Introduction

Orogeny involves the building of mountains by plate tectonic processes in which the lithosphere is deformed under compression. Several lines of evidence mark the Palaeoproterozoic as the time when mountain building commenced in the forms which have continued to the present day. This is evident in a rapid increase in the frequency of collisional orogens, an increase in the preservation of continental crust as recorded by the earliest large lateral plate motions, the earliest global subduction network at 2 Ga, the predominance of steep subduction zones, and the first appearance of ophiolites about 2 Ga1,2,3,4,5,6. The Palaeoproterozoic also saw a marked increase in the biomass in the oceans, and consequent burial of organic carbon in the crust, which are recorded in the geochemical anomalies of the Lomagundi-Jatuli and Shunga events7,8,9. The earliest Palaeoproterozoic carbon burial occurred at the passive margins of fragmenting supercratons as plate tectonics emerged10. The increase in abundance may have been a unique response to the c. 2.3 Ga Great Oxidation Event (GOE), and/or a series of regional responses to widespread igneous activity that provided increased nutrients7,11. The result was prolific oxygen-tolerant cyanobacteria that left globally widespread carbon-rich sediment. An analysis of the time interval between the burial of organic carbon and orogeny indicates that the developments of high biomass and plate tectonics are linked. The additional carbon allowed easier deformation of the crust, in a manner that built mountain belts, and thereby plate margins characteristic of modern plate tectonics.

Results and discussion

Lubricating the growth of orogens: organic matter and mountain building

The formation of plate margins, where orogens develop, is enhanced by sediment, which confers low strength. Wet sediment and metasediment increase the lubrication of subduction12,13. Sediment containing carbonaceous matter is particularly effective in allowing deformation to occur, due to the ductile nature of organic-rich shales, overpressure weakening due to maturation of organic matter, and metamorphism to low-friction graphite14. Sediments rich in organic matter are relatively common in collisional orogens due to the incorporation of thrust platform sequences14. Consequently, numerous studies identify organic material as a critical control on the weakening of faulted rocks in orogens15,16,17,18. Only small amounts (few%) of carbon are required to dominate the frictional strength of the rock and allow seismic slip19. Upon graphitization of the carbon, the frictional strength is further reduced to about 0.3 times the normal stress for carbonaceous pelitic rocks20. Critically, low frictional strength enhances thrust-stacking in collisional orogens21, thus promoting crustal thickening during mountain building22.

Direct evidence for the deformation potential of highly carbonaceous rocks in orogens is found in an examination of major detachment surfaces in the younger Phanerozoic record. Twenty examples of thrust detachments that supported orogenic deformation (10 s to 100 s km extent), aged Cambrian to Paleogene, are consistently facilitated by shales containing a few percent TOC (Supplementary Table 1). The critical importance of this high level of organic carbon concentration to allow deformation is evident in a data set of Devonian shales (n = 102), where shearing occurred in shales with mean TOC 2.8%, while no shearing occurred in shales with mean TOC 1.4 %23. When highly carbonaceous rocks occur on a global basis, their contribution to orogenic deformation is similarly widespread. Following the oceanic anoxic events of the Cretaceous, the resultant black shales became detachment surfaces in numerous orogens including in the Rockies, the Andes, Svalbard, central Europe, Indonesia, and Japan (Supplementary Table 1).

Given the link between organic matter and deformation evident in younger rocks, the scene was set for collisional orogenesis at ~2 Ga (Fig. 1) by the exceptional accumulation of organic carbon during the mid-Palaeoproterozoic (Figs. 1, 2). Abundant carbon is evident from a peak in black shale deposition24, and from the record of the world’s richest (highest % TOC) graphite deposits25. The maximum carbon contents in carbonaceous rocks in the Palaeoproterozoic orogens are consistently above 10%, and many are above 20% (Table 1). These values are much greater than typical values for carbonaceous rocks in Phanerozoic orogens (Supplementary Table 1). Most of the richest graphite deposits are Palaeoproterozoic25.

Fig. 1: Palaeoproterozoic orogens, distributed globally, exhibiting significant deformation within highly carbonaceous rocks.
figure 1

Orogenies all commenced 2.2 to 1.8 Ga. Details in Table 1. Orogens shown relative to distribution of Proterozoic shields47.

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Fig. 2: Occurrence of carbonaceous sediment and orogeny through time.
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Sediments (curves coloured blue) were recorded by graphite deposits weighted by mean carbon content25, the abundance of black shales24, and stromatolite occurrences as a measure of carbonate platform abundance29. Orogeny (curves coloured red) was recorded by the incidence of metamorphism35, abundance of orogens dated by the onset of deformation34, and preserved length of orogens34. Peak occurrence in sediment input coincides with a peak in orogenic activity, emphasized by the toned bar at 2.0 to 1.8 Ga.

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Table 1 Carbon contents of carbonaceous units which accommodated deformation in orogens.
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The marine cyanobacteria that are assumed to be responsible for enhanced carbon burial underwent several important developments following the GOE. Cyanobacteria became larger after the GOE, up to 50 µm diameter from <3 µm previously, and then developed sheaths26, potentially increasing the mass of cellular carbon that was buried. A 10 µm diameter cell has about 25 times as much carbon as a 2.5 µm cell, based on established relationships27. At this range of cell sizes, larger phytoplankton cells would also settle through the water column significantly faster, and would be more likely to flocculate8. This would enhance the burial of carbon by restricting the time when oxidation could occur28.

In many cases, the carbonaceous sediments were accompanied by platform limestones. The stromatolite record shows a peak at ~1.9 Ga, consistent with widespread sedimentation on broad shallow platforms29. The passage of anomalous quantities of carbonate into subduction zones at this time is recorded by widely distributed carbonatites in Palaeoproterozoic orogens30,31. The abundant carbonate would have further reduced friction in the Palaeoproterozoic crust32. Supracrustal sediments engender thin-skinned tectonics, which is characterized by high degrees of crustal shortening22,33. The wide distribution of supracrustal sediments deposited at ~2.0 Ga was therefore available to confer a high degree of shortening to the immediately subsequent orogens.

Palaeoproterozoic orogens: widespread mountain building

A peak in orogenesis during the Palaeoproterozoic at ~2 Ga is marked by a large number of individual orogens34, the overall preserved orogen length34, and a high incidence of metamorphism35. The first widespread formation of high mountains is inferred at this time, from a peak in the production of S-type granites from 1.95-1.65 Ga and a high incidence of metamorphic rocks36.

Global deformation of the Palaeoproterozoic lithosphere is shown by the response to abundant carbon in numerous individual orogens. Twenty orogens with very highly carbonaceous rocks, of Palaeoproterozoic age distributed worldwide (Fig. 1), each record deformation focussed in pelitic/graphitic metasediments, including thrusting, imbrication, isoclinal folding, and shear zones that characterise crustal shortening (Table 1, Supplementary Note 1). The maximum period between sedimentation and deformation in the Palaeoproterozoic data set (measured from the onset of sedimentation to end of orogeny) was mostly less than 200 Myr, notwithstanding uncertainties in dating (Fig. 3; Table 1). The period ranges from 60 to 120 Myr for five orogens in North America, where more age data is available, suggesting that this is a good guide to typical times for consumption of oceanic lithosphere in the Palaeoproterozoic. Most subducting lithosphere today is Late Cretaceous or younger, with a mean age of about 65 Ma37, and the time interval between Phanerozoic shale sedimentation and orogeny is usually less than 200 Myr and often less than 100 Myr (Supplementary Table 1). The consumption of seafloor sediment in the Palaeoproterozoic was, thus, at a similar rate to today, which has several implications. The similarity is consistent with the establishment of global subduction, and cycling of carbon via subduction zones, at ~2 Ga3. As the carbon contents of the sediment were anomalously high in the Palaeoproterozoic, the flux of carbon into subduction zones was greater, and hence deformation could take place more readily than had been possible hitherto. As inferred above, an anomalous episode of global carbon concentration on the ocean floor can support deformation by decollement in multiple orogens within 100 Myr. The availability of abundant carbon on the Palaeoproterozoic seafloor therefore allowed widespread orogenesis, as occurred subsequently in the Phanerozoic. The increasing database of ages implies a consistent speed of subduction, which supports the inference2 that Palaeoproterozoic plate convergence occurred at a similar rate to the present day.

Fig. 3: Chronology of carbonaceous sediments and deformation in 20 Palaeoproterozoic orogens.
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Details in Table 1. The maximum interval between carbonaceous rocks (blue colour) and orogenic deformation (purple colour) is consistently <200 Myr, comparable to Phanerozoic orogens.

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The 20 examples chosen are all collisional orogens, involving continent-continent collision, or they have a mixed accretionary and collisional history34. The role played by carbonaceous rocks in contractional deformation is evident in cross-sections through Palaeoproterozoic orogenic belts, which show imbricate thrust slices detached in pelitic/graphitic sediments, often stacked near-vertically (Fig. 4).

Fig. 4: Cross-sections through Palaeoproterozoic orogenic belts, deformed within pelitic/graphitic sediments.
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Sections in (a) Kimban, (b) Wopmay, (c) Trans-Hudson, and (d) Magondi orogens show imbricate thrust slices detached in carbonaceous sediments, commonly stacked near-vertically48,49,50,51.

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The role of graphitic sediments in mountain building

In at least fifteen of the twenty orogens in Fig. 1, the deformed rocks contain graphite ore bodies. In ore-grade graphite, the organic carbon content is very high, enriched above the levels in unaltered carbonaceous sediments. In some cases, enrichment is a consequence of the mobilization of organic carbon during deformation and metamorphism38. In some other Precambrian graphite deposits, carbon is added from mantle carbon dioxide or due to the metamorphic decarbonation of marbles39. These alternatives can be distinguished by the carbon isotopic composition of the graphite, which would be relatively light or heavy, respectively39. All of the twenty orogens have known isotopic compositions for their graphitic sediments, which are consistently light (Supplementary Note 1, Fig. 5), indicating a biological origin for the carbon in the orogens rather than from an abiotic source.

Fig. 5: Isotopic composition of carbon (δ13C ‰) in carbonaceous rocks in twenty orogens.
figure 5

Orogens are shown in Fig. 1 and listed in Table 1. All data fall in range characteristic for biological carbon; none fall in range characteristic for magmatic carbon. Data sources in Supplementary Note 1.

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Evidence that the graphitic sediments localized and enhanced deformation takes two forms. Firstly, the concentration of graphite on shear surfaces is a direct consequence of carbon mobility during fault slip15,16,17,18. In some cases (Fig. 4), there is evidence for structural thickening of the graphite due to local redistribution (e.g. in Kimban and Ketilidian orogens), and graphite is a noted component in subduction zones (e.g. in Trans-North China and Penokean orogens). Secondly, data from relatively young plate margins, including the Pacific rim40, show that fault slip in graphitic rocks can cause a decrease in order in the graphite structure. This so-called strain-induced amorphization of graphite19,40 is an exception to the assumed irreversibility of the graphitisation process. The anomalous disorder indicates a potential signature for graphite that facilitated deformation in the deep geological record. Graphite that had been involved in deformation could be less ordered (lower maturation and inferred palaeotemperature) than the background graphitic metasediment.

Spectroscopic data from graphite in several Palaeoproterozoic orogens38,41,42,43, including the Pine Creek Orogen (Australia), Birimian Orogen (Ghana), Minas Orogen (Brazil), and Svecofennian Orogen (Finland), distinguish between a component in the country rock representing metamorphosed organic matter, and a component in fault rocks which was mobilized from the country rock (Fig. 6). Exceptionally, the mobilized graphite in all four orogens is less ordered than the country rock, as deduced from standard measurements using Raman spectroscopy and/or X-ray diffraction, expressed as palaeotemperature-equivalents for comparison. The consistent pattern of disorder in mobilized carbon in fault rocks in the Palaeoproterozoic orogens strongly indicates that the carbon was directly involved in the Palaeoproterozoic deformation.

Fig. 6: Comparison of degree of order in graphitic components in metamorphic country rock and fault rock.
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Order is expressed as palaeotemperature equivalent, which increases with an order. Data are shown for four Palaeoproterozoic orogens38,41,42,43, and sheared rocks in young Hidaka Orogen40 and laboratory experiment52 for comparison. Orogens show decrease in order in fault rocks (red colour) relative to the original metasediment (black colour).

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The long-term legacy of the Palaeoproterozoic

As the consequence of carbon burial in the Palaeoproterozoic seas was the incorporation of refractory graphite during orogenesis, much carbon became locked into the geological cycle. This left a legacy for mountain building through subsequent geological history. The properties that enhanced deformation and crustal shortening during the Palaeoproterozoic are just as likely to do so where 2 Ga sediments are present in much younger orogens. Accordingly, degrees of crustal shortening in Phanerozoic orogens are high where they are rooted in Palaeoproterozoic crust22. In the Himalayas, Palaeogene thrusting was focussed in the graphitic sediments of the ~2 Ga Lesser Himalaya Sequences, which had already focussed crustal shortening during the Palaeoproterozoic44. The intervening 2 Gyr record includes, for example, detachment on Palaeoproterozoic shale units during the 1.0 Ga Grenvillian Orogeny in Labrador45 and the 0.43 Ga Caledonian Orogeny in Norway46. Pressure-temperature (P-T) trajectories for Palaeoproterozoic granulitic metapelites (Fig. 7) reflect reactivation involving compressional uplift of the granulites, either during normal exhumation or during a younger orogenic event. Palaeoproterozoic pelites were reactivated during the main periods of orogenesis over the last 2 Gyr (Fig. 8, Table 2). The exceptional Palaeoproterozoic biomass was thus reflected not only in mountain building during the Palaeoproterozoic but also in mountain building ever since.

Fig. 7: Pressure-temperature (P-T) trajectories for pelitic granulites in Palaeoproterozoic orogens.
figure 7

Typically, trajectories pass from hot (>800 °C) temperatures (red circles) during crustal thickening to cooler temperatures (blue circles) during exhumation. Note range of timescales from thickening to exhumation from rapid to over a billion years. Trajectories from published sources53,54,55,56,57,58,59,60.

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Fig. 8: Reactivation of Palaeoproterozoic pelites during younger orogenies.
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Pelites deformed during a subduction-related collision in the Palaeoproterozoic were deformed further during a younger continental collision, particularly during episodes of widespread orogenesis34 (yellow colour). Peaks in orogen distribution are expressed as numbers of orogen lengths per 100-Myr bins5. Details of orogenies in Table 2.

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Table 2 Examples of reworking of Palaeoproterozoic metapelites during deformation in younger orogenies (Fig. 6).
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Methods

The relationship between carbon burial and plate margin deformation was evidenced by the review of data available in the published literature.

The link was demonstrated in young (Phanerozoic) rocks using 20 case studies where detachment on organic-rich beds is documented. The case studies were characterised using the age of sedimentation, carbon contents of sediment, and age of deformation. The characterization was repeated for 20 Palaeoproterozoic orogens, with the additional documentation of organic carbon isotope composition to prove biogenicity. The periods between sedimentation and deformation were then compared between the Phanerozoic and Palaeoproterozoic data sets.

Strain composition of graphite was inferred using published spectroscopy data for carbonaceous matter in 4 Palaeoproterozoic orogens where components in the country rocks and remobilized in fault rocks were both available.