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# Seafloor spreading evolution in response to continental growth

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The growth of the continental crust has shaped the evolution of the Earth from its interior to its fluid envelopes. Continents have played a major role in the evolution of global tectonics through their interaction with mantle convection. The feedback between continents and mantle convection has been studied for the past 25 years, but it is only recently that the dynamic influence of continents on seafl oor spreading can be explored thanks to progress in convection modeling. In this work, we investigate how continental size impacts seafloor spreading activity with state-of-the-art three-dimensional spherical convection models. We show that increasing the continental area forces higher production rates of new seafloor with stronger fluctuations. As a consequence, the average age of the seafl oor decreases with increasing continental area. This study suggests that mantle heat loss experienced significant fluctuations through continental growth and reinforces the estimate of <10% continental growth since the late Archean.
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GEOLOGY
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March 2014
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INTRODUCTION
The nature of seaﬂ oor spreading is critical for
many aspects of Earth sciences like mantle de-
gassing (Tajika, 1998), sea level (Gafﬁ n, 1987;
Flament et al., 2008), ocean chemistry (Ber-
ner and Kothavala, 2001), and Earth’s cooling
(Labrosse and Jaupart, 2007). More than 50 yr
of data collection and kinematic reconstruction
efforts have led to signiﬁ cant improvements in
plate tectonic modeling back to 200 Ma (Pilger,
1982; Kominz, 1984; Rowley and Lottes, 1988;
Scotese et al., 1988; Müller et al., 1997; Lith-
gow-Bertelloni and Richards, 1998; Seton et al.,
2012). Models have been proposed for earlier
times back to the Paleozoic (Stampﬂ i and Borel,
2002), but such works remain challenging be-
cause they are naturally limited by the preserva-
tion of very old seaﬂ oor. As a consequence, it is
very difﬁ cult to estimate how seaﬂ oor spread-
ing operated in the Paleozoic and Precambrian.
Remnants of ancient obduction sequences
are suggested, dating back to the late Archean
(Kusky et al., 2001) and even to the early Ar-
chean (Polat et al., 2002). Geochemical studies
have proposed that the Jack Hills (Australia)
zircons (more than 4 b.y. old) could be the earli-
est hints of seaﬂ oor spreading (Harrison et al.,
2005). Therefore, seaﬂ oor spreading could have
operated while continents represented a smaller
area than today (de Wit, 1998).
Continental growth, along with mantle cool-
ing, is a fundamental process that has shaped
Precambrian mantle dynamics. Indeed, the con-
tinental area, occupying now around 30% of the
surface, may have been as low as 10% in the
Mesoarchean (Taylor and McLennan, 1995).
The tempo of continental growth is, however,
still debated: some models favor massive growth
between 2.7 and 2.3 Ga (Taylor and McLennan,
1995), while others propose an earlier growth
In this paper, we investigate how the size of
continents inﬂ uences the properties of seaﬂ oor
cal convection models with plate-like behavior
and continental lithosphere. The calculations
presented here show that continental growth
enhances seaﬂ oor production and the time de-
pendence of spreading. Our study suggests that
mantle heat loss experienced signiﬁ cant ﬂ uctua-
tions through continental growth and reinforces
the estimate of <10% continental growth since
the late Archean (Schubert and Reymer, 1985).
CONVECTION MODELS WITH
CONTINENTS AND SEAFLOOR
The numerical models employed in this study
are 3-D spherical convection models built on
successive generations of software that started
with the Cartesian models of Tackley (2000)
incorporating pseudo-plasticity, before being
extended to spherical geometry (van Heck and
Tackley, 2008). They now include a basic model
of continental lithosphere (Yoshida, 2010; Rolf
and Tackley, 2011), modeled as thick and buoy-
ant rafts, being 100 times more viscous than
oceanic lithosphere. In this study, we computed
numerical solutions in spherical geometry with
a spherical cap–shaped continent covering 10%,
30%, 50%, or 70% of the surface (both of the
latter percentages are probably higher than ever
existed on Earth) and without any continent.
The numerical models used here are technically
similar to those in Rolf et al. (2012) and Coltice
et al. (2012), with a resolution reaching 40 km
vertically in the top boundary layer. This resolu-
tion appears relatively crude because 1 km reso-
lution needs to be reached to resolve present-
day Earth plate boundaries (Gurnis et al., 2004),
but the rheology employed here produces broad
and diffuse plate boundaries that are resolved
here (a computation with 30% continent and
improved resolution of 30 km reproduced the
results). Although the presented models are state
of the art and converge toward tectonic models
(Coltice et al., 2013), improvements in rheology
and resolution are required for a direct compari-
son with the Earth. Also, the Rayleigh number
of our models (based on the temperature drop
over the surface boundary layer) is 106, 10–100
times lower than expected for the Earth. There-
fore, time in the numerical solutions is scaled
such that the model with 30% continents has a
present-day Earth’s transit time of 85 m.y., as
described in Gurnis and Davies (1986). The
models are purely internally heated; a uniform,
time-independent internal heating rate of ~3 ×
1012 W kg–1 is used to obtain a 1300 K drop
over the lithosphere for 30% continental area
with our parameters. In a previous study, no sig-
niﬁ cant differences in the following results were
observed for the problem addressed here when
14% of basal heat ﬂ ow was introduced with
multiple continents covering 30% of the surface
(Coltice et al., 2012).
The models presented are >3-b.y.-long con-
vection solutions at statistical steady state, so
they can be used to obtain a statistical descrip-
tion of the production rate of new seaﬂ oor and
average seaﬂ oor age for a given conﬁ guration
of continental area. The area-age distribution
of the seaﬂ oor in the models is computed by
converting heat ﬂ ow into age assuming a half-
space cooling model, following the approach of
Labrosse and Jaupart (2007) as used previously
in Coltice et al. (2012).
RESULTS
We studied the evolution of the area-age dis-
tribution of the seaﬂ oor in models with a con-
tinent covering 10%, 30%, 50%, and 70% of
the surface of the model, and in a model with
0% continental area. The snapshots in Figure 1
were chosen to represent area-age distributions
closest to box-shaped in order to make a com-
parison between the models. The proportion of
the seaﬂ oor is the same for any given age un-
til it eventually drops down for the maximum
age. Such a distribution is expected in a case
of steady-state convection without a continent:
cold instabilities start to sink once a critical age
is reached, corresponding to the maximum age,
and no younger subduction is expected. How-
ever, mantle convection is not steady-state and
continents impose a geometrical constraint on
surface tectonics. Hence, over the course of a
Seaﬂ oor spreading evolution in response to continental growth
N. Coltice1,2, T. Rolf3, and P.J. Tackley3
1Laboratoire de Géologie de Lyon, Université Lyon 1, Ecole Normale Supérieure de Lyon, Université de Lyon, 69007 Lyon, France
2Institut Universitaire de France, 103, Bd Saint Michel, 75005 Paris, France
3Institute of Geophysics, ETH Zurich, 8092 Zurich, Switzerland
ABSTRACT
The growth of the continental crust has shaped the evolution of the Earth from its interior
to its ﬂ uid envelopes. Continents have played a major role in the evolution of global tecton-
ics through their interaction with mantle convection. The feedback between continents and
mantle convection has been studied for the past 25 years, but it is only recently that the
dynamic inﬂ uence of continents on seaﬂ oor spreading can be explored thanks to progress
in convection modeling. In this work, we investigate how continental size impacts seaﬂ oor
spreading activity with state-of-the-art three-dimensional spherical convection models. We
show that increasing the continental area forces higher production rates of new seaﬂ oor
with stronger ﬂ uctuations. As a consequence, the average age of the seaﬂ oor decreases with
increasing continental area. This study suggests that mantle heat loss experienced signiﬁ -
cant ﬂ uctuations through continental growth and reinforces the estimate of <10% continen-
tal growth since the late Archean.
GEOLOGY, March 2014; v. 42; no. 3; p. 1–4
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doi:10.1130/G35062.1
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Published online XX Month 2013
© 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
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GEOLOGY
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280
Age of Oceanic Lithosphere [m.y.]
0 40 80 120 160 200 240 280
Age of seafloor (m.y.)
0
1
2
3
4
5
6
Area per unit age (km2yr–1)
0 40 80 120 160 200 240 280
Age of seafloor (m.y.)
0
1
2
3
4
5
6
Area per unit age (km2yr–1)
0 50 100 150 200 250 300
Age of seafloor (m.y.)
0
1
2
3
4
5
Area per unit age (km2yr–1)
0 40 80 120 160 200 240 280
Age of seafloor (m.y.)
0
1
2
3
4
5
6
Area per unit age (km2yr–1)
Figure 1. Snapshots of convection solutions with increasing continent size (in gray) increasing from top to
bottom (0%, 10%, 30%, and 70% of surface). Left column represents seaﬂ oor age maps, and right column
represents corresponding area-age distributions.
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calculation, the area-age distribution is mostly
linearly to exponentially decreasing once a con-
tinent is present in our calculations (Coltice et
al., 2012, 2013). The seaﬂ oor age maps, con-
cordantly with the area-age distributions, show
that younger seaﬂ oor ages dominate the system
when continental area is increased. The time
series of the seaﬂ oor production rate for the
50% continent case displays a long episode of
periodicity, during which the seaﬂ oor produc-
tion rate varies between the highest and lowest
values, that other models do not show. This pe-
culiar feature may be caused by the speciﬁ c con-
tinental conﬁ guration, representing a spherical
harmonic of degree one, which also corresponds
to the dominant wavelength of convection with
plate-like behavior and a large continent (Zhong
et al., 2007; Rolf et al., 2012).
As shown in Figure 1, the production rates of
new seaﬂ oor are similar in the different snap-
shots and lower than average. However, in most
cases this production rate is dependent on the
continental area as shown in Figure 2. Indeed, it
increases linearly with continental area, and the
standard deviation of the production rates also
increases proportionally (Fig. 3). The ﬂ uctua-
tions of seaﬂ oor spreading are hence enhanced
by increasing continental lithosphere and favor
linearly decreasing to exponentially decaying
area-age distributions.
As a consequence, the average age of ocean
basins in our models decreases linearly with
increasing continental area (Fig. 3). The aver-
age age obtained with 30% continental cover is
63 Ma, consistently close to that of the Earth at
62 Ma (Müller et al., 1997). In this case, the av-
erage seaﬂ oor age reaches a maximum of 80 Ma
and a minimum of 43 Ma. The case with 50%
continental cover differs from the others, proba-
bly because of the peculiar regularity of the ﬂ ow
as already described above, but we think it is an
outcome of this unique conﬁ guration.
DISCUSSION AND CONCLUSIONS
The models show that increasing the con-
tinental area results in higher production rate
of new seaﬂ oor. Continental lithosphere repre-
sents a thick conductive lid, hence the growth of
the continents reduces the area over which the
mantle can efﬁ ciently cool. As a consequence,
the oceanic heat ﬂ ow per unit of area and the
production rate of new seaﬂ oor are higher for a
smaller oceanic area for a given amount of heat
sources. The area-age distribution is thus in-
creasingly dominated by young seaﬂ oor ages as
continents grow, the average age of ocean basins
decreasing accordingly.
The calculations presented here conﬁ rm
spreading rate can vary by a factor of two as in
recent tectonic and convection models (see dis-
cussion and references in Coltice et al., 2013).
They also suggest that continental growth en-
hances the time dependence of seaﬂ oor spread-
ing. Moresi and Solomatov (1998) already men-
tioned a stronger time dependence of convection
when continental lithosphere is present. Time
dependence of the production rate has a large
impact on the consumption of seaﬂ oor (Becker
et al., 2009), hence the area-age distribution
changes notably with time from a box shape
with reduced spreading to a decay shape with
substantial spreading (Coltice et al., 2012).
Our models have limitations as discussed
earlier, but we are conﬁ dent our results will
hold with improved treatment of shear localiza-
tion in the future because predictions of these
models already converge toward plate tectonic
models (Coltice et al., 2013). Previous studies
show that the multiplicity of continents does
not affect the average seaﬂ oor production rate
by more than 10%, but reduces by at least 30%
the magnitude of the ﬂ uctuations (Coltice et
al., 2012). Future work on the roles of multiple
continents, layered viscosity, continental area
evolution, core heating, and the existence of
deep chemical heterogeneities will reﬁ ne this
rst attempt to characterize seaﬂ oor spreading
through continental growth.
The distribution of seaﬂ oor ages is funda-
mental for modeling heat ﬂ ow or continental
freeboard. When the production of new seaﬂ oor
is high and the area-age distribution features a
decay with increasing age, both heat ﬂ ow and
sea level are high. Our results corroborate the
work of Labrosse and Jaupart (2007), suggest-
ing that strong heat ﬂ ow uctuations and the
changing shape of area-age distributions have to
be taken into account when modeling the small
changes of heat loss through the Precambrian
(Herzberg et al., 2010).
Continental freeboard has been modeled as
resulting mainly from the competition between
two processes (Schubert and Reymer, 1985;
Galer, 1991; Flament et al., 2008): continental
growth reduces the size of oceanic basins and
hence promotes ﬂ ooding, in contrast to mantle
cooling which deepens ocean basins. The hy-
pothesis of a constant freeboard since the late
Archean (Eriksson, 1999; Miller et al., 2005)
has been used to constrain crustal growth to
a limited amount during this period, so as to
compensate for the deepening of ocean basins
subsequent to mantle cooling (Schubert and
Reymer, 1985). Our results show that the ef-
fect of continental growth on seaﬂ oor spreading
further prevents ocean basin deepening because
younger seaﬂ oor is favored when continental
area increases. For a present-day continental
area, switching from a box-shape area-age dis-
tribution to a linearly decreasing area-age distri-
bution corresponds to ~400 m of sea-level rise
(Labrosse and Jaupart, 2007), which is equivalent
to a growth of continents from 70% to 100% of
their present-day volume (Galer, 1991). Hence,
our study strongly reinforces the estimate of
<10% continental growth since the late Archean
(Schubert and Reymer, 1985). Progressive emer-
gence of continental landmasses throughout the
Archean (Eriksson et al., 2006; Flament et al.,
2008; Pons et al., 2011; Arndt and Nisbet, 2012)
suggests that continental growth occurred early
within the Earth’s history.
ACKNOWLEDGMENTS
We thank Carolina Lithgow-Bertelloni and two
anonymous reviewers for their fruitful comments,
as well as the journal editor James Spotila, for con-
structive advice that improved the manuscript. This
research has received funding from Institut Univer-
sitaire de France and Crystal2Plate, a FP-7 funded
Marie Curie Action under grant agreement number
PITN-GA-2008-215353. Calculations were per-
formed on MONTE ROSA, the high-performance
computing cluster of the Swiss National Supercom-
puting Centre (CSCS), under project ID s272.
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No continent
10% continent
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70% continent
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Revised manuscript received 26 November 2013
Manuscript accepted 3 December 2013
Printed in USA
... Indeed, the ocean basin volume based on reconstructed seafloor age distributions since 140 Ma compares favorably with the observed long-term trend of global sea level over the same time [M€ uller et al., 2008]. Additionally, the amount of Earth's surface covered by continents also drives significant variation of seafloor age distribution [Coltice et al., 2014] but the corresponding effect on global MOR depth has yet to be investigated. This study uses mantle convection models to address the question of how the depth of the MOR may have changed over geologic time. ...
... Pseudoplasticity is implemented through a stress dependence of the viscosity with a yield stress [Coltice et al., 2012[Coltice et al., , 2014. We choose a nondimensional value of 15,000 to produce a plate-like behavior and largescale convective flow. ...
... From the solutions at statistical steady state, we compute the dynamic evolutions for integration times over 4 Gyr (scaling time with present-day transit time) of the models that are analyzed in this study. The age of the seafloor is computed from the value of the heat flow [Coltice et al., 2012[Coltice et al., , 2014. Since small-scale convection is limited in these models, this approximation is effective. ...
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... A second important difference between our model and Earth's continental crust is that our heat-blanket covers the whole system, while Earth's continents cover only 30%-40% of Earth's surface (Taylor & McLennan, 1995). The effects of continent coverage on the convective system have been investigated with both numerical simulations (Coltice et al., 2014;Cooper et al., 2006;Jellinek & Lenardic, 2009;Lenardic et al., 2005;Whitehead & Behn, 2015) and laboratory experiments (Guillou & Jaupart, 1995;Jellinek & Lenardic, 2009;Lenardic et al., 2005). A major finding is the existence of two different regimes depending on the continent coverage . ...
... When applied to Earth, one may expect a very moderate effect of continent coverage on the surface heat flux, as Earth should remain throughout its evolution in the high heat flux regime. Nevertheless, partial coverage of continents should still impact mantle dynamics by inducing a large scale motion in the system (Guillou & Jaupart, 1995) and by increasing the production rate of sea floor (Coltice et al., 2014). Moreover, continental drift may affect the convective motions in the mantle by focusing cold downwellings on the edge of continents and disrupting the convective cells (Whitehead & Behn, 2015). ...
Article
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Earth's continental crust is characterized by a strong enrichment in long‐lived radioactive isotopes. Recent estimates suggest that the continental crust contributes to 33% of the heat released at the surface of the Earth, while occupying less than 1% of the mantle. This distinctive feature has profound implications for the underlying mantle by impacting its thermal structure and heat transfer. However, the effects of a continental crust enriched in heat‐producing elements on the underlying mantle have not yet been systematically investigated. Here, we conduct a preliminary investigation by considering a simplified convective system consisting in a mixed heated fluid where all the internal heating is concentrated in a top layer of thickness dHL (referred to as “heat‐blanketed convection”). We perform 24 numerical simulations in three dimensional Cartesian geometry for four specific set‐ups and various values of dHL. Our results suggest that the effects of the heated layer strongly depend on its thickness relative to the thickness of the thermal boundary layer (δTBL) in the homogeneous heating case (dHL = 1.0). More specifically, for dHL > δTBL, the effects induced by the heated layer are quite modest, while, for dHL < δTBL, the properties of the convective system are strongly altered as dHL decreases. In particular, the surface heat flux and convective vigor are significantly enhanced for very thin heated layers compared to the case dHL = 1.0. The vertical distribution of heat producing elements may therefore play a key role in mantle dynamics. For Earth, the presence of continents should however not affect significantly the surface heat flux, and thus the Earth's cooling rate.
... When applied to Earth, one may expect a very moderate effect of continent coverage on the surface heat flux, as Earth should remain throughout its evolution in the high heat flux regime. Nevertheless, partial coverage of continents should still impact mantle dynamics by inducing a large scale motion in the system (Guillou & Jaupart, 1995) and by increasing the production rate of sea floor (Coltice et al., 2014). Moreover, continental drift may affect the convective motions in the mantle by focusing cold downwellings on the edge of continents and disrupting the convective cells (Whitehead & Behn, 2015). ...
... face(Taylor & McLennan, 1995). The effects of continent coverage on the convective system have been investigated with both numerical simulations(Lenardic et al., 2005;Cooper et al., 2006; Jellinek & Lenardic, 2009;Coltice et al., 2014;Whitehead & Behn, 2015) and laboratory experiments(Guillou & Jaupart, 1995; Lenardic et al., 2005; Jellinek & Lenardic, 2009). ...
Preprint
Earth's continents are characterized by a strong enrichment in long-lived radioactive isotopes. Recent estimates suggest that they contribute to 33\% of the heat released at the surface of the Earth, while occupying less than 1\% of the mantle. This distinctive feature has profound implications for the underlying mantle by impacting its thermal structure and heat transfer. However, the effects of a continental crust enriched in heat-producing elements on the underlying mantle have not yet been systematically investigated. Here, we conduct a preliminary investigation by considering a simplified convective system consisting in a mixed heated fluid where all the internal heating is concentrated in a top layer of thickness $d_{HL}$ (referred to as "heat-blanketed convection"). We perform 24 numerical simulations in 3D Cartesian geometry for four specific set-ups and various values of $d_{HL}$. Our results suggest that the effects of the heated layer strongly depend on its thickness relative to the thickness of the thermal boundary layer ($\delta_{TBL}$) in the homogeneous heating case ($d_{HL} = 1.0$). More specifically, for $d_{HL} > \delta_{TBL}$, the effects induced by the heated layer are quite modest, while, for $d_{HL} < \delta_{TBL}$, the properties of the convective system are strongly altered as $d_{HL}$ decreases. In particular, the surface heat flux and convective vigour are significantly enhanced for very thin heated layers compared to the case $d_{HL} = 1.0$. The vertical distribution of heat producing elements may therefore play a key role on mantle dynamics. For Earth, the presence of continents should however not affect significantly the surface heat flux, and thus the Earth's cooling rate.
... Coltice et al. (2013) also show that these models display a significant time-dependence of the average spreading rate, which alter the age vs. area distribution of ocean basins. Increasing continental area enhances spreading rate and associated fluctuations ( Coltice et al., 2014). Plate reorganisations produce spreading rate changes that can reach a factor of 2 ( Coltice et al., 2013), along with variations of the length of mid ocean ridges, and changes in the shape of the area vs. seafloor age distribution ( Fig. 12). ...
Article
The concept of interplay between mantle convection and tectonics goes back to about a century ago, with the proposal that convection currents in the Earth’s mantle drive continental drift and deformation (Holmes, 1931). Since this time, plate tectonics theory has established itself as the fundamental framework to study surface deformation, with the remarkable ability to encompass geological and geophysical observations. Mantle convection modeling has progressed to the point that connections with plate tectonics can be made, pushing the idea that tectonics is a surface expression of the global dynamics of one single system: the mantle-lithosphere system. Here, we present our perspective, as modelers, on the dynamics behind global tectonics with a focus on the importance of self-organisation. We first present an overview of the links between mantle convection and tectonics at the present-day, examining observations such as kinematics, stress and deformation. Despite the numerous achievements of geodynamic studies, this section sheds light on the lack of self-organisation of the models used, which precludes investigations on feedbacks and evolution of the mantle-lithosphere system. Therefore, we review the modeling strategies, often focused on rheology, that aim at taking into account self-organisation. The fundamental objective is that plate-like behaviour emerges self-consistently in convection models. We then proceed with the presentation of studies of continental drift, seafloor spreading and plate tectonics in convection models allowing for feedbacks between surface tectonics and mantle dynamics. We discuss the approximation of the rheology of the lithosphere used in these models (pseudo-plastic rheology), for which empirical parameters differ from those obtained in experiments. In this section, we analyse in detail a state-of-the-art 3D spherical convection calculation, which exhibits fundamental tectonic features (continental drift, one-sided subduction, trench and ridge evolution, transform shear zones, small-scale convection, and plume tectonics). This example leads to a discussion where we try to answer the question: can mantle convection models transcend the limitations of plate tectonics theory?
... As younger age of the oceanic crust is an important control on the eustatic rise (e.g., Mü ller et al., 2008;Anderson and Anderson, 2010), Precambrian global sea level might have reached (at least, episodically) a high position relative to the Phanerozoic and, in particular, the Paleogene, which features much larger plates (Seton et al., 2012). Such an interpretation is in agreement (at least, in part) with the outcome of modeling undertaken recently by Coltice et al. (2014). The above-mentioned and other peculiarities of Precambrian tectonics (including, first of all, the activity of mantle plumes (Gargaud et al., 2012;Gerya, 2014)) likely influenced global sea-level change on a large scale; possible changes in crust production may also induce a cyclic pattern of these changes (Eriksson et al., 2004(Eriksson et al., , 2005. ...
Article
Global sea level has changed cyclically throughout Earth's history due to a variety of mechanisms that operate on a variety of timescales. Here we attempt to place constraints on the "actual" number of sea-level cycles that can be interpreted directly from the eustatic reconstructions. We apply an interpretative algorithm to Paleogene sea level records and identify three orders of eustatic cycles longer than 1. Ma. However, the three-ordered cyclicity might not represent cycles of global eustatic change. First, only cycles of the highest of the established orders (with a timescale of 10. s of Ma) are coherent among different sea-level records. Second, the interpreted cycles are not supported by the compilation of the regional maximum flooding surfaces. Third, climatic history alone cannot explain the eustatic changes. Fourth, the interpreted cyclicity differs significantly from what is known about the tectonic control of eustasy. Fifth, there may be other orders higher than those established. The problem is rooted in (1) the fact that eustatic curves might not necessarily reflect global events (e.g., fluctuations shown on these curves may be artifacts related to regional tectonic activity) and (2) the possible weakness of Paleogene (especially Eocene) eustatic cyclicity and its significant "overprint" by regional tectonic activity. Our attempted analysis claims for significant improvement of the available eustatic reconstructions. Unfortunately, the regional stratigraphical data remain still insufficient to develop any alternative eustatic curve that can be further interpreted to understand the number of "actual" cycle orders.
... Modern numerical studies of mantle convection have addressed many of the unexplored complexities from the earlier studies, including: nonlinear temperature-dependent rheology (Torrance and Turcotte, 1971;Parmentier et al., 1976;Richter et al., 1983;Solomatov, 1995); compressibility (Jarvis and Mc Kenzie, 1980;Leng and Zhong, 2008;King et al., 2010); three-dimensional geometry (cf. Gable et al., 1991;Tackley et al., 1993;Lowman et al., 2001Lowman et al., , 2003Lowman et al., , 2004; self-consistent equations of state King, 1994, 1998;Nakagawa et al., 2009); spherical geometry (Schubert and Zebib, 1980;Hager and O'Connell, 1981;Bercovici et al., 1989;Tackley et al., 1993;Bunge et al., 1997;Wen and Anderson, 1997;Zhong et al., 2000); and the role of plates and slabs (Gurnis and Hager, 1988;Gur-nis and Zhong, 1991;Zhong and Gurnis, 1992;King and Hager, 1994;Bercovici, 1995;King and Ita, 1995;Christensen, 1996Christensen, , 2001Chen and King, 1998;Trompert and Hansen, 1998;Tackley, 2000;Bercovici, 2003;Billen and Gurnis, 2003;Billen and Hirth, 2007;van Heck andTackley, 2008, 2011;Billen, 2008Billen, , 2010Lee and King, 2011;Coltice et al., 2013Coltice et al., , 2014. ...
Chapter
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Calculations of mantle convection generally use constant rates of internal heating and time-invariant core-mantle boundary temperature. In contrast, parameterized convection calculations, sometimes called thermal history calculations, allow these properties to vary with time but only provide a single average temperature for the entire mantle. Here I consider three-dimensional spherical convection calculations that run for the age of the Earth with heat-producing elements that decrease with time, a cooling core boundary condition, and a mobile lid. The calculations begin with a moderately hot initial temperature, consistent with a relatively short accretion time for the formation of the planet. I fi nd that the choice of a mobile or stagnant lid has the most signifi cant effect on the average temperature as a function of time in the models. However, the choice of mobile versus stagnant lid has less of an effect on the distribution of hot and cold anomalies within the mantle, or planform. I fi nd the same low-degree (one upwelling or two upwelling) temperature structures in the mobile-lid calculations that have previously been found in stagnant-lid calculations. While having less of an effect on the mean mantle temperature, the viscosity of the asthenosphere has a profound effect on the pattern of temperature anomalies, even in the deep mantle. If the asthenosphere is weaker than the upper mantle by more than an order of magnitude, then the low-degree (one or two giant upwellings) pattern of temperature anomalies results. If the asthenosphere is less than an order of magnitude weaker than the upper mantle, then the pattern of temperature anomalies has narrow cylindrical upwellings and cold downgoing sheets. The low-degree pattern of temperature anomalies is more consistent with the plate model than the plume model.
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The basic quantitative distinction between global oceanic ridge volume and the global rate of seafloor generation is made fully explicit. From this, the question of inversion over time from the former quantity into the latter is then posed using a generalized expression to approximate global subduction zone distribution. Two numerical methods are described. Then, assuming the hypothesis that long-term (108 yr) eustatic sealevel change is due primarily to changing ridge volume, an inversion of a widely cited Phanerozoic sealevel curve (Vail) is also presented. The approach taken here is expected to be of direct importance for quantitative models of the carbonate-silicate cycle which seek to develop scenarios for atmospheric carbon dioxide variation over geologic time scales. Indeed, the testing of sealevel inversion, as performed here, may ultimately come from its degree of correspondence with past climate variation.-Author
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A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
Article
I review geologic evidence for Archean plate tectonic processes within the framework of geophysical, geochemical and experimental observations. For the Late Archean (2.5–3.0 Ga), the evidence is primarily based on the rich database of the Superior craton; for the Early Archean (3.0–4.0 Ga), data from the Pilbara and Kaapvaal cratons are examined. Data from other old cratons are used as supplementary evidence. The verdict is that there is a robust consensus on the validity of using plate tectonic boundary processes to decipher the Late Archean rock record; and it also confirms that such processes dominated the early Archean.
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Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present-day plate motions. A fuller understanding, however, can be derived from consideration of the history of plate motions. Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate-driving forces. We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions. The present-day field is not equipartitioned in poloidal and toroidal components; toroidal motions account for only one third of the total. The toroidal/poloidal ratio has changed substantially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone. We develop a self-consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) calculating the induced plate motions for each stage of the Cenozoic. The “slab” heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core-mantle boundary. The slab model predicts the observed geoid extremely well, although comparison between predicted and observed dynamic topography is ambiguous. The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present. The plate motion model itself successfully predicts Cenozoic plate motions (global correlations of 0.7–0.9) for mantle viscosity structures that are consistent with a variety of geophysical studies. We conclude that the main plate-driving forces come from subducted slabs (>90%), with forces due to lithospheric effects (e.g., oceanic plate thickening) providing a very minor component (<10%). For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs. Unfortunately, we cannot reproduce the toroidal/poloidal partitioning ratios observed for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries. The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawaiian-Emperor seamount chain. Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian-Emperor bend. However, we find that this collision has no effect on the motion of the Pacific plate, implying that important plate boundary effects are missing in our models. Future progress in understanding global plate motions requires (1) more complete plate reconstruction information, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc mantle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity constraints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change.
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For 50 years of data collection and kinematic reconstruction efforts, plate models have provided alternative scenarios for plate motions and seafloor spreading for the past 200 My. However, these efforts are naturally limited by the incomplete preservation of very old seafloor, and therefore the time- dependence of the production of new seafloor is controversial. There is no consensus on how much it has varied in the past 200 My, and how it could have fluctuated over longer timescales. We explore how seafloor spreading and continental drift evolve over long geological periods using independently derived models: a recently developed geodynamic modelling approach and state-of-the-art plate reconstructions. Both kinematic reconstructions and geodynamic models converge on variations by a factor of 2 in the rate of production of new seafloor over a Wilson cycle, with concomitant changes of the shape of the area– age distribution of the seafloor between end members of rectangular, triangular and skewed distributions. Convection models show that significant fluctuations over longer periods (∼1 Gy) should exist, involving changes in ridge length and global tectonic reorganisations. Although independent, both convection models and kinematic reconstructions suggest that changes in ridge length are at least as significant as spreading rate fluctuations in driving changes in the seafloor area–age distribution through time.
Article
Continents slowly drift at the top of the mantle, sometimes colliding, splitting and aggregating. The evolutions of the continent configuration, as well as oceanic plate tectonics, are surface expressions of mantle convection and closely linked to the thermal state of the mantle; however, quantitative studies are so far lacking. In the present study we use 3D spherical numerical simulations with self-consistently generated plates and compositionally and rheologically distinct continents floating at the top of the mantle in order to investigate the feedbacks between continental drift, oceanic plate tectonics and the thermal state of the Earth's mantle, by using different continent configurations ranging from one supercontinent to six small continents. With the presence of a supercontinent we find a strong time-dependence of the oceanic surface heat flow and suboceanic mantle temperature, driven by the generation of new plate boundaries. Very large oceanic plates correlate with periods of hot suboceanic mantle, while the mantle below smaller oceanic plates tends to be colder. Temperature fluctuations of subcontinental mantle are significantly smaller than in oceanic regions and are caused by a time-variable efficiency of thermal insulation of the continental convection cell. With the presence of multiple continents the temperature below individual continents is generally lower than below supercontinent and is more time-dependent, with fluctuations as large as 15% that are caused by continental assembly and dispersal. The periods featuring a hot subcontinental mantle correlate with strong clustering of the continents and periods characterized by cold subcontinental mantle, at which it can even be colder than suboceanic mantle, with a more dispersed continent configuration. Our findings with multiple continents imply that periods of partial melting and strong magmatic activity inside the continents, which may contribute to continental rifting and pronounced growth of continental crust, might be episodic processes related to the supercontinent cycle. Finally, we observe an influence of continents on the wavelength of convection: for a given strength of the lithosphere we observe longer-wavelength flow components, when continents are present. This observation is regardless of the number of continents, but most pronounced for a single supercontinent.
Article
Interpretation of available isotopic ages and of published geologic maps of igneous rocks in eastern Australia indicates a north-south time transgressive pattern of cessation of igneous activity along a curvilinear trace, beginning about 35 Ma at 20 °S and extending to the present, near 37 °S. However, volcanic activity began 70-60 Ma over the length of the Highlands and persisted through the early Cenozoic until progressive cessation began. The latitudinal rate of termination is compatible with that predicted by plate reconstructions relative to the Hawaiian 'hotspot', but the trend of the trace is incompatible with that predicted unless the Hawaiian-Emperor bend is somewhat greater than 50 m.y. in age relative to the LaBrecque et al. [1977] time scale. The volcanic pattern appears to be incompatible with a hotspot or plume hypothesis. Paleostress and contemporary stress indicators as well as the volcanic pattern support a model in which the trace forms due to intraplate extension normal to the trend of the trace, resulting in pressure release and melting at the base of the lithosphere, beginning in the late Cretaceous. This stress field persisted until 35 Ma when progressive reorientation of the stress field occurred. Cessation of igneous activity reflected onset of compression normal to the trend of the trace. As a consequence, a migrating stress node is recorded in the progressive pattern of extinction of volcanic activity along the trace. Evidence from other 'hotspot' traces, suggests that they too represent intraplate stress controls. It is not necessary to postulate deep sources (mantle plumes) for the origin of hotspot traces; intraplate stress due to drag, plate margin forces, thermal contraction, and nonsphericity of the earth can also reflect 'absolute' motion.