Paul F. Hoffman Series

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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

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. Nous croyons other orogenic belts. In the current The Coastal batholith of Peru out- que la séquence majeure de plutonisme paradigm, from the Gulf of Guayaquil crops within the Cordillera Occidental, et d’intrusion de dykes qui a succédé à southward, a long-lived arc complex a range of towering peaks rising paral- la collision finale à 73–62 Ma doit être formed within and upon the continen- lel to the Pacific coast and separated reliée à une rupture de la plaque tal lip above the eastward-dipping sub- from it by a narrow desert plain, some GEOSCIENCE CANADA Volume 41 2014 257

80ºW 74ºW proterozoic to Paleozoic metamorphic rocks, including ~600 Ma orthogneiss, Ecuador Peru Cretaceous West Peruvian Trough cut by Ordovician plutons, interpreted basin to represent part of a magmatic arc s platform e o pe o (Chew et al. 2007), and overlain by two ap ll t m o ll

O m O sequences of metamorphosed Paleo- A Metamorphic basement zoic rocks: an older sequence deposit- & Paleozoic cover S ed and metamorphosed between 450 HuancabambaS Tahúin Deectiono and 420 Ma; and a younger sequence terrane u 8 Ma Cordillera terrane Blanca batholith deposited after 320 Ma and metamor- i th h ~105-90 Ma phosed at 310 Ma (Cardona et al. Lancones basin Lancones basin 2009). A 300 km-long discontinuous harzburgite Late Cretaceous band of dismembered and serpen- blueschist 8ºS 8ºS m Marañon fold- eclogite tinized Neoproterozoic ophiolite –

thrust belt Paleozoic buried to pressures of 12.5 ± 1 kbar plutons and metamorphosed during the Outer shelf Ordovician, thrust over much less E high a s deformed and lower grade Carbonifer- st exploration well P ous metasedimentary rocks, and pre- e M ru served in part as klippen – outcrops in a uv P A P ra ia A the western part of the complex (Cas- a ñ n m c l t c o ro e i n u troviejo et al. 2009, 2010; Willner et al. gh ri c h h c c Huarmey h C ca 2010; Tassanari et al. 2011). There are o a m -Cañete Lima m also extensive tracts of Carboniferous pl Trough ex and Permo–Triassic plutonic rocks

O (Chew et al. 2007; Mišković et al. c e a 2009). 14ºS n 14ºS To the southwest are sedimentary rocks of the Late Jurassic–Creta- ceous West Peruvian Trough (Wilson Coastal batholith 1963). The trough (Figure 1), devel- LT Tithonian-Albian Arequipa oped upon rocks of the Marañon com- Casma-Cañete groups plex, comprises an eastward-tapering ax xis wedge of marine sedimentary rocks thin Mesozoic cover s o on Arequipa block f m with a west-facing sandstone and car- od de Mesoproterozoic rn bonate-dominated shelf – discon- tr Arequipa block en formably capped by latest Cretaceous Arequipa block ch Chile to Paleocene, red, cross-bedded sand- 76ºW stone derived from the southwest– and a thicker sandstone–shale–limestone Figure 1. Geological sketch map of western Peru illustrating the general relations basinal facies to the southwest (Scher- between Tithonian–Albian rocks of the Casma Group deposited in Huarmey renberg et al. 2012). In the traditional basin, the Coastal batholith, and the West Peruvian Trough. Location of the Early interpretation, the westernmost parts Cretaceous subduction complex (harzburgite–blueschist–eclogite), which predated of the trough contain several intercon- the events in this paper as the rocks there were exhumed during the Early Creta- nected subbasins such as the Huarmey ceous, is from Feininger (1980). i–Illascas massif; m–Isla Macabi; h–Isla de las in the north and Rio Cañete to the Hormigas de Afuera; l–Isla Lobera; LT–Lake Titicaca. south, both of which were filled with 5000–9000 m of Cretaceous, domi- 15–150 km wide. To the northeast sits the Amazon are rocks of the Suban- nantly Albian, submarine basaltic, high plateau country and the dean region, an area of Tertiary basins andesitic, and dacitic volcanic rocks, Cordilleras Central and Oriental, which overlying thick sequences of Phanero- collectively referred to as the Casma merge southward with the Cordillera zoic rocks and basement, which to the Group (Cobbing 1976, 1978, 1985). Occidental in central Peru. Deep west become progressively involved in The main difference between the two canyons dissect the ranges and provide thick-skinned basement-involved subbasins is that in the Rio Cañete excellent cross-sections of the geology, thrusts and thin-skinned thrusts (Math- Basin to the south is a thick section of whereas the high plateau has a gently alone and Montoya 1995; Pfiffner and Early Cretaceous sedimentary rocks rolling topography. To the east of the Gonzalez 2013). Farther southwest, in that predate the volcanic rocks, other- Andes lies the jungle-covered, but the Cordillera Oriental, lies the wise the basins were stratigraphically apparently oil-rich, Amazon basin. Marañon complex (Figure 1), which is and structurally continuous and could Emerging from the jungles of a northwest-trending belt of Late Neo- be considered one basin (Cobbing 258

1978). These westernmost subbasins SW NE are generally interpreted to be part of a single marginal basin that encom- Tapacocha Zone 6 km wide West Peruvian passed all of the West Peruvian Huarmey-Cañete isoclinal folds, greenschist grade, Trough (Atherton et al. 1985a). intense cleavage basin The boundary between the aquagene subaerial eastern, sedimentary part of the basin tuff pyroclastics Basin Platform and the western volcanic-dominated carbonate limestonelimestone sequence has long been known to be Arequipa abrupt. In his broad reconnaissance, block Wilson (1963) called the change rapid; shale but in his subsequent work (Wilson et Marañon block al. 1967), he realized that it was a fault- pillow shale ed contact. Cobbing (1976) recognized basalt ? that not only stratigraphy, but also modified fro structures within the two sectors were Figure 2. Schematic cross-section showing the commonly accepted relations with- different, and so termed the eastern in the West Peruvian-Huarmey marginal basin (modified from Cobbing 1976). We part a miogeosyncline and the western have added the Topacocha zone from Myers (1974). volcanic facies as eugeosynclinal. Myers (1974, 1975) described the boundary between the two as a tectonic line, part as the shallowing-upward marine andesitic lava piles, up to 1.8 km thick, which he named the Tapacocha Axis. to nonmarine Casapalca Formation, are intercalated with aquagene tuff and Cobbing (1978), while recognizing the which sits disconformably on the older associated sedimentary rocks, whereas importance of the Tapacocha Axis, stratigraphic units (Scherrenberg et al. to the east calc-alkaline andesitic to considered a more eastward fault, the 2012). Paleocene and younger, broadly dacitic lavas, subaerial cooling units of Cordillera Blanca fault, as the critical folded, terrestrial, intermediate to ignimbrite, and lesser quantities of tectonic line, because he thought that siliceous lavas and tuffs, along with clastic sedimentary rocks dominate the the Tapacocha Axis wasn’t a persistent related volcaniclastic rocks of the sections (Myers 1974, Cobbing 1976; structure to both north and south. Calipuy Group sit disconformably atop Atherton et al. 1985a). The facies indi- The rocks of the Huarmey- rocks of the fold–thrust belt, the cate that the basin shoaled eastward. Cañete basins were intruded by the batholith, and its wall rocks (Atherton As stated earlier, the Coastal batholith (Figure 1), which et al. 1985b; Figure 18 of Pfiffner and Huarmey-Cañete basin was considered ranges in age from 105 Ma to 62 Ma Gonzalez 2013). to be the western part of a larger mar- (Mukasa 1986). It is one of the great ginal basin, known as the West Peru- Cordilleran-type batholiths of the The Batholithic Envelope vian Trough or basin (Figure 2), locat- world and has been extensively studied The main bulk of the Coastal batholith ed to the east where it is entirely sedi- by a British group headed by Pitcher (Figure 1) was emplaced between mentary and developed along the west- and Cobbing (Cobbing and Pitcher about 100 Ma and 60 Ma into 5–9 km ern side of a Paleozoic basement block 1972a, b; Cobbing et al. 1981; Cobbing of volcanic and volcaniclastic rocks of known as the Marañon complex (Wil- and Pitcher 1983; Pitcher et al. 1985). the Casma Group, which are generally son 1963; Mégard 1987; Cardona et al. The basement for the Haurmey-Cañete hypothesized to have been deposited 2009). Rocks within the basin are sand- Trough and the Coastal batholith in within the actively subsiding, intercon- stone, shale, limestone and marl that the south is the Mesoproterozoic Are- nected, linear basins of the Huarmey- were deposited continuously from the quipa terrane (Shackleton et al. 1979; Cañete Trough (Cobbing 1978; Ather- Late Jurassic to the Late Cretaceous Loewy et al. 2004; Casquet et al. 2010), ton et al. 1985a; Pitcher 1993). The age (Mégard 1984; Scherrenberg et al. whereas to the north, basement is of the bedded rocks ranges from 2012). Sedimentary rocks within the unknown on-land as the Arequipa Tithonian to Albian, but the bulk of basin formed an eastward-tapering block strikes into the Pacific Ocean; the section was erupted and deposited wedge and are divided into a thin, east- however, recent exploration wells and rapidly during the Latest Aptian and ern platformal succession and a thick studies of sparse offshore islands (Fig- Albian (Myers 1974; Cobbing 1976; western sequence separated by a west- ure 1) discovered that the Arequipa Atherton et al. 1983, 1985a; de Haller side down, syn-sedimentary fault block continues northward on the et al. 2006). Magmatism apparently known as the Chonta fault (Scherren- outer shelf high (Romero et al. 2013). ceased coincidentally with a period of berg et al. 2012). The western edge of Rocks of the West Peruvian deformation in the Late Albian (Ather- the westward-deepening basin is Trough were detached from their base- ton et al. 1985a; Atherton and Webb abruptly truncated – and separated ment, folded, and thrust eastward dur- 1989). from rocks of the Huarmey-Cañete ing the Late Cretaceous and Early Pale- Within the western part of the basin – by an obscure and mostly cov- ocene, coincident with inversion of the basin (Figure 2), several 2 km-thick ered zone, in many places also invaded basin and development of a wide- sections of tholeiitic basalt and associ- and obliterated by plutons, known as spread foredeep sequence preserved in ated hyaloclastite, along with massive the Tapacocha Axis (Figure 2), a > 6 GEOSCIENCE CANADA Volume 41 2014 259

From Regan, 1985 b. mafic and siliceous magmas (Mukasa 1986). Although the plutons and their metamorphic haloes clearly crosscut structures in the country rock, some are cut by zones of intense ductile Casma LimaLima deformation and therefore were N intruded during the deformation Granitoid rocks Early mafic Wall rock intrusions 100 km (Regan 1985). The mafic bodies are

c. a. from Myers, 1974 variable high-Al gabbro with bulk

5 km N 2 km compositions that approximate olivine Granitoids N th oli basalt (Regan 1985). Both field and Blastomylonite th ba Hb diorite l geochemical evidence, such as pro- ta Mafic & siliceous s foundly different trace element charac- oa dykes C Microgranite teristics and lack of any major element Gabbro/diorite continua, indicate that rocks of the Plag-Hb mafic suite are not comagmatic with porphyry the younger granitoid rocks of the Pillow lava Pillow lavas, hyaloclastite & breccia andesitic aquagene tuff Coastal batholith, but instead record a Globe breccia Sedimentary rocks separate intrusive event (McCourt Flows, tuffs, Andesitic lavas, tuff & breccia and dacitic ignimbrite 1981; Regan 1985).

From Pitcher and Bussell, 1985 Submarine pyroclastic rocks The Coastal Batholith Figure 3. (a) Geological sketch map modified from Myers (1974) illustrating the The Coastal batholith (Figure 1) is a two generations of folds that appear to predate the emplacement of the Coastal 2000 km long linear batholith com- batholith; (b) sketch map from Regan (1985) showing the distribution of the early posed of a great number, perhaps 1000 tholeiitic mafic intrusions; (c) geological map showing one area of a batholith-par- or more, individual plutons (Cobbing allel dyke swarm related to the early mafic intrusions (modified from Pitcher and et al. 1981; Pitcher 1985). There were Bussell 1985). two major pulses of plutonism within the batholith that postdated the suite km wide belt of isoclinal folds, green- west, was deposited. At least 600 m of of mafic to intermediate plutons and schist-facies metamorphism, and subaerial andesitic to basaltic lava and dykes emplaced at 105–101 Ma. The intense cleavage development (Myers pyroclastic rocks sit unconformably first occurred from about 91 Ma to 82 1974). upon the folded rocks (Myers 1974; Ma (Wilson 1975; Mukasa 1986). Fol- Magmatism within the Atherton et al. 1985a; Atherton and lowing a break of 9–10 m.y. another Huarmey-Cañete basins stopped dur- Webb 1989). The post-folding volcanic suite was intruded in the range 73–62 ing the late Albian and the rocks were rocks thicken westward, are likely Ma (Cobbing et al. 1981; Mukasa variably folded, in places isoclinally 100–82 Ma, and contain clasts of gran- 1986). (Figure 3a). Myers (1974) described ite and porphyry not known from pre- Those interested in the petro- two orthogonal fold sets: NNW, paral- folding rocks (Atherton et al. 1985a). graphic details of the batholith will lel to the basinal axes, and NE, normal undoubtedly find the descriptions and to them (Figure 3a). Both sets plunge The Early Mafic Intrusions maps in the memoirs by Cobbing et al. gently. The NNW set of folds is locally Rocks of the Huarmey-Cañete basin (1981) and Pitcher et al. (1985) to be isoclinal, verges NE, is in places over- are riddled with a diverse suite of without equal and the descriptions of turned, and has acicular hornblende ~105–101 Ma magnesian, calcic to the batholith here are summarized porphyroblasts aligned with fold axes. calc-alkaline mafic intrusions (Figure from those contributions augmented Myers used deformed ammonites to 3b), dominantly peridotite, gabbro and by other papers where noted. Most of show that even gently dipping beds diorite, which collectively comprise the plutons of the batholith are com- had in excess of 20% shortening. The about 16% of the Coastal batholith posed of quartz diorite, tonalite, gran- ages of both sets of folds are tightly (McCourt 1981; Regan 1985; Mukasa odiorite, and monzogranite, with two constrained as they postdate the main 1986). The intrusions form sills, dykes, periods of coeval batholith-parallel phase of Albian magmatism but pre- small plugs, plutons several tens of km dyke swarms (Pitcher 1978). In a gen- date a suite of mafic intrusions, dated long, and impressive basin-parallel eral sense, and like other Cordilleran at about 105–101 Ma (Mukasa 1986). mafic dyke swarms (Figure 3c) that batholiths, magmatism became more The sedimentary facies east of predate emplacement of most grani- siliceous with time. the Tapacocha belt within the West toid plutons of the Coastal batholith Originally the plutons were Peruvian Trough show neither evi- (Regan 1985; Pitcher and Bussell divided into genetically related units dence of this deformation nor an 1985). Locally, some granodiorite and and super-units based on spatial, tem- influx of immature sediment until the monzogranite plutons are as old as the poral, textural and modal variations, Late Cretaceous–Paleocene when flu- more mafic bodies indicating close but U–Pb dating (Mukasa 1986) vio-deltaic sediment, shed from the temporal and spatial relations between showed a more complex situation with 260 much younger plutons intruding older 50 8 Plutonic Pulses suites. In general, it is exceedingly diffi- a. <58 Ma - E dip arc rocks e. 73-62 Ma - slab failure 40 94-82 Ma - arc? cult to relate various plutonic phases in c alcic 105-101 Ma - slab failure 6 Cordilleran batholiths by fractionation shoshonitic 30 and/or assimilation of wall rock: they high-K are more complex assemblages (e.g. 4 a l k a 20 2 l i c

Memeti et al. 2012). Some workers, K O (wt %) a lk medium-K a li most notably Atherton et al. (1979) -c al ci c 2 10 c al alc believed, based largely on low initial - 100*Q/(Q+Or+Ab+An) Q’ kaline 87Sr/86Sr, that rocks of the Coastal 0 0 102030405060708090100 0 batholith came directly from the man- 50 55 60 65 70 75 80 tle; however, Cordilleran batholiths do ANOR - 100*(An/(Or+An)) SiO (wt %) not occur in regions where the crust is 2 oceanic and there are several ways to 10 b. <60 Ma - Arc-Related (N=4) create rocks with low initial 87Sr/86Sr 100 (Hildebrand and Bowring 1984). In the 5 c 10 lci case of the Coastal batholith, it ca li- ka e al in f.

22 l appears clear that the early mafic mag- 0 ka 1 al lc- mas are predominantly mantle-derived, ca 73-62 Ma - Slab Failure c 100 lci SiO2= 50-65 wt.% (N=25) but even those are associated with ca -5 more siliceous magmas. The main bulk = (NaMALI O+K O-CaO) 10 of material within the Coastal batholith likely represents complex 1 1 73-62 Ma - Slab Failure c. mixtures of heterogeneous mantle with n SiO2= 65-70 wt.% (N=25) .9 roa 100 highly variable, but dominantly inter- fer .8 mediate, crustal material. Fe* 10 Following the early 91–82 Ma .7 period well-documented in the exten- 1 total.6 total 73-62 Ma - Slab Failure sive Lima and Arequipa sectors of the sian SiO2= 70-78 wt.% (N=23) agne 100 batholith, a second period of magma- .5 m

Fe* = (FeOFe* /(FeO + MgO)) 10 tism followed in the 73–62 Ma time- .4 Sample/Primitive Mantle frame (Pitcher 1985, 1993; Mukasa 1986). This period of magmatism 45 50 55 60 65 70 75 80 1 SiO (wt %) includes the great centred complexes 2 (e) 94-82 Ma 100 Arc-Related (N=19) and major swarms of batholith-parallel 3 dykes that just predate the centred complexes. The age of dykes were Peraluminous 10 bracketed to be between 73 and 71 Ma 2 1 and the younger ring complexes and 105-101 Ma centred intrusions were emplaced 100 Slab Failure (N=7) between 71 and 62 Ma, with the bulk Al/(Na + K) Metaluminous 1 10 of magmatism 70 ± 2 Ma (Mukasa Peralkaline d. 1986). Minor amounts of volcanism 1 0.5 1.0 1.5 occurred at about 70 Ma (Polliand et Al/(Ca - 1.67*P + Na + K) al. 2005) and it may have been related to the suite of centred complexes. Figure 4. Geochemistry of the plutonic groups of the Coastal batholth plotted on: The centred complexes are (a) CIPW normative Q’ (100*(Q/(Q+Or+Ab+An)) versus ANOR 10–30 km wide and composed of indi- (100*(An/(Or+An)) classification diagram of Streckeisen and LeMaitre (1979) vidual plutons and ring-dykes of gab- showing compositional trends for different representative types of plutonic suites from Whalen and Frost (2013); (b) Na2O+K2O–CaO (or MALI) vs. SiO2 and (c) bro and diorite intruded by granodior- total total ite and granite, in places granophyric FeO /(FeO + MgO) (or Fe*) vs. SiO2 granitic rock classification diagrams of (Bussell 1985). The dimensions of Frost et al. (2001). The boundary between ferroan and magnesian plutons was individual plutons with radii from 6–10 modified as suggested by Frost and Frost (2008); (d) a Al saturation index (ASI) km, the presence of ring dykes, and [molecular Al/(Ca–1.67*P+Na+K)] versus molecular Al/(Na+K) (inverted peral- the overall centred nature of the com- kaline index) diagram; (e) a SiO2 vs. K2O diagram with suite subdivisions after plexes, led them to be interpreted as LeMaitre (1989) (low-, medium-, high-K) and Peccerillo and Taylor (1976) (high-K, sub-cauldron intrusions (Bussell et al. shoshonitic); and (f) primitive mantle normalized extended element plots. Normal- 1976; Bussell 1985). izing factors are from Sun and McDonough (1989). Analyses are from Pitcher et al. Granitoid plutons of the (1985). Coastal batholith range from calcic, GEOSCIENCE CANADA Volume 41 2014 261

Figure 5. Stratigraphic sections of the West Peruvian Trough from Scherrenberg et al. (2012) compared and contrasted with simultaneous, or time-correlative, units and events in the Huarmey-Cañete trough basin to the southwest. The figure shows that the rock packages in each basin were unlikely to have been deposited within the same basin, or even adjacent to one another, during the Cretaceous and were more likely to have been juxtaposed during the Late Cretaceous–Early Tertiary. calc-alkaline to alkali-calcic (Figure 4a malized plots (Figure 4f) are features all of which are located west of the and b). Based on their Fe* vs. silica val- of granitoid rocks formed within arc Tapacocha zone, thinly bedded silici- ues (Figure 4c), all but four high silica settings or derived from crustal clastic and limey rocks accumulated to samples from the Coastal batholith are sources. Coastal batholith variations in the east in the West Peruvian basin magnesian or oxidized, a distinctive (La/Y)N, Sr/Y, and Y bear both on the (Figure 5) to a thickness of at least feature of granitoid rocks formed at nature of the protoliths and the P–T 4000 m in the western off-shelf facies subducting margins (Frost et al. 2001). conditions of magma generation and about half that farther east on the In general, the mainly metaluminous (Wang et al. 2007). The Sr/Y < 40, west-facing platform (Wilson 1963; compositions (Figure 4d), amphibole- (La/Y)CN < 20, and a lack of pro- Scherrenberg et al. 2012; Pfiffner and bearing mineralogy, and common nounced positive Sr anomalies on Gonzalez 2013). The rocks span most igneous microgranitic enclaves within extended element plots (Figure 4f) of the Cretaceous (Figure 5) and there more mafic end-members of suites (cf. exhibited by all Coastal batholith grani- were no significant breaks in sedimen- Pitcher et al. 1985) indicate the Coastal toid rocks, indicate formation within tation until the uppermost Cretaceous batholith comprises I-type granitic relatively thin crust under P–T condi- when the platformal sequence was dis- rocks derived from infracrustal sources tions with residual plagioclase and no conformably overlain by a thin (Chappell and Stephens 1988). The residual garnet (Wang et al. 2007). sequence of calcareous marl capped by Coastal batholith includes medium-K, red cross-bedded sandstone, collective- high-K and shoshonitic compositions Marañon Fold–Thrust Belt ly interpreted to represent foredeep fill and is consistently enriched in large- During deposition and deformation of (Scherrenberg et al. 2012). ion lithophile elements (LILE) (Figure rocks within the Huarmey-Cañete The foredeep developed when 4e). LILE-enrichment and negative Ta basin, as well as during the subsequent rocks of the Cretaceous West Peruvian anomalies on extended element nor- emplacement of the Coastal batholith, basin were detached from lower struc- 262 tural levels along a major décollement, Also outcropping in northern that the magmatic rocks are the prod- folded, and thrust to the NE to create Peru–southern Ecuador, is the Creta- uct of an arc built on continental crust. what is known as the Marañon ceous Lancones basin (Figure 1), which Consider that the average thickness of fold–thrust belt (Scherrenberg et al. comprises a broadly folded, metallifer- basalt formed at oceanic spreading 2012, 2014). The belt is well developed ous volcanic section ranging in age ridges is about 0.5 km (Moores 1982; between the Tapacocha zone and the from ~105–90 Ma, just younger than Dick et al. 2006) as basaltic eruptions Marañon complex and places western most of the Huarmey basin; hosts Late only occur within the 1–3 km wide rift basinal facies rocks over more easterly Cretaceous plutons correlated with valley, which spreads outward and platformal facies rocks. The structure those of the Coastal batholith; and sits cools (Moore et al. 1974). ranges from tightly folded and isoclinal structurally between the Amotape and Thick sequences of arc rocks with steep axial fabrics to shallowly Olmos massifs (Jaillard et al. 1999; erupted and deposited within a basin dipping beds with shallowly dipping Winter et al. 2010). The earliest mag- are common as most continental arcs northeast-vergent thrust faults (Pfiffner matism, ~105–100 Ma, was a bimodal, form within subsiding depressions or and Gonzalez 2013; Scherrenberg et al. tholeiitic to calc-alkaline basalt-domi- basins on crust of average or below 2014). The thrust belt continues south- nated sequence of pillow basalt with average thickness (Levi and Aguirre ward into Bolivia (McQuarrie 2002). associated volcanic massive sulphide 1981; Hildebrand and Bowring 1984; The age of the deformation is deposits and low Nb and Y siliceous Busby-Spera 1988; Busby 2012). Mod- bracketed to be latest Cretaceous to rocks; whereas younger 99–90 Ma ern examples include the Cascades, Early Tertiary based in part on 53 Ma magmatism involved both tholeiitic where the volcanoes sit in a half volcanic rocks of the Calipuy Forma- and calc-alkaline basalt, andesite and graben; the low-standing Alaskan tion that unconformably overlie rocks more siliceous lava (Winter 2008). Peninsula where volcanoes such as of the thrust belt (Atherton et al. Because the Lancones basin volcanic Augustine sit within Cook Inlet; the 1985b; Petford and Atherton 1994) as rocks are only openly folded and range Kamchatka Peninsula of eastern Russia well as the Late Cretaceous–Early Ter- in age from ~105 Ma to 90 Ma, they where majestic stratovolcanoes sit in tiary age of the Casapalca molasse appear to be correlative with thin post- huge fault-bounded troughs; New (Scherrenberg et al. 2012). To the 105–100 Ma deformed volcanic rocks Zealand where the Taupo zone is south, rocks of the Arequipa block in the Huarmey basin and the early actively extending as calderas and stra- were also thrust to the NE over Meso- mafic suite within the Coastal tocones form; and the Central Ameri- zoic sedimentary rocks prior to the batholith. can arc where volcanoes are aligned in emplacement of a 62 Ma granitoid plu- Seismic refraction, seismic a long linear depression. The modern ton (Ellison et al. 1989). reflection and gravity data collected in Andes are an exception to the pattern. the northern sector at about 9ºS cou- Furthermore, the stratigraphy within Other Considerations pled with drill cores and outcrops on pendants and wall rocks of Cordilleran Although the southern part of the islands suggest that basement to the batholiths provides no evidence of Coastal batholith and its wall rocks are west of the Huarmey basin on the thick crust as they too sat at, or below, underlain by Mesoproterozoic base- Outer Shelf High is composed of sea level during volcanism. The Sierra ment of the Arequipa block, the nature crystalline rocks cut by intrusions Nevada and Peninsular Ranges of basement beneath the northern half (Jones 1981; Thornburg and Kulm batholiths of North America were was until recently uncertain, but now is 1981; Couch et al. 1981), now known both low-standing during arc magma- thought to be Mesoproterozoic Are- to be part of the Arequipa block tism as documented by marine sedi- quipa block just as to the south (Romero et al. 2013). Just to the east mentary rocks as young as 100 Ma (Romero et al. 2013). Metamorphic and beneath rocks of the Huarmey interbedded with the volcanic rocks rocks of Paleozoic age outcrop in basin, seismic and gravity data show an (Fife et al. 1967; Allison 1974; Nokle- extreme NW Peru (Cardona et al. arch-like structure of rock with a den- berg 1983; Wetmore et al. 2005; Busby 2009) and appear to continue north- sity of 3.0 g/cm3 that was interpreted et al. 2006; Saleeby et al. 2008; Memeti ward into Ecuador (Figure 1) where to be the result of crustal rupture and et al. 2010; Centeno-García et al. they are juxtaposed against Mesozoic upwelling of mantle material into the 2011). Similarly, the Casma arc volcanic eclogite and blueschist in the Amotape crust (Jones 1981; Couch et al. 1981). rocks described here were marine prior massif (Feininger 1980). The Paleozoic to 105 Ma (Cobbing 1978, 1985; rocks of northern Peru and southern INTERPRETATION Atherton et al. 1985a) as were those of Ecuador apparently constitute base- While the Huarmey-Cañete basins are the Jurassic–Cretaceous Ocoite arc in ment within the Tahuín terrane, inter- generally considered to represent the northern Chile (Levi and Aguirre 1981; preted to be exotic with respect to western part of a marginal basin (Cob- Åberg et al. 1984). South America (Feininger 1987). The bing 1978; Atherton et al. 1983, 1985a; Although the Huarmey-Cañete relation between the northern terranes Atherton and Webb 1989; Pitcher basin and the West Peruvian Trough and the Arequipa block is unclear as 1993), the tremendous thickness, some are currently adjacent to one another, exposure is poor but they must have 5–9 km, of basalt and andesite, cou- the lack of volcanic debris, coupled been joined prior to emplacement of pled with voluminous intermediate to with the complete absence of the 100 the Coastal batholith, which crosscuts siliceous ignimbrite and exposed Meso- Ma deformation within the West Peru- both basement units. proterozoic basement suggest to us vian Trough, preclude it being adjacent GEOSCIENCE CANADA Volume 41 2014 263 to the Huarmey-Cañete basin during density of the oceanic lithosphere and transected by an arc-parallel strike- its formation (Figure 4). Similarly, as causes the lower plate to break, pre- slip fault, common to zones of oblique the emplacement of the voluminous dominantly by viscous necking (Duretz convergence (Fitch 1972) and that Coastal batholith occurred at the same et al. 2012) at its weakest point, and meridional migration during the colli- time as sedimentation in the trough, sink into the mantle. This failure allows sion separated the two. Such a scenario one would expect some effect, such as asthenosphere to upwell through the would explain the NE cross-folds that uplift, on the western edge of the tear, melt adiabatically, and rise into the folded the earlier NNW isoclines yet basin at 100 Ma, due to increased heat collision zone, where it interacts with predated the early mafic suite (Figure flux, but none is reported: the defor- the crust. The resulting magmas, which 3a). mation in the eastern trough is much form linear arrays above tears in the Hildebrand (2013) recognized later, during the Late Cretaceous to descending slab, are linear upwellings that several Cordilleran batholiths of Early Tertiary. Lastly, the facies of the flowing through the breach in the slab, North America, such as the Peninsular two basins are incompatible because but are compositionally heterogeneous Ranges, Sierran and British Columbia the West Peruvian Trough deepened as they reflect differences within both Coast batholiths were composed of westward whereas the Huarmey-Cañete the mantle and crust and well as vari- two phases: an early arc phase and a shoaled to subaerial eastward (Figure able amounts of melting and mixing. younger post-deformational phase, 2). Thus, we conclude that the They commonly overlap the terminal which he interpreted to represent slab- Huarmey-Cañete arc is exotic with stages of deformation and are highly failure magmatism due not only to its respect to its current position in west- metalliferous (Solomon 1990; Cloos et post-deformational timing, but also to ern Peru. To the east we see no evi- al. 2005; Hildebrand 2009, 2013). the exhumation and erosion related to dence of another magmatic belt of the Because the early tholeiitic doubling of the crust during collision. appropriate age that might be inter- mafic suite – as well as temporally The age of collisional deformation in preted as a Cretaceous arc, so there is equivalent basalt of the Lancones the North American batholiths is no reason a priori for subduction basin – was emplaced during, to just about 100 Ma – the same as in the beneath South America to have been after, deformation and appears unrelat- Coastal batholith of Peru. easterly during most of the Cretaceous. ed to rocks erupted and emplaced both One batholithic segment in This model is similar to that proposed before and after them, we interpret North America has long been recog- recently by Pfiffner and Gonzalez them to represent the earliest slab fail- nized as an out-of-place orphaned (2013). ure magmas. Similarly, based on hafni- block: the Salinian block (Ross 1978), Arcs generally shut down, are um isotopes in zircon (Polliand et al. located just west of the San Andreas deformed and thickened when they 2005), Maastrichtian volcanic rocks fault in central California. There, collide with another arc, microconti- near Lima are thought to have little amphibolite-granulite facies gneiss and nent or continent (Moores and Twiss crustal input and may also be slab fail- schist lacking pure quartzite and car- 1985). Therefore, we attribute the mag- ure volcanic rocks related to the bonate rocks characteristic of the matic shutdown and the ~105 Ma iso- younger Late Cretaceous–Early Terti- North American margin (Ross 1977) clinal folding of rocks within the basin ary collision. Thus, in the case of the are cut by 100–82 Ma plutons ranging to collision of the Huarmey-Cañete arc Coastal batholith both slab failure vol- in composition from gabbro to gran- with an unknown block or terrane. canic rocks and plutons exist. Nearly odiorite (Mattinson 1978, 1990; Kistler Interestingly, the Peninsular Ranges identical sequences of magmatism, and Champion 2001; Kidder et al. batholith of Baja California, the Sierra which occurred as deformation was 2003; Chapman et al. 2014), which is Nevada Batholith of California and the waning and with similar plutonic rock similar to the setting and magmatism Coast Plutonic Complex of British compositions and temporal relations, of slab failure sectors in the other Columbia all have sutures of this age were documented for the Silurian of batholiths. Thus, the Salinian block within them (Hildebrand 2013), sug- western Newfoundland by Whalen et appears to be missing its western arc gesting some original continuity. al. (2006) and the Pliocene of the East- component (Page 1970, 1982) and, During collisions the compet- ern Carpathians (Gîrbacea and Frisch with 100–82 Ma magmatism, appears ing buoyancy of the lower plate conti- 1998). In both cases, mafic break-off to represent the slab failure half of a nental crust and the negative buoyancy magmas rose rapidly into the collision Cordilleran batholith, which makes it a of the attached oceanic slab ultimately zone and were followed by a longer reasonable candidate for the exhumed lead to slab failure (McKenzie 1969; period of calc-alkaline to shoshonitic eastern half of the Coastal batholith of Isacks and Molnar 1969; Roeder 1973; magmatism. Peru with its Arequipa–Antofalla base- Price and Audley-Charles 1987; Sacks The eastward-vergent folds ment (Loewy et al. 2004). In fact, pale- and Secor 1990; Davies and von within rocks of the Huarmey-Cañete omagnetic data from Upper Cretaceous Blanckenburg 1995; Davies 2002; arc suggest, but do not prove, that the and Paleocene sedimentary rocks of Atherton and Ghani 2002;). This is lower plate lay to the east: nevertheless, the Salinian block indicate that they because the buoyancy forces resisting as stated, there is no evidence for were deposited 2800–2100 ± 500 km the subduction of continental litho- deformation or arc magmatism of this south of their present location (Cham- sphere are as large as those pulling age in cratonic Peru. Therefore, we pion et al. 1984), although there is oceanic lithosphere downward (Cloos suggest that the Arequipa block and its some controversy about the data based et al. 2005). Eventually, the greater unknown collider were both offshore on study of different rocks (Whidden 264

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Figure 6. Interpretive balanced cross-sections through the central Andean fold–thrust belt after McQuarrie (2002) illustrating Arequipa block sitting atop inferred South American crust. et al. 1998). Nevertheless, paleontologi- quipa terrane. Whatever the ultimate South America (McQuarrie 2002; cal data suggest that the fauna of nature of the magmatism, the ocean DeCelles and Horton 2003). Salinia are a reasonable match with closed during the Late Cretaceous– Following the 82 Ma shut- those in the Peninsular Ranges of Early Tertiary when the Arequipa down of magmatism in the Coastal southern California (Elder and Saul block, with its volcanic cover and plu- batholith, a second period of magma- 1993), which are considered to be far tonic complexes, docked with western tism started some 9–10 m.y. later and traveled (Hagstrum et al. 1985). Wher- South America to create the eastward- appears to have postdated the accre- ever they were during the mid-Creta- vergent Marañon fold–thrust belt. The tion of the Huarmey rocks and the ceous, we know of no other blocks in eastward vergence of the fold–thrust first phase of the Coastal batholith to the Cordillera that match as well as belt, the absence of Mesozoic magma- the South American craton. This peri- Salinia and the Coastal batholith of tism older than 72 Ma in this region of od of magmatism was characterized by Peru. cratonic South America, and the many voluminous and widespread batholith- Although it is difficult to sepa- plutons of this age range in the parallel, intermediate composition dyke rate slab failure magmas from subse- Coastal batholith (Mukasa 1986) com- swarms in the age range 73–71 Ma and quent arc magmas, except that slab fail- bine to demonstrate westward subduc- centred complexes emplaced between ure magmas are syn- to closely post- tion of the South American craton 72 Ma and 64 Ma (Pitcher 1985; collisional, in the Peruvian case given beneath the Arequipa terrane. A west- Mukasa 1986). The dyke swarms attest the differences in major and trace ele- ward subduction scheme beneath the to widespread batholith-normal exten- ment trends, and the 10 m.y. gap Arequipa block is also supported by a sion during their emplacement, which between the mafic intrusions and the Late Cretaceous–Early Tertiary fore- Pitcher and Bussell (1985) suggested 91–82 Ma bodies, it is possible that deep and east-vergent thrust belt to the occurred during uplift or doming of only the early mafic suite represents south in the Altiplano of Bolivia the region as a whole. We interpret slab failure magmatism and that plu- where, as documented by structure sec- these magmatic products as the direct tons of the 91–82 Ma suite formed tions (Figure 6) and drill holes, the result of slab failure of the west-dip- above a westward-dipping subduction Arequipa block sits along the western ping South American lithosphere dur- zone that lay beneath an ocean margin of the fold–thrust belt and ing the Late Cretaceous–Early Tertiary between South America and the Are- atop the western edge of cratonic collision that produced the Maraña GEOSCIENCE CANADA Volume 41 2014 265 fold–thrust belt. If correct, the pre- seem thinner to the south and so there to calcic-alkalic, whereas the slab- and post-collisional plutons and dykes may have been less extension within failure groups are more variable constrain the collision to have been 77 the region. and include some alkali-calcic to ± 5 Ma. alkalic and shoshonitic composi- It thus appears that the CHEMISTRY tions (Figure 4). Also, the 73–62 Coastal batholith contains two periods We recognize that the Coastal batholith Ma slab-failure group includes of slab-failure magmatism with a peri- has served as one of the archetypical some ferroan (reduced) samples od of arc magmatism sandwiched Cordilleran batholiths generally (Figure 4), considered to reflect an between them: (1) an older period in hypothesized by geologists to have intra-plate setting (Frost et al. the range 105–101 Ma; and (2) a been generated by subduction of 2001). younger period that started at 77 ± 5 oceanic lithosphere beneath a conti- 3. There do not appear to be obvious Ma and terminated at about 62 Ma. nental margin (Pitcher et al. 1985) and differences in normalized trace ele- The concurrence of two periods of so served as a model, both petrograph- ment abundances or patterns slab-failure magmatism and a period of ically and geochemically, for similar between arc-related and slab-fail- subduction-related plutonism in the belts of all ages worldwide. Analyses ure plutonic groups (Figure 4). same linear trend over such a long from the Coastal batholith are by However, zirconium, in > 60 wt. period of time is most likely related to today’s standards incomplete, and most % silica arc-related samples, ranges the narrow width of the Huarmey- of the principal workers have retired or from 84 to 222 ppm, versus 49 to Coastal batholith–Arequipa block and died. That said, we decided to take a 454 ppm in slab-failure samples, its history of subduction and collision. look at the existing chemistry (Pitcher which yielded zircon saturation It should serve as a cautionary tale for et al. 1985) to see if there are signifi- temperature (Watson and Harrison those geologists who argue that arcs cant differences between the various 1983) ranges of 721° to 785°C, oscillate back and forth between com- settings we outlined. We believe it and 629° to 866°C, respectively. pression and extension simply due to important because the centred com- This suggests that granitoid mag- slab dip, obliquity, or convergence rate plexes of the Coastal batholith, here mas generated during slab-failure (Ducea 2001; Ramos 2009, 2010a; suggested to be of slab-failure origin, likely formed under higher partial DeCelles et al. 2009; Paterson et al. are quite similar in overall form and melting temperatures than those 2012b). In the case of the Arequipa composition to post-collisional phases formed within an arc setting. terrane the variations are due to com- of North American batholiths, such as The apparent differences plex plate interactions that led to three the La Posta Suite of the Peninsular between the arc-related and slab-failure periods of subduction, two collisions, Ranges batholith (Silver and Chappell plutonic groups may reflect fundamen- and two periods of slab-failure mag- 1988; Clinkenbeard and Walawender tal differences in petrogenetic process- matism. Thus, the answer to Shackle- 1989; Walawender et al. 1990; Kim- es. The greater compositional range ton’s insightful question, while complex brough et al. 2001; Lee et al. 2007) and and higher temperatures of the slab- in detail, relates more to the narrow plutons of the Sierran Crest magmatic failure plutonic groups may indicate nature of the Arequipa terrane due to event within the Sierra Nevada greater thermal and material input the loss of its eastern half, and two batholith (Bateman 1992; Coleman and from the mantle during their genera- periods of slab failure with consequent Glazner 1998; Paterson et al. 2012a, b). tion. Their range to more alkalic com- upwelling mantle: it was twice the While we strongly believe that positions may reflect input from crustal lid to linear mantle upwelling. the best evidence for slab-failure mag- enriched asthenospheric mantle materi- Following terminal collision at 77 ± 5 matism is its timing in that it overlaps al during slab failure versus depleted- Ma and an initial period of slab-failure with and just follows collision, we nev- mantle input to the arc-related plutonic magmatism, new eastward subduction ertheless examined various plots of the groups. The association of zoned led to magmatism of the Calipuy For- data as shown on Figure 4 to ascertain intrusions in the 77 ± 5 Ma slab-failure mation at 53 Ma even farther eastward if there are visible differences between group may reflect these magmas being and for the first time during the Creta- arc-type and slab-failure plutonic higher temperature melts, as indicated ceous on rocks of the South American events within the Coastal batholith. We by zircon saturation temperatures. Such craton (Figure 5). suggest the following first-order differ- melts would tend to rise higher in the Most workers have related the ences. crust and contain little or no restite, broad arch of dense rocks within the 1. Arc-related Coastal batholith plu- rendering them less viscous, thus facili- crust of the Western Cordillera to rep- tonic groups span lower silica tating mixing and/or crystal fractiona- resent upwelling mantle during erup- ranges (~14 wt.% for the 94–82 tion/differentiation. In contrast, lower tion and extension of the crust related Ma group and ~5 wt.% for the < temperature arc-related granitoid intru- to the Huarmey basin (Atherton and 60 Ma group) than the slab-failure sion would tend to form from restite- Webb 1989) however, upwelling mantle groups (~ 22 wt.% for the rich and more viscous magmas that resulting from two periods of slab fail- <105–101 Ma group and ~25 cannot readily fractionate or mix with ure postdated the terminal collision wt.% for the 73–62 Ma group) other magmas. In any case, if the origi- and could also have caused such a fea- (Figure 4). nal samples or powders could be locat- ture. Why it doesn’t occur to the south 2. The arc-related plutonic groups ed, and funds were available, it would is unclear, but the volcanic sequences are medium- to high-K and calcic be worthwhile to analyse them for a 266

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1000 km n Crato GreatGreat arcaarc CoastalCoasoastaal babatholithatholthotholithlli h OOcOcoiteoiti e ararcc Patagonia batholith Colombiaoolombiollombio iaa LIP LIPP AArequipa-Antofallarequipu papa-Antofallaffalla CChileniahhileniahilhi e a Rocas Verdes basin K K K K K K K K v Cv C K K D K K i MCi D D AD AD K K

D AD

Figure 7. Geological map of the Andes with volcanic cover removed to illustrate the four main Late Cretaceous–Early Tertiary collisional regions discussed in the text and the close spatial relationship between Cretaceous foredeep deposits in green and Paleozoic–Precambrian basement of western South America. Olive green labeled C is accreted LIP and Great Arc as shown in detail on Figure 7. Modified from Schenk et al. (1997). full spectrum of trace elements by duction beneath western South Ameri- the exotic terranes (Sempere et al. modern methods. ca. 1997). Within the Colombia–Ecuador To the south the composite REGIONAL CONSIDERATIONS sector to the north, west-dipping sub- Mesoproterozoic Arequipa–Antofalla The realization that the 2000 km-long duction of cratonic South America block continues into Bolivia and Chile Coastal batholith and its basement(s) beneath the Great Arc of the (Loewy et al. 2004; Ramos 2008, were exotic with respect to South Caribbean and the Colombian– 2010b) and there sits atop South America during the Cretaceous caused Caribbean oceanic plateau (Figure 8) American cratonic crust (Figure 9) us to re-examine the geology over the led to Campanian (~75 Ma) emplace- directly west of Upper Cretaceous– entire length of western South Ameri- ment of arc fragments and pieces of Early Tertiary foredeep deposits ca looking at known similarities and oceanic plateau upon western South (DeCelles and Horton 2003; Arriagada differences, and provides the final key America throughout western Colombia et al. 2006) and a fold–thrust belt (Fig- to unlocking the tectonic development and the northern three-quarters of ure 6) with about 330 km of shorten- of the margin during the Cretaceous. western Ecuador (Altamira-Areyán ing (McQuarrie 2002; McQuarrie et al. In this section we discuss briefly our 2009; Pindell and Kennan 2009; Jaillard 2005). The occurrence of the exotic findings for 3 additional sectors (Figure et al. 2009; Villagómez and Spikings allochthons coincident with the thick- 7) and then go on to compare them 2013). To the east a foreland basin, est parts of the Andean crust (All- with recent analyses of North Ameri- extending from the Caribbean Sea as mendinger et al. 1997) suggests that ca. We then present a new model that far south as Bolivia, developed during much, if not all, of the crustal thicken- contrasts with the more popular the Late Cretaceous–Early Paleocene ing might be accounted for by partial hypothesis of long-lived eastward sub- as a response to the thrust loading of westward subduction of the South GEOSCIENCE CANADA Volume 41 2014 267

880ºW0ºW 70ºW70ºW American margin and emplacement of n SouthSouth ea bea the allochthons atop it. Some younger, an S BlueschistBlueschist arib AmericaAmerica ibbe C Car AArcrc even modern, thrusts near the eastern RomeralRomeral GreaterGreater s topographic break might relate to grav- 10ºN10ºN LIP erers in situsitu ffafaultault PanamaPanama ve itational spreading of the overthick- PanamaPanama sliversssliv HuancabambaHuancabamba ened and uplifted collisional crust. PalestinaPalestina faultffaault Still farther south, relations are a n complex and incompletely resolved, a PANAMANIAPPAANAMANIA c 55ºNºN e i c 86-6586-65 MaMa arcs,arcss,, plateaux,plateauxx,, but a >10 km thick synclinal accumula- O r

forearcs,ffoorearcs, 8888 MMaa ColombiaColombia tion of Jurassic to Late Cretaceous vol- c e fi PPlateaulateau i canic and allied rocks, collectively c m a termed the Ocoite arc, sits uncon- P mb Colu ia CARIBBEANCARIBBEAN LLIPIP A 00ºº Ecuad or 995-885-88 MaMa plateauplateau bbasaltasalt formably upon deformed and meta- ooff interoceanicinteroceanic ooriginrigin h morphosed Paleozoic rocks rocks of t Chilenia, a Neoproterozoic basement GREATGREAATT ARCARC u

r terrrane (Levi and Aguirre 1981; Åberg o d CCretaceousretaceous aarc,rc, bblueschist,lueschist, a u r uuadua eer o c PPePer LB E ssedimentaryedimentary rocks,rocks, ChauchaChaucha et al. 1984; Ramos 2010b). Basement S 55ºSºS outcrops are scarce and their relations OTHEROOTTHER TTERRANESERRANES 500500 kmkm to the more easterly Cuyania terrane Amotape,Amotape, Olmas,Olmas, blueschist,blueschist, eeclogite,clogite, TahuínTTaahuín terrane,terrane, are obscure (Ramos 2010b). East of 1105-9005-90 MMaa LanconesLancones basinbasin fromfrom KKennanennan & PPindell,indell, 22009009 the volcanic rocks a well-developed, but largely eroded, foredeep basin and Figure 8. Geological sketch map showing the distribution of terrane fragments in easterly vergent fold–thrust belt had the Colombian and Ecuadorian sector of the Andes and an exploded view for developed by the Late Cretaceous greater clarity. Modified from Kennan and Pindell (2009). As discussed in the text, (Morabito and Ramos 2012). The belt part of the more extensive Great Arc of the Caribbean and the Colombian– (Figure 10) is broken into several Caribbean oceanic plateau were emplaced at about 75 Ma above the west-facing pieces by younger faults and was erod- passive margin of South America. LB–Lancones basin ed during younger exhumation: different sectors have different names such 15º15º 30º as Malargue, Chos Malal, Agrio, and Aluminé fold–thrust belts (Cobbold

60 crustalcrustal and Rossello 2003; Howell et al. 2005; Thin-skinnedThin-skinned 400 thicknessthicckness Thick-skinnedThick-sskinned Ramos and Folguera 2005; Zapata and thrustthrust bbeltselts innk kmm tthrusthrusst bbeltselts Folguera 2006). East of the Neuquén Subandean 50 Precordillera, a N–S linear band of late 65º 60 SB kinematic andesitic to rhyodacitic vol- 70 SSPP canic rocks and subvolcanic porphyry, o 60 collectively known as the Neunauco iplan Puna 0 Alt Belt, were emplaced between 75 ± 3 7070 Ma and about 60 Ma (Morabito and 00m0 3000m3000300 PreCordilleraPreeCCoorddilleilleerra Ramos 2012; Spagnuolo et al. 2012). 500 ? 40 ? ? Because they were intruded at the tail end of deformation we interpret them Pacifi CuzcoCuuzco c Oc as slab-failure magmas formed when ean the westward-subducting slab failed PrecambrianPPrrreeccaambrriian basementbasement during the collision. FamatinaFamatina In the Patagonian Andes the 70º70º CCuyanauyana TThThick-skinnedhicckkk---sskkiiinnned bbeltselttss 157–75 Ma Patagonian batholith was CChileniahilenia SB:SB: SantaSanta BaBarbararbara emplaced atop the South American ArequipaArequipa AntofallaAntofalla SPSP:: SierrasSierras PPampeanasampeanas craton during the Late LimaL Cretaceous–Early Paleocene by closure of the oceanic Rocas Verdes basin along a series of eastward-vergent, Figure 9. A generalized map showing distribution of thin- and thick-skinned westward-dipping thrust faults (Figure thrust belts, distribution of basement blocks, and crustal thicknesses in the Alti- 11), which contain slices of ophiolite plano & Puna regions of the central Andes illustrating extent of the exotic Are- and document westward subduction quipa–Antofalla blocks; darker areas approximate basement outcrops. Modified (Dalziel et al. 1974; Wilson 1991; Krae- from Allmendinger et al. (1997) and Ramos (2008). mer 2003; Ghiglione et al. 2010; Mal- oney et al. 2011). Eastward propagat- ing orogenic wedges of the Magellanes 268

continentalcontinenti ttalal ddepdepositionepositiitiioni South American lithosphere during the MMainain Cretaceous. In many cases basement EE.. L. CCordilleraordillera LowerLower & EEarlyarly LLateate PPaleogenealeogene MMiddleiddle & MMiddleiddle CCret.ret. CCret.ret. CCret.ret. CCret.ret. UUpperpper within these terranes is poorly JurJurassicassic CentralCentral JJurassicurassic PaleozoicPaleozoic DDepressionepression exposed, but where observed is commonly Mesoproterozoic. The original provenance of the terranes is obscure.

HEMISPHERIC IMPLICATIONS 74º70º It has not escaped our attention that the deformational scheme outlined s ChosChos MMalalalal c

i ffold-thrustold-thrust a a here for South America is strikingly

r r

e e

a n bbeltelt

l l l

c l

i i o l similar to that of the North American i

r o s

o s v

C d d Laramide event (Hildebrand 2013, e 75±3 MMaa - 60 MMaa s

e r e

p p p p p p volcanicvolcanic rocksrocks

38º38º 2014), but before discussing the North

o o o e n r

e e e and pporphyryorphyry P a

D c

c s

a M American geology, we need briefly

a a

i t c t o

i e

e n

fi mention some new tomographic r r AgrioAgrio ffold-old-

c C

- - thrustthrust bbeltelt

O advances arising from improved com-

o o

r r c

e u in puting power and high-resolution seis- as

a J B

n : é n u mic arrays, such as USArray (Williams a q u r e

N e AluminéAluminé l et al. 2010), and also from recently l i fold-thrustfoldld- ththrustt

d r 40ºº beltbelt developed kinematic models for plate

l motions in deep mantle reference 125-75125-75 MMaa a l f t si s a plutonicplutonic s a r a frames based on seafloor spreading t m o rocksrocks an C n oni history, hotspot migration, paleomag- e volcanovolcano g Pata C orth N 100 km km netism, and moving continents (Müller et al. 1993; O’Neill et al. 2005; Torsvik Figure 10. Schematic cross-section at ~33ºS through the Mesozoic arc terrane of et al. 2008a, b; Doubrovine et al. 2012; central Chile illustrating its overall synclinal form and its location relative to the Shepard et al. 2012; Seton et al. 2012). Late Cretaceous–Early Tertiary fold–thrust belts. The 75 ± 3 Ma volcanics and Sigloch and Mihalynuk (2013) hypabyssal porphyries intruded and overlie the thrust faults and are here interpret- analysed tomography of the ’fast’ ed to represent slab-failure magmas. Generalized cross-section from Åberg et al. regions in the mid-mantle beneath 1984; Levi and Aguirre 1981. Map compiled from sources cited in text. In addition North America, readily interpreted as to the fold–thrust belt in the Neuquén foreland basin, the Main Cordillera is also a folded relict oceanic slabs (Grand et al. Late Cretaceous eastward-vergent thrust stack that at 33ºS forms the highest and 1997), and combined the paleogeo- narrowest section, known as the Aconcagua thin-skinned fold–thrust belt (see Fig- graphical models for the Jurassic–Early ure 8 in Ramos et al. 2004). Horizontal lines are post-collisional fault-bounded Cretaceous westward migration of depressions. North America during the opening of the Atlantic Ocean with observations and Malvinas basins developed in front strewn along its transform margins that the largest mantle anomaly, which of the thrusts during the Campanian (Garrett et al. 1987). is a steeply inclined slab wall in the and were subsequently incorporated It appears then that during the transition zone and lower mantle that into the thrust sheets (Macellari et al. Late Cretaceous–Early Tertiary, large extends for over 40° of latitude 1989; Suarez et al. 2000; Olivero and crystalline terranes, containing Juras- beneath eastern North America Malumián 2008; Ghiglione et al. 2010). sic–Cretaceous magmatic arcs, were (Sigloch 2011), must have formed dur- Magmatism within the batholith con- emplaced upon the entire west coast of ing westward, not eastward, subduc- tinued to the time of collision at ~75 South America, and not necessarily tion. Eastward subduction beneath Ma then stopped. Much younger plu- only during the Paleozoic as commonly North America during westerly migra- tons relate to eastward, post-collisional hypothesized (Vaughan et al. 2005; tion of North America as the Atlantic subduction. Ramos 2008, 2009, 2010a). In our Ocean opened implies that both the Just as the Great Arc of the alternative model, eastward-vergent trench and continent would have been Caribbean passed between North and Late Cretaceous–Early Tertiary coupled and migrated westward South America to form the Antillean fold–thrust belts and associated east- together, and thus have left an inclined, arc today, the Scotia arc (Barker et al. wardly migrating foredeep basins not vertical, slab. Hildebrand (2014) 1991) represents a Pacific realm that developed on cratonic South America found that the Sigloch and Mihalynuk migrated into the Atlantic Ocean nearly synchronously with collision of (2013) model, once corrected for pale- (Alvarez 1982; Pugh and Convey 2000) a composite arc-bearing ribbon conti- omagnetic inclination errors (Kent and through the gap between South Ameri- nent. When coupled with the accretion Irving 2010), matched the time and ca and Antarctica as originally suggest- of the ribbon continent, and the lack place for initial impingement of the ed by Moores (1970). Eastward migra- of arc magmatism on South America, Cordilleran Ribbon Continent in the tion of the arc left scattered traces they indicate westward subduction of Great Basin region during the Sevier GEOSCIENCE CANADA Volume 41 2014 269

Thrusting was apparently SW NE ongoing from Sevier to Laramide time 150 Ma Yaghan Fm Zapata Fm within the North American margin of the Great Basin sector, but it was much subdued (e.g. DeCelles and South Am ? erica Coogan 2006; Yonkee and Weil 2011). Intense deformation and metamorphism occurred within the Great Basin between 85 and 75 Ma (Camilleri et al. 1997; McGrew et al. 2000) and the classic thick-skinned deformation of the Colorado Plateau region started East-vergent 75 Ma Magellanic during the Maastrichtian and continued fold-thrust belt into the Tertiary (Dickinson et al. 1988; foredeep Patagonia Lawton 2008). In pre-San Andreas paleogeo- batholith graphic reconstructions (Powell 1993), South America another zone of Laramide deformation strikes obliquely across Arizona and through the Southern California Trans- Figure 11. Schematic cross section of Rocas Verdes basin and its closure illustrat- verse Ranges (Figure 12) where it is ing the unknown width of the basin and the westerly subduction of South Ameri- truncated at the coast. Orocopia and can cratonic crust during its closure. Modified from Maloney et al. (2011). Pelona schist (Jacobson et al. 2007) is associated with this zone, which suggests that it had once been joined with event, and some 20 m.y. later the more well-defined geophysical and detrital the similar Swakane gneiss of the widespread terminal collision during zircon provenance break (Ridgway et North Cascades (Matzel et al. 2004; the Laramide event. al. 2002; Trop and Ridgway 2007; Hildebrand 2013, 2014). Similarly, a The mantle tomography fits Hults et al. 2013). Late Cretaceous– major swarm of Late Cretaceous–Early well with known geology, for all along Early Tertiary northward-vergent Tertiary post-collisional plutons – con- the entire western margin of North thrusts and folds also deformed Early sidered by Hildebrand (2013) to have America, eastward-vergent thrusts and Cretaceous features in northern Alaska, been generated by slab failure – trend- foredeep basins developed on the cra- including apparent basement duplexing ing through the Transverse Ranges, the ton during the Late Cretaceous–Early in the Brooks Range (Moore et al. Mojave and Sonoran deserts, and on Tertiary. Unaware of the mantle 1997). through western Mexico would also tomography arguments, Hildebrand In the Canadian segment, match with similar age plutons of the (2009, 2013) and Johnston (2008) pre- rocks of the North American conti- Cascades and Idaho (Figure 12). sented models to explain the thrusts nental terrace were separated from A continuous Late Creta- and foredeep, based entirely on geolog- their basement along a detachment ceous–Paleocene foreland fold–thrust ical observations, in which the North located within Cambrian shale, folded, belt and related foredeep occur American margin was partially sub- and thrust eastward to form the Rocky throughout north–central and eastern ducted to the west beneath an exotic, Mountain fold–thrust belt during the Mexico just west of the Gulf of Mexi- arc-bearing ribbon continent. The sub- Late Cretaceous–Early Tertiary (Price co (Eguiliz de Antuñano et al. 2000). It sequent publication of the tomograph- and Mountjoy 1970; Price 1981; Price formed during the terminal accretion ic model thus provided entirely inde- and Fermor 1985; Fermor and Moffat of the Guerrero superterrane sector of pendent verification of the westerly 1992). A thick, and northward migrat- the Cordilleran Ribbon Continent subduction model. We briefly describe ing, clastic wedge of Campanian–Pale- (Tardy et al. 1994; Centeno-Garcia et some key regions (Figure 12) from ocene age developed to the east in the al. 2008, 2011). The western margin of north to south as examples to illustrate foreland basin during this deformation Oaxaquia and Mixteca were deformed the breadth of terminal Laramide colli- (Catuneanu et al. 2000; Ross et al. in the Late Cretaceous–Early Tertiary sion. 2005; Larson et al. 2006). in a dominantly east-vergent In Alaska, rocks of the Farther west in the Coast Plu- fold–thrust belt (Suter 1984, 1987; Valanginian–Cenomanian Kahiltna tonic complex (Figure 12) a linear band Hennings 1994; Fitz-Díaz et al. 2012) basin were metamorphosed and thrust of exhumation is found along with a and the Tampico–Misantla foredeep northward at ~74 Ma, coincident with major firestorm of Late Cretaceous– developed in front of the advancing development of the Campanian–Maas- Early Tertiary plutons (Armstrong thrusts (Busch and Gavela 1978). trichtian Cantwell basin, a thrust-top 1988), which Hildebrand (2009, 2013) We step aside to mention that basin formed during the deformation related to slab failure following accre- some 20–25 m.y. earlier at 100 Ma, and caused by the accretion of Wrangellia tion of Wrangellia at about 80 Ma (see a bit farther west than the Laramide to the paleo-Alaskan margin along a also Gehrels et al. 2009). deformational front, the Santiago 270

Peak–Alisitos arc of the Peninsular Ranges batholith – built upon a varied

M basement, ranging in age from Protero- zoic to Jurassic (Shaw et al. 2014; Premo et al. 2014; Kistler et al. 2014), and torn from the western margin of

tant areas (red the ribbon continent at about 140 Ma (Lawton and McMillan 1999; Mauel et

aska; CP–Colorado al. 2011; Peryam et al. 2012) – collided MX with a west-facing Lower Cretaceous carbonate platform, known as the

cident with the swarm of Late Guerrero–Morelos platform in the south and the Sonoran shelf in the from Armstrong (1988). north (LaPierre et al. 1992; Monod et al. 1994; González-Léon et al. 2008). Because the basement within the arc terrane contained fragments of crust, such as the Antler platform and Cabor- ca terrane (Ketner 1986; Gastil et al. S 1991; Stewart 2005; Hildebrand 2009, 2013; Premo et al. 2010), derived from CP the ribbon continent, and ultimately

T from the ’lost’ SW corner of North America, possibly during the transition from Pangea B to A (Irving 1977; Kent and Muttoni 2003; Irving 2004), make GB arguments tying the Guerrero superterrane to North America prior to the

CR Laramide non-definitive. Terranes derived from this part of Laurentia should contain large quantities of Grenville age zircon grains reflecting gneiss Swakane their proximity to that belt, which was rich in Grenvillian basement (Hoffman 1989). Returning to the summary of the Laramide event we note that to the south of Guerrero superterrane sector, lies the Zongolica fold–thrust belt (Figure 12), which involved thrusting of deeper water sedimentary rocks eastward over the reefal carbonate- dominated Cordoba platform during the Santonian–Campanian (Nieto- Samaniego et al. 2006). In the Cuicate- co terrane of southern Mexico, Maas- trichtian schist, greenstone, gabbro, Area of Area Paleocene exhumation and serpentinite were thrust eastward over red beds of the Maya terrane during the latest Cretaceous–Paleocene (Pérez-Gutiérrez et al. 2009). At the southern end of the Maya block (Fig- ure 12), a west-facing carbonate-domi- K nated platform sitting on basement of the Maya block was drowned during NA

Sketch map showing extent of map showing Sketch Cordilleran Ribbon Continent (green) in North America after Hildebrand (2013, 2014) and some impor the uppermost Campanian, buried by orogenic flysch during the Maastricht- ian–Danian (Fourcade et al. 1994), and overthrust by ultramafic nappes. Rocks Figure 12. stars) with documented Laramide-age deformation as discussed in text. The zone of exhumation, more or less coin major Paleocene Late Cretaceous collision in Coast Plutonic complex slab-failure plutons (Hildebrand 2009), following Cretaceous–Early Tertiary Mountain fold–thrustCR–Rocky MX–Mexican fold–thrust belt; GB–Great Basin; K–Kahiltna block; basin; M–Maya belt; NA–Northern Al Ranges. Plateau; S–Sonoran Desert; T–Transverse of the lower plate crystalline basement GEOSCIENCE CANADA Volume 41 2014 271 were metamorphosed to eclogite at 76 Relict westward-dipping Ma, which implies that part of the Pacific slab overrun by South ast North American margin was subducted E e Sou America migrating west Ris Am th er to a depth greater than 60 km at about ica that time and exhumed to amphibolite le nt Sinking grade a million years later (Martens et a le m nt mantle r a M al. 2012), presumably by slab failure. e m p r i p e d Even farther south, in the U - w PGZ R A o i t L d l rotated Chortis block, Rogers et al. PP a g n e t PGZ i (2007) documented a Late Cretaceous c belt of southeast-dipping imbricate thrusts, which they interpreted to rep- T u n

Inner z

resent the accretion of the Caribbean o o s Core

arc system to the Chortis block (see a J also Pindell et al. 2005; Pindell and O u re Kennan 2009; Ratschbacher et al. ter Co

2009). The arc-bearing block continues A

PGZ f

PGZ c

through its diachronous collision zone a Geoid high a

with the Bahamian Bank of North n

i PP Geoid low u

America represented on Cuba and His-

w Sinking m e PP

0 k Post-perovskite paniola, through the Virgin Islands N mantle 66

PGZ Plume generation

(Schrecengost 2010) to its still active R

S zone

W r

m u S

Antillian segment before reaching I n

d modified from Burke 2011; Trønnes 2010 i a northern South America, where it was n diachronously deformed along the coastline from west to east (Ostos et Figure 13. This model presented here is testable by mantle tomography, for if cor- al. 2005). That the Antillean arc is part rect there should be a relict vertical slab wall beneath eastern South America and of the Great Arc is supported by the the western South Atlantic, just as exists beneath eastern North America. The presence of Jurassic oceanic basement occurrence of Late Cretaceous–Early Tertiary accretion by westward subduction and chert at La Desirade (Mattinson et over the length of both North and South America, and its composite nature, sug- al. 2008; Montgomery and Kerr 2009). gest that the Cordilleran Ribbon Continent developed over a zone of major down- Overall, the Laramide event welling into the mantle where various blocks were swept together during the Meso- was more or less synchronous from zoic. We speculate that this lengthy subduction complex sat west of Tuzo and east Alaska to Tierra del Fuego and, based of Jason, the two long lived zones of mantle upwelling, and separated the Pantha- on the mantle tomography and east- lassic oceanic plates from the Pacific oceanic plates as shown in this equatorial sec- ward vergence of thrusts, it is inferred tion. Modified from Trønnes (2010) and Burke (2011). that the Americas were the lower plate in a collision with an arc-bearing block, formed the boundary between the Pan- Mihalynuk 2013), exists beneath east- interpreted for North America as a thalassic and Pacific plates, which if ern South America and the western more or less continuous ribbon conti- true, means that the Laramide event South Atlantic. Modern mantle tomog- nent (Johnston 2001, 2008; Moores et represents the final demise of the Pan- raphy should be able to resolve it. al. 2002; Hildebrand 2009, 2013). thalassic Ocean. This implies that the The Cordilleran Ribbon Con- Adding the South American sector to dominant mode for the Panthalassic tinent is a composite terrane that is it makes it one of the longest recog- closure was subduction away from the composed of many disparate frag- nized orogenic belts of any age. The Americas, as independently confirmed ments, yet these fragments were amal- alternative hypothesis for magmatic for North America by deep mantle gamated prior to terminal collision shutdown and upper plate thrusting, tomography (Sigloch and Mihalynuk with the Americas during the Laramide that of an eastward-dipping flat slab 2013). Easterly subduction beneath the event. This implies that the ribbon carrying high-standing buoyant continents began only once the Pan- continent was assembled above a major plateaux and ridges (Gutscher et al. thalassic lithosphere was consumed. zone of mantle down-welling. If Tuzo 2000; von Huene and Ranero 2009), is The Antillean and Scotian arcs repre- and Jason represent zones of long- simply unavailable as a viable mecha- sent the only active relics of the west- lived mantle upwelling (Burke and nism over the strike-length observed. erly dipping subduction regime. Torsvik 2004; Torsvik et al. 2008a, b; Based on the ages of arc mag- Our model for South America Burke 2011) then we surmise that the matism within it, the bulk of the rib- predicts that a mid-mantle vertical fast Cordilleran Ribbon Continent devel- bon continent was amalgamated during anomaly, comparable to that beneath oped above a complementary zone of the Jurassic at around the time when, eastern North America, and represent- down-welling between the two and or just after, the Pacific plates formed ing a west-dipping oceanic slab over- represents a first-order feature of (Hildebrand 2013). Thus, we suspect run by the continent as it migrated Mesozoic Earth (Figure 13). that the ribbon continent may have westward (Sigloch 2011; Sigloch and 272

CONCLUSIONS which started by about 53 Ma. REFERENCES 1. Five to nine km of eastward-shoal- 8. Similar temporal and spatial rela- Åberg, G., Aguirre, L., Levi, B., and Nys- ing, volcanic and volcaniclastic tions exist over the entire western tröm, J.O., 1984, Spreading-subsidence rocks were deposited within the Andes and we interpret them to and generation of ensialic marginal Huarmey-Cañete basin, mainly indicate that a ribbon continent basins: an example from the early Cre- during the Albian, and then collided with western South Amer- taceous of central Chile, in Kokelaar, between 105 and 100 Ma were ica during the Late Cretaceous– B.P., and Howells, M.F., eds., Marginal Basin Geology: Volcanic and Associat- deformed and intruded by syn- to Early Tertiary above a west-dip- ed Sedimentary and Tectonic Process- post-kinematic mafic-intermediate ping subduction zone. The model es in Modern and Ancient Marginal magmas at 100 Ma. predicts that there is a vertical slab Basins: Geological Society, London, 2. The Huarmey-Cañete volcanic– (fast zone) in the mid-mantle Special Publications, v. 16, p. 185–193, volcaniclastic sequence (Casma beneath eastern South America http://dx.doi.org/10.1144/GSL.SP.19 Group) is interpreted to represent and the western South Atlantic, 84.016.01.14. an arc that collided with an just as occurs beneath eastern Allison, F.C., 1974, The type Alisitos for- unknown block or terrane at North America (Sigloch 2011; mation (Cretaceous, Aptian–Albian) of 105–100 Ma. This block may have Sigloch and Mihalynuk 2013). Baja California and its bivalve fauna, in been the Salinian block of west- 9. The South American ribbon conti- Gastil, G., and Lillegraven, J., eds., central California. nent was part of a much longer Geology of Peninsular California, composite ribbon that included AAPG-SEPM-SEG Pacific Section 3. Contemporaneously with, and 49th Annual Meeting Field Trip immediately after, the collision, the the North American ribbon conti- Guidebook, p. 20– 59. area was intruded by calcic to alka- nent, Rubia, and the Antillean and Allmendinger, R.W., Jordan, T.E., Kay, line gabbro, strike-parallel bimodal Scotian arcs. S.M., and Isacks, B.L., 1997, The evo- dyke swarms, and more siliceous 10. The composite nature of the lution of the Altiplano-Puna plateau plutons collectively interpreted as Cordilleran Ribbon Continent, of the central Andes: Annual Review slab-failure magmatism. which spanned the hemisphere of Earth and Planetary Sciences, v. 25, 4. During the Tithonian–Albian peri- from north to south, likely formed p. 139–174, http://dx.doi.org/ od of arc volcanism, the Late along the boundary of the Pantha- 10.1146/annurev.earth.25.1.139. Albian folding, and emplacement lassic and Pacific oceanic plates at Altamira-Areyán, A., 2009, The ribbon of plutons and dykes of the a zone of long-lived mantle down- continent of northwestern South 105–82 Ma Coastal batholith, the welling into which arcs and ter- America: Unpublished PhD thesis, ranes were swept and amalgamated The University of Houston, Houston, area presently located to the east TX, 193 p. and occupied by rocks of the West throughout the Mesozoic. Alvarez, W., 1982, Geological evidence for Peruvian basin formed a quiescent 11. Following collision of the the geographical pattern of mantle west-facing carbonate–clastic plat- Cordilleran Ribbon Continent with return flow and the driving mecha- form to basin succession without the still westward migrating Ameri- nism of plate tectonics: Journal of volcanic debris or lacunas. The cas, the current regime of east- Geophysical Research, v. 87, p. incompatibility between the two ward subduction of Pacific plates 6697–6710, http://dx.doi.org/ basins indicates that the Huarmey beneath North and South America 10.1029/JB087iB08p06697. arc–Coastal batholith and their commenced. Armstrong, R.L., 1988, Mesozoic and early Arequipa basement were exotic Cenozoic magmatic evolution of the with respect to the West Peruvian ACKNOWLEDGEMENTS Canadian Cordillera, in Clark, S.P., Jr., basin. Discussions with Paul Hoffman were Burchfiel, B.C., and Suppe, J., eds., Processes in Continental Lithospheric 5. The east-vergent thrusts and the helpful and we are especially pleased to Deformation: Geological Society of lack of an arc on South America contribute to this volume honouring America Special Papers, v. 218, p. indicate that the Huarmey arc– him, as Hoffman has been a 40-year 55–92, http://dx.doi.org/ Coastal batholith–Arequipa block mentor and friend to the senior author. 10.1130/SPE218-p55. collided with South America at 77 For this South American study Arriagada, C., Cobbold, P.R., and Roperch, ± 5 Ma above a west-dipping sub- Eldridge Moores provided the inspira- P., 2006, Salar de Atacama basin: A duction zone to form the east-ver- tion to examine the geology there. Dis- record of compressional tectonics in gent Marañon fold–thrust belt. cussions with him were always encour- the central Andes since the mid-Creta- 6. After collision an intense swarm of aging and enlightening. Alexandre ceous: Tectonics, v. 25, TC1008, 73–71 Ma belt-parallel dykes fol- Zagorevski (GSC) provided positive http://dx.doi.org/10.1029/2004TC00 lowed closely by 72–62 Ma centred input on a draft of the manuscript. 1770. complexes and ring dykes intruded The manuscript was reviewed by two Atherton, M.P., and Ghani, A.A., 2002, Slab breakoff: A model for Caledonian the collision zone and is interpret- anonymous reviewers and Adrian late granite syn-collisional magmatism ed as slab-failure magmatism. Pfiffner, whose review we found espe- in the orthotectonic (metamorphic) 7. Volcanism of the Calipuy Group cially constructive. This paper is anoth- zone of Scotland and Donegal, Ire- postdated the thrusting and is er in our line of self-funded studies. land: Lithos, v. 62, p. 65–85, interpreted to be the initial mag- http://dx.doi.org/10.1016/S0024- matism of eastward subduction, 4937(02)00111-1. GEOSCIENCE CANADA Volume 41 2014 273

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