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 conti- nental 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/135/2/207/4885609/gsjgs.135.2.0207.pdf by guest on 29 September 2021 208 E. J. Cobbing 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 con- sidered 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.