Stratigraphic and provenance analysis of Triassic rock units between 28-29° S, northern Chile: implications on the tectonic and paleogeographic evolution of the southwestern margin of Gondwana

. Triassic rock units of northern Chile (28-29° S) record the transition, both in time and space, between two major orogenies that affected the southwestern margin of South America, the Gondwanian and Andean orogenies. The geodynamic configuration of the margin during this transition is still a matter of debate, particularly whether subduction was interrupted or continued under different parameters in between the orogenies. In order to evaluate these hypotheses by understanding the paleogeographic evolution of the margin, this work synthesizes recent stratigraphical, structural and geochronological data from northern Chile (28-29° S), along with detrital zircon analysis and detritus characterization of the two main siliciclastic Triassic basins present in the area. A detailed study of the evolution of the San Félix and the Canto del Agua basins, their source areas, and exhumation processes of the margin recognizes two stages of intra-arc/ forearc basins system development separated by a Carnian unconformity. The first stage (Lopingian-uppermost Middle Triassic) develops an eastern intra-arc basin, which is represented by the volcaniclastic rocks included in the Guanaco Sonso Formation and the roots of the volcanic arc represented by Chollay Plutonic Complex, bounded to the east by a Pennsylvanian-Cisuralian basement block. The forearc basin for this stage is constituted by two graben depocenter, separated by a topographic high, of marine to transitional depositional environment and proximal sediment sources. The eastern graben is filled by conglomerates and turbiditic rocks grouped in Members M1 to M4 of the San Félix Formation, and the western graben, by sedimentary and volcanic rocks of the lower section of the Canto del Agua Formation. The second stage (Norian-Rhaetian) involves an eastern intra-arc basin, represented by the volcanic rocks of the La Totora Formation that seals the exhumed roots of the magmatic arc developed in the previous stage, and a marine to transitional forearc basin to the west, represented by the sedimentary rocks of M5 member of the San Félix Formation and the upper section of the Canto del Agua Formation. These two successions show basal fluvial conglomerates unconformably overlying Anisian prodelta deposits of the first stage, recording a major base level drop of the forearc basin. Sonso y el Complejo Plutónico Chollay que representa las raíces del arco volcánico, limitada al oeste por un bloque de basamento pensilvánico-cisuraliense. El antearco de esta etapa se constituye por depocentros tipo graben, separados por un alto topográfico, de ambiente depositacional marino a transicional y fuentes de aporte sedimentario proximales. El graben oriental está relleno de rocas conglomerádicas y turbidíticas agrupadas en los miembros M1 a M4 de la Formación San Félix, mientras el graben occidental, de rocas sedimentarias y volcánicas de la sección inferior de la Formación Canto del Agua. La segunda etapa (Noriense-Rhaetiense) involucra una cuenca de intrarco, representada por las rocas volcánicas de la Formación La Totora que sella las raíces exhumadas del arco volcánico que se desarrolló en la etapa anterior, y una cuenca de antearco de ambiente marino a transicional, representada por las rocas sedimentarias del Miembro M5 de la Formación San Félix y la sección superior de la Formación Canto del Agua. Estas dos sucesiones presentan conglomerados fluviales basales que cubren discordantemente depósitos de prodelta de edad anisiana de la etapa anterior, registrando una gran caída del nivel de base en el antearco durante el carniense. Palabras clave: Triásico, Gondwana, Paleogeografía, Cuenca de intrarco, Cuenca de antearco, Discordancia del Carniense.


Introduction
In the southwestern South American margin Early Triassic to Early Jurassic rock units record the transition, both in time and space, between two major orogenies, the Gondwanean and the Andean. The changes that occurred between these orogenies include: westward shift of the magmatic locus, modifications in deformation style, magmatism and metamorphism (Fig. 1). These changes reveal an important contrast in the geodynamical configuration of the continental margin. The current hypotheses about the geodynamic setting of northern Chile during this period are divergent (Fig. 1 inset). On one hand, one model postulates continental rifting and arrested subduction (Mpodozis and Kay, 1992). For this model, some authors propose that during the Triassic-Lower Jurassic there were extensional basins with fast subsidence characterized by continental and marine sedimentation with abundant volcanoclastic rocks of felsic to intermediate composition ( Fig. 1; Charrier, 1979;Uliana and Biddle, 1988;Franzese and Spalletti, 2001). The NW-SE to NNW-SSE orientations of these basins would be controlled by the reactivation of Paleozoic structures inherited from the accretionary history of the margin (Ramos, 1994;Franzese and Spalletti, 2001). On the other hand, recent studies based on geochemical data postulate a setting with continuous subduction under extensional or transtensive tectonic regime (Vásquez et al., 2011;Del Rey et al., 2016;Coloma et al., 2017). Most of these hypotheses are used as generalized models for the whole southwestern Gondwana margin; nevertheless, stratigraphical and geochronological data to support both hypotheses is rather limited.
In northern Chile, between the 28° and 29° S, Permian to Lower Jurassic rocks crop out widely from the coastline to the Andean water divide (i.e., Chile-Argentina international border, Fig. 1). These rock units include clastic sedimentary successions, volcanic successions, and plutonic and metamorphic complexes (Reutter, 1974;Moscoso and Covacevich, 1982;Ribba et al., 1988;Mpodozis and Kay, 1992;Welkner et al., 2006;Murillo et al., 2013;Salazar and Coloma, 2016;Figs. 2 and 3). Such wide variety of Permian-Triassic rocks offers a unique record for the surface and subsurface geological processes that took place in the Gondwanean margin during the end of the Gondwanides and the birth of the early Andean Orogenic Cycle.
In this contribution, we present a complete revision of the Permian to Triassic stratigraphy of northern Chile between 28° and 29° S, both in the Frontal and Coastal cordilleras. Then, through U-Pb detrital zircons analysis and sediment characterization, we study their depositional ages, possible sources and genetic relationship between different basins, to then elucidate the exhumation history of the margin. Finally, we study the paleogeographic evolution in this area through a comparison between the different Triassic rock units and evaluate the possible geodynamic configuration of the margin.

Pre-Triassic Stratigraphy
The oldest rocks in the study area are of Paleozoic age. These units outcrop as N-S oriented belts both in the Frontal and Coastal Cordilleras. Pre-Triassic rock units on the Frontal Cordillera constitute the basement where the San Félix, Guanaco Sonso and La Totora basins developed, while the ones outcropping in the Coastal Cordillera nested the Canto del Agua basin.

Ordovician-Missisipian
The oldest rock in the studied segment correspond to those included in the El Cepo and El Tránsito Metamorphic Complexes (ECMC and ETMC, respectively), cropping out in the western part of Frontal Cordillera, and the Las Placetas Formation, in the eastern part (Fig. 2). The ECMC is exposed as small isolated xenoliths of phyllites and mica schists within Pennsylvanian plutons in the southernmost part of the study area (Fig. 2) whose maximal depositional age is Ordovician (Ortiz and Merino, 2015;  FIG. 1. Location and regional geological context of the study area. Features distribution in Chile and Argentina modified from Charrier et al. (2007) and Franzese and Spalletti (2001), Gondwana reconstruction modified from Torsvik and Cocks (2013) and Castillo et al. (2017) and references therein. .   Arévalo and Welkner (2008), Salazar et al. (2013), Ortiz and Merino (2015), Creixell et al. (2016) and Salazar and Coloma (2016).  (7); 221±3 Ma (5); 218±2 Ma (2) and metapelites with quartz, muscovite, chlorite and biotite as main minerals, whose fossil content indicates a Late Devonian to early Carbonifeous age (Haplostigma furquei Frenguelli) which is consistent with published maximum depositional ages (Table 1; Salazar and Coloma, 2016) and contact relationships (Fig. 4A).
The ETMC is composed of metabasites, quartzmica schists, nodular albite-schists, quartz-amphibole schists, quartzites and marbles and has been interpreted as a petrotectonic association of an accretionary prism (Ribba et al., 1988) with a maximum depositional age of 370 Ma (Table 1; Álvarez et al., 2011). Bahlburg et al. (2009) also reported U-Pb ages on detrital zircon rims for the ETMC from an uncertain location, between 250 and 370 Ma. This younger age distribution is more similar to that obtained from the overlying Triassic San Félix Formation (see section 3.2.1), and therefore this data is not considered in the discussion.

Pennsylvanian to Cisuralian
Rock units of this age crop out in the western part of the Frontal Cordillera forming a NS belt of plutonic and volcanic rocks, defined as the Chanchoquín Plutonic Complex, the Quebrada Pinte Diorites (protolith of the El Portillo Mylonite) and the Cerro Bayo Formation ( Fig. 2; Salazar et al., 2013). The Cerro Bayo Formation is composed of rhyolitic to dacitic volcaniclastics with U-Pb zircon ages ranging from 301 to 325 Ma (Table 1; Salazar 2012;Maksaev et al., 2014), and is intruded by the penecontemporaneous Chanchoquín Plutonic Complex, composed by coarse grained granodiorites, monzogranites and foliated tonalites with U-Pb zircon ages from 286 to 296 Ma (Table 1; Fig. 2;Pankhurst et al., 1996;Salazar et al., 2013). The easternmost remnant of this plutonism is the Quebrada Pinte Diorites, outcropping east of the Pinte fault (Fig. 2), with U-Pb zircon ages of 324 and 326 Ma (Table 1; Ortiz and Merino, 2015;Murillo et al., 2013) which is one of the protoliths of Middle Triassic El Portillo Mylonites (Table 1; Fig. 3; Murillo et al., 2013).

Cisuralian to Lopingian
Eastward of the Carboniferous magmatic belt a Permian N-S trending magmatic belt crops out composed by isolated outcrops or rhyodacitic pyroclastic rocks (Laguna Chica Formation), tonalitic intrusives (Quebrada El Pintado and Quebrada Las Cañas tonalites) and metamorphic rocks as xenoliths within the Triassic Chollay Plutonic Complex (La Pampa Gneisses; Fig. 2; Table 1). The Laguna Chica Formation is a dacitic to rhyolitic volcaniclastic unit that lies unconformably on the Las Placetas Formation rocks, is intruded by the Triassic Chollay Plutonic Complex, and has reported U-Pb zircon ages between 264 and 276 Ma (Salazar and Coloma, 2016). Middle to upper Permian tonalitic units have U-Pb zircon ages ranging from 255 to 270 Ma , commonly show penetrative magmatic foliation marked by orientation of plagioclase, amphiboles and biotite crystals and also constitute a protolith of the El Portillo Mylonites. Metamorphic rocks of this age are assigned to the La Pampa Gneisses (Ribba, 1985;Salazar et al., 2013), sillimanite and cordierite bearing gneisses where Álvarez et al. (2013) interpreted a peak of metamorphic conditions between 5.1 and 5.6 kbar and 709-779 °C at 268±2 Ma (Fig. 2).

Pennsylvanian to Cisuralian
The pre-Triassic basement in the Coastal Cordillera in the studied segment is composed by two units, the Punta de Choros Metamorphic Complex (PCMC, Carboniferous) (Creixell et al., 2012) and the sedimentary rocks of the Llanos del Chocolate Beds (Carboniferous-Permian) (Welkner et al., 2006;Creixell et al., 2016). The PCMC consists of an association of mica schist, metaturbidites and metabasites with a very low to low grade of metamorphism up until greenschist facies with local occurrences of amphibolites facies (Creixell et al., 2012;Navarro, 2013). The former reports a metamorphic peak at 319±3 Ma, based on amphibole Ar/Ar age and a U-Pb detrital zircon maximum depositional age of 334±6 Ma for its protolith, which indicates a deposition age during the Carboniferous. The Llanos del Chocolate Beds is preserved directly to the east of the PCMC as a thick (ca. 1,300 m) sedimentary sequence (Creixell et al., 2016). This unit is composed by conglomerates, sandstones, mudstones, limestones and locally exposed dacite domes and tuffs with minor or no metamorphism (Creixell et al., 2016). Two crystallization zircon U-Pb ages obtained from dacite lavas of 304±3 and 319±3 Ma and four zircon U-Pb detrital maximum depositional ages between 273 and 292 Ma indicate an upper Carboniferous to Permian age for the sequence (Creixell et al., 2016). These ages are consistent with Permian brachiopod fauna recognized near to the base of this unit (Welkner et al., 2006).

Stratigraphy
The filling of this basin is exclusively made up by the sedimentary rocks of the San Félix Formation, a NS-striking siliciclastic wedge, reaching up to 6 km in thickness, that crops out along the Del Carmen River (Fig. 4;Reutter, 1974;Ribba, 1985;Salazar, 2012) and is composed by 5 members, M1 through M5, from bottom to top (Ribba, 1985). Members M1 and M5 are dominantly conglomeratic, though the latter also include volcanic intercalations, members M2 and M4 are turbiditic successions, and M3 is composed of sandstones and conglomerates.
Northward and southward of its type locality, the San Félix town, the thickness of the San Félix Formation dramatically diminishes as its members M1 to M4 progressively onlap onto the metamorphic basement ( Figs.1 and 4). To the west, the San Félix Formation is truncated by the Late Cretaceous to Tertiary west vergent San Félix Thrust    4D) and the ETMC (Fig. 4C), to the west and east of the Las Pircas fault, respectively, and it laterally interfingers towards the northeast with the lower section of the La Totora Formation. Fossil marine fauna from member M4 indicate an Anisian age (Fig. 4;Zeil, 1958;Barthel, 1958), which is consistent with the Norian age assigned to the M5 Member by Salazar et al. (2013). The only constraint for the initiation of deposition of the San Félix Formation previous to this work is the underlying Pennsylvanian Cerro Bayo Formation (Fig. 4C). Two U-Pb crystallization zircon ages and one detrital zircon age indicate a Norian (middle Upper Triassic) age for the M5 member (Table 1), which is also consistent with paleoflora age determinations (Mohr and Schöner, 1985;Salazar et al., 2013).

Basin evolution and depositional environment
The areal distribution and stratigraphic relationships of the different members of the San Félix Formation led Salazar et al. (2013)  Sedimentological studies interpret the San Félix Formation (Triassic) as a marine fan delta depositional system, where conglomeratic members M1 and M5 correspond to delta plain fluvial system, fine grained M2 and M4 members correspond to prodelta environments and the sandy to conglomeratic M3 member as the upper part of the delta slope, with episodic subaerial exposure (Reutter, 1974;Ribba, 1985;Bell and Suárez, 1994;Padel et al., 2012;Salazar et al., 2013). Published geochronological data show a depositional gap between members M4 and M5 of at least 20 Ma (Zeil, 1958;Barthel, 1958;Ribba, 1985;Salazar et al., 2013;Fig. 3) which suggests that the topmost M5 member of the formation represents a f luvial system within a different geological context, likely influenced by the volcanic processes related to the La Totora Formation, since these two units laterally interfinger (Table 1).

Stratigraphy
The units involved in the configuration of this basin are the Guanaco Sonso Formation, the Chollay Plutonic Complex and the El Portillo Mylonites. The volcaniclastic Guanaco Sonso Formation corresponds to the filling of the basin, which is contemporaneous and partially intruded by the granitic Chollay Plutonic Complex. Both of these units are bounded to the west by the mylonitic NS trending belt of the El Portillo Mylonites.
Guanaco Sonso Formation. The Guanaco Sonso Formation (Martin et al., 1999) consists in a volcaniclastic succession cropping out in the eastern part of the Frontal Cordillera (Fig. 2). It covers, in an angular unconformity, the Late Devonian Las Placetas Forma-tion (Fig. 4A) and the Guadalupian Laguna Chica Formation, and is intruded by penecontemporaneous granites of the Chollay Plutonic Complex. It is best exposed close to the Chile-Argentina international border, where it reaches nearly 300 m in thickness and is unconformably covered by Paleocene to Miocene strata. It is composed of basalts at the base, and dacitic to rhyolitic tuffs and lavas with sandstones, conglomerates and mudstone intercalations towards the top (Salazar and Coloma, 2016).
The Guanaco Sonso Formation was originally assigned a Guadalupian (middle Permian) age, based on multigrain U-Pb zircon ages (Martin et al., 1999;Charchaflié, 2003), however, recently, a series of single grain LA-ICP-MS U-Pb zircon ages obtained by Ortiz and Merino (2015) and Salazar and Coloma (2016), including an age in its type locality, allowed these authors to reassign this unit to an age range between 254 and 237 Ma, arguing that the presence of inherited populations of Cisularian to Guadalupian age identified in their analysis, would account for the older multigrain ages previously published.
Chollay Plutonic Complex. The Chollay Plutonic Complex is the most widely exposed unit in the Chilean  Frontal Cordillera (Fig. 2). It intrudes Devonian metasediments of the Las Placetas Formation (Fig. 4A) and, in a minor extent, the penecontemporaneous Guanaco Sonso Formation. This plutonic complex is intruded by Late Triassic Plutons of the Colorado syenogranites and Cenozoic granitoids, as is unconformably covered by the Late Triassic La Totora Formation and younger stratified units ( Fig. 2; Salazar et al., 2013;Salazar and Coloma, 2016). It is composed of diorites, tonalities, granodiorites, monzogranites and syenogranites, being monzogranites the dominant lithology, which is commonly coarse-grained with abundant graphic and perthitic textures. Biotite and amphibole are the dominant ferromagnesian phases Salazar and Coloma, 2016).
Nine U-Pb zircon ages obtained by several authors in the study area (Álvarez et al., 2011;Salazar et al., 2013;Maksaev et al., 2014;Salazar and Coloma, 2016) allows to constraint the construction of this plutonic complex between 248 and 237 Ma (Lower-Middle Triassic) (Table 1).
El Portillo Mylonites. El Portillo Mylonites, was described by Ribba et al. (1988) as a thin 10 km long fringe of mylonitic rocks located east of the Pinte fault trace (Fig. 2). This unit includes mylonitized portions of the Quebrada Pinte Diorites and Las Cañas Tonalites , which are host rocks of the Chollay Plutonic Complex, and is unconformably covered by the Late Triassic La Totora Formation (Fig. 4B).
Lithologically, it is composed by bands of metadiorites, metatonalites and metagranodiorites crosscut by granitic bands, all of these present variable degrees of mylonitization and with a dominant vertical and NS oriented foliation with left-lateral shear sense (Murillo et al., 2013).
Two SHRIMP U-Pb ages on zircon nuclei, obtained by Murillo et al. (2013)

Basin depositional environment
Published studies focused on depositional processes and environment for the rocks of the Guanaco Sonso Formation (Lower to Middle Triassic) are scarce (López et al., 2015;Salazar and Coloma, 2016). Based on volcanic and siliciclastic facies analysis, López et al. (2015) interpreted a continental environment with fluvial, alluvial and lacustrine facies associations carrying fossil remains of hydrophilic flora. The volcanic deposits, rhyolitic to basalticandesite in composition, evidence hydromagmatic eruptive mechanisms and a strong control exerted by a syndepositional set of bivergent normal fault system of low displacement and wavelength (~15 m and ~50 m, respectively), as the result of low elastic thickness for the basement of the studied volcanic pile during its accumulation (López et al., 2015;Salazar and Coloma, 2016). The latter is interpreted by these authors as consequence of the shallow emplacement of granitic magma bodies of the contemporaneous Chollay Plutonic Complex ( Fig. 3; Table 1), which is consistent with the intrusion relationship observed between these two units in some places (Fig. 2) and the section exposed at the Valeriano river (Fig. 4A). These features are characteristic of caldera environments developed over batholith construction sites (Zimmerer and McIntosh, 2012;Branney and Acocella, 2015), as is drainage isolation represented by the lacustrine environment. The Chollay Plutonic Complex, underlying the Guanaco Sonso basin, is bounded to the west by the contemporaneous El Portillo Mylonites (EPM, Table 1), which is a 251-240 Ma left-lateral shear zone (Murillo et al., 2013;Salazar et al., 2013;Hervé et al., 2014) affecting older Carboniferous to Permian plutons ( Fig. 2; Table 1).
This unit is composed of andesitic lavas with aphanitic, amygdaloidal and porphyritic texture, andesitic breccia and minor tuffs.
U-Pb zircon ages range from 210 to 221 Ma Maksaev et al., 2014;Salazar and Coloma, 2016; Table 1) and are consistent with the contact relationships described above. Older U-Pb zircon ages in a basaltic tuff of the basal layer yielded Carboniferous and Early Triassic populations (Maksaev et al., 2014), however, the contact relationships of the unit and composition of the sampled rock, suggest these are inherited ages, not reflecting a depositional age for the unit.
The La Totora Formation is slightly younger than both the Colorado syenogranites, within the study area ( Fig. 3 and Table 1; Salazar and Coloma, 2016), and the rhyolitic and dacitic rocks of the Pastos Blancos Formation (232 and 221 Ma; Ortiz and Merino, 2015), located a few km further south.

Basin Configuration
The La Totora basin is a NS elongated depocenter infilled by the dominantly andesitic volcanics of the La Totora Formation (Upper Triassic; 210-221 Ma) and accommodated by normal faults ( Fig. 4E; Salazar et al., 2013). The maximum accommodation of this basin occurs as a NS trending axis, immediately east of the Pinte fault (Fig. 2). Along this maximum accommodation domain the volcanic rocks of the La Totora Formation unconformably covers the mylonitic contact (El Portillo Mylonites) between the Chollay Plutonic Complex (Lower to Middle Triassic), to the east, and Carboniferous to early Permian intrusives to the west (Fig. 2). West of the Paleozoic block, the La Totora Formation laterally interfingers with the fluvial sedimentary M5 member of the San Félix Formation, which unconformably covers the prodelta deposits of Anisian (Middle Triassic) M4 Member (Bell and Suárez, 1994;Salazar et al., 2013). Along the eastern shoulder of the La Totora basin the volcanic rocks of the La Totora Formation covers mostly the Chollay Plutonic Complex, while further east, near the international border, it locally covers the volcanics of the Guanaco Sonso Formation (Salazar and Coloma, 2016).

Stratigraphy
The Canto del Agua Formation (Moscoso and Covacevich, 1982) consists of a marine, clastic and volcanic succession, with their outcrops discontinuously distributed along the Coastal Cordillera between the 28°30᾽ and 29°10᾽ S. At its type locality, it was described as a 2,100 m thick succession, carrying Anisian, Norian and Early Jurassic fossil fauna, that lies unconformably on the Punta de Choros Metamorphic Complex (Carboniferous) and is covered by andesites and basaltic andesites of Jurassic age (La Negra Formation). At the base of a succession originally assigned to this unit at the Chañaral Creek (29° S), Welkner et al. (2006) described Permian brachiopod fauna that led them to define a restricted basal section as Llanos del Chocolate Beds. Recently the contact between Llanos del Chocolate Beds and Canto del Agua Formation has been displaced further upsection at the Chañaral Creek, based on Carboniferous to Permian U-Pb zircon ages obtained by Creixell et al. (2016). The latter implies a major stratigraphic discontinuity between Llanos del Chocolate beds and the Canto del Agua Formation which has not been recognized before nor characterized on outcrop.

Basin depositional environment
This unit was initially described by Moscoso (1979) as a marine sequence, on the basis of its lithology and fossil fauna. The distribution of the unit along two localities in the study area suggests it was deposited in distinct depocenters. In the type locality, Moscoso and Covacevich (1982) estimated a total thickness of 2,100 m, whereas in the south, near El Molle mine (29º S), thickness estimations remain under doubt since Creixell et al. (2016), on the basis of new U-Pb geochronology, demonstrated that an important portion of the sequence first assigned to this unit, is older and attributed to the Carboniferous to Permian Llanos del Chocolate Beds (Table 1). According to Suárez and Bell (1992), the Canto del Agua basin was developed in a submarine section of a coarse-grained fan delta, and Arévalo and Welkner (2008) proposed that the basin was bounded by two normal faults of WNW strike, active during deposition.

San Félix Formation
Fully developed thickness of the San Félix Formation is exposed exclusively in the hanging wall of the west vergent San Félix fault (section location c in Fig. 2), while only a condensed 350 m thick section is exposed in the footwall of the same fault, at the southernmost part of the Carmen river (section location b in Fig. 2), which was sampled for detrital zircon analysis (Figs. 6, 7). The latter section is directly overlying late Carboniferous acid volcanics of the Cerro Bayo Formation ( Fig. 2; Table 1). From bottom to top this sequence is made of a basal section of tabular pebbly sandstones, arkosic to lithic in composition, whose lithic component include volcaniclastic, metamorphic and felsic intrusive lithologies and a silicic lithic ash tuff bed. Towards the top of this basal section, the conglomeratic component increases forming clast supported cobbly conglomerates lenses, with concave erosive bases, rounded to subrounded clasts of mainly felsic tuffs and intrusives, and minor metamorphic rocks. Overlying this section there are 70 m of alternating dark mudstones and grey sandstones, the latter occasionally showing groove marks, mudclasts and centimetric diagenetic calcareous nodules. Feldspathic litharenites and wackas cover the former section in an angular unconformity, varying to conglomeratic sandstones towards the top. The lithic component of these sandstones is mainly felsic volcaniclastics, schists and andesites, and the minerals are quartz, plagioclase and muscovite. These beds also carry fossil flora remnants of Dicroidium odontoperoides and Dicroidium zuberi. A younger section of alternating beds of mudstones and sandstones covers previous sandstones, with a middle section of sandstones and pebbly sandstones, with filled channels structures, cross stratification, mudclasts and abundant felsic volcaniclastics and intrusives and minor metamorphic clasts. Finally this section ends with beds of clast supported conglomerates and sandstones, with abundant concave erosive bases and clasts of felsic composition (volcanic and intrusive) and of polycrystalline quartz. Sandstones are more abundant towards the top, and they are coarse grained, with angular grains and abundant quartz and amphibole.

Canto del Agua Formation
In its type locality, near de Plomiza mine (Fig. 2), this unit onlaps the metamorphic basement ( Fig. 2; Table 1). From bottom to top, the section is composed of parallel laminated muddy sandstone with mudstone intercalations bearing Daonella sp.; covered by a section of brown conglomeratic sandstone with subrounded metapelitic clasts (Fig. 5). The upper part of the section is composed of glassy lapilli tuffs and fine-grained sandstone lenses, covered by polimictic orthoconglomerates with clasts of metapelites, acid volcanics and andesites, bearing minor reworked fossil trunks of Calamites sp. Over the conglomerates lies a rhythmic sequence of fissile mudstones that vary to sandstones towards the top, followed by mudstones with minor intercalations of altered volcanic rocks and sandstone to the roof. Above that, there are recrystallized limestones with abundant fossils fragments such as echinoderms, brachiopods, indeterminate shell fragments, allochemical clasts, peloids and minor terrigenous quartz fragments. The top of the section corresponds to a rhythmic sequence of mudstone and muddy sandstone with parallel lamination and f lame structures. The sandstones present allochemical components such as peloids, shell fragments and terrigenous components such as feldspar and quartz fragments.
The base of the section of the Canto del Agua Formation at the El Molle mine (Fig. 2) is made up of matrix-supported conglomerates with acid volcanic clasts and a coarse grained sandstone matrix, intercalated with minor conglomeratic and coarse grained sandstones. Clast-supported conglomerate with rounded to sub/rounded, imbricated, poorly-sorted clasts cover the previous strata and are intercalated with centimetric lenses of coarse-grained sandstones. Clasts correspond to rhyolites, acid volcanic rocks, tuffs, microdiorites and metamorphic quartz. The upper beds are coarse-grained cross-bedding sandstone and conglomerates, with clasts of acid volcanics, tuffs and metamorphic lithics. At the top, sandstones and mudstones with parallel lamination, normalgraded and trough cross-bedding bearing bivalves and indeterminate fossils of pelecypodes crop out. The sandstones are composed of tuffaceous and metamorphic lithics, quartz, plagioclase and white mica.

Detrital zircon U-Pb Geochronology
U-Pb data and methodology are included in the supplementary material (Supplementary material 1 and 2, respectively), while concordia and density population diagrams are shown in figures 6 and 7.

Relative probability Number
Age (

CPV-12-29X
La Puntilla on the top of San Félix Formation
A sandstone from the bottom of the type locality section (CPV-12-88X) yielded five unmixed Permian-Carboniferous age peaks (n=94)

San Félix Formation
An age range between upper Anisian (~242 Ma) and upper Ladinian (~237 Ma) has been assigned to the M4 member from its fossil content (Daonella sturi and Daonella lommeli; Zeil, 1958;McRoberts, 2010). Maximum depositional ages from detrital zircons are represented by the youngest age populations of 253.8±1.9 and 250.3±2.6 Ma, from samples at the bottom and top of the La Puntilla section, respectively (Figs. 6 and 7). These results suggest that the deposition of members M1 to M4 of the San Félix Formation began after 255 Ma and extended through the Ladinian (242-237 Ma, Cohen et al., 2013).

Canto del Agua Formation
At its type locality, the base of the Canto del Agua Formation bear Daonella; whose biostratigraphic age varies between upper Anisian through Ladinian (McRoberts, 2010), whereas its topmost part bears Psiloceras (Calcoceras) sp. and Arnioceras sp. of Hettangian and Sinemurian ages (199-190 Ma;Moscoso and Covacevich, 1982). Maximum depositional ages for three samples on the same section are represented by the youngest detrital zircon age populations of 264.5±5.3 Ma (base) and 211.9± 3.4 Ma (middle) and 212.8±7.3 Ma (top; Figs. 6 and 7). The maximum depositional age for the lower part of the section is consistent with and upper Anisian to Ladinian age of deposition evidenced by its fossil content. Both the biostratigraphical and maximum depositional ages obtained in the upper section are significantly younger, Norian to Sinemurian, than the lower part of this formation. These contrasting ages would allow dividing the unit in two different sequences, the first deposited in the Middle Triassic and a youngest deposited in the Norian to Hettangian. Further south, at the El Molle mine locality (Fig. 2), a maximum depositional age of 216.4±2.4 Ma was obtained in the middle part of the exposed section. This age is consistent with the upper part of the section exposed in the type locality.

Stratigraphic correlations
Maximum depositional age of 255 Ma, along with the marine to transitional depositional environments and the absence of Upper Triassic detrital ages in the La Puntilla section, suggest that members M1 to M4 of the San Félix Formation can be correlated to the lower section of the Canto del Agua Formation at the Coastal Cordillera (Fig. 5). On the other hand, coincident geochronological data from the upper section of Canto del Agua Formation and the Member 5 of San Félix Formation allows us to correlate the regression evidenced by the establishment of fluvial depositional systems during the Late Triassic on both areas (Fig. 5).

San Félix basin
San Félix Formation shows, along the whole sequence, a relatively homogenous lithic component for sandstones and conglomerates, with variable amounts of metamorphic rocks, felsic tuffs, intrusives and minor andesites. Sandstones are texturally immature to submature, which indicates relatively local sources. The detrital zircon U-Pb age distributions also show similar age patterns along the sequence, which consists of three well defined age populations of 240-260, 280-300 and 310-335 Ma, with scarce older ages in the middle of the section (Fig. 7).
The zircon pattern distribution for the older than 335 Ma ages is similar to those found in the El Tránsito Metamorphic Complex (ETMC; Álvarez et al., 2011), which is unconformably covered by the San Félix Formation ( Fig. 2 (Zappettini et al., 2015). However, the rocks of the EPM are unconformably covered by the Totora Formation (Fig. 4B, Table 1), indicating they were exhumed before the Norian, probably providing Carboniferous zircons to the Triassic drainage systems. The lower Permian age population is the most important component throughout the La Puntilla section and it most likely reflects the erosion of the Chanchoquín Plutonic Complex (285-304 Ma), which is unconformably covered by the San Félix Formation ( Fig. 2 and Table 1) and constitutes the footwall of the eastern border fault of the San Félix basin (Las Pircas fault). The third and youngest age population (240-260 Ma) is broadly contemporaneous with the Guanaco Sonso Formations ( Fig. 3 and Table 1). The abundant felsic volcaniclastic component of the Guanaco Sonso Formation indicates highly explosive eruptions, which may have contributed juvenile zircons to the basin.
In summary, sources for sediment to the San Félix basin are located eastward of the San Félix Formation, and furthermore, non-contemporaneous sources can be constrained to the basement block bounded by the Las Pircas fault, in the west, and the EPM in the east.

Canto del Agua basin
At its type section at Plomiza Mine the detrital zircon ages of the Canto del Agua Formation are grouped in a major population between 300 and 260 Ma (Fig. 7), with minor clusters around 330 and 480 Ma, while the nature of the lithic components is dominated by metapelitic clasts and quartz grains. The most prominent age population correlates with the age range of the Llanos del Chocolate Beds (Table 1), which are exposed 80 km south of this locality (Fig. 2). The interbedded felsic volcanics present in the Llanos del Chocolate Beds could have contributed to the subarkosic nature of the sandstones. The 330 Ma and older ages are most likely populations inherited from the metamorphic complexes underlying the Canto del Agua Formation (Punta de Choros Metamorphic Complex or PCMC in table 1; Bahlburg et al., 2009;Álvarez et al., 2011).
The middle and top parts of the type section roughly show the same age peaks recognized at the bottom of the section, though less prominent and with a Late Triassic age population. Given that no Late Triassic volcanic units have been recognized in the Coastal Cordillera, a possible source for such population could be the tuffaceous deposits underlying the analyzed horizons (Fig. 5). A volcanic input to the basin is evidenced by the increased contribution of felsic and andesitic volcanic fragments up section. The section exposed at the El Molle mine locality shows the same age populations than the upper part of the type section, with the exceptions of a Lopingian to Lower Triassic population (ca. 259-247 Ma), absent in the type locality, and that the Late Triassic population is slightly older (216-224 Ma). The former cannot be related to any known source of such age in the Coastal Cordillera. In the Frontal Cordillera, stratigraphic relationships show plutonic and volcanic rocks of this age (Chollay Plutonic Complex and Guanaco Sonso Formation; Fig. 3). These rock units were exhumed before the Norian (Table 1), therefore, providing a possible source for this population of zircons. The Norian population could be attributed to acidic volcanism in the Frontal Cordillera exposed immediately south of the study area (Pastos Blancos Formation; Table 1).
In conclusion, the sediment contribution for the Canto del Agua Formation is dominated by local sources. An exception, are the rocks cropping out near El Molle locality, which are equivalent to the upper part of the type section and suggest some contribution from the Frontal Cordillera, both by drainage processes and by atmospheric transport from volcanic eruptions.

Gondwana margin evolution during the Triassic at 28-29° S
Triassic basins in the southwestern Gondwanean margin have been previously interpreted as a continuous NNW oriented horst and graben system encompassing both Argentinian Ischigualasto and Cuyo basins and Chilean marine to transitional Triassic basins distributed along the margin (Charrier, 1979). However such scenario is not completely compatible with the presence of a NS trending magmatic belt in the Frontal Cordillera, composed of granitic batholiths (Chollay Plutonic Complex), a volcaniclastic basin (Guanaco Sonso Formation), both with continental magmatic arc isotopic and geochemical signatures (Del Rey et al., 2016;Coloma et al., 2017), and with the presence of NS mylonitic shear zones (EPM). These features suggest the mechanical decoupling of the clastic rift basin systems located west (Chile) and east (Argentina) of the magmatic belt. On the other hand, Early to Middle Triassic basins located west of the arc, are bounded by contemporaneous N-S faults (such as the Las Pircas fault and El Portillo Mylonites) rather than by the NNW orientation of Argentinean basins (Fig. 1). Under such considerations, in the studied segment, Triassic volcanic basins located in the present Frontal Cordillera and Triassic sedimentary basins in Chile are more likely to have developed as NS trending intra-arc and forearc basins respectively. This is consistent with the structural style shown in offshore seismic reflection studies at 31-33° S (Contreras-Reyes et al., 2014, where, in spite that the ages of the reflectors are poorly constrained, the sedimentary cover fills a series of N-S trending grabens and hemigrabens in the upper slope, that were tectonically inverted in recent (Andean) compressive pulses and gravitational collapses.
Geochronological data and stratigraphic relationships exposed above (Fig. 3) identify two stages during the Triassic evolution of the study area, separated by a regional unconformity of Carnian age, a first latest Permian-Anisian stage and a second Late Triassic stage (Fig. 3). Such unconformity coincides with the Middle to Late Triassic exhumation of mélanges between 26 and 42° S along the Chilean coastline (Kato and Godoy, 2015).

First stage
Between the Lopingian and the uppermost Middle Triassic (Ladinian), the studied segment presented three depocenters separated by two topographic highs. The eastern depocenter, an intra-arc basin, accommodated the Guanaco Sonso Formation, a mainly volcaniclastic succession whose well developed lacustrine facies relate to a closed drainage network and a local base level (Fig. 8). This depocenter was developed penecontemporanously with the incremental construction of the Chollay Plutonic Complex conforming the coupled magmatic system of a caldera type volcanic complex (Salazar and Coloma, 2016;López et al., 2015). The Chollay Plutonic Complex is flanked westward by the EPM (Fig. 4B), a ductile shear zone with left-lateral displacement recorded at 242 Ma (Murillo et al., 2013; Table 1) and whose surface expression could have been the structures bounding the Guanaco Sonso basin to the west (Fig. 8).
The San Félix forearc basin is bounded to the east by the Las Pircas fault and the western block of the EPM, which is composed of Pennsylvanian to Cisuralian igneous rocks (Chanchoquín Plutonic Complex) intruding the ETMC. This block is the footwall of the Las Pircas normal fault, hence, it constitutes a relatively uplifted block and the main source for the sedimentary rocks of the San Félix basin, as depicted by lithic components, detrital zircon patterns and textural maturity of the San Félix Formation sedimentary rocks. Lopingian to Middle Triassic (Ladinian) detrital zircon input to this unit is interpreted as the contribution of highly explosive eruptions related to the Guanaco Sonso basin evolution that could have episodically poured juvenile zircons into the drainage system feeding the San Félix basin (Fig. 8). Maximum accommodation of Members M1 to M4 is reached between Las Pircas and the San Félix faults (Fig. 2), suggesting these faults could have been initially bounding a graben structure ( Fig. 8) but they were reversely reactivated during Andean orogenesis.
Sedimentary provenance analysis on the San Félix and the lower Canto del Agua Formations evidences local sources for both depocenters, which suggests emerged landmasses offshore from the San Félix basin coastline (Fig. 8). Possible present-day analogues for such landmasses can be recognized as longitudinal island belts or bathymetric highs exhumed as footwalls of normal faults developed in the inner wedge of some accretionary prisms (e.g., Wang and Hu, 2006;Gallen et al., 2014).

The Carnian unconformity
The regional unconformity documented at the base of the Upper Triassic units in the study area evidences a regional exhumation event taking place after the Anisian.
In the Coastal Cordillera, the unconformity between the fluvial upper section of the Canto del Agua Formation (Norian) and the underlying Anisian-Ladinian delta deposits of the Lower section reveals only a minor exhumation that doesn't expose deep crustal levels. In the Frontal Cordillera, however, the unconformity documented at the base of the La Totora and the M5 Member of the San Félix Formation reveals marked EW variations in magnitude of exhumation. West of the Paleozoic block, the exhumation is similar to the observe in the Coastal Cordillera as it exposes Anisian marine sediments, while to the east, the exhumation is maximum, exposing Anisian intrusives and mylonites. Further east, exhumation gradually diminishes to expose Lopingian to Anisian volcanics. This lateral variation is similar to the one described for the Pastos Blancos Formation (233-221 Ma) immediately south of the study area, where its maximum thickness is located covering the contacts between Paleozoic intrusives to the west and Triassic intrusives to the east, and Triassic volcanics in its easternmost outcrops (Ortiz and Merino, 2015).
The E-W variation on exhumation magnitude in the Frontal Cordillera evidences the growth of a regional NS trending antiform. The axis of this antiform follows a regional shear zone along the contact between the Chollay and the Chanchoquín plutonic complexes (Figs. 2 and 8). Along the hinge of this antiform, the volcanotectonic depression of the La Totora basin is later developed during the second stage (Fig. 8). This antiform could be explained by passive rift extension, that is, thermal bulging by heat input from the mantle leading to normal faulting and magmatism. Such scenario is consistent with the mantle like magma source for Late Triassic magmatism described at the study area (Hervé et al., 2014;González et al., 2018).
On the other hand, in the Coastal Cordillera, this unconformity has been related to a transpressional exhumation event between Middle to Late Triassic along the Chilean coast (Kato and Godoy, 2015;Godoy, 1985). The latter is consistent with a major shift on the global motion of Gondwana at around 230 Ma, from a counterclockwise rotation with an Euler pole located in northernmost South America to a westward drift of the whole supercontinent (Torsvik et al., 2012;Matthews et al., 2016).

Second stage
During the Norian and Rhaetian, andesitic lavas and tuffs of the La Totora Formation were accommodated within a volcanotectonic depression at the present-day Frontal Cordillera (Fig. 4E), as part of the intra-arc basin (Fig. 8). These deposits seal the activity of two major structural features developed in the previous stage, the Las Pircas fault (Fig. 4C) and the El Portillo Mylonites shear zone (Figs. 8 and 4B). In the forearc basin, its volcanic deposits laterally interfingers with the M5 Member fluvial deposits (Fig. 8), hence, recording a westward migration of the coastline towards the Canto Del Agua depocenter (Fig. 8). On the other hand, the volcanic activity that took place in the Frontal Cordillera is recorded as felsic pyroclastic deposits in the Coastal Cordillera, probably derived from the Pastos Blancos Formation related volcanism, located a few km south of the study area (Table 1). During this stage, the facies arrangement in the Canto del Agua depocenter shows a local transgression, not evidenced in the San Félix Depocenter (Fig. 7). Such transgression may be related to local extensional tectonism evidenced in the contemporaneous fault assisted emplacement of the Carrizal Bajo Plutonic Complex ( Fig. 8; Grocott et al., 2009).

Major unconformities and their regional correlations
The Carboniferous to Triassic stratigraphic record of the outward Pangea margin shows global discontinuities that mark major shifts in the geodynamic configuration of the supercontinent at 280, 260 and 230 Ma (Riel et al., 2018). In the studied segment of the southwestern margin of Gondwana, recent studies and the results of this work allow to better constraint the timing and distribution of these unconformities (Fig. 9).
The Cisuralian unconformity that places the Laguna Chica Formation over deformed Devonian sediments in the Frontal Cordillera correlates well with a regional unconformity defined in the San Rafael Block (Argentina), where Cisuralian volcanics of the Choiyoi Group overlie deformed Carboniferous sediments (Rocha-Campos et al., 2011;Fig. 9). This unconformity has been referred to as the San Rafael tectonic phase (Azcuy and Caminos, 1987), and has been well documented also in the Argentine Frontal Cordillera and Precordillera provinces (Fig. 9). Its presence in the Coastal Cordillera is uncertain, however, it might be represented in the unconformity by which the Llanos del Chocolate beds cover the penecontemporaneous PCMC near the Chañaral Creek, in an structurally complex accretionary prism environment (Figs. 2 and 9; Creixell et al., 2016).
A Lopingian-Triassic unconformity is described in the Chilean Frontal Cordillera, by which members M1 to M4 of the San Félix Formation overlie Carboniferous-Cisuralian igneous rocks and Devonian metasediments in the west, and the Guanaco Sonso Formation covers Devonian metasediments and Guadalupian volcanics, in the east (Figs. 2 and 9). This unconformity correlates with the one described at the base of the Cuyo and Uspallata groups in Argentina, where rift related clastic sediments cover Permian volcanics of the Choiyoi Group (Fig. 9). Its presence in the Argentine Frontal Cordillera is not clear since Late Triassic Rancho de Lata Formation directly overlies volcanic rocks assigned to the Choiyoi Group, although the age of the latter is poorly constrained in that area (Fig. 9), hence the presence of Triassic volcanics equivalent to the Guanaco Sonso Formation cannot be discarded, nor a discontinuity in this underlying volcanic pile. In the Coastal Cordillera, this unconformity is not well geochronologically constrained, however, in consideration of the Lower to Middle Triassic age assigned in this work to the lower section of the Canto del Agua Formation in the Plomiza mine area (Fig. 2), it should be equivalent to the basal unconformity through which it overlies Carboniferous-Permian metasedimentary rocks (PCMC).
Finally, the Carnian unconformity described in this work in the Chilean Coastal and Frontal Cordilleras could be equivalent to the basal unconformity of the Rancho de Lata Formation in the Argentinean Frontal Cordillera (Fig. 9), and to changes on the accommodation structures and environmental conditions in the Precordillera (Cuyo basin) recorded in the Marachemill unit (Barredo et al., 2012). Recently, this unconformity has been well constrained in norther Chile at the base of the Domeyko basin (Espinoza et al., 2018).
This regional framework for the main sequences of the Carboniferous-Triassic rock record, will allow to better study the timing and spatial variations of tectonic processes for each evolutionary stage of the southwestern margin of Gondwana and its possible relationship with global geodynamic events.

Conclusions
Between 28° and 29° S, the transition between the Gondwanean and Andean Orogenies during the Triassic is made up of two stages of an intra-arc/forearc basins system, separated by a Carnian unconformity recognized in the Frontal Cordillera and inferred in the Coastal Cordillera in Chile.
During the first stage (Lopingian-uppermost Middle Triassic), the intra-arc basin is represented by the Guanaco Sonso Formation (253-237 Ma), rooted by the Chollay Plutonic Complex (237-248 Ma) and bounded to the west by a left-lateral shear zone recorded in the EPM (242 Ma). The forearc basin is represented by two extensional marine to transitional depocenters separated by topographic highs, the San Félix basin to the east and the Canto 1 Salazar et al., 2013. 2 Maksaev et al., 2014. 3 Ortiz and Merino, 2015. 4 Salazar and Coloma, 2016. 5 Sato et al., 2015. 6 Rocha-Campos et al., 2011. 7 Domeier et al., 2011. 8 Gómez et al., 2015. 9 Ottone et al., 2014. 10 Strazzere and Gregori 2011. 11 Spalletti et al., 2008. 12 Ávila et al., 2006. 13 Barredo et al., 2012. 14 Creixell et al., 2016   FIG. 9. Regional correlations for lithostratigraphic units and unconformities (based on timescale by Cohen et al., 2013). del Agua basin to the west, filled by the homonymous formations. This stage is recorded in members M1 to M4 of the San Félix Formation and in the lower section of the Canto del Agua Formation, both of which carry Anisian fossil fauna and where maximum depositional ages of 254 and 265 Ma were obtained from detrital zircon analyses, respectively. Detrital zircons, clast composition and sediment maturity indicate local sources for both depocenters. Particularly, for the San Félix Formation, the main source of sediment is the Triassic-Paleozoic block located at the footwall of the normal Las Pircas fault that bounds the San Félix basin to the east, with minor inputs from the volcanic activity related to the Guanaco Sonso basin evolution.
The Carnian unconformity shows a major exhumation in the Chilean Frontal Cordillera where it gets to expose the Chollay Plutonic Complex (237-248 Ma) and the El Portillo Mylonites (242 Ma), while in the forearc basin it is recorded by a an erosional unconformity where Norian fluvial deposits overlie Anisian prodelta deposits in both, San Félix and Canto del Agua formations (Fig. 5).
During the second stage (Norian-Rhaetian), lavas and pyroclastic deposits from La Totora Formation and the Member 5 of the San Félix Formation sealed the activity of the Las Pircas fault and the El Portillo Mylonites shear zone. At the Coastal Cordillera, the volcanic activity present in Canto del Agua Formation is recorded by acid pyroclastic deposits probably derived from the Pastos Blancos Formation related volcanism. Furthermore, the upper sedimentary levels of Canto del Agua depocenter recorded a local transgression, which is absent in the San Félix Formation, and coincides with local extensional emplacement of the Carrizal Bajo Plutonic Complex.
Regionally, the Carnian unconformity has not been documented before as such, although it is suggested to correlate with other unconformities at the bases of Late Triassic units in the Frontal Cordillera.