Structural Styles of Subandean Fold and Thrust Belt of Peru and Southern Ecuador

Antenor M. Aleman

Along-strike variations in structural styles of the east-verging Subandean fold and thrust belt (SAFTB) in Peru and southern Ecuador are controlled by the presence or absence of thick Late Permian to Jurassic evaporite sequences rather than changes in subducting plate geometries as has been suggested previously for the Andes. Salt distribution and thickness have not only controlled the styles and segmentation along the SAFTB but also have been important factors in strike variations across the belt. The southern Ecuador SAFTB lacks significant evaporite units and is characterized by thick-skinned deformation that encompasses high-angle reverse faults, and broad, low-amplitude folds. The style changes to thin-skinned deformation near 2°S lat. and it is well illustrated in the Santiago and Huallaga basins where thick evaporite units are present. This segment is characterized by a major decollement on the salt, grabens formed by salt withdrawal from reactivation of thrust faults as listric normal faults, salt piercement at or near synclinal axes, and periclines and asymmetric folds. The frontal thrust of this thin-skinned segment consists of box, overturned and upright folds above shallow salt domes, and by a major backthrust at the mountain front. This segment extends to 10°30^primeS lat., near Oxapampa, Peru, where the thin-skinned SAFTB is narrow and changes across strike to a thick-skinned deformation as the evaporite units thin and disappear eastward. South of 10°30^primeS lat., a new thick-skinned deformation segment is present in souther Peru and characterizes most of the deformation in the SAFTB of the Ucayali and Madre De Dios basins.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.