A STEP IN THE FORMATION OF THE HUANCABAMBA DEFLECTION IN THE ANDES OF PERU AND ECUADOR

Of the two Andean oroclines, the origin of the Huancabamba deflection between the central and northern Andes is uncertain. It has been linked to trans-Amazonian crustal breaks, and inferred to be a primary paleogeographic feature. A geodynamic analysis of the Huancabamba deflection, however, suggests that the formation of the Early Cretaceous Lancones Basin in northern Perú is a critical step in the life of the orocline. This basin is part of the intra-arc and back-arc rift developed along the length of the Andes. The NE-striking Lancones Basin is flanked on the east by the Proterozoic Olmos massif, an outlier of the Amazon craton, and to the west by the Amotape Range, which is underlain by allochthonous(?) Precambrian and Paleozoic rocks that arrived prior to the Jurassic. The syn-rift basaltic Ereo, La Bocana and Lancones Formations of Albian age and the sag-phase turbidite sequences of the Copa Sombrero Group of Albian to Cenomanian age filled the basin. NW-SE extension formed the Lancones Basin and WNW to NW-striking sinistral strike-slip faults bound the southern margin. In central Perú, age-equivalent intra-arc and back-arc rift basins strike NW and result from SW-NE extension; cross-basin faults generally strike NE. Marine volcanic rocks on the west and sedimentary rocks on the east filled the basins. As the central Peruvian basins approach the Huancabamba deflection, they split with one arm continuing into the Lancones Basin and an inland arm east of the Olmos massif. Between northern and central Perú, WNW-striking faults controlled basin development. These faults are oblique to the rift basin in central Perú, are normal to the Lancones Basin, and link with sinistral strike-slip faults along the southern margin of the Lancones Basin. The WNW-striking sinistral strike slip faults allowed the northern Andes to rotate clockwise away from the central Andes during rifting in the Cretaceous. A continental promontory at the future site of the Huancabamba deflection was the result. Tertiary shortening, collision of an oceanic terrane in Ecuador, and a 40°clockwise rotation in northern Perú accentuated the orocline.