Cretaceous deformation of the southern Central Andes: synorogenic growth strata in the Neuquén Group (35º 30′-37ºS)

LUCAS M. FENNELL; ANDRÉS FOLGUERA; MAXIMILIANO NAIPAUER; GUIDO GIANNI

BASIN RESEARCH

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2015 p. 1 - 22

0950-091X

The Neuquén Group is an Upper Cretaceous continental sedimentary unit exhumed during the last late Miocene contractional phase occurred in the Southern Central Andes, allowing a direct field observation and study of the depositional geometries. The identification of syngrowth strata on these units surrounding the structures that comprise the frontal parts of the Andes, sedimentological provenance analyses and U-Pb dating of detrital components, allowed the definition of a synorogenic unit that coexisted with the uplift of the early Andean orogen since ~102 Ma, maximum age obtained in this work, compatible with previous assignments. The definition of a wedge top area in this foreland basin system, where growth strata were described, permitted to identify a Late Cretaceous orogenic front and foredeep area, whose location and amplitude contrast with previous hypotheses. This wedge top area was solely fed from the paleo-Andes with small populations coming from sources in the foreland zone that are interpreted as a recycling in Jurassic and Lower Cretaceous sections, which contrasts with other analyses performed at the foredeep zone that have mixed sources. In particular, Permian sources are interpreted as coming directly from the cores of the basement structures, where Neopaleozoic sections are exposed, next to the synorogenic sedimentation, implying a strong incision in Late Cretaceous times with an exhumation structural level similar to the present. The maximum recognized advance for this Late Cretaceous deformation in the study area is approximately 500 km east of the Pacific trench, which constitutes an anomaly compared to neighbor segments where Late Cretaceous deformations were found considerably retracted. The geodynamic context of the sedimentation of this unit is interpreted as produced under the westward fast moving of South America, colliding with two consecutive mid-ocean ridges during a period of important plate reorganization. The subduction of young, anhydrous, buoyant lithosphere would have produced changes in the subduction geometry, reflected first by an arc waning/gap and subsequently by an arc migration that coexisted with synorogenic sedimentation. These magmatic and deformational processes would be the product of a shallow subduction regime, following previous proposals, which occurred in Late Cretaceous times, synchronous to the sedimentation of the Neuquén Group.