Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab

Journal of South American Earth Sciences 15 (2002) 39±57

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Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean ¯at-slab Suzanne Mahlburg Kay a,*, Constantino Mpodozis b a

b

Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY 14853, USA Servicio Nacional de GeologõÂa y MinerõÂa, Avenida Santa Maria 0104, C.C. Casilla, 10465 Santiago, Chile Received 1 November 2001; accepted 1 November 2001

Abstract Changes in the Neogene (Miocene to Recent) magmatic style of the modern Chilean ¯at-slab region and its margins (27±348S) re¯ect both variations in Nazca±South American convergence parameters and the effects of subduction of the Juan FernaÂndez Ridge hotspot track. These effects are superimposed on an initially thicker arc crust in a more compressional tectonic regime in the north than in the south. Distinctive magmatic periods across the region re¯ect relatively steep subduction from ,27 to 20 Ma, frontal arc migration associated with forearc subduction erosion from ,20 to 16 Ma, compressional deformation and andesitic volcanism from ,15 to 9 Ma, a peak of slab shallowing associated with frontal arc migration and arc broadening from ,8 to 4 Ma, and termination of volcanism over the ¯at-slab by ,5 Ma. Changes in magmatic style and distinctive chemical signatures in ,20 to 16 Ma volcanic rocks coincide with a marginwide transition from a more extensional to a more compressional tectonic regime linked to changing Nazca±South American convergence parameters. North to south differences in ,15 to 9 Ma magmas partially re¯ect the southward migration of the northeast-trending arm of the subducting Juan FernaÂndez Ridge, whose effect arrived in the ¯at-slab region at ,14 Ma. The termination of andesitic arc volcanism across most of the region at ,9 Ma and extreme shallowing of the slab beneath the central ¯at-slab coincides with the arrival and eastward propagation of the east± west-trending segment of the Juan FernaÂndez Ridge. Overall, the subduction of the Juan FernaÂndez Ridge is a perturbation that causes extreme shallowing in part of a regionally shallowing subduction zone. Chemical signatures of the erupted magmas re¯ect the changing mantle and crustal con®guration above the shallowing subduction zone. Those with adakitic-like signatures are derived from subduction zone ma®c magmas contaminated by garnet granulitic to eclogitic facies crust in the thickened crust or by crust incorporated into the mantle as a result of forearc subduction erosion. There is no evidence that any of these magmas were generated by melting of the subducted slab. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Neogene; Magma; Tectonic regime

1. Introduction The purpose of this paper is to use magmatism as a tool in deciphering the Neogene tectonic evolution and causes of the shallow subduction zone beneath the modern Andean Chilean ¯at-slab from 28 to 338S latitude. In recent years, it has been established that shallow subduction regimes such as this are not related to extreme convergence rates or subduction of very young ocean crust (Cloos, 1993). This is exempli®ed by Nazca±South American plate interactions in the Chilean ¯at-slab region, where the convergence rate is near 8.7 cm/year and the subducting oceanic crust is now Eocene in age. As elsewhere, shallow subduction seems favored by subduction of intermediate-age oceanic crust * Corresponding author. E-mail addresses: [email protected] (S. Mahlburg Kay), [email protected] (C. Mpodozis).

and rapid, but not extreme, convergence rates. Another often-quoted factor is the association of shallow subduction zones with the subduction of thickened oceanic crust, particularly seamounts or oceanic plateaus. This association was ®rst made for the Chilean ¯at-slab by Nur and Ben-Avraham (1981) and Pilger (1981, 1984), who postulated that slabshallowing was triggered by the subduction of the aseismic Juan FernaÂndez Ridge. More recently, Kirby et al. (1996), Gutscher et al. (2000a,b), and YanÄez et al. (2001), among others, have strongly advocated this viewpoint. The focus here is to analyze which aspects of the magmatic and tectonic history of the Chilean ¯at-slab can be attributed to subduction of the Juan FernaÂndez Ridge and which must be explained by other causes. The principal magmatic units of the modern Chilean ¯at-slab region in Chile and Argentina are analyzed with respect to their spatial, temporal, and chemical variation. The region discussed extends from 278S in the north to 348S in the

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Fig. 1. Map of Central Andes showing Chilean and Peruvian ¯at-slab segments relative to the Chile trench, contours to the Benioff Zone from Cahill and Isacks (1992), principal geographic regions, and Late Miocene to Recent volcanic centers (black dots). Figure modi®ed from Kay et al. (1999). Boxed area is region shown in Fig. 2.

south (Fig. 1). General transects to the south, on the southern margin, across the middle, and on the northern margin of the modern ¯at-slab region are examined (Fig. 2). Major magmatic units are summarized in Table 1. Understanding the sources and evolution of magmatism over shallow

subduction zones is particularly important, as a major effect of the shallowing process is to diminish the volume of the magma-producing mantle wedge over the shallowing slab. Changes in mantle thermal structure eventually cause volcanism to terminate over the shallowest parts of the slab. Geochemical signatures of evolving Neogene Chilean ¯atslab magmatic units provide clues to the mantle and crustal source regions associated with the shallowing subduction zone. The diagrams in Figs. 3±5 summarize key chemical characteristics for more than 1000 samples from Late Cenozoic plutonic and volcanic sequences across the ¯at-slab region (references in Fig. 3). The K2O±SiO2 plots in Fig. 3 provide a guide to rock types and differentiate the sequences into the low-, medium-, and high-K and shoshonitic arc series. The trace element ratio plots in Figs. 4 and 5 provide insights into magma-forming processes and source regions. Comparisons are optimized at similar silica contents with signatures of ma®c magmas being more re¯ective of mantle processes and those of silicic lavas re¯ective of crustal processes. Plots of light (La/Sm) versus heavy (Sm/Yb) REE ratios (Fig. 4) serve as a guide to pressure- and temperature-sensitive residual mineral assemblages at depth that equilibrated with the erupted magmas. Plots of La/Ta versus Ba/Ta ratios (Fig. 5) serve as a guide to subduction zone components in the magma source region. La/Ta ratios .25 are generally considered characteristic of arc mantle sources. Very high La/Ta ratios re¯ect differences in solubility of La (light REE element) and Ta in ¯uids, as well as oxidizing, hydrous environments that preferentially stabilize oxide phases that incorporate Ti-group elements (Ta, Nb). Ba is transported into the mantle wedge by subduction zone ¯uids. High Ba/Ta ratios are typical of magmas with a strong subduction zone signature.

Fig. 2. Map of ¯at-slab region showing locations of principal volcanic centers in the Chilean ¯at-slab relative to contours to the Benioff zone from Cahill and Isacks (1992), the Precordillera (gray with black stripes), Sierras Pampeanas (gray), and track of Juan FernaÂndez Ridge from YanÄez et al. (2001). Volcanic belts and centers color coded for age: Early Miocene 27±18 Ma (horizontal back strips), Middle to Late Miocene 16±9 Ma (gray and dotted), and Late Miocene±Pliocene 7±0 Ma (black). White squares are Early Miocene backarc basaltic ¯ows.

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Table 1 Summary of central Andean Chilean ¯at-slab and bordering regions magmatic events (27±348S latitude) Age (Ma)

El Teniente Region

, 4 to 0

SVZÐTupungato to Maipo forearc Cachapoal lavas 3±2 Late Hornblende Dikes

7±5

Teniente Porphyry Young plutons 6±7 HuincaÂn Group 7±5

Aconcagua/San Luis

Flat-slabÐRamadas/El Indio

Bonete, Pissis, southern CVZ Ojos del Salado Laguna Verde Ignimbrites 4±3

San Luis/El Morro ,2

San LuisÐ6

Maricunga/Bonete/Valle Ancho

Vallecito Group 6±5 Pocho Volcanic Group 7±5

Jotabeche/Pircas Negras Units C. Negro/Fandango/old Pissis Final Farallon Negro Famatina Mogotes Group 5

Late TamboÐ8 Precordillera centers Cerro Blanco

Copiapo dacite complex 8±7 Dos Hermanos/backarc 8 Farallon Negro Group/most 8±6 9 Ma Laguna Aparejos basalt

Aconcagua Center

Tambo Group Fm. 12.7±9 Ramadas/PiraÂmides 13±9 Precordillera centers

Copiapo dacite complex 11±10 Cadillal/Gold Porphyry 14±10 backarc ignimbrites

Teniente Magmatic Complex Maqui Chico/plutons 14.5±12

Old Aconcagua

C8 de Las ToÂrtolas Gm. 17±14 Calingasta Valley centers

Maricunga 17±14 Valle Ancho Ignimbrite

20±17

Gap

Farellones Fm. Manquehue Paramillos/Colorado 19±16

In®ernillo plutons Farellones Fm. near 328S ?

Virtual gap

26±21

La Obra pluton 20 Coya MachalõÂ/Abanico Fms.

Abanico Fm.

DonÄa Ana Group Escabroso Fm. 21±18 Tilito Fm. 27±23 MaÂquinas Basalt 22

La Coipa//Refugio Segerstrom Basalt 23

9±7

Teniente Magmatic Complex Upper Sewell Unit/plutons 9±7

13±9

Teniente Magmatic Complex Lower Sewell Unit/plutons ,10 HuincaÂn/Molle Groups 13±9

18±13

Fig. 3. Plot of K2O versus SiO2 for Neogene magmatic units over and on the margins of the modern Chilean ¯at-slab. Data from Kay et al. (1987, 1988, 1991, 1994, 1999), Kay and Gordillo (1994), Mpodozis et al. (1995, 1997), Cristallini et al. (1994), Ramos et al. (1989, 1996a), Kurtz et al. (1997), SelleÂs (1999a), Vatin-Perignon et al. (1996), Baldauf (1997), and Kay and Mpodozis (unpublished). See text for discussion.

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Fig. 4. Plot of La/Sm versus Sm/Yb for Neogene magmatic units over and on the margins of the modern Chilean ¯at-slab. Data sources as in Fig. 3. See text for discussion.

The diagrams also show that some Chilean ¯at-slab region magmatic units have the chemical characteristics of rocks popularly called adakites (SiO2 . 56%; Al2O3 . 15%; Sr . 400 ppm; Yb , 1.9 ppm; La/Yb , . 2; Sm/Yb . 3; see Drummond and Defant (1990)). Originally, this term was intended for magmas such as those from Adak, Alaska (Kay, 1978), which were interpreted as melts from the downgoing slab that subsequently were modi®ed in the overlying mantle. Using this framework, Gutscher et al. (2000a) argued that slab melting is also a major process over shallow subduction zones and reinterpret some Chilean ¯at-slab magmatic units as slabmelts. The problem with this model is that the adakite de®nition used by Drummond and Defant (1990) allows essentially any andesitic to dacitic magma equilibrated with a garnet-bearing mineral residue to be called an adakite, whether that garnet be in the downgoing slab, the thickened continental crust (Kay et al., 1987, 1991; Hildreth and Moorbath, 1988), or crust mechanically removed from beneath the arc or forearc margin (Stern, 1991; Kay and Mpodozis, 1999, 2000). As discussed later, Neogene Chilean ¯at-slab adakite-like signatures are best interpreted as related to residual garnet in the thickened

lower crust or, in forearc crust, recycled into the mantle by forearc subduction erosion.

2. Spatial and temporal characteristics of Neogene magmatic rocks over the Chilean ¯at-slab and its margins 2.1. Late Oligocene/early Miocene 27±21 Ma The ®rst period considered is the Late Oligocene to Early Miocene (,27 to 21 Ma). Magmatic units of this age were emplaced just after the change from slow, very oblique to fast, nearly normal Nazca±South American convergence that marks the beginning of the modern Andean cycle (Pardo-Casas and Molnar, 1987). They erupted along the entire length of the modern shallow subduction zone from north of 278S to south of 338S latitude (Figs. 2 and 6) with activity largely con®ned to the arc and near-backarc region. Calc-alkaline arc lavas in the north evolved in a thicker crust than tholeiitic arc lavas to the south (Kay et al., 1999). Ma®c backarc lavas erupted east of the Maricunga Belt (Kay et al.,

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Fig. 5. Plot of Ba/Ta versus La/Ta for Neogene magmatic units over and on the margins of the modern Chilean ¯at-slab. Typical arc magmatic rocks have La/ Ta ratios greater than 25 along with high Ba/Ta ratios. Data sources as in Fig. 3. See text for discussion.

1994), in the El Indio region (Kay et al., 1991), and in the El Teniente region (Kay et al., 1999) are best related to moderately transtensional tectonic conditions associated with oblique extensional basins. Arc magmatic rocks at the northern end of the ¯at-slab region in the Maricunga Belt (26±288S, Fig. 2) are generally concentrated in the Cerros Bravos, La Coipa, and Refugio region lava dome complexes. Most are medium- to high-K andesites to dacites (Fig. 3) with moderately sloping REE patterns (Fig. 4, La/Yb ˆ 7±20) and typical arc-like La/Ta ratios (25±50, Fig. 5) (Kay et al., 1994, 1999; Mpodozis et al., 1995). Backarc units to the east include the basaltic to ma®c andesitic Segerstrom lavas, which have transitional to backarc-like chemical signatures (La/Ta ˆ 20±25; Fig. 5; Kay et al., 1999). Farther south, from 288S to almost 328S, Early Miocene magmatism was dominated by dacitic to rhyodacitic ignimbrites capped by basaltic andesitic to ma®c andesitic lavas. Units near 298S include the Cantarito ignimbrites and the Cerro Pulido andesitic center (Kay et al., 1991). The best studied is the DonÄa Ana Group (Maksaev et al., 1984; Kay et al., 1987, 1991) near 308S where Martin et al. (1997) assigned age limits of 27±23 Ma to the dominantly silicic andesitic to rhyodacitic Tilito Formation and 21±18 Ma to

the basaltic to ma®c andesitic Escabroso Formation. A mild discordance between these units is consistent with a period of deformation near 20 Ma. Ongoing studies suggest a similar pattern could extend from 288S to at least the Aconcagua River valley near 328S (Rivano et al., 1990). These lavas are generally medium- to high-K calc-alkaline andesites to rhyodacites with arc-like La/Ta ratios (20±50, Fig. 5) and relatively ¯at REE patterns (most La/Yb , 15 and Sm/ Yb , 2.5, Fig. 4). The least arc-like (lowest La/Ta ratios) are the Tilito rhyodacitic units, which include a component derived from recycled continental crust (Kay et al., 1987, 1991). The trace element characteristics of DonÄa Ana Group magmas indicate equilibration with pyroxene and amphibole-bearing residual assemblages in normal to slightly thickened arc crust (Kay et al., 1991). Their general characteristics are similar to those of modern transitional Southern Volcanic Zone lavas from ,34 to 368S (Fig. 6), where crustal thicknesses are ,40 km and alkali basalts occur in the backarc (Hildreth and Moorbath 1988; Tormey et al., 1991; Fig. 2). The contemporaneous MaÂquinas basalts erupted in the backarc east of the El Indo belt have intraplate La/Ta ratios (15±18, Fig. 5) consistent with eruption in an extensional backarc environment over a steeply dipping subduction zone (Kay et al., 1991).

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Fig. 6. Spatial and temporal summary of principal Late Oligocene to Recent magmatic units on the Andean arc front from 26 to 378S showing times of frontal arc migration and general correlations of magmatic groups with similar chemistry. Chemical types are based on general correlations with magmas erupting in the modern southern, transitional, and northern Southern Volcanic Zone (Hildreth and Moorbath, 1988; Tormey et al., 1991), the southern Central Volcanic Zone (Mpodozis et al., 1996), and the Vallecito/Pircas Negras/Jotabeche group in the Maricunga Belt (Kay et al., 1999). Fig. 6 shows that major changes in magma types generally coincide with times of arc migration. Magmatic units and their chemistry are discussed in the text.

Early Miocene lavas south of 328S span a broad compositional range and tend to have tholeiitic differentiation trends. Those in the Los Andes/Santiago region in Chile, near 32.5±338S, are generally included in the Abanico Formation (Gana et al., 1999; SelleÂs, 1999a), whereas those east of the El Teniente region, near 33.5±34.58S, are described as the Coya MachalõÂ Formation (Charrier et al., 1996). Geochemical characteristics of the Abanico lavas near 338S are transitional between those of the DonÄa Ana Group and the low- to mid-K basaltic to dacitic (47±67% SiO2, Fig. 3) Coya MachalõÂ ¯ows, dikes, sills, and volcaniclastic units to the south. Among the Coya MachalõÂ units are the least potassic, most tholeiitic Neogene lavas with the ¯attest REE patterns (Sm/Yb ratios , 2, Fig. 4) in the studied region. Their arc-like character is indicated by high La/Ta (30± 60) and Ba/Ta (500±2000; Fig. 5) ratios (Kay et al., 1995, 1991). Their REE and other trace element characteristics are consistent with a low-pressure, pyroxene-dominated residual mineral assemblage (Kay et al., 1999) similar to that of modern southern Central Volcanic Zone lavas south of ,388S (Figs. 3±5) where crustal thicknesses are ,30 to 35 km (see Hildreth and Moorbath, 1988; Tormey et al., 1991; Fig. 6). Lavas in the eastern Coya MachalõÂ belt have lower La/Ta ratios and are generally analogous to Pleistocene/Holocene alkali basalts in the backarc south of ,388S. 2.2. Latest Early Miocene 20±16 Ma The latest Early Miocene was a time of transition from an

extensional or transtensional tectonic regime to a more compressional one. This period is marked by near-magmatic lulls in some regions and eruption of lavas with distinctive chemical characteristics in others. The period ended with the eastward migration of the magmatic front in the south and a general eastward broadening of the arc elsewhere. Kay et al. (1987, 1988, 1991, 1999) and Kay and Abbruzzi (1996) used the eastward broadening of the arc, the occurrence of high-angle thrusting in the Main Cordillera (Maksaev et al., 1984), the inception of deformation in the Precordillera near 18 Ma (Jordan et al., 1993), and the termination of ma®c backarc volcanism to argue for initial shallowing of the subduction zone beneath the central modern ¯at-slab by ,18 Ma. A summary of magmatism from north to south follows. Magmatic activity in the Maricunga Belt arc in the north was considerably reduced from 21 to 17 Ma compared with the earlier Early Miocene period. Only a few small ,21 to 17 Ma dacitic dome complexes associated with pyroclastic rings (near 25853 0 S) and isolated, dacitic ignimbrite remnants are known (Kay et al., 1994; Mpodozis et al., 1995). This near-magmatic lull ended with widespread volcanism that began with the eruption of hornblende-bearing andesitic lavas from the Jotabeche Norte center near 27.68S (hornblende Ar/Ar age of 18.3 ^ 0.7 in McKee et al. (1994); all other ages ,16.4 Ma) and the Ojos de Maricunga/Pastillos stratovolcanic cluster near 278S (see age summaries in Kay et al. (1994) and Mpodozis et al. (1995)). Some of these pre-15 Ma andesitic lavas are notable for their high La/Yb (near 26±35), Sm/Yb (5±6.2, Fig.

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Fig. 7. Series of maps showing temporal progression of magmatic units across the Chilean ¯at-slab and its margins relative to the temporal migration of the position of the Juan FernaÂndez Ridge, after YanÄez et al. (2001). Position of ridge is shown by bold black line; dashed lines show approximate extent of region expected to be in¯uenced by ridge subduction. Continental margin is shown for reference. Open circles are frontal arc centers; solid circles are backarc centers. See also Fig. 2.

4), and La/Ta (50±56, Fig. 5) ratios compared with older and younger Miocene lavas from the Maricunga Belt (Kay et al., 1994). Between the southern end of the Maricunga Belt and the Aconcagua region (,28 to 328S), the late Early Miocene is marked by eruption of ma®c andesitic ¯ows and the emplacement of shallow level intrusives (Maksaev et al., 1984; Rivano et al., 1985, 1990). Magmatic units between 29 and 318S include the ma®c andesitic ¯ows of the 21± 18 Ma Escabroso Formation (Martin et al., 1997) and the shallow-level andesitic intrusives of the 18±15 (?)Ma In®er-

nillo Unit (Maksaev et al., 1984; Kay et al., 1987). The mostly medium-K Escabroso ¯ows, with their relatively ¯at REE patterns (La/Yb ˆ 7±13, Sm/Yb ˆ 2±3, Fig. 4) and arc-like La/Ta ratios (29±53, Fig. 5), are typical of arc ma®c magmas evolved in a normal thickness crust. They are genetically associated with the more crustally contaminated dacitic to rhyolitic Tilito Formation (Kay et al., 1987, 1991). In contrast, the In®ernillo intrusives, with their lower La/Ta ratios (20±27, Fig. 5) and markedly higher K, U (4±6), and Th (6±16) concentrations, re¯ect a different source history (Kay et al., 1987, 1991). These still

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poorly understood andesites, like those in the modern Central Volcanic Zone Tinguiririca volcano near 34.98S (Hildreth and Moorbath, 1988), require a high K, U, and Th component that is dif®cult to explain by contamination of ma®c magmas in the overlying crust (Kay et al., 1987, 1991). Yet farther south near 338S, the Early Miocene is marked by an eastward displacement of the frontal arc from the Santiago to the Farellones region (Figs. 2, 6, and 7). Some magmatic units in this region have distinctive chemical features. Among these are the 20±18.8 Ma andesitic (Cerro Manquehue) intrusions near Santiago, analyzed by SelleÂs (1999a,b), that have distinctively higher Sr concentrations (550±775 ppm) and steeper REE patterns than the Early Miocene Abanico lavas that they intrude (La/ Yb ˆ 15±28 versus ,8, Sm/Yb ˆ 3±5.3 versus ,2.5; see Fig. 4). Chemical analyses by Vatin-Perignon et al. (1996) show the same pattern. SelleÂs (1999b) discusses the possibility that the higher pressure residual mineralogy implied by their REE patterns could be related to slab melting (Drummond and Defant, 1990), but such a model seems inconsistent with contemporaneous subduction of the relatively old and cold Paleogene oceanic crust (YanÄez et al., 2001). Farther east in Argentina, Latest Early Miocene ,16 to 15 Ma lavas in the Cerro Aconcagua region (Kay et al., 1991; Ramos et al., 1996a) are also notable for their steeper REE patterns (La/Yb ˆ 19±22, Sm/Yb ˆ 3.7±4.2) and higher Sr (to 840 ppm) contents compared with nearby Miocene units (11±10 Ma lavas from Cerro Aconcagua have La/Yb ˆ 10±13, Sm/Yb ˆ 2±2.3; Fig. 4). To the north, near 328S in the Cordillera del Limite on the Argentine/Chilean border (Fig. 2), andesitic lavas with distinctive, steep, heavy REE patterns (Sm/Yb ˆ 3.4±6.1, labeled `Limite' in Fig. 4) analyzed by Cristallini et al. (1994), Ramos et al. (1996a), and Kay and Rivano (unpublished) could also be of Early Miocene age. To the east, Early Miocene 20±16 Ma basaltic to dacitic volcanic units erupted in a broadened arc that extended into the western Precordillera (Figs. 2 and 7). Centers occur in the Cerro Colorado/Paramillos region in the south (18.9± 16.2 Ma; Kay et al., 1991); west of Barreal (20.1 ^ 2.5 Ma), possibly in the Cerro Ullun Complex in the central Precordillera (Leveratto, 1976); and at Tocota (18.3 ^ 2.5 Ma) and Cerro Negro (17 ^ 5 Ma) east of GualilaÂn in the north (Leveratto, 1976). Some of these magmatic units are marked by very high La/Ta (44±83; Fig. 5) and La/Sm ratios (3.2± 7.2; Fig. 4) and extreme Sr (850±1152 ppm) and Ba (670± 1700 ppm) concentrations (Kay et al., 1988, 1991). Their relatively ¯at, heavy REE patterns (Sm/Yb ˆ 2.5±3.0) do not require a high-pressure residual assemblage, in contrast with the suggestion of Carrasquero (1997) that they have a source component derived from melting of thickened ma®c continental crust or the downgoing slab. Farther south, along the main arc near 348S, a virtual magmatic lull occurred in the El Teniente region from

,19 to 16 Ma. Kurtz et al. (1997) argue for a period of uplift and crustal thickening associated with compressional deformation at this time. Their argument is based on Ar/Ar mineral ages for the La Obra pluton and chemical data (particularly REE data; see Fig. 4) that indicate that Early Miocene Coya Machalõ lavas equilibrated at lower pressures in a thinner crust than did post-16 Ma Teniente Volcanic Complex lavas (see also Kay et al., 1999). 2.3. Middle Miocene to early Late Miocene 15±9 Ma The Middle Miocene to early Late Miocene marks the peak of andesitic eruptions from stratovolcano and dome complexes along the length of the modern ¯at-slab and its margins. Volcanism at these centers occurred during a general time of compressional deformation, as evidenced by periods of crustal shortening in the Aconcagua (MalarguÈe) and La Ramada fold and thrust belts in the Main Cordillera, as well as the Precordillera fold and thrust belts in the foreland (Jordan et al., 1993; Ramos et al., 1996a,b; Godoy et al., 1999; Cristallini and Ramos, 2000). Crustal thickening beneath the arc linked to periods of crustal shortening is re¯ected in the chemistry of the arc magmas (Kay et al., 1987, 1991; Kay and Abbruzzi, 1996). North to south variations in magmatic timing, style, and chemistry re¯ect regional differences in crustal and lithospheric thicknesses. After the 20±17 Ma volcanic low, Maricunga Belt volcanism began again in earnest with the eruption of 16± 9 Ma stratavolcanic/dome complexes. These centers can be divided into two groups. The ®rst, with ages from 16 to 13 Ma, includes the Dona IneÂs volcano in the north, the Ojos de Maricunga/Pastillos centers in the central part, and the Cadillal complex in the south (Kay et al., 1994; Mpodozis et al., 1995). Lavas from these centers are typically mid- to high-K andesites with La/Yb and Sm/Yb ratios that are intermediate between those of pre-20 Ma and post11 Ma Maricunga Belt volcanic units (Fig. 4). In detail, the 14±10 Ma lavas have a smaller range of Sm/Yb (2.9±4.5) and La/Ta (25±45) ratios than do the 17±15 Ma lavas (Sm/ Yb ˆ 3.3±6.0, La/Ta ˆ 25±58). The second group includes the 11±9 Ma high-K silicic andesitic to dacitic units of the large Copiapo ignimbrite/dome complex and smaller centers near Nevado de Jotabeche. These units have generally steeper REE patterns (Sm/Yb ˆ 4.7±6.21; Fig. 4) and higher La/Ta (34±55; Fig. 5) ratios than the earlier group (Kay et al., 1994). Their eruption marks the beginning of the change in magmatic style to the ma®c lava ¯ows and silicic dome complexes that typify the Late Miocene units of the Maricunga Belt. To the east of the Maricunga Belt, backarc dacitic ignimbrites erupted in the western part of the Valle Ancho (Fig. 2) at ,15 Ma and, in the central part, at ,11 to 10 Ma. Their La/Ta ratios and REE patterns are similar to those of 15± 10 Ma Maricunga Belt lavas. This period of backarc volcanism ends with the eruption of ,9 Ma backarc basaltic

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andesites with strong arc signatures (La/Ta ,55; Fig. 5) just east of the Valle Ancho (Mpodozis et al., 1997; Kay and Mpodozis, 2000). Continuing south to 28±29.58S, Middle to Late Miocene frontal arc volcanism appears scarce, consistent with an ,18 to 7 Ma volcanic lull in this region (Mpodozis and Kay, unpublished data). Farther south in the El Indio region (29.5±318S), Middle Miocene volcanism is associated with the large Cerro de Las ToÂrtolas stratovolcano and the small SancarroÂn center. On the basis of extensive K/Ar dating in Chile, Martin et al. (1997) grouped the Middle to Late Miocene units in this region into a 17±14 Ma Cerro de las ToÂrtolas Formation dominated by lava ¯ows and a younger 12.7±10 Ma Tambo Formation dominated by intermediate to felsic tuffs. In general, the chemical signatures of these units overlap those of similar-age Maricunga Belt units (Figs. 3±6). Despite age uncertainties, at least some of the ,14 Ma ¯ows are chemically like the 14±10 Ma Cadillal ¯ows in the Maricunga Belt, whereas some of the ,12 Ma ¯ows (Sm/Yb ˆ 5±7.4; La/Ta ˆ 26±63; Figs. 4 and 5; Kay et al., 1991) are like the Copiapo dacites. Andesitic volcanism terminated in this region with the eruption of ,8 Ma ¯ows in Argentina that are chemically like those that erupted at ,14 Ma (Kay and Mpodozis, unpublished data). Farther east, minor backarc andesitic to dacitic units were emplaced in the Argentine Precordillera. These units include an ignimbrite/dome complex at Cerro Ullun (K/Ar biotite age of 10.2 ^ 0.8 Ma) and a dacitic porphyry (K/Ar biotite age of 11.0 ^ 0.5 Ma) east of Rodeo (see summaries in Leveratto, 1976; Kay et al., 1988; Kay and Abbruzzi, 1996). Continuing south, the 12.7±10.7 Ma La Ramada stratovolcano complex, the 11.4±9 Ma PiraÂmide region centers near 328S, and the 10.3±9 Ma Cerro Aconcagua stratovolcano complex near 338S erupted during this period (Kay et al., 1991; Cristallini et al., 1994; Ramos et al., 1996a). The eruptions of these centers essentially mark the end of arc volcanism in this region. Lavas from these centers are typically mid-K to low high-K hornblende-bearing andesites (Fig. 3) with arc-like La/Ta ratios (25±45; Fig. 5) and moderate heavy REE ratios (Sm/Yb ˆ 2±4; Fig. 4). Their ¯atter REE patterns relative to the contemporaneous Tambo volcanic units farther north are consistent with a lower pressure residual assemblage in a thinner crust in this region. Their eruption just after a ,14 Ma (Paranense) marine ingression that reached the easternmost part of the Cordillera at this latitude (PeÂrez and Ramos, 1996) also appears consistent with a relatively thinner crust at this time. South of 328S, extensive Middle Miocene andesitic centers also erupted east of the Early Miocene volcanic front. Lavas east of Santiago near the Argentine border are generally included in the Farellones Formation (see Beccar et al., 1986; Rivano et al., 1990). Those that erupted near 348S in the El Teniente region, some 30 km east of the Early Miocene Coya Machalõ arc front, are included in the 15±7 Ma Teniente Volcanic Complex (Godoy et al., 1999).

47

They can be divided into three subgroups (Table 1; Kurtz et al., 1997; Kay et al., 1995, 1999). The oldest 14.5±12.2 Ma Maqui Chico subgroup (50±75% SiO2; Fig. 3) is distinguished by a tholeiitic differentiation trend, ¯at heavy REE patterns (Sm/Yb ˆ 1.9±2.9 averaging 2.4; Fig. 4), high La/Sm ratios (up to 10 re¯ect residual titanite) in rhyodacitic units, and low La/Ta ratios (20±48 averaging 34; Fig. 5). Units in the ,11 to 9.5 Ma Lower Sewell and 9.3±7 Ma Upper Sewell subgroups show more calc-alkaline differentiation trends, higher La/Ta ratios (40±79 averaging 56), and steeper heavy REE patterns (Sm/Yb ˆ 2.1±3.1 averaging 2.9). This temporal trend is consistent with the increasing stability of residual hornblende in a progressively higher pressure, more hydrous magmatic environment in a thickening crust (Kay et al., 1999). Contemporaneous granodioritic plutons with ages from 15 to 8 Ma generally have overlapping chemical characteristics with the lavas (see Kurtz et al., 1997). Backarc magmas with ages from ,13.9 to 5.6 Ma (HuincaÂn group) also erupted in the MalarguÈe fold and thrust belt in Argentina east of the El Teniente region (Baldauf, 1997; Baldauf et al., 1997). Their higher K contents (Fig. 3) and lower La/Ta (Fig. 5) ratios relative to contemporaneous El Teniente region lavas are like those expected in backarc lavas, whereas their REE characteristics generally overlap those of the Maqui Chico and Lower Sewell units (Fig. 4). The waning of thrusting in the western backarc in association with initiation of backarc magmatism at ,14 Ma and eastward migration of the thrust front from ,14 to 10 Ma (Baldauf 1997; Baldauf et al., 1997) support linked backarc shortening and crustal thickening beneath the arc. Such a sequence is consistent with the evolution of successively younger Teniente Volcanic Complex units in a progressively thicker arc crust, as well as with the argument by Kurtz et al. (1997) that El Teniente region plutons were progressively emplaced at depth, as ascending magmas had increasing dif®culty traversing the thickening crust. 2.4. Late Miocene to Recent 9±0 Ma The Late Miocene marks the essential end of andesitic magmatism at ,9 Ma in the main arc of the ¯at-slab (28± 338S), with the last vestiges of volcanism occurring at isolated dacitic centers that expired at ,5 Ma (Maksaev et al., 1984; Kay et al., 1987). On the northern and southern margins of the ¯at-slab, the volcanic front migrated eastward between 7 and 3 Ma with ,50 km of migration between 33 and 34.58S (Kay et al., 1995; Kay, 1996) and ,50 km between 27 and 288S (Kay et al., 1999). Backarc magmatism also attained its greatest eastward extension in this period, with activity in the Pocho ®eld in the Sierras de CoÂrdoba lasting from ,7.9 to 4.5 Ma (Kay and Gordillo, 1994). By 2 Ma, volcanism had terminated across most of the ¯at-slab, with remaining activity occurring near the margins. The period is marked by chemically distinctive

48

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Fig. 8. Lithospheric scale cross-section across ¯at-slab region after Kay et al. (1999) showing principal processes affecting distribution and chemistry of arc magmatic units. See text for discussion.

magmatic rocks (Figs. 3±5) that erupted as the arc front migrated or shut off over the shallowing subduction zone. In the Maricunga region, Latest Miocene volcanism is represented by the small 8±7 Ma Copiapo stratocone, the 8±5 Ma rhyodacitic Jotabeche caldera complex, and the coeval Pircas Negras ma®c andesite ¯ows (Mpodozis et al., 1995). All of these units are characterized by steep, heavy REE patterns (Sm/Yb ˆ 5±10; Fig. 4) and high to extreme La/Ta ratios (40±90; Fig. 5). The most extreme ratios occur in the 7±5 Ma Jotabeche rhyodacites and the ,8 Ma Dos Hermanos ma®c andesites (Kay et al., 1994, 1999). Maricunga arc volcanism terminated at 7±5 Ma as magmatic activity increased in the former backarc. Some of the backarc lavas erupted between 8 and 5 Ma are also characterized by steep REE patterns and high La/Ta ratios (Figs. 4 and 5). By 3 Ma, the arc front was well established to the east in the southern Central Volcanic Zone (Ojos del Salado region near 278S), where lavas characterized by moderate REE slopes (Sm/Yb ˆ 2.5±3; Fig. 4) and arclike La/Ta ratios (25±45; see ,2 Ma ®eld in Fig. 5) erupted (Mpodozis et al., 1996). Farther south, between 27 and 28.58S, Late Miocene to Pliocene magmatic activity also expanded eastward but then terminated at ,3 to 2 Ma in the Cerro Bonete±Nevado Pissis region with the eruption of a bimodal suite of ma®c andesites and rhyodacites with high Sm/Yb (4±10) ratios (Kay and Mpodozis, 2000). Farther south, between 28 and 32.58S, andesitic volcanism essentially ceased at ,9 to 8 Ma in the Main Cordillera. The eruption of the 6±5 Ma dacitic Vacas Heladas ignimbrite of the Vallecito Formation, which represents the only signi®cant subsequent activity (Maksaev et al., 1984; Ramos et al., 1989; Kay et al., 1991). These high-K rhyodacites are characterized by moderately steep, heavy REE patterns (Sm/Yb ˆ 4±5; Fig. 4) and relatively low, but still arc-like, La/Ta ratios (20±30; Fig. 5).

The rest of the 9±0 Ma magmatic story of the Chilean ¯at-slab is in the Precordillera and the Sierras Pampeanas in the backarc (Fig. 2). Notable centers include the 8.5±5.2 Ma FaralloÂn Negro volcanic cluster (Sasso and Clark, 1998), the ,5 Ma porphyries and domes in the Sierra de Famatina (Losada-CalderoÂn et al., 1994), the 6.3 ^ 0.7 Ma Cerro Blanco center in the Precordillera (Kay et al., 1988; Kay and Abbruzzi, 1996), the 7.9±4.5 Ma Pocho volcanic ®eld in the Sierra de Cordoba (Kay and Gordillo, 1994), and the 6.4 Ma and 2.6±1.9 Ma centers in the Sierra de San Luis (Ramos et al., 1991). The most distinctive of these units are the high-K to shoshonitic-like Pocho and San Luis lavas, which are characterized by moderate Sm/Yb (1.8±2.9 at Pocho; 2.6±3.1 at San Luis) and high La/Ta (20±41 at Pocho; 39±63 at San Luis) ratios for backarc lavas (Figs. 3±5; see Kay and Gordillo, 1994). The FaralloÂn Negro lavas have similar Sm/Yb ratios (2.4± 2.5) but lower La/Ta ratios (18; Kay et al., 1988; Sasso and Clark, 1998). Continuing south to 33.5±34.58S, Late Miocene magmatic activity in the Teniente Magmatic Complex generally waned after the emplacement of the Upper Sewell lavas and plutons at ,7 Ma (Kay et al., 1995, 1999; Kurtz et al., 1997). The ®nal magmatic units include the Los Bronces/Rio Blanco tourmaline breccias at 7±4.9 Ma (Warnaars et al., 1985; Stern and Skewes, 1995), the El Teniente region Braden Breccia at ,4.7 Ma (Cuadra, 1986), and the `Late Hornblende' dikes at ,3.8 to 2.8 Ma. At the same time, the magmatic front was migrating eastward, as marked by the intrusion of high-K granodiorites in the ,6.6 to 5.5 Ma Young Plutonic Complex (Kurtz et al., 1997). These plutons, like the magmatic units in the dying arc, are characterized by markedly high Sm/Yb (.3; Fig. 4) and La/Ta (.35; Fig. 5) ratios (Kurtz et al., 1997, 1995,

S. Mahlburg Kay, C. Mpodozis / Journal of South American Earth Sciences 15 (2002) 39±57

1999). Farther west, the ,2.3 Ma forearc Cachapoal ma®c andesites, which have lower heavy REE (Sm/Yb ˆ 2.8) and La/Ta ratios (28±33), were emplaced as the northern Southern Volcanic Zone arc stabilized near its modern position (Stern, 1989; Stern and Skewes, 1995). The chemistry of the Southern Volcanic Zone centers (Hildreth and Moorbath, 1988; Tormey et al., 1991) overlaps that of the Miocene Cerro de las ToÂrtolas group near 308S (Fig. 6). 3. Controls on magmatic patterns A key question is what tectonic factors control the spatial, temporal, and geochemical patterns of the Neogene magmatic rocks over the Chilean ¯at-slab. Important considerations are the changing shape and thermal condition of the mantle wedge over the subducting plate, the pattern of crustal and continental lithospheric thinning and thickening, the role of subduction erosion of the forearc crust, and the process of eastward migration of the arc magmatic front (see Fig. 8). Controls can be internal and external. Internal controls to consider include the preexisting composition, rigidity, and state of stress of the lithosphere and crust through which the magmas erupt. Potentially signi®cant external controls are marginwide events associated with changes in convergence parameters and local effects related to changes in the physical nature of the subducting oceanic lithosphere. A much debated factor in the Chilean ¯at-slab region is the effect of the subduction of the Juan FernaÂndez Ridge on the shallowing of the subduction zone and the magmatic and structural history of the region. As a ®rst step in evaluating the effects of ridge subduction on magmatism across the ¯at-slab, major Neogene centers were plotted on the temporal maps in YanÄez et al. (2001), which show the path of the Juan FernaÂndez hotspot track on the subducted Nazca plate for the last 24 Ma (Fig. 7). The modern position of the hotspot track is shown on Fig. 2. In making their reconstructions, YanÄez et al. (2001) compensated for the westward motion of the coast (Valparaiso, Chile, shown for reference) relative to a ®xed Juan FernaÂndez hotspot. These maps are most accurate in the oceanic region and lose resolution beneath the continent, because projecting the ridge to the surface requires assuming the shape of the subduction zone through time. The locations of the magmatic centers on the maps in Fig. 7 are plotted in their modern position relative to Valparaiso, Chile. No compensation has been made for shortening of the continental crust, because this effect is minor compared with projection errors. In addition, major shortening in a given region largely predates emplacement of the magmatic centers east of where that shortening occurred. In studying controls of ¯at-slab magmatism, an important parameter to consider is the width over which the subducted Juan FernaÂndez Ridge segment affects the downgoing slab.

49

One approach is to relate the width of the affected region to that of the subducted ridge inferred from the segment still on the modern sea ¯oor. The width of the Juan FernaÂndez Ridge is ,110 km in the vicinity of the O'Higgens seamount, near 73±748W (based on bathymetry in von Huene et al. (1997)), and reaches a maximum of ,75 km farther west (based on anomalies on GEOSAT marine gravity charts; see McAdoo and Marks (1992)). In between, the width of the ridge essentially decreases to zero. In arguing that the Chilean ¯at-slab segment is principally due to the subduction of the Juan FernaÂndez Ridge, Gutscher et al. (2000b) assume that the subducted ridge beneath the modern ¯at-slab had the maximum width of the segment still on the Nazca plate. Their general model, based on the wider and thicker Nazca Ridge that is subducting beneath Peru, indicates that a 200 km wide, 18 km thick plateau is likely to produce a ¯at-slab segment 310 km across. This width, which is an absolute maximum for the Juan FernaÂndez Ridge, is indicated by the dashed lines on the maps in Fig. 7. This modeled 310 km width can be compared to the width of the effects commonly associated with the Chilean ¯atslab. The ®rst is the arc volcanic gap, which extends some 550±660 km from the Ojos del Salado Volcano at 278S or the ,2 Ma Incapillo caldera at 288S, to the Tupungato center near 338S. Another comparison is the shape of the seismic contours to the subducting slab, which are gradational to the north and bend sharply to the south on the margins of the ¯at-slab region (Cahill and Isacks, 1992; see Figs. 1 and 2). The perpendicular distance in map view in Fig. 2 from the ridge track center (YanÄez et al., 2001) to the 125 km contour to the south is some 80 km, whereas that to the 150 km is some 125±150 km. The latter is in good agreement with the half-width of the 310 km broad affected region modeled by Gutscher et al. (2000b). Finally, the southern end of the Sierras Pampeanas uplifts, which is often taken as the southern expression of the ¯atslab, is some 350±400 km south of the modeled ridge center, far south of the 160 km modeled half-width. The complex interaction of the Sierras Pampeanas and the southern Puna ranges, the transitional shape of the contours, and the bend in the subducted ridge make an appraisal to the north more dif®cult. In Section 4, the magmatic history of the ¯at-slab is revisited, and possible links to the regional tectonic evolution are examined. Different scenarios are required to explain magmatic and tectonic events before and after the arrival of the Juan FernaÂndez Ridge in the Chilean ¯at-slab region at ,14 Ma. 4. Magmatic and tectonic events prior to the arrival of the Juan FernaÂndez ridge Both old (Pilger, 1981) and new (YanÄez et al., 2001) reconstructions of the Juan FernaÂndez Ridge track on the

50

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subducting Nazca plate show that pre-14 Ma magmatic and tectonic patterns across the Chilean ¯at-slab region cannot be related to the Juan FernaÂndez Ridge. For the period between 25 and 16 Ma, the YanÄez et al. (2001) reconstructions in Fig. 7 show that the Juan FernaÂndez Ridge was subducting beneath the Andean margin near 198S at 22 Ma and near 218S at 18 Ma. Even with a 280±300 km wide effect per side, the ridge is too far north to be a factor in events in the modern ¯at-slab region. As such, variations in magmatic characteristics and tectonic style in the Chilean ¯at-slab region at this time must be associated with other factors, such as external causes related to changes in rate and direction of convergence of the South American and Nazca plate (Tebbens and Cande, 1997). A clue to the causes of these changes comes from evidence of a transition to a more compressional tectonic regime from the mildly extensional to neutral one that existed from ,27 to 20 Ma. This transition is evident in the El Teniente region, where Kurtz et al. (1997) and Godoy et al. (1999) argued for tectonic inversion and uplift at ,20 to 18 Ma. Similarly, Maksaev et al. (1984) demonstrated an episode of high-angle reverse faulting near 308S, and Jordan et al. (1993) argued for the inception of Precordillera thrusting at ,18 Ma. The termination of backarc extension during this time is consistent with the end of intraplate-like volcanism east of both the El Teniente (Charrier et al., 1996) and the El Indio (Kay et al., 1991) belts, as well as weak, arc-like, ma®c volcanism east of the Maricunga Belt (Kay et al., 1999). Other notable late Early Miocene events include the eastward expansion of arc magmatic centers into the Calingasta/Uspallata Valley and the western Precordillera (Kay et al., 1988) and the eruption of magmas with distinctly steep REE patterns or high K, Th, and U contents in the main arc. Additional evidence for change comes from the major reduction in arc volcanism between 20 and 16 Ma and the essential magmatic lulls in the El Teniente region (Kurtz et al., 1997; Godoy et al., 1999) and Maricunga Belt (Kay et al., 1994; Mpodozis et al., 1995). Finally, the period from 19 to 16 Ma was marked by the eastward migration of the arc front and a change to higher pressure residual minerals in equilibrium with post ,17 to 16 Ma magmas across the entire region (Fig. 6). Evidence for arc migration extends from ,36 to 308S (Kay, 1996; Kay et al., 1987), well beyond the limits of the modern Chilean ¯at-slab region (see Fig. 6). This migration is shown by the Middle Miocene arc front being some 20 km east of the Coya MachalõÂ arc in the El Teniente region; along the Argentine border in the Farellones front, east of Santiago; and at the Cerro de Las ToÂrtolas, east of the Dona Ana front, in the El Indio region. Although the arc did not migrate in the Maricunga Belt, vigorous arc volcanism reinitiated there at ,16 Ma (Kay et al., 1994). Kay et al. (1987, 1991, 1999) used a subset of these factors to argue for initial shallowing of the Chilean ¯atslab as early as ,18 Ma, leaving open the question of the

driving force. Among likely causes are changes in convergence parameters and/or subduction of older regions of thickened oceanic crust not associated with the Juan FernaÂndez Ridge. Along the ¯at-slab region, magmatic and tectonic indicators suggest a more compressional Early Miocene tectonic regime in the north than in the south. As discussed, calc-alkaline arc magmas and ma®c backarc lavas erupted through thicker crust in the Maricunga Belt (Kay et al., 1994; Mpodozis et al., 1995) than did tholeiitic arc and intraplate-like backarc lavas that were emplaced in mildly extensional basins in the El Teniente region (Kurtz et al., 1997; Kay et al., 1999; Godoy et al., 1999). Evidence for an even more extensional regime in the arc south of El Teniente comes from a ,25 to 20 Ma magmatic trend (see Fig. 6), basinal evolutionary patterns, and normal faults on seismic lines as far south as 428S (MunÄoz et al., 2000; Jordan et al., 2001). The regional effects of these magmatic and tectonic variations seem best explained by changes in convergence parameters superimposed on local variations along the preexisting margin. Those working offshore have noted an important period of plate reorganization in the modern Paci®c Ocean region near 20 Ma (Tebbens and Cande, 1997). This change is re¯ected in convergence models like those by Pardo-Casas and Molnar (1987) and Somoza (1998), which show important changes along the Andean margin at ,20 Ma. Their models (at 22 and 408S latitude) show a period of relatively rapid (,12 to 15 cm/year), nearly orthogonal (within 108) convergence from 25 to 20 Ma that replaced a long period of much slower, more oblique dextral Late Oligocene convergence. The Pardo-Casas and Molnar (1987) model shows a subsequent small obliquity increase at an increased rate (,18 cm/year) from 20 to 10 Ma, whereas the Somoza (1998) model (at 228S) shows a small rate decrease (to ,12 cm/year) with a dextral obliquity of ,308 from 20 to 15 Ma and ,158 from 15 to 10 Ma. In contrast, models by Cande and Leslie (1986) and Cande (personal communication) show a slight decrease in dextral obliquity after ,19 Ma. Differences in these convergence models result from uncertainties in interpretations of the marine magnetic record and need to be assessed in conjunction with magmatic and tectonic episodes along the continental margin. Using this approach, Jordan et al. (2001) suggested that reinitiation of southern Andean volcanism at ,27 to 25 Ma, in conjunction with a change from slow, oblique to more rapid, orthogonal convergence, can be reconciled with an extensional environment from ,27 to 20 Ma in the arc south of 348S. In their model, extension is attributed to thermal collapse of the crust above a thinning lithosphere as arc magmatism was reestablished after a nearly 10million-year lull. An alternative model is the formation of a series of pull-apart basins or oblique-extensional basins associated with a slightly more oblique convergence regime, as is the modern case in the southern Andes. Superposition of an oblique extensional regime on a preexisting thicker

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crust and lithosphere in the Maricunga Belt near 27±288S rather than in the El Teniente region near 348S provides a reasonable explanation for ,27 to 20 Ma magmatic and tectonic patterns across the Chilean ¯at-slab and its boundaries. A series of pull-apart basins provides an explanation for the discontinuous ma®c backarc magmatic pattern. The subsequent 20±16 Ma magmatic and tectonic regime seems best explained by a shift to a more compressional regime associated with more orthogonal convergence. Such a model ®ts with unpublished convergence parameters from Cande (personal communication). This change would mark the beginning of the long period of Miocene compressional deformation and tectonic inversion along much of the margin. A change in convergence direction requires a mechanical adjustment between the two converging plates that can explain a period of reduced magmatic activity in the late Early Miocene followed by voluminous Middle Miocene volcanism in a reestablished arc farther to the east. Backarc magmatic activity stretching from the Valle Ancho into the eastern Precordillera would then be a response to displacement of the arc front and initial shallowing of the subduction zone. An alternative model is that the ,20 to 16 Ma period of reduced magmatic activity was a time of high obliquity, as indicated by Somoza (1998). The distinctive transient REE ratios (high Sm/Yb; Fig. 4), which are indicative of high pressure mineral residuals in late Early Miocene ¯at-slab magmas (near Santiago and Aconcagua, in the Cordillera del Limite near 328S, in Maricunga Belt Jotabeche Norte lavas), are dif®cult to explain by systematic changes in crustal thickness using lower crustal contamination models like the MASH (melting, assimilation, storage, homogenization) model of Hildreth and Moorbath (1988). Although such a model works well to explain long-term systematic REE changes in Early to Late Miocene Cordillera ¯at-slab magmas (Kay et al., 1987, 1991; Kay and Abbruzzi, 1996), the transient nature of high pressure residual assemblages in equilibrium with the latest Early Miocene lavas require unreasonable changes in crustal thicknesses over a short period of time. A better explanation is that these changing REE patterns, along with transient high La/Ta ratios (Fig. 5), are related to the transport of tectonically removed forearc continental crust into the oxidized, hydrated mantle source region above the slab by processes associated with forearc subduction erosion (von Huene and Scholl, 1991; Stern, 1991; Kay and Mpodozis, 1999; see Fig. 2). Such a process is favored by mechanical adjustments above a migrating arc front triggered by changes in convergence vectors along the continental margin. High Th and U contents in the In®ernillo Unit could also result from this process. 5. Post-15 Ma magmatism and subduction of the Juan FernaÂndez ridge The period from 14 to 9 Ma marks the arrival of the Juan

51

FernaÂndez Ridge, carried on the subducting Nazca plate, into the Chilean ¯at-slab region. This is a time of thrusting related to compressional shortening and the peak of andesitic arc volcanism along the entire ¯at-slab region. Except for small differences in velocity, proposed convergence models essentially agree for 15±5 Ma. Somoza (1998) shows the most extreme change at 15 Ma with a shift from an obliquity of ,298 from 20 to 15 Ma to ,128 from 15 to 10 Ma. Looking at the Juan FernaÂndez ridge track, several ®rstorder correlations seem reasonable as the northeast-sweeping arm moves southward from ,14 to 10 Ma and the more nearly east-west segment begins subducting beneath the margin after ,10 Ma (Fig. 7). 5.1. Arrival of the northeast-trending arm of the ridge 14± 10 Ma Following the reconstructions of YanÄez et al. (2001) and assuming that the ridge affects a region some 150±200 km on a side, the ridge was near the northern Maricunga Belt at ,14 Ma and passed beneath the Copiapo center in the Maricunga Belt at ,12 to 10 Ma. The approach and passage of the ridge center through the Maricunga arc correlates with the transition from a stratovolcanic chain to a large isolated dacitic ignimbrite complex (Copiapo Complex) at ,11 Ma. The eruption of lavas with steep REE patterns (high Sm/Yb ratiosÐCopiapo group) and high La/Ta ratios coincide with this change. Continuing south, the ridge passed beneath a possible Middle to Late Miocene volcanic gap between the southern Maricunga and northern El Indio belts and arrived in the El Indio region by 12±10 Ma. Such a gap might be explained by subduction of a large volcanic center on the Juan FernaÂndez hotspot track. In the El Indio region, the arrival of the ridge approximately coincides with the change from the andesite-dominated 17±14 Ma Cerro de Las ToÂrtolas formation to the dacite-dominated 12.7±10 Ma Tambo formation. As in the Maricunga Belt, the El Tambo lavas with the steepest REE patterns and highest La/Ta ratios erupted as this change occurred. Because the ridge axis was still too far north to in¯uence the magmas erupting from the La Ramada and Cerro Aconcagua centers, these hornblende-bearing andesitic and dacitic units are like those that erupted from the Middle Miocene Cadillal and Cerro de Las ToÂrtolas centers. The termination of frontal arc andesitic volcanism at 9 Ma from the Maricunga Belt to Aconcagua occurred as the east±west segment of the ridge approached (Fig. 7). The in¯uence of the approaching ridge could also be tied to magmatic events in the backarc. In the Maricunga backarc, the Valle Ancho ignimbrites erupted at ,15 to 14 Ma on the outer edge of the ridge as it was arriving. Other centers farther east and south formed as the ridge migrated south. These include small eruptions in the FaralloÂn Negro region (12.56 ^ 0.36 to 8.59 ^ 0.10 Ma; Sasso and Clark, 1998) and in the Precordillera in the GualilaÂn (11.0 ^ 0.5 Ma) and Ullun (10.2 ^ 0.8 Ma) regions (Leveratto, 1976; Kay et al., 1988; Kay and Abbruzzi, 1996). The

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approach of the ridge beneath the Precordillera correlates with the 12±10 Ma compressional shortening peak for that region inferred by Jordan et al. (1993). Nevertheless, not all of the magmatic and tectonic effects in and on the margins of the ¯at-slab region can be explained by ridge subduction. In particular, weaker parallel events in the Teniente region near 348S occur south of the area that is reasonably affected by the ridge. Magmatic and geochemical parallels include higher Sm/Yb ratios in the 10 Ma Lower Sewell than in the 14.5±12 Ma Maqui Chico lavas, which implies a temporal increase in the pressure of the mineral residue in equilibrium with these magmas. Tectonic parallels include a Late Middle Miocene compressional deformation in accord with trace element differences related to contemporaneous crustal thickening associated with compressional shortening (Kurtz et al., 1997; Kay et al., 1999; Godoy et al., 1999). In accord with this, Baldauf et al. (1997) used 14±9 Ma lavas in the backarc MalarguÈe fold and thrust belt near 358S to demonstrate Miocene backarc shortening at this time. The age of this deformation is similar to that inferred for the Precordillera (Jordan et al., 1993). As such, crustal shortening and associated thickening of the arc crust are associated with a general compressional regime that extended beyond the region affected by the subduction of the Juan FernaÂndez Ridge (see Fig. 6). Farther north, the effects of ridge subduction are superimposed on this regional stress ®eld and preexisting lithospheric conditions. The arrival of the ridge seems to be a perturbation that enhanced, but was not the only driving force for crustal shortening. 5.2. Arrival of the nearly E±W-trending ridge segment 9± 0 Ma The main effects of the subduction of the east±westtrending ridge on the modern ¯at-slab region began at ,9 to 8 Ma as andesitic volcanism terminated in the frontal arc along much of the length of the ¯at-slab. North of ,318S, effects related to the arrival of the east±west-trending segment are dif®cult to distinguish from those of the backarc progression of the northeast-sweeping arm. In the Maricunga region, the former Maricunga arc front shut off at ,7 Ma in the Copiapo center and at ,5 Ma in the Jotabeche caldera farther south. In both areas, the arc front shifted eastward toward the position of the modern southern Central Volcanic Zone front. The time of arc migration corresponds to the passing of the NE-trending prong of the ridge. The unusual chemistry of magmas (Figs. 3±5) erupted at this time is highlighted by a need for a very high pressure residual mineralogy in equilibrium with ma®c andesite ¯ows near Cerro Dos Hermanos (Sm/ Yb ˆ 7±8) and the Jotabeche rhyodacites (Sm/Yb ˆ 8± 10). Kay and Mpodozis (1999, 2000) interpreted the extreme REE patterns, along with the extreme La/Ta (90± 100) ratios in these units, as evidence of an episode of forearc subduction erosion as the slab shallowed and the

arc front migrated eastward. Shallowing of the slab over the subducting ridge is also consistent with eastward broadening of the arc, which led to the eruption of the large FaralloÂn Negro complex at ,8.5 to 6.9 Ma (Sasso and Clark, 1998). By 4 Ma, the NE-trending ridge segment had passed, allowing the modern southern Central Volcanic Zone to be stabilized before 2 Ma. In the Pissis/Bonete region at the extreme southern end of the Central Volcanic Zone and closer to the east±west-trending limb of the subducting ridge, volcanism terminated at ,2 Ma (Kay and Mpodozis, 2000). Farther south in the El Indio region, andesitic arc volcanism ended at ,8 Ma, and essentially all volcanism terminated with the eruption of the dacitic Vacas Heladas (Vallecito Fm.) ignimbrite at 6 to 5 Ma. Contrasts with events related to arc migration in the Maricunga Belt could be due to a shorter segment of the NE-trending part of the ridge passing beneath the arc and a position closer to the axis of the arriving east±west segment (Fig. 7). Farther east, the ,7 Ma backarc Cerro Blanco center appears to have erupted as the bend in the ridge passed below the eastern Precordillera. Striking contrasts in the trace element and isotopic signatures of the Main Cordillera Vacas Heladas and Precordillera Cerro Blanco silicic magmas re¯ect contamination of arc magmas by very different continental basement components in separate Paleozoic terranes (Kay and Abbruzzi, 1996). Frontal arc volcanism also essentially terminated at ,9 Ma between 32 and 338S with the last eruptions of the La Ramada/Cerro Aconcagua/PiraÂmide centers. This termination coincides with the bend in the ridge arriving beneath the frontal arc (Fig. 7). The lack of contemporaneous backarc magmatism east of the La Ramada and Cerro Aconcagua centers could be due to the bend in the ridge not extending this far south. Magmatic characteristics of the El Teniente region just south of the ¯at-slab are more dif®cult to place in the context of ridge subduction, because they evolved outside of a 200 km swath from the ridge center (Fig. 7). As in earlier times, there are magmatic and tectonic parallels with events to the north. In particular, trace element changes between the Lower and Upper Sewell groups at ,9 to 8 Ma (Kay et al., 1995, 1999; Kay and Mpodozis, 2001) coincide with the termination of andesitic volcanism in the ¯at-slab Main Cordillera to the north. After 7 Ma, andesitic volcanism essentially terminated in the El Teniente region as the arc front migrated eastward (6.6±5.5 Ma Young Plutonic Complex; Kurtz et al., 1997). This shift occurred as major compressional deformation affected the El Teniente region (Kurtz et al., 1997; Godoy et al., 1999). Steeper REE patterns (high Sm/Yb ratios) in latest Miocene/early Pliocene El Teniente region magmatic units indicate a transition to a higher-pressure residual mineralogy, as do those for similar age magmatic units on the northern margin of the ¯at-slab (Kay et al., 1999). Overall, eastward migration of the arc magmatic front from the Teniente Volcanic

S. Mahlburg Kay, C. Mpodozis / Journal of South American Earth Sciences 15 (2002) 39±57

Complex to the present Southern Volcanic Zone between ,7±5 and 4±3 Ma is coincident with arc front migration from the Maricunga Belt to the southern Central Volcanic Zone near 27±288S (Fig. 6). The ,8 to 4 Ma arc migration events in the Maricunga Belt and El Teniente region and the termination of volcanism over much of the modern ¯at-slab frontal arc are events that coincide with the subduction of the kink of the ridge axis and the arrival of the east±west ridge segment (Fig. 7). However, the timing of these events is consistent with ridge subduction as only a perturbation of a much larger set of driving forces. Evidence comes from contemporaneous events that occurred over large segments of the Andean margin outside the Chilean ¯at-slab, including the widely discussed ,7 Ma compressional event over the shallowing Peruvian ¯at-slab (SeÂbrier and Soler, 1991; Noble and McKee, 1999), the eruption of massive 7±6 Ma ignimbrites in the central Puna±Altiplano plateau (Fig. 1), and the initiation of ma®c volcanism in the southern Puna (see Kay et al., 1999). The driving force for changes near 7±5 Ma again appears to be related to variations in Nazca±South American convergence parameters. Tebbens and Cande (1997) note that plate reorganization at chron 3A (,5.5 Ma) is synchronous with an extended interval of plate boundary reorganization in the northeast Paci®c. Likewise, Atwater and Stock (1999) note the importance of a 7±5 Ma change in North American±Paci®c convergence parameters in interpreting events in the western United States. These changes likely affect the entire Paci®c region. Unfortunately, Nazca±South American convergence models are not suf®ciently re®ned to access detailed Nazca±South America convergence changes at this time. Among existing analyses, Somoza (1998) discusses a possible 12±8 cm/year decrease in convergence velocity and an increase in dextral convergence near 5 Ma. Cande (personal communication) shows a large change in obliquity and a decrease in rate from ,9 to 6 cm/year near 9.5 Ma followed by a rate increase to 8 cm/ year at 4 Ma. Using data from Brozena (1986) and SeÂbrier and Soler (1991) note an increase in Atlantic half-spreading rates from ,7 to 5 Ma in analyzing events affecting the Peruvian ¯at-slab region. 5.3. Ridge subduction and backarc volcanism 9±0 Ma The signi®cant expansion of backarc volcanism across the modern ¯at-slab region after 12 Ma also should be considered in light of the subducting Juan FernaÂndez Ridge track. As discussed, the initial broadening of the arc near 12 Ma includes backarc events at FaralloÂn Negro at 12.6 Ma and the Precordillera at 12±10 Ma. These centers are at the easternmost edge of the region possibly affected by ridge subduction at 12 Ma, given a 300±400 km wide path of in¯uence and the reconstruction in Fig. 7. They are within the region by 10 Ma. The more extensive expansion of backarc volcanic

53

centers at ,8 to 5 Ma occurs as the bend between the east±west- and northeast-trending segments of the ridge was arriving and subducting beneath the region (Fig. 7). Given uncertainties in the ridge position, most of these centers fall in a ,350 km broad zone centered on the ridge axis. Many are concentrated in the backarc of the Maricunga Belt and El Indio regions. These include the 7±5 Ma Valle Ancho and Bonete region centers in the migrating frontal arc above the northeast-trending ridge arm and the Cerro Blanco center in the Precordillera over the bend in the ridge axis to the south. Units in the far backarc include the 8.6±6.8 Ma FaralloÂn Negro centers, the 7.9±4.5 Ma Pocho centers in the Sierra de Cordoba, and the ,5 Ma Mogotes Group centers in the Sierra de Famatina (Losada-CalderoÂn et al., 1994). Given the ridge positions in Fig. 7, the FaralloÂn Negro centers were near or just south of the ridge track at ,8 to 6 Ma, whereas the older Pocho centers were quite far to the south. By 5±4 Ma, backarc volcanism essentially ended at this latitude, except for centers in the Famatina and Pocho ®elds over the opposing sides of the kink in the ridge axis. The spatial extent of the far backarc magmas can be interpreted to re¯ect the geographic limits of the hydrated mantle wedge over the subducting slab (Fig. 8). Association of the high-K FaralloÂn Negro, Pocho, and San Luis centers with the southward migrating shallow subduction zone seems a reasonable way to explain why these centers exist. However, a simple link to the Juan FernaÂndez Ridge is unclear for the 7.5 ^ 0.5 Ma Pocho and the 6±2 Ma San Luis centers, as they erupted south of the region reasonably affected by the ridge, according to Fig. 7. One solution for the Pocho centers is that the east±west ridge segment was farther east than is shown by ,8 Ma. This would ®t with the uplift of the Sierras Pampeanas being directly related to the shallowing of the subduction zone (Jordan et al., 1983). Such an explanation does not work for the San Luis centers, which are some 200 km south of the modern projection of the ridge axis (Fig. 2). Given a relation to shallow subduction, their location suggests that magma generation occurred at the outermost reaches of the region in¯uenced by the shallow subduction. Such an explanation is consistent with their parental magmas forming at depths of ,200 km or more, in accord with phlogopite-breakdown triggering the formation of these high-K magmas (see Kay and Gordillo, 1994). 6. Flat-slab adakites from melting of thickened or tectonically removed crust Gutscher et al. (2000a) proposed that slab melting is a major process in producing magmas with steep REE patterns (adakites) in shallow subduction zones. One of their type cases was the Chilean ¯at-slab region, for which they proposed that magmas erupted between 10 and 4 Ma could be attributed to slab melting. However, the slab-melting process seems enigmatic in a shallow subduction zone,

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which should produce a cooler, thermal structure in the mantle. Furthermore, previous thermal models have shown that slab melting occurs only when the subducting slab is less than ,7 Ma (Peacock et al., 1994), in direct con¯ict with the Eocene age of the Nazca plate oceanic crust that is subducting beneath the Chilean ¯at-slab (YanÄez et al., 2001). To surmount these problem, Gutscher et al. (2000a) argued that a slab moving through the asthenosphere heats at the same rate whether it descends vertically or moves horizontally through the mantle. By assuming a shallow 1200 8C mantle isotherm at 60 km well to the east of the volcanic front, they were able to produce a model with temperatures of 700±800 8C at the top of the slab, which is suf®cient for ¯uid-present partial melting of subducted basaltic oceanic crust. However, there are various problems in using this model to explain the 10±4 Ma magmas over the Chilean ¯at-slab region as slabmelts. The ®rst is to justify a preexisting thin continental lithosphere (,60 km) with a 1200 8C isotherm at shallow depth for hundreds of kilometers east of the volcanic line. Another is that not all of the Chilean ¯at-slab magmas interpreted as slabmelts have adakitic geochemical signatures (e.g. Pocho and Precordillera lavas, Figs. 3±5), and the distribution of those that do is not consistent with a slab-melting model. Finally, the adakitic signatures that occur in Main Cordillera ¯at-slab magmatic units are better explained by contamination of mantle-derived arc magmas at the base of the thickened eclogitic facies continental crust. Following the Chilean ¯at-slab cross-section in Gutscher et al. (2000a), units they interpret as slabmelts include the 10±8 Ma Tambo andesites and the 6±5 Ma Vallecito rhyodacites in the Main Cordillera, the Cerro Blanco dacites in the Precordillera, and the Pocho lavas in the Sierra de Cordoba (see Fig. 8). Starting in the east, the only slabmelt-like characteristic of the Pocho lavas is their high Sr contents; their heavy REE patterns (Sm/Yb) are relatively ¯at and their Yb concentrations are normal (Kay and Gordillo, 1994; see Fig. 4). A major problem with interpreting them as slabmelts is the presence of basaltic to ma®c andesitic lavas, which are inconsistent with any slabmelting model. In addition, the Pocho centers are ,700 km east of the trench, not ,500 km, as shown by Gutscher et al. (2000a). Moving westward, the Precordillera Cerro Blanco lavas also lack the steep Sm/Yb ratios associated with slab melts (Kay et al., 1988). Only the El Tambo and Vallecito lavas in the Main Cordillera have chemical characteristics that fall within the de®nition of adakite (Sr . 600 ppm, low Yb, steep La/Yb and Sm/Yb ratios). However, these lavas erupted through the thickest crust over the coolest part of the ¯at-slab and are better explained by contamination of arc-derived ma®c magmas in a thickened crust (Kay et al., 1991). As discussed by Kay and Abbruzzi (1996), the crustal thickening needed to account for residual garnet in the lower crust is consistent with models for crustal shortening across the ¯at-slab region

(Maksaev et al., 1984; Allmendinger et al., 1990; Cristallini and Ramos, 2000). Finally, the eruption of slabmelts in the terminal stages of frontal arc volcanism is contrary to any expected slab-melting model. 7. Conclusions 1. Late Oligocene to Early Miocene volcanism across the modern Chilean ¯at-slab region of the Andes is characterized by voluminous magmas erupted in association with intra-arc basins and sporadic occurrences of backarc basaltic to ma®c andesitic volcanism. The tectonic setting is consistent with oblique extensional basins. This regime is superimposed on an initially thicker crust in the Maricunga region to the north than in the El Teniente region to the south, where manifestations of extension are the strongest. 2. The latest Early Miocene spread of volcanism and deformation into the Calingasta/Uspallata Valley and Precordillera over the Chilean ¯at-slab region, which has been interpreted to signal initial shallowing of the subduction zone, is unrelated to the subduction of the Juan FernaÂndez Ridge, which was far to the north at the time. These events, which are roughly synchronous with the end of backarc extension and the initiation of compressional deformation and eastward arc migration along this and other parts of the Andean margin, correlate with a change in Nazca±South American convergence parameters. Virtual volcanic gaps and unusual chemical signatures in Chilean ¯at-slab magmatic units erupted at this time can be explained by components introduced into the mantle through forearc subduction erosion as the arc front migrated eastward. Magmatic activity in the reestablished arc to the east was dominated by ,16 to 8 Ma andesitic stratovolcanoes erupted in a compressional tectonic environment. 3. The ®rst effects of the subduction of the Juan FernaÂndez Ridge are recorded as the northeast-trending branch of the ridge arrived in the northern ¯at-slab region at ,12 to 10 Ma. Events at that time include a transition in volcanic style from a chain of andesitic stratovolcanic centers to isolated dacitic dome complexes and an increase in backarc volcanism. Subsequently, andesitic volcanism terminated along the ¯at-slab margin at ,9 to 8 Ma as the east±west-trending branch of the ridge track advanced eastward near 318S. 4. Eastward migration of the arc front on the northern and southern margins of the ¯at-slab region between ,7 and 4 Ma and the termination of dacitic volcanism in the central ¯at-slab at ,6 to 5 Ma occurred as important magmatic and compressional events took place elsewhere along the Andean margin. This period is a time of plate reorganization elsewhere in the Paci®c region, suggesting that, as in the Early Miocene, changes in convergence parameters could be a major driving force

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for arc migration. The subduction of the Juan FernaÂndez Ridge appears to be a perturbation that leads to pronounced shallowing of the subduction zone over the Chilean ¯at-slab. Distinctive transient chemical changes in the magmas erupted in the migrating arcs (e.g. high Sm/Yb and La/Ta ratios) are best associated with forearc subduction erosion. Magmatic differences on the northern and southern margins of the ¯at-slab re¯ect a thicker crust and longer period of ridge subduction on the northern margin. 5. Although the latest Miocene to Pliocene FaralloÂn Negro, Pocho, and San Luis magmatic centers in the far backarc are best related to shallowing of the subduction zone, some of them erupted outside the region thought to be directly in¯uenced by the subducting Juan FernaÂndez Ridge. Their location suggests that magma generation occurred at the outer limits of the effects of mantle hydration over the shallowing slab. The San Luis lavas erupted at least 200 km above this slab. 6. Neogene magmas erupted in the ¯at-slab region, like those in the modern Southern and Central Volcanic Zones, are crustally contaminated mantle melts that formed in the hydrated mantle wedge above the subducting slab. Adakite-like trace element signatures, which signal equilibration with garnet-bearing residual mineral assemblages, developed as mantle-derived melts were contaminated in thickened garnet granulite to eclogitic facies lower crust or by crust removed from the base of the forearc crust though processes of forearc subduction erosion (particularly during arc migration). There is no evidence to support any of these magmas being generated as slabmelts, consistent with the relatively cold thermal regime expected over a shallowing subduction zone.

Acknowledgements Discussions with Victor Ramos, Estanislao Godoy, Beatriz Coira, Moyra Gardeweg, Sergio Rivano, Reynaldo Charrier, Ernesto Cristallini, Robert Kay, and participants in the Cornell Andes project are particularly acknowledged, as are reviews by Leopoldo Lopez-Escobar, Carlos Rapela, and Gonzalo YanÄez. In the last 10 years, principal funding for this project has come from the US/NSF (92-05042), the Chilean FONDECYT (grants 149/92, 1950025, 199009), the Chilean geological survey (SERNAGEOMIN), and CODELCO. References Allmendinger, R.W., Figueroa, D., Snyder, D., Beer, J., Mpodozis, C., Isacks, B.L., 1990. Foreland shortening and crustal balancing in the Andes at 308S latitude. Tectonics 9, 789±809. Atwater, T., Stock, J., 1999. Paci®c±North America plate tectonics of the Neogene southwestern United States; an update. International Geology Review 40, 372±402.

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Baldauf, P.E., 1997. Timing of the uplift of the Cordillera Principal, Mendoza Province, Argentina. PhD thesis, George Washington University, Washington, DC, 356 pp. Baldauf, P.E., Stephens, G.C., Nullo, F.E., Combina, A., Kunk, M., 1997. Tertiary uplift, magmatism, and sedimentation of the Andes, southern Mendoza Province, Argentina. Geological Society of America Abstracts with Programs 29, 48. Beccar, I., Vergara, M., Munizaga, F., 1986. Edades K±Ar de la FormacioÂn Farellones en el CordoÂn del Cerro La Parva, Cordillera de los Andes de Santiago, Chile. Revista GeoloÂgica de Chile 28± 29, 109±113. Brozena, J.M., 1986. Temporal and spatial variability of sea¯oor spreading processes in the northern South Atlantic. Journal of Geophysical Research 91, 497±510. Cahill, T.A., Isacks, B.L., 1992. Seismicity and shape of the subducted Nazca Plate. Journal of Geophysical Research 97 (17), 503±517 see also page 529. Cande, S.C., Leslie, R.B., 1986. Late Cenozoic tectonics of the southern Chile Trench. Journal of Geophysical Research 91, 471±496. Carrasquero, S.I., 1997. Volcanismo de arco en el area del por®do cuprifero Paramillos sur, Uspallata, Mendoza, Argentina. XIII Congreso GeoloÂgico Argentino y III Congreso de Exploracion de Hidrocarburos, Buenos Aires, vol. 1, pp. 95±100. Charrier, R., Wyss, A.R., Flynn, J.J., Swisher III, C.C., Norell, M.A., Zapatta, F., McKenna, M.C., Novaceck, M.J., 1996. New evidence for late Mesozoic±early Cenozoic evolution of the Chilean Andes in the upper Tinguiririca valley (358S), central Chile. Journal of South American Earth Sciences 9, 1±30. Cloos, M., 1993. Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges and seamounts. Bulletin of the Geological Society of America 105, 545±562. Cristallini, E.O., Ramos, V.A., 2000. Thick-skinned and thin-skinned thrusting in the La Ramada fold and thrust belt: crustal evolution of the High Andes of San Juan, Argentina (328S L). Tectonophysics 317, 205±235. Cristallini, E.O., Kay, S.M., Ramos, V.A., 1994. Las volcanitas Cretacicas y Terciarias de la cordillera del Limite, Argentino±Chileno a los 328 de latitud sur. Seventh Congreso GeoloÂgico Chileno, ConcepcioÂn, Chile, vol. 2, pp. 1311±1315. Cuadra, P., 1986. GeocronologõÂa K±Ar del yacimiento El Teniente y aÂreas adyacenters. Revista GeoloÂgica de Chile 27, 3±26. Drummond, M.S., Defant, M.J., 1990. A model for trondjemite±tonalite± dacite genesis and crustal growth via slab melting: Archean to modern comparisons. Journal of Geophysical Research 95, 21503±21521. Gana, P., Selles, D., Wall, R., 1999. Area Titil-Santiago, Region Metropolitana, Mapas Geologicos 11, SERNAGEOMIN, Subdireccion Nacional de GeologõÂa y MinerõÂa, Santiago, Chile. Godoy, E., YaÂnÄez, G., Vera, E., 1999. Inversion of an Oligocene volcanotectonic basin and uplifting of its superimposed Miocene magmatic arc in the Central Chilean Andes: ®rst seismic and gravity evidences. Tectonophysics 306, 217±236. Gutscher, M., Maury, R., Eissen, J.-P., Bourdon, E., 2000a. Can slab melting be caused by ¯at subduction? Geology 28, 535±538. Gutscher, M.A., Spakman, W., Bkjwaard, H., Engdahl, E.R., 2000b. Geodynamics of ¯at subduction: seismicity and tomographic constraints from the Andean margin. Tectonics 19, 814±833. Hildreth, W., Moorbath, S., 1988. Crustal contributions to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrology 98, 455±489. von Huene, R., Scholl, D.W., 1991. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust. Reviews of Geophysics 29, 279±316. von Huene, R., CorvalaÂn, J., Fleuh, E.R., Hinz, K., Korstgard, J., Ranero, C.R., Weinrebe, W., 1997. Tectonic control of the subducting Juan Fernandez Ridge on the Andean margin near Valpariso, Chile. Tectonics 16, 474±488.

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