Abstract

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.

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