Cretaceous deformation of the southern Central Andes: synorogenic growth strata in the Neuquén Group (35° 30′–37° S)

Abstract

The Neuquén Group is an Upper Cretaceous continental sedimentary unit exhumed during the latest Miocene contractional phase occurred in the southern Central Andes, allowing a direct field observation and study of the depositional geometries. The identification of growth strata on these units surrounding the structures of the frontal parts of the Andes, sedimentological 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 ca. 100 Ma, maximum age obtained in this work, compatible with previous assignments and constrained in the top by the deposition of the Malargüe Group, in the Maastrichtian (ca. 72 Ma). 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 mostly fed from the paleo‐Andes with small populations coming from sources in the cratonic area 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 recognised advance for this Late Cretaceous deformation in the study area is approximately 500 km east of the Pacific trench, which constitutes an anomaly compared with neighbour 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 reorganisation. 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.

Exhumation of the Neuquén Basin in the southern Central Andes (Malargüe fold and thrust belt) from field data and low-temperature thermochronology

“…It is proposed that i) rapid and anomalously wide migration of the orogenic front towards the continental interior follows arc expansion that modifies the thermal and therefore the mechanical structure of the lithosphere, favoring shortening; followed by ii) shut down of arc magmatism producing a volcanic arc gap, and consequent "refrigeration" of the upper plate through removal of the mantle wedge and lowering of basal heat flow. Fossil shallow subduction settings along western South America have been proposed on similar grounds: anomalous foreland deformation and arc expansions/migrations (James and Sacks, 1999;Kay et al, 2006;Ramos and Folguera, 2005;Kay and Coira, 2009;Espinoza et al, 2010;Ramírez de Arellano et al, 2011;González Guillot et al, 2011;Spagnuolo et al, 2012;Gianni et al, 2015, Fennell et al, 2015.…”

“…subduction of young oceanic lithosphere associated with mid ocean ridges , Fennell et al, 2015. In particular, the described anomalous geometry of the Miocene magmatism over the present retroarc zone, following a NE trend, and its chemistry for the 17-15 Ma magmatic period, could indicate the presence of an asthenospheric anomaly at the beginning of the Payenia shallow subduction zone (see Folguera et al, 2015).…”

“…Recently, there has been increasing evidence and discussion about the role of shallow and flat subduction settings in the construction of the Andes and North American cordillera (Murphy et al, 1998;James and Sacks, 1999;Ramos and Folguera, 2009;Kay and Coira, 2009;De Celles et al, 2009;Martinod et al, 2010;Dávila and Cárter, 2013;Gianni et al, 2015, Fennell et al, 2015. These models have to a certain extent, explained the pattern of magmatic variability through time as a product of changes in the angle of subduction.…”

“…In particular, Late Cretaceous eastward migration of the arc may be the result of slab shallowing, perhaps related to subduction of an active spreading center (Fennell et al, 2015).…”

“…Inset in the lower right-hand corner indicates the categories in which exhumation ages derived from fission track data were divided. The deformational front is based on Gianni et al (2015) and Fennell et al (2015). Note that most of thermochronological data are located on the western Andean side.…”