The Gastre Fault System: an intra-plate boundary during the initial rifting of Gondwana

RAPELA, C.W.; PANKHURST, R.J; OVIEDO, E.S.

COMUNICACIONES - DEPARTAMENTO DE GEOLOGIA. FACULTAD DE CIENCIAS FISICAS Y MATEMATICAS. UNIVERSIDAD DE CHILE

Universidad de Chile

Lugar: Santiago; Año: 1991 vol. 42 p. 186 - 193

0069-357X

The Gastre Fault System (GFS) is a prominent NW-SE shear zone in the Northpatagonian Massif, with individual faults inferred to occupy a zone at least 30 km wide (Fig. 1) (Coira et al., 1975). It has been recently beer shown that the GFS is spatially and genetically related to the Batholith of Central Patagonia, a Late Triassic-Early Jurassic plutonic-volcanic calcalkaline belt (Rapela et al., 1991). The GFS constitutes a remarkably significant geological boundary (Rapela & Pankhurst, 1991): 1) It is the southern limit of the Permo-Triassic granite-rhyolite province (see Rapela & Kay, 1988; Kay et al., 1989). The northwesterly projection of the GFS cuts the coast of Chile at 38 S, just south of both the southernmost exposures of late Paleozoic granitoids in the Southern Coastal Batholith (Herv et al, 1987) and the southernmost exposures of the continental Triassic sediments along the Coastal Cordillera. On the other hand, it is the northern limit of the Mesozoic Cenozoic Patagonian batholith. 3) It appears to mark a significant change in the nature of the Paleozoic basement rocks of the Coastal Cordillera, with the occurrence of rocks of oceanic provenance (cherts and MORB-type basalts) only to the south of this point (Herve et al., 1981; Herve, 1988). 4) There is also a distinction, in term of age and chemical characteristics between the early Mesozoic rhyolitic suites of extra-Andean Patagonia north and south of the GFS. Those within or to the north are older (; 180- 230 Ma) and in some cases have alkalic signatures (eg. Marifil Group, Haller et al., 1990), whereas those to ¡he south (Chon-Aike Formation) are younger (; 155-175 Ma, De Barrio, 1989) and mostly calc-alkaline (Uliana & Biddle, 1987). There is no evidence of recent seismicity in the GFS, but epicenters along the Chilean coast south of 38 S are displaced westwards relative to those to the north and the concentration of epicenters near its Pacific termination is higher than in neighboring sectors of the Chilean coast (Corvalan, 1981). The GFS as a geological boundary is interpreted as a crustal discontinuity representing a late Triassic-early Jurassic intraplate boundary associated with the initial rifting and break-up phase of Gondwana (Rapela & Pankhurst, op. cit). In this paper we examine ¡he correspondence of ¡he GFS with similar structures in ¡he Malvinas/Falkland-Plateau (MFP), as well as ¡he compatibility of this hypothesis with the available paleomagnetic evidence. Geological considerations implicit in ¡he model have been discussed elsewhere (Rapela & Pankhurst, op. cit.).