Age, spreading rates, and spreading asymmetry of the world's ocean crust

TL;DR: In this article, the authors present a digital model of the age, spreading rate, and asymmetry at each grid node by linear interpolation between adjacent seafloor isochrons in the direction of spreading.

Abstract: We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

Summary

Introduction

• The gridded seafloor isochrons of Müller et al. [1997] represent a widely used resource in a variety of fields ranging from marine geology and geophysics, global seismology, geodynamics and education.
• Here the authors present a new set of grids, including a complete grid of oceanic crustal ages without gaps, based on a revised set of global plate rotations (Supplementary Table 1), as well as gridded sea floor spreading half rates and spreading asymmetry.

3. Error analysis

• An age uncertainty grid is constructed following the approach of Müller et al. [1997], but in addition based on reconstructing ~40,000 magnetic anomaly identifications to their conjugate ridge flanks for constraining the uncertainty in isochron locations and geometries (Fig. 1b).
• The authors 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.
• The authors smooth this grid using a cosine arch filter (5° full width) and add the result to the initial splined grid of age errors.
• Their age estimates along large-offset fracture zones may be more uncertain than at small-offset fracture zones or on "normal" ocean crust.

4. Spreading rate and asymmetry

• Based on their seafloor isochrons and rotation model, the authors have calculated seafloor spreading half rates (Fig. 3a) and relative proportions of crustal accretion (i.e. spreading asymmetry) on conjugate ridge flanks (Fig. 3b).
• Half spreading rates are based on stage rotations computed from their set of finite reconstruction rotations, and spreading asymmetry estimates are based on determining the percentage of crustal accretion between pairs of adjacent isochrons by dividing the angular distance between them by the half-stage rotation angle.
• Both the central North Atlantic and the western Pacific ridges show a sharp pulse of fast spreading rates around 150 Ma.
• This sharp increase in rates may reflect a potential time scale miscalibration.

5. Regional review of tectonic reconstructions

• Compared with the digital isochrons of Müller et al. [1997], their model of the distribution of oceanic crustal age, has been improved considerably.
• This model is supported by geophysical characteristics of the Ionian and east Mediterranean basins (e.g. isostatic equilibrium, seismic velocities, elastic thickness), suggesting that the age of the seafloor must be older than Early Jurassic [Stampfli and Borel, 2002].
• An alternative model for the interpretation of magnetic lineations in this area was presented by Robb et al. [2005]; however, these authors did not develop a plate tectonic model or derive plate rotations, which makes it difficult to test their model as an alternative.
• Following Stampfli and Borel [2002] and Heine et al. [2004] their model includes reconstructions of the major plates constituting the eastern Neo-Tethys ocean.
• The remainder of the Indian Ocean isochrons and reconstructions are taken from Müller et al.

6. Oceanic residual basement depth

• The origin of oceanic residual basement depth anomalies is still controversial.
• In their model two major parameters that govern the asymmetries in depth-age behavior of oceanic lithosphere are absolute plate motion velocities, determining shear-induced asthenospheric flow, and the locations of hotspots near mid-ocean ridges, causing pressure-induced flow towards and along ridges.
• The authors present a set of digital residual basement depth grids accompanied by maps of past subduction zone locations since 140 Ma and seismic tomography cross-sections that may prove useful for investigating the processes associated with residual depth anomalies.
• In these experiments, the boundary layer cools by conduction and then becomes unstable once its local Rayleigh number exceeds a critical value.

7. Origin of crustal accretion asymmetries and basement depth anomalies

• Beyond the many improvements in their digital model of seafloor isochrons summarized above, an innovation presented here are the global grids of sea floor spreading half rates and spreading asymmetries (Fig. 3 and Figs, 4-9), as well as their maps of past subduction zone locations since 140 Ma, paired with shear-wave seismic tomography cross-sections (Fig. 12).
• Instead it appears that asthenospheric flow from hotspots to nearby mid-ocean ridges exerts a larger control on spreading asymmetries than absolute ridge migration velocities, as the authors observe deficits in crustal accretion on ridge flanks close to mantle plumes (Fig. 3b).
• These crustal accretion asymmetries along the East Pacific Rise have resulted in long-term excess accretion on the Nazca Plate, implying consecutive westward ridge jump/propagation events.
• The authors have labelled the areas outlined above in Fig. 11a, and assembled a set of maps of past subduction zone locations paired with seismic tomography cross sections from Ritsema and van Heijst [2000] and Ritsema et al.[2004] (Fig. 12).

7. Conclusions

• Gridded oceanic crustal ages, spreading half-rates, asymmetries and depth anomalies combined with a global relative and absolute plate motion model have a wide range of applications.
• The authors have provided an example how these data sets can be used to decipher the origin of spreading asymmetries and basement depth anomalies.
• Exactly when this ridge may have existed and how it may have continued to the north and linked with either the Izanagi-Pacific or the Farallon Pacific ridge is currently not known.
• Due to the wide spacing of their isochrons and many remaining uncertainties in the observed age-area distribution in the Pacific, their isochrons in this region remain work in progress.

Full text

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site
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Geochemistry, Geophysics, Geosystems
April 2008; VOLUME 9 : NIL_18-NIL_36
http://dx.doi.org/10.1029/2007GC001743
© 2008 American Geophysical Union
An edited version of this paper was published by AGU.
Archimer http://www.ifremer.fr/docelec/
Archive Institutionnelle de l’Ifremer
Age, spreading rates, and spreading asymmetry of the world's ocean
crust
R. Dietmar Müller
1, *
, Maria Sdrolias
1
, Carmen Gaina
2
and Walter R. Roest
3
1
EarthByte Group, School of Geosciences, Building H11, The University of Sydney, NSW 2006, Australia
2
Center for Geodynamics, Geological Survey of Norway, Leiv Eirikssons vei 39, N-7491, Trondheim, Norway
3
Département Géosciences Marines, Ifremer, BP 70, 29280 Plouzané, France
*: Corresponding author : R. Dietmar Müller, email address : d.muller@usyd.edu.au
Abstract:
We present four companion digital models of the age, age uncertainty, spreading rates, and spreading
asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution.
The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc
basins. The age, spreading rate, and asymmetry at each grid node are determined by linear
interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor
between the oldest identified magnetic anomalies and continental crust are interpolated by geological
estimates of the ages of passive continental margin segments. The age uncertainties for grid cells
coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge
flanks, are based on the difference between gridded age and observed age. The uncertainties are also
a function of the distance of a given grid cell to the nearest age observation and the proximity to
fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently
related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward
hot spots. We also use the new age grid to compute global residual basement depth grids from the
difference between observed oceanic basement depth and predicted depth using three alternative
age-depth relationships. The new set of grids helps to investigate prominent negative depth
anomalies, which may be alternatively related to subducted slab material descending in the mantle or
to asthenospheric flow. A combination of our digital grids and the associated relative and absolute
plate motion model with seismic tomography and mantle convection model outputs represents a
valuable set of tools to investigate geodynamic problems.
Keywords: digital isochrons; ocean floor; plate kinematic; geodynamic; seafloor spreading.

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site
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1. Introduction
The gridded seafloor isochrons of Müller et al. [1997] represent a widely used resource in a
variety of fields ranging from marine geology and geophysics, global seismology,
geodynamics and education. However, this grid contains many gaps corresponding to ocean
floor that was poorly mapped at the time when the grid was assembled. They include parts of
the southern and central eastern Indian Ocean, parts of the Late Cretaceous ocean crust in
the southwest Pacific and many back-arc basins. Also, no spreading rate or spreading
asymmetry grids were constructed at that time. Here we present a new set of grids, including
a complete grid of oceanic crustal ages without gaps, based on a revised set of global plate
rotations (Supplementary Table 1), as well as gridded sea floor spreading half rates and
spreading asymmetry.
2. Methodology
The locations and geometry of mid ocean ridges through time are represented by seafloor
isochrons, reconstructed on the basis of marine magnetic anomaly identifications and
fracture zones identified from marine gravity anomalies [Sandwell and Smith, 1997] and a
global set of finite rotations (Supp. Table 1). We follow the interpolation technique outlined by
Müller et al. [1997], and use the timescales of Cande and Kent [1995] and Gradstein et al.
[1994]. In the main ocean basins, our seafloor isochrons are constructed for the same chrons
as those used by Müller et al. [1997], namely chrons 5o (10.9 Ma), 6o (20.1 Ma), 13y (33.1
Ma), 18o (40.1 Ma), 21o (47.9 Ma), 25y (55.9 Ma), 31y (67.7 Ma), 34y (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). However, in back-arc basins, as well as in the Coral and Tasman seas, we construct
isochrons at

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denser intervals, using chrons 1o (0.8 Ma), 2Ay (2.6 Ma), 3n4o (5 Ma), 5o (10.9 Ma), 5Dy (17.3
Ma), 6o (20.1 Ma), 6By (22.6Ma), 7o (25.2 Ma), 8o (26.6 Ma), 9o (28.0 Ma), 10o (28.7 Ma),
11y (29.4 Ma), 13y (33.1 Ma), 15o (35.0 Ma), 16y (35.3 Ma), 17o (38.1 Ma),18o (40.1 Ma), 19o
(41.5 Ma), 20o (43.8 Ma), 21o (47.9 Ma), 22o (49.7 Ma), 24o (53.3 Ma), 25y (55.9 Ma), 26o
(57.9 Ma), 27y (60.9 Ma), 30y (65.6 Ma), 32y (71.1 Ma), 33y (73.6 Ma), 33o (79.1 Ma), 34y
(83.5 Ma), where "y" stands for young end of chron and "o" for old end of chron. The resulting
gridded ages of the ocean floor are shown in Fig. 1a.
3. Error analysis
An age uncertainty grid is constructed following the approach of Müller et al. [1997], but in
addition based on reconstructing ~40,000 magnetic anomaly identifications to their conjugate
ridge flanks for constraining the uncertainty in isochron locations and geometries (Fig. 1b). This
satisfies the requirement that any given seafloor isochron is based on magnetic anomaly data
from both conjugate ridge flanks, at least where both flanks are preserved. The accuracy of the
age grid varies not only due to the spatially irregular distribution of ship track data in the oceans,
but also due to the existence of the Cretaceous Normal Superchron from about 118 to 83 Ma.
Other sources of errors are given by our chosen spacing of isochrons as listed before, between
which we interpolated linearly. We assume that age grid errors depend on the distance to the
nearest data points and to a lesser extent on 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 (Fig. 2) 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

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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 ~45000 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. We set a generous upper limit
for acceptable errors as 15 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 containing crust formed in the
Cretaceous Normal Superchron, such as in the southwest Pacific Ocean and offshore eastern
India (Fig. 1b)..
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

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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, 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 merged
grid of all uncertainties is finally smoothed with a 0.8 degree full width cosine arch filter and is
shown in Figure 1b.
4. Spreading rate and asymmetry
Based on our seafloor isochrons and rotation model, we have calculated seafloor spreading half
rates (Fig. 3a) and relative proportions of crustal accretion (i.e. spreading asymmetry) on
conjugate ridge flanks (Fig. 3b). Half spreading rates are based on stage rotations computed
from our set of finite reconstruction rotations, and spreading asymmetry estimates are based on
determining the percentage of crustal accretion between pairs of adjacent isochrons by dividing
the angular distance between them by the half-stage rotation angle. As a result, relative crustal
accretion rates of conjugate plates (Fig. 3b) can be computed by linear interpolation along
isochrons and gridding, following the same methodology as that used for gridding isochron ages.
In areas where only one of two ridge flanks is preserved, computed spreading asymmetries
correspond to local deviations in individual spreading corridors from the average distance
between two isochrons. We have masked those areas in the Pacific crustal accretion grid that
lack conjugate ridge flanks. However, we have not masked the abyssal plains west of Australia
in our grid showing crustal accretion percentages (Fig. 3b) because several well-known large
ridge jumps away from Australia, which transferred large areas of Indian ocean crust to the
Australian Plate [Mihut and Müller, 1998; Müller, et al., 2000], are outlined well by mapping
deviations from the average distance between adjacent isochrons. Spreading velocities and the

Frequently asked questions

Q1. What are the contributions in "Age, spreading rates, and spreading asymmetry of the world's ocean crust" ?

The authors present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world 's ocean basins as geographic and Mercator grids with 2 arc min resolution. 

Q2. What are the future works in "Age, spreading rates, and spreading asymmetry of the world's ocean crust" ?

For instance, the authors hope to test Taylor 's [ 2006 ] suggestion in the future that the Ontong Java, Manihiki and Hikurangi plateaus formed as one contiguous large igneous province. 

Q3. Why are the isochrons in the Pacific region still work in progress?

Due to the wide spacing of their isochrons and many remaining uncertainties in the observed age-area distribution in the Pacific, their isochrons in this region remain work in progress. 

Q4. What are the applications of the gridded oceanic crustal ages?

Gridded oceanic crustal ages, spreading half-rates, asymmetries and depth anomalies combined with a global relative and absolute plate motion model have a wide range of applications. 

Q5. Why did they use the model by Cande et al.?

due to problems associated with a rotation pole located close to the geographic South Pole as they suggest (see discussion in Müller et al. [2007]) the authors use the model by Cande et al. [2000]. 

Q6. What is the way to describe the accretion of the ridges?

In the central North Atlantic the authors find cumulative excess accretion of only 1% on North America during the last 130 m.y., even though the ridge migration rate is three times that in the South Atlantic. 

Q7. What is the reason that some subducted slab material is still present in the upper mantle?

At the AAD there is an absence of Tertiary slab material, and the reason that some Cretaceous slab material is still present in the upper mantle is rather that the southeast Indian Ridge, which is intersecting the subducted Phoenix slab material roughly at right angles, has drawn up some of the subducted slab material from just above the transition zone [Gurnis, et al., 1998]. 

Q8. What is the effect of the oblique opening on the north Atlantic ridge?

the oblique opening at extremely slow rates (Fig. 4) may have resulted in amagmatic extension and mantle exhumation and serpentinization, as suggested by seismic refraction data [Reid and Jackson, 1997]. 

Q9. What are the results of the asymmetric subduction model?

These results may be useful as constraints for subduction modelsincluding absolute and relative plate motions, subduction hinge kinematics, mantle convection and mantle wedge properties to better understand active margin processes. 

Q10. What is the simplest way to describe the decaying oscillations?

The new material then cools again by conduction, until it in turn becomes unstable,resulting in a series of decaying oscillations about an asymptotic steady-state value, as reflected in Crosby's [2007] age-depth curve (Fig. 10). 

Q11. What is the effect of asthenospheric flow on spreading asymmetries?

Instead it appears that asthenospheric flow from hotspots to nearby mid-ocean ridges exerts a larger control on spreading asymmetries than absolute ridge migration velocities, as the authors observe deficits in crustal accretion on ridge flanks close to mantle plumes (Fig. 3b). 

Q12. What is the origin of the oceanic basement depth anomalies?

Following the idea that spreading asymmetry, basement depth anomaly and asthenospheric flow normal to the ridge axis may be causally connected, the relationship between long-term asymmetries in spreading and asymmetries in oceanic basement subsidence can be evaluated, but this requires the construction of a residual basement depth grid, based on selected age-depth curves (Fig. 10). 

Q13. What is the common description of the depth anomalies?

The depth anomalies off the east coast of the USA ((EC) and the Bay of Bengal (BB) are associated with Cretaceous (and older) subducted slab material in the lower mantle only, whereas the Gulf of Mexico (GM), the Argentine Basin (AB), the "hourglass shaped" depth anomaly associated with the Australian Antarctic Discordance (AAD) and the Philippine Sea (PS) depth anomaly (Fig. 11a, b) are associated with subducted slab material both above and below the upper/lower mantle transition zone (Fig. 12).