ABSTRACT The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.

Abstract The Diapir Fold Zone of the Carpathians is the most prolific onshore hydrocarbon area in Romania. Structural complexity, mainly due to the presence of salt, combined with poor seismic quality near and below the salt lead to contrasting structural models in the area. To gain insights into the mid-Miocene tectonic evolution, structural geometries and the effects of penetrative strain, we ran dual décollement scaled sandbox models with layered brittle and ductile materials. Results of two analogue models (20 and 33% shortening) revealed that the onset of the deformation sequence was mainly characterized by layer-parallel shortening. As shortening continued, a foreland-verging sequence of supra-salt detachment folds and sub-salt duplexes evolved. The sub-salt duplexes are located directly below the crests of the detachment folds, as the development of these large wavelength anticlines was related to sub-salt deformation. Salt flow was another controlling factor of the deformation style, as salt accumulated in the anticlinal cores and increased the coupling in the supra-salt synclinal axis. Our results offer insights into the effects of salt on the kinematic evolution of this area, help to predict geometries in areas of poor seismic quality, and highlight the important contribution of penetrative strain on deformation and reservoir quality.

Abstract A series of analogue models were run to investigate oblique inversion of pre-existing grabens when overprinted by later shortening and the effect of these grabens on development of contractional structures. Obliquity angle ( α ) defining the initial trend of pre-existing grabens relative to the shortening direction, was systematically changed from 0°, 10°, 20°, 30°, 40°, 50°, 65° and 90°. Different structural styles are shown in different models and also in sections cutting across different parts of the models. Model results show that existence of multi-grabens enhances lateral discontinuity of overprinted thrusts in map view. With increasing the obliquity angle, more and longer lateral ramps developed sub-parallel to the graben trends. The pre-existing grabens were apparently rotated from their initial trends during shortening. Some of the normal faults bounding the grabens were partially inverted and resulted in bulging of the syn- and post-rift graben fill sediments. Most normal faults were displaced and rotated by thrusting, and provided relatively weak zones for propagation of thrusts. By comparing with observations from Qingxi graben in western China and from the SW Taiwan fold-and-thrust belt, where oblique inversion occurred, model results can be used to interpret unclear relationships between thrusts and pre-existing extensional structures during superimposed deformation.

Abstract: Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation. This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers. The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).