An official website of the United States government
Magnetitite deposits like El Laco (Chile) are rare and have controversial origins. An unusual magnetitite lava flow overlying a rhyolite unit occurs in the north-central Alaska Range and originally covered ~ 750 km2 of the Miocene Nenana basin. Dating of the rhyolite and relationships between the magnetitite and sedimentary rocks indicate that both are of Late Miocene age. The magnetitite flow is mainly magnetite with some post-eruptive alteration to hematite. Both the rhyolite flow and the magnetitite flow are vesicular, but the magnetitite flow also has small, millimetre-scale columnar jointing. The vesicular zones in the magnetitite flow grade into massive rock on the scale of a thin section, suggesting a degassing lava origin. Samples of the magnetitite flow contain between 12 and 26 wt.% SiO2 and between 45 and 75 wt.% FeO. Rare earth elements (REE) and trace elements from the magnetitite and rhyolite have similar patterns but with lesser abundance in the magnetitite. Both the rhyolite and the magnetitite have light-REE-enriched REE profiles with negative Eu anomalies. Electron microscopic analysis shows that most of the silica and trace element content of the magnetitite flow comes from very finely disseminated silicate minerals and glass in the magnetite. This suggests that the magnetitite was derived from a magma that had undergone unmixing into a silica-rich phase and an iron-rich phase prior to its eruption. Fractures and vesicles within the magnetitite flow contain minor rhyolitic glass and minerals suggesting that the rhyolite magma invaded columnar joints in the solidified magnetitite flow, and is a subvolcanic sill-like body at the studied locality. The magnetitite flow erupted prior to the emplacement of the rhyolite, which may be extrusive on a regional scale. The features of the Nenana magnetitite, and its geological relationships, are consistent with genetic models that invoke unmixing of magma into immiscible Fe-rich and Si-rich liquids during ascent.
more »
« less
The last deglaciation of Alaska and a new benchmark 10Be moraine chronology from the western Alaska Range
- Award ID(s):
- 1853705
- PAR ID:
- 10331813
- Date Published:
- Journal Name:
- Quaternary Science Reviews
- Volume:
- 287
- Issue:
- C
- ISSN:
- 0277-3791
- Page Range / eLocation ID:
- 107549
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract A shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading.more » « less
-
Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an ∼700 km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262 m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the R/V Sikuliaq in May and July 2018 and retrieved on two cruises aboard the R/V Sikuliaq and R/V Langseth in August–September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May–June 2019, and an active-source seismic survey on the R/V Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 Mw 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories.more » « less
-
Abstract Comprehensive observations of surface wave anisotropy across Alaska and the Aleutian subduction zone would help to improve understanding of its tectonics, mantle dynamics, and earthquake risk. We produce such observations, using stations from the USArray Transportable Array, regional networks across Alaska, and the Alaska Amphibious Community Seismic Experiment in the Alaska‐Aleutian subduction zone both onshore and offshore. Our data include Rayleigh and Love wave phase dispersion from earthquakes (28–85 s) and ambient noise two‐ and three‐station interferometry (8–50 s). Compared with using two‐station interferometry alone, three‐station interferometry significantly improves the signal‐to‐noise ratio and approximately doubles the number of measurements retained. Average differences between both isotropic and anisotropic tomographic maps constructed from different methods lie within their uncertainties, which is justification for combining the measurements. The composite tomographic maps include Rayleigh wave isotropy and azimuthal anisotropy from 8 to 85 s both onshore and offshore, and onshore Love wave isotropy from 8 to 80 s. In the Alaska‐Aleutian subduction zone, Rayleigh wave fast directions vary from trench parallel to perpendicular and back to parallel with increasing periods, apparently reflecting the effect of the subducted Pacific Plate. The tomographic maps provide a basis for inferring the 3‐D anisotropic crustal and uppermost mantle structure across Alaska and the Aleutian subduction zone.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.quascirev.2022.107549
