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Hawaii Institute of Geophysics Contribution No. 1102

With the exception of the lower continental slope, or inner wall of the Peru Trench, normal faulting characteristic of extensional tectonics predominates in the shallow structure from the outer rise on the Nazca plate, across the trench, the contenental margin, the Coastal province, and into the High Cordillera province of the Andes of northern Peru. Marine seismic reflection and gravity traverses are used to trace the major basins, horsts, and large faults seaward from where they have been mapped on land and in the subsurface. Although these large-scale structures can be traced geophysically, neither single-channel nor multichannel seismic records reveal the wealth of detail of block faults and olistostrome-like low-angle slides known from subsurface studies of the onshore and offshore oil fields in the fore-arc basin along the coast at Talara, Peru.

The subduction process has been efficient during the Cenozoic at the Peruvian margin. The accretionary wedge under the inner wall of the trench is small compared to the volume of sediment that entered the trench. Moreover, plutons near the coast and the trends that strike seaward in the structures of the igneous and metamorphic basement there indicate that a substantial width of the lower outer edge of the continental crust has been removed. The evidence strongly suggests that the crust has been stoped away by subduction.

Whereas coastal Peru is underlain by a metamorphic, plutonic, and volcanic basement that is termed continental, coastal Ecuador has a belt of ophiolitic rocks regarded as former oceanic crust. The Dolores-Guayaquil Megashear separates the two terranes. It is probably a transform-fault boundary setting aside the ophiolites of coastal Ecuador and Colombia as a mini-plate. The Progreso Basin lies beneath the Gulf of Guayaquil and contains hydrocarbons. It was formed as a pull-apart basin by dextral movements along the Dolores-Guayaquil Megashear where that fault strikes westward into the trench. Banco Peru may be a horst of mafic or ultramafic basement left as a slice when the Progreso Basin was formed.

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