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_Journal of Structural Geology 32 (2010) 464-477_ _Andean sinistral transpression and kinematic partitioning in South Georgia_ _Michael L. Curtis*, Michael J. Flowerdew, Teal R. Riley, Martin J. Whitehouse, J. Stephen Daly_ _British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, UK; Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden; UCD School of Geological Sciences, University College Dublin, Bel?eld, Dublin 4, Ireland_ _Article history: Received 16 October 2009; Received in revised form 26 January 2010; Accepted 17 February 2010; Available online 23 February 2010_ _Keywords: Geochronology Fold appression Back-arc basin Polyphase deformation_ _Abstract_ _The island of South Georgia exposes remnants of a Late Jurassic to Early Cretaceous Andean magmatic arc and marginal basin system that was compressively deformed during the mid-Cretaceous main Andean Orogeny, forming widespread NW-SE trending folds and a coaxial penetrative cleavage displaying a predominantly NE-SW stretching lineation._ _Detailed structural studies of the Cooper Bay to Cape Vahsel area reveal intense, mid-Cretaceous polyphase deformation strongly influenced by sinistral strike-slip shear parallel to the NW-SE regional structural grain, and along a major pre-existing fault. We interpret this as the partitioned wrench component of bulk transpressional deformation. The relationship between fold axial plane orientation and interlimb angle of widely distributed mesoscale folds is consistent with counterclockwise rotation and fold appression due to sinistral simple shear deformation, suggesting kinematic strain partitioning of the wrench component was highly efficient. Locally, modification of steep tectonic anisotropies to shallow inclinations during D2 deformation induced imperfect or inefficient partitioning with fold arrays exhibiting characteristic transpressional deformation._ _Our partitioned transpression model for main Andean deformation of South Georgia fits well with tectonic interpretations of the Cordillera Darwin, Patagonia._ _Crown Copyright ? 2010. Published by Elsevier Ltd. All rights reserved._ _1. Introduction_ _The island of South Georgia lies approximately 1800 km east of the southern tip of South America along the Scotia arc, which forms a complex locally emergent submarine ridge linking the southernmost Andes via South Georgia, the South Sandwich Islands volcanic arc, South Orkney and South Shetland Islands, and finally the Antarctic Peninsula (Fig. 1). Established stratigraphical, igneous and tectonic correlations between South Georgia and Tierra del Fuego indicate that prior to the Late Oligocene to recent evolution of the Scotia Sea, South Georgia was situated at the southern tip of South America where it formed an extension of the Andean magmatic arc and marginal basin system._ _The elongate, Late Jurassic to Early Cretaceous Andean marginal basin exposed in southernmost Patagonia and South Georgia is known as the Rocas Verdes marginal basin. During the late-Early Cretaceous to mid-Cretaceous a combination of increased sea-floor spreading rates and a change in the South American pole of rotation relative to Africa is considered to have increased the convergence rate between the Pacific and South American plates, resulting in compressive deformation and inversion of the Rocas Verdes basin during the mid-Cretaceous main Andean Orogeny. In South Georgia mid-Cretaceous deformation is manifest as widespread and locally intense folding of Early Cretaceous basin fill deposits juxtaposed against basement rocks of the Drygalski Fjord Complex along a regional shear zone referred to as the Cooper Bay Dislocation Zone._ _In this contribution we present new structural, kinematic and geochronological data from the Cooper BayeCape Vahsel region of South Georgia, providing a detailed model for the kinematic evolution of adjacent basin fill deposits strongly influenced by a component of sinistral wrench tectonics. We propose that main Andean deformation of South Georgia was transpressional in character with efficient kinematic strain partitioning focussing the wrench component along a pre-existing major tectonic boundary._ _2. Geology and stratigraphy of South Georgia_ _The southern tip of South Georgia is composed of two basement complexes separated by an inferred fault (Fig. 2). The Drygalski Fjord Complex (DFC) is characterised by an assemblage of highly deformed paragneisses and metasedimentary rocks intruded by extensive tholeiitic plutons. Age constraints within the DFC are poor, with K-Ar dates from olivine gabbros suggesting an Early Jurassic age. The Larsen Harbour Complex (s.s.) is interpreted to be an ophiolite sequence representing local development of mac?o oceanic crust prior to 150 ? 1 Ma. Together this basement assemblage is interpreted as a Gondwana margin accretionary complex, of uncertain age, that underwent crustal thinning during Middle to Late Jurassic times and is inferred to underlie much of the island._ _The remainder and majority of rock exposure on South Georgia is formed by two laterally equivalent turbidite sequences deposited in an Early Cretaceous back-arc basin. The Cumberland Bay Formation contains diagnostic Early Cretaceous marine macrofossils and forms an 8 km thick succession of andesitic volcaniclastic greywackes derived from a volcanic island arc, while the more siliciclastic Sandebugten Formation is inferred to be contemporaneous and derived from the continental margin of the back-arc basin. A third metasedimentary succession, the Cooper Bay Formation, is restricted to the most southeasterly promontory of South Georgia and based on geochemical data considered a facies variant of the Cumberland Bay Formation, albeit derived from a more basic source._ _Deformation of greywacke sequences occurred predominantly at low metamorphic grade along the northeast coast increasing to biotite zone of greenschist facies in the Cooper Bay Formation adjacent to the Drygalski Fjord Complex. Large scale northwest-southeast trending chevron folds dominate Early Cretaceous greywacke successions with a predominantly north-northeast-south-southwest mineral lineation developed along a ubiquitous axial planar cleavage. K-Ar whole rock dating of geographically widespread slate samples from the Cumberland Bay and Sandebugten formations range between 51 and 135 Ma, with a best estimate for minimum age of deformation and metamorphism being 82-91 Ma. This estimate is consistent with geological history of Rocas Verdes marginal basin in southern Patagonia suggesting Early Cretaceous back-arc basin fill of South Georgia was deformed during mid-Cretaceous main Andean orogeny._ _One of the most significant structural components of the South Georgia microcontinental block is the regional scale Cooper Bay Shear Zone, previously referred to as the Cooper Bay Dislocation Zone. This juxtaposes Early Cretaceous greywacke successions against basement complexes displaying complex tectonic history._ Ключевые слова: greenschist facies, simple shear, transpressional, fold tightening, shear, structural data, earth, bay formation, society, strain, image, grunow, strain partitioning, biotite, transpression, cvsz, plane, ?eld, wa, fold axis, decay constant, minimum age, transtensional eld, chiglione, series, non-coaxial component, mineral, shallow inclination, holdsworth, geological society, southwestern margin, patagonia, cape, hinge, fuegian andes, circle, storey, rb sr, early, fold hinge, analysis, cretaceous, doi, geology, interlimb, broad range, white, main, structural evolution, folding, mylonite, nordsim facility, axial plane, band, journal, specic range, dalziel, sinistral sense, elsevier, area, axial, trouw, antarctic, vahsel, journal structural, university, form, cape vahsel, rock, natural history, age calculation, margin, shear band, interlimb angle, dott, formation, clayton, island, mineral lineation, partitioning, simple, sinistral, cooper bay, grain, open circle, southwest, south, cunningham, deformation, inferred, sample yield, structural geology, tectonics, granite sample, fold symmetry, fold axial, southwest margin, mair, data, fault, south georgia, andes, path, age, lled circle, ma, metabasic schist, marginal basin, mylonite foliation, kinematic partitioning, geological, andean, scotia arc, swedish museum, shear zone, pankhurst, isoplot ludwig, tight fold, wrench component, macroscale fold, society london, tectonic correlation, zone, kinematic evolution, rotation, jones, fold, geochemistry, cumberland bay, antarctica, relative, mesoscale, antarctic peninsula, geological evolution, concordia age, curtis journal, cordillera darwin, ? ma, bulletin, tight, loss, northeast margin, sinistral shear, geochronology, component, fold tightness, tectonic, tectonic evolution, scale, lawver, bay, displaying, fildani, su?rez, development, whitehouse, london, georgia, inherited grain, ludwig, strain path, cooper, southernmost andes, angle, journal structural geology, evolution, zircon, stone, foliation, inefcient partitioning, arc, curtis, long, southernmost, corfu, tanner, south america, kinematic, bay shear, tanner thomson, tectonophysics, structure, open, metasedimentary rock, granite, lineation, signicant component, mesoscale fold, nelson, macroscale, macdonald, southern, range, fold attitude, fold appression, structural, cvsz structure, dewey, counter-clockwise rotation, main andean, complex, vahsel shear, symmetry, fold data, clockwise-oblique angle, eds, teyssier, pole, cbsz, basin