Michel Villeneuve

Camille Rossignol

Rossana Martini

Jean-Jacques Cornée

Sylvie Bourquin

New insights into the Triassic sedimentary environment of the eastern parts of the Song Da and Sam Nua basins alongside the Indosinian Song Ma suture, Northern Vietnam

Keywords: Stratigraphy, Foraminifers, Sedimentology, Carbonate platforms, Indosinian orogeny, Northern Vietnam

This study documents the sedimentology, structures, and foraminifer content of Triassic formations from the eastern part of the Truong Son Belt (Northern Vietnam). Sedimentary rocks in the eastern Song Da and Sam Nua basins occur on both sides of the Middle Triassic 2 Song Ma suture zone that separates the Indochina Block from the South China Block. The main outcomes of our study include detailed stratigraphic successions that we have established for the two basins. New foraminifer determinations indicate a Middle to Late Triassic age for the carbonate deposition. The depositional settings of the carbonate deposits and their deformation patterns support a southward "subduction" beneath the Indochina Block, ending with the collision with the South China Block during the Middle Triassic. Moreover, our new biostratigraphic dating of the stratigraphic successions can be used to compare the Triassic carbonate platforms from Northern Vietnam with other platforms elsewhere, including those that have already been well documented in Indonesia.

Introduction

During the Triassic, extensive carbonate platforms developed in different basins in Southeast Asia [START_REF] Khuc | Triassic correlation of the Southeast Asian mainland[END_REF][START_REF] Payne | Environmental and Biological Controls on the Initiation and Growth of a Middle Triassic (Anisian) Reef Complex on the Great Bank of Guizhou, Guizhou Province, China[END_REF][START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]Lerhmann et al., 2015), including those in Northern Vietnam [START_REF] Khuc | Triassic correlation of the Southeast Asian mainland[END_REF][START_REF] Martini | Occurrence of Triassic foraminifers in carbonates deposits from the Ninh Binh Area (North Vietnam)[END_REF][START_REF] Algeo | The Permian-Triassic boundary at Nhi Tao, Vietnam: Evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform[END_REF][START_REF] Son | The Permian-Triassic boundary sections in northern Vietnam (Nhi Tao and Lung Cam sections): Carbon-isotope excursion and elemental variations indicate major anoxic event[END_REF][START_REF] Komatsu | Radiation of Middle Triassic bivalve: Bivalve assemblages characterized by infaunal and semi-infaunal burrowers in a storm-and wave-dominated shelf, An Chau Basin, North Vietnam[END_REF][START_REF] Metcalfe | Changhsingian (Late Permian) conodonts from Son La, northwest Vietnam and their stratigraphic and tectonic implications[END_REF]. These platforms provide important information that can be used to document the global environmental evolution and biotic recovery after the end-Permian mass extinction [START_REF] Algeo | The Permian-Triassic boundary at Nhi Tao, Vietnam: Evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform[END_REF][START_REF] Son | The Permian-Triassic boundary sections in northern Vietnam (Nhi Tao and Lung Cam sections): Carbon-isotope excursion and elemental variations indicate major anoxic event[END_REF][START_REF] Komatsu | Radiation of Middle Triassic bivalve: Bivalve assemblages characterized by infaunal and semi-infaunal burrowers in a storm-and wave-dominated shelf, An Chau Basin, North Vietnam[END_REF][START_REF] Metcalfe | Changhsingian (Late Permian) conodonts from Son La, northwest Vietnam and their stratigraphic and tectonic implications[END_REF]. However, the precise stratigraphic framework for the Northern Vietnam Triassic carbonate platforms is still incompletely characterized [START_REF] Shi | A Changhsingian (Late Permian) brachiopod fauna from Son La, northwest Vietnam[END_REF][START_REF] Martini | Occurrence of Triassic foraminifers in carbonates deposits from the Ninh Binh Area (North Vietnam)[END_REF][START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF][START_REF] Rossignol | The volcaniclastic series from the Luang Prabang Basin, Laos: A witness of a triassic magmatic arc ?[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF], making it difficult to make correlations and comparisons with other platforms of similar age worldwide and in Southeast Asia.

The Early and Middle Triassic carbonate platforms of Northern Vietnam developed in foreland basins associated with the onset and growing of the Indosinian orogen from the Early to the Late Triassic [START_REF] Lepvrier | Indosinian tectonics in Vietnam[END_REF][START_REF] Vuong | U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper Paleozoic paleotethyan lithospheric remnants[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Faure | Triassic tectonics of the southern margin of the South China Block[END_REF][START_REF] Roger | Emplacement and cooling of the Dien Bien Phu granitic complex: Implications for the tectonic evolution of the Dien Bien Phu Fault (Truong Son Belt, NW Vietnam)[END_REF][START_REF] Halpin | U-Pb zircon geochronology and geochemistry from NE Vietnam: A "tectonically disputed" territory between the Indochina and South China blocks[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF]. The studied areas concern the Song Da Basin, located to the southwest of the Indosinian orogen, and the Sam Nua Basin, bordering the orogen to the northeast. These basins are separated by the Song Ma Ophiolitic Suture Zone, which is located along the Song Ma Fault (Fig. 1). This suture zone corresponds to the boundary between two continental blocks [START_REF] Ridd | South-East Asia as a part of Gondwanaland[END_REF][START_REF] Metcalfe | Origin and assembly of south-east Asian continental terranes[END_REF][START_REF] Metcalfe | Tectonic framework and Phanerozoic evolution of Sundaland[END_REF][START_REF] Lepvrier | Indosinian tectonics in Vietnam[END_REF][START_REF] Vuong | U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper Paleozoic paleotethyan lithospheric remnants[END_REF]Faure et al., 2014, 2016, andreferences therein): the South China Block (SCB) to the north and the Indochina Block (IB) to the south (Fig. 1). The collision between these two blocks probably occurred during the Triassic [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF][START_REF] Lepvrier | Indosinian tectonics in Vietnam[END_REF][START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Zhang | Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone between the South China and Indochina Blocks[END_REF][START_REF] Kamvong | Adakites in the Truong Son and Loei fold belts, Thailand and Laos: Genesis and implications for geodynamics and metallogeny[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Roger | Emplacement and cooling of the Dien Bien Phu granitic complex: Implications for the tectonic evolution of the Dien Bien Phu Fault (Truong Son Belt, NW Vietnam)[END_REF][START_REF] Halpin | U-Pb zircon geochronology and geochemistry from NE Vietnam: A "tectonically disputed" territory between the Indochina and South China blocks[END_REF][START_REF] Hieu | Stages of late Paleozoic to early Mesozoic magmatism in the Song Ma belt, NW Vietnam: evidence from zircon U-Pb geochronology and Hf isotope composition[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF][START_REF] Thanh | Late Permian-Triassic granitic rocks of Vietnam: the Muong Lat example[END_REF], even though various other ages such as Silurian-Devonian [START_REF] Thanh | Fish suggests continental connections between the Indochina and South China blocks in Middle Devonian time[END_REF][START_REF] Findlay | The Structural Setting of the Song Ma Region, Vietnam and the Indochina-South China Plate Boundary Problem[END_REF][START_REF] Thanh | Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block[END_REF] or Carboniferous [START_REF] Metcalfe | Tectonic framework and Phanerozoic evolution of Sundaland[END_REF][START_REF] Metcalfe | Changhsingian (Late Permian) conodonts from Son La, northwest Vietnam and their stratigraphic and tectonic implications[END_REF]) have been proposed.

A general stratigraphic framework for the Song Da and Sam Nua basins has been presented by [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF], in which the carbonate deposits of these two basins were succinctly described. The present study, based on 16 cross-sections located in the eastern part of the Song Da and Sam Nua basins, focuses on the structural, stratigraphic, and sedimentological characteristics of these marine Triassic formations, with a particular emphasis on the carbonate platform deposits. We then compare the main characteristics of the Northern Vietnam platforms with other Triassic formations located in eastern Indonesia (Banda area; [START_REF] Villeneuve | Upper Triassic shallow water limestones in the Sinta Ridge (Banda Sea, Indonesia)[END_REF][START_REF] Villeneuve | Upper Triassic shallow water limestones in the Sinta Ridge (Banda Sea, Indonesia)[END_REF]Martini et al., 1997[START_REF] Martini | Upper Triassic carbonate deposits of Seram (Indonesia): palaegeographic and geodynamic implications[END_REF].

Geological setting

The Indochina Peninsula is comprised of several continental blocks and volcanic arcs, separated by faults or sutures (Fig. 1). These continental blocks and volcanic arcs amalgamated together during various tectonic events that occurred from the Paleozoic to the Cenozoic [START_REF] Ridd | South-East Asia as a part of Gondwanaland[END_REF][START_REF] Burrett | Plate tectonics and the fusion of Asia[END_REF][START_REF] Metcalfe | Origin and assembly of south-east Asian continental terranes[END_REF][START_REF] Metcalfe | Tectonic framework and Phanerozoic evolution of Sundaland[END_REF][START_REF] Metcalfe | Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys[END_REF][START_REF] Carter | Understanding Mesozoic accretion in Southeast Asia: Significance of Triassic thermotectonism (Indosinian orogeny) in Vietnam[END_REF][START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF][START_REF] Carter | Was the Indosinian orogeny a Triassic mountain building or a thermotectonic reactivation event[END_REF][START_REF] Ferrari | An alternative plate tectonic model for the Palaeozoic-Early Mesozoic Palaeotethyan evolution of Southeast Asia (Northern Thailand-Burma)[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Faure | Triassic tectonics of the southern margin of the South China Block[END_REF][START_REF] Burrett | The configuration of Greater Gondwana-Evidence from LA ICPMS, U-Pb geochronology of detrital zircons from the Palaeozoic and Mesozoic of[END_REF][START_REF] Halpin | U-Pb zircon geochronology and geochemistry from NE Vietnam: A "tectonically disputed" territory between the Indochina and South China blocks[END_REF].

In Northern Vietnam (Fig. 1), Cenozoic tectonic events are linked to the collision between the Indian subcontinent and the Asian plate and are marked by the Red River Fault (RRF) activity [START_REF] Leloup | The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina[END_REF]. The Song Ma Suture, separating the Song Da Basin (southern part of the SCB) from the Truong Son Belt (northern part of the IB), is related to the Middle-Late Triassic Indosinian tectonic event [START_REF] Lepvrier | Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) 40 Ar-39 Ar Triassic ages and Cretaceous to Cenozoic overprints[END_REF].

Many geodynamic models have been proposed to account for the development of the Indosinian orogeny [START_REF] Fromaget | Note préliminaire sur la stratigraphie des formations secondaires et sur l'âge des mouvements majeurs en Indochine[END_REF][START_REF] Fromaget | Le Trias dans la partie Nord-Ouest du Synclinal de Sam Neua (Tonkin et Laos)[END_REF][START_REF] Ridd | South-East Asia as a part of Gondwanaland[END_REF][START_REF] Burrett | Plate tectonics and the fusion of Asia[END_REF][START_REF] Metcalfe | Origin and assembly of south-east Asian continental terranes[END_REF][START_REF] Metcalfe | Tectonic framework and Phanerozoic evolution of Sundaland[END_REF][START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF][START_REF] Ferrari | An alternative plate tectonic model for the Palaeozoic-Early Mesozoic Palaeotethyan evolution of Southeast Asia (Northern Thailand-Burma)[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Faure | Triassic tectonics of the southern margin of the South China Block[END_REF][START_REF] Burrett | The configuration of Greater Gondwana-Evidence from LA ICPMS, U-Pb geochronology of detrital zircons from the Palaeozoic and Mesozoic of[END_REF][START_REF] Halpin | U-Pb zircon geochronology and geochemistry from NE Vietnam: A "tectonically disputed" territory between the Indochina and South China blocks[END_REF]and references therein). In particular, many studies have focused on the Song Ma Suture [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF][START_REF] Lepvrier | Indosinian tectonics in Vietnam[END_REF][START_REF] Nakano | Triassic eclogite from northern Vietnam: inferences and geological significance[END_REF][START_REF] Lepvrier | Northdirected Triassic nappes in Northeastern Vietnam (East Bac Bo)[END_REF][START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Vuong | U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper Paleozoic paleotethyan lithospheric remnants[END_REF][START_REF] Roger | Emplacement and cooling of the Dien Bien Phu granitic complex: Implications for the tectonic evolution of the Dien Bien Phu Fault (Truong Son Belt, NW Vietnam)[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Faure | Triassic tectonics of the southern margin of the South China Block[END_REF][START_REF] Lai | The Western Ailaoshan Volcanic Belts and their SE Asia connection: A new tectonic model for the Eastern Indochina Block[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF][START_REF] Yan | U-Pb isotope geochronology and geochemistry of granites from Hainan Island (northern South China Sea margin): Constraints on late Paleozoic-Mesozoic tectonic evolution[END_REF]. Taking the stratigraphic evolution of the sedimentary basins flanking the Indosinian orogen into account, [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF] proposed a classical collisional system between the SCB and IB following the southward subduction, beneath the IB, of an oceanic crust from the Permian to the Middle Triassic period. Associated with this subduction, a volcanic arc emplaced onto the northern margin of the IB. Associated with this magmatic arc, the Sam Nua Basin developed as a retroarc foreland basin. The Song Da Basin, located in the southern part of the SCB, remained in a passive margin position until the beginning of the collision between the SCB and IB, when it was incorporated in a foreland system (as defined by [START_REF] Catuneanu | Retroarc foreland systems-evolution through time[END_REF].

Northern Vietnam (Fig. 2A) is comprised of three main domains (Tran Van Tri, 1973).

A northern domain is delimited to the south by the RRF and mainly comprises Cambrian to Silurian deposits along with subordinate Permian and Triassic formations (Fig. 2A). A central domain is delimited to the north by the RRF and to the south by the Song Ma Suture (Fig. 2A). The Song Da Basin is located in this central domain, which is mainly characterized by NW-SE structures. To the south of the Song Da Basin, the Song Ma Suture consists of an ophiolitic mélange comprising lenses of serpentinized peridotite, gabbro, plagiogranite, diabase, and basalt enclosed within metasedimentary rocks [START_REF] Hutchison | Ophiolite in Southeast Asia[END_REF]Trung et al., 2006;[START_REF] Thanh | Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block[END_REF][START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Vuong | U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper Paleozoic paleotethyan lithospheric remnants[END_REF][START_REF] Zhang | U-Pb dating and tectonic implication of ophiolite and metabasite from the Song Ma suture zone, northern Vietnam[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF]. Geochemical analyses indicate normal to enriched mid-ocean-ridge basalt affinities (Trung et al., 2006;[START_REF] Zhang | Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone between the South China and Indochina Blocks[END_REF][START_REF] Zhang | U-Pb dating and tectonic implication of ophiolite and metabasite from the Song Ma suture zone, northern Vietnam[END_REF] or supra-subduction zone ophiolite affinities [START_REF] Thanh | Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block[END_REF] for the mafic rocks. The Song Ma Suture underwent eclogite and highpressure granulite facies metamorphism [START_REF] Nakano | Discovery of high pressure granulite facies metamorphism in northern Vietnam: Constraints on the Permo-Triassic Indochinese continental collision tectonics[END_REF][START_REF] Nakano | Triassic eclogite from northern Vietnam: inferences and geological significance[END_REF][START_REF] Zhang | Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone between the South China and Indochina Blocks[END_REF] during the Middle to the Late Triassic.

A southern domain located to the south of the Song Ma Suture hosts the Sam Nua Basin. This southern domain is marked by NW-SE structures to the west and SW-NE structures to the east. The northwestern part of the Sam Nua Basin contains various calkalkaline magmatic rocks, which are either interbedded (volcanic) or intrusive (plutonic) into the Triassic sedimentary successions [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF][START_REF] Hoa | Permo-Triassic magmatism and metallogeny of Northern Vietnam in relation to the Emeishan plume[END_REF][START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF]. These calk-alkaline magmatic rocks are interpreted as being emplaced in an arc setting [START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF][START_REF] Hieu | Stages of late Paleozoic to early Mesozoic magmatism in the Song Ma belt, NW Vietnam: evidence from zircon U-Pb geochronology and Hf isotope composition[END_REF].

Several unconformities corresponding to different tectonic and/or orogenic events have been recognized in the Cambrian to the Neogene lithostratigraphic successions of Northern Vietnam (Fig. 2B). In particular, a major unconformity, referred to as the Indosinian unconformity, occurred during the Middle to Late Triassic [START_REF] Deprat | Les zones plissées intermédiaires entre le Yunnan et le Haut-Tonkin[END_REF][START_REF] Fromaget | Note préliminaire sur la stratigraphie des formations secondaires et sur l'âge des mouvements majeurs en Indochine[END_REF][START_REF] Fromaget | Le Trias dans la partie Nord-Ouest du Synclinal de Sam Neua (Tonkin et Laos)[END_REF][START_REF] Lacassin | Unconformity of red sandstones in north Vietnam: field evidence for Indosinian orogeny in northern Indochina?[END_REF][START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF].

The stratigraphic succession of the Triassic deposits of the Song Da and Sam Nua basins has been established by Vu [START_REF] Khuc | Paleontological Atlas of Vietnam, Mollusca 3[END_REF], who differentiated three formations with respect to their macrofossil content such as mollusks, brachiopods, and ammonites. The Induan/Olenekian Tan Lac Formation is comprised of mainly shales, sandstones, and marls, and a minor amount of carbonates and volcaniclastic materials [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF]. Various fossils, including bivalve, ammonoid, gastropod, and echinoderm remains indicate deposition during the Early Triassic in a marine environment. Above the Tan Lac Formation, the Anisian Dong Giao Formation is made up of limestones containing brachiopods, bivalves, ammonoids [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF], and foraminifers [START_REF] Martini | Occurrence of Triassic foraminifers in carbonates deposits from the Ninh Binh Area (North Vietnam)[END_REF]. Above the Dong Giao Formation, the Song Boi Formation, which is similar to the Napeng Formation of Myanmar [START_REF] Brunnschweiler | Contributions to the Post-Silurian geology of Burma (Northern Shan States and Karen State)[END_REF], comprises a basal conglomerate followed by sandstones, siltstones, and shales [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF]. Bivalve [START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF] and ammonoid fossils [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF] imply a marine depositional environment. Based on the fossil content, a Ladinian to Carnian age is attributed to the Song Boi Formation [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF]. However, the ammonoid assemblage described in the upper part of this formation [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF] is probably of lower Norian age, instead of Ladinian-Carnian [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF].

The current study aims to provide precise data on the stratigraphy, sedimentology, and structural arrangement of the Triassic formations of these eastern parts of Vietnam. For this purpose, we performed detailed investigations in two areas of the Song Da Basin, between the cities of Hoa Binh and Ninh Binh (Fig. 3A) and around the city of Ninh Binh (Fig. 3B). We also conducted similar investigations in the region of Vinh (Fig. 3C), located in the Sam Nua Basin.

Methodology

Structural investigations

Various localities in the Song Da and Sam Nua basins have been examined for structural analyses. Sixteen cross-sections (designated by Roman numbers in Fig. 3 and subsequent figures) were investigated: 12 are in the Song Da Basin (Fig. 3A and3B) and four are in the Sam Nua Basin (Fig. 3C). The cross-sections were oriented perpendicularly to the main structures. In the Song Da Basin, we conducted fieldwork along the road from Hoa Binh to Ninh Binh and in the Ninh Binh massif, also referred to as the "Dry Halong". In the Sam Nua Basin, most of the investigations were realized along the coastal roads between Thanh Hoa (to the north) and Vinh (to the south). In each basin, the different stratigraphic units identified (hereafter referred to as "members" and designated by lowercase letters and black circled numbers in Fig. 4 and subsequent figures) are correlated using intersecting cross-sections (e.g., sections VI and IX; Fig. 3B). When this was not possible (e.g., sections XV and XVI; Fig. 3A), the correlations between the different members were performed using the biostratigraphy (foraminifers) from the samples collected along these sections. The red-circled numbers in Fig. 4A and subsequent figures designate these samples.

Petrography and biostratigraphy

Limestone samples were collected from the best-preserved portions of the outcrops, avoiding recrystallized and fractured lithologies as much as possible. A total of 163 samples were collected: 131 from the Song Da Basin (Fig. 3A) and 32 from the Sam Nua Basin (Fig. 3C).

The microfacies and biostratigraphic analyses are based on the observation of more than 39 thin sections (2.3 × 4 cm) under transmitted light (Zeiss Axioskop microscope). Additionally, eight samples were imaged by cathodoluminescence (CL) microscopy to reveal both specific sedimentological features and microfossils not visible in transmitted light. A Cl8200 MK5optical CL with a cold cathode was used with beam conditions of 15 kV at 50-60 mA and an unfocused beam of approximately 1 cm. The observation chamber had a residual pressure of 80 mTorr. The samples were not coated. The parameters were kept fixed to avoid additional treatment. No filter or standard was used for image calibration. Microfacies were determined according to the sedimentary components and textures using the classification by [START_REF] Dunham | Classification of carbonate rocks according to depositional textures[END_REF], supplemented by that of [START_REF] Embry | A Late Devonian reef tract on northeastern Banks Island, NWT[END_REF]. A total of 19 samples were examined for their microfossil contents (mainly foraminifers). Only two were devoid of microfossils and 17 could be used for biostratigraphic determinations, among which nine were from the Song Da Basin and eight were from the Sam Nua Basin.

The Song Da Basin

The Song Da Basin, as defined by [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF], extends from Ninh Binh (SE) to the Dien Bien Phu Fault (NW) (Fig. 2). It is bounded to the south by the Song Da Fault and to the north by the RRF and comprises a heterogeneous basement made up of metamorphic and sedimentary rocks of various ages (Than and Khuc, 2012, and references therein).

Cross-section analyses and lithostratigraphy of the units

The investigated sections are located along the road from Hoa Binh to Ninh Binh (Fig. 3A) and in the Ninh Binh massif (Fig. 3B). Twelve cross-sections have been logged in this part of the Song Da Basin.

The S-N oriented cross-section A-B (Fig. 4, combined sections II and V in Fig. 3A) is located between the Ha Binh (or Ha Trunc) village to the south and the Phu Ly village to the north. The Ordovician-Silurian basement is exposed to the south, around the Ha Binh village.

The youngest deposits, corresponding to the upper levels of the upper limestone members (members 24 and 25), are exposed to the north around the Phu Ly village. Several structures have successively been investigated, namely, from south to north: the Bim Son and Dong Giao synclines, the Tam Diep sheets, the "Dry Halong" massif (Ninh Xuan massif), exhibiting a suite of gentle synclines and anticlines, and the Cu Gian Khuat structure with a reverse fold (V25). The southern Ordovician-Silurian massif consists of a succession of anticlines and synclines showing a strong deformation. The main shrinking process is absorbed by the numerous thrusts that occur mainly out of the main carbonate massifs, allowing to reconstruct a detailed lithostratigraphic succession. However, owing to the numerous thrusts and the scarcity of exposures, the complete succession requires some correlations between several cross-sections.

The synthetic stratigraphic succession contains five units (I to V) and 24 members.

Unit I corresponds to the Ordovician-Silurian basement exposed around the Ha Binh village.

Unit II is mainly exposed along cross-section VI (Fig. 5A), where seven members (denoted b to h in Figs. 5B, 5C, and 5D) have been distinguished. The first member of this unit (member a) is not exposed in this section but in section XV.

Unit III is well exposed along cross-section XV (Man Duc village, Fig. 6A) to the south of the Muong Khen crossroad (Fig. 3B). Three members (denoted a, d, and e in Fig. 6B) have been distinguished in Unit III.

Units IV and V are exposed along cross-section X (Hao Lu, Fig. 7A). Unit IV comprises only two members (denoted e and f in Fig. 7B) and Unit V is made up of two other members (denoted a and b in Fig. 7B).

The 12 lithostratigraphic logs (I to XVI), derived from the logging of the 12 sections, are shown in Fig. 8. To facilitate the correlations between the sedimentary sections, we attributed the same number (from 1 to 24) to each member of each unit. The thickness of each unit can be estimated despite the local tectonic effects, except for Unit I. Units II, III, IV and V are more than 200 m, 300 m, 300 m, and 350-400 m thick, respectively. One of the main goals of our work is to provide similar marker levels for every section across the studied area.

Biostratigraphy

In the investigated area, only two radiometric studies have been performed by U-Pb dating on zircon grains. The first one was conducted on a granitic pluton that crosscuts the limestones of the Dong Giao Formation (Mong et al., 2004), which yielded an age of 242.4 ± 2.2 Ma [START_REF] Hoa | Permo-Triassic magmatism and metallogeny of Northern Vietnam in relation to the Emeishan plume[END_REF]. The second radiometric study was conducted on sandstones from the lower Tan Lac Formation of the Muong Khem area (Unit II, section VI, Fig. 6) by [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF], which gave a maximum depositional age of 243.1 ± 2.3 Ma. Thus, the Tan Lac Formation appears to be deposited after the youngest zircon age (243.1 ± 2.3 Ma), indicating a Middle Triassic deposition. However, our biostratigraphic investigations based on benthic foraminifers gave an Olenekian/Induan age for samples V10, V140, and V143 (members 16 and 17); an Anisian age for sample V140 (member 17); a late Anisian age for samples V144 (member 18), V93 (member 19), and V12, V95, and V98 (member 20); and an Anisian/Ladinian age for sample V78 (member 21). Members 19, 23, and 24 did not yield any suitable microfossils for biostratigraphic dating. A list of foraminifers related to these samples is presented in Annex I. Consequently, a synthetic chronostratigraphic log can be proposed by merging lithostratigraphic and biostratigraphic data (Fig. 9). Units II (members 3 to 10) and III (members 11 to 15) are considered as Early Triassic; Unit IV (members 16 to 21) and the base of Unit V (member 22) are ascribed to the Middle Triassic (Anisian/Ladinian). The upper part of Unit V was deposited between the Ladinian and the Late Triassic.

Petrology, diagenesis, and sedimentology

Unit I. Because of the small number of outcrops, only two facies have been recognized in the Paleozoic basement: (1) tuffs, shales, sandstones and strongly deformed quartzites (Fig. 10A) and ( 2) thick beds of gray limestones.

Unit II. From the base to the top, member 3 yielded samples V8b and V136, which correspond to alternations of red shales and sandstones; members 4 and 5 are composed of gray limestones with bioturbations and a lumachel containing gastropods (V92); member 9 corresponds to an alternation of marls and nodular limestones (V90), and member 10 consists of limestones (V91) (Fig. 10B).

Unit III. This unit comprises shales interbedded with limestones, limestone beds increase upward. From the base to the top, the unit is made up of member 11 with conglomerates and massive, black, laminated limestones (V9 and V137), member 12 with micritic black limestones (V131), and member 13 with gray pink bioclastic limestones (V132). Members 14 and 15 are made up of black limestones with bioclasts. Thin beds of bioturbated shales containing a lumachel of gastropods are associated with a succession of gray marls and nodular limestones.

Unit IV. The base of this unit consists of member 16, which is composed of black laminated limestones (V10 and V141) (see also [START_REF] Martini | Occurrence of Triassic foraminifers in carbonates deposits from the Ninh Binh Area (North Vietnam)[END_REF] showing microbialitic structures.

Above is member 18, which is made up of black limestones with remnants of Dasycladacean algae (V144). Member 19 is composed of dark gray micritic limestones with recrystallized microfossils (V93), and member 20 consists of homogeneous micritic limestones with foraminifers (V12, V94, V95, V96, and V97). The top of Unit IV consists of member 21, which is made up of recrystallized limestones with foraminifers (V78). Unit V. Two members compose this unit: at the base, member 22 is made up of bioclastic limestones (V80). Above, member 23 is made up of black micritic limestones (V82) (Fig. 10C).

In summary, the lithological characteristics and the microfacies occurring in units II to V indicate that the limestones deposited on a wide, shallow water carbonate shelf, under low energy conditions. For Unit II, the terrigenous input, together with bioturbations and gastropods, could indicate a protected coastline. The algal laminations and conglomerates in the basal part of Unit III indicate temporary and local intertidal conditions. Accordingly, we propose a deposition on a low-angle carbonate ramp located in a marginal basin. The thickness of the deposits (hundreds of meters), together with constant shallow water settings along the entire succession, suggests that the basin underwent a rapid subsidence. Moreover, the Anisian carbonate ramp is younger in age from south to north.

Tectonics

The tectonic styles are closely related to the lithology of the units. Unit I (V13 and Vm1)

shows very strong deformations in shales and tuffs (Fig. 10A). The fold axes are oriented E-W (N90°) and axial foliation is nearly vertical. This is likely related to Triassic structural styles because they are not found in the overlying sedimentary deposits.

Units II and III show beds with gentle dips to the south (N120°/SW50°), whereas station V20 displays a thrust to the north, which seems to be a characteristic of this unit. In Unit IV (V76 to V78, cross-section X, Fig. 7), the beds gently dip to the north. Cross-section A-B (Fig. 4) exhibits large open folds in the carbonate platform. However, locality V23A (Fig. 10D) displays internal thrusting from east to west. These thrusts are associated with north-south axial folds similar to the Dinh anticline (cross-section VI, Fig. 4). Unit V (crosssection X, Fig. 7) shows gentle dips to the north. However, the thrust here is from north to south. To summarize, two types of structures are observed: large NW-SE folds, following the main trend of the belt, and secondary N-S folds, associated with northward thrusting and indicating an E-W shrinking. The main movements are toward the north. Nevertheless, southward, reverse faults in the limestone units are also present and could indicate a "jurassian-like" folding style associated with a "décollement" process at the shale/carbonate contact.

Summary

In their last paper devoted to this basin, [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF] took the entire basin into consideration and delineated five formations with, from the base to the top: the Co Noi (equivalent to the Tan Lac Formation) (Induan/Olenekian), the Dong Giao (Anisian), the Nam Than (early Ladinian), the Song Boi (early Norian), and the Suoi Bang (late Norian/Rhaetian).

The Dong Giao Formation mainly consists of limestones. Other formations are essentially made of siliciclastic and argillaceous members. In the eastern area of central-northern Vietnam, only the upper parts of the Co Noi Formation (or Tan Lac Formation) and the Dong Giao Formation have been distinguished based on the stratigraphic framework provided by Rossignol et al. (2018) (after Tri and[START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF] and [START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF]). Figure 10 from [START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF] presents a simplified succession of the Dong Giao Formation, which corresponds to our Unit IV and members 16 to 21.

The depositional environments of Units II to IV are indicative of a marine passive margin setting with regular subsidence. However, a moderate tectonic impact and a weak metamorphism indicate that this passive margin subsequently underwent "jurassian-like" deformations, similar to those described in the Jurassian Belt in front of the Alpine Belt, which is not consistent with a collisional environment. The main tectonic shrinking was accommodated by thrusts that are well developed in units I and III but moderately developed in units IV and V.

The Sam Nua Basin

The Sam Nua Basin is an elongated NW-SE trending basin located south of the Song Da Basin. It is delimited to the south by the Song Ca Fault, to the north by the Song Ma Fault, and to the west by the Dien Bien Phu Fault (Fig. 2). The basement of the basin comprises Neoproterozoic to late Cambrian metasedimentary rocks with high-grade metamorphism and late Silurian to Devonian metasedimentary rocks with low-grade metamorphism and strong deformations [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF]. The Nam Co area, between the Song Da and the Sam Nua basins, comprises eclogites that underwent a high granulitic metamorphism [START_REF] Nakano | Discovery of high pressure granulite facies metamorphism in northern Vietnam: Constraints on the Permo-Triassic Indochinese continental collision tectonics[END_REF][START_REF] Nakano | Triassic eclogite from northern Vietnam: inferences and geological significance[END_REF]. These units represent the internal zone of an orogen ascribed to the collision between the IB and SCB [START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF]. The Sam Nua Basin sediments include calkalkaline magmatic rocks interpreted as being emplaced in an arc setting [START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF].

Cross-section analyses and lithostratigraphy of the units

Four main cross-sections oriented perpendicularly to the main structures have been studied in the eastern part of the Sam Nua Basin (Fig. 3C).

Cross-section XI (Fig. 11). This SSW-NNE section is located between Phung Luat to the south and Ho Truong to the north. It intersects several limestone structures: the Yen Li monocline, the Cau Gia (Quynh Lu and Dien Bien samples) anticline, the My Hung syncline, the Nui Thong Lim anticline, and the Hoang Mai and Nui Bom synclines. The local succession is composed of three units: Unit II with one member (denoted a in Fig. 11B), Unit IV with five members (denoted a and c to f in Fig. 11B), and Unit V with two members (denoted a and b in Fig. 11B). The thickness scales for the units are given in Fig. 11.

Cross-section XII (Fig. 12). This NW-SE cross-section is located between the Nghia Dan and Quynh Long villages. From west to east, three main structures are present: the Ru chop Binh monocline with an eastward dip, the Dien Binh syncline, and the Quynh Lu anticline. Two main lithologies have been recognized: green claystones and gray limestones. The lithological succession exposed in this area contains three units. From the base to the top, these units are:

Unit II with two members (denoted a and c in Fig. 12B), Unit III with one member (denoted a in Fig. 12B), and Unit IV with two members (denoted a and c in Fig. 12B).

Cross-section XIII (Fig. 13). This WSW-ENE section, extends from the village of Do Luong to the village of Dien Chau. From the west to the east, the section crosses the Hao Son syncline, the Vinh Thanh monocline with a dip toward the east, and a metamorphic basement close to the Dien Chau crossroad. The change of the main structural directions, from NE-SW to NW-SE, occurs between the localities of Vinh Thanh and Hao Son. The local lithological succession comprises a metamorphic basement ascribed to Unit I and limestones interbedded with marls ascribed to Unit III, which is made up of five members (denoted a to f in Fig. 13B).

Cross-section XIV. This NW-SE cross-section is following the road joining the villages of Do Luong, Nam Dan and Vinh. This section exhibits metamorphosed rocks, with a general dip to the west and a vertical foliation. No fossils have been found and no radiometric data are available to date these supposed Triassic formations (Tran Van Tri, 1973). We assigned these formations to Unit I, although we recognize that a metamorphic marker cannot be used for any stratigraphic correlations.

Seventeen members can be defined in the Sam Nua Basin based on correlations and comparisons between the cross-sections and their lithological successions. With regards to the Song Da Basin, the same number has been attributed to the members of each cross-section (Fig. 14).

The total thickness of the Sam Nua Basin succession cannot be precisely evaluated due to the lack of sequences related to tectonic events. However, the estimated thickness of Unit II is more than 300 m, whereas units III, IV and V are approximately 400 m, 300 m, and 300 m thick, respectively.

Biostratigraphy

As far as we are aware, no radiometric data are available in the eastern part of the Sam Nua Basin, except for a dated rhyolite exposed in the southern part of the basin, which gives an age of 251.9 ± 1.7 Ma [START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF]. Thus, the chronostratigraphic attribution is based on macrofossils and foraminifers. The foraminifer association assigns sample V101 (Vinh Thanh locality, cross-section XIII, Unit III, member 10) to the Olenekian/Anisian. Samples V104 to V106 (Quinh Lu locality, cross-section XII, Unit IV, member 12), V122 (Hoang Mai locality, cross-section XI, Unit IV, member 13), and V126 (Ho Thuong locality, cross-section XI, Unit limestone. Member 13 (V122) is a gray micritic limestone. Members 14 (V126) and 15 (V127) are made up of gray bioclastic limestones. Unit V. Member 16 (V120 and V121) is comprised of bioconstructed algal limestones (Fig. 15C) and member 17 (V127 and V128) is made up of bioclastic limestones.

The CL analyses performed on the samples from the two basins reveal close similarities between the diagenetic evolution of the limestones from the Song Da and Sam Nua basins. Two main comparable diagenetic phases have been recognized in the Sam Nua Basin (e.g., Unit III, member 10, V102) and the Song Da Basin (e.g., Unit IV, member 21, sample V95). The first diagenetic phase affected the sediments before their complete lithification. This early diagenetic synsedimentary event is linked to the circulation of an autochthonous interstitial fluid in a geochemically closed system. The permeability was significantly reduced during this phase, preventing further fluid circulation and late cement precipitation. A second diagenetic phase, corresponding to a late diagenetic event, is only recognizable in the fractures related to late tectonic activity and/or in the rare dissolution cavities (i.e., dissolved organisms).

Tectonics

The main structures evidenced in this part of the basin exhibit NW-SE or N-S directions. The tectonic style is very similar to the one exposed in the Song Da Basin with a succession of anticlines, monoclines, and synclines. In this part of the Sam Nua Basin, the dips of the beds are larger (50 to 60°) than those identified in the Song Da Basin. Remnants of NE-SW structures occur in the main NW-SE bands, as observed in the V127 outcrop, in the Hoang Mai and Ho Thuong localities (corresponding to samples V121 and V126). Moreover, we observed remnants of N-S foliation in the Hung Nguyen quarry close to the city of Vinh, and a secondary foliation oriented NW-SE cross cutting NE-SW structures of Vinh Than (crosssection XII). This suggests that the NE-SW structures are older than the NW-SE ones.

Outcrop V109 (cross-sections XI and XII) displays a good example of these NE-SW folds (Fig. 15D).

Summary

Our investigations show that the Triassic formations of the Sam Nua Basin present a basal, mainly siliciclastic unit, as well as two upper units that are mainly calcareous. As in the Song Da Basin, five units, comprising 17 members, have been defined. The carbonate deposits in the Sam Nua Basin are younger from south to north (Fig. 14), which could indicate a transgressive pattern with a retrogradation from south to north. The southern part is more metamorphosed than the northern one. The deformations indicate that the NE-SW structures are possibly older than the NW-SE ones, at least in the southern part of the study area.

Correlations with the central and western part of this basin [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF] allow us to attribute units II and III to the Dong Trau Formation. However, it is important to note that Unit III is more calcareous than the other deposits of the Dong Trau Formation described elsewhere (e.g., [START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF][START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF]. Unit IV likely corresponds to the upper part of the Dong Trau Formation and/or to the Hoang Mai Formation [START_REF] Son | Geology and mineral resources of Muong Kha -Son La sheet (F-48-XXV and F-48-XXVI), department of geology and minerals of Vietnam[END_REF]. Unit V could be younger than the Dong Trau Formation. The counterpart of the Suoi Bang Formation (late Norian/Rhaetian) has not been documented in this eastern part of the basin.

Discussion

There are very few differences between the stratigraphic successions of the opposite margins of the Sam Nua and the Song Da basins. Both comprise Anisian to Carnian carbonate platforms with black micritic limestones at the base and light gray, mainly bioclastic limestones toward the top. However, we note that black micritic members can occur elsewhere at the top of the Song Da Basin successions. Both carbonate platforms exhibit similar sedimentary characteristics and biostratigraphic contents, including foraminifers, oncoids, red nodules, diverse bioclasts, algal laminations, and bioturbations. The bioclastic facies are interbedded with apparently azoic micritic members. Both sedimentary successions have similar large thicknesses: 1200 m for the Song Da Basin and ca. 1300 m for the Sam Nua Basin. Finally, both reflect rather similar depositional environments corresponding to subtidal to supratidal conditions in an inner platform. The thickness of the deposits and their overall similar depositional environments suggests that both platforms underwent steady subsidence.

Even if we cannot demonstrate that the Sam Nua Basin belongs to an active margin, with remnants of a calk-alkaline volcanic arc as assumed by several authors [START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF], we note that the carbonates in the Anisian to Carnian Sam Nua margin retrograded toward the north, similar to the Anisian/Carnian carbonate platform of the Song Da Basin (Rossignol et al., 2018 and Figs. 18 and 19 within). This favors the hypothesis of a convergence and suturing of the two margins.

The Sam Nua margin uplifted northward while the Song Da margin was drowned northward (uplifted from south to north). This hypothesis has already been put forward for the time equivalent Nanpanjiang carbonate platform (Lehrmann et al., 2005a), which Lehrmann et al. (2005b) have suggested is being progressively drowned northward under the influence of the growing Indosinian orogen (Lehrmann et al., 2005b). Even if our study does not allow to clearly identify an accretionary prism or a volcanic arc (despite the occurrence of some volcanic remnants in Unit II in the Sam Nua Basin), the timing, depositional environment, and subsidence patterns evidenced in the studied areas are consistent with a southward subduction of the Song Ma sea floor followed by a collision between the IB and SCB [START_REF] Lepvrier | Northdirected Triassic nappes in Northeastern Vietnam (East Bac Bo)[END_REF][START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Vuong | U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper Paleozoic paleotethyan lithospheric remnants[END_REF][START_REF] Roger | Emplacement and cooling of the Dien Bien Phu granitic complex: Implications for the tectonic evolution of the Dien Bien Phu Fault (Truong Son Belt, NW Vietnam)[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Faure | Triassic tectonics of the southern margin of the South China Block[END_REF]Lai et al., 2014;Lehrmann et al., 2015b;[START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF]. This southward subduction is consistent with the location of a continental arc, marked by the occurrence of calk-alkaline rocks [START_REF] Liu | Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications[END_REF][START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF][START_REF] Shi | Zircon U/Pb ages and geochemistry of granitoids in the Truong Son Terrane, Vietnam. Tectonic and metallogenic implications[END_REF][START_REF] Hieu | Stages of late Paleozoic to early Mesozoic magmatism in the Song Ma belt, NW Vietnam: evidence from zircon U-Pb geochronology and Hf isotope composition[END_REF] onto the northern IB. In addition, no significant unconformity has been identified between units II and III (Fig. 10B), supporting the hypothesis that a major tectonic event did not occur before the deposition of Unit III.

Another aim of this study is to improve our knowledge about the Triassic carbonate platforms of Northern Vietnam in order to compare them with other Triassic carbonate platforms in Southeast Asia and Indonesia. Indonesian Triassic carbonate platforms have been reported in eastern Sulawesi (Martini et al., 1997), Sinta Ridge in the Banda Sea [START_REF] Villeneuve | Upper Triassic shallow water limestones in the Sinta Ridge (Banda Sea, Indonesia)[END_REF], as well as the islands of Seram [START_REF] Martini | Upper Triassic carbonate deposits of Seram (Indonesia): palaegeographic and geodynamic implications[END_REF]

Conclusions

This study reports new structural, sedimentological, and biostratigraphic (foraminifers) data for the Triassic formations of the eastern parts of the Song Da and Sam Nua basins based on an examination of four and 12 cross-sections in the Sam Nua Basin and Song Da Basin, respectively. The main results are as follows:

1) Two new detailed synthetic stratigraphic successions of the Triassic formations are provided. These stratigraphic successions have been established through consistent correlations between the different cross-sections. Five units and 23 members have been characterized for the eastern part of the Song Da Basin, and five units and 17 members have been identified in the Sam Nua Basin.

2) New biostratigraphic investigations based on benthic foraminifers improved the chrono-stratigraphic constraints for both basins, allowing us to compare the evolution of the carbonate platforms in the Song Da and Sam Nua basins from the Ladinian to the Rhaetian.

3) Sedimentological analyses provide relevant information on the depositional environments of these formations, reflecting two distinct margins that successively underwent low and high energy conditions. 4) Structural investigations point to differences in the relationships between the NW-SE and NE-SW structures.

5)

Tectonic studies argue for a northward deformation vergence, favoring a northward obduction of the Song Ma rift and thus a northward collision between the IB and SCB.

However, the evolution of the carbonate platforms alone cannot be used to support any of the geodynamic models although a northward migration of the deposits is not inconsistent with a northward collision.

6) Comparison of the Northern Vietnamese Triassic carbonate platforms with other

Triassic carbonate platforms exposed in surrounding countries, particularly those of eastern Sulawesi (eastern Indonesia), should be developed to improve our paleogeographic and geodynamic knowledge of the whole of Southeast Asia during the Triassic period. 7) Additionally, our micropaleontological studies could help researchers understand the biological recovery that followed the end-Permian biological crisis. [START_REF] Metcalfe | Tectonic framework and Phanerozoic evolution of Sundaland[END_REF][START_REF] Metcalfe | Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys[END_REF][START_REF] Rossignol | The volcaniclastic series from the Luang Prabang Basin, Laos: A witness of a triassic magmatic arc ?[END_REF][START_REF] Rossignol | The Indosinian orogeny: A perspective from sedimentary archives of North Vietnam[END_REF]. A. Simplified geological map (after Tran Van Tri, 1973 and[START_REF] Faure | The South China block-Indochina collision: where, when, and how?[END_REF]. The black rectangles indicate the areas investigated in this study. B. Simplified stratigraphic column of Northern Vietnam (after [START_REF] Tri | Geological map of Vietnam (the North part[END_REF][START_REF] Tri | Geology and Earth Resources of Vietnam[END_REF]Thanh and[START_REF] Thanh | Stratigraphy Units of Vietnam[END_REF]. The wavy lines indicate the main unconformities. This synthetic stratigraphic succession is based on local stratigraphic successions.

Figure captions

Corresponding cross-sections are indicated on the right side. Note that only the key biostratigraphic samples have been reported (see text for discussion). The stratigraphic chart and numerical ages are after [START_REF] Cohen | The ICS International Chronostratigraphic Chart[END_REF]. The thicknesses of the levels are estimates. The different members identified along the Song Da Basin have been correlated using intersecting cross-sections, where possible (Fig. 3C) and through their biostratigraphic content (foraminifers; see text for discussion). An-La: undifferentiated Anisian-Lanidian stages. The stratigraphic chart and numerical ages are after [START_REF] Cohen | The ICS International Chronostratigraphic Chart[END_REF].

  , Buru and Misool. There, the main Triassic platforms developed in the Late Triassic (late Norian to Rhaetian) and were part of a discontinuous shallow water reef platform. The carbonates deposited in these platforms contain foraminifers and palynomorphs with Australian/Southern Tethys affinities. The thickness of the carbonate deposits is less than 250 m (250 m in eastern Sulawesi and 200 m on Seram Island). These Norian to Rhaetian reef carbonate platforms rest upon Upper Carnian to Norian micritic or marly limestones and are capped by latest Triassic to Early Jurassic intertidal to pelagic deposits. According to Villeneuve et al. (2010), the block comprising these platforms came from a northern extension of the Irian Jaya Block. With respect to the eastern Indonesian Triassic platforms, the Vietnamese ones are older, thicker and reflect margins with higher subsidence. The contrasting subsidence patterns shown by the northern Vietnam and Indonesian basins likely reflect different tectonic settings and/or a different rheology of the basements of these basins.

Figure 1 .

 1 Figure 1. Location of the main blocks and sutures in the Indochina Peninsula.

Figure 2 .

 2 Figure 2. Simplified geological map of Northern Vietnam.

Figure 3 .

 3 Figure 3. Location of the cross-sections within the investigated areas.

Figure 4 .

 4 Figure 4. Cross-section (II and V) from Ha Binh to Cau Gian Khuat.

Figure 5 .

 5 Figure 5. Cross-section VI in the Dinh mountain area.

Figure 6 .

 6 Figure 6. Cross-section XV in the Man Duc area.

Figure 7 .

 7 Figure 7. Cross-section X in the area of the Hao Lu temples (Lei temples).

Figure 8 .

 8 Figure 8. Correlation between the members identified in the Song Da Basin.

Figure 9 .

 9 Figure 9. Synthetic stratigraphic series of the Song Da Basin.

Figure 10 .

 10 Figure 10. Photographs of the representative outcrops from the Song Da Basin.

Figure 11 .

 11 Figure 11. Cross-section XI between Phung Luat and Ho Thuong.

Figure 12 .

 12 Figure 12. Cross-section XII between Nghia Dan and Quinh Long.

Figure 13 .

 13 Figure 13. Cross-section XIII between Do Luong and the Dien Chau road.

Figure 14 .

 14 Figure 14. Correlation between the members identified in the Sam Nua Basin.

Figure 15 .

 15 Figure 15. Photographs of the representative outcrops from the Sam Nua Basin.

Acknowledgements

This research is part of the "Carbonate platforms of the Permian/Triassic in Vietnam" program, involving the University of Hanoi (Vietnam) and the University of Provence (now Aix-Marseille University). This work was supported by the CNRS (France), the SNSF (Switzerland) and the Geological Department at the University of Hanoi, with the participation of the University of Geneva (Switzerland) and Geosciences Rennes (France). We acknowledge professors Tong Dzui Tanh, Ta Ton Trang, and Nghi Tran of the University of Hanoi for their participation in the "Carbonate platforms of the Permian/Triassic in Vietnam" program. C.R. acknowledges support from the FAPESP (Fundação Amparo à Pesquisa do Estado de São Paulo; processos 2018/02645-2 and 2015/16235-2). The English version of the manuscript was post-edited by S. Mullin.

Appendices


Najib Souissi
email: souissi.najib@gmail.com

Catherine Mabru

Chedly Bradai

Keywords: Squeeze pressure, 2017 Al alloy, Aging temperature, Microstructure, Mechanical properties

Aging response investigation of 2017 Al alloy processed by gravity and squeeze casting Suitable pressure levels and heat treatment of casting wrought aluminum alloy are key process parameters in making components with better metallurgical properties. For this purpose, the effect of applied pressure and aging treatment on the microstructure and the mechanical properties of an industrial 2017 aluminum alloy manufactured by gravity and direct squeeze casting processes are investigated in this paper. After treating to various aging states, the correlation between these parameters and the characteristics of the cast alloy has been analyzed and discussed.

ties of squeeze cast 6061 Al alloy and found that the strength of the alloy decreases with the presence of the (FeCrMn) 3 SiAl 12 phase and increases with that of the finely dispersed Mg 2 Si phase. Zhang et al. [8] reported that the heat treatment caused an improvement of the tensile strength and elongation of SC Al-Cu based alloy. However, reports on aging temperature effect of SC wrought Al alloy are very few. It must be noted that precipitation behavior is known to have an important influence on the metallurgical properties of Al alloys.

Al-4 wt.%Cu (2xxx series) alloys are a precipitation hardening in which Cu promotes the formation of h-Al 2 Cu precipitates. The decomposition of the supersaturated solid solution (SSS) is believed to proceed in the following steps: SSS ? Guinier-Preston (GP) zones ? h'' ? h' ? h-Al 2 Cu [9]. Obviously, the development of high-performance alloys requires a great understanding of the relationship between processing parameters and mechanical properties through a quantitative prediction of the precipitation behavior. More specifically, wrought 2017 aluminum alloy, which is widely used in aircraft and aerospace structures [10,11], has gained a growing interest in it especially when formed by the SC technique [12 -15]. Moreover, it is reported that the combination of squeeze pressure and heat treatment offers several advantages that make wrought aluminum alloys acquire improved mechanical properties [6,8]. An understanding of the interrelationships between these parameters is particularly important since the mechanical properties of casting components are highly correlated with the final microstructure and crystallographic texture. Thus, the main objective of the present work is to investigate the effects of the aging temperature and to develop a suitable age-hardening heat-treatment process for gravity die casting (GC) and SC wrought 2017 Al alloy. It focuses on the relationship between the microstructure and the mechanical behavior of the material.

Experimental procedure

Material and specimen preparation

The casting experiments were conducted using a 2017 A aluminum alloy, which is widely used as wrought product in various mechanical areas [START_REF] Vargel | The most common wrought aluminum alloys[END_REF][START_REF]Metallic materials and elements for aerospace vehicle structures[END_REF]. Its chemical composition, as determined by spectral analysis, is shown in Table 1.

It was received as an extruded bar of diameter 80 mm and melted at 700 8C in a graphite crucible using an electric resistance furnace. The casting process was carried out

Introduction

Among the different techniques described for materials forming, the squeeze casting (SC) process is used to produce high integrity and low cost aluminum alloy [1]. During the solidification of the melt, the direct action of pressure activates two principal phenomena that are not generally observed in traditional die casting methods. First, the pressure generates a high cooling rate due to the reduction of the air gap between the metal and the die wall, hence, there is a much larger effective contact area [2]. In addition to that, the temperature of the liquidus could rise upon application of pressure, leading to a higher degree of undercooling [3]. The change in the nucleation ratio and cooling rate would increase, which creates a finer grain size [3]. The second phenomenon caused by SC is the fact that direct applied pressure inhibits defects associated with shrinkage cavities and porosity formation [4]. This eliminates casting drawbacks and results in a better quality of casting products with a much higher density. On the other hand, heat treatment processing is considered as a promising method to improve the quality of aluminum alloys. Yet, some research studies investigating the effect of heat treatment process on casting wrought Al alloys have been reported in literature [5 -8]. In this regard, Kim et al. [6] studied the effect of aging times on the microstructure and mechanical properties of 7075 Al alloy. The results show that the fine-grained structures and the uniform distribution of eutectics at the periphery of the grain boundaries caused significant changes in hardness and tensile strength of specimens. Skolianos et al. [7] assessed the influence of T6 heat treatment on the microstructure and tensile proper-through a solid mold and a punch made from heat treated H13 steel.

The direct SC experiments were conducted on a hydraulic press, where the pressure on the molten metal is kept at 90 MPa until the end of solidification. In GC, molten alloy was poured directly into the die without external pressure. The mold was preheated up to 200 8C. The as-cast billets were rod-shaped; 23 mm in diameter and 110 mm in length. The choice of casting parameters was based on previous works [12 -14]. Afterwards, the billets were solution treated at 500 8C for 8 h followed by water quenching. In order to investigate the effects of aging temperature and the applied pressure on the microstructure and mechanical properties, some quenched specimens were placed in the furnace (6 h) for the artificial aging process at different temperatures. Figure 1 shows an overall scheme of the elaboration process and the experimental set-up followed in the present study.

DSC analyses

The differential scanning calorimetry (DSC) experiments were conducted on a SETARAM DSC-92 instrument for temperatures ranging from 25 to 600 8C at a heating rate of 10 K min -1 . The specimens were turned to disks of 4 mm diameter and 6 mm high and placed in a hermetic alloy sealed aluminum capsule.

Microstructural characterization

The X-ray diffraction (XRD) measurements were performed using a Philips PW 1800 with Cu-K a radiation.

The fracture surfaces of the tensile specimens were examined with a JEOL 6460 LV scanning electron microscope (SEM).

Mechanical tests

Vickers microhardness (HV) tests were performed with a MEKTON hardness tester, at a load of 300 g applied for 30 s. Five measurements were taken at randomly selected points on the middle transverse section of the billet with a diamond pyramidal indenter. Tensile tests were carried out at room temperature using an MTS-810 universal testing machine with a crosshead speed 2 mm min -1 . In order to ensure the reproducibility of the tensile results, three tests were performed for each condition.

Results and discussion

The DSC thermograms and the Vickers microhardness measurements, as a function of aging temperature after water quenching of both GC and SC specimens, are plotted in Fig. 2. It is clearly shown that the microhardness of SC alloy is higher than those of the GC alloy. This is a result of the change in microstructure and the densification of an as-cast alloy caused by the applied pressure [6]. The first intense exothermic signal centered at around 70 8Ci sa ssigned to the formation of GP zones while the following weak endothermic peaks (around 85 8C) to their dissolution. The wide exothermic peak centered at 170 8C is associated with h" phase precipitation. These metastable precipitates are responsible for the maximum microhardness values [18], and this is commonly observed at 180 8C. It was also noticed that the h" peak of the SC (90 MPa) sample is larger than that of the GC (0 MPa), which is consistent with its higher microhardness. However, the wide exothermic peaks at 320 8C are ascribed to the formation of incoherent h' precipitates. Thus, the formation of the latter leads to a substantial decrease in microhardness [START_REF] Birol | [END_REF]. Added to that, the intense endothermic peaks at 519 8C for GC and 528 8C for SC specimens are attributed to the melting of the intermetallic compound h-Al 2 Cu [19]. Moreover, the thermal effects associated with h' ? h transformation may not be sufficient to exhibit a separate signal during the DSC scans.

As shown in Fig. 2b, this indicates that the peak relative to the melting of h phase is shifted toward higher temperatures with respect to that of GC specimen. This can be attributed to the increase in eutectic temperature in correlation with the increase in pressure [6]. X-ray diffractograms of the three states of GC and SC samples are illustrated in Fig. 3. The XRD patterns of both cast alloys under 0 MPa (Fig. 3a) and 90 MPa (Fig. 3b) reveal the Bragg reflections of the soluble Al 2 Cu and insoluble Al 12 (Fe,Mn,Cu) 3 Si intermetallic phases. Other residual phases are expected to be dissolved into the matrix by solution treatment. These diffraction results are consistent with those published by Birol [START_REF] Birol | [END_REF]20,21]. It is also seen that the intensity of the diffraction peaks related to the Al 2 Cu phase tends to broaden with aging temperature, confirming the reported precipitation sequence. This indicates that, during the aging process, the Cu separated out in the form of Al 2 Cu which are predominant precipitates in the alloy. Figure 4 presents the results of the SEM and EDX (energy dispersive X-ray) analysis of the sample aged at 180 8C prepared by gravity and squeeze casting process, respectively. The SEM-image shows a wide intermetallic precipitation distributed among the grain boundaries, as already observed in the optical micrographs. Also, a lot of shrinkage microporosity is seen in gravity samples (Fig. 4a), while nearly no porosity is observed in squeeze casting samples (Fig. 4b). The predominant phases were identified by EDX (Fig. 4a' and Fig. 4b') analysis once again to be the h-Al 2 Cu intermetallic phases in both GC and SC samples aged at 180 8C.

The tensile properties such as ultimate tensile strength (UTS), yield strength (YS) and elongation percent (El%) of the GC and SC specimens in the different states I (quenched), II (quenched + aged 180 8C) and III (quenched + aged 320 8C) are compiled in Table 2. This shows that the GC specimens have low tensile properties than the SC specimens since the intensity of the applied pressure has a significant influence on the mechanical properties of the 2017 Al alloy [6,13]. The effect of aging temperature on tensile properties showed that the aging temperature of 180 8C (state II) presents the highest values of UTS and YS and the lowest value of El%. The enhancement of UTS and YS are likely due to the higher precipitates density of metastable Al 2 Cu and insoluble Al 12 (Fe,Mn,Cu) 3 Si precipitates. These precipitates reduce the dislocation movement and thus a higher stress is required for its bowing. It is observed that the best combination of properties, i. e. high UTS (298 MPa) and YS (223 MPa), is achieved when aging and pressure are at 180 8C and 90 MPa, respectively. In addition as expected, the El% varies inversely with the other tensile features. It is thus fair to conclude that the variation of tensile properties with squeeze pressure and heat treatment shows a good synchronized effect with microhardness measurements. This variation is the result of microstructural changes that have been observed previously.

Figure 5 shows typical fracture surfaces of the tensile specimens. On a macroscopic scale, the tensile fracture surface of the GC (0 MPa) specimens is at around 908 angle relative to the load direction for uniaxial tension, which is the characteristic of a brittle fracture surface. However, the SC (90 MPa) specimens are considered to be ductile and typically fail at approximately 458 with respect to the load direction of the uniaxial tension.

High magnification examination of the fracture surface of the GC specimens shows that the existence of microporosity (see arrows in GC) and the coarse grain size become the principal source of low ductility. This is a typical feature of intergranular fracture paths. Obviously, with no applied pressure, the alloy exhibits shrinkage problems associated with gas porosity due to the low interdendritic fluidity of the molten material [22]. The tensile fracture surface of the SC specimens corresponds to transgranular fracture and reveals that microvoids mostly disappear and that equiaxed dimples occupy a large area. Inclusions and precipitates that initiate the ductile rupture are also observed in the fractography. However, the Al 2 Cu intermetallic compounds that inevitably exist at the grain boundaries (see arrows in SC), lead to early fractures by creating notch effects during the tensile test [23]. It is also revealed that the breakage of intermetallic phases into smaller parts facilitates the dislocation motion. Generally, it can be concluded that the existence of cavities and higher density of precipitates is the principal source of lower ductility.

Conclusion

In this study, the effect of squeeze pressure combined with the artificial aging process on the microstructure and the mechanical properties of the cast 2017 A aluminum alloy have been quantified. The conclusions of this work can be summarized as follows: 1. Microhardness evolution of the investigated alloy at various aging temperatures shows that the microhardness increases at first and then decreases as aging temperature increases. The maximum and the minimum values are attained when aging at 180 8C and 320 8C, respectively. In addition, the differential scanning calorimetry (DSC) analysis reveals the transformations of the different phases dissolved in the Al matrix. 2. After aging at 180 8C, the microstructure of both SC and GC alloys was characterized by a high density of metastables precipitates, which corresponds to the maximum value of microhardness. 3. The rise of UTS, YS, microhardness and the fall of ductility at an aging temperature of 180 8C is more likely due to the precipitation of hard Al 2 Cu phase and the elimination of gas porosity by applied pressure. These results are in good agreement with the microstructural evolution according to the different aging temperatures. 4. The examination via SEM fractography and elongation results of both SC and GC alloys indicates that the fracture mode becomes more ductile at 320 8C.

Fig. 1 .

 1 Fig. 1. Experimental set up flowchart.

Fig. 2 .Fig. 3 .

 23 Fig. 2. (a) Microhardness variations and DSC thermograms of GC and SC specimens. (b) Magnification of the DSC curves showing the melting peaks of h phases.

Fig. 4 .

 4 Fig. 4. SEM images and corresponding EDX analysis of: (a, a') gravity, and (b, b') squeeze cast samples each aged at 180 8C.

Fig. 5 .

 5 Fig. 5. SEM micrographs of tensile fracture of the selected samples: I, II and III under 0 MPa and 90 MPa.

Table 1 .

 1 Chemical composition of 2017 A aluminum alloy.

	Elements	Cu	Mg	Si	Fe	Mn	Ni	Pb	Zn	Cr	Al
	Mass %	4.47	0.4	0.52	0.65	0.37	0.1	0.03	0.25	0.1	Bal.

Table 2 .

 2 Tensile properties results of the investigated alloy in the different metallurgical states.

	Process	Metallurgical state	UTS (MPa)	YS (MPa)	El (%)
	GC	I (quenched)	200	125	1.7
	(0 MPa)	II (aged 180 8C)	219	175	1.2
		III (aged 320 8C)	175	118	2.5
	SC	I (quenched)	278	199	2.2
	(90 MPa)	II (aged 180 8C)	298	223	1.85
		III (aged 320 8C)	276	121	3.1