ALONG STRIKE SEGMENTATION OF THRUST-RELATED DEFORMATION ALONG THE UCAYALI-CAMISEA FORELAND BASIN (PERU)

The Ucayali-Camisea basin is located at the Peruvian foreland in the transition zone between the north (narrow) to south (wide) Central Andes (i.e. Abancay Deflexion). The overall structure of the Ucayali-Camisea basin is controlled by two types of foreland verging thrust systems. The north section of the basin is characterised by Paleozoic basement-related structures showing a southward increasing degree of tectonic reactivation during the Neogene main Andean orogeny. The central and southern sections show pervasive thrusting detaching on extensional back-arc to extensional-intra-cratonic Silurian-Devonian units. Oblique ramp systems related to hinterland evolution limits to the north areas of major Silurian-Devonian sedimentation as well as the transition from thick-skinned (basement-related) to thin-skinned (detachment-related) tectonics. Thrust tectonic styles resulting from the Miocene-Present Andean orogeny are controlled by: 1) The geometry of re-activated thrust faults and ramp systems, which defines the zones affected by thin and thick tectonics; 2) The thickness of the pre-deformational sedimentary package, which controls the vertical segmentation of deformation; 3) The thickness of the relatively incompetent Silurian-Devonian units lying above the basement, which controls the width of the deformation zone and promotes an increase of deformation; and 4) Basement inflexion points (probably related to pre-existing extensional faulting) that control the location of major thrust systems and basal wedging. The deformation front is highly segmented. At least five structural styles can be distinguished: 1) Basement-related structures; 2) Fault nucleation; 3) Tectonic wedging and passive uplift; 4) Fault-propagation folds; and 5) Piggy-back cannibalisation of thrust systems.

The southward increase of shortening and fold-and-thrust belt width from north to south is promoted by the increase of thickness of the Silurian-Devonian units. The thickness of the early Paleocene units seems an important factor in controlling fold-and-thrust belt deformation and width at the scale of the Andean foreland.