David B. Stone

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time-averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long-lived core-mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low-latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low-quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two-parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

Paleoclimatic forcing of magnetic susceptibility variations in Alaskan loess during the late Quaternary

Visual matches and statistical tests suggest correlations between marine isotope curves, retrodictive solar insolation at lat 65°N, and magnetic susceptibility profiles through late Quaternary age Alaskan loess sections. The susceptibility changes largely appear to reflect variability in magnetite content due to climatically controlled changes in wind intensity and competence. Magnetic susceptibility profiles through massive loess can provide stratigraphic context for intercalated paleosols and tephras. A prominent paleosol correlated with marine isotope stage 5 occurs several metres above the Old Crow ash in loess sections, indicating that this important tephra is older than suggested by thermoluminescence dates, and may have been deposited ca. 215 ±25 ka.

Isotope studies in large river basins: A new global research focus

Rivers are an important linkage in the global hydrological cycle, returning about 35%of continental precipitation to the oceans. Rivers are also the most important source of water for human use. Much of the worlds population lives along large rivers, relying on them for trade, transportation, industry, agriculture, and domestic water supplies. The resulting pressure has led to the extreme regulation of some river systems, and often a degradation of water quantity and quality For sustainable management of water supply agriculture, flood-drought cycles, and ecosystem and human health, there is a basic need for improving the scientific understanding of water cycling processes in river basins, and the ability to detect and predict impacts of climate change and water resources development.

Tectonic implications of Alaska Peninsula paleomagnetic data

Abstract Recently determined paleomagnetic pole positions from rocks of Cretaceous and Eocene age from southern and southwestern Alaska are displaced from the pole positions derived from rocks of similar age from cratonic North America. These Alaskan data, when combined with previously determined Alaskan Jurassic paleopoles, mark out a paleomagnetic polar wander path very different from that for North America, but similar in trend to that for the Pacific. By constraining Alaska to have always been adjacent to the North American plate boundary, the paleomagnetic data have been used to reconstruct possible paleogeographies. The error limits in the paleomagnetic data are large, thus allowing a wide range of reconstructions, but to satisfy the data at all, South Alaska must have moved north and rotated in a clockwise direction with respect to North America in Mesozoic and/or Early Tertiary times. Movement of this sort is compatible with the relative plate motions predicted for the NE Pacific.

Varve counting vs. tephrochronology and 137Cs and 210Pb dating: A comparative test at Skilak Lake, Alaska

The age of recently deposited sediments in Skilak Lake has previously been estimated only by counting [open quotes]varves[close quotes]. We measured the sedimentation rate of Skilak Lake using two radionuclides, [sup 137]Cs and [sup 210]Pb, and found it to be about an order of magnitude lower than previous estimates based on varve counts. We also identified several tephras through a combination of visual inspection, core X-radiographs, observation of variations of the magnetic susceptibility, intensity of magnetization of the unconsolidated sediments, and microprobe analyses of volcanic glass shards. Tephrochronologic dates using matches with the Katmai 1912 tephra and an Augustine tephra from 500-550 yr B.P. are in good agreement with the sedimentation rate estimates based on the radioisotope data, and indicate that the rhythmite layers at Skilak Lake are not annual varves. These new estimates of sedimentation rate reaffirm that care is needed in varve dating and require that earlier work on sunspot and climate changes thought to have been recorded in the Skilak Lake sediments be reevaluated. 16 refs., 6 figs., 1 tab.

Crustal structure of the active margin, south central Alaska: An interpretation of seismic refraction data from the Trans‐Alaska crustal transect

Seismic refraction and wide-angle reflection data from the U.S. Geological Surveys Trans-Alaska Crustal Transect are used to investigate the upper crustal structure of southcentral Alaska and to develop a model using two-dimensional asymptotic ray theory. The data considered here are from the N-S 126-km Cordova Peak refraction profile, which was positioned to cross the Chugach and Prince William terranes. These two tectonostratigraphic terranes form part of a large accretionary complex related to the Alaska subduction zone. Interpretation of data from the Cordova Peak profile indicates systematically higher velocities in Chugach terrane rocks as compared with Prince William terrane rocks at comparable depths. Unconsolidated sediment and glacial ice overlie Chugach terrane rocks Of unusually high compressional velocities (4.7–7.0 km/s) in the upper 10 km of crust. A northward thickening metasedimentary and metavolcanic flysch sequence, having an average velocity of 6.0 km/s, is underlain by metavolcanic mafic basement rocks having an average velocity of 6.6 km/s. The upper 10 km of the Prince William terrane have compressional velocities ranging from 3.7 to 6.2 km/s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic Prince William terrane basement rocks. The upper crustal velocity structures of the Chugach and Prince William terranes are distinct to at least 10 km depth, which suggests that the Contact fault zone is a boundary which extends to at least 10 km. Midcrustal layers beneath the Chugach terrane contain two velocity reversals which may indicate the presence of off-scraped oceanic sediment, overpressurized zones, shear zones, or some combination of these possibilities. Data from energy sampling midcrustal layers beneath the Prince William terrane do not provide similar evidence for low-high velocity zones in that area.

A 500-year-long record of tephra falls from Redoubt Volcano and other volcanoes in upper Cook Inlet, Alaska

Abstract Volcanic ash layers preserved in glacial-lacustrine sediments at Skilak Lake on the Kenai Peninsula of southcentral Alaska constitute a record of eruptions at Redoubt Volcano and other Alaskan volcanoes which affected the upper Cook Inlet area during the last 500 years. High-resolution magnetic susceptibility profiling delineates similar sequences of tephra layers in several 1-m-long lake sediment cores. Electron microprobe analyses of glass shards from the tephras indicate correlation of some ash layers with known reference tephras from the source volcanoes, while other ash layers record previously unknown prehistoric eruptions. Skilak Lake cores contain ash from the historic 1912 Katmai eruption, the 1902 Redoubt eruption, and the 1883 Mount St. Augustine eruption as well as prehistoric ash layers erupted from Crater Peak at Mt. Spurr ca. 250–350 years ago, from Redoubt Volcano at ca. 300–400 years ago and again at ca. 350–450 years ago, and a 500-year-old ash from Mount St. Augustine. Still older tephras from Redoubt Volcano and Crater Peak at Mt. Spurr are found lower in the cores. The cores indicate that volcanoes in the Cook Inlet area have erupted every 10–35 years during the 20th century, and ash falls have occurred at Skilak Lake at least once every 50–100 years for the last 500 years, with Redoubt, Spurr, and Augustine Volcanoes being the most important sources of tephra.

Cooperative program helps decipher tectonics of northeastern Russia

Recently obtained paleomagnetic, geochronologic, and seismic data are greatly improving our understanding of the tectonics of northeastern Russia, which lies in an accretionary zone between the stable platforms of the North American and Eurasian plates. In particular, the present-day plate boundaries and motions have been clarified and a model has been developed for the accretionary history of the poorly understood Mesozoic fold belts of northeastern Russia. The evolution of northeastern Russia appears to be similar to that of Alaska and the Canadian Cordillera, and many similar tectonic and stratigraphic features can be identified in all three regions.

New data on plate tectonics of Alaska

Abstract Geologic evidence indicates that Alaska probably consists of a series of eugeosynclinal belts that may represent several events of continental development. Recent paleomagnetic data indicate that since Jurassic time, a block of southern Alaska that includes at least the Alaska Peninsula and Aleutian Range has rotated approximately 59° clockwise and has moved northward approximately 19° of latitude. It is unknown how many similar events might have occurred previously. An extensive search for active faults in south-central Alaska did not locate a welldefined Pacific—North American plate boundary north of the Fairweather fault. Seismic data indicate a Benioff zone involving the Pacific plate, which is dipping northwest at a low angle beneath much of southern Alaska. This portion of Alaska, which is south of the Denali fault, appears to be partially coupled to the Pacific plate beneath. The Denali fault is usually regarded as an extension of the Queen Charlotte Islands transform fault, but the transfer of motion from the Denali fault to the Aleutian trench may occur at lower crustal depths in the Mt. McKinley area. Seismic data and deformation associated with the 1964 Alaska earthquake imply present activity along the eastern Aleutian trench, although the activity and the trench become poorly defined south and east of Prince William Sound. The plate-boundary connection between Mt. McKinley and the Queen Charlotte Islands fault is poorly defined, but may occur along the Totschunda fault system in southern Alaska, along a complex system of thrust faults, or along another fault system in southern Alaska; it may be poorly defined because it is geologically young and is still developing. Evidence of recent fault activity north of the Denali fault gives a similarly complicated picture and suggests that transfer of stress may occur along the McGinnis Glacier, Healy Creek, Donnelly Dome, Clearwater Lake, and other active faults. Active faults farther north may or may not result from plate interaction along these boundaries. Evidence of recent fault activity has been found within the southern partially coupled block along the Castle Mountain, Hanning Bay, Patton Bay, and other faults.

Bering Sea-Aleutian Arc, Alaska

The Aleutian Island Arc forms part of the northern boundary of the Pacific Ocean, stretching from the Alaska Peninsula in the east to the Kamchatka Peninsula in the west (Fig. 1). The islands of the arc are located on the crest of a large ridge that follows the same trend as the islands. South of the Aleutian Ridge is the Aleutian Trench, which again connects from south of the Alaska Peninsula in the east through to the Kuril-Kamchatka Trench in the west. The trench is about 3400 km from end to end.