Unconformity of red sandstones in north Vietnam: field evidencefor Indosinian orogeny in northern Indochina?

Robin Lacassin{*, P. Herve Leloup{, Phan Trong Trinh{ and Paul Tapponnier{{Laboratoire de Tectonique, MeÂcanique de la LithospheÁre; UMR 7578 CNRS, Institut de Physique du Globe de Paris (IPGP), 4,

Place Jussieu, 75252 Paris Cedex 05, France; {Institut of Geological Sciences, National Center for Natural Sciences and Technology,

Hanoi, Vietnam

Northern Vietnam, `locus typicus' ofIndosinian orogenesis

North and east of the Himalayas, theAsian crust has been structured by sev-eral collisional events (e.g. AlleÁ gre etal., 1984; SengoÈ r andHsuÈ , 1984;Hutch-inson, 1989). Amongst these events, theIndosinian or Cimmeride orogeny isthought to have affected a large regionfrom northeastern Tibet to mainlandSEAsia, in themiddle Triassic (Deprat,1914b; Fromaget, 1941; SengoÈ r andHsuÈ , 1984; Hutchinson, 1989). Thisconcept of large collisional deforma-tions of Triassic age comes from thepioneering work of Deprat (1914a,1914b), who suggested that the Black-River region of northernVietnamwas amajor edifice of refolded thrust nappesaffecting the Palaeozoic and Triassicrocks. Together with the possible exis-tence of discordances within the Trias-sic series (Fromaget, 1941; Fromaget,1952), the main argument for a Triassicage of nappe emplacement, is the ex-istence of red conglomerates, sand-stones and pelites reported to be syn-tectonic and to lie unconformably ondeformed Triassic sediments. Hence,the concept of Indosinian orogenesishas been extended to the whole Indo-china peninsula, southwestern Chinaand eastern Tibet (Fromaget, 1941;SengoÈ r and HsuÈ , 1984).In the Yunnan province of China, it

was thus quite surprising to find verylittle evidence for such Triassic com-

pressional deformations along the RedRiverFault zone, andonboth sides of it(Tapponnier et al., 1990). There, mostof the visible deformations are clearlyof Cenozoic age (SchaÈ rer et al., 1990;SchaÈ rer et al., 1994; Leloup et al., 1995).In central Yunnan and Sichuan basins,there is no clear angular unconformitywithin the Triassic to Cretaceous seriesand the Triassic rocks are generallyfolded or sheared together with theJurassic±Cretaceous redbeds (Tappon-nier et al., 1990; Leloup et al., 1995). Innorth-west Vietnam, `locus typicus' ofIndosinian orogenesis, there is no pub-lished, clear factual description of suchangular unconformity. These observa-tions have led us to question the realsignificance and importance of this or-ogenesis (Tapponnier et al., 1996).

The Indosinian unconformity in theBlack-River region

In northern and central Vietnam, theclearest descriptions of tectonic struc-tures are those of Deprat (e.g. Deprat,1914a, 1914b, 1914c). His detailed andfactual cross-sections proved to behighly reliable in the field, a conclusionwe failed to reach for the interpretativesections and descriptions of Fromaget(1941). In central Vietnam, Depratclearly describes the post-Hercynianunconformity of lower Triassic poly-genic conglomerates deposited ontogneisses (Deprat, 1914c). In his studiesof the black river region, comparablefield evidence for a syn-Triassic, post-compressional unconformity is lessclear. Deprat's detailed geological sec-tions of the Song Da region, south of

the so-called Black-River nappes,show the reportedly unconformablered-bed series pinched in synclines,and folded like the Triassic limestones(Deprat, 1914a, 1914b). As no detailedfield description of this hypotheticalunconformity has been published sinceDeprat, we attempt here to reinvesti-gate the structure of some of these redbed basins.

Geological setting of the Song Da region

Along the Red River Fault Zone(RRFZ) in NW Vietnam, left-lateralshear has occurred mainly in the high-grade mylonitic core of the Dai NuiCon Voi (Leloup et al., 1995, 1997).South and north of this core, severalstrike-slip faults and shear zones alsoaccount for significant left-lateral mo-tion (Fig. 1). About 100 km south-westof theRRFZ, the SongDa (Black river)has entrenched a deep valley in foldedPalaeozoic and Triassic sediments.Upper Palaeozoic lower±middle Trias-sic sedimentation is considered to haveoccurred in a subsident basin, the `SongDa rift', together with basaltic volcan-ism and intrusions of upper Permian toearly Triassic age (Hutchinson, 1989).For SengoÈ r and HsuÈ (1984), this zonewould correspond to the major indosi-nian suture zone. The rocks of the SongDa region have been affected by anintense N±S to NE±SW shorteningand most of the structures, includingthrusts, shear zones and tight folds, arepresently steep. Where present, linea-tions and shear criteria attest to nearlyvertical stretching and upthrust move-ments on such steep structures.

106 #1998 Blackwell Science Ltd

ABSTRACTWe re-examine several unconformity sites in northwesternVietnam where the Indosinian orogenesis has been defined.Field observations in the Black-River (Song Da) region, near SonLa, demonstrate that red sandtones and conglomerates,reportedly of Cretaceous age, occur unconformably aboveschistosed epimetamorphic middle-Triassic sediments. Amesozoic unconformity thus really exists in this region, thoughthe age of deformation is poorly contrained (upper Triassic to

upper Cretaceous). In most places, however, the unconformity isstrongly deformed, folded Triassic limestones are often thrustedonto the Cretaceous rocks. This implies that Tertiarydeformations probably due to the India±Asia collision havebeen strong in the northern part of the Indochina block.

Terra Nova, 10, 106±111, 1998

AhedBhedChedDhedRef markerFig markerTable markerRef endRef start

*Correspondence: E-mail: lacassin@ipgp.

jussieu.fr; http://www.ipgp.jussieu.fr/depts/

tecto.html

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South of the Song Da valley, in theSon La and Moc Chau region, a dis-continuous stripe of red beds (Fig. 1b),reportedly ofCretaceous age, ismappeddiscordantlyon the foldedTriassic series(General Department of Mines andGeology of the Socialist Republic ofVietnam, 1988; Nguyen Xuan Bao etal.1978;BuiPhuMy et al.1978;NguyenVinh et al., 1978; Trang Danh Tuyet et

al. 1978). Geological maps and satelliteimages suggest that this stripe is cut orboundedby strike-slip faults and thrusts(Fig. 1b). We report here detailed ob-servations along two key sections:North of Son La (section 1, Fig. 2a)and across the Yen Chau basin (section2,Fig. 2c),which allowus toassess in thefield the relationships between the redbeds and the Triassic rocks.

The basal unconformity of the red bedseries north of Son La

The first section (Fig. 2a), from theSong Da River to Son La, shows redconglomerates and sandstones, mappedas Cretaceous, outcropping in a smallbasin within the Triassic series. To-wards the SW, the middle-Triassiclimestones (Anisian according to De-

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Terra Nova, Vol 10, No. 2, 106±111 Unconformity of red sandstones, Vietnam . R. Lacassin et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DaiNui

ConVoi

SongMa

anticlinorium

Phan Si

Pang

Tulevolcanics

Son La

LC

DBP

YB

YCMC

103°E

22°N

22°N

21°N

104°E 105°E

104°E 105°E

1

2

50 km

Fig. 1 Location and tectonic sketch of north-western Vietnam and adjacent regions. (a) Location of NW Vietnam in tectonicframework of continental SE Asia. Arrows show finite sense of displacement along Cenozoic strike-slip faults. Hatched areascorrespond to inferredTriassic fold and thrust belts strongly affected by subsequent Tertiary deformations. (b) Tectonic sketchmap ofSongDa (Black-River) region showing SonLa±MocChau red bedbasins and locationof geological sections ofFig. 2 (circled numbers)(after General Department of Mines and Geology of the Socialist Republic of Vietnam 1988; General Geological Department of theDemocratic Republic of Vietnam 1973). The Red River Fault (R.R.F.) and Dai Nui Con Voi metamorphic core mark the Cenozoictransform boundary between South China and Indochina blocks (Leloup et al., 1995). DBP, Dien Bien Phu; LC, Lao Cai; MC,MocChau; YB, Yen Bai; YC, Yen Chau, D.B.P.F., Dien Bien Phu Foult.

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prat, 1914c; Nguyen Vinh, 1978; TrangDanh Tuyet, 1978) are tightly folded ina series of anticlines and synclineswhose axes trend NW±SE to WNW±ESE. These limestones probably over-thrust the red bed basin of Muong Da(Fig. 2a), though contact between thetwo units has not been observed di-rectly in the field. The red sandstonesand conglomerates of this basin dip 15±308 SW on average. Towards the NE,they overlie unconformably a series ofschists, sandstones and quartzites, ofTriassic age. On a single outcrop alongthe road (UTM coordinates 48Q VJ001 691) one can observe the red con-glomeratic beds, dipping 20±308 SSW,lying directly on top of schistose, stee-ply dipping sandstones and schists(Figs 2b and 3a).Up to the Song Da valley, the rest of

the section shows a nearly verticalflyschoid series with shale, schist, greyand yellowish sandstone and quartzite,reportedly of Ladinian age (NguyenVinh, 1978; Trang Danh Tuyet, 1978).These rocks are epimetamorphic andaffected by fracture or slaty cleavages(Fig. 3b). The bedding planes trendN120±1308E and dip steeply to theNE or SW, while the cleavage generallydips to the NE (Fig. 3b). The constantbedding (S0)±cleavage (S1) relation-ships, together with polarity criteria(graded bedding, shape of refracted

cleavage, load and flute casts) suggestthat this nearly vertical series is roughlymonoclinal (Fig. 2a). The northern-most part of the section, near the SongDa valley, shows mylonitic metatuffsand rhyolites, with a N1108E-trendingschistosity dipping to the NE or nearlyvertical. These rocks are probablythrust southwestward onto the flysch-oid series.

Evidence for shortening of the red bedseries

The second section crosses the southernborder of the Yen Chau red-bed basin,& 50 kmSEof SonLa (Fig. 2c).Hills ofmiddle-Triassic limestones, dipping tothe SW, bound the basin to the south.These limestones are thrust towards thenorth onto the red conglomerate, sand-stone and pelitic series. A 250-m thickzone of sheared limestone, pelite andsandstonemarks the thrust contact andshows intense boudinage, microfoldingand small reverse shear-zones (Fig. 2c).Overturned tonearly vertical thick bedsof red conglomerate, also cut by reversefaults and shear zones, outcrop justbelow this sheared zone (Fig. 3c, d).These conglomerates bear mainly lime-stone and sandstone pebbles, with a fewquartz pebbles. We have not foundpebbles of plutonic or metamorphicrocks. Towards the north and upward,

the series become less conglomeraticand grades into fine-grained sand-stones and pelites that form the middlepart of the basin. South of the village ofChien Sang the sandstones dip & 208NE while they dip gently to the SSWnorth of the road from Son La to MocChau (Fig. 2c). We have not reachedthe northern border of the basin alongthis section. Nevertheless, betweenYenChau and Moc Chau, the morphologyof the Yen Chau basin on satelliteimages and in the field implies thatthrusts may also follow this northernborder (Fig. 1b). The Yen Chau red-bed basin thus appears to be a pinchedsyncline bounded on both sides by re-verse faults.

Discussion

The detailed field observations we re-port here definitively demonstrate thatthe red bed series of the Black-Riverregion have been deposited on pre-viously deformed Triassic rocks. Thatthe sandstones and schists located be-low the unconformity are steep andschistose strongly supports the ideathat the red-beds are not onlappingonto a block-faulted region, as in cen-tral Yunnan (Leloup et al., 1995), butinstead postdate a compressional tec-tonic event. Note however, that theunderlying rocks are only epimeta-

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Unconformity of red sandstones, Vietnam . R. Lacassin et al. Terra Nova, Vol 10, No. 2, 106±111. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 2 Geological sections across red bed basins of Song Da region. (a) Section along unpaved road from Son La to Song Da river(section 1, Fig. 1b); NE to the left. (b) Detail of unconformity of conglomerates onto schistosed sandstones and shales (outcrop of Fig.3a). (c) Section of southern border and middle part of Yen Chau basin (section 2, Fig. 1b), NE to the right. Main road from Son La toMoc Chau is in middle of section.

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morphic and that the red conglomerateis not a polygenic molasse bearing a lotof metamorphic and plutonic pebbles.This could imply that, in the Black-River region, the pre-unconformitycompressional event has not broughtto the surface deep metamorphic units,such as those typically found in internalparts of a mountain belt.

From available data, it is difficult toassign a unequivocal age to this com-pressional event.During the first half ofthe century, the red beds were consid-ered to be upper Triassic to lower Lias-sic. It was the so-called `terrain rouge'inferred to be `infralias' and syntectonicby Deprat (1914a, 1914b), then by Fro-maget (1941). A Cretaceous age is now

ascribed to these red-beds, that couldperhaps be as young as upper Cretac-eous to lower Cenozoic (General de-partment of mines and geology of thesocialist republic ofVietnam1988;Gen-eral Geological Department of the De-mocratic Republic of Vietnam, 1973;Phan Cu Tien et al., 1989). Indeed, theyreportedly `contain Cenozoic remains

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Terra Nova, Vol 10, No. 2, 106±111 Unconformity of red sandstones, Vietnam . R. Lacassin et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 3 (a) View of red bed unconformity along Son La-Song Da section (Fig. 2a, b). The dotted white line outlines the base ofconglomeratic beds (congl.) above nearly vertical to overturned schists and sandstones (sch.); S0 is bedding. (b) Detail of schistosedsandstones (left part ofFigs 2b and 3a). Spaced fracture cleavage in coarse sandstone beds (left) grades into penetrative slaty cleavage inschists (middle). Curved shape of cleavage, due to refractionmore important in coarser parts of sandstone beds (left), outlines graddedbedding and is used as polarity criteria. (c) Nearly vertical red conglomerates at southern border of Yen Chau basin (Fig. 2c). (d)Detailed view of conglomeratic beds of Fig. 3(c). Pebbles, rounded to angular, are mostly sandstone and limestone. Hammer handleoutlines bedding.

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and a freshwater Bivalvia (Unio sf.grabaui) of Late Cretaceous' (Phan CuTien et al., 1989; p.56). The possibleages for the deformation of the under-lying Triassic rocks thus range betweenthe middle±upper Triassic and prob-ably the uppermost Cretaceous. In anycase, the unconformable red beds can-not be younger than the volcanic andhypovolcanic complex that overprintthe unconformity south of the Phan SiPang range (Fig. 1). One Ar±Ar radio-metric result (Sun Li Chung et al. 1997)on phlogopite-rich lavas (cocites) sug-gests an age of about 30 Myr for thiscomplex. Triassic ages of metamorph-ism and deformation have been ob-tained from the SongMa anticlinorium(& 30 km south of Son La, Fig. 1b) andfarther south in central Vietnam (Mal-uski et al., 1995; Lepvrier et al., 1997;Maluski et al., 1997). This supports thehypothesis of an Indosinian (upperTriassic) deformation in the black riverregion. We cannot exclude, however,the existence of a Jurassic orCretaceousevent also suggested by some radio-chronological results in the region(Maluski et al., 1997). At most, thisdeformation could even be related tothe onset of the India±Asia collision, asit is inferred in Tibet for the deforma-tion of the Takena series below theunconformable Linzizong formation(Tapponnier et al., 1986).It is clear, however, that the basal

unconformity, and the red-beds them-selves are involved in the fold andthrust edifice of theBlack-River region.This implies that post-Cretaceousshortening, likely an effect of the In-dia±Asia collision, has been strong inthe northern part of the Indochinablock. In the foldedTriassic limestones,the proportion of shortening that wasacquired before and after red-bed de-position remains unclear. Note, how-ever, that the orientation of fold axes inthe limestones and the post red-bedthrusts have the same direction, andhave linked geometries at places, im-plying that they could be due to thesame Tertiary tectonic event.At the scale of mainland SE Asia

(Fig. 1a), the probably Triassic, BlackRiver tectonic belt could tentatively becorrelated with the Songpan-Garzefold and thrust belt of eastern Tibet,western Sichuan and northern Yunnan(Mattauer et al., 1992; She Fa Chen etal., 1995). In the Songpan-Garze beltand in the western Sichuan basin, the

occurrence of Indosinian shortening isattested to by an upper Triassic uncon-formity (She Fa Chen et al., 1995) andof Jurassic granites cross-cutting foldedTriassic rocks (Calassou, 1994). In thisbelt, which is also strongly affected byTertiary shortening (Calassou, 1994;Lacassin et al., 1996), the other majorreported unconformities are of Ceno-zoic age (She Fa Chen et al., 1995). Inthis hypothesis, the southern part of theSongpan Garze belt would have beencut and displaced 400±700 km towardsthe south by Tertiary sinistral motionalong the Red River Fault zone.

Acknowledgements

Field work was part of a cooperationprogram between the Centre National dela Recherche Scientifique (CNRS-INSU,France) and the National Center for Nat-ural Sciences and Technology (Vietnam).We thank Nguyen Trong Yem for support-ing this work, J.P. Cogne , H. Maluski, J.Marcoux and U. SchaÈ rer for fruitful dis-cussions, J. Malavieille and an anonymousreferee for reviewing the paper. IPGPcontribution n81558.

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accepted 21 July 1998

#1998 Blackwell Science Ltd 111

Terra Nova, Vol 10, No. 2, 106±111 Unconformity of red sandstones, Vietnam . R. Lacassin et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Paper 178 Disc