Robert B. Forbes

Radiometric Ages of Kodiak Seamount and Giacomini Guyot, Gulf of Alaska: Implications for Circum-Pacific Tectonics

Kodiak Seamount and Giacomini Guyot have been dated at 22.6 � 1.1 and 19.9 � 1.0 [2σ (standard deviation)] x 106 years, respectively. Concordant whole-rock and plagioclase potassium-argon dates and fission-track apatite ages demonstrate that significant quantities of excess radiogenic 40Ar are not present in the dated samples. These seamounts are the northwesternmost edifices of the Pratt-Welker chain, which cuts obliquely across magnetic anomaly patterns in an older northeastern Pacific sea floor. The older of the two dated seamounts is in the Aleutian Trench, apparently about to be subducted. If one assumes that seamounts are generated by plate motion over a fixed hot spot in the mantle, a Pacific-plate motion of 6.6 centimeters per year during early Miocene time may be calculated.

Blueschists of the Kodiak Islands, Alaska: An extension of the Seldovia schist terrane

The Kodiak Islands are composed of a series of northeast-trending belts of schists and deep-sea rock types that are interpreted as having been accreted to the continental margin during several discrete phases of subduction since early Mesozoic time. The Kodiak Islands schist terrane is the oldest of these accretionary belts and crops out discontinuously along the northwest side of the islands. Metamorphic rocks in this belt include quartz-mica schist, marble, metachert, greenschist, blueschist, and epidote amphibolite; the rocks yield Early Jurassic K-Ar mineral ages. These ages apparently provide a measure for the age of emplacement of the Kodiak Islands schists and are consistent with independently determined age estimates based on (1) biostratigraphy of the associated forearc basin deposits and (2) K-Ar ages from the associated plutonic arc on the Alaska Peninsula. Similarities in rock types, mineral ages, and tectonic setting indicate that the Kodiak Islands schist terrane is the southwestern extension of the Seldovia blueschists of the Kenai Peninsula.

K40/Ar40 Age Relations of the Coast Range Batholith and Related Rocks of the Juneau Ice Field Area, Alaska

K 40 /Ar 40 whole-rock, hornblende and biotite ages were determined for samples of crystalline schists and quartz monzonitic intrusives along a northeasterly traverse from Blackerby Ridge, 4 miles north of Juneau, Alaska, to the Alaska-Canada Boundary. Whole-rock and biotite ages of the quartz monzonite composing the East Marginal Pluton of the Coast Range Batholith, range from 46.9 to 52.8 m.y. Hornblende and biotite ages of migmatitic gneisses range from 50.4 to 56.1 m.y.; 57 to 60 m.y. K 40 / Ar 40 ages were determined for biotite and hornblende from crystalline schists in almandine amphibolite facies terrane on Blackerby Ridge. Apparent hornblende and biotite K 40 / Ar 40 ages increase to the southwest in the crystalline schists on the south margin of the bathohth. Hornblende ages are older than those determined forco-existent biotite from parent migmatitic gneisses. These data suggest that synkinematic metamorphism occurred in Late Cretaceous or early Tertiary time in the West Marginal Belt, and that the quartz monzonite intrusives of the East Marginal Pluton were emplaced during the Eocene period.

Dredged Trachyte and Basalt from Kodiak Seamount and the Adjacent Aleutian Trench, Alaska

Blocky fragments of aegirine-augite trachyte (with accompanying icerafted gravels.) were recovered from the upper slopes of Kodiak Seamount in several dredge hauls. An alkali basalt pillow segment was also dredged from a moatlike depression, at a depth of 5000 meters, near the west base of the seamount. These retrievals confirm the volcanic origin of Kodiak Seamount and further support the view of Engel, Engel, and Havens that the higher elevations of seamounts are composed of alkali basalts or related variants.

Ultrabasic inclusion from the basalts of the Hut Point area, Ross Island, Antarctica

The basaltic rocks of the Hut Point area contain a diverse suite of inclusions. The xenolithic nature of many sedimentary, metamorphic and igneous inclusion types is clearly displayed, but previous studies have left considerable doubt as to whether the ultrabasic inclusions are noncognate xenoliths, and possible relicts from the mantle brought to the surface in a magma originating in this zone; or rather of cognate and/or cumulative origin.Olivine grains in the dunite and enstatite bearing peridotite inclusions commonly display undulose extinction and strain bands (translation laminae), and preferred indicatrix orientations expressed as B γ girdles. Enstatite frequently contains clinopyroxene exsolution laminae, and many grains are deformed and/or disrupted by shear zones.Weak B γ olivine fabrics have also been detected in the titanaugite peridotites and pyroxenites, but no evidence of post-crystallization deformation was detected. Titanaugite also occurs as phenocrysts (xenocrysts?) and in the groundmass of the host basalt chemical and optical data suggest that the titanaugite grains in the basalt and the inclusions are genetically related.These preliminary findings suggest that the titanaugite bearing inclusions may be of cognate origin, and that the dunite and enstatite bearing peridotite fragments are more likely noncognate xenoliths.Field and laboratory studies of this problem are continuing as supported by a grant from the National Science Foundation.

A Geological and Geophysical Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska

The Pilgrim Springs geothermal area, located about 75 km north of Nome, was the subject of an intensive, reconnaissance-level geophysical and geological study during a 90-day period in the summer of 1979. The thermal springs are located in a northeast-oriented, oval area of thawed ground approximately 1.5 km{sup 2} in size, bordered on the north by the Pilgrim River. A second, much smaller, thermal anomaly was discovered about 3 km northeast of the main thawed area. Continuous permafrost in the surrounding region is on the order of 100 m thick. Present surface thermal spring discharge is {approx} 4.2 x 10{sup -3} m{sup 3} s{sup -1} (67 gallons/minute) of alkali-chloride-type water at a temperature of 81 C. The reason for its high salinity is not yet understood because of conflicting evidence for seawater vs. other possible water sources. Preliminary Na-K-Ca geothermometry suggests deep reservoir temperatures approaching 150 C, but interpretation of these results is difficult because of their dependence on an unknown water mixing history. Based on these estimates, and present surface and drill hole water temperatures, Pilgrim Springs would be classified as an intermediate-temperature, liquid-dominated geothermal system.

Radar and infrared remote sensing of geothermal features at Pilgrim Springs, Alaska

High-altitude radar and thermal imagery collected by the NASA research aircraft WB57F were used to examine the structural setting and distribution of radiant temperatures of geothermal anomalies in the Pilgrim Springs, Alaska area. Like-polarized radar imagery with perpendicular look directions provides the best structural data for lineament analysis, although more than half the mapped lineaments are easily detectable on conventional aerial photography. Radiometer data and imagery from a thermal scanner were used to evaluate radiant surface temperatures, which ranged from 3 to 17 C. The evening imagery, which utilized density-slicing techniques, detected thermal anomalies associated with geothermal heat sources. The study indicates that high-altitude predawn thermal imagery may be able to locate relatively large areas of hot ground in site-specific studies in the vegetated Alaskan terrain. This imagery will probably not detect gentle lateral gradients.