Abstract

The marked eastern swing in the Darby thrust, at the southern end of Snider Basin, coincides with the subsurface impingement of La Barge Platform on the frontal thrusts of the Cordilleran thrust belt. Four groups of limestone rocks from the Darby-Hogsback thrust sheet, from both north and south of the impingement with La Barge Platform, were studied in order to quantify the strain effects of the uplift of the Moxa Arch. Group I rocks are from north of the swing in the Darby thrust where the Moxa Arch lies directly below the surface trace of the Darby thrust. Groups 2 and 3 are from Packsaddle and Deadline ridges respectively where the Darby-Hogsback thrust is localized on a monocline above the basement shear zone at the southwest margin of La Barge Platform. Group 4 rocks are from Hogsback Ridge which lies west of the Moxa Arch. Several rocks in each group show the Turner (1953) maximum compression axes as bedding parallel; the Turner axes in each case oriented normal to the trend of the footwall ramp encountered. "Girdling" of the Turner compression axes, in a plane normal to the footwall ramp, might indicate progressive reorientation of the maximum principal stress with respect to bedding as the ramp was encountered. In certain cases stress was transmitted across rather than along bedding. Some rocks in Groups 2, 3, and 4 show bedding-normal Turner compression and greatest principal shortening axes. These results suggest that these rocks 1) were not within the load carrying units of the thrust sheet or 2) were along the outer arc of a folded, mechanically thick unit where interlayer slip was inhibited and have primarily recorded bending strains. Nearly vertical Turner compression axes, especially south of Snider Basin, might indicate regional uplift or broad folding. The Moxa Arch seems to have indirectly determined the location of the Darby thrust. The position of the Darby thrust was controlled by the cover-rock geometry that formed during deformation of the basement and subsequent thrusting.