Nature - 15.08.2019

reSeArCH Letter

global tectonic regime during the Palaeoproterozoic era^4. Here we

focus on the broader, continuous global trends in metamorphic T/P

outlined above.

From the statistical evaluation of the distributions of metamorphic

T/P through time presented here, it is hypothesized that the mod-

ern style of plate tectonics—characterized by a distinctly bimodal

distribution of metamorphic T/P—developed gradually (Fig.  3 ). This

hypothesis can be considered an alternative to: (1) hypotheses that

infer plate tectonics to have begun abruptly in the Neoproterozoic era,

based on the apparently sudden appearance of blueschist and ultrahigh-

pressure (UHP) metamorphism about 0.7 Gyr ago^7 ; (2) hypotheses

that infer that a plate tectonic regime similar to the modern one has

been operative since the Palaeoproterozoic era (about 2  Gyr ago)^10 –^12 at

the latest, and; (3) the hypothesis of transient (≤0.3 Gyr) modern-like

plate tectonic behaviour in the Palaeoproterozoic era, before true plate

tectonics began in the Neoproterozoic era^1. The hypothesis presented

here, of a gradual but continuous transition in tectonic style since about

2.5 Gyr ago, is similar to the hypothesized gradual onset of plate tec-

tonics between 3.2 and 2.5 Gyr ago proposed in ref.^2 , but extends that

gradual change in tectonic style through to the modern era. We argue

that the most plausible mechanism for the hypothesized gradual change

in plate tectonic style since the Archaean eon is secular cooling of the

upper mantle (Fig.  3 ).

Changes in the temperature of the upper mantle affect not only the

thermal state of the crust but also the thickness and density of oceanic

lithosphere. Under the higher upper-mantle temperatures that are

thought to have existed during the Proterozoic and Archaean eons^21 –^23 ,

the degree of decompression melting at mid-ocean ridges is predicted

to have been higher, resulting in thicker oceanic lithosphere^23 ,^24 (as

much as 135  km 2.5 Gyr ago versus 60  km today)^25 , which could have

remained buoyant (relative to the underlying asthenosphere) for sub-

stantially longer than modern oceanic lithosphere (as much as 0.1 Gyr

to attain neutral buoyancy 2.5 Gyr ago versus 0.01–0.03 Gyr today)^25.

Greater thickness and buoyancy of oceanic lithosphere before 1  Gyr

ago might have favoured more uniformly shallow (less steeply dip-

ping) subduction and overall higher thermal gradients in subduction

environments, similar to the moderate-T/P metamorphism (green-

schist–amphibolite–eclogite series) associated with modern collisional

orogenesis.

A possible young analogue for shallower, hotter subduction meta-

morphism that might have been more prevalent before 1  Gyr ago is the

greenschist–amphibolite-facies Orocopia–Pelona–Rand schist (OPRS)

of southern California, which records T/P (500–650 °C GPa−^1 )^26 com-

parable to many eclogites, high-pressure granulites and amphibolites

in the Palaeo- and Mesoproterozoic eras (2.5–1.0 Gyr ago; Fig.  3 ,

Extended Data Fig. 2). The OPRS is thought to have formed in response

to a transition from steeper, colder subduction (Franciscan-type) to

shallower, hotter subduction related to the incoming of an oceanic

plateau (thicker, more buoyant oceanic lithosphere)^26. Consideration

of similar >1-Gyr-old rocks as plausibly related to subduction, rather

than focusing only on blueschist and UHP metamorphism, might offer

new insights into subduction processes (oceanic and continental) on

the early Earth. Many 2.5–1.0-Gyr-old orogenic belts—including

the Grenville, Sveconorwegian, Trans-North China, Trans-Hudson,

Eburnean, Ubendian–Usagaran and Belomorian belts—preserve

bimodal distributions of metamorphic rocks, with the lowest-T/P

rocks characterized by T/P similar to that of the OPRS (about 500–

650 °C GPa−^1 ; Extended Data Fig. 2)^26. These contrast the blueschist

and UHP metamorphism that are common on modern Earth. We

hypothesize that modern-style plate tectonics might have developed as

the time to neutral buoyancy became substantially less than the average

age of oceanic lithosphere, favouring colder, steeper subduction of the

type most common in modern subduction zones.

This study focuses on metamorphic T/P—a proxy for the ther-

mal gradients of different tectonic environments—as a more reliable

indicator of the tectonic regime than either T or P. However, another

1,350

1,200

1,500

1,650

Mantle potential

22

, T

(°C)

Metamorphic

T/P

(°C GPa

–1)

Age (Gyr)

1,350

1,200

1,500

1,650

Mantle potential

21

, T

(°C)

Increase in diversity of

metamorphic environments

Increase in diversity and overall

cooling of metamorphic environments

Kumdy-Kol,

Kokchetav Massif,

Kazakhstan

Nordre Stromfjord,

Nagssugtoqidian orogen,

Greenland

Fada N’Gourma greenstone belt,

Eburnean orogen, Burkina Faso

Kovik tectonic window, Trans-Hudson orogen, Canada

Eclogite xenolith in carbonatite,

Trans-North-China orogen, China

0 0.5 1.0 1.5 2.0 2.5 3.0 3. 54 .0

2,000

1,200

600

300

150

Fig. 3 | The range of metamorphic T/P (blue symbols) has become

increasingly varied through time, with its average value decreasing

since about 2  Gyr ago. Metamorphic rocks that fall outside this trend

(anomalously low T/P for their age) are labelled. The overall lowering of

metamorphic T/P (and increase in metamorphic diversity) through time

coincides with secular cooling of the upper mantle. Large brown symbols

are from ref.^21 , with ages modified as described in ref.^30. Small brown

symbols are from ref.^22 , modified for direct comparison with ref.^21 , as

described in Methods. Both datasets are shown with best-fit quadratic

regressions and 95% prediction intervals.

380 | NAtUre | VOL 572 | 15 AUGUSt 2019