The Song Ma Zone [17,20,59] consists of several northwest-striking fault-bounded units (Fig. 4). Westwards, this zone and the associated Song Da Zone, which are bounded by the Song Da Fault, deflect clockwise as a result of the tertiary dextral motion of the Dien Bien Phu Fault.

The axial unit of the Song Ma Zone is represented by metasediments of the Song Ma and Ham Rong Formations [4,5,54], assigned to the Cambrian–Lower Ordovician. It encloses tectonically sheared lenses of mafic to ultramafic rocks of ophiolitic nature, and accordingly it has been interpreted as the trace of an oceanic suture between Indochina and South China [14,20,26,27,51,60]. To the south, bounded by the Song Ma Fault, is exposed the undeformed Triassic Song Ma Granite, which intrudes laterally the metamorphic Su Lu Formation made of sillimanite–garnet micaschists and amphibolites. This intrusion and, more to the west, the equivalent Dien Bien Granitic Complex are unconformably covered by Upper Triassic (Norian) conglomerates and sandstones. The northern unit is made of different metamorphic formations which form a broad antiform, called the Song Ma anticlinorium [59]. The core of the antiform, crossed by the road from Son La City to Chieng Khuong, is occupied by the Late Proterozoic–Lower Cambrian Nam Co Formation [4,5,54], made of micaceous pelitic schists. Going to the west, the structure evolves into a system of NNW-trending antiforms and synforms separated by submeridian faults, which are crossed by the road from Thuan Chuan to Dien Bien through Tuan Giao and by the Nam Co River. In the area of Muong Ang, the axial part of a relatively flat and broad synform is occupied by the Devonian metamorphic, but still fossiliferous, Pa Ham Formation [5,54], containing corals.

North of the Song Ma zone, the Song Da zone is classically interpreted as a continental rift [20] that differentiated within a carbonate environment represented by the Early Permian Da Mai Formation. In the rift zone, the sedimentary succession is dominated by terrigenous facies rocks up to the Lower Triassic. Mafic effusive formations of Permian age (Yen Duyet, Cam Thuy and Vietnam Formations) [4,54] are interbedded in the series. In particular, komatiite rocks, equivalent to those known in South China (Emenshian volcanic province) and related to a mantle-plume activity, have been described in the area [19,46,47]. In the Middle Triassic, the sedimentation becomes calcareous, but involves interbeds of felsic Anisian calc-alkaline rocks (rhyolite, tuffs) of the Dong Trau Formation. Similar felsic rocks of the Song Da Formation, west of the Dien Bien Fault, could have been emplaced as soon as the Permian [54]. The Upper Triassic is represented by terrigenous formations. Such Upper Triassic clastics and also Cretaceous red sandstones that rest unconformably above schistosed epimetamorphic Middle Triassic formations demonstrate the influence of a late Indosinian episode of deformation in the Song Da Zone [24].

In the Song Ma zone, petrologic evidence demonstrates that the Honvang serpentinic body, consisting of massive serpentinite with chromian spinel derived from lherzolitic harzburgite, altered gabbro of MORB-like affinity and some high Al-type chromitite, represents a remnant of palaeo-oceanic lithosphere [60]. The surrounding metamorphic rocks, which enclose the disrupted ophiolitic material, reach amphibolite facies, with muscovite, biotite, garnet, and sillimanite assemblages. Occurrences of HP mineral assemblages that had been suspected [17] were not confirmed until the recent discovery of relics of eclogite facies rocks enclosed within schists [36,37].

Extensive geochronological investigations by Ar–Ar method on synkinematic minerals from mylonites in the Nam Co and Song Ma Formations display ages of 240–250 Ma [26,29].

According to the descriptions by Findlay [14–16] for the Son La-Chieng Khuong and Co Ma sections of the Song Ma antiform, the tectonometamorphic evolution is polyphased, and includes the development of several planar and linear structures. S1 is related to a metamorphic layering, well expressed in orthogneisses and in metabasic rocks; in the metasediments, the dominant schistosity S2 overprints an earlier lithological/metamorphic quartz–mica layering (S1), developing axial planar to isoclinal microfolds. In turn, S1 and S2 fabrics are affected by a steep crenulation cleavage S3. The associated L3 lineation, as well as mesoscopic F3 folds that refold the previous foliations, corresponds to the general northwestern strike of the antiform. This map-scale structure therefore could appear as being formed during a late Indosinian tectonic stage.

Our investigations at the southern limb of the antiform have revealed the existence of steeply-dipping gneissic ultramylonites with subhorizontal stretching lineation. The accompanying asymmetrical criteria associated with stretching lineation consistently indicate a dextral sense of shear (Fig. 3A–D). This strike-slip shearing is corroborated by the occurrence of sheath folds and also by the development of steep-axe microfolds, as already mentioned [14]. In our interpretation, the arching is not the result of a superimposed episode of deformation, but is likely synchronous with the dextral shearing and the development of mylonites along the Song Ma suture, being related to the same dextral transpressional kinematics. Sheath folds, associated with the map-scale antiform, which were attributed to a D3 event by Findlay [14,15], are therefore more likely formed during an initial episode, synchronously with the dextral shearing. The north–south system of antiforms and synforms that develops west of Tuan Giao has been formed in the same way. In the axial part of the synform near Muong Ang, the Devonian marbles, associated with calcschists, exhibit a marked stretching lineation and boudinage structure that both indicate an axial transport to the NNW (Fig. 3E). At the eastern border of the synform, the Nam Co Fm and the intensively folded Devonian marbles overthrust the Permian lavas of the Song Da Zone through a ductile contact. As it is exposed in the upper course of Nam Muc River and north of Tuan Giao, these greenschist-facies aphanitic lavas are progressively sheared, as a result of Indosinian movements.

The western part of the Kontum massif, at short distance from the Lao-Vietnamese boundary, is crossed by the major north–south-striking Po Ko Fault, which belongs, as the Dak To Kan Fault, to a system of submeridian faults that developed parallel to the Kontum continental margin. From Phuoc Son to the South of Pleikan, the steeply dipping Po Ko Fault coincides with the Dak Sé valley and, after the Loxo pass, with the upper course of the Krong Po Ko. South of Pleikan, the main fault splays into several secondary faults. More to the south, the trace disappears beneath the Quaternary Pleiku basalts plateau. To the north of Phuoc Son, the main fault is relayed by a set of subparallel fault segments, but the extension of the system is unclear.

In between Pleikan to Phuoc Son, recent fault scarps, abrupt reliefs, and other morphotectonic features attest to the recent activity of the Po Ko fault through normal and strike-slip movements. This is confirmed by the development of pseudotachylites and by the existence of uplifted Quaternary conglomerate deposits in a small graben formed at the emplacement of an overlapping zone at the Loxo pass.

On the major part of its length, the course of the Po Ko Fault coincides with a sheared palaeosuture zone, delineating contrasting terranes (Fig. 5). The suture is underlined by discontinuous outcrops of a variety of ophiolitic rocks that are referred to the Pleiweik Complex [6,7,54]. The age of this oceanic material remains unsettled yet, due to the absence of associated datable cherts and appropriate radiometric dating. The complex consists generally of elongated and sheared lenses of gabbros, pyroxenites, and peridotites that are partially amphibolized and serpentinized at their margins [58]. Good outcrops can be examined in the area of the Loxo pass and on the banks of the Yaly artificial lake, where gabbros and pyroxenites are exposed as rather massive layers or are intensively sheared as a result of strike-slip ductile movements (Fig. 6).

Rocks belonging to the magmatic and metamorphic Dien Binh Complex are distributed west of the Po Ko Suture (Figs. 5 and 6). The western extension of these terranes cannot be clarified, because they are largely intruded by granitic rocks or covered by Permian volcanic lavas or volcano-detritic Triassic sediments. More to the west, they disappear beneath the Mesozoic sedimentary strata of the Khorat Basin. In addition, this zone, which is close to the Cambodia-Lao-Vietnamese boundary, is a restricted area, quite difficult to be accessed for geological investigations. Good outcrops are provided by the new Ho Chi Minh road from the North of Pleikan (Ngoc Hoi) to Dak Gley, and by the banks of the Po Ko River in the vicinity of Dakto (nearby the ancient military landing stripe), nearby the Dien Binh village, along the Dak Psi River. Other good outcrops exist more to the west near Sathay, along Dak Sir.

The Dien Binh Complex [6,7,18,54] consists of amphibolite-facies igneous rocks, made of porphyroid granodioritic to dioritic bodies, fine-grained aplitic veins, and mafic layers or dykes. This magmatic material intrudes into foliated metasedimentary rocks, represented by paragneisses, sillimanite-bearing micaschists, and quartzites. A weak foliation, underlined by the planar arrangement of biotite, can be recognized in the magmatic intrusions (Fig. 7G), parallel to the foliation of the host rocks. The mafic parts, transformed into amphibolites, are stretched and boudinated, showing asymmetrical kinematic criteria (Fig. 7H). Regionally, the trace of the foliation swings from north–south to WNW–ESE in the segment comprised between Pleikan and Dakto, while the dip to the west and SSW is moderate to steep. A later generation of magmatic material, made of granite, aplitic veins and rhyolites, cuts through this complex, as evidenced by the field relations provided by various outcrops (Fig. 6). Unmetamorphosed dacitic lava flows, which belong to the Permian Chu Prong Formation, lie subhorizontally on the Complex, nearby the road to Sathay. Recently discovered Lower Devonian fossiliferous carbonate platform sediments of the Cu Brei Formation rest unconformably, through basal conglomerates, on the Dien Binh granitic material [55].

East of the Po Ko Fault, metamorphic and related intrusive rocks of the Kannack and Ngoc Linh Complexes are exposed [18,45]. Recent petrological investigations have shown that both have experienced granulitic conditions up to ultra-high temperature metamorphism [35,38,41,42], before being partly retrogressed and migmatized. Conversely, the Kham Duc Complex, which developed north of the massif and is separated from the Ngoc Linh Complex by the Quang Ngai–Phuoc Son and Trabong shear zones [8], is only composed of epidote amphibolite and amphibolite facies rocks [18,45].

• • The Kannack Complex, in the southeastern part of the massif, is represented by various granulitic rocks and mineral assemblages, including primary or retrograde paragenesis [35,38,41,42,61]. The main rock types are foliated Al-rich pelitic to semi-pelitic banded gneisses (khondalitic and kinzigitic gneisses), and more massive felsic gneisses; mafic gneisses, quartzo-feldspathic and calc-silicate rocks occur as interlayers or lenses. Essentially represented in the Song Ba valley, the magmatic rocks of the Plei Man Ko Formation [9,10,18] consist of garnet-bearing granites and orthopyroxene-bearing granites equivalent to charnockites, with, in particular, enderbites and fine-grained gabbroic to noritic rocks [44]. According to the field relations, they intrude the host granulite-facies gneiss: either they parallelise or cut through the foliation, being themselves foliated or massive [27]. They appear as coeval intrusive rocks that could be derived respectively from pelitic and mafic granulites [44]. Both the granulites and charnockites have suffered partial melting, as shown by the development of anactectic leucosomes.
• • The Ngoc Linh Complex (Song Ré and Tac Po Formations), in the northern and northwestern part of the area, is mainly represented by amphibolite-facies rocks (garnet amphibolite, biotite-hornblende gneiss), which are locally extensively migmatized (as in the Song Ré area). Recently, eclogitic ultrahigh-temperature-facies mafic granulitic rocks, represented by blocks and lenses within mylonitized felsic gneisses [35,38], have been identified in the western Ngoc Linh Complex in the area of Poko and Dak To Khan.

The so-called Caledonian Orogeny, although this name is inappropriate to be applied to Asia, has affected different parts of China, and in particular the South China block [2]. In Vietnam, the existence of a Lower Palaeozoic tectonic event is not clearly established. Ordovician ages of 465–470 Ma characterize the former HT granulitic episode in the eastern Kontum Massif. In western Kontum, on the western side of the Po Ko Suture, crystallisation ages of 400 to 450 Ma are displayed by the granodioritic intrusions of the Dien Binh Complex [33,61]. Similar protolith ages of 418–407 Ma have been reported from the Indosinian orthogneiss of the Dailoc Massif, west of Da Nang [1], and a 428-Ma crystallisation age is recorded by the orthogneiss of the Song Chay Massif, in North Vietnam [49]. It is highly probable that such a magmatic event is largely distributed throughout Vietnam, being the equivalent of the magmatic intrusions that are well known in the South China Block [62].

Devonian formations, which are exposed in northern Vietnam and north-central Vietnam, and further south in south-central Vietnam as an isolated outcrop over the western Dien Binh unit of the Kontum Massif [55], rest unconformably on older rocks. Whatever their facies, the Lower Devonian deposits, as it is clearly mentioned in the description of the stratigraphic units of Vietnam [54], exhibit a general basal unconformable contact. Only in some areas, they are in continuity with the Upper Silurian. Although further investigations are needed, such a relationship suggests that Indochina, as South China, has been subjected to the ‘Caledonian’ orogenic event.

The Devonian succession is itself variably deformed. Either, the Devonian rocks are highly deformed when they were involved in the Indosinian shear zones of the Truong Son Belt, as for example in the Song Ma Zone, or they are preserved from intense deformation and only slightly folded and almost unmetamorphosed within the segments comprised between the shear zones.

The Lower Devonian succession, in the eastern zone of Vietnam, north and south of the Song Ma Suture and in South China, displays similar red facies and contains the same Vertebrate fauna [21,22,48,53,63]. Westwards, the terrigenous to near-shore facies evolves progressively into mixed and carbonate-platform marine environments. These similarities imply biogeographical land connection between the Indochina and South China blocks, and therefore the absence of any oceanic barrier at that period of time. Accordingly, this means that Indochina and South China were parts of the same landmass as soon as the Late Silurian [21,22,48,53,63]. If any older oceanic gap would have already existed along Song Ma, the connection of the two previously distinct blocks could likely be the result of a ‘Caledonian’ tectonic event. A further separation of Indochina and South China by oceanic opening of a Palaeotethyan branch along Song Ma could then only be established after the Devonian. Upwards within the Devonian succession, from the uppermost Lower Devonian, marine conditions were progressively generalized. As already suggested, the development of endemic forms at that time could be due to block-faulting and the differentiation of a broad rift [22], which possibly continued to develop into an oceanic basin.

The distribution of the metamorphic rocks throughout the Kontum Massif show significant variations from east to west and from north to south, according to their metamorphic grade.

The highest-grade granulite-facies metamorphism is restricted to the western Kannack and Ngoc Linh Complexes with thermobarometric estimates of 900–990 °C and 1.03–1.50 GPa [35,37,41–43]. Eclogitic (1.8 GPa) ultrahigh-temperature conditions are recorded in mafic granulites, as part of a prograde metamorphism [35]. In the eastern Kannack and Ngoc Linh area, estimated P–T conditions, corresponding to granulite- to amphibolite-facies metamorphism, correspond to medium to high temperatures and low pressures (700–900 °C and 0.50 to 0.84 GPa), with temperature decrease in the northeast. In the northern Kham Duc Complex, mineral assemblages, which correspond to an amphibolite- to epidote amphibolite-facies metamorphism, indicate low to medium temperatures and moderate to high pressures [38].

The highest grade UHT and HP metamorphic rocks in western Kannack and Ngoc Linh Complexes show a similar clockwise P–T path [35,38,42], with a near-isothermal decompression following the peak metamorphism and a final near-isobaric cooling. A comparable clockwise shape is displayed by the Kham Duc Complex [37]. Retrograde conditions variably affect the massif. Primary assemblages are either well preserved, while in other areas they are replaced by a secondary paragenesis that could be the result of a low-grade superimposed event [30].

Geochronological data, obtained by various methods on granulite-facies rocks collected throughout the Kannack and Ngoc Linh Complexes, have revealed the existence of two age groups, corresponding to two episodes of HT metamorphism [50]. All the charnockitic rocks of the Song Ba area yield Triassic ages. The older Lower Palaeozoic granulitic-facies ages characterize the eastern part of the Kannack Complex, whereas Triassic ages are restricted to the Song Ba area in the central and eastern parts of the Ngoc Link Complex, close to the Po Ko Suture Zone.

In the Dien Binh Complex, U–Pb dating on the magmatic intrusions also indicates Ordovician–Silurian ages, while the Ar–Ar method provides younger, but non-Indosinian ages, on the same rocks. These ages have been interpreted as differential cooling ages. A partial reset, due to a later thermal overprint that could be relevant to an Indosinian event is unlikely, as has been discussed [33]. A speculated distinct intermediate event has been inferred, but until now has not been documented elsewhere in Indochina.

Whereas the U–Pb and Ar–Ar Ordovician–Silurian ages reflect the climax of the original granulite facies metamorphism, the second age group is related to the Indosinian overprint resulting in a partial or total reset. In eastern Kannack, the granulite-facies rocks yield a preserved Ordovician–Silurian age. The total metamorphic resetting of the primary HT granulitic event by the Indosinian thermal event can be explained, in the central Song Ba part of the Kannack Complex, by the huge amount of the Triassic charnockitic intrusions, providing a sufficient reheating [50]. Conversely, the Indosinian high-grade granulitic conditions recorded in the Ngoc Linh Complex, east of the Po Ko Suture, require another explanation, within a collisional setting zone [38,42]. Taking into account the structural and kinematic data collected in this suture zone, together with the distribution of ages and metamorphic grade in the granulite-facies rocks, a west-dipping subduction along the Po Ko suture and a subsequent collision provide the needed HP conditions. Although the oceanic material of the suture has not been dated, Triassic ages found for the high-pressure granulite metamorphism [38] are in favour of a westward subduction process of a branch of the Palaeotethys, leading to continental subduction and suturing in the Early Triassic [27].

This interpretation is corroborated by the emplacement of granitic intrusions and effusive rocks along the Po Ko Zone, on the western Dien Binh side. The Late Carboniferous–Early Permian volcano-sedimentary Dak Lin Formation, including dacite, andesite, and basaltic tholeiitic lavas, crops out in the Northwest of the city of Buon Ma Thuot. Similarly calc-alkaline rocks (tuffs, rhyolite, dacite and andesite) of the Chu Prong Formation, referred to the Upper Permian–Lower Triassic, are exposed in the area of Plei Ku and Sa Thay (Figs. 5 and 6), and are related to a volcanic-arc environment. Part of these subduction-related rocks corresponds to pre- or syn-collisional emplacement, and is consequently deformed. Some other bodies of granite and effusives are undeformed, cutting through the Indosinian foliated rocks: they correspond to a postcollisional setting. A granitic intrusion of 204-Ma age marks the upper limit of the shearing age along the Po Ko zone [27]. Along the Quang Ngai shear zone, to the north, the shearing was completed by 222 Ma, as documented by a crosscutting granitic intrusion (coordinates:15° 09′; 108° 42′) [61].

This geodynamic setting also explains why the Dien Binh Complex, located in the upper plate, displays preserved pre-Indosinian ages.