Restricted to the Arenig. a magmatic arc became active farther W in the western ... western Puna during the Arenig which transformed the Puna basin into a ...
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LITHOSPHERIC MODELINGOF THE ORDOVICJAN FORELAND BASIN IN THE PUNA OF NW ARGENTINA: IMPLICATIONS FOR THE INTERPRETATION OF EARLY PALEOZOIC TERRANES
Heinrich Bahlburg
Geologisch-PalaontologischesInstitut, Universitiit Heidelberg, Im Neuenheimer Feld 234,6900 Heidelberg, Germany.
La Cuenca Ordovicica dela Puna septentrional argentina se desarrollo de una posicion de tras-arc0UMa de antepais. Modelos litosfericos demuestran que la interpretacion original basada en datos sedimentologicos, volcanol6gicos yel dlculo de tazas de subsidencia tectonica corresponde procesos a fisicos vigentesen la evolucibn de cuencas de antepais. Sin embargo, los resultados dela modelacion soportan dos procesos e interpretaciones distintos:(i) La evolucion dela Cuenca controlada por compresion asociadaunaaumento de margen pasivo del espesor dela litosfera, O (ii) la construccion deun arc0 sobre corteza continental un anterior. Las dos alternativas tienen consecuencias geologicas distintas referiendose al moshico de los terranes delPaleozoico temprano.
K E Y WORDS
Argentina, Ordovician, basin modeling, terranes
INTRODUCTION
MoststudiesofEarlyPaleozoicsedimentarybasins in NW Argentina and N Chilè led to qualitative reconstructions of their geotectonic evolution. Here I present the results of a lithospheric modeling approach of the Ordovician basinin the Puna of NW Argentina whichcombines towards a quantitative interpretation data obatined in the field. theoretical considerations with
Tm EAIUYPALAEOZOIC OF BASIN N W ARGJWTINA M e r the Pampean Orogenyin the early Cambrian, the Early Paleozoic evolution of marine basinsstarts in the Late Cambrian. The Late Cambrian Meson Group overlies the Pampean Orogen with an erosional of shallow marine. partly tidally influenced unconformity. It is c. 3000 m thick and consists predominantly quartz-sandstones and shales. They were deposited on a west-facing marine platform (Kumpa and Sanchez, 1988). With a fhrther erosional unconformity, the Meson Group is overstepped to the E and W by the of the Santa Victoria Group (Latest Cambrian to Early Llanvirn). quartz-rich sandstones and shales
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Restricted to the Arenig. a magmaticarc became active fartherW in the western Puna region andin N Chile. It is mainly evidenced by basaltic to andesitic lavas of geochemical volcanic arc affinity, and associated volcanoclastic apron depositsof c. 3500 m thickness (Koukharsky et al., 1988; Breitlueuz et al., 1989; Baldburg, 1990). At this time, the Puna basin and the siliciclastic platfom of the Santa Victoria Group were in a back-arc position. Tectonic subsidence analysis indicates a major loading event in the western Puna duringthe Arenig which transformed the Puna basin into a foreland basin (Bahlburg,1990). This was accompanied by the emergence of the Cordillera Oriental in the Early Llanvirn. In the Middle Ordovician, a c. 3500 rn thick volcanoclastic turbiditecomplex was deposited in the Puna basin. Presently available data suggest that the Arenig arc had been extinctby this time. In modification of earlier subduction modcls for the Andean marginin the Ordovician (Coira etal., 1982; HervC et al., 19&7),the evolution of the Ordovician basin in the puna and Cordillera Oriental \vas interpreted as connected to the collision of the Arequipa Terrane (Ipamos, 1988; Forsytke et al., 1993), leading to the Early Ordovician arc being thrust eastward over its bacl-arc basin pahlburg, 1990). This interpretation \vas abletoexplaintectoniesubsidencerates of upto 600 m/h/Ia associatedwith high sedimentationrates in the developingforelandbasin.Horvever,there is no positiveevidenceof the respective thsust fault,amd alternative interpretations have be to tested. According to the recently revived hypothesis of the Late Proterozoic supercontinent in the Cambrian (Bond et al., 1984; Dalziel, 1991; Dalla Salda et al., 1992), eastern NorthAmerica rifted awayfrom western South h e r i c a during the Cambrian.This may be reflected in W Argentina by the Late CambrianBarly Ordovicianpotentialrift-to-driftsuccession in theArgentinianCordilleraOrientalrepresented by the M e s h and Santa Victoria Groups (see above). Consequentlyit would be lkely that a passive margin stage in the Late Chmbrian and possibly Tremadoc preceded the development of the h e n i g arc as part of an active margin. In order to test both interpretations, an infinite beam elastic plate lithospherie loading model \vas the amlution of the Ordovician Puna applied whichis able to take accounl of 1" order features obsewed in basin in N W Argentina. T m scenarios were testet: (i) that of lithospheric thickening through thrustingof the existing arc over its back-arc basin (Tectonic Scenario), and (ii) that of arc construction on a previously ). details of the model will be discmsed elsewhere (Baldburg and passive margin (Arc S ~ e ~ r i oNmerical Furlong, in prep.).
Arc Scenario: The fïrst loading eventin the Arenig (mas. 6000 m load thiclmess) is taken to be dominated by early volcanic products of basic chemistry with a density of 2900 kg/m3. The sediment shed from this evolving source is taken to be of eorresponding increased density,i.e 2700 kg/m3. On the far side of the basin, sedimentinput reflects rnetaworphic and plutonicsources, the sedimentary rocks of the mtern Puna basin are quartz-rich, and their densityis assumed as 2500 kg/m3. At the time of the second loading event in the Llanvim (rnax. 2000 rn loadthickness), the arc is assumedtohavewolvedtouppercrustal con~positiomwith a density of 2800 kg/m3, with a corresponding density of the eroded sediment of 2600 kg/m3. For the third tirnestepin thc Llandeilono additional loading eventis assumed as there is no record of active volcanism. The sediment is taken to reflect erosion to deeper levels of the arc, cutting into granitoih, and a densityof 2500 kg/m3 is used. Tectonic Scenario: Throughout this scenario, an upper crustal density of 2800 kg/m3 is assigned to the tectonic loads. Eroded sedimentin al1 timesteps ha5 a density of 2500 kg/m3.No tectonic loads were added in the subsidence rate at this time in the Llandeilolime step as tectonic subsidence data indicate a decrease (Baldburg. 1990).
The results obtainedin the two different scenarios show only very minor diKerences between thern. This is not surprising as the hvo runs dBer only in the small density differences assigned to the various loads. These differences play only an insignificant role in comparison to the ovemding interplay between the size of the loads and the flexvral properties of the loaded lithosphere. Therefore it appears to be justifkd to ignore these minor discrepancics in the following discussion.
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Nodes (1 Node = 15 km) Fig. 1: Arc Scenario, results of timestep2 (Llanvirn), assuming aflesural rigidity of the lithosphereof 10’’ Nm. baseline: top the of lithosphere before loading; flexed baseline:of the top lithosphere after loading. As the main conclusion from the mode1 scenarios it can be stated that under both assumptions the geometry and facies patterns of the Ordovician Puna basin were reproduced wellfor a total load thickness of mm. 8000 m, loadingalithosphere with a flemral rigidityof 10’’Nm, representing an equivalentelastic thickness of 13 km. This is a fitting value either for a thinned back-arc crust orYoung a leading edgeat a previously passive margin (Karner and Watts, 1983; Erickson, 1993). The results are inconsequential in so far as no preference for either of the scenarios can be derived. However, they have distinctly different NW Argentina andN .Chile. implications for interpretations of the geotectonic evolution of According to Ramos (1988), the Arequipa Terrane docked to. the Pampeanas Terrane in the Ocloyic Orogeny at the end of the Ordovician. Although thereis no positive evidenceof a major Ordovician suture in this region, the terrane boundary behveen the Pampeanas Terrane in the E, and the Arequipa Terranein the W is assumed to be located along the border behveen the Cordillera Oriental the andPuna, or within the Puna. In either case, the Arenig arc would reside on the Arequipa Terrane. This interpretation corresponds to the Tectonic Scenario presented above. If, however, the Arenig arc was constructed on the previously passive margin of the Pampeanas Terrane. as is inherent to the Arc Scenario, the arc would reside on the former trailingedge of this terrane. The suture with the Arequipa Terrane would accordingly coincide with the position of the Ordovician subduction zoneW of the Cordon de Lila in N Chile (Niemeyer, 1989). This interpretation is supported by the facies patterns and geometries of the Ordovician sedimentary rocks in the Cordillera Oriental which represent, from E to W, an unintermpted shelf-slopebasin section (Bahlburg, 1991). Further field studies should be directed towards identlfylng the actual position of the suture between the Pampeanas and Arequipa Terranes.
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BAHLBURG, H., 1990, The Ordovician basin in the Puna of NW Argentina and N Chile: geodynamic evolution from back-arc to foreland basin.GeotektonischeForschungen 75, 1-107. BAHLBURG, H., 1991, The Ordovician back-arc to foreland suceessor basin in the Argentinian-Chilean Puna: tectonos&menm trends and sea-levelchanges. In: Sedirnentation, Tectonics, and Eusfasy (Ediied by Macdonald, D.I.W.) . S'ecial Publications of the international Association of Seditnentologists 12, 465-484. BOND, G.C., NICICESON, P.A. and KOVLIEPIZ, M.A., 1984, Breakupof a supercontinentbetween 625 and 555 Ma: new evidence and implications for continental histories. Earth and Planetay Scieme Letters '70, . 325-345. B T e E I m U Z , C., BAHLBURG, H., DELAKOWITZ, B. and PICHOWIAK, S., 1989, Volcanic eventsin the Paleozoic central Andes. Jourr-nal ofSouth Arnericatr Earth Sciences 2, 171-189. COWA, B., DAVLBSON, J., ~ O D O Z I S C. , and RAMBS, V., 1982, Tectonic and magmatic evolution of the Andes of'norlhern Argentina and Chile. Earth Science Reviews 18,303-332. DALLASALDA,L.,DALZIEL,I.W.D.,CINGOLANI, C. and VARELA, R, 1992,DidtheTaconic Appalachians continue into southem South America? Geologv 20, 1059-1062. DALZIEL, I.W.D., 1991, Pacifie mmgins of Lawentia and East Antardca-Australia as a conjugate riIl pair: evidence amd implicationsfor an Eocmbrian supercontinent. Geo/ugv 19,598-601. ENCKSON, S.G., 1993,Sedimentloading,lithospheric flexure, andsubduction initiation at passive margim. Geology 21, 125-128. FORSYTHE,B.D., DAVLBSON, J., M€"9DOZIS, C., and JESINCEY, C., 1993, Loever Paleozoic relativc motion of the Arequipa bloek and Gondwana: paleomagnetie evidence frorn Sierra de Almeida nofihem of Chile. Tectonics 12,219-236. HERV6,F., GQDOY, E., P W A , M.A., lUMQS, V. W E L A , C., MPODOZIS, C. and DAWSON, J., 1987, A general view on the Chilm-Argentime Andes, with emphasis on their m l y histoq. American Geophysical Union Geodynalnic Series 18, 97-1 13. K O U I ~ S K Y M., , COIRA, B., BARBER,E. and HANNING, M., 1988, Geoquimica de vulcanitas ordovicicasde la h a (Argentina) y mus implicaneias tecibnicas. J< Congres0 Geoldgico Chileno 4, 1137-158. Karner, G.D. and Watts, A.B., 1983, Gravity anomalies and flexwe of the lithosphereat mountain ramges. Journal of Geoplysical Research 88, B4,449-477. K W A , M. and SmCHEZ, MC., 1988, Geolog and sedimentologyof the Cambrian Grupo Mesbn @W Argentina). IR: Thesouthern CentralAndes:contributionstostructure and evolution of cm active continental margin (Edited by H. Bahlburg, C. Breitkem, and P. Giese). Lecture Notes in Earth Sciences 14,39-54. NlEM€YER, H., 1989, El complejoigneo-sedimentariodelCordondeLila, region deAntofagasta: Significadoteethico. Revista Geoldgica de Chile 16, 163-181. RAMOS, V.A., 1988. Late Proterozoie-Emly Paleozoic ofSouth America a collisional kistorgr. Episodes 11, 168-174.
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