Structure of the range-front region of the northeastern Brooks Range, TITLE: Ar^f-i^ Naf-innal w-MHI-ifp Kpf.igp fANWR) , Alaska

The structural style of the range-front region of the northeastern Brooks Range in ANWR is strongly controlled by 1) a series of detachment horizons in both pre-Mississippian rocks and the unconformably overlying Mississippian and younger cover sequence and 2) variations in the lithology and structural competency of the rocks between these detachment horizons. These variables strongly influence both vertical and lateral changes in structural style. The major detachment horizons which define the structural stratigraphy are 1) within the pre-Mississippian sequence, 2) in the Mississippian Kayak Shale (where present), 3) in the Jurassic to Cretaceous Kingak Shale (where present), and 4) in the Lower Cretaceous pebble shale unit. The Brooks Range of northwestern ANWR is dominated by a series of narrow, linear, doubly-plunging anticlinoria, whereas in northeastern ANWR it is characterized by only two broad and strongly arcuate anticlinoria. In both areas the anticlinoria are controlled by the geometry of a duplex bounded by a floor thrust in pre-Mississippian rocks and a roof thrust in the Kayak Shale. In the west, where the pre-Mississippian partially consists of structurally competent carbonates of the Katakturuk Dolomite and Nanook Limestone, each anticlinorium marks a single horse in the duplex. However, in the east, pre-Mississippian rocks are relatively incompetent and each anticlinorium is cored by multiple horses. In the west, shortening above the roof thrust is by detachment folding between the Kayak and Kingak Shales, except where the Kayak Shale detachment horizon is depositionally absent to the north, in the Sadlerochit Mountains. In contrast, in eastern ANWR shortening above the roof thrust is by major thrust duplication of the entire sequence between the Kayak and Kingak Shales, perhaps due to lithology and thickness changes within the Kayak Shale. The Devonian Okpilak batholith marks the boundary between the eastern and western structural provinces. The thrust-controlled range front of eastern ANWR extends north of the batholith, suggesting that the batholith itself is underlain by a thrust fault. Jurassic and younger rocks have been eroded or are covered in most of the region, but display a distinctive structural style where they are locally well exposed in northwestern ANWR. Rocks above the pebble shale detachment horizon consist largely of shales, so their structure is characterized by complex and closely spaced folds and thrust faults. Beneath the pebble shale, the Lower Cretaceous Kemik Sandstone commonly forms duplexes where it is underlain by the Kingak Shale detachment horizon. Duplexes do not occur wher the Kingak Shale is absent beneath the Kemik Sandstone due to erosion at the Lower Cretaceous unconformity. The Kemik Sandstone duplexes are exposed locally around the margins of the Sadlerochit Mountains. This suggests the duplexes above the Kingak Shale formed before the Sadlerochit Mountains anticline, a major structure formed above a detachment in the pre-Mississippian sequence. These duplexes and other eastnortheast-trending structures in Mississippian and younger rocks suggest a northnorthwest transport direction. These structures are cut by later east-trending structures which involve pre-Mississippian rocks. These probably are controlled by a pre-existing east-trending structural grain within the pre-Mississippian rocks, and reflect a shift to a deeper detachment level over time.