39 Structural Geology (Posters)

Added due to amount of discipline abstracts submitted.

Wednesday, 11 March 2026
9:00 AM - 12:00 PM
RCC, Lower Level Hall

Evolving Damage: Architecture of the Douglas Fault, Wisconsin, and Mechanical Implications for Inversion of the Mid-Continent Rift

Paper number: 39-5
Program: Technical Sessions
Time: 9:00 AM – 9:00 AM

Authors

Abstract

Fault damage zone architecture strongly influences rupture mechanics, fluid flow, and crustal strength, serving as pre-existing weaknesses that may reactivate under latter tectonic regimes. However, fault zones may be poorly oriented with respect to subsequent stress states and exhibit high frictional strength. Processes like stress rotation in the fault damage zone, chemical and mechanical alteration of fault rocks, and fault healing and sealing are all hypothesized to promote fault slip under unfavorable stress conditions. 

Here, we present new structural observations from the Douglas Fault in northwestern Wisconsin to demonstrate how progressive rock failure is recorded in damage-zone architecture during early normal faulting conditions associated with the breakup of Laurentia and subsequent inversion of the 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 results indicate that reactivation and inversion of inherited rift faults localize progressive failure into highly anisotropic hanging-wall zones, producing composite networks that both weaken the crust and promote fluid connectivity during rupture. Under these conditions, the elevated fluid pressures needed to invert a steeply dipping Douglas Fault would require sealing of fractures in the damage zone prior to inversion. Reactivation may have been further promoted by the formation of frictionally weak gouges in the fault core during the initial stages of slip. Tracking fracture intensity gradients, cross-cutting fracture sets, and evolving mechanical properties within damage zones offers a powerful archive of progressive failure history along intraplate faults, and can inform physically based models of reactivation potential.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411873

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