Structural and stratigraphic evolution of Shira Mountains, central Ucayali Basin, Peru?

The Ucayali Basin is a Peruvian sub-Andean basin that initially formed during the extensive tectonics of the Early Paleozoic. Originally, the Ucayali Basin was part of a larger basin that extended east of the current Andean chain along the Peruvian territory. Subsequently, this large basin was divided into many smaller sub-Basins during the Andean Orogeny. Today, the basin covers an area of about 140,000 km2, and it is morphologically defined by two well-differentiated structural features: the sub- Andean fold and thrust belt (SFTB) to the west and the Amazon plain and Brazilian shield to the east. It is limited to the north and south by the Contaya and Fitzcarrald Arches respectively, the Andes to the west and the Brazilian Shield to the east. These structural features acted as favorable elements to add sediments and to contribute to the structural development of this basin. The sedimentary section of the basin varies in thickness from 1 to 10 km, with ages of strata ranging from the Paleozoic to Quaternary. The strata were deposited in deep and shallow marine as well as transitional and fluvial continental environments. The most important phase of marine sedimentation was initiated with the transgression of the Cretaceous sea (Aptian ?Albian) over the irregular paleogeography defined by morphologic highs and peneplains. Tectonic features of the basin show structural deformations parallel to the Andean front, where overturned structures are observed. These are commonly cut by thrusts and laterally displaced by strike-slip faults. To better understand the development of the Shira Mountains in the central part of the Ucayali Basin, the structural and stratigraphic relationships were mapped out using a dense grid of 2D seismic reflection data and well log control. Three regional EW cross sections were constructed and restored to the top of the Cretaceous to determine the nature of deformation and faulting during the Paleozoic and Mesozoic. The reconstructions show that Shira Mountains fault was initially a major normal fault bounding a half graben. The fault was reactivated by later compression as a thick-skinned thrust fault that detaches between 21 and 24 km depth. Reactivation occurred during Upper Miocene between 7.2 and 5.3 Ma, corresponding to the Quechua 3 compressive phase of Andean Orogeny. The shortening of the central Ucayali Basin determined by the reconstructed cross sections ranges between 3 and 5.5%.