the Anticlinorium contains rock units consonant with an accretionary complex. In addition there are two areas to the southwest and along trend of the Anticlinorium, the Dien Lu district and Nui Nua ultramafic massif, which have been included as part of the Anticlinorium. As mapped by the Vietnamese Geological Survey, the Song Ma Anticlinorium consists of the Nam Co Formation in the core, and the Song Ma and Pa Ham formations on the flanks; our traverses crossed these units. The Nam Co Formation has been assigned a Proterozoic age (see Tran Van Tri, 1979). It is overlain with a supposed unconformity by the Song Ma Formation, which has been attributed a Middle Cambrian age, and by metamorphosed rocks of the Pa Ham Formation assigned an ?OrdovicianDevonian age (Tran Dang Tuyet, 1978). The Nam Co Formation is dominated by highly micaceous, low- to high-

grade pelitic schists. The Song Ma Formation is a series of metabasites, metagreywacke, pelitic schist and limestone and marble beds ranging in grade from greenschist to amphibolite facies. Polyphasal metamorphism and deformation have destroyed the original sedimentary and volcanic textures. The Pa Ham Formation has been divided into two units, phl and ph2,by the Vietnamese Geological Survey. Both units are found on both flanks of the Anticlinorium where they are metamorphosed and multiply deformed. On the southern side of the Anticlinorium these units contain thick and extensive limestone/marblehorizons which form prominent strike ridges. Unit phl is a sedimentary sequence which also contains distinctivebeds of white siliceous conglomerate and sandstone as well as quartz-sericite shale and black, thin bedded limestone. Where seen by us, this unit showed the same metamorphic and structural history as in the older rocks. Unit ph2is described in the Vietnamese Geological Survey maps as "basic effusives", this unit was not seen by us. In the Song Ma Anticlinorium these three formations are supposed to be separated by unconformities (Tran Dang Tuyet, 1978; Phan Son, 1978; Tran Van Tri, 1979). In our traverses the three formations showed the same metamorphic and structural history and thus we saw no evidence for these unconformities. The Song Ma Formation is intercalated with infaulted ultramafic lenses (Bo Xinh Complex) which are commonly serpentinised and in part altered to magnesite,and a heterogeneous banded orthogneissdescribed as a plagiogranite and referred to as Posen Complex. The Bo Xinh Complex has been correlated with the ultra-mafic rocks at Nui Nua (see Tran Van Tri, 1979 for summary). Although multiple metamorphism and deformation have obscured the original stratigraphic relationships in the Song Ma Anticlinorium where seen by us, the general character of the rock units resembles that of metamorphosed greenstone belts elsewhere (eg. Sulitjelma (Norway), Nicolson and Rutland, 1969; Findlay, 1980; Boyle et al., 1985; northern Victoria Land, Antarctica (see GANOVEX Team, 1984 and Findlay, 1991a for reviews); and Victoria, Australia (see Crawford and Keys, 1978)) which have been interpreted as island-arc subduction complexes. Further to this interpretation the general lithological character, stratigraphy and probable age of the rocks in the Song Ma Anticlinorium bear a remarkably close comparison to early Palaeozoic island-arc sequences in Tasmania, northern Victoria Land and NW Nelson, (New Zealand) and such a possible correlation should not be ignored (see Findlay, 1989, 1991a, 1991b). The stratigraphy in the Dien Lu district (Fig. 3) differs from that of the main body of the Anticlinorium. Here, nonmetamorphosed, fossiliferous oolitic Lower to Middle Cambrian limestone beds (Dinh Minh Mong, 1978) and a singly cleaved, volcanic lithicwacke sandstone series, both of which form the Dien Lu Formation, are juxtpposed north (Tran Van Tri, 1973) of pelitic schists of the Song Ma Formation. These rocks are overlain apparentlyunconformably @inh Minh Gondwam Research, V l , No.

I, 1997

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STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

Fig. 3. Geology of Dien Lu region, simplified form Dinh Minh Mong (1978). Legend as in Fig. 2.

Mong, 1978) by non-metamorphic rocks assigned ?Ordovician to Devonian ages (undivided Pa Ham Formation, (unit 0,-D, ph); and the Devonian Nam Pia Formation). Southwest of Dien Lu, pelitic schists and possibly Ordovician marble horizons typical of the southern flank of the Anticlinorium crop out in the Nam Niem river where they (Dinh Minh Mong, 1978) form the southeast continuation of the Song Ma Anticlinorium (see Fig. 2). The relationships at Dien Lu are critical and are discussed more fully later, the contact between the Dien Lu Formation and the schists of the Song Ma Formation must be a fault, although the contact is not exposed. Likewise, although the non-metamorphosed rocks of the Pa Ham Formation are mapped (Dinh Minh Mong, 1978) as overlying this contact unconformably, the unconformity is not clearly exposed where seen by us. The Nui Nua ultramafic massif has also been included in the Song Ma Anticlinorium by Vietnamese workers (eg. Tran Van Tri, 1979). This ultramafic body occupies about 60 km2, has not been metamorphosed and displays cumulus textures where seen by us near Co Dinh (Fig. 4). This intrusive body is thus very different from the small serpentinisedand magnetised ultramafic bodies in the southern flank of the Song Ma Anticlinorium. Furthermore, the only deformation seen by us Gondwana Research. V l , No. I , 1997

105 I 30

10$45'

- 15'9

'0

-19 40 Quaternary

I..'.1U l t r a m a f i c l ' I 1

Cretaceous congl.

Triassic s i l s t n . - m Permo-Carb\\pevonian

nPermian clastics =Cambrian

limestn.

schist

Fig. 4. Geological Map, Nui Nua. Crosses show localities for fault striation data.

in the Nui Nua body are small faults confined to the northeastern margin, although shear zones are known also in the northwestern part of the body (Nguyen Ngoc Lien, pers.

16

R. H. FINDLAY AND PHAN TRONG TRlNH

comm., 1993). This lack of deformation contrasts markedly with that seen in the ultramafic bodies in the southern flank of the anticlinorium. Therefore, the ultramafic body at Nui Nua is probably younger than those in the Song Ma Anticlinorium, and it may be a Permian intrusive emplaced tectonically by younger faulting rather than being an integral part of the Song Ma Formation.

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 palaeontological data are from the non-metamorphosed Dien Lu Formation which is a shelf sequence dominated by oolitic limestone and marly beds and is not a correlate of the Song Ma Formation. The ramifications of this are discussed more fully later.

Geochronology and age of rocks in Song Ma Anticlirioriurii

Boundary Fault Systems and Fault Kinematics

It is difficult to assess the ages assigned to the rocks within the Song Ma Anticlinorium as the geochronologic data are limited and the palaeontological data are reported in Vietnamese i n the Vietnamese Geological Survey Explanatory Notes accompanying the 1 :200000 map series. However, these geological maps show the principal fossil localities which allows some assessment of the palaeontological data. The only available geochronological data are three WAr 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 lo be a thesis in Russian written by Phan Truong Thi ( 1978) and whose data and abstract were cited by Nguyen Ngoc Lien (1980). The original manuscript is not available to the authors. The 360 Ma age is from the thermal aureole of a granitoid intruding the schists of the Anticlinorium. The 455 Ma and 425 Ma agcs may be taken as syn-to post-metamorphic cooling ages and may be interpreted as: (1) a metamorphic event with a minimum age of455 Ma overprinted by a metamorphic event with a minimum age of425 Ma; or (2) a metamorphic minimum agc of 455 Ma partially reset at 425 Ma by Late Devonian granitoid intrusion or delayed uplift; or (3) the metamorphic minimum agc is 425 Ma, but there are local excess argon complications which would give older ages such as the 455 Ma age. Whatever interpretation is accepted, 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 the non-metamorphosed rocks of this unit are mapped as ?Ordovician-Devonian at Dien Lu (Dinh Minh Mong, 1978). Tran Van Tri (1979) cites lnouyia sp., Metanomocare sp., Utiidae and Daniesellidae from the Song Ma Formation (see Phan Kim Ngan ( 1 972) for source data, in Vietnamese). The only fossil localities shown in the 1 :200000 geological maps of the Song Ma Anticlinorium are in the Dien Lu Formation in the inlier in the Dien Lu district, with an additional fossil locality lying in correlates of the non-metamorphic rocks of the Dien Lu Formation some 15-18 km to the NW (Dinh Minh Mong, 1978). We note here that in the 1:1000000 geological map of northern Vietnam (Tran Van Tri, 1973) the distribution of Song Ma Formation and Dien Lu Formation differs from that in the 1:200000 Ninh Binh geological map (Dinh Minh Mong, 1978). Our work in the Dien Lu inlier confirms the rock distribution shown in Tran Van Tri (1973). This, and the distribution of fossil localities shown in the Dien Lu inlier by

The Song Ma Anticlinorium is bound to the north by the Song Da Fault, and to the south by the Song Ma Fault, both of which are long and extend across northern Vietnam. In order to determine the kinematics of these faults we discuss also regional faulting to the northwest and southeast of the Song Ma Anticlinorium. Our fault data are from the Son La-Moc Chau region which includes the Nam Sap area, the Nam Muoi Fault (Fig. 5) and the Song Da Fault (Song Da region), and from the northern and southern flanks of the Song Ma Anticlinorium.

North of Song Ma Anticlinoriurn In the Moc Chau-Nam Sap region (Fig. 6) there occurs a narrow belt of folded Cretaceous continental red beds (Phan Son, 1978) flanked by Triassic carbonate horizons. The Nam Sap is a doubly-closed valley draining to the north through a gorge incised in the Triassic limestone sequences. The Cretaceous beds form the floor of the valley, whereas the Triassic beds form the mountain tops flanking the valley, and the contact between the Triassic and Cretaceous units must be a flat thrust as it follows the contour. This thrust can be traced for 15-25 km northwest of Moc Chau towards Yen Chau. To the north of Yen Chau this thrust and the Cretaceous beds are overthrust by the Devonian Permian and Devonian units of the Ta Khoa region (Phan Son, 1978). The dips and strikes in the Vietnamese geological map of the Nam Sap region (Phan Son, 1978) indicate a series of anticlinal and synclinal folds parallel to the Nam Sap, which is confirmed by the mapped distribution of the Triassic units and by bedding measurements in the Cretaceous units. The map sheet also shows that, north of the Nam Sap, small areas are occupied by Cretaceous rocks; and the contours on this geological map show that these areas of Cretaceous rock lie in valley floors whereas the Triassic beds form the high ground. We interpret this as supporting a thrust relationship where the upper plate consists of Triassic beds. Thus the Song Da region must contain a regional fold-thrust belt of post-Cretaceous age (the term post-Cretaceous, rather than Tertiary, is used as we do not know the younger limit to age of this thrusting, and the region is seismically active (Nguyen Ngoc Thuy, 1991;Nguyen Dinh Xuyen, 1991; Le Than Hoi et al., 1991). In the thuan Chau-Son La district, the 1:200000 Son La map sheet shows a complex array of faults (Fig. 5 ) one of which, the Nam Moui Fault was followed for palaeostress analysis of faulting. Near Muong La, east of Thuan Chau, this fault makes Gondwana Research, V l , No. I , 1997

STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

\ \

\.\

a

S c h i st,gne i ss, am phi b o Ii t e o f ?Proterozoic, Palaeozoic &Cenozoic age

17

Proterozoic ig n eous co m p Iex es

m P e r m o Triassic Cambrian Triassic non metamorphic rocks x granitoid

nu

Cretaceous :.: Neogene

\\

Fig. 5 . Map of major faults in northern Vietnam from Tran Van Tri (1973). The map shows broad lithologies only. Note possible C-S style, or curved Riedel R shears, between the Song Hong Fault and Tu Le region, and boudinage of the Proterozoic igneous complexes of the Phan Si Pan area (south of Lao Cai) on Riedel R shears. Given that thrusting has probably occurred on the other major faults, the Tu Le/Song DdSong Ma region could be regarded as an interplate flower structure related to sinistral strike-slip motion on the Song Hong-Song Chay Zone.

a westward bend along which has occurred 10 km of sinistral slip as indicated by themapped offset of Triassic beds Restoring this 10 km of strike-slip opens up a void 7-10 km wide (Fig. 7) along the northwest-striking fault segment, clearly, thrusting must have occurred during faulting along on this segment of the Nam Moui Fault. Gondwnnn Resenrch. V l , No. I , 1997

This observation led to careful study of the 1:200000 Son La and adjacent 1:200000 geological map sheets which were examined for offset marker beds. Where such were identified, the photocopied maps were cut along the faults and the faults reversed till a best fit of marker beds was obtained. From these cut-ups, it is clear that a minimum shortening of 30-50% has

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R. H. FINDLAY AND PHAN TRONG TRINH

--

=$0-

...-........

FAULT

-THR*sT

ANTI c L I NE

. I .

.......

---%--

SYN CLINE

NATIONAL HIGHWAY

Fig. 6 . Simplified geological map of Nam Sap - Moc Chau region, from Nguyen Xuan Bao and Phan Cu Tien (1978). to show interpretation as thrust - fold complex.

occurred across parts of the Song Da region by combined sinistral strike-slip and reverse faulting. It is reasonable to expect that such shortening is confined neither to the faults' where offsets could be identified from the 1:20oooOmap sheets, nor to these map sheets alone. At Phu Yen (Fig. 2) there is additional unequivocal evidence for regional thrust faulting .Here, (Fig. 8) the road crosses the boundary between east dipping Cretaceous conglomeratebeds on the northwest side of the road and the east dipping Triassic marine limestones on the southeast side of the rod. The Triassic beds clearly overly the Cretaceous units, and a thrust contact is the only logical interpretation. The geometry of the Triassic limestone beds here (Fig. 8) is easily explained as a thrust duplex.

Boundary faults, northern flank of Song Ma Anticlinorium

The boundary between the Song Da region and the Song Ma Anticlinorium is the extensive northwest-trending Song Da Fault, which dips 60" northeast, (Nguyen Ngoc Thuy, 1991; Nguyen Dinh Xuyen, 1991; Le Than Hoi et al., 1991; Pham Khoan et al., 1995). We crossed this fault near Thuan Chau and at Ban Buon. The Song Da Fault is located but not exposed about 3 km south of Thuan Chau, where fossiliferous (Nguyen Dinh Hop, pers comm., 1993) Cambrian slates of the Song Da region are juxtaposed closely against polydeformed greenschist facies rocks of the Song Ma Anticlinorium. Some 2 km further south (grid ref. 65 1673,1:200000 Son La geological map) the Thuan Gondwana Research, RI. No. I , 1997

STRUCTURALSETTING OF SONG MA REGION, VIETNAM

19

10 3'45'

ORDOVICIAN

DEVONIAN

a

CARB:PERM.

PERMIAN

TRIASSIC

Fig. 7. Sketch geological map, from 1:200000 Dien Bien Phu geological map, of Muong La - Thuan Chau - Ban Muong area to show effect of removing the 10 km of left lateral slip on Nam Muoi Fault at Muon La. The Triassic beds here contain a clear marker horizon which can be fitted across the fault on reversal of sinistral slip. Similar exercises on the 1:200000 Van Yen, Son La, Dien Bien Phu and Yen Bai geological maps indicate similar major crustal shortening.

.. .. .. . .

1

\

Fig. 8. Sketch from field notes and photographs of the geology north of Phu Yen. Cretaceous beds on west side of road dip below Triassic beds to the east. If the Triassic limestone outcrops were originally one unit, then Triassic beds must form a series of thrust duplexes. Gondwana Research, V l , No. I , 1997

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R. H. FINDLAY AND PHAN TRONG TRINH

Chau-Co Ma road crosses a black graphitic schist containing bands of cataclasite. This cataclasite marks the southeastern faulted boundary between metamorphosed units of the ?Ordovician-Devonian Pa Ham Formation and the Song Ma

1

Fig. 9. Geometry of faulting at grid reference 651673 (note : this locality appears to be 200-300m south of where Phan Son ( I 978) has mapped this fault contact). Arrow indicates trend 0 , . Inset is from field sketch.

The Song Da Fault is probably more complex than shown by Phan Son (1978)as it splays in the Ban Buon region. The southern splay is exposed at about grid reference 95 1399 (Son La 1:200000 geological map) some 2.5 km due south of Ban Buon where the non-metamorphic rocks of the Pa Ham Formation are juxtamposed against the Song Ma Formation. Although the contact between the two formations is not exposed, a 6m thick fault breccia cuts the Pa Ham Formation and strikes northwest. The fault breccia contains mesoscopic reverse faults and a deformed cherty conglomerate nearby contains shear bands and asymmetrically deformed pebbles whose geometry shows sinistral shear. Within the adjacent Song Ma Formation post-schistosity kinks indicate reverse faulting, as do kink-bands elsewhere in the Song Ma Formation. The northern splay of the Song Da Fault passes through Ban Boun where the Devonian rocks of the Song Da rzgion are faulted againist the non-metamorphosed Ordovician units to the southwest. Here, there occurs a graphitic fault-rock containing asymmetric phacoids and a C-S fabric which dips steeply northeast and indicates reverse slip (Fig. lo). Also present are sinistral drag folds, and the kinematic picture here is the same as those at described in Figures 8 and 9, with a low NE plunging 0, and a reverse fault situation.

Formation to the southeast. The graphitic schist is folded into southeast verging folds, and small-scale faults indicate reverse slip (Fig. 9). As this fault zone is very extensive (Phan Son, 1978) and trends parallel to the Song Da Fault some 1.5-2 km northwest, then the Song Da Fault may also have a reverse component of slip.

Fig. 1 I . Thrust faults in northern limb of Song Ma Anticlinorium, road from Thuan Chau to Co Ma. Here two measured thrusts are shown and Cplanes (broken great circle). The stippled area is enclosed by three measurements of the S-fabric associated with the C-planes and o1is shown by the solid triangle (X marks the theoretical slip line derived from o?(solid circle)).

Faulting within and south of the Song Ma Anticlinorium Fig. 10. Geometry of faulting at Ban Buon, showing S-Fabric in siltstones (dashed lines; a scaly cleavage) and C-planes (faults, heavy lines). The broken great circles are the C-planes, the solid great circle is the S-fabric. Solid triangle, probable 0,; solid square, probable 0,; solid circle, 02.Assumes of slip on C planes at 90" to 02.

Thrusting is not only evident north of the Anticlinorium but thrusts are exposed within the northern limb of the Anticlinorium along the road between Thuan Chau and Co Ma. We noted three northeast dipping thrusts in the region of Gondwana Research, VI,No. 1. I997

21

STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

approximately map grid reference 600650 to 625652 on the Dien Bien Phu 1 :200000 geological map (the co-ordinates are

of topographic maps makes it difficult to show the relationships accurately; field sketches are presented in Fig. 12.

I

View S E f r o m 2

f

-

(ap p r 0 x)

est. 1.5 km

distortion p ossi ble

t o Thu.8n Chau

t o S6ng M i i v a l l e y

Devo n Ian 1i m estone OGabbro

O M i c a schist [VvlMetabasite HMarble

a&

'97

Fig. 12. Field sketch map and field sketches of geology at Co Ma. Note that there is uncertainty as to scale and precise orientation as the 1:SOOOO topographic map was not available to the author during fieldwork and the 1:200000 Dien Bien Phu geological map does not show the location of Co Ma.

approximate as this map does not show to road to Co Ma). These thrusts (Fig. 11) are associated with kink folding in the lower plate, and post-F, kink folds occur through much of the Nam Co Formation along the Thuan Chau-Co Ma road. The geometry of these kink folds is discussed later and is consistent with overthrusting from the northeast. Further evidence for shortening across the Song Ma Anticlinorium is seen at Co Ma where topographic relationships demand that schists of the Nam Co Formation and metabasites and marbles of the Song Ma Formation are thrust over fossiliferous but thermally metamorphosed Devonian limestone beds intruded by a galena-bearing gabbro. The unavailability Gondwana Research, % I tNo. I , 1997

In the Song Ma township region, the Vietnamese Dien Bien Phu and Son La 1:200000 geological maps (Tran Dan Tuyet, 1978; Phan Son, 1978) show numerous NW striking faults cutting the southern flank of the Song Ma Anticlinorium. From considerations of possible mapped offsets of the Posen Complex, some of the faults in the southern flank of the Song Ma Anticlinorium appear to have dextral offsets of 50-70 km (Son La 1:200000 geological map; Phan Son, 1978) Alternatively, it could be argued that apparent offsets of the metamorphosed Pa Ham Formation indicate sinistral offsets of as great as 10 km. Clearly, in the Song Ma valley the mapped distribution of rock units is not diagnostic of slip sense, thrusting could also easily explain in the arrangement of units.

22

R. H. FINDLAY AND PHAN TRONG TRlNH

I

Fig. 13, Fault geometry, grid reference 697365, Co Phuong Creek, Double lines, trend of Song Ma Anticlinorium;heavy great circle, major displacement zone, light great circle, Riedel R shears. Solid triangle-0,; open circle-0,; solid square-0,.

We investigated two of these NW trending faults in our traverse up Co Phuong creek, near Song Ma township. At grid reference 697365 (Son La 1:200000 geological map) Phan Son (1978) shows a foliated shear zone, and in this we found Riedel Shear structures showing dextral slip geometries (Fig. 13). Further south, at approximately grid reference 607350 (Son La 1:200000geological map) there occurs a zone of shearing about 100 m wide which includes evidence for ductile (C-S) and cataclastic deformation. The C-S fabric confirms that sinistral slip has been important whereas possible Riedel R faults in the cataclasite are consistent with dextral slip. Thus contrast between the sense of slip between the brittle and ductile features might indicate an early sinistral phase followed by a later dextral overprint. In the Dien Lu region of the Song Ma Anticlinorium (Fig. 2), the observed relations between topography, rock unit distribution and the faults mapped here (Fig. 3) (Dinh Minh Mong, 1978) convince us that thrust faulting has produced a series of thrust fault duplexes. We have attempted a reconstruction Of the pre-fault geology in the Dien Lu It is clear (Figs. 2 and 14) that beds in the southern part of the inlier can be matched if thrusting is assumed. It is less easy to determine the pre-fault arrangement west of Dien Lu (Fig. 14)

Fig. 14. Reconstruction of pre-fault geology, Dien Lu area. Criteria for reconstruction were alignment of marker beds in and adjacent the Dien Lu inlier, and the generation of a consistent structure in beds to west of Dien Lu where there were no clear offsets. Legend as in Fig. 3.

Gondwana Research, V l , No. 1,1997

STRUCTURAL SETTING OF SONG MA REGION, VETNAM

but assuming that the domal style of folds mapped in the Dien Lu inlier (Tran Van Tri, 1973; Dinh Minh Mong, 1978) also occur to the west then a similar structural style to the Dien Lu inlier may be obtained by treating the major faults as sinistral reverse faults (Fig. 14). We have not attempted to unravel the structures in the Permo-Triassic to the northeast of Dien Lu but note that Dinh Minh Mong (1978) illustrates regional, tight, NW-trending folds in these beds and these folds are cut by numerous NW-trending faults. Our reconstruction of the pre-fault geology (Fig. 14) aligns schist now west of the Dien Lu inlier almost along strike of the schist in the Dien Lu inlier. It also highlights the ?Ordovician-Devonian (non-metamorphosed Pa Ham Formation) unconformity and a Carboniferous-Permian unconformity overlying the Pa Ham Formation. The PermoTriassic beds onlap across the Craboniferous-Permian units to rest locally on both the Pa Ham Formation and the Devonian Nam Pia Formation. The Song Ma Fault forms the southern boundary of the Song Ma Anticlinorium. According to the 1:200000 Vietnamese geological maps (Tran Dang Tuyet, 1978; Phan Son, 1978; Dinh Minh Mong, 1978), offsets of Devonian and Triassic granitoids could indicate 120 km of left lateral 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 20" and 60",the Song Ma Fault must be a sinistral reverse fault. The Dien Bien Phu i:200000 geological map ( T r a Dang Tuyet, 1978) reveals that, as with the Song Da region, the faults in and south of the Song Ma Anticlinorium are not only extensive but some are markedly sinuous. One such fault, about 20 km south west of Song Ma town, and south of the Song Ma Fault, makes a pronounced westward bend (Fig. 15) analogous to that reported in the Muong Lam area southeast of Thuan Chau. Based on the certainty of thrusting in the Song Ma Anticlinorium and to the north (our data; Nguyen Ngoc Thuy. 1991; Nguyen Dinh Xuyen, 1991; Le Than Hoi et al, 1991), we interprete this swing as a sinistral lateral ramp offsetting an important but unnamed thrust. The available structural data in this area point to regional folding along a northwest trend and the structural data in the Dien Bien Phu sheet show that these folds have been overridden by the thrust, as is evident elsewhere in the study region. Relations between Folding, Kinking and Faulting The Song Ma and Song Da regions are dominated by NWtrending folds and faults (1:200000 geological map series, Geological Survey of Vietnam) which lie parallel to the Song Ma Anticlinorium. As F, in the Anticlinorium produced a metamorphic fabric, it most likely occurred at or before the

Fig. 15. Geology of region southwest of Song Ma town to illustrate probable thrust with lateral ramp. Gondwana Research, V l , No. I , 1997

23

24

R. H. FINDLAY AND PHAN TRONG TRINH

1

Fig. 16. Kink band geometries from Song Ma Anticlinoriurn.Light great circle - general distribution of poles to kink bands; twin straight lines trend of Song MaAnticlinoriurn;solid circles - poles to reverse kinks; open circles - sinistral kinks; solid triangles - dextral kinks; open squares - normal kinks; small x - kink bands, slip sense not recorded; great circles, dashed, dotted, with teeth - reverse conjugate kink bands (A, B); great circles, dashed, dotted, arrows - probable conjugate strike-slip kinks (C, D); large crosses - 0 , derived from A and B and C and D; arrows-general trend of 0 , for ready comparison with Figs. 9, 10, 1 I and 13.

youngest WAr agc of 425 Ma. The NW-trending folds in the Song Da region fold Triassic beds and hence are not related to F,. As the F, anticlinorial arch of the Song Ma Anticlinorium follows the trend of the regional folds of the Song Da region, then i t is reasonable to argue that this NW-trending F, Anticlinorium is one of these regional folds. Post-S, kink bands are common within the Anticlinorium. These structures are kinematically equivalent to faults and their geometry is indicated in Fig. 16. Briefly, reverse kink bands dominate and confirm compression across the Anticlinorium; the orientation of these structures mimics that of the small scale reversc faults and thrusts described later. Similarly, the two strike-slip kink bands labelled A and B in Fig. 16 confirm compression across the Anticlinorium. A regional palaeostress field may be determined from this array if all reverse kink bands are treated as contemporaneous. These kink bands define a great circle of poles, intimating a common 02,which lies moreor-less in the trend of the Song Ma Anticlinorium. Thus 0,and o,can be determined as the faults are reverse; 0 ,plunges about 25" to 20" There is a close similarity between the geometries of the reverse kink bands and major faults in and around the Song Ma Anticlinorium. There is also a close similarity between the palaeostress fields derived from the kink bands, from considerations of the geometries of the minor structures in the fault zones seen by us (Figs. 9, 10, 11 and 13), and the

palaeostress directions derived from fault-striation data (see next section). The geometrical similarities alone are a sound argument that the kinking in the Song Ma Anticlinorium is related to the regional faulting, and therefore we regard these kinks as very young structures related to the regional faulting. The Cretaceous rocks of the Nam Sap region are folded into a NW-trending syncline (Nguyen Xuan Bao and Phan Cu Tien, 1978) and our observations on the NE flank of this structure, about 7km NW of Moc Chau, confirm that the NE limb dips steeply in contrast to the SW limb which dips northeast at 10-25"(Nguyen Xuan Bao and Phan Cu Tien, 1978). This SW-verging structure shares the geometry of the F, Song Ma Anticlinorium, the regional folds affecting the Triassic sequences of the Song Da region, and the inferred pre-thrust folds in the Dien Lu region. Therefore, it is logical to conclude that all these structures formed during the same event and are younger than Late Cretaceous, as confirmed by the cross-section in Nguyen Xuan Bao and Phan Cu Tien (1978) and Dinh Minh Mong (1978). The NW-trending folds are cut by thrusts and probably transpressive sinistral strike-slip faults. It is logical to argue that these structures are related as both involve shortening in more-or-less the same direction. We therefore argue that the regional NW trending folds are an early ductile shortening strain which was followed by localisation of strain to reverse/ thrust and transpressive sinistral strike-slip faults. This event is a post-Cretaceous and not an Indosinian event and we will argue in the next section that it relates to Oligocene motion along the Song Hong Fault.

Palaeostress Analysis of Faulting Approach

Palaeostress analysis using striated faults is an accepted tool for analysis of fault kinematics. Although the technique relies on several assumptions that do not appear to be valid for real rocks (eg see Pollard and Saltzer, 1993; Dupin et al., 1993; Cashman and Ellis, 1994), both numerous local and regional studies (Arthaud, 1969; Angelier, 1979, 1984; Gauthier and Angelier, 1985; Berry and Banks, 1985; Berry, 1989; Phan Trong Trinh, 1993;Findlay, 1996) have demonstrated the value of this technique. Phan Trong Trinh and colleagues have carried out a regional palaeostress analysis in northern Vietnam (Lacassin et al., 1994; Phan Trong Trinh et al., 1991; Nguyen Trong Yem et. al., 1991; PhanTrong Trinh et al., 1994a, 1994b) and this work provides a regional background for our work at Nui Nua, along the Nam Muoi Fault, and in the Song Ma district. We analysed our data using the methods of Etchecopar et al. (1981) using a programme prepared by R.F. Berry, University of Tasmania (see Berry and Banks (1985) and Berry (1989) for explanation of method). A major limitation to any stress analysis is that, in heavily faulted mobile zones suffering progressive deformation, strainpartitioning can lead to local preservation of the earlier strain Gondwana Research, W,No. I , 1997

25

STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

history and thus differences in the calculated stress tensor from site to site can be interpreted as distinct tectonic events rather than rotation of material surfaces during movement within the fault system or local rotation of the stress tensor during the movement history. These consideration apply to all approaches to determining stress systems from local outcrops within a broad zone of deformation and are difficult to resolve without consideration of the broad movement picture. Through consideration of the regional picture developed by Phan Trong Trinh et al. (1994a), we are able to place our more local study in its context and thus can avoid this problem. Thc analytical program we used allows the operator to: (1) tcst different percentages of the faults being studied to determine the best-fit coherent stress system; this is given by inspection of the distribution of faults falling in the range of 0.0 to 0.3 or 0.4 radians away from the ideal solution (end column in Tablcs 2,3 and 4); and (2) clean from the data-file these best-fit faults in order to test the remaining non-fitting faults to see which percentage of them give a coherent palaeostress. This “cleaning” technique can be used to test for

possible multiple fault events. In addition, the slip-senses of the faults can be altered. Whilst there is no point in altering slip-senses where these can be determined unequivocally in the field, it does allow testing faults whose slip-senses are unknown or unccrtain to see if such faults could fit the best-fit stress system. Clearly, such tests must only be carried out with reference to the known field-data as the program allows a freedom of manipulation adequate to create a coherent answer, irrespective of the regional field evidence. We believe we have avoided this problem, and where we have so manipulated the data we recognise it by citing the results as “possible” (eg. see Table 2); these “possible” results are regarded as permissive and not definitive.

Field Data and Results Our field data are presented in Figs. 17, 18, 19,20, 21 and 22 and the reader is referred to the extensive figure captions and to Tables 1-4. Table 1 is a summary of our conclusions drawn from visual inspection of the faults orientations and slip-senses; visual

Table I . Summary of conclusions as to possible interpretation of the field-data, based on inspection of stereo nets shown in Figs. 14 to 19 inclusive. Fault Zonehrea and Figure

Nam Muoi, grid 597938 61 I943 Fig. 18A

General trend minor faults

Slip sense (S-sinistral D-dextral R-reverse N-normal)

Comments

Conclusions

N NE ESE

S S D

N & ESE consistent with NW o1to produce reverse on NE but which are sinistral and consistent with NNE,.

2 generations: SE o1 NNE o1

E N

S

E sinistral consistent with NE 0 ,

NE 0 ,

Narn Muoi grid 690800 Fig. 18C

NW ESE N NE

R S S? D?

Reverse & ESE Sinistral , 0 , NE. N & NE possibly conjugate, E-W shortening.

NE ol, & SE to E 0 ,

Nam Muoi, grid 803693 Fig. 17A

SE NE SSE NE

R S,D S

SE reverse, NE o1& NE reverse, NW ol, or both have oblique slip with W ol. SSE dextral NE 0 , ; NE sinistral NE 0 , ; SSE sinistral ESE 0 ,

2 generations: NE o1& ESE

Nam Muoi, 178 grid 704695

E NE ESE N

S ?N,S DR

Dominantly sinistral reverse oblique slip on steep to shallow E faults consistent with NNE to NE 0 , ; N o1would also produce dextral reverse on SE faults, and sinistral on NW dipping NE fault.

Dominantly N to NE 0 ,

Bridge 4 kin from Thuan Chau, NW trending fault, Fig. 19

SE SSE E

D, minor R ? ?

SE faults follow fabric, flattened prolate pebbles follow slickenlines (See Fig. 17A) and extensional fibres in tension gashes

NW o1 consistent with dextral slip on SE and E faults and tension gashlfabiic geometry

Song Ma region Fig. 20

NW N

R&N D?, R

NE sinistral consistent with N to NE ol. N dextral consistent with NE ol.

NE o1and ESE to SE 0 , probable

Nam Muoi, I8B grid 656836

Gondwann Reseurch, K l , No. I , 1997

?

R

?

01

26

R. H. FINDLAY AND PHAN TRONG TRINH

Nui Nua, Fig. 22A

Nui Nua, Fig. 22B

Nui Nua, Fig. 22C

NE E

S D

E dextral consistent with SE 0,.E trending shear band in quarry in gneiss at Chien Gang has dxtral slip. NW reverse consistent with NE ol. NW normal may be due to downslope gravity-driven slip

NW, NE dip NW, steep dip N ENE NW, NE dip WNW, SW dip NE NNE NN W

R D D S R R S S J D

NW, NE dip reverse faults consistent with NE ol, NW & N steep dextral and ENE sinistral could be conjugate, N to NNE o1

NE o1

NW & WNW reverse faults consistent with NE o1NE sinistral consistent with N to NE ol. NE dextral consistent with shallow W ol. NE sinistral and NNW dextral could indicate N ol.

Possibly 2 generations: NE 0,& E-W

NW, SW & NE dip WNW W

R, N

SW, W and NE dipping thrusts consistent with NE ol, S dipping sinistral consistent with NE to E ol. Normal fault trends NW, uncertain significance. Not compatible with thrusts unless related to extension (non-plane strain) during N-S contraction on NE-SW trending thrusts.

N to NE 0, most probable

R S

Table 2. Bulk analysis of all striated faults of known slip-sense. Best fit iteration (BFI) number

Minimization % No. of faults

I 2 3 4 5 6 7 8 9 10 II

75 % 75% 70% 65% 60% 65% 70% 60% 65% 50% 65%

I68 I46 123 I00 82 67 49 37 26 17 10

6?

61 Plunge"

Trend" Plunge"

13 10 3 5 22 26 33 31 35 33 77

020 I70 02 I I39 349 345 186 I96 I I9 I I8 009

71 9 5 6 55 52 17 10

151 262 291 230 222 217 084

10

023 239 147

100

39 10

R

6 3

Trend"

Plunge"

Trend"

14 76 54 82 25 27 52 57 53 34 08

286 033 145 01 I 090 089 330 355 280 001 239

.73 .7 I .15 .89 38 .86 .28 .50 .37 .30 .60

No. in 0.0 to 0.3 Rd range 0. I to 0.2 0.2 to 0.3 0.0 to 0. I 14 13 10 9 10 13 9 4 4 4

5

9 II 13 9 6 3 1

4 2 3 0

5 I1 7 10 11

3 3 3 3 0 1

Table 3. Palaeostress, northern Vietnam Son La and Song Ma districts. Fault Zone/ Locality

Minimization % No. of faults

Nam Moui FZ all data possibility I possibility 2 possibility 3 possibility 4 Nam Muoi FZ 803693 Narn Muoi FZ 704695 possibility 1 possibility 2 possibility 3 Thuan Chau Bridge possibility 1 possibility 2 possibility 3 Song Ma Anticlinorium possibility

Plunge"

61

6 2

Trend" Plunge"

Trend"

Plunge"

6,

R Trend" 1 I4

No. in 0.0 to 0.3 Rd range 0.0 to 0.1 0.1 to 0.2 0.2 to 0.3

80%, 70%. 70%, 80%,

21 21 21 21

05 00 02

020 020 020 020

66 75 71 70

266 272 291 286

22 14 20 20

Ill 112 112

.96 .92 .95 .95

80%.

26

01

352

42

083

48

262

.20

5

3

65%, 60%, 60%,

13 13 13

24 34 02

I95 I67 219

34 39 39

088 045 127

46 33 51

313 283 312

.74 .74 .66

4 4 4

2 2 2

70%, 70%, 70%,

14 14 14

77 00 75

29 1 I67 295

05 78 07

179 258 180

12 12 14

088 076 088

37 .59 .93

6 4 4

3 3 0

0 3 3

50%.

13

30

I84

07

278

61

022

.39

5

7

0

10

Gondwana Research,

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STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

27

Table 4. Palaeostress, from Nui Nua ultramafic massif, northern Vietnam. Faulty Zone/ Locality Nui Nua (Bulk data) Group 1 Group 2 Group 3 Nui Nua SE Group 1 Group 2 Nui Nua NW 1 Group 1 Group 2 Nui Nua NW 2 Group I

Minimization % No. of faults

6, Plunge"

63

62 Trend" Plunge"

Trend"

R

Plunge"

Trend"

No. in 0.0 to 0.3 Rd range 0.0 to 0.1 0.1 to 0.2 0.2 to 0.3

85%. 85%. 65%,

92 57 44

02 25 05

020 024 135

07 10 11

290 289 266

82 63 78

121 180 020

0.19 0.65 .37

12 7 8

7 5 6

4

65%, 50%,

36 15

29 53

008 029

05 25

276 261

60 26

177 156

.13 .36

9 4

6

2

6 2

60%. 55%

35 19

06 03

I34 296

41 02

229 026

49 87

038 158

.39 .94

6 6

74 3

5 1

60%

21

07

018

54

117

35

283

.16

6

3

3

5 4

Fig. 17. Fault orientation data from Muon Lam sector of Nam Muoi Fault. Solid circles - reverse faults; open circles-normal faults; solid squares-sinistral faults; open squares-dextral faults; solid triangles-unknown slipsense; great circles with teeth-thrusts; m o w s show sense of strike-slip. Solid circles on great circles, faults with striations. Net A- From grid reference 803693. Faults shown in general orientation only; a southeast-northwest 0 , would be permitted. Net B- From grid reference 704695.

Fig. 18. Fault orientation data from Ban Giang region of Nam Muoi Fault. Great circles are faults; solid circles are striations, sense of motion shown. Net A- from grid references 597938 and 611943. Net B- from grid reference 656836. Net C- from grid reference 690800.

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R. H. FINDLAY AND PHAN TRONG TRINH

inspection of fault orientations and slip-senses support the interpretation that faulting along the Nam Muoi Fault, in the Song Ma Valley, and at Nui Nua is controlled principally by a N to NE oriented shallowly plunging 0 ,with evidence for a ESE shallowly plunging ol. Tables 2,3, and 4 summarise our analyses using the method of Etchecopar et al. (1981). The bulk analysis of all our data (Table 2) indicate a shallowly plunging, NNE trending o1 involved in either strike slip or reverse slip; a shallowly plunging NW trending o1involved in reverse slip; and an ESE trending o1 involved in oblique reverse slip. Given a general NW strike of regional faulting this would produce (NNE and NW 0 , )dextral reverse slip and (ESE o1) sinistral reverse slip. Figs. 17 and 18 summarise our field data along the Nam Muoi Fault (Fig. 5) between Son La and Ban Giang. This fault lies parallel to the Song Da Fault and the fault zone juxtaposes (Trang Dang Tuyet, 1978) Cretaceous beds against parts of the Permian komatiite-bearing (Poliakov et al., 1991)sequence. We analysed separately data from two localities in the segment of the Nam Muoi Fault where it trends E-W between Muong La and Son La (grid refs. 803693 and 704695, Figs. 17 A and 17B, see Table 3) and shows a sinistral offset of 10 km (see earlier). We combined our limited outcrop data from grid references 597938, 611943, 656836 and 690888 (Ban Giang area) to generate a data set of 21 faults (see Fig. 18) which are shown as “combined Nam Muoi FZ” in Table 3. These data give an indication as to what stress tensors might have given rise to the faults measured in the Ban Giang area and thus allow comparison with other results. At grid reference 803693 (Fig. 17A) on the Nam Muoi Fault (Table 3) we have evidence for a north-trending 0 , and an

oblique slip situation with a low R value (0,approaches ol) for 11 out of 26 faults. A grid reference 704695 (Fig. 17 B) on the Nam Muoi Fault (Table 3) we have the possibility of either or both a NNE o1 and an SE 0,;again, the orientation of 0, demands oblique reverse slip and R has 0,>o,. The 4 possiblc stress iensors from the Ban Giang area (Fig. 18; Table 3) indicate a possible NE o1but the high R value means we cannot distinguish between normal and reverse slip (ol = 02);this could be a result of the small data set as the misorientation of 2 faults (10%)from the ideal orientation could have an adverse affect. At Thuan Chau Bridge (Fig . 19;Table 3), both normal and reverse faulting are shown amplify pebbles/cleavage/gashes by possibilities 1 and 3 (ol steep, o1 2 0 7 )whereas possibility 2 is a strike-slip situation with o1plunging horizontally to the SSE and o2steep (R = 0.59 and 0 , > 0, > 0 , > ). Possibility 2 agrees with the results calculated from the geometry fine tension gashes and elongatcd pebbles in the sheared pelite here (Fig. 19 A) and with the observed dextral slip on the ESE trending faults (Fig. 19 B). Possible normal faulting has occurred on the NW to NNE trending faults and may be compatible with possibilities 2 and 3 in Table 2. In the Song Ma Valley, our 13 faults (Fig. 20; Table 3) are from scattered localities around the Song Ma-Chieng Khuong and Co Ma regions. As such they are a very poor data set for this kind of analysis as we cannot constrain local conditions and 13 faults are too few to yield a statistically sensible result over such a large area. However, we can produce a possible Ntrending o1 in a reverse fault situation which is not an unreasonable answer given the overall regional pattern of

A

Fig. 19. Fault orientation data from Thuan Chau bridge. Net A- Relationship between cleavage, tension fractures and prolate pebbles. Solid circles- pebble lineation; stippled area - general slaty cleavage orientation ; NW-SE great circles are fine, hard-to-measure tension fractures with quartz extension fibres normal to tension gash walls. Note the pebbles appear to follow the trend of the slickenlines and the cleavage that of the predominant (dextral) striated faults (Fig. 19A). This suggests a kinematic relationship between faulting, cleavage and pebble extension. 0 , shown. Net B- Faults and slickenlines, motion shown. The NW trending normal faults are consistent with the tension gash orientation and could be extensional faults related to dextral slip on the WNW trending faults.

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STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

Fig. 20. Fault orientation data from southern limb of the Song Ma Anticlinorium. Note normal faulting on NW trend dextral slip on WNW trend compare (Fig. 19). Note NNE compatible reverse fault and possibly incompatible NNE dextral faults.

faulting. However, the possibility shown should not be used in isolation to prove reverse faulting in the Song Ma Valley; it is permissive and not definitive. Striated faults are common along the SE flank of the ultramafic massif at Nui Nua, and our data set here is relatively large (Figs. 21 and 22). However, the structural setting at Nui Nua is less clear than along the flnaks of the Song Ma Anticlinorium (Fig. 4). The Nui Nua massif is probably juxtaposed on its NE flank against a Devonian and younger sedimentary series by a NW trending fault (Fig. 4). The SW flank of the massif is faulted against schists and Permo-Triassic beds by a possible thrust, given the relationship between topography and the outcrop pattern.

Fig. 21. Poles to all faults (A) and striations (B) from Nui Nua.

Gondwana Researcli. V l , No. I , 1997

29

For our analysis at Nui Nua we collected data from three localities (Fig. 4; Table 4) and carried out a bulk analysis of all the data followed by local analyses from the 3 localities. Both our bulk analysis and our locality analyses indicate 0,has a steep to shallow plunge dominately to the N to NNE and a subordinate shallow plunge to the ESE and WNW. The R values for the Nui Nua data set are variable. For the Group 1 bulk data 1 (Table 4), for the data from Nui Nua SE Group 1 and for the data from Nui Nua NW2 Group 1 R is low and o22 03;that is these data could indicate either or both a strike-slip or reverse fault situation, or possibly non-plane strain. For Nui Nua NW Group 2, R = 0.94 and 0 ,= o3 effectively; this indicates either reverse or normal faulting. The SE to ESE-trending o1(05"to 135", 06" to 134"and 03" to 296"; Table 4) is probably involved with thrusting (0,plunges at ll", 25"and 02") and in two of the three cases R = 0.37 and 0.39. The R value of 0.94 in the case of the result where 0, plunges 3" to 296" (Table 4) could be an indication that the sample size tested (55% of 19 readings) is too small for a coherent result given the variability of the fault orientations in this group. The other results (0,to NE) have intermediate plunges for 0,(10"to 54") and are suggestiveof oblique reverseslip.

Summary of Fault Kinematics There is clear regional evidence for sinistral transpressive strike-slip on the regional NW trending faults. This faulting cuts Cretaceous and in some instances Tertiary beds (Phan Trong Trinh et al., 1994a, 1994b). Studies of different kinds concerning the mesoscopic faults and kink bands point to two likely stress fields, one with 0, ranging between NNW and NE and consistent with reverse possibly dextral oblique slip, and the other with 0,oriented ESE and consistent with sinistral reverse oblique slip.

30

R. H. FINDLAY AND PHAN TRONG TRINH

B

I

Fig. 22. Fault orientation data at Nui Nua. Stippled area - encloses thrust faults (great circles with teeth); summary orientation of strike-slip faults shown by great circle plots with arrows for sense of motion. Net A- Nui Nua Southeast. Symbols as for Fig. 17. Net B- Nui Nua Northwest 1. Symbols as for Fig. 17. Net C- Nui Nua Northwest 2. Symbols as for Fig. 17.

Our palaeostress analysis shows that the NNW-NE 0, dominates. It is reasonable to argue that the latest phase of regional fault motion would yield the greatest number of slickenlines on the minor faults as reactivation of earlier faults would occur and their previous movement signatures would have been destroyed. Therefore, we propose that the N N W NE o1is the younger. Phan Trong Trinh et al. (1994a) present palaeostress maps for Miocene and Plio-Quaternary times in northern Vietnam. Their data show a shallowly plunging regional E-directed o1 for Miocene times and a shallowly plunging N-directed 0,for Plio-Quaternary times. Our data fit this general regional pattern although we cannot constrain absolute time of faulting beyond saying it is post-Cretaceous. The regional faults across northern Vietnam trend NW to WNW. A shallowly plunging E-directed 0,would produce sinistral slip with some reverse motion on the regional faults whereas a shallowly plunging N-directed o1would produce reverse motion with some dextral slip. The regional fold orientation is consistent with this history; a shallowly plunging E-directed o1and steep 0,would produce transpressional folds trending NW which would be flattened and overthrust when o1swung to a northerly direction. Our data include evidence for an ESE to SE directed 0,. Whether this is a local perturbation on the general Miocene Edirected o1we do not know; if it represented the regional picture it could well lead to sinistral slip across faults trending moreor-less NW and possible extension on faults trending WNW. Given the spread and consistency of the data points for the Miocene in Phan Trong Trinh et al. (1994a), then we suggest that out ESE to SE trending o1may not reflect fully the regional Miocene stress tensor.

Discussion What is the Song Ma Anticlinorium?

The Song Ma Anticlinorium has been shown as a subduction

zone between the South China and Indochina plates (Tran Van Tri, 1979) and has been described by Bunopas and Vella (1978), Bunopas (1981), Maranate and Vella (1986), Sengor et al. (1988) and Metcalfe (1993) as the boundary between these two plates. As the southern flank of the Anticlinorium consists of fault-slices of metagreywacke, metabasite, marble, ultramafics and plagiogranite, it could be interpreted as the metamorphosed relic of a fore-arc and island arc complex. The high-grade metamorphic assemblages (?kyanite plus staurolite) could be used to argue for deep burial by subduction. The Song Ma Anticlinorium is intruded by Devonian granitoids and the schists have a minimum metamorphic age of 425 Ma. The Anticlinorium overthrusts Triassic granitoids to the south along the Song Ma Fault, which is seismically active. This fault follows the trend of the numerous post-Cretaceous faults in Vietnam and we consider it to be one of these many CenozoicPresent structures. These data confirm: (1) that the protolith of the Song Ma Anticlinorium, which had been metamorphosed by Silurian times, cannot have been deposited in an Indosinian (PermoTriassic) subduction zone between the South China and Indochina plates; (2) that 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 Permian times. The geological relationships at Dien Lu are critical to the Palaeozoic geology of northern Vietnam. At Dien Lu, schists of the Song Ma Anticlinorium are juxtaposed against Lower to Middle Cambrian oolitic limestone beds and nonmetamorphosed volcaniclastic lithicwacke of the Dien Lu Formation. The minimum age of the schists is 455-425 Ma, given the available WAr data (see earlier). Both the schists and the oolitic limestone beds are overlain unconformably by non-metamorphosed limestones mapped as the ?OrdovicianDevonian Pa Ham Formation (Dinh Minh Mong, 1978). Given the minimum age of metamorphism in the schists as Middle Gondwana Research, V l , No. I , 1997

STRUCTURAL SETTING OF SONG MA REGION, VIETNAM

Ordovician-Silurian, these metamorphic rocks cannot be overlain unconformably by the older Dien Lu Formation. Therefore, the two units must be juxtaposed by a fault which predates the unconformably overlying and folded Pa Ham Formation Therefore, this fault pre-dates the post-Cretaceous F, deformation which folded the Song Ma Anticlinorium but post-dates the metamorphism in the schists of the Song Ma Anticlinorium. The Dien Lu Formation contains a Lower to Middle Cambrian fauna (Tong Bien Tap, 1978; see also Tran Van Tri, 1979) which occurs also in rocks of similar type and age on the South China plate in the Thai Nguyen region (Phan Cu Tien, pers. comm., 1993) and which are shown as correlates of the Dien Lu Formation in Tran Van Tri (1 973). This has three consequences: (1 ) the Dien Lu foramtion at Dien Lu must form part of the South China plate; (2) there is a cryptic suture at Dien Lu between a previously metamorphosed possible islandarc sequence (Song Ma and Nam Co Formations) and a nonmetamorphosed shelf sequence (Dien Lu Formation) and this cryptic suture may have been the South China plate boundary before deposition of the Pa Ham Formation; (3) as the Dien Lu inlier is cut by probably Oligocene faults which include thrusts first formed in an Oligocene thrust-fold belt then the Song Ma Anticlinorium and the South China plate sequence in the Dien Lu inlier is probably allochthonous. The above also re-confirms that the Song Ma Anticlinorium cannot be the site of an Indosinian subduction zone along the margin of the South China plate. Rather, the Song Ma Anticlinorium is most likely a relic of a metamorphosed subduction-related sequence (Song Ma terrane) that docked with the South China plate after metamorphism (post-425 Ma?) but before deposition of the non-metamorphosed rocks of the Pa Ham Formation. These conclusions also clarify the situation with regard to recent discoveries of Devonian fish at Ly Hoa, well south of the Song Ma Fault (Tong Dzuy Thanh et al., 1996). These fish 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 to and formed part of the South China plate before deposition of the Pa Ham Formation, then the fish-bearing rocks at Ly Hoa could well have been deposited on what was the South China plate in Givetian?-Frasnian times. Then what is the South ChinaAndochina Plate boundary? The Vietnamese Geological Survey's mapping programme before 1978 defined many extensive NW trending faults between Da Nang and the Song Hong. As shown on the Vietnamese Geological Survey's 1 :200000 map series, and in Tran Van Tri (1979) and Phan Cu Tien (1989), these faults commonly cut Cretaceous beds and some cut Cenozoic beds. These faults lie parallel to the Song Hong Fault on which there appears to have been sinistral strike-slip motion of Gondwuttci Reseriich, V I , No. I, 1997

31

between 300 km (Rangin, Pers. comm., 1995) and at least 500 (Tapponnier et a]., 1990).According to Tapponnier et a]. (1990) this displacement occurred in Oligocene times, and was followed by Quaternary dextral slip. Regional palaeostress studies by Phan Trong Trinh et al. (1994a, 1994b) have shown that between the Song Hong Fault and the Song Da region the Miocene regional stress field involved E-W compression, whereas in Plio-Quaternary times regional stress involved N-S to NE-SW compression and E-W extension. Our data are in fair agreement with these results. In addition to this, we conclude that not only did faulting involve thrusting and strike-slip, but also regional folding, including the syn-F,thrusting and folding of the Song Ma Anticlinorium. Given the generally parallel NW orientation of the major faults between Dang Nang (Fig. 1) and the Song Hong (see earlier discussion) then the palaeostress data predict major regional transpressional sinistral strike-slip in Oligo-Miocene times and dominantly reverse dextral slip in Plio-Quaternary times. The conclusion is in agreement with the relatively local considerations of Tapponnier et al. (1990) which concern the Song Hong Fault. Our results demand modification of the previous idea that the Song Hong Fault is the sole boundary between the South China and Indochina plates. Whilst this fault may have been the major locus of strain in Oligo-Miocene times, the plate boundary i s best regarded as a very broad plate boundary zone between the Song Hong and Da Nang and which clearly contains elements of the South China plate. Therefore to ask which fault is the plate boundary is a simplistic and probably irrelevant question. Rather, any discussion of the South China/Indochina plate boundary involves two questions. Firstly, where do we define the modern plate boundary in the very broad plate boundary zone between the Song Hong and Da Nang; perhaps the answer is the southern limit, the Song Ca Fault (Fig. I), of the tectonic melange that is the northern half of Vietnam. Secondly, we need to define the age of the plate boundary about which we speak; the Dien Lu inlier contains an ancient cryptic suture between a Cambrian fragment of the South China plate and a previously metamorphosed terrane, formed in a Cambrian island-arc setting elsewhere, which docked with the South China plate between Silurian and Devonian times . There may be other similar examples in the plate boundary complex of northern Vietnam.

Acknowledgements Field work was carried out in 1993 through the invitation of Dr. Phan Cu Tien (Director, Research Institute of Geology and Mineral Resources, Geological Survey of Vietnam) to R.H. Findlay and was sponsored by the Australian Department of Industry, Trade and Commerce. BHP(Asia) Pty Ltd is thanked also for encouragement and assistance. R.H. Findlay was accompanied variously in the field by Drs. Phan Cu Tien

32

R. H. FINDLAY AND PHAN TRONG TRINH

(Director) and Nguyen Ngoc Lien and Messrs Nguyen Thu Gaio, Nguyen Dinh Hop, Pham Binh, and Vo Xuan Dinh of

RIGMR. This project forms a contribution to IGCP 321, “Gondwana Dispersion and Asian Accretion” and IGCP 383, “Palaeostress, Geodynamics and Neotectonics and Natural Hazard Studies in west Pacific/Asia regions”. Dr. I. Metcalfe, IGCP 321 coleader, University of New England, is thanked for his support.

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