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The geological eon from about 4.0 to 2.5 billion years ago. Continental crust composition and tectonic processes varied during this interval.

Authors: Ming Tang, Kang Chen, Roberta L. Rudnick.

Science 351, 372–375. Uses Ni/Co and Cr/Zn ratios to infer a change in upper continental crust composition during the Archean; the inferred timing of global plate tectonics is an interpretation.

Research author on the generation and evolution of continental crust.

A long-lived stable part of a continent, commonly with a thick, depleted lithospheric mantle keel; that keel is distinct from the continental crust.

Horizontal contraction of continental crust, commonly accommodated by folding and thrust faults and contributing to tectonic thickening.

Research author on petrology and the formation of ancient continental crust.

Authors: Roberta L. Rudnick, David M. Fountain.

Reviews of Geophysics 33, 267–309. Synthesizes seismic, geochemistry and petrological evidence for heterogeneous upper, middle and lower continental crust.

Choose an ocean–continent convergent margin. The essential relationship is recycling of oceanic lithosphere by subduction, together with production and modification of crust in a volcanic arc.

The incoming tectonic plate consists of thin oceanic crust, chiefly basalt and gabbro, above colder lithospheric mantle. As that oceanic lithosphere approaches the subduction zone, bending produces the subduction trench. Its cold interior can become denser than the surrounding mantle; the resulting slab pull helps sustain subduction. The descending slab carries both oceanic crust and lithospheric mantle into the interior. Thus the recycled unit is an entire portion of lithosphere, rather than just its crust.

At the subduction trench, some incoming sediment is scraped off and stacked by thrust faults into an accretionary prism. Material can also be added beneath the overriding plate by tectonic underplating. A forearc basin collects sediment between the accretionary prism and the volcanic arc. This is an accreting example: other convergent margins lose upper-plate material through tectonic erosion, so an accretionary prism is not universal.

The cold slab undergoes increasing pressure and metamorphism. Conversion of basaltic rocks and gabbro towards eclogite increases their density, while slab dehydration releases water from hydrated minerals. The rising fluid lowers the solidus of hot peridotite in the mantle wedge, causing flux melting. The usual source of arc magma is therefore the fluid-modified mantle wedge; extensive slab melting is not required. Thermal-petrological calculations show that the depth and amount of slab dehydration depend on the incoming slab's thermal structure.^[ref]

Buoyant magma rises through the overriding plate, freezes in igneous intrusions or erupts in the volcanic arc. Fractional crystallization, interaction with existing continental crust and repeated intrusion can produce more evolved arc magmas and add new crust. Some material merely reworks old crust, and simultaneous tectonic erosion can remove it: volcanism alone does not measure net continental growth. Tectonic shortening, folding and thrust faults can thicken and raise the upper plate.

The Wadati-Benioff zone marks earthquakes along and within the cold descending slab. Shallow interface earthquakes reflect unstable slip where the plates are coupled; deeper earthquakes require mechanisms such as dehydration embrittlement and are not simply dry shallow friction operating at every depth. In contrast, the hot mantle wedge deforms mainly by material creep. Together, the subduction trench, accretionary prism, forearc basin, volcanic arc and inclined seismicity connect surface features to the processes recycling and creating crust.

Slab pull is generally the largest direct driving contribution for plates attached to substantial cold descending slabs. Cooling makes oceanic lithosphere negatively buoyant; sinking slabs transmit a pulling force to the rest of the tectonic plate. The comparison is a force balance, so the following driving and resisting processes both matter.

The first two gravitational contributions ultimately belong to the same coupled mantle convection system: the plates are its cold upper thermal boundary layer. They are not passive rafts on a prescribed conveyor belt. A plate with little attached descending slab need not have slab pull as its largest local driver, and a large gross slab pull can be partly consumed by bending and interface resistance. Geological observations and force comparisons nonetheless support the general dominance of slab pull for rapidly moving subducting plates.^[ref]

Use a rock strength envelope: at each depth plot the smaller differential stress needed for frictional failure and for sustained ductile material creep at a specified strain rate. Shallow rock generally strengthens with depth because effective confining pressure increases; sufficiently deep hot rock weakens because thermally activated material creep becomes efficient.

At shallow depth, Byerlee's law or a Mohr-Coulomb failure criterion gives a relation of the formwhere $C$ is cohesion, $\mu$ a coefficient of friction, ${\sigma }_n$ the compressive normal stress and $P_f$ the pore pressure. Increasing overburden generally increases effective stress, hence the frictional strength. Conversion from fault shear stress to differential stress depends on the stress regime and fault orientation; a sketch should not label a universal numerical slope. High pore pressure or weak fault gouge can lower the frictional strength substantially.

A simplified dislocation creep relation isHere $Q$ is activation energy, $V^{\ast }$ activation volume, $R$ the molar gas constant and $T$ absolute temperature; $A$ and $n$ depend on composition and deformation mechanism. At fixed strain rate, increasing temperature usually decreases the required stress strongly. The competing effects of pressure, water weakening of rocks, grain size and composition also matter. The intersection with frictional failure gives a brittle-ductile transition, not a change from solid to liquid.

For the Indian continental interior, a useful first sketch has an upper-crustal strength peak, a weaker lower continental crust, and renewed strength in lithospheric mantle beneath the Moho. Quartz-bearing rocks can undergo ductile material creep at conditions where colder, drier olivine-rich mantle retains appreciable strength. This creates a possible two-peak, or “jelly sandwich”, rock strength envelope. With further depth the mantle also weakens. A cold craton can retain strong lithospheric mantle to much greater depths than a tectonically heated continental region.

For the central Pacific, show thin mafic oceanic crust above a relatively thick layer of strong lithospheric mantle. There need not be a conspicuous weak lower-crustal layer. Much of the integrated strength can reside below the Moho, with a broad strength peak followed by ductile weakening towards the asthenosphere. Cooling and age control the thickness: young hot oceanic lithosphere would give a much shallower and weaker profile than an old central-Pacific plate.

The plotted Moho depths of about 38 km and 7 km, and all plotted stresses, are illustrative assumptions. “India” alone does not specify a unique geothermal gradient, water content or strain rate, and the Pacific includes plates of different ages. The robust comparison is the possible crust–mantle layering in a continent versus the thin oceanic crust and mantle-dominated strength of an old oceanic plate. The Moho is a compositional boundary; neither it nor the deepest small plotted strength is automatically the seismic lithosphere-asthenosphere boundary.

A useful idealization is a thin sedimentary cover over a felsic upper continental crust, an intermediate middle continental crust, and a more mafic lower continental crust, ending at the Mohorovičić discontinuity at about 35–40 km. The boundaries within the continental crust are approximate compositional divisions, not three universal sharp interfaces.

The important trend is increasing average mafic character with depth, rather than a change from granite directly to peridotite. Granulite describes a metamorphic facies, not a unique chemical composition. The synthesis in Nature and composition of the continental crust: A lower crustal perspective by Roberta L. Rudnick and David M. Fountain combines seismic and petrological evidence for these distinctions.

The expected broad contrast is a less mature, commonly thinner early juvenile continental crust, with more mafic material and abundant sodic tonalite-trondhjemite-granodiorite, rather than the more differentiated, potassium-rich granitoid component characteristic of much Phanerozoic continental crust. This is a secular tendency, not a single stratified column applicable throughout the Archean.

Archean terrains commonly expose tonalite-trondhjemite-granodiorite gneiss associated with greenstone belts containing basalt, komatiite and metasedimentary rocks. Many later granitoids are richer in potassium, including granite and granodiorite. The proportions changed during the Archean: a reconstruction from trace element ratios in sedimentary rocks infers a markedly mafic upper continental crust before about 3.0 Ga and a more felsic upper continental crust by 2.5 Ga. Thus “all Archean continental crust was mafic” would be wrong. A preserved tonalite-trondhjemite-granodiorite-dominated nucleus can be strongly felsic, including at depth; its composition is not identical to that of its original juvenile precursor.^[ref]

Thickness needs a stated comparison. An influential seismic compilation found preserved Archean crust about 27–40 km thick, commonly without the thick high-velocity basal layer of its 40–55 km thick Proterozoic comparison provinces. That supports a relatively thin endmember, but is not a measurement of every original Archean continental crust, nor a demonstration that every Archean region was thinner than today's 35–40 km reference column. Tectonic thickening, magmatic underplating, erosion, delamination and preservation bias all alter what survives. The very thick lithospheric mantle beneath a craton must also not be mistaken for thick continental crust.^[ref]

The transferable answer is therefore more mafic early juvenile precursors, widespread sodic tonalite-trondhjemite-granodiorite production, and variable thickness evolving through time; late Archean felsic nuclei can already resemble substantial modern continental crust.

Today the usual first melt is basaltic melt from the mantle wedge; in a hotter Archean subduction zone, melting of hydrated basaltic crust itself could generate sodic felsic melt of tonalite-trondhjemite-granodiorite composition. Distinguish a primary melt from the evolved melt ultimately added to the upper continental crust.

On the modern Phanerozoic Earth:

This adds juvenile material derived from the mantle; simply remelting old continental crust is reworking, not net juvenile addition. The amount and depth of slab water release depend on its thermal history, as quantified in Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide by Peter E. van Keken and others.

For the hotter Archean endmember posited in the question, a hotter mantle and a warm descending plate make it easier for hydrated basaltic crust to reach its solidus. Its partial melting can directly make silica-rich, sodium-rich tonalite-trondhjemite-granodiorite liquids, leaving a more mafic residue. Residual garnet and amphibole, and the presence or absence of residual plagioclase, help determine the rare-earth element and strontium signatures. These are not ultramafic liquids merely because the surrounding mantle was hotter.

The contrast is therefore commonly fluid-fluxed mantle melting followed by differentiation today, versus a greater potential contribution from direct melting of hydrated mafic crust in the hotter case. Tonalite-trondhjemite-granodiorite is not unique evidence for subduction: melting of thickened mafic crust can produce similar liquids. Nor are all modern arcs devoid of slab melting. These qualifications are central to The composite Archaean grey gneisses: Petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth by Jean-François Moyen.

Cascadia subducts a young, warm plate and lacks the enormous plateau and crustal root of the central Andes; the central Andean Nazca Plate system also supports much deeper intraslab earthquakes. These are regional comparisons: neither western margin is uniform along its length.

First, the Juan de Fuca Plate entering Cascadia is young, typically of order 10 million years old, because its spreading ridge lies close to the margin. Its lithosphere is comparatively thin and warm. Much of the Nazca Plate entering the central Andes is older and colder. A cold plate carries low temperatures farther into the mantle, sustaining deep intraslab seismicity; the warm Cascadia slab loses its brittle seismogenic character much shallower. This concerns earthquakes within the slab, not the size of megathrust earthquakes: both margins can generate very large plate-interface events. The official warm-slab synthesis treats Cascadia as an endmember.^[ref]

Second, the central Andes have undergone major sustained crustal shortening and tectonic thickening, building a high plateau and locally roughly 60–70 km thick continental crust. Cascadia has an active volcanic arc and mountains but no equivalent continuous central-Andean plateau and crustal root. The difference reflects the accumulated history of convergence, upper-plate deformation, slab dip, trench motion and mechanical coupling, not just the instantaneous existence of subduction. The Andes include flat-slab segments with gaps in arc volcanism as well as normally dipping segments.^[ref][ref]

A young warm incoming plate and a faster, colder incoming plate can therefore generate different thermal structures, dehydration patterns and deformation histories. Plate age alone is insufficient to predict the thickness of the overriding continental crust.

Use chemical signatures to reconstruct processes, then test tectonic context. Geochemistry can establish melting conditions, surface alteration and recycling, but plate tectonics is a kinematic system. I would sample dated suites across a mapped geological succession and compare competing origins, rather than declare an isolated “arc-like” analysis proof of a global regime. Radiometric dating must identify whether the measured age records crystallization, later metamorphism, alteration or inheritance.

Hydrous magmatism and selective element transfer. Analyze fresh basalt, associated intrusive rocks and differentiated volcanic suites for major elements, trace elements and isotope ratios. Relative enrichment of fluid-mobile elements such as barium, strontium and lead, together with relative depletion of niobium and tantalum, can support addition of slab-derived components to a depleted mantle wedge. Test coherent variations in barium/niobium, thorium/niobium and rare-earth element patterns against modeled fluid addition, sediment melting, source depletion and fractional crystallization. Do not treat these ratios as fixed universal cutoffs. Crustal contamination, remelting of previously arc-modified crust, residual rutile or other source effects can create some similar patterns. Altered rocks can also gain mobile elements long after their formation.

The case becomes stronger if the chemical evolution occurs in a plausible succession of oceanic, early forearc and mature arc magmas, with supporting structure and geochronology. The key evidence would be correlated process changes in an independently reconstructed margin, not resemblance to one modern basalt plot. A geochemical discrimination diagram is a comparison tool; its modern training associations are not a proof that only those settings could exist on a hotter Earth.

High-pressure melting and the fate of mafic crust. Tonalite-trondhjemite-granodiorite suites with high strontium/yttrium or lanthanum/ytterbium and low heavy rare-earth element abundances can indicate melting with residual garnet and little plagioclase. Model the combined major element composition, residual phases and melt fraction to estimate the pressure, hydration and source of melting. This could support melting of buried hydrous oceanic crust. However, basal melting of thickened crust, delamination and nonplate vertical burial can produce related conditions. High strontium/yttrium alone neither proves a descending slab nor dates the first subduction. Conversely, hotter ancient subduction might not produce exactly the same magmatic products as modern cold slabs.^[ref]

Radiogenic isotopes distinguish new extraction from reworking. Couple zircon uranium-lead dating with hafnium isotope ratios, and whole-rock samarium-neodymium isotope ratios with strontium isotope ratios or lead isotope ratios. These systems can constrain contributions from depleted mantle, older continental crust and altered surface reservoirs. For a closed parent-daughter system,where $P(t)$ is the present parent abundance, $D^{\ast }$ the radiogenic daughter accumulated since closure, and $\lambda$ the radioactive decay constant. Initial daughter composition, closure and mixing must be constrained before interpreting an age. A crustal extraction model age depends on a reference mantle evolution and subsequent history; it is not automatically a rock's crystallization age or the date of a tectonic transition.

Juvenile signatures can indicate addition from the mantle, while evolved signatures can indicate reworking of older crust; neither is exclusive to a particular tectonic regime. Spatially organized juvenile additions at a reconstructed margin, followed by collision and reworking, are more diagnostic than an undifferentiated global age histogram. Work combining zircon hafnium isotope ratios, oxygen isotope ratios and age distributions illustrates how crustal generation, reworking and preservation can be separated conceptually, while remaining dependent on sampling and model assumptions.^[ref]

Stable isotopes can identify a visit to the surface. Zircon oxygen isotope ratios substantially different from an appropriate mantle reference may record melting of material that previously underwent low-temperature water-rock interaction. This would support surface alteration followed by burial or remelting. It would not, by itself, distinguish subduction from burial under a thick volcanic pile or another form of recycling. Alteration, metamorphic exchange, inherited cores and analytical domains need checking. Multiple stable isotope systems, including boron isotope ratios in appropriate rocks, can strengthen a fluid-transfer interpretation if their fractionation and later resetting are modeled.

Deep samples provide an independent archive. If dated diamond inclusions or mantle xenoliths preserve eclogite compositions together with a surface-alteration chemical signature, they could demonstrate transfer of crustal material into deep lithospheric mantle. Dating the relevant mineral assemblage and distinguishing inclusion growth from later host transport are essential. A primary diamond-inclusion study used coupled chemical and isotopic observations to infer changes in deep recycling; the general lesson is to test a transport history, not to make the first preserved sample an infallible onset clock.^[ref] Delamination or other sinking processes remain alternatives unless structure and regional history establish plate convergence.

For each candidate signal, I would seek an older comparison population, quantify alteration and analytical uncertainty, reproduce the pattern in several independent regions, and combine chemistry with the tests in the other parts. A securely dated demonstration of subduction shows that it operated by that time in that region; it does not demonstrate that it began then everywhere. A secular shift toward more frequent, spatially coherent signatures can support expansion of the regime, but only after correcting for the survival of its archive.

Test geological associations and their chronology. The most persuasive hypothetical record would combine rigid-block relative motion, creation of oceanic lithosphere, its destruction at a coherent margin, and repeated assembly and dispersal of continents. I would compile independently dated rock and structural occurrences in age bins, map their associations, and distinguish primary formation from later emplacement. The aim is to evaluate alternative tectonic regimes, not simply to collect the oldest example of a familiar name.

Oceanic lithosphere and sutures. An ophiolite association containing residual peridotite, gabbro, sheeted dikes and submarine basalt could demonstrate construction of an oceanic section. Its case for plate tectonics strengthens if it is associated with an oceanward sedimentary succession, a metamorphic sole, an accretionary prism or mélange, and eventual emplacement along a suture zone between formerly distinct blocks. Date construction, metamorphic heating and emplacement separately. A dismembered collection of ultramafic and mafic rocks need not be a complete ophiolite; an isolated volcanic pile or greenstone belt can also arise in a plateau or vertical-overturn setting. Hotter early mantle may produce oceanic sections unlike a modern template.

Cold burial beside a hot arc. Blueschist and eclogite with well-constrained low temperature/pressure ratios would support unusually deep burial before substantial heating. Coesite or metamorphic diamond in a suitable continental crust protolith could establish ultrahigh-pressure metamorphism, consistent with deep continental burial and return. Reconstruct pressure-temperature-time paths using mineral assemblages, zoning, reaction textures and radiometric dating; a peak pressure without its timing and burial geometry gives a weaker constraint. Eclogite alone only indicates appropriate pressure-temperature-composition conditions and may also form at the base of thick crust.

An approximately coeval, spatially organized pair of low thermal-gradient and high thermal-gradient metamorphic belts would be stronger evidence for juxtaposition of a cold descending plate and a hotter volcanic arc or hinterland. Test the original spatial relation rather than pairing unrelated rocks brought together much later. A statistical study of metamorphic temperature/pressure distributions illustrates how gradual widening and bimodality can be used to evaluate changing tectonic behavior. The ratio is an apparent metamorphic thermal gradient, not necessarily a directly measured steady geothermal gradient.^[ref]

The absence of old blueschist is not a decisive absence of old subduction. A hotter thermal regime may suppress its production, and later heating can transform it. The test should therefore include the distribution of pressure-temperature-time paths predicted by early-planet models, rather than demanding the youngest modern mineral assemblages at all ages.

Kinematic structures and basin evolution. Long, coherent thrust belts, imbricated sediments, folds, regionally consistent shear sense, major strike-slip faults, and suture zones can identify lateral shortening and relative displacement. A reconstructed ocean-facing volcanic arc, forearc basin and accretionary prism with a common polarity is more compelling than isolated shortening. Mountain building alone is not exclusive to plate tectonics: vertical instabilities and impacts can also deform rock. The length scale, sequence, strain partitioning and linkage between structures matter.

For repeated opening and closing, seek a dated progression from continental rifting, through a subsiding passive margin and an intervening oceanic assemblage, to convergence, continental collision and renewed rifting. Matching stratigraphy, sedimentary provenance and structural restoration on opposing blocks can test whether a reconstructed Wilson cycle is geometrically possible. Paleomagnetism from independently dated, demonstrably primary magnetizations could show differential latitudinal motion and rotation of separate blocks. Relative motion between blocks is more useful than motion of one block, which could include true polar wander. Remagnetization, inclination shallowing and longitude ambiguity must be assessed.

Secular changes require an archive model. Compile the incidence, spatial scale and recurrence of these associations through time, normalized where possible by exposed rock area, sampling effort and dating opportunities. Include detrital zircon and sedimentary provenance where they preserve information from eroded source rocks, while recognizing zircon fertility and sedimentary sorting biases. Increased abundance of plate-like associations in several independent regions could support their transition from local episodes to a widespread regime. A sharp break is only one possible outcome; a progressive increase in persistence or geographical coverage would support gradual establishment.

A useful conceptual accounting iswith additional redistribution between age bins if ages are reset or misassigned. This is not a calibrated dating law: it shows why an observed first appearance need not equal a true first formation. Subduction consumes oceanic lithosphere; erosion, metamorphism, tectonic reworking and burial selectively remove or obscure evidence. The oldest archive is especially incomplete, and some apparently diagnostic assemblages only become likely as the planet cools. An age peak may measure enhanced preservation during continental collision rather than an increased production rate.

State the chronological inference precisely. If an unambiguous assemblage forms at time $t_e$, then the required process was operating by $t_e$; an onset time measured forward from planetary formation obeys $t_{\mathrm{onset}}\le t_e$. If both are expressed as ages before present, the inequality reverses: the onset age is at least as old as the assemblage age. Constraining how much earlier onset could have occurred requires well-sampled older records and a credible prediction that the sought evidence would have formed and survived. Several coeval, independent records consistent with sustained horizontal recycling give a stronger inference of widespread plate tectonics than the oldest ambiguous occurrence of any single rock type. Combined with geodynamic modelling and geochemistry, the defensible answer would report a range for first local activity and a potentially different range for establishment of a persistent global system.

A near-surface layer of sedimentary rocks over crystalline basement; its thickness can be zero and is not a fixed fraction of continental crust.

A family of felsic plutonic rocks prominent in studies of early continental crust. Melting hydrous mafic sources can generate related compositions, but the rock name alone does not establish subduction.