Title:
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
Author(s):
Publication Year:
2008
Abstract:
The Neoproterozoic-Cambrian Kaoko Belt is an orogen-scale (800x 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. Keywords: Transpression, Shear zones, Strain, Stretching lineations, Mid-crust processes, West Gondwana, Amalgamation, Dispersal.
Publication Title:
Gondwana Research
Volume:
13
Issue:
1
Pages:
45-85
Item Type:
Journal Article
Language:
en

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