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

Abstract

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.

Structural overprint of a late Paleozoic accretionary system in north-central Chile (34°-35°S) during post-accretional deformation

“…Along two prominent traverses from E to W, i.e., the Río Maule transect (Godoy, 1970;Richter et al, 2007) and along the abandoned railway line east of Pichilemu, it can be observed that a second foliation S 2 continuously evolves from a widely spaced foliation parallel to subhorizontal west-vergent open F 2 -folds ( Figs. 2A, B) with a penetrative transposition foliation S 2 that becomes increasingly fl attened towards the west, where it becomes the dominant regional foliation in the Western Series.…”

“…For comparison of the results of both methods the conventional octahedral shear strain γ oct was determined. Both methods and their application are described in detail in Richter et al (2007). Along the Río Maule traverse in the southern part of the study area, these authors documented a continuous structural transition between the Eastern and Western Series caused by continuous variation of the strain parameters.…”

“…The Western Series was overprinted at high pressure greenschist facies apart from local blueschist facies conditions (350-420°C, 7-11 kbar), whereas the Eastern Series was contemporaneously overprinted at 3 kbar with progressively rising temperature of 400°-720°C towards a batholith in the east that intruded in the east representing the Late Palaeozoic magmatic arc (Willner, 2005). It was shown by Richter (1977) and Richter et al (2007) that the two major units represent two different principal modes of accretion: initial frontal accretion in the Eastern Series that gradually changes to basal accretion (underplating to the base of the wedge) in the Western Series. Particularly, the traverse east of Constitución (35°30'S) was shown to represent a complete and coherent architecture of the accretionary system (Willner, 2005;Richter et al, 2007).…”

“…1) González-Bonorino (1971) and Aguirre et al (1972) fi rst proposed that this basement represents deep-seated parts of a palaeo-accretionary wedge within a paired metamorphic belt, which was active in late Paleozoic times (Hervé et al, 1984). Whereas the architecture of this accretionary system was strongly overprinted in northern Chile during Mesozoic times, it is largely well preserved in central Chile, where its characteristics have recently been studied in detail (Glodny et al, 2005(Glodny et al, , 2006Willner, 2005;Willner et al, 2005;Richter et al, 2007). Traditionally two units of contrasting structural evolution are recognized in central Chile (Hervé et al, 1984): The Western Series of mixed oceanic and continental rocks is regarded as a high pressure/low temperature unit, whereas the Eastern Series composed of metagreywackes only experienced a low pressure/high temperature metamorphic overprint related to intrusion of an arc batholith.…”

“…It was shown by Richter (1977) and Richter et al (2007) that the two major units represent two different principal modes of accretion: initial frontal accretion in the Eastern Series that gradually changes to basal accretion (underplating to the base of the wedge) in the Western Series. Particularly, the traverse east of Constitución (35°30'S) was shown to represent a complete and coherent architecture of the accretionary system (Willner, 2005;Richter et al, 2007). Particularly in northern Chile, episodic destruction of the continental margin by shortening-related subduction erosion during Mesozoic times is supposed to have affected the metamorphic basement (e.g., Scheuber et al, 1994;Scheuber and González, 1999).…”