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International audience; This work first presents field structural analysis, anisotropy of magnetic susceptibility (AMS) measurements, and kinematic and microstructural studies on the Neoproterozoic Pengguan complex located in the middle segment of the Longmenshan thrust belt (LMTB), NE Tibet. These investigations indicate that the Pengguan complex is a heterogeneous unit with a ductilely deformed NW domain and an undeformed SE domain, rather than a single homogeneous body as previously thought. The NW part of the Pengguan complex is constrained by top-to-the-NW shearing along its NW boundary and top-to-theSE shearing along its SE boundary, where it imbricates and overrides the SE domain. Two orogen-perpendicular gravity models not only support the imbricated shape of the Pengguan complex but also reveal an imbrication of high-density material hidden below the Paleozoic rocks on the west of the LMTB. Regionally, this suggests a basement-slice-imbricated structure that developed along the margin of the Yangtze Block, as shown by the regional gravity anomaly map, together with the published nearby seismic profile and the distribution of orogen-parallel Neoproterozoic complexes. Integrating the previously published ages of the NW normal faulting and of the SE directed thrusting, the locally fast exhumation rate, and the lithological characteristics of the sediments in the LMTB front, we interpret the basement-slice-imbricated structure as the result of southeastward thrusting of the basement slices during the Late Jurassic-Early Cretaceous. This architecture makes a significant contribution to the crustal thickening of the LMTB during the Mesozoic, and therefore, the Cenozoic thickening of the Longmenshan belt might be less important than often suggested.

International audience

International audience; Detrital zircons from Cretaceous micaschist, Late Eocene-Earliest Oligocene sandstone and Early Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian (Neoproterozoic), Ordovician and Variscan (Carboniferous) events widespread in Western and Central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A distinguishable group of Triassic zircons, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the Early Oligocene "Schistes à Blocs" formation and the Brianconnais “Flysch Noir”. In contrast, this age group is absent in Late Eocene-Earliest Oligocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene sandstone were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in Early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units. Based on previous studies and our new data, we argue that the the Brianconnais Zone was likely a paleorelief since the Middle Eocene that accounts for the lack of detritus from the Adriatic units. Contemporary sediments were accumulated in the foredeep of the Adriatic Plate. From Oligocene time onwards, the blockage was cut through after a regional uplifting, and thus the Internal Zone started to provide detritus into the western flexural basins.

International audience; The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite-frequency kernel method, we present a new high-resolution tomography model using P wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region and fundamental pinpoints for the analysis of Cenozoic magmatism, (U)HP metamorphism, and Alpine topography. Our results document the lateral continuity of the European slab from the Western Alps to the central Alps, and the downdip slab continuity beneath the central Alps, ruling out the hypothesis of slab break off to explain Cenozoic Alpine magmatism. A low-velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab break offs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered.

Extensive organic-matter (OM) rich facies (black shales) occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections (Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province) straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon (TOC), pyrite sulphur, and different species of Fe, including dithionite-extractable Fe (FeD), pyrite Fe (FeP), HCI-extractable Fe (FeH), and total Fe (FeT), in black shales, as well as other redox proxies, such as the S/C ratio, the ratio between highly reactive Fe (FeHR = FeD + FeP) and FeT, and the Fep/(FeP + FeH) ratio, known as the degree of pyritization (DOP). In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios (0.20-0.77; avg. 0.45), high DOP values (0.21-0.72; avg. 0.54), and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low FeHR/FeT ratios (0.10-0.35; avg. 0.21), low DOP values (0.11-0.40; avg. 0.28), and S/C values are clustering on the normal marine value (S/C = 0.36). The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios (0.32-0.49; avg. 0.41), high DOP values (0.46-0.68; avg. 0.53) and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post-glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios (0.00-0.08, avg. 0.02), low FeHR/FeT ratios (0.10-0.37, avg. 0.27), low DOP values (0.01-0.27, avg. 0.10), and TOC contents (0.72%-4.27%, avg. 2.55%). Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high (0.94-9.32%, avg. 4.44%), but low (0.35%-2.12%, avg. 1.29%) in the mid-early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.

Exhumation of deeply buried ultrahigh-pressure (UHP) terranes remains a puzzle in understanding lithospheric dynamic process. New evidence that constrains the exhumation process of the Dabie UHP terrane includes the following. (1) Structural data record the architecture of the Dabie UHP terrane. Three slices are recognized—i.e., from top to bottom, the Susong blueschist slice, the Huangzhen-Huangweihe (HH) HP slice, and the Jinheqiao-Shuanghe-Bixiling (JSB) UHP slice. These slices have been stacked and domed. (2) Geophysical data depict two offsets in the Moho. Offset-I separates the Dabie UHP terrane from the North China craton, and has served as a subduction channel, as well as an exhumation channel. Offset-II, separating the Dabie UHP terrane from the foreland belt, developed due to geodynamic regime reorganization. (3) Geochronological data reveal that the HP slice was exhumed earlier that the UHP slice. A northward-younging exhumation polarity is implied, with younger exhumation age closer to the exh...

International audience; Fold-and-thrust belts are hot topics in the research of orogens. However, the single geological or geophysical discipline often provides multisolutions on the geometry of a fold-and-thrust belt. We introduce here a multidisciplinary method combining geological and geophysical methods, take the Hutubi River section as a case to study the geometry of the fold-and-thrust belt in the northern piedmont of Tianshan. Firstly, we have carefully taken geological survey and got structural data on the surface. Secondly, petroleum seismic profile was re-interpreted based on the surface data and drilled wells. However, the seismic profile is absent in the contact zone between the mountain and the Junggar basin. We therefore carried out gravity measurements and forward modeling along this profile with the densities of the basement and sediments. The result shows that the northern flank fault of Tianshan doesn't exist along the Hutubi River profile, and the basin sediments could be continually followed from the basin to the mountain interior. This indicates that the tectonic structures in the contact zones between the mountain and the basin are variable, as the western section in the Jingou River section shows that the Tianshan basement thrusts northward on the basin sediments. Based on the balance-section technology, the restored section shows 4. 8 km of shortening. Comparing with previous results, this indicates the heterogeneous deformation along the northern piedmont of Tianshan. It also implies that this multidisciplinary method could be widely used in the fold-and-thrust belt.

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic melange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are...