Abstract

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east.

Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States.

Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

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