Alison B. Till

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska–Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska–Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700–540 Ma, with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440–400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic–Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska–Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska–Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska–Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

Thrust involvement of metamorphic rocks, southwestern Brooks Range, Alaska

Most models for the tectonic history of the western Brooks Range treat Proterozoic and lower Paleozoic metamorphic rocks exposed in the southern part of the range as passive structural basement vertically uplifted late in the Mesozoic orogenic episode. Mapping in the metamorphic rocks shows that they can be divided into two structurally and metamorphically distinct belts, both of which were folded and thrust during the orogeny . Recognition of these belts and the nature of the contact separating them is critical to construction of accurate tectonic models of the Brooks Range fold and thrust belt.

The geologic history of Redoubt Volcano, Alaska

Abstract Redoubt Volcano is a composite cone built on continental crust at the northeast end of the Aleutian arc. Magmas erupted at Redoubt are medium-K calc-alkaline basalts, andesites, and dacites. The eruptive history of the volcano can be divided into four parts: the early explosive stage, early cone-building stage, late cone-building stage, and post-glacial stage. The most silicic products of the volcano were erupted during the early explosive stage about 0.888 Ma and include pumiceous pyroclastic flow deposits, block-and-ash flow deposits, and a dome or shallow intrusive complex. Basalt and basaltic andesite lava flows and scoria and ash flows were produced during the early cone-building stage, which was underway by 0.340 Ma. During the late cone-building stage, andesitic lava flows and block-and-ash flows were emplaced. Airfall deposits produced during post-glacial eruptions are silicic andesite in composition. Since the early cone-building stage, magmas have become progressively more silicic, but none are as silicic as those in the early explosive stage. Limited Pb and Sr isotopic data suggest that Redoubt magmas were contaminated by North American continental crust.

Detrital blueschist-Facies metamorphic mineral assemblages in Early Cretaceous sediments of the foreland basin of the Brooks Range, Alaska, and implications for orogenic evolution

Detrital metamorphic minerals found in the foreland basin of the Brooks Range were derived from blueschist-facies rocks that now occupy the ranges metamorphic core. The metamorphic minerals occur in the Torok Formation and the Nanushuk Group; they are contained in sediments at least as old as middle Albian in age, and may have arrived earlier. The source of the minerals is the blueschist-facies schist belt, the structurally highest, more internal of two structurally and metamorphically distinct belts that occupy the metamorphic core of the Brooks Range. The blueschist facies rocks of the schist belt were exhumed between Late Jurassic and middle Albian time, during the major pulse of contractional deformation in the Brooks Range orogeny, rather than during late stage extensional collapse or postcontractional isostatic rebound, as suggested by other workers. The processes by which the blueschist-facies rocks were uplifted and denuded must have operated during contractional tectonism. Post peak metamorphic structures in the blueschist-facies rocks support their uplift by thrust systems; their unroofing apparently occurred by erosion and possibly by small-scale extensional faulting.

Nd- and Sr-Isotope Evidence for Proterozoic and Paleozoic Crustal Evolution in the Brooks Range, Northern Alaska

Exposures of Proterozoic crust in Alaska are rare; however, isotopic data from younger rocks derived from unexposed basement suggest that Proterozoic (or older) crust may underlie large areas of northern Alaska. Late Proterozoic orthogneisses, Late Proterozoic to Cambrian metasedimentary rocks, and Devonian granitic orthogneisses in the western Brooks Range have Nd-and Sr-isotope compositions consistent with derivation, at least in part, from Middle to Early Proterozoic crust. Proterozoic metasedimentary rocks have calculated Nd crustal residence ages of 2.0 Ga and are intruded by Late Proterozoic metagranites with somewhat more radiogenic initial isotopic compositions than the metasedimentary rocks at the time of intrusion. Initial isotopic compositions of Devonian metagranitic rocks range from

Proterozoic Geochronological Links between the Farewell, Kilbuck, and Arctic Alaska Terranes

\epsilon_{Nd} = -6.5 to 1.6

Geology of Seward Peninsula and Saint Lawrence Island

. The least radiogenic values are compatible with nearly pure crustal melts of a source with isotopic composition similar to what exposed Proterozoic crust, as sampled by the metasedimentary rocks and Late Proterozoic metagranites, would have had at 370 Ma. The range of observed isotopic compositions may be explained by mixing mantle-derived magmas, or melts of juvenile crust, with melts of Proterozoic crust in varying proportions. Nd isotope systematics of the Late Proterozoic metasedimentary rocks are identical to reported values from Early Proterozoic metaplutonic rocks in the Idono Complex and Kilbuck terrane in SW Alaska, and to clastic sedimentary rocks from the Belt Supergroup in Montana. Isotopic compositions of sampled metagranitic and metasedimentary rocks are not compatible with derivation exclusively from crust with the age and isotope characteristics of the Yukon-Tanana terrane. Very limited isotopic data from other continental domains within North America leave open the possibility of affinity with crust exposed in NW Canada, the Canadian Arctic Islands, or the NW Canadian Cordillera.

Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska

A synthesis of Late Jurassic and Early Cretaceous collision-related metamorphic events in the Arctic Alaska–Chukotka microplate clarifies its likely movement history during opening of the Amerasian and Canada basins. Comprehensive tectonic reconstructions of basin opening have been problematic, in part, because of the large size of the microplate, uncertainties in the location and kinematics of structures bounding the microplate, and lack of information on its internal deformation history. Many reconstructions have treated Arctic Alaska and Chukotka as a single crustal entity largely on the basis of similarities in their Mesozoic structural trends and similar late Proterozoic and early Paleozoic histories. Others have located Chukotka near Siberia during the Triassic and Jurassic, on the basis of detrital zircon age populations, and suggested that it was Arctic Alaska alone that rotated. The Mesozoic metamorphic histories of Arctic Alaska and Chukotka can be used to test the validity of these two approaches. A synthesis of the distribution, character, and timing of metamorphic events reveals substantial differences in the histories of the southern margin of the microplate in Chukotka in comparison to Arctic Alaska and places specific limitations on tectonic reconstructions. During the Late Jurassic and earliest Cretaceous, the Arctic Alaska margin was subducted to the south, while the Chukotka margin was the upper plate of a north-dipping subduction zone or a zone of transpression. An early Aptian blueschist- and greenschist-facies belt records the most profound crustal thickening event in the evolution of the orogen. It may have resulted in thicknesses of 50–60 km and was likely the cause of flexural subsidence in the foredeep of the Brooks Range. This event involved northern Alaska and northeasternmost Chukotka; it did not involve central and western Chukotka. Arctic Alaska and Chukotka evolved separately until the Aptian thickening event, which was likely a result of the rotation of Arctic Alaska into central and western Chukotka. In northeastern Chukotka, the thickened rocks are separated from the relatively little thickened continental crust of the remainder of Chukotka by the oceanic rocks of the Kolyuchin-Mechigmen zone. The zone is a candidate for an Early Cretaceous suture that separated most of Chukotka from northeast Chukotka and Alaska. Albian patterns of magmatism, metamorphism, and deformation in Chukotka and the Seward Peninsula may represent an example of escape tectonics that developed in response to final amalgamation of Chukotka with Eurasia.