Tertiary tectonics of the Border Ranges Fault System, north-central Chugach Mountains, Alaska: Sedimentation, deformation and uplift along the inboard edge of a subduction complex

In south-central Alaska the Border Ranges Fault system (BRFS) separates lower Paleogene rocks of a forearc basin sequence from a Cretaceous subduction complex. In a north-central part of the Chugach Mountains the upper Paleocene-lower Eocene Chickaloon Formation was deposited along the seaward margin of the forearc basin as an alluvial fan complex. A field study combining geologic mapping of a ;Structural studies indicate that wrench folds developed with their axes subparallel to the direction of maximum incremental stretching and were rotated and tightened into subparallelism with adjacent faults. During a late increment of transtensive deformation, early wrenchfolds were over printed by younger wrench folds that make a ;In early Paleogene time the BRFS comprised a steeply-arcward dipping system of normal-separation faults that periodically uplifted metamorphic rocks of the subduction complex. The Chickaloon Formation was locally derived by erosion of this subduction complex and has probably been displaced laterally by no more than a few tens of kilometers of dextral-slip faulting of post-late Paleocene and pre-early late Eocene age. The oroclinal bend of southern Alaska is post-early Eocene in age. Episodes of forearc uplift were probably driven by isostatic uplift of a structurally "overthickened" accretionary prism after discrete episodes of sediment underplating. The present north-central Chugach Mountains were last rapidly uplifted in early Miocene time.