Paleozoic structural controls on shortening transfer in the Subandean foreland thrust sytem, Ene and southern Ucayali basins, Peru

The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep-seated décollements within the basement related to thick-skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin-skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross-section balancing shows an along-strike homogenous horizontal shortening of ∼56 km (∼30%) across the Ene-southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N-S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generate major décollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene-southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region.