The Itacaiúnas Belt on the southeastern margin of the Amazonian Craton comprises two E-W-trending Archaean fault zones (Carajás and Cinzento strike-slip systems) developed sub-parallel to a broad zone of earlier ductile shearing. Older basement rocks display a pervasive E-W-trending mylonitic fabric, formed at high temperatures during sinistral transpression c. 2.8 Ga. A sequence of younger Archaean cover rocks was deformed under low greenschist facies conditions during a second phase of sinistral transpression associated with retrogression and reworking of basement fabrics. All these rocks are overlain unconformably by very low grade to unmetamorphosed volcanic and sedimentary rocks of Late Archaean-Early Proterozoic age. The Carajás and Cinzento strike-slip fault systems formed during subsequent regional dextral transtension (c. 2.6-2.7 Ga) which down-faulted cover sequence rocks into dilational fault jogs. Later sinistral transpression (>c. 1.9 Ga) partially inverted these dilation zones, producing oblique compressional faults and folds evident mainly in cover rocks. Renewed fracturing, regional emplacement of granitic plutons and swarms of dykes c. 1.8 Ga were followed by fault-controlled deposition of immature sandstones and conglomerates. Phanerozoic activity is limited, although some faults display evidence of minor neotectonic activity. The waning influence of the basement architecture and decreasing intensity of later reactivations is consistent with lithosphere-scale weakening with a finite life span.
The partitioning of deformation into wrench- and contraction-dominated deformation domains is a widely reported but poorly described phenomenon in ancient transpression zones. This paper documents spectacularly exposed examples of such partitioning from the Southern Uplands terrane in SE Scotland (Berwickshire), which was deformed during late Llandovery to Wenlock time. A well-exposed coastal section from Eyemouth to Burnmouth preserves a broadly homoclinal sequence in which a highly heterogeneous array of contemporaneous structures formed during regional triclinic transpression. The deformation involved components of NW–SE contraction with subvertical extension, top-to-the-SE thrusting and top-to-the-SW sinistral shear. In the northern third of the section studied these components are partitioned into a series of fault-bounded, metre- to kilometre-scale structural domains that contain geometrically and kinematically distinct assemblages of variably curvilinear folds, strike-slip detachments and locally transecting cleavages. The structures are all broadly contemporaneous and, in individual domains, record either non-coaxial contractional- or sinistral wrench-dominated strains. Similar highly heterogeneous, domainal structural patterns are likely to be found in other regions of oblique convergence in both ancient and modern settings.
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