Ocean Drilling Program (ODP) Site 677 provided excellent material for high resolution stable isotope analysis of both benthonic and planktonic foraminifera through the entire Pleistocene and upper Pliocene. The oxygen isotope record is readily correlated with the SPECMAP stack (Imbrieet al.1984) and with the record from DSDP 607 (Ruddimanet al.1986) but a significantly better match with orbital models is obtained by departing from the timescale proposed by these authors below Stage 16 (620 000 years). It is the stronger contribution from the precession signal in the record from ODP Site 677 that provides the basis for the revised timescale. Our proposed modification to the timescale would imply that the currently adopted radiometric dates for the Matuyama–Brunhes boundary, the Jaramillo and Olduvai Subchrons and the Gauss–Matuyama boundary underestimate their true astronomical ages by between 5 and 7%.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300020782

An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Ariantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points

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A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

Introduction to geomagnetism Ferromagnetic minerals Origins of natural remanent magnetism Sampling, measurement, and display of NRM Paleomagnetic stability Statistics of paleomagnetic data Paleomagnetic poles Special topics in rock magnetism Geochronologic applications Applications to paleogeography Applications to regional tectonics Appendix: Derivations

Paleomagnetism: Magnetic Domains to Geologic Terranes

We analyze five high-resolution time series spanning the last 1.65 m.y.: benthic foraminiferal δ18O and δ13O, percent CaCO3, and estimated sea surface temperature (SST) at North Atlantic Deep Sea Drilling Project site 607 and percent CaCO3 at site 609. Each record is a multicore composite verified for continuity by splicing among multiple holes. These climatic indices portray changes in northern hemisphere ice sheet size and in North Atlantic surface and deep circulation. By tuning obliquity and precession components in the δ18O record to orbital variations, we have devised a time scale (TP607) for the entire Pleistocene that agrees in age with all K/Ar-dated magnetic reversals to within 1.5%. The Brunhes time scale is taken from Imbrie et al. [1984], except for differences near the stage 17/16 transition (0.70 to 0.64 Ma). All indicators show a similar evolution from the Matuyama to the Brunhes chrons: orbital eccentricity and precession responses increased in amplitude; those at orbital obliquity decreased. The change in dominance from obliquity to eccentricity occurred over several hundred thousand years, with fastest changes around 0.7 to 0.6 Ma. The coherent, in-phase responses of δ18O, δ13O, CaCO3 and SST at these rhythms indicate that northern hemisphere ice volume changes have controlled most of the North Atlantic surface-ocean and deep-ocean responses for the last 1.6 m.y. The δ13O, percent CaCO3, and SST records at site 607 also show prominent changes at low frequencies, including a prominent long-wavelength oscillation toward glacial conditions that is centered between 0.9 and 0.6 Ma. These changes appear to be associated neither with orbital forcing nor with changes in ice volume.

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Pleistocene evolution: Northern hemisphere ice sheets and North Atlantic Ocean

The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture ( 50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as endmembers. Time scales estimated for the formation of >500 km of crystal-poor rhyolite range from 10 to 10 years, within the estimated residence times of mushes in the upper crust (>10 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions.

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On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes

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.

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

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.

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

Tholeiitic diabase dikes that trend northwest-southeast, parallel to the coastline, are common in northwestern Liberia. K-Ar whole-rock and mineral ages determined from dikes that intrude Precambrian crystalline rocks are discordant and range from 186 to 1,213 m.y. Incremental heating experiments on three neutron-irradiated samples of these rocks give “saddle-shaped” 40 Ar/ 39 Ar release diagrams that reach minima of less than 300 m.y. at intermediate temperatures and that do not fit a 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar isochron. K-Ar ages determined from diabase dikes and sills that intrude Paleozoic sedimentary rocks near the coast are all within the range 173 to 192 m.y. 40 Ar/ 39 Ar incremental heating data for one of these samples gives a plateau age and a 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar isochron age that are concordant with the conventional K-Ar age. The conventional and 40 Ar/ 39 Ar K-Ar data show that the dikes intruding Precambrian basement rocks contain large and variable amounts of excess 40 Ar, whereas the diabase intruding Paleozoic sandstone does not. All of the intrusions are probably earliest Jurassic in age. Mean paleomagnetic directions in six dikes and sills that intrude sedimentary rocks are nearly parallel to mean paleomagnetic directions in 19 dikes that intrude Precambrian rock, further evidence for contemporaneity. The paleomagnetic pole derived from all 25 diabase units is at lat 68° N., long 242° E., with α 95 = 5°, in close agreement with other Mesozoic paleomagnetic poles from the African continent. A mean paleomagnetic pole for northwest Africa has been calculated using these data and published paleomagnetic directions from 19 other intrusive rock units that have similar radiometric ages in Morocco and Sierra Leone. This pole is compared with another paleomagnetic pole calculated from published data from 16 localities in igneous rocks of latest Triassic to earliest Jurassic age distributed from Nova Scotia to Pennsylvania. The comparison shows that, with the African and North American continents in their present positions, the two poles differ by 44° of arc, but when the continents are restored to the predrift configuration proposed by Bullard and others (1965), the angular difference diminishes to 3°. This coincidence of paleomagnetic poles provides an earliest limit of 180 ± 10 m.y. for the separation of Africa from North America.

Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting

A highly detailed record of both the direction and intensity of the Earth's magnetic field as it reverses has been obtained from a Miocene volcanic sequence. The transitional field is low in intensity and is typically non-axisymmetric. Geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur, suggesting that liquid velocity within the Earth's core increases during geomagnetic reversals.

How the geomagnetic field vector reverses polarity

Upper Triassic volcanogenic rocks of the Seven Devils Group and Huntington Formation of Brooks (1979b) in northeastern Oregon record two episodes of magnetization. The older magnetization, which predates Late Jurassic deformation of the volcanic arc terrane comprising the Seven Devils Group, was probably acquired in Late Triassic time. The inclinations of both normal and reversed polarity magnetizations indicate that the Triassic rocks originated approximately 18° from the paleoequator. The results are consistent with paleomagnetic data from similar Triassic rock assemblages on Vancouver Island and in south central Alaska. We interpret these assemblages as displaced fragments of Wrangellia, a once coherent late Paleozoic volcanic arc and early Mesozoic volcanic plateau that was torn apart and carried north by plate motion along the continental margin. We interpret the younger magnetization, which fails the fold test, as a widespread thermochemical overprint caused by Late Jurassic and Early Cretaceous plutonism. This magnetization yields a paleomagnetic pole at 60°N, 295°E (α95 = 7°), and is similar to the pole obtained from plutons that intrude the Seven Devils Group. The angular difference between the pole from the plutons and the Late Jurassic-Early Cretaceous reference pole for stable North America suggests that the Seven Devils volcanic arc has undergone a large clockwise rotation (66°±21°). The rotation may reflect the last stages of the arc's accretion to the continental margin or later large-scale block movements in the Pacific Northwest.

C. Sherman Grommé

Papers

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

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

Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting

How the geomagnetic field vector reverses polarity

Paleomagnetism and Mesozoic tectonics of the Seven Devils Volcanic Arc in northeastern Oregon