Influence of the architecture of magma-poor hyperextended rifted margins on orogens produced by the closure of narrow versus wide oceans

Orogens resulting from the closure of narrow oceans, such as the Alps or the Pyrenees, usually lack voluminous synsubduction and synorogenic magmatism. Such orogenies are essentially controlled by mechanical processes in which the initial architecture of the original rifted margins strongly controls the architecture of the orogen. In this paper we first provide a synthesis of the structure, dimensions, and lithology of hyperextended rift systems and oceans, based on recent seismic and petrologic data. We then investigate how rift-related inheritance influences crustal characteristics and mantle geochemistry of orogens related to the closure of narrow oceans, and compare them to orogens resulting from the closure of wide and/or mature oceans. Our results show that narrow oceans usually lack a mature spreading system forming Penrose-type oceanic crust (i.e., 6–7-km-thick basaltic oceanic crust typical of steady-state spreading systems; see Anonymous, 1972), in contrast to wide oceans. However, there is statistically no difference in the structural and lithological architecture of their passive continental margins. Thus, the main difference between narrow and wide oceans is whether the margins are separated by a significant amount of oceanic crust and underlying depleted mantle. In addition, due to the lack of significant magmatism during the closure of narrow oceans, the mantle wedge is likely to remain relatively fertile compared to the wedge above long-lasting subduction of wide oceans. This difference in mantle composition may dictate the magmatic budget of subsequent orogenic collapse or rifting events.