FRACTURE EVOLUTION IN A FOLD-AND-THRUST BELT AND THE ASSOCIATED DEFORMED FORELAND BASIN: AN EXAMPLE FROM THE NORTHEASTERN BROOKS RANGE AND COLVILLE BASIN, ALASKA

Detailed structural mapping, geochronology and geothermometry along a surface to subsurface transect in the northeastern Brooks Range fold-and-thrust belt and foreland Colville basin provide insight into how fractures developed as host rocks were incorporated into fold-and-thrust deformation. Dominant regional structure in the study area is characterized by a north-vergent duplex with a basal decollement in the Mississippian Kayak Shale and a roof thrust in the Jurassic-Cretaceous Kingak Shale.

Detailed mapping of pre-Mississippian to Lower Cretaceous rocks constrains the structural setting and relative timing of fracture formation. Structural domains were defined based on position within the regional duplex: 1) D1: floor thrust hanging wall, 2) D2: floor thrust footwall, and 3) D3: above roof thrust. Fractures documented include (earliest to latest): 1) filled N-S striking, Type I fractures, 2) filled E-W striking, Type II fractures, 3) unfilled N-S striking, Type I fractures, and 4) unfilled E-W striking, Type II fractures. Both filled sets occur in each domain, but are absent in rocks above the Kayak Shale in D3. Unfilled sets are pervasive throughout the stratigraphy and in each domain. The filled Type I and II sets are interpreted to be the result of burial, while the unfilled Type I and II sets probably initiated during uplift.

Fluid inclusion analysis, thermal maturity indicators, and fission-track thermochronology provide limits on the timing and conditions during deformation and fracture formation and filling. Homogenization temperatures for Type I and II filled fractures that formed prior to or during deformation indicate that each set in each domain filled near maximum temperatures and burial depths as estimated from vitrinite reflectance values (D1 ~190°C, ~7.6 km; D2 ~175°C, ~7.0 km; D3 ~179°C, ~7.2 km; assuming a geothermal gradient of 25°C/km ). Crack seal textures and bent fibers in quartz and calcite cements suggest syn-kinematic crystal growth. Type I and II unfilled sets formed at shallower depths and/or in the absence of fluids. Apatite fission-track data from Mississippian to Jurassic samples indicate that folds now exposed at the surface south of the present range front formed at ~45 Ma (D1), followed by deeper, basement-involved thrusting at ~35 Ma (D2) and farther north at ~23 Ma (D3).