Data from LANDSAT images, published geologic and geophysical reports, and field observations are used to document eight major transverse boundaries that divide the northern Andes into segments about 125 km long. Most of the segment boundaries have no apparent relation to structure on the Nazca plate being subducted, and may instead represent fundamental and long-lasting structures of the continent. Subduction of the Carnegie Ridge results in increased uplift of the Andes, widespread and chemically diverse volcanism, and major faulting and seismicity. Subduction of a highly fractured zone in southern Ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style—from andesitic stratovolcanoes to rhyolitic ash-flow sheets. Recognition of the segmented structure of the northern Andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to Subduction.

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