Peru, Bolivia and Northern Chile) from Eocene to Present

Tectonics and uplift in Central Andes (Peru, Bolivia and Northern Chile) from Eocene to present

Abstract : The analysis of sedimenta y and volcank records, exposed in southern Peru, Bolivia, and norfhem Chile, allow us to establish the chronological evolution of Central Andes from Upper Eocene to Present. This analysis is based on fieid observations and a re-evaluation of the available geological data. It gives evidence for six discrete compressionai tectonic puises that are dated : Upper Eocene (ca 42 Ma), Upper Oligocene (ca 26-28 Ma). lower Miocene (ca 15- 17 Ma). middle Miocene (ca 10 Ma). Upper Miocene (ca 7 Ma) and early Ouaternary (ca 2 Ma), respectively. The magnitude of shortening and geographical extent of these compressional phases are highly variable. ln particular. the lower and middle Miocene compressional pulses could correspond to deformational climaxes chiefly characterized by compressionai tectonics. Generally these compressional pulses appear to be coeval with periods of high convergence rate. Moreover. available structural data on these phases suggest that their directions of shortening were roughly parallel to the orientation of convergence. Between these compressional pulses, basin infiliings take place.; they are highly variable in thickness and composition and these differences are in agreement with the different mechanics that may be put forward to explain the formations of Andean basin. Consequently they are indicative of the stress regimes that prevail between the compressional pulses. This stress regime should be mainly tensionai in the Altiplano, Western Cordillera and Fore-Arc basins. On the contrary, it is essentially compressional in the Subandean Lowlands. Magmatic activity has occurred in the High Andes since at least Upper Oligocene time (ca 25 Ma). The magnitude of this magmatic activity appears to be related to the convergence rate. Magmatism was maximum between 25 and 6 Ma, i.e., contemporaneously with the period that is characterized by a higher rate of convergence than the present-day period. Observations performed along the Pacifie Lowlands. on Neogene morphological surfaces. show clearly that the high cordilieran topography formed between roughly26 and 6 Ma, i.e., during the Miocene period of high convergence rate. This Andean uplift has been discontinuous and roughly coeval with the compressional pulses.

Key words : Tectonics - Uplift - Timing - Cenozoic - Central Andes - Peru - Bolivia - Chile.

%surnkj : Tectonique et soulèvement dans les Andes Centrales (Pérou,~Bolivie et Nord Chili) depuis 1’Eocène jusqu’à l’actuel. L’analyse des séries sédimentaires et volcaniques, qui affleurent dans le Pérou sud, la Bolivie et le Chili nord, permet d’établir lëvolution chronologique des Andes Centrales de I’Eocène à l‘actuel. Cette analyse est basée sur des observations de terrains et une ré-évaluation des données géologiques disponibles. Elle met en évidence six phases tectoniques compressives. datées respectivement de I’Éocène supérieur (- 42 Ma)< de I’Oligocène supérieur (- 26-28 Ma), du Miocène inférieur (= 15- 17 Ma), du Miocène moyen (= 10 Ma). du Miocène supérieur (= 7 Ma) et du Quaternaire ancien (~2 Ma). Le raccourcissement et la répartition géographique de ces phases sont très variables. En

1 UA 730 CNRS, Laboratoire de Géologie Dynamique Interne. Bat 509, Université Paris Sud, 91405 Orsay Cedex. Il2 ORSTOM, 213. rue Lafayette‘ 75010 Paris. (3) FUNVIS/S, A.P. 189.2, Caracas. Venezuela.

Géodynamique 3 (I-2). 1988 : 85-106 85 M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS particuirer. les phases compressives du Miocène inférieur et moyen pourraient correspondre à des pics de deformations dans une penode de tectonique essentiellement compressive. En général ces phases de tectoniques compressives apparaissent contemporames des périodes à taux de convergence élevés. D’ailleurs. les rares données structurales disponibles suggèrent que les drrections de raccourcksement sont approximativement parallèles à la directron de la convergence entre les plaques Nazca et Amerique du Sud. Entre ces phases se produit le remplissage des bassins sédimentaires andins. Ces remplissages sont très variables en épaisseur et en composition et ces différences sont en accord avec les différents mécanismes qui peuvent etre invoques pour expliquer la formatlon des bassins andins. En conséquence. ils sont indicatifs des régimes de contrainte qur prévalent entre les phases compressives Ce régime de contrainte est plutôt en extension dans IAltiplano. dans la Cordrllere Occidentale et dans les bassins de fore-arc. En revanche, il est essentiellement compressif dans les collines sub-andines. L activité magmatique est localisée dans les Hautes-Andes au moins depuis I’Oligocène (= 25 Ma). L Ïmportance de cette activité magmatique paraît être liée à IÏmportance du taux de la convergence. Le magmatrsme a eté maximal entre 25 et 6 Ma c est-à-dire contemporain de la période qui est caractérisée par un taux de convergence plus élevé que l’actuel. Les observatrons réalisees le long de la partie émergée du fore-arc, sur des surfaces morphologiques néogènes, montrent clairement que la haute topographie cordilléraine s’est formee approximativement entre 26 et 6 Ma. c’est-à-dire durant la periode miocène de fort taux de convergence. Ce soulèvement andin a été discontinu et a peu près contemporain des phases compressives Mots-clés : Tectonique - Soulèvement - Chronologre - Cénozoïque - Andes- Centrales - Pérou - Bolivie - Chili

Resumen : Tectonica y levantamiento en 10s Andes Centrales (Peru, Bolivia y Norte de Chile) desde el Eoceno. El analisis de las series sedimentanas y volcanrcas en el Sur de Peru. Oeste de Bolivia y Norte de Chile nos permite de establecer la cronologia de la evolucion gendmamica de /os Andes Centrales desde el Eoceno superior. El estudio se realizo mediante trabajos de campo y tambren compilaciones y evaluaciones de 10s datos publrcados. Seis eventos tectonrcos compresrvos pueden distmguirse Eoceno superior (~~42 Ma). Oligoceno superior (262B Ma). Mioceno inferior (15- 17 Ma), Mioceno media (= 10 Ma], Miocene superior (- 7 Ma], y Cuaternario antiguo (~2 Ma) La intensidad y el area abarcado por cada uno de estos eventos compresivos es muy variable Generalmente estas fases tectonicas compresivas se producen durante 10s periodos de alta velocidad de las placas convergentes ademas 10s escasos datos estructurales parecen mdrcar que las drreciones de acortamrento estan paralelas con la direcion de convergencia de las placas de Nazca y America del Sur. El relleno de las cuencas sedimentarias andinas occure entre estas fases. Los relenos varian tanto en potencia como en naturaleza y estas drferencias reflejan 10s ambiantes tectonicos que regian entre /os fases compresivas. Se nota que /OS esfuerzos son esencialmente extensionales en el Altiplano. en la Cordillera Occidental y en las cuencas del ante-arco. En cambra son mayormente compresivos en el Sub-andino. La actividad magrnatica esta localisada en 10s Altos Andes. por 10 menos desde el Oligoceno ; el magmatismo ha sido mas activa entre 25 y 6 Ma. y su intensidad parece relacionada con la velocrdad de las placas en convergencia. Las observaciones, a Jo largo de las partes emergtdas del ante-arco. de las paleotopografias neogenas indican que la alta superficie cordillerana se hizo entre 26 y 6 Ma. es decir durante el periodo mroceno de alta velocidad de convergencra. El levantamiento andino se ha producido de manera discontmuo, con breves pcrtados de levantamrento mas o menos sincronos de las fases tectonicas compressivas.

Palabras claves : Tectonica - Levantamiento - Cronograma - Cenozoico - Andes Centrales - Peru - Bolrvia - Chile norte.

IIiTRODUCTION from field observations and from a re-evaluation of the available geological information. Our studies indi- High plateaus, such as Tibet or Central Andes, that are cate clearly that the Andean style of deformation has related to convergent setting are clearly characterized been variable, the best evidence of stress variation by a tensronal deviatoric state of stress (ARMIJO et a/., being the occurrence of several angular unconformi- 1986 : SEBRIER el al., 1985 ; LAVENU, 1986 : MERCIER et ties of regional extent which show that compressional a/. 1987). One of the major problems is thus to structures have been formed discontinuously. Assu- understand how therr high topography has been ming that angular unconformities included within formed. The aim of this paper is thus to report data on overlapping time-spans are contemporaneous, six the formation of the Andean Cordillera. in southern discrete compressional pulses cari be distingukhed Peru, western Bolivia and northern Chile, Le., between since 40-45 Ma B.P. In addition, morphological obser- about 140 and 206. Data presented here proceed vations show that these tectonic pulses appear roug-

86 Géodynamique3 (1-2). 1988 : 85-106 Tectonics and uplifi in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present

hly coeval with discontinuous phases of uplift which the Pacifie Coast. Region 2 constitutes the high are chiefly responsible for the formation of the Cordil- Andean plateau whose mean elevation is over 4 km ; leran topography between 26 and 7 Ma. it is subdivided into three morphostructural units (fig. 2) : the Western Cordillera, the Altiplano. and the Eastern Cordillera. Region 3 is composed of the Subandean zone and of the Amazonian Foreland CHRONOLOGICAL AND SEDIMENTOLOGICAL plain (fig. 2). Due to the scarcity of the data in region 3. EVOLUTION we have grouped them as the Subandean Lowlands. All these six morphostructural units formed during the In southern Peru, Bolivia and northern Chile (fig. 1). Meso-Cenozoic evolution of the Central Andes. Their present-day topography from the Peru-Chile trench to main characteristics are summarized below. the Brazilian shield defines three parallel major regions that characterize the Central Andes : (1) the The Pacifie Coast Andean Fore-Arc, (2) the High Andes, and (3) the The Pacifie Coast corresponds to the uplifted eastem Andean Foreland. Region 1 cari be subdivided into edge of the Cenozoic marine Upper slope basins two morphostructural zones (fig. 2) : the submerged which are limited to the East by the Coastal Cordillera. Andean slope and the emerged Pacifie Lowlands that This latter constitutes a 1 000 to 1 500 m high scarp are separated by the basement uplift of the Coastal which overlooks the Pacifie Ocean, indicating that a Cordillera. Lying eastward of the trench, between major fault zone lies oceanward from the coastline depth of 7 Ooo and 2000 m, the lower and middle (BRUGGEN, 1950 ; PASKOFF, 1978 ; SÉ~RIER et a/., 1979 ; slope is almost exempt of sediment and if an accre- MACHARÉ et al., 1986). In contrast, its continental side tionai’y piism is present it is very small and restricted toward the Pacifie Piemont is generally fairly flat, with to the lowermost siope (von HUENE et a/., 1985). The its top being tut by flat topography that corresponds Upper slope, shallower than a depth of 2000 m, to a Ter-Gary morphological surface : the Coastal corresponds to the outer fore-arc basins (MACHARE et Tarapaca Pediplain in northern Chile (MORTIMER et a/., 1986). We have analyzed only the emerged eas- SARL. 1972) and the Coastal Cordillera Top Surface in tern edge of these slope basins which trop out along southern Peru (S~BRIER eta/., 1979). The substratum of

Fig. 1. - Map showing main hydrographie features. active volcanoes (black triangles) and localities napes used in the text. Dolted lrnes indicate boundanes between Peru. Chile, Bolivia and Brazil

Géodynamique 3 (l-2). 1988 : 85-l 06 87 M. SÉBRIER, A. LWENU, M. FORNARI, J.-P. SOULAS

FI~ 2 - Morpho-srructural zonatlon sketch Line with attached open triangles Indicates the 8x1s of the Peru-ChIle trench

Cenozoic deposits is made of Precambrian and Pa- onto mobile continental shelf or Upper slope basins leozoic rocks locally overlain by Mesozoic marine that were episodically uplifted and slightly deformed volcanic formations. by compressional tectonics. Scattered deposits of at least five major Cenozoic In the northernmostpart of South Pet-u, the Ica region marine cycles trop out along the Pacifie Coast ; they (140s) exhibits the most comprehensive marine re- have yielded the following ages : Upper Eocene. cord. In fact this area shows affinities with the subsi- Upper Oligocene-lower Miocene. Upper Miocene, ding marine fore-arc of central Peru (MACHAR~ et a/., Pliocene, and Pleistocene-Holocene (fig. 3 and 4). 1986). From bottom to top the following marine for- These Cenozoic marine cycles are separated by mation are observed (fig. 4). unconformities, locally angular, and have tut several marine terraces that give a notched aspect to the The Paracas Formation (PETERSEN, 1954 ; NEVVELL, coastal scarp. At several localities, these terraces are 1956 ; RUEGG, 1956) consists some 600 m of yellow- covered with deposits exhibiting retrogradational ochre sandstones, shales and siltstones intercalated depositional structures, which clearly indicate that the with limestones. and reworked tuffs. These beds have terraces have been tut by marine transgressions onto yielded Upper Eocene fossils, with the reported a subsident margin. Generally. the huge coastal scarp Oligocene ones belonging to the unconformable has prevented Cenozoic transgressions to progress Caballas formation (MACHARE et a/.. 1986). inland. and as a consequence nearly all the remaining The Caba/as formation ( MACHARÉ et a/., 1986) consists Andean domain has been continental throughout the of some 300 m of shales and sandstones. An Upper Cenozoic. Each Cenozoic marine cycle is characteri- Oligocene-lower Miocene age is supported by pa- zed by a vertical evolution that from bottom to top leontological determinations made on these deposits. show decreasing clast sizes and bed thickness. Brec- cias, conglomerates, coarse sandstones are seen The Lower Pisco formation (de MUIZON and DEVRIES, toward the bottom while lutites, shales, clayish sands 1985 ; de MUIZON and BELLON, 1986) consists of about are exposed toward the top. Much evidence of basi- 500 m of upper Miocene beds that overlie uncomfor- nal instabilities are also observed (submarine sliding. mably the Caballas formation (MACHARE et a/., 1986). syn-sedimentary faults.. ). All these observations indi- It is made of yellowish sandstones, lutites. diatomites cate that Cenozoic marine beds were thus deposited and limestones (PETERSEN. 1954 ; RUEGG. 1956).

88 Géod)‘/Jd/7J/t&E 3 (l-2). 1988 85-106 PACIFIC PIEDMONT WESTERN CORDILLERA SUD ANDEAN ZONE

Fluvial Terraces

\/---d-\~- PAGORENE MAZUKO

------A--

CHUNTACALA

IPURURO ------

VOLCANICS

.-.-----./. __------

MOQUEGUA

Fig. 3 - Chronological table showing the correlations of the Cenozoic formations and the timing of the compressionai deformations. names in block letters (i.e : PunoJ represent regronal formation. Italie prints are used for local formations or facies (i.e : Nazca). Radiometric ages are located inside circles, they are in mrllron years , more precisions are given in the text. Marine formations are represented by stippled areas. Undulating lines represent the compressional tectonic phases, where they are dotted the krnd of geometric relation between the formations is not known M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS

6.5 18 Diable 9 16.9 Chuntac ‘f”i4.2 OxaYa 23 Hua ylil Altos 25 de 22.8 Moque Azapa Pica 59

Fig. 4 - Llthologrcal columns of the Pacifie Coast and Lowlands sections, A PISCO (14oS. 76oW) : B Camana (16030’s. 73oW) , C : Caraveli (16r~S. 7303O’W) D Moquegua (17030’s. 71oW) E : Arica (18030’s 7OoW) F Mamltîa (2OoS, 691130’). 1 : Conglomerates 2 Sandstones : 3 ..Pelltlc rocks 4 Carbonates : 5 Gypsum : 6 : Lavas ; 7 : Tuffs : 8 Fossilbearing level. Numbers Radlometric dates rn Million Years

The Upper Pisco formatlon (de MUIZON et and DEVRIES. Pacifie Piemont (fig. 2) in Peru and the Longitudinal 1985) overlie unconformably the Lower Pisco forma- Depression in Chile. Its flat topography, i.e., South tion. It consists of some 150 m of sandstones and Peruvian G Pampas )) or Chilean (( Pampa del Tamaru- siltstones intercalated with diatomites, tuffs and eva- gal )), is gently inciined seaward and incised by deep porites. Radiometric and paleontological determina- canyons. Its substratum is similar to that of the tions give a Pliocene age to this formation (de MUIZON, Coastal Cordillera, but Mesozoic marine and early 1979 : de MUIZON and BELLON. 1980). Tertiary continental volcanic rocks and their plutonic Pleistocene marine terraces trop out only along the counterparts are much more common. coastal scarp. Their great elevation that reaches The Pacifie Lowland basins are slightly asymetric. As approximately 600-700 m is interpreted as a conse- mentioned above (see § « The Pacifie Coast ))) their quence of the aseismic Nazca ridge subduction contact with the Coastal Cordillera in general is not beneath this part of the fore-arc (MACHARE et a/., 1986. faulted, excepted in the Chilean Antofagasta region In centra/ southem Peu, around Camana (1603O’S). 230s) where the east-facing Atacama fault is exposed the following formations cari be observed (fig. 4) : l OKADA, 1971). In contrast, the eastern edge of these The Camana formation (RIVERA, 1950 ; RÜEGG, 1952 : Pacifie Lowland basins is generally faulted by a PECHO and MORALES, 1969) is composed of 600-800 m west-facing. often east-dipping major fault zone that of yellowish lutites. sandstones, and limestones that separates them from the High Andes, the upthrown have yielded Upper Oligocene to lower Miocene block. The Pacifie Lowland basins are thought to have fossils (RUEGG. 1952 ; DROOGER, 19% ; PARDO in PECHO initiated approximately during Oligocene time, be- and MORALES, 1969). It is thus a lateral-equivalent of cause their deposits are overlying unconformably the Caballas formation. This transgression went over volcanic beds that are related to a Paleocene to the Coastal Cordillera, cutting its top surface (S~BRIER Eocene (only known in northern Chile) magmatic arc et a/., 1979). and invaded the Pacifie Piedmont in the (LAUGHLIN et a/.. 1968 : OUIRT et a/., 1971 ; SILLITOE, vicinity of Caraveli (SEBRIER et a/., 1982 ; HUAMAN, 1981 ; COIRA et a/., 1982 ; VATIN-PERIGNON et a/., 1982). 1985) These basins are filled with unfossiliferous continental clastic deposits. which are interbedded with nume- The La Planchada formation (LAHARIE, 1975 ; BEAUDET rous ignimbritic tuffs that allow generally these depo- et a/., 1976) consists of sandstones, sands, lutites, and sits to be dated. The sedimentary evolution cari be conglomerates of Pliocene age. It overlies unconfor- divided into two major periods : (1) from Oligocene mably the Camana formation, being inset within it. Its to lower-middle Miocene. basins are characterized by outcrops are seen up to an elevation of some 250 m. aggradational processes while, (2) from Upper Mio- In Northernmost Chile, between Arica (18030’s) and cene to Present. degradation prevails In detail. major Antofagasta (23030’s). Miocene and Pliocene marine aggradation ceased during lower Miocene (ca 17 Ma) beds are known along the Coast. The largest outcrops in southern Peru (SEBRIER et a/.. 1982 ; TOSDAL et a/., are exposed to the North of Antofagasta where the 1984 ; HUAMAN, 1985). while it ended after 12 Ma and Pleistocene Mejillones Formation overlies unconfor- priorto 10 Ma in northern Chile (MORTIMER, 1973). This mably the Miocene-Pliocene La Portada Formation discrepancy may reflect climatic differences or limted (FERRARI~ and DI B~ASE, 1978). knowledge of the Longitudinal Depression filling. In the Pacifie Lowlands, i.e., in the inner .fore-arc The Pacifie Lowlands basins, the Oligocene to Present evolution cari be The Pacifie Lowlands is a 70 km wide unit that consti- summarized as a succession of five series (fig. 3 and tutes a series of elongated basins known as the 4).

90 Gé&ynamique 3 (1-2). 1988 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present

(1) The Lower Oligocene series are characterized by fluvial terraces that were deposited while rivers were clastic red beds intercalated with many evaporitic down-cutting present-day valleys within Pliocene beds (mainly gypsum). They are known in Peru, as the Series 4 (SÉBRIER et a/., 1982). Lower Moquegua Formation (STEINMANN, 1929 ; sÉ- BRIER et aL 1979 ; MAROCCO et a/., 1986), and in Chile as the Azapa (SALAS et a/., 1966), Sichal (MAKSAEV, The Western Cordillera 1979 and San Pedro-Tambores Formations (LAHSEN, The Western Cordillera is approximately 150 km wide 1982 l Only two of these formations, located in the and forms a high plateau with a mean elevation of Precordillera nearby the boundary between the Wes- 4 500 m. On its top, 5 OOO- to 6000 m high calc- tern Cordillera and the Longitudinal Depression (22oS, alkaline active volcanoes lie ; they correspond to the 69oW). have in fact yielded K/Ar radiometric ages : the main magmatic arc. This last one is characterized by Sichal, 34.7fl Ma (MAKSAEV, 1978) and the San the pilling up of Cenozoic volcanic materials. The Pedro, 28+6 Ma (TRAVISANY, 1978). substratum of Cenozoic volcanics consists mainly of (2) The Upper Oligocene to lower Miocene series Mesozoic marine clastic rocks that are cropping out in few places : in the bottom of deep canyon valleys or correspond to Piemont deposits of gravels conglomerates and sandstones intercalated with gypsum and at the top of paleo-relief. ignimbritic tuffs. These beds unconformably. overlie In the Western Cordillera. lithologies and thicknesses series 1. They are known, in Pet-u as the Upper are highly variable (fig. 5). However two main volcanic Moquegua Formation (STEJNMANN, 1929 ; SÉBRIER et groups, Oligo-Miocene and Plio-Pleistocene, may be a/., 1982 ; TOSDAL et al., 1984) and the Huaylillas distinguished (fig. 3 and 5). The Oligo-Miocene group Formation (WILSON and GARCIA, 1962 ; SÉBRIER et al., corresponds, in southern Peru to the Tacaza group, 1979 ; TOSDAL et al., 1981), and in Chile as the Oxaya dated between 24.2 Ma at Paso Huaylillas (17045’s) Formation (SALAS et al., 1966) ; or the Tarriarugal on the Peru-Chile border (SÉBRIER etal., 1983 ; BELLON, gravels (GALLI and DINGMAN, 1962). In fact the Upper written com.), and 8.9 Ma at Cailloma (FORNARI et a/, Moquegua Formation corresponds mainly to clastic 1983) ; in Bolivia it corresponds to the Mauri, Abarba facies while the Huaylillas Formation is mainly made and Upper Quehua Formations dated between 26 and of ignimbritic tuffs that have yielded ages ranging 10.5 Ma (EVERNDEN etal., 1966 and‘1977 ; KUSSMAUL et bétween 25 and 17 Ma (MORTIMER et a/., 1974 ; BELLON al., 1973 ; LAVENU, 1986). The Plio-Pleistocene group and LEFEVRE, 1976 ; BAKER and FRANCIS, 1978 ; NOBLE et corresponds, in southern Peru, to the Barroso Group al.. 1979 ; TOSDAL et al., 1981 ; COIRA et al., 1982 ; and to various local formations such as the Cerke, LAHSEN, 1982 ; VATIN-PÉRIGNON et al., 1982 ; S~BRIER et Perez and Chararïa (LAVENU. 1986). Chronological data al.. 1983). In southern Peru around Caraveli (16oS), a on northern Chile, reported by LAHSEN (1982). also marine ingression is interbedded in the Upper Mo- agree with a subdivision in two main groups. The quegua Formation (SÉBRIER et al., 1979 ; HUAMAN, Maure Formation of southern Peru (MENDIVIL, 1965) is 1985). it is dated on plagioclase at 25.5-r 1 Ma and on a Miocene lacustrine facies interbedded within the biotite at 24.5kO.8 (NOBLE et al., 1985) ; these are the Tacaza group. In fact, several Pliocene lacustrine beds only Cenozoic marine beds known in the whole inner are erroneously mapped as Maure, and we suggest to fore-arc basins of southern Peru and northern Chile. name them Pati Formation as the Pati beds are conformably overlying lower Barroso andesites dated (3) The middle to Upper Miocene series is characteri- at 4.4 Ma (BELLON and LEFÈVRE, 1976). Pleistocene zed by grave1 deposits intercalated with ignimbitic lacustrine beds such as the Capillune Formation tuffs and andesites dated between 7 and 15 Ma (MENDIVIL, 1977) are interbedded within the Upper (MORTIMER et al., 1974 ; TOSDAL et al., 1981 ; LAHSEN, Barroso Formation. 1982 ; HUAMAN, 1985). Miocene series 3 rest unconformably on Upper Oligocene-lower Miocene se- The occurrence of lower Oligocene volcanics is still ries 2. In addition in southern Peru, they partly fill dubious in the Western Cordillera. This epoch ap- broad valleys that were excavated within series 2. In pears to be characterized instead by continental northern Chile they correspond to a wide erosional clastic red beds such as the Sichal Formation of surface known as the Tamarugal pediplain (MORTIMER northern Chile dated at 34.7 Ma (MAKSAEV, 1978) or the and SARI~, 1975). Series 3 beds are known as the Lower Quehua of southwestern Bolivia (KUSSMAUL et Chuntacala Formation in Peru (MANRIQUE and PLAZOL- al., 1977). In southern Peru the Pichu Formation is LES, 1974 ; TOSDAL et al., 1981 and 1984 ; SÉBRIER et al., older than 16 Ma (SÉBRIER et al., 1983). It could thus be 1982) and the Diablo Formation in northern Chile. either lower Miocene, i.e., ,coeval with the lower part of the Tacaza Group (Huilacollo and Tingopalca vol- (4) The Pliocene series is characterized by facies canics) or Oligocene as considered by AUDEBAUD et a/. similarto those of Miocene series 3. In addition, these (1976). Pliocene beds partly fil1 deep canyons incised within Miocene series 3 during uppermost Miocene time (M~RTIMER, 1973 : LAtiAfw, 1975 ; sCBRER et a/., 1979 The Altiplano and 1982 ; TOSDAL et al., 1981 and 1984). The Altiplano (fig. 2) ii a high endoreic plain situated (5) The Pleistocene series eonsists of three stepped at .a mean elevation of nearly 4 000 m, i.e. slightly

Géodynamique 3 (I-2). 1988 : 85-106 91 M. SÉBRIER. A. LAVENU, M. FORNARI, J.-P. SOULAS

iipaz (2X ‘67oW) S$nbols‘ds in frg. 4 below the level of the Western and Eastern Cordillera. is roughly coeval with the Mauri Formation tow&d the It is 150 km wide and about 1 500 km long, extending Western Cordillera. The Pliocene period corresponds from southeast of Cuzco (140s) to the Puna of Argen- mainly to lacustrine deposits interbedded with ignim- tina (270s). The Altiplano is thus restricted to the britic tuffs dated between 7.6 and 3.27 Ma (EVERNDEN central part of Central Andes. During the whole et a/., 1966 and 1977 ; CLAPPERTON. 1979 ; GRANT et a/., Cenozoic era the Altiplano basin has been strongly 1979 : LAVENU. 1986). The Pleistocene period is cha- subsiding ; the debris resulting from the erosion of racterized by the deposit of the Paleo-Titicaca and the surrounding Cordilleras and from volcanic emis- Poopo lacustrine beds, what form a tier of five lacus- sions have accumulated there, lying on a substratum trine terraces, each separated from the next by a made mainly of lower Paleozoic and Cretaceous major incision (LAVENU, 1984). Major deposits corres- rocks. pond to the Ulloma Formation, known also as Azan- The Altiplano basin is characterized by very thick garo Formation on the Peruvian Altiplano. These continental series of Cenozoic deposits (fig. 6). The lacustrine beds are coeval with fluvial terraces and most comprehensive sedimentological record is seen morainic deposits. In the northern part of the Bolivia Altiplano (Coro-Coro Each time that detailed field studies have been per- and Umala areas). There, the Cenozoic series could formed, the reported thicknesses appeared to be reach 21 500 m (reported in LAVENU, 1986). However, over-estimated (LAVENU, 1986). Nevertheless, the the Cenozoic series has not been deposited conti- Miocene Coro-Coro series have a thickness of the nuously : in fact. five main depositional periods cari order of 10000 m. Therefore. even if the total thick- be distinguised, separated by folding and related ness must be lowered, the Miocene epoch is charac- unconformities (fig. 3 and 6). The lowermost one is terized by a high rate of deposition. Paleocene-Eocene in age and ends with the Late Eocene Tihuanaku Formation (MARTINE~, 1980). The On the Peruvian Altiplano. the Ayaviri area show : following period is lower Oligocene in age, dated (1) the Eocene Mu%~ni Formation (AUDEBAUD et a/., near its top at Kollu Kollu at 33.6 Ma (EVERNDEN et a/.. 1973) ; (2) the Oligocene Puno or Ayaviri Formation 1966) Then, the Upper Oligocene to Upper Miocene (CHANOVE et a/., 1969 ; AUDEBAUD and VAIN-PERIGNON, period is characterized by a higher rate of deposition 1974) ; (3) The Lower Tinajani Volcanics, dated bet- (i.e., subsidence rate) : an average of more than ween 19 and 27 Ma (BONHOMME et a/,. 1985) ; (4) The 0.5 mm per year that could reach in some members Lower Thajani Formation, dated between 14 and 1 mm peryear (LAVENU, 1986). Radiometric dates have 18 Ma (BONHOMME et a/.. 1985) ; (4) The Upper Tina- been obtained only from its Upper part, between 8 jani Formation, attributed to Upper Miocene (AUDE- and 11.7 Ma (EVERNDEN et a/.. 1966 and 1977 ; LAVENU, BAUD et a/.. 1976) : and (5) The Barroso Group. In the 1986). This Upper Oligocene to Upper Miocene period Puno area. NEWELL (1949) reported a 7 000 m thickness

92 Géodynamique 3 (I-2). 1988 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present for the Oligocene Puno Formation, it is unconforma- The Eastern Cordillera bly covered by the Vilque latites dated between 4.2 and 6 Ma (BELLON and LEFÉVRE, 1976 ; KANEOKA and The Eastern Cordillera (fig. 2) is a 4 000 m high plateau GUEVARA, 1984 ; FERAUD, written com.), which in turn about 150 km wide with some glaciated range sum- are covered by the Pleistocene lacustrine beds of the mits that are over 6 000 m. Durinq the whole Cenozoic Paleo-Titicaca. lake. era it was an uplifting area ii which some small

\ \ 26 \ \ 33 \ \ \ \ \ 5.4-6.4

Frailes

4-18 9-28

---

Fig. 6. - Lithological columns of the Altiplano sections ; N : Lipez (220% 67oW) : 0 : Ayaviri (15%. 7003O’W) ; P : Puno (16% 7OoW) : 0 : Coro Coro (1703O’S. 6803O’W) ; R : Tambo Tambillo (19030’s. 67030’W). Cm : Middle Cretaceous : L.T.V. LowerTinajani volcanics : Q : Quaternary. Symbols as in fig. 4

Géodynamique 3 (1-2). 1988 : 85-106 93 M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS intermontane basins formed. Their substratum is two narrow zones : the 50 km wide Subandean zone made of Paleozoic and Mesozoic rocks intruded by and the 50 to 100 km wide Amazonian Foreland plain. plutonic material. The Eastern Cordillera corresponds The Subandean zone is an hilly area, with elevations to the locus of the (( inner magmatic arc )) (CLARK etal., ranging between 400 and ‘1 000 m that corresponds to 1983). and since upper Oligocene time. it was charac- a Cenozoic fold and thrust belt that has been defor- terized by the occurrence of both mantle and crustal med mainly in the late Miocene. The Amazonian derived magmatism. foreland plain corresponds to the present-day area of In southeastern Peru. near Macusani, the Crucero sedimentation and approximately to the easternmost basin is characterized by sedimentary series (fig. 7) front of the Andean deformations. that are lithologically similar to those of the Altiplano Due to unfavourable field conditions, the Subandean (LAUBACHER et a/., 1987). They consist of : (1) The geology is still poorly known. From Upper Cretaceous Oligo-Miocene Caycone continental Formation, and to Present it is characterized by five series (fig. 3 and (2) the Pliocene Arcoaja Formation dated at 3.9 Ma 9). (1) Upper Cretaceous series correspond to sand- (LAUBACHER et a/., 1984a). The whole series js uncon- stones and Maestrichtian marine Iimestones (DAVILA formably covered by Pleistocene fluvial terraces and and PONCE DE LEON, 1971). (2) Paleogene series cor- outwash to morainic deposits. The middle part of the respond in southern Peru to the Huayabamba and in Caycone Formation contains sub-alkaline basalts and Bolivia to the Quendeque and Bororigua Formations crustal derived ignimbritic tuffs dated between 15.9 (PARDO et a/., 1973 ; SANJINES and JIMENEZ, 1976 ; and 24.8 Ma (LAUBACHER eta/., in press). In the vicinity MARTINE~, 1980). (3) Oligo-Miocene series are formed of Macusani, the Macusani ignimbritic tuffs, dated in southern Peru by the lpururo, and in Bolivia by the between 4.2 and 9 Ma (BARNES et a/., 1970 : NOBLE et Charqui, and lower Oligocene to Miocene Petaca a/., 1984). fil1 a paleo-valley oriented toward the Formations (PARDO etal., 1973 ; SANJINES and JIMENEZ, Amazonian foothills. 1976 ; REYES, 1976). (4) Neogene series correspond, In the Cochabamba basin (fig. 7), continental red in southern Peru to the Pagorene (PARDO et a/., 1973) beds, attributed to the Paleogene-Miocene epoch, or Mazuko Formations (LNJBACHER et a/.. 1984b) and are unconformably overlain by Pliocene lacustrine in Bolivia to the Pliocene Upper Chaco conglomerates beds. These ares are folded and covered by Pleisto- (MARTINE~, 1980). (5) Pleistocene to Present series are cene fluvial terraces (LAVENU, 1986). In the Potosi area composed of fluvial terraces. (fig. 7), volcanic and sedimentary bed, dated between 18.8 and 21.9 Ma (EVERNDEN et a/., 1966 and 1977 : GRANT et a/., 1979). are tut by the Ag-bearing Cerro Rico stock, dated at about 14 Ma (GRANT et a/., 1979). TIMING OF COMPRESSIONAL TECTONIC PHA- The previous formations are unconformably covered SES by the Los Frailes ignimbritic tuffs dated at 7.4 Ma (GRANT et a/., 1979). In the vicinity of La Paz, the In Central Andes, STEINMANN (1929) distinguished Cordillera Real and Quimsa Cruz exhibit intrusives three main compressional tectonic phases during the bodies (fig. 7) dated between 19.2 and 28.4 Ma Andean orogenic cycle : the Peruvian, Incaic, and (MCBRIDE et a[. 1983). Quechua Phases. These are dated Upper Cretaceous (Santonian). Upper Eocene, and uppermost Miocene respectively (AUB~~IN et a/., 1973 : Au~Ef3Aut1 et a/., The Subandean Lowlands 1973 ; MÉGARD, 1973, 1978 : DALMAYRAC et a/., 1977, The Subandean Lowlands corresponds to the Ama- 1980; MARTINE~, 1978; MAKSAEV, 1978; NOBLE et a/., zonian piedmont of the Andes. They formed a subsi- 1979). Nevertheless the above-mentionned authors ding trough in which the debris resulting from the and additional works (MORTIMER, 1974 ; SOULAS, 1975, erosion of the Andean Cordillera accumulate (fig. 8). 1977 ; KUSSMAUL et a/., 1977 ; SÉBRIER et a/., 1979,1980, These Subandean Lowlands cari be subdivided into 1982 ; COIRA et a/., 1982 ; LAHSEN et a/., 1982 ; MÉGARD,

Pli0 Cayconi 22.9 7.4 24.8 Mi0 18.8 Mondragon PS 21.9 Agoa Dolce

Fig. 7 - Lithological columns of the Eastern Cordillera sections : S : Crucero (14030’s. 7OoW) T : Cordlllera Real-Quimça Cruz (lôn30’S. 68oW) U Cochabamba (17030’s. 66oW) V Potosl (19~3O’S. 66oW). Pg : Paleogene Mio : Miocene : PIIO : Pllocene ; Q : Quaternary Symbols as In fig. 4

94 Géodynamique 3 (l-2). 1988 : 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present

Quebrodo ssw Rio Dos de Moyo I Rio Morcoooto I Nusiniscoto Mozukb Sonto Rose NEE

Cordillère Bos.Sin 4nticlinol Plaine l l Orientale - de Puente - Collines Sub Andines - do Quinte Mil Inombori Modre de Dios m -3000

0 10 km L-5-J

Fig. 8. - Cross section in the southern Peru Subandean Lowlands along the Inambari river. PI : Lower Paleozoic metamorphic rocks with Upper Oligocene dioritic intrnsion : Kr : Upper Cretaceous sandstones ; Pg : Paleogene red beds ; Ng : Neogene grey beds. Dot pattern : Early Quaternary of the Quinte Mil basin and Upper Quaternary of the Madre de D~OS river plain

1984 ; MÉGARD et a/., 1984) emphasize that the Andean is bracketed (MAKSAEV. 19791 between 34 Ma (SichaI evolution is characterized by more than three tectonic Formation) and 42.2 Ma (Icanche Formation).’ As in pulses. Even if their spatial distribution and related central Peru it is prior to 41 Ma (NOBLE eta/., 1979) this deformational amplitude are strongly variable, at least tectonic pulse should be therefore dated at about 9 discrete pulses cari be distinguished with the follo- 42Ma. . wing ages : Albian (Mochica phase), Santonian (Pe- This Upper Eocene compressional pulse has produ- ruvian phase), uppermost Cretaceous-Paleocene, ced the westfacing Putina imbricated thrust zone Upper Eocene (lncaic phase), Upper Oligocene (Ay- (IAUBACHER, 1978) that is located along the northern mara phase), lower Miocene, middle Miocene, Upper Miocene (Quechua phase), Upper Pliocene-early Quaternary.

T Pliocene The upper Eocene compressional tedonic -----_U.Charqui phase, Fl (ca 42 Ma) T Upper This tectonic phase is one of the major compressional Miocene pulse of the Central Andes ; it is observed in most of Andean domain (AUBOUIN et a/., 1973 ; AUDEBAUD et a/., 1973 ; MEGARD, 1973, 1984 and 1985 ; DALMAYRAC et a/., 1980 ; MARTINE~, 1980 ; COIRA et a/., 1982 ; LAH- SEN, 1982 ; MALUMIAN and RAMOS, 1984 ; CAMINOS et a/., 1985). In the studied area it is demonstrated by the unconformities seen between (fig. 3) : the Oligocene Puno and Eocene Mufiani Formations of the Peruvian Altiplano (CHANOVE et a/., .1969) ; and the lower Oligocene Petaca and Paleogene Huayabamba For- T Lower mations of the Bolivian Sub-Andes (SANJINES and Oligocene JIMENEZ, 1976). In the Peruvian Pacifie Piedmont, the unconformity between the Paleocene Toquepala vol- canites and the undated Lower Moquegua Forma- tions may be due to this Upper Eocene tectonic pulse. On the Bolivian Altiplano it is not clearly demonstrated, but if it occurred there, its major effect could be the sedimentation of the Coniri conglomerates bet- 0 ween the Tihuanaku and Kollu Kollu beds. Fig. 9. - Lithological columns of the Subandean Lowlands sections ; W : Madre de Dios (130s. 7003O’W) ; X : Beni (16%. 67oW) : In the Upper Rio Loa Precordillera of northern Chile Y : Santa Cruz (18% 6303O’W). Cs : Upper Cretaceous : Q : (21030’s) the age of the Upper Eocene tectonic pulse Quaternary. Symbols as in fig. 4

Géodynamique 3 (l-2). 1988 : 85-106 95 M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS

shore of the Titicaca lake, on the boundary separating to the Oligocene epoch (AUDEBAUD et a/., 1976 ; SE- the Eastern Cordillera from the Altiplano. We have not BRIER et a/., 1979 and 1982) ; (3) in the northern performed structural observations of the Upper Eo- Chilean Longitudinal depression, between the lower cene deformations. nevertheless available data (MÉ- Miocene Oxaya-Altos de Pica Formations and the GARD, 1973 and 1978 ; DALMAYRAC et a/.. 1977 and underlying red beds dated at 2826 Ma in the Ata- 1980 : MARTINE~. 1980) suggest that the shortening cama district (TRAVISANY, 1978). direction trends roughly NE-SW, i.e.. approximately This tectonic pulse is also reported : in Colombia at parallel north the Upper Eocene convergence of the about 24 Ma (DUQUE-CARO. 1976) : in north-central Farallon and South American plates (PILGER, 1983). Chile (290-310s) prior to 27 Ma (MAKSAEV et a/., 1984) : and NW Peru (MACHARE et a/.. 1986). In central Peru, its evidence is still unclear. Structural analysis per- The Upper Oligocene compressional tectonic formed in the Pacifie Piedmont of southern Peru pulse, F2 (ca 26-28 Ma) (SEBRIER et a/., 1982 ; HUAMAN, 1985 : MACHARC et a/., The best evidence for an Oligocene compressional 1986) indicate a roughly NNE-SSW trending direction phase is found in southwestern Bolivia (fig. 6). where of shortening, i.e., approximately parallel with the the lower Miocene Upper Quehua Formation dated at Upper Oligocene (between 26 and 28 Ma) direction of 24 Ma overlies with an angular unconformity the convergence (PILGER, 1983). This compressional Oligocene lower Quehua Formation dated at 22.9 Ma phase corresponds thus with the beginning of the fast (WSSMAUL et a/.. 1975). Although these old ages convergence period (Pilger, 1983) that extended up to should be recalculated using new decay constants, Upper Miocene time (ca 6-7 Ma) ; this latter period they indicate a tectonic pulse located approximately being coeval with an increase of volcanic activity. Due in the Upper Oligocene. In the other areas. mapping to its large areal extent and because it is clearly and regional studies support this conclusion. On the demonstrated on the Bolivian Altiplano we propose to Bolivian Altiplano (fig. 3)‘ MARTINE~ (1978) reports a name this Upper Oligocene compressional tectonics : compressional pulse bracketed between 34 and the Aymara phase. 26 Ma according to EVERNDEN’S (1966) radiometric dating. In northern South Peru. at the boundary between the Western Cordillera and the Altiplano. an The lower Miocene compressional tectonic angular unconformity is located between an Oligo- pulse, F3 (ca 15-17 Ma) cene magmatic arc, dated at 32.5-34.7 Ma, near Tin- This phase is presently poorly known because, either taya (NOBLE et a/.. 1984). and the Tacaza volcanic the unconformable overlying formations are impreci- Formation dated at 27.2+3 Ma near Antabamba (PE- sely dated. or the reported unconformities are un- CHO, 1981). Likewise. on the central Peruvian Alti- clear. Whatever the case, structures related to this plano, in the vicinity of Ayaviri (fig. 6). the Lower tectonic pulse (monoclinal folds and reverse faults) Tinajani Volcanics, dated between 19 and 27 Ma appear to be small and the main effect of this phase (BUNHOMME et a/.. 1985). rest unconformably on the is exhibited by the evolution of landform (see sec- . Oligocene Puno Formation (AUDEBAUD and VATIN-PE- tion 5). In the Peruvian Pacifie Piedmont, this phase is RIGNON, 1974) ; in addition, nearby Puno the Tacaza roughly coeval with the large valley incision th,at took Formation is seen unconformably on the Puno one place between the lower Miocene Huaylillas and (NEWELL, 1949). In the Western Cordillera. evidence for middle-Upper Miocene Chuntacala Formations (SC- an Oligocene phase in unclear because the Pichu BRIER et a/.. 1979 and 1982 : TOSDAL et a/., 1981 and Formation remains undated. If its proposed Oligo- 1984, HUAMAN, 1985). cene age (AUDEBAUD eta/., 1976) is confirmed. thus its unconformity with the Tacaza Formation (MAROCCO In fact this valley incision corresponds to a major and DEL PINO, 1966) should be due to an Oligocene change : aggradation was previously the main land- pulse. In the Ichurïa-Characato area. where the un- form process within the Pacifie Piedmont whereas, conformity has been observed, the oldest ages yiel- erosion (i.e., uplift) has subsequently been the domi- ded by the Tacaza Formation are about 16 Ma (VATIN- nant one. PERIGNON et a/., 1982 ; SÉBRIER et a/.. 1983). they indi- This compressional pulse should be (fig. 3) : (1) on cate that the Tacaza-Pichu unconformity is at least the Pacifie Piedmont bracketed between 14.2 and lower Miocene in age. In the Central Andean Fore- 17.6 Ma (SÉBRIER et a/.. 1982) ; (2) in the Western Arc. the Oligocene compressional tectonics is shown Cordillera prior to the 15.8 Ma Tintinave ignimbritic by the following unconformities (fig. 4) : (1) in the Ica tuff (SEBRIER et a/., 1983) ; and (3) on the Peruvian region. between the Upper Oligocene Caballas and Altiplano bracketed between 18 and 19 Ma (BON- Upper Eocene Paracas marine Formations (MACHARE HOMME etal.. 1985a). In the Eastern Cordillera, the 15.9 et a/., 1986 and 1988) ; (2) in the Peruvian Pacifie ignimbritic tuff of the Crucero basin could be uncon- Piedmont. between the lower Miocene-Upper Huaylil- formable with the Caycone volcanics dated at 22.8 Ma las-Upper Moquegua Formations dated between 17.6 (BONHOMME eta/,, 1985b). This compressional event is and 25.3 Ma (TOSDAL et a/., 1981 : NOBLE et a/., 1985) bracketed : in south central Peru (120-1303O’S), bet- and the underlying lower Moquegua that is attributed ween 14 and 17 Ma (IVIÉGARD et a/.. 1984) : in north

Geodynamique 3 (I-2). 1988 : 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present

central Chile (290-31oS), between 16.6 and 18.9 Ma Peru (MÉGARD et a/,, 1984 : BONNOT, 1984), in northern (MAKSAEV et a/., 1984) ; and in central Chile (330-360s , Chile (LAHSEN, 1982). in the Argentine Puna (SCHWAB between 14 and 21 Ma (DRAKE et a/., 1976 and 1982 i and LIPPOLT, 1974), and in Central Chile (DRAKE et a/.. In Colombia evidence exists for compressional de- 1976 and 1982). formations at about 15 Ma (DUQUE-CARO, 1974).There- The Pacifie Coast and Piedmont structures are repre- fore this lower Miocene compressional event, ap- sented by reverse and strike-slip faults, as also by pears to be bracketed between 15-17 Ma. monoclinal flexures. The Western Cordillera and the Altiplano are characterized by folds and by reverse Middle Miocene compressional tectonic phase, and strike-slip faults. In the Eastern Cordillera mainly F4 (ca 10 Ma) faults are observed, whilst in the Sub-Andean zone folds and thrusts predominate. As AUDEBAUD et a/., As for the lower Miocene compressional pulse there (1976) ; LAUBAC.HER (1978) ; MARTINE~ (1978) and si- is only little evidence for a middle Miocene tectonic BRIER et a/., (1979) have already emphasized, this pulse. In the Peruvian Pacifie Piedmont (fig. 4). re- compressionai tectonic phase displays a preponde- verse faulting, affecting the Chuntacala Formation, rance of faulting over folding. Structures associated are dated between 9.4 and 10.8 Ma (HUAMAN, 1985). with Late Miocene movements bear witness to the Similarly in the North Chilean Longitudinal Depres- importance of strike-slip tectonics in the deformation sion (fig. 4), the Diablo Formation, dated at 9 Ma, of the High Andes continental trust. rests unconformably on the Oxaya-Altos de Pica Formations dated between 15.5 and 23 Ma (MORTIMER Results obtained by structural analysis seem com- plex. According to FORNARI et a/., (1978) the compres- et a/., 1974). The unconformity located between the sional stress axis of this phase would be E-W trending Upper and Lower Tinajani (AUDEBAUD and VATIN-PÉRI- in the Western Cordillera of South Peru. The same GNON, 1974) should correspond to this tectonic pulse direction of shortening is proposed for the Upper (fig. 3). Miocene compressional deformations of the Peruvian In central Peru this compressional pulse is dated Fore-Arc (HUAMAN, 1985 ; MACHARE et a/., 1986). They between 9 and 10 Ma (MEGARD et a/., 1984). Evidence are thus identical to that found by SOULAS (1975) for of compressional deformations are also mentioned in this same phase, in Central Peru, to the North of the Colombia (DUQUE-CARO, 1974) and in northwestern Abancay deflection. On the other hand, on the Boli- Argentina (ALLMENDIGER, 1986). vian Altiplano. the disposition of (( en échelon v folds In the Pacifie Piedmont of southernmost Peru, the in the Miocene beds, with an average orientation of middle Miocene phase is coeval with an erosional the axes of NllOoE, along the San Andres fault period that formed stepped pediplains (TOSDAL et a/., striking N1450E is in agreement with a right lateral 1984 ; see section K Andean uplift )). Although the horizontal displacement related to an approximately lower and middle Miocene compressional phases are N-S trending shortening. Similarly results from the poorly demonstrated it is unlikely that all the com- Crucero basin (LAUBACHER et a/., 1987) suggests a N-S pressional deformations have been produced by a trending shortening. There are thus three possible single tectonic phase. Nevertheless these two com- interpretations : (1) Either, the stress pattern varies pressional pulses may be interpreted as climax events between the Western Cordillera of South Peru and the belonging to a Miocene period that would be charac- Bolivian Altiplano. (2) Orthis phase is subdivided into terized by compressional deformations. The few two compressional pulses, N-S and E-W trending structural data on these two compressional pulses respectively. (3) Orthis is a geometrical consequence suggest that shortening was oriented roughly E-W of the Andean oroclinal bends, as we proposed for (HUAMAN, 1985). the early Quaternary tectonic pulses.

Late Miocene compressional tectonic phase, F5 (ca 7 Ma) Late Pliocene-Early Quaternary compressional This late Miocene compressional phase is best dated tectonic movements, F6 (ca 2 Ma) on the Bolivian Altiplano where it took place between This phase was first reported in Bolivia where it is 7.4 Ma and 8 Ma (fig. 3 and 6)‘. Elsewhere, it is also prior to 2.5 Ma (EVERNDEN, 1966 ; MARTINET 1980). relatively well dated. It falls before 7.2 Ma in the Now it is known in virtually the whole Central Andes Western Cordillera, 5.7 Ma in the Peruvian Altiplano (SÉBRIER et a/.. 1979 and 1982 ; S~BRIER and MACHARE, and 3.9 Ma in the Eastern Cordillera. In the Pacifie 1980 ; BLANC, 1984 ; BONNOT, 1984 ; MALUMIAN and Piedmont and on the Pacifie Coast it is placed before RAMOS, 1984 ; ALLMENDIGER, 1986 ; LAVENU, 1986). the Pliocene deposits (fig. 3). In the Sub-Andean Nevertheless, in the Western Cordillera it is still not zone, the reputedly Plio-Pleistocene Pagorene Forma- clearly characterized. Likewise in the Sub-Andean tion is posterior to this phase. zone, it is often difficult to distinguish its effects from This Upper Miocene compressional pulse is reported those of Late Miocene tectonics (fig. 3). However, in along the whole Andean chain. It is dated at about the southermost part of the studied region, these two 6-7 Ma : in Colombia (VAN HOUTEN, 1976). in central tectonic phases have clearly been separated due to

Géodynamique 3 (I-2). 1988 : 85-106 97 M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS the occurrence of two angular unconformities of located (fig. 10). In the Central Andes of South Peru, regronal extent (AHLFELD and BRANISA, 1960). North Chile, and Bolivia, compressional deformations The effects of these Latest Neogene compressional that occurred from Oligocene to Present would result movements are generally local. In the Pacifie Coast from five discrete compressional tectonic phases. and Piedmont this compressional tectonics produced They are dated respectively from Upper Oligocene, essentially monoclinal folds. reverse and strike slip Lower Miocene. Middle Miocene, Late Miocene and faults and also warping. These kind of structures Upper Pliocene-Early Quaternary. Presented data affect the marine Upper Pliocene and detritic Early could suggest that these compressional tectonic Quaternary at Puente Hamani near to Pisco and also phases could be short-lived, may be less than a the continental Pliocene at Toran near to Camana mi//ion years each, at least for the two most recent. (fig. 1). On the Altiplano, this tectonics has produced This observation supports the idea of crisis in the folds. reverse and strike slip faults. It affects the geodynamic process and give évidence for change in lacustrine Pliocene of Curahuara and La Paz regions the stress regime. and also latitic flows of the Puno region (Atuncolla fault, 20 km to the NW of Puno). In the Eastern Cordillera, these movements locally produced folds associated with reverse and strike slip faults. This is SEDIMENTATION AND TECTONICS the case in the Cochabamba basin (LAVENU and BALLI- VIAN, 1980) where the lacustrine Pliocene deposits are Throughout the whole studied period, the Andes slightly folded. In the Sub-Andean foot-hills, these display continuously the characteristics of a mobile compressional movements mainly generated folds zone. As a matter of fact there is a strong subsidence associated with reverse faults. During Upper Plio- in the Altiplano basin. particularly during the Oligo- cene-Early Quaternaty tectonic movements, strike- Miocene. The analysis of Coastal marine deposits. slip faulting has been the major deformational pro- Western Cordillera volcanics. and Subandean conti- cess of the High Andes trust, inducing thus slight nental deposits support also subsident conditions. In local folding. addition, uplifting areas are contemporaneous of the Cenozoic sedimentation ; the Eastern Cordillera, the Wherever structural analysis has been performed. ‘it indicates : (1) A roughly E-W trending compressional boundaries between the Altiplano and the Western Cordillera, and between the Western Cordillera and stress axis for the older and more extended puise. the Fore-Arc. Tectonic activity occurs therefore bet- Thus the major faults with an (( Andean strike )) ween the discrete compressional pulses. (NW-SE) have tendency to left lateral horizontal displacement, e.j., the San Andres fault on the Boli- During Quaternary and Present, the Andean state vian Altrplano. (2) A roughly N-S trending compres- tectonics is characterized by N-S trending extension sional stress axis for the younger pulse. Thus the in the High Andes and Fore-Arc, and by E-W trending (( Andean strike )) faults have tendency to right lateral compression in the Subandean Lowlands and at the horizontal displacement ; e.j.. the Laguna Pomacan- contact between the Nazca and South American chi fault located 60 Km to the SE of Cuzco (fig. 1). In Plates (SÉBRIER et a/., 1982, 1985, and 1987 ; BONNOT. the South-Peruvian Sub-Andean foot-hills, this pulse 1984 ; LAVENU, 1986 : MACHARE et a/., 1986). During seems responsible of folding and faulting that corres- Pliocene, the same tectonic outlines prevail, but the pond tu a roughly N-S trending shortening (LAUBA- extension is oriented between E-W and NE-SW (SÉ- CHER et a/, 1984b ; SEBRIER et a/., 1985). BRIER et a/., 1982 : BONNOT, 1984 : LAVENU, 1986 ; MA- CHARÉ et a/,, 1986). For earlier periods structural in- In fact, the occurrence of two different kinematics formation is too scarce to establish what was the may be due to a single discrete tectonic pulse during Oligo-Miocene state of stress between the compres- which the compressional stress axis alternates from N-S to E-W. As it is suggested by theoritical model- sional pulses. Sedimentological observations on the ling of angular plate boundary (SHIMAZAKI eta/., 1978 : Andean basins provide only some suggestions. KATO ef a/.. 1980) this alternation may result from the The sedimentological records of the Andean basins Andean oroclinal bends : the Santa Cruz (1805) and show that their thicknesses, and consequently their Huancabamba (50s) deflexions. subsidence rates, are. highly variable (fig. 4, 5. 6, 7, and 9). The Fore-Arc basins are the less subsident Subsequently, during Pleistocene time, a few com- (maximum being of the order of 1 mm per year),.and pressional episodes occurred whose compressional they may be interpreted as slight crustal undulatrons stress axis would seem to be about NW-SE (LAVENU. caused by the shallower convex geometry of the 1978). Whatever the interest of these few compres- Benioff zone (MACHARE et a/.. 1986). The Subandean sional episodes, the conspicuous Pleistocene and basins exhibit subsidence rates comparable to those Present tectonics is roughly N-S extensional (SÉBRIER of the Fore-Arc basins. However these Subandean et a/, 1985 : LAVENU, 1986). basins correspond indeed to compressional Foreland From Oligocene to Present. the observed structures basins. As other Foreland basins. they should be emphasize the predominance of faulting and the interpreted as resulting from lithospheric flexure due occurrence of folding where the thickest series are to thrust loading (DICKINSON, 1977). The High Andes

98 Géodynamique 3 [l-2). 1988 : 85-106 COASTAL PACIFIC CORDILLERA PIEDEMONT WESTERN CORDILLERA ALTIPLANO EASTERN CORDILLERA SUB ANDEAN ZONE sw < NE

Fig. 10. - Synthetical geological profiles across the studied area. The upper one is located in southern Peru and the lowerone in northernmost Chile and central Bolivia. 1 Oligo-Mio-Pliocene marine sediments ,2 : Oligo-Miocene continental detritic sediments’; 3 : Oligocene to Lower Miocene continental detritic sediments ; 4 : Oligo-Miocene volcanic and volcano-clastic sediments ; 5 Miocene terrigenous and lacustrine deposits ; 6 : Pliocene conglomeratic and lacustrine deposits ; 7 : Plio-Pleistocene tuffs and lavas ; 8 : Quaternary volcanoes ; 9 : Oligo-Miocene intrusive bodies M. SÉBRIER, A. LAVENU, M. FORNARI, J.-P. SOULAS basins have characteristics that vary according to their Oligo-Miocene volcanism. In addition, a peculiar morphostructural locations. The small Eastern Cordil- magmatism is found in the Eastern Cordillera (see lera basins are clearly exceptional and related to section 1.5). it is characterized both by crustal and intracordilleran fault zones. Their tectonic regime may mantle derived volcanic and plutonic rocks (GRANT et be interpreted as resulting either from normal faul- a/., 1979 : MCBRIDE et a/.. 1983 ; WUBACHER et a/., in ting. as the Bolivian Cochabamba basins (LAVENCI et press). a/.. 1986). or from strike-slip faulting, as the South- The compressional tectonic phases appear to control Peruvian Crucero basin (LAUBACHER et a/., in press) the great features of magmatic evolution. In the The Western Cordillera basins are located within the studied zone, volcanic emissions occurred mainly magmatic arc. Although very thick series have been between compressional pulses (fig. 3). Howeverthere reported (3 000 m of Tacaza. according to NEWELL, is some evidence that volcanic activity cari occur at 1949). they may not be indicative of rapid subsidence the same time than compressional deformation as it rates. In fact these thick volcanic series cari be due to is shown in the Ayacucho basin during the Upper the pilling up of volcanic materials or to the infilling of Miocene pulse (MEGARD et a/., 1984). In the Altiplano canyon valleys blocked by volcanoes. The Altiplano and Eastern Cordillera the ages of batholiths or stocks basin exhibits the thlckest sedimentary series : during support two periods of main intrusive activity : about Oligo-Miocene time subsidence rate is at least of 19 to 26 Ma and about 9 to 14 Ma (EVERNDEN et a/.. 0.5 mm per year and could reach 1 mm per year 1966,1977 : AUDEBAUD et a/., 1979 ; GRANT et a/., 1979 ; (LAVENII. 1986). It corresponds to a sedimentary MCBRIDE et a/., 1983). Thus these intrusive activities trough located between the main magmatic arc (Wes- extended out of compressional phases ; however tern Cordillera) and the inner magmatic arc (Eastern they seems to initiate during compressional phases Cordillera). However its origin is still unclear. Whate- and to follow their intrusive process later. ver the case, the strong Oligo-Miocene volcanic activity of the Western Cordillera and the high sedi- The strong Oligo-Miocene magmatic activity of the mentary accumulation of the Altiplano are likely to be High Andes is coeval with the 26 to 6 Ma period of related to a dominantly tensional stress regime due fast convergence rate that is reported by PILGER (1983 either to extensional (i.e., (( rift 1) basins) or strike-slip and 1984). It is also contemporaneous with a period (i.e , (l pull-apart )) basins) deformations. that is characterized by several compressional pulses (see section (( Timing of compressional tectonic phases »). Conversely, during Quaternary and Pliocene Andean magmatism is clearly related with extensional MAGMATISM AND TECTONICS tectonics (SÉBRIER et a/., 1985 ; LAVENU. 1986). There- fore, Andean magmatic activity cari occur contemporaneously with both tensional or compressional de- In Central Andes, a strong volcanic activity has been formations. In this late case, in fact compressional occurring from Oligocene to Present. This magmatic deformations may be due to strike-slip faulting and activity is a conspicuous feature of the Andean chain. thus locally transtensional deformations should During Meso-Cenozoic evolution, a magmatic east- control volcanic emissions. ward migration is obsetved. Notwithstanding, it is not a continuous process. There are three locations for magmatic activity : Pacifie Coast and adjacent Pied- mont during Mesozoic till Upper Cretaceous, limit ANDEAN UPLIFT between Pacifie Piedmont and Western Cordillera from Upper Cretaceous till Upper Eocene, Western In southern Peru and Bolivia northern Chile, the Andes Cordillera and Altiplano from Oligocene to Present. form a high plateau whose average altitude is over The jumps in the location of magmatism occurred 4 000 m. There are only a few summits of the Western roughly at the same time as the major compressional and Eastern Cordilleras which overtop 6 000 m. Like- pulses : Peruvian phase (Upper Cretaceous) and wise, the Altiplano between these two cordilleras is Incaic phase (Upper Eocene). However, from Oligo- itself at an average altitude of almost 4 000 m. Thus cene to Present, migration is not clearly obsen/ed with Central Andes exhibit a flat topography on their top. compressional tectonic phases. In fact. it seems that Early authors attributed this conspicuous feature, to a the volcanic activity points have a certain stability. raised ancient morphological surface : this is BOW- Obviously some periods of more intense activity are MAN’S (1916) Puna surface. From then questions known but are not well studied. they could be roughly began to be asked about how and when this Puna Ilmitated by the compressional phases. During Plio- sut-face had been raised to more than 4 000 m. This Quaternary time, LEFEVRE (1979) has determined that has proved to be a very ticklish problem to salve the potash amount in volcanic rocks increases with because the Puna surface is polygenic (DOLLFUS. the distance to the oceanic trench and the depth of 1973). In the studied region remnants of volcanic the Benioff zone, Thus the volcanism is talc-alkaline plateau, evidence of ancient morphological surfaces. in the Western Cordillera to shoshonitic in the Alti- range in age from 53 Ma (MORTIMER, 1973) to Quater- plano. The same observation seems to exist for nary ! In addition. the initial altitudes of these surface

Géodynamique 3 (1-Z). 1988 : 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present are unknown, hence it is bot possible to estimate the thus a strong erosional event. This erosional event amplitude of uplift, and consequently its velocity. The bears witness of a desiquilibrium in a fairly stable commonly accepted idea is a very recent and still morphological system. It indicates therefore a rapid active uplift. Actually, this problem is better approa- uplift of the Andean topography. This remarkable ched by the study of the Andean Piedmonts. The phenomenon is coeval with compressional tectonic Amazonian lowlands are too eroded and too covered phases (see sections N The Pacifie lowlands N and by the forest to allow detailed observations SO that the N Timing of compressional tectonic phases ))) : Middle desertic Pacifie Lowlands are much more illustrative. Miocene (incision into SI), Late Miocene (incision On the Coast and Pacifie Piedmont, SÉBRIER et a/. into S2) and Pleistocene (incision into S3). The Andes (1979 and 1982) have distinguished three flat paleoto- have thus risen mainly during or slightly after each pographies : Sl, S2 and S3. They are respectively compressional puise. The amount of uplift between attributed to Lower Miocene, Upper Miocene and two surfaces is equal at least to the difference of Upper Pliocene-Early Quaternary. Each one is separa- height between the older one and the bottom of the ted from the older one by a major incision of drainage incision prior to the younger one. Thus the Middle (fig. 11). Miocene uplift is at least 400 m, and the Late Miocene one is about 1 000 m. The Quaternary uplift is of the SI corresponds to an aggradational pediplain formed order of 200-300m as shown by the altitude of on the topmost surface of a Piedmont accumulation Pleistocene marine terraces and present valleys inci- (Upper Moquegua Formation) covered by acidic tuffs sion in Pacifie Piedmont. (Huaylillas and Oxaya Formations, see 1.2 and fig. 4). From Middle Miocene to Present, S~BRIER eta/. (1979) SZcorresponds to the top of wide paleovalleys fillings (Chuntacala Formation, see G The Pacifie Lowlands 1)) have estimated a minimum average speed of uplift of or to the Tamarugal pediplain (Diablo Formation, see 0.15 mm per year. However this rate does not take G The Pacifie Lowlands »). S2 is incised roughly about into account the discontinuity of the phenomenon, 400 m into Sl. In detail, S2 is formed by three stepped nor the probable subsidence of the Andes during certain periods. It is best then to try to estimate the stages of alluvial deposits (TOSDAL et a/., 1984). average velocity of uplift pulses. The Lrate Miocene S3 corresponds to an aggradational pediplain formed compressional tectonic phase is the only one which on the top of valleyfillings (Pliocene alluvial deposits). presently allow to do such an estimation. The com- The difference of height between S2 and S3 is roughly pressional pulse occurred about at 7 Ma. Rather at the of about 500 m. However the canyon incision poste- same time a strong uplift allowed a valley excavation rior to S2 and prior to S3 is nearly 1 000 m deep. at least to their present day level (SÉBRIER et a/., 1982). The formation of these flat paleotopographies have This incision took place very rapidly since it was required a relative tectonic stability. Between each completed. both in Peru and northern Chile, at the one, it is necessary to produce a major incision, and endmost Miocene, about 6 Ma B.P. (see section N The

S km

a b Fig. Il. - Morpho-stratigraphie sketch of southern Peru Andean Pacifie Piedmont showing the Cenozoic morphological surfaces. SI, S2. and S3. Tectonic structures (faults and monoclinal folds) have not been represented. Pc : Pliocene conglomeratic deposits : Pm : Pliocene marine sediments : V2 : Upper Miocene acidic tuffs (Chuntacala Formation) VI : Upper Oligocene-Lower Miocene acidic tuffs (Huaylillas Formation] : M2 Upper Oligocene-Lower Miocene continental sediments (Upper Moquegua Formation) : M2m : Upper Oligocene-Lower Miocene manne intercalation of the Upper Moquegua Formation , Ml Oligocene continental sediments (Lower Moquegua Formation) : 2 Pre-Oligocene substratum Thick dotted line indicate the location of the fault zone limiting the Western Cordrllera from the Piedmont

GéodynamIque 3 (1-2). 1988 : 85-106 101 M. SÉBRIER, A. LAVENU, M. FORNARI. J.-P. SOULAS

Pacifie Lowlands 1)). Thus the available time for the minimum height (Hm) at the level of the Western incision is roughly 1 Ma. The amount of uplift being Cordillera cari be estimated : about 1 000 m in 1 Ma, the Late Miocene àverage HmlL. Sin 10%2000 m uplift rate is 1 mm/year. This shows that the Andes had already risen conside- Using the Pacifie Piedmont morphological surface it is rably and could not have been less than some 2 000 m also possible to have the order of magnitude of the high at about l-7-20 Ma. This lower Miocene Andean minimum altitude of the Andes at the beginning of the topography should be due to the Paleogene geody- Neogene. As we said SI surface corresponds to an namic evolution. aggradational pediplain. Presently this kind of broad Finally in Central Andes, two scales of vertical move- Piedmont pediplain has deposits slope angle which is ments must be distinguished : (1)-Uplifting pulses roughly of the order of 10 or even less. In addition this that involve the whole of Andean Cordillera and are pediplarn was in relation with sea level as a marine roughly coeval with compressional tectonic pulses. intercalation is seen within the Upper Moquegua (2)-Differential vertical movements along Andean formatlon (see (( The Pacifie Lowlands 1)). Besides, fault zones that are coeval with sedimentation and analysis of the Coastal Cordillera scarp (SEBRIER et a/., volcanic activity. The figure 12 illustrates these diffe- 1979 and 1982) indicates that the Miocene coastline rential vertical movements and the minimum altitude was roughly nearby the present day one. Thus the of the Andes during three epochs : Lower Oligocene, length (L) of this pediplain, which cari be followed up Lower Miocene and Pliocene. The progressive uplift to the Pacifie Coast, was at very least equal to the of Andean chain and the fact that the Eastern Cordil- distance between the Pacifie Coast and the Western lera has always behaved as a positive zone Will be Cordillera edge, that is to say 100 km. Therefore, the noticed.

OLIGOCENE 35 N1A

MIOCENE 20 MA

PLIOCENE 4 MA

6 Km.

APROXIMATIVE SCALE 50 Km 0L

Fig 12 - Schematic profiles showing the Cenozolc evolution of Central Andes. 1 manne sediments : 2 terrigenous continental deposits : 3 continental detntlc deposits 4 lacustnne sedlments , 5 : volcanic and volcano-clastic deposits : 6 volcanic deposits (tuffs and lavas) lying DB morpholoqlcal surfaces 7 : rntrusive bodies , 8 olistolitlc facies. Eilack arrows show differential vertrcal movements. size of arrows in relation with the estimated amplitude of vertical movements Active faults coeval with sedimentation are indicated by a couple of thin half arrows Stippled area for pre-Oligocene substratum.

102 Géodynamique 3 (I-Z), 1988 : 85-106 Tectonics and uplift in Central Andes (Peru, Bolivia and Northem Chile) from Eocene to present

CONCLUSIONS 1UATERN/ ii -e F.6 PLIOCEb ÏË In southern Peru, Bolivia, and northern Chile, field -tt F.5 observations and re-evaluation of the geological data .-Y; F.4 allow to establish the chronological evolution of the MIOCENE Andean morphostructural zones and to correlate 8 F.3 them. This analysis gives evidence for six discrete compressional tectonic pulses that are dated respectively : Upper Eocene (ca 42 Ma), Upper Oligocene (ca

26-28 Ma), lower Miocene (ca 15-17 Ma), middle )LIGOCENE Miocene (ca 10 Ma), Upper Miocene (ca 7 Ma), and early Quaternary (ca 2 Ma). These discrete pulses appear to correspond to high convergence rate pe- EOCENE riods (fig. 13). Available structural data on these -+ F.l compressional phases suggest their shortening direc- A tion is roughly parallel to the convergence orientation. Basinal infillings are highly variable. They are in Fig. 13. - Chronology of compressional events. FI to F6, that have agreement with the various processes that may put affected Central Andes since Late Eocene time. Vertical thick forward to explain the formation of Andean basins. segments show the time-ranges of these tectonic events. Conver- These basins are preferentially compressional in the gence rate between Nazca and South American plates from PILGER (1983). space between dotted lines represents uncertarnities on Subandean Lowlands and mainly tensional in the convergence rate calculated by PARDO-CASAS and MOLNAR (1987). High Andes and Pacifie Lowlands. Magmatic activity Equatorial Atlantic half spreading rate from BROZENA (1986). Thick is located in the High Andes since at least Upper dashed line indicates the period of high arc magmatic activity Oligocene (ca 26 Ma). It is coeval both with compres- Magnetic anomalies and chronostratigraphy from BEGGREN et a/, (1985). sional and extensional deformations. In Central Ari- des, this magmatic activity is maximum between 26 and 6 Ma, i.e., contemporaneously with the Miocene has been produced both by tectonic shortening and period of high convergence rate (fig. 13). It appears by magmatic accretion. thus that the magnitude of magmatic activity is related with the convergence rate. Observations performed Acknowledgements on the Neogene morphological surfaces show clearly that the high cordilleran topography has been formed We thank A.H. CLARK. F. MEGARD. R. MAROCCO. G. LAURACHER and between roughly 26 and 6 Ma, i.e., during the period D.C. NOBLE for useful discussions. This work has been carried out within the context of the scientific cooperation contract concluded of Miocene high convergence rate and strong mag- between the Peruvian Geophyslcal Institute (IGP). the French Insti- matic activity. These morphological data indicate also tute of Andean Studies (IFEA) and UA 730 CNRS (Université Paris that the uplift of Central Andes has been produced by Sud-Orsay). This study has also been performed under the auspi- discontinuous’ event that are coeval with the discrete ces of the ORSTOM missions of Peru and Bolivia. We thank these three organisations for the material aid that they have afforded us. compressional tectonic phases. This Andean uplift is Our research has also benefited from the financial support of the old in part, as at the end of Miocene, the present-day INSU ATP Geodynamique and Projet Andes and from the DGRST altitude had rather been reached. All these data are specific action 77.7.1922. We thank J. MERCIER for critically reading thus in agreement with the fact .that the Western the manuscript and Peter MOLNAR for his valuable review. Cordillera uplift is due to a crustal thickening which Manuscrit accepté par le Comité de Rédaction le 10 janvier 19&5.

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