Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile

Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S1. As the F1 folds became slightly overturned to the west, S1 was folded about west-vergent open F2 folds and an S2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by Rf /ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-tothe- east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c. 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c. 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.researc

Structural contacts in the Late Paleozoic accretionary wedge of central Chile and their tectonic significance for the evolution of the accretionary complex

The Chilean accretionary wedge is part of a Late Paleozoic subduction complex that developed during subduction of the Pacific plate underneath South America. The wedge is commonly subdivided into a structurally lower Western Series and an upper Eastern Series. Understanding the contact between both series has been a long standing problem and is fundamental for the understanding of the evolution of the wedge system. We show the progressive development of structures and finite strain from the least-deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to higher grade schist of the Western Series at the Pacific coast...conferenc

High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone related to the Tapo ultramafic body, Eastern Cordillera of Central Peru

A discontinuos belt of elongated ultramafic rock bodies (mostly serpentinites) occurs in the Eastern Cordillera of the central Peruvian Andes. One of the main occurrences is the Tapo Massif, a lense-shaped serpentinite body, ~2 km x 5 km, comprising small podiform chromitite deposits (Castroviejo et al., 2009) and bands or lenses of garnet-amphibolite, both strongly sheared and thrust upon the upper Paleozoic sediments of the Ambo Group (Fig. 1). Metabasite geochemistry suggests a mid-ocean ridge or an ocean island protolith. The whole sequence can be interpreted as a disrupted ophiolitic complex (Castroviejo et al., 2010). The geological setting of the Tapo occurrence is described by J.F. Rodrigues et al. (2010). To get information about its geotectonic setting we applied new geothermobarometric techniques to the garnet amphibolite. Finding representative samples with an adequate mineralogy to apply these techniques is in this case a difficult task. A common problem is the almost ubiquitous overprinting by serpentinisation or retrograde metamorphism and, locally, by metasomatism or alteration enhanced by deformation, producing a variety of rock types, as rodingites, birbirites and listvaenites. Nevertheless, careful sampling followed by petrographic examination of the rocks allowed to identify some samples in which useful assemblages are present

Crustal Evolution of the Northeast Laurentian Margin and the Peri-Gondwanan Microcontinent Ganderia Prior to and During Closure of the Iapetus Ocean: Detrital Zircon U–Pb and Hf Isotope Evidence from Newfoundland

Detrital zircon populations in sedimentary rocks from the Laurentian margin and the accreted microcontinent Ganderia on both sides of the main Iapetus suture (Red Indian Line) in central Newfoundland have been studied by combined U–Pb and Lu–Hf isotope analyses. Variation in εHf(t) values with age of zircon populations of distal provenance (>900 Ma) reflect the crustal evolution within the source continents: in zircon derived from Laurentia, episodes of juvenile magma production in the source could be detected at 1.00 – 1.65 and 2.55 – 3.00 Ga, and mixing of juvenile and recycled crust in continental magmatic arcs occurred at 0.95 – 1.40, 1.45 – 1.60, 1.65 – 2.05 and 2.55 – 2.75 Ga. These ages are consistent with the crustal history of northeastern Laurentia. Similarly, zircon of distal provenance from Ganderia reveals times of juvenile magma production in the source at 0.70 – 0.90, 1.40 – 1.75, 1.85 – 2.40 and 2.7 – 3.5 Ga, and episodes of mixing juvenile and recycled crust at 0.95 – 1.35, 1.45 – 1.60, 1.70 – 2.15 and 2.6 – 2.8 Ga. These data reflect the crustal evolution in the present northern part of Amazonia, its likely source craton.      The evolution of magmatic arcs at the margins of both continents can be studied in a similar way using detrital zircon having a proximal provenance (<900 Ma). In contrast to the Laurentian margin, Ganderia is characterized by development of Neoproterozoic – Cambrian continental arcs (ca. 500 – 670 Ma) that were built on the margin of Gondwana. εHf(t) values indicate recycling of Neo- and Mesoproterozoic crust. During and following accretion of the various elements of Ganderia to Laurentia, the syn-tectonic Late Ordovician to Silurian sedimentary rocks deposited on the upper plate (composite Laurentia) continued showing only detritus derived from Laurentia. These sedimentary rocks contain detrital zircon from Iapetan juvenile, continental and successor arcs that were active between ca. 440 and 550 Ma, and from continuing magmatic activity until 423 Ma. Arrival of the first Laurentian detritus at the outermost part of Ganderia indicates that the Iapetus ocean was closed at ca. 452 Ma. The magmatic arcs along the former Laurentian margin in Newfoundland evolved differently. In the northwestern part, εHf(t) values point to recycling of Mesoproterozoic and Paleoproterozoic crust. In the southwest, εHf(t) values indicate addition of juvenile crust, recycling of Mesoproterozoic crust and mixing with juvenile magma. SOMMAIRELes populations de zircons détritiques des roches sédimentaires issus de la marge laurentienne et du microcontinent d’accrétion de Ganderia, des deux côtés de la principale suture Iapetus (linéation de Red Indian) dans le centre de Terre-Neuve, ont été étudiés par analyses combinées U–Pb et Lu–Hf.  Les variations des valeurs εHf(t) en fonction de l’âge des populations de zircons distaux (>900 Ma) reflètent l’évolution de la croûte des continents sources : les zircons de Laurentie ont permis de détecter des épisodes magmatiques juvéniles dans la source entre 1,00 - 1,5, et 2,55 - 3,00 Ga, ainsi que des épisodes de mélange de croûte juvénile avec des croûtes d’arcs magmatiques continentaux recyclés entre 0,95 – 1,40, 1,45 – 1,60, 1,65 – 2,05, et 2,55 – 2,75 Ga.  Ces datations correspondent bien à l’histoire de la croûte de la portion nord-est de la Laurentie.  De même, le zircon distal de Ganderia révèle des épisodes de production de magmas juvéniles dans la source entre 0,70 - 0,90, 1,40 - 1,75, 1,85 - 2,40, et 2,7 - 3,5 Ga, ainsi que des épisodes de mélanges de matériaux juvéniles et de croûtes recyclés entre 0,95 - 1,35, 1,45 - 1,60, 1,70 - 2,15, et 2,6 - 2,8 Ga.  Ces données reflètent l’évolution de la croûte dans la portion nord actuelle de l’Amazonie, son craton source probable.     L’évolution des arcs magmatiques à la marge de ces deux continents peuvent être étudiées de la même manière en utilisant le zircon détritique proximal (<900 Ma).  Contrairement à la marge laurentienne, celle de Ganderia est caractérisée par le développement d’arcs continentaux Néoprotéozoïque-Cambrien (env. 500 – 670 Ma) qui se sont constitués à la marge du Gondvana.  Les valeurs de εHf(t) indiquent un recyclage de la croûte au Néoprotérozoïque et au Mésoprotérozoïque.  Durant et après l’accrétion des divers éléments de Ganderia et de la Laurentie, les roches sédimentaires syntectoniques de la fin de l’Ordovicien et du Silurien qui se sont déposées sur la portion supérieure de la plaque (Laurentie composite) ne montrent toujours que des débris provenant de la Laurentie.  Ces roches sédimentaires renferment des zircons détritiques juvéniles iapétiques, et d’arcs continentaux et d’arcs subséquents, qui ont été actifs entreentre (env. 440 et 550 Ma) et  d’une activité magmatique continue jusqu’à 423 Ma.  L’apport des premiers débris à la marge extrême de Ganderia indique que l’océan s’est fermée il y a env. 452 Ma.  Les arcs magmatiques le long de l'ancienne marge laurentienne à Terre-Neuve ont évolué différemment.  Dans la portion nord-ouest, les valeurs de εHf(t) indiquent un recyclage de la croûte au Mésoprotérozoïque et au Paléoprotérozoïque.  Dans la portion sud-ouest, les valeurs de εHf(t) indiquent l’ajout d’une croûte juvénile, un recyclage de la croûte mésoprotérozoïque et un mélange avec un magma juvénile

Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Perú (Tarma province)

High-pressure conditions of 11–13 kbar/500–540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm–Nd mineral-whole rock isochron at 465 ± 24 Ma dates fluid influx at peak temperatures of ~600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12–13 °C/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Marañon Complex. Here, low PT conditions at 2.4–2.6 kbar, 300–330 °C were estimated for a phyllite–greenschist assemblage representing a contrasting metamorphic geotherm of 32–40 °C/km characteristic for a magmatic arc environment

Significado de las ofiolitas neoproterozoicas en la Cordillera Oriental del Perú

Los cinco trabajos precedentes (Rodrigues et al. 2010, a y b, Tassinari et al. 2010, Castroviejo et al. 2010 b, Willner et al. 2010) han tratado distintos aspectos de los terrenos ultramáficos conocidos en la COr, Cordillera Oriental, cuya ubicación, destacada en rojo, puede encontrarse en la figura 1. Es el momento de intentar una síntesis que, mediante la integración y la discusión de los resultados hasta ahora obtenidos, permita alcanzar una interpretación y, al menos, esbozar respuestas a algunas cuestiones fundamentales acerca del basamento pre-andino de la Cordillera. En primer lugar es pertinente examinar, a la vista de los nuevos datos presentados, la cuestión esencial: ¿se trata realmente de ofiolitas? Luego, habrán de plantearse cuestiones como el emplazamiento y la historia geológica de los terrenos implicados, en el marco de la evolución del supercontinente Rodinia y de la Cadena Andina, para finalmente tratar de comprender su significado geotectónic

Significado de las ofiolitas neoproterozoicas en la Cordillera Oriental del Perú

Los cinco trabajos precedentes (Rodrigues et al. 2010, a y b, Tassinari et al. 2010, Castroviejo et al. 2010 b, Willner et al. 2010) han tratado distintos aspectos de los terrenos ultramáficos conocidos en la COr, Cordillera Oriental, cuya ubicación, destacada en rojo, puede encontrarse en la figura 1. Es el momento de intentar una síntesis que, mediante la integración y la discusión de los resultados hasta ahora obtenidos, permita alcanzar una interpretación y, al menos, esbozar respuestas a algunas cuestiones fundamentales acerca del basamento pre-andino de la Cordillera. En primer lugar es pertinente examinar, a la vista de los nuevos datos presentados, la cuestión esencial: ¿se trata realmente de ofiolitas? Luego, habrán de plantearse cuestiones como el emplazamiento y la historia geológica de los terrenos implicados, en el marco de la evolución del supercontinente Rodinia y de la Cadena Andina, para finalmente tratar de comprender su significado geotectónic

Significado de las ofiolitas Neoproterozoicas de la Cordillera Oriental del Perú (9°30'-11°30')

Los cinco trabajos precedentes (Rodrigues et al. 2010, a y b, Tassinari et al. 2010, Castroviejo et al. 2010 b, Willner et al. 2010) han tratado distintos aspectos de los terrenos ultramáficos conocidos en la COr, Cordillera Oriental, cuya ubicación, destacada en rojo, puede encontrarse en la figura 1. Es el momento de intentar una síntesis que, mediante la integración y la discusión de los resultados hasta ahora obtenidos, permita alcanzar una interpretación y, al menos, esbozar respuestas a algunas cuestiones fundamentales acerca del basamento pre-andino de la Cordillera. En primer lugar es pertinente examinar, a la vista de los nuevos datos presentados, la cuestión esencial: ¿se trata realmente de ofiolitas? Luego, habrán de plantearse cuestiones como el emplazamiento y la historia geológica de los terrenos implicados, en el marco de la evolución del supercontinente Rodinia y de la Cadena Andina, para finalmente tratar de comprender su significado geotectónico