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(Ordovícico) En El Segmento Andino Central Del Orógeno Terra Australis

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(Ordovícico) En El Segmento Andino Central Del Orógeno Terra Australis XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S9_011 La tectónica acrecional oclóyica (Ordovícico) en el segmento andino central del orógeno Terra Australis Astini R.A.1, Martina F.1 1Laboratorio de Análisis de Cuencas, CICTERRA-Universidad Nacional de Córdoba, Av. Velez Sarsfield 1611, 2º piso, of. 7, X5016GCA Córdoba, Argentina. [email protected] En el marco del orógeno acrecional de Terra Australis [1] se reconocen intervalos temporales con acreción de terrenos e interrupción momentánea de la subducción que afectan diferencialmente al margen protoandino y permiten reconocer segmentos con características e historias contrastadas en los Andes. Sobre la base de una revisión conceptual y bibliográfica y de nuevos estudios estratigráficos, geoquímicos e isotópicos en la región del antepaís andino central se propone un mecanismo acrecional alternativo para la etapa de orogénesis ordovícica, que es la de mayor extensión que haya afectado al margen proto-pacífico antes de la orogenia gondwánica, con que finaliza la historia acrecional paleozoica. Dentro de la etapa de orogénesis oclóyica se propone separar la colisión del terreno de Precordillera [2] en el segmento andino central entre 27°30’ y 36°30’ LS aproximadamente (~1000 km de longitud), de la acreción de un bloque alargado (sliver o ribbon terrane) de basamento mesoproterozoico de mucho mayor extensión longitudinal (superior los 2000 km) (Fig. 1) que, de alguna manera, ha sido denominado Occidentalia [3]. Este último incluiría en los Andes Centrales a los terrenos de basamento mesoproterozoico correspondientes a las Sierras Pampeanas Occidentales, ubicados al oeste del cinturón de Famatina y su continuación septentrional en la Puna Catamarqueña (afloramientos de Casadero Grande) y en los terrenos de Arequipa-Antofalla. Mientras que el terreno de Precordillera, de origen Lauréntico, incluye una cubierta sedimentaria exótica a Gondwana, el sliver de basamento mesoproterozoico (Fig. 1) sería presuntamente parautócotono, presentando una clara impronta de magmatismo y metamorfismo ordovícicos [4, 5, 6, 7, 8, 9, 10,], diferente del que caracteriza al resto del margen superior del Gondwana Occidental, donde se construyó el arco magmático Famatiniano y las cuencas de retroarco [11, 12]. Ambos terrenos habrían sido 1 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 acrecionados al margen de Gondwana durante el Ordovícico constituyendo una constelación de terrenos [13], pero cada uno habría generado diferentes respuestas en las respectivas placas superiores (Fig. 2). En este sentido, vale remarcar que los pequeños plutones graníticos de tipo I reconocidos en las Sierras Pampeanas Occidentales [7, 8, 9] permiten sostener que dicho terreno fue parte de una placa alta en el sistema convergente, diferente del borde de Gondwana que posee una impronta cámbrica, compatible con el episodio pampeano. La presencia de magmatismo ordovícico (y metamorfismo) en las Sierras Pampeanas Occidentales (y su continuación septentrional) constituye una evidencia difícil de reconciliar con la hipótesis de Cuyania como un terreno único. En las Provincias de San Juan y La Rioja (antepaís andino actual) el lineamiento de Valle Fértil representaría el contacto entre el margen de Gondwana y el terreno parautóctono, mientras que el límite entre este último y la Precordillera estaría en el borde occidental de la Sierra de Pie de Palo [14]. De acuerdo con esta hipótesis, el terreno compuesto de Cuyania [15] representaría en realidad al conjunto de terrenos que se acrecionaron durante el Ordovícico, sin necesariamente responder a una misma dirección de convergencia. La acreción casi contemporánea (durante el Ordovícico) de al menos estos dos bloques indica la complejidad del orógeno Terra Australis, donde interactuaba una constelación de terrenos, que permitirían explicar una cierta diacronía y diferente magnitud de las diversas expresiones orogénicas (magmatismo, metamorfismo y cuencas asociadas). A pesar de la compleja interacción entre ambos terrenos en los Andes Centrales del sur una estructura de rumbo NW, compatible con tectónica de indentación (Fig. 1), estaría representada por la faja de deformación de Jagüé y la fractura de Valle Ancho con extensión hacia Chile. Estos lineamientos, que han sido activos controles de la estucturación preandina y andina, no sólo registran un desplazamiento de rumbo sinestral [16] y rotación de bloques menores con sentidos contrapuestos [17] al sur y norte, sino que tanto el retroarco como los niveles de exhumación son fundamentalmente diferentes, pudiendo indicar, asimismo, importantes componentes de cinemáticas de rumbo. Esta nueva hipótesis abre perspectivas para los testeos estructurales y correlaciones isotópicos y petrológicas que se están llevando a cabo en la región. Referencias [1] Cawood, P.A. (2005) Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoporterozoic and Paleozoic. Earth-Science Reviews, vol. 69, 249-279. [2] Astini, R.A., Benedetto, J.L., Vaccari, N.E. (1995) The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A 2 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 geodynamic model. Geological Society of America Bulletin, vol. 107, 253-273. [3] Dalla Salda, L., Cingolani, C.A., Varela, R. (1992) Paleozoic belt of the Andes and southwestern South America: Result of Laurentia-Gondwana collision? Geology, vol. 20, 617-620. [4] Loewy, S.L, Connelly, J.N, Dalziel, I.W.D. (2004) An orphaned basement block: the Arequipa-Antofalla Basement of the central Andean margin of South America. Geololgical Society of America Bulletin, vol. 116,171-87. [5] Chew, D.M., Schaltegger, U., Košler, J., Whitehouse, M.J., Gutjahr, M., Spikings, R.A., Miškovic A. (2007). U-Pb geochronologic evidence for the evolution of the Gondwanan margin of the northcentral Andes. Geological Society of America Bulletin Vol. 119, 697-711. [6] Cardona. A., Cordani, U.G., Ruiz, J., Valencia, V.A., Armstrong, R., Chef, D., Nutran, A., Sánchez, A.W. (2009) U-Pb zircon geochronology and Nd isotopic signatures of the pre-Mesozoic metamorphic basement of the Eastern Peruvian Andes: Growth and provenance of a Late Neoproterozoic to Carboniferous accretionary orogen on the northwest margin of Gondwana. Journal of Geology, vol. 117, 285-305. [7] Casquet, C., Baldo, E., Pankhurst, R.J., Rapela, C.W., Galindo, C., Fanning, C.M., Saavedra, J. (2001) Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo. Geology Vol. 29(8), 703-706. [8] Varela, R., Sato, A.M., Basei, M.A.S., Siga Jr., O. (2003) Proterozoico medio y Paleozoico inferior de la sierra de Umango, antepaís andino (29°S), Argentina: Edades U- Pb y caracterizaciones isotópicas. Revista Geológica de Chile Vol. 30(2), 265-284. [9] Vujovich, G.I., Van Staal, C.R., Davis, W. (2004). Age constraints on the tectonic evolution and provenance of the Pie de Palo Complex, Cuyania composite terrane and the Famatinian orogeny in the Sierra de Pie de Palo, San Juan, Argentina. Gondwana Research Vol. 7(4), 1041-1056. [10] Ramos, V.A., Dallmeyer, R.D., Vujovich, G. (1998). Time constraints on the Early Palaeozoic docking of the Precordillera, central Argentina, Geological Society Special Publication Vol. 142, 143-158. [11] Astini, R.A., Dávila, F.M. (2004) Ordovician back arc foreland and Ocloyic thrust belt development on the western Gondwana margin as a response to Precordillera terrane accretion. Tectonics, vol. 23, TC4008 1-19. [12] Büttner, S.H. (2009) The Ordovician Sierras Pampeanas-Puna basin connection: Basement thinning and basin formation in the Proto-Andean back-arc. Tectonophysics (en prensa). [13] Astini, R.A., Thomas, W.A., McClelland, W.C. (2005) Provenance of Middle Ordovician detritus in the Argentine Precordillera. Academia Nacional de Ciencias, Gondwana 12, Abstracts, p. 51, Mendoza. [14] Van Staal, C.R., Vujovich, G., Davis, W. (2005) Evolution of structures in a continental subdution channel: an example from the Sierra de Pie de Palo, Cuyania 3 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 terrane, Argentina. Gondwana 12, Resúmenes: 362. Mendoza. [15] Ramos, V.A., Vujovich, G., Dallmeyer, R.D. (1996) Los klippes y ventanas tectónicas preándicas de la Sierra de Pie de Palo (San Juan): edad e implicancias tectónicas. XII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos, Actas V: 377-391. [16] Martina, F., Astini, R.A. (2009) Geología de la región del río Bonete: extremo norte del terreno de Precordillera. Revista de la Asociación Geológica Argentina, en prensa. [17] Spagnuolo, C. M. (2009) Evolución paleogeográfica del Noroeste argentino en el Paleozoico temprano en base a estudios paleomagnéticos. Tesis Doctoral, Inédita. Universidad de Buenos Aires, p. 1-277. Figura 2: Modelo en dos pasos (a y b) de doble subducción asumiendo que a la latitud de San Juan el terreno compuesto de Cuyania estaría formado por la amalgamación del terreno exótico de Precordillera y un terreno presumiblemente parautóctono compuesto por un basamento mesoproterozoico al que se sobreimpone una señal magmática y metamórfica ordovícica. Figura 1: Mapa de terrenos propuestos para el margen occidental de Gondwana mostrando la separación de la Precordillera de un terreno alargado de basamento mesoproterozoico (sliver), presuntamente parautóctono. Entre La Rioja y Mendoza el conjunto ha sido involucrado en Cuyania. Nótese la indentación por el emplazamiento de la Precordillera. 4.
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