REGIONAL BRITTLE DEFORMATION, STRAIN, AND PALEOSTRESS TRAJECTORIES, TETON RANGE, WYOMING USA

We analyzed opening-mode fracture size, spacing, strain, and orientation patterns in the Middle Cambrian Flathead sandstone throughout the Teton Range, Wyoming. Fluid inclusion assemblage (FIA) temperatures from fracture cements tie fracture formation to burial history. Scanlines and high definition SEM-Cathodoluminescense image analysis provided fracture statistics for each field area in addition to illuminating fracture cement textures. Fractures are aligned normal to bedding in three main sets; Set A, B, and C; these strike, N, NNE, and WNW respectively. Other orientations of fractures have locally developed in subsidiary sets adjacent to Laramide reverse faults and young normal faults; such as, the South-striking Teton Normal Fault that bounds the range to the East. Fracture heights are typically bed-bounded and have opening displacement sizes ranging from 0.01 to 1 mm. Large fractures having heights of tens of centimeters to meters include both barren joints and quartz lined or quartz filled fractures. By focusing on the small size fraction of the fracture population, we systematically restrict analysis to fractures that demonstrably formed in the subsurface, eliminating those that are likely a consequence of exhumation or other near-surface effects. Fracture feature statistics reflect regional deformation; folds, faults and fractures associated with NNW-trending reverse faults were active during the Laramide orogeny while deformation associated with N- and WNW-striking extensional normal faults likely initiated in the Eocene and continues development today. Independently derived burial history curves have been calculated for the Flathead Formation throughout the Tetons. FIA analysis of fracture cements provided essential information for understanding fluid-rock interactions during fracture population development around diverse types of structural regimes; paleo -temperature, -pressure, and -fluid composition conditions are then utilized to understand absolute fracture timing. Our results demonstrate the utility of microfractures in quartz-rich sandstones as systematic regional guides to understanding brittle deformation as paleostress indicators which can help better predict spatial and temporal fracture patterns.