ALBERT

Description: Since the first application of cosmogenic nuclides to the study of glacial history and processes in 1990, increasing numbers of studies have used a variety of cosmogenic isotopes to quantify the exposure age and erosion rate of glaciated landscapes. However, obtaining chronological data from glaciated landscapes once covered by cold-based, nonerosive ice is challenging because these surfaces violate assumptions associated with simple cosmogenic exposure dating. Nonerosive glacial ice fails to completely remove nuclides produced during previous periods of exposure, leaving behind rock surfaces with complex, multigenetic nuclide inventories. Here, we constrain the glacial history, landscape evolution, and efficiency of subglacial erosion in the Pangnirtung Fiord region of southern Baffin Island using over 300 paired analyses of in situ cosmogenic 10 Be and 26 Al. Simple exposure ages are 6.3–160 ka for 10 Be ( n = 152) and 4.3–124 ka for 26 Al ( n = 153). Paired bedrock-boulder samples have discordant ages, simple exposure ages generally increase with elevation, 10 Be and 26 Al ages for the same sample disagree, and both boulders and bedrock yield multimodal age distributions—all patterns suggestive of limited subglacial erosion. Measured 26 Al/ 10 Be ratios indicate that about one third of the samples in the data set experienced at least one period of pre-Holocene exposure followed by burial with limited erosion. Modeled two-isotope minimum-limiting exposure durations are as high as hundreds of thousands of years, and minimum-limiting burial durations range up to millions of years, implying that parts of southern Baffin Island have been preserved beneath nonerosive glacial ice for much, if not all, of the Quaternary. A subset of the samples contains few nuclides inherited from prior periods of exposure and is thus useful for constraining the chronology of the most recent deglaciation. Using these samples, we infer that deglaciation of most of the landscape occurred ca. 11.7 ka and that the Duval moraines, a prominent feature of the last deglaciation, formed ca. 11.2 ka.

Description: Field evidence, map compilation, geochemistry, geochronology, and potential field data document six intervals of Cretaceous magmatism in the central Sverdrup Basin. These are: (1) Hauterivian (ca. 130 Ma) volcaniclastic deposition in the lower Isachsen Formation; (2) 126.6 ± 1.2 Ma (U-Pb zircon) gabbroic intrusion; (3) 120.8 ± 0.8 Ma (U-Pb baddeleyite) diabasic intrusion; (4) 105.40 ± 0.22 Ma (U-Pb detrital zircon) pyroclastic deposition at the top of the Invincible Point Member, Christopher Formation; (5) upper Albian (ca. 103 Ma) pillow and hydroclastic breccia in the upper Christopher Formation; and (6) uppermost Albian (ca. 101 Ma) volcanic breccia and scoria in the Hassel Formation. Whole-rock geochemical data show that these magmatic rocks are similar to previously documented High Arctic large igneous province tholeiitic basalts, but analyses of fresh glass in tuffs reveal evolved ferroandesite to dacite compositions not recorded in whole-rock data. Approximate ages of saucer-shaped sills inferred from the relationship of sill width to depth of emplacement suggest at least three intervals of sill emplacement between 130 and 120 Ma. The new data show that volcanism in the Sverdrup Basin was of greater spatial extent, and that magmatism occurred more frequently, than was previously recognized. Comparison of the new central Sverdrup Basin data and interpretations with other data sets from the Sverdrup Basin, Svalbard, and Franz Josef Land suggests that High Arctic large igneous province magmatism occurred over a more extended period of time in the central Sverdrup Basin than in other regions.

Description: Th-Pb dating of monazite in xenoliths of low-temperature metamorphic eclogite facies rocks from diatremes of the Navajo volcanic field in the center of the Colorado Plateau (southwest United States) yields ages of ca. 28 Ma. Because monazite is not a primary phase in basic igneous or metamorphic rocks, but introduced during metasomatism, we suggest that the fluid responsible for monazite growth was derived from prograde metamorphic dehydration reactions in serpentinites and related rocks in the subducted Farallon plate. These fluids hydrated the overlying sub-plateau lithospheric mantle, consuming garnet (thus mobilizing rare earth elements) and lowering mantle density and increasing volume, contributing to the uplift of the Colorado Plateau in early Oligocene time.

Description: A narrow (~7 km wide) fold and thrust belt in west Texas that represents the northernmost extent of a Grenville-age collisional belt along the southern margin of Laurentia (Grenville Front), records a complex history of deformation and associated fluid flow. The Streeruwitz thrust that emplaced ca. 1.35 Ga high-grade metamorphic rocks over ca. 1.25 Ga foreland sedimentary and volcanic rocks postdates polyphase deformation in the footwall and is complexly folded into domes and basins. Four phases of tectonism, recording a changing kinematic setting, affected the area and formed: (1) pre-Streeruwitz ductile polyphase folds (F 1 –F 3 ) and associated foliations (S 1 –S 2 ) consistent with northward tectonic transport; (2) dextral oblique-slip, high-angle, west-northwest–trending faults associated with upright vertical sheath folds (F 4 ); (3) Streeruwitz and related subsidiary imbricate thrusts that truncate F 1 –F 4 folds at a high angle and cause localized folding (F 5 ) consistent with north-northeast to northeastward tectonic transport; and (4) complex southeast- and northwest-trending domes and basins (F 6 ) of the thrusts, resulting from continual Grenville-age transpression. Fluids with an evolving chemistry over time were channelized along the thrusts, metasomatically altering the adjacent rocks. Early siliceous fluids caused replacement of mafic dikes and dolostones that preserve F 1 and S 1 and formation of extensive talc bodies with talc aligned axial planar to F 2 , forming the dominant S 2 fabric. Initial thrusting at depth produced mylonites in both footwall (syn-S 2 ) and hanging-wall rocks that were later brecciated in the final stage of thrusting along the Streeruwitz thrust. Further evolution of fluids along the thrusts is recorded in altered rocks adjacent to thrusts, breccias, and veins, starting with silica- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced footwall rocks and cemented breccias in both the hanging wall and footwall. This study documents a previously unrecognized complex structural, metamorphic, and metasomatic history, and fluid evolution in the foreland. This history, coupled with differences from that in the overriding older metamorphic rocks, requires a new kinematic model for the southern margin of Laurentia. In addition, the disparity in deformation timing and kinematic evolution between west Texas and the central Texas Llano uplift requires active subduction in west Texas after collision in central Texas. We propose that collision of a north-verging continental indenter with southern Laurentia initially occurred in the Llano area (ca. 1150–1120 Ma) and that continued subduction along strike caused clockwise rotation of the indenting continent and collision in west Texas (ca. 1060–980 Ma).

Description: Olivine, orthopyroxene, and spinel compositions within seafloor peridotites yield important information about the nature of Earth’s mantle. Major element compositions of these minerals can be used to calculate oxygen fugacity, a thermodynamic property critical to understanding phase equilibria in the upper mantle. This study examines how hydrothermal alteration at the seafloor influences peridotite chemistry. The Tonga Trench (South Pacific Ocean) exposes lithospheric forearc peridotites that range from highly altered to completely unaltered and provides an ideal sample suite for investigating the effect of alteration on spinel peridotite major element chemistry and calculated oxygen fugacity. Using the Tonga peridotites, we develop a qualitative alteration scale rooted in traditional point-counting methodology. We show that high degrees of serpentinization do not affect mineral parameters such as forsterite number in olivine, iron site occupancy in orthopyroxene, and Fe 3+ /Fe ratio in spinel. Additionally, while serpentinization is a redox reaction that leaves behind an oxidized residue, the oxygen fugacity recorded by mantle minerals is unaffected by nearby low-temperature serpentinization. As a result, oxygen fugacity measured by spinel oxybarometry in seafloor peridotites is representative of mantle processes, rather than an artifact of late-stage seafloor alteration.

Description: Over the past decades, much research has focused on the mid-Cretaceous greenhouse climate, the formation of widespread organic-rich black shales, and cooling intervals from low- to mid-latitude sections. Data from the High Arctic, however, are limited. In this paper, we present high-resolution geochemical records for an ~1.8-km-thick sedimentary succession exposed on Axel Heiberg Island in the Canadian Arctic Archipelago at a paleolatitude of ~71°N. For the first time, we have data constraints for the timing and magnitude of most major Oceanic Anoxic Events (OAEs) in brackish-water (OAE1a) and shelf (OAE1b and OAE2) settings in the mid-Cretaceous High Arctic. These are consistent with carbon-climate perturbations reported from deep-water records of lower latitudes. Glendonite beds are observed in the upper Aptian to lower Albian, covering an interval of ~6 m.y. between 118 and 112 Ma. Although the formation of glendonites is still under discussion, these well-dated occurrences may support the existence of cool shelf waters in the High Arctic Sverdrup Basin at this time, coeval with recent geochemical data from the subtropical Atlantic indicating a drop in sea-surface temperature of nearly 4 °C.

Description: 〈span〉The Alaska Range, the topographic signature of the Denali fault, has an unusual physiography, with the Nenana River sourced from the south side of the divide and traversing along the range front some distance before heading north across the mountain range. Previous researchers suggested that a change from south-flowing to north-flowing drainage occurred at ca. 6 Ma, or early-middle Miocene, during initial phases of Alaska Range uplift. We applied 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating of detrital micas from modern river sediment (proxy for basinwide source) and strata of the Neogene Tanana Basin (sink for the paleo–Nenana River), located along the northern front of the Alaska Range, to further investigate this hypothesis. In addition, we acquired 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar muscovite ages from bedrock for additional source constraints and compared our results to regional geochronology data sets and geological mapping. During the earliest Miocene, the paleo–Nenana River likely flowed south. By the early Miocene, the paleo–Nenana River flowed to the north. During the middle Miocene, drainage reorganization continued, suggesting a variable history of rock uplift in the Alaska Range. By the late Miocene, sediment recycling occurred as the southern extent of the Tanana Basin was uplifted and eroded. The modern Nenana River near Cantwell has a muscovite age signature different than the Tanana Basin strata, implying continued drainage reorganization after the deposition of the Pliocene Nenana Gravel. In summary, the Nenana River drainage changed direction to north-flowing by ca. 18 Ma, driven by tectonism. Drainage reorganization continues today, demonstrating that strike-slip fault transpressive orogens can have complex paleodrainage histories.〈/span〉

Description: Valley networks are some of the strongest lines of evidence for extensive fluvial activity on early (Noachian; 〉3.7 Ga) Mars. However, their purported absence on certain ancient terrains, such as Arabia Terra, is at variance with patterns of precipitation as predicted by "warm and wet" climate models. This disagreement has contributed to the development of an alternative "icy highlands" scenario, whereby valley networks were formed by the melting of highland ice sheets. Here, we show through regional mapping that Arabia Terra shows evidence for extensive networks of sinuous ridges. We interpret these ridge features as inverted fluvial channels that formed in the Noachian, before being subject to burial and exhumation. The inverted channels developed on extensive aggrading flood plains. As the inverted channels are both sourced in, and traverse across, Arabia Terra, their formation is inconsistent with discrete, localized sources of water, such as meltwater from highland ice sheets. Our results are instead more consistent with an early Mars that supported widespread precipitation and runoff.

Description: The tectonic setting of northern Laurentia prior to the opening of the Arctic Ocean is the subject of numerous tectonic models. By better understanding the provenance of detrital zircon in the Canadian Arctic prior to rifting, both the prerift tectonic setting and timing of rifting can be better elucidated. In the Sverdrup Basin, two distinct provenance assemblages are identified from new detrital-zircon U-Pb data from Lower Triassic to Lower Jurassic strata in combination with previously published detrital-zircon data. The first assemblage comprises an age spectrum identical to that of the Devonian clastic wedge in the Canadian Arctic and is termed the recycled source. In contrast, the second assemblage is dominated by a broad spectrum of near syndepositional Permian–Triassic ages derived from north of the basin and is termed the active margin source. Triassic strata of Yukon and Arctic Alaska exhibit a similar dual provenance signature, whereas in northeastern Russia, Chukotka contains only the active margin source. Complementary hafnium isotopic data on Permian–Triassic zircon have Hf values that are consistent with the common evolved crustal signature of the Devonian clastic wedge detrital-zircon grains and Neoproterozoic–Paleozoic basement rocks in the Arctic Alaska–Chukotka microcontinent. Furthermore, newly identified volcanic ash beds throughout the Triassic section from the northern part of the Sverdrup Basin, along with abundant Permian–Triassic detrital zircon, suggest a protracted history of magmatism to the north of the basin. We interpret that these zircons were sourced from a magmatically active region to the north of the Sverdrup Basin, and in the context of a rotational model for opening of Amerasia Basin, this was probably part of a convergent margin fringing northern Laurentia from the northern Cordillera along the outboard edge of Arctic Alaska and Chukotka terranes. In Early Jurassic strata, Permian–Triassic zircons decrease substantially, implying the diminution of the active margin as a sediment source as initial rifting isolated the Permian–Triassic source from the Sverdrup Basin.