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. Fractures can provide useful information about migration pathways, as well as thermal and fluid conditions during fracture formation. 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 fracture fill, and apatite fission-track (AFT) ages. In domain I, an overturned, asymmetric, north-vergent anticline is the surface expression of an anticlinal stack of Carboniferous platform carbonates. Dense fracturing during folding may have allowed fluid migration before filling with calcite cement. AFT ages from overlying, Neocomian, proximal basin deposits were thermally reset following burial and thermal models suggest that cooling may have begun as early as ~90 Ma. Domains II and IV are characterized by the lack of filled fractures. Domain II, immediately north of domain I, consists of clastic foreland basin deposits deformed into open, symmetric folds that are detached from the underlying rocks of domain I. To the north, domain IV consists of shallow marine and non-marine basin deposits that are deformed into kilometer-scale open folds. These folds are detached from the underlying mudstones of domain III. The lack of fracture fill in domains II and IV indicates dry, shallow fracturing that is interpreted as the result of regional stress and local deformation. AFT ages from domains II and IV record regional cooling at approximately 60 Ma. Domain III is located stratigraphically between the units of domains II and IV and contains rare sandstone beds within mechanically weak marine shale and siltstone. Domain III is deformed into meter-scale, south-vergent structures, with the more competent beds containing both filled and unfilled fractures. AFT ages from domain III were not reset and yield depositional grain ages of approximately 100 Ma. AFT ages, structural position, and deformational style of domain III are consistent with a zone of regional back-thrusting with significant displacement between domains II and IV.