Letter
https://doi.org/10.1038/s41586-019-1462-2Metamorphism and the evolution of plate tectonics
robert M. Holder1,2*, Daniel r. Viete^1 , Michael Brown^3 & tim e. Johnson4,5
Earth’s mantle convection, which facilitates planetary heat loss,
is manifested at the surface as present-day plate tectonics^1. When
plate tectonics emerged and how it has evolved through time are
two of the most fundamental and challenging questions in Earth
science^1 –^4. Metamorphic rocks—rocks that have experienced solid-
state mineral transformations due to changes in pressure (P) and
temperature (T)—record periods of burial, heating, exhumation
and cooling that reflect the tectonic environments in which they
formed^5 ,^6. Changes in the global distribution of metamorphic
(P, T) conditions in the continental crust through time might
therefore reflect the secular evolution of Earth’s tectonic processes.
On modern Earth, convergent plate margins are characterized by
metamorphic rocks that show a bimodal distribution of apparent
thermal gradients (temperature change with depth; parameterized
here as metamorphic T/P) in the form of paired metamorphic belts^5 ,
which is attributed to metamorphism near (low T/P) and away from
(high T/P) subduction zones^5 ,^6. Here we show that Earth’s modern
plate tectonic regime has developed gradually with secular cooling
of the mantle since the Neoarchaean era, 2.5 billion years ago. We
evaluate the emergence of bimodal metamorphism (as a proxy for
secular change in plate tectonics) using a statistical evaluation of
the distributions of metamorphic T/P through time. We find that
the distribution of metamorphic T/P has gradually become wider
and more distinctly bimodal from the Neoarchaean era to the
present day, and the average metamorphic T/P has decreased since
the Palaeoproterozoic era. Our results contrast with studies that
inferred an abrupt transition in tectonic style in the Neoproterozoic
era (about 0.7 billion years ago^1 ,^7 ,^8 ) or that suggested that modern
plate tectonics has operated since the Palaeoproterozoic era (about
two billion years ago^9 –^12 ) at the latest.
The theory of plate tectonics can explain the assembly and break-up
of supercontinents, how mountain ranges and major mineral deposits
form^13 , and perhaps even why there is life on Earth^14 ,^15. However, it
is unclear how plate tectonics emerged and why it does not occur on
other planets in the Solar System. Although there is broad agreement
that plate tectonics has been dominant during the last billion years of
our planet’s history, how and when it emerged, and how it has evolved
through time, are disputed^2 ,^3 ,^6 ,^7. A diagnostic feature of plate tectonics
on modern Earth is the bimodal distribution of metamorphic tempera-
tures and pressures (Fig. 1 ), which is expressed in paired metamorphic
belts^5 ,^6 and is key to the identification of the past operation of plate
tectonics from the rock record. Here we present a statistical evaluation
of metamorphic T/P through Earth’s history, with the purpose of doc-
umenting the emergence and evolution of the bimodal distribution of
metamorphic T/P as a proxy for the emergence and evolution of Earth’s
plate tectonic regime. We show that the modern bimodal distribution
of metamorphic T/P, and therefore modern plate tectonics, developed
gradually since the end of the Neoarchaean era, about 2.5 Gyr ago. We
hypothesize that the development of modern plate tectonics is linked to
secular cooling of the mantle and associated changes in the thickness,
buoyancy and rheology of oceanic lithosphere, resulting in evolution
in the styles of both subduction and collisional orogenesis.
We assess changes in the global distribution of metamorphic T/P
using a database of age and (P, T) conditions of metamorphic rocks
from 564 localities from ref.^4. The data define distinct modes centred
at about 2.6, 1.8, 1.0, 0.5 and <0.2 Gyr ago^4 (Extended Data Fig. 1).(^1) Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA. (^2) Department of Earth and Environmental Sciences, University of Michigan, Ann
Arbor, MI, USA.^3 Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD, USA.^4 School of Earth and Planetary Sciences, The Institute for Geoscience
Research (TIGeR), Space Science and Technology Centre, Curtin University, Perth, Western Australia, Australia.^5 State Key Laboratory of Geological Processes and Mineral Resources, China
University of Geosciences, Wuhan, China. *e-mail: [email protected]
log[T/P (°C GPa–1)]
KDE
500 °C
GPa
–1
500 °C GPa
–1
KDE
Metamorphic rock
compilation
150 °C GPa
–1
1,500 °C
GPa–1
T (°C)
P (GPa)
Frequency
a
b
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
0
5
10
15
20
25
30
200 300 400 500 600 700 800 900 1,000 1,100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Fig. 1 | Metamorphism in the last 0.2 Gyr is characterized by a bimodal
distribution of apparent metamorphic thermal gradients, T/P. a, Kernel
density estimates (KDE) of metamorphic (P, T) conditions in rocks
younger than 0.2 Gyr (ref.^4 ). The red line (500 °C GPa−^1 ) represents the
dividing line between the two modes shown in b. b, Histogram and KDE
of metamorphic T/P, plotted logarithmically to illustrate more clearly
the bimodal distribution shown in a. This bimodal distribution of T/P
is manifest in paired metamorphic belts and is considered a diagnostic
feature of plate tectonics.
378 | NAtUre | VOL 572 | 15 AUGUSt 2019