Geology topics

Sherman Gromme

Publications and source records attributed to Sherman Gromme.

7 recordsLinked to original sources

USGS2012

Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low

Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long‐term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Geomagnetic paleointensities by the Thelliers' method from submarine pillow basalts: Effects of seafloor weathering

Measurements of geomagnetic paleointensity using the Thelliers' double‐heating method in vacuum have been made on 10 specimens of submarine pillow basalt obtained from 7 fragments dredged from localities 700,000 years old or younger. In the magnetic minerals, the titanium/iron ratio parameter x and the cation deficiency (oxidation) parameter x were determined by X‐ray diffraction and Curie temperature measurement. Fresh material ( z ≅ 0) provided excellent results: most of the natural remanent magnetization (NRM) could be thermally demagnetized before the magnetic minerals became altered, and the NRM‐TRM lines were straight and well constrained, and geologically reasonable paleointensities were obtained. Somewhat oxidized material ( z ≅ 0.2) also provided apparently valid paleointensities: values were similar to those from fresh specimens cut from the same fragments, although only half or less of the NRM could be thermally demagnetized before alteration of the magnetic minerals. More highly oxidized material ( z ≅ 0.6) gave a result seriously in error: the paleointensity value is much too low, because of continuous disproportionation of titanomaghemite during the heating experiments and because seafloor weathering had decreased the NRM intensity. From limited published data, the extent of oxidation of titanomagnetite to cation deficient titanomaghemite in pillow basalt exposed on the seafloor appears to be approximately z = 0.3 at 0.2–0.5 m.y., z = 0.6 at 1 m.y., and z = 0.8–1.0 at 10–100 m.y. This implies that valid paleointensities can be obtained from exposed submarine basalt, but only if the basalt is younger than a few hundred thousand years. Equally good paleointensities were obtained from strongly magnetized (L‐type) basalt and moderately magnetized (L‐type) basalt. The degree of low‐temperature oxidation of cubic iron‐titanium oxides in submarine basalts correlates very well with the diminution of amplitude of linear magnetic anomalies when both are compared as a function of crustal age. Systematic radial variation of Curie temperature is a primary feature of submarine basalt pillows, so that estimation of the oxidation parameter z from the Curie temperature alone by assuming a value for x can be in error. Reasonably precise and self‐consistent values of both x and z can be obtained if both the cubic cell dimension and the Curie temperature of the cubic oxide are measured.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Geomagnetic paleointensities from excursion sequences in lavas on Oahu, Hawaii

Paleomagnetic data demonstrating three late Tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of Oahu, Hawaii were published by R. R. Doell and G. B. Dalrymple in 1973. We have determined geomagnetic paleointensities by the Thelliers' method for 14 lavas from the three sites. During these experiments, considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others. Moreover, we often observed the production of spurious high‐temperature chemical remanent magnetization during the Thellier experiments. An analysis of this particularly troublesome problem is presented. Two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole, which suggests that these excursions represent aborted reversals or fragments of reversals. At the third site, however, the paleointensity did not become low as the field diverged. This excursion may reflect the variation of a large nondipole source near Hawaii.

Hawaii↗

USGS2011

Updated paleomagnetic pole from Cretaceous plutonic rocks of the Sierra Nevada, California: Tectonic displacement of the Sierra Nevada block

We report remanent magnetization measurements from 13 sites in Cretaceous plutonic rocks in the northern Sierra Nevada (38°N–39.5°N). By increasing the number of available paleomagnetic sites, the new data tighten constraints on the displacement history of the Sierra Nevada block and its pre-extensional position relative to interior North America. We collected samples in freshly exposed outcrops along four highway transects. The rocks include diorite, granodiorite, and tonalite with potassium-argon ages (hornblende) ranging from 100 Ma to 83 Ma. By combining our results with previous paleomagnetic determinations from the central and southern Sierra Nevada (excluding sites from the rotated southern tip east of the White Wolf–Kern Canyon fault system), we find a mean paleomagnetic pole of 70.5°N, 188.2°E, A 95 = 2.6° ( N = 26, Fisher concentration parameter, K = 118). Thermal demagnetization indicates that the characteristic remanence is generally unblocked in a narrow range within 35 °C of the Curie temperature of pure magnetite. Small apparent polar wander during the Cretaceous normal-polarity superchron, plus prolonged acquisition of remanence at the site level, may account for the low dispersion of virtual geomagnetic poles and relatively large K value. Tilt estimates based on overlapping sediments, stream gradients, and thermochronology of the Sierra Nevada plutons vary from 0° to 3° down to the southwest. Without tilt correction, the mean paleomagnetic pole for the Sierra Nevada is essentially coincident with the North American reference pole during the Cretaceous stillstand (125 Ma to 80 Ma). At 95% confidence, the apparent latitude shift is 1.1° ± 3.0° (positive northward), and the apparent rotation is negligible, 0.0° ± 4.7°. Correcting for each degree of tilt, which is limited to 3° on geologic evidence, increases the rotation anomaly 2.2° counterclockwise, while the apparent latitude shift remains unchanged.

California↗

USGS2010

Time-averaged paleomagnetic field at the equator: Complete data and results from the Galapagos Islands, Ecuador

We present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the Galapagos Islands, Ecuador, collected by Allan Cox in 1964–1965 but nearly all previously unpublished. The islands are located in the eastern Pacific Ocean within 1.4° of latitude from the equator and range in age from historically erupted to 3 Ma, mostly determined by published K-Ar and 3He isotopic dating. The number of sites collected on each island ranges from 1 to 28, for a total of 186. After combining duplicate site mean directions, 149 are used for an overall mean direction and 8 represent excursions and one reversal path. Divided by geomagnetic polarity chron, 110 site means are Brunhes or Jaramillo (normal polarity), 27 are Matuyama (reversed polarity), and 12 are Gauss (both polarities). We have completed the magnetic cleaning that was commenced in the late 1960s. Secondary (mostly viscous) magnetizations were nearly all removed by alternating field demagnetization at 10 mT. We have used the so-called blanket cleaning method, generally at 10 mT. All sites were in basalt flows and gave good paleomagnetic results; none was rejected in toto, and only a few core specimens were magnetically unsatisfactory. Nearly all sites had eight independently oriented cores, and within-site angular standard deviations of directions range from 1° to 8°. We used both Fisher and Bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian, while the corresponding virtual pole (VGP) populations are essentially circularly distributed. The paleomagnetic poles, calculated as the means of VGPs, are as follows: Brunhes and Jaramillo, north latitude = 86.9°, east longitude = 245.1°, and 95% confidence radius A 95 = 1.9°; Matuyama, latitude = 87.2°, longitude = 158.2°, and A 95 = 3.8°; Gauss, latitude = 83.0°, longitude = 204.7°, and A 95 = 7.0°. These paleomagnetic poles do not differ significantly from one another, but the Brunhes and Jaramillo combined pole is significantly near-sided with respect to the Galapagos, as is the overall mean pole, which is at latitude = 87.1° and longitude = 227.6°, with A 95 = 1.7°. Omitting the excursion and reversal path data, the overall angular standard deviation of VGPs is 11.7° with lower and upper 95% confidence limits of 10.8° and 12.7°, respectively, in good agreement with previously published values for near-equatorial sampling latitudes.

Galapagos Islands↗

USGS2010

Paleomagnetism of the Siberian flood basalts of the Noril'sk area: A constraint on eruption duration

The volcanic sequence of the Noril'sk area, northern Siberia, provides the most complete section of early Siberian flood-basalt volcanism. Paleomagnetic measurements for more than 4000 samples of lava and tuff indicate that nearly all of this >3500-m-thick sequence was laid down during one interval of normal magnetic polarity. Lavas of the lower third of this sequence are cut by the ore-bearing Noril'sk-I intrusion, which has an age of 251 Ma, identical to that of the Permian-Triassic boundary. Thus, the normal-polarity interval represented by this sequence is inferred to be the first of the Triassic Period. Eruption of this enormous volume of material in a relatively brief period coincident with the earth's greatest mass extinction requires that all aspects of Siberian flood-basalt volcanism be evaluated carefully as possibly contributing to that catastrophe.

Noril'sk Area, Siberia↗

USGS2007

Stratigraphy, paleomagnetism, and anisotropy of magnetic susceptibility of the Miocene Stanislaus Group, central Sierra Nevada and Sweetwater Mountains, California and Nevada

Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post–9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = −26.1°, D = 163.1°, and α 95 = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = −62.8°, D = 159.9°, α 95 = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, α 95 = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, α 95 = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of ∼10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial flow. AMS-derived flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

California, Nevada↗