Several types of structures are observed in the Precambrian, Mesozoic and Cenozoic rocks of theAmazon region, which represent the major features of the neotectonic framework developed since theMiocene. They controlled the sediment deposition of the Upper Tertiary and Quaternary, as well as haveinfluenced the development of the present landform patterns and drainage systems. Transpressive andtranstensive areas are recognized based on their nature and geometry, and related to two main episodes oftranscurrent displacement of Miocene/Pliocene and Upper Pleistocene /Recent ages. Sets of E-W, ENEWSWand NE-SW right-hand strike-slip faults are present in most of these areas. These sets are linked bynormal faults trending NW-SE and NNW-SSE, or by thrust faults trendig NE-SW and ENE-WSW,depending upon their geometry. Large areas with N-S trending younger normal faults are also observed.Earthquakes, the phenomenon of “fallen lands”, fluvial channels migration, hot springs, etc., are related toareas where some of these faults remain active. All these structures are related to an intraplate E-W righthandshear system induced by the rotation of South American Plate towards west.

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The Marajó Basin area presents geologic and geomorphologic features chiefly due to the Mesozoic extension and post-Miocene neotectonics. The extension event with an Early and a Late Cretaceous phases originated four sub-basins that constitutes the Marajó Basin, with a thick continental clastic sequence showing marine influence. NW and NNW normal faults and NE and ENE strike-slip faults controlled the basin geometry. The extension, related to the Equatorial Atlantic opening, propagated into the continent along crustal weakness zones of the Precambrian Tumucumaque, Amapá and Araguaia orogenic belts. The neotectonic event is a strike-slip regime which developed transtensional basins filled in by Upper Tertiary shallow marine (Pirabas Formation) and transitional sequences (Barreiras Group), followed by Quaternary fluvial deposits and transitional sequences derived from the Amazon and Tocantins rivers and the Marajoara estuary. The current landscape has a typical estuarine morphology. The coast morphology presents sea-cliffs on transitional Upper Tertiary sequences, while inwards dominate hills sustained by Mid-Pleistocene lateritic crust, with a flat erosive surface at 70 m. In the eastern Marajó Island several generations of paleochannels associated with fluvial-estuarine sequences are recognized, while a fluvial-marine plain is widespread on its western side.

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TH E PRECAMBRIAN OF NATI VID ADE REGIO N, GOlAS, BRAZIL. Thi s~per presents the results of the geological in vestigation performed on the Preca mbrian rock s at the Nativi dede regio n (Northe rn Goi4s). Archean tcnalites, mafic rock s, and supracrus tals of ancient evolution not yet reconstituted were modified by strong progressive non-coaxial ductil e deformation, with displacement, imbrication. sheari ng. recrytallizatioh in high to medium amphi bolit e facies. and some migma tiza tio n. These mod ified rocks gra de to gra nite-greenstone terrai n in the easte rn part of the area . All these modified rocks and unaffected gneisses are assembled in the Goiano Complex . The mass displacement occurred towards SSE and the myloniti c foliation has ON30 d irection, what is interpreted as related to a not yet located oblique ramp . This impo rtant tectoni c event is adm ited to be of Late Archean age . Afterwards. high angle ducti le shear zones developed, with 10N20 direction, sinistral movement. and recrystallizat ion under green schist facies conditions. This event seems to be of Early Proterozoic age and introduced modifications o n the ear lier structural geometry. An ensialic basin could be developed and clo sed in some interval of the, 1.850-4 50 Ma period; its sedimentary fillin g, the Natividad e Group, accumulated on stable shelf environme nt. was folded with ON20 trending axis and slightly transformed by low grade metamorphism. T ranscurrent SON60 dextral and 3OON3 15 sinistra l faults also could form, cutting this supracrus tal sequence and the older Goiano Com plex.

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