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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.

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.