Tectonic Evolution of the Contaya Arch Ucyali Basin, Peru

The Contaya arch is an elongated topographic high that divides the Huallaga, Maranon and Ucayali basins in the Peruvian Amazonian plain. Its position well into the foreland basin and well inland from the main Andean thrust belt has proven to be enigmatic. Although it is often considered to be a single structural feature with a single origin, we show here that the Contaya arch is composed of distinct structures with different structural styles and different geologic histories: The main structures include the Contamana high, Contaya high, and Moa divisor. The Contamana high is limited by high angle reverse faults with a NW-SE orientation parallel to the Andean fold and thrust belt. This structure formed in the Tertiary, most probably in the Miocene after deposition of the Pozo Formation sands and shales. To the east of this is the Contaya high. This structure originally formed during the Triassic-Jurassic and was later reactivated in the Tertiary. The easternmost structure, the Moa divisor, separates the Ucayali basin from the Acre basin in Brazil. It is bounded by a high-angle, thick-skinned reverse fault. It appears to be a reactivated normal fault that formed in the Paleozoic during rifting and deposition of the Mitu Formation. From 10 to 4 Ma the subducted Nazca ridge was located beneath the Peruvian fold and thrust belt in the area where the Contamana high, Contaya high and Moa divisor are located. We suggest that the uplift of the Moa Divisor and the Contamana high as NW-SE oriented structures bounded by high-angle, thickskinned reverse faults and the reactivation of the Contaya high during the Miocene is related to the subduction of the Nazca ridge from 10 Ma to present.