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Supplementary material to “New Internet
software aids paleomagnetic analysis and
plate tectonic reconstructions”
Antonio Schettino and Christopher R. Scotese
For additional information, contact Antonio Schettino, Dipartimento Nuove Tecnologie, Istituto
Tecnico Industriale "E. Molinari", Via Crescenzago 110, 20132 Milano, Italy; E-mail:
[email protected]
Citation:
Schettino, A., and C. R. Scotese (2001), New Internet software aids paleomagnetic analysis and
plate tectonic reconstructions, Eos Trans. AGU, 82(45).
A new method for the generation of synthetic and smoothed APWPs was recently proposed by
Schettino and Scotese [2000]. A synthetic APWP for a given reference plate is based on the
transfer of paleopoles from several continents to the reference plate via the global plate tectonic
rotation model. For example, paleopoles from North America, Australia, etc., may be combined
with African paleopoles to generate a "composite" African APWP. In fact, if the total
reconstruction poles for any continent with respect to the reference plate are known, then each
"foreign" paleopole can be rotated into the coordinates of the reference continent and can
contribute to the construction of a composite APWP. This method has been applied by several
authors during the last 2 decades [e.g., Besse and Curtillot, 1991; Bocharova and Scotese, 1993].
Various attempts alternative to the conventional sliding time-window approach have been less
successful at smoothing APW paths [e.g., Jupp and Kent, 1987]. Our new smoothing method
[Schettino and Scotese, 2000] is simple and much more intuitive with respect to other
mathematical treatments. In this method, a spline curve is fit to the plot of paleodeclinations to
give the best estimate of declination though time. In a similar fashion, a spline curve is fit
through the plot of paleolatitudes to give the best estimate of latitude through time. A synthetic
APWP, which does not depend upon the selected reference point, is then generated by
recombining information from the independently smoothed paleolatitudinal and declination
curves.
A two-part set of software tools developed at the Istituto Tecnico Industriale "E. Molinari" of
Milan, Italy, reflects this new smoothing method. The first part, the Paleomagnetic Subsystem
(PS), comprises applications for manipulating a modified version of the Global Paleomagnetic
Database (GPMDB), searching tables, generating listings of quality paleopoles according to
filtering parameters such as plate identification, producing plots and spline regression curves of
declination and paleolatitude for a reference site, and generating smoothed Apparent Polar
Wander Paths (APWPs). All these tools, as well as the specific implementation of the GPMDB,
have been designed for use in the field of plate tectonic modeling. The Plate Reconstruction
Subsystem (PRS) comprises at present a single application that generates global maps of the past
configuration of the continents and ocean basins in several map projections from Early Jurassic
to recent time. For all these reconstructions, the reference continent, central Africa, is oriented
according to a synthetic and smoothed APWP in a standard paleomagnetic reference frame.
The tools are accessible by URL at: http://www.itis-molinari.mi.it/Geo.html. A complete set of
static paleogeographic maps for the entire Phanerozoic can be found at the Paleomap Project
Web site: http://www.scotese.com/.
Software Implementation
The programs were developed as Common Gateway Interface (CGI) executables to be run on a
Windows NT Web Server. For best performance, all the applications were written in C/C++,
adapting corresponding modules of a desktop software package for plate tectonic modeling
designed by the first author [Schettino, 1998]. Some of these programs generate HTML pages
containing links to related results. For instance, a page containing information about a rock unit
also contains a link to the corresponding journal reference as well as to related paleopoles. This
method allows easy navigation across the GPMDB tables. Other applications also generate flat
ASCII tables that can be downloaded and used by other programs. The graphic output (plots,
APWPs, or paleogeographic maps) consists of downloadable images in Portable Network
Graphics (PNG) format. This new, powerful graphic system is a patent-free replacement for
GIFs. For more information about this format visit the W3C site at
http://www.w3.org/Graphics/PNG/ or the official PNG site at http://www.libpng.org/pub/png/.
The Paleomagnetic Subsystem (PS)
The modified version of the GPMDB used by the paleomagnetic subsystem includes only three
of the six main tables that are defined in the original database [Lock and McElhinny, 1991].
These are the Reference, Rockunit, and Pmagresult tables. Two other "stand-alone" help tables
from the original GPMDB, Information and Timescale, were also incorporated to give further
information about rock units and paleopoles. An additional table, not included in the standard
GPMDB release, contains data about the set of Mesozoic and Cenozoic tectonic elements and
plates that were identified by the authors. Each block is defined as a rigid tectonic unit of
continental lithosphere bounded by faults, or, in few cases, by folds, with an independent
tectonic history during the Mesozoic or the Cenozoic. The Plates table includes 223 tectonic
blocks or plates defined in the global and regional paleotectonic models of Scotese [1990] and
Schettino and Scotese [2000]. This data set will be further discussed in two publications being
prepared by Schettino and Scotese. Figure 1 illustrates the structure of this implementation of the
GPMDB.
Fig. 1. Structure of the paleomagnetic database installed on the Web server. Arrows represent one-to-many
relationships.
The main difference between this version and the original version of the GPMDB is in the
paleopole grouping criterion. In the original data set of Lock and McElhinny [1991], paleopoles
are grouped by continent, and each result may be assigned to a terrane. Terranes are in turn
grouped by continent. This continent/terrane model does not easily integrate with existing global
plate tectonic models that describe the motion of a large number of independently moving
tectonic elements. These tectonic elements may or may not correspond to terranes defined in the
GPMDB. For this reason it is difficult to combine paleopoles from different continents to
produce composite APWPs. Our implementation allows a better integration of paleomagnetic
data and plate tectonic modeling and reconstruction tools.
The Web server described in this paper provides independent paleolatitude or declination plots,
as well as synthetic and smoothed APWPs, by the method described above. All of these tools are
included in the paleomagnetic subsystem. In an example of paleolatitude plot for a point at (0N,
25E) in Central Africa (Figure 2a), 165 filtered paleopoles with an age range 0-200 Ma from
Central Africa, Northeast Africa, Madagascar, Somalia, India, Arabia, Australia, Antarctica, the
Brazilian Craton, Parana Block, and Patagonia were used to generate the curve. Filtering was
performed according to the following reliability criteria (default values): B
sites), N/B
4 (mean number of samples per site), ED95
4 (number of
15o (95% confidence interval),
DEMAGCODE
2 (cleaning procedure code),
t
20 Ma (half-interval of age
uncertainty). A subsequent post-filtering procedure was then applied to exclude paleopoles that
gave a paleolatitude determination of more than 10o above or below the spline regression curve.
Figure 2b shows an example of declination plot for the same reference point and set of plates. In
this instance, 189 filtered paleopoles in the age range 0-200 Ma were used in the regression
analysis. Finally, Figure 2c illustrates the APWP resulting from these paleolatitude and
declination spline regression plots.
Fig. 2. (caption pending)
The Plate Reconstruction Subsystem
Plate tectonic reconstruction maps require the association between a) a rotation model; b) a
compilation of tectonic elements of continental lithosphere; and c) a compilation of additional
paleogeographic features that are to be displayed (for example, mountains, magmatic provinces,
age of the ocean floor, etc.).
A rotation model is built starting from a set of identified marine magnetic anomalies and
associated fracture zones. Fracture zone trends determine, for each time interval, the location of
the instantaneous Euler pole of rotation of a plate with respect to a conjugate plate. Magnetic
anomalies determine the amplitude of motion, that is, the size of the angular velocity vector.
However, actual rotation models include finite total reconstruction poles at anomaly times rather
than instantaneous poles. These rotations represent the total rotation poles and angles of rotation
that must be applied to a tectonic element to get the relative positions with respect to a reference
plate at the anomaly times. Rotation models are tree structures that include total reconstruction
poles for each pair of conjugate plates. Collisional settings at regional scale require a special
treatment, because relative motions cannot be calculated using magnetic anomalies and fracture
zones. In such difficult areas--for example, the Mediterranean region--indirect methods of
analysis must be applied.
The rotation model used by the Web server is partly based on a digital compilation of oceanic
isochrons of Royer et al. [1992] and the continental tectonic elements of the PALEOMAP
Project [Scotese, 1990]. The North Atlantic and Arctic regions were modeled according to the
rotation parameters of Rowley and Lottes [1988]. The "root" continent, Central Africa, was
oriented according to a synthetic APWP in a standard paleomagnetic reference frame. Finally,
the global model was integrated in some cases by detailed regional scale models of, for example,
the Mediterranean, southeast Asia, and southwest Pacific.
We use a digital compilation of 223 tectonic elements for the Mesozoic and the Cenozoic. These
blocks form the basic layer of any reconstruction and represent the distribution of the continental
lithosphere through geologic time. The most difficult task was to obtain a reliable unstretched
Continent-Ocean Boundary (COB) when a block was partly bounded by oceanic crust. This task
was accomplished using gravity anomaly data [Sandwell and Smith, 1997] and, whenever
possible, Moho depth and basement profiles generated using the Cornell Interactive Mapping
Server (accessible by URL: http://atlas.geo.cornell.edu/ima.html).
Finally, additional paleogeographic features that can be viewed through the Web server are the
age of the ocean floor and the distribution of Large Igneous Provinces (LIPs). A legend is
available in a separate window. Apart from the above-mentioned digital compilation of tectonic
elements of continental crust, the maps can display up to 2074 outlines of LIPs [Coffin and
Eldholm, 1994] and 1031 polygons representing blocks of oceanic crust bounded by transform
faults and isochrons. The LIP data set is a modified version of the digital compilation available
via anonymous ftp from the University of Texas Institute for Geophysics
[ftp://ftp.ig.utexas.edu/pub/LIPS/]. Most of the 1031 elements of oceanic crust were extracted
from the latest release (version 1.5) of the digital ocean floor age grid [M�t>ller et al., 1997],
using the method described by Schettino [1999]. A complete display of the age of oceanic areas
on both sides of a spreading center can only be obtained when the reconstruction time coincides
with one of the anomaly times defined in the rotation model. These anomalies are: A5 (10.9 Ma),
A6 (20.1 Ma), A13 (33.1 Ma), A18 (40.1 Ma), A21 (47.9 Ma), A25 (55.1 Ma), A31 (67.7 Ma),
A34 (83.5 Ma), M0 (120.4 Ma), M4 (126.7 Ma), M10 (131.9 Ma), M16 (139.6 Ma), M21 (147.7
Ma), and M25 (154.3 Ma). Figure 3 shows a sample paleogeographic map generated by the Web
server for anomaly A13. Map projection is Mollweide with center of projection at (30S,180E).
Fig. 3. Plate reconstruction for anomaly A13 (33.1 Ma). Map projection is an oblique Mollweide, with center of
projection at 30S, 180E.
Future Plans
Future plans include a Java interface for visualizing velocity fields and flow lines, a system for
selecting the display area, and the capability of selecting other reference systems (for example,
hot-spot frames of reference).
References
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