Proterozoic–Early Paleozoic evolution in western South America—a discussion” in: Tectonophysics, 354: 121–137 (2002

Proterozoic–Early Paleozoic evolution in western South America—a discussion” in: Tectonophysics, 354: 121–137 (2002

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  Proterozoic–Early Paleozoic evolution in western SouthAmerica—a discussion $ Florencio G. Acen˜olaza a,1 , Hubert Miller   b, *, Alejandro J. Toselli a,1 a   Instituto Superior de Correlacio´ n Geolo´  gica, Universidad Nacional de Tucuma´ n, Miguel Lillo 205, 4000 San Miguel de Tucuma´ n, Argentina  b  Institut fu¨ r Allgemeine und Angewandte Geologie, Ludwig-Maximilians-Universita¨ t Mu¨ nchen, Luisenstraße 37, 80333 Mu¨ nchen, Germany Accepted 7 June 2002 Abstract The hypothesis of exotic terranes in Peru´, Bolivia, Argentina and Chile generated discussions on the mode of transfer andextent of accretional events that may have occurred in the southern Andes during the Late Proterozoic–Early Paleozoic.Initially, a tectogenesis based on autochthonous mobile fold belts was discussed. Following ideas emphasised the fragmentationof the supercontinent Rodinia, Laurentia moving along the West Gondwana border and colliding with the Gondwana westernmargin. The most important effect of this Laurentia/Gondwana relationship was attributed to the Argentine Precordillera (or Cuyania) terrane splitting off from Laurentia and docking to Gondwana in the Early Paleozoic. In this study, the most citedarguments for this Laurentia/Precordillera relationship are discussed, emphasising paleontological considerations. It is shownthat these arguments do not exclude a close srcinal vicinity of the Precordillera terrane to Gondwana.The Precordillera terrane is suggested to be part of a hypothetical platform, which developed between South America, Africaand Antarctica (SAFRAN platform), and which was displaced to its actual position by transcurrent faults. The collisional eventsin the Sierras Pampeanas ensued from strike–slip movements and were responsible for the S and I type transpressionalmagmatism along the Pampean and Famatinian terranes. The final result of this continent-parallel movement of terrane slices issimilar to that of a terrane split off from Laurentia, but the first-named way of formation easier explains the general continuity of  plate convergence at the western border of Gondwana than the Laurentia/Precordillera connection does. D  2002 Elsevier Science B.V. All rights reserved.  Keywords:  Gondwana; Proto-Pacific; Paleozoic; Terranes 1. Introduction Until 15 years ago, the central and southern Andeshave been considered to be more or less autochtho-nous and coherent within the Gondwana continent.Sedimentation and magmatism seemed to be bound toa more or less continuous subduction-controlled tec-tonic regime. Only the Arequipa Massif of southernPeru and the Madre de Dios Archipelago of the southChilean Patagonian fiord region were thought to beallochthonous (Fig. 1), but their provenance was not  much discussed in the literature.Within this general frame, the Argentine Precordil-lera of Mendoza and San Juan (in short: Precordillera) 0040-1951/02/$ - see front matter   D  2002 Elsevier Science B.V. All rights reserved.PII: S0040-1951(02)00295-0 $ Thispaperwasacceptedforpublicationaftercompletionofthespecial issue ‘‘Andean Geodynamics’’, Tectonophysics 345 (1–4). * Corresponding author. Fax: +49-89-2180-6514.  E-mail addresses: (F.G. Acen˜olaza,A.J. Toselli), (H. Miller). 1 Fax: 354 (2002) 121–137  Fig. 1. Regional geological units of southern South America as mentioned in the text.  F.G. Acen˜ olaza et al. / Tectonophysics 354 (2002) 121–137  122  was mostly considered ‘‘exotic’’ due to the important fossil-rich Cambro-Ordovician limestone series,unknown in other parts of the Andes. However, therewas only little discussion on its provenance or  regional relations until Hoffman (1991) and Dalziel (1992) proposed close relationships between Lauren-tia and West Gondwana in the Early Phanerozoic.Since then, the Precordillera is considered to represent a piece of Laurentia, which was split off when thiscontinent was situated close to southwestern Gond-wana, or even collided with it in the Early Paleozoic(Astini et al., 1995; Dalziel, 1997; Dalla Salda et al.,1993; Thomas and Astini, 1996, 1999; Ramos et al.,1998; Pankhurst and Rapela, 1998). Only Acen˜olazaand Toselli (1988, 2000) suggested the Precordillerato be of Gondwanian srcin and to have reached its present-day position by coast-parallel transpressionalmovements.Many arguments seem to favour the nowadaysclassical hypothesis of a Laurentia-derived Precordil-lera. The most cited are:  the Grenvillian age of the surrounding and under-lying crystalline basement and its Pb-isotopy;  the supposed proximity of Laurentia to West Gondwana within Rodinia;  the ‘‘missing piece’’ within the Cambro-Ordovi-cian platform of Texas;  the Ordovician K-bentonites found in the Precor-dillera, in Laurentia and Baltica;  the rift indicating ocean bottom ultramafic rocks at the west side of the Precordillera; and  the dominant affinity of the trilobite faunas of thePrecordillera to those of Laurentia.We will show—with focus on the paleontologicalevidence—that all these data can be explained inanother way. This paper gives a synthesis of the datawe consider important to justify a para-autochthonoussrcin of the Precordillera terrane. We are aware that the model presented in this paper does not agree withthe most widely accepted ideas; however, we areconvinced that the srcin and history of movement of the Precordillera is better explained by our modelthan by the aforementioned hypotheses.Since the amount of publications on the Precordil-lera terrane’s relations to Laurentia increased im-mensely during the past decade, we regret that thetask of this paper cannot be the full quotation,description and discussion of all of them. 2. The position of the Precordillera Lithologic differences indicate that the Precordil-lera terrane, the product of a typical passive margin, tothe east is in structural contact with the Sierra deFamatina (Fig. 1), which is an active margin. This geographic situation is geologically incongruent andwas intensely discussed (Ramos, 1988; Acen˜olaza and Toselli, 1988; Baldis et al., 1993; and many others).On account of geological evidence, we supposethat collision and docking of the Precordillera along-side the Sierra de Famatina did not occur in theOrdovician (Llandeilian). The carbonate facies indi-cates for the Precordillera low latitude position andmarine warm water conditions. On the contrary, thenearly carbonate-free facies of the Sierra de Famatinaspeaks for a middle- to high-latitute position withtemperate water. Further, the Precordillera’s nearlycontinuous stratigraphic sequence from Upper Ordo-vician to Devonian has no correspondence within theSierra de Famatina. It is only sure that the present  position was reached by the end of the FamatinianCycle (Upper Devonian). This is documented by the Neopaleozoic clastic post-orogenic sedimentary rocksof the Paganzo basin and equivalents, which weredeposited when the Precordillera and Famatina ter-ranes were in contact.Expansion of the ocean floor may follow a pattern perpendicular to the corresponding subduction zones.However, oblique and slightly rotational movementsare very frequent, as observable for many places, suchas the present Nazca and Cocos plates west of LatinAmerica (see, e.g. Barckhausen et al., 1998). In this way, ‘‘microplates’’ are generated, which are dis- placed alongside a continent by transform move-ments. Such phenomena are well known from theCenozoic displacements along the Liquin˜e–Ofquifracture zone in southern Chile and the San Andreasfault in California. In ‘‘terranes’’ of the western North-American Cordillera (e.g. Salinia, Stikinia, Wangeliaterranes; Windley, 1995), several examples of trans-  pressional interaction of oblique convergent platesand continental margins along strike–slip movementsare also reported.  F.G. Acen˜ olaza et al. / Tectonophysics 354 (2002) 121–137   123  If we accept that today’s lithospheric processes areapplicable to the whole Phanerozoic, there is no doubt that different models may explain how two terraneswith contrasting geological history were juxtaposed. 3. Gondwana and its Paleo-Pacific borderlands It is generally accepted that Gondwana existed at least since the Late Neoproterozoic. According to the Fig. 2. Cratons, intercratonic basins and orogens in West Gondwana at Vendian/Early Cambrian times. The hypothetical microcontinent SAFRAN is situated between South America, Africa and Antarctica.  F.G. Acen˜ olaza et al. / Tectonophysics 354 (2002) 121–137  124  general concept, South America, Africa, Antarctica,Australia, India and Madagascar formed one super-continent belonging South America and Africa toWest Gondwana, and the remaining parts to East Gondwana. The two blocks are separated by a mobile belt, the ‘‘Mozambique orogen’’, which has beenactive in the Neoproterozoic, and which runs throughthe eastern margin of Af rica and the western border of today’s East Antarctica (Fig. 2).The Mozambique Belt srcinated from the closingof an ocean at the end of the Proterozoic. At that time,and during the Early Paleozoic, many basins andorogens developed on the western border of Gond-wana. An example is the ‘‘Samfrau geosyncline’’,described and defined by Du Toit (1927), who tried to explain the relation between South America, Africaand Australia dur ing the Paleozoic. A similar  modelwas presented by Acen˜olaza and Miller (1982) show-ing the continuity of the Pampean orogen betweenArgentina and South Africa, Antarctica and Australia positioned at the Gondwana border to the Proto-Pacific.Taking into account the relations among the Pro-terozoic and Paleozoic outcrops between South Amer-ica, Africa and East Antarctica, we deduce theexistence of a former area (‘‘Safran’’; Fig. 2) placedamidst these continents. This area conjecturally pro-vided the terranes, which then formed continentalfragments like the Precordillera, the Argentine MarinePlatform, Malvinas Islands and Patagonia, having been displaced to the ‘‘north’’ along the South Amer-ican cr atons during rotation of the East Gondwana Block  (Acen˜olaza and Toselli, 2000). Dalziel (1997, p. 23 and following) describes a very similar proce-dure of the Precordillera’s provenance and wandering,with the only difference that he calls it part of Laurentia, not part of  SAFRAN as a division of  Gondwana. Adie (1952), Mitchell et al. (1986), Mar- shall (1994) and Curtis and Hyam (1998) showed that  the Malvinas belonged to a sector close to the CapeFold Belt and were later displaced to their present  position. Paleomagnetic data support the idea of their srcin near Aghullas (southern Atlantic Ocean) withinthe platform, with a later displacement and rotation.At the same time, the varied terranes now composingWest Antarctica may have rotated to join East Ant-arctica and to close the gap between East Antarctica and the South American cratons (Fig. 3). Fig. 3. The roˆle of transcurrent faults in the development of West Antarctica. (A) Mid to Late Cambrian after  Curtis (2001), modified. EWM: Ellsworth–Whitmore block. (B) Pensacola Mountains and rotation of Ellsworth Mountains along dextral faults. After Schmidt and Rowley, inCurtis and Storey (1996).  F.G. Acen˜ olaza et al. / Tectonophysics 354 (2002) 121–137   125
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