Eric S. Gottlieb

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.

Closing the Canada Basin: Detrital zircon geochronology relationships between the North Slope of Arctic Alaska and the Franklinian mobile belt of Arctic Canada

Constraining the pre-opening paleogeography of the Canadian and Alaskan margins of the Canada Basin is a first-order objective in resolving the plate tectonic evolution of the Amerasia Basin of the Arctic Ocean. The most widely accepted model for opening of the Canada Basin involves counterclockwise rotation of Arctic Alaska away from Arctic Canada about a pole of rotation in the Mackenzie Delta region, although numerous other kinematic models have been proposed. The rotation model is tested using detrital zircon U-Pb geochronology of 12 samples from Middle Mississippian to Early-Middle Jurassic strata (Ellesmerian and lower Beaufortian megasequences) obtained from wells and outcrop along Alaska’s North Slope. These northerly-derived strata were deposited in fluvial to nearshore marine environments along the south-facing (present-day) shelf margin of the Arctic Alaska Basin and contain 360–390 Ma, 415–470 Ma, 500–750 Ma, 0.9–2.1 Ga, and 2.4–3.2 Ga zircon populations. Detrital zircon age populations in Ellesmerian and lower Beaufortian strata are remarkably similar to detrital zircon populations from Devonian foreland clastic wedge strata in the Canadian Arctic Islands and northern Yukon Territory. A paleogeographic setting in which Arctic Alaska received sediments recycled from the Devonian foreland clastic wedge and underlying Franklinian Basin strata is most consistent with the model of [Embry (1990)][1] in which northern Alaska lay within the foreland fold and thrust belt of the Franklinian mobile belt prior to the opening of the Canada Basin. The sequences that are inferred to have been the long-lived source region for Ellesmerian and lower Beaufortian strata were uplifted by Paleozoic (predominantly Late Devonian) deformation that has been documented along the Canadian and Alaskan margins. Triassic and Jurassic strata deposited along the Arctic Canada, Arctic Alaska, and northern Yukon shelves have detrital zircon ages that are significantly older than the youngest detrital zircon ages (Mesozoic) in coeval strata that were deposited west of Hanna Trough and north of the Sverdrup Basin axis, supporting continuity of these bathymetric features prior to opening of the Canada Basin. [1]: #ref-32

Detrital zircon geochronology of Neoproterozoic–Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave–Snow Lake fault hypothesis

Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the >2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2σ) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.

Neoproterozoic basement history of Wrangel Island and Arctic Chukotka: integrated insights from zircon U–Pb, O and Hf isotopic studies

Abstract The pre-Cenozoic kinematic and tectonic history of the Arctic Alaska Chukotka (AAC) terrane is not well known. The difficulties in assessing the history of the AAC terrane are predominantly due to a lack of comprehensive knowledge about the composition and age of its basement. During the Mesozoic, the AAC terrane was involved in crustal shortening, followed by magmatism and extension with localized high-grade metamorphism and partial melting, all of which obscured its pre-orogenic geological relationships. New zircon geochronology and isotope geochemistry results from Wrangel Island and western Chukotka basement rocks establish and strengthen intra- and inter-terrane lithological and tectonic correlations of the AAC terrane. Zircon U–Pb ages of five granitic and one volcanic sample from greenschist facies rocks on Wrangel Island range between 620±6 and 711±4 Ma, whereas two samples from the migmatitic basement of the Velitkenay massif near the Arctic coast of Chukotka yield 612±7 and 661±11 Ma ages. The age spectrum (0.95–2.0 Ga with a peak at 1.1 Ga and minor 2.5–2.7 Ga) and trace element geochemistry of inherited detrital zircons in a 703±5 Ma granodiorite on Wrangel Island suggests a Grenville–Sveconorwegian provenance for metasedimentary strata in the Wrangel Complex basement and correlates with the detrital zircon spectra of strata from Arctic Alaska and Pearya. Temporal patterns of zircon inheritance and O–Hf isotopes are consistent with Cryogenian–Ediacaran AAC magmatism in a peripheral/external orogenic setting (i.e. a fringing arc on rifted continental margin crust).

Deformational history and thermochronology of Wrangel Island, East Siberian Shelf and coastal Chukotka, Arctic Russia

Abstract In Arctic Russia, south of Wrangel Island, Jura–Cretaceous fold belt structures are cut by c. 108–100 Ma plutonic rocks and a c. 103 Ma migmatitic complex (U–Pb, zircon) that cooled by c. 96 Ma (40Ar/39Ar biotite); the structures are unconformably overlain by c. 88 Ma and younger (U–Pb, zircon) volcanic rocks. Wrangel Island, with a similar stratigraphy and added exposure of Neoproterozoic basement rocks, was thought to represent the westwards continuation of the Jura–Cretaceous Brookian thrust belt of Alaska. A penetrative, high-strain, S-dipping foliation formed during north–south stretching in Triassic and older rocks, with stretched pebble aspect ratios of c. 2:1:0.5 to 10:1:0.1. Deformation was at greenschist facies (chlorite+white mica; biotite at depth; temperature c. 300–450°C). Microstructures suggest deformation mostly by pure shear and north–south stretching; the quartz textures and lattice preferred orientations suggest temperatures of c. 300–450°C. 40Ar/39Ar K-feldspar spectra (n=1) and muscovite (n=3) (total gas ages c. 611–514 Ma) in Neoproterozoic basement rocks are consistent with a short thermal pulse during deformation at 105–100 Ma. Apatite fission track ages (n=7) indicate cooling to near-surface conditions at c. 95 Ma. The shared thermal histories of Wrangel Island and Chukotka suggest that Wrangel deformation is related to post-shortening, north–south extension, not to fold–thrust belt deformation. Seismic data (line AR-5) indicate a sharp Moho and strong sub-horizontal reflectivity in the lower and middle crust beneath the region. Wrangel Island probably represents a crustal-scale extensional boudin between the North Chukchi and Longa basins.

ZIRCON U-PB AGES AND PETROLOGIC EVOLUTION OF THE ENGLISH PEAK GRANITIC PLUTON: JURASSIC CRUSTAL GROWTH IN NORTHWESTERN CALIFORNIA

In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1–2-km-diameter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10–15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20–25 km for the early satellitic plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207Pb-corrected 206Pb/238U ages, 95% confidence level) from seven samples in the complex via laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc. INTRODUCTION TO THE REGIONAL GEOLOGY The iconic Klamath Mountains Province provides a classic example of the Phanerozoic growth of transitional continental crust. Its imbricated lithotectonic belts testify to successive (1) arrival of far-traveled oceanic crust and overlying deep-sea sediments, (2) underflow and off-loading along the convergent plate junction, (3) suprasubduction-zone arc generation, and (4) erosional supply of arc debris to the accreting ophiolitic basement terranes. This late Paleozoic–Mesozoic development of northern California was typified by chiefly margin-parallel slip, episodic suturing of ophiolitic complexes and their spatially associated chert-argillite terranes, concurrent construction of vol canicplutonic arcs, plus the aprons of derived clastic sediments (Saleeby, 1981, 1982, 1983; Ernst et al., 2008). The terrane assembly of the Klamath Mountains has been correlated with the northern Sierran Foothills based on similar rock types, structures, terrane ages, the oceanward assembly of successively younger geologic units, and their times of orogeny (Davis, 1969; Davis et al., 1980; Wright and Fahan, 1988; Wright and Wyld, 1994; Irwin, 2003). However, the accreted Sierran Foothill terranes stand nearly vertically, whereas the Klamath thrust sheets root gently to the east. Scattered, chiefly calc-alkaline igneous activity characterized the Late Triassic margin, but most lithologic sections of late Paleozoic–early Mesozoic age are oceanic in their genesis. However, a major Andean arc began to form in the Sierra Nevada and Klamath Mountains by ca. 175 Ma during transpressive eastward underflow of oceanic lithosphere (Dunne et al., 1998; Irwin, 2003; Dickinson, 2008). This magmatic arc shed clastic detritus into the realm of the accreting ophiolitic basement terranes of both the Klamath Mountains and Sierran Foothills (Miller and Saleeby, 1995; Scherer et al., 2006). Figure 1 presents the regional geology of the Klamath Mountains Province, as well as published geochronology of various mafic-to-felsic plutons. The orogen is a terrane collage consisting of four major lithotectonic zones. From east to west, these are the Eastern Klamath Belt, the Central Metamorphic Belt, GEOSPHERE GEOSPHERE; v. 12, no. 5 doi:10.1130/GES01340.1 6 figures; 1 table; 1 supplemental file CORRESPONDENCE: wernst@ stanford .edu CITATION: Ernst, W.G., Gottlieb, E.S., Barnes, C.G., and Hourigan, J.K., 2016, Zircon U-Pb ages and petrologic evolution of the English Peak granitic pluton: Jurassic crustal growth in northwestern California: Geosphere, v. 12, no. 5, p. 1422–1436, doi: 10 .1130 /GES01340.1. Received 30 March 2016 Revision received 8 July 2016 Accepted 23 August 2016 Published online 9 September 2016 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.