Along-strike segmentation of the Andean foreland: causes and consequences

ELSEVIER

Tectonophysics 301 (1999) 75–94

Along-strike segmentation of the Andean foreland: causes and consequences J. Kley a,Ł , C.R. Monaldi b , J.A. Salfity b a

Geologisches Institut, Universita¨t Karlsruhe, P.O. Box 6980, D-76128 Karlsruhe, Germany b Universidad Nacional de Salta-CONICET, Buenos Aires 177, 4400 Salta, Argentina Received 6 February 1998; accepted 14 September 1998

Abstract Thrust belts of Tertiary age characterize the east flank of the Andes and part of the adjacent foreland from Venezuela to the southern tip of South America. This foreland deformation comprises three basically different structural styles: (1) thin-skinned thrust belts detached within the sedimentary cover, (2) thick-skinned thrust belts with an inferred basal detachment in the basement, at 10–20 km depth and (3) foreland basement thrusts which possibly affect the entire crust. Alternating foreland structural styles along the orogen, often with sharp boundaries, produce a segmentation of the Andean foreland. The changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the South American plate, for instance: thin-skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic=crystalline basement unaffected by Mesozoic extension. In areas of Mesozoic rifting, normal faults have been reactivated during Tertiary contraction, resulting in the formation of thick-skinned thrust belts. The basement becomes involved even if potential de´collement levels are present. Deep-seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover. The three basic structural styles are unequally efficient in terms of shortening: Thin-skinned belts are typically shortened by 40–70%, thick-skinned belts by 20–35%, and areas of foreland thrusts by less than 10%. The structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening. Available estimates of shortening along the Central Andes actually suggest that shortening does not vary steadily, but has several maxima and minima. Strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments (‘flat slabs’), but closely with lithospheric structure: The most strongly shortened segment coincides with thick mantle lithosphere, suggesting that shortening of the edge of the South American plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the Andes. This underthrusting of thick lithosphere may have caused steepening of a formerly ‘flat’ slab in the Arica bend region to the ‘normal’ 25–30º dip presently observed.  1999 Elsevier Science B.V. All rights reserved. Keywords: Andes; forelands; fold and thrust belts; thin-skinned tectonics; basement tectonics; reactivation

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author. Tel.: C49-721-608-2941; Fax: C49-721-608-2138; E-mail: [email protected]

0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 2 3 - 6

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1. Introduction Along most of their 7500 km length, the eastern flank of the Andean orogen and part of the adjacent foreland exhibit thrust structures of Tertiary (mostly Neogene) age. However, the structural styles of this foreland deformation vary markedly along strike. In most cases, the boundaries between regions of different structural style are well defined, producing a segmentation of the Andean foreland. This segmentation of the South American plate in part coincides with the segmented geometry of the subducted Nazca plate, which has ‘normally’ (25– 30º) and subhorizontally dipping segments (the ‘flat slab regions’; Barazangi and Isacks, 1976; Cahill and Isacks, 1992). This was interpreted as evidence for a direct influence of subducted plate geometry on the style of foreland deformation (Jordan et al., 1983; Pilger, 1981). However, the segmentation of Tertiary age within the South American plate also correlates with older stratigraphic and structural inhomogeneities (Allmendinger et al., 1983; Allmendinger and Gubbels, 1996), suggesting that these discontinuities, among other factors, exert a strong control on the dip of the subducted plate. Finally, varying degrees and different styles of foreland shortening were related to the thermal structure and mechanical properties of the South American lithosphere (Isacks, 1988; Watts et al., 1995). Tectonic shortening is now widely accepted as an important process in Andean orogeny (Sheffels, 1990; Schmitz, 1994). A large amount of this shortening is accommodated by the foreland thrust belts. The variations in the style and magnitude of foreland deformation can therefore be expected to have a significant effect on bulk shortening across the Andes and thus on the overall architecture and deep structure of the orogen. In this paper, we first describe the varying styles of foreland deformation along the entire orogen from western Venezuela to Tierra del Fuego and then try to evaluate the relative importance of the different controlling factors proposed.

We will also attempt to relate the history of deformation in the Central Andes to the evolution of crustal and lithospheric structure.

2. Structural styles of foreland deformation The term ‘foreland deformation’ will here be used collectively for all deformation occurring on the continentward slope of the mountain belt (the foreland fold-and-thrust belt) and within the foreland basin. The internal part of the thrust belt, where rocks deformed prior to the Tertiary orogeny are involved in thrusting, will be termed ‘central belt’. Three principal styles of foreland deformation can be distinguished in the Andes. (1) Thin-skinned fold-and-thrust belts with structures detached along a de´collement level within the sedimentary cover. Their map pattern is characterized by long, continuous thrust faults and anticlines at regular spacing. Shortening of the cover in the foreland belt must be balanced by extensive overthrusts of basement sheets in the central belt (Fig. 1a). Thin-skinned belts are strongly shortened, usually by 40–70% of their original width. (2) Thick-skinned thrust belts with a supposed de´collement in basement rocks at mid-crustal (10– 20 km) depths. Fold-thrust structures are relatively broad, short and often have irregular strike directions. Transverse structures are common. Some of these belts occur at the orogenic front in a position similar to thin-skinned belts (Fig. 1b), but others are situated far away from the mountain belt. Percent shortening in well-developed thick-skinned thrust belts is markedly less than in thin-skinned belts and typically ranges from 20–35%. (3) Laramide or Pampeanas-type basement thrusts which possibly cut the entire crust, as indicated by deep-reaching seismic activity (Jordan and Allmendinger, 1986) (Fig. 1c). These thrusts are widely spaced, with associated anticlines of very large wavelength. When occurring over broad areas, they tend

Fig. 1. Examples of the three basic structural styles of Andean foreland deformation in map view. (a) Thin-skinned thrust belt: Subandean Ranges of southern Bolivia and northern Argentina. (b) Thick-skinned thrust belt: Santa Barbara System, northern Argentina. (c) Foreland basement thrusts: Sierras Pampeanas, Argentina. Note different scales. Locations in Fig. 2a.

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to form irregular, anastomosing map patterns. Shortening in areas of basement thrusts typically does not exceed 10%.

3. Segments of the Andean foreland The distribution of the three basic structural styles of foreland deformation along the orogen is shown in Fig. 2a. No simple pattern is evident, but some principal observations can be made: Thin-skinned and thick-skinned thrust belts are mutually exclusive in a single segment of the foreland, alternating only along strike, but not across the strike of the foreland belt. In contrast, basement thrusts occur in a variety of settings, and commonly in conjunction with the other styles. Two of the three well-developed zones of basement thrusts occur on the foreland side of thin-skinned thrust belts (Peru and northern Argentina). Also, areas of thin-skinned thrusting may later become affected by basement thrusts as a result of the piggy-back propagation of basement-cover thrusts (e.g. Interandean Zone of southern Bolivia; Fig. 1a and Fig. 3). A switch from thin-skinned deformation to basement thrusts can even occur several times as deformation propagates cratonward. In some cases, foreland basement thrusts pass laterally into basement nappes of the central belt (Cutucu´ uplift, Ecuador=Peru, and Shira uplift, southern Peru) suggesting that the latter may evolve from foreland basement thrusts (cf. Hatcher and Hooper, 1992). Only rarely do deep-seated basement thrusts form the exclusive style of deformation (eastern Colombia and western Venezuela). Based on the occurrence, sometimes in combination, of the basic structural styles described above (Fig. 2a), the Andean foreland can be subdivided into ten segments, each exhibiting a more or less uniform structural configuration (Fig. 2b): (1) Me´rida: This segment includes the Sierra Nevada de Santa Marta, Sierra de Perija´ and the Me´rida Andes. Large basement thrusts underlying these ranges are the dominant style of deformation (Kellogg and Bonini, 1982; Laubscher, 1987), with no clear separation between a main Cordillera and foreland region. Range-parallel strike-slip faults are important in the Me´rida Andes (Schubert et al., 1992).

(2) Magdalena: Comprises the Magdalena valley and the Eastern Cordillera of Colombia. The Magdalena valley is underlain by a belt of backthrusts towards the Central Cordillera (Butler and Schamel, 1988), whereas the Eastern Cordillera is a thick-skinned thrust belt with local thin-skinned deformation (Dengo and Covey, 1993; Cooper et al., 1995; Roeder and Chamberlain, 1995). (3) Napo: Major foreland deformation is restricted to a single trend of broad, basement-cored anticlines [Napo and Cutucu´ uplifts; (Tschopp, 1953; Baldock, 1982)], which are overthrust in the west by rocks of the internal zone (Southern end of the Garzo´n Massif and Cordillera Real). (4) Huallaga: Comprises a broad, salt-related thinskinned fold belt and several basement thrusts which fringe its eastern edge and possibly underlie the thin-skinned structures in some places (Alema´n and Marksteiner, 1993; Mathalone and Montoya, 1995). (5) Madre de Dios: Similar to the Napo segment, the internal zone overthrusts a little deformed foreland with a narrow belt of thin-skinned(?) structures. In the SE, an almost unfaulted, foreland-dipping monocline underlies the mountain front (Ingeomin, 1975; Mathalone and Montoya, 1995). (6) Beni-Pilcomayo: Comprises the wide, welldeveloped thin-skinned belts of the Bolivian and northernmost Argentinian Subandean Ranges (Roeder, 1988; Baby et al., 1992, 1993; Dunn et al., 1995). Basement thrusts are interpreted to underlie internal parts of the foreland thrust belt (Kley, 1996). (7) Pampeanas: This segment is characterized by a complex array of thin-skinned thrust belts and basement thrusts. Thin-skinned belts include the Aconcagua thrust belt (Ramos, 1988) and most of the Argentine Precordillera (Jordan et al., 1993), whereas basement thrusts characterize the Frontal Cordillera (Ramos et al., 1996) and the eastern margin of the Puna plateau (Allmendinger, 1986). A large zone of basement thrusts occurs in the foreland (Sierras Pampeanas, (Jordan and Allmendinger, 1986). In the north, the Sierras Pampeanas give way to the thick-skinned thrust belt of the Santa Barbara System (Monaldi and Kley, 1997) and Eastern Cordillera (Grier et al., 1991). (8) Neuque´n: This segment has a thick-skinned thrust belt with localized thin-skinned deformation (Malargu¨e fold belt and its southern continuation

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Fig. 2. (a) Main structural and morphological units of the Andean foreland and adjacent central belts. Boxes show the locations of Fig. 1a–c. (b) Distribution of the different styles of foreland deformation along the Andes and the foreland segments defined in this article. Also shown are the locations of the cross-sections in Fig. 3 and the areas of near horizontal subduction (‘flat slabs’).

into the Neuque´n basin; Vin˜es, 1990; Manceda and Figueroa, 1995) bordering on the central belt. (9) Chubut: A narrow belt of moderately to intensely folded and imbricated sediments is overthrust by plutonic and metamorphic rocks of the central belt (Ramos and Corte´s, 1984). An important zone of strike-slip faulting runs parallel to the central

belt (Cembrano et al., 1996). Inversion of Mesozoic normal faults occurs in the foreland far to the east [San Bernardo fold belt (Homovc et al., 1995)]. (10) Magallanes: Comprises the well-developed thin-skinned fold-and-thrust belt that extends from 47ºS to the eastern tip of Tierra del Fuego (Ramos, 1989; Alvarez-Marro´n et al., 1993).

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4. Causes of style variations It is evident that segmentation of the foreland occurs at a smaller scale than segmentation of the subducted plate, implying that features within the South American plate should play a role in determining the foreland structural style. Many regional studies suggest that the stratigraphy and pre-Neogene tectonic history of individual areas exert an important control on the development of distinct structural styles. South of 30ºS, we have adopted the segmentation proposed by Mpodozis and Ramos (1989), who also pointed out that there is a close correlation between differences in the Jurassic to Cretaceous stratigraphic and tectonic evolution and the Neogene to Recent structure of the individual segments. Analysis of the setting in which each of the three basic structural styles tends to develop suggests the following relationships: Thin-skinned fold-and-thrust belts depend on the existence of a more or less conformable sedimentary cover at least some 3 km thick. However, a strong basement that can be underthrust integrally under the orogen appears to be equally important. In the Andes, the widespread Mesozoic extension structures are particularly prominent weak zones which prompt the basement to become involved in thrusting. This is most clearly seen where thin-skinned thrust belts merge laterally into thick-skinned belts. Such transitions occur at the Pilcomayo=Pampeanas and Pampeanas=Neuque´n segment boundaries. In

both cases, the basal detachment horizon of the thinskinned belt continues across the segment boundary or resumes after a short interruption (Fig. 4). Nonetheless, in the realm of Mesozoic normal faults the detachment level is used locally at best, while the first order structures are determined by thrust reactivation of normal faults (Ramos et al., 1996; Monaldi and Kley, 1997). Indeed, the data presently available suggest that all Andean thick-skinned thrust belts result from the inversion of Mesozoic rift basins [Eastern Cordillera of Colombia (Colletta et al., 1990); Santa Barbara System (Salfity et al., 1993) and Eastern Cordillera (Grier et al., 1991), Cuyo basin (Dellape´ and Hegedus, 1995), Malargu¨e fold belt (Manceda and Figueroa, 1995) and San Bernardo fold belt (Homovc et al., 1995); all Argentina]. However, it is interesting to note that Mesozoic extension also seems to have set the stage for several thinskinned thrust belts. This is the case where extension was concentrated in marginal basins, while a more uniform (flexural?) subsidence characterized the adjacent continent, allowing for platform sediments to accumulate. Upon inversion of the marginal basin, the platform sequences were telescoped and stacked to form thin-skinned thrust belts (Fig. 5a). Interestingly, these are the oldest thin-skinned belts of the Andean cycle. They include the Maran˜on thrust belt (Late Eocene; Me´gard, 1984) that evolved from the west Peruvian trough (Atherton et al., 1983), the Aconcagua thrust belt (Early to Middle Miocene; Ramos and Cortes, 1993) that evolved from a plat-

Fig. 3 (pp. 81–83). Structural cross-sections in the different foreland segments (note different scales). (a) Me´rida segment: The basement thrusts of the Me´rida Andes. Near-surface geology after Macellari (1982); depth to Moho after De Toni and Kellogg (1993). (b) Magdalena segment: Thick-skinned thrust belt of the Magdalena valley and Eastern Cordillera, Colombia. Slightly modified from Colletta et al. (1990). (c) Napo segment: basement thrusts underlying the Napo and Cutucu´ uplifts. Modified from Tschopp (1953). (d) Huallaga segment: Salt-related thin-skinned Santiago thrust belt. Slightly modified from Alema´n and Marksteiner (1993). (e) Madre de Dios segment: Basement thrusts (?) at the deformation front. Near-surface geology slightly modified from Se´brier et al. (1988). Deep structure and restored section by the authors. (f) Beni–Pilcomayo segment: Thin-skinned thrust belt of the Subandean Ranges, Bolivia. Subandean Ranges from Baby et al. (1992) and Dunn et al. (1995), slightly modified; Interandean Zone and Eastern Cordillera from Kley (1996). (g), (h) Pampeanas segment: (g) Thick-skinned thrust belt of the Santa Barbara System. (h) Thin-skinned thrust belt of western and central Precordillera; basement thrusts of eastern Precordillera and Sierras Pampeanas. Slightly modified from Allmendinger et al. (1990) and Zapata and Allmendinger (1996). (i) Neuque´n segment: Thick-skinned Malargu¨e thrust belt. From Manceda and Figueroa (1995). (j) Chubut segment: Poorly developed thin-skinned belt. Near-surface geology from Ramos and Corte´s (1984); deep structure and restored section by the authors. (k) Magallanes segment: Thin-skinned thrust belt; basement involved in transition to central belt. External zone from Alvarez-Marro´n et al. (1993), slightly modified. Near-surface geology of internal zone from Klepeis (1994); deep structure and restored section by the authors. Stratigraphic symbols: PE D Precambrian; Pz D Paleozoic; E D Cambrian; O D Ordovician; S D Silurian; D D Devonian; C D Carboniferous; Mz D Mesozoic; Tr D Triassic; J D Jurassic; K D Cretaceous; T D Tertiary; Pg D Paleogene; Ng D Neogene; Q D Quaternary.

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Fig. 3 (continued).

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Fig. 3 (continued).

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Fig. 4. (a) Pre-Late Cretaceous geology at the Pilcomayo=Pampeanas segment boundary. Note the broad basement arch underlying the ENE-trending branch of the Salta group rift basin and the coincidence of the Santa Barbara System (SBS) with an NNE-trending branch. (b) Cross-section through the map in (a). The southward termination of the thin-skinned Subandean thrust belt (SA) is caused by truncation of Silurian and Devonian strata against the basement arch. Although the potential detachment level in Silurian shales resumes farther south, reactivation of Cretaceous normal faults and thick-skinned deformation are favored. Data from Bianucci and Homovc (1982), Salfity (1982) and Di Persia et al. (1991).

form east of the Central Chile ‘aborted’ marginal basin (Mpodozis and Ramos, 1989), and the Magallanes thrust belt (Paleocene to Oligocene; Winslow, 1981) that formed adjacent to the Rocas Verdes marginal basin (Dalziel et al., 1974) (Fig. 6). In contrast, extensional faulting that affected wide areas in the continental interior promoted the formation of thick-skinned thrust belts (Figs. 5b, 6). The conditions for the development of foreland basement thrusts are more difficult to understand. The transition from the thick-skinned thrust belts of the Santa Barbara System and Eastern Cordillera to the basement thrusts of the Sierras Pampeanas

coincides with the border of the Cretaceous rift basin in which the thick-skinned belts developed (Gonza´lez and Mon, 1996). Basement thrusts tend to be associated with older structural highs of both local (few tens of km) to regional (several hundreds of km) extent. An example at local scale is the Shira uplift in Peru (Mathalone and Montoya, 1995), an example at regional scale is the region of the Sierras Pampeanas. The thin sedimentary cover associated with long-standing high areas and absence of pronounced Mesozoic extension structures may be reasons for basement thrusts to form. However, while this provides a possible explanation for

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Fig. 5. Different modes of continental extension produce different styles of foreland deformation upon later inversion. (a) Extension is concentrated in a narrow marginal basin flanked by subsiding but largely unfaulted platforms. Inversion results in thin-skinned thrusting of the platform sedimentary cover. (b) Extension affects a wide swath of the continental interior, creating a wide ‘continental embayment’. Inversion results in normal fault reactivation and creates a thick-skinned thrust belt. See Fig. 6 for actual examples.

the particular structural style of the areas affected by basement thrusts, it does not explain why those areas become involved in foreland deformation at all. There is a conspicuous link between basement thrusts and thin-skinned thrust belts: Two of the three major areas of basement thrusting in the Andes, in the Huallaga and Pampeanas segments, lie forelandward of thin-skinned thrust belts. Where both types of deformation are well dated, basement thrusts and thin-skinned thrusts can be seen to be partly coeval, with basement thrusts nucleating during the late stages of thin-skinned deformation (Strecker et al., 1989; Jordan et al., 1993). In both Peru and Argentina, the thrust front of the thin-skinned belts has propagated up to the external limit of their respective

de´collement levels (Mathalone and Montoya, 1995; Zapata and Allmendinger, 1996), causing an increase in shear strength at the base of the thrust wedge and probably a drop in strain rates. This ‘blocking’ of the thin-skinned thrust belts and an associated raise in the horizontal stress acting on the foreland crust may be another important factor in the formation of basement thrusts. The earlier proposed correlation of flat subduction with basement thrusts (Jordan et al., 1983; Pilger, 1981) is somewhat weakened if the entire orogen is considered. Although the major Peruvian and Argentinian examples do occur in the flat slab regions, minor basement thrusts persist north of the northern flat slab (Fig. 2b). More significantly, the spectacular

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Fig. 6. Correlation of marginal basins with thin-skinned thrust belts and of broad ‘continental embayments’ with thick-skinned thrust belts in southern South America. Shoreline gives maximum marine extension in Lower Jurassic to Lower Cretaceous interval [modified from Uliana and Biddle (1988)]. Central Chile basin from Mpodozis and Ramos (1989). The Mesozoic magmatic arc is not shown.

basement thrusts of the Me´rida segment overlie the slowly subducting Caribbean plate which presently dips at 25–30º (Malave´ and Sua´rez, 1995). However, these structures differ from their flat slab counterparts in some important respects. Basement thrusts are the dominant, if not exclusive, style of Andean deformation in the Me´rida segment, although the area has been affected by marked Mesozoic extension. The displacements on the individual thrusts are unusually large, and percent shortening is probably as high as 25% or more (Kellogg and Bonini, 1982; Laubscher, 1987; Dengo and Covey, 1993). Thus, despite the overall structural similarities with other Andean examples, the basement thrusts of the Me´rida segment might represent a different type of deformation. However, the strongest argument against a direct control on foreland structural style by

subducted plate geometry is the coincidence of segmentation in the lower plate with older stratigraphic and structural features of the South American plate. Unless this coincidence is purely accidental, we are forced to conclude that in some way the properties of the overriding plate have influenced the development of the flat slab segments.

5. Consequences of foreland segmentation In contrast to earlier held views, crustal thickening by thrust tectonics is now widely accepted as an important process in Andean orogeny. A substantial portion of the total shortening across the Andes is interpreted to have been accommodated by the foreland thrust belts (Roeder, 1988; Baby et al., 1996;

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Kley et al., 1997) and the different foreland structural styles accommodate quite different amounts of shortening. The style variations of foreland deformation should therefore have a marked effect on bulk shortening across the Andes. The potential effects of shortening variations on the entire subduction system were first discussed by Isacks (1988) who argued that the less shortened sectors of the South American plate will more rapidly override the subducted plate and hence flatten its dip. This hypothesis provides a mechanism how discontinuities of the overriding plate can cause segmentation of the subducted plate. The variations of shortening along the edge of the South American plate were ascribed by Isacks (1988) to a varying width of lithosphere thermally weakened by the underlying asthenospheric wedge. Numerical models by Wdowinski and Bock (1994) also suggest that the width of the orogen, and particularly of the high plateau of the Central Andes, is essentially a function of the width of a preexist-

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ing thermal anomaly. However, the close control of foreland structural style by pre-Tertiary stratigraphic and structural features discussed above indicates that varying foreland shortening mainly reflects such inherited inhomogeneities of the South American plate, as suggested for the bend region by Sheffels (1995). This is corroborated by the observation that not only the along-strike segmentation of the foreland coincides with older features within the South American plate, but also the cross-strike structural segmentation of the thrust belt: From central Peru to northern Argentina, the limit between the central belt and the foreland thrust belt follows the external (eastern to northeastern) edge of pre-Andean deformation, and is closely followed by the edge of the high plateau (Fig. 7). The rapid changes of foreland structural style and sharp segment boundaries along the Central Andes suggest that shortening might also vary in a rather discontinuous and unsystematic manner. Currently available shortening estimates from geologic data in

Fig. 7. The northeastern to eastern limit of terrains deformed in the Ordovician to Jurassic=Cretaceous interval closely coincides with the eastern edge of high topography [thick 3000 m contour line; from Isacks (1988)] illustrating the paleostructural and stratigraphic control on the architecture and topography of the modern Andes. The correlation breaks down south of the El Toro lineament, where Cretaceous rocks rest directly on metamorphic basement defrormed during an earlier orogenic cycle. Data from Ingeomin (1975), Pareja et al. (1978), Kennan (1993) and own observations.

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Fig. 8. Variations in the amount of foreland shortening along the Central Andes. The shortening values shown are not representative for the entire segments defined in Fig. 2, but refer to the transects indicated. Note that shortening does not decrease steadily from the maximum in the bend region, but has two additional submaxima in Central Peru and the Argentinian Precordillera. The flat slab regions show no close correlation with the magnitude of foreland shortening. Shortening estimates from Allmendinger (1986), Allmendinger et al. (1990), Grier et al. (1991), Baby et al. (1993), Manceda and Figueroa (1995), Zapata and Allmendinger (1996), Kley et al. (1997), Monaldi and Kley (1997). Additional estimates by the authors based on geology by Tschopp (1953), Koch (1962) and Se´brier et al. (1988).

fact indicate that the strongly shortened bend region is flanked on both sides by shortening minima which separate it from two lesser shortening peaks corresponding to the Peruvian Subandean Zone and the Argentinian Precordillera, respectively (Kley and

Monaldi, 1998; Fig. 8). The two minima correspond to the Madre de Dios segment and the northern end of the Pampeanas segment where the foreland thrust belt is poorly developed or missing (cf. Fig. 3e). The loss in foreland shortening may in part be compen-

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sated by increased shortening in the internal zones (Allmendinger and Gubbels, 1996; Kley, 1996), but a full compensation appears unlikely in the light of presently available structural data. The variations in foreland shortening should therefore correspond to similar, if more subdued, variations in bulk shortening across the orogen. The flat slab regions do not appear to coincide with the least shortened segments of the overriding plate but rather with shortening submaxima, suggesting that ‘flattening out’ by rigid upper plate segments cannot be the prime reason for the flat slabs to develop. On the other hand, the shortening variations closely correlate with changes in the lithospheric structure of the South American plate as derived from the attenuation of seismic waves (Whitman et al., 1992). According to these data, which cover the entire area of normal subduction and the adjacent transitions to the flat slab segments, most of the Beni–Pilcomayo segment is underlain by a lithospheric mantle about 200 km thick. The lithosphere is thin only under the active magmatic arc. Both at the northern and southern segment boundaries, however, the region of thin lithosphere widens towards the foreland (Fig. 9). These regions coincide with the areas of strongest shortening gradient in the foreland belt, suggesting that much of the thick lithosphere in the central part may in fact result from overthrusting of the Andes over the Brazilian shield (cf. the discussion in Whitman et al., 1996). It must be noted, however, that the western edge of thick lithosphere will not be changed in shape by crustal shortening. The present pattern of lithospheric thickness beneath the Andes therefore reflects the superposition of differential shortening on an older(?) lithospheric structure. The coincidence of thin lithosphere in the foreland with the area affected by Cretaceous rifting (Fig. 9) suggests that thin lithosphere may have been inherited from Mesozoic extension. The potential effect of 150 km of foreland shortening on the mantle lithosphere is demonstrated in Fig. 10 by taking the actual situation back in time to the moment before the onset of substantial overthrusting in the foreland belt (ca. 7 Ma). Crustal shortening is shown simplified as a single transcrustal thrust. Since there is no evidence for thrust duplication of the Moho in the Andes, we assume a detachment at the base of the crust. Foreland shortening there-

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Fig. 9. The rapid transitions from thick to thin lithosphere under the Central Andes as derived from seismic wave attenuation data correlate with the areas of strongest shortening gradient in the foreland belt, suggesting that thick cratonic lithosphere has underthrust the Andes from the east in the central segment. The coincidence of the eastward salient of thin lithosphere near 25ºS with the thick-skinned thrust belt of the Santa Barbara System (SBS) may indicate a contribution to thinned lithosphere by Mesozoic extension.

fore results in the trench approaching the edge of thick cratonic lithosphere, narrowing both the asthenospheric wedge and the magmatic arc. This is consistent with the magmatic history north of 24ºS, where major volcanism receded from a broad area that reached far to the east to the narrow trend of the actual volcanic chain in late Miocene time (McBride et al., 1983; Coira et al., 1993; Whitman et al., 1996; Allmendinger et al., 1997). The persistence of small mafic centers and He isotopic evidence for mantle melting at depth throughout the region were interpreted to indicate a thin lithospheric mantle right across the Altiplano (Hoke et al., 1994; Lamb et al., 1997), but are here tentatively attributed to asthenospheric material ‘trapped’ above the underthrusting cold lithosphere. Fig. 10 furthermore suggests that underthrusting of the mantle lithosphere may have caused steepening of the subducted plate. Prior to foreland shortening, the asthenospheric wedge would have been wide enough to accommodate a slab dipping as shallow as it dips today at the northern end of the flat slab region (28ºS). This implies that the

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Fig. 10. Potential effects of strong foreland shortening on the mantle lithosphere and the geometry of the subducted plate at 21ºS. (a) Accelerated plate convergence since 25 Ma causes gradual flattening of the subducted plate and establishment of a broad magmatic arc. The lithosphere thins by advection of heat. Moderate shortening of the upper plate (max. few tens of km) takes place within the growing orogen. (b) By about 7 Ma, the existence of a wide asthenospheric wedge is suggested by large ignimbrite eruptions far to the east of the trench. This wide asthenospheric wedge may have accommodated a slab dipping as shallow as in the flat slab segment at 28ºS today. (c) Present day situation (from Whitman et al., 1992, modified). Foreland shortening by 150 km has caused underthrusting of thick mantle lithosphere under the orogen. The ignimbrite centers have been shut off as the magmatic arc retreated with the narrowing asthenospheric wedge. The underthrusting lithosphere has forced the subducted plate to steepen again. Note that stages (a) and (b) were constructed by restoring 150 km of foreland shortening on the present day situation. For clarity, no changes to the mantle lithosphere besides those caused by underthrusting are shown between stages (b) and (c). It is not suggested that lithospheric mantle thickness beneath the Andes was actually zero from 7 Ma to the present.

subducted plate may initially have gone through a stage of general flattening, both in the central segment which dips steeper today and in the adjacent flat slab regions, probably caused by the acceleration of convergence between South America and the Nazca plate at about 25 Ma (Pardo-Casas and Molnar, 1987). During this stage, which lasted until about 7 Ma, a broad asthenospheric wedge developed by a combination of convective removal of the

lithosphere and, possibly, delamination. The onset of strong foreland shortening in late Miocene time and concomitant underthrusting of thick mantle lithosphere would then have forced the slab to steepen again in the Beni–Pilcomayo segment. In contrast, thinning of the lithosphere by convection and delamination (Kay and Abbruzzi, 1996) and flattening of the slab would have continued in the flat slab areas, where the foreland was little deformed.

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6. Discussion A control on foreland structural style by particular stratigraphic and structural settings is evident. However, it is more difficult to understand this control in terms of mechanical properties. We suggest that thinskinned thrust belts, thick-skinned thrust belts and foreland basement thrusts can essentially be viewed as different modes of basement deformation. Both the basement thrusts of the central belts bordering on thin-skinned thrust belts and foreland basement thrusts may nucleate in a similar way as very deep reaching (transcrustal?) fault zones. The importance of a thick sedimentary cover is to allow such faults to flatten out and to accumulate very large amounts of displacement which are accommodated ahead of the advancing basement thrust sheet by formation of a thin-skinned thrust belt. In contrast, in areas of thin sedimentary cover the individual faults directly reach the surface, resulting in rapid buildup of topographic relief at low fault displacements. This will counteract large movement on single faults and instead favor the nucleation of additional basement thrusts. The major thrusts of thick-skinned belts appear to be restricted to the upper crust from the beginning, probably due to reactivation of the extensional detachments of the rift zones in which they form or to higher heat flow in those areas, with faulting confined to the brittle upper crust. In thick-skinned belts, reactivation of the normal faults appears to require less work than the nucleation of a new, throughgoing de´collement, suggesting that the shear strength of the normal fault zones must have remained very low for several tens of million years. If variations of the sedimentary cover and of basement structure are the prime reason for the strong segmentation of Tertiary structure, the deformational style of the Andean foreland is essentially controlled by crustal features. Other authors have proposed that the mechanical properties of the entire lithosphere determined the changes from thin-skinned to basement-involved foreland deformation. Watts et al. (1995) calculated along strike changes in flexural rigidity from gravity data and suggested that flexurally strong lithosphere in the bend region favored thin-skinned thrusting, whereas basement-involved thrusting characterized the weaker lithosphere of the adjacent areas. Watts et al. (1995) and Whitman

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et al. (1996) argued that a thin and hot foreland lithosphere would shorten internally instead of being underthrust under the orogen, resulting in basementinvolved foreland structures. However, it is important to note that foreland basement thrusts frequently occur forelandward of thin-skinned thrust belts and that both deformation styles act simultaneously. It seems easier to interpret this in terms of stratigraphic variations than in terms of weak or strong lithosphere. Nevertheless, it is possible that long-standing differences in lithospheric properties of adjacent segments translate into stratigraphic differences that will ultimately control the structural style of the foreland.

7. Conclusions Foreland deformation along the Andes shows marked and rapid changes in structural style. Three basic structural styles can be observed: Thin-skinned thrust belts, thick-skinned thrust belts and foreland basement thrusts. Although the basement is involved in both thick-skinned belts and foreland basement thrusts, the two styles differ in depth of faulting and hence the wavelength and spacing of their structures. Thin-skinned thrust belts developed in regions of thick (>3 km) sedimentary cover outside the areas affected by strong backarc extension in Mesozoic time. In contrast, thick-skinned thrust belts formed within the Mesozoic extensional basins and show extensive thrust reactivation of normal faults. Foreland basement thrusts have more varied settings, but tend to occur on the external (foreland) side of thin-skinned thrust belts in areas of relatively thin sedimentary cover. The close control on foreland structural style by variations in the stratigraphy and earlier tectonic evolution of individual segments suggests that these inherited features also play a dominant role in determining the amount of Tertiary foreland shortening. From geological data it appears that foreland shortening along the Andes has several maxima corresponding to thin-skinned thrust belts separated by minima related to areas of poorly developed foreland belts. There is no close correlation between the variations in foreland shortening and the changes in dip of the subducted Nazca plate from ‘normal’ to subhorizontal. However, the region of strongest foreland shortening in the Central An-

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des closely correlates with thick mantle lithosphere under the orogen, suggesting that underthrusting of thick cratonic lithospheric mantle beneath the Andes has been important there. This underthrusting appears to be reflected in the magmatic evolution of the Altiplano plateau which is marked by a considerable narrowing of the magmatic arc since late Miocene time. Furthermore, thick lithosphere underthrust from the east may have led to steepening the slab dip to the presently observed 25–30º from a shallower position since late Miocene time.

Acknowledgements We thank H. Kocks for assistance in drawing the illustrations. Critical reviews by S. Lamb and V. Ramos helped us to improve the manuscript. J.K.’s research in Bolivia and Argentina was funded by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Collaborative Research Center 267 ‘Deformation Processes in the Andes’ and through projects Kl 495=1-1 to 1-3.

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