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A new type of structural association comprising periclinal folds is present in thick-bedded calcareous mudstones and siltstones of the Early Cretaceous Hekou Group in the Lanzhou Basin, NW China. The periclinal fold system consists of meso-scale, open anticlines and synclines with fold axial trends in two perpendicular orientations (NE-SW and NW-SE) and two sets of conjugate thrust faults that form intersections in the same two orientations. Folded and faulted layers of the periclinal fold system are not sedimentary bedding surfaces but sub-parallel curviplanar foliations formed by compaction of the original strata soon after deposition. Many folded curviplanar foliations connect with the unfolded, conjugate thrust faults, indicating that faulting was active during and/or soon after folding. The NE-SW and NW-SE-trending fold axes and fault planes indicate bilateral, subhorizontal contraction acting simultaneously during the formation of the periclinal fold system. In some cases, converging slickenlines on curviplanar foliation planes within anticline cores point to localized isotropic contraction probably caused by scaping fluids and increased hydrostatic pressure. Similarities with the better-known polygonal fault system include development in fine-grained sediments and layer boundness, whereas main differences are a dominance of contractional faults in the periclinal fold system versus extensional faults in the polygonal fault system and a higher consolidation of sediments needed for the development of the periclinal fold system. This study suggests that collision between the Lhasa and Qiangtang Blocks combined with oblique convergence and subsequent collision along the southeastern margin of the Eurasian plate following compaction-related volume loss of fine-grained sediments led to the formation of the periclinal fold system in the Hekou Group in the Late Cretaceous. • A type of structural association named as "Periclinal Fold System" is first reported. • The Periclinal Fold System develops in thick-bedded mudstones and siltstones. • Compaction fabrics are folded in the Periclinal Fold System, but not bedding planes. • The Periclinal Fold System is formed in the constrictional setting. [ABSTRACT FROM AUTHOR]

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic‐early Mesozoic deformation events have been distinguished: (1) nearly north‐south‐striking ductile thrusting along the eastern Alxa Block in the Late Devonian; (2) nearly east‐west‐trending brittle top‐to‐the‐south thrusting in the Mid‐Late Permian; (3) nearly east‐west‐trending initially ductile then brittle dextral transtension with 30–40 km of displacement in the Late Permian; and (4) northeast‐trending ductile sinistral shearing with 120–125 km of displacement in the Mid‐Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri‐eastern Gondwana block. Parallel east‐west‐trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the Paleo‐Asian Ocean in the Mid‐Late Permian. The Late Permian east‐west‐trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west; this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt (CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far‐field tectonic effects related to plate margin tectonic activities (e.g., the closure of the Mongolian‐Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India‐Eurasia collision). The large‐scale brittle and ductile strike‐slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic‐early Mesozoic deformation events have been distinguished: (1) nearly north‐south‐striking ductile thrusting along the eastern Alxa Block in the Late Devonian; (2) nearly east‐west‐trending brittle top‐to‐the‐south thrusting in the Mid‐Late Permian; (3) nearly east‐west‐trending initially ductile then brittle dextral transtension with 30–40 km of displacement in the Late Permian; and (4) northeast‐trending ductile sinistral shearing with 120–125 km of displacement in the Mid‐Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri‐eastern Gondwana block. Parallel east‐west‐trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the Paleo‐Asian Ocean in the Mid‐Late Permian. The Late Permian east‐west‐trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west; this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt (CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far‐field tectonic effects related to plate margin tectonic activities (e.g., the closure of the Mongolian‐Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India‐Eurasia collision). The large‐scale brittle and ductile strike‐slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]