Syntectonic magnetization of the mid-Palaeozoic Sierra Grande Formation: Further constraints on the tectonic evolution of Patagonia

Journal of the Geological Society, Jan 1998 by Rapalini, A E

Abstract: A palaeomagnetic study was carried out in the Silurian Devonian elastic sedimentary rocks of the Sierra Grande Formation, exposed in northeastern Patagonia (41.6oS, 65.3oW). Thirteen sites (n=88) were located on opposite limbs of a syncline-anticline structure. Stepwise thermal demagnetization permitted the identification of a very stable magnetic component of reversed polarity carried by hematite. Stepwise performance of the fold test yielded negative results both in situ and after 100% bedding correction, but positive after partial unfolding (19%). This indicates a syntectonic origin for the isolated magnetization. A pole position was computed for the partially (19%) corrected remanence: SG3: 77.3oS. 310.7oE,8p=7.7,, 8m=6.6o, N=13. Its position is coincident with late Early to Late Permian palaeomagnetic poles from South America, suggesting that age for the previously undated folding of the Sierra Grande sequence and therefore for the main tectonic event that affected the northern boundary of Patagonia. Palaeozoic palaeomagnetic poles from Patagonia obtained to date agree with those from Gondwana of Devonian or younger age, suggesting that Patagonia did not undergo important displacements relative to South America since those times. This and the Permian age of deformation determined in this study invalidates tectonic models involving collision of a far-travelled Patagonia with Gondwana in the mid- or Late Palaeozoic.

Keywords: Patagonia, Palaeozoic, Gondwana, palaeomagnetism.

Patagonia is the extreme southern part of South America. Its peculiar geological features have attracted the attention of many geologists since the first decades of this century. In particular, Keidel (1925) and Windhausen (1931) suggested that Patagonia may have evolved apart from the rest of the South American continent during part of the Phanerozoic. In plate-tectonic terms this is to say that Patagonia may be considered as a suspect terrane. In the last two decades several hypotheses were proposed suggesting an allochthonous nature of the Patagonian block (Dalmayrac et al. 1980; Martinez 1980; Ramos 1984, 1988; Selles Martinez 1988). These were mainly based on considerations of several geological and tectonic features around the northern boundary of this terrane. However, some of these models cannot sustain thorough scrutiny (see Ramos 1984 for a discussion of different models). More recently, Dalla Salda et al. (1992) presented a radically different hypothesis, proposing that the western areas of Patagonia and most of western Argentina were part of an allochthonous terrane with Laurentian affinities ('Occidentalia'), that was accreted to South America in Ordovician times. According to this hypothesis, eastern areas of Patagonia would be part of the older cratonic nuclei of South America. The older ages of accretion of `western Patagonia' proposed in this model imply an essentially autochthonous character for the whole of Patagonia in mid- and Late Palaeozoic times.

The most consistent and favoured of the 'allochthonous' models that consider Patagonia as a single block is that proposed by Ramos (1984), which suggests that this terrane underwent a frontal collision with the southern margin of Gondwana during the Late Palaeozoic. It has also been speculated that Patagonia was part of a ring of accreted terranes that collided with southern Gondwana in the Late Palaeozoic (Kay et aL 1989; Kay 1993). According to Ramos's (1984) model (Fig. 1), an ocean was consumed between the northern boundary of Patagonia and the southern Gondwana margin by subduction beneath Patagonia, until collision between both continental blocks in the mid- or Late Palaeozoic. Several geological features around the northern boundary of Patagonia can be explained by this model (Fig. 1): (i) the Ventana (or Sierras Australes) fold belt, located north of the Patagonian boundary, where Palaeozoic platform sediments were highly deformed with NE vergence during the Permian; (ii) mid- and Late Palaeozoic calcalkaline magmatism on the northern Patagonian margin, interpreted as the magmatic arc developed prior to the collision; (iii) tectonic deformation of mid-Palaeozoic sediments in NE Patagonia, assumed to have happened in the Late Devonian-Early Carboniferous; (iv) a PermoTriassic rhyolitic plateau on La Pampa province, just north of Patagonia, interpreted as post-collisional volcanics.

Despite this, the model has remained controversial for several reasons: (i) lack of a suture with obducted ocean floor between both lithospheric blocks; (ii) no biostratigraphic or palaeoclimatic evidence suggesting different palaeogeographies for Patagonia and South America in the Late Palaeozoic; (iii) comparable Late Proterozoic to Early Palaeozoic basement rocks in northern Patagonia and the Pampean Ranges in central Argentina (e.g. Varela et al. 1991), suggesting crustal continuity between both; (iv) unclear location of the hypothetical boundary in western Argentina and Chile.