Michael W. McElhinny

Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana

Paleomagnetic data from East Gondwana (Australia, Antarctica, and India) and Laurentia are interpreted to demonstrate that the two continents were juxtaposed in the Rodinia supercontinent by 1050 Ma. They began to separate after 725 Ma, allowing the formation of the Pacific Ocean. The low-latitude Rapitan and Sturtian glaciations occurred during the rifting that led to continental breakup. East Gondwana remained in low latitudes for the rest of the Neoproterozoic, while Laurentia moved to polar latitudes by 580 Ma. During the Vendian, a wide Pacific Ocean separated the two continental land masses. The younger Marinoan, Ice Brook, and Varangian glaciations in the early Vendian preceded a second continental breakup in the late Vendian, causing formation of the eastern margin of Laurentia and rejuvenation of its western margin. Paleomagnetic data indicate that Gondwana was not fully assembled until the end of the Neoproterozoic, possibly as late as Middle Cambrian.

Paleozoic terranes of eastern Australia and the drift history of Gondwana

Abstract Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.

Permo-Triassic magnetostratigraphy in China: northern Tarim

Abstract The upper boundary of the Permo-Carboniferous Reversed Polarity Superchron has been identified in a palaeomagnetic study of the Permo-Triassic of the northern part of the Tarim Basin, China. This boundary serves as an important marker horizon for correlation with other Permo-Triassic sequences both in China and world-wide. A Permo-Triassic palaeomagnetic pole for the Tarim Block is estimated to be at 71.8°N, 187.6°E. Comparison with similar age poles from the adjacent blocks of China and Asia suggests that the Tarim was widely separated from the Sino-Korean Block in Permo-Triassic times but was not yet sutured to Kazakhstan.

The myth of the Pacific dipole window

The angular dispersion of virtual geomagnetic poles (VGP) from lava flows is often cited as having been anomalously low in the Pacific during the Brunhes epoch because the dispersion from Hawaiian lavas is said to be much lower than measured elsewhere. This led to the concept of the Pacific dipole window or Pacific non-dipole low. Because lavas tend to be erupted in bursts of activity, many of the Hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations. Geochronological controls, as are available for Hawaii, have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field. Unfortunately, in that analysis the angular dispersion of VGPs was calculated about the mean VGP instead of about the spin axis, and thus underestimated the dispersion. We have therefore reanalyzed the relevant data using the following criteria. (1) Only those lavas with α95 < 10° have been considered. (2) A latitude-dependent cut-off angle is used to eliminate those vectors that are not part of the normal secular variation. (3) We have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector, because suitable geochronological controls are rarely available. Application of this new method to the Hawaiian data shows excellent agreement with the results from using geochronological controls. Where possible we have therefore included this method in our overall analysis. As expected, the resulting global data are compatible with a Fisher distribution about the spin axis. Brunhes age data from Hawaii (N = 96 independent measurements) give a VGP angular dispersion about the spin axis of SF = 12.4°, and for the Pacific region as a whole SF = 12.5° (N = 190) between latitudes 15° and 30° (north or south). These values are the same as SF = 12.4° (N = 160) calculated for the rest of the world in the same latitude range. This clearly demonstrates that the hypothesis of the Pacific dipole window may confidently be rejected.

Global paleomagnetic database supplement number one: update to 1992

This is the first Supplement to the Manual, first published inSurveys in Geophysics in 1991 and issued also as a separate volume, for the operation of the Global Paleomagnetic Database (GPMDB) using ORACL E. Minor changes have been made to the database structure as foreshadowed in the Manual and major extensions have been made to the MENU involving a new set of command files. These and other changes are detailed and should be read in conjunction with the original Manual. This latest Version 2.2 of the the GPMDB now contains over 7000 results with over 2600 references and covers all published data world-wide up to the end of 1992. Diskettes containing the new data set and accompanying program files may be obtained, as before, from World Data Center A in Boulder, Colorado. Both the updated data set and program files completely replace the original Version 1.4 released in August 1991.

Global Paleomagnetic Data Base developed into its visual form

The Global Paleomagnetic Database (GPMDB) created and developed by M.W. McElhinny and J. Lock is used by researchers all over the world. The well-known version of this data base is in Microsoft Access format, a user-friendly interface that requires no programming skills. The next step in developing these data bases lies in the visualization of data and the integration of the paleomagnetic data with Geographical Information Systems (GIS). One of the most popular GIS software applications among Earth scientists is Arc View, which was developed by the Environmental Systems Research Institute (ESRI). A new Arc View 3.1 application visualizes the GPMDB and provides a range of new possibilities for interactive search, analysis, and plate tectonic applications of paleomagnetic data.

Oracle Hardware and Software

To install and run ORACLE for MS-DOS, your computer must meet these minimum requirements which are for Version 5.1C in 1990. These requirements are subject to change without notice in future versions.

Database Queries with SQL*Plus

The language used to access the database is SQL (Structured Query Language—pronounced SEQUEL). Data are retrieved from the database through queries. SQL is an English-like language that can be used to build queries of substantial complexity and capability. Users with little or no experience can learn SQL’s basic features very quickly. The aim of this Section is to familiarise you with these basics and provide experience working with the GPMDB. It is not a comprehensive coverage of all ORACLE’S features, for this refer to the ORACLE documentation (SQL Language Reference Manual, SQL*Plus Reference Manual) and work through the tutorials provided in the ‘SQL*Plus User’s Guide’.

Database Installation and Maintenance

It is advisable for each PC system using the ORACLE RDBMS that a single person be appointed as the Database Administrator (DBA). The DBA should be responsible for managing and maintaining the GPMDB. The DBA should have sole authority to install or re-initialise the ORACLE RDBMS, expand the database, load, update and backup the GPMDB, and grant userID’s (often with different privileges). This chapter is directed at your DBA.

Database Queries with SQL*Menu and SQL*Forms

A special Application Menu called PMAG has been developed for use with the GPMDB. This Menu was automatically imported into the ORACLE system when the GPMDB was first loaded.