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C. Sherman Gromme

Publications and source records attributed to C. Sherman Gromme.

15 recordsLinked to original sources
USGS2013

Paleomagnetic contributions to the Klamath Mountains terrane puzzle-a new piece from the Ironside Mountain batholith, northern California

We obtained paleomagnetic samples from six sites within the Middle Jurassic Ironside Mountain batholith (~170 Ma), which constitutes the structurally lowest part of the Western Hayfork terrane, in the Klamath Mountains province of northern California and southern Oregon. Structural attitudes measured in the coeval Hayfork Bally Meta-andesite were used to correct paleomagnetic data from the batholith. Comparing the corrected paleomagnetic pole with a 170-Ma reference pole for North America indicates 73.5° ± 10.6° of clockwise rotation relative to the craton. Nearly one-half of this rotation may have occurred before the terrane accreted to the composite Klamath province at ~168 Ma. No latitudinal displacement of the batholith was detected.

Oregon, Washington↗
USGS2012

Paleomagnetic data from the Coso Range, California and current status of the Cobb Mountain normal geomagnetic polarity event

Two basalt flows which erupted about 1.08 m.y. ago in the Coso Range, California, have normal magnetic polarity and thus provide additional evidence for the Cobb Mountain normal polarity event. A review of available data confirms that this event was of geomagnetic origin. A mean age of 1.10 ± 0.02 m.y. B.P. for the Cobb Mountain normal polarity event was found to best fit all available radiometric and geologic data.

California↗
USGS2012

Paleomagnetic results from the Shasta Bally Plutonic Belt in the Klamath Mountains Province, northern California

Available paleomagnetic data show approximately 100° of clockwise rotation for Permian and Triassic strata of the Eastern Klamath terrane. Jurassic strata of this terrane are rotated approximately 60° clockwise, which is comparable to rotations reported for Jurassic plutons that occur elsewhere in the Klamath Mountains province. Paleomagnetic data obtained during the present study from the Shasta Bally belt of Cretaceous plutons indicate 25.7° ± 13.6° of clockwise rotation for the province since Early Cretaceous time (≃ 136 Ma). The waning stages of rotation at the time of emplacement of the Shasta Bally belt plutons, which was closely followed by deposition of basal strata (Lower Cretaceous) of the Great Valley sequence, probably represents completion of accretion of the province to cratonic North America.

California↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition, 2. Field intensity variations and discussion of reversal models

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long‐term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root‐mean‐square of the three field components X , Y , and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr −1 , which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Paleomagnetism and K-Ar ages of volcanic rocks from Long Valley caldera, California

Paleomagnetic measurements and K‐Ar age determinations on volcanic rocks from Long Valley caldera, California, have enabled further refinement of eruptive activity within this large silicic volcanic center. K‐Ar age determinations show that postcaldera volcanic eruptions began 0.73 m.y. ago and continued periodically until about 50,000 years ago. The eruptions were not temporally random but tended to occur in distinct episodes separated by periods of quiescence. Volcanism in the western half of the caldera was particularly intense between 0.15 and 0.50 m.y. ago, when many units ranging in composition from basalt to rhyolite were erupted. An average of the virtual geomagnetic poles for 33 units from the caldera yields a paleomagnetic pole at 89.7°N, 138.4°E, (α 95 = 5.1°), which is indistinguishable from the earth's rotational axis. The ancient geomagnetic field dispersion about this mean pole is 16.0°, with upper and lower limits of 19.3° and 13.6°, respectively.

California↗
USGS2012

Paleomagnetism of Middle Tertiary volcanic rocks from the Western Cascade Series, northern California

The Western Cascade Series (WCS) is a 3.5‐km‐thick, crudely homoclinal (east dipping) calcalkaline volcanic sequence of mid‐Oligocene to early Miocene age that crops out near the southern tip of the Cascade Range in northern California. The mean direction of remanent magnetization in the WCS is D, 4.9°; I, 57.6° (N, 53; k, 14.4; α 95 , 5.3°). When compared to a reference direction for the North American craton, the WCS direction indicates that the southern Cascade Range has rotated 14.0° ± 9.0° since the WCS accumulated. A difference in mean direction between the lower and upper halves of the WCS suggests that much of this rotation occurred during the late Oligocene. Six other paleomagnetic studies of rock units of an age roughly comparable to the WCS also are available from western Oregon, northwestern California, and southwestern Washington. All show statistically significant clockwise rotation (inclinations of all but one are concordant). Comparison of directions indicates that the Pacific Northwest did not rotate as a rigid body, but neither did it behave as a collection of small, independently rotating domains. The amount of rotation found throughout the area increases to the west or northwest, suggesting a driving force for rotation operating at the continental margin.

California↗
USGS2012

Paleomagnetism of the Tertiary Clarno Formation of central Oregon and its significance for the tectonic history of the Pacific Northwest

The Clarno Formation, a mostly Eocene and partly early Oligocene sequence of andesitic lavas and volcaniclastic rocks, is the oldest Tertiary formation exposed in north central Oregon. Remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84°N, 278°E with a 95% confidence cone of 7°. Comparison of this pole with the North American reference pole for Eocene time indicates that the Clarno Formation has rotated 16° clockwise with an uncertainty of 10° but has undergone no significant latitudinal displacement. Further comparison with paleomagnetic data from the Miocene Columbia River Basalt Group and Steens Basalt shows that large‐scale block rotations in the eastern Columbia Embayment ceased sometime between 38 m.y. and 15 m.y. A paleogeographic reconstruction for 38 m.y. is offered which is consistent with the observed rotation of the Clarno and with the other paleomagnetic data from the Pacific Northwest, which are briefly reviewed. This reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre‐Miocene Tertiary rocks of the Pacific Northwest by an extensional tectonic model. As a consequence, only part of the rotations in the pre‐Tertiary rocks of the eastern Columbia Embayment need to be the result of accretionary tectonics. The initiation of magmatism in the eastern Columbia Embayment that is represented by the Clarno Formation itself may have been contemporaneous with the beginning of crustal extension in the Pacific Northwest.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Paleomagnetic study of the Eastern Klamath terrane, California, and implications for the tectonic history of the Klamath Mountains Province

Paleomagnetic study of Permian through Jurassic volcanic and sedimentary strata of the Eastern Klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary. Similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane, in general, has behaved as a single rigid block. Paleomagnetic data indicate that the volcanic island arc represented by this terrane, the nucleus of the province, was facing toward the present southwest during late Paleozoic time, although its orientation during earlier periods is unknown. Whether the arc was separated from the North American craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data. The declination anomalies for both Permian and Triassic strata are similar (average = 106° ± 12°), so we infer that clockwise rotation of the late Paleozoic arc did not begin until latest Triassic or earliest Jurassic time. The arc may have completed its initial rotation with respect to stable North America by Middle Jurassic time. After some retrograde motion, the arc was again facing west by the Late Jurassic, by which time some of the more westerly terranes of the province had become attached to the Eastern Klamath terrane. The composite Klamath Mountains terranes continued to rotate until the final 60° of clockwise rotation was nearly complete by the Early Cretaceous. Coincidence of the waning stages of rotation, at about 136 Ma, with the beginning of deposition of the basal Great Valley sequence onto the Klamath basement probably represents the completion of accretion of the Klamath Mountains terranes to the North American continent. Nearly all the rotation occurred while the Klamath Mountains terranes were part of a converging oceanic plate, with only about 20° of rotation in mid‐Tertiary time during Basin and Range extension. No data currently available show evidence for any significant latitudinal displacement of any Klamath Mountains terranes relative to cratonic North America.

California↗
USGS1991

Concordant paleolatitudes from ophiolite sequences in the northern California Coast Ranges, U.S.A.

Paleomagnetic data have been obtained from two ophiolite sequences in the northern California Coast Ranges: from Mount Diablo in the San Francisco Bay area and from Potter Valley, north of Clear Lake. The ophiolite exposed at Mount Diablo is part of the late Middle to Late Jurassic Coast Range ophiolite, and that exposed near Potter Valley is Late Jurassic to perhaps Early Cretaceous in age and occurs within the Franciscan assemblage. Data from the sheeted-dike complex at Mount Diablo show these rocks to be strongly overprinted, probably following uplift and erosion of the ophiolite. Samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30.7°N, 159.5°E with α 95 = 5.6°. A comparison of this pole with the Jurassic apparent polar wander path for North America indicates that the ophiolite has rotated 45° ± 7° counterclockwise relative to the craton and has not been latitudinally displaced. The diabase and pillow basalt in Potter Valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79.0°N, 61.5°E with α 95 = 6.4°. This result indicates that the ophiolite at Potter Valley has rotated approximately 29° ± 8° clockwise, and has undergone little or no latitudinal displacement. Because of the predominantly northeastward transport of oceanic plates converging with the western margin of North America since middle Mesozoic time, the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin.

California↗
USGS1984

Implications of paleomagnetism for the tectonic history of the Eastern Klamath and related terranes in California and Oregon

Paleomagnetic study of Permian to Jurassic volcanic and sedimentary strata of the Eastern Klamath terrane has shown the remanent magnetism of these rocks to be prefolding and primary. The Permian and Triassic rocks are both indicated to have rotated 100° clockwise, while the Jurassic strata have rotated 60° clockwise. The respective amounts of rotation for Permian and Jurassic strata are similar along two widely-spaced transects of the arcuate Eastern Klamath terrane, which indicates rigid-block rotation rather than oroclinal bending. These results suggest that the Permian and Triassic strata began to rotate during Late Triassic or Early Jurassic time, possibly in response to the beginning of accretion of the Eastern Klamath terrane to the continental margin. Measurements on superjacent Cretaceous strata indicate that the rotation of the Eastern Klamath terrane as well as other parts of the Klamath Mountains province was virtually compete by Early Cretaceous time. The virtual cessation of rotation probably marked the completion of the accretion of the Eastern Klamath terrane to North America. Neither these nor other data currently available show evidence for any significant latitudinal displacement of the Klamath Mountains terranes.

Pacific Section S.E.P.M.↗
USGS1976

Natural remanent magnetization, mag¬netic properties, and oxidation of titanomagnetite in basaltic rocks from DSDP Leg 34

We have made paleomagnetic measurements and investigated the magnetic mineralogy of 24 samples of basalt and diabase from Sites 319, 320, and 321 on the Nazca plate. Sample distributions, locations, and ages of overlying sediments are as follows: Site 319, Hole 319, lat 13°01.04'S, long 101°31.46'W, 2 samples, early Miocene (N8); Site 319, Hole 319A, (same coordinates and age), 12 samples. Site 320, Hole 320B, lat 9°00.40'S, long 83°31.8O'W, 2 samples, late Oligocene (N2); Site 321, lat 12°01.29'S, long 81°54.24'W, 8 samples, late Eocene (P16). The objectives were (1) to investigate the stability of the natural remanent magnetization (NRM), (2) to evaluate an improved method of alternating-field (AF) demagnetization, (3) to determine the directions of the magnetically stable components of NRM and the approximate in-situ intensities of NRM, (4) to identify the ferrimagnetic minerals and determine their degree of oxidation, and (5) to relate this oxidation to the magnetic characteristics of the minerals.

Initial reports of the Deep Sea Drilling Project↗
USGS1973

A comparison of three methods of determining geomagnetic paleointensities

Paleointensity studies by the methods of the Thelliers, Wilson, and van Zijl were compared for adjacent specimens from each of five basaltic lava flows. For three of the flows, the actual paleointensity is independently known within ±6%. All specimens had Curie temperatures above 500°C, and those from four of the flows had lower Curie temperatures as well, indicating they probably underwent varying degrees of high-temperature oxidation and disproportionation during initial cooling. These four suffered irreversible changes in their blocking temperature spectra when heated in air above 400 to 500°C. Nonetheless, the paleointensities derived by the Thelliers' method below these temperatures agree with the actual values within the uncertainties. The values obtained by the other methods are not as accurate or reliable. In four out of five lavas, A. F. cleaning of 100 to 200 oe prior to each measurement of remanence in the Thelliers' method degraded the results for determination of paleointensity.

Journal of Geomagnetism & Geoelectricity↗
USGS1970

Magnetic studies of lunar samples

The remanent magnetism of a lunar type C breccia sample includes a large viscous component with a time constant of several hours, and a high coercivity remanence, possibly acquired by impact processes on the lunar surface. Ilmenite(?) and metallic iron in breccias, and ferrous and metallic iron in glass beads separated from lunar fines (type D) were identified by high-field and low-temperature experiments. The iron appears to occur in a wide range of grain sizes including the single domain and multidomain states.

Science↗