CONSTRAINTS ON FRACTURE EVOLUTION IN THE NORTHEASTERN BROOKS RANGE FOLD-AND-THRUST BELT 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 Colville foreland basin provide insight into how fractures developed as host rocks were incorporated into fold-and-thrust deformation. These fractures have implications for fractured reservoirs and potential migration pathways. Dominant regional structure in the study area is characterized by a north-vergent duplex with a basal decollement in pre-Mississippian basement rocks and a roof thrust in the Mississippian Kayak Shale. The Ellesmerian cover sequence accommodates shortening by detachment faulting and folding. Structural domains I, II, and III were defined based on mechanical stratigraphy and are bounded by detachment units.

Detailed mapping of pre-Mississippian to Lower Cretaceous rocks constrains the structural setting and relative timing of fracture formation. Fractures documented include (earliest to latest): 1) a N-S striking filled set, 2) an E-W striking filled set, 3) a N-S striking unfilled set, and 4) an E-W striking unfilled set. Both unfilled fracture sets occur throughout the stratigraphy and in each domain, while the filled sets occur throughout Domains I and II but not in strata higher than the Cretaceous Kingak Shale in Domain III. The filled sets are interpreted to have formed during burial and the unfilled sets probably initiated during uplift.

Fluid inclusion analyses, thermal maturity indicators, and fission-track thermochronology provide limits on the timing and conditions during deformation and fracture formation and filling. Homogenization temperatures from fluid inclusions in filled fractures indicate that fluids were migrating through the fractures near maximum temperatures and burial depths as estimated from vitrinite reflectance values. Syn-kinematic cement textures suggest cementation occurred synchronously with fracture formation. In contrast, 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 (Domain I), followed by deeper, basement-involved thrusting at ~35 Ma (Domain II) and farther north at ~25 Ma (Domain III).