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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Tectonophysics 460 (2008) 178–185 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Tectonic rotations in the Deseado Massif, southern Patagonia, during the breakup of Western Gondwana Rubén Somoza a,b,⁎, Haroldo Vizán a,b, Graeme K. Taylor c a CONICET, Argentina b Departamento de Ciencias Geológicas, FCEyN, Universidad de Buenos Aires, Argentina c School of Earth, Ocean and Environmental Sciences, University of Plymouth, UK article info abstract Article history: Paleomagnetic investigation in the Deseado Massif, southern Patagonia, suggests that Triassic sedimentary Received 13 May 2008 rocks carry a latest Triassic to Jurassic remagnetization and that earliest Jurassic plutonic complexes carry a Received in revised form 4 August 2008 reversed polarity magnetization of thermoremanent origin. Despite uncertainties in the timing of the Accepted 5 August 2008 observed remanence in the Triassic rocks and the lack of paleohorizontal control on the plutonic complexes, Available online 15 August 2008 comparison of the derived pole positions with the most reliable Late Triassic–Jurassic apparent polar wander Keywords: paths indicates that the study areas underwent significant clockwise vertical-axis rotation. In contrast, Patagonia paleomagnetic results from mid-Cretaceous rocks in the region indicate no rotation. The observed crustal Jurassic rotations in the Deseado Massif are thus bracketed to have occurred between Jurassic and Early Cretaceous Gondwana breakup times, documenting southern Patagonian deformation during the breakup of Western Gondwana and then Paleomagnetism enlarging the regional record of clockwise rotations associated with this event. These results suggest a more complex than previously supposed tectonic evolution of this part of South America. © 2008 Elsevier B.V. All rights reserved. 1. Introduction context, all reconstructions that leave the FMI in continuity with the Cape foldbelt for Early Jurassic times require or imply a dislocation The relationship between Patagonia and the Falkland–Malvinas within or offshore of present-day Patagonia (e.g. Fig. 1). Islands (FMI) is a controversial theme in Gondwana paleogeography. However, evidence for such a microplate boundary is not obvious. Simple closure of the South Atlantic, based on continental outlines or There is no tectonic record from Patagonia and the Argentine shelf to continent–ocean boundary reconstructions and assuming a rigid suggest the presence of a cryptic tectonic suture (e.g. Biddle et al., southern South America (e.g. Rabinowitz and LaBrecque, 1979), leaves 1996; Ramos, 1996). In particular, the structural style and the the FMI south of southern Africa. In contrast, paleomagnetic data disposition of the basins surrounding the islands seem to be from dolerite dykes in the FMI led to postulate that the islands, incompatible with major microplate interactions, at least since pre ~180 Ma, were located in a position adjacent to and eastward of the Callovian times (e.g. Platt and Philip, 1995; Biddle et al., 1996; Cape foldbelt of South Africa (Mitchell et al., 1986; Taylor and Shaw, Richards et al., 1996; Ross et al., 1996). However, paleomagnetic data 1989; Musset and Taylor, 1994). This reconstructed position had from the North Patagonian Massif (NPM) have shown the occurrence previously been proposed by Adie (1952) and after the paleomagnetic of clockwise tectonic rotations during the Early Cretaceous reconstruction has received widespread support on stratigraphic, (Geuna et al., 2000), which are kinematically compatible with the sedimentological and structural grounds (e.g. Curtis and Hyam, 1998; rotation recorded in the Jurassic dolerites from the FMI. The possibility Thomson, 1998; Jacobs et al., 1999; Trewin et al., 2002). The paleogeo- that these Patagonian rotations may have a more regional expression graphic scenario implies that the FMI (or Lafonian) microplate rotated led us to carry out a paleomagnetic sampling of Mesozoic rocks in the ~100° clockwise away from southern Africa during the dismemberment Deseado Massif, the closest continental area to the FMI. The results of the supercontinent (Ben-Avraham et al.,1993; Marshall,1994; Barker, suggest that the sampling localities have also undergone substantial, 1999; Storey et al., 1999; MacDonald et al., 2003). The implication, tacit pre-Aptian clockwise rotations, thus enlarging the record of rotational or otherwise in many of these studies, is of an independent FMI deformation in Patagonia during the breakup of Western Gondwana. microplate and therefore there should be a plate boundary between this microplate and its nearest neighbour, southern Patagonia. In this 2. Geological setting of the Deseado Massif and paleomagnetic sampling ⁎ Corresponding author. Departamento de Ciencias Geológicas, FCEyN, Universidad de Buenos Aires, Argentina. Tel./fax: +54 11 4788 3439. The tectonic history of the Deseado Massif was recently summar- E-mail address: [email protected] (R. Somoza). ized by Ramos (2002). Small and sparse outcrops of Lower Paleozoic 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.08.004 Author's personal copy R. Somoza et al. / Tectonophysics 460 (2008) 178–185 179 high-K, calcalkaline monzonites at ~200 Ma (Pankhurst et al., 1993; Rapela and Pankhurst, 1996; Márquez et al., 2002). Widespread Middle to Upper Jurassic lavas, ignimbrites, and volcaniclastics (Bajo Pobre, Chon Aike and La Matilde formations, respectively) accumu- lated under the extensional conditions that preceded the opening of the South Atlantic Ocean (Gust et al., 1985; Homovc and Constantini, 2001). Resting on a regional unconformity of supposed Early Cretaceous age, flat-lying Aptian and younger rocks document the relative tectonic stability that characterizes the later stages of the geological history of this region. One of the original aims of this fieldwork was to re-sample the Jurassic Chon Aike Formation and re-assess earlier results (e.g. Valencio and Vilas, 1970; Creer et al., 1972; Vilas, 1974) but we were unable to locate suitable outcrops that would have provided a reliable and meaningful result. Indeed, most of the visited Chon Aike localities, Fig. 1. Early Jurassic configuration of Gondwana proposed by Dalziel et al. (2000). Several of the crustal blocks between the main continents are denoted, Patagonia (PT), including the study area of Valencio and Vilas (1970), were composed Falklands/Malvinas (FMI), Ellsworth Mountains (EM), and Antarctica Peninsula (AP). of just one or two thick ignimbrites, which would not have allowed Note that the space problems derived from the correlation between the Falklands reliable averaging of paleosecular variation (PSV) of the paleomag- Islands and the Cape foldbelt (CFB) are resolved by dividing Patagonia into independent netic field. Likewise, geographically widespread sampling of sites crustal blocks. The location of the Gastre Fault System (GFS) is also shown. from, potentially, different structural blocks would not have allowed us to recognize effects of PSV versus relative block rotation nor plutonic and metamorphic rocks yield limited insight into the nature multiple repeat sampling of the same individual ignimbrites. In of its basement (Giacosa, 1996; Ramos, 2002). In Permo-Triassic times order to try to recognise and define the magnitude of any large scale the region was subjected to rifting (Homovc and Constantini, 2001) crustal block rotation, the study was then re-focussed upon ~200 Ma which culminated with the emplacement of localized intrusions of calcalkaline plutonic complexes (Pankhurst et al., 1993; Rapela and Fig. 2. Map of the surveyed area in the Deseado Massif showing the sampling localities: a) El Tranquilo, b) La Calandria and La Juanita, c) La Leona. Inset shows the location of the studied area, dot indicates the area in the North Patagonian Massif studied by Geuna et al. (2000). Author's personal copy 180 R. Somoza et al. / Tectonophysics 460 (2008) 178–185 1993; Courtillot et al., 1994; Torsvik et al., 2001) and South America does not escape these uncertainties. Proposed APW paths based on South American data have all included a significant number of Patagonian poles whose consideration to analyze our results, given that we are trying to test for deformation in the region, would be inappropriate. The alternative is therefore to use recently published synthetic APW paths transferred to South American coordinates (Fig. 3). Differences between the resultant synthetic curves arise in part from different pole selection criteria but more significantly differences in assumptions about Pangea's paleogeography. The reference curves are as follows. APW curve A is a combined European and North American APW path (Torsvik et