EVOLUTION AND TIMING OF FRACTURES AND RELATED MAP-SCALE STRUCTURES OF THE CENTRAL BROOKS RANGE FOLD-AND-THRUST BELT, NORTHERN ALASKA

Fractures form in foreland basin rocks during their progressive incorporation into a fold-and-thrust belt and subsequent uplift. These fracture systems contain useful information about migration pathways and fractured reservoirs. This study focuses on the south to north transition from pre-orogenic carbonates near the Brooks Range mountain front northward into synorogenic clastic foreland basin deposits. Four distinct structural domains are characterized by differences in fracture density, the presence or absence of fill, the sense of structural vergence, and apatite fission-track cooling ages. In domain I, just north of the mountain front, an overturned, asymmetric, north-vergent anticline is the surface expression of an anticlinal stack of Carboniferous platform carbonates with dense concentrations of fractures that served as fluid migration pathways before being closed by calcite cement. Apatite fission-track cooling ages from Permian rocks in the back limb of the structure indicate that the overlying units were never buried deeply enough to become thermally reset. To the north, domains II and IV are characterized by the lack of filled fractures. In domain II, proximal clastic foreland basin deposits are deformed into open, symmetric folds that are structurally detached from the underlying rocks of domain I. In domain IV, shallow marine and non-marine basin deposits form kilometer-scale open folds that are structurally detached from the underlying shales of domain III. The lack of fracture fill in domains II and IV indicate shallow, dry fracturing related to regional in-situ stress and local deformation. Domains II and IV both yield apatite cooling ages consistent with a previously recognized regional cooling event at around 60 Ma. Domain III is located structurally and stratigraphically between the units of domains II and IV and contains both filled and unfilled fractures in rare coherent beds within mechanically weak, marine shale, siltstone, and subordinate fine-grained sandstone that deform into meter-scale, south-vergent structures. Detrital apatites in domain III are not reset and record ages of around 100 Ma. Thermal observations and the structural position of domain III, between the thermally reset domains II and IV are consistent with it being a zone of regional back-thrusting.