ALBERT

Description: An orocline is a thrust belt or orogen that is curved in map-view due to it having been bent or buckled about a vertical axis of rotation. Two distinct types of oroclines are recognized: progressive and secondary. Progressive oroclines are restricted to the scale of a thrust sheet to thrust belt, are thin-skinned, and develop during thrust sheet emplacement. Secondary oroclines are larger, occurring at the scale of an orogen, and are plate-scale features that affect crust and lithospheric mantle. Unlike progressive oroclines, which develop during initial orogenesis and in response to the same orogen-perpendicular stress responsible for thrust sheet emplacement, secondary oroclines are extra-orogenic, developing after initial orogenesis and in response to an orogen-parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development. We present case studies of the Wyoming Salient, a progressive orocline that characterizes the Sevier thrust belt of the western United States, and the coupled Cantabrian and Central Iberian oroclines, which are linked secondary oroclines affecting the Variscan orogen of Iberia. The vertical-axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis. Detailed three-dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role, if any, of primary curvature in progressive oroclines. The use of tectonic vectors, such as paleocurrent directions, provides a means of recognizing and characterizing the initial geometry of secondary oroclines. Because secondary oroclines involve the entire lithosphere, detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling.

Description: 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.