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Recent earthquakes in Japan and California vividly remind us of the fact that the Earth is an active planet, and large parts of its crust, called tectonic plates, are moving on its surface. The deep ocean basins form key regions for deciphering past and present plate motions, because the ocean floor records polarity reversals of the Earth's magnetic field in its rocks. In some ways it is similar to a giant tape recorder. Correlation with a geological time-scale then allows us to estimate the time when particular portions of the ocean floor were created by volcanic activity along mid-ocean ridges.
The digital ocean floor age map resulted from a collaborative study between the University of Sydney, Australia; the Geological Survey of Canada; Scripps Institution of Oceanography in La Jolla, California; the Laboratoire de Geodynamique, Villefrance sur Mer, France and the University of Texas, Austin. This map and the data it is based on provide a means to reconstruct continents in their past configurations.
Knowledge of plate motions not only leads to a better understanding of global and regional earthquake potential, for example in British Columbia, but also to a better understanding of the tectonic processes that are responsible for the formation of the large sedimentary basins that contain vast quantities of oil and gas in Canada's offshore regions.
by R. Dietmar Müller, Walter R. Roest, Jean-Yves Royer, Lisa M.
Gahagan, and John G. Sclater (May, 1996).
R. Dietmar Müller
Department of Geology and Geophysics, University of Sydney, Australia
Walter R. Roest
Geological Survey of Canada, Ottawa, Canada
Jean-Yves Royer
Lab. de Géodynamique, Villefranche Sur Mer, France
Lisa M. Gahagan
Institute for Geophysics, University of Texas, Austin, Texas
John G. Sclater
Scripps Institution of Oceanography, La Jolla, California
Abstract
Introduction
Ocean Floor Isochrons and Plate Boundaries
Interpolation of Isochrons and Gridding
Accuracy
Conclusions
Acknowledgments
References
Addresses and e-mail of authors
We have created a digital age grid of the ocean floor with a grid node
interval of 6 arc-minutes using a self-consistent set of global isochrons
and associated plate reconstruction poles. The age at each grid node was
determined by linear interpolation between adjacent isochrons in the direction
of spreading. Ages for ocean floor between the oldest identified magnetic
anomalies and continental crust were interpolated by estimating the ages
of passive continental margin segments from geological data and published
plate models. We have constructed an age grid with error estimates for
each grid cell as a function of (1) the error of ocean floor ages identified
from magnetic anomalies along ship tracks and the age of the corresponding
grid cells in our age grid, (2) the distance of a given grid cell to the
nearest magnetic anomaly identification, and (3) the gradient of the age
grid, i.e. larger errors are associated with high age gradients at fracture
zones or other age discontinuities. Future applications of this digital
grid include studies of the thermal and elastic structure of the lithosphere,
the heat loss of the Earth, ridge-push forces through time, asymmetry of
spreading, and providing constraints for seismic tomography and mantle
convection models.
The age of the ocean floor is an important parameter in the study of plate
tectonic processes. An accurate digital age grid is essential for many
studies, including plate kinematics, studies of plate driving forces, mantle
dynamics, ocean floor roughness and paleoceanography. Several analog maps
of the age of the ocean floor have been compiled using magnetic anomaly
data [e.g., Sclater et al., 1981; Larson et al., 1985]. A digital version
of the latter map was produced by Cazenave et al. [1988], at a grid interval
of half a degree (approx. 55 km). Recent improvements in identifications
of magnetic anomalies and plate kinematic models, especially aided by dense
gravity data from satellite altimetry, permit a more detailed description
of the spreading process, and have initiated the construction of a more
detailed age grid.
We have constructed a global set of isochrons for the ocean basins corresponding
to magnetic anomalies 5, 6, 13, 18, 21, 25,31, 34, M0, M4, M10, M16, M21,
and M25 based on a global plate reconstruction model, magnetic anomaly
identifications and fracture zones [see also Royer et al., 1992]. The geomagnetic
time scale of Cande and Kent [1995] was used for anomalies younger than
chron 34 (83 Ma), the time scale from Gradstein et al. [1994] for older
times. Isochrons were constructed by plotting reconstructed magnetic anomaly
and fracture zone picks, as well as selected small circles computed from
stage rotation poles for each isochron time, keeping one plate fixed. Then
best-fit continuous isochrons were constructed, connected by transforms,
in the framework of one fixed plate [see also Müller et al., 1991].
A complete set of isochrons for all conjugate plate pairs was derived by
rotation of every isochron to their present day position.
Construction of a complete age grid also requires knowledge of the present
day plate boundary geometry. The boundaries shown in Figure 1 have been
compiled based on a marine gravity grid from Geosat exact repeat mission,
Geodetic Mission, and ERS-1 satellite altimetry data [Sandwell et al.,
1994], bathymetric data, and Earthquake epicenters. There is a significant
area of ocean floor that is older than the oldest mapped isochrons. In
order to estimate ages for the oldest ocean floor in ocean basins bounded
by passive margins, we assigned ages to continental margin segments based
on geological data and published plate models. The regional boundaries
between continental and oceanic crust have been compiled in Müller
and Roest [1992] (North and central North Atlantic), Nürnberg and
Müller [1992] (South Atlantic), and Royer et al. [1992] (Indian Ocean).
South of 60°S a dense grid of Geosat Geodetic Mission data [Sandwell
et al., 1994] has been used to better locate boundaries between continental
and oceanic crust in remote areas such as the Antarctic continental margin.
In order to create a smooth grid of ocean floor ages that maintains all
sharp age discontinuities at fracture zones, we first create a set of densely
interpolated isochrons. We assume that the spreading direction between
two adjacent isochrons is given by a constant stage pole of motion, derived
from our plate kinematic model. We also assume that the spreading velocity
between two adjacent isochrons is constant, and that consequently the age
varies linearly in the direction of spreading on a given ridge flank. To
simplify the calculations, each pair of adjacent isochrons is transformed
to a coordinate system in which the stage pole of motion between the two
isochrons is moved onto the geographic north pole [Roest et al.,1992].
Then intermediate isochrons were linearly interpolated along plate flow-lines.
This is equivalent to interpolation along small circles about the stage
pole. The complete set of isochrons for each stage was subsequently rotated
back into the geographic reference frame. This was done for each isochron
pair on each plate pair.
To interpolate the ages onto a regular grid, we assume that the isochrons
are continuous, which is implemented by densely interpolating between observation
points along each isochron. A minimum curvature routine is used to obtain
age values on a regular grid at a resolution of 0.1 degrees, equivalent
to 6 arc-minutes. Areas of the ocean floor with insufficient data coverage
were blanked out in the grid. We included data from selected back-arc basins,
where data coverage is sufficient and available to us. The resolution of
our grid for these areas is typically reduced by a factor of 10 with respect
to the oceanic grid, i.e. the resolution in back-arc basins does not exceed
1 degree, and provides merely a rough estimate of the age distribution
in these basins. The resulting grid is shown in Figure
1a.
The accuracy of the age grid varies considerably due to the spatially irregular
distribution of ship track data in the oceans. Other sources of errors
are given by our chosen spacing of isochrons as listed before, between
which we interpolated linearly. These stages are especially long during
long time intervals without changes in the polarity of the Earth's magnetic
field such as the Cretaceous Magnetic Quiet Zone from about 118 to 83 Ma.
We assume that age grid errors depend on the distance to the nearest data
points and the proximity to fracture zones. In order to estimate the accuracy
of our age grid, we construct a grid with age-error estimates for each
grid cell dependent on (1) the error of ocean floor ages identified from
magnetic anomalies along ship tracks and the age of the corresponding grid
cells in our age grid, (2) the distance of a given grid node to the nearest
magnetic anomaly identification, and (3) the gradient of the age grid,
i.e. larger errors are associated with high age gradients across fracture
zones or other age discontinuities. The latter also reflects that, due
to the interpolation process, uncertainty in the magnetic anomaly will
induce larger age errors in regions of slow spreading rates than in regions
of fast spreading rates.
We first compute the age differences between ~30000 interpreted magnetic
anomaly ages and the ages from our digital age grid, and investigate the
size and distribution of the resulting age errors. We find that the majority
of errors are smaller than 1 m.y. and errors larger than 10 m.y. are mostly
due to erroneously labeled or interpreted data points. Therefore we set
an upper limit for acceptable errors as 10 m.y. As a lower limit we arbitrarily
choose 0.5 m.y., since we do not expect to resolve errors smaller than
0.5 m.y. given the uncertainty in the timescales used. We grid the remaining
age errors by using continuous curvature splines in tension.
The constraints on the ages in our global age grid generally decrease with
increasing distance to the nearest interpreted magnetic anomaly data point.
Areas without interpreted magnetic anomalies include east-west spreading
mid-ocean ridges in low latitudes such a the equatorial Atlantic ocean,
where the remanent magnetic field vectors are nearly parallel to the mid-ocean
ridge and cause very small magnetic anomalies, and areas with sparse data
coverage such as some remote areas in the southern ocean. In order to address
the "tectonic reconstruction uncertainties" for these areas,
we create a grid containing the distance of a given grid cell to the nearest
data point, ranging from zero at the magnetic anomaly data points to 10
at distances of 1000 km and larger. We smooth this grid using a cosine
arch filter (5° full width) and add the result to the initial splined
grid of age errors.
Fracture zones are usually several tens of km wide, containing highly fractured
and/or serpentinized ocean crust. Age estimates may be uncertain especially
near large-offset fracture zones, which are more severely affected by changes
in spreading direction than small-offset fracture zones. Consequently,
our age estimates along large-offset fracture zones may be more uncertain
than at small-offset fracture zones or on "normal" ocean crust.
Large-offset fracture zones are easily identified in the age grid by computing
the gradient of the age grid. We identify the age gradients associated
with medium- to large offset fracture zones, set the gradients of "normal"
ocean crust to zero, smooth the result with a 3x3 moving average filter,
and scale the grid to range from one to two. After multiplying the error
grid with the smoothed age gradients along fracture zones, we have not
altered the errors associated with "normal" ocean floor, and
increased the errors at fracture zones by a factor between one and two,
depending on the magnitude of the age gradient. The resulting grid of age
uncertainties is shown in Figure 1b.
The digital age grid presented here is the first of its kind, because (1)
in the past ages of the ocean floor have only been available on analog
maps, with the exception of a digitized version of Larson et al.'s [1985]
age map produced by Cazenave et al. [1988] at a relatively coarse grid
interval of 0.5°, (2) our grid is based on a self-consistent global
plate model, and (3) it is accompanied by a grid estimating the uncertainties
of the gridded ages. A shortcoming of our error analysis at present is
that it does not include the uncertainties of the plate rotations. We hope
to include this parameter in the next age grid generation.
This work was made possible by the contributors to the former Paleoceanographic
Mapping Project (POMP, University of Texas, Austin) who released data that
served as partial input for constructing the isochrons, POMP industry sponsors
for financial support to RDM, LMG and JYR, and by the PLATES industry sponsors
through support to LMG. Construction of the age grid was started at the
Scripps Institute of Oceanography while the senior author was supported
by a graduate and a post-doctoral fellowship. JYR acknowledges support
by the CNRS (Centre National de la Recherche Scientifique). CONOCO/Canada
provided funds for publishing color figures. The GMT software system from
P. Wessel and W.H.F. Smith was invaluable in performing the age error analysis,
and for producing the Figures.
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D. Nürnberg, and J.G. Sclater, A global isochron chart, Univ. of Texas
Inst. for Geophysics Tech. Rep., 117, 1992.
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L. M. Gahagan, Institute for Geophysics, University of Texas, 8701 Mopac
Boulevard, Austin, TX 78759-8345 (e-mail: lisa@utig.ig.utexas.edu)
R. D. Müller, Department of Geology and Geophysics, Building F05,
University of Sydney, N.S.W. 2006, Australia (e-mail: dietmar@es.su.oz.au)
W. R. Roest, Geological Survey of Canada, 615 Booth Street, Room 235,
Ottawa ON, K1A 0E9, Canada (e-mail: roest@agg.NRCan.gc.ca)
J.-Y. Royer, Lab. de Géodynamique, O.O.V., La Darse, B.P. 48,
06230 Villefranche Sur Mer, France (e-mail: royer@es.su.oz.au)
J. G. Sclater,, Scripps Institution of Oceanography, UCSD, 9500 Gilman
Drive, La Jolla, CA 92093-0215 (e-mail: sclater@bullard.ucsd.edu)
