Numerical Constraints on Folding and Thrusting in Jiudong Basin: Implication for the Northeastward Growth of Qinghai–Tibetan Plateau

Under regional tectonic shortening in the northern margin of Qinghai–Tibetan Plateau, the fold-and-thrust belts composed of four thrust faults (North Qilian-Shan, North Yumu-Shan, South Heli-Shan, and North Jintanan-Shan) formed from southwest to northeast discontinuously sequentially in the Jiudong Basin area during Late Cenozoic. Meanwhile, the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges were formed successively, as the Earth's local surface was unequally uplifted. In this study, based on geological and geophysical observations, a simple two-dimensional elastic-plastic numerical finite element method model for a southwest-northeast section in Jiudong Basin is successfully established to simulate the spatiotemporal evolution of the local fold-and-thrust belts. Results show that the computed equivalent plastic strain concentration zones and the four observed thrust faults are consistently correlated in spatial position orientation and time sequence. The simulated upper-surface deformation is congruent with the observed topographic peaks and uplift sequences of the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges. This study provides a geodynamic basis for understanding the growth mechanism of the northern margin of Qinghai–Tibetan Plateau under tectonic horizontal shortening. Also, we provide a thorough sensitivity analysis for the model parameters of this particular geologic setting. Our sensitivity simulations, considering systematic case variations about the regional geometrical-material parameters, suggest the manifestation of three different possible evolution patterns of fold-and-thrust belts for a wedge above a decollement layer, with wedge plastic deformation migrating from 1) thick to thin end (well-known), 2) thin to thick end, and 3) both ends to middle. Finally, our results suggest that in this region, further growth of mountain ranges is expected to continue in the future.

Pre-folding fracturing in a foredeep environment: insights from the Carseolani Mountains (central Apennines, Italy)

Bedding-perpendicular joints striking parallel (longitudinal) and perpendicular (transverse) to both the axis of the hosting anticline and the trend of the foredeep-belt system are widely recognized in fold-and-thrust belts. Their occurrence has been commonly attributed to folding-related processes, such as syn-folding outer-arc extension, although they can also be consistent with a pre-folding foredeep-related fracturing stage. Here we report the pre-folding fracture pattern affecting the Pietrasecca Anticline, in the central Apennines (Italy), resolved by a detailed field structural analysis. Field observations, scan-lines and interpretation of virtual outcrops were used to study the intensity, distribution and the orientations of fracture pattern along the anticline. The fracture pattern of the Pietrasecca Anticline consists of longitudinal and transverse joints, oriented approximately perpendicular to bedding, and of a pre-folding longitudinal pressure-solution cleavage set, which is oblique to bedding regardless of the bedding dip. Cross-cutting relationships show that joints predated the development of the pressure-solution cleavage. Furthermore, joint intensity does not relate to the structural position along the anticline. Taken together, these observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth. Our work further demonstrates that joints striking parallel and orthogonal to the main fold axis do not necessarily represent syn-folding deformation structures.

Impact of mechanical stratification on the structural style of the Lublin Basin, SE Poland: results of seismic interpretation and implications for quantification of deformation within the frontal parts of thin-skinned fold-and-thrust belts

We demonstrate how lithological and mechanical stratification of Ediacaran–Carboniferous sedimentary package governs strain partitioning in the Lublin Basin (LB) which was incorporated in the marginal portion of the Variscan fold-and-thrust belt. Based on the geometry of seismic reflectors, the pre-Permian–Mesozoic sedimentary sequence was subdivided into two structural complexes differing in structural style. The lower one reveals forelandward-vergent imbrication, while the upper one comprises fold train, second-order deformations, and multiple local detachments. Lithological composition of the upper structural complex controlled geometry, kinematics, and position of compressional deformations in stratigraphic profile. System of foreland-vergent thrusts which links lower and upper detachment developed due to efficiency of simple shear operating in heterogeneous clastic-carbonate-evaporitic strata of the Lower–Upper Devonian age. Internal homogeneity promoted the formation of conjugate sets of thrusts in Silurian shales and Upper Devonian limestones. Structural seismic interpretation combined with sequential restoration revealed localised thickening of Devonian strata and up to 5% difference in length of Devonian horizons. This mismatch is interpreted as a manifestation of distributed shortening, including layer-parallel shortening (LPS), which operated before or synchronously to the initiation of folding. The amount of distributed strain is comparable with numbers obtained in external parts of other fold-and-thrust belts. The outcomes derived from this study may act as a benchmark for studying variability in a structural style of multilayered sequences which were incorporated in the external portion of other fold-and-thrust belts.

Basin evolution in response to flat subduction in the Altiplano

This contribution assesses models for basin formation in the Altiplano. New magnetostratigraphy, palynology, and 40Ar/39Ar and U-Pb geochronology from the central Corque Syncline demonstrate that the 7.4 km-thick section was deposited between 36.7 and 18.7 Ma. The base of the section post-dates exhumation in both the Western and Eastern cordilleras, precluding deposition in a classic retroarc foreland basin setting. Rotated paleomagnetic vectors indicate counterclockwise rotation of 0.8°/Myr since the early Oligocene. Detrital zircon provenance data confirm previous interpretations of Eocene–early Oligocene derivation from the Western Cordillera and a subsequent switch to an Eastern Cordilleran source. Flexural modeling indicates that loads consistent with paleoelevation estimates cannot account for all subsidence. Rather, the timing and magnitude of subsidence is consistent with Eocene emplacement and Oligocene–early Miocene resteepening of a flat slab. Integration of the magmatic, basin, and deformation history provides a coherent model of the effects of flat subduction on the overriding plate. In this model flat subduction controlled basin formation in the upper plate, with subsidence enhanced in front of the zone of flat subduction but reduced over the crest of the flat slab. We conclude that the Altiplano was conditioned for plateau formation by Eocene–Oligocene flat subduction.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-beltsSupplementary material:https://doi.org/10.6084/m9.figshare.c.5664345

Unconformity development in retroarc foreland basins: Implications for the geodynamics of Andean-type margins

Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, adjacent fold-thrust belt, and broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to nondepositional or erosional conditions in which the interruption of subsidence precludes net sediment accumulation. This study explores the genesis of long duration (>1–20 Myr) unconformities and condensed stratigraphic sections by considering modern and ancient examples from the Andes. These cases highlight potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (a) shortening-induced uplift in the frontal thrust belt and proximal foreland; (b) growth and advance of a broad, low-relief flexural forebulge; (c) uplift of intraforeland basement blocks; (d) tectonic quiescence with regional isostatic rebound; (e) cessation of thrust loading and flexural subsidence during oblique convergence; (f) diminished accommodation or sediment supply due to changes in sea level, climate, erosion, or transport; (g) basinwide uplift during flat-slab subduction; and (h) dynamic uplift associated with slab window formation, slab breakoff, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, paleoenvironmental conditions, and duration of unconformities and condensed sections.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Gravitational sliding or tectonic thrusting?: Examples and field recognition in the Miura-Boso subduction zone prism

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.