Late Paleozoic to Jurassic chronostratigraphy of coastal southern Peru: Temporal evolution of sedimentation along an active margin

Accepted Manuscript Late Paleozoic to Jurassic chronostratigraphy of coastal southern Peru: Temporal evolution of sedimentation along an active margin F. Boekhout, T. Sempere, R. Spikings, U. Schaltegger PII:

S0895-9811(13)00091-6

DOI:

10.1016/j.jsames.2013.07.003

Reference:

SAMES 1190

To appear in:

Journal of South American Earth Sciences

Received Date: 10 March 2012 Accepted Date: 12 July 2013

Please cite this article as: Boekhout, F., Sempere, T., Spikings, R., Schaltegger, U., Late Paleozoic to Jurassic chronostratigraphy of coastal southern Peru: Temporal evolution of sedimentation along an active margin, Journal of South American Earth Sciences (2013), doi: 10.1016/j.jsames.2013.07.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

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We simplify the south Peruvian Carboniferous to Jurassic stratigraphic units The Yamayo Group spans Late Visean to the Late Triassic We date the local onset of subduction-related magmatism to 216 Ma The top of this volcanic unit illustrates extension-related trenchward arc migration This study summarizes tectonic switching events along the south Peruvian margin

*Manuscript Click here to view linked References

Late Paleozoic to Jurassic chronostratigraphy of southern

Peru:

temporal

sedimentation along an active margin F. Boekhout1*, T. Sempere2, R. Spikings1, U.Schaltegger1

evolution

of

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coastal

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1. Section of Earth and Environmental Sciences, Rue des Maraîchers 13, CH-1205 Geneva, Switzerland

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* Present address: Case 115, IMPMC, 4 place Jussieu, 75005 Paris, Cedex 05, France 2. University of Grenoble 1, IRD, CNRS, BP 53, F-38041 Grenoble Cedex 9, France

Corresponding author: F. Boekhout ([email protected]) Tel. ++3301 44 27 52 21| Fax ++3301 44 27 51 52

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Keywords: geochronology; Peru; stratigraphy; magmatism; zircon

Abstract

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We present an integrated geochronological and sedimentological study that significantly revises the basin and magmatic history associated with lithospheric thinning in southern coastal Peru (15–18°S) since the onset of subduction at ~530 Ma.

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Until now, estimating the age of the sedimentary and volcanic rocks has heavily relied on paleontologic determinations. Our new geochronological data, combined with numerous field observations, provide the first robust constraints on their chronostratigraphy, which is discussed in the light of biostratigraphical attributions. A detailed review of the existing local units simplifies the current stratigraphic nomenclature and clarifies its absolute chronology using zircon U-Pb ages. We

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observe that the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru consists of two first-order units, namely (1) the Yamayo Group, a sedimentary succession of variable (0–2 km) thickness, with apparently no nearby volcanic lateral

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equivalent, and (2) the overlying Yura Group, consisting of a lower, 1–6 km-thick volcanic and volcaniclastic unit, the Chocolate Formation, and an upper, 1–2 km-thick

sedimentary succession that are in markedly diachronous contact across the coeval arc

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and back-arc. We date the local base of the Chocolate Formation, and thus of the Yura Group, to 216 Ma, and show that the underlying Yamayo Group spans a >110 Myr-

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long time interval, from at least the Late Visean to the Late Triassic, and is apparently devoid of significant internal discontinuities. The age of the top of the Chocolate Formation, i.e. of the volcanic arc pile, varies from ~194 Ma to less than ~135 Ma across the study area. We suggest that this simplified and updated stratigraphic

1. Introduction

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framework can be reliably used as a reference for future studies.

A well-defined chronostratigraphy is crucial to accurately reconstruct the

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evolution of an active continental margin and its neighbouring continental mass. This appears particularly relevant for the Late Paleozoic to Jurassic, during which the assembly of Pangaea and its subsequent breakup occurred. Because Pangaea was

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entirely surrounded by subduction zones (e.g. Collins (2003)), improved temporal constraints on the evolution of the active continental margins should provide original insights into the history of its assembly and breakup. We address this issue by studying the present-day, emerged fore-arc of southern Peru (Fig. 1). New and reliable chronostratigraphic constraints were acquired for the Late Paleozoic to Jurassic by U-Pb dating of detrital and magmatic zircons using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) 2

and Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS). The stratigraphy of this region remained poorly constrained due to the

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fragmented preservation of the Late Paleozoic to Jurassic stratigraphic succession and severe alteration of its volcanic rocks. Current chronostratigraphic knowledge of this 3–10 km thick, dominantly sedimentary pile (Fig. 2), which overlies the basement of

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the Arequipa Massif in discontinuous outcrops, has mainly relied on sparse and

sometimes poorly constrained biostratigraphical determinations. Furthermore, initial

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mapping in the 1960’s led to the definition of a plethora of units with little subsequent agreement about their possible correlation and age. The main objectives of this paper are to review all previous biostratigraphical and sedimentological work carried out in southern coastal Peru (15° to 18°S), and constrain the duration and extent of periods of sedimentation and arc magmatism using our U-Pb zircon age data acquired from

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the pre-Andean margin of southern Peru, from the Carboniferous to the Jurassic. We find that the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru consists of two first-order units, which are the Yamayo Group and the overlying

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Yura Group (Fig. 2). The Yamayo Group consists of a sedimentary succession of variable (0–2 km) thickness, with apparently no nearby volcanic lateral equivalent. In contrast, the Yura Group consists of a lower, 1–6 km thick volcanic and volcaniclastic

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unit, the Chocolate Formation, and an upper, 1–2 km thick sedimentary succession, that are in diachronous contact across the coeval arc and backarc. The ages of these previously relied on paleontologic analyses, and we present geochronological data that provide the first robust constraints on their chronostratigraphy. Strata in the Yamayo Group were thought to have accumulated during the Late Paleozoic to Triassic (Bellido and Guevara, 1963; Pino et al., 2004; Wilson and

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García, 1962), but lack volcanic rocks, precluding direct isotopic geochronological analyses. Although ash layers were reported in Carboniferous units from other locations in Peru (Mégard, 1978), they are apparently missing from possibly coeval

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sequences in southern Peru. Permo-Triassic stratigraphic successions have not been indisputably identified in this region. Sedimentary detritus may offer a more widely representative record of events along an ancient active margin in this case where only

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limited outcrops of magmatic rocks are preserved. We investigated the erosional

products of these ancient arc systems using U-Pb zircon (detrital and magmatic)

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geochronology, (Dickinson and Gehrels, 2009) thus deciphering the geochronological history of this part of the Andean margin.

The lowest part of the overlying Yura Group is the Chocolate Formation, which was previously considered to be Late Triassic to Jurassic in age (Sempere et al., 2002), and consists of a volcanic and volcaniclastic succession. Most age estimates on

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these rocks are based on biostratigraphy due to severe alteration. We provide the first dating of the very base and top of this key unit, and present U-Pb ages of detrital

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zircons extracted from younger sedimentary units within the Yura Group.

2. Magmatic history of the Peruvian margin Along the West Gondwana segment of the Terra Australis Orogen, subduction of

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oceanic crust is believed to have started at ~530 Ma (Early Cambrian) on the basis of U-Pb ages for emplacement of metaluminous calc-alkaline granitoids (Rapela et al., 1998; Cawood, 2005; Chew et al., 2007). The sedimentary evolution of coastal southern Peru has therefore been greatly influenced by the presence of an active arc since that time. An Ordovician magmatic belt was recorded along large parts of this margin (Bellizzia and Pimentel, 1994; Pankhurst et al., 2000; Cawood, 2005;

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Vaughan and Pankhurst, 2006; Bahlburg et al., 2009). In the northern and central Peruvian Eastern Cordillera a number of plutonic bodies record evidence of the existence of an Ordovician arc with U-Pb zircon ages between 474 - 442 Ma (Chew et

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al. (2007). Bahlburg et al. (2006) demonstrated that the southern segment of the present-day Eastern Cordillera of Peru evolved in a back-arc setting during the

Ordovician. Within the so-called “Arequipa Massif”, underlying coastal southern Peru

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(Fig. 1), calc-alkaline granitoids were emplaced with a peak in plutonic activity

between 472 and 460 Ma (Loewy et al., 2004; Mukasa and Henry, 1990; Casquet et

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al., 2010). Recently Romero et al., 2013 found that the Ordovician arc related magmatism in coastal Peru extends up to Isla de las Hormigas in the offshore at the latitude of Lima.

An active continental margin is presumed to have existed in Peru until the Early

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Devonian (Bahlburg et al., 2009; Chew et al., 2007), after which a gap in magmatism is observed possibly due to changing plate kinematics. Magmatic arc activity was reestablished in the Carboniferous, evidenced by magmatic back–arc activity recorded

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in the central and northern segment of the Eastern Cordillera of Peru by emplacement of I-type subduction-related granitic magmatism at ~342–285 Ma (Mišković et al., 2009) and andesitic lava flows associated with the Mississipian Ambo Group (Chew

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et al., 2007; Mišković et al., 2009). The oldest Carboniferous magmatism in Peru is recorded at ~345 Ma in the northern Eastern Cordillera (Chew et al., 2007). . The end of this Late Paleozoic tectonic cycle in Peru is marked by a major regional metamorphic event and deformation with the emplacement of partially migmatized Permian to Triassic granitoids within the south-central segment of the Eastern Cordillera of Peru between 10°S and 13°S (285–223 Ma; Mišković et al., 2009).

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Subduction continued in the Late Triassic – Early Jurassic along large parts of the Andean margin, and has continued until today in what is known as the Andean cycle

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(Haschke et al., 2006; Ramos and Aleman, 2000). Demouy et al. (2012) recently obtained U-Pb zircon ages on calc-alkaline plutonic rocks between 196 – 175 Ma

(Fig. 1), close to the city of Arequipa (Fig. 1), indicating that the magmatic arc was

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located more inland in the Early Jurassic, relative to the Late Jurassic Ilo Batholith

that fringes the present day coastline (Boekhout et al., 2012). A period of major

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subduction-related extension with basin formation and bimodal magmatism was installed along most of the Peruvian Andes in the Jurassic (Ramos and Aleman, 2000; Sempere et al., 2002; Boekhout et al., 2012; Demouy et al. , 2012). This latter extensional process has been related to a change in the absolute motion of South

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America relative to the subducting plate (Ramos, 2010 and refs therein).

The record of subduction related plutonism since ~530 Ma is not continuous in coastal southern Peru (Boekhout et al., 2013). With the exception of the Ordovician

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plutons that intrude Grenvillian aged migmatites, the only plutonic units that have been recognised in coastal southern Peru are the composite Jurassic-Cretaceous calcalkaline Ilo Batholith that crops out close to the town of Ilo, and yields ages zircon U-

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Pb of 174–152 Ma and 110–106 Ma (Fig. 1). No abundant Carboniferous, Permian or Triassic plutonic magmatic units are known in this area (Mukasa and Henry, 1990). Romero et al., 2013 show a likely outboard position of the arc with respect to the present day coastline during both Permian and Early Cretaceous. Even though the magmatic record is not completely preserved, the coeval sedimentation can be

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observed along large parts of the south Peruvian margin giving insight in the evolution of this long lasting active margin.

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3. Review of regional stratigraphy of coastal southern Peru, and sampling We present a review of the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru, and propose a simplification of the current stratigraphic nomenclature.

within the framework of this simplified stratigraphy.

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For reasons of clarity, we discuss this nomenclature and describe our sample locations

The stratigraphy of southern coastal Peru has been studied by numerous

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authors (e.g. Jenks (1948); Benavides (1962); Wilson and García (1962); Bellido and Guevara (1963), Vicente (1981), Romeuf et al., (1995); Sempere et al., (2002); Pino et al., (2004), resulting in the definition of many different units, with little subsequent agreement about their possible correlation and age. A synthesis of existing

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descriptions and data concerning the Late Paleozoic to Jurassic stratigraphy renders it eminently clear that precise geochronological constraints are of paramount importance.

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Several key locations (Fig. 1) have been studied in the current work, in an attempt to arrive at a general consensus regarding the stratigraphy of southern Peru. Previous stratigraphic descriptions of volcanic rocks and fossiliferous sediments were

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initially used in our approach due to the paucity of previously available, reliable geochronological data. Regionally, a metamorphic basement is overlain by a thick stratigraphic succession composed of sedimentary and volcanic units of Late Paleozoic to Jurassic age. The basement consists of Grenvillian (mainly migmatites) and rare Ordovician metamorphic rocks that are locally intruded by Ordovician plutons (Wasteneys et al., 1995; Martignole and J., 2003; Loewy et al., 2004; Casquet, 2010). The overlying 7

stratigraphic succession, as we will define below, consists of the Yamayo Group (Bellido and Guevara, 1963) and the younger Yura Group (Sempere et al., 2002). To facilitate the discussion and review of previous work we will already adopt this

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stratigraphic division into the overview of previous work in this section (Fig. 2). Given the stratigraphic thicknesses involved, the Late Paleozoic to Jurassic

succession is invariably fragmented into incomplete sections and outcrops scattered

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across southern Peru, which initially complicated its recognition and analysis. These stratigraphic units dispersed along the present-day forearc of southern Peru were

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measured and sampled to establish temporal and paleostructural relationships between geographically distinct regions. A series of localities were chosen based on section continuity, presence of biostratigraphical constraints, and the quality of outcrops (Fig. 1).

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3.1 Basement of the Arequipa massif

Basement rocks are exposed in many areas of the present-day forearc of southern Peru, in patches that markedly decrease in size from the coastline to the

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piedmont of the Western Cordillera. They dominantly consist of Grenvillian-aged migmatites and other metamorphic rocks, which are referred to as the Arequipa Massif, and are fragmented into faulted blocks. The Grenvillian-aged rocks record

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reworking of Paleoproterozoic crust during an orogenic event at 1.0–1.2 Ga (Loewy et al., 2004; Wasteneys et al., 1995). The basement of the Arequipa massif has undergone a complex, polycyclic

magmatic and metamorphic evolution from the Early Proterozoic to the Early Paleozoic. Loewy et al. (2004) proposed that the Arequipa Massif accreted to South America at ~1.05 Ga ago during the Sunsás Orogeny, when the combined ~1.9–1.8

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and 1.5–1.4 Ga juvenile crust of the Arequipa-Antofalla terrane collided with Amazonia. The allochtonous character of the Arequipa Massif relative to cratonic South America was confirmed using U-Pb zircon geochronology and Pb isotope

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systematics (Loewy et al., 2004; Mamani et al., 2010). The largest domain within the Arequipa basement is the Camaná–Mollendo Block (MCB) (Casquet et al., 2010; Martignole and J., 2003), which consists of high-grade rocks that underwent

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granulite-facies UHT metamorphism (Fig. 1).

Ordovician intrusions with U-Pb ages ranging between ~473-440 Ma (Loewy et al.,

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2004) locally occur in the Grenvillian-aged basement of southern Peru, forming the San Nicolás Batholith (Mukasa and Henry, 1990), and part of the larger-scale Famatinian Arc (e.g. Chew et al., 2007). The main Famatinian magmatic pulse was accompanied by metamorphism and deformation, with metamorphism beginning as early as ~497 Ma (Casquet et al., 2010; Loewy et al., 2004). Granite 09FB01 was

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taken from an undated plutonic body in the northern part of the Paracas peninsula (“A” inset in Fig. 1) in an attempt to constrain the continuation of this Famatinian

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plutonic belt along the present day margin.

3.2 Yamayo Group

The Yamayo Group (Bellido and Guevara, 1963) was defined in the lower

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Tambo valley (El Fiscal area), where it overlies the basement and underlies the Chocolate Formation with stratigraphic contacts (“B” in Fig. 1). We propose to use the term “Yamayo Group” from now on to include all sedimentary strata and units that postdate the basement and predate the Chocolate Formation. This group generally crops out incompletely, lacking either its lower part (commonly due to faulting) or its upper part, but this is not the case in the El Fiscal area where it was initially defined.

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The Yamayo Group consists of an apparently exclusively sedimentary succession that is overlain by the dominantly volcanic Chocolate Formation. The Yamayo Group overlies the basement with a stratigraphic or, more commonly,

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normal-fault contact (Taipe, 2004), and may locally be entirely absent, in which case the Chocolate Formation directly overlies the basement, as at Cerro Verde (Fig.1).

The Yamayo Group may have a thickness of more than several hundred meters, and

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may be thicker than 1.5 km in several areas. The sedimentary succession occupies a

position between the basement and the Chocolate Formation, although it crops out in

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disjunctive and often incomplete sections, and presents facies variations between them. Because of the initial difficulty to assess the relationships of these sections to each other, strata belonging to this succession were described under a variety of names (e.g. Yamayo Group, Machani Fm, Torán Fm, Pocoma conglomerates, Cocachacra mudstones), to which a variety of ages (reviewed below) were tentatively

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assigned on the basis of fossil findings, stratigraphic position or assumptions. Furthermore, local formational names were later substituted by names defined in classical but remote areas of Peru, depending on the age suggested by local fossils or

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facies (e.g. the Cabanillas, Ambo, Tarma and/or Mitu formations or groups). This situation explains the current apparent complexity of the Late Paleozoic to Triassic nomenclature in this region of Peru, which this paper simplifies and clarifies. This

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complexity is especially apparent when published geological maps are considered. The Yamayo Group is exposed in thick, largely continuous siliciclastic

sections in the Mal Paso, Estique Pampa and Chala areas, which include paleontologically constrained Carboniferous strata and underlie the Chocolate Formation (Fig. 1). Although the base of each of these sections is truncated by a fault, they provide an opportunity to constrain the chronology of these units and their

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duration. Sedimentary facies in the Yamayo Group are dominantly siliciclastic, ranging from conglomerates to mudstones, with an overall dominance of sandstones. A few limestone beds occur in some areas (e.g., Mal Paso, Atico, Ocoña) and have

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locally yielded a marine fauna suggesting a Late Paleozoic age (Pino et al., 2004). The depositional environment ranges from alluvial fan and alluvial plain to shallowmarine and deltaic. The occurrence of very-fine-grained siliceous deposits,

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reminiscent of bedded chert, in the Ilo area (Pocoma, Quebrada Guaneros, Río

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Osmore), suggests that localized deep-water environments existed in the basin (Fig.1).

3.2.1 El Fiscal and Cocachacra area (“B” in Fig. 1)

Bellido and Guevara (1963) defined the Yamayo Group in the El Fiscal area, to describe the ~400m thick succession of sedimentary strata that overlies the basement and underlies the Chocolate Formation, and tentatively assigned a Late

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Triassic – Early Jurassic age to it. A marine faunule was later found in this same unit, from which brachiopods were utilized to assign an Emsian age (Boucot et al., 1980), leading the Peruvian Geological Survey to reassign the unit to the Devonian

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Cabanillas Group. However, the Cabanillas Group was originally defined in the Altiplano, ~210 km NE of El Fiscal, but the “Devonian” faunule from the El Fiscal area indicates a warm-water environment and its composition differs markedly from

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the Devonian cold-water fauna typical of Bolivia and Andean southern Peru (Boucot et al., 1980). Furthermore, detrital zircons from a sandstone sample from this sequence yielded several Carboniferous ages and a youngest age of 297 ± 12 Ma (Reimann et al., 2010), indicating a stratigraphic age that is clearly younger than Devonian.

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Two samples (09FB119, 09FB120) were collected from the Yamayo Group in the El Fiscal area, and both yielded Carboniferous maximum ages (see below).

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Bellido and Guevara (1963) mapped the Cocachacra Mudstones, located 5 km south of El Fiscal (“B” in Fig. 1.), a ~100 m-thick informal unit which they assigned to the Permian, approximately 4km SW of where they mapped the Yamayo Group.

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We observed that these mudstones are siliceous, in fault contact with the nearby Grenvillian-aged migmatites, and are intruded by plutons belonging to the Ilo

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batholith (Boekhout et al., 2012). Given their location and the areal geology, we conclude that these restricted outcrops should be considered as a lateral facies variation, including fine-grained siliceous deposits, within the Yamayo Group. Because of their unfavourable facies for U-Pb detrital zircon studies, these deposits

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were not sampled.

3.2.2 Pocoma area (“C” in Fig. 1.)

About 22 km north of Ilo, the metamorphic rocks dated as Ordovician by

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Casquet et al. (2010) are postdated by an ~700m thick, fining-upward succession of conglomerates, sandstones and siliceous siltsones and mudstones, which conformably underlie the Chocolate Formation to the north, and thus locally represent the Yamayo

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Group. Bellido and Guevara (1963) mapped this clastic sedimentary unit as the “Pocoma conglomerates”, another informal unit which they tentatively assigned to the Permian.

Arenites 09FB30 and 09FB31 are from the very base of this succession, postdate the local basement and are likely to be located close to the local base of the Yamayo

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Group (Fig. 2). Both have yielded Early Devonian maximum stratigraphic ages (see below).

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3.2.3 Mal Paso area (“D” in Fig. 1.) The Yamayo Group is represented here by the Machani Formation. This >1.5

km thick unit was defined in the Mal Paso area, ~48 km NNE of Tacna, by Wilson

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and García (1962), who tentatively assigned it to the Triassic as they observed that it conformably underlies the Chocolate Formation (Figs. 1 and 3). We confirm that no

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evidence suggestive of a chronostratigraphic hiatus (as a marked erosional surface or signs of paleo-weathering) can be observed at the contact between the Yamayo Group and the Chocolate Fm. in this location. The base of the Machani Formation is in fault contact with a complex consisting of undated basement gneisses that is intruded by a coarse-grained, leucocratic, 2-mica granite from which a sample was collected

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(09FB82) yielding a Late Triassic age (see below).

The lowermost part of the exposed Machani Formation consists of conglomerates and conglomeratic sandstones, which grade upwards into a thick,

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monotonous alternation of sandstones, siltstones and mudstones of several hundreds of meters. The middle part of the unit includes medium- to fine-grained sandstones and mudstones that yielded Nothorhacopteris and Tomiodendron plant fossils (Pino et

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al., 2004), which indicate a Carboniferous age. A few limestone beds, including a Late Paleozoic mollusc fauna, occur ~10-20 m above this plant horizon and underlie other beds with abundant plant debris (Pino et al., 2004). One of these limestone beds yielded bivalves determined as Myalina pliopetina (Salinas, 1985), a species thought to indicate the earliest Permian (Newell, 1942). The upper part of the Machani Formation in the Mal Paso area is dominated by white-coloured, coarse- to medium-

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grained sandstones. Two arenites (09FB71 and 09FB77) from this upper Machani Formation yielded Triassic maximum stratigraphic ages (see below).

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3.2.4 Estique Pampa area (“E” in Fig. 1.) The Mal Paso and Estique Pampa areas are only ~6 km apart, but are

separated by a regional-scale fault zone (Fig. 1). The faulted base of a >500m thick

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monotonous succession of well-bedded sandstones and subordinate mudstones occurs ~1 km southwest of the village of Estique Pampa. This sedimentary succession is

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conformably overlain by a >2 km thick sequence of volcanic and volcaniclastic strata (Fig. 3). Although they were initially ascribed to the Jurassic Yura and Late Cretaceous-Paleogene Toquepala groups, respectively (Wilson and García, 1962), these two characteristic units clearly represent the local expressions of the Yamayo Group (upper Machani Formation, with somewhat coarser facies than at the nearby

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type-locality) and Chocolate Formation, respectively, as we demonstrate below. Reimann et al. (2010) considered the sedimentary succession as belonging to the Devonian Cabanillas Group and sampled the middle part of the outcropping section,

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from which they recovered several Late Paleozoic zircons, with a youngest detrital zircon age of 355 ± 11 Ma. We collected samples just below and above the YamayoChocolate contact on the road to Tacna, 3.5 km south of Estique Pampa (Figs. 1 and

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3). As in the Mal Paso area, we observed no evidence of a hiatus at this contact. An ignimbrite (09FB88) just above the contact (Fig. 4a) and the uppermost sandstone below the contact (09FB87) were collected (Fig. 3).

3.2.5 Torán area, Majes valley (“F” in Fig. 1.)

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In the Torán area, ~100 km west of Arequipa (Fig.1), the basement is overlain by greenish to brown-coloured conglomerates and sandstones (east bank of Majes river), which grade upwards into better-bedded, finer-grained strata (west bank of

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Majes river); trough cross-bedding in some sandstone beds indicate paleo-currents towards the NW. This >1.5 km thick succession was labelled “Torán Formation” and tentatively assigned to the Devonian (Paredes, 1969; Morales, 1997) although no

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fossils have been found. This assumption of a Devonian age led to subsequent attribution of this sequence to the Devonian Cabanillas Group (e.g Palacios et al.,

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(1995); Reimann et al., (2010)). Because this non-metamorphic sedimentary succession overlies the basement, it is considered here as belonging to the Yamayo Group.

Medium-grained sandstones from the Torán Formation (east bank) yielded a youngest zircon age of 482 ± 13 Ma (Reimann et al., 2010). A medium- to fine-

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grained sandstone (09FB125), sampled from the same area, yielded a Carboniferous maximum stratigraphic age (see below).

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3.2.6 Ocoña area (from Atico to La Chira; “G” in Fig. 1.) The Ocoña area is centered on the lower Ocoña valley, ~100 km west of Arequipa (Fig.1), but extends over ~100 km, from ~20 km WNW of Atico in the west, to La

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Chira, ~26 km WNW of Camaná, in the east. This elongated area nearly connects with the Puerto Viejo – Chala area more to the west. Along this Ocoña area (Fig. 1), a dominantly reddish, >1–2 km thick, sedimentary succession crops out in two elongated strips, where it overlies the basement with a “younger-on-older”-type tectonic contact associated with multiple evidence of extensional tectonics (Taipe, 2004). Due to local fossil contents and lithological aspects, this unit was initially

15

mapped by Mendívil and Castillo (1960) and Bellido and Narváez (1960) as encompassing the Ambo Group, Tarma Formation, and Mitu Formation, although this nomenclature was defined in central Peru, ~600 km to the NNW, where it applies to a

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markedly different lithological succession. These type-successions for Carboniferous to Triassic are situated in the Eastern Cordillera of central Peru and consist of four

distinct units (e.g. Mégard, 1978), which are: the Ambo Group (Mississippian; mainly

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sandstones and conglomerates rich in plant debris, and volcanic rocks; 0.2–1.2 kmthick), the Tarma Group (Pennsylvanian; mainly shales, limestones, and sandstones;

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0.3–2 km-thick), the Copacabana Group (Early Permian; limestones and shales; 0.2–2 km-thick), and the Mitu Group (Late Permian to Late Triassic; reddish to purplebrown continental deposits, ranging from conglomerates to mudstones, and volcanic rocks; 0.4–3 km-thick).

In contrast, the succession observed in the Ocoña area dominantly consists of

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reddish-brown, alluvial to shallow-marine strata ranging from coarse conglomerates to mudstones and commonly including plant debris, some of which were determined as Pennsylvanian in age (Odontopteris sp., and Sphenopsida indet: Morales, 1997;

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Odontopteris may however reach the Early Permian (Taylor et al., 2009). To the WNW (Atico area), these sandstone-dominated reddish-brown facies are underlain by brown to grey siltstones and mudstones intercalated with limestone beds, which

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yielded marine fossils of Pennsylvanian to Early Permian age (Bellido and Narváez, 1960). To the ESE (La Chira), the outcropping strata consist of poorly-bedded, dark marly siltstones that include a diverse marine fauna of Pennsylvanian age (Pecho and Morales, 1969). Taken separately, the relatively abundant plant debris, Pennsylvanian fossils and the dominant reddish colour are arguably reminiscent, respectively, of the Ambo, Tarma, and Mitu groups of central Peru, but the stratigraphic order of

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succession does not apply, and no equivalent of the Copacabana Group can be identified. These marked differences and clear incompatibilities with the stratigraphy of central Peru strongly argue for using a simplified regional nomenclature within the

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Yamayo Group. In the Atico area, the outcropping succession is ~2.1 km-thick, mostly reddishbrown and dominantly consists of fine-grained siliciclastic deposits and limestones in

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its lower part, of sandstones in its middle part, and of conglomerates in its upper part.

The lower part, exposed along the northern rim of the Atico strip, mainly consists of

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brown and greenish-grey mudstones and siltstones, but locally includes conglomerates derived from the north (with clasts of quartzites, volcanic rocks, granites, migmatites) that grade into coarse- to fine-grained sandstones. Interbedded mudstones commonly include indeterminable vegetal debris and may be relatively rich in organic remains. Some limestone beds also occur, which yielded Triticites fusulinids, corals, and

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bivalves (Bellido and Narváez, 1960), implying a marine environment at least sporadically and/or locally. Conglomerates are especially abundant in the upper part of the succession, where beds are commonly 1-3 m thick and display growth-strata

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geometries that are likely related to the nearby detachment at the contact with the basement (along the southern rim of the strip). Paleo-current data here indicate drainage from the south. Clasts include gneisses and migmatites, white quartz, pink

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granites, altered volcanic rocks and grey limestones. Volcanic and quartzite clasts are generally subrounded, whereas metamorphic, plutonic and limestone clasts are dominantly subangular to angular, reflecting a closer provenance. The occurrence of volcanic clasts, which may reach 30 cm in size, in both the lower and upper part of this Atico succession imply that volcanic rocks (of unknown age) were cropping out in the area during the time the basin was active. However, the fact that apparently no

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volcanic rocks are interbedded in the Yamayo Group strongly suggests either that no volcanic arc was active during the time of deposition, or, if one was active, that it was located outboard of the present southern Peruvian margin. Isotopic age dating of these

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clasts has been precluded by their andesitic composition and severe degree of alteration.

The Yamayo Group is >1.1 km-thick in the lower Ocoña valley, and consists

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of conglomerates, coarse- to fine-grained lithic sandstones, siltstones and mudstones, and, toward the east, limestone and scarce gypsum beds. West of the Ocoña river, the

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conglomerates are spatially closely associated with a detachment fault that marks the southern rim of the Ocoña strip (similar to Atico), and rapidly grade northwards into reddish-brown sandstones and mudstones (the latter including very locally thin beds of dark limestone). The conglomerates are thick-bedded to massive; clasts are generally rounded to sub-angular, up to 30 cm in size, and include greenish-grey to

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dark gneisses, white to pink granitoids, and white quartz. Dispersed plant debris is commonly found in the sandstone and mudstones. East of the river, facies are finergrained and consist of grey to green siltstones and mudstones, with subordinate

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limestone and rare gypsum beds, suggesting that the basin deepened eastwards, at least in this area. Indeed, 22 km more to the ESE, at La Chira, dark marly siltstones yielded a diverse marine fauna of Pennsylvanian age, including brachiopods

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(Buxtonia peruviana, Derbyia sp., Dictyoclostus sp.), molluscs (Allorisma sp., Worthenia sp.), as well as crinoid and bryozoan fragments (Pecho and Morales, 1969).

A medium- to fine-grained red-brown sandstone was sampled (09FB132) in the lower Ocoña valley (west bank), from deformed sedimentary strata located a few

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meters above the detachment that separates them from the basement (Fig. 4b). The youngest age obtained from zircons within this sample is Carboniferous (see below).

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3.2.7 Puerto Viejo – Chala area (“H” in Fig. 1.) The Puerto Viejo – Chala area is characterized by a very thick section that crops out in several segments. The exposed sedimentary succession consists of the

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Yamayo Group, Chocolate Formation, and given the ages documented below, a

dominantly volcanic time-equivalent of the Yura Group that cannot be readily

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distinguished from the Chocolate Formation sensu stricto (see below). The entire succession dips towards the NNW at ~35° (albeit with local variations), and its base is exposed at Puerto Viejo, where it is faulted against a cataclastic, white coarse-grained granite of probable Ordovician age, which is intruded into nearby Grenvillian-aged migmatites.

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The Yamayo Group is exposed along ~4 km of coastline between Puerto Viejo and Punta Saguas (Fig. 1 and 3). The contact with the overlying Chocolate Formation is covered along this coastline, although the Chala and Chaparra geological maps

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(Olchauski, 1980) suggest that it is exposed ~1 km north of the Punta Saguas outcrops. This assumption yields a thickness of the Yamayo Group between the base of the Chocolate Formation and the fault at the base of its exposed section of ~2.3 km.

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The exposed lowermost part of the Yamayo Group consists of a monotonous

pile of well-bedded blackish siltstones, mudstones and fine-grained sandstones. Wellpreserved plant remains belonging to the genera Nothorhacopteris and Tomiodendron were found in dark siltstones a few meters away from the basal fault (Fig. 4c), placing the bottom of the Puerto Viejo – Chala section in the Carboniferous. Sample 09FB16 was collected from a medium- to fine-grained sandstone bed a few meters above the

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Nothorhacopteris-Tomiodendron levels, and yielded a late Visean maximum age (see below). The section along the coastline exhibits a slowly coarsening-upward succession dominated by dark siltstones and sandstones, in which no marine fossils

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have been found. Sandstones become dominant ~1.2 km above the base of the section, and show an overall continuous increase in grain size over a few hundreds of meters. No significant unconformity was observed in the entire succession. Facies in the

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upper 400 m of the exposed section are dominated by coarser, reddish deposits,

ranging from coarse lithic sandstones to very coarse conglomerates (Fig. 4d), with

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beds thickening upwards and some cross-bedding. This coarsening-upward sequence of coarse sandstones and conglomerates is interpreted to reflect rapid progradation of an alluvial fan system. Clasts of pebble to cobble size include Grenvillian-aged metamorphic rocks and granitoids resembling the Ordovician granitoids known in the nearby Atico area, as well as rounded hornblendite pebbles. Sample 09FB17 was

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collected in the northern half of the Punta Saguas outcrop, probably a few hundreds of meters below the Chocolate Formation, and yielded a mid-Triassic, maximum

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stratigraphic age (see below).

3.2.9 Paracas area (“A” in Fig. 1.) Carboniferous sedimentary strata, assigned to the Ambo Group due to the

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occurrence of Nothorhacopteris and Tomiodendron plant fossils (Pfefferkorn and Alleman, 2001), crop out at La Mina, a restricted (<5 km2) area of the Paracas Peninsula and the only known Late Paleozoic locality in the area. These Carboniferous strata are overlain by, and locally in fault contact with, Cenozoic sedimentary rocks. Although incomplete, this sequence is supposed to be in stratigraphic and/or tectonic contact with the Precambrian basement, which crops out

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in several other areas of the peninsula. A medium- to fine-grained sandstone (09FB152) was sampled near the base of the outcropping Carboniferous sequence at La Mina (Fig. 4e), and yielded an Early Devonian maximum stratigraphic age (see

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below). This peninsula also includes the undated intrusions of inferred Ordovician age that was sampled (09FB01) to constrain the extent of the Ordovician arc (see below).

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3.3 Yura Group

The Yura Group (sensu Sempere et al., 2002) is composed of a lower,

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dominantly volcanic unit, the Chocolate Formation, and an upper marine sedimentary succession. Fossils found in many of the sedimentary units have provided most of the time constraints on the Yura Group chronostratigraphy that have been available until now.

The Jurassic to Neocomian reference units for southern Peru were defined by

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Jenks (1948) in the Yura valley, located ~25 km NW of Arequipa (I in Fig.1). They include the Chocolate, Socosani, Puente, Cachíos and Labra formations (the latter including the Gramadal and Hualhuani units). Because of lithological variations, a

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different nomenclature was defined by Wilson and García (1962) in the Tacna region, which include the Junerata (= Chocolate), Pelado, San Francisco, Ataspaca and Chachacumane formations. Later comprehensive studies include Vicente (1981) and

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for the Arequipa region, Salinas (1985) and Pino et al. (2004) for the Tacna region. Following Sempere et al. (2002), all these units are grouped together into the Yura Group s.l. as they reflect a continuous evolution of the Arequipa-Tacna basin. As underlined above, the Chocolate Formation (Jenks, 1948) forms a very thick, dominantly volcanic interval within the otherwise dominantly siliciclastic Late Paleozoic through Jurassic stratigraphic record of coastal southern Peru. This unit is

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known in many areas along coastal southern Peru, and it is commonly several kilometers thick and mostly consists of volcanic and volcaniclastic strata. Volcanic rocks include lava flows, breccias and pyroclastites, and range in composition from

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basalts to rhyolites. The Chocolate Formation sharply overlies the Yamayo Group (this contact is exposed in only a few areas). In contrast, it is diachronously overlain by the Yura Group (see below), although local contacts can generally be described as

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sharp.

Despite the key role that Mesozoic volcanism plays in understanding the early

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evolution of the onset of Andean subduction, the precise age of this thick volcanic succession has remained unknown until now, mainly because of the strong alteration affecting these rocks. The main stratigraphic constraints on the volcanic rocks ascribed to the Chocolate Formation have been provided by the over- and underlying sedimentary units. The base of the Chocolate Formation is however rarely exposed

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(Martínez et al., 2005). Because no apparent stratigraphic hiatus, marked by an erosional or weathering surface, was observed between the Chocolate Formation and underlying strata including a Carboniferous flora in the Tacna region, it has been

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suggested that the base of this unit might locally be as old as Late Carboniferous (Pino et al., 2004). We confirm below that no significant stratigraphic hiatus exists between the Chocolate Formation and underlying Yamayo Group, but demonstrate

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that this contact is in fact Late Triassic in age, based on our dating of sample 09FB88 (Fig. 1 and 3) from the very base of the Chocolate Formation in the Estique Pampa section (see above), and on Middle to Late Triassic maximum ages obtained from the underlying upper part of the Yamayo Group in the same area, and at Punta Saguas. The Junerata (= Chocolate) Formation in the Tacna region, i.e. in southernmost Peru, is conformably overlain by fossiliferous limestones (the Pelado

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Formation) that yielded ammonites ranging from the early Sinemurian (~195 Ma) to the mid-Toarcian (Wilson and García, 1962; Salinas, 1985; Alván, 2009). The outcrops only expose the upper 300 m of the local Chocolate Formation in the

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classical Yura area near Arequipa (Fig. 3), which consist of a fining-upward sequence of dominantly purplish-brown, volcaniclastic conglomerates, sandstones, siltstones and mudstones (Acosta et al., 2008) This succession was deposited in alluvial to

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coastal plain environments that probably fringed the active volcanic arc at that time.

The upper part of this volcaniclastic sequence in the section near Hacienda Socosani

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includes a ~2m thick greenish calcareous intercalation, in which ammonites of late early Sinemurian age (~194 Ma) were found (Acosta et al., 2008; Alván, 2009; Jenks, 1948; Vicente, 1981). The uppermost Chocolate Formation in the same section consists of volcaniclastic, distal alluvial deposits that are sharply overlain by fossiliferous, shallow-marine limestones that define the Socosani Formation (Fig. 3).

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On the basis of ammonites, the age of the Socosani Formation was considered to span the Toarcian – early Bajocian interval (Vicente, 1981; 1982; Acosta et al., 2009; Alván, 2009); however, the very base of the Socosani Formation at its type-locality is

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marked by a calcareous ash bed that we sampled (09FB63; Fig. 4f and 3) and date hereafter as 186 ± 1 Ma, which is Pliensbachian by current standards (Gradstein et al., 2012).

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The upper parts of the volcanic-dominated sequences in the Puerto Grau and

Chucarapi coastal areas were suggested to be younger than the upper Chocolate Formation in the Arequipa inland area, as they are conformably overlain by ammonite-bearing strata of Bajocian to Callovian age (Bellido and Guevara, 1963; Jaén et al., 1963; Narváez, 1964). The Chocolate Formation in those areas is described in detail and dated by Boekhout et al. (2012) in relationship to the intrusion

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of the Ilo Batholith. At Ite, zircon from an andesitic lava flow (09FB98) was dated 172.3 ± 2.1 Ma by U-Pb using LA-ICPMS (Bajocian, Gradstein et al., 2012); this andesite was too altered to be considered for geochemistry. Additionally, a 40

Ar-39Ar age of 157.2 ± 0.4 Ma was obtained from a

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plagioclase plateau

hypovolcanic intrusion at Chala (Romeuf et al., 1995). Near Chucarapi in the lower Tambo valley, zircons from sandstones overlying the Chocolate Formation yielded

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maximum stratigraphic ages of 165.0 ± 3.6 and 161.9 ± 3.6 Ma, with youngest cluster ages of 169.9 ± 0.6 and 167.0 ± 0.7 Ma, respectively (summarized in Fig.1; Boekhout

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et al., 2012). Another U-Pb maximum detrital zircon age in the same range of 165.5 ± 1.9 Ma was reported in a volcano-sedimentary section at Alto El Madero, ~25 km north of Ilo.

A ≥7 km thick pile of volcanic rocks and interbedded volcaniclastic sediments,

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including many magmatic sills, was ascribed to the Chocolate Formation (Olchauski, 1980 ) in the region of Chala. Magmatic rocks are calc-alkaline and subductionrelated (Romeuf et al., 1995; Martínez et al., 2005). Mafic flows in the upper part of

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the unit yielded whole rock

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Ar-39Ar ages of 177.1 ± 2.2 and 157.3 ± 1.4 Ma

(Roperch and Carlier, (1992) and a plagioclase

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Ar-39Ar age of 164.1 ± 0.3 Ma

(Romeuf et al., 1995), demonstrating that the upper Chocolate Formation near Chala

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is a volcanic time-equivalent of the mid-Jurassic sedimentary succession that crops out in the Yura area. The section studied in this work crosses the upper Chocolate Formation along

the Quebrada Huacllaco, located ~6 km ESE of Chala, but is not well exposed due to the Neogene sedimentary cover. The base of this section exhibits a 50-100 m-thick tectonically disturbed zone characterized by magma mingling textures accompanied

24

by numerous generations of faults and folds at metric and decimetric scales. Stratigraphic continuity, however, improves up section. This part of the Chocolate Formation consists of a thick sequence of basaltic andesite flows intercalated with

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ignimbrites, welded lapilli tuffs, volcaniclastic sandstones, conglomerates and breccias. Various generations of cross-cutting stocks, sills and dykes also occur. Lava

flows and subvolcanic mafic intrusives have rather similar petrographic and

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mineralogic characteristics consisting of phenocrysts of plagioclase, clinopyroxene

and sometimes pseudomorphosed olivine, with Fe-Ti oxides as phenocrysts and

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microphenocrysts. The intersertal groundmass is composed of microliths of the same minerals and glass. Two volcanic samples were collected to better constrain the age of volcanic activity in this succession: andesitic ignimbrite 07FB66, located in the middle of the volcanic pile (Fig. 3), yielded an Aalenian age; dacitic ignimbrite 07FB02, located ~130 m above sample 07FB66 (Fig. 3), yielded a precise, earliest

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Bajocian age (see below).

At Chala a sandstone-dominated succession crops out within the upper part of the Chocolate Formation. Some sandstone beds include scattered subangular volcanic

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cobbles up to 50 cm in size, implying coeval erosion of volcanic rocks on high-angle slopes, nearby. Wave ripples and stromatolitic structures locally testify to a shallowwater environment. Sample 09FB150 was collected from one of these sandstones

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(Figs. 4g) and yielded a Valanginian youngest cluster age, with a Hauterivian maxium stratigraphic age on the youngest single grain (see below). The Chocolate Formation mostly accumulated where the coeval main volcanic

arc was located. The marked thickness and age variations probably reflect variations in the location and magmatic production of the arc. The sedimentary units that overlie

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the Chocolate Formation can be grouped into three diachronous macrofacies, which also reflect these variations: (1) Volcaniclastic (arc-derived) deposits overlie the Chocolate Formation. Shallow-

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water deposits are calcareous and are represented by the Socosani and Pelado formations. The overlying Puente Formation consists of greywackes, which are

reworked arc material that was deposited as turbidites (Vicente et al., 1982). These

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deposits accumulated in the Jurassic forearc basin and along the southwestern slope of the backarc basin.

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(2) Deeper-water deposits accumulated in the axial region of the backarc basin, grading laterally into coeval deposits in the southwestern and northeastern slopes. They dominantly include shaly facies and fine-grained turbidites, and are represented by the Cachíos Formation (and San Francisco and Ataspaca formations). (3) The basinal, shaly deposits are gradationally overlain by “deltaic” quartz-

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arenites, which prograde from the north and northeast. These usually very clean sandstones are represented by the Labra (and Chachacumane) Formation. Paleocurrent data unambiguously show that they were derived from the continental

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“Amazonian” region, and thus provide average “samples” of all rocks located in that direction during that time. However, it is likely that large explosive eruptions occurred along the coeval arc, and that distal volcanic deposits were also reworked

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into these sandstones. Sample (09FB124) was collected from the lowermost Labra Formation (Fig. 4h), in the Majes valley, thought to be Oxfordian in age (e.g., Sempere et al., 2002), and yielded an Oxfordian maximum stratigraphic age (see below).

4. Analytical methods

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A U-Pb detrital zircon LA-ICP-MS study was carried out on 14 sandstones in the Peruvian Coastal Cordillera (assigned to the Devonian, Carboniferous and Jurassic) to

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constrain their maximum stratigraphic age. One ash bed and three ignimbrites were dated from the stratigraphic levels assigned to the Jurassic Chocolate Formation by LA-ICP-MS and CA-ID-TIMS. Additionally, two plutonic bodies were analyzed by

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LA-ICP-MS to investigate their relationship with the known Jurassic plutonic rocks.

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Geochemical analyses were performed on some of the dated magmatic rocks.

4.1 Geochemistry

All geochemical analyses were carried out at the Institute of Mineralogy and Geochemistry, University of Lausanne. 7 whole rock samples were crushed and

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grinded using a jaw crusher and agate-ring mill, lithium tetraborate glass discs were prepared and major and some trace elements were determined by X-ray fluorescence spectrometry using a Philips PW 2400 spectrometer. The BHVO-1 basaltic standard

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was used for accuracy control. Abundances of REE and additional trace elements (e.g. Th, U, Ta, Cs, Hf) were analysed by laser ablation inductively coupled plasma massspectrometry (LA-ICP-MS) on the lithium tetraborate discs using an ELAN 6100

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DRC ICP-MS (Perkiun Elmer) interfaced to a GeoLas 200M 193nm excimer ablation system (Lambda Physik). Operating conditions of the ablation system included a 10 Hz repetition rate, 120 µm spot size and c. 10 J/cm2 on-sample energy density. Helium was used as the carrier gas. The acquisition times for the background and the ablation interval were c. 70 and 35 s, respectively. Dwell times per isotope ranged from 10 to 20 ms, and peak-hopping mode was employed. The ThO+/Th+ and

27

Ba2+/Ba+ ratios were optimized to 0.0036 and 0.0142, respectively. The SRM 612 synthetic glass standard from NIST was used for external standardisation. The average element abundances in it were taken from (Pearce et al., 1997). Ca42 served as

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an internal standard. Raw data were reduced off-line using LAMTRACE, a Lotus 1-23 spreadsheet written by Simon Jackson (Macquarie University, Australia). For each

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sample 3 measurements were acquired and results were then averaged.

4.2 Zircon U-Pb geochronology

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Several kilograms of rock specimens were crushed and zircons were extracted from the sub-300 µm size fraction using a Wilfley table, a Frantz magnetic separator and gravimetric separation using di-iodomethane with a density of 3.32 g/cm3. A side slope of 2° on the Frantz magnetic separator was used to separate the least magnetic zircons for ID-TIMS analysis (see below). All zircons selected for analysis were

4.2.1 LA-ICP-MS

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handpicked in ethanol using a binocular microscope.

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Zircons extracted from 14 sandstones, 2 plutonic rocks, an ash layer and 2 ignimbrites were dated using the U-Pb LA-ICP-MS method. 15-20 zircon grains per plutonic and volcanic sample and 70 – 110 zircons per detrital sample were mounted into epoxy

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resin blocks and polished to obtain flat surfaces. The zircon grains were characterized by cathodoluminescence imaging on a CamScan MV2300 SEM (Institute of Geology and Paleontology, University of Lausanne) to reveal their internal structure. Pb206U238 and Pb207-U235 dates were obtained using a 193-nm excimer ablation system UP193FX (ESI) interfaced to an Element XR sector field, single-collector ICP-MS (Thermo Scientific) at the Institute of Mineralogy and Geochemistry, University of

28

Lausanne. Operating conditions were similar to those described in Ulianov et al. (2012) and included a 35 µm spot size combined with a relatively low on-sample energy density of 2.2-2.3 J/cm2 and a repetition rate of 5 Hz to minimize the

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fractionation. A GJ-1 standard zircon (206Pb/238U age of 600.5 ± 0.4 Ma; Schaltegger et al., unpublished) was used for external standardisation. The 91500 standard was measured along with sample zircons on a routine basis to control the accuracy of

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results. The ratio-of-the-mean intensities data reduction method was used to calculate

the mean Pb/U concentration ratio (Ulianov et al., 2012). No quantitative common

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lead correction was applied, a qualitative control of the intensities for masses 202 and 204 and a careful inspection of the cathodoluminescence images were used instead, following the approach of Jackson (2004). Uncertainties of

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Pb/238U mean ages are

representative of internal uncertainty only, no variance of secondary standard data has been propagated onto the uncertainty of the mean ages. U–Pb data are plotted in

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concordia diagrams as 2σ error ellipses (see online Appendix A). Only analytically concordant points were used to calculated mean 206Pb/238U ages.

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4.2.2 CA-ID-TIMS

One sample of the Chocolate Formation was dated by CA-ID-TIMS (07FB02). The methodology used for zircon annealing and leaching, dissolution and chemical

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separation of Pb and U is the same as that described in Schoene et al. (2010). Zircons were analyzed at the University of Geneva on a TRITON thermal ionization mass spectrometer using the EARTHTIME

205

Pb-233U-235U tracer solution. U-Pb data are

plotted as 2σ error ellipses using ISOPLOT (Ludwig, 2003).

5. Results

29

5.1 U-Pb zircon geochronology Locations of all samples can be found in Figure 1, and all U-Pb data are plotted in

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Figure 5. Errors are reported as 2σ. Detailed results of U–Pb analysis can be found in the Supplementary data, Table A1. Results will be discussed from northwest to

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southeast along the present day coastline.

5.1.1 Paracas peninsula

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A medium- to fine-grained sandstone sampled at La Mina (09FB152; Fig. 5a), yielded a maximum single zircon age of 407.7 ± 7.7 Ma (Early Devonian). A majority of the zircons are Ordovician (n = 53) with 238U/206Pb ages between 456.7 ± 11.0 and 491.7 ± 4.7 Ma, indicating dominant erosional input from the Famatinian arc. A few older zircons yielded ages between ~750 and ~1914 (n = 11). The occurrence of

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Nothorhacopteris and Tomiodendron plant fossils provides a tighter age constraint than the maximum stratigraphic age indicated by the detrital zircons. Based on biostratigraphy, this sample has a Carboniferous age, which postdates the maximum

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stratigraphic age obtained from the detrital zircon U-Pb ages by ~70 Ma. The peninsula hosts intrusions of which granite sample 09FB01 (Fig. 5b) yielded an age of 461.1 ± 1.8 Ma (n = 17, MSWD = 2.3).

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6.1.2 Chala

Sandstone 09FB16 (Fig. 5c) was collected at Puerto Viejo from the lowermost, dominantly fine-grained part of the exposed section of the Yamayo Group. All zircons, except for 5 grains, yielded Proterozoic ages ranging between ~1020 and ~1900 Ma (n = 56). The 5 youngest grains have a mean age of 335.9 ± 5.3 Ma (n = 5,

30

MSWD = 0.22), and the youngest single zircon yields an age of 332.5 ± 3.3 Ma. This Late Visean maximum stratigraphic age corroborates the age assigned to the plant fossils that occur a few meters below (Fig. 3).

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Sandstone 09FB17 was collected at Punta Saguas from the coarse conglomeratic uppermost part of the exposed section of the Yamayo Group. The four youngest

zircons are mid-Triassic, of which the average age of the youngest three grains is

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233.3 ± 1.5 Ma (n = 3, MSWD = 0.022), and one zircon yields a slightly older age of

238.9 ± 2.7 Ma; the youngest single grain has an age of 233.1 ± 2.7 Ma. In contrast to

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sandstone 09FB16, located ~2 km down section, there is not only basement detritus present (between ~900 and ~1850 Ma, n = 68), but also Ordovician zircon grains, with ages ranging between 416 and 490 Ma (n = 11). The appearance of Ordovician zircons is not surprising given the change in source input observed between the finegrained lowermost and conglomeratic uppermost parts of this ~2 km thick section.

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Andesitic ignimbrite 07FB66 was collected from the middle portion of the local Chocolate Formation s.l. at Chala (Fig. 5e). Few zircons could be separated and dated by LA-ICPMS, yielding an Aalenian age of 173.3 ± 1.3 Ma (n = 3, MSWD = 0.71),

Group.

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rendering this portion of the Chocolate Formation s.l. time-equivalent to the Yura

Dacitic ignimbrite 07FB02 (Fig. 5f) is stratigraphically located ~130 m above

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andesite 07FB66 in the same volcanic sequence, and yields a precise, earliest Bajocian CA-ID-TIMS zircon age of 171.22 ± 0.15 Ma (n = 4, MSWD = 1.04), corroborating its stratigraphic position. Sample 09FB150 (Fig. 5g) was taken from stratified sandstones interbedded within the upper Chocolate Formation s.l. near Chala. Two Grenvillian-aged zircons and two Ordovician aged zircons were recorded. All other zircons in this sample fall

31

into two younger age groups. The oldest of these two age clusters yields ages between 165.4 ± 3.3 and 180.1 ± 2.7 (n = 10) suggesting that these grains were eroded from lateral equivalents of the underlying volcanic sequence dated by ignimbrites 07FB66

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and 07FB02. A younger range of zircon ages was obtained between a youngest single zircon of 134.8 ± 2.2 Ma and 149.6 ± 1.4 (n = 24).The rather large spread in the

remaining detrital ages is indicative of multiple source rocks. This sample displays an

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apparent age gap between ~165 and ~150 Ma. The record of detrital zircon ages

between ca. 135 and ca. 150 Ma suggests that volcanic arc activity continued until the

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latest Jurassic and Early Cretaceous near Chala. Such areas may have included the middle Ocoña valley, where plutonic rocks belonging to the Coastal Batholith yielded 40

Ar-39Ar plateau ages of 137.5 ± 1.8 and 135.7 ± 1.7 Ma (Schildgen et al., 2009), but

others were possibly located within regions outboard of the present day coastline.

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6.1.3 Ocoña

Zircons from sandstone 09FB132 (Fig. 5h) yielded Early Paleozoic ages in the range 455–506 Ma (n = 32). A few older zircons yielded local Grenvillian metamorphic

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basement ages. The sample also yielded one Carboniferous zircon with an age of 340.0 ± 6.4 Ma, which is consistent with the Pennsylvanian age assigned to plant fossils from the same sequence and area (Morales, 1997), and with Taipe’s (2004)

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correlation of this unit with Pennsylvanian marine deposits at La Chira, ~50 km more to the ESE (Pecho and Morales, 1969).

6.1.4 Majes valley Sandstone 09FB125 (Fig 5i) was collected from the lower part of the Yamayo Group (Torán Formation, on the east bank of the Majes river valley). The youngest

32

age cluster yields an age of 337.1 ± 1.4 Ma (n = 24, MSWD = 1.4). Two zircons were significantly younger, with ages of 322.7 ± 4.5 Ma and 315.8 ± 5.1 Ma. Older zircons (n = 9) range from 447.0 ± 12.9 Ma to 506.0 ± 12.7 Ma. This sample exhibits a gap in

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zircon ages between 447.0 and 345.7 Ma. Other older grains are distributed between ~1050 and ~1850 Ma (n = 23).

Upstream in the same valley, sandstone 09FB124 (Fig. 5j) was collected in the

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lowermost part of the Labra Formation (Yura Group), which is thought to be

Oxfordian in age (e.g., Sempere et al., 2002). Paleocurrent data from this sandstone-

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dominated unit indicate provenance from the north and northeast, i.e. not from the main arc; these sandstones may however have reworked explosive volcanic products erupted by the arc into the back-arc, where the Labra Formation accumulated. A youngest age group yields ages between 156.2 ± 2.3 - 162.9 ± 1.5 Ma (n = 6) with a youngest single zircon age of 156.2 ± 2.3 Ma. These ages are Oxfordian (Gradstein et

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al., 2012). In contrast to sandstone 09FB125 from the Yamayo Group, this sample from the Yura Group shows some scattered detrital zircon ages that range from 241.3 ± 2.3 to 274.4 ± 3.2 Ma (n = 12). The majority of the older grains have ages between

6.1.5 Yura

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806.5 ± 6.9 and ~2150 Ma (n = 21).

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A ash layer sampled at the very base of the Socosani Formation (sample 09FB63; Fig. 5k) yielded a mid-Pliensbachian (Gradstein et al., 2004) mean age of 185.8 ± 1.0 Ma (n = 12, MSWD = 1.4). Three older zircons are considered to be inherited and have been excluded from the mean age calculation.

6.1.6 El Fiscal

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Two sandstones (09FB119, 09FB120) were collected from the type section of the Yamayo Group, in its area of definition, although both yielded few zircons. The youngest single grain of sandstone 09FB119 (Fig. 5l) yielded an age of 336.5 ±

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6.4 Ma. Six Ordovician to Cambrian grains with an age range between 459.3 ± 4.8 and 531.4 ± 5.9 Ma, and eleven basement grains ranging between ~991 and ~1880 Ma, were dated.

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All six zircons separated from sandstone 09FB120 (Fig. 5m) were derived from the basement, and yield ages between ~1000 and ~1460 Ma, except for one grain with an

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age of 320.1 ± 4.0 Ma.

6.1.7 Pocoma

The conglomeratic arenites that postdate the metamorphic rocks 22 km north of Ilo, and are apparently located in the lowermost part of the local Yamayo Group, yielded

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Early Devonian maximum stratigraphic ages.

Conglomerate 09FB30 (Fig. 5n) is stratigraphically located above conglomerate 09FB31, and yields a youngest group with a mean age of 411.0 ± 4.4 Ma (n = 25,

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MSWD = 0.12), and a youngest single zircon of 402.2 ± 6.5 Ma. Nine Early Paleozoic grains range in age between 429.3 ± 5.5 and 483.5 ± 5.9 Ma, whereas older grains range from 630.2 ± 6.3 to ~1900 Ma (n = 13).

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Conglomerate 09FB31 (Fig. 5o) yields a mean age of 413.8 ± 1.1 Ma (n = 56, MSWD = 2.7), with youngest single zircon of 402.6 ± 6.9 Ma. Eight older grains with ages ranging from ~850 and ~1880 Ma, were found.

6.1.8 Estique Pampa

34

Minor inheritance was found in an ignimbrite at the base of the Chocolate Formation (09FB88; Figs. 3h and 5p): 39 zircons out of the 50 recovered form the youngest population with a mean age of 215.9 ± 0.8 Ma (n = 39, MSWD = 1.8). Of the other 11

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zircons, one yields a slightly older age of 235.1 ± 2.2 Ma, three Paleozoic grains yields ages between 410.8 ± 4.4 and 475.3 ± 3.4 Ma and the remaining seven grains are basement detritus with ages between ~1065 and 1830 Ma.

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Sandstone 09FB87, sampled just below the contact between the Chocolate

Formation with the underlying, uppermost sedimentary rocks of the Machani Fm.

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(09FB87, Fig. 5q), yielded zircons that span a rather continuous age range between 444.1 ± 4.6 to 552.1 ± 5.6 Ma (n = 40). The youngest single zircon in this sample yielded an age of 218.8 ± 3.3 Ma, which is slightly older than the age of the conformably overlying ignimbrite and thus indicates the most likely conformable contact between the units. Eighteen older grains were found, of which four zircons are

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Carboniferous with ages between 328.2 ± 3.9 to 345.4 ± 2.8 Ma (n = 4), two grains are 401.7 ± 4.2 and 402.8 ± 3.8 Ma and the twelve oldest grains record ages between

6.1.9 Mal Paso

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481.7 ± 4.7 and ~2060 Ma.

Two medium- to fine-grained sandstones were sampled from the upper part of the

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Machani Fm., underlying the Chocolate formation (09FB71, Fig. 5r; and 09FB77, Fig. 5s), in the Mal Paso section, where the Machani Formation was initially defined (Wilson and García, 1962). The youngest age group from sandstone 09FB77 has a middle Triassic mean age of 238.6 ± 3.3 Ma (n = 7, MSWD = 1.2), with a youngest single zircon age of 230.2 ± 11.4 Ma. Twenty slightly older grains yield ages that range between 246.2 ± 5.4 and

35

268.9 ± 4 Ma. An older group has a mean age of 290.6 ± 2.4 Ma (n = 13, MSWD = 1.4). Another, older cluster, yields an age of 478.1 ± 3.9 Ma (n = 14, MSWD = 1.8). Almost all other grains yield Grenvillian or older ages (n = 24), except for a few

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zircons between 503.9 ± 7.9 and 778.8 ± 14.4 Ma (n = 5). The youngest detrital zircon obtained from sandstone 09FB71 yielded a maximum stratigraphic age of 234.0 ± 6.1 Ma. Twenty-one older grains yield ages that range

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between 245.8 ± 4.2 and 316.4 ± 4.3 Ma, while twelve Ordovician grains yield ages

between 441.8 ± 4.7 and 495.1 ± 5.5 Ma. Even older zircons have ages that are either

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characteristic of Grenvillian orogenesis, or older (n = 22).

A 2-mica granite (09FB82, Fig 5t) that intrudes basement gneisses in this area yielded a Late Triassic mean age of 220.7 ± 1.6 Ma (n = 14, MSWD = 4.5). Two outliers were not included in the mean as they showed signs of inheritance.

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6.2 Whole-rock geochemistry and tectonic affinity

Whole-rock XRF and LA-ICPMS analyses are presented in Table 1, although the

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geochemical dataset is limited due to the severe alteration of the rocks. Five ignimbrites of the Chocolate Fm. were taken in the Chala section (07FB02, 07FB65, 07FB66, 09FB149) and one andesitic lava flow near Ilo (09FB92). Samples 07FB02

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and 09FB149 are dacitic in composition, samples 07FB65 and 09FB92 are andesitic, and sample 07FB66 is rhyolitic. Samples 07FB02 and 07FB66 yielded U-Pb ages of 171.22 ± 0.15 and 173.3 ± 1.3 Ma, respectively (Fig. 3). An Ordovician granite in the Paracas peninsula (09FB01) and a Triassic 2-mica granite intruding the local basement in the Mal Paso area, were also analyzed. The other analyzed samples remain undated. When plotted together, all samples follow the same high LILE/HFSE

36

trace-element trend and have trace element anomalies characteristic of subduction zone processes in both primitive mantle and chondrite normalized element plots (Fig. 6). The REE patterns show enrichment of LREE relative to HREE (La/Yb = 5-10).

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Negative Nb-Ta anomalies are observed in all samples, although most prominently in the Jurassic volcanic rocks. Negative Eu anomalies are present in all samples, with the

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most negative anomaly for the Ordovician granite.

7. Stratigraphic implications

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All concordant detrital U-Pb zircon ages, together with previous detrital analyses performed in the studied region have been combined in Figure 7, giving an overview of the new stratigraphic constraints obtained for southern coastal Peru. The youngest zircon ages are reported and are interpreted in combination with field observations and biostratigraphical determinations (summarized in Table 2 and 3 for

7.1 Yamayo Group

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the Yamayo and Yura Group respectively).

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We redefine the Yamayo Group as all of the sedimentary strata that postdates the basement, and predates the Chocolate Formation. This definition is supported by our geochronological results (Table 2 and 3), which suggest that the Yamayo Group spans

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between the mid-Carboniferous to the Late Triassic. No support was found for the Emsian age (~407-398 Ma) reported on the basis of

brachiopods (Boucot et al., 1980) in the El Fiscal – Cocachacra area, where the Yamayo Group was initially defined (Bellido and Guevara, 1963). The youngest ages obtained from zircons extracted from sandstones in this area yielded Carboniferous ages of 336.5 ± 6.4 Ma and 320.1 ± 4.0 Ma, indicating that these sedimentary rocks

37

are younger than currently assumed. Reimann et al., (2010) report youngest zircon ages of 297 ± 12 and 301 ± 14 Ma, Carboniferous ages of 323 ± 8 and 350 ± 5 Ma, from the same area (Fig. 1), These ages show that the Yamayo Group in the El Fiscal

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– Cocachacra area includes strata that were deposited during or after the Pennsylvanian, or even Early Permian. Because all these Carboniferous or Early

Permian zircons were obtained from three sandstones that were sampled randomly, it

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is doubtful that the Devonian age assigned to the local brachiopod fauna is correct, and it is more likely that these fossils are Carboniferous or Permian. According to P.

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Racheboeuf (pers. comm. to T.S., 2009), the Cocachacra brachiopod determined as Acrospirifer cf. A. atlanticus by Boucot et al. (1980) is similar to Alispirifer cf. transversus, described by Angiolini et al. (2003) from the Early Pennsylvanian (~318307 Ma) of Colombia. This insight and our results are mutually consistent, making clear that the El Fiscal–Cocachacra brachiopod fauna requires revision, and

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independently suggests that its age is Pennsylvanian, accounting for differences between in the local assemblage, and the Devonian brachiopod fauna documented in the nearby Altiplano.

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Our results contribute to the knowledge on Pennsylvanian sedimentation in coastal southern Peru. Previously reported Devonian strata exposed near Torán, in the Majes valley, also yielded a youngest zircon age of 315.8 ± 5.1 Ma, and 24 zircons form an

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age cluster with a mean age of 337.1 ± 1.4 Ma: a Pennsylvanian age is thus also likely, at least in part, for this thick sandstone-dominated succession. Furthermore, Pennsylvanian marine fossils occur at La Chira, ~54 km WSW of Torán (Pecho and Morales, 1969). A Late Pennsylvanian flora (Odontopteris, which may range into the Early Permian (Taylor et al., 2009) occurs in the lower Ocoña valley, ~24 km WNW of La Chira (Morales, 1997), close to where sandstone 09FB132 yielded an early

38

Visean (340.0 ± 6.4 Ma) maximum stratigraphic age. Pennsylvanian or Early Permian foraminifers (Triticites) occur in limestone beds in the lower section of the Yamayo Group in the lower Atico valley (Bellido and Narváez, 1960). Thus Pennsylvanian,

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and possibly Early Permian deposition is well documented from at least Cocachacra to Atico. In contrast with the partly coeval carbonate-dominated Copacabana Formation of western Bolivia and Andean southern Peru (e.g., Sempere, 1995),

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Pennsylvanian sedimentation in coastal southern Peru was dominantly siliciclastic.

The Nothorhacopteris flora is considered to be typical of the Ambo Group in Peru

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(e.g., Palacios et al., 1995), and to indicate a late Visean to earliest Serpukhovian age (Iannuzzi and Pfefferkorn, 2002), i.e. ~335-325 Ma (Gradstein et al., 2012). However, Nothorhacopteris floras thrived in temperate paleoenvironemnts and thus migrated across South America during the Carboniferous, from the northwest in the late Visean – earliest Serpukhovian (~335-325 Ma) to the southeast in the mid-Pennsylvanian

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(~312-307 Ma; Iannuzzi and Rösler, 2000). Furthermore, the Ambo Group post-dates metamorphism and Late Pennsylvanian intrusions (Chew et al., 2007) in northern Peru. Tree-sized (Tomiodendron) and smaller (Cyclostigma, Lepidodendropsis)

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lycophytes are dominant in this unit (Iannuzzi and Pfefferkorn, 2002). Outcrops containing the Nothorhacopteris flora in coastal southern Peru are scarce, and only three occurrences are known, at La Mina, Puerto Viejo, and Mal Paso (see above).

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At Puerto Viejo, sandstone 09FB16 was collected close to well-preserved

Nothorhacopteris and Tomiodendron fossils, and yielded a late Visean maximum stratigraphic age (youngest zircon 332.5 ± 3.3 Ma; youngest cluster 335.9 ± 5.3 Ma), which is in close agreement with the ~335-325 Ma age range assigned to this flora by Iannuzzi and Pfefferkorn (2002).

39

At La Mina, sample 09FB152 yielded an Early Devonian maximum stratigraphic age (407.7 ± 7.7 Ma) although it was also collected close to the occurrence of Nothorhacopteris and Tomiodendron fossils (Iannuzzi et al., 1997), which indicate a

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~335-325 Ma stratigraphic age (Iannuzzi and Pfefferkorn, 2002). Such a time gap demonstrates that in the Yamayo Group, maximum stratigraphic ages obtained from

detrital zircons may be separated from the stratigraphic ages by as much as 70 Myr or

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more. Therefore, the Early Devonian maximum stratigraphic ages obtained at Pocoma

from sandstones 09FB30 (youngest zircon 402.2 ± 6.5 Ma; youngest cluster 411.0 ±

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4.4 Ma) and 09FB31 (youngest zircon 402.6 ± 6.9 Ma; youngest cluster 413.8 ± 1.1 Ma) cannot be readily interpreted as evidence for Devonian strata within the Yamayo Group. Because (1) no unconformity between would-be Devonian and Carboniferous deposits was observed in the field, (2) Pocoma lies only ~57 km southeast of El Fiscal, and (3) the fact that Devonian maximum stratigraphic ages were documented

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at La Mina, even though we know from fossil evidence the real stratigraphic age of these sediments is Carboniferous. We adopt here the more parsimonious hypothesis that the strata in Pocoma were also deposited during the Carboniferous. Furthermore,

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the presence of only one dominant Devonian age peak in each of the sandstones from Pocoma implies a very local source, as almost no detritus derived from the surrounding Ordovician basement is recorded. These results suggest that the

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Famatinian arc continued until the Early Devonian in this region. Early Devonian zircons were also recorded by Reimann et al. (2010) in southern Peru. At Mal Paso, sandstones 09FB71 and 09FB77 were collected in the upper part of

the Yamayo Group, located several tens of metres above limestones bearing molluscs of possible earliest Permian age (Salinas, 1985), which in turn overlie a 10-20 m local level that yielded a limited Nothorhacopteris-Tomiodendron flora (Pino et al., 2004).

40

Both samples yielded Middle Triassic maximum stratigraphic ages of 234.0 ± 6.1 Ma (with 21 grains between 245.8 ± 4.2 and 316.4 ± 4.3 Ma) and 230.2 ± 11.4 Ma (with a younger cluster of ~238.6 ± 3.3 Ma). These Carboniferous to Triassic ages match by

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the ages recovered from sample 09FB87 in the Estique Pampa section, ~5 km northeast of Mal Paso, namely a youngest zircon age of 218.8 ± 3.3 Ma and 4 grains distributed between 328.2 ± 3.9 and 345.4 ± 2.8 Ma. These ages show that the

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sequence exposed in this section is not equivalent to the Cabanillas Group of the Altiplano.

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The Yamayo Group is overlain in the Mal Paso - Estique Pampa area by a 216 Maold ignimbrite at the base of the Chocolate Formation, and hence the uppermost part of the Yamayo Group was deposited in the Middle to Late Triassic. This conclusion fits with results obtained from the sample from Punta Saguas, 460 km WNW of Mal Paso, which also yielded a Middle Triassic maximum age of 233.3 ± 1.5 Ma (mean of

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the three youngest grains), also in the uppermost portion of the Yamayo Group. A combination of our results and the biostratigraphical constraints demonstrate that the Yamayo Group includes strata of late Visean – earliest Serpukhovian (with a

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Nothorhacopteris flora), Pennsylvanian (and possibly Early Permian), and Middle to Late Triassic age. Therefore, it is possible that the Yamayo Group includes strata of all or most time periods within the Carboniferous to Late Triassic interval, as

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suggested by the great thickness, monotony and apparent continuity of most available sections of the Yamayo Group. Although this unit spans at least the time period between ~330 and ~216 Ma, no significant discontinuity has been observed. We are unable to reliably assess the time at which the Yamayo Group started to be deposited. As demonstrated by the occurrence of the Nothorhacopteris flora, sedimentation commenced before the late Visean or earliest Serpukovian, i.e. before

41

~335-325 Ma. Furthermore, it is possible that deposition began after the latest Devonian – earliest Carboniferous, as this time interval is generally marked in South America at these latitudes by a depositional unconformity (e.g., Mégard, 1978;

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Sempere, 1995). A surprising result of this study is the demonstration of Triassic sedimentation in coastal southern Peru. Although the existence of Triassic strata had been previously

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inferred (Bellido and Guevara, 1963), the fact that sedimentary rocks that host Middle

Triassic zircons underlie the Late Triassic base of the Chocolate Formation confirms

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the conjecture of Bellido and Guevara (1963). Our data demonstrate the existence of Triassic sedimentary rocks in the Puerto Viejo – Chala (09FB17) and the Mal Paso (09FB71, 09FB77) sections. In the latter location, the conformable contact between an early Triassic ignimbrite (09FB87) and the underlying sedimentary rocks is now dated to ~216 Ma.

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Sandstone 09FB17, with a maximum stratigraphic age of 233.1 ± 2.7 Ma does not contain detritus derived from a Carboniferous arc in the detrital zircon U-Pb age record. Interestingly, detritus of the Ordovician arc (n = 11), absent in the

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stratigraphically lower sample 09FB16, is recorded by the detrital zircon U-Pb age spectrum of sample 09FB17. This corroborates field observations that are diagnostic of a drastic change in source input, and could indicate that there was exhumation of

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remnants of the Ordovician arc, after the Carboniferous. Sandstones 09FB71 and 09FB77, from the Tacna region, yield maximum Triassic

stratigraphic ages of 234.0 ± 6.1 and 230.2 ± 11.4 Ma, respectively. Both samples exhibit zircon ages that range between ~315 and ~230 Ma. These samples, in contrast to samples in the Chala area for example, yield a continuous range of zircon ages between the Carboniferous and the Early Triassic. Possibly these detrital zircons were

42

derived from the Eastern Cordillera of Peru where granitic magmatism during ~342– 285 Ma and younger partially migmatized granitoids between 10°S and 13°S of Permian and Triassic age (285–223 Ma) have been reported (Mišković et al., 2009).

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Another possibility is that they were derived from a magmatic arc located offshore the present-day coastline.

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7.2 Yura Group

The zircon U-Pb age of 215.9 ± 0.8 Ma obtained from an ignimbrite (09FB88) at the

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very base of the Chocolate Formation in the Estique Pampa section demonstrates that the volcanism in the Chocolate Formation started in the earliest Norian at this locality. The single detrital zircon dated at 218.8 ± 3.3 Ma from the non-volcanic sedimentary rocks (09FB87) that conformably and immediately underlie the ignimbrite confirms the absence of a significant chronological hiatus between the Yamayo Group and the

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Chocolate Formation, as these two ages overlap within error. Two samples from the non-volcanic sedimentary rocks (09FB71 and 09FB77), which underlie the Chocolate Formation (i.e., from the upper Yamayo Group) in the nearby Mal Paso area yielded

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Middle Triassic zircons with ages of 234.0 ± 6.1 and 230.2 ± 11.4 Ma, respectively, and the latter also yielded an age cluster with a mean age of 238.6 ± 3.3 Ma (n = 7). The 215.9 ± 0.8 Ma age obtained at the very base of the Chocolate Formation at

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Estique Pampa is only slightly younger than the 220.7 ± 1.6 Ma age obtained from a leucocratic 2-mica granite (09FB82) that intrudes the basement gneisses at Mal Paso, and exhibits an arc signature. This near-synchroneity suggests that eruption at ~216 Ma was cogenetic with nearby felsic plutonism dated at ~221 Ma, and indicates that arc magmatism abruptly developed in the Late Triassic in the Tacna area. This age furthermore confirms that in this area, the contact between the basement intruded by a

43

Triassic granite and the nearby Carboniferous to Triassic Machani Formation is faulted. Late Early Sinemurian ammonites in the Pelado (= Socosani) Formation (Salinas,

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1985) in the Tacna region place a minimum age of ~199.5 Ma to the top of the underlying Junerata (= Chocolate) Formation lavas. Schaltegger et al. (2008) dated

the base of the Sinemurian to 199.5 Ma, in northern Peru. This constrains the volcanic

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accumulations in this section to between 215.9 ± 0.8 and ~199.5 Ma, i.e. ~16 Myr

duration. The top of the Chocolate Formation is now dated at ~186 Ma in the Yura

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valley, ~250 km northwest of the Tacna region, by an ash layer located at the very base of the overlying Socosani Formation, demonstrating that the top of the Chocolate Formation is strongly diachronic across the study area. This is confirmed by the fact that in the region extending from Chucarapi (Cocachacra) to Puerto Grau, along the present-day coastline, the Chocolate Formation includes a lava flow dated at ~172

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Ma, which was partly coeval with local calc-alkaline plutonism ranging from 172 to 152 Ma. Volcaniclastic sandstones overlying the Chocolate Formation at Chucarapi yielded maximum stratigraphic ages of 165 and 162 Ma (Boekhout et al., 2012).

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The occurrence of Middle Triassic zircons in coarse conglomerates below the base of the Chocolate Formation in the Chala area at Punta Saguas strongly suggests that the Late Triassic resumption of arc magmatism found in the Tacna region is likely to

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have occurred along the entire subduction segment corresponding to present-day southern Peru. The Chocolate Formation in this area includes calc-alkaline ignimbrites and lava flows dated at ~177 Ma (Roperch and Carlier, 1992); 173 and 171 Ma (this study); 164 Ma (Romeuf et al., 1995), and 157 Ma (Roperch and Carlier, 1992). Siliciclastic deposits intercalated in the upper Chocolate Formation near Chala

44

yielded a maximum stratigraphic age of ~135 Ma, indicating that arc volcanism continued into the Early Cretaceous, in this area. Our data thus indicate that the shortest interval spanned by the Chocolate Formation

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is ~216-199.5 Ma in the Tacna region, and also suggest that the longest is ~216-<135 Ma in the Chala region. This implies that the order of magnitude of volcanic accumulation rates in the Chocolate arc was ~100 m/Myr. We stress that this estimate

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is imprecise due to the generally high uncertainty on the thickness of this unit.

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8. Implications on the geodynamic evolution of southern Peru

The western margin of South America has been the site of almost continuous subduction since at least the Cambrian (Ramos and Aleman, 2000). This initially resulted in major calc-alkaline intrusions and metamorphism between ~515 - 400 Ma (Famatinian arc), and gave rise to plutons in this age range (472 - 460 Ma) along the

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present day coastline of the south Peruvian coast, of which we dated the most northerly exposed suite in along the present day Peruvian coast, on the Paracas Peninsular at 461 ± 1.8 Ma Ma. Numerous partly coeval Ordovician plutons have

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been recognized in the Eastern Cordillera of Peru (474 - 442 Ma) (Chew et al., 2007; Mišković et al., 2009). The presence of a rigid basement block (Arequipa Terrane) may have focused Ordovician back-arc magmatism in the region of its continental

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suture with the palaeo-continental margin of South America, while the main arc was located along the present day coastline. The depositional environment in coastal southern Peru during sedimentation within the Yamayo Group (Carboniferous – Triassic) ranges from alluvial fan and alluvial plain to shallow-marine and deltaic. No plutonic activity coeval with these sedimentary rocks is observed along the southern Peruvian coast, except for one late

45

Triassic pluton (221 Ma) near Estique Pampa (this work). However, Carboniferous to Permian plutonic rocks were emplaced in the Eastern Cordillera of Peru between ~336-223 Ma, in the same location as the Faminatinian back-arc plutons (Mišković et

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al., 2009). A combination of i) the trend of the Ordovician arc along the present day coastline, ii) accretionary orogens in general young oceanward during supercontinent

break-up (Cawood et al., 2009; Kemp et al., 2009), and iii) the evidence in the Eastern

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Cordillera for an extensional setting throughout the Carboniferous to Triassic period, we argue that arc axes and accretionary prisms during this period might well be

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located outboard of the present day coastline of southern Peru.

Subduction erosion has removed at least 110 km of coastline perpendicular to the margin since 150 Ma (Clift et al., 2003; Stern, 2011), possibly accounting for the absence of Carboniferous to Triassic main arc plutonic rocks along coastal southern Peru. This hypothesis is similar to that proposed by (López-Gamundí and Breitkreuz,

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1997) for central and northern Chile and would place the Yamayo Group sedimentary rocks in a marginal basin position during a dominantly extensional accretionary arc setting that prevailed during the Carboniferous to Late Triassic along the whole

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margin of South America (e.g. Alasino et al., 2012). Generally, arc – trench sandstones (forearc basins – accretionary prisms) contain a significantly higher proportion of young arc-derived zircons, compared to old cratonic grains (Cawood et

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al., 2009), which is not the case in the sedimentary rocks of the Yamayo Group, supporting they may have been deposited in a marginal basin environment. More sedimentological and geochronological work is however necessary to establish a detailed paleogeographic basin reconstruction related to a possible outboard migrating arc. Volcanic and volcaniclastic rocks of the Chocolate Formation were deposited between 216 – 134 Ma in coastal southern Peru, and overlies the Yamayo Group.

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The main arc axis migrated landward in the Late Triassic, assuming an outboard location for the older magmatic arcs. No plutons were emplaced in the back-arc related Eastern Cordillera between 220 and 200 Ma (Mišković et al., 2009). Therefore

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this period of local magmatic quiescence in the Eastern Cordillera of southern Peru is partly coeval with the onset of arc magmatism overlying the Yamayo group

sedimentary rocks in coastal southern Peru, possibly indicating compressional period

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with landward arc migration. This landward arc migration can be recognised in the stratrigraphic record by the boundary between the Yamayo and the Yura Formation.

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Plutonic calc-alkaline bodies with ages between 196 to 176 Ma were emplaced close to the town of Arequipa (Demouy et al., 2012; Fig. 1), after which an extensional regime once again prevailed during the Jurassic, with the consequent (younging outboard) emplacement of calc-alkaline plutons from the Ilo Batholith between 174 and 152 Ma along the present day coastline (Boekhout et al., 2012).

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An extensional to transtensional regime has prevailed (Polliand et al., 2005) from the Early Cretaceous until the opening of the South Atlantic Ocean at ~110 Ma (Boekhout et al., 2012). Subsequently, dominantly landward arc migration of the arc

2010).

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axis has been reported until the arc reached its present day position (Mamani et al.,

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Potential mechanisms for a switch from an extensional to a compressional accretionary setting, such as that observed in the Late Triassic and Cretaceous, include: (1) flattening of the subducted slab, causing an increase in the length of the seismically locked zone and increasing horizontal stress in the upper plate (reference Gutscher here), (2) accretion of buoyant oceanic lithosphere (e.g. Spikings et al., 2010), and (3) plate reorganization causing an increase in convergence across the

47

boundary (Cawood 2009). The effects of flat-slab subduction (Ramos and Folguera, 2009) and terrane accretion should be spatially limited to the region of either the flat slab or the accretion zone, which in turn should result in short-lived orogenesis and/or

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diachronous events that migrate along the convergent margin. There is no evidence for terrane accretion in southern Peru in the Late Triassic and no clear sign of a

diachronous migrational event along the margin, implying that the observed arc

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migration might not be spatially limited to southern Peru. We favor a late Triassic increase in convergence rates across the Pacific margin around 220 Ma, coeval with

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the initial break-up of Pangaea.

Summary

Knowledge of the Carboniferous to Jurassic stratigraphic record of coastal southern

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Peru appears considerably improved by the dating of the local base of the Chocolate Formation to 216 Ma, and by the recognition of the underlying Yamayo Group as one major sedimentary unit spanning at least the late Visean to Late Triassic time interval.

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We observe that the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru consists of two first-order units, namely (1) the Yamayo Group, a sedimentary succession of variable (0–2 km) thickness, with apparently no nearby volcanic lateral

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equivalent, and (2) the overlying Yura Group, consisting of a lower, 1–6 km-thick volcanic and volcaniclastic unit, the Chocolate Formation, and an upper, 1–2 km-thick sedimentary succession, that are in markedly diachronous contact across the coeval arc and back-arc. The apparently abrupt inception of volcanism in the study area reflects the sharp development of a magmatic arc where only sedimentary deposits had previously accumulated during more than 110 Myr.

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This new chronostratigraphic framework gives insight in arc migrational events along this part of the Gondwana margin. The location of the Carboniferious to Triassic arcs, developed in an extensional environment outboard of the present day coastline, now

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seems likely, placing the Yamayo Group in a marginal basin position. The Late Triassic landward migration of magmatic arc activity with the initial onset Chocolate

Formation could be related to an increase of the convergence rate along the margin,

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temporally producing the switching from a negative to positive trench roll-back velocity. The Jurassic period was once again dominated by an extensional tectonic

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setting, during which trenchward arc migration can be observed from the Arequipa region (196 – 176 Ma) to the present day coastline (172 – 152 Ma). This study therefore implies that tectonic switching events played an important role along the South Peruvian margin responsible for the present-day spatial distribution (and absence) of magmatic arcs. Combined magmatic and detrital U-Pb zircons ages

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within a well-defined stratigraphic and geodynamic framework allow a more accurate reconstruction of the Late Paleozoic to Mesozoic history of the south Peruvian

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margin.

Acknowledgements

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This study was funded by the Swiss National Science Foundation. The manuscript was greatly improved by comments from reviewers R.J. Pankhurst and V. Ramos. We thank A. Ulianov for precious technical support during LA-ICP-MS analyses and F. Capponi for whole-rock XRF analyses.

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Figure captions

Figure 1. Map of southern Peru indicating the studied sedimentary successions (A-I).

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The local basement coincides with the ´Arequipa Massif´ underlying the present-day fore-arc and Western Cordillera. The Mollendo-Camaná Block (MCB) forms a major part of the forearc and is intruded by Ordovician plutonic rocks. The following

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locations were studied: (A) Paracas area, La Mina; (B) El Fiscal and Cocachacra area

(C) Pocoma area; (D) Mal Paso area; (E) Estique Pampa area; (F) Majes area; (G)

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Ocoña valley; (H) Chala; (I) Yura.

Figure 2. Schematic overview of previous stratigraphic units in southern Peru (modified after Vicente (1981) and Pino et al. (2004)). Detailed descriptions of the

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sedimentary units can be found in section 3.

Figure 3. Stratigraphic column and obtained geochronological constraints on the sediments and volcaniclastic sequences in the Chala, Yura and Estique Pampa/Mal

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Paso areas. The ages obtained by Romeuf et al. (1995) and Roperch and Carlier (1992) were not obtained in the same section that we measured, and are therefore not included in the Chala stratigraphic column. (1.Socosani type locality, 2. Pelado, San

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Francisco, Ataspaca, and Chachacumane formations).

Figure 4. Field photographs of the coastal Cordillera of southern Peru. a) The ~12 mthick ignimbrite that forms the base of the Chocolate Formation near Tacna; b) Syntectonic Yamayo Group overlying the basement (separated by a detachment fault) near Ocoña; c) Nothorhacopteris-Tomiodendron flora, Peruvian ´sol´ as scale; d)

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Coarsening-upward sequence at the base of the exposed section of the Yamayo Group at Puerto Viejo of coarse sandstones and conglomerates within an alluvial fan system; e) Outcrop of the ´La Mina´ sequence on the Paracas peninsula near the occurence of

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Tomiodendron plant fossils; f) Socosani Formation at its type-locality near Yura, marked by a calcareous ash bed; g) Sandstones overlying andesitic and basaltic lava

flows in the upper Chocolate Formation near Chala; h) Lowermost Labra Formation

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in the Majes valley (Oxfordian).

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Figure 5. Plots of all geochronological data. All sandstones were measured by LAICP-MS. For the other samples the U-Pb dating method used is indicated in the left top corner of the plot.

a) Sandstone from the bottom of the exposed section of the Yamayo Formation at La Mina, Paracas peninsula, near the occurence of Tomiodendron and Nothorhacopteris

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plant fossils; b) Granite along the northern shore of the Paracas peninsula; c) and d) Sandstones from the base of the exposed section of the Yamayo Group at Puerto Viejo near Chala; e) and f) Ignimbrites from the upper Chocolate Formation near

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Chala; g) Sandstone from the top of the Chocolate Formation near Chala; h) Sandstone from the syntectonic Yamayo Group overlying the basement (separated by a detachment fault; i) Sandstone from Yamayo Group in the Majes valley; j)

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Sandstone from the bottom of Socosani formation in the Majes valley; k) Ash layer from the top of the Chocolate Formation in the Yura section; l) and m) Sandstone from Yamayo Group in the El Fiscal area; n) and o) Sandstones from the base of the Yamayo Group at Pocoma; p) Ignimbrite from the very base of the Chocolate Formation in the Estique Pampa area; q) r) and s) Sandstones from the top Yamayo

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Group near Estique Pampa and Mal Paso t) coarse-grained, leucocratic, 2-mica granite near Mal Paso.

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Figure 6. a) Primitive normalized trace element patters for dated and undated plutonic rocks of Ordovician, Triassic and Jurassic in the present day fore-arc of southern

Peru. Normalizing values taken from Sun and McDonough (1989). In both diagrams

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the light grey continuous bar indicates the values of the geochemical data set of the Jurassic plutons (174 – 152 Ma) of the Ilo Batholith (Boekhout et al., 2012) b)

plutonic rocks as in figure 6a.

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Chondrite normalized REE diagrams (Sun and McDonough, 1989) for the same

Figure 7. Figure summarizing all geochronological data obtained in the different locations. Letters in squares refer to photos in figure 3. Letters in circles refer to the

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geochronological data in figure 5.

Table 1. Whole rock data for volcanic and plutonic rocks in coastal southern Peru.

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Major element oxides in wt.%; trace elements in ppm. Lithology: 2MG, 2-mica granite; DL, dioritic lava flow; RI, rhyolitic ignimbrite, AL, andesitic lava flow; G, granite; AI, andesitic ignimbrite.

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Table 2. Summary of geochronological result of the Yamayo Group arranged by decreasing youngest zircon.

Table 3. Summary of geochronological results of the Yura Group arranged by decreasing youngest cluster age.

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SC

RI PT

generation through time and space. Geological Society of America Bulletin 122, 162-182. Martignole, J., J., M., 2003. Regional-scale Grenvillian-age UHT metamorphism in the Mollendo-Camana block (basement of the Peruvian Andes). Journal of Metamorphic Geology 21, 99-120. Martínez, W., Cervantes, J., Romero, D., Sempere, T., 2005. The Late Paleozoic - Early Mesozoic Chocolate Formation of Southern Peru: New data and interpretations. 6th International Symposium on Andean Geodynamics, 490-492. Mégard, F., 1978. Etude géologique des Andes du Pérou central. Memoir ORSTROM 86, 310 p. Mendívil, S., Castillo, W., 1960. Geología del cuadrángulo de Ocoña. Comisión de la Carta Geológica Nacional, Lima, 1(3). Mišković, A., Spikings, R.A., Chew, D.M., Košler, J., Ulianov, A., Schaltegger, U., 2009. Tectonomagmatic evolution of Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic contraints from the Peruvian Eastern Cordilleran granitoids. Geological Society of America Bulletin 121, 1298-1324. Morales, G., 1997. Nuevos datos geocronologicos y biostratigraphicos del macizo antiguo de Arequipa. IX Congreso Peruano de Geología, 365 - 369. Mukasa, S.B., Henry, D.J., 1990. The San Nicolás batholith of coastal Peru: early Paleozoic continental arc or continental rift magmatism? Journal of the Geological Society, London 147, 27-39. Murphy, J.B., Nance, R.D., 2008. The Pangea conundrum. Geology 36, 703-706. Narváez, S., 1964. Geología de los cuadrángulos de Ilo y Locumba. Carta Geológica Nacional, Boletín 6 Serie A, 75p. Newell, N.D., 1942. Lower Paleozoic pelecypods: Mytilacea. University of Kansas Publications 10(2), 1-115. Olchauski, L.E., 1980 Geología de los cuadrángulos de Jaqui, Coracora, Chala y Chaparra. Buletín del Instituto de Geología, Minería y Metalurgía, 34(A). Palacios, O., Sánchez, A., Herrera, F., 1995. Geología del Perú. Lima, INGEMMET, Boletín 55, 143 p. Pankhurst, R.J., Rapela, C.W., Fanning, C.M., Marquez, M., 2006. Gondwanide continental collision and the origin of Patagonia. Earth-Science Reviews 76(3-4), 235-257. Pankhurst, R.J., Riley, T.R., Fanning, C.M., Kelley, S.P., 2000. Episodic silicic volcanism in Patagonia and the Antarctic Peninsula: Chronology of magmatism associated with the break-up of Gondwana. Journal of Petrology 41, 605 - 625. Paredes, J., 1969. Geología de los cuadrángulos de Camaná y La Yesera. Carta Geológica Nacional, 72 p. Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P., 1997. A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115-144. Pecho, V., Morales, G., 1969. Geología de los cuadrángulos de Camaná y La Yesera. Bol. Carta Geológica Nacional 21, 72 p. Pfefferkorn, H.W., Alleman, V., 2001. The Carboniferous of Paracas - A synthesis, XI Congreso Peruano de Geologia Lima, pp. 43. Pino, A., Sempere, T., Jacay, J., Fornari, M., 2004. Estratigrafía, Paleogeografía y paleotectόnica del intervalo Paleozoico superior - Cretáceo inferior en el area de Mal Paso - Palca (Tacna), Nuevas contrubuciones del IRD y sus contrapartes al conocimiento geolόgico del sur del Peru. Sociedad geologica del Perú, Lima, pp. 1544.

AC C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

55

EP

TE D

M AN U

SC

RI PT

Polliand, M., Schaltegger, U., Frank, M., Fontboté, L., 2005. Formation of intra-arc volcanosedimentary basins in the western flank of the central Peruvian Andes during Late Cretaceous oblique subduction: field evidence and constraints from UPb ages and Hf isotopes. International Journal of Earth Sciences 94, 231-242. Ramos VA, Aleman A. 2000. Tectonic evolution of the Andes. In Tectonic Evolution of South America, Cordani U.J, Milani E.J, Thomaz Filho A, Campos DA (eds). International Geological Congress, Rio de Janeiro 635–685 Ramos, V.A., Folguera, A., 2009. Andean flat-slab subduction through time. Geological society of london special publications 327, 31 - 54. Ramos, V.A., 2010. The tectonic regime along the Andes: present-day and Mesozoic regimes. Geological Journal 45, 2 – 25 Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., 1998. Early evolution of the Proto-Andean margin of South America. Geology 26, 707-710. Reimann, C., Spiske, M., Bahlburg, H., Lopez, S., Carlotto, V., 2010. Geodynamic evolution of the early Paleozoic Western Gondwana margin 14°–17°S reflected by the detritus of the Devonian and Ordovician basins of southern Peru and northern Bolivia. Gondwana Research 18/2-3, 370-384. Romero, D., Valencia, K., Alarcόn, P., Ramos, V.A., 2013. The offshore basement of Peru: Evidence for different igneous and metamorhpic domains in the forearc. Journal of South American Earth Sciences 42, 47-60. Romeuf, N., Aguirre, L., Soler, P., Féraud, G., Jaillard, E., Ruffet, G., 1995. Middle Jurassic volcanism in the Northern and Central Andes. Revista Geologica de Chile 22, 245– 259. Roperch, P., Carlier, G., 1992. Paleomagnetism of Mesozoic rocks from the cerntral andes of Southern Peru: importance of rotations in the development of the Bolivian Orocline. Journal of Gephysical Research 97, 17,233 - 17,249. Salinas, E., 1985. Evolución paleogeográfica del sur del Perú a la luz de los métodos de análisis sedimentológicos de las series del departamento de Tacna. Universidad Nacional San Agustín de Arequipa, tesis de grado,205 p. Schaltegger, U., Guex, J., Bartolini, A., Schoene, B., Ovtcharova, M., 2008. Precise U-Pb age constraints for end-Triassic mass extinction, its correlation to volcanism and Hettangian post-extension recovery. Earth and Planetary Science Letters267, 266275. Schildgen, T.F., Ehlers, T.A., Whipp Jr., D. M., van Soest., M.C., Whipple, K.X., Hodges, K.V., 2009. Quantifying canyon incision and Andean Plateau surface uplift, southwest Peru: A thermochronometer and numerical modeling approach. Journal of Geophysical Research: Earth Surface 114,F04014, doi:10.1029/2009JF001305. Schoene, B., Latkoczy, C., Schaltegger, U., Gunther, D., 2010. A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMSTEA). Geochimica et Cosmochimica 74, 7144-7159. Sempere T., 1995. Phanerozoic evolution of Bolivia. In: Tankard A.J., Suarez-Soruco S., and Welsink H.J., eds., Petroleum basins of South America. American Association of Petroleum Geologists Memoir 62, 207-230 Sempere, T., Carlier, G., Soler, P., Fornari, M., Carlotto, V., Jacay, J., 2002. Late PermianMiddle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics. Tectonophysics 345(1-4), 153-181. Senshu, H., Maruyama, S., Rino, S., Santosh, M., 2009. Role of tonalite-trodhjemite-granite (TTG) crust subduction on the mechanism of supercontinent breakup. Gondwana Research 15, 433-422. Stern, C.R., 2011. Subduction erosion: Rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle. Gondwana Research 20, 284-308.

AC C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

56

EP

TE D

M AN U

SC

RI PT

Taipe, E., 2004. Détachements en extension et sédimentation: les bassins Pennsylvaniens d´Atico et Ocoña (Sud-Pérou). Mémoire de DEA, Université Paul-Sabatier (Toulouse), 38 p. Taylor, T.N., Taylor, E.L., Krings, M., 2009. Paleobotany: The biology and evolution of fossil plants. Academic Press, 1252 p. Ulianov, A., Müntener, O., Schaltegger, U., Bussy, F., 2012. The data treatment dependent variability of U-Pb zircon ages obtained using mono-collector, sector field, laser ablation ICP-MS. Journal of Analytical Atomic Spectrometry 27, 663-676. Vaughan, A.P.M., Pankhurst, R.J., 2008. Tectonic overview of the West Gondwana margin. Gondwana Research 13, 150-162. Veevers, J.J., 2004. Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating. Earth-Science Reviews 68, 1-132. Vicente, J.-C., 1981. Elementos de la estratigrafía Mesozóica sur-peruana. Volkheimer, W. and Musacchio, E.A. (Eds.) Cuencas sedimentarias del Jurásico y Cretácico de Comité Sudamericano del Jurásico y Cretácico America del Sur 1, 319 - 351. Vicente, J.-C., Beaudoin, B., Chavez, A., Leon, I., 1982. La cuenca de Arequipa (Sur Peru) durante el Jurasico–Cretacico inferior. V Congreso Latinoamericano de Geologia 1, 121–153. Villagómez, D., Spikings, R., Magna, R., Kammer, A., Winkler, W., Beltrán, A, 2011. Geochronology, geochemistry and tectonic evolution os the Western and Central cordilleras of Colombia. Lithos 125, 875-896. Wasteneys, H.A., Clark, A.H., Farrar, E., Langridge, R.J., 1995. Grenvillian granulite-facies metamorphism in the Arequipa Massif, Peru: a Laurentia-Gondwana link. Earth and Planetary science letters 132, 63-73. Wilson, J., García, W., 1962. Geología de los cuadrángulos de Pachía y Palca (Hojas 36-v y 36-x). Comisión Carta Geológica Nacional (ahora INGEMMET), boletín 4.

AC C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

57

74°00´ Lomas

for

Chala

71°00´

rc

We s

río Majes

MCB

Atico

Pacific Ocean

Aplao

I F

Ocoña G

17°00´

te

Yura

Hacienda Socosani

La Chira Camaná

rn

Arequipa

fo

re

B

Cocachacra

N Peru

c

ill

er

a

E

río Osmore

M AN U

Ite

Pachía

09FB120: max age 320.1 Ma

La Yarada

max age 297 Ma1

0´

09FB116: max age 165.0 Ma* 09FB15: max age 161.9 Ma* 09FB98: 172.3 Ma* 07FB26: max age 165.5 Ma*

EP

Alto El Madero

Pocoma

Ite

71

°0

20 km

0´

°3

0´

°0

Ilo

0´

09FB30: max age 402.2 Ma 09FB31: max age 402.6 Ma

Mollendo

72

Puerto Grau

72

AC C

Chucarapi Cocachacra

CHILE

Arica

TE D

El Fiscal

D

Tacna

°0

17

Estique Pampa

Mal Paso

100 km

09FB119: max age 336.5 Ma

Pacific Ocean

rd

Puno

Moquegua

Ilo

0

Co

Lago Titicaca

no

Quebrada Guaneros

ar

SC

C

A

la

Cerro Verde

Mollendo

Lima

Juliaca

tip

Punta Saguas - Puerto Viejo

18°00´

70°00´

Al

N

16°00´

72°00´

ea

H

A

73°00´

RI PT

Figure 75°00´ 1

Younger sedimentary cover Volcanic rocks of the Chocolate Formation (Triassic- Jurassic) intruded by the Ilo Batholith (Jurassic - Cretaceous) Yamayo Group Grenvillian basement with Ordovician intrusives

lowest part of Yura Group Sample location (A-I)

*data from Boekhout et al. (2012) 1

data from Reimann et al. (2010)

Figure 2

shallowing- and thickening-upward marine quartz-arenites, and subordinate shales

Labra Fm

turbiditic greywackes

Socosani Fm

SC

shallow-marine carbonates

M AN U

Chocolate Fm

volcanic rocks and volcaniclastic deposits

TE D

Yamayo Group

local alluvial-fan deposits

EP

diverse sedimentary strata ranging from alluvial to marine deposits

basement

AC C

Yura Group

Puente Fm

RI PT

basinal and slope deposits, dominantly fine-grained

Cachíos Fm

Grenvillian migmatites and metamorphites, Ordovician granites and local metamorphites

limestones dominant

volcanic & volcaniclastic deposits shales dominant conglomerates dominant sandstones dominant

500 m

Figure 3

09FB63 (185.8 ± 1.0 Ma)

100 m

RI PT M AN U

SC

200 m

1000 m

plutonic rocks black shales sandstone metamorphic basement

09FB16 (max. 332.5 ± 3.3 Ma) Ordovician plutons

volcanic rocks plutonic rocks Nothorhacopteris flora ammonite biostratigraphy

Paleozoic

AC C

09FB17 (max 233.1 ± 2.7 Ma)

2000 m

Proterozoic

Sinemurian

09FB88 (215.85 ± 0.82 Ma) 09FB87 (218.8 ± 3.3 Ma) 09FB77 (max 230.2 ± 11.4 Ma)

?

volcano clastic sediments

0m

3000 m

Car b b on if e r ou s Perm. Yamayo G ro up

EP

0m

Proterozoic

2

TE D

Yu r a Gr o u p

Ch o c o l a t e F o r m a t io n

3000 m

Jurassi c

07FB66 (173.3 ± 1.3 Ma) 07FB65

M e s o z o i c

Ch o c o l a t e F o r m a tio n

Jurassi c

Yu r a Gr o u p

09FB149

2000 m

Yamay o Gr ou p

Car b b on if e r ou s

Pa l e o z o i c

Permian

Triassic

M e s o z o i c

4000 m 07FB02 (171.22 ± 0.15 Ma)

Juras s i c

300 m

C hoco l at e F o r m a t io n

1

Triassi c

09FB150 (max. 134.8 ± 2.2.Ma)

Location: Estique Pampa/Mal Paso

Yu r a Gr o u p

Location: Yura

M e s o z o i c

Location: Chala

1000 m

0m

09FB71 (max. 234 ± 6.4 Ma)

09FB82 (220.7 ± 1.6 Ma)

Figure 4

a

c te Fm

SC

RI PT

ola Choc

S

N

roup

yo G Yama

TE D

b

M AN U

Tomiodendron sp.

EP

Basement

AC C

d

e

SE

NW

up

a Yam

ro yo G

Yam

ayo

Gr

ou

p

Figure 4

f

Socosani Fm

shell fragments

RI PT

Chocolate Fm

EP

M AN U s

w

va

flo

la

AC C

h

Fm

e lat

o

oc

Ch

TE D

ic ast l c ni s lca ent o v im sed

SC

g

NE

SW

up

Yur

ro aG

Figure 5

Paracas peninsula

Paracas peninsula

A 09FB152 (sst), n=66

B 09FB01 (granite)

youngest zircon: 407.7 ± 7.7 Ma Frequency 60

206

0.077

456.7 ± 11 - 491.7 ± 4.7 Ma (n = 50)

50

Pb/ 238U data-point error ellipses are 2σ LA-ICP-MS

474

0.076

470

0.075

30

0.074

20

0.073

462

RI PT

40

466

458

454

407.7 ± 7.7 Ma

Mean 206Pb/238U age: 461.1 ± 1.8 (n = 17) MSWD = 2.3

450

0.072

10

446

0

500

1000

1500

0.071 0.48

SC

0

2000

age (Ma)

Chala

0.52

0.56

0.60

0.64 207

Chala

D 09FB17 (sst), n = 85

youngest zircon: 332.5 ± 3.3 Ma Frequency

youngest zircon: 233.1 ± 2.7 Ma Frequency

335.9 ± 5.3 Ma (n = 5, MSWD = 0.22)

25 20

~1000 - ~1900 Ma (n = 56)

TE D

15

M AN U

C 09FB16 (sst), n=61

10

0 0

500

1000

EP

5

1500

20

233.3 ± 1.5 Ma (n = 3, MSWD = 0.0022)

15

416.2 - 490.3 Ma (n = 11) ~900 - ~1850 Ma (n = 68)

10

5

0 0

2000

500

1000

1500

Chala

AC C

age (Ma)

E

Pb/

0.0280

238

F

Chala 07FB02 (ignimbrite)

206

Udata-point error ellipses are 2σ

LA-ICP-MS

2000

age (Ma)

07FB66 (ignimbrite) 206

Pb/ 235U

Pb/ 238U

0.02704

data-point error ellipses are 2σ

ID-TIMS

176

0.02698

0.0275 174

0.02692

171 0.0270

172

Mean 206Pb/238U age: 173.3 ± 1.3 Ma (n = 3) MSWD = 0.71

170

0.0265 0.17

0.18

0.19

0.20

0.21

207

Pb/ 235U

Mean 206Pb/238U age: 171.22 ± 0.15 Ma (n = 4) MSWD = 1.04

0.02686 0.02680 0.180

0.185

0.190

207

Pb/ 235U

Figure 5

G Chala 09FB150 (sst), n=38

Ocoña H 09FB132 (sst), n=39

youngest zircon: 134.8 ± 2.2 Ma 14 Frequency 134.8 ± 2.2 12 149.6 ± 1.4 Ma (n = 24) 30

35

10

30

8

25

20

6

15

4

165.4 ± 3.3 180.1 ± 2.7 Ma (n = 10)

0 5

20 15

2

10

340 ± 6.4 Ma

10 130

140

150

160

170

180

190

5

0 200

600

1200

2000

0

age (Ma)

Majes valley

500

1000

2000 age (Ma)

1500

Majes valley

J

Sample 09FB125 (sst), n=58 youngest zircon: 315.8 ± 5.1 Ma Frequency

SC

0

0

Sample 09FB124 (sst), n=59 youngest zircon: 156.2 ± 2.3 Ma Frequency

M AN U

I

455.3 ± 7.5 - 506 ± 12 Ma (n = 32)

RI PT

25

youngest zircon: 340 ± 6.4 Ma Frequency

14

30 25

337.1 ± 1.4 Ma (n = 24, MSWD = 1.4)

20

447 ± 12.9 Ma - 506 ± 12.7 Ma (n = 9)

12

156.2 ± 2.3 162.9 ± 1.5 Ma (n = 6)

10

~570 Ma

8

TE D

15

0

500

1000

EP

5 0

6

~1050 - ~1850 Ma (n = 23)

10

1500

2 0

2000

age (Ma)

09FB63 (ash layer)

0.0315

LA-ICP-MS

AC C

K Yura

~800 - ~2150 Ma (n = 21)

4

L

0

500

1000

1500

2000

age (Ma)

El Fiscal 09FB119 (sst), (n = 18)

youngest zircon: 336.5 ± 6.4 Ma Frequency

198

194

0.0305

6

459.3 ± 4.8 - 531.4 ± 5.9 Ma (n = 6) 336.5 ± 6.4 Ma

5

190

~991 and ~1880 Ma (n = 11)

4

0.0295 186

3 182

Mean 206Pb/238U age: 185.8 ± 1.0 (n = 12) MSWD = 1.4

0.0285 178

0.0275 0.17

0.19

0.21

0.23

2 1

0.25 0

0

500

1000

1500

2000

2500

age (Ma)

FigureEl 5 Fiscal M 09FB120 (sst), n = 6 youngest zircon: 320. 1 ± 4 Ma Frequency

Pocoma

N 09FB30 (sst), n = 47

youngest zircon:402.2 ± 6.5 Ma Frequency

3 30

320. 1 ± 4 Ma

25

2

411.0±4.4 Ma (n = 25), MSWD = 0.12

20 15

1

10 5

age (Ma)

age (Ma) 0

0 600

1000

1400

1800

0

Pocoma

500

1500

2000

Estique Pampa

O 09FB31 (sst), n = 72

P 09FB88 (ignimbrite), 215.85 Ma (n=40)

youngest zircon: 402.6 ± 6.9 Ma Frequency

Frequency

70

50

60 40

215.85 ± 0.82 Ma (n = 39, MSWD = 1.8)

40

30

30

M AN U

SC

413.8 ±1.1 Ma (n = 56, MSWD = 2.6)

50

1000

RI PT

0 200

20

20

10

10

age (Ma)

0

age (Ma)

0

0

500

1000

1500

2000

TE D

youngest zircon: 218.8 ± 3.3 Ma Frequency 218 ± 3.3 Ma

R

AC C

10 5 0 0

500

1000

1500

09FB71 (sst), n = 58 youngest zircon: 234.0 ± 6.1 Ma 234.0 ± 6.1 Ma

245.8 ± 4.2 - 316.4 ± 4.3 Ma (n = 21)

15 10 5

age (Ma)

age (Ma) 0

2000

0

Mal Paso

T

09FB77 (sst), n = 72

S youngest zircon: 230.2 ± 11.4 Ma Frequency

500

1000

238.6 ± 3.3 Ma (n = 7, MSWD = 1.2) 246.2 ± 5.4 - 268.9 ± 4 Ma

16 14 12

09FB82 (2 mica-leucogranite)

242 238

0.0375

234

0.0365

230

0.0355 222

8

0.0345

6

218 214

0.0335

4 2

age (Ma)

0 0

500

1000

1500

2000

2000

226

478.1 ± 3.9 Ma (n = 14, MSWD = 1.8)

10

1500

Mal Paso

0.0385

18

2000

1500

20

444.1 ± 4.6 552.1 ± 5.6 Ma (n = 40)

15

1000

Frequency

EP

20

500

Mal Paso

Estique Pampa

Q 09FB87 (sst), n = 61

25

0

2500

0.0325

210

0.19 0.21 0.23

Mean 206Pb/238U age: 220.7 ± 1.6 (n = 14) MSWD 4.5 0.25 0.27 0.29 =0.31

Figure 6 1000

EP

10

IP T

US

CR

100

M A

TE D

Jurassic undated (07FB65, 09FB92, 09FB149)

100

b

N Rock/Chondrites

Triassic (09FB82) Ordovician (09FB01) Jurassic dated (07FB66, 07FB02)

10

AC C

Rock/Primative Mantle

a

1

1 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Hb Er Tm Yb Lu

Paracas

150 Ma

Jurassic

SC

C 332.5 Ma

H

340.0 Ma

400 Ma

Q R

S

?

e

* Maximum detrital zircon ages from Reimann et al. 2010 **Bahlburg et al. 2009

M 320.1 Ma L 336.5 Ma

359* Ma

N O

Arequipa Massif

218.8 Ma 230.2 Ma 234.0 Ma

297* Ma

B Famatinian arc: 513 - 402 Ma**

Proterozoic

h

P 215.9 Ma T 220 Ma

AC C

EP

407.7 Ma

173.5* Ma

M AN U

TE D

Permian

I 315.8 Ma

a A 350 Ma

g

K 185.8 Ma

233.1 Ma

c 300 Ma

Carboniferous

161.9* Ma 165.0* Ma

RI PT

F 171.2 Ma E 173.3 Ma

D

Estique Pampa/ Mal Paso

f 156.2 Ma J

200 Ma

Devonian

Yamayo Group

El Fiscal/ Pocoma

Yura

Ilo Batholith: 174 - 152 Ma

134.8 Ma

G

250 Ma

P a l e o z o i c

Majes Ocoña

Chala

d

Triassic

Chocolate Formation

Yura Group

M e s o z o i c

Figure 7

402.2 Ma 402.6 Ma

Table 1

Table 1 sample

09FB82

age (Ma)

07FB66

09FB01

171

173

07FB65

09FB92

09FB149

461 Jurassic

Jurassic

Jurassic

latitude

-17.622683

-15.873991

-15.8760109

-13.806983

-15.8787689

-17.9917

-16.43915

longitude

-70.058850 -74.1913452

-74.1922455

-76.294133

-74.1922073

-70.854933

-73.118867

lithology

2MG

DL

RI

G

AI

A

A

72.66

63.17

77.11

75.56

56.45

57.01

69.16

TiO2

0.25

0.66

0.30

0.17

0.99

1.14

0.50

Al2O3

14.38

15.86

10.67

12.26

16.10

16.84

Fe2O3

0.94

5.77

2.06

0.89

8.11

5.07

MnO

0.01

0.08

0.01

0.01

0.17

0.03

MgO

0.09

2.05

0.42

0.10

3.85

3.12

CaO

0.54

0.48

0.16

0.11

4.44

3.85

Na2O

2.33

5.30

2.79

1.13

3.60

4.13

K2O

4.93

4.11

4.94

6.88

4.28

5.79

P2O5

0.10

0.15

0.05

0.02

0.30

LOI

1.47

1.54

0.78

0.98

1.40

total

97.70

99.16

99.28

98.10

99.68

Sc

6.40

14.49

6.55

V

28.76

91.30

37.94

Mn

0.01

0.06

0.01

Co

1.50

6.37

1.10

4.05

6.10

10.03

Zn

12.52

36.09

5.18

Ga

12.96

14.63

7.31

Rb

166.03

79.92

Sr

116.31

113.20 28.66

131.52

212.81

Nb

12.56

8.57

Mo

1.10

1.13

Cs

3.19

0.52

Ba

971.62

736.55

La

34.40

21.57

Ce

65.04

Pr

7.19

Nd

25.97

Sm

47.94

0.01

0.04

0.06

0.05

SC

11.04

1.21

1.66

0.84

99.28

99.26

1.69

26.79

18.27

6.08

14.32

210.89

204.64

26.55

0.00

0.15

0.03

0.01

0.15

23.66

3.09

0.28

4.92

4.09

7.92

7.39

52.14

9.79

14.25

14.00

15.57

14.18

76.64

76.39

163.83

136.96

101.67

156.48

39.21

31.59

272.44

324.50

74.30

16.35

17.82

27.33

28.11

14.81

134.71

236.77

191.73

259.68

149.19

4.22

18.80

6.35

5.52

4.08

0.47

0.80

1.36

0.76

0.73

0.34

1.09

1.35

1.14

0.56

457.04

240.76

918.22

1752.56

1801.72

11.45

40.40

23.77

3.94

33.99

26.74

71.76

52.01

10.33

37.13

TE D

21.29

Zr

3.23

4.52

EP

Y

13.13

0.63

M AN U

Cu

RI PT

SiO2

5.05

2.87

8.96

6.38

2.04

8.28

19.91

11.44

31.26

27.07

12.69

33.94

AC C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

07FB02 215

4.72

4.19

2.58

5.50

5.71

4.43

6.02

0.90

0.88

0.40

0.20

1.14

0.79

1.03

4.26

3.99

2.25

4.04

5.28

5.00

4.88

0.61

0.65

0.39

0.56

0.73

0.73

0.58

3.84

4.96

2.56

3.76

4.92

4.54

3.07

0.76

1.00

0.57

0.73

1.04

0.99

0.59

2.16

3.18

1.94

2.14

2.89

2.89

1.57

Tm

0.32

0.50

0.28

0.37

0.41

0.42

0.23

Yb

2.36

3.18

2.18

2.74

2.89

2.95

1.70

Lu

0.34

0.51

0.32

0.42

0.42

0.49

0.28

Hf

3.57

5.66

3.70

6.10

5.38

6.71

3.88

Ta

0.95

0.54

0.26

1.13

0.40

0.43

0.28

Pb

25.51

10.21

1.53

4.89

6.40

0.94

1.24

Eu Gd Tb Dy Ho Er

ACCEPTED MANUSCRIPT

Table 2

locality

GPS coordinates sample #

AC C

Paracas peninsula -13.919

max. Unit ascribed by previous paleontologic youngest age group youngest zircon stratigraphic author(s) age age

-76.349 09FB152

Ambo Gp

Pocoma

-17.449

-71.381

09FB31

Pocoma

-17.449

-71.381

09FB30

Pocoma informal unit

Ocoña

-16.403

-73.139 09FB132

Ambo Gp / Tarma Fm

El Fiscal

-17.047

-71.723 09FB119 Yamayo Gp / Cabanillas Gp

Chala

-16

El Fiscal

-16.985

-71.748 09FB120 Yamayo Gp / Cabanillas Gp

Majes valley

-16.313

-72.454 09FB125

Mal Paso

-17.609

-70.061

09FB71

upper Machani Fm

Chala

-15.967

-74.041

09FB17

not described nor mapped

Mal Paso

-17.68

-70.087

09FB77

upper Machani Fm

Estique Pampa

-17.57

-70.032

09FB87

uppermost Machani Fm

-74.026

Pocoma informal unit

EP

09FB16

TE D Ambo Gp

Cabanillas Gp

Late 456.7 ± 11 - 491.7 ± Mississippian 407.7 ± 7.7 Ma Early Devonian 4.7 Ma (n = 53) flora

proposed stratigraphic age (see text)

Late Mississippian

—

413.8 ± 1.1 Ma (n = 56, MSWD = 2.7)

402.6 ± 6.9 Ma Early Devonian

Carboniferous?

—

411.0 ± 4.4 Ma (n = 25, MSWD = 0.12)

402.2 ± 6.5 Ma Early Devonian

Carboniferous?

Late Pennsylvania n flora

455 - 506 Ma (n = 32)

Emsian 459- 531 Ma (n = 6) brachiopods

M AN U

Late 335.9 ± 5.3 Ma (n = Mississippian 5, MSWD = 0.22) flora

340.0 ± 6.4 Ma

early Visean

Pennsylvanian

336.5 ± 6.4 Ma

mid-Visean

Pennsylvanian or Permian

332.5 ± 3.3 Ma

late Visean

Late Mississippian

Emsian brachiopods

~1000 - ~1460 Ma (n = 5)

320.1 ± 4.0 Ma

Serpukhovian/ Pennsylvanian

Pennsylvanian or Permian

—

337.1 ± 1.4 Ma (n = 24, MSWD = 1.4)

315.8 ± 5.1 Ma

Serpukhovian/ Pennsylvanian

Pennsylvanian

above a Late 245.8 ± 4.2 - 316.4 ± Mississippian 234.0 ± 6.1 Ma Middle Triassic 4.3 Ma (n = 21) flora

Middle or Late Triassic

SC

Middle or Late Triassic

above a Late 238.6 ± 3.3 Ma (n = Mississippian 230.2 ± 11.4 Ma Middle Triassic 7, MSWD = 1.2) flora

Middle or Late Triassic

—

233.3 ± 1.5 Ma (n = 3, MSWD = 0.022)

RI PT

Middle Triassic (Ladinian)

—

233.1 ± 2.7 Ma

328.2 ± 3.9 - 345.4 ± 218.8 ± 3.3 Ma 2.8 Ma (n = 4)

Late Triassic

Late Triassic

Table 3

locality

GPS coordinates

AC C

paleontologic sample # unit adscribed by previous author(s) age

youngest cluster

youngest zircon

zircon max. age

proposed stratigraphic age

Estique Pampa

-17.569867

-70.031733

09FB88

ignimbrite at base of Chocolate Fm

—

215.85 ± 0.82 Ma

earliest Norian

216 Ma

Yura

-16.2747

-71.730833

09FB63

ash bed at base of Socosani Fm

below Toarcian ammonites

185.8 ± 1.0 Ma

Pliensbachian

186 Ma

-15.8760109

-74.1922455

07FB66

ignimbrite in Chocolate Fm

—

173.3 ± 1.3 Ma

Aalenian

173 Ma

-15.873991

-74.1913452

07FB02

ignimbrite in Chocolate Fm

—

171.22 ± 0.15 Ma

Bajocian

171 Ma

156.2 ± 2.3 Ma

Oxfordian

Oxfordian

134.8 ± 2.2 Ma

Hauterivian

Neocomian

Chala Chala

EP

TE D

Majes valley

-16.183967

-72.466567

09FB124

Chala

-15.871

-74.2481

09FB150

above Callovian and below 152.6 ± 2.3 - 162.9 sandstone in lowermost Labra Fm Tithonian ± 1.5 Ma (n = 6) ammonites

M AN U

sandstone in Chocolate Fm

—

134.8 ± 2.2 - 149.6 ± 1.4 Ma (n = 24)

SC

RI PT