Detailed field studies of 12 small basement-cored folds in the Rocky Mountain foreland from southwestern Montana to northern New Mexico indicate that there was considerable variation in the degree of deformation of the basement rocks during Late Cretaceous and Paleocene folding. This variation may be characterized by two end-member styles of basement behavior (mode 1 and mode 2). In mode 1 folds basement deformation is confined to a narrow zone of cataclasis adjacent to a single fault, the cover rocks are significantly thinned on the forelimb of the fold and have a small carbonate to clastic rock ratio (<0.2) in the lower 300 m (1000 ft) of section, the basement-cover contact on the forelimb is a fault, and the interlimb angle is 60° or less. In mode 2 folds basement deformation occurs in a broad zone between the principal fault and the anticlinal hinge surface, which is a fault in several structures. The basement deformation occurs as slip on sets of closely-spaced fractures, as flexural slip on preexisting foliation oriented subparallel to bedding, as axial surface-parallel slip on foliation, or as pervasive cataclasis. The cover rocks in mode 2 structures maintain nearly constant thickness through the fold, have a carbonate to clastic rock ratio that is relatively high in the lower 300 m (1000 ft) of section (>0.4), and are in stratigraphic (as opposed to fault) contact with the basement on the forelimb. The axial surface penetrates the basement, the interlimb angle is >90°, and backthrusts are common. Most existing folds will have characteristics of mode 1 and mode 2 to varying degrees; for example, a basement-cover interface that is part fault and part stratigraphic contact on the forelimb, an intermediate interlimb angle (60°–90°), moderate thinning of cover rocks on the forelimb, and deformed forelimb basement with an intermediate thickness. The style of basement-cored folds depends partly on the nature and orientation of prefolding basement fabric and the competence of the cover rocks. Well-foliated basement rocks that have foliation oriented subparallel to bedding or that have foliation in a “favorable” orientation for hinge-surface-parallel slip produce mode 2 folds, as do cover-rock sections with high carbonate to clastic rock ratios. Relatively isotropic basement rocks with low carbonate to clastic ratios produce mode 1 folds. Other factors that probably control the style are degree of influence of earlier faulting and the taper of the hanging-wall basement wedge; however, observations of the 12 folds in this study are inconclusive regarding the importance of these factors. Confining pressure and temperature are important only insofar as they determine the overall mechanical behavior of the basement and the cover rocks. Total variation in overburden (2.5–5 km) during initial deformation has not permitted basement behavior to deviate from the brittle field. In progressive deformation of mode 1 structures, a relatively competent basement block is forced into relatively incompetent cover, resulting in no significant basement deformation. In mode 2 structures a relatively incompetent basement block is forced against a relatively competent cover. The basement deforms by generation of an anticlinal hinge surface that migrates away from the fault. Faults can propagate into the cover along the synclinal hinge, across the forelimb, or along the anticlinal hinge surface. In the latter two cases fault-dip changes can produce backthrusts.

The distribution of Laramide strain in the Precambrian basement rocks of four small Rocky Mountain foreland folds was controlled by lithologies and orientations of preexisting foliation in the faulted forelimbs. Features of brittle deformation that developed in the basement were faults, sets of parallel, conjugate, or anastomosing fractures, zones of penetrative grain cracks and intergrain slip without grain-size reduction, and local zones of cataclasis or incipient mylonitization. In the London Hills anticline of Montana, foliation in amphibolite and gneiss was nearly parallel to bedding in cover rocks prior to folding. The foliation in the forelimb was rotated and deformed by layer-parallel slip between a forelimb thrust and a fault in a diabase dike located at the hinge zone of the anticline. In the Sheephead Mountain anticline of Wyoming, the forelimb fault cut foliation in quartzofeldspathic gneiss at a high angle. Penetrative brittle deformation occurred in the forelimb as a wide zone of fractures parallel to a forelimb thrust at the basement-cover contact, but the basement rocks were not folded. In the Gnat Hollow anticline-syncline, Colorado, foliation in interlayered granite, schist, and gneiss dipped about 20° more steeply than the forelimb thrust. Brittle deformation was confined to the fault zone and a small region in the core of the forelimb anticline. In the Romero Hills anticline, New Mexico, foliation in the basement was parallel to thrusts that cut both basement and cover rocks. Slip on foliation surfaces near the thrusts was pervasive and simple shear on foliation was distributed across the backlimb and forelimb of the structure. In comparing the four structures, preexisting foliation surfaces were most active during Laramide deformation where they paralleled forelimb faults, and were least active where foliation was at a high angle to forelimb faults or was not rotated into the forelimb orientation. If the angle between faults and foliation is about 15° or more, or if foliation is not nearly parallel to cover-rock layers, preexisting foliation appears to have exerted little influence on Laramide strain patterns in the foreland folds.

Three basement-cored anticlines in the southern Bridger Range and Canyon Mountain anticline and Squaw Creek fault in the northern Gallatin Range were analyzed by comparing foliations, mesoscopic faults, and slickenside lineations in basement rocks to Laramide structures in Phanerozoic cover strata. In regions where the pre-Laramide angle of discordance between Archean metamorphic foliation and bedding in Phanerozoic cover rocks was low, less than 25° (Bridger Range anticlines and at Canyon Mountain anticline), the basement was rotated congruently with cover strata during Laramide folding. This was accomplished by localized flexural slip on basement foliation surfaces. However, the more commonly observed style of deformation involved segmentation of the basement into internally undeformed, rigid, macrogranular (up to 12 m thick) domains that were displaced relative to one another along zones of cataclastic deformation. This style of basement deformation commonly occurred in regions where complex Archean fold patterns in the basement impeded foliation-parallel slip. At Squaw Creek, the angle of discordance between Phanerozoic cover strata and basement foliation was high (up to 80°) prior to Laramide folding. The absence of rotated foliation surfaces concordant with cover rocks and the presence of a planar basement-cover interface indicate a nonrotational basement response. Therefore, the structural attitude of Archean metamorphic foliation with respect to the basement-cover contact appears to have been a major factor in controlling the geometry of Laramide basement deformation in the cores of these anticlines. Congruent rotations of rigid, macrogranular basement blocks and cover rocks predominated where the angle of discordance was low (e.g., western anticline, Bridger Range), whereas a nonrotational response was observed where the angle of discordance was high (Squaw Creek).

The Scarface thrust of the western Madison Range, Montana, is a 17° west-dipping Late Cretaceous thrust that places Archean gneisses over a complexly folded panel of Phanerozoic sedimentary rocks. The Archean-Cambrian contact on the footwall of the Scarface thrust is nearly vertical, and both bedding in the cover and foliation in the gneisses near the contact were rotated by 38° during folding. Paleozoic rocks up section in the footwall are overturned, with an axial surface that dips less than 10° west. The Scarface thrust is locally folded over lower Paleozoic rocks on the footwall. Folding was produced by post-Scarface thrust movement on a minor east-dipping splay fault that follows bedding in Devonian rocks. Of the two dominant shear fracture and fault sets in the basement (strikes and dips of N52°W, 47°NE; N20°W, 50°SW), the northeast-dipping set is parallel to foliation and reflects a strong influence of foliation on basement deformation. Intergranular fractures nucleated at the tips of biotite grains. Narrow zones of cataclasis containing shredded biotite formed along the intergranular fractures. Advanced stages of deformation were accompanied by formation of thicker zones of wavy, foliated cataclasites defined by dark seams of comminuted biotite, feldspar, and quartz. The recumbent footwall syncline is superimposed on the west limb of a large, more open syncline in Paleozoic and Mesozoic rocks. We are unable to resolve which fold formed first. Faulting sequences are also equivocal. The Scarface thrust may have been emplaced as a shallowly dipping sheet, or it may have been steeper initially and rotated during movement on the structurally lower Beaver Creek thrust.

Understanding the kinematic development of basement-involved compressive structures that form at low temperature is dependent, in part, on gaining a better factual understanding of their deformational behavior. Results presented here show that the response of crystalline basement to deformation is incongruous among different structures in Colorado and Wyoming. At Big Thompson anticline and Rattlesnake Mountain anticline, Precambrian basement was not rotated in the anticlinal hinge during Tertiary folding. At both Banner Mountain and at a minor fold on Casper Mountain the basement has been rotated near the anticlinal hinge by as much as 26°. In the steep limb of all four of these monoclinal structures the basement is in fault contact with the stratified cover. At two sites, the Five Springs Creek area of the Bighorn Mountains and Casper Mountain, evidence for greater rotation of basement is clear. Precambrian dikes at Five Springs exhibit rotation of as much as 85° across the Laramide anticlinal hinge. Although the anticline at Casper Mountain shows minor folding, in the footwall a depositional contact on basement is rotated 50° from regional dip. These structures (Big Thompson anticline, Casper Mountain, and Five Springs) show minor fractures in the basement that apparently do not control the kinematics of basement deformation, but do indicate the stress field orientation. Typically σ 1 is nearly horizontal and normal to the fold axis, σ 2 is parallel to the fold axis, and σ 3 is very steep. This is the stress field normally associated with thrust faulting.

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.

The EA thrust system, northwestern Wyoming, consists of a southwest-directed, Precambrian-cored thrust sheet that formed above the northeast-dipping EA and North EA thrusts. The Precambrian-Cambrian contact is broadly folded, both at the leading edge of the Precambrian-cored portion of the system, and within the thrust sheet. The leading edge of the thrust sheet collapsed by slip along normal faults. Deformation of Precambrian rocks in the thrust sheet occurred by slip along northeast and steeply southwest–dipping faults. Deformation mechanisms in the faults include brittle fracture, cataclasis, and syntectonic alteration of feldspar to clays. Slip was concentrated in gouge zones and foliated cataclasites; relatively small amounts of slip occurred along narrow fracture surfaces. Faults in Precambrian rocks bound relatively intact, nondeformed blocks that range in size from 100 m to greater than 1 km. The combining of documented deformation mechanisms with kinematic restorations of cross sections shows that the broadly folded form resulted from slip along reverse faults within Precambrian rocks. The broad folds may mark the early stage of thrust-sheet formation, and subsequent slip on thrusts cut the limb of the Precambrian-cored fold to translate the thrust sheet southwestward over sedimentary rocks in the Wind River basin. Folded rocks may also have developed in the hanging wall of the EA thrusts, with simultaneous fold growth and thrust slip. Restorations also account for collapse of the thrust sheet and show that the primary reverse fault responsible for emplacement of the thrust sheet dipped 40° to 50° northeast, and the present change in dip of the fault is due mostly to hanging-wall collapse by slip along normal faults.

The Jakeys Fork fold and fault structure is a Precambrian granite-gneiss–cored anticline in the hanging wall of the steeply west dipping Ross Lakes shear zone. The Ross Lakes shear zone juxtaposes Precambrian rocks in its hanging wall and steeply dipping to overturned Cambrian and Ordovician sedimentary rocks in its footwall. Precambrian and Cambrian Flathead Sandstone in the hanging wall of the Ross Lakes shear zone are folded in an open, kinklike fold defined by gently and steeply dipping panels of Cambrian Flathead Sandstone. Precambrian rocks in the core of the hanging-wall anticline deformed by slip on a narrow fault in the hinge zone of the fold and by randomly oriented slip on narrow, discrete faults of many orientations on either side of the hinge zone. Cambrian Flathead Sandstone was welded to the Precambrian rocks during folding, and detachment occurred in the Cambrian Gros Ventre Formation. The Ross Lakes shear zone formed by brecciation and cataclasis. Precambrian rocks in the footwall of the Ross Lakes shear zone deformed by small amounts of slip on steeply west dipping faults, which resulted in a steepening of the contact between Precambrian rocks and the overlying Cambrian Flathead Sandstone. Faults in Precambrian rocks in the footwall of the Ross Lakes shear zone bound regions of undeformed Precambrian rock with widely spaced fracture sets. Cambrian Gros Ventre and younger formations are folded into a tight, overturned footwall syncline with its steep limb parallel to the Ross Lakes shear zone.

The Wind River Mountain Range is one of the most prominent Laramide uplifts of the Rocky Mountains deformed foreland. Precambrian rocks in the core of the Wind River anticline were deformed inhomogeneously along a three-dimensional network of brittle and brittle-ductile deformation zones, which are present at all scales, allowing shortening of the basement core by an amount equivalent to that of the Phanerozoic cover. Along the northeast flank of the anticline where the cover sequence was thin (< 5 km), the underlying basement was deformed under brittle conditions, forming an anastomosing network of brittle deformation zones. The Torrey Creek zone is one such large brittle zone that is well exposed. It probably grew into a major zone because of its location between two different lithologies. Shearing within the zone occurred by distributed slip on numerous gently-dipping conjugate fractures with unidirectional slip. The zone has a complex deformation history of successive phases of shearing (involving grain-size reduction by stable and unstable fracturing, gouging, and wear) and dilation (involving vein precipitation) that allowed it to grow in thickness with increasing displacement. Both the development of and sliding on a network of such deformation zones require a significant amount of energy, and are important components of the total deformation in the Wind River Mountains.

Comparisons of geometric relations, microtextures, and the nature of fluid-rock interaction for deformed basement rocks in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, reveal regional variations in structural style. Basement within the Wind River Range, Wyoming, is deformed by large-scale reverse faults and by intervening major brittle deformation zones (BDZs). Slip along faults and BDZs produced 30% regional shortening by bulk pure shear. Basement away from BDZs is cut by widely spaced fractures, but fracture intensity increases toward BDZs, and complex fracture networks occur along BDZ margins. BDZs contain anastomosing zones of breccia and cataclasite that developed by accumulation of fine-grained matrix due mostly to intergranular cracking and production of wear fragments along shear fractures. Small amounts of chlorite, clay, and calcite record local alteration along major BDZs. Cataclasis appears to be the dominant deformation mechanism, although very fine-grained foliated cataclasite along major faults may record a switch to pressure solution, particulate flow, or grain boundary sliding. Basement within the northern Wasatch Range, Utah, is deformed by large-scale imbricate thrust faults, by intervening networks of major ductile deformation zones (DDZs), and locally by networks of minor DDZs. Slip along thrusts and DDZs produced 60% shortening, with regional-scale simple shear and significant components of pure shear within individual thrust sheets. Basement rock away from DDZs is cut by widely spaced fractures and displays limited alteration. A transition zone along DDZ margins displays variable alteration concentrated along complex fracture and microcrack networks. Variably deformed grains and mineral-filled veins record temporally overlapping cataclasis, fluid-rock interaction, and plastic deformation. DDZs contain phyllonite that formed by accumulation of fine-grained matrix due to pervasive alteration, plastic deformation, and periodic microcracking. Quartz in DDZs is highly strained and recrystallized, and feldspar is almost completely altered to foliated aggregates of mica. Cataclasite is mixed with phyllonite along major thrust fault zones. Concentration of deformation within BDZs and DDZs reflects overall strain softening along these zones relative to the host rock. Relations between displacements and thicknesses of zones record growth of BDZs and DDZs with time, and may reflect periods of strain hardening during progressive deformation. The behavior of any particular zone is determined by a number of competing strain softening and hardening processes that are related to grain size reduction, fluid-rock interaction, and geometric evolution of internal structures.

The kinematics of east-striking faults during the Laramide orogeny in central Wyoming are problematic. These faults are commonly interpreted as thrusts accommodating north-south shortening. In addition, they have been interpreted to postdate northwest-striking faults that accommodate northeast-southwest shortening. Results of systematic mapping, in conjunction with a detailed kinematic study, in the west Owl Creek Mountains demonstrate that the high-angle, east-striking North Owl Creek fault is dominantly left slip. The fault is linked kinematically with the low-angle Mud Creek thrust in the western Owl Creek Mountains fault system to the east and with the low-angle Black Mountain thrust in the Washakie thrust system 50 km to the west. The role of the fault was to transfer east-northeast–west-southwest shortening between the Washakie thrust system and the west Owl Creek Mountains fault system during Laramide shortening. A protracted deformation history is required to explain the development of the North Owl Creek fault system. The system is interpreted to have formed by propagation of two lateral ramps: one linking the Mud Creek thrust, the other linking the Black Mountain thrust. Field relations also indicate that initiation of the system was probably not controlled by the orientation of Precambrian shear zones, dikes, or foliations. Instead, they indicate that Precambrian structures and Paleozoic strata have been rotated adjacent to high-angle faults in the North Owl Creek fault system during left-slip motion.

Laramide monoclines occur in the Phanerozoic rocks in the Grand Canyon region above a block-faulted, northeastward-tilted Precambrian foundation composed of crystalline metamorphic rocks; the crystalline rocks are mantled by northeastward-thickening wedges of Middle and Late Proterozoic Grand Canyon Supergroup sedimentary and volcanic rocks. The structural character of the Precambrian complex is one of heterogeneous lithologies and is highly anisotropic, owing primarily to imprinted Middle and Late Proterozoic north-, northeast-, and northwest-trending reverse and normal faults, and secondarily to foliation within the metamorphics and layered fabrics within the Grand Canyon Supergroup. Most Grand Canyon monoclines are underlain by a single, west-dipping, high-angle, Precambrian normal fault that was reactivated during Laramide west-southwest–east-northeast compression. Laramide reverse slip was opposite in sense to the Precambrian motion. The structural anisotropies provided by these faults are the single most influential element that localized deformation of the Laramide monoclines. The Laramide monoclines are wholly contained within the Phanerozoic section in areas directly underlain by Precambrian crystalline rocks. However, the anticlinal axial surfaces of the monoclines extend downward into the hanging walls which contain ductile Precambrian sedimentary rocks. The extent of such folding is proportional to the thickness of the Precambrian sedimentary rocks. Folding of the ductile Precambrian strata in the hanging wall caused the profiles of the monoclines to broaden laterally into the hanging walls. Structural anisotropies associated with strong foliation in the Precambrian crystalline rocks and layering in the Precambrian Grand Canyon Supergroup did not influence appreciably Laramide deformation in the Grand Canyon region.

The Fra Cristobal Range in south-central New Mexico exposes different styles of deformation variably affecting Precambrian crystalline rocks and Paleozoic, Mesozoic, and Cenozoic cover strata. Three main structural domains are recognized: I is a thick-skinned domain (basement-involved) along the northwest flank of the range; II is a thin-skinned domain along the southwest flank of the range; and III is a combined thick- and thin-skinned domain along the east flank of the range. Domains with thick-skinned deformation are characterized by fault-propagation folds cored by basement blocks bounded by reverse or thrust faults. These domains are segmented by transverse faults into two subdomain types: (1) east-vergent, overturned folds cored by a basement block with corners rounded by cataclastic flow; and (2) upright folds overthrust by basement that is imbricated by east-directed thrust faults. Thin-skinned deformation involves only Paleozoic cover strata and is characterized by decollement-style thrust faults, duplexes, and related upright to overturned folds. Assuming a balance of basement shortening along strike, it is likely that a basement uplift formed west of domain II but is now buried in the Rio Grande rift. Domain II structures developed either as thrusting propagated from basement into cover, by out-of-syncline thrusting east of the uplift, or by gravity sliding off the uplift. Domain III is characterized by a large, east-vergent, overturned monocline (probably basement cored at depth); the overturned limb was subsequently overthrust by its western subhorizontal limb (similar to a cross-crestal fold). A two-stage development is suggested: (1) basement-cored fault-propagation folding in domain III, and (2) shortening in domains I and II to the west and late-stage cross-crestal faulting of the domain III fold. Behavior of basement rocks during deformation includes: (1) shortening and uplift of basement by reverse faulting, (2) cataclastic flow of basement on fault-block corners allowing folding of the basement-cover contact, (3) longitudinal segmentation of some domains by transverse faults, and (4) possible control of basement faulting by preexisting weaknesses.

Subsurface reflection seismic and borehole data, combined with surface geologic mapping, provide a comprehensive data base for structural analysis of the central Rocky Mountain foreland province. These data, supplemented by analogue clay-model studies, constrain geometric and kinematic interpretations of the basement-involved thrust-generated folds (thrust folds) that formed structural traps for petroleum accumulations within the foreland basins. Case studies of selected intrabasin oil-field anticlines, illustrated with seismic profiles, structural cross sections, and structural contour maps, define immature, intermediate, and mature thrust folds. Because net slip is greatest at the sediment-basement contact, fold-generating thrusts must have nucleated within basement. Thrust-plane reflections on seismic profiles show that these thrusts are nearly planar within basement, and developed at an angle of between 20° and 35° to the basement surface. The thrusts then propagated and steepened upward, usually accompanied by backlimb rotation. Overlying Phanerozoic sediments were lifted and stretched over rising hanging-wall basement blocks and were ultimately offset by the propagating thrusts along the forelimbs of growing anticlines. In advanced stages of development, tapered hanging-wall basement wedges sustained significant finite strain, while in the footwalls, the basement remained relatively undeformed. Synthetic and antithetic detachment thrusts within the sedimentary column, footwall thrust wedges at the basement level, terminal tear faults, and shallow crestal extensional faults are common secondary structures. The basic elements of the thrust-fold model are incorporated in a true-scale, northeast-southwest structural transect drawn across Wyoming.

Strain in sedimentary rocks is linked to deformation of underlying basement rocks during the formation of basement-involved folds. Strains are represented by an array of structures such as lift-off folds, thrust faults, heterogeneous thickness changes, extensional faults, and boudinage. Detachment surfaces define the boundaries of structural lithic units within sedimentary rocks that are characterized by different deformation styles. A kinematic model is presented to investigate how strain distribution in folds is controlled by basement deformation. The model examines folds that form above a basement block that is displaced on a single reverse fault. Rocks in the fold undergo layer-parallel shortening (and thickening) and/or extension (and thinning). Extensional strains increase as fault angle and fault slip increase. As the basement fault propagates upsection strains will vary in the hanging wall and footwall of the fault and in unfaulted beds upsection from the fault tip. Results predicted by the modeling compare favorably with folds in the Rocky Mountain foreland province.