Éditeur(s) : HAL CCSD Seoul National University
Résumé : http://www.episodes.org/backissues/302/abstract2.htm
International audience
The Trans-North China Belt (TNCB) is a Paleoproterozoic collisional orogen (ca. 1.9-1.8 Ga) responsible for the amalgamation of the North China Craton. Detail field works in Lüliangshan, Hengshan, Wutaishan and Fuping massifs where the belt is well exposed, allow us to draw new tectonic map and crustal-scale cross sections. The available petrologic, radiometric, geochronologic data are integrated in a geodynamic evolution scheme for this orogen. The Low Grade Mafic Unit (LGMU) is interpreted as an ophiolitic nappe rooted in a suture zone located in the western part of the Lüliangshan. This ophiolitic nappe overthrusts to the SE upon the Orthogneiss-Volcanites Unit (OVU) that consists of a bimodal volcanicsedimentary series metamorphosed under amphibolite facies conditions intruded by calcalkaline orthogneiss. The OVU is a composite Neoarchean-Paleoproterozoic magmatic arc developed during two stages (ca. 2500 and 2100 Ma) upon a continental basement corresponding to the western extension of the Neoarchean Fuping massif. The OVU overthrusts to the SE the Fuping massif along the Longquanguan shear zone. This stack of nappe, coeval with an amphibolite facies metamorphism, is dated at ca 1880 Ma. Subsequently, the metamorphic series experienced a widespread migmatization at 1850 Ma and was intruded by post-orogenic plutons dated at 1800 Ma. The weakly to unmetamorphosed Hutuo Supergroup unconformably overlies the metamorphosed and ductilely deformed units (OVU and LGMU), but it is also involved in a second tectonic phase developed in subsurface conditions. These structural features lead us to question the ca 2090 Ma age attributed to the Hutuo supergroup. Moreover, in the Fuping massif, several structural and magmatic lines of evidence argue for an earlier orogenic event at ca 2100 Ma that we relate to an older west-directed subduction below the Fuping Block. The Taihangshan Fault might be the location of a possible suture zone between the Fuping Block and an eastern one. A geodynamic model, at variance with previous ones, is proposed to account for the formation of the TNCB. In this scheme, three Archean continents, namely from West to East, the Ordos, Fuping and Eastern Blocks are separated by the Lüliang and Taihang Oceans. The closure of the Taihang Ocean at ca 2100 Ma by westward subduction below the Fuping Block accounts for the arc magmatism and the 2100 Ma orogeny. The second collision at 1900-1880 Ma between the Fuping and Ordos blocks is responsible for the main structural, metamorphic and magmatic features of the Trans-North China Belt.
ISSN: 0705-3797

insu-00152972
https://hal-insu.archives-ouvertes.fr/insu-00152972
https://hal-insu.archives-ouvertes.fr/insu-00152972/document
https://hal-insu.archives-ouvertes.fr/insu-00152972/file/Episodes_TNCB_Faure_et_al.pdf

Éditeur(s) : HAL CCSD
Résumé : International audience
In the external Western Alps, the collision started with the shortening of the proximal European (Dauphinois) margin. One peculiar feature is the crustal thickening that finally led to the exhumation of the basement External Crystalline Massifs (ECM) during the Neogene. In these massifs, various age of shortening, structural style and sequence of deformation have been proposed leading to different kinematics along the Alpine arc that in turn do not imply the same crustal/lithospheric strength. In this contribution, we present new structural and microstructural data along with Ar/Ar ages from field studies in the Oisans-Grandes Rousses massifs, between Grenoble and Briançon). From balanced cross-sections, we show that about half of the external zone crustal shortening is accommodated within the ECM with similar Ar/Ar ages in all massifs. Furthermore, the shear zones present very large thicknesses (several hundred meters) and do not reactivated either the inherited Liassic normal faults or the Variscan foliation. Finally, the main shortening structures are localized beneath inherited Liassic syn-rift basins. These results suggest that the crust was weak during its shortening. Such behavior is most likely due to the combination of several factors: the crust burial (below the overthrusting internal units) down to about 10km, the presence of weak inherited syn-rift basins, and the strain softening in shear zones that evolve in mica-rich phyllonites. These results are consistent with the studies in the Aar massif and should most likely apply to the Mont Blanc massif, whose relationships with its cover are still debated.
EGU General Assembly
Vienne, Austria

insu-00839426
https://hal-insu.archives-ouvertes.fr/insu-00839426
BIBCODE : 2013EGUGA..15.8837B

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : International audience
The Jurassic Mirdita ophiolite crops out in the western branch of the Hellenide-Dinaride ophiolite belt in the Balkan Peninsula; it represents a remnant of the Tethyan oceanic lithosphere developed between the Apulian and Pelagonian microcontinents. Emplaced during the Cretaceous on the western margin of Pelagonia, the Mirdita ophiolite was involved in southwestward thrusting of the Hellenide-Dinaride tectonic units during the Eocene alpine tectonics. It was little affected, however, by these alpine events as marked by nearly horizontal Cretaceous limestone deposits overlying the ophiolite. A continuous section of Middle Jurassic oceanic crust, including thin gabbros, a N-S-trending sheeted dike complex, and extrusive rocks, is exposed in the central part of the ophiolite and thickens eastward within a regional, N-S-trending synform. Peridotite massifs exposed along the western and eastern edges of this synform show major structural and petrological differences. The eastern ultramafic domain has a harzburgitic mantle exhibiting a high-temperature asthenospheric foliation dipping steeply to moderately to the west. Major chromite deposits are restricted to these eastern ultramafic massifs. The western ultramafic domain has a zoned mantle, with asthenospheric harzburgite exposed at its western margin, progressively replaced eastward by plagioclase-peridotites that were highly strained at low temperatures, and that are bounded by amphibole-peridotites along with crustal rocks. The occurrence of plagioclase is ascribed to melt impregnation processes that occurred immediately before or during intense lithospheric deformation. The gabbros in the eastern massifs are thicker (1-2 km and layered, whereas they are highly thin and discontinuous in the western massifs. Mantle peridotites of the western massifs locally are in direct contact with the overlying diabase and volcanic rocks along ductile shear zones. Development of epidote-amphibolite facies metamorphism in upper-crustal rocks was produced by hydrothermal alteration in the oceanic realm, and was intense and widespread in the western domain. The Mirdita ophiolite likely originated in a short-lived, narrow ocean basin, which was closed during the Middle Jurassic by eastward overthrusting of the western domain, and westward subduction of the eastern domain. This constrictional phase was followed by dextral oblique convergence between the Pelagonian and Apulian microcontinents. This model implies a parautochtonous origin of the Mirdita ophiolite between these two microcontinents.
ISSN: 0020-6814

hal-00456063
https://hal.archives-ouvertes.fr/hal-00456063
DOI : 10.1080/00206810902823982

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We analyze the kinematics of fault tip folding at the front of a fold-and-thrust wedge using a sandbox experiment. The analog model consists of sand layers intercalated with low-friction glass bead layers, deposited in a glass-sided experimental device and with a total thickness h = 4.8 cm. A computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation. Active deformation at the tip of the forward propagating basal décollement is monitored along the cross section with a high-resolution CCD camera, and the displacement field between pairs of images is measured from the optical flow technique. In the early stage, when cumulative shortening is less than about h/10, slip along the décollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage of detachment tip folding, horizontal displacements decrease linearly with distance toward the foreland. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined axial surfaces. When the cumulative slip on the décollement exceeds about h/10, deformation tends to localize on a few discrete shear bands at the front of the system, until shortening exceeds h/8 and deformation gets fully localized on a single emergent frontal ramp. The fault geometry subsequently evolves to a sigmoid shape and the hanging wall deforms by simple shear as it overthrusts the flat ramp system. As long as strain localization is not fully established, the sand layers experience a combination of thickening and horizontal shortening, which induces gradual limb rotation. The observed kinematics can be reduced to simple analytical expressions that can be used to restore fault tip folds, relate finite deformation to incremental folding, and derive shortening rates from deformed geomorphic markers or growth strata.
ISSN: 2169-9313

hal-00138061
https://hal.archives-ouvertes.fr/hal-00138061
https://hal.archives-ouvertes.fr/hal-00138061/document
https://hal.archives-ouvertes.fr/hal-00138061/file/Bernard_et_al-2007-Journalsur Geophysical_Research__Solid_Earth_%281978-2012%29.pdf
DOI : 10.1029/2005JB004149

Éditeur(s) : HAL CCSD
Résumé : International audience
Timor Island was at time considered as an example of "accretionary prism" linked to the collision between the Australian block and the Banda arc. However, its geological evolution is more complex. Five main superimposed structural units are distinguished in West Timor. The today structure is the result of three main tectonic events that occurred during the Late Oligocene, Late Early Pliocene and Late Pliocene-Early Pleistocene times, respectively. Our field investigations in the 1990 to 2000 decade completed with geochemical analyses and K-Ar datings (Jurassic and Miocene ages) of magmatism allow to precise the geodynamic evolution of Timor that can be summarized as follows: a first block was detached from Gondwana (unit 2) and drifted to the Asiatic margin until the Late Oligocene when it collided with the Asiatic active margin (unit 3). Then, the new block formed by both 2 and 3 units drifted to the south during the Miocene and the Early Pliocene until it collided with the Australian margin (ASM), by the Late Early Pliocene. Then, the Australian and Timor blocks moved together towards the North-North East during the Late Pliocene until they collided with the Banda fore-arc (unit 4). Later on (Pleistocene), Timor island was capped by the "autochthon" (unit 5) and then on (Quaternary?) by the Banda volcanic arc northward thrusted over the South Banda basin. Taking in consideration its close relationships with both the Australian plate and the Eurasian one. Timor may be considered as a key area for building this geodynamical scenario of Indonesia.
Bulletin Société Géologique de France

insu-00933548
https://hal-insu.archives-ouvertes.fr/insu-00933548
DOI : 10.2113/gssgfbull.184.6.569