Abstract
Whether modern-style plate tectonics operated on early Earth is debated due to a paucity of definitive records of large-scale plate convergence, subduction, and collision in the Archean geological record. Archean Alpine-style sub-horizontal fold/thrust nappes in the Precambrian basement of China contain a Mariana-type subduction-initiation sequence of mid-ocean ridge basalt blocks in a 1600-kilometer-long mélange belt, overthrusting picritic-boninitic and island-arc tholeiite bearing nappes, in turn emplaced over a passive margin capping an ancient Archean continental fragment. Picrite-boninite and tholeiite units are 2698 ± 30 million years old marking the age of subduction initiation, with nappes emplaced over the passive margin at 2520 million years ago. Here, we show the life cycle of the subduction zone and ocean spanned circa 178 million years; conservative plate velocities of 2 centimeters per year yield a lateral transport distance of subducted oceanic crust of 3560 kilometers, providing direct positive evidence for horizontal plate tectonics in the Archean.
Highlights
Whether modern-style plate tectonics operated on early Earth is debated due to a paucity of definitive records of large-scale plate convergence, subduction, and collision in the Archean geological record
The lower stack of nappes includes circa 2698 ± 30 Ma island-arc tholeiites and high-MgO, high-SiO2, low-Ti picritic-boninitic volcanic rocks, hallmark signatures of Mariana-type subduction initiation in extant and Phanerozoic arcs[7,8]. The latter is intruded by circa 2692 ± 35 Ma evolved trondhjemitic dikes, similar to mid-upper crustal tonalities-trondhjemites of intra-oceanic arcs[9,10], with an age difference from subduction initiation to arc magmatism similar to that in Mariana-type subduction-initiation events[7,8,11], This forearc subduction-initiation sequence is overthrust by an orogenic mélange belt, that extends for more than 1600 km along strike, containing a wide variety of blocks including midocean ridge basalt (MORB)-type ophiolitic rocks[12,13], in a highly-deformed metasedimentary matrix
The Central Orogenic Belt (COB) of the Neoarchean North China Craton (NCC) (Fig. 1a) is a 1600-km-long Neoarchean collisional orogen formed between an intra-oceanic arc terrane in the west, and the ancient continental Eastern Block in the east[12,13,18,19,20]
Summary
Whether modern-style plate tectonics operated on early Earth is debated due to a paucity of definitive records of large-scale plate convergence, subduction, and collision in the Archean geological record. We report unequivocal hallmark evidence for Archean subduction and large-scale horizontal plate motions, culminating in the emplacement of a series of subhorizontal fold nappes with regional-scale overturned limbs, onto a formerly distant continental margin These structures are considered diagnostic indicators of horizontal plate translations in modern orogens. The remarkable high degree of similarity between orogenic structure, zonation, and contained rock units and their relationships in the Archean Central Orogenic Belt and the Alpine and Appalachian systems, clearly formed by plate tectonics, suggests that plate tectonics is the most reasonable mechanism by which to interpret this Archean sequence We use this clear example of subhorizontal Alpine-style forearc nappes thrust over a continental margin in the Archean to make quantifications of the translational length-scales of Archean plate motions.
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