Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at ∼38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between ∼306 and ∼286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to ∼250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.     

Morphology and geology of the continental shelf and upper slope of southern Central Chile (33°S–43°S)

The continental shelf and slope of southern Central Chile have been subject to a number of international as well as Chilean research campaigns over the last 30 years. This work summarizes the geologic setting of the southern Central Chilean Continental shelf (33°S–43°S) using recently published geophysical, seismological, sedimentological and bio-geochemical data. Additionally, unpublished data such as reflection seismic profiles, swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated. The outcome is an overview of the current knowledge about the geology of the southern Central Chilean shelf and upper slope. We observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope, due to local redistribution of fluvial input, mainly governed by bottom currents and submarine canyons and highly productive upwelling zones. Shelf basins show highly variable thickness of Oligocene-Quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land. Seismic velocity studies indicate that a paleo-accretionary complex that is sandwiched between the present, relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern Central Chile.     

Complete recycling of a magmatic arc: evidence from chemical and isotopic composition of Quaternary trench sediments in Chile (36°–40°S)

Marine Quaternary trench and slope sediments were sampled along the margin of the Southern Andes, Chile between 36° and 40°S. Major and trace element contents indicate only minor influence of weathering and transport fractionation. The whole rock composition of the sediments is similar to the average rock of the Cretaceous to Holocene magmatic arc of this section of the southern volcanic zone. Sr, Nd, and Pb isotope composition of the sediments also resembles closely the average composition of the magmatic arc. The contribution of compositionally distinct Palaeozoic crust, which makes up most of the volume of the forearc, is ~0–20% crustal Sr, Nd, and Pb according to the isotope record of the trench and slope sediments. Input of sediments from the continent into the subduction system was dominated by detritus from the magmatic arc at least for the last 20 My on the basis of the Oligocene to Holocene exhumation history of the margin.     

Late Eocene to Early Miocene Andean uplift inferred from detrital zircon fission track and U–Pb dating of Cenozoic forearc sediments (15–18°S)

Timing, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from ∼50 to ∼4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon U–Pb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and facies changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at ∼25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19°S already at ∼35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at ∼40, (iii) a flat-subduction period at around ∼40 to ∼30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma.     

Cretaceous reactivation and intensified erosion in the Archean–Proterozoic Limpopo Belt,demonstrated by apatite fission track thermochronology

Cratons are generally assumed to be regions of long-lasting tectonic stability. In particular the study of the Phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore. This fact is even more pronounced in terranes lacking Mesozoic or younger penetrative structural fabrics and metamorphic overprinting. Our study in the Limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks, and has profound implications for the crustal evolution of the Zimbabwe Craton.Apatite fission track data from 35 samples taken along two transects, in the southern edge of the Zimbabwe Craton and in the Central Zone of the Limpopo Belt, suggest that extensive regions experienced kilometer-scale exhumation in two discrete events, as recently as the Cretaceous. The first occurred at around 130 Ma, and the second at around 90 Ma. Basin subsidence and sedimentation loads on the Mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material. Further, the results indicate that young and old “surfaces” (in a geomorphological sense) may be structurally juxtaposed in regions of high elevation in Zimbabwe. This is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation.     

Evolution of a Cambrian active continental margin: The Delamerian–Lachlan connection in southeastern Australia from a zircon perspective

The Early Palaeozoic Ross–Delamerian orogenic belt is considered to have formed as an active margin facing the palaeo-Pacific Ocean with some island arc collisions, as in Tasmania (Australia) and Northern Victoria Land (Antarctica), followed by terminal deformation and cessation of active convergence. On the Cambrian eastern margin of Australia adjacent to the Delamerian Fold Belt, island arc and backarc basin crust was formed and is now preserved in the Lachlan Fold Belt and is consistent with a spatial link between the Delamerian and Lachlan orogens. The Delamerian–Lachlan connection is tested with new zircon data. Metamorphic zircons from a basic eclogite sample from the Franklin Metamorphic Complex in the Tyennan region of central Tasmania have rare earth element signatures showing that eclogite metamorphism occurred at ~ 510 Ma, consistent with island arc–passive margin collision during the Delamerian(− Tyennan) Orogeny. U–Pb ages of detrital zircons have been determined from two samples of Ordovician sandstones in the Lachlan Fold Belt at Melville Point (south coast of New South Wales) and the Howqua River (western Tabberabbera Zone of eastern Victoria). These rocks were chosen because they are the first major clastic influx at the base of the Ordovician ‘Bengal-fan’ scale turbidite pile. The samples show the same prominent peaks as previously found elsewhere (600–500 Ma Pacific-Gondwana and the 1300–1000 Ma Grenville–Gondwana signatures) reflecting supercontinent formation. We highlight the presence of ~ 500 Ma non-rounded, simple zircons indicating clastic input most likely from igneous rocks formed during the Delamerian and Ross Orogenies. We consider that the most probable source of the Ordovician turbidites was in East Antarctica adjacent to the Ross Orogen rather than reflecting long distance transport from the Transgondwanan Supermountain (i.e. East African Orogen). Together with other provenance indicators such as detrital mica ages, this is a confirmation of the Delamerian–Lachlan connection.     

Thermal history of the Krishna–Godavari basin,India: Constraints from apatite fission track thermochronology and organic maturity data

The Krishna–Godavari (KG) basin, a passive margin Late Carboniferous to Holocene basin along the rifted east coast of India, includes the deltaic and inter-deltaic regions of the Krishna and Godavari rivers onshore and extends into the offshore. It is one of India’s premier hydrocarbon-bearing basins. In an attempt to better understand the thermal history of the basin, apatite fission track (AFT) data has been obtained from six exploration wells (five onshore and one offshore). AFT thermal history models as well as other thermal indicators e.g. vitrinite reflectance (VR), Rock–Eval T_max data reveal that the host rocks are currently at their maximum post-depositional temperatures and that any possible heating related to small-scale tectonism or rifting episodes in the basin bears little significance on the maturation of the sediments. In the case of one borehole (M-1) however, the organic maturity data reveals a period of Oligocene cooling across an unconformity when ∼1000 m of section was eroded due to falling sea-level. This information offers the potential for improved basin modeling of the KG basin.     

Middle to late Triassic mélange exhumation along a pre-Andean transpressional fault system: coastal Chile (26°–42° S)

Mélanges occur as discontinuous, mappable, units along an extensive N–S-trending, steeply dipping zone of distributed shear in metamorphic complexes along the coast of central Chile. Large mélange zones, from north to south, near Chañaral, Los Vilos, Pichilemu, and Chiloé Island, contain variations in lithologic and structural detail, but are consistent in exhibiting cross-cutting fabric features indicating a progressive transition from earlier ductile to more brittle deformation. In the Infiernillo mélange near Pichilemu, Permian to Early Triassic, sub-horizontal schistosity planes of the Western Series schist are disrupted, incorporated into, and uplifted along high-angle, N–S- to NNE–SSW-trending brittle–ductile shears. Mylonitic and cataclastic zones within the mélange matrix indicate active lateral shear during cumulative exhumation from depths exceeding 12 km in some areas. Exotic lithologies, such as Carboniferous mafic amphibolite and blueschist, formed during earlier Gondwanide subduction, match well with similar rocks in the Bahia Mansa to Los Pabilos region 750 km to the south, suggesting possible dextral offset. The development of the Middle to Late Triassic, N–S=trending, near-vertical shear zones formed weaknesses in the crust facilitating later fault localization, gravitational collapse, and subduction erosion along the continental margin. The length and linearity of this zone of lateral movement, coincident with a general hiatus of regional arc magmatism during the Middle to Late Triassic, is consistent with large-scale dextral transpression, or possible transform movement, during highly oblique NNE–SSW convergence along the pre-Andean (Gondwana) margin. The resultant margin parallel N–S-trending shear planes may be exploited by seismically active faults along the present coastal area of Chile. The palaeo-tectonic setting during the transitional period between earlier Gondwanide (Devonian to Permian) and later Andean (Late Jurassic to present) subduction may have had some similarity to the presently active San Andreas transform system of California.     

Mesozoic to Cenozoic retroarc basin evolution during changes in tectonic regime,southern Central Andes (31–33°S): Insights from zircon U-Pb geochronology

The southern Central Andes of Argentina and Chile (27–40°S) are the product of deformation, arc magmatism, and basin evolution above a long-lived subduction system. With sufficient timing and provenance constraints, Andean stratigraphic and structural records enable delineation of Mesozoic-Cenozoic variations in subsidence and tectonic regime. For the La Ramada Basin in the High Andes at ∼31–33°S, new assessments of provenance and depositional age provided by detrital zircon U-Pb geochronology help resolve deformational patterns and subsidence mechanisms over the past ∼200 Myr. Marine and nonmarine clastic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback. Subsequent sediment delivery from the Coastal Cordillera corresponded with initial flexural accommodation in the La Ramada Basin during Andean shortening of late Early Cretaceous to Late Cretaceous age. The architecture of the foreland basin was influenced by the distribution of precursor extensional depocenters, suggesting that inherited basin geometries provided important controls on later flexural subsidence and basin evolution. Following latest Cretaceous to early Paleogene tectonic quiescence and a depositional hiatus, newly dated deposits in the western La Ramada Basin provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension (development of the Abanico Basin, principally in Chile, at ∼28–44°S). Inversion of this late Paleogene extensional basin system during Neogene compression indicates the southern Central Andes were produced by at least two punctuated episodes of shortening and uplift of Late Cretaceous and Neogene age.     

Calbuco Volcano and minor eruptive centers distributed along the Liquiñe-Ofqui Fault Zone,Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20 S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO₂ contents in the 55–60% range, and have comparatively low K₂O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO₂ contents, low ¹⁴³Nd/¹⁴⁴Nd and high ⁸⁷Sr/⁸⁶Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.     

Restoring the Silurian to Carboniferous northern active continental margin of the Mongol–Okhotsk Ocean in Mongolia: Hangay–Hentey accretionary wedge and seamount collision

Trapped between the Siberian Craton to the north, and the Tarim and North China blocks to the south, the Mongol–Okhotsk Belt represents a young portion of the Central Asian Orogenic Belt. The Andean-type orogeny was caused by the closure of the Mongol–Okhotsk Ocean, whose timing and mode of geodynamic evolution still needs to be clarified in more detail. This paper investigates the lithostratigraphic units of the Gorkhi Formation, which are comprised in the Hangay–Hentey terrane representing the accretionary prism of the northern continental margin of the Mongol–Okhotsk Ocean. In the tectonically disrupted slices of the Gorkhi Formation we identify a typical ocean plate stratigraphy with pelagic to hemipelagic and turbiditic sequences. Locally, shallow marine limestones are found associated with mafic volcanic rocks. XRF and LA–ICPMS measurements of these volcanic rocks clearly indicate an OIB signature. The geochemistry together with lithostratigraphic observations permit us to reconstruct the evolution of a seamount, ranging from its growth in shallow water depths and drowning due to subduction-related subsidence linked with subsequent radiolarian chert, hemipelagic shale and turbiditic sandstone cover sedimentation. Calc-alkaline felsic dikes later intruded the accreted silt and sandstone deposits. Standard sandstone provenance analysis, detrital zircon U–Pb laser ablation ICP-MS dating and Hf isotope ratios in the sandstones document a prevailing continental volcanic arc sourcing during Carboniferous from the northern margin of the Mongol–Okhotsk Ocean.     

Late thermal events based on zircon fission track ages in northeastern New South Wales and southeastern Queensland: Links to Sydney Basin seismicity?

Zircon, concentrated from basaltic terrains in northeastern New South Wales and southeastern Queensland, reveals some unexpectedly young fission track peaks. Between 2 to 13 Ma in age, these peaks are younger than known Tertiary basaltic ages from these regions which match older fission track peaks. Analysis of the fission track data suggests that the young dates are probably not reset ages due to recent heating events such as bush fires, but more likely mark thermal resetting by later volcanic eruptions.

The young ages decrease southwards from Queensland through northern New South Wales and trend toward seismic zones within the Sydney Basin in the Newcastle, Blue Mountains and Illawarra regions. A model based on northward motion of the Australian plate over a hot asthenospheric source (0.75° latitude/Ma over 12 Ma)) predicts the positions of most young zircon ages to within ± 70 km in latitude when projected from seismic sites at Newcastle and Bowral‐Robertson.

A minor hot spot source is proposed, some 250 km across, which triggered isolated basaltic and zircon‐bearing eruptions every few million years and now underlies the southern Sydney Basin. This would bring Sydney Basin seismicity into line with other seismic zones known at present hot spot positions across southeastern Australia and the Tasman Sea. It raises questions concerning activation of local seismicity, potential for volcanic risk and distribution of young uplift in the Sydney region. Similar studies are needed to test other puzzling seismic zones (e.g. the Dalton‐Gunning Zone).     

Ordovician passive continental margin magmatism in the Central-European Variscides: U–Pb zircon data from the SE part of the Karkonosze-Izera Massif, Sudetes, SW Poland

Approximately 500-Ma-old orthogneisses are widespread in the eastern part of the Variscan belt and are commonly interpreted to have intruded mica-schist series of assumed Neoproterozoic age. New SHRIMP zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the Karkonosze-Izera Massif (SW Poland) indicate that they are late Cambrian/early Ordovician rather than Neoproterozoic in age, based on the zircon age spectra distributed mainly between ca. 500 and 660 Ma (with a few Proterozoic inherited minimum ages of ca. 970 and 1,825 Ma). Younger zircon dates, dispersed between ca. 412 and 464 Ma, are interpreted as a result of Pb-loss likely caused by subsequent metamorphism. Consequently, the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca. 500-Ma-old orthogneiss protoliths (with the pooled concordia age of 487 ± 8 Ma interpreted as the best approximation of the protolith intrusive age). Field relationships, petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca. 500 Ma granite intrusions, and indicate that their recent close proximity is the result of tectonic juxtaposition. However, both the mica schists enclosing the bimodal metavolcanic rocks, and the orthogneisses, are interpreted to represent a Cambro-Ordovician passive continental margin sequence being part of the Saxothuringian domain. They are tectonically overlain to the east by HP/T metamorphic units, comprising MORB-type metaigneous rocks, and delineating a tectonic suture separating the Saxothuringian block in the west from an assumed continental block (Tepla-Barrandian) to the south-east.     

Paleoseismicity and seismic hazard in southern Patagonia (Argentina-Chile; 50°–55°S) and the role of the Magallanes-Fagnano transform fault

Patagonia, including the island of Tierra del Fuego, lies in southernmost South America at the junction of the South American, Antarctic, and Scotia tectonic plates. Historical and instrumental records have documented several local earthquakes of damaging magnitude, posing a threat to the rapidly growing population of 300,000 and the expanding industrial and service infrastructure. Short and inaccurate instrumental records of local seismic events and a diffuse epicenter distribution not clearly related to the recognized seismogenic structures have hindered an adequate evaluation of the seismic hazard for this region. To improve this situation, a paleoseismological study was carried out on two gravelly strandplains on the Atlantic coast of Patagonia. Surveying combined ground-probing radar, vertical electric sounding, and seismic refraction. Coseismic normal faults buried beneath the strandplain bodies were revealed and related to the morphology of the strandplains. The faults have probable ages between 0.9 and 6.4 kyr BP and a recurrence rate of about 1 kyr. The more likely source for these structures is the Magallanes-Fagnano fault, a continental transform fault that crosses Tierra del Fuego. The distance of more than 300 km from the buried coseismic structures to the trace of the Magallanes-Fagnano fault argues for high-magnitude earthquake activity on this fault throughout the Holocene. Urban development on soft glacial and alluvial substrates increases the hazard.     

Paleocene–early Eocene post-subduction magmatism in Sikhote-Alin(Far East Russia):New constraints for the tectonic history of the Izanagi-Pacific ridge and the East Asian continental margin

New isotopic,geochemical and geochronological data justify the widespread occurrence of middle Paleocene to early Eocene(60–53 Ma) post-subduction felsic magmatism across the entire Sikhote-Alin territory(southeastern Russia),conform with previous observations in Northeast China,the southern Korean Peninsula,and the Inner Zone of Japan.This igneous activity in East Asia coincided with the reactivation(after tectonic quiescence between ～93–60 Ma) of left-lateral strike-slip displacements along the Tan-Lu and Central Sikhote-Alin faults and with the post-60 Ma cessation of subduction/accretion recorded in the Shimanto belt of SW Japan.The SikhoteAlin post-subduction igneous A-type rocks present diverse mineralogical and geochemical features that suggest interactions of the subducting plate with anhydrous mantle upwelling through slab tears in the continental margin.The middle Paleocene–early Eocene magmatism is not related to subduction but is synchronous with strikeslip tectonics and the termination of accretionary prism development,suggesting a shift in tectonic regime from oceanic plate subduction at a convergent margin to parallel sliding and initiation of a transform continental margin.These new observations are inconsistent with the current tectonic model of 60–50 Ma Izanagi-Pacific ridge subduction beneath East Asian continental margin.     

The crustal structure beneath the Central Andean forearc and magmatic arc as derived from seismic studies — the PISCO 94 experiment in northern Chile (21°–23°S)

In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35–45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high v_p/v_s ratios in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high v_p/v_s ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.     

The Mercedario rift system in the principal Cordillera of Argentina and Chile (32° SL)

Recent studies carried out in the High Andes of central-western Argentina in the provinces of San Juan and Mendoza have established its stratigraphic and structural evolution. This paper presents new data on the Triassic–Early Jurassic rift system, the depositional sequences, and a synthesis of the tectonic evolution of the region, along with a correlation with the Chilean continental margin.The paleogeographic evolution of the Cordillera Principal at these latitudes is controlled by the development of the Mercedario rift system. This rift began with the sedimentation of synrift deposits of the Rancho de Lata Formation, during the Rhetian (about 190 Ma). Subsidence was driven by normal faults, locally preserved in spite of the severe tectonic inversion of the Andes during the Cenozoic. Different authors have emphasized that an important extension dominated the transition between the Triassic and Jurassic periods along the magmatic arc in the Coastal Cordillera of Chile on the western side of the Andes. Extension was related to the bimodal magmatism that characterized the evolution of this segment (30°–33° SL). The granitic plutonism and the associated mafic volcanism indicate that they were controlled by extension during 220–200 Ma. The first subduction related granitoids at these latitudes are 170 Ma old (Bathonian).The geometry of the Mercedario rift system may be reconstructed by the pattern of the normal faults. Rifting was followed by a thermal subsidence that expanded the original area of sedimentation and controlled the paleogeography of the Los Patillos Formation during Pliensbachian to early Callovian times. This period of cooling and thermal subsidence is correlated with magmatic quiescence in the continental margin. The evolution of the basin closely matches the magmatic history of the Chilean continental margin. Subduction at the continental margin began in the Bathonian, together with deposition of the upper section of Los Patillos Formation.Arc magmatism shifted to the Cordillera Principal during the Kimmeridgian, where it is represented by the volcanic and volcaniclastic deposits of Tordillo Formation.Early Mesozoic evolution of the Andean system at these latitudes is, thus, reconstructed by a comparative analysis of these two adjacent regions, driven by a common tectonic regime, but through different geological processes.     

Miocene to recent history of the western Altiplano in northern Chile revealed by lacustrine sediments of the Lauca basin (18°15′–18°40′ S/69°30′–69°05′W)

The intramontane Lauca Basin at the western margin of the northern Chilean Altiplano lies to the west of and is topographically isolated from the well-known Plio-Pleistocene lake system of fluvio-lacustrine origin that covers the Bolivian Altiplano from Lake Titicaca to the north for more than 800 km to the Salar de Uyuni in the south. The Lauca Basin is filled by a sequence of some 120 m of mainly upper Miocene to Pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin. Volcanic rocks, partly pyroelastic, provide useful marker horizons. In the first period (6–4 Ma) of its evolution, the Lago Lauca was a shallow ephemeral lake. Evaporites indicate temporarily closed conditions. After 4 Ma the lake changed to a perennial water body surrounded by alluvial plains. In the late Pleistocene and Holocene (2-0 Ma) there was only marginal deposition of alluvial and glacial sediments. The basin formed as a half-graben or by pull-apart between 10 and 15 Ma (tectonic displacement of the basal ignimbrite sequence during the Quechua Phase) and 6.2 Ma (maximum K/Ar ages of biotites of tuff horizons in the deepest part of the basin). Apart from this early basin formation, there has been surprisingly little displacement during the past 6 Ma close to the Western Cordillera of the Altiplano. Also, climate indicators (pollen, evaporites, sedimentary facies) suggest that an arid climate has existed for the past 6 Ma on the Altiplano. Together, these pieces of evidence indicate the absence of large scale block-faulting, tilt and major uplift during the past 5–6 Ma in this area.