Letter reSeArCH
important secular change in metamorphism relates to a large increase
in the maximum pressure of metamorphism through the Proterozoic
eon (2.5–0.55 Gyr ago), resulting in the widespread occurrence of con-
tinental UHP metamorphism beginning in the Neoproterozoic era,
about 0.7 Gyr ago^4. The T/P values of UHP rocks are consistent with
the gradual change proposed here (Figs. 2 , 3 ), but the reason for their
higher maximum pressures requires additional comment. Although
many mechanisms have been invoked to explain the formation and
exhumation of continental UHP rocks^27 , their observation at Earth’s
surface in general requires that: (1) positively buoyant material be
carried to depth; and (2) the downward force acting on the material
subsequently ceases (for example, owing to physical separation from
the denser material driving subduction or foundering), allowing the
material to return towards Earth’s surface. The geodynamic controls
on the formation of UHP rocks have been addressed through numer-
ical experiments by ref.^28 , which hypothesized that the formation
and exhumation of UHP rocks might have been precluded by shal-
lower slab breakoff associated with a hotter mantle (and rheologically
weaker plates) earlier in Earth’s history. Although further simulations
are needed to understand slab breakoff during subduction, these results
suggest that the increase in maximum metamorphic pressure, the emer-
gence of a distinct low-T/P mode of metamorphism (Fig. 2 ) and the
overall decrease in metamorphic T/P since the end of the Neoarchaean
era, 2.5 Gyr ago (Fig. 2 ), can be all linked to gradual cooling of the
upper mantle (Fig. 3 ).
In summary, we present a statistical evaluation of metamorphic T/P
through Earth’s history, with the purpose of documenting the gradual
emergence of the modern bimodal distribution of metamorphic T/P
as a proxy for the emergence of Earth’s modern plate tectonic regime.
This approach is rooted in the classical concept of paired metamor-
phic belts^5 as a diagnostic feature of plate tectonics, while leaving open
the possibility that, owing to evolution in the style of plate tectonics^29 ,
paired metamorphism in Earth’s past might have been characterized
by different apparent thermal gradients than today. We show that the
modern bimodal distribution of metamorphic T/P developed gradu-
ally since the end of the Neoarchaean era, about 2.5 Gyr ago, and that
globally bimodal metamorphism emerged before either blueschist or
UHP metamorphism in the geological record. We hypothesize that
both the development of bimodal metamorphism and the appearance
of blueschist and UHP metamorphism are linked to secular cooling
of the mantle and to associated changes in the thickness, buoyancy
and rheology of oceanic lithosphere, resulting in evolution in the
styles of both subduction and collisional orogenesis. Importantly, this
and other hypotheses concerning changes in tectonic style (or lack
thereof)^1 ,^4 ,^6 ,^7 ,^10 –^12 ,^19 through time are testable through further numer-
ical modelling and continued examination of the Precambrian meta-
morphic rock record.
Online content
Any methods, additional references, Nature Research reporting summaries,
source data, extended data, supplementary information, acknowledgements, peer
review information; details of author contributions and competing interests; and
statements of data and code availability are available at https://doi.org/10.1038/
s41586-019-1462-2.
Received: 18 January 2019; Accepted: 30 May 2019;
Published online 7 August 2019.
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