Henri Maluski

Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing

Abstract A new subdivision of the pre-Permian rocks of the Bohemian Massif into terranes is based on recent structural, kinematic and radiometric studies and on a comparison with the French Variscan Belt (mainly the Massif Central). Major tectonothermal events are attributed to the Variscan (390–300 Ma) and the terrane boundaries trending roughly SW-NE to SSW-NNE to Variscan thrusts or ductile strike-slip faults. On a NW-SE section from the Saxothuringian area to the Moravian Carboniferous foredeep, the Bohemian Massif appears as a fan-like divergent orogen with two high-grade metamorphic belts (the Saxothuringian with northwest vergence and the Moldanubian with an east to southeast vergence) separated by a less metamorphic and much less eroded block, the Barrandian. From southeast to northwest the belt may be divided into six terranes: Moravian, Drosendorf and Gfohl (previously grouped in the Moldanubian zone), Barrandian, Munchberg-Tepla and Saxothuringian. These terranes have all suffered different geological histories and are now separated by major thrusts of strike-slip faults. This configuration is the result of a complex Variscan history, probably including an early subduction and closure of an oceanic domain followed by obduction, continental collision, continental subduction and strike-slip faulting lasting more than 80 Ma.

Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) 40Ar39Ar Triassic ages and Cretaceous to Cenozoic overprints

Abstract Several NW-trending ductile and partly mylonitic shear zones cross the Truong Son belt of Central Vietnam, along the Song Ma and Song Ca valleys and, north of the Kontum block, from Da Nang to Aluoi and Khe Sanh. High-grade metamorphic rocks of amphibolite facies, showing a retrograde evolution and consisting of ortho- and paragneisses, metavolcanics, amphibolites, marbles and quartzites are exposed along these structures. They display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near-horizontal to gently plunging mineral stretching lineation, indicating a strike-slip tectonic regime of deformation. Along the southernmost fault zone at least, various and consistent kinematic indicators, including SC structures, asymmetric tails of porphyroclasts, prove that the strain and metamorphism have been generated by a phase of dominantly non-coaxial deformation with a dextral sense of shear. 40Ar39Ar dating, applied on the high-grade metamorphic rocks minerals occurring along these zones, provide plateau cooling ages, closely around 245 Ma, establishing that this event took place in the lowermost Triassic as an early phase of the Indosinian orogeny. The existence of Indosinian movements in Vietnam, as they have been defined by previous authors during the early century, is now accurately confirmed and this is the first insight in the occurrence of ductile strike-slip tectonics of Indosinian age along NW-SE fault zones. Well-expressed in Central Vietnam is a Cretaceous thermal and deformational overprint, marked by epimetamorphism, which took place between 90 and 120 Ma, as attested by low-temperature degassing ages. This Cretaceous event is not found further north in the Song Ma zone where younger ages appear as a result of the influence of Cenozoic shear movements along the Red River fault which displays 20–30 Ma ages. On the northern flank of Song Ca, in the Bu Khang-Phu Hoat core complex, comparable Oligocene-Lower Miocene ages (20–35 Ma), yielded by biotite and phlogopite, reflect a strong overprinting and attest to a rapid uplift of the basement.

Ar-Ar and fission-track ages in the Song Chay Massif: Early Triassic and Cenozoic tectonics in northern Vietnam

The Song Chay Massif is the northeasternmost metamorphic complex in Vietnam, to the east of the Red River Shear Zone. It shows a large antiformal structure involving orthogneisses and migmatites overlain, on its northern flank, by muscovite bearing marbles. An E–W striking fault bounds the dome to the South. Kinematic indicators along a S–N section reveal top-to-the-N shear sense along the interface between the orthogneissic core and the overlying metasediments. Radiometric ages were obtained by the 40Ar–39Ar method using purified mica separates. Across the dome ages range from 236 Ma at the southern edge to 160 Ma in the core, attesting to a strong imprint in the Early Triassic time. A clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24 Ma) that obtained in the nearby Red River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high temperature deformation of the Red River Shear Zone took place. The final exhumation of the Song Chay orthogneiss constrained by fission-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south. Timing is similar to that of exhumation in the Red River Shear zone.

Oligo‐Miocene midcrustal subhorizontal shear zone in Indochina

New structural observations of Oligo-Miocene deformation in north Vietnam along the Red River Shear Zone suggest that left-lateral strike-slip shear was restricted to the upper and middle crust above a horizontal shear zone. Left-lateral shear deformation is associated with low-pressure-low-temperature parageneses. High-temperature deformation is restricted to zones where the foliation has a low dip in the core of the Dai Nui Con Voi antiformal dome. These observations complete those made earlier in the Bu Khang extensional dome farther south. Above the horizontal shear zone left-lateral transpression was active during the first stage and changed to transtension some 33 Myr ago. Purely extensional metamorphic domes (Bu Khang) or transtensional domes (Dai Nui Con Voi) were exhumed during this younger stage. Our observations plead for caution when interpreting strike-slip structures at lithospheric scale.

Geodynamic Significance of the Kontum Massif in Central Vietnam: Composite 40Ar/39Ar and U‐Pb Ages from Paleozoic to Triassic

The Kontum massif of south‐central Vietnam has long been classified as lower continental crust of Precambrian (Archean) age. U‐Pb zircon and 40Ar/39Ar biotite ages from charnockites and amphibolite facies rocks from this basement, however, yield much younger ages and imply at least two Paleozoic thermal events. Syntectonic charnockites from the Kannack complex and the southern Kontum massif record Permo‐Triassic magmatism (U‐Pb: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Oligocene-Miocene Bu Khang extensional gneiss dome in Vietnam: Geodynamic implications

249\pm 2

Geochemistry and 40 Ar- 39 Ar geochronology of the mafic metavolcanic rocks from the Rychory Mountains Complex (West Sudetes, Bohemian Massif); palaeotectonic significance

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