Norman J. Silberling

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic melange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values ( Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Collision tectonics in the Cordillera of western N America: examples from Alaska

Summary The western edge of N America grew during the Mesozoic and Early Cenozoic by the addition of numerous accreted terranes consisting of fragments of island arcs, scraps of oceanic basins, seamounts, continental rise deposits and pieces of continents. The result of the accretionary process was production of new continental crust of extraordinary heterogeneity and structural complexity. This complexity is best portrayed in Alaska, where virtually 95% of the state was assembled during the Cretaceous, with attendant folding, large-scale thrust faulting, penetrative deformation and regional metamorphism. In this brief report, we discuss evidence for large-scale displacement of Wrangellia and for its time of incorporation into the Alaskan tectonic collage. The structures produced during accretion of this large coherent body are contrasted with those produced during coeval collapse of interval flysch basins that lay N of Wrangellia and with those associated with emplacement of enormous thrust sheets of oceanic rocks on top of continental platforms in Central and Northern Alaska. Despite compelling evidence for thousands of km of relative displacement between accreted terranes and between the terranes and the N American Craton, the resulting collage carries few readily discernible signals that allow application of currently popular simple plate tectonic models.

Reinterpretation of lower Mesozoic rocks on the Chilkat Peninsula, Alaska, as a displaced fragment of Wrangellia

The southern Chilkat Peninsula is underlain by low-grade metamorphic rocks consisting of a thick unit of greenstone, in part of Carnian age, that is locally overlain by an attenuated section of calcareous sedimentary rocks and chart of latest Carnian to late Norian age and a thick flysch unit of Jurassic and/or Cretaceous age. The Triassic part of this sequence has previously been inferred to be part of the Taku terrane. However, its lithology, age, fossils, and regional distribution indicate that it is probably a sliver of Wrangellia that was offset from the eastern Alaska Range to its present position by at least 350 km dextral displacement on the Denali fault.

Mesozoic Accretionary Tectonics of Alaska: ABSTRACT

Most of Alaska represents an enormous mosaic of allochthonous tectono-stratigraphic terranes, each characterized by a distinctive pre-Tertiary stratigraphic and tectonic history. More than 40 terranes presently are recognized, ranging in size from thousands of square kilometers to unique tectonic blocks having outcrop areas of only a few square kilometers. With a single exception, all of these pre-Tertiary terranes evidently are allochthonous with respect to North America and to one another. Some are best interpreted as displaced parts of the continental margin, but others--particularly in southern Alaska--may be exotic to North America. Paleomagnetic studies show that some terranes such as Wrangellia, have been transported as much as 3,000 km. Amalgamation of different terranes prior to final emplacement can be documented or inferred in a few places, but most of Alaska was tectonically assembled from individual lithosphere fragments and microplates during late Mesozoic time by complex accretion. Various schemes have been proposed for rotation and/or offset of northern Alaska into its present position. In south-central Alaska, southwest-trending terranes generally parallel the present-day Aleutian trench system. Outcrop patterns of rocks within these terranes point to mainly convergent late Mesozoic accretionary tectonics involving the development of nappes. Subsequent strike-slip faulting of these terranes is relatively minor, but original Mesozoic accretion of these belts may have involved large-scale transform displacemen along the northwest-trending parts of the Tintina trench and ancestral Denali fault systems. Significant tectonic displacement continues only along the active strike-slip faults in southeast Alaska and accretion is limited to the mechanically related Aleutian trench system. End_of_Article - Last_Page 784------------