MELANGE AND LATE PALEOZOIC-TRIASSIC(?) CHERT, LIMESTONE, AND PILLOW BASALT ALONG A TERRANE SUTURE ZONE SOUTH OF THE DENALI FAULT, REINDEER HILLS, ALASKA

Melange in the Reindeer Hills of south-central Alaska consists of disrupted flysch, strained conglomerate (chert, argillite, quartzite clasts), and deformed felsic dikes with exotic blocks of Silurian-Devonian limestone and metabasalt of unknown age. The melange matrix has an early-stage bedding-parallel slaty cleavage and a scaly cleavage wrapped around cm- to m-sized phacoids of mudstone, siltstone, and sandstone. Small-scale faults, folds, and crenulation cleavage crosscut the early cleavage and bedding and reveal an episode of late-stage NNW-SSE shortening and ~E-W dextral strike-slip faulting. The limestone and metabasalt blocks are up to 10’s of m across and are in fault-contact with the melange. Structurally overlying the melange is ~850 m of black, gray, and light green radiolarian chert with thin interbeds of sandy bioclastic limestone and shale capped by ~50 m of pillow basalt. The limestone beds yield Mississippian to Permian fossils. The pillow basalt is depleted in the rare earth elements similar to that of mid-ocean-ridge basalts (La(n)/Yb(n)=0.9, La(n) 1.6 x chondrite).

An increase in deformation intensity in the zone between the chert and melange is evidence that the chert-melange contact is a fault. The entire chert sequence is folded into a tight ENE-trending synform, parallel to the chert-melange fault contact and consistent with the NNW-SSE shortening recorded in the melange. If the flysch and conglomerate in the melange were derived from the adjacent Jurassic-Cretaceous Kahiltna assemblage, then the melange is post-early Late Cretaceous in age and probably formed in a suture zone between the Alaska margin and the accreting Wrangellia composite terrane. The exotic blocks may be fragments of the nearby Mystic or Dillinger terranes. The chert-pillow basalt sequence has oceanic affinity and could represent a seamount fragment or possibly a fragment of North American continental margin analogous to the Mystic or McKinley terranes, which both include Late Paleozoic radiolarian chert. The dextral strike-slip faults in the melange indicate that the late-stage NNW-SSE shortening was related to oblique-slip along the suture zone, consistent with Late Cretaceous-early Tertiary dextral simple shear along the Denali fault which trends ~E-W along the north side of the Reindeer Hills.