Extrusion-related extensional signatures under high-pressure nappe compressional regime

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CARLOS H. G. CARVALHO AND MARIO C. CAMPOS-NETO

Instituto de Geociências, USP, São Paulo, SP.

Presented by ANTONIO C. ROCHA-CAMPOS

The Neoproterozoic nappe system exposed south of the São Francisco Craton underwent a minimum of 200 km of near-horizontal northeastward displacement. This nappe system comprises the south-southeast sector of Brasília Belt and piles up three tectonic settings (from top to bottom): the roots of a neoproterozoic magmatic arc (Socorro-Guaxupé Nappe); high-pression metassedimentary nappes (Três Pontas-Varginha, Carmo da Cachoeira, Aiuruoca-Andrelândia, Pouso Alto and Carvalhos klippe); parautochthonous medium-pressure nappes and duplexes.

In the southeast of Aiuruoca-Andrelândia Nappe, a brittle-ductile sinistral shear zone (Liberdade Shear Zone) limits the southern Alagoa Migmatites and may represent a lateral ramp for the eastern Lima Duarte Nappe. Close to Alagoa (MG), this shear zone turns E-W, breaking the nappe hind portion, in a southward normal-ductile movement.

North of this shear zone, the garnet-sillimatite bearing metassedimentary sequence lays over a gnaissic-diatexitic basement and is highly sliced by strata-like tourmaline-bearing granites. In the metassediments, main foliation S₂ is transposed by a normal-fault related thick protomilonitic S₂' foliation. In the gnaisses, southward transport is marked by shear-sense indicators such as S-C foliations and quartz sigmoids. A low-temperature, late brittle thrust system, overlaps the extensional structures.

Heading north, the end of this structural-metamorphic domain is marked by an antiform in gnaissic diatexites. A normal fault with average northeastward dip limits the fold and juxtaposes upper kianite-staurolite bearing micaxists. From this limit, the structural pattern of Aiuruoca-Andrelândia Nappe is homogeneous, with sin-metamorphic ductile shear-sense indicators to E-NE.

The alternating of extensional structures in a general compressive regime and in the hind portion of high-pressure nappe, is associated with the extrusion process and with the search, in the collisional orogen, of isostatic balance. — (December 14, 2001).

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Supported by FAPESP 00/11390-4 and 98/15624-8.

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002