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, S 1 . As the F 1 folds became slightly overturned to the west, S 1 was folded about west-vergent open F 2 folds and an S 2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S 2 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 R f /ö 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.

ABSTRACT. In the Coastal Cordillera of central Chile a coherently preserved architecture of a late Paleozoic accretionary prism is exposed at 36°-35°S in close spatial association with a neighbouring area at 34°-35°S, where it is strongly modifi ed by post-accretional processes. Syn-and post-accretional structures can be distinguished relatively easily in this region studying the deviations from the original architecture. South of 35°S a transitional contact between two major units is observed, which refl ects a continuous change of the mode of accretion in the accretionary wedge before ~305 Ma: the structurally overlying metagreywacke of the Eastern Series exhibits structures typical of frontal accretion, i.e., subvertical chevron folds of bedding planes with an axial-plane foliation S 1 . With increasing fi nite strain structurally downwards, open F 2 folds develop associated with a S 2 -foliation which becomes gradually fl attened as it rotates into a subhorizontal orientation. S 2 is the penetrative transposition foliation in the structurally underlying Western Series. It affects the continent-derived metagreywacke series as well as metabasite intercalations of oceanic origin and was formed during basal accretion. This principal evolution of the accretionary system places fi rm constraints on the original architecture also in regions where it was destructed after accretion. Accretion ceased at ~225 Ma, when a major tectonic change from a convergent to an extensional/strike-slip regime occurred. Although the development of the margin in central Chile is largely characterized by extension during Mesozoic and Cenozoic times, two pronounced episodes involving shortening of the forearc particularly affected the Western Series north of 35°S: 1. Expressions of strike-slip activity during Jurassic times involve local steepening of the originally fl at S 2 -foliation planes, local rotation of the stretching lineation L 2 into the N-S direction, tight upright folding of the S 2 foliation and refolding about steep axes with associated vertical cataclastic left-lateral shear zones. 2. The left-lateral reverse Pichilemu-Vichuquén fault at the boundary between both units is a prominent brittle structure that formed at ~100 Ma concomitant with basin closure and acceleration of exhumation rates in the forearc. Similar contractional structures occur along the coast further north, where both units partly disappeared by subduction erosion most likely during these deformation episodes. The transition between nearly unaffected accretion systems in the south and disrupted and partly subducted ones in the north occurs at 35°S. (34°-35°S)... (34°-35°S), durante deformación posacrecional. En la Cordillera de la Costa de Chile, de 36° a 35°S, se encuentra expuesta la arquitectura coherentemente preservada de un prisma de acreción del Paleozoico tardío, en cercana vecindad a un área entre los 34° y 35°S, donde el mismo está fuertemente modifi cado por procesos postacrecionales. Estudiando las desviaciones de la arquitectura original,...

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We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31-32 • S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10-100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40 Ar-39 Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174-165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40 Ar-39 Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ∼130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ∼1400 km length.

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