| Summary: | 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.
|
|---|