A critique of the analogy between Archaean and Phanerozoic tectonics based on regional mapping of the Mesozoic-Cenozoic plate convergent zone in the Makran, Iran

Presentations by W.B. Hamilton in 1995 and 1998 are cited, in which he listed objections to plate tectonics (as we know it in the Phanerozoic) having operated in the Archaean. The Iranian Makran, which comprises both mainly Mesozoic developments including five ophiolites in the Inner Makran and two Cenozoic accretionary prisms (Makran and Saravan), is discussed in comparison with possible Archaean analogues, in the light of the last two of Hamilton’s four statements:(1) rted Archaean plate tectonics indicators superficially resemble actual Phanerozoic indicators when taken in isolation from their association” ean assemblage are profoundly different” ve ophiolites in the Makran are compared with possible analogous eruptive associations of greenstone belts. The absence from Archaean assemblage of melanges like the Coloured Melange of the Makran, and also the absence of sheeted dykes, is noted. The areal extend of the ophiolites appears to be an order greater than all but rare individual Archaean coherent eruptive assemblages (e.g. 165 km×35 km) and Archaean pillow lavas may represent quite shallow depths of effusion, whereas sediment intercalations in the Makran are of deepwater facies. The long-lived eruptivity of individual Makran ophiolites is noted (e.g. from 190 to 60 Ma). Granitic rocks are restricted in the Makran to minor developments (pods, net veins) or layers in ophiolites, and chains of irregular granodiorite plutons inland from the two subduction fronts: there are no domical intrusions of Archaean type. Blueschists are locally developed in the Makran, but are unknown in the Archaean. Cenozoic accretionary prisms of the Makran, the scale of development of turbidites and overlying neritic sediments in the Makran accretionary prism, and turbidites in the Saravan accretionary prism, is remarkable: individual ‘packets’ between listric faults are more than 10,000 m thick, an order greater than most turbidite sequences in the Archaean (e.g. Mt. Belches, Western Australia), though such thicknesses are known in the Slave Province, Canada. These sequences are disrupted tectonically by listric faults, and by dislocation related to faulting and folding, such dislocation where intense and widespread producing wide tracts of sedimentary melange spatially related to the faults. Exotic blocks of immense size are scattered through the dislocated sediments and melange, squeezed up from below. The scale of the sedimentary prisms themselves is greater than any sedimentary expanse seen in the Archaean. Post-tectonic fanglomerate developments are superimposed on the folded rocks of the prisms, with individual fans measuring more than 5000 m apparent thickness in sections, something not matched in the Archaean. not all Hamilton’s objections are supported, there do appear to be fundamental differences between Phanerozoic plate tectonic convergent zones and anything recorded in the Archaean. It has been remarked that not all such zones include accretionary prisms, but nothing at all like the prisms of the Makran appears to be revealed anywhere in Archaean terrains. Suggestions that the ophiolites and greenstone belts represent different levels of erosion (i.e. the Archaean exposes only ophiolite roots), are manifestly absurd, for both expose sea-floor eruptives and sediments together with their substrate. The conclusion is reached that simply taking geochemical signatures of rocks as indicators of uniformitarian analogy with modern plate tectonics is facile. Hamilton was certainly correct on stressing the importance of the observed geological associations being considered in a holistic manner together with the geochemical signatures.