During the Cretaceous to Eocene the W. part of N. Ecuador received the tholeiitic and calk-alkaline magmatic products of one volcanic arc, which built up on a thin or dense basement, at least partly of oceanic origin. Accumulation of this arc was preceded by early Cretaceous volcanicity and flexuring in the E. and was brought to a close by late Senonian to Eocene orogenic movements. The partly ensimatic arc of N. Ecuador contrasts markedly with the totally ensialic arc of S. Ecuador and Peru, and suggests an important change in basement type near 2°30'S (the latitude of Guayaquil). Oceanic basement in N. Ecuador was probably trapped in the Lower Cretaceous by initiation of subduction at a trench in approximately the same relative offshore position as at present.

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