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Mainland France is part of a plate interior with a strong structural heritage, undergoing a low rate of deformation, where destructive earthquakes can nevertheless occur. In this paper, we emphasize that the knowledge of active faults is still largely fragmentary, and that significant efforts are needed to generate robust data, in particular on the numerous faults, that still lack any study. This is the aim of the “Failles ACTives France” (FACT) axis launched in the framework of the Transverse Seismicity Action (ATS) of the Resif-Epos consortium. We present some recent investigations carried out along suspected active faults in mountainous areas, their forelands and remote lowlands, which implement new approaches and new tools, and allow characterizing their Quaternary activity.

In the rejuvenated mountain front, preexisting basement structures are often reactivated and interact with the subsequent thin‐skinned deformation. How the deep structures affect the shallower ones is key to establishing the processes and mechanisms for the foreland fold‐and‐thrust system. We presented an exceptional case study on the structural inheritance between the deep Mesozoic strike‐slip faults and the shallow Cenozoic contractional folds from the Northern Tianshan foreland basin, Northwest China, using high‐resolution 2‐D and 3‐D seismic data. Based on the interpretation of seismic data and progressive restoration, our study illustrated the NW‐trending Ai‐Ka strike‐slip faults controlled a dextral shear zone, which initiated the Gaoquan restraining bend in the basement during Jurassic. Later, these strike‐slip structures, close to the mountain front, were reactivated during the N‐S Mio‐Pliocene contraction, and folded the upper décollements that characterized the localization of thin‐skinned deformation. In contrast, in the further foreland, nonreactive strike‐slip faults controlled basal décollement pinch‐out, which localizes the thin‐skinned deformation, resulting in en échelon folds that trace the strike of the deep strike‐slip faults. The onset time of each anticline shows that the thin‐skinned deformation first extended laterally and then propagated further north, resulting in ca. 7 km shortening along the whole foreland. Moreover, the shortening rate decreased eastward from 0.90 to 1.46 mm/yr along the Gaoquan‐Kayindike structural line to 0.24–0.37 mm/yr along the Dunan structural line as the Sikeshu depression, constrained by the NW‐trending Ai‐Ka strike‐slip fault, narrowed eastward. This feature implies that the width of the depression may control the amount of displacement propagation. Key Points: The geometry and kinematics of Mesozoic transpressional and transtensional structures were documented using 3‐D seismic dataThe Mesozoic strike‐slip faults affected the localization of Cenozoic thin‐skinned folding by creating pinch‐out or folds in the décollementCenozoic forelandward folding consumed 7 km S‐N shortening. Shortening rate decreased from 0.90–1.46 to 0.24–0.37 mm/yr eastward [ABSTRACT FROM AUTHOR]

We conducted a series of analogue experiments on shortening of a brittle cover (dry sand) above a deep, thin, frictional detachment (glass microbeads). In some experiments, the cover was homogeneous, entirely brittle. In others, there was a thin viscous silicone layer (representing salt) embedded at mid height into the cover, and initially located in the foreland of the fold-and-thrust belt. Our goal was to determine whether or not the presence of such a décollement in the cover could have an impact on the mechanics and kinematics of the underlying subsalt thrusts. Results confirm that, once the front of the foldbelt reached the hinterland salt pinch out, the kinematics of the deeper belt changed drastically: its front stopped propagating forward, and most of the subsequent shortening was accommodated by a larger-than-normal slip along the foremost and youngest deep thrust, while, above the salt décollement, the deformation front propagated very fast, creating a very low surface slope. We hypothesize that it is the gentle sub-critical surface slope associated with rocksalt’s low viscosity that prevents the build-up of an overall surface slope steep enough to allow the underlying, deep foldbelt to continue propagating forward. Finally, one experiment in which only one half of the width of the model comprised an interbedded viscous décollement has shown that the kinematics of the deep thrust was affected even in the adjacent salt-free domain.

[1] The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ~1 Ma and ~23.6 Ma, with some uncertainty below ~21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ~16-15 Ma and ~11-10 Ma. Rock magnetic parameters also undergo important changes at ~16-15 Ma and ~11-10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ~16-15 Ma and ~11-10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (