Closing Pandora's Box: Additional Insights on Inclination Bias Using a Random Walk Approach

Abstract A fundamental working assumption in paleomagnetic studies is that the Earth's magnetic field averages to a geocentric axial dipole (GAD) when sufficiently sampled. One of the main tools for evaluating the GAD hypothesis in pre-Cenozoic times is based on the distribution of inclination values. Recent studies of inclination-only data show a bias towards low inclination and a number of alternative explanations were forwarded to explain this bias. The inclination only analysis relies on the fact that the planet has been adequately sampled in a spatially and/or temporally random manner. A recent paper argued that the inclination only studies might misrepresent the field because the extant global paleomagnetic database does not provide an adequate sampling of the field. In this study, we examine other sources of bias in the database. We find that the apparent contributions of quadrupolar and octupolar fields may depend upon the binning procedure used. For example, the Cenozoic database can be favorably compared to GAD when assigned to temporal bins based on geologic periods, but is decidedly non-GAD when averaged on a finer temporal scale. We also demonstrate that the Paleozoic inclination distribution may result from a regional sampling bias and we quantitatively assess the probability that the Precambrian global paleomagnetic dataset sufficiently integrates the time-averaged Earth's magnetic field. Our analysis suggests that the extant inclination database contains myriad forms of bias and may not represent the Earth's magnetic field. Unfortunately, the analysis cannot rule out the existence of persistent nondipolar fields. The global paleomagnetic database does indeed show a rather consistent bias towards low-inclination values (median inclination is 40° versus 49° for the GAD). Models of the earth's magnetic field and the thermal evolution of the planet may yield additional clues regarding its GAD or non-GAD nature

Heat flow at the Platanares, Honduras, geothermal site

Three boreholes, PLTG-1, PLTG-2 and PLTG-3, were drilled in the Platanares, Honduras geothermal system to evaluate the geothermal energy potential of the site. The maximum reservoir temperature was previously estimated at 225-240[deg]C using various types of chemical and isotopic geothermometry. Geothermal gradients of 139-239[deg]C/km, calculated from two segments of the temperature-depth profile for borehole PLTG-2, were used to project a minimum depth to the geothermal reservoir of 1.2-1.7 km. Borehole PLTG-1 exhibited an erratic temperature distribution attributed to fluid movement through a series of isolated horizontal and subhorizontal fractures. The maximum measured temperature in borehole PLTG-1 was 150.4[deg]C, and in PLTG-2 the maximum measured temperature was 104.3[deg]C. PLTG-3 was drilled after this study and the maximum recorded temperature of 165[deg]C is similar to the temperature encountered in PLTG-1.Heat flow values of 392 mWm-2 and 266 mWm-2 represent the first directly-measured heat flow values for Honduras and northen Central America. Radioactive heat generation, based on gamma-ray analyses of uranium, thorium and potassium in five core samples, is less than 2.0 [mu]Wm-3 and does not appear to be a major source of the high heat flow. Several authors have proposed a variety of extensional tectonic environments for western Honduras and these heat flow values, along with published estimates of heat flow, are supportive of this type of tectonic regime.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29424/1/0000503.pd

Late Proterozoic paleomagnetism and tectonic models: a critical appraisal

Interpretations of Proterozoic orogenic belts in terms of plate tectonic processes have been widely divergent. Published models range between the extremes of no relative motions between continental nuclei (implying ensialic orogenic processes) on the one hand, to large-scale relative motions with oceans opening and closing (resulting in continent-continent collisions) on the other hand. Paleomagnetic data can, in theory, contribute significantly to this debate; however, as shown in this paper, several tectonic interpretations on the basis of paleomagnetic data have been premature. A critical continent in many of the previous models is Africa. In order to test hypotheses, for instance, for the late Proterozoic-Cambrian Pan African orogeny, a compilation of paleopoles has been made for Africa, with age ranges falling fully or partially within the interval of 1150 to 500 Ma. A quantitative comparison of the quality of this African dataset with the Phanerozoic poles for North American and Europe shows that the late Proterozoic paleopoles of Africa generally have very low reliability. It appears that the data from other Gondwana continents are equally unreliable and even less abundant. This means that currently the dataset of Gondwanaland cannot be used with confidence for the testing of tectonic models such as the Precambrian supercontinent, at least for the time after 1150 Ma. Well-dated late Proterozoic paleopoles from the three cratonic nuclei within Africa (Congo, Kalahari, West Africa) define relatively short apparent polar wander path segments, but each with different age ranges. This implies that they cannot be compared with each other to test relative motions between the cratonic nuclei and that a choice between ensialic and ensimatic models for the Pan African orogenic belts cannot yet be based on paleomagnetic data. While this does not imply that the tectonic models (e.g. those of Piper and McWilliams) are wrong, it does mean that substantial paleomagnetic support for them will have to wait more and higher-quality paleopole determinations with better dating precision.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29100/1/0000136.pd

Paleomagnetism of the Late Archean Nyanzian System, western Kenya

The Nyanzian System lavas of western Kenya are believed to be the oldest rocks of the Tanzanian Craton. Intrusive age relationships suggest an age [ges]2850 Ma although direct attempts at dating the Nyanzian have produced disparate results. Our study involves a suite of samples collected from the Nyanzian basalts, pillow basalts, andesites and rhyolites from sixteen sites in western Kenya. These rocks yield a tilt-corrected paleomagnetic pole at 14[deg]N, 150[deg]E (K=59, dp=5[deg], dm=7[deg]). This pole is constrained to be older than the first (D1) deformation (>2472+/-30 Ma) by positive fold, conglomerate and reversal tests. Analysis of the paleomagnetic data base for three African cratonic nuclei (Tanzanian, Kaapvaal/Zimbabwe and West Africa) for the time period from 2.0 Ga to 3.0 Ga demonstrates a paucity of well-dated poles, although there are several poles from the Kaapvaal/Zimbabwe and Tanzanian Cratons which allow "spot-readings" of their relative positions. We demonstrate, based on these data, that the Kaapvaal/Zimbabwe and Tanzanian Cratons were drifting independently at ~ 2875 Ma, ~ 2700 Ma and ~ 2450 Ma. This independent motion of the Tanzanian and Kaapvaal/Zimbabwe Cratons indicates that previously proposed models involving African cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31278/1/0000184.pd

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous^Early Tertiary apparent polar wander (APW) paths (300^40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous^Early Jurassic and Late CretaceousT ertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland's position is problematic in a Bullard et al. fit, because of a Late Triassic^Early Jurassic regime of compression ( s 300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland`locked' to Europe for the Late Paleozoic^Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous. ß 2001 Published by Elsevier Science B.V

The magnificent seven : A proposal for modest revision of the Van der Voo (1990) quality index

Thirty years ago, Rob Van der Voo proposed an elegant and simple system for evaluating the quality of paleomagnetic data. As a second-year Ph.D. student, the lead author remembers Rob waxing philosophical about the need to have an appropriate, but not overly rigid evaluation system. The end result was a 7-point system that assigned a (1) or (0) for any paleomagnetic result based on objective criteria. The goal was never to reject or blindly accept any particular result, but merely to indicate the degree of quality for any paleomagnetic pole. At the time, the global paleomagnetic database was burgeoning and it was deemed useful to rank older paleo magnetic results with the newer data being developed in modern laboratories. Van der Voo's, 1990 paper launched a silent revolution in paleomagnetism. Researchers began to evaluate their data against those seven criteria with the anticipation that reviewers would be similarly critical. Today, paleomagnetism is a mature science. Our methods, analyses, and results are more sophisticated than they were 30 years ago. Therefore, we feel it is appropriate to revisit the Van der Voo (1990) criteria in light of those developments. We hope to honor the intention of the original paper by keeping the criteria simple and easy to evaluate while also acknowledging the advances in science. This paper aims to update the criteria and modernize the process. We base our changes on advances in paleomagnetism and geochronology with a faithful adherence to the simplicity of the original publication. We offer the "Reliability" or "R" index as the next generation of the Van der Voo "Quality" or "Q" index. The new R-criteria evaluate seven different information items for each paleomagnetic pole including age, statistical requirements, identification of magnetic carriers, field tests, structural integrity, presence of reversals and an evaluation for possible remagnetization.Peer reviewe

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at http://www.lsst.org/lsst/sciboo

A paleomagnetic analysis of Cambrian true polar wander, Earth planet

Abstract The latest Neoproterozoic through Cambrian is one of the most remarkable intervals in geologic time. Tectonically, the period from 580 to 490 Ma marks a time of rapid plate reorganization following the final stages of supercontinental breakup and Gondwana assembly. The apparent speed at which this reorganization occurred led some to propose a link between tectonic events, biologic changes and climatic changes. One of the more intriguing proposals is that the tectonic changes were triggered by an episode of inertial interchange true polar wander (IITPW) which resulted in a rapid (6º=m.y.) shift of the spin axis relative to the geographic reference frame. IITPW is a special case of true polar wander (TPW) that makes specific demands on the length of apparent polar wander paths (APWPs) recording the motion. Specifically, each path must allow for ¾90º of synchronous motion during the interval from 523 to 508 Ma. A review of paleomagnetic data for Laurentia, Baltica, Siberia and Gondwana indicates that none of the APWPs approaches the necessary length, each path is of a different length and the apparent motions are non-synchronous. Collectively, these observations negate the premise of a Cambrian IITPW event. Since the IITPW hypothesis was proposed as an alternative to rapid plate motion of Laurentia and Gondwana during the Neoproterozoic-Cambrian interval, any alternative model must account for this rapid motion. I suggest that a reasonable explanation for 'anomalously' high rates of plate motion for some continents, possibly on the order of 20-40 cm yr 1 , is enhanced plate motion driven by lower-mantle thermal anomalies and possibly true polar wander. In fact, the enhanced plate motions driven by these lower-mantle sources may provide a dynamic feedback triggering true polar wander

Strange attractors, spiritual interlopers and lonely wanderers: The search for pre-Pangean supercontinents

The observation is made that there are very strong similarities between the supercontinents Columbia, Rodinia and Pangea. If plate tectonics was operating over the past 2.5 billion years of Earth history, and dominated by extroversion and introversion of ocean basins, it would be unusual for three supercontinents to resemble one another so closely. The term ‘strange attractor’ is applied to landmasses that form a coherent geometry in all three supercontinents. Baltica, Laurentia and Siberia form a group of ‘strange attractors’ as do the elements of East Gondwana (India, Australia, Antarctica, Madagascar). The elements of “West Gondwana” are positioned as a slightly looser amalgam of cratonic blocks in all three supercontinents and are referred to as ‘spiritual interlopers’. Relatively few landmasses (the South China, North China, Kalahari and perhaps Tarim cratons) are positioned in distinct locations within each of the three supercontinents and these are referred to as ‘lonely wanderers’. There may be several explanations for why these supercontinents show such remarkable similarities. One possibility is that modern-style plate tectonics did not begin until the late Neoproterozoic and horizontal motions were restricted and a vertical style of ‘lid tectonics’ dominated. If motions were limited for most of the Proterozoic, it would explain the remarkable similarities seen in the Columbia and Rodinia supercontinents, but would still require the strange attractors to rift, drift and return to approximately the same geometry within Pangea. A second possibility is that our views of older supercontinents are shaped by well-known connections documented for the most recent supercontinent, Pangea. It is intriguing that three of the four ‘lonely wanderers’ (Tarim, North China, South China) did not unite until just before, or slightly after the breakup of Pangea. The fourth ‘lonely wanderer’, the Kalahari (and core Kaapvaal) craton has a somewhat unique Archean-age geology compared to its nearest neighbors in Gondwana, but very similar to that in western Australia