The geometry of Laramide foreland uplifts near the basement-cover interface provides a critical test of models for basement-involved foreland structures. Limits and controls on basement folding are well defined by deflections of the basement-cover unconformity in the northeastern Front Range of Colorado, Rattlesnake Mountain anticline of northern Wyoming, and northern Teton Range of northwestern Wyoming. Where basement overlies sedimentary strata on a single reverse fault, basement folding is largely limited to the area within 1 km of the fault. In this area, distributed high-angle faulting bends the unconformity in the hanging-wall basement tip up to 20° toward the fault. Folding of the hanging-wall tip increases with progressive fault slip and decreased fault dip. Smaller structures commonly show little basement folding, contradicting the fold then fault sequence predicted by the fold-thrust structural model. The footwall basement is either undeformed or bent downward by hanging-wall loading, with upward folding limited to the fault’s narrow zone of cataclastic flow. Where fault splays break the basement into one or more fault-bounded wedges, overlying strata form more open folds without necessitating penetrative basement folding. The hanging-wall basement commonly shows little folding because the weak basement tip, defined by the main fault and the hanging-wall unconformity, is often left behind in the footwall as a subthrust wedge, filling the gap between the flexed sedimentary strata and the footwall basement block. The downward-narrowing zones of deformation with their variably curved fold surfaces and heterogeneous strain are consistent with fault-propagation folding by distributed shear in a triangular shear zone (trishear). Heterogeneous trishear models predict either extensive basement folding in the hanging wall (footwall-fixed trishear) or no basement folding in the hanging wall (hanging-wall–fixed trishear). These trishear modes form a continuum of basement behavior, with the strength of the hanging-wall basement tip controlling the shear-zone location and the extent of basement deformation. Structures with lower angle thrusts show more basement folding because their basement tips are narrowly tapered and weak, fixing the triangular shear zones with respect to the footwall. Structures with higher angle reverse faults show less basement folding because their basement tips are blunt and stronger, fixing the triangular shear zones with respect to the hanging wall.

Abstract Trishear is a kinematic model of fault-propagation folding in which the decrease in displacement along the fault is accommodated by deformation in a triangular shear zone radiating from the tip line. This model has garnered increasing acceptance, particularly for cases where parallel kink-fold models do not work (e.g., footwall synclines, lateral and vertical changes in bedding thickness, and orientation). The articulation of the model in terms of velocity fields has enabled systematic explorations of the parameters controlling the trishear geometry; rapid, objective application of trishear to the simulation of real structures; and application of the model in three dimensions. The model has highlighted the importance of a parameter not unique to trishear, the propagation to slip ratio, which has a profound effect on fold geometry and is fundamental to understanding all types of fault-related folds. The drive to understand the significance of such parameters has instigated the application of several mechanical modeling strategies. Block-motion viscous, finite-element, and discrete-element analyses have all provided insight into trishearlike fault-propagation folds. Clearly, from these models, trishear most successfully simulates fold geometries where significant layered anisotropy is absent and the material is incompressible. Despite these modeling efforts, the significance of the trishear apical angle remains elusive. Trishear has been applied to a variety of real-world problems, including growth strata analysis, potential fracture distribution, paleoseismology, and even seismic hazard analysis.

Abstract Mostly the Palaeozoic and Mesozoic basins of North Africa have generally followed, and reworked, earlier basement trends formed by: (1) the NW-SE accretion of continental and oceanic terranes onto a Pan-African nucleus in northeastern Africa, and (2) the collision of this amalgam of accretionary terranes with the West African Craton. During the Upper Precambrian Pan-African Orogeny, the West African Craton formed a rigid block which indented this amalgam of accreted mobile belts to form much of North Africa. Intrusion of this indentor into North Africa caused the expulsion of narrow, triangular-shaped blocks of lithosphere to the north and south in a tectonic style very similar to the Miocene-Pliocene deformation of Tibet. Expulsion reactivated the earlier shear zones to form an anastomosing pattern of steeply dipping shears with left and right lateral sense of displacement. Left lateral shear also affected the northern edge of the West African Craton during this process of indentation. Subsequent rifting of the Pan-African mountain belt resulted in a series of grabens, which were infilled with Upper Precambrian-Cambrian molasse. These are the precursor basins for the Palaeozoic sediments which cover much of North Africa. The effects of rifting continued into the Cambro-Ordovician in the western basins. During the Silurian-Devonian many of the rifts were reworked. A new basin formed in the Atlas and Anti-Atlas, related to the growth of the proto-Tethyan Ocean. Basin inversion characterizes the Palaeozoic structures of the western Atlas and Anti-Atlas, producing thickened crust and a large mountain belt during the Carboniferous. Foreland basins formed on either side of this mountain belt and both the mountains and the adjacent basins were compartmentalized by WNW-ESE-trending transfer zones. Pan-African structures, within the African Plate, were reworked with further indentation of the West African Craton into Pan-African crust. The craton was pushed eastward, generating a left lateral shear couple along its northern margin. NW–SE-trending faults were reworked as dominantly left lateral strike-slip faults and N-S-trending fault blocks were rotated slightly in a clockwise sense. There was probably further lateral expulsion of lithosphere, ahead of the NE–SW-trending front of the indentor, reworking earlier N-S-trending shear zones. The North African Palaeozoic basins were inverted during the Hercynian-Appalachian Orogeny. In the Ahnet Basin the shortening was approximately NNE-SSW, perpendicular to the trend of the structures. This inversion was particularly marked in the Ougarta-Ahnet Basin where it produced a series of open to closed, north-south to NW-SE-trending folds above reactivated basement faults. During the Mesozoic, the Hercynian-Appalachian mountain belt underwent extension to produce deep rift basins infilled with continental sediments and some volcanics. The High Adas formed as an arm to the Atlantic Basin. Transfer zones have a WNW-ESE trend, indicating that this was the main extension direction, similar to that in western and southwestern Europe. In northeastern Algeria, the orientation of the Mesozoic grabens suggests reworking of the basement fabric formed by Pan-African accretionary tectonics. The structures appear to die out toward the southwest into a broad transfer zone with some NW-SE-trending faults. The northeastern edge of the basin is obscured by later rift basins in the eastern Mediterranean. The Palaeozoic faults of the Amguid Spur, overlying one of the major shear zones of the Hoggar, formed a structural high throughout the Mesozoic with probably several pulses of inversion. An important episode of inversion occurred during the Aptian-Albian with the development of anticlines and associated reverse faults. Crustal extension associated with block faulting occurred in the Sirte Basin of Libya during the Mid- and Late Cretaceous. The block faults trend NNW-SSE to NW-SE, cross-cutting earlier Palaeozoic fold structures at a high angle but possibly parallel to some of the basement shear zones. The faults form the tips of a rift basin which opened between Sicily and Tunisia in the central Mediterranean. The Cretaceous faults have a component of right lateral displacement as well as normal fault movements. The Mesozoic basins of the High and Middle Atlas were inverted during the Late Cretaceous-Early Oligocene. The displacement direction, as seen from the transfer systems, was NW-SE, almost perpendicular to the Middle Atlas, but at a lower angle with the High and Sahara Atlas, which must have had components of oblique or right lateral movement. Minor effects of this inversion are reported from the Saharan basins.