146-5 Fractures and Fluid Pathways: Quantitative Characterization of the Reactivated Douglas Fault Damage Zone, Wisconsin

Abstract:

Fault damage zones exert a first order control on subsurface fluid flow by governing fracture connectivity, permeability anisotropy, and the distribution of deformation surrounding fault cores. Consequently, damage zone architecture influences rupture mechanics, fluid flow, and crustal strength during fault evolution and reactivation through multiple tectonic events. Here, we present quantitative structural observations from the Douglas Fault in northwestern Wisconsin, which integrate detailed outcrop mapping and fracture network analysis, to demonstrate how progressive rock failure is recorded in damage-zone architecture during early normal faulting conditions and subsequent inversion of the North American Midcontinent Rift. 

At multiple study sites, the steeply dipping Douglas Fault thrusts highly fractured basaltic hanging wall rocks of the Chengwatana volcanics over less damaged Bayfield Group sandstones in the footwall. Mapping and thin-section analyses reveal a strongly asymmetric damage zone, with a >100 m wide zone of pervasive multi-generational fractures  in the hanging wall, including opening-mode veins, shear fractures, and microcrack networks. Cumulative fracture-frequency plots display a steep inner-zone intensity gradient that transitions outward to a broad, diffuse outer damage zone with perpendicular distance from the fault core, and fracture orientations define two dominant sets: NE–SW trending, subvertical extension fractures with deformed calcite veins and an overprinting NW–SE striking set of shear to hybrid fractures. The footwall rocks record lower fracture density and compactional microcracking consistent with distributed deformation in semi-consolidated sedimentary deposits. These structural gradients record progressive strain localization while preserving evidence for multiple phases of deformation. The resulting fracture architectures define mechanically distinct domains which are expected to exhibit contrasting permeability anisotropy and fluid-flow behavior during the fault’s history.

By quantitatively linking fracture intensity, orientation, connectivity, and structures across the Douglas Fault damage zone, this study demonstrates how outcrop scale structural observations can inform conceptual and numerical models of fault controlled permeability. For example, the observed fracture architecture further suggests that successful inversion of the steeply dipping Douglas Fault likely required transient fluid overpressure and localized reductions in fault strength, potentially requiring progressive fracture sealing and frictionally weak fault gouge generated during earlier extensional slip.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.