Siniktanneyak Mountain (SM) is a klippe of the Brooks Range ophiolite, the structurally highest sheet in the N-directed Brooks Range thrust belt, in the central Brooks Range. SM layering dips steeply to the north-northwest and has a stratigraphic thickness of up to 12 km. From southeast to northwest, SM consists 2 km of ultramafic rocks, 6 km of layered cumulate gabbro, and 4 km of isotropic gabbro, diabase dikes and basaltic lavas. Intermediate to felsic intrusions cross-cut the isotropic gabbro and diabase dikes, but also cut the basal ultramafic rocks. Basaltic lavas and underlying diabase dikes have island arc tholeiite chemistry with low chondrite-normalized La/Yb (~2) and moderate normalized Nb-Ta depletion relative to La. Cross-cutting intermediate to felsic intrusions also have arc-type trace element chemistry but distinctly higher La/Yb (~5) and are not co-genetic with the basaltic lavas and diabase dikes. Scattered high-Mg boninite dikes also cut isotropic gabbro and cumulate gabbro, and rocks of this composition are characteristic of early magmatism in modern island arcs of the western Pacific Ocean. Three new SHRIMP-RG U-Pb zircon ages of intermediate and felsic intrusions at SM, as well as two from Misheguk and Asik Mountains 200 km to the west, range from 161.1 +/- 2.0 to 163.7 +/- 2.6 Ma, all within error of each other. Zircon eHf(t) are juvenile and range from +16 (Late Jurassic depleted mantle) to +7; the most felsic dikes have the lowest eHf(t). By comparison Late Jurassic tonalite (WSU LA-ICPMS zircon U-Pb age: 146.9 +/- 1.2 Ma) from the Koyukuk arc terrane south of the Brooks Range yielded similarly juvenile eHf(t) (+11 to +13) and a similar trace element signature to the dated Brooks Range ophiolite samples. The arc tholeiite lavas and sheeted dikes, along with boninite dikes, suggest the Brooks Range ophiolite was generated in an extensional setting during arc initiation (forearc of the Koyukuk arc system?). The dated intermediate and felsic intrusive phases in the Brooks Range ophiolite are younger than the main mafic intrusive and volcanic section, but how much younger is uncertain.

First posted August 29, 2016 For additional information, contact: Geology, Minerals, Energy, and Geophysics Science CenterU.S. Geological Survey345 Middlefield Road, MS 901Menlo Park, CA 94025-3591http://geomaps.wr.usgs.gov/gmeg/ The structural architecture of Alaska is the product of a complex history of tectonism that occurred along the Cordilleran and Arctic margins of North America through interactions with ancient and modern ocean plates and with continental elements derived from Laurentia, Siberia, and Baltica. To unravel the tectonic history of Alaska, we constructed maps showing the age, distribution, structural style, and kinematics of contractional and penetrative extensional deformation in Alaska north of latitude 60° N. at a scale of 1:5,000,000. These maps use the Geologic Map of the Arctic (Harrison and others, 2011) as a base map and follow the guidelines in the Tectonic Map of the Arctic project (Petrov and others, 2013) for construction, including use of the International Commission on Stratigraphy time scale (Cohen and others, 2013) divided into 20 time intervals. We find evidence for deformation in 14 of the 20 time intervals and present maps showing the known or probable extent of deformation for each time interval. Maps and descriptions of deformational style, age constraints, kinematics, and information sources for each deformational episode are discussed in the text and are reported in tabular form. This report also contains maps showing the lithologies and structural geology of Alaska, a terrane map, and the distribution of tectonically important units including post-tectonic sedimentary basins, accretionary complexes, ophiolites, metamorphic rocks.These new maps show that most deformational belts in Alaska are relatively young features, having developed during the late Mesozoic and Cenozoic. The oldest episode of deformation recognized anywhere in Alaska is found in the basement of the Farewell terrane (~1.75 Ga). Paleozoic and early Mesozoic deformational events, including Devonian deformation in the Arctic Alaska terrane, Pennsylvanian deformation in the Alexander terrane, Permian deformation in the Yukon Composite (Klondike orogeny) and Farewell terranes (Browns Fork orogeny), Early and Late Jurassic deformation in the Peninsular-Wrangellia terranes, and Early Cretaceous deformation in northern Alaska (early Brookian orogeny) show that within-terrane amalgamation events occurred prior to assembly of Alaska. Widespread episodes of deformation in the Late Cretaceous and early Cenozoic, in contrast, affected multiple terranes, indicating they occurred during or following the time of assembly of most of Alaska.The primary deformational event in northern Alaska was the Late Jurassic and Early Cretaceous (early) Brookian orogeny, which affected most terranes north and west of the early Cenozoic Tintina, Victoria Creek, Kaltag, and Poorman dextral-slip faults in central Alaska. In southern Alaska, formation of the southern Alaska accretionary complex (Chugach, Prince William, Yakutat terranes) and associated magmatism in the Peninsular-Wrangellia terrane began near the Triassic-Jurassic boundary and continued episodically throughout the remainder of the Mesozoic and the Cenozoic. The collision of these terranes with the Farewell and Yukon Composite terranes in central Alaska is recorded by contractional deformation that emanated from the intervening basins in the Late Cretaceous. The boundary between northern and central Alaska is constrained to late Early Cretaceous but is enigmatic and not obviously marked by contractional deformation. Early Cenozoic shortening and transpressional deformation is the most widespread event recorded in Alaska and produced the widespread late Brookian orogenic event in northern Alaska. Middle and late Cenozoic shortening and transpression is significant in southern Alaska inboard of the underthrusting Yakutat terrane at the Pacific margin subduction zone as well as in northeastern Alaska.