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.

The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cenozoic time, its influence on monsoon intensity, and its reflection of the dynamics of continental plateaus. Models of plateau growth vary in time, from pre-India-Asia collision (e.g., Ϸ100 Ma ago) to gradual uplift after the India-Asia collision (e.g., Ϸ55 Ma ago) and to more recent abrupt uplift (<7 Ma ago), and vary in space, from northward stepwise growth of topography to simultaneous surface uplift across the plateau. Here, we improve that understanding by presenting geologic and geophysical data from north-central Tibet, including magnetostratigraphy, sedimentology, paleocurrent measurements, and 40 Ar/ 39 Ar and fission-track studies, to show that the central plateau was elevated by 40 Ma ago. Regions south and north of the central plateau gained elevation significantly later. During Eocene time, the northern boundary of the protoplateau was in the region of the Tanggula Shan. Elevation gain started in pre-Eocene time in the Lhasa and Qiangtang terranes and expanded throughout the Neogene toward its present southern and northern margins in the Himalaya and Qilian Shan.climate ͉ tectonics ͉ magnetostratigraphy ͉ Hoh Xil Basin ͉ Cenozoic T he Tibetan Plateau is the most extensive region of elevated topography in the world (Fig. 1). How such high topography, which should have an effect on climate, monsoon intensity, and ocean chemistry (1-5), has developed through geologic time remains disputed. Various lines of investigation, including evidence from the initiation of rift basins (6), potassium-rich (K-rich) volcanism (7), tectonogeomorphic studies of fluvial systems and drainage basins (8), thermochronologic studies (9), upper-crustal deformation histories (10, 11), stratigraphic and magnetostratigraphic studies of sediment accumulation rates (12), paleobotany (13), and oxygen isotope-based paleoaltimetry (14-22), have suggested different uplift histories. Authors of recent geologic studies (11) have proposed that significant crustal thickening (and by inference, surface uplift) in the Qiangtang terrane occurred in the Early Cretaceous [Ϸ145 mega-annum (Ma) age], followed by major crustal thickening within the Lhasa terrane between Ϸ100 and 50 Ma ago. This hypothesis remains disputed (23). Other models of plateau growth range from Oligocene (e.g., Ϸ30 Ma ago) gradual surface uplift (7) to more recent (Ͻ7 Ma ago) and abrupt surface uplift (24), with oblique stepwise growth of elevation northward and eastward after the India-Eurasia collision (7,20,25,26). With few exceptions (e.g., see refs. 11 and 27), most of these models focus on data from the Himalaya and southern Tibet and remain relatively unconstrained by geologic data from the interior of the Tibetan Plateau.The Hoh Xil Basin (HXB) of the north-central Tibetan Plateau (Figs. 1 and 2) is the most widespread exposure of Paleogene sediments on the high plateau and contains Ͼ5,000...

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We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time‐averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long‐lived core‐mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low‐latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low‐quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two‐parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

A variety of rock types from eighteen volcanic units of the western United States were studied by Thellier's method. Ninety‐five NRM‐TRM curves were determined, and paleo‐intensities are estimated from twelve of the units. Each paleo‐intensity, on the average, represents the mean of values derived from five separate samples. Two paleo‐intensities have standard deviations of the mean less than 5%, two between 5 and 10% and eight between 10 and 20%; no estimate is made for the remaining six units because of great internal inconsistency of the data or insufficient work. The largest ratio of paleo‐intensity to the present field intensity at the same location is 1.1, the lowest 0.2. The 0.2 value is for a Miocene transition zone, supporting the hypothesis that the intensity decreases during a field reversal. Low ratios were found for a few other units. One such Pliocene unit also has an anomalous direction of magnetization, suggesting the field may have been in the act of reversing when the unit was magnetized 7.2±0.3 m.y. ago. The direction is normal for the other units with low ratios; one of these is probably less than 10,000 years old and thus cannot be associated with a reversal. Samples of dacite from the 1915 eruption of Mt. Lassen behave in a suprising and probably atypical manner; simple comparison of the NRM to the total TRM yields an experimental value much closer to the known intensity than that determined by Thellier's method.