DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT: PRELIMINARY RESULTS OF STRAIN ANALYSIS

Strain analysis, in concert with complimentary paleomagnetic and fracture studies, are underway to determine the 3D kinematic evolution of the Wyoming salient of the Sevier fold-thrust belt. Preliminary strain data are reported here for the Jurassic Twin Creek and Triassic Ankareh formations. Mesoscopic to microscopic strain is accommodated by a variety of structures, which depends on lithology. Micritic limestone displays spaced cleavage and veins; bioclastic limestone contains plastically deformed fossils; redbeds have rough cleavage; and well bedded sandstone displays minor fold and fault networks (including wedge faults and oblique-slip, cross striking faults that produce differential shear and minor longitudinal extension). Two dimensional strain values were estimated using mass balance relations for spaced cleavage in micritic limestone, deformed crinoids in bioclastic limestone, and reduction spots in redbeds. Three-dimensional strain was estimated using 2D values for three perpendicular planes for selected samples. The XY strain plane and cleavage are about perpendicular to bedding in most areas, reflecting early layer-parallel shortening. Principal shortening (Z) directions (and cleavage poles) fan over 90 degrees around salient, roughly perpendicular to structural trends, reflecting a combination of initial curvature and vertical axis rotation of early fabrics. Principal shortening values vary from <10% in the frontal Hogsback thrust sheet where cleavage is weak to absent, to 15 to 30% in the more interior Crawford sheet where cleavage intensity is moderate to strong. Minor longitudinal extension parallel to structural trends is widespread, possibly reflecting a component of radial transport and development of secondary curvature. In general, strain intensity increases in more interior thrust sheets, and westward within individual sheets. Strain intensity in Ankareh redbeds and Twin Creek limestones are similar in many areas, but locally the Ankareh appears to have lower strain, which may reflect unrecognized minor faulting, or detachments that separate packages with differential shortening. Regional patterns indicate that internal strain is a significant component of total deformation in many areas, and should be taken into consideration when constructing and restoring cross sections.