Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The Yakutat-St. Elias collision in SE Alaska and adjacent Canada represents a prime example of present-day tectonics associated with an indentor corner. Its eastern syntaxis is marked by high exhumation, a sharp structural bend, and strain concentration at the transition from shortening to oblique transpression. Here we present GPS velocity and strain rate fields that cover the syntaxis, including 11 new stations in the core of the St. Elias Mountains. These data are corrected for transient deformation (glacial isostatic adjustment and postseismic and interseismic loading) to produce residual velocities and strain rates representative of long-term tectonics. The main features of these velocity and strain rate fields are a peak in strain rates (strain knot) in the syntaxis at the junction between the main fault systems and a rapid rotation from convergence-parallel to convergence-normal orientations of the velocities and shortening axes around the syntaxis, leading to shortening across the southern Denali Fault. These features are consistent with the strain and tectonic patterns expected near an indentor corner at the transition from shortening to transpression, with a combination of diffuse and localized deformation. The GPS velocities and strain rates show diffuse deformation around the syntaxis, from pure convergence-parallel shortening in the orogenic wedge to oblique extension that accommodates the strain rotation at the front of the syntaxis. This indentor-corner model also results in a near-zero strike-slip rate on the southern Denali Fault and shows no clear evidence for a throughgoing fault hypothesized to link the Fairweather and Totschunda Faults.
ISSN: 0148-0227

hal-01243510
https://hal.archives-ouvertes.fr/hal-01243510
DOI : 10.1002/2014JB011842

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
We analyze focal mechanisms of shallow-intermediate earthquakes in a NW-SE transect along the western Betic Cordillera and Alboran Sea, and deep earthquakes located in the central Betics to constrain the state of stress at the Gibraltar Arc slow convergence area. Shallow earthquakes (< 40 km) are preferably clustered at the mountain front. A general NW-SE horizontal compression is compatible with the convergence, and NW plunging compression axes are in agreement with frontal thrust activity. Toward the Alboran Sea the earthquakes are progressively deeper, reaching intermediate depths (40-120 km) near the coast and supporting the present-day activity of subduction only in this area. The Iberian continental crust concentrates most of the intermediate seismicity and is forced to partially sink into the mantle, probably through the pull of the oceanic slab. This downdip pull together with the buoyancy of the Iberian continental crust produces the slab curvature, downdip extension in the external arc of the continental slab, and downdip compression in the inner arc. T axes highly dipping to the southeast at 90-120 km depth occur at the oceanic/continental transition. Deep earthquakes (> 620 km) show very similar focal mechanisms, fitting the general slab behavior of resistance to further descent at the 660 km discontinuity. Seismicity features evidence the present-day stress distribution in a context of transition from subduction to continental collision.
ISSN: 1525-2027

hal-00669889
https://hal.archives-ouvertes.fr/hal-00669889
DOI : 10.1029/2011GC003824

Éditeur(s) : HAL CCSD
Résumé : International audience
Sedimentary basins, through the sedimentary successions and the nature of the deposits, reflect the geology of thearea from which the sediments were derived and thus provide valuable record of hinterland tectonism. As the collisionbetween the South China and the Indochina blocks (i.e. the Indosinian orogeny) is still the object of a numberof controversies regarding, for instance, its timing and the polarity of the subduction, the sedimentary basins associatedwith this mountain belt are likely to provide clues to reconstruct its geodynamic evolution. However, boththe Sam Nua Basin (located to the south of the inner zones of the Indosinian orogeny and the Song Ma ophiolites)and the Song Da Basin (located to the north of the inner zones), northern Vietnam, are still lacking importantinformation regarding the depositional environments and the ages of the main formations that they contain.Using sedimentological and dating analyses (foraminifers biostratigraphy and U-Pb dating on detrital zircon), weprovide a new stratigraphic framework for these basins and propose a geodynamic evolution of the present-daynorthern Vietnam.During the Early Triassic, the Sam Nua Basin was mainly supplied by volcaniclastic sediments originating froman active volcanic activity. Geochemical investigations, combined with sedimentological and structural analyses,support an arc-related setting for this magmatism. This magmatic arc resulted from the subduction of a south dippingoceanic slab that once separated the South China from the Indochina blocks. During the Middle to the LateTriassic, the Sam Nua Basin underwent erosion that lead to the formation of a major unconformity, termed theIndosinian unconformity. This unconformity is interpreted to result from the erosion of the Indosinian mountainbelt, built after the continental collision between the South China and the Indochina blocks. Later, during the LateTriassic, the Sam Nua Basin experienced the deposition of very coarse material, emplaced under continental settingand representing the product of the erosion of the Indosinian mountain belt.To the North, the Song Da Basin is characterized by strongly diachronous deposits over a basal unconformitydeveloped at the expense of volcanic and volcaniclastic deposits related to the Emeishan Large Igneous Province.The sedimentary succession indicates a foreland setting during the Early to the Middle Triassic, which contrastswith the commonly assumed rift setting for these sediments. Thus, the Song Da Basin documents the formation ofthe Indosinian thrust belt, located immediately to the South of the basin.
European Geosciences Union General Assembly 2017
Vienne, Austria

insu-01510331
https://hal-insu.archives-ouvertes.fr/insu-01510331

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We have analyzed shear wave splitting recorded by portable and permanent broadband and long-period stations located in the eastern United States. Teleseismic shear waves (SKS, SKKS, and PKS) were used to retrieve the splitting parameters: the orientation of the fast wave polarization plane ϕ and the delay time δt. In total, 120 seismic events were processed, allowing for more than 600 splitting measurements. Within the Appalachians, stations located in the western (external) part are characterized by δt≈1s and ϕ trending N50°– 70°E in the south and central regions and N30°–40°E in the north, closely following the trend of the orogenic belt in these areas. The transition region between north and central is characterized by δt≈1–1.3 s and by E-W trending ϕ that are at a high angle to the regional geologic trend. Measurements at two stations located in the eastern (internal) part of the belt indicate very weak anisotropy. The large-scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate. Splitting along the southern and eastern margins of the continent is consistent with that expected for Grenvillian deformation, an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out. Within the cratonic core, the correlation between δt and lithospheric thickness suggests a lithospheric anisotropy. Smaller-length-scale variations also argue for a significant contribution of lithospheric structures. The fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern United States, i.e., the Grenville and Appalachian orogenies and the subsequent rifting of the North Atlantic Ocean. Our observations in the western Appalachians suggest that the anisotropy may be preserved since the Grenvillian orogeny. The absence of detectable splitting in the two stations in the eastern Appalachians is attributed to the igneous intrusions related to the Atlantic rifting. The measurements in the transition between the northern and central southern Appalachians, constitute an intriguing anomaly, whose E-W ϕ have little obvious relation to the regional surface geology. We suggest two possible causes: (1) the local dominance of asthenospheric flow, motivated by the proximity of a pervasive low-velocity anomaly and (2) lithospheric deformation in a transcontinental strike-slip fault zone active during the Appalachian collision.
ISSN: 2169-9313

hal-01388859
http://hal.univ-reunion.fr/hal-01388859
http://hal.univ-reunion.fr/hal-01388859/document
http://hal.univ-reunion.fr/hal-01388859/file/Barruol_JGR_1997.pdf
DOI : 10.1029/96JB03800

Éditeur(s) : HAL CCSD
Résumé : Poster
The Lesser Caucasus in Armenia is an active volcanic and tectonic zone which resulted from the collision of the Arabian and the Eurasian plates since Neogene times. The During Quaternary, Lesser Caucasus was uplifted (0.3 mm/yr; Mitchell and Westaway, 1999) and experienced extensional tectonics times. Large lakes developed in graben structures. The diatomitic sequences of the Shamb paleo-lake (South Armenia) offer a rare opportunity to give new insights of Western Asia paleo-climate. Based on macroflora analysis, Bruch and Gabrielyan (2002) proposed a cooling and drying general climate trend through Pleistocene times in relation with a general uplift of the chain. Several questions have to be answer for this poorly investigated region. Did the climate record humid glacials and arid interglacials as suggested northward in Kazakhstan? What are the vegetation and climate responses to orbital parameters and to the monsoon? Moreover the lesser Caucasus is known as the entrance way used by the first hominids in Eurasia during Pleistocene time. How was the environment at this time? We present an integrated palynological, 40Ar/39Ar isotopic and magnetostratigraphic study for the most complete section (Joannin et al., in press). 40Ar/39Ar dating of two volcaniclastic layers provided ages of 1.24 ± 0.03 and 1.16 ± 0.02 Ma (2. Magnetostratigraphic data show that the entire Shamb section is of reversed polarity which correlates with part of the Matuyama period (1.785-1.070 Ma). Pollen assemblages and macroremains diversity revealed an alternation of glacial and interglacial phases that are compared with climate changes inferred from the global isotopic curve. The Shamb section ranges from approximately 1.300 to 1.080 Ma in age (marine isotopic stages 40 to 31). The vegetation of the Lesser Caucasus developed in a mosaic pattern in a Pleistocene continental, mostly arid climate, similar to the present-day climate. The vegetation changes record a dominant climate response to the obliquity orbital parameter. The influence of precession could not be established from the Shamb data. Pollen and macroflora both indicate that glacial periods were cold and dry and that interglacials were warm with local humidity. The early Pleistocene climatic model for Western Asia is thus similar to the climatic model for the Mediterranean area. Bruch, A., Gabrielyan, I.G., 2002. Quantitative data of the Neogene climatic development in Armenia and Nakhichevan. Acta Universitatis Carolinae – Geologica 46, 41–48. Joannin, S., Cornée, J.J., Münch, P., Fornari, M., Vasiliev, J., Krijgsman, W., Nahapetyan, S., Gabrielyan, Y., Ollivier, V., Roiron, P., Chataîgnier, C. In press. Early Pleistocene climatic cycles in continental deposits of the Lesser Caucasus of Armenia inferred from palynology, magnetostratigraphy, and 40Ar/39Ar dating. Earth and Planetary Science Letters. Mitchell, J., Westaway, R., 1999. Chronology of Neogene and Quaternary uplift and magmatism in the Caucasus: constraints from K-Ar dating of volcanism in Armenia. Tectonophysics 304, 157–186.
EGU Eurasian Climate and Tectonics
Vienne, Austria

hal-00481799
https://hal.archives-ouvertes.fr/hal-00481799

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
P>Most fault-related pseudotachylytes in the Val Sesia area, Ivrea zone, NW Italian Alps, occur in two areas: near the Canavese Line in the Kinzigite Formation paragneisses and in an elongate belt in the Mafic Complex gabbros away from the Canavese Line. Ar-40/Ar-39 dating indicates that pseudotachylytes from the Kinzigite Formation were formed in the late Eocene, likely during the early stages of the collision between the NW corner of the Adriatic indenter and the European margin. Pseudotachylytes from the Mafic Complex were formed under a transpressive strain regime, as suggested by kinematic indicators, and yield Early Cretaceous (Albian) Ar-40/Ar-39 ages. Their formation is possibly related to shortening during the inception of the tectonic inversion of the Adriatic passive margin.
ISSN: 0954-4879

hal-00617518
https://hal.archives-ouvertes.fr/hal-00617518
DOI : 10.1111/j.1365-3121.2011.00994.x