PASSIVE MARGIN-LIKE SEQUENCES IN AMALGAMATING ARCHIPELAGOS SIGNAL CONTINENTAL GROWTH FROM ARCHEAN TO PRESENT

Constraints on how and when continental margin sequences first developed is a key indicator of the growth and stability of early protocontinents and evolution of plate tectonics through time. We report a well-preserved Neoarchean (2560 - 2510 Ma) passive margin-type shallow-water sequence from the Precambrian basement of North China, in the North China Craton (NCC), documenting its sedimentary facies, structures, and detrital zircon geochronology. Tectonostratigraphic study demonstrated the autochthonous sequence records a transgressive setting from coastal (Unit 2) to shallow-marine platform facies (Unit 3), deposited over a granitic-tonalitic gneiss of a microcontinental basement (Unit 1). The allochthon (Unit 4), overthrust the autochthonous sequence, and closed a pre-existing ocean by 2500 Ma. Structural studies show that the SSE-directed ductile shearing demonstrated in the sedimentary sequence is consistent to the dominant deformation episode forming the Alpine-style fold-thrust belts thrust over the studied sequence. Detrital zircon analysis reveals the sedimentation take place in an accretionary orogen, and MDA calculation constrained the passive margin-type sequence initiated by 2560 Ma and was terminated by emplacement of nappes between 2510 and 2500 Ma. We propose that this passive-margin-like sequence was generated by subsidence of the back-side of a previously accreted arc: the amalgamating arcs of the EB formed a protocontinent, whose trailing margins subsided as most modern passive margin do, but after accretion and cessation of volcanism, instead of after continental rifting, generating similar thermal-subsidence related platformal or continental shelf-like environments, and remaining temporarily stable before the next arc collided and joined the amalgam 50 Ma later. This sequence shows that some of Earth’s oldest passive margins formed on relatively quiet back-sides of archipelagos in accretionary orogens, during rapid growth of continents through arc amalgamation in the late Archean. Post-collisional subsidence of archipelago back-sides represent a novel subsidence mechanism, distinct from the traditional understanding of passive margins generated by continental rifting, signaling initial protocontinental maturity on the early Earth.