Adolph Yonkee

Reconstructing the kinematic evolution of curved mountain belts: Internal strain patterns in the Wyoming salient, Sevier thrust belt, U.S.A.

Analyses of mesoscopic structural and strain patterns in red beds of the Triassic Ankareh Formation and limestones of the Jurassic Twin Creek Formation, in concert with complementary paleomagnetic studies, constrain the three-dimensional kinematic evolution of curved fold-and-thrust systems in the Wyoming salient of the Sevier thrust belt. Spaced cleavage, fracture and vein networks, minor folds, and minor faults in limestones and red beds accommodated early layer-parallel shortening (LPS) concentrated in front of the growing thrust wedge, along with minor strike-parallel extension and wrench shear related to the development of orogenic curvature. Strain, estimated using mass balance relations for cleavage seams, crinoid ossicles in bioclastic limestone, and reduction spots in red beds, displays systematic regional patterns. Principal shortening directions are subperpendicular to structural trend around the salient, reflecting a combination of primary curvature and secondary rotation of early LPS fabrics. LPS magnitudes vary from <5% in central parts of the frontal Hogsback thrust system, where cleavage is absent, to 10%–30% in the more interior Crawford thrust system, where cleavage intensity is moderate to strong; strain also increases toward the salient ends. Internal strain is a significant component of total deformation and should be considered when restoring cross sections. Strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting.

Anisotropy of magnetic susceptibility in weakly deformed red beds from the Wyoming salient, Sevier thrust belt: Relations to layer-parallel shortening and orogenic curvature

Anisotropy of magnetic susceptibility (AMS) and structural studies of red beds in the Wyoming salient were completed to evaluate relations of magnetic fabrics to layer-parallel shortening and vertical-axis rotation in curved fold-thrust systems. The red beds display cleavage, fractures, veins, minor folds, and minor faults that accommodated widespread early layer-parallel shortening and minor strike-parallel extension. Magnetic susceptibility is carried mostly by paramagnetic phyllosilicates and ferromagnetic hematite that have composite fabrics related to sedimentary deposition, diagenesis, and tectonic processes. Anisotropy of magnetic susceptibility fabrics range from distinctly oblate ellipsoids parallel to bedding that reflect dominant sedimentary fabrics (type 1), to moderately oblate ellipsoids with weak magnetic lineations roughly parallel to the intersection of weak layer-parallel shortening fabrics and bedding (type 2), to triaxial and prolate ellipsoids with distinct magnetic lineations parallel to the intersection of moderate layer-parallel shortening fabrics and bedding (type 3). Type 1 sites occur mostly in the central, frontal part of the salient where layer-parallel shortening is 15%. Magnetic lineations are subparallel to structural trend and exhibit a tangential pattern around curved fold-thrust systems. Regional patterns of anisotropy of magnetic susceptibility are broadly similar to patterns of finite strain estimated from reduction spots. Combined with paleomagnetic data, anisotropy of magnetic susceptibility data indicate that early layer-parallel shortening fabrics started with minor primary curvature and then underwent significant vertical-axis rotation during large-scale thrusting. Correlations with finite strain, structural, and paleomagnetic data sets indicate that analysis of anisotropy of magnetic susceptibility in weakly deformed red beds is useful for evaluating kinematic evolution of thrust systems.

Reconstructing the kinematic evolution of curved mountain belts: A paleomagnetic study of Triassic red beds from the Wyoming salient, Sevier thrust belt, U.S.A.

Determining the kinematic history and mechanics of curved fold-and-thrust belts is fundamental to understanding the tectonic evolution of mountain systems. To better understand the development of a classic curved fold-and-thrust belt, we completed an integrated paleomagnetic and strain study of the Wyoming salient. Paleomagnetic data are reported here from 154 sites collected from red beds of the Triassic Ankareh Formation in the salient and nine sites collected from the relatively stable foreland. Red beds display three components with distinctly different magnetic behaviors: (1) a near-primary Triassic magnetization carried by hematite that is stable up to 680 °C (Tr component, 91 sites); (2) a Cretaceous chemical remagnetization carried partly by magnetite (K component, 32 sites); and (3) a recent viscous magnetization that is mostly removed by 350 °C. Site mean vectors for the Tr and K components show a high degree of scatter from expected Triassic and Cretaceous reference directions, suggesting significant tilt and rotation subsequent to magnetization acquisition. Restoration of tilt and folding for individual site means results in well-clustered shallow and moderate inclinations for the Tr and K components, respectively, and in variable declinations related to systematic vertical-axis rotations. Statistical analysis of declinations for both components indicates that ∼75% of present-day salient curvature resulted from secondary rotation, and ∼25% of primary curvature was likely related to sedimentary basin architecture. Analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets (∼80%) compared to the frontal thrust sheets (∼65%), suggesting that rotations were concentrated near the leading edge of the propagating fold-and-thrust wedge, with only minor additional rotation of internal sheets. Transfer zones, oblique ramps, and more deformed overturned fold limbs display locally more complex patterns, which can be understood through careful structural analysis. When combined with internal strain data and regional structural relations, paleomagnetic data support a kinematic model of a progressive arc with curved thrust-slip paths and differential shortening that rotated early layer-parallel shortening fabrics and produced minor strike-parallel extension. This kinematic history likely reflects a combination of processes, including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient, presence of a weak basal detachment and fault-zone weakening that favored lower taper, and buttressing by Laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient.

Towards a better understanding of the influence of basement heterogeneities and lithospheric coupling on foreland deformation: A structural and paleomagnetic study of Laramide deformation in the southern Bighorn Arch, Wyoming

Integrated structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic analyses of sedimentary cover rocks along variably oriented major faults and en echelon fold systems in the southern Bighorn Arch, Wyoming, were undertaken to test kinematic and mechanical models of Laramide thick-skin deformation. The Laramide foreland is characterized by an anastomosing network of basement-cored arches and associated cover folds that trend overall NW-SE, but in detail are curved, range from N-S to E-W trending, and form both right- and left-stepping en echelon systems. Development of variably trending Laramide arches has been variously attributed to temporal changes in stress directions, wrench faulting, and localization of deformation related to basement heterogeneities during regional SW-NE shortening. Within the southern Bighorn Arch, widespread but limited layer-parallel shortening (LPS) was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries early in the deformation history. LPS directions vary from perpendicular to acute with local fold structural trends, consistent with a single shortening episode. Although internal strain is limited, weak AMS lineations defined by kinked and rotated phyllosilicates are widely developed and consistently perpendicular to LPS directions. Structurally restored paleomagnetic declinations record only limited, non-systematic vertical-axis rotations, indicating that wrenching was not an important component of the deformation field during development of the southern Bighorn Arch and that curvature of the arch was a primary feature. Palinspastically restored LPS directions are on average WSW-ENE, but display local deflections related to heterogeneities of underlying basement blocks and proximity to major faults, some of which were localized along Precambrian shear zones and igneous dikes. Crustal shortening patterns across the Laramide foreland are interpreted to reflect deformation partitioning in response to a single far-field shortening direction partly related to flat-slab subduction along with effects of pre-existing basement weaknesses and strain softening during progressive faulting.

Tectonic evolution of a Laramide transverse structural zone: Sweetwater Arch, south central Wyoming

Structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data record patterns of layer-parallel shortening (LPS), vertical-axis rotation, and regional fault-fold evolution across the Sweetwater Arch, a major west to WNW trending, basement-cored Laramide uplift in Wyoming. The southern arch flank is bounded by a WNW striking reverse fault zone that imbricated basement and cover rocks, the northern flank is bounded by a west striking fault zone with a component of strike-slip and NW trending en echelon folds, and the eastern plunge transitions into an area of multiple-trending faults and folds. Synorogenic strata record major arch uplift from Maastrichtian to Early Eocene time, followed by arch collapse. LPS, with development of systematic minor fault sets and AMS lineations, preceded large-scale folding. LPS directions, estimated from both minor fault and AMS data, were oriented WSW along the northern flank, subparallel to Laramide regional shortening, but were refracted to the SSW along the southern flank, and to the west along the eastern arch plunge. Additional minor faults developed along steep fold limbs during continued shortening, with directions remaining SSW along the southern flank but becoming more variable along the eastern plunge where an increasingly heterogeneous stress field developed as additional faults were activated along basement heterogeneities. Vertical-axis rotation was limited along the arch flanks, whereas the eastern plunge underwent counterclockwise rotation. Deflections in shortening directions were partly related to basement heterogeneities, including weak supracrustal belts on the arch flanks, a strong granitic core, and local reactivation of Precambrian shear zones.