GEOSCIENCE CANADA Volume 41 2014 255 PAUL F. HOFFMAN SERIES considered by previous workers to rep- its Mesoproterozoic basement resent western and eastern parts remained offshore until 77 ± 5 Ma respectively of the same marginal when it collided with, and was basin. The Huarmey-Cañete Trough, emplaced upon, the partially subducted which sits on Mesoproterozoic base- western margin of South America to ment of the Arequipa block, was filled form the east-vergent Marañon with up to 9 km of Tithonian to fold–thrust belt. A major pulse of Albian tholeiitic–calc-alkaline volcanic 73–62 Ma plutonism and dyke and volcaniclastic rocks. It shoaled to emplacement followed terminal colli- subaerial eastward. At 105–101 Ma the sion and is interpreted to have been rocks were tightly folded and intruded related to slab failure of the west-dip- during and just after the deformation ping South American lithosphere. Mag- Arc and Slab-Failure by a suite of 103 ± 2 Ma mafic intru- matism, 53 Ma and younger, followed Magmatism in Cordilleran sions, and later in the interval 94–82 terminal collision and was generated by Ma by probable subduction-related eastward subduction of Pacific oceanic Batholiths I – The plutons of the Coastal batholith. The lithosphere beneath South America. Cretaceous Coastal West Peruvian Trough, which sits on Similar spatial and temporal Batholith of Peru and its Paleozoic metamorphic basement, relations exist over the length of both comprised a west-facing siliciclastic- Americas and represent the terminal Role in South American carbonate platform and adjacent basin collision of an arc-bearing ribbon con- Orogenesis and filled with up to 5 km of sandstone, tinent with the Americas during the shale, marl and thinly bedded lime- Late Cretaceous–Early Tertiary Hemispheric Subduction stone deposited continuously through- Laramide event. It thus separated long- Flip out the Cretaceous. Rocks of the West standing westward subduction from Peruvian Trough were detached from the younger period of eastward sub- Robert S. Hildebrand1 and Joseph their basement, folded and thrust east- duction characteristic of today. We B. Whalen2 ward during the Late Cretaceous–Early speculate that the Cordilleran Ribbon Tertiary. Because the facies and facing Continent formed during the Mesozoic 1Department of Earth and Planetary directions of the two basins are incom- over a major zone of downwelling Sciences patible, and their development and between Tuzo and Jason along the University of California, Davis, California subjacent basements also distinct, the boundary of Panthalassic and Pacific 95616-8605, USA two basins could not have developed oceanic plates. E-mail: [email protected] adjacent to one another. Based on thickness, composi- SOMMAIRE 2Geological Survey of Canada tion and magmatic style, we interpret Nous avons étudié les relations spa- 601 Booth St., Ottawa, Ontario the magmatism of the Huarmey- tiales et temporales des unités de K1A 0E8, Canada Cañete Trough to represent a magmat- roches dans la portion ouest de la ic arc that shut down at about 105 Ma Cordillère du Pérou, où deux bassins SUMMARY when the arc collided with an crétacés, la fosse d’accumulation de We examined the temporal and spatial unknown terrane. We relate subsequent Huarmey-Cañete et la fosse d’accumu- relations of rock units within the West- magmatism of the early 103 ± 2 Ma lation péruvienne de l’ouest, ont été ern Cordillera of Peru where two Cre- syntectonic mafic intrusions and dyke perçues par des auteurs précédents taceous basins, the Huarmey-Cañete swarms to slab failure. The Huarmey- comme les portions ouest et est d’un and the West Peruvian Trough, were Cañete-Coastal batholithic block and même bassin de marge. La fosse de Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.047 © 2014 GAC/AGC® 256 Huarmey-Cañete, qui repose sur le lithosphérique sud-américaine à duction zone (Pitcher 1993; Jaillard and socle mésoprotérozoïque du bloc pendage ouest. Le magmatisme de 53 Soler 1996). In this hypothesis, while d’Arequipa, a été comblée par des Ma et plus récent qui a succédé à la subduction remained easterly at least couches de roches volcaniques tholéi- collision finale, a été généré par la sub- since the Jurassic, variable stresses tiques – calco-alcalines de l’Albien au duction vers l’est de la lithosphère within the margin, generally attributed Thithonien atteignant 9 km d’épaisseur. océanique du Pacifique sous to changes in slab dip and obliquity, led Vers l’est, l’ensemble a fini par former l’Amérique du Sud. to arc migration and shut-down as well des hauts fonds. Vers 105 à 101 Ma, Des relations temporelles et as periods of extension, which formed les roches ont été plissées fortement spatiales similaires qui existent tout le basins, and periods of compression, puis recoupées par une suite d’intru- long des deux Amériques représentent which created fold–thrust belts (for sions vers 103 ± 2 Ma, durant et juste la collision terminale d’un ruban conti- example Kay and Mpodozis 2002; après la déformation, et plus tard dans nental d’arcs avec les Amériques durant Ramos and Folguera 2005; Ramos and l’intervalle 94 – 82 Ma, probablement la phase tectonique laramienne de la fin Kay 2006; Mosquera and Ramos 2006; par des plutons de subduction du du Crétacé–début du Tertiaire. Elle a Ramos 2009, 2010a). batholite côtier. Quant à la fosse d’ac- donc séparé la subduction vers l’ouest Because the Andes have long cumulation péruvienne de l’ouest, elle de longue date de la période de sub- served as a living proxy for under- repose sur un socle métamorphique duction vers l’est plus jeune carac- standing the geology of the North paléozoïque, et elle est constituée d’une térisant la situation actuelle. Nous American Cordillera (Hamilton 1969a, plateforme silicoclastique – carbonate à considérons que le ruban continental b; DeCelles et al. 2009), and because pente ouest et d’un bassin contigu de la Cordillère s’est constitué durant le some researchers have recently chal- comblé par des grès, des schistes, des Mésozoïque au-dessus d’une zone lenged the idea of long-lived easterly marnes et des calcaires finement lam- majeure de convection descendante subduction beneath North America inés atteignant 5 km d’épaisseur et qui entre Tuzo et Jason, le long de la limite (Moores et al. 2002, 2006; Johnston se sont déposés en continu durant tout entre les plaques océaniques Panthalas- 2008; Hildebrand 2009, 2013), we le Crétacé. Les roches de la fosse d’ac- sique et Pacifique. decided to undertake a careful re- cumulation péruvienne de l’ouest ont examination of existing data for west- été décollées de leur socle, plissées et INTRODUCTION ern South America to better under- charriées vers l’est durant la fin du Cré- In the 1972 discussion of the now stand the analogs. In this contribution tacé et le début du Tertiaire. Parce que classic paper, The Coastal batholith of we focus on the Peruvian sector of the les facies et les profondeurs de sédi- Peru, Robert M. Shackleton, student of margin (Figure 1) where intrusions of mentation de ces deux fosses d’accu- African geology and Royal Fellow, the Coastal batholith constitute an mulation dont incompatibles, et que asked the authors why the granitic and immense, dominantly Late Cretaceous leur développement et leur socle sont related plutonic rocks of the batholith batholith, built mostly of tonalitic and différents, ces deux fosses ne peuvent rose along a narrowly restricted zone granodioritic plutons that collectively pas s’être développées côte à côte. over a long period of time (Cobbing form a linear 60 km-wide band extend- À cause de l’épaisseur accu- and Pitcher 1972a). Because the plu- ing over 2000 km within the core of mulée, de sa composition et du style de tons were undated at that time, Pitcher the Western Cordillera (Pitcher 1978, son magmatisme, nous pensons que la couldn’t answer the question, but he 1985, 1993). Since the landmark publi- fosse d’accumulation de Huarmey- did say that such a finding was indeed cations of Cobbing and Pitcher, the Cañete représente un arc magmatique surprising. To this day Shackleton’s batholith has been interpreted to be qui s’est éteinte vers 105 Ma, lorsque query remains unanswered, and given the direct result of long-lived eastward l’arc est entré en collision avec un ter- the plethora of published papers on subduction of oceanic lithosphere rane inconnu. Nous pensons que le arc migration due to slab flattening and beneath the South American continen- magmatisme subséquent aux premières rollback over the past couple of tal margin (Pitcher 1978; Cobbing et al. intrusions mafiques syntectoniques et decades, the question remains on 1981; Pitcher et al. 1985). Here we aux réseaux de dykes de 103 ± 2 Ma point: Why was magmatism of the revisit the batholith and its setting to sont à mettre au compte d’une rupture Coastal batholith (Figure 1), which we suggest an alternative explanation for de plaque. Le bloc Huarmey-Cañete- now know represents more than 40 its origin, which we believe finally batholitique côtier et son socle méso- m.y. of intense magmatism, confined answers the question Shackleton posed protérozoïque sont demeurés au large to such a narrow band? some 40 years ago. We end by briefly jusqu’à 77 ± 5 Ma, moment où il est Over the four decades since, discussing how our new model affects entré en collision et a été poussé par- the Andes have become the standard the existing paradigm for Andean and dessus la marge ouest sud-américaine example of an orogenic belt formed at hemispheric orogenesis. partiellement subduite, pour ainsi for- a subducting plate margin, and so form mer la zone de chevauchement de ver- a template for the interpretation of REGIONAL SETTING gence est de Marañon.