Search USGS⌕ Search

Geology topics

G. B. Dalrymple

Publications and source records attributed to G. B. Dalrymple.

30 records · Page 2Linked to original sources
USGS1972

Hawaiian-emperor chain and its relation to cenozoic circumpacific tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction. © 1972, The Geological Society of America, Inc.

Hawaii↗
USGS1970

Thermoluminescence of lunar samples

Appreciable natural thermoluminescence with glow curve peaks at about 350 degrees centigrade for lunar fines and breccias and above 400 degrees centigrade for crystalline rocks has been recognized in lunar samples. Plagioclase has been identified as the principal carrier of thermoluminescence, and the difference in peak temperatures indicates compositional or structural differences between the feldspars of the different rock types. The present thermoluminescence in the lunar samples is probably the result of a dynamic equilibrium between acquisition from radiation and loss in the lunar thermal environment. A progressive change in the glow curves of core samples with depth below the surface suggests the use of thermoluminescence disequilibrium to detect surfaces buried by recent surface activity, and it also indicates that the lunar diurnal temperature variation penetrates to at least 10.5 centimeters.

Science↗
USGS1969

Experimental introduction of excess Ar40 into a granitic melt

Samples of a Precambrian granite were melted in sealed capsules to produce a radiogenic Ar 40 atmosphere over the melt. The amount of Ar 40 incorporated in the quenched charge was then determined. Under these experimental conditions the amount of argon dissolved in the quenched melt was appreciable and could be an important source of error in potassiumargon dating.

Contributions to Mineralogy and Petrology↗
USGS1969

40Ar/36Ar analyses of historic lava flows

The ratio 40 Ar/ 36 Ar was measured for 26 subaerial historic lava flows. Approximately one-third of the samples had 40 Ar/ 36 Ar ratios either higher or lower than the atmospheric value of 295.5 at the 95% confidence level. Excess radiogenic 40 Ar in five flows ranged from about 1 × 10 −13 to 1.5 × 10 −12 mol/g. Possible excess 36 Ar in three flows was on the order of 10 −16 to 10 −15 mol/g. Upper 95% confidence limits for excess 40 Ar in samples with normal 40 Ar/ 36 Ar ratios are generally less than 3 × 10 −13 mol/g. The origin of the excess 36 Ar is unknown but it may be due either to the incorporation of primitive argon that has been stored in the mantle in very low potassium environments or to enrichment in 36 Ar as atmospheric argon diffuses into the rocks after they cool.

Earth and Planetary Science Letters↗
USGS1967

Geomagnetic polarity epochs: Nunivak Island, Alaska

New paleomagnetic and potassium-argon dating measurements have been made of basalt flows from Nunivak Island, Alaska, with the following results. (1) The best estimate of the age of the Brunhes/Matuyama polarity epoch boundary is found to be 0.694 m.y. (2) The best estimate of the age of the Gauss/Gilbert boundary is 3.32 m.y. (3) Three normally magnetized flows with ages from 0.93 to 0.88 m.y. are in accord with previous estimates of the age and duration of the Jaramillo normal event. (4) One normally magnetized flow with an age of 1.65 ± 0.09 m.y. supplies additional evidence for the Gilsá normal event. (5) Two new normal events are identified within the Gilbert reversed epoch, the “Cochiti normal event” with an age of 3.7 m.y. and the “Nunivak normal event” with an age of 4.1 m.y.

Alaska↗
USGS1967

Pliocene geomagnetic polarity epochs

A paleomagnetic and K-Ar dating study of 44 upper Miocene and Pliocene volcanic units from the western United States suggests that the frequency of reversals of the earth's magnetic field during Pliocene time may have been comparable with that of the last 3.6 m.y. Although the data are too limited to permit the formal naming of any new polarity epochs or events, four polarity transitions have been identified: the W10 R/N boundary at 3.7 ± 0.1m.y. , the A12 N/R boundary at 4.9 ± 0.1m.y. , the W32 N/R boundary at 9.0 ± 0.2m.y. , and the W36 R/N boundary at 10.8 ± 0.3 − 1.0m.y . The loss of absolute resolution of K-Ar dating in older rocks indicates that the use of well defined stratigraphic successions to identify and date polarity transitions will be important in the study of Pliocene and older reversals.

Earth and Planetary Science Letters↗
USGS1967

K Ar and Rb Sr measurements on P-207, the U.S.G.S. interlaboratory standard muscovite

Potassium-argon analyses of P-207 in twenty-four laboratories and rubidium-strontium analyses in ten laboratories indicate that for material of this typo the average interlaboratory precision for KAr ages is 2.6% and for RbSr ages is 3.4%, and the average intralaboratory precision for KAr ages is 1.6% and for RbSr ages is 2.9%.

Geochimica et Cosmochimica Acta↗
USGS1966

Geomagnetic polarity epochs: A new polarity event and the age of the Brunhes-Matuyama boundary

Recent paleomagnetic-radiometric data from six rhyolite domes in the Valles Caldera, New Mexico, indicate that the last change in polarity of the earth's magnetic field from reversed to normal (the Brunhes-Matuyama boundary) occurred at about 0.7 million years ago. A previously undiscovered geomagnetic polarity event, herein named the "Jaramillo normal event," occurred about 0.9 million years ago.

New Mexico↗