Andean Structural Styles: A Seismic Atlas

Andean Structural Styles: A Seismic Atlas is a comprehensive reference illustrating the variability in structural styles and hydrocarbon traps that exist in the Andean chain. The Andean chain, stretching over more than 5,000 km (3,000 mi) from Venezuela to Argentina, contains a large number of sedimentary basins which have developed in a wide range of tectonic settings. Some of these basins are highly mature, with hydrocarbon production from Paleozoic, Mesozoic, and Cenozoic sedimentary sequences, while others are still underexplored.

Andean Structural Styles: A Seismic Atlas covers topics including fold types, thrust faults, triangle zones, inversion structures, synorogenic deposits, and growth stratal geometries. These topics are illustrated by thirty-two seismic examples interpreted and uninterpreted, covering most of the Andean basins, and five chapters reviewing the structural styles of the Andes, the complexity of processing seismic in these settings, how analogue models help in the interpretation, and several outcrop analogues. This reference is invaluable to both hydrocarbon exploration of the Andes and researchers and students in the fields of exploration geology and structural geology. Also, those teaching structural geology and seismic interpretation will find a valuable resource with lots of uninterpreted seismic examples that can be used in their lectures.

• Includes a vast collection of high-quality, color images
• Features case studies covering the entirety of the Andes Mountain chain
• Presents high-quality seismic data that was previously only available to oil companies

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 26, 2022
• ISBN: 9780323859585

Book preview

Section I

Introduction

Outline

Chapter 1. Tectonic inheritance and structural styles in the Andean fold-thrust belt and foreland basin

Chapter 2. Seismic imaging in fold-and-thrust belts

Chapter 3. Analogue modeling as a tool to assist seismic structural interpretation in the Andean fold-and-thrust belt

Chapter 1: Tectonic inheritance and structural styles in the Andean fold-thrust belt and foreland basin

Brian K. Horton ¹ , and Andres Folguera ²       ¹Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, United States      ²Instituto de Estudios Andinos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires-CONICET, Buenos Aires, Argentina

Abstract

Andean orogenesis is expressed in the diverse deformational records of crustal structures and sedimentary basins in western South America. Here we summarize retroarc structural styles within the Andean orogenic belt and foreland basin system through consideration of regional contractional fault geometries, their kinematic interactions with other structures, and the comparative involvement of crystalline basement and sedimentary cover rocks. In assessing the controls on structural style, we emphasize the importance of precursor conditions and employ the concept of tectonic inheritance to identify four factors that influence Andean deformation. (1) Structural inheritance involves the reactivation of preexisting faults or basement fabrics and accompanying inversion of sedimentary basins. (2) Stratigraphic inheritance is exemplified by the preferential localization of interconnected thin-skinned structures above regional décollements developed in wedge-shaped stratigraphic packages versus isolated basement-involved thick-skinned fault structures formed in provinces with limited cover strata. (3) Rheological properties guide the activation of new structures by means of the integrated strength, rock and mineral composition, fluid content and pressure, and associated mechanical heterogeneities and anisotropies that define crustal and lithospheric architecture. (4) Thermal structure in the form of initial temperature conditions and later thermal perturbations (such as cooling/heating episodes related to arc magmatism, subducting slab dynamics, or lithospheric removal) can promote inboard advance or outboard retreat of deformation. We propose that evaluation of tectonic inheritance will help explain variations in Andean structural styles that do not bear simple relationships to the history of plate convergence, subduction, and magmatism along the western margin of South America.

Keywords

Andes; Basin; Basin inversion; Faults; Fold-thrust belts; Foreland basins; Inheritance; Orogeny; South America; Subduction; Tectonics

Acknowledgments

Introduction

Tectonic framework

Modern configuration

Orogenic history

Time-space variability

Rationale: tectonic inheritance

Structural inheritance

Stratigraphic inheritance

Rheological inheritance

Thermal inheritance

Andean structural styles

Décollement-style fold-thrust systems

Basement-involved block uplifts

Inversion of pre-Andean basins

Inversion of Andean extensional basins

Backthrust belts

Salt-involved belts

Summary and discussion

Different modes of tectonic inheritance

Tectonic drivers and controls

References

Introduction

The Andes are a long-lived orogenic system with diverse structural and stratigraphic records that reflect the wide variety of deformational processes along convergent plate boundaries. The Andean fold-thrust belt and foreland basin system (Fig. 1.1) have been generated during sustained ocean-continent convergence, subduction, continental arc magmatism, and shortening along the western edge of the South American plate (Ramos, 1999, 2009; Folguera et al., 2015; Horton, 2018a). Although the Andean forearc and magmatic arc experienced strike-slip deformation, the retroarc sector has been controlled by principally trench-perpendicular compressional stresses that induced regional horizontal shortening, crustal thickening, growth of a large crustal root, construction of isostatically compensated topography, and cratonward advance of an evolving flexural load and foreland basin (James, 1971; Isacks, 1988; Beck and Zandt, 2002; DeCelles and Horton, 2003).

Cretaceous-Cenozoic shortening has been accommodated by diverse structural elements that affect Phanerozoic cover strata, igneous rocks of mostly Paleozoic–Mesozoic age, and mechanical basement of Precambrian and locally Paleozoic age (including crystalline igneous and metamorphic rocks with minor amounts of metasedimentary and metavolcanic rocks). Fundamental to the Andean retroarc zone are integrated ramp-flat fold-thrust systems characterized by geometrically and kinematically linked fold and generally low-angle thrust structures developed in supracrustal rocks above regional décollements (Schmitz, 1994; Schmitz and Kley, 1997; Baby et al., 1997; Kley et al., 1999; McQuarrie, 2002a). These thin-skinned structures contrast with isolated high-angle reverse faults that penetrate to deeper crustal levels and potentially accommodate greater degrees of oblique transpressional deformation (Jordan and Allmendinger, 1986; Ramos et al., 2002). Within the orogen, newly formed normal faults are relatively uncommon and show limited displacement, but have played an important role in Andean evolution as signs of orogenic collapse (Folguera et al., 2009; Giovanni et al., 2010; Ramos et al., 2014) or diminished mechanical coupling between the subducting and overriding plates (Ramos, 2010; Mpodozis and Cornejo, 2012; Horton and Fuentes, 2016; Horton, 2018b).

Following influential studies of the Appalachians, North American Cordillera, and Laramide province (Rodgers, 1971; Coney, 1976; Dickinson and Snyder, 1978; Erslev, 1993), structural styles in contractional orogenic belts are routinely classified into thin-skinned fold-thrust systems restricted to cover strata versus thick-skinned basement-involved deformational provinces (Jordan et al., 1983; Rodgers, 1987; Pfiffner, 2017). Some studies suggest that binary categorization into these two modes fails to represent the range of structural styles in the Andean fold-thrust belt and foreland basin (e.g., Allmendinger and Gubbels, 1996; Kley et al., 1999; Ramos et al., 2002, 2004; Giambiagi et al., 2012). Specifically, most thrust systems with regional décollements ultimately involve crystalline basement rocks at depth in trailing hinterland segments, thus precluding strict discrimination of thin-skinned versus thick-skinned systems purely on the basis of basement involvement. Nevertheless, careful documentation of basement involvement and contrasts between thin- and thick-skinned systems provides insights into the structural evolution of major orogenic systems (e.g., Frizon de Lamotte et al., 2000; Meng and Zhang, 2000; Molinaro et al., 2005; Butler et al., 2006; Yonkee and Weil, 2015; Muñoz, 2019). In addition to the classification of thin-versus thick-skinned geometries, a more encompassing scheme could recognize that many attributes intrinsic to the orogenic system—so-called inherited elements—operate to varying degrees to guide the geometric and kinematic evolution of contractional orogenic systems.

Here we explore different factors that influence the structural styles of the Andean retroarc fold-thrust belt and foreland basin. Employing the concept of tectonic inheritance, we highlight the roles of: (a) preexisting structural features such as faults and basement fabrics prone to subsequent strain localization; (b) inherited stratigraphic architecture and lithologic units; (c) crustal/lithospheric rheology; and (d) thermal framework, including thermal perturbations. Our motivation is to expand the discussion beyond purely structural inheritance and build a broader appreciation for these different controls while recognizing their potential temporal and spatial overlap during the evolution of the Andean thrust belt and foreland basin.

Although we seek to promote a wider perspective on inheritance and structural styles, several issues are beyond the scope of this presentation. We do not discuss estimates of the precise onset and rates of Andean orogenesis, which span from Cretaceous to late Cenozoic and have been presented elsewhere (e.g., Ramos, 1999, 2009; DeCelles and Horton, 2003; McQuarrie et al., 2005; Oncken et al., 2006; Horton, 2018a, 2018b). We also do not review the many calculations of total orogenic shortening from crustal-scale balanced cross sections, which show distinct variations along strike (e.g., Baby et al., 1997; Kley and Monaldi, 1998; Kley et al., 1999; McQuarrie, 2002a; Giambiagi et al., 2012, 2015). Rather than assessing such regional kinematic variations and their implications for surface topography, we focus on the theme of tectonic inheritance and underscore key templates or structural analogs that will help researchers evaluate the potential controls on Andean structural styles for specific targets of investigation.

Figure 1.1  Tectonic map and DEM of western South America showing plate boundaries, Andean magmatic arc, zones of flat slab subduction, the topographic front of the Andean fold-thrust belt (black line), the foreland deformation front (red line), modern stress orientations from earthquake focal mechanisms, and selected segments of the retroarc foreland basin system. Modified from Ramos and Folguera (2009) and Hortan (2018a,b).

Tectonic framework

Modern configuration

The modern plate tectonic configuration of western South America (Fig. 1.1) involves an ocean-continent convergent plate boundary with a subduction zone defined by the east-dipping oceanic Nazca slab. The trench-parallel Andean magmatic arc is composed of four discontinuous segments (the northern, central, southern, and austral volcanic zones) divided by separate regions of late Cenozoic flat-slab subduction or slab-window generation, including the Colombian/Bucuramanga flat slab (2–8°N), Peruvian flat slab (5–15°S), Chilean/Pampean flat slab (27–33°S), and the Patagonian slab window adjacent to the Nazca-Antarctica spreading ridge (45–48°S) (Barazangi and Isacks, 1976; Jordan et al., 1983; Ramos, 1999; Breitsprecher and Thorkelson, 2008; Ramos and Folguera, 2009; Wagner et al., 2017). The Andes Mountains form a continuous topographic barrier that is the product of retroarc crustal thickening driven by horizontal shortening in a compressional tectonic regime. This shortening has been accommodated in the retroarc fold-thrust belt by principally dip-slip contractional structures, as reflected in earthquake focal mechanisms and modern stress measurements (e.g., Assumpção et al., 2016). Many thrust-belt structures are geometrically and kinematically linked to a middle to upper crustal décollement that is considered to underlie the entire Andean orogenic belt (Baby et al., 1997; McQuarrie, 2002a; Giambiagi et al., 2012, 2015).

The Andean topographic front marks the sharp boundary between the fold-thrust belt and the low plains of the foreland basin system, in which regional isostatic (flexural) subsidence has generated accommodation space for large volumes of clastic sediment eroded from the Andean orogen (Horton and DeCelles, 1997; Mora et al., 2010a). Additionally, isolated topographic highs within the broad foreland lowlands, although disconnected from Andean topography, represent the easternmost expression of crustal shortening (i.e., the foreland deformation front), such as in the western Amazonian foreland of Peru (5–15°S) and the Sierras Pampeanas of central Argentina (27–33°S). In many regions, Andean structural and stratigraphic records indicate a long-term cratonward advance of arc magmatism, fold-thrust deformation, and foreland basin subsidence (Rutland, 1971; Isacks, 1988; DeCelles and Horton, 2003; Gómez et al., 2005; Haschke et al., 2006; Carrapa and DeCelles, 2008; Bayona et al., 2008; Folguera and Ramos, 2011; Horton et al., 2020; Capaldi et al., 2020, 2021). This systematic pattern, however, is not well developed in other regions (particularly within narrow segments of the orogen such as the northern Andes of Ecuador and southern Colombia at 2°N–5°S and the southern Andes of Chile and Argentina at 34–44°S and 50–55°S) where nonuniform and unsteady processes have been promoted by changes in slab dynamics, fluctuations in tectonic regime, or variations imparted by tectonic inheritance (e.g., Mpodozis and Ramos, 1990; Cooper et al., 1995; Colletta et al., 1997; Charrier et al., 2002; Parra et al., 2012; McGroder et al., 2015; Folguera et al., 2015). Further complexities are introduced by spatially variable climatic conditions that can set up internal feedbacks between erosion and deformation style that vary along the length of the ∼8000km long orogenic belt (Masek et al., 1994; Horton, 1999; Montgomery et al., 2001; Strecker et al., 2007; McQuarrie et al., 2008; Mora et al., 2008; Ghiglione et al., 2019).

Orogenic history

Mesozoic–Cenozoic subduction along the western edge of South American generated a long-lived continental magmatic arc and retroarc region with diverse structures and basin systems. A fundamental shift at ∼100Ma from extensional or neutral tectonic regimes to chiefly retroarc shortening followed ∼130–120Ma opening of the south Atlantic Ocean and westward advance of the South American plate away from the African plate (Coney and Evenchick, 1994; Horton, 2018a; Gianni et al., 2018). After this stage of Gondwana breakup, plate convergence involved westward absolute motion of South American above subducted east-dipping oceanic lithosphere, with limited accretion of oceanic materials along the northernmost and southernmost Andean margin (Dalziel, 1986; Megard, 1989).

Construction of the Andes is attributed to Late Cretaceous-Cenozoic retroarc crustal shortening in which the inboard (eastward) advance of the deformation front demonstrates the time-transgressive history of crustal thickening and associated flexural foreland basin development. Nazca-South American plate convergence was generally orthogonal to the roughly north-trending plate margin (Maloney et al., 2013; Muller et al., 2016). Oblique components of plate convergence have been largely accommodated in the arc and forearc regions (e.g., Fitch, 1972), with most structures in the retroarc fold-thrust belt recording plane-strain conditions marked by thrust/reverse focal mechanisms with minimal strike-slip displacement (Fig. 1.1). Similarly, extension has been confined to either the highest segments of the orogenic belt, where gravitational collapse is possible (e.g., Dalmayrac and Molnar, 1981), or the thrust belt hinterland to forearc region where diminished mechanical coupling along the subduction interface may allow extension during slab rollback (Meade and Conrad, 2008; Martinod et al., 2010).

Time-space variability

Despite the shared Mesozoic–Cenozoic plate tectonic history of roughly orthogonal convergence and noncollisional processes along the length of western South America, Andean orogenesis has involved substantial variability in tectonic regime, deformation magnitude, and basin evolution. Late Cretaceous-Cenozoic variations in tectonic regime include alternations between sustained regional compression and shorter episodes of minor extension or neutral stress conditions (Ramos, 2010; Horton, 2018b). Additional temporal variations may be manifest as discrete episodes of accelerated or decelerated Andean deformation and foreland subsidence (e.g., Vergani et al., 1995; Jaillard et al., 2000; Gómez et al., 2005; Oncken et al., 2006; Bayona et al., 2008; Parra et al., 2009; Buford Parks and McQuarrie, 2019; Echaurren et al., 2019). Spatial variations are readily apparent in the modern Andean orogen (Fig. 1.1) and include along-strike fluctuations in topography (height and width of the orogen), crustal thickness, shortening magnitude, and arc magmatism.

Several spatial variations in the Andes correlate with the current plate tectonic configuration. Along the length of the plate margin, the modern distances from the trench to (A) the Andean topographic front (the thrust belt-foreland basin boundary) and (B) the foreland deformation front (easternmost surface or subsurface expression of Cenozoic shortening) are highly variable along strike (Fig. 1.2) (Horton et al., 2022). The trench-normal distances span from 300 to 800km, with the most pronounced inboard position of the topographic and deformation fronts recognized in four separate provinces. The maximum inboard advance of topography and deformation is defined in the central Andes, where the Andean orogen attains its greatest width, crustal thickness, total horizontal shortening, and mean elevation. The other three provinces overlap with the Peruvian flat slab (5–15°S), the Chilean/Pampean flat slab (27–33°S), and the Patagonian slab window where the Nazca-Antarctica spreading ridge (Chile Ridge) intersects the Andean margin (45–48°S).

Rationale: tectonic inheritance

We propose that structural styles in the Andes and many contractional orogenic belts may be considered in the broad context of tectonic inheritance (Fig. 1.3). This rationale highlights the influence of four parameters: (1) preexisting structures, (2) precursor stratigraphic architecture, (3) rheological and strength properties, and (4) crustal/lithospheric thermal configuration. These categories are not mutually exclusive, but are specified here in order to appreciate the range of potential explanations for differences in structural style. In this rationale, the concept of inheritance need not be limited strictly to cases of structural inheritance that involve fault reactivation and inversion of former sedimentary basins.

We suggest that the four types of inheritance presented here may apply at different spatial and temporal scales. This approach recognizes contrasting processes in (a) upper versus lower crustal regions, (b) supracrustal cover strata versus mechanical basement, (c) more felsic (weaker) versus more mafic (stronger) composition, and (d) early orogenic (thin crust) versus late orogenic (thick crust) conditions.

Structural inheritance

Structural inheritance represents the influence of preexisting faults or basement fabrics on structural style (Fig. 1.3A). Structural inheritance is readily recognized in cases of fault reactivation and basin inversion in upper crustal levels (e.g., McClay, 1995; Lowell, 1995; Cooper and Warren, 2010; Bonini et al., 2012). Many Andean faults are spatially colocated or closely parallel to preexisting faults and basement fabrics. A frequent Andean situation involves the reactivation of Mesozoic extensional faults as thrust or reverse faults during Cenozoic shortening. In such cases, a pre-Andean structural low delineated by a hangingwall extensional basin becomes later elevated during Andean shortening, inverting the original basin. Basin inversion is prevalent in surface and subsurface examples throughout the Andes, particularly in the eastern foothills of Colombia (2–7°N; Cooper et al., 1995; Mora et al., 2009; Tesón et al., 2013; Teixell et al., 2015; Mora et al., 2020; Costantino et al., 2021), the foreland provinces of Ecuador and Peru (0–13°S; Balkwill et al., 1995; Baby et al., 2013; McGroder et al., 2015; Zamora and Gil, 2018; McClay et al., 2018), the Salta rift system of northernmost Argentina (22–27°S; Grier et al., 1991; Comínguez and Ramos, 1995; Kley and Monaldi, 2002; Kley et al., 2005; Carrera et al., 2006; Monaldi et al., 2008); and the Neuquén Basin of central Argentina (34–40°S; Manceda and Figueroa, 1995; Mosquera and Ramos, 2006; Giambiagi et al., 2008, 2022; Mescua et al., 2014; Fuentes et al., 2016).

Contractional inversion of an ancestral extensional basin can be recognized by: a sharp contrast between thick hangingwall stratigraphic units and thinner correlative footwall strata; the presence of fault-proximal facies (including coarse-grained alluvial fan or fan-delta deposits) preferentially developed in hangingwall areas adjacent to the original normal fault; and the spatial coincidence, intersection, or overlap of older normal and younger thrust/reverse faults. Unequivocal demonstration of fault reactivation can be challenging, in that newly formed thrust faults may also nucleate within the extensional basin in close proximity but not precisely along original normal faults.

Figure 1.2  A comparative plot showing along-strike (latitudinal) variations in Andean orogenic width and retroarc shortening. Orogenic width is defined as the trench-normal (cross-strike) distance between the South American subduction trench and the Andean deformation front, with delineation of three separate provinces of basement-involved foreland deformation: (1) Peruvian foreland (5–15°S), (2) Sierras Pampeanas foreland (27–33°S), and (3) northern Patagonian foreland (45–48°S). From Horton et al. (2022).

Characteristic structures accompanying basin inversion (Fig. 1.3A) include arrowhead or harpoon thrust-fold geometries defined by reactivated faults with synextensional stratal wedge (half-graben) geometries preserved in hangingwall anticlines; and buttress-related folds formed in hanging walls where steeply dipping fault segments and/or rigid footwall basement rocks impede slip along the reactivated fault surface (Badley et al., 1989; McClay, 1995; Carrera et al., 2006; Granado and Ruh, 2019; Martínez et al., 2021). Listric normal faults are prone to partial reactivation with shortening-induced displacement focused along the gently dipping, deeper segments of the fault rather than the steeper shallow segments. In such cases, deformation at shallower levels may be manifest in newly formed footwall splay or shortcut faults geometrically linked to the deeper reactivated master fault (Mora and Parra, 2008; Amilibia et al., 2008; Carrera and Muñoz, 2013; Tesón et al., 2013; Fuentes et al., 2016; Mora et al., 2020). Several examples of fault reactivation and basin inversion are presented in this volume (e.g., Zamora and Carter, 2022; Martínez and Fuentes, 2022; Giambiagi et al., 2022; Cortés et al., 2022).

Figure 1.3  A series of cross sections depicting four modes of tectonic inheritance. (A) Structural inheritance, in which shortening involves (1) reactivation of a preexisting normal fault with accompanying basin inversion, and (2) fault nucleation along igneous and/or metamorphic basement fabrics. (B) Stratigraphic inheritance represented by (1) upper-crustal shortening accommodated by interconnected ramp-flat (décollement-style) thin-skinned structures involving cratonward-tapering cover strata versus (2) basement-involved thick-skinned shortening along isolated fault structures that penetrate to deeper crustal levels. (C) Rheological inheritance, in which variations in yield strength profiles (as well as mineral/rock composition, fluid content/pressure, and resulting heterogeneities and anisotropies) promote (1) a weak mid-crustal décollement with associated thin-skinned fold-thrust belt versus (2) a uniformly strong profile with thick-skinned crustal-scale basement faults. (D) Thermal inheritance, showing the thermal structure along an ocean-continent convergent margin involving (1) a steep subduction zone with a warm, weak overriding plate undergoing high-magnitude horizontal shortening in a retroarc fold-thrust belt and (2) a flat slab subduction zone involving a cool, rigid overriding plate in which crustal-scale contractional structures develop toward the craton within the plate interior.

Andean structural reactivation is not restricted to Mesozoic normal faults, as inherited Paleozoic and Precambrian structures have also been reactivated (e.g., Carrera and Muñoz, 2013; Giambiagi et al., 2014; Perez et al., 2016a). A less-common illustration of structural inheritance involves strain localization of Andean structures along metamorphic and/or igneous fabrics (Fig. 1.3A). These fabrics constitute preexisting planes of weakness (such as foliation, schistosity, or cleavage planes) that are preferentially activated during regional shortening. Most examples of this process involve pre-Andean weaknesses within crystalline basement or intrusive belts (González Bonorino, 1950; Mosquera and Ramos, 2006; Hongn et al., 2010; Giambiagi et al., 2011), but Paleozoic foliation or cleavage fabrics within sedimentary rocks also have been inferred to guide later Andean structures (Laubacher, 1978; Dalmayrac et al., 1980; Martinez, 1980).

An important additional form of structural inheritance involves large-scale Andean reactivation of regional mid- to upper-crustal extensional detachments. For example, the basement detachment and affiliated normal faults within the ∼250km wide Cretaceous Salta rift system were reactivated during Cenozoic shortening, creating a series of parallel ranges within the Puna and Santa Barbara regions of northernmost Argentina (22–27°S; Grier et al., 1991; Kley and Monaldi, 2002; Pearson et al., 2013). The regional integration and geometric linkage of these structures rule out oversimplified interpretations of these features as isolated, independent basement uplifts analogous to crustal-scale structures of the Sierras Pampeanas province (27–33°S) (e.g., Montero-López et al., 2018).

Stratigraphic inheritance

Stratigraphic inheritance refers to the impact of predeformational stratigraphic architecture on structural style (Fig. 1.3B). In contractional systems, a spatial correspondence has been observed between the inherited stratigraphic framework and the existence and geometry of regional décollements and interconnected ramp-flat structures developed in preexisting supracrustal cover strata (Lageson and Schmitt, 1994; Lawton et al., 1994; Boyer, 1995; Mitra, 1997; Espurt et al., 2008; Yonkee and Weil, 2015; Parker and Pearson, 2021). Specifically, an asymmetric foreland-tapering stratigraphic prism that is thicker in the hinterland and thinner in the foreland routinely correlates with a deformational style involving a master regional décollement beneath a fold-thrust belt commonly characterized by a leading imbricate fan and trailing zone of duplex structures (Fig. 1.3B). For such stratigraphic wedges, the principal décollement is situated at or near the basement-cover interface, with potential additional detachment horizons present at higher structural levels within the cover succession. In nearly all Andean cases, the main décollement is linked to a major footwall ramp in the trailing part of the fold-thrust belt that penetrates into the underlying basement, although these basement rocks may remain at depth with no surface exposure (e.g., Kley, 1996; Allmendinger and Zapata, 2000).

Two key examples include foreland-directed (east-vergent) structures of the Interandean and Subandean fold-thrust belt in the central Andes of southern Peru, Bolivia, and northernmost Argentina (13–23°S; Mingramm et al., 1979; Baby et al., 1995, 1997; Dunn et al., 1995; Kley, 1996; Moretti et al., 1996; Echavarria et al., 2003; Anderson et al., 2017, 2018; Fuentes et al., 2018; Rojas Vera et al., 2019), and retroarc thrust structures in the Precordillera of the southern central Andes of west-central Argentina (29–33°S; von Gosen, 1992; Ramos et al., 1996; Cristallini and Ramos, 2000; Allmendinger and Judge, 2014). These zones contain hinterland-thickening Paleozoic stratigraphic packages in which a gently west-dipping décollement forms along inherited stratigraphic contacts within the Paleozoic succession and along the basal interface with Precambrian basement rocks.

In selected Andean regions, comparable geometries are expressed in Mesozoic strata capping mechanical basement of late Paleozoic age. Such cases include an array of imbricate fan, duplex, and backthrust structures, as recognized in the Tierra del Fuego region of the southernmost Andes (53–56°S; Alvarez-Marrón et al., 1993; Kraemer, 2003) and in the hinterland Aconcagua and La Ramada segments of the southern central Andes (Cegarra and Ramos, 1996; Giambiagi et al.,