Structure and strain variation at mid-crustal levels in a transpressional orogen: A review of Kaoko Belt structure and the character of West Gondwana amalgamation and dispersal

Abstract

The Neoproterozoic-Cambrian Kaoko Belt is an orogen-scale (800 × 180 km) transpressional system important in the amalgamation of West Gondwana. Mid-crustal transpression at amphibolite to granulite facies conditions is dominated by two major, > 400 km exposed, strike-slip shear zones bounding a 20-40 km wide high-grade Orogen Core. To the east, a deeply buried nappe-dominated Escape Zone has inverted metamorphic sequence and verges outwards onto a platformal foreland. To the west, an arc-like Neoproterozoic Coastal Terrane was amalgamated and variably reworked during transpression. The major Purros and Three Palms Mylonite Zones have calculated shear displacements on the order of 120-180 km. These shear zones are moderately to steeply dipping mylonite zones of 1-5 km width, are arcuate and curvilinear in map view and show along-strike variation in slip kinematics. Also highly curved in vertical section, the shear zones define a flower to half-flower geometry for the Orogen Core. An oblique network of mylonitic shear zones, akin to Riedel shears, links the major shear zones and defines regional-scale shear lozenges internally deformed by tight upright folding and shear fabrics. These shear zones create domains in the Orogen Core with varying dominance of pure shear (in shear lozenges) and simple shear (in shear zones). However, absence of dip-slip domains and the smoothly continuous traces of sub-horizontal to shallow and acute, oblique stretching lineations across all parts of the belt, preclude marked kinematic partitioning and the internal part of the belt resembles large-scale triclinic shear. Clast aspect ratios, boudin train extension, sheath fold aspect geometry, degree of rotation of planes producing flanking folds, composite S-C foliations, pressure fringes on pyrite and garnet porphyroclasts provide a semi-quantitative measure of strain intensity. Average strain ratios are X/Z > 40:1 for the major shear zones, X/Z > 12:1 for the Orogen Core, X/Z > 8:1 for the Escape Zone and X/Z > 3:1 for the Coastal Terrane. A more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index. Foliation traces have a sigmoidal pattern in the Orogen Core, swinging from sub-parallel to the boundary shear zones to higher acute angles in the internal parts. Deformation character also varies from upright open folding in amphibolite facies domains in the north, upright tight chevron folding in a low-grade central domain, to a high-grade domain of tight to isoclinal inter-folded basement and cover, with inclination decreasing towards the south. The Kaoko Belt is a well-exposed sector of an extensive (3000 km long), broad (400 km) arcuate orogenic system "Adamastor Orogen" that amalgamated West Gondwana, bringing the South American (Sao Francisco and Rio de la Plata Cratons) and African (Kalahari and Congo Cratons) components together. Though a complex system, most sectors involved oblique collision and accretion of magmatic arcs of 660-610 Ma age, followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 Ma, with shear zones remaining active until ∼ 530 Ma. This E-W amalgamation immediately pre-dates the final N-S amalgamation of Gondwana along the Kuunga Orogen between 535 and 510 Ma. The large-scale Adamastor Orogen, consisting of Kaoko, Dom Feliciano, Ribeira, Araçuai and West-Congo mobile belts, also shows broadly similar and symmetric structural architecture throughout. The high-grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form. Either side of the internal zone containing amalgamated arcs and high-grade core, are nappe-fold and thrust belts that rework attenuated passive margin basement and Adamastor Ocean sediments and structures verge outward at moderate to high-angles onto both foreland margins. The Kaoko Belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater "Adamastor Orogen"; such as metamorphic zonation and heterogeneous distribution of deformation style, flow regime and highly variable degrees of reworking strain and recrystallization. This highly partitioned and steep structural grain localized lithospheric extension and rifting 415 Ma later during breakup and dispersal of Gondwana. © 2007 International Association for Gondwana Research.