Abstract Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U‐Th‐Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA ‐ ICP ‐ MS U‐Th‐Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common‐Pb component. The use of measured Pb‐isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common‐Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208 Pb/ 232 Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.
Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2 Ga and 2.5 Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780–2710 Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700–2440 Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.
Les zones de subduction representent des zones d'evolution cles dans la comprehension des processus geologiques majeurs actifs sur notre planete. En particulier, la caracterisation et l'evolution des magmas generes dans les zones d'arcs oceaniques et la relation entre leur genese et les processus de croissance crustale sont des axes primordiaux. Cette etude est focalisee sur la portion d'arc oceanique exhumee du Kohistan (Nord Pakistan) qui constitue un laboratoire exceptionnel pour apprehender cette problematique. Basee sur une approche geochimique multi-methodes (elements majeurs et en trace, isotopes), cette etude a permis d'etablir : 1/ l'absence de relation genetique directe entre les roches ultrabasiques de la racine de l'arc et la section crustale sus-jacente. La formation de la sequence ultrabasique de Jijal se produit a ~117 Ma via une reaction de type magma-roche entre des liquides de type boninitique (appauvris en terres rares), et le manteau lithospherique de type MORB-Indien ; 2/ un modele geodynamique en trois stades majeurs resumant l'evolution de l'arc oceanique et de la subduction sur une periode d'environ 30 Ma. Ce modele debute par l'initiation de la subduction et la formation de l'arc volcanique s.s. (1er stade) ; le 2eme stade correspond a un evenement thermique majeur. Il est represente par un sous-placage important de magmas et une granulitisation intense de la base de l'arc. Le recyclage de la croute inferieure cumulative et residuelle, dans le manteau sous-jacent, se produit durant cette etape vraisemblablement suite a des processus d'erosion thermo-mecanique. Le dernier stade, entre 95 Ma et 85 Ma, scelle la fin du fonctionnement de la zone de subduction et correspond a une periode amagmatique suivie par une breve reprise du magmatisme avant la collision de l'arc apres 85 Ma. Enfin, une modelisation numerique comparative avec la croute continentale globale met en evidence la composition significativement plus basique de la croute de l'arc insulaire du Kohistan. De fortes similitudes sont en revanche observees entre la section d'arc etudiee et la croute continentale inferieure.
Detrital zircon spectra reflect the tectonic setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
