The Andean Geosyncline in Peru, and Its Distinction from Alpine Geosynclines

Jl geol. Soc. Lond. Vol. 135. 1978, pp. 207-218, 4 figs. Printed in Northern Ireland.

The Andean geosyncline in Peru, and its distinction from Alpine geosynclines

E. J. Cobbing

SUMMARY: The Andean geosyncline in Peru is divided into faulted strips of crust. This has resulted in the isolation of basins of deposition which have both subsided and deformed independently. Five basins were formed, two of which were filled with sediments and pertain to the miogeosyncline, two with volcanics form the eugoesyncline, and a fifth composed of both volcanics and sediments forms a miogeanticlinal horst between the two. A volcano-plutonic chain was superimposed upon the fractured continental crust and the eugeosyncline was filled with material derived from the volcanoes and also from plutons of the rising batholith. The miogeosynclinal basins were filled with material derived from the continental hinterland. The eugeosyncline was deformed during the Upper Albian and the Coastal Batholith was emplaced during the Upper Cretaceous while sedimentation continued in the miogeosyncline. Deformation of the miogeosyncline occurred during the Palaeocene. Aubouin's distinction of Andean from Alpine chains is confirmed. Andean chains are characterised by andesite volcanics and tonalite batholiths, whereas Alpine chains contain ophiolite belts and sedimentary flysch. The differences may reflect their different tectonic setting, the Andes having developed under a convergent regime between a continent and a large ocean while the. Alpine chains may have evolved under a regime involving the opening and closing of small ocean basins between continents. It is suggested that andesite volcanics and tonalite batholiths are characteristic indicators of the subduction process at continental margins.

This paper attempts to give a synoptic account of the emerged which will be summarised here. It has been major geological features of the Western Cordillera of found convenient to use the geosynclinal nomencla- Peru between the Olmos Arch in the north and Nazca ture established by Kay (1951) and elaborated by in the south, and to discuss these features in a plate Aubouin (1965). tectonic setting. The 1 : 500,000 map of Northern Peru compiled by the author and published by IGS The development of the West (DOS 1973) and which covers the greater part of the area is the result of long-term work by many geolo- Peruvian Trough gists, both Peruvian and British, who have been en- gaged in mapping part of Peru under a programme of Wilson (1963) defined the West Peruvian Trough as technical co-operation between the Instituto de lying between the Marafion Geanticline and a western Geologia y Mineria del Peru and the Institute of tectonic land offshore of the present day coastline. He Geological Sciences supported by the Ministry of observed that the western part of the trough was Overseas Development, London. This map forms the composed of volcanics and the eastern part of sedi- basis of the present study but subsequent work, which ments. He described the sedimentary sequence and is still in progress, has also been included. noted that a rapid change in facies occurred across a The area falls within the West Peruvian Trough as line running along the continental divide which sepa- defined by Wilson (1963), comprising a mainly vol- rated the West Peruvian Trough from the Marafion canic belt to the west (a eugeosyncline) and a sedimen- Geanticline. He later established (1967) that this hinge tary belt to the east (a miogeosyncline). line was fault controlled, the fault system having be- The sedimentary stratigraphy of Northern Peru is haved as normal faults during the Cretaceous but with well known from the work of Benavides-Caceres Tertiary deformations and uplift it became resurgent (1956) and Wilson (1963), and on the basis of this as high angle reverse faults. work it has been possible to reconstruct the Cobbing (1973a) confirmed the existence of the palaeogeography with a fair degree of confidence. The eastern hinge line. He referred to the sedimentary part stratigraphy of the volcanic sector is poorly known of the trough as a miogeosyncline and the volcanic because of the intractable nature of the volcanic se- part as a eugeosyncline. He also noted that the struc- quences themselves and because over large areas they tures within the sedimentary and volcanic belts were are obliterated by the tonalite plutons of the coastal different and established that within the miogeosyn- batholith. However, Myers (1974) established the cline the structure was that of an anticlinorium. local stratigraphy near Huarmey and from this and Myers (1974) described the stratigraphy of the vol- subsequent work (Webb 1976) a total picture has canic sector of the trough at the latitude of Huarmey

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and referred to it as an eugeosyncline. He also Peruvian Trough, two of which pertain to the suggested that a tectonic line separated the eugeosyn- miogeosyncline and two to the eugeosyncline. cline from the miogeosyncline and he called this the Tapacocha Axis. At the point where he described it this axis corresponded both to the eastern limit of the The Pongos Basin eugeosyncline and to the eastern limit of the batholith. Myers (1975) considered that the West Peruvian The stratigraphy of this basin was described by Trough was divided into blocks by a system of block Benavides-Caceres (1956). It is separated from the faulting in the underlying basement. He called the Marafion Geanticline to the east by the eastern western tectonic land of Wilson (1963) the Paracas boundary fault, while to the north and west the lower Block, the Marafion Geanticline the Marafion Block, clastic members of the sequence overlap on to the the sector between the Paracas Block and the Olmos Arch. The basin was initiated during the Upper Tapacocha Axis the Paramonga Block, and that be- Neocomian and prior to that epoch it formed a posi- tween the Tapacocha Axis and the Marafion Block the tive area with respect to the Chavin Basin immediately Chavin Block. to the south. However, from the Hauterivian stage The Chavin Block corresponded to the miogeosyn- upwards the two basins were continuously connected. cline and the Paramonga Block to the eugeosyncline. Subsidence in the Pongos Basin was greater between He also noted a difference in tectonic style between the the Cenomanian and Santonian, and it contains the two areas; within the Paramonga Block (eugeosyn- thickest section for this period in the whole of Peru. cline) the anticlines were broad and open but the The sequence consists mainly of carbonates but with synclines were narrow and tight, while within the considerable shaly and marly admixtures. The Pongos Chavin Block narrow ribbon like folds were formed Basin and the Chavin Basin together form the with axes traceable for 100 km or more. miogeosyncline. It is proposed to retain much of the systems of Wilson and Myers but with certain differences. It is considered that the most important tectonic line within the The Chavin Basin West Peruvian Trough was not the Tapacocha Axis but the Cordillera Blanca Fault. This line maintains a cons- This basin forms the dominant part of the tant distance from the coast, and south of the latitude miogeosyncline. It was initiated during the Tithonian of Lima, where the miogeosyncline peters out, it forms and its maximum subsidence was during that stage. It the eastern boundary of the trough against the is separated from the Marafion Geanticline to the east Marafion Geanticline. The Tapacocha Axis is not per- by the eastern boundary fault system and from the sistent but dies out both to north and south. Where it miogeosyncline to the west by the Cordillera Blanca is present, however, the area between it and the Fault. The eastern boundary faults are arranged in an Cordillera Blanca Fault corresponds to a positive ridge en echelon manner so that the outcrop width of the separating the eugeosyncline from the miogeosyncline. basin broadens northwards and narrows southwards. The deposits on this ridge were thin compared to The fault systems meet at about the latitude of Lima those in the troughs to either side and consisted of and the basin dies out at this point. During the Titho- sediments of Hauterivian to Aptian age in the north- nian and most of the Neocomian the basin was ern part and volcanics of similar age in the south. The bounded both to the east and north by land which was whole ridge is here referred to as a miogeanticline. the source of the Chicama Formation, a thick se- It is considered that the division of the West Peru- quence of black mudstones in which both ammonites vian Trough into block faulted strips by Myers (1975) and plant remains occur. Dark, current bedded quart- is fundamentally correct, but that there are more zites are abundant in the upper part of the formation. blocks than he recognised. This block faulting within There was a connection to the miogeanticline during the trough has resulted in the isolation of strips of the Neocomian, and there may have been one during crust which have then behaved independently of one the Tithonian, but this is less certain. The connection another. As a result, different basins of deposition to the miogeanticline may also have extended to the have been formed within the trough. Each of these eugeosyncline. basins may have been interconnected throughout the From the Hauterivian onward there was continuous Cretaceous or at different times within the Cretaceous, connection with the other basins within the West and for this reason the stratigraphic sequences in Peruvian Trough and also with the Cretaceous se- adjacent basins are commonly similar. Each basin quences then being deposited on the Maranon Geanti- behaved independently, however, with the main subsi- cline so that the sequences in both of the miogeosyncli- dence taking place at different times in different ba- nal basins and on the Marafion Geanticline are similar, sins, with the result that the stratigraphic thicknesses varying only in the thickness of the formations in the for any one period are very different in each basin. different areas. Sequences below the Albian were Five basins have been recognised within the West mainly clastic, while from the Albian upwards deposits

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were mainly carbonates. The pre-Albian clastics were whether any are present. Neocomian sediments and considered by Wilson to have been deposited in a Tithonian volcanics are exposed near Lima but this is deltaic environment, while the post-Albian carbonates in a zone which is transitional to the Rio Cafiete Basin were formed in shallow seas which received little and it may be that these strata die out further north in terrigenous material except in the case of the Pongos the Huarmey Basin proper. Nevertheless it is con- Basin. sidered that the Huarmey Basin was initiated during the Tithonian but that its main subsidence occurred The Rio Santa Basin during the Albian. Myers (1974) described a sequence of about 5000 m The basin is located upon the miogeanticlinal block of pillow lavas, andesitic volcaniclastics and interca- bounded by the Cordillera Blanca Fault and the lated sediments. Ammonites throughout the sequence Tapacocha Axis. The block formed a positive horst indicated a late Middle Albian age. Webb (1976) with respect to the adjacent miogeosynclinal and showed that many of the volcaniclastic units thin to eugeosynclinal basins during most of Cretaceous time, the east and thicken to the west and that the size of but during the Valanginian to Aptian interval it sub- the contained clasts also diminishes to the east. She sided, allowing a thick sequence of shales to accumu- therefore suggested that the volcaniclastic units were late in the northern part and an equally thick sequence derived from a chain of island volcanoes located at the of volcanics in the southern. There may have been a boundary between the Huarmey Basin and the connection across the miogeanticline during the Titho- Paracas Eugeanticline. She also showed that the nian, but no deposits of this age have been found and pillow lavas were fed by dykes, and accordingly it it is considered that if they exist they are probably is now possible to suggest a model for the filling thin. of the Huarmey Basin. The question of the transition from sediments to Domes and lenses of pillow lavas were built on to volcanics along this structure is important, for it is the floor of the basin by dykes which were fed by the considered likely that a chain of volcanoes may have rising gabbro plutons aligned along the western edge been located along the Tapacocha Axis. The vol- of the Coastal Batholith. At the same time volcaniclas- canoes, which were present only along the southern tic turbidites and lava flows were derived from a part of the axis, were active during the Upper Neoco- volcanic chain at the western edge of the eugeosyn- mian, so that the Carhuaz Formation which is present cline. Another chain of island volcanoes (more acid in in the north was entirely replaced by volcanics in the composition) was located along the Tapacocha Axis south. These volcanics were composed mainly of and from this a sequence of coarse andesitic and pyroclastic andesites and dacites and were probably dacitic breccias and tufts was derived, the Churin more acid in composition than the volcanics in the facies of the Casma Group (Webb 1976), formerly eugeosyncline to the west. named the Lancha Formation (Cobbing 1973). Within There is an upward transition from the Carhuaz the variously derived volcanic deposits, shales, shales to the Senal Cochapunta Formation of Myers silicified shales and quartzites occur, generally thin but (1974), which is composed of silicified tufts, dark locally up to 1000 m thick, and generally lenticular silicified siltstones and thin carbonates. This forms a in form. transitional sequence to the eugeosyncline and it seems to be a facies particularly associated with the Tapacocha Axis. The Rio Cahete Basin The Rio Cafiete Basin is structurally and strati- The Huarmey Basin graphically continuous with the Huarmey Basin. Both the miogeosyncline and the miogeanticline die out at The Huarmey Basin and the Rio Cafiete Basin the latitude of Lima and the eugeosyncline as exemp- further south both pertain to the eugeosyncline of this lified by the Rio Cafiete Basin is correspondingly paper. The stratigraphy of the Huarmey Basin was broader here than it is to the north, occupying the first established by Myers (1974) and elaborated by entire width of the West Peruvian Trough. The basin Webb (1976). The basin is transitional to the east is bounded to the east against the Marafion Geanti- through the Senal Cochapunta Formation to the cline by the continuation of the Cordillera Blanca miogeanticline. Its relationship to the Paracas Eugean- Fault, but the nature of the facies changes across this ticline is not known, but it is presumed to rest uncon- hinge line have not yet been established. The principal formably on that structure. To the south it is transi- point of difference between the Rio Cafiete and Huar- tional to the Rio Cafiete Basin and to the north it rests mey Basins is that in the former a thick section of unconformably upon folded Chicama shales (Cossio Neocomian to Aptian sediments underlies the Albian 1964). volcanics, which are of similar thickness and lithology Within the limits of the basin no rocks older than to those in the Huarmey Basin. These sediments com- Albian have been identified and it may be questioned prise the massive cherty limestones of the Imperial

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Formation and dark shales and quartzites of the Coniacian and show that the formations in the Morro Solar Formation. These shales are generally eugeosyncline, which were folded prior to the em- similar to those of the Carhuaz and Chicama Forma- placement of the Coastal Batholith, were deformed tions and were probably deposited under the same during an Upper Albian folding episode. Stratigraphic kind of deltaic conditions. The limestones of the knowledge does not conflict with this, since no fossils Imperial Formation, however, have no equivalent lith- younger than late middle Albian have been found. ologies in the other basins. The structural implications, however, are considerable. The Morro Solar shales are underlain by the Puente It would seem that the eugeosyncline was folded Piedra volcanics at Lima and consequently the basin is and that much of the Coastal Batholith was emplaced considered to have been in existence at least from the during a period when sedimentation was still proceed- Tithonian onward. ing in the miogeosyncline. The Albian volcanics of the Casma Group are simi- Sedimentation continued in the miogeosyncline until lar in every respect to those in the Huarmey Basin and the end of the Cretaceous, when the red beds of the are of about the same thickness. The main subsidence Chota and Casapala formations were laid down with therefore probably occurred during the Albian. The slight alscomormity upon the Celendin limestone. thick Neocomian to Aptian section, however, shows These red beds were derived from the west and may that substantial subsidence also occurred at this time, have resulted from uplift in the eugeosyncline. so it would seem that steady subsidence occurred over Folding in the miogeosyncline occurred during the a longer period in the Rio Cafiete Basin than in the Palaeocene and affected the entire Cretaceous se- Huarmey Basin. A similar facies change to that seen in quence and the Chota and Casapala formations, but the Huarmey Basin occurs across the basin within the not the overlying Calipuy volcanics which were laid Casma volcanic group. The volcanics on the eastern down upon an erosion surface which extended right side of the basin are of intermediate to acid composi- across the miogeosyncline, the eugeosyncline and tion and are more fragmental, including coarse pyro- the Marafion Geanticline. clastic material. As in the Huarmey Basin these were It is clear, therefore, that the eugeosyncline and the probably derived from a chain of island volcanoes miogeosyncline were deformed separately at different located not at the Tapacocha Axis, which had died times. The simplest way of explaining this is to sup- out, but along the prolongation of the Cordillera pose that the crustal strips which contained the Blanca Fault. eugeosynclinal basins not only subsided differently The Rio Cafiete Basin extends southwards and is with respect to adjacent strips bu were also uplifted at known to be physically continuous with basins in which an earlier time, the folding within the thick cover mainly Jurassic sediments and volcanics are exposed. being consequent upon the uplift. The detailed correlation with these sequences has not The concept of dividing the West Peruvian Trough yet been made. It is possible that the Morro Solar into crustal blocks which isolated different basins of Formation may be in part equivalent to the shales of deposition which subsided differentially is also useful the Yura Group which extend from the Tithonian into in understanding the deformation history. Evidently the Neocomian. If this is so it is possible that Cretace- the blocks were uplifted and deformed independently ous rocks exposed in the Rio Cafiete Basin may be in the same way that they subsided independently. underlain by a considerable thickness of Upper It is interesting to note that the gross structures also Jurassic volcanics. correspond in large measure to the form of the princi- A section of the Jurassic sequences further south pal basins of deposition. A major anticlinorium, for occurs at Nazca, where Mesozoic formations uncon- example, runs down the axis of the Chavin Basin and formably overlie the Marcona Formation, which is of is curved in the north in conformity with the form of Lower Palaeozoic or older age, and which in turn lies the basin; a synclinorium corresponds to the Huarmey unconformably upon the ancient gneiss complex which Basin, while another anticlinorium is located on the forms the Coastal Cordillera and the Paracas Eugean- Rio Cafiete Basin. ticline. Myers (1975) observed that structures in the Huar- Structure mey Basin took the form of broad anticlines and pinched synclines, and he attributed this style of fold- Until recently it was thought that there was one ing to the upward propagation of deep shearing in the principal phase of deformation in Peru during the basement into a thick and rigid cover. Conversely Palaeocene which was followed by subordinate phases Wilson (1967), Coney (1971) and Cobbing (1973a) throughout the Tertiary. The radioisotope work of have emphasised that the style of structure in the Wilson (1975) demonstrated that the actual situation Chavin Basin depended on the presence of a thick is more complex; normal tonalite members of the basal shale formation which deformed in a ductile Coastal Batholith gave radioisotope ages ranging from manner enabling the sedimentary carapace to deform 93-84 Ma and the ages on the gabbros ranged from independently of the underlying basement. It is con- 95-105 Ma. These ages range from the Albian to the sidered likely that with uplift the shales in the lower

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part of the sequence flowed upwards and outwards contributed to the formation of the geosynclinal fur- towards the flanks. rows. This conjecture is reinforced by the fact that the A similar type of deformation has occurred within Cordillera Blanca batholith is emplaced along the the Rio Ca~ete Basin, where the Morro Solar shales same fault which defined the deepest part of the and Imperial limestones, though folded with the over- miogeosyncline and maximum Tithonian sedimenta- lying Casma volcanics, are locally disharmonic. It is tion. Descriptions of part of the Coastal Batholith and this detachment of the ductile sediments against un- comments on the several batholiths are given derlying and overlying rigid material which enabled elsewhere (Cobbing & Pitcher 1972a, Pitcher 1975). the anticlinoria to form. Plutonism The basement underlying the The Mesozoic and Cenozoic plutonic bodies (Fig. 1) geosynclinal belt have an extremely linear nature (Pitcher 1972). It has been postulated (Pitcher 1972) that this is related to Although no rocks older than Upper Jurassic are faulting at depth, of the same character as that which exposed within the confines of the geosyncline the

Cordillera Blanca Tapacocha ~ Axis ~. Batholith and ..... ,. .q÷¢,~atholith~ Fault

\ o~ \ \ \,\

oo? • Nazca

Sediments

Volcanics

Intrusive

Ancient gneiss complex

? ~?o 2?? 3OOk~

FIG. 1. Outline of the Mesozoic geosyncline and its relationship to the main palaeotectonic elements.

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Anticlinorium

,/ Synclinorium

0 100 200 3o0km i I

1. Pongos Basin 2. Chavin Basin 3. Rio Santa Basin 4. Huarmey Basin 5 Rio Ca~ete Basin

LIFE OF BASIN 1. Neocomian to Palaeocene 2. Tithonian to Campanian 3. Tithonian to Albian 4. Tithonian to Albian 5. Tithonian to Albian

MAIN PERIOD OF SUBSIDENCE TYPE OF FILL 1. Cenomanian to Sa'ltonian Sediments ~.. 2. Tithonian Sediments 3. Vatanginian to Aptian Sediments and volcanics 4. Albian Volcanics 5. Neocomian to Albian Volcanics and sediments

FIG. 2. Schematic isopachyte map to show the main basins of deposition within the geosyncline, and also that the gross structure is related to basin morphology. nature of the underlying basement may be inferred Cambrian age and, because of its extensive develop- with a fair degree of confidence. Both to the north and ment throughout the Andes, to be the earliest essen- east of the miogeosyncline in the Marafion Geanticline tial element of the Andean chain (Cobbing 1972). and the Olmos Arch, metamorphic rocks (principally There is no doubt that at least part of the geosynclinal pelitic schists and quartzites) are exposed which dis- zone is underlain by this metamorphic belt. play chlorite- to amphibolite-facies metamorphism At the latitude of Nazca metamorphic rocks of the and polyphase folding. They contain few marker hori- Coastal Cordillera have been dated at 1800 Ma (Cob- zons and are not known to contain volcanic rocks, bing et al. 1977). They consist of both metasedimen- while the gross structures are parallel to the Andes. In tary and granitic gneisses and contain structures com- places they are overlain discordantly by Ordovician pletely discordant to the later Andean trends. It has sediments and though the age of metamorphism is not been suggested that they belong to the ancient Trans- known it is evidently pre-Ordovician. Amazonian nucleus of South America (Cobbing et al. The belt is considered to be of late Precambrian or 1977). This ancient basement forms the floor of the

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WEST PERUVIAN TROUGH EAST PERUVIAN TROUGH ~-~ -~ Paracas Geanticline t ~ MaraRon r Geanticline Brazilian shield EU MIO ! i Continentalcrust

LOWER AND MIDDLE ALBIAN

Tapacocha Axis

UPPER ALBIAN

Cordillera Blanca Fault

f f

/ / PALAEOCENE

FIG. 3. Cartoon illustrating the formation of the West Peruvian Trough by block faulting of the continental crust and the deformation of the cover consequent upon differential uplift of the subsident blocks along resurgent faults. The eugeosyncline is filled with material derived from volcanoes located along the bounding faults of the geosynclinal block. Pillow lava lenses are fed by dykes originating from gabbro plutons, black. Sediments dotted, tonalite plutons hatched.

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eugeosyncline at this latitude and it is presumed that not affect the miogeosyncline until the Tertiary. The the same holds true north of Nazca where the Coastal deformation, however, occurred independently in Cordillera is submerged. blocks rather than as a wave. It is thus evident that the basement underlying the The tectonic style of the Andes differs from that of geosynclinal furrows is of two distinct types, an oldei the Alps in that the folding is upright and there has pre-Andean cratonic type and a younger post-cratonic never been any development of nappes. Just such a Andean type. difference in style was used by Crook (1969) as one of The depositional troughs are simply block faulted, the bases of distinction between 'Pacific' and 'Atlantic' ribbon like strips which have subsided differentially at geosynclines. different times, the subsiding troughs being filled with The nature of the fill in the volcanic furrow is also whatever material was available. In the case of the different; in the Andean case it consists of basaltic miogeosyncline this was mainly clastic sediment de- andesite and andesite, whereas in Aubouin's model it rived principally from the Marafion Geanticline (Wil- is ophiolitic with a sedimentary flysch. The Andean fill son 1963), while in the eugeosyncline it was a combi- is, however, similar to that of the Fraser eugeosyncli- nation of sediments from the eugeanticlinal ridge and nal belt of the North American Cordillera (Kay 1951). volcanic material from volcanic islands superimposed The gross morphological organisation of the or- upon the continental margin above a subduction zone. thogeosyncline is identical to that of Aubouin's model, The origin of the block faulting within the compo- as is the tectonic polarity. The sedimentary polarity is site basement, however, is by no means clear. It is similar with respect to initiation of the furrows, but possible that it is simply the reaction of the brittle different with respect to the time of subsidence. The crust to convergent subduction. On the other hand, tectonic style and the volcanic fill of the internal the structures may have already been present as a furrow are different. result of earlier orogenic activity and may have merely It may be questioned whether absolute conformity been reactivated. A feature of the Andean, and indeed to the complete scheme developed by Aubouin is a of all circum-Pacific fold belts, is the presence of necessary condition for all geosynclines, and whether, pre-Mesozoic fold belts upon which the Mesozoic at this scale, nature ever repeats itself precisely. Kay structures have been superimposed. The fact that (1951) certainly favoured a looser framework, stating these older fold belts are circum-Pacific may suggest that 'the rocks in the volcanic belts in some instances that they were related to an older Pacific Ocean. merge directly into those in the non-volcanic belts in a single geosyncline that would be classed as a eugeosyncline on the outer border and a miogeosyn- Comparison of the Andean Or- cline on the inner'. thogeosyncline with the model of Aubouin 1965 The Andes: a liminary or a geosynclinal chain The most complete discussion of the classic or- thogeosyncline with its eugeosynclinal-miogeo- Aubouin and his co-workers in Chile developed a synclinal pair is that of Aubouin (1965). From different nomenclature which mirrored in every re- the above description it is clear that all the major spect the classical geosynclinal nomenclature. This morphological units of Aubouin's classic or- nomenclature was applied by Aubouin (1972) to the thogeosyncline are present in the Peruvian Andes. Peruvian situation. However, the model also requires both sedimentary Their viewpoint was that the Andes is not a and tectonic polarity and here there are certain differ- geosynclinal chain but a liminal chain. Their reasons ences. were that the Andes occupy a liminal (i.e. marginal) The polarity in development of the furrows required position with respect to the ocean, that the fill of the by Aubouin, with the internal (eugeosynclinal) furrow internal furrow is different in all respects from the developing before the external (miogeosynclinal) fur- Alpine example, that the plutonism and volcanicity are row, is not precisely followed in the Andean case. different, and that the tectonics are simple as opposed Both furrows were initiated during the Upper Jurassic, to the Alpine nappes. but the main external furrow has its greatest develop- Aubouin considered that all chains were originally ment during the Tithonian, while the internal furrow liminal (marginal) to a continental mass. In the Alps was greater in the Albian. In Chile the sedimentary the opposed liminal chains became converted to bilim- and volcanic furrows seem to exhibit the required inal and multiliminal chains with the closing of Tethys. polarity of formation (Vicente 1972). However, since Aubouin (1972) considered that the The tectonic polarity required by Aubouin's model Magellan Cordillera in the Southern Andes, and the certainly exists in the Andes, for deformation occurred Caribbean Cordillera in the Northern Andes were during the Cretaceous in the eugeosyncline and did geosynclinal (in the Alpine sense) it is clear that his

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objection to the word geosyncline in the Andean crust, it would seem that subduction has been opera- context does not stem from its liminal (marginal) tive more or less continuously along this plate bound- position but from the character of the fill of the ary, certainly from the Albian (and probably from the internal furrow. Lower Jurassic) until the present time. A long period Sectors of the Andes which Aubouin referred to as of subduction is also required to account for the large liminal are characterised by thick sequences of vol- area of oceanic crust east of the East Pacific Rise canics of intermediate to acid composition. They also which has disappeared. have great volumes of granodioritic material disposed Subduction is evidently a process of ocean plate in batholiths and are virtually devoid of a sedimentary consumption which may result in the ultimate closure flysch facies. Alpine chains, in contrast, are character- of the ocean concerned. It follows that a sure indica- ised principally by flysch which is derived from the tion of the former location of a fold belt at the margin most internal position (i.e. the eugeanticline), and of a continent and an open ocean would be the ophiolites, which usually form an early part of the presence of abundant andesitic volcanics and tonalite sequence; they also lack batholiths. plutons. Conversely, when these are absent, one must Aubouin noted a further distinction of critical im- surely question whether the fold belt was ever situated portance between Andean and Alpine chains in that at the border of an open ocean which has been closed the former are contained within sialic crust, while the by subduction. In fact they are generally lacking in latter rest in part upon oceanic crust incorporated later Alpine fold belts and one is bound to consider the into the mountain chain. possibility that the present ophiolite strips represent It is known that the eugeanticlines which supplied nothing more than rather narrow oceanic seaways, the flysch to the Alpine eugeosynclinal furrows consist perhaps analogous to the Red Sea. It also seems of Hercynian basement and therefore the oceanic do- possible that subduction may not have been an impor- main which supplied the observed ophiolites was most tant process in the closing of these oceanic strips. probably of a very restricted character. It is thus rather There are three ways of closing an ocean: subduc- surprising to find that Aubouin (1972), having clarified tion, obduction and tectonic shortening. Evidently the distinction between Alpine and Andean chains, subduction is the only effective way of closing a large should, in his concluding paragraphs, revert to the idea ocean, for either of the other two methods would that Alpine chains originated at an Atlantic type mar- result in ophiolite belts hundreds of miles wide which gin. Although strips of oceanic crust have been incor- are not known to exist. Consequently, if the suggestion porated into the fold belts, it does not seem likely that made above (that subduction is not an important these ever represented an open ocean of Atlantic type. mechanism in the Alpine case) is correct, it would While agreeing with most of the factual differences follow that the oceanic seaways were closed either by between Andean and Alpine chains noted by Au- obduction or tectonic shortening. Similar views were bouin, one hesitates before accepting the restriction of expressed by Helwig (1974), who observed that 'The the term geosyncline to the Alps alone and to those crust of continental rifts, rhombochasms, spheno- sectors of mountain chains characterised by sedimen- chasms, marginal basins, oceanic arcs and remnant tary flysch and ophiolites. It is considered that the basins is more abundantly preserved in orogenic belts differences are sufficiently indicated by the prefixes than is normal, ridge-generated oceanic crust'. It is Andean or Alpine rather than by the introduction of a interesting to note that the Magellan Cordillera, at the new term which would tend to confuse discussion southern tip of the Andes, and the only part to be about other geosynclines which may be neither An- classified by Aubouin (1972) as an eugeosyncline dean nor Alpine. Under these circumstances the (Alpine sense), owes its eugeosynclinal character to purely descriptive geosynclinal nomenclature retains the incorporation of a marginal basin into the certain advantages. Andean chain (Dalziel et al. 1974). The Alpine geosynclines could thus be envisaged as having developed along the margins of a mosaic of The tectonic setting of the small plates (Glangeaud 1968), bounded by narrow Andes and the Alps oceanic strips. That a solution of this type might be possible instead of that requiring a situation between a The principal difference between Andean and Alpine continent and an open ocean was hinted at by Au- geosynclines lies in the type of vulcanicity, ophiolitic in bouin (1972, p. 460). 'Cependant il par~it peu proba- the Alpine case and andesitic in the Andean. It has ble que l'on puisse rendre compte de l'rvolution des been observed by Aubouin (1972) and many others, deux modules liminaires et grosynclinal par un that this andesitic vulcanicity is a circum-Pacific mrcanisme identique: la difference n'est pas seulement phenomenon and is in most cases related to the pres- due au rapprochement final de deux masses continen- ence of a subduction zone. tales dan le cas du second; les deux mod6les Accepting that andesitic vulcanicity and granitoid sont originaux des le drbut de leur 6volution plutonism are associated with subduction of oceanic palrogrographique'.

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It would seem probable that the Andean chain troughs. This view has been usefully summarised by developed under a regime of subduction at the margin Clark et al. (1976), who state that 'The earlier Jurass- between a large continent and a large ocean, but that ic and Lower Cretaceous episodes of vulcanism and the Alpine chains developed under a much more subjacent plutonism led to the construction of island complex regime involving the opening and closing of arcs upon the outer edge of the sialic crustal margin'. small ocean basins between two continents. Whatever the merits of applying island arc terminology to a continental chain, it is clear that in general terms the geological situation is one of island arc type activity superimposed upon a continental margin. This The status ot geosyndinal has resulted in the development of special features, nomenclature both magmatic and sedimentary, which are specific to this tectonic situation. With the advent of plate tectonics there have been Theoretically it would be desirable to study the many calls to abandon geosynclinal nomenclature in conditions of formation of the Cretaceous Andes by favour of one which reflects more accurately the vari- analogy with other areas where subduction of oceanic ous stages of plate interaction. Thus, 'Plate tectonics crust beneath continental crust is occurring at present, have rendered geosynclinal terminology largely obso- and where volcanic material is being deposited under lete, but the stratigraphic and tectonic relations previ- marine conditions. Such areas are few, the only one of ously observed in geosynclinal successions can now be any considerable extent being part of the Sunda island easily explained by analogy with modern tectonic set- arc. Until such areas have been studied it will not be tings' (Mitchell & Garson 1976). possible to formulate an actualistic model of any preci- One cannot quarrel with the principle of describing sion. geological features in actualistic terms, since this is no It is probable, however, that studies of the volcanic more than applying the principle of uniformitarianism and sedimentary sequences in the Andes themselves which, though questioned from time to time, has re- will provide the basis for an actualistic model, but mained one of the cornerstones of geology. Neverthe- although some studies of volcanic and sedimentary less if the actualistic model is subsequently shown to process have been made (Webb 1976, Wilson 1963), be incorrect the attempt will not have served any these do not yet provide an adequate basis for a useful purpose. complete model. The Andean (Cordilleran) orogen is particularly im- Thus although the Andean chain may be understood portant because it is widely held to represent a general in the light of its tectonic setting, an actualistic case which can be, and indeed has been, applied to nomenclature for the depositional trough is not yet other orogenic belts, some as old as the Archean (Key available. Under these circumstances it is considered et al. 1976). This general view is not disputed and is advantageous to use the purely descriptive geosyncli- indeed felt to be perfectly correct; the difficulty arises hal nomenclature. in establishing those principles which are truly charac- The terms eugeosyncline and miogeosyncline are teristic and may be legitimately applied to older instantly recognisable by the majority of geologists as orogens. implying thick volcanic sequences on the one hand and The least complicated sector of the Andes is that sedimentary sequences on the other. Different kinds of which comprises Peru and the greater part of Chile, geosynclines doubtless develop under different plate commonly referred to as the Central Andes, and this tectonic regimes (as discussed above), but this need paper has described an important part of that orogen. entail no confusion if the regime is specified. Neither trench deposits nor marginal basins are Moreover, if there is a real doubt as to the kind of characteristic of this sector; where they occur, either in regime involved, as is commonly the case with older the Andes or in other Cordilleran chains, they may be orogens, the use of a geosynclinal nomenclature en- considered as departures from the norm, as estab- sures a degree of objectivity. It is, for example, by no lished in the Central Andes, and additional to the means clear what the relationship of the Palaeozoic normal calcalkaline volcano-plutonic chain. Andes was to the plate boundaries of that time. The earlier plate tectonic syntheses were exclusively It is salutary to reflect that, in the area described, ocean-based and propounded the view that Cordille- there is no indication from the sediments themselves ran orogenic belts were characterised by thick turbidi- that they were deposited at a convergent plate bound- tic sequences on oceanic crust which became accreted ary. Only the volcanics of the eugeosyncline indicate to the continent by thermal doming over a subduction this. In the Andean case the situation is clear enough zone (Dewey & Bird 1970). This model attracted because of the presence of the Pacific Ocean, the deep widespread comment but has now been superseded by sea trench and the inclined seismic zone, but if it were one which shows older continental crust extending as to be imagined that a similar fold belt lay deep within far as the Peru/Chile trench with the accumulation of a continent it might be quite difficult to reconstruct the sedimentary and volcanic sequences in block faulted original plate tectonic regime with certainty.

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In conclusion it is considered that the continued use materially improving an early draft, and J. J. Wilson for his of the term geosyncline is fully justified on practical assistance with the stratigraphy. grounds. That this is widely felt to be so is indicated by The paper is published with the permission of the Director of the Institute of Geological Sciences and of the Director of the number of papers published annually in which the the Instituto de Geologia y Mineria del Peru, to whom I am word geosyncline figures in the title. particularly indebted for logistic support over an extended ACKNOWLEDGEMENTS. I thank L. Aguirre, W. S. Pitcher, A. period. Bussell and my colleagues in IGS for critically reading and

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Received 21 June 1977. E. J. COBBING, Institute of Geological Sciences, 154 Clerkenweli Road, London ECIR 5DU.

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