Sedimentary record of plate coupling and decoupling during growth of the Andes

Geochronologic, provenance, and sediment accumulation records from the long-lived (>100 m.y.) retroarc basin at the transition from the central to southern Andes provide improved resolution to examine the duration and controls on mixed-mode deformation and an enigmatic foreland depositional hiatus. Detrital zircon U-Pb ages for the Malargüe and Neuquén basin systems of western Argentina reveal shifts in exhumation and accumulation compatible with magmatic-arc and thrust-belt sources during unsteady Cretaceous–Neogene deformation. Fully developed foreland basin conditions were only achieved during separate periods of Late Cretaceous and Neogene shortening contemporaneous with possible episodes of enhanced coupling between a westward-advancing South American plate and the subducting Nazca slab. Separating these two contractional episodes is a 20–40 m.y. phase of reduced sedimentation and unconformity development, potentially signifying a neutral to extensional mode across the retroarc hinterland to forearc region during diminished plate coupling. We propose that the Andean orogen and its foreland and forearc basins have always been sensitive to variations in subduction dynamics, such that regional shifts in slab buoyancy and subduction geometry (particularly slab dip) superimposed on plate-scale shifts in convergence have governed mechanical coupling along the plate boundary and resulting fluctuations among contractional, extensional, and neutral tectonic regimes.


INTRODUCTION
Transient shifts in geodynamic coupling along subduction boundaries (Dewey, 1980;Royden, 1993;Wells et al., 2012) may regulate the evolution of convergent-margin sedimentary basins.With sufficient age control, these basins can provide valuable constraints on the inception and duration of possible alternating modes of upper plate shortening, extension, and neutral conditions that govern orogenic growth (e.g., Ramos, 2010;Mpodozis and Cornejo, 2012).For South America (Fig. 1), it is generally accepted that mid-Cretaceous separation from Africa marked a fundamental shift to a chiefly compressive Andean orogenic regime (Dalziel et al., 1974;Mpodozis and Ramos, 1990;Coney and Evenchick, 1994).However, evidence for periodic lulls, punctuated extension, and pulsed shortening suggests a complex interplay among contrasting modes of deformation (Jordan et al., 2001;Folguera et al., 2015), potentially driven by phases of variable subduction and plate coupling.
Uncertainties on the onset and pace of Andean deformation are compounded by limited constraints on depositional timing and potentially significant unconformities, as well as incomplete documentation of responsible structures.Although long-term construction of the Andes is routinely attributed to a single switch to regional shortening (e.g., Wilson, 1991;DeCelles and Horton, 2003;Balgord and Carrapa, 2016), the central to southern Andean history of Argentina and Chile has involved proposals for mixed-mode deformation throughout Mesozoic-Cenozoic time (Maloney et al., 2013).In the retroarc Neuquén Basin and Malargüe fold-thrust belt (Fig. 2), a critical sedimentary package and regional unconformity of uncertain duration define a lengthy late Mesozoic to mid-Cenozoic transitional period of questionable tectonic conditions.We present geochronologic, stratigraphic, and sediment accumulation results in support of two distinct and broadly separated phases of Late Cretaceous and Neogene shortening, crustal thickening, and flexural foreland basin development in west-central Argentina.This seemingly anomalous record of fluctuating tectonic regimes can be reconciled with patterns of plate coupling and decoupling, highlighting an underappreciated sensitivity of retroarc basins to multiple shifts in deformation along Andean-type convergent margins.

GEOLOGIC SETTING
The transition between the central and southern Andes is situated above a 30° east-dipping segment of the subducting Nazca slab (Fig. 1).At 33°-41°S, the retroarc zone encompasses the northern Neuquén Basin and Malargüe foldthrust belt (Fig. 2), which contain thin-skinned thrust systems and basement-involved block uplifts superimposed on Mesozoic extensional structures, with a modest 10-40 km of total eastwest shortening (Manceda and Figueroa, 1995;Giambiagi et al., 2012).

U-Pb GEOCHRONOLOGY
Zircon U-Pb ages for seven sandstone and three tuffaceous sandstone samples provide provenance and chronostratigraphic constraints for the Upper Cretaceous-Neogene succession (Fig. 4; Table DR1 in the GSA Data Repository 1 ).Laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) results were generated following methods outlined for the University of Arizona LaserChron Center 1 GSA Data Repository item 2016210, U-Pb geochronological results, is available online at www .geosociety.org/pubs /ft2016.htm,or on request from editing@geosociety.org.
(Tucson, Arizona, USA) (Gehrels et al., 2008).Compiled U-Pb age spectra for the Cenomanian-Campanian Neuquén Group (Tunik et al., 2010) show a prevailing Cretaceous population with subordinate Jurassic, Permian, and older Paleozoic ages (Fig. 4).The overlying Campanian-Paleogene Malargüe Group records replacement of these diverse populations by a narrow distribution of ages in which five samples from the Pircala and Coihueco Formations display unimodal latest Cretaceous to middle Eocene populations.These samples exhibit a systematic upsection decrease in the youngest age populations, from 57.3 ± 0.5 Ma to 41.1 ± 1.1 Ma (Fig. 4).U-Pb age distributions for five samples from overlying Neogene basin fill show the introduction of a diverse assemblage spanning the range of age populations expressed in underlying samples, but with an increase in late Paleozoic basement populations (330-250 Ma).Above the disconformity, the basal conglomerate of the Neogene Agua de la Piedra Formation (Rodados Lustrosos unit; Fig. 3) contains a youngest age population of 18.9 ± 0.6 Ma, and overlying units (Loma Fiera and Tristeza Formations) yield youngest ages of 10-8 Ma (Fig. 4).  3 and 4).Thickness data from seven sites, numbered from west to east, constrain sediment accumulation histories (Fig. 5).

PROVENANCE AND CHRONOSTRATIGRAPHY
U-Pb results provide provenance signatures of the activation and variable contribution of magmatic-arc and shortening-related source regions, as well as absolute ages for Cenozoic stratigraphic units above and below the foreland disconformity.
(1) The Upper Cretaceous Neuquén Group (Figs. 3 and 4) was sourced mainly from the Late Cretaceous magmatic arc in the west, recycled Jurassic extensional basin fill, and late Paleozoic basement in the east, matching evidence for shortening-related exhumation (Tunik et al., 2010;Mescua et al., 2013).
(2) The overlying Malargüe Group (Figs. 3 and 4) shows derivation from magmatic-arc sources of latest Cretaceous-Eocene age, with restricted input from fold-thrust belt or basement sources.The youngest populations are considered to represent the age of deposition, consistent with a nearly exclusive magmatic-arc source (e.g., Horton et al., 2015).These ages demonstrate a late Paleocene through middle Eocene signal, indicating prolonged ca.57-41 Ma deposition (Fig. 4).
(3) Above the disconformity, Neogene basin fill (Figs. 3 and 4) contains diverse ages compatible with derivation principally from the foldthrust belt and inverted basement blocks (Giambiagi et al., 2012).The youngest ages for the Agua de la Piedra Formation yield a ca.19 Ma basal age, requiring a ca.41-19 Ma hiatus (Fig. 4).Overlying units show youngest ages of 10-8 Ma, in accordance with rapid Miocene synorogenic accumulation in a proximal foreland basin.

SEDIMENT ACCUMULATION HISTORY
The provenance variations and stratigraphic hiatus quantified here correspond to an unsteady multiphase accumulation history.Industry agethickness data from five wells and two surface localities in the northern Neuquén Basin (Fig. 5) reveal (1) Late Triassic-Early Cretaceous accumulation consistent with extensional and postextensional subsidence (Manceda and Figueroa, 1995); (2) a Late Cretaceous onset of rapid accumulation during early Andean shortening; (3) Paleocene to earliest Miocene slow accumulation, then disconformity development; and (4) Miocene-Quaternary rapid accumulation during late Andean shortening.
The unsteady post-100 Ma history of rapid, then slow, then rapid accumulation (Fig. 5) is potentially indicative of long-term passage of a flexural forebulge (e.g., Horton et al., 2001;DeCelles and Horton, 2003;Fuentes et al., 2011), particularly in light of the long-duration (>20 m.y.) foreland disconformity.However, the corresponding lack of evidence for significant Paleogene shortening, exhumation, thrust-belt provenance, and rapid flexural accommodation requires an alternative explanation.Given the synchroneity with late middle Eocene to earliest Miocene extension farther west (Burns et al., 2006;Charrier et al., 2007;Rojas Vera et al., 2014), we interpret the hiatus to reflect a period of no thrust loading, no flexural accommodation, and sediment bypass in the foreland, possibly amplified by minor isostatic rebound due to erosional unloading (e.g., Legarreta and Uliana, 1991).The preceding Late Cretaceous and subsequent Neogene phases of high retroarc accommodation suggest a complex history of fluctuating deformation configurations contrary to most models of Andean orogenesis.

SUMMARY AND IMPLICATIONS
Provenance and accumulation records for the Andean retroarc basin (34°-37°S) reveal Late Cretaceous and Neogene phases of shorteninginduced flexural subsidence separated by a protracted period of little to no foreland accommodation.After Late Cretaceous flexure, a ca.60-40 Ma phase of reduced accumulation was succeeded by the ca.40-20 Ma hiatus identified here, and then renewed Neogene flexural subsidence .The hiatus coincided with a distinctive record of late middle Eocene to earliest Miocene extensional basin development in the adjacent Andean hinterland, magmatic arc, and forearc (Loncopué, Abanico, and Cura-Mallín basins; Fig. 2).We attribute the long-term variable pattern to (1) crustal shortening and flexural loading during enhanced plate coupling (Late Cretaceous and Neogene; Fig. 6A) alternating with protracted intervals of limited shortening and foreland accommodation during (2) neutral (ca.60-40 Ma; Fig. 6B) to (3) extensional regimes (ca.40-20 Ma; Fig. 6C) driven by diminished plate coupling.
Alternating phases of shortening-induced flexure and tectonic quiescence may seem broadly applicable across the Andes, but such discrete phases need not be synchronous or margin wide.In fact, late middle Eocene to earliest Mio cene extension appears to have been confined to the Chile-Argentina segment at 30°-40°S.Therefore, although changes in plate coupling could represent continent-scale (>5000-7000 km along margin) variations in plate convergence, upper plate advance, and age or thickness of the subducting slab (Sobolev and Babeyko, 2005;Schellart, 2008;Capitanio et al., 2011;Maloney et al., 2013), we contend that transient variations in slab dip, as commonly dictated by buoyancy and thickened oceanic crust at aseismic ridges or plateaus (Fig. 1) (Gutscher, 2002;Ramos and Folguera, 2009), provide the most viable mode of regulating the degree of coupling at smaller length scales (<1000-2000 km) along Andeantype margins.

Figure
Figure 4. Detrital zircon U-Pb age distributions depicted as age probability functions (shaded curves) and age histograms (open rectangles); shaded vertical bars denote the young age populations for Upper Cretaceous, Paleogene, and Neogene samples.

Figure 5 .
Figure 5. Stratigraphic age versus thickness diagram showing sediment accumulation curves for seven sites in the northern Neuquén Basin (Fig. 2).Note the three-phase (rapidslow-rapid) post-100 Ma accumulation history.

Figure 6 .
Figure 6.Schematic cross sections illustrating proposed modes of variable plate coupling and basin genesis along Andean-type plate margins, with inferred regional stress regime (large arrows), faulting, and retroarc sediment dispersal patterns.A: Coupled margin with upper plate shortening.B: Neutral margin with no upper plate deformation.C: Decoupled margin with upper plate extension.