Geology topics

Heather A. Bleick

Publications and source records attributed to Heather A. Bleick.

9 recordsLinked to original sources

USGS2017

Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages

In support of regional geologic framework studies, we obtained 50 new argon-40/argon-39 ( 40 Ar/ 39 Ar) ages and 33 new uranium-lead (U-Pb) ages from igneous rocks of southwestern Alaska. Most of the samples are from the Sleetmute and Taylor Mountains quadrangles; smaller collections or individual samples are from the Bethel, Candle, Dillingham, Goodnews Bay, Holy Cross, Iditarod, Kantishna River, Lake Clark, Lime Hills, McGrath, Medfra, Talkeetna, and Tanana quadrangles. A U-Pb zircon age of 317.7±0.6 million years (Ma) reveals the presence of Pennsylvanian intermediate igneous (probably volcanic) rocks in the Tikchik terrane, Bethel quadrangle. A U-Pb zircon age of 229.5±0.2 Ma from gabbro intruding the Rampart Group of the Angayucham-Tozitna terrane, Tanana quadrangle, confirms and tightens a previously cited Triassic age for this intrusive suite. A fresh mafic dike in Goodnews Bay quadrangle yielded a 40 Ar/ 39 Ar whole rock age of 155.0±1.9 Ma; this establishes a Jurassic or older age for the previously unconstrained (Paleozoic? to Mesozoic?) sandstone unit that it intrudes. A thick felsic tuff in the Gemuk Group in Taylor Mountains quadrangle yielded a U-Pb zircon age of 153.0±2.0 Ma, extending the age of magmatism in this part of the Togiak terrane back into the Late Jurassic. We report three new U-Pb zircon ages between 120 and 110 Ma—112.0±0.9 Ma from syenite in the Candle quadrangle, 114.9±0.3 Ma from orthogneiss assigned to the Ruby terrane in Iditarod quadrangle, and 116.6±0.1 Ma from a gabbro of the Dishna River mafic-ultramafic complex in Iditarod quadrangle. The latter result requires a substantial age revision, from Triassic to Cretaceous, for at least some rocks that have been mapped as the Dishna River mafic-ultramafic complex. A tuff in the Upper Cretaceous Kuskokwim Group yielded a U-Pb zircon (sensitive high-resolution ion microprobe, SHRIMP) age of 88.3±1.0 Ma; we speculate that the eruptive source was an arc along the trend of the Pebble porphyry copper deposit along the Gulf of Alaska continental margin. More than half of the new ages fall between 75 and 65 Ma, confirming the existence, based on conventional potassium-argon (K-Ar) ages, of a 70-Ma igneous flare-up across southwestern Alaska. Our new ages hint that during this pulse, the locus of magmatism shifted toward the Gulf of Alaska, that is, toward a more outboard position. This shift is consistent with the hypothesis that magmatism was the product of rollback of a subducted slab, which at that time would have been the Resurrection Plate. Intrusive rocks in the Taylor Mountains and Sleetmute quadrangles in the age range of 63 to 59 Ma were emplaced shortly before the onset of ridge subduction as dated by near-trench plutons in the adjacent part of the Chugach accretionary complex. Southwestern Alaska at this time would have been positioned above a very young subducted slab belonging to the Resurrection Plate; magmas, in this scenario, were generated near the edge of the slab window related to ridge subduction. A 56.3±0.2 Ma granite in Taylor Mountains quadrangle and a 54.7±0.7 Ma ashfall tuff in McGrath quadrangle were likely emplaced above the Resurrection-Kula slab window, which by this time is inferred to have entered the region. Another ashfall tuff in McGrath quadrangle, at 42.8±0.5 Ma, likely belongs to the Meshik Arc, the product of renewed subduction after inferred passage of the slab window. A 49.0±0.3-Ma rhyolite in Taylor Mountains quadrangle is about the age of the transition from slab window to renewed subduction. Two plutons in the western Alaska Range, at 31.8±0.4 and 30.9±0.6 Ma, belong to a suite of gabbro to peralkaline granite of unknown origin. Finally, a 4.6±0.1-Ma basalt from a flow in Taylor Mountains quadrangle belongs to the Neogene basaltic province of western Alaska. These rocks were erupted in a distal retroarc setting; the cause of magmatism is unknown.

Alaska↗

USGS2014

238U-230Th dating of chevkinite in high-silica rhyolites from La Primavera and Yellowstone calderas

Application of 238 U- 230 Th disequilibrium dating of accessory minerals with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical evolution of magma within the framework of absolute time. Chevkinite, a Th-rich accessory mineral that occurs in peralkaline and metaluminous rhyolites, may be particularly useful as a chronometer of crystallization and differentiation because its composition may reflect the chemical changes of its host melt. Ion microprobe 128 U- 230 Th dating of single chevkinite microphenocrysts from pre- and post-caldera La Primavera, Mexico, rhyolites yields model crystallization ages that are within 10's of k.y. of their corresponding K-Ar ages of ca. 125 ka to 85 ka, while chevkinite microphenocrysts from a post-caldera Yellowstone, USA, rhyolite yield a range of ages from ca. 110 ka to 250 ka, which is indistinguishable from the age distribution of coexisting zircon. Internal chevkinite-zircon isochrons from La Primavera yield Pleistocene ages with ~5% precision due to the nearly two order difference in Th/U between both minerals. Coupling chevkinite 238 U- 230 Th ages and compositional analyses reveals a secular trend of Th/U and rare earth elements recorded in Yellowstone rhyolite, likely reflecting progressive compositional evolution of host magma. The relatively short timescale between chevkinite-zircon crystallization and eruption suggests that crystal-poor rhyolites at La Primavera were erupted shortly after differentiation and/or reheating. These results indicate that 238 U- 230 Th dating of chevkinite via ion microprobe analysis may be used to date crystallization and chemical evolution of silicic magmas.

Jalisco, Wyoming↗

USGS2014

U-Pb detrital zircon geochronology as evidence for the origin of the Nome Complex, northern Alaska, and implications for regional and trans-Arctic correlations

Detrital zircons from the Nome Complex, a metamorphic terrane in northern Alaska, reveal important constraints on the early Paleozoic history of the Arctic Alaska–Chukotka terrane, a microcontinental block with an origin exotic to Laurentia. Twenty-two samples (17 in this study, five previously published) produce three detrital zircon population patterns (called themes), indicating that at least three distinguishable source areas contributed to the metamorphic protolith. Detrital zircon populations from metamorphosed rift-related mafic volcaniclastic rocks, a lithologic subunit of the Nome Complex, contain a dominant population of 740–550 Ma zircons. Samples from three other lithologic units yielded populations dominated by early Paleozoic zircons and characterized by a large population of 450–420 Ma zircons. A few samples, taken from two different lithologic units, yielded populations dominated by Mesoproterozoic zircons (most around 1.25–0.9 Ga) and lacked zircons younger than 900 Ma. None of the 22 samples contained more than a few Archean zircons. The ages of the youngest detrital zircon populations indicate that little of the protolith for the Nome Complex can be as old as Proterozoic, as previously thought. Further, a significant part of the protolith sequence is Devonian or younger; these rocks are likely correlative with Devonian or Mississippian units in the Brooks Range, specifically marine parts of the Endicott or Lisburne Groups. Based on detrital zircon data, limiting factors can be placed on the paleogeographic history of the Nome Complex and associated parts of the Arctic Alaska–Chukotka terrane: (1) 740–550 Ma zircons were deposited in a rift-related basin formed on a continental margin in the early Paleozoic; at least some of those zircons may have been sourced from local basement; (2) a transition to new sediment sources is reflected in Devonian or younger protoliths with the appearance of 450–420 Ma and 1.25–0.9 Ga detrital zircons; and (3) 450–420 Ma and 1.25–0.9 Ga zircons may have been supplied from sources outside the Arctic Alaska–Chukotka terrane.

Alaska↗

USGS2014

A sight "fearfully grand": eruptions of Lassen Peak, California, 1914 to 1917

On May 22, 1915, a large explosive eruption at the summit of Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as 280 miles to the east. This explosion was the most powerful in a series of eruptions during 1914–17 that were the last to occur in the Cascade Range before the 1980 eruption of Mount St. Helens, Washington. A century after the Lassen eruptions, work by U.S. Geological Survey (USGS) scientists in cooperation with the National Park Service is shedding new light on these events.

California↗

USGS2013

Andesites of the 2009 eruption of Redoubt Volcano, Alaska

Crystal-rich andesites that erupted from Redoubt Volcano in 2009 range from 57.5 to 62.5 wt.% SiO 2 and have phenocryst and phenocryst-melt relations consistent with staging in the upper crust. Early explosive products are low-silica andesites (LSA, < 58 wt.% SiO 2 ) that ascended from deeper crustal levels during or before the 6 months of precursory activity, but a broad subsequent succession to more evolved and cooler products, and predominantly effusive dome growth, are interpreted to result from progressive mobilization and mixing with differentiated magmas tapped from pre-2009 Redoubt intrusions at ~ 3–6 km depth. Initial explosions on March 23–28 ejected predominantly LSA with a uniform phenocryst assemblage of high-Al amphibole, ~ An 70 plagioclase, ortho- and clinopyroxene, FeTi oxides (890 to 960 °C), and traces of magmatic sulfide. Melt in the dominant microlite-poor LSA was compositionally uniform dacite (67–68 wt.% SiO 2 ) but ranged to rhyolite with greater microlite growth. Minor amounts of intermediate- to high-silica andesite (ISA, HSA; 59–62.5 wt.% SiO 2 ) also erupted during the early explosions and most carried rhyolitic melt (72–74 wt.% SiO 2 ). A lava dome grew following the initial tephra-producing events but was destroyed by an explosion on April 4. Ejecta from the April 4 explosion consists entirely of ISA and HSA, as does a subsequent lava dome that grew April 4–July 1; LSA was absent. Andesites from the April 4 event and from the final dome had pre-eruptive temperatures of 725–840 °C (FeTi oxides) and highly evolved matrix liquids (77–80 wt.% SiO 2 ), including in rare microlite-free pyroclasts. ISA has mixed populations of phenocrysts suggesting it is a hybrid between HSA and LSA. The last lavas from the 2009 eruption, effused May 1–July 1, are distinctly depleted in P 2 O 5 , consistent with low temperatures and high degrees of crystallization including apatite. Plagioclase–melt hygrometry and comparison to phase equilibrium experiments are consistent with pre-eruptive storage of all three magma types at 100–160 MPa (4–6 km depth), if they were close to H 2 O-saturation, coincident with the locus of shallow syn-eruptive seismicity. Deeper storage would be indicated if the magmas were CO 2 -rich. Relatively coarse-grained clinopyroxene-rich reaction rims on many LSA amphibole phenocrysts may result from slow ascent to, or storage at, depths shallow enough for the onset of appreciable H 2 O exsolution, consistent with pre-eruptive staging in the uppermost crust. We interpret that the 2009 LSA ascended from depth during the 8 or more months prior to the first eruption, but that the magma stalled and accumulated in the upper crust where its phenocryst rim and melt compositions were established. Ascent of LSA through stagnant mushy intrusions residual from earlier Redoubt activity mobilized differentiated magma pockets and interstitial liquids represented by HSA, and as LSA–HSA hybrids represented by ISA, that fed the subsequently erupted lava domes.

Alaska↗

USGS2013

Volcano–ice interactions precursory to the 2009 eruption of Redoubt Volcano, Alaska

In late summer of 2008, after nearly 20 years of quiescence, Redoubt Volcano began to show signs of abnormal heat flow in its summit crater. In the months that followed, the excess heat triggered melting and ablation of Redoubt's glaciers, beginning at the summit and propagating to lower elevations as the unrest accelerated. A variety of morphological changes were observed, including the creation of ice cauldrons, areas of wide-spread subsidence, punctures in the ice carved out by steam, and deposition from debris flows. In this paper, we use visual observations, satellite data, and a high resolution digital elevation model of the volcanic edifice to calculate ice loss at Redoubt as a function of time. Our aim is to establish from this time series a proxy for heat flow that can be compared to other data sets collected along the same time interval. Our study area consists of the Drift glacier, which flows from the summit crater down the volcano's north slope, and makes up about one quarter of Redoubt's total ice volume of ~ 4 km 3 . The upper part of the Drift glacier covers the area of recent volcanism, making this part of ice mass most susceptible to the effect of volcanic heating. Moreover, melt water and other flows are channeled down the Drift glacier drainage by topography, leaving the remainder of Redoubt's ice mantle relatively unaffected. The rate of ice loss averaged around 0.1 m 3 /s over the last four months of 2008, accelerated to over twenty times this value by February 2009, and peaked at greater than 22 m 3 /s, just prior to the first major explosion on March 22, 2009. We estimate a cumulative ice loss over this period of about 35 million cubic meters (M m 3 ).

Alaska↗

USGS2012

Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska

New geochronologic and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed ~60 Ma metamorphic rocks with the base of the Arkose Ridge Formation. U-Pb (analyzed by Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG)) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3±0.3 (previously considered a Jurassic unit), 79.1±1.0, 76.1±0.9, 75.8±0.7, 72.5±0.4, 71.9±0.3, 70.5±0.2, and 67.3±0.2 Ma. The cooling of these plutons occurred between 72 and 66 Ma (zircon fission track (FT) closure ~225°C). 40 Ar/ 39 Ar analyses of hornblende, white mica, and biotite fall into this range (Harlan and others, 2003). New apatite FT data collected on a west-to-east transect reveal sequential exhumation of fault blocks at 62.8±2.9, 54±2.5, 52.6±2.8, and 44.4±2.2 Ma. Plutonic clasts accumulated in the Paleocene Arkose Ridge Formation to the south. Detrital zircon (DZ) ages from the formation reflect this provenance: a new sample yielded one grain at 61 Ma, a dominant peak at 76 Ma, and minor peaks at 70, 80, 88, and 92 Ma. The oldest zircon is 181 Ma. Our apatite FT ages range from 35.1 to 50.9 Ma. Greenschist facies rocks now sit structurally between the plutonic rocks and the Arkose Ridge Formation. They are separated from plutonic rocks by the vertical Hatcher Pass fault and from the sedimentary rocks by a detachment fault. Ar cooling ages (Harlan and others, 2003) and new zircon FT ages for these rocks are concordant at 61-57 Ma, synchronous with deposition of the Arkose Ridge Formation. A cooling age of ~46 Ma came from one apatite FT sample. The metamorphic protolith (previously considered Jurassic) was deposited at or after 75 Ma based on new DZ data. The probability curve has a major peak from 76 to 102 Ma, minor peaks at 186, 197, 213, 303, 346, and 1,828, and two discordant grains at ~2,700 Ma. This is similar to DZ populations in the Valdez Group. The short period of time between deposition, metamorphism, and exhumation are consistent with metamorphism in a subduction-zone setting. Ductile and brittle structures in the metamorphic rocks are consistent with exhumation in a transtensional setting.

Alaska↗

USGS2011

Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data

This 1:500,000-scale geologic map depicts the bedrock geology of Seward Peninsula, western Alaska, on the North American side of the Bering Strait. The map encompasses all of the Teller, Nome, Solomon, and Bendeleben 1:250,000-scale quadrangles, and parts of the Shishmaref, Kotzebue, Candle, and Norton Bay 1:250,000-scale quadrangles (sh. 1; sh. 2). The geologic map is presented on Sheet 1. The pamphlet includes an introductory text, detailed unit descriptions, tables of geochronologic data, and an appendix containing conodont (microfossil) data and a text explaining those data. Sheet 2 shows metamorphic and tectonic units, conodont color alteration indices, key metamorphic minerals, and locations of geochronology samples listed in the pamphlet. The map area covers 74,000 km 2 , an area slightly larger than West Virginia or Ireland.

Alaska↗

USGS2010

Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data

This 1:500,000-scale geologic map depicts the bedrock geology of Seward Peninsula, western Alaska, on the North American side of the Bering Strait. The map encompasses all of the Teller, Nome, Solomon, and Bendeleben 1:250,000-scale quadrangles, and parts of the Shishmaref, Kotzebue, Candle, and Norton Bay 1:250,000-scale quadrangles (sheet 1; sheet 2). The geologic map is presented on Sheet 1. The pamphlet includes an introductory text, unit descriptions, tables of geochronologic data, and an appendix containing conodont (microfossil) data and a text about those data. Sheet 2 shows metamorphic and tectonic units, conodont color alteration indices, key metamorphic minerals, and locations of geochronology samples listed in the pamphlet.

Alaska↗