Late Oligocene–early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of southern Chile: Implications for the tectonic development of the North Patagonian Andes

Alfonso Encinas1,†, Andrés Folguera2, Verónica Oliveros1, Lizet De Girolamo Del Mauro1, Francisca Tapia1, Ricardo Riffo1, Francisco Hervé3,4, Kenneth L. Finger5, Victor A. Valencia6, Guido Gianni2, and Orlando Álvarez7,8 1Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción 4030000, Chile 2Instituto de Estudios Andinos “Don Pablo Groeber,” Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires–CONICET, Buenos Aires C1428EHA, Argentina 3Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago 6511228, Chile 4Carrera de Geologia, Universidad Andres Bello, Salvador Sanfuentes 2357, Santiago 837-0211, Chile 5University of California Museum of Paleontology, Berkeley, California 94720, USA 6School of the Environment, Washington State University, Pullman,Washington 99164, USA 7Instituto Geofísico Sismológico “Ing. Fernando Volponi”, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad de San Juan, San Juan 5400, Argentina 8Consejo Nacional de Investigaciones Cientifícas y Técnicas, CONICET, Buenos Aires C1033AAJ, Argentina

ABSTRACT tribute the origin of the Traiguén Basin to a by weak coupling and extension resulting from transient period of slab rollback and vigor- subduction of old oceanic crust. The Chilean margin has been used as the ous asthenospheric wedge circulation that The Chilean margin has not been in a perma- model of an ocean-continent convergent sys- was caused by an increase in trench-normal nent state of compression during its long his- tem dominated by compression and active convergence rate at ca. 26–28 Ma and that tory (e.g., Mpodozis and Ramos, 1990). After mountain building as a consequence of the resulted in a regional event of extension and a rifting event in the Triassic, subduction was strong mechanical coupling between the up- widespread volcanism. reestablished in the Jurassic under extensional per and the lower plates. The Andean Cor- conditions (Mpodozis and Ramos, 1990). As a dillera, however, shows evidence of alternat- INTRODUCTION consequence, a series of interconnected backarc ing phases of compressional and extensional basins developed east of the new volcanic arc deformation. Volcano-sedimentary marine The East Pacific or Chilean margin is the arche- and were filled with marine and continental sed- strata in the Aysén region of southern Chile typal example of an ocean-continent convergent imentary strata (Mpodozis and Ramos, 1990; contribute to an understanding of the causes system (e.g., Dewey and Bird, 1970; Charrier et Mpodozis and Cornejo, 2012). By the early Late of extensional tectonics and crustal thinning al., 2002). Subduction along this margin has been Cretaceous, a shift to a contractional tectonic that occurred in the Andean orogeny be- taking place continuously at least since the Early regime resulted from the accelerated westward cause these deposits constitute the only reli- Jurassic (Mpodozis and Ramos, 1990). A distinc- drift of South America in response to the final able record of submarine suprasubduction tive feature of the Chilean margin is the Andean opening of the South Atlantic Ocean (Russo and volcanism during the Cenozoic in southern Cordillera, the longest and highest active moun- Silver, 1996; Mpodozis and Cornejo, 2012). South America. In order to discern the age tain chain in a subduction setting, which devel- The backarc basins that had developed in the and tectono-sedimentary setting of these oped as a consequence of crustal shortening and previous extensional regime were tectonically strata, referred to as the Traiguén Forma- magmatic addition (Jordan et al., 1983; Charrier inverted, and the Andean Cordillera and its tion, we integrated sedimentology, ichnol- et al., 2002). associated foreland basins started to develop ogy, petrography, geochemistry, structural The Chilean margin differs markedly from the (Mpodozis and Ramos, 1990). geology, foraminiferal micropaleontology, West Pacific margin, which is characterized by a The archetypal contractional “Chilean subduc- and U-Pb geochronology. Our results indi- series of intraoceanic arcs and associated exten- tion mode” described by Uyeda and Kanamori cate that the Traiguén Formation was de- sional marginal basins usually floored by mafic (1979) seems to have predominated during the posited in a deep-marine extensional basin crust (Bartholomew and Tarney, 1984; Mpodozis Late Cretaceous–Holocene interval, whereas the during the late Oligocene–earliest Miocene. and Ramos, 1990). Uyeda and Kanamori (1979) evolution of the Chilean margin during the Juras- The geochemistry and petrography of the related the differences between both margins sic–Early Cretaceous would have resembled the pillow basalts suggest that they formed in a to the relatively young and buoyant oceanic “Marianas subduction mode” (Mpodozis and convergent margin on a thinned crust rather crust that generates strong mechanical coupling Ramos, 1990). However, geologic evidence indi- than at an oceanic spreading center. We at- between the upper and the lower plates and char- cates that the Andean margin has not been in a acterizes the “Chilean subduction mode.” In con- continuous state of compression since the Late †[email protected] trast, the “Marianas subduction mode” is typified Cretaceous, as pulses of major tectonic short-

GSA Bulletin; Month/Month 2015; v. 128; no. X/X; p. 1–17; doi:10.1130/B31303.1; 9 figures; Data Repository item 2015373.

For permission to copy, contact [email protected] 1 © 2015 Geological Society of America Encinas et al. ening have alternated with intervals character- is the only known example of suprasubduction the North Patagonian Andes and on the eastern ized by moderate shortening or extension (e.g., submarine volcanism in southern South Amer- Chonos Islands (Pankhurst et al., 1999); and Mpodozis and Ramos, 1990; Martinod et al., ica during the Cenozoic. (3) Mesozoic and Cenozoic sedimentary and 2010). In addition, despite rather uniform con- The present contribution investigates the tec- volcanic rocks, which crop out principally on vergence conditions along the Chilean margin tono-sedimentary setting of the Traiguén For- the eastern flank of the Andes and the Precor- for the same periods of time, the tectonic evolu- mation, integrating information from sedimen- dillera (Suárez and De La Cruz., 2000; Ramos tion of the Andean Cordillera shows remarkable tology, ichnology, petrography, geochemistry, and Ghiglione, 2008). Less extensive are late latitudinal differences. Throughout most of the structural geology, foraminiferal micropaleon- Oligocene–early Miocene volcano-sedimentary Cenozoic, the Andes of southern Peru and north- tology, and U-Pb geochronology. Based on our rocks of the Traiguén Formation exposed in a ern Chile (~15°S–27°S) have been subjected to findings and previous geologic studies in this N-S belt between the metamorphic basement ongoing compression (e.g., Oncken et al., 2006, region, we delve into the causes of subsidence and the North Patagonian Batholith (Espinoza and references therein), while the Andes of cen- and submarine volcanism in the Traiguén Basin and Fuenzalida, 1971; Bobenrieth et al., 1983; tral and southern Chile (~33°S–46°S) have had and explore the relation of this event to Ceno- Hervé et al., 1995) and late Oligocene–Miocene alternating episodes of compressional and exten- zoic plate kinematics along the Chilean margin. marine rocks that occur in small outcrops across sional tectonics (e.g., Jordan et al., 2001; Charrier the forearc, the North Patagonian Andes, and the et al., 2002; Orts et al., 2012; García Morabito GEOLOGIC SETTING Precordillera (De Vries et al., 1984; Frassinetti and Ramos, 2012). The causes of variations in and Covacevich, 1999; Encinas et al., 2014, and the tectonic regime both in time and space dur- The Traiguén Formation (~44°S–46°S) is references therein). ing Andean history are still not well understood located on the forearc and western flank of and have been ascribed to several different the North Patagonian Andes (Figs. 1 and 2). Previous Studies on the Traiguén causes, including changes in the convergence This segment of the Andean Cordillera extends Formation rate between the oceanic and continental plates between ~38°S and 47°S and presents signifi- (e.g., Pardo-Casas and Molnar, 1987), variations cant differences when compared with the Cen- The Traiguén Formation is a marine volcano- in the absolute westward motion of South Amer- tral Andes to the north, including lower eleva- sedimentary unit that crops out on the islands ica (e.g., Silver et al., 1998), changes in interplate tion (1–2 km) and width (~300 km), reduced and part of the mainland area of the Aysén coupling (Lamb and Davis, 2003; Yáñez and crustal thickness (~40 km), and less shorten- region of southern Chile between ~44°S and Cembrano, 2004), and modifications of the dip ing (12–25 km) (Mpodozis and Ramos, 1990; 46°S (Figs. 1 and 2; Espinoza and Fuenzalida, angle of the subducting slab (Jordan et al., 1983; Hervé, 1994; Ramos and Ghiglione, 2008; Orts 1971; Bobenrieth et al., 1983; Hervé et al., Martinod et al., 2010). et al., 2012). Also distinctive of the North Pata- 1995). This unit was defined by Espinoza and The volcano-sedimentary marine deposits gonian Andes is an ~1000 km, trench-parallel, Fuenzalida (1971), who designated Traiguén in the Aysén region of southern Chile (~45°S; dextral strike-slip fault system referred to as the Island as its type locality. Subsequently, Boben- Fig. 1) are pertinent to our quest to better under- Liquiñe-Ofqui fault zone (Hervé 1994; Cem- rieth et al. (1983) included in the Traiguén For- stand what caused the alternating intervals of brano et al., 2002). An active spreading center, mation the volcano-sedimentary rocks that crop compression and extension that characterize the Chile Rise, is currently subducting at ~46°S out in the northern part of the study area (Mag- the tectonic evolution of some segments of the and defines the Chile triple junction between the dalena Island and neighboring localities), which Andes during the Cenozoic. These strata crop Nazca, South American, and Antarctic plates Fuenzalida and Etchart (1975) had previously out in the present forearc of the cited region (Cande and Leslie, 1986). described as the Puyuhuapi Formation. and constitute a succession of basaltic pillow Three N-S–trending physiographic units The Traiguén Formation consists of inter- lavas interbedded with marine sandstones and characterize the Chilean margin between ~33°S bedded pillow basalts, tuffs, breccias, sand- siltstones ascribed to the Traiguén Formation and 44°S: the Coastal Cordillera, the Central stones, and shales (Espinoza and Fuenzalida, (Espinoza and Fuenzalida, 1971; Hervé et al., Depression, and the Main Andean Cordillera. 1971; Bobenrieth et al., 1983; Hervé et al., 1995; Silva, 2003). This unit was originally The Coastal Cordillera and the Central Depres- 1995). Rocks of this unit in the northern part of thought to be Early Cretaceous (Fuenzalida and sion are not well defined in the study area, the study area are metamorphosed to greenschist Etchart, 1975), but subsequent studies favor the where the latter gradually narrows to the south and amphibolite grades (Hervé et al., 1995; Silva, Cenozoic, although a refined age has remained and disappears at ~46°S–47°S (Cembrano et al., 2003). Although the Traiguén Formation overlies elusive because this formation has been ascribed 2002; Silva, 2003). The Chonos Islands, with schists of the Paleozoic–Triassic metamorphic to the Miocene (Céspedes, 1975), late Oligocene elevations up to ~1000 m, are remnants of the basement at Magdalena Island (Bobenrieth et al., (Hervé et al., 2001), and Eocene–middle Mio- Coastal Cordillera after the deep glacial incision 1983; Hervé et al., 1995), its actual stratigraphic cene (Silva et al., 2003). The origin of the Trai- of this range during the Pleistocene and subse- thickness has not been determined because its guén Basin is also debated, having been attrib- quent flooding of the valleys as sea level rose in exposures are limited to small coastal outcrops uted to extensional tectonics (Bartholomew and the Holocene. In this region, fjords and islands among the lush vegetation that characterizes Tarney, 1984; Jordan et al., 2001), or to strike-slip characterize the Central Depression. this area. The Traiguén Formation is intruded by tectonics (Hervé et al., 1995). Submarine volca- The most extensive rocks in the study area numerous basic aphanitic and doleritic dikes and nism in the Traiguén Basin during the Cenozoic are (1) Paleozoic–Triassic metamorphic rocks, locally by plutonic bodies of gabbroic to grano- is significant because it indicates important which crop out on most of the Chonos Islands dioritic composition (Hervé et al., 1995). crustal thinning (Mpodozis and Cornejo, 2012) and also on the western flank of the Andes in The age of the Traiguén Formation has been and refutes the notion of a “Chilean subduction the southern part of the study area (Hervé et al., a matter of debate. Stiefel (1970) tentatively mode” in a permanent state of contraction as 2003); (2) Jurassic, Cretaceous, Eocene, and assigned rocks belonging to this unit to the envisaged by Uyeda and Kanamori (1979). Also Miocene plutonic rocks of the North Patago- Triassic–Early Tertiary interval. Fuenzalida singular is the fact that the Traiguén Formation nian Batholith, which crop out along most of and Etchart (1975) considered it to be Early

2 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile

73ºW 72ºW 71ºW 75ºW 74ºW74º La Junta

Melimoyu -44ºS

Archipiélago de los Chonos Puyuhuapi N IISLASLA MAGDMAGDAALENLENA aleda

Isla Ipún Puerto Cisnes Canal Mor

-45ºS Ji Maca A

N Puerto Aysén

É cean Coyhaique

Isla Clemente AIGU TR

A ARGENTIN SL I Península aci c O Gallegos Hudson P Balmaceda

-46ºS cean

a-Buenos Aires Paci c O Península rer OFZ al Car e de Taitao L

Lago Gener Chil

Argentina Puerto Guadal Campos de Hielo Norte Península Golfo de -47ºS Tres Montes Penas 0 50 Km Cochrane

Pliocene- Holocene glacial, uvial and marine deposits Lower Cretaceous volcanic and sedimentary rocks

Pleistocene-Holocene volcanic rocks Jurassic volcanic and sedimentary rocks

Miocene-Pliocene Taitao Ophiolite Miocene plutonic rocks

Miocene-Pliocene continental sedimentary rocks Eocene plutonic rocks

Upper Oligocene - Miocene marine sedimentary rocks (Ipún, G. Chaicayán, Guadal and Vargas Formations) Upper Cretaceous plutonic rocks

Upper Oligocene- Lower Miocene marine Lower Cretaceous plutonic rocks volcano-sedimentary rocks (Traiguén Formation)

Paleogene volcanic and sedimentary rocks Jurassic plutonic rocks

Lower Oligocene volcanic breccia of Mitahues Island Paleozoic-Triassic metamorphic rocks

Eocene - Miocene basaltic rocks Strike-slip faults Border

Figure 1. Regional geologic map. Inset is the area shown in Figure 2. Figure is after Segemar (1994, 1995); Pankhurst et al. (1999); Thomson (2002); Sernageomín (2003); Silva (2003). LOFZ—Liquiñe-Ofqui fault zone.

Geological Society of America Bulletin, v. 126, no. XX/XX 3 Encinas et al.

73°45’W 73°30’W exposures of the Traiguén Formation are lim- sharp and sometimes show load and flame struc- ited to short coastal cliffs. The basal and upper tures and pseudonodules. Mineralogical compo- contacts of this unit at its type locality are not sition of this facies is similar to that of sandstone Isla Mitahues visible. The sedimentary rocks of the Traiguén in facies T. Isla Formation comprise interbeds of sandstone and Quemada 40 siltstone, with minor conglomerate and breccia. Interpretation Las They are locally interrupted by volcanic rocks, Structureless sandstones are commonly inter- 80 Mentas -45º30’S principally pillow basalts. We identified five preted as deposited by turbidity currents based

N sedimentary facies, as described next. on the common association of this facies with

Isla Luz É

U classic turbidites (Posamentier and Walker,

G

Isla I Rhythmically Interbedded Sandstone and 2006); however, other authors have postulated Humos Isla Ana A Siltstone (T) alternative depositional mechanisms, such as

R

T

sandy debris flows (see Weimer and Slatt, 2007,

A

L Description and references therein). The presence of load

S I This facies is composed of medium- to very and flame structures and pseudonodules indi- 20 OFZ coarse-grained sandstone rhythmically interbed- cates rapid deposition. L Isla Rojas ded with siltstone (Fig. 3A). Beds are typically -45º45’S centimeters to decimeters thick, and they form Siltstone and Very Fine-Grained Sandstone partial Bouma cycles where sandstone predomi- (Sls) Isla McPherson nates and is locally amalgamated. Sandstone 15 typically shows Tb-c (parallel-convolute or, Description more rarely, ripple cross-lamination; Fig. 3B), This facies is composed of dark-gray siltstone

25 Km Ta-b (massive-parallel), Tb (parallel), or Ta-c and very fine-grained sandstone that locally (massive-convolute) divisions. Sandstone bases form intervals a few meters thick and show rela- Figure 2. Map of study area showing geo- are typically sharp, but load and flame structures tively well-defined stratification, parallel lami- logic units and sample localities. See Figure occur at the contact with some siltstone beds, nation, and, more rarely, asymmetrical ripple 1 for key to the geologic units. LOFZ— and some sandstone strata are crosscut by injec- marks. This facies is locally interbedded with Liquiñe-Ofqui fault zone. tion of the underlying siltstone (Fig. 3C). Fossils classic turbidites (T) and conglomerates (Cg). are generally absent from these facies, although some of the siltstones contain foraminifera and Interpretation Cretaceous based on the occurrence of poorly fragments of indeterminate macrofossils. Sand- The small sizes of the outcrops in which we preserved marine fossils and on the erroneous stone is primarily composed of quartz, plagio- observed this facies prevent a refined interpreta- assumption that it was intruded by Late Creta- clase, and biotite. tion, but the presence of interbedded turbidites ceous plutonic rocks, which are now known to indicates that it was probably deposited by distal be Miocene in age (Pankhurst et al., 1999). Cés- Interpretation turbidity currents and hemipelagic fallout. pedes (1975) suggested a Miocene age for the The occurrence of rhythmically interbedded Traiguén Formation based on planktonic fora- sandstone and siltstone forming Bouma cycles Conglomerate (Cg) minifera. Hervé et al. (1995) dated sedimentary is characteristic of classic turbidites (Bouma, and volcanic rocks of the Traiguén Formation at 1962; Posamentier and Walker, 2006). The com- Description Magdalena Island with the Rb-Sr method and mon presence of convolute lamination, as well This facies consists of conglomerate beds that obtained two imprecise ages of 20 ± 28 Ma and as the occurrence of load and flame structures are typically decimeters thick and usually inter- 20 ± 26 Ma, but they noted that these most likely and siltstone injections crosscutting the overly- bedded with turbidites, sandstones, siltstones, are the ages of metamorphism. Hervé et al. ing beds, indicate rapid deposition of sediment or lava flows. Basal contacts are sharp or ero- (2001) dated detrital zircons from a sandstone that caused trapping of pore fluid and deforma- sive. The conglomerates are clast-supported and sampled from Traiguén Island and obtained a tion of primary structures (Posamentier and contain moderately to well-sorted, angular to 26.0 ± 1.5 Ma U-Pb sensitive high-resolution Walker, 2006). subrounded clasts typically centimeters to deci- ion microprobe (SHRIMP) maximum age for meters in size. Its clasts are mostly pyroclastic this unit. Silva et al. (2003) signaled, however, Massive Sandstone (Ms) lithics and more rarely andesite, basalt, or intra- that the basal Traiguén Formation could be as clasts of sandstone or siltstone. Typically, the old as Eocene based on the 38–15 Ma K/Ar ages Description conglomerate is normally graded with subtle obtained from a suite of dikes that they consid- This facies consists of medium- to coarse- stratification, and it often grades upward into ered to be contemporaneous with the Traiguén grained massive (structureless) sandstone that tuffaceous sandstone. Formation volcanic rocks. may locally display zones of indistinct parallel lamination. Individual beds are up to a meter Interpretation SEDIMENTARY FACIES OF THE thick and locally form amalgamated stacks Normal grading indicates transport by turbu- TRAIGUÉN FORMATION typically separated by thin partings of parallel- lent flows where grains moved freely relative laminated sandstones. This facies is interbedded to one another. Clast-supported conglomerates Fieldwork was carried out on Traiguén Island with classic turbidites (facies T), conglomerates present in deep-marine successions are typically and neighboring islands (Fig. 2). Due to the (facies Cg), or siltstones and very fine sand- interpreted as having been deposited by turbid- thick vegetation that characterizes the area, stones (facies Sil). Basal contacts are commonly ity currents (see Posamentier and Walker, 2006).

4 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile

A B

C D

E F

Figure 3. Characteristic facies and trace fossils of the sedimentary rocks of the Traiguén Formation. (A) Rhythmically inter- bedded sandstone and siltstone (facies T) at Las Mentas (Traiguén Island). (B) Detail of turbidite showing well-developed con- volute lamination at the top. McPherson Island. (C) Turbidites showing sandstone strata crosscut by upward injection of the underlying siltstone. McPherson Island. (D) Intraformation breccia (facies Sl) with folded stratified block made of siltstone at Las Mentas (Traiguén Island). (E) Synsedimentary folded sandstone and siltstone. The fold’s structure is partially delineated but difficult to follow due to intense deformation. McPherson Island. (F) Phycosiphon incertum in siltstone bed at Las Mentas (Traiguén Island). Scale is 1 cm.

Geological Society of America Bulletin, v. 126, no. XX/XX 5 Encinas et al.

The occurrence of rip-up clasts is evidence of 4D). The pillows are well preserved, and the lowtails” attributed to water interaction and turbulent scouring of the substrate immediately majority of them are <1 m in diameter. Some of undercooling during the final stages of crystal prior to deposition. The dominance of pyroclas- them show a thin aphanitic rim that is slightly growth after the emplacement of the lava flows tic lithics in these facies suggests pyroclastic more altered than the rest of the pillow; these (Lofgren et al., 1974). flows transformed into water-supported turbid- may be relicts of the glassy outer shell character- All studied samples and most of the volcanic ity flows (see Cas and Wright, 1991). istic of water-magma interaction (Németh and rocks of the Traiguén Formation are altered to Martin, 2007). The pillows also exhibit different some extent, but even though the alteration can Slumps (Sl) types of surface features related to lava displace- be penetrative, their igneous textures and some ment or water-magma interaction (Németh and original minerals are preserved. Pumpellyite and Description Martin, 2007), such as symmetrical spreading prehnite are the most abundant secondary min- This facies is made up of chaotic, unbed- cracks (Fig. 4C), contraction cracks (Fig. 4D), eral phases, as indicated by DRX (X-ray diffrac- ded units that can be subdivided into intrafor- and ropy wrinkles (Fig. 4B). Occasionally, flame tion) analyses (Tapia, 2015); they occur mainly mational breccia and synsedimentary folded structures occur in the contact between basaltic as the infilling of amygdules, in which they can strata. Intraformational breccia is massive, very flows and sedimentary rocks, evidencing the be intergrown with calcite, quartz, or chlorite poorly sorted, and several meters thick. Clasts load effect of the volcanic rocks on wet sedi- and form veinlets and aggregates replacing the are angular to subrounded, range in size from ments. Massive, columnar-jointed flows lacking original interpillow material. Plagioclase phe- centimeters to several meters, are randomly ori- pillows occur locally. Hyaloclastites are rarer nocrysts are partially replaced by sericite, clays, ented, and are typically siltstone, tuff, and more than lava flows and are characterized by frac- and minor chlorite. Seen in hand specimen, the rarely basalt. Stratified blocks of siltstone, some tured basalt fragments with curviplanar surfaces milky luster of the feldspar phenocrysts suggests of them folded (Fig. 3D), or interbedded pyro- and a glass-rich matrix partially to completely the Ca-Na exchange characteristic of the albiti- clastic breccia and tuff are common. Some of replaced by palagonite that gives the rock its zation processes of low-grade metamorphism the large siltstone clasts contain benthic fora- characteristic greenish-brown color. Pyroclastic (Frey and Robinson, 1999). Clinopyroxenes are minifera indicative of bathyal depths. Synsedi- breccias and tuffs, often of more intermediate completely to partially replaced by chlorite and mentary folded sandstone and siltstone strata composition than the effusive rocks, are locally minor quartz or calcite, whereas olivines are (Fig. 3E) form intervals a few meters thick that interbedded with lavas and sedimentary rocks. completely replaced by iddingsite and bowling- overlie and underlie flat-bedded successions. Dikes and sills intrude the volcanic and sedi- ite. Plagioclase microliths in the groundmass are mentary strata. Some of the dikes with pillow altered to sericite and clay, and the glass is devit- Interpretation structures are connected to the lava flows, which rified to aggregates of chlorite/smectite, titanite, As noted by Posamentier and Walker (2006), indicates their contemporaneity. and Fe-oxyhydroxides. At the outcrop scale, the slumps consisting of intraformational breccias Volcanic rocks assigned to the Traiguén For- palagonitization process is inferred from the with stratified blocks were probably derived mation by Bobenrieth et al. (1983) also occur on brownish green color of the devitrified matrix of from collapse of submarine channel walls. Large the islands west and north of Traiguén Island, hyaloclastites. However, palagonite is very diffi- blocks were probably rafted on top of mass- including Humos, Luz, and Mitahues (Fig. cult to recognize in petrographic sections, and it transport complexes that traveled short distances 2). The rocks at these localities include basal- is thought to occur only within the groundmass, because the transport of stratified megaclasts in a tic lavas and volcanic breccia with abundant where it is almost indistinguishable from pure turbulent medium or across long distances would basaltic dikes and sills. These volcanics are not smectite aggregates. probably result in their disintegration (Posa- interbedded with sedimentary rocks, nor do they Table DR2 lists the major- and trace-element mentier and Walker, 2006). The folded nature show features indicative of subaqueous deposi- contents in the studied samples along with the of some of the siltstone megaclasts indicates tion. The U-Pb age of 32.2 Ma that we obtained isotopic ratios of Sr, Nd, and Pb (see footnote 1). that they were unconsolidated, and the pres- for sample Isla Mitahues 1.1 (see following) The analyzed rocks would plot in the basaltic- ence of bathyal foraminifera suggests that they suggests that they are older than the submarine trachyandesite field of a classification diagram were eroded from deposits of deep-water origin. lavas on Traiguén and its nearby islands. of Le Maitre (1989) based on their alkalis (N2O,

Sandstone and siltstone strata with synsedimen- The petrography of the Traiguén Formation K2O) and silica (SiO2) contents. However, when tary folding were formed by sliding of uncon- lavas described here is detailed in Table DR1.1 plotted in diagrams based on the composition solidated turbidites that were previously depos- The rocks are porphyritic basalts with up to of more immobile elements (high field strength ited in the basin and subsequently displaced 20% phenocrysts of plagioclase, clinopyroxene, elements [HFSEs]), such as the Nb/Y versus downslope (Posamentier and Walker, 2006). or mafic minerals completely altered to second- Zr/Ti diagram for altered basalts (Winchester ary phases and Fe-Ti oxides. The groundmass and Floyd, 1977), all of the samples plot in the PETROGRAPHY AND GEOCHEMISTRY has an intersertal or intergranular texture with area of subalkaline basalts. This agrees with OF THE VOLCANIC ROCKS primary plagioclase and clinopyroxene and the observed primary mineralogy devoid of secondary devitrified glass. The original mafic an alkali-rich phase. The alkalis, Fe, and MgO Lava flows and related volcanic and subvol- phenocrysts most likely were olivine and clino- contents of most samples correspond to the calc- canic rocks crop out in several localities along pyroxene, with the latter more abundant. Phe- alkaline series in the AFM (K2O+N2O-FeO*- the coast of Traiguén and neighboring islands nocrysts exhibit skeletal textures and “swal- Mg) classification of Irvine and Baragar (1971); (Figs. 1 and 2). The lava flows are locally inter- only three samples plot over the line separat- bedded with sedimentary rocks (Fig. 4A) and 1GSA Data Repository item 2015373, petrography ing the calc-alkaline and tholeiitic fields. Since exhibit features that are commonly associated of volcanic and volcaniclastic rocks, elemental and iso- there is little variation on the SiO content in the topic composition of basalts, U-Pb LA-ICPMS zircon 2 with subaqueous volcanism, such as the pillows data, and foraminiferal data from the Traiguén Forma- analyzed samples, most of the other elements do observed in outcrops on the Rojas, Traiguén, tion, is available at http://www​.geosociety​.org/pubs​ ​ not reflect trends when compared to silica abun- Ana, and Acuao Islands (Figs. 4A, 4B, 4C, and /ft2015.htm or by request to [email protected]. dances; however, elements such as Ca, Mg, Al,

6 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile

A B

lv

br sd pl

C D

Figure 4. Volcanic rocks of the Traiguén Formation. (A) Sandstone (sd) and breccia (br) with basaltic clasts onlapping lava- flow deposits with well-developed pillows (pl). The three rocks are covered by a blocky lava flow (lv); Rojas Island. (B) Large lava flow (megapillow?) surrounded by pillows up to 1 m in size. The arrow on the left flank of the outcrop points to surface ridges interpreted as ropy wrinkles; Traiguén Island. (C) Cross section of several pillows showing light-colored rims inter- preted as altered, quenched surfaces of the flow. Symmetrical spreading cracks are visible in the pillow outer part; Ana Island. (D) Contraction cracks around pillows; Acuao Island.

Ni, and Cr decrease with increasing Zr, whereas ied rocks is rather homogeneous, with 87Sr/86Sr Although the poor preservation of trace fos- Na, K, P, Ti, Rb, Ba, Sr, Cu, rare earth elements ranging 0.70467–0.705943, 143Nd/144Nd ranging sils precludes a refined interpretation, the asso- (REEs), and HFSEs increase their abundance 0.512904–0.512983, and 206Pb/204Pb, 207Pb/204Pb, ciation of Phycosiphon incertum and Chon- with increasing Zr. If Zr is taken as a differen- and 208Pb/204Pb ranging 18.482–18.577, 15.586– drites isp. in the silty intervals is ascribed to tiation indicator, the evolution of the volcanic 15.626, and 38.343–38.468, respectively (Figs. the Zoophycos ichnofacies, which is character- rocks would have been controlled by fractional 5C and 5D). ized by low oxygen levels associated with high crystallization of olivine, clinopyroxene, and organic detritus in quiet-water settings, and plagioclase. TRACE FOSSILS which is typical of outer-shelf to slope settings The primitive mantle–normalized values for (Frey and Pemberton, 1984). In the case of the large ion lithophile elements (LILEs), HFSEs, Trace fossils in the sedimentary strata of the Traiguén Formation, the Zoophycos ichnofacies and REEs show a pattern of decreasing content Traiguén Formation are scarce and generally characterizes interturbidite intervals of coloni- toward more compatible elements (Fig. 5A) show poor preservation. Thalassinoides isp. zation. Monospecific suites of Thalassinoides along with a marked Nb-Ta negative anomaly forms monospecific suites characterized by isp. forming extensive horizontal networks (relative to LILEs). Chondrite-normalized REE extensive horizontal networks at the top of some characterize the Skolithos ichnofacies, which values show low abundances, flat patterns, and sandstone strata. Taenidium? isp. and Scolicia is typical of high-energy shallow-marine envi- no Eu anomalies (Fig. 5B), which are consis- isp. are also present, but rare, in sandstone. Phy- ronments, but is also common in deep-water tent with the less-differentiated character of the cosiphon incertum (Fig. 3F) and Chondrites isp. settings where it reflects local environmental basalts. The isotopic composition of the stud- occur in some siltstone beds. conditions such as high energy, high levels of

Geological Society of America Bulletin, v. 126, no. XX/XX 7 Encinas et al.

AB 100

le 100 nt Ma

ve Chondrite /P

miti 10 ck ri 10 /P Ro ck Ro

1 1 Rb Th Nb K Ce Pr P Zr Eu Dy Yb Ce Nd Sm Gd Dy Er Yb Cs Ba U Ta La Pb Sr Nd Sm Ti Y Lu La Pr Pm Eu Tb Ho Tm Lu 16.0 )

Ga

0.5135 55 UCC DMM Seawater alteration C (4. D 15.8 chron eo

G )

Mantle Ar Pb BSE EMII (1.77 Ga Nd NHRL

0.5130 204 LCC

/ 15.6 144

ra Pb y MORB Nd/

207 EMI 143 BSE 15.4 0.5125 DMM 15.2 EMI 0.5120 15.0 0.7010.702 0.7030.704 0.7050.706 0.7070.708 0.709 15 16 17 18 19 20 21 22 87Sr/86Sr 206Pb/204Pb 600 Hf/3 Ti/V=10 ARC Ti/V=20 A = N-MORB EFCFB 500 VAT B = E-MORB MORB

C = OIB (Rift) 400 Ti/V=50 A D = Arc-basalts

V 300 B

200 Ti/V=100 D

100 C

0 0510 15 20 25 Th Ta Ti/1000 Traiguén Fm basalts (this work) Abanico Fm (Muñoz et al. 2006) Neuquén back-arc volcanism (Kay and Copeland 2006) Coastal magmatic belt (Muñoz et al. 2000) Traiguén Fm basalts (Silva 2003) Ventana Fm (Kay et al. 2007)

Figure 5. Geochemical plots of the studied rocks and other relevant volcanic units from central and south-central Chile and Argentina. (A) Primitive mantle–normalized contents of trace elements (normalizing values from Sun and McDonough, 1989). (B) Chondrite-normalized contents of rare earth elements (REEs; normalizing values from Sun and McDonough, 1989). (C) 143Nd/144Nd vs. 87Sr/86Sr diagram showing mantle domains (after Zinderl and Hart, 1986). (D) 207Pb/204Pb vs. 206Pb/204Pb diagram showing mantle domains (after Zinderl and Hart, 1986). (E) Tectonic discrimination diagram for basalts (Wood, 1980). (F) Tectonic discrimination diagram for basalts (Shervais, 1982). N-, E-MORB—normal, enriched mid-ocean ridge basalts; OIB—oceanic-island basalts; VAT— volcanic arc tholeiites; CFB—continental flood basalts; DMM—depleted mantle; BSE—bulk silicate earth; EMI- EMII—enriched mantle I and II; UCC-LCC—upper-lower continental crust; ARC—arc rocks. The rocks from the Coastal magmatic belt (Muñoz et al., 2000), the Abanico Formation (Muñoz et al., 2006), and the Magdalena Island basalts (Silva et al., 2003; this work) represent the arc domain (i.e., magmatism proximal to the plate mar- gin). The rocks from the Neuquén Basin backarc volcanism (Kay and Copeland, 2006) and the Ventana Formation (Kay et al., 2007) represent the backarc domain (i.e., the domain distal to the plate margin).

8 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile oxygenation, sandy substrate, and an abundance U and Th were monitored by comparison to of suspended organic particles (Buatois and NIST 610 trace-element glass standard. Two López-Angriman, 1992). The Skolithos ichno- natural zircon standards were used: Plesovice, facies in the Traiguén Formation appears to be with an age of 337.13 ± 0.37 Ma (Sláma et al., associated with the rapid deposition of sand. 2008), and FC-1, with an age of 1099.0 ± 1.4 Ma (Paces and Miller, 1993). U-Pb ages were FORAMINIFERA calculated and plots were generated using Iso- plot (Ludwig, 2003; Fig. 7). The U-Pb zircon A single rock sample was recovered by Cés- age errors for a set of analyses are reported pedes (1975) from the northwestern part of Figure 6. Large benthic agglutinated fora- using the quadratic sum of the analytical error Traiguén Island at Las Mentas (Fig. 2). The minifera in rock samples from Las Mentas plus the total systematic error (Valencia et al., foraminifera in that sample (Sh-159) were (Traiguén Island). Left is Ammodiscus sp.; 2005). Results are presented in Table DR5 (see studied by Mario Ernst (in Céspedes, 1975), right is Bathysiphon sp. footnote 1). who identified 23 benthic and two planktic species among the mostly poorly preserved Results specimens and concluded that the assemblage DR4 [see footnote 1]). The specimens all show was Miocene and possibly bathyal (Tables some degree of diagenetic alteration that mask Volcanic rock samples are characterized by DR3 and DR4 [see footnote 1]). Our revision potentially species-diagnostic features. These clear pink euhedral zircon crystals, whereas of his foraminifera list indicates that about half two benthic agglutinant genera, especially when sandstones included heterogeneous zircon pop- of the benthic taxa have upper-depth limits on represented by such large specimens, are most ulations presenting euhedral-subeuhedral to the slope. Bathysiphon eocenicus and Melonis characteristic of upper-middle bathyal to lower- subrounded grains that ranged from clear pink pompilioides suggest that deposition may have bathyal water depths but do not have biostrati- to dark red-brown. CL images showed a great been at lower bathyal (>2000 m) depths. Osorio graphic value in this study (Finger, 2013). variation from single igneous-related oscil- (1989) reexamined this material and recorded latory growth patterns to complex patterns. eight benthic taxa identified only to genus, U-Pb GEOCHRONOLOGY Almost all of the grains are characterized by plus 16 planktic species. Osorio assigned an Th/U ratios >0.1 (Table DR5 [see footnote 1]). upper-lower to lower-middle Miocene age Methodology Zircons from sample Las Mentas 1.1 (lapilli based on the concurrence of planktic species tuff, Table DR6 [see footnote 1]) from Traiguén that had their first or last appearances in zone In order to determine the age of deposition Island are dominantly late Oligocene and early N8 (Tables DR3 and DR4 [see footnote 1]). He and sedimentary provenance of the Traiguén Miocene. The calculated TuffZirc age (Ludwig, also interpreted the depth of the assemblage as Formation, we ran U-Pb analyses on 953 zir- 2003) yielded a 206Pb/238U age of 23.4 ± 0.3Ma upper bathyal because of its abundant planktic con grains extracted from one pyroclastic (n = 77, 2s; Fig. 7). Older zircon ages of ca. 28 specimens and agglutinated benthic specimens, rock, one volcanic rock, and six sandstones Ma are also present but in minor proportion. and the absence of miliolids. (for location of samples, see Fig. 2). Heavy A volcanic breccia (Table DR6 [see foot- According to current biostratigraphic data mineral concentrates of the <350 µm fraction note 1]) was collected in the Mitahues Island (BouDagher-Fadel, 2015), the Sh-159 plank- from eight rock samples were separated using (Isla Mitahues 1.1), and over 96% of the 122 tic foraminiferal assemblage listed by Osorio traditional techniques at ZirChron LLC (Tuc- zircon analyses yielded an early Oligocene (1989) indicates disparate ages. The concurrent son, Arizona). Zircons from the nonmagnetic age (206Pb/238U TuffZirc age of 32.2 ± 0.4 Ma, range of most of the assemblage is early middle fraction were mounted on a 1-in.-diameter 2s; Fig. 7). Miocene zone N11, which includes the first (~2.5-cm-diameter) epoxy puck and then pol- Detrital zircons from the Las Mentas 3.5 appearance of Globigerina bulbosa and the last ished to expose the granular surface. After sandstone showed a major peak at ca. 23 Ma appearance of Globorotalia peripheroacuta, but cathodoluminescence (CL) imaging at the Uni- and smaller peaks in the Cretaceous, Paleozoic, his assemblage includes Globigerina binaiensis versity of Idaho, the laser ablation–inductively and Precambrian (Fig. 7). The Las Mentas 12.1 and Globigerina winkleri, both of which have coupled plasma–mass spectrometry (LA-ICP- sandstone was also characterized by a young last appearance datums in early Miocene zone MS) U-Pb analyses were conducted at Wash- peak (ca. 26 Ma maximum depositional age) and N5, and Globigerina tripartite, which has an ington State University (Pullman, Washington) older peaks of 127, 280, and 1127 Ma (Fig. 7). Eocene–Oligocene range. using a New Wave Nd:YAG ultraviolet 213 nm In contrast to the Las Mentas samples, the Our thin sections of seven rock samples were laser coupled to a Thermo Scientific Element Isla Ana 1.1 sandstone lacked an early Mio- devoid of foraminifera. To isolate foraminifera 2 single-collector, double-focusing, magnetic cene population (Fig. 7), although the sample for study, we processed seven siltstone samples sector ICP-MS. Operating procedures and included relatively similar Cretaceous to Pre- from different stratigraphic sections of the Trai- parameters are similar to those described in cambrian peaks of 127, 287, 476, and 1085 Ma guén Formation in the study area, employing a Chang et al. (2006). Laser spot size and repeti- and older Mesoproterozoic and Paleoprotero- variety of methods. Soaking in H2O2 and HCl tion rate were 30 µm and 10 Hz, respectively. zoic zircon ages. failed to disaggregate the rocks, but immersion Each analysis consisted of a short blank analy- Sandstone samples from Isla Rojas (1.1 and in 50% HF for 20 minutes succeeded in freeing sis followed by 250 sweeps through masses 6.5) and McPherson 2.2 showed similar zir- some specimens. In addition, we chiselled out 202, 204, 206, 207, 208, 232, 235, and 238, con populations as those in the other samples, a few large specimens with a minimal amount taking ~30 s. Time-independent fractionation although Isla Rojas 1.1 and McPherson 2.2 had of matrix from three rock samples. Our efforts was corrected by normalizing U/Pb and Pb/ rare late Oligocene 25–26 Ma age zircons (n = 2 enabled us to recognize only Ammodiscus sp. Pb ratios of the unknowns to the zircon stan- and 1), and Isla Rojas 6.5 lacked the early Mio- and Bathysiphon sp. (Fig. 6; Tables DR3 and dards (Chang et al., 2006). Concentrations of cene detrital zircon component (Fig. 7).

Geological Society of America Bulletin, v. 126, no. XX/XX 9 Encinas et al.

60 23.4 Ma 60 LAS MENTAS 1.1 23.2 Ma LAS MENTAS 3.5 50 23.4 +/- 0.3 Ma 50 23.2+/- 0.4 Ma n=77 n=39 40 2 σ 40 2 σ 30 30

20 20

10 10 292 Ma 0 0

18 20 22 24 26 28 30 32 34 36 0 400 800 1200 1600

7 14 127 Ma LAS MENTAS 12.1 ISLA ANA 1.1 26 Ma 6 287 Ma 12 26.2 +2.2/- 2.7 Ma Max Depo ~127 Ma 456-477 Ma 1086 Ma n=7 5 n=6 10 126 Ma 2 σ 4 8 276 Ma 574 Ma 6 470 Ma 3 700-720 Ma 2 Relative probability 4 1006 Ma 2 1

0 0 0 500 1000 15002000 2500 3000 0 400 800120016002000 2400 2800

18 18 128 Ma 16 25 Ma ISLA ROJAS 1.1 ISLA ROJAS 6.5 16 Max Depo ~128 Ma 14 127 Ma Max Depo ~25 Ma n=2 14 n>15

Number of analyses 12 12 10 10 359-408 Ma 382-407 Ma 252 Ma 8 8 476 Ma 272 Ma 1055 Ma 289 Ma 6 467-554 Ma 6 525 Ma 1075 Ma 1370 Ma 4 1099 Ma 4 585Ma 1237 Ma 1460 Ma 2 2 0 0 0 400 800 1200 1600 2000 2400 2800 0 400 800 1200 1600 2000 2400 2800

25 140 126 Ma McPHERSON 2.2 32.2 Ma ISLA MITAHUES 1.1 Max Depo ~126 Ma 120 32.2 +0.4/-0.4 Ma 20 n=18 n=105 100 2σ 15 80

477-581 Ma 60 10 267 Ma 1050-1085 Ma 40 390 Ma 5 650 Ma 987 Ma 20

0 0 0 400 800 1200 16002000 24002800 0 100 200 300 400 500 600 700800 Age (Ma) Age (Ma) Figure 7. U-Pb ages and probability density plots for sandstones and tuffs from the Traiguén Formation and the Mitahues Island volcanic breccia.

EVIDENCE OF SYNEXTENSIONAL normal faults are associated with the injection addition, the lapilli tuff of sample Las Mentas SEDIMENTATION of basaltic material (Fig. 8B). This observation 1.1 (Table DR6 [see footnote 1]), which was implies that normal faults are related to crustal- interbedded with sedimentary rocks in this unit, Extensional faults cut sedimentary and vol- tectonic processes and not merely to gravita- yielded an age of 23.4 ± 0.3 Ma. These new data canic rocks of the Traiguén Formation in sev- tional spreading. agree with the ca. 26 Ma U-Pb SHRIMP maxi- eral outcrops of this unit. Normal faults are mum depositional age for a sandstone sample centimeters to meters long and at 70°–90° to DISCUSSION from the northwestern corner of the Traiguén the beds, which they offset up to 3 m (Fig. 8A). Island reported by Hervé et al. (2001). Taken The listric geometry of some fault planes pro- Age of the Traiguén Formation together, the consistency of these data may vokes the rotation of the hanging wall by a few constrain the age of the Traiguén Formation to degrees (Figs. 8A, 8C, and 8D). In general, nor- The youngest detrital zircon populations in the late Oligocene–earliest Miocene; however, mal faults show higher displacements in older four of the six sandstone samples from the Trai- there is no complete section of this unit to con- strata than in younger strata, implying that guén Formation yielded dates between ca. 23 firm whether it spans a wider interval of time. fault movement was synchronous with sedi- and 26 Ma, indicating a late Oligocene–earliest A minimum age for the Traiguén Formation is mentation (Figs. 8C and 8D). In some cases, Miocene maximum depositional age (Fig. 7). In probably constrained by the ca. 20 Ma age of

10 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile

A

offset

B C C

pillows fault contact

basaltic lavas

D

Figure 8. (A) Metric-scale normal faults in the Traiguén Formation strata on McPherson Island (see Fig. 2 for loca- tion). (B) Normal faults offsetting turbidites in the Traiguén Formation and associated with the injection of basal- tic lava with pillow structures that indicate subsurface emplacement; on anonymous island close to northwestern Traiguén Island. (C–D) Metric-scale synextensional growth strata in turbidites of the Traiguén Formation on Rojas Island. Note that normal faults accommodate maximum thicknesses at the footwalls and that displacement is higher in the older strata and diminishes toward the top. a pluton that intrudes the formation on Mag- mation were Late Cretaceous, but subsequent ous with volcanism of the Traiguén Formation; dalena Island (Pankhurst et al., 1999; Hervé et studies revealed them to be Miocene (Pankhurst some of these dikes do not intrude this unit and al., 2001). et al., 1999). Hervé et al. (1995) obtained two could be older, whereas those that crosscut the A review of previous age assignments for Rb-Sr ages of 20 ± 28 Ma and 20 ± 26 Ma from Traiguén Formation strata could be younger. the Traiguén Formation that were obtained by sedimentary and volcanic rocks on Magdalena Finally, the planktic foraminiferal assemblage other methods reveals some disagreements with Island, but they noted these ages most likely of Traiguén Island sample Sh-159, assigned our data. The Early Cretaceous age assigned correspond to their metamorphism. Dikes crop- to the Miocene by Mario Ernst (in Céspedes, to this unit by Fuenzalida and Etchart (1975) ping out in the Aysén region were dated by K/Ar 1975), was restricted by Osorio (1989) to the was principally based on the assumption that at 38–15 Ma (Silva et al., 2003, and references lower–middle Miocene (Table DR4 [see foot- the plutonic rocks intruding the Traiguén For- therein) and considered to be contemporane- note 1]). Their determinations must be taken

Geological Society of America Bulletin, v. 126, no. XX/XX 11 Encinas et al. with caution, as both noted poor preservation. by the occurrence of the large benthic foramin- is absent in our samples) probably was derived In addition, the species listed by Osorio (1989) ifera of the typically bathyal genera Ammodis- from granitic rocks of the Peninsula Gallegos indicate discordant biostratigraphic ranges. cus and Bathysiphon found in this study, as well and Isla Clemente (Pankhurst et al., 1999), west The Traiguén Formation was originally as other taxa reported by Céspedes (1975) and of the Traiguén Basin (Fig. 1), as these are the defined by Espinoza and Fuenzalida (1971) on Osorio (1989). The occurrence of the Zoophy- only rocks of that age that crop out in the region. Traiguén Island. Bobenrieth et al. (1983) later cos ichnofacies in fine-grained strata of this unit According to their major-element contents, augmented this unit with the volcano-sedimen- is also consistent with the sedimentologic data the basalts of the Traiguén Formation would have tary rocks of Magdalena Island and neighboring (Frey and Pemberton, 1984). All of the fossil mostly a calc-alkaline affinity, but their abundant localities (Fig. 1). Yet, there are no reliable dates evidence from the Traiguén Formation, how- secondary mineral phases (e.g., pumpellyite, epi- for the successions that crop out in the northern ever, must be taken with caution due to the poor dote, chlorite, prehnite, and white mica) could localities, as noted herein. Given the occurrence preservation. have significantly changed the FeO/MgO ratios, of pillow metabasalts assigned to the Paleozoic Analysis of provenance based on the age of as well as the alkalis. In two samples (Mentas 3.1 accretionary complex (Pankhurst et al., 1992) of detrital zircon populations from our samples and Ana 1.4), the K2O and TiO2 concentrations the North Patagonian Andes at ~42°S, we can- (Fig. 7; Table DR5 [see footnote 1]) and the are particularly low. In all the analyzed basalts, not discard the possibility that the volcano-sed- sample studied by Hervé et al. (2001) sug- the LILEs exhibit larger dispersion than the imentary succession of Magdalena Island pre- gests that the Traiguén Formation sediment HSFEs, likely due to the hydrothermal alteration dates that of Traiguén Island. However, Hervé was derived from emerged areas located east and/or low-grade metamorphism that affected et al. (1995) defended the hypothesis that these and west of this unit (Fig. 1), which would sup- the volcanic rocks; therefore, a reliable approach rocks are at least Tertiary based on the initial port the idea that the outcrops of the Traiguén to clarifying magmatic sources or tectonic set- 87Sr/86Sr ratio of 0.7049 ± 0.0005 for the slate Formation correspond, at least approximately, ting must be used on the basis of HFSE and REE isochron, and they also noted the unconformable with the original configuration of the basin. abundances. Most of the Traiguén Formation contact over the Paleozoic basement. Another This inference, however, is uncertain because basalts plot in the field of arc basalts or arc tho- uncertainty is the correlation of the Traiguén the geologic configuration of this region dur- leiites, although two sample plots are close to the Formation with the volcanic rocks exposed ing deposition of the Traiguén Formation was mid-ocean-ridge basalt (MORB) field (Figs. 5E west and north of Traiguén Island (e.g., islands subsequently modified by high exhumation and 5F), thus confirming the subduction-related of Humos, Luz, and Mitahues; Figs. 1 and 2). rates that gave way to the erosion of most of nature of the volcanism. Isotopes of Sr and Nd Unlike the Traiguén Formation, these rocks are the Mesozoic–Cenozoic volcano-sedimentary suggest that the source of the Traiguén magma not interbedded with sedimentary strata and cover (Thomson, 2002; Adriasola et al., 2006) was a combination of a depleted mantle and a lack features typical of subaqueous deposition. as well as the emplacement of the Miocene more-enriched component such as the conti- A U-Pb age of 32.2 Ma obtained from a volca- plutons of the Northern Patagonian Batholith nental crust (Fig. 5C). The interaction between nic breccia on Mitahues Island (Fig. 7) strongly (Pankhurst et al., 1999). The seven samples of magma and seawater is inferred from the decou- supports the notion that they are unrelated to the the Traiguén Formation analyzed in this study pling between the isotopic systems, which is Traiguén Formation. This date is also important yielded late Oligocene–early Miocene, Early indicated by the marked shifting of 87Sr/86Sr because it is the only reported evidence of early Cretaceous, Triassic, Permian, Ordovician, ratios toward more radiogenic values. Radio- Oligocene magmatism in the area. Cambrian, and Proterozoic zircon ages. The genic Sr likely was incorporated from seawater In summary, U-Pb ages obtained in this study youngest late Oligocene–early Miocene group in the secondary mineral phases, whereas the and that of Hervé et al. (2001) indicate that likely derives from volcanism contemporane- magmatic isotope composition of insoluble Nd the volcano-sedimentary marine rocks of the ous with deposition of the Traiguén Formation. remained unchanged. Isotopes of Pb suggest the Traiguén Formation at its type locality were Early Cretaceous zircons probably derive from addition of sediments to the magmatic source, as deposited between ca. 23 and 26 Ma (late Oli- plutonic rocks of the same age that extensively the 207Pb content is slightly higher than that of gocene–earliest Miocene). Whether the early crop out east and west of the Traiguén Forma- the MORB (Fig. 5D) source, requiring a crustal Oligocene volcanic breccia of Mitahues Island tion exposures. Triassic and older zircons were component mixed with a hypothetical depleted forms part of the initial stage of volcanism in probably recycled from the metamorphic rocks subarc mantle. the Traiguén Basin or constitutes a different widely exposed in the area, principally west of The geochemistry of the basalts in the Trai- depositional sequence is uncertain. Also unclear the Traiguén outcrops. A notable feature is the guén Formation is similar to other late Oligo- is the correlation of the pillowed metabasalts of limited representation of Triassic zircons, which cene–early Miocene volcanic rocks of central Magdalena Island and its neighboring area with are abundant in the Chonos metamorphic com- and south-central Chile and Argentina, which the Traiguén Formation. plex (also noted by Hervé et al., 2001). Hervé et have been interpreted as having been deposited al. (2001) cited the presence of late Oligocene, in an extensional tectonic regime (e.g., Muñoz Tectono-Sedimentary Setting Late Cretaceous, earliest Jurassic, Permian, and et al., 2000; Jordan et al., 2001; Kay and Cope- older Paleozoic and Proterozoic detrital zircon land, 2006; Aragón et al., 2011, 2013; Litvak The occurrence of syndepositional normal populations in their IT983 sample from the et al., 2014). However, the Traiguén volcanics faults in the Traiguén Formation indicates that northwestern part of Traiguén Island. Noting show a more marked subduction signature. extension generated basin subsidence. The pres- an absence of Early Cretaceous zircons in their When compared to the contemporaneous volca- ence of well-preserved pillow lavas and hyalo- sample, they proposed that the plutonic rocks of nic units in the arc (closer to the plate margin) clastites indicates that volcanism was subaque- this age, which presently crop out extensively and backarc (farther east of the plate margin) ous. The interbedded sedimentary rocks show in the area, were buried during deposition of domains, LILE and HFSE contents in the Trai- facies characteristic of a deep-marine environ- the Traiguén Formation, but our results do not guén basalts show trends similar to those rocks ment such as classic turbidites and slump depos- support that hypothesis. They also noted that located near the plate margin (Figs. 5A and its. Deep-marine deposition is further supported the Late Cretaceous zircon population (which 5E), being more abundant in LILEs (with the

12 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile exception of two samples that have low K, Sr, a submarine environment. A similar secondary guén Formation accumulated around 23–26 Ma, and Ba) and significant negative Nb-Ta anoma- mineralogy pattern has been also described by when convergence was more orthogonal (Pardo lies. The asthenospheric mantle, either depleted Silva (2003) for the basalts of Traiguén Island. Casas and Molnar, 1987; Somoza, 1998). In or oceanic-island basalt (OIB)-like, with little We interpret this association as the prehnite- addition, although the long-term nature, timing, modification from a subducted slab, has been pumpellyite facies of low-grade metamorphism and causes of lateral motion along the Liquiñe- invoked as the main source for the majority of rather than the hydrothermal alteration observed Ofqui fault zone remain poorly understood, late Oligocene–early Miocene magmas (Muñoz in the mid-ocean-ridge seafloor, where smectite, most data support the notions that this fault et al., 2000; Silva, 2003; Kay and Copeland, celadonite, carbonates, zeolites, Fe-oxyhydrox- system formed after ca. 14 Ma and deforma- 2006; Kay et al., 2007; Bruni et al., 2008; ides, and minor pyrite are the dominant phases tion is predominately transpressional (see Cem- Aragón et al., 2013). However, it is not possi- due to the circulation of seawater heated by the brano et al., 2002; Thomson, 2002; Encinas et ble to attribute their origin solely to MORB- or magmatic system (Frey and Robinson, 1999). al., 2013a). OIB-like sources from the geochemistry of the In fact, Oligocene–early Miocene volcanic Muñoz et al. (2000) and Jordan et al. (2001) studied samples (Figs. 5E and 5F), and the slab rocks of the Abanico Formation show a similar described a regional episode of extensional tec- component is required to achieve their LILE prehnite-pumpellyite facies but were deposited tonics that affected the Chilean margin between compositions. Some authors have interpreted under subaerial conditions (Muñoz et al., 2006). 33°S and 46°S during the late Oligocene–early this as an inherited signal in the mantle source The basalts of the Traiguén Formation from the Miocene. This event resulted in a series of or the lithosphere from previous subduction pro- Magdalena Island area (Fig. 1), on the other extensional basins that extended from the pres- cesses (Muñoz et al., 2000; Aragón et al., 2013), hand, differ from those on Traiguen Island by ent Chilean coast to the retroarc in Argentina possibilities that cannot be discarded for the having mineral assemblages that represent the and filled with thick successions of volcanic and Traiguén basalts. On the other hand, the Nd and greenschist, epidote-amphibolite, and amphibo- sedimentary rocks. Correlation of the Traiguén Pb isotopic composition of the Traiguén basalts lite metamorphic facies. These were interpreted Formation with the late Oligocene–early Mio- suggests a depleted to slightly enriched mantle as diastathermal or contact metamorphism fea- cene volcano-sedimentary deposits that crop out source, which agrees with the models that pro- tures by Hervé et al. (1993), but Hervé et al. in such a large area indicates that subsidence pose a slab window (Aragón et al., 2011, 2013) (1995) and Silva (2003) later suggested the pos- and volcanism were likely caused by a more or invigorated asthenospheric circulation in the sibility that the metamorphism was an ocean- regional event than localized activity along the mantle wedge (Muñoz et al., 2000). floor off-axis process. Liquiñe-Ofqui fault zone. Supporting this idea, The lack of a significant crustal contribution Morata et al. (2005) and Bruni et al. (2008) to the Traiguén Formation magmatism is also a Relation to Plate Convergence obtained geochemical signatures suggestive feature shared with contemporaneous volcanic of extensional tectonics from late Oligocene– units, as evidenced by the isotopic composition Our data indicate that the deep-marine vol- early Miocene igneous rocks that crop out in and REE contents of the Traiguén basalts (Figs. cano-sedimentary rocks of the Traiguén For- the North Patagonian Andes and the retroarc at 5B, 5C, and 5D). Furthermore, the Traiguén mation at its type locality were deposited in a the same latitude as the Traiguén Basin. Also basalts plot in the island-arc or arc tholeiites suprasubduction extensional basin during the noteworthy is the occurrence of early Miocene domains in several discrimination diagrams, late Oligocene–early Miocene. Extension gen- (23–19 Ma) volcano-sedimentary deposits of suggesting that the suprasubduction zone was erated an attenuated crust that, according to the Ventana Formation in the North Patagonian located within a thinned crust and supporting the conventional isostatic analyses, should have Andes at 41°S–42°S, which consist of fossilifer- idea that a large portion of the Andean margin attained a thickness that did not exceed 33 km ous marine beds interbedded with basaltic rocks (~33°S–46°S) was under an extensional tectonic in order to allow the marine ingression (Intro- (Bechis et al., 2014), the latter of which show regime that stretched the continental crust (e.g., caso et al., 2000). However, extension does not geochemical features indicative of a thinned Muñoz et al., 2000; Jordan et al., 2001). The appear to have been sufficient to generate oce- crust (Bechis et al., 2014; Litvak et al., 2014). geochemical signature of the Tertiary? metaba- anic crust, as indicated by the geochemistry and The causes of extension and widespread vol- salts from Magdalena Island, which Bobenrieth metamorphic facies of the Traiguén basalts. canism during the late Oligocene–early Miocene et al. (1983) included in the Traiguén Forma- The causes of subsidence and subaqueous in central and south-central Chile and Argentina tion, suggest that they may have originated in a volcanism in the Traiguén Basin and its relation are not well understood. Jordan et al. (2001) MORB-like tectonic environment (Hervé et al., with plate convergence kinematics are debated. related this significant event in the history of 1995; Silva et al., 2003), a possibility that can- Bobenrieth et al. (1983) indicated that the Trai- the Chilean margin to the major plate reorga- not be discarded for the Traiguén basalts. Yet, guén Formation formed in a forearc basin that nization in the southeast Pacific that resulted in both sets of samples share similar characteris- had an oceanic volcanic arc and a marginal a shift from slower, more oblique South Amer- tics, such as LILE enrichment over HFSE and basin to the east, where the Cretaceous volcano- ica–Farallón convergence to more rapid, near- Pb isotopic composition of an enriched mantle sedimentary Coyhaique Formation was depos- normal South America–Nazca convergence at source (Figs. 5A, 5C, and 5D), which are often ited. That scenario was based in the erroneous ca. 26–28 Ma (see Pardo Casas and Molnar, associated with subduction magmatism. assumption that the Traiguén Formation is 1987; Somoza, 1998). According to Jordan et As indicated before, the secondary mineral Mesozoic. Hervé et al. (1995) suggested that the al. (2001), this change gave way to a transient phases present in the volcanic rocks (pumpel- Traiguén Formation may have been deposited state of slab configuration and mantle circula- lyite, prehnite, epidote, chlorite, calcite, quartz, in pull-apart or asymmetric extensional basins tion, which generated a wide zone of volcanic iddingsite, fine-grained white mica, and titanite) (related to the dextral motion of the nearby activity and extension, and this is inconsistent and their occurrence as infillings of amygdules strike-slip Liquiñe-Ofqui fault zone) as a conse- with models that link high convergence rates to or veins, replacing phenocrysts or interpillow quence of oblique plate convergence between 45 strong mechanical coupling between the upper material, and in the groundmass are features and 25 Ma. To the contrary, our results indicate and lower plates (e.g., Pardo-Casas and Molnar, that suggest very low-grade metamorphism in that the volcano-sedimentary rocks of the Trai- 1987). Alternatively, Muñoz et al. (2000) pro-

Geological Society of America Bulletin, v. 126, no. XX/XX 13 Encinas et al. posed that the change in subduction geometry and the increase in trench-normal convergence rate during the late Oligocene induced a tran- sient period of vigorous asthenospheric-wedge circulation, that triggered the slab rollback of the subducting Nazca plate that resulted in regional extension and widespread volcanism. Transient steepening of the subduction angle caused the westward expansion of the magmatic activity that gave way to the “mid-Tertiary Coastal magmatic belt” (Muñoz et al., 2000). After the middle Miocene, the subduction angle decreased, causing contractional deformation, inversion of the extensional basins, and the shift of magmatic activity to its current position in the Main Andean Cordillera (Muñoz et al., 2000). Similarly, Kay and Copeland (2006) ascribed extensional conditions during this period to near-normal Nazca–South America plate con- vergence and slab rollback, which they related to slow relative motion between South America and the underlying mantle. In contrast to pre- vious hypotheses, Aragón et al. (2011, 2013) related synextensional volcanism in northern Patagonia to the collision of the Farallon-Aluk active ridge, which, according to these authors, evolved into a Farallon-Patagonia transform plate margin during the Paleogene to early Mio- cene subsequent to the detachment and sinking of the Aluk plate and the opening of a large slab window. The aforementioned discrepancies indicate the necessity of further study to better under- stand the causes of regional extension and widespread volcanism during the late Oligo- cene–early Miocene. Nevertheless, we believe the model proposed by Muñoz et al. (2000) accounts for all the characteristic features of this interval because slab rollback, accelerated asthe- nospheric-wedge circulation, and steepening of the subduction angle can explain the general- ized extension and crustal thinning, widespread volcanism with a mixture of slab signature and pristine mantle sources, and occurrence of igne- ous rocks in the coastal area (Fig. 9). Maximum Figure 9. Cross sections showing the main tectonic phases that have impacted the study area amounts of extension and crustal thinning on the since the late Mesozoic and the inferred changes in the slab geometry during the Oligocene– Chilean margin during the late Oligocene–early Miocene (based on model of Muñoz et al., 2000; see text for further explanation). (1) Late Miocene occurred in the Traiguén and Ventana Cretaceous compression. (2) Late Oligocene–early Miocene extension, widespread volcanism, basins, as indicated by the occurrence of subma- and Pacific and Atlantic marine transgressions triggered by slab rollback, intensified astheno- rine volcanism in those areas. spheric wedge circulation, and the steepening of the subduction angle. (3) Middle–late Miocene compression, inversion of the extensional basins, and the return of magmatic activity to its Implications for the Evolution of the North current position in the Main Andean Cordillera caused by the decrease of the subduction angle. Patagonian Andes

The late Oligocene–early Miocene exten- when major plate reorganization related to cene–Eocene, the North Patagonian Andes sional regime that gave way to submarine vol- the breakup of southern Gondwana resulted experienced synextensional uplift, according to canism in the Traiguén Basin represents a sin- in the tectonic inversion of the Jurassic–Early Aragón et al. (2011, 2013). Regional extension gular event in the development of the North Cretaceous extensional basins (Mpodozis and in this chain during the late Oligocene–early Patagonian Andes. This mountain chain began Ramos, 1990; Suárez and De La Cruz, 2000; Miocene is evidenced by the onset of submarine developing during the late Early Cretaceous Folguera and Ramos, 2011). During the Paleo- volcanism in the Traiguén and Ventana basins,

14 Geological Society of America Bulletin, v. 126, no. XX/XX Late Oligocene–early Miocene submarine volcanism in southern Chile the presence of synextensional strata observed Encinas et al., 2013a, 2013b, 2014; Bechis et and the connection between the Pacific and the in seismic profiles and outcrops (Jordan et al., al., 2014). Also debated is the possibility of an Atlantic oceans between ~41°S and 47°S. The 2001; Folguera et al., 2010; García Morabito Atlantic–Pacific connection during deposition subsequent phase of compressive tectonics that and Ramos, 2012; Orts et al., 2012; Bechis of these strata (e.g., Bertels, 1980; Ramos, 1982; started around 16 Ma resulted in the emersion, et al., 2014) and the geochemistry of the vol- Encinas et al., 2013a, 2014; Bechis et al., 2014). uplift, and deformation of the late Oligocene– canic rocks, which reflects a complex mixture Recently, Encinas et al. (2014) reported the early Miocene marine strata concurrent with the of slab signature and pristine mantle sources presence of both Pacific and Atlantic molluscan growth of the North Patagonian Andes. (e.g., Muñoz et al., 2000; Jordan et al., 2001; taxa in late Oligocene–early Miocene marine Morata et al., 2005; Kay and Copeland, 2006; strata of the La Cascada and Vargas Formations CONCLUSIONS Bruni et al., 2008; Aragón et al., 2011; Litvak (43°S–44°S), supporting the existence of a tran- et al., 2014). An important phase of compres- sient connection between the two oceans. The The results of our study indicate that the sive and transpressive deformation and Andean precise pathway of this connection, however, volcano-sedimentary Traiguén Formation was uplift that commenced in the early–middle remains uncertain (see Bechis et al., 2014; Enci- deposited in a deep-marine extensional basin Miocene gave way to the inversion of the late nas et al., 2014; references therein). during the late Oligocene–earliest Miocene. Oligocene–early Miocene basins and to the The similar interpretations of a late Oligo­ The geochemical and petrographic analyses present configuration of the North Patagonian cene–early Miocene age for the described marine suggest that the basalts of this unit represent the Andes (Suárez and De La Cruz, 2000; Thom- deposits and their widespread occurrence from activity of subduction-related volcanism over a son, 2002; Adriasola et al., 2006; Blisniuk et al., the Pacific to the Atlantic coasts of Patagonia thinned crust. Although we cannot discard the 2005; Ramos and Ghiglione, 2008). Thus, the suggest that they reflect the same tectonic events. role of the strike-slip Liquiñe-Ofqui fault sys- tectonic history of the North Patagonian Andes Most studies suggest that these strata were depos- tem during the deposition of the Traiguén For- includes two notable phases of compression and ited under extensional regimes (Elgueta et al., mation, correlation of this unit with volcanic uplift during the late Early Cretaceous and Neo- 2000; Jordan et al., 2001; Giacosa et al., 2004; and sedimentary rocks deposited in extensional gene separated by moderate extension during Melnick and Echtler, 2006), although some stud- basins widely distributed in Chile and Argentina the Paleocene–Eocene and maximum regional ies favor compressive deformation (Orts et al., between ~33°S and 46°S indicates that the Trai- extension and subsidence in the late Oligocene– 2012; Becerra et al., 2013). Bechis et al. (2014) guén Basin was likely formed as a consequence early Miocene (Fig. 9). proposed that the marine transgressions in the of a regional episode of extension and wide- A debated issue concerning the tectonic and Río Foyel area (41°S–44°S) were related to the spread volcanism during the late Oligocene– paleogeographic evolution of the North Pata- late Oligocene–early Miocene extensional tec- early Miocene. This event has been attributed to gonian Andes is the origin of the late Cenozoic tonic stage proposed by Muñoz et al. (2000) and a transient period of slab rollback and intensi- marine deposits exposed in the forearc, Main Jordan et al. (2001). They noted that the oldest fied asthenospheric wedge circulation resulting Andean Cordillera, and retroarc of this region marine strata in the region are ca. 23–19 Ma and from an increase in trench-normal convergence (e.g., Ramos, 1982; Frassinetti and Covacev- interbedded with basaltic rocks of the Ventana rate at ca. 26–28 Ma. Progressive extension and ich, 1999; Encinas et al., 2008; Bechis et al., Formation, evidencing the connection between crustal thinning after the formation of the Trai- 2014). Late Cenozoic marine deposits in the subsidence and crustal thinning related to the guén Basin allowed a marine transgression of Chilean forearc crop out in several locali- extensional tectonics ascribed to deposition of Pacific and Atlantic origin in Patagonia during ties between ~33°S and 47°S (De Vries et al., this unit (Rapela et al., 1983; Kay and Copeland, the early Miocene. Compressive tectonics that 1984; Elgueta et al., 2000; Encinas et al., 2008, 2006; Bechis et al., 2014; Litvak et al., 2014). began around 16 Ma resulted in the inversion 2012). Coeval marine strata occur in small, dis- Progressive extension caused general subsidence of the Traiguén Basin concurrent with the uplift connected outcrops in the western and eastern of the region that led to the marine incursion of the North Patagonian Andes. The Traiguén flanks of the North Patagonian Andes between and deposition of the Río Foyel Formation and Formation constitutes the only reliable record of ~40°S and 44°S at Lago Ranco, Ayacara, Río equivalent units (Bechis et al., 2014). The late submarine suprasubduction volcanism during Foyel, and neighboring localities (Ramos, Oligocene–earliest Miocene (26–23 Ma) vol- the Cenozoic in southern South America and 1982; Orts et al., 2012; Encinas et al., 2013a, cano-sedimentary marine deposits of the Trai- our study of this unit confirms the importance of 2013b, 2014; Bechis et al., 2014; references guén Formation appear to be coeval with most extensional tectonics along the Chilean margin therein). Late Cenozoic marine deposits of of the continental volcano-sedimentary deposits during the late Oligocene–early Miocene. Atlantic origin, popularly known as the “Patag- of south-central Chile and Argentina described oniense,” are exposed in several localities from by Muñoz et al. (2000) and Jordan et al. (2001), ACKNOWLEDGMENTS the Atlantic coast of Argentina to the eastern and slightly older than most of the late Cenozoic This research was funded by Fondecyt projects flank of the North Patagonian Andes at Guadal, marine deposits of Patagonia (Azúa et al., 2012; 1110914 and 1151146 of Conicyt. We appreciate the Chile (~47°S; Ramos, 1982; Frassinetti and Cuitiño et al., 2012; Orts et al., 2012; Encinas et support provided by the Corporación Nacional For- estal of Chile (CONAF), the Consejo de Monumen- Covacevich, 1999; Cuitiño et al., 2012). al., 2013a, 2013b, 2014; Bechis et al., 2014). In tos Nacionales de Chile (CMN), the Traiguén Island The age and correlation of these marine agreement with the Bechis et al. (2014) model, Huilliche community “Nahuelquín-Delgado,” the as- deposits have been discussed for several years we consider that the late Oligocene–early Mio- sistance of the boat Petrel and its crew, and the help of (Ramos, 1982; Frassinetti and Covacevich, 1999; cene extensional event proposed by Muñoz et Pablo Azúa during fieldwork. We are grateful to Nancy Gutiérrez et al., 2012; Orts et al., 2012; Finger, al. (2000) and Jordan et al. (2001) for the area Riggs, Constantino Mpodozis, and Eugenio Aragón for their helpful suggestions and constructive com- 2013; Encinas et al., 2008, 2013a, 2013b, 2014; between ~33°S and 46°S generated maximum ments, which considerably improved the manuscript. 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