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Australia: The Land Where Time Began |
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Oceanic Crustal Thickness Decrease After Breakup of Pangaea
Since the Archaean 2.5 Ga
the mantle of the Earth has cooled by 6-11oC every 100 My.
The loss of surface heat that led to this drop in temperature may have
been enhanced by the processes of
plate tectonics,
such as the breakup of continents, the continuous formation of oceanic
lithosphere at the mid-ocean ridges and subduction at deep-sea trenches.
In this study Van Avendonk et al.
used a compilation of marine seismic refraction data gathered from ocean
basins globally to analyse thickness changes of ocean crust over time.
They found that oceanic crust that formed about 170 Ma in the Middle
Jurassic is on
average 1.7 km thicker than crust that is being produced at the present
at mid-ocean ridge systems. If the thicker
oceanic crust
from the Jurassic is caused by higher mantle temperature, the upper
mantle may have cooled by 15-20oC per 100 My over this period
of time. It is suggested by the difference between this and the cooling
rate over the long term that modern plate tectonics coincide with grater
heat loss from the mantle. Van Avendonk et
al. also found that the
oceanic crustal thickness increase with the age of a plate is stronger
in the Atlantic Ocean
and the Indian Ocean
compared with the
Pacific Ocean. The idea that the temperature of the upper mantle in
the Jurassic was higher in the wake of the fragmentation of the
supercontinent
Pangaea as an effect of
continental insulation is supported by this observation.
It is suggested by the geochemistry of ancient
basalts that the long term
cooling rate of the mantle is no more than about11oC every
100 My (Herzberg, Condie & Korenaga, 2010); Condie, Aster & van Hunen,
2016). The role of the continents is a factor in the evolution of the
loss of heat of the Earth over time. The mantle beneath large continents
can be insulated for some time, which would lead to increased mantle
temperatures (Gurnis, 1988; Whittaker et al., 2008). If there is
evidence that formation and dispersal of Pangaea had an effect on the
mantle beneath it more insight would be gained into the importance of
these plate tectonic events. Van Avendonk et
al. used measurements of
oceanic crustal thickness as a proxy for the temperature of the mantle
to test these ideas because the thicker oceanic crust is thought to form
when temperature of the mantle is higher. In particular, about 60 % of
the crust of the Earth has formed by decompression melting of mantle
upwelling that is relatively hot beneath the mid-ocean ridges (Müller,
Sdrolias, Gaina & Roest, 2008). It has been shown by numerical models of
seafloor spreading (Bown & White, 1994) there will be a decrease in the
amount of melting of the mantle when the full rate of spreading is less
than 20 mm per year, and the a potential temperature of the mantle will
lead to a higher degree of partial melting. Variations in the thickness
of young oceanic crust are constrained by marine refraction data to
between 3 km and 8 km, which is consistent with the numerical models
(White et al., 2001). Some evidence for a small increase in the
thickness of the oceanic crust with age is provided by seismic data
(McClain & Atallah, 1986), though historically there have been few
measurements of older oceanic crust. Seismic refraction studies of
subduction zones
and the deep seafloor adjacent to passive margins have provided better
global coverage with estimates of the thickness of oceanic crust of
various ages. Insight has been provided by these additional data into
the processes of seafloor spreading in the past, assuming there has not
been significant overprinting by later geologic events.
Van Avendonk et al. have
shown the geographical distribution of thickness measurements of oceanic
crust based on seismic refraction data gathered over the last 40 years.
The relationship between age and thickness of oceanic crust in the
Pacific, Atlantic and Indian oceans can be evaluated because of these
seismic constraints on seafloor dating from170 Ma to recent (0.3 Ma).
All the measurements were made on seafloor that formed at a full
spreading rate of 30 mm per year or more, as crust that is generated at
slower rates can be anomalously thin
(White et al., 2001). The list of 234 estimates of crustal thickness and
their literature sources are listed in the supplementary Table 1. A
large amount of scatter with a small positive correlation is shown by
the distribution of seismic crustal thickness data as a function of age.
Oceanic crust has been shown by a least squares regression to have
decreased over time by 10 ± 1 m per My on average, which totals 23 %
since 170 Ma. The linear least-squares fit of the crustal thickness
measurements has an x2 statistic of 1.9, where a data fit
that is comparable to the uncertainties would be x2=1.0. That
there are also spatial variations in the thickness of the oceanic crust
at any given age, which are not modelled in this study, is shown by the
additional scatter in the data. It is indicated by the linear regression
that has a coefficient of determination
R2 of 0.28 that
its variance is clearly smaller than that of the crustal thickness that
is best fitting, though this improvement is modest.
Implications for global tectonics
Deep mantle overturning and cooling may have been enhanced by the
partial insulation of the mantle of the Earth by large continents
(Lenardic, Moresi, Jellinek & Manga, 2005), leading to a large decrease
in the temperature of the mantle over the last several hundred million
years. Asthenospheric mantle that had been thermally insulated beneath
Pangaea (Gurnis, 1988) was emplaced in the young Atlantic and
Indian Ocean, as Pangaea was breaking up in the early Jurassic. Thicker
oceanic crust than that formed at the mid-ocean ridges bordering the
Pacific Plate was initially generated by the hot upper mantle. Mantle
overturn is slow beneath the Atlantic and Indian oceans compared with
that beneath the Pacific Ocean where it is more vigorous, because the
lithosphere beneath the Atlantic and Indian oceans is fixed to tectonic
plates that have deep continental keels. A result of this is that the
thermal state of these 2 areas may have evolved differently over time.
It appears that young oceanic crust, less than 1 My old, is on average
1.3 ± 0.2 km thicker in the
Pacific Ocean than in the Atlantic and Indian oceans, even though
measurements of crust that formed at spreading rates of less than 20 mm
per year were excluded from this study. Van Avendonk et
al. suggest that this
discrepancy is explained best by differences in average upper mantle
temperature that are basin wide.
Van Avendonk et al. inferred
a decrease rate of oceanic crustal thickening over time in the Pacific
Ocean of 8 ± 1 m per My is
slightly larger than an earlier estimate of 7 m per My that had been
based on fewer seismic refraction data (McClain & Atallah, 1986). The
oldest west Pacific Ocean oceanic crust dating to the mid-Jurassic (170
Ma) is on average 1.4 km thicker than the crust that is newly produced
at the East Pacific Rise. 170 Ma the mantle would possibly have been
between 18oC and 30oC hotter in the Jurassic,
which is a temperature anomaly that is smaller than the 50-60oC
that has been predicted from the geochemistry of the oldest Pacific
basalts (Humler, Langmuir & Daux, 1999; Fisk & Kelly, 2002). Van
Avendonk et al. suggest
regional anomalies in the temperature of the mantle beneath the
mid-ocean ridge system (Gale, Langmuir & Dalton, 2014) may account for
this mismatch, as there are not many sites from the Ocean Drilling
Program on old seafloor where it would be possible to make such a
comparison. Enough buoyancy is provided by the estimated thickening of
the Pacific oceanic crust with age to explain roughly 300 m of the
flattening that has been observed of seafloor depth as a function of age
(Parsons & Sclater, 1977), an amount that is comparable to the depth of
the seafloor misfit of current plate cooling models for the Pacific
Ocean (Hillier, 2010). Long-term changes in sea level may have been
influenced by the difference in isostasy of the oceanic plate over time
(Miller et al., 2005).
The mantle of the incipient Atlantic Ocean offshore from the US has
recently been reported to have been 150oC higher than beneath
modern mid-ocean
ridges (Brandt, Regelous, Beier & Haase, 2013) which would be
consistent with the crust being much thicker than what has been observed
in the seismic data. In the western central Atlantic Ocean the seafloor
dating to the Jurassic is locally smooth (Whittaker et al., 2008;
Minshull, 1999), which Van Avendonk et
al. suggest lends proof that
at the time of its formation the mantle beneath it was unusually hot 30
My after the continents of North America and Africa separated. A seismic
refraction study has shown that in this area the oceanic crust is
locally 8 km thick (Minshull et al., 1991), which is more easily
explained by a past mantle potential temperature approximately 50oC
higher than that of the modern Mid-Atlantic Ridge system. There is a
large amount of scatter arising from lateral heterogeneity in the
temperature and composition of the mantle that can be seen in the
long-term signal of cooling of the mantle in samples of oceanic basalt
or data from marine seismic refraction data (Gale, Langmuir & Dalton,
2014; Niu & O’Hara, 2008).
It has been debated whether there had been an arrival of a deep mantle
plume offshore from the southeast US about 200 Ma that accompanied the
rifting and breakup of Pangaea (Wilson, 1997; McHone, 2000; Janney &
Castillo, 2001). During early seafloor spreading at that location the
elevated mantle temperature may be consistent with a plume, though the
strong positive correlation between the thickness of the oceanic crust
and the age of the tectonic plate applies more generally to the Atlantic
and Indian oceans. Prior to breakup of the supercontinent the
continental insulation of the mantle beneath central Pangaea warmed the
asthenosphere (Humler & Besse, 2002), which contributed to the extensive
syn-rift magmatism between the eastern US and Africa (Holbrook& Kelemen,
1993), leading to the formation of Jurassic oceanic crust that is on
average thicker than the crust that is generated along the Mid-Atlantic
Ridge of the present.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |