Magnetic Reversal Time Scale Research Articles

Geomagnetic secular variation in Late Weichselian Allerød sediments from Nr. Lyngby (Denmark)

Palaeomagnetic measurements on 400 specimens from lake sediments exposed in the cliff of the classic Late Glacial Allerød site at Nørre Lyngby in North Jutland, Denmark, are presented. Two profiles in the 7 m sequence of sand, silt and gyttja, spanning the time interval between c. 12 000 and c. 10 700 BP show about 5 cycles in the declination and about 2 cycles in inclination. Secular variation features as observed at this site are also recognizable at sites in southern Sweden and Soviet Karelia. Comparisons with Holocene records indicate that the short time-scale behaviour (i.e. < 103 y) of the geomagnetic field appears to have been similar since 14 000 BP, i.e. for a period considerably longer than the timescale of the variations themselves, thus suggesting that this type of behaviour is a permanent feature of the geomagnetic field. These secular variation features may be useful in local as well as more regional stratigraphical correlations for the Late Glacial and Holocene on a much more detailed timescale than is obtained from the magnetic reversal timescale used for older materials.

Twelve Key 20th-century Discoveries in the Geosciences

This paper reviews 12 major discoveries in geology and geophysics during the 20th century, reasonably mature and discrete with respect to subject, time, or discoverer. The Textbook of Geology (Geikie, 1903) and Understanding the Earth (Brown and others, 1992) are used as benchmarks, supplemented by Yoder's 0007-1992 “Timetable of Petrology” (1993). The discovery of the radioactive decay law by Rutherford and Soddy was fundamental, bearing on the age of the Earth, internal energy, nature of the crust, and a quantitative geologic time scale. Discoveries of the internal structure of the Earth, investigated seismically, included definition of the core boundary (Gutenberg and Weichert), the inner core (Lehmann), the mantle/crust boundary (Mohorovic), the lithosphere, and the asthenosphere. Deep structure of the continental crust, revealed since 1975 by reflection profiling and study of exposed sections, has been found to be drastically different from previous concepts. The magnetic reversal time scale was developed by correlating reversed magnetization in terrestrial and marine rocks with radiometric and stratigraphic data, interpreted in light of sea-floor spreading. Sea-floor spreading, the key element of plate-tectonic theory, discovered by several independent lines of inquiry, has been directly confirmed by space-geodesy measurements of plate motion and intraplate rigidity in the Pacific Basin. Elastic rebound, the cause of shallowfocus earthquakes, was proposed after the 1906 California earthquake by Reid, using geodetic evidence; deep-focus events are still not understood. The mechanism of overthrust faulting, a major problem as late as the 1950s, was discovered by Hubbert and Rubey, who demonstrated that high fluid pressure could reduce normal stress to the point that gravity sliding would be effective. X-ray diffraction by the crystal lattice was discovered in 1912, and its applications revolutionized mineralogy, confirming solid solution and clarifying many crystallographic phenomena. The origins of basaltic and granitic magmas, completely unknown in 1900, were definitively explained for basalts by Bowen as partial melting of a peridotitic mantle, and for most granitic magmas as partial melting of crustal rocks by Tuttle and Bowen. Space exploration has revealed common patterns of planetary differentiation, including a first (or felsic) differentiation forming global primitive crusts, followed by a second (or basaltic) differentiation, including continued basaltic magmatism. Another major result of space exploration and related research has been discovery of the importance of impact cratering, proposed as being responsible for formation of continental nuclei, the first ocean basins, and many mass extinctions in the geologic record. The most important scientific achievement of the 20th century is suggested to be the discovery of the DNA structure, revealing not only the molecular basis for all life on Earth but a critical line of investigation, formation of RNA, for study of the origin of life.

Effects of ridge crest‐trench interaction on Antarctic‐Phoenix Spreading: Forces on a young subducting plate

Precise measurements of spreading rates on marine magnetic profiles collected to the west of the Antarctic Peninsula have enabled some consideration of the forces governing plate motion, since Antarctic—Phoenix motion has been controlled by the local rather than global force balance over the last 35 m.y. The total effective driving force per unit length of trench is calculated to have ranged between 2.6 and 3.6×1012 N/m, which is much less than is commonly thought necessary to support subduction. Conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench. The low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed. Driving forces show a strong correlation with observed spreading rates, which indicates that resistive forces were largely velocity dependent. Fluid migration up the subduction zone may elevate temperatures in and around the shear zone, reducing resistive forces below the levels required by purely conductive models. Changes in convergence rate may affect the depths of both the brittle/ductile deformation boundary and the basalt/eclogite phase change, causing a negative feedback which would appear as a velocity‐dependent resistive force. The different driving forces acting on the NE and SW parts of the Phoenix plate, as a consequence of older oceanic lithosphere at the trench in the NE, caused Antarctic–Phoenix spreading to take place about a near pole to the SW since 21 m.y. ago, and ultimately resulted in disruption of the Phoenix plate about 9 m.y. ago. Spreading rates decreased abruptly about 6 m.y. ago, probably because of E‐W compression across the long transform faults bounding the Phoenix plate. However, spreading on the last three segments of the Antarctic–Phoenix Ridge continued at least until 4 m.y. ago. Either spreading stopped progressively from SW to NE, or the final stage took place about a very near pole to the SW. A magnetic quiet zone extends up to 95 km from the margin between the Tula Fracture Zone and the North Anvers Fracture Zone, and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision. The absence of a magnetic quiet zone associated with the most recent ridge crest–trench collisions suggests a change in sedimentary regime during the late Miocene. Anomalously fast apparent spreading rates between 23 and 21 m.y. ago are thought to indicate an error in this part of the magnetic reversal time scale.

An Introductory-Geology Exercise on the Polar-Reversal Time Scale

The exercise provided gives students an opportunity to work with data related to Earth's magnetic polarity, and leads them through a series of tabulations, plots, and interpretations. The data consist of 150 sets, each of which includes a magnetic declination, an age, and a geographic location. The generalized sequence of class activities is as follows: 1) considering a question raised in the exercise; 2) plotting some aspect of the data; 3) interpreting the resulting plot, and; 4) going on to additional questions. Eventually the students are led to the conclusion that Earth's magnetic field has reversed itself periodically, and they then work out the magnetic reversal time scale for the last 4.5 million years. All materials necessary for the exercise are provided, including an instructor's guide, question and answer sheets, and graphs.

Interpretations of the magnetostratigraphy of the Hadar hominid site, Ethiopia

As part of an investigation to determine the age of the hominidbearing strata of the Hadar Formation of Ethiopia we have determined the magnetostratigraphy of two separate sections of the strata. Two major zones of reversed polarity occur within the predominantly normally magnetized sequence, one at the top and the other in the middle. There are difficulties with the absolute age determinations of volcanic units within the strata which result in uncertainties in correlating the Hadar magnetostratigraphy with the standard magnetic reversal time scale. The age of the upper part of the section is established by the BKT-2 tuff at ∼2.8 Myr (refs 1–4). The K–Ar dating of the Kadada Moumou basalt which lies in the middle of the sequence yields variable results ranging from 3.1 to 3.6 Myr (refs 1–4). Regional geochemical5 and bio-stratigraphical6 studies suggest the younger age for the Kadada Moumou basalt. We show here that this is also the age that results in the most consistent interpretation of the observed magneto-stratigraphy of the formation, which is that the Hadar Formation was deposited during the Gauss Normal epoch and that the two reversals represent the Kaena and Mammoth events within that epoch.

Biostratigraphy and magnetostratigraphy, late Pliocene rocks, 111 Ranch, Arizona

Fossiliferous strata exposed along the northwestern flank of Dry Mountain on the 111 Ranch, 27 km southeast of Safford, Graham County, Arizona, were dated by means of isotopic methods and the magnetic-reversal time scale. A 100-m fossiliferous interval contains the later part of the Gauss Chron and the early part (pre-Olduvai) of the Matuyama Chron. Zircon fission-track ages on an ash bed just below the Gauss-Matuyama boundary support this identification. Owing to these age constraints, the fossil mammals at the 111 Ranch locality are all constrained within the later part of the Blancan Mammal Age rather than the latest Blancan and early Irvingtonian, as previously stated in the literature. They are, therefore, synchronous with certain localities (Wolf Ranch and California Inst. Technology) in the Saint David Formation of the San Pedro Valley 130 km to the southwest of Dry Mountain. The horse Nannippus persisted in the 111 Ranch locality at least until the end of the Gauss Chron, in confirmation of data from the San Pedro Valley. A diverse group of equine and asinine equids is evident in the 111 Ranch fauna, which argues for an early divergence of these major branches of horse phylogeny. The 111 Ranch fauna records the first appearance of three neotropical immigrants in the southwest, including the sloth Glossotherium , the glyptodont Glyptotherium , and the capybara Neochoerus . A palearctic immigrant, the vole Synaptomys , is also present.

Magnetic lineations in the Pacific Jurassic quiet zone

Magnetic anomalies of low amplitude (<100 gammas) are present in the Jurassic magnetic quiet zone of the western Pacific Ocean. These small anomalies are lineated and can be correlated among the Phoenix, Hawaiian and Japanese lineation patterns. Thus, they represent seafloor spreading that recorded some sort of magnetic field phenomena prior to magnetic anomaly M25 at 153 m.y. B.P. The most likely possibility is that they represent a series of late Jurassic magnetic field reversals that occurred during a period of anomalously low magnetic field intensity. We propose a time scale of magnetic reversals between 153 and 158 m.y. B.P. to account for these anomalies and suggest that the dipole magnetic field intensity increased by a factor of about four from 160 to 140 m.y. B.P. in the late Jurassic.

Revised magnetic polarity time scale for Late Cretaceous and Cenozoic time

A revision of the Heirtzler and others magnetic reversal time scale is presented. In addition to incorporating published studies which have increased the resolution and accuracy of their time scale, we have revised the relative lengths of anomalies 4A to 5 and 29 to 34 and have eliminated anomaly 14. We have calibrated the time scale by choosing an age of 3.32 m.y. B.P. for the older reversal boundary of anomaly 2A and 64.9 m.y. B.P. for the older reversal boundary of anomaly 29. The resulting magnetic reversal time scale is in reasonable agreement with the biostratigraphic ages from Deep Sea Drilling Project (DSDP) drill holes.

Magnetic results from basalts and sediments from the Nazca Plate

THE drilling sites for leg 34 of the Deep Sea Drilling Project (DSDP) were in the Nazca Plate (Fig. 1), which was formed by the spreading from two ridges, the fossil Galapagos Rise and the currently active East Pacific Rise. Spreading half rates on the East Pacific Rise vary from 8 to 10 cm−1 (refs 1–3) and are the highest on any presently active ridge. Fast spreading is associated with subdued basement topography, a situation that enables structural provinces within the Nazca Plate to be clearly delineated. The near equatorial latitudes of the sites with poor magnetic anomaly development mean, however, that the determination of the ages of sites 319 and 320 with respect to the magnetic reversal time scale will be precluded. Table 1 shows the basement ages for the sites, estimated from the overlying sediments.

A revised time scale of magnetic reversals for the Early Cretaceous and Late Jurassic

A magnetic reversal block model for the Early Cretaceous-Late Jurassic period was developed from four closely spaced profiles across the Hawaiian lineation pattern by Hilde (1973) and Hilde et al. (1974). Larson (1974) independently developed an improved model of this reversal period by reanalyzing the data presented by Larson and Chase (1972) plus a profile collected during Deep Sea Drilling Project (DSDP) leg 32. These two models are similar and contain nearly twice as many reversals as the original late Mesozoic reversal model proposed by Larson and Pitman (1972). These models are combined in this paper and are the basis for a revised magnetic reversal stratigraphy for the Early Cretaceous-Late Jurassic. The model is placed in a framework of geologic time by analyzing magnetic data in the vicinity of DSDP drill holes that reached volcanic basement on various late Mesozoic lineation patterns. The magnetic ages of these sites are plotted as a function of paleontologic ages at the bottom of the holes to determine a revised time scale of magnetic reversals for the Early Cretaceous-Late Jurassic. Fundamental to this analysis are the assumptions that the Hawaiian lineation pattern was generated at a constant spreading rate and that the paleontologic ages in the DSDP holes closely approximate the basement ages. This analysis shows that the M reversal pattern spans the Aptian to Oxfordian stages and ranges in chronologic age from 107 to 153 m.y.

World-Wide Correlation of Mesozoic Magnetic Anomalies, and Its Implications

In the course of correlating three sets of Mesozoic magnetic lineations in the western Pacific (the Phoenix, Japanese, and Hawaiian lineations), Larson and Chase (1972) determined a paleomagnetic pole for the Pacific plate for the Early Cretaceous. Using this pole we have derived a magnetic reversal model for the Hawaiian lineation set. We then have used this model to correlate the entire Hawaiian lineation set to the entire Keathley lineation set in the western North Atlantic. On the basis of these correlations and drill holes associated with the lineation patterns, we have extended the geomagnetic reversal time scale back to the base of the Late Jurassic (162 m.y. B.P.). A period of reversals occurred corresponding to the Hawaiian and Keathley lineations from 150 o t 110 m.y. B.P., and these reversals are bracketed by long periods of dominantly normal polarity (the Cretaceous and Jurassic magnetic quiet zones). This magnetic reversal time scale significantly alters previous notions of the timing and origin of sea-floor spreading features in the Atlantic Ocean. It implies that the Bay of Biscay opened sometime during the interval between 150 and 110 m.y. B.P.; that drift in the South Atlantic was initiated at sometime during the interval from 110 to 85 m.y. B.P. (probably close to 110 m.y. B.P.); and that the seaward portion of the marginal quiet zones of the eastern United States and northwestern Africa resulted from sea-floor spreading during the Late Jurassic period of dominantly normal magnetic polarity prior to 150 m.y. B.P. In the Pacific during the late Mesozoic, spreading was occurring from at least five spreading centers joined at two triple points. The vast majority of the Pacific Basin today is occupied by only the Pacific-plate side of these spreading patterns. This implies that an area equal to most of the Pacific Basin has been subducted beneath the surrounding continents since the Early Cretaceous. Our magnetic reversal time scale calls for a rapid pulse of spreading from about 110 to 85 m.y. B.P. at all the spreading centers in both the Atlantic and Pacific Oceans. This implies a pulse of rapid subduction around the rim of the Pacific that we relate to episodes of large-scale plutonism in eastern Asia, western Antarctica, New Zealand, the southern Andes, and western North America during the Late Cretaceous.