Mechanisms and Kinematics of a Precambrian-Cored Fold and Fault Structure: Jakey\u27s Fork, Wyoming

The Jakeys Fork fold and fault structure is a Precambrian granite-gneiss–cored anticline in the hanging wall of the steeply west dipping Ross Lakes shear zone. The Ross Lakes shear zone juxtaposes Precambrian rocks in its hanging wall and steeply dipping to overturned Cambrian and Ordovician sedimentary rocks in its footwall. Precambrian and Cambrian Flathead Sandstone in the hanging wall of the Ross Lakes shear zone are folded in an open, kinklike fold defined by gently and steeply dipping panels of Cambrian Flathead Sandstone. Precambrian rocks in the core of the hanging-wall anticline deformed by slip on a narrow fault in the hinge zone of the fold and by randomly oriented slip on narrow, discrete faults of many orientations on either side of the hinge zone. Cambrian Flathead Sandstone was welded to the Precambrian rocks during folding, and detachment occurred in the Cambrian Gros Ventre Formation. The Ross Lakes shear zone formed by brecciation and cataclasis. Precambrian rocks in the footwall of the Ross Lakes shear zone deformed by small amounts of slip on steeply west dipping faults, which resulted in a steepening of the contact between Precambrian rocks and the overlying Cambrian Flathead Sandstone. Faults in Precambrian rocks in the footwall of the Ross Lakes shear zone bound regions of undeformed Precambrian rock with widely spaced fracture sets. Cambrian Gros Ventre and younger formations are folded into a tight, overturned footwall syncline with its steep limb parallel to the Ross Lakes shear zone

Mechanical Behavior of Basement Rocks, Scarface Thrust, Central Madison Range, Montana

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