SEISMICITY AND VARIATIONS OF THE CRUSTAL TENSIONAL STATE OF THE RETRO-ARC IN THE SOUTHERN CENTRAL ANDES DURING THE LAST 5 MA ( S)

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U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10 CONGRESO GEOLÓGICO CHILENO 2003 SEISMICITY AND VARIATIONS OF THE CRUSTAL TENSIONAL STATE OF THE RETRO-ARC IN THE

1 U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10 CONGRESO GEOLÓGICO CHILENO 2003 SEISMICITY AND VARIATIONS OF THE CRUSTAL TENSIONAL STATE OF THE RETRO-ARC IN THE SOUTHERN CENTRAL ANDES DURING THE LAST 5 MA ( S) FOLGUERA, A. 1, ARAUJO, M. 2, RAMOS, V. A. 1, MELNICK, D. 3, HERMANNS, R. 3, GARCÍA MORABITO, E. 1, BOHM, M. 3 1 Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina folguera@gl.fcen.uba.ar, andes@gl.fcen.uba.ar, legolas@yahoo.com.ar 2 Instituto Nacional de Previsión Sísmica, San Juan, Argentina maraujo@inpres.gov.ar 3 GeoForschungsZentrum, Potsdam, Germany melnick@gfz.potsdam.de, hermanns@gfz.potsdam.de, mirjam@gfz.potsdam.de INTRODUCTION The inner sector of the Neuquén fold and thrust belt, located in the Southern Central Andes ( S) (Fig. 1) is probably one of the most simple areas of the entire Andes, in a structural point of view (10, 11, 12, 13, 14, 15). Geology is almost indistinguishable on both sides of the water divide, and the development of reverse faulting has been only localised in broad segments of this feature (4, 6, 8). This chain does not seem to have been formed in an Andean classical way and moreover, it seems to have been destroyed by extensive normal faulting in more than one single episode of extension (3, 4). This is one of the few areas where recent tectonics in the Andes has been governed by an extensional regime during the last five million years. The evidence of an extensional control in the Early Pliocene and Quaternary accumulations allows to discuss the causes of this anomalous behaviour in this part of the Andean orogen, which shows a close relation to the distribution of the main volcanic axis during the Neogene as a function of potential changes in the Wadati- Benioff geometry (3, 7, 8, 12). A series of field recognisances have been made during the last years, where there have been gathered a large amount of data. Extensive mapping and regional interpretations based on TM and photographs, as well as digital elevation models, have allowed the selection of the main structures, to identify during the field examination. The Neuquén Andes are the eastern foothills of the Main Cordillera between 36 and 40 S (Fig. 1). The work area is comprised between and 39 S, between the Río Neuquén to the north at Chos Malal and the Río Agrio, in the Las Lajas area. To the west the area is part of the Chilean Andean slope. To the east it encompasses the western outcrops of the Neuquén embayment and the Late Cenozoic Loncopué and Andacollo troughs. Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.

DYNAMICS OF THE ARC FRONT AND TECTONIC FRAME There is a striking pattern in the distribution of the Cenozoic volcanic rocks across the present analysed segment of the Neuquén Andes.

2 DYNAMICS OF THE ARC FRONT AND TECTONIC FRAME There is a striking pattern in the distribution of the Cenozoic volcanic rocks across the present analysed segment of the Neuquén Andes. This fact called the attention of Mpodozis and Ramos (1989) (7), who emphasised that in contrast to other segments like Northern and Central Chile (N of 27 S and N of 33 ), here there is not a volcanic expansion or migration of the magmatic belts toward the foreland during the formation of the fold-and-thrust belt. These authors recognised an oscillatory behaviour of the different volcanic belts, being one of the few segments where the belt migrated toward the trench, as shown later on by Stern (1989) (12). Depending on the considered lapse, two discrete jumps and shifts of the volcanic front may be differentiated during the Cenozoic. The first jump to the trench is comprended between the Early Paleogene and the Late Paleogene (11). This movement is determined between the Serie Andesítica, a very external volcanic arc corresponding to the Paleocene-Eocene lapse, and the magmatic front defined by a series of intrusives corresponding to an Upper Oligocene to Lower Miocene basin, the Cura Mallín Basin (Fig. 2). The youngest one corresponds to the lapse between the Lower Pliocene volcanic centers and the Upper Quaternary ones (8, 12), even though previously a broadening of the arc had taken place between the Late Miocene and the Early Pliocene (5). The Upper Miocene volcanic front and the Lower Pliocene front were almost coincident, and they have only differed in relation to their

3 amplitude, making the Lower Pliocene to Quaternary the second discrete registered jump during the Cenozoic (8, 12). New field data has shown the existence of an extensional control in the Lower Pliocene volcanic accumulations (4), known as Cola de Zorro Formation in the Andes of Neuquén in Argentina and in the VIII Región in Chile (9, 13). Some extensional depocenters have accumulated more than a thousand meter thick successions. Extension has continued into the Quaternary, with the formation of a series of volcanic depocenters in the inner retro-arc (4). We postulate that the extension was triggered by a gravitationally induced post-orogenic collapse of a previous orogen, formed during compression associated with the main phase of tectonic inversion (12-8 My) of the Cura Mallín basin (25-10 My) (13, 15). Although the Cola the Zorro basin and Quaternary retro-arc depocenters are superimposed to the Upper Miocene orogenic structure, their extension is limited between S (14). The extensional collapse was caused by negative trench-roll back velocity, and steepening of the Nazca plate, evidenced by backward migration of the volcanic arc. Extension in this section of the Southern Andes has continued for some time into the Quaternary, as shown by extensive normal scarps with morphological expression and profuse monogenetic volcanism in the inner retro-arc located in longitudinal depressions parallel to the Andean front.

4 Even though little is known about the dynamics of the Andean front at these latitudes, what seems to be an anomalous fact is that deep troughs, filled by volcanic materials, are flanking the topographic Andean break (10). Two depressions can be clearly distinguished, the Andacollo and the Loncopué troughs, which are part of the same trend. Both are rectilineous basins aligned with the westernmost outcrops of the Neuquén basin at S. Its younger fill spans from Early Pliocene volcanics to Quaternary basalts, only separated by a locally developed angular unconformity. The Quaternary volcanics are mainly monogenetic mafic andesites to basalts. ANALYSIS OF SEISMICITY The hypocenters corresponding to the fore-arc, intra-arc and retro-arc, between 36 and 40 S show a series of remarkable patterns clearly distinguishable (Fig. 3): First, the superficial seismicity is mostly located in the fore-arc area (2). Second, in this area two clear sources, one related to the Wadati-Benioff zone, and other of superficial origin are clearly individualised. Third, the retro-arc is characterised by a much more diffuse area of earthquakes. Fourth, the forearc displays a Wadati-Benioff zone with two mean dip values, 10 and 30 to the east (2). Fifth, the area of more diffuse pattern of seismicity corresponds to a steeper segment of around 40 to the east. Sixth, the intra-crust seismic retro-arc activity develops north of S, and to the south is mainly restricted to the intra-arc (1) as the more external seismicity of the segment (Fig. 3). DISCUSSION The analysis of the seismic distribution reveals that the Wadati-Benioff zone is formed by two main segments. A 30 east dipping in the fore-arc region (2) and a not well defined 40 one correlated with a diffuse seismic pattern affecting the lower crust and lithospheric mantle. The series of westward jumps which have experienced the volcanic front would have been the response to the steepening of the Wadati-Benioff zone, reaching 40 as an exceptional value between 5 and probably 2 Ma. After this period, the volcanic arc became concentrated in a line as a consequence of the incipient shallowing (+10 ) of the plate which affected only up to the forearc and intra-arc zone. The retro-arc has inherited the Lower Pliocene and Lower Quaternary Wadati-Benioff zone, which will be gradually replaced in time. Therefore the Lower Pliocene-Lower Quaternary extensional depocenters would have incipiently begun to be inverted in the inner retro-arc, as a consequence of a shallower Wadati-Benioff zone than the one where they have been created over. CONCLUSIONS 1) Intra-crust retro-arc seismicity is only relevant north of S. South of this latitude the Andean orogenic front is defined at the intra-arc position, while to the north it shifts to the east in association with the inner retro-arc as evidenced by crustal seismicity. 2) The Wadati-Benioff zone could have had a different geometry with evidence of a rather steeper configuration 2 Ma ago before the "normal" present period of subduction at these latitudes. This would explain the development of intra-arc extensional depocenters between and 39 S during Early Pliocene and Early Quaternary.

6 ACKNOWLEDGEMENTS This study has been made with funding from PICT 06729/99 of Agencia Nacional de Promoción Científica y Tecnológica to V.A. Ramos. REFERENCES 1. Barrientos, S. y Acevedo, P Seismological aspects of the Lonquimay (Chile) volcanic eruption. Journal of Volcanology and Geothermal Research. Vol. 53. p Bohm, M., Lüth S., Echtler H., Asch G., Bataille K., Bruhn C., Rietbrock A. y Wigger P The Southern Andes between 36 and 40 S latitude: seismicity and average seismic velocities. Tectonophysics. Vol p Folguera, A., Ramos, V.A. y Melnick, D Partición de la deformación en la zona del arco volcánico de los Andes neuquinos (36º-39ºS) en los últimos 30 millones de años. Revista Geológica de Chile. Vol. 29. No. 2. p Folguera, A., Ramos, V.A. y Melnick, D. (en prensa). Recurrencia en el desarrollo de cuencas de intraarco. Cordillera Neuquina (37 30 ). Revista de la Asociación Geológica Argentina. 5. Kay, S.M Tertiary to Recent transient shallow subduction zones in the Central and Southern Andes. Quince Congreso Geológico Argentino. El Calafate. Argentina. Actas 3. p Melnick, D.; Folguera, A.; Rosenau, M.; Echtler, H. y Potent, S Tectonics from the Northern segment of the Liquiñe- Ofqui fault system (37-39 S), Patagonian Andes. Fifth International Symposium of Andean Geodynamics, Abstracts. Toulouse. Francia. p Mpodozis, C. y Ramos, V.A The Andes of Chile and Argentina. Ericksen, G.E., Cañas Pinochet, M.T. y Reinemud, J.A. (Eds.), In Geology of the Andes and its relation to Hydrocarbon and Mineral Resources. Circumpacific Council for Energy and Mineral Resources, Earth Science Series. Vol. 11. p Muñoz, J. y Stern, C The Quaternary volcanic belt of the southern continental margin of South America: Transverse structural and petrochemical variations across the segment between 38 and 39 S. Journal of South American Earth Science. Vol. 1. No. 2. p Niemeyer, H. y Muñoz, J Geología de la hoja 57 Laguna de La Laja, Región del Bío-Bío. Servicio Nacional de Geología y Minería. Mapa 1: Santiago de Chile. 10. Ramos, V.A Estructura. Rolleri E.O. (Ed.), In Geología y recursos naturales de la Provincia del Neuquén, Relatorio Séptimo Congreso Geológico Argentino. p Ramos, V.A., Estructura del sector occidental de la faja plegada y corrida del Agrio, Cuenca Neuquina Argentina. Décimo Congreso Latinoamericano de Geología. Vol. 2. p Buenos Aires. 12. Stern, C Pliocene to present migration of the volcanic front, Andean Southern Volcanic Front. Revista Geológica de Chile. Vol. 16. No. 2. p Suárez, M. y C. Emparán Hoja Curacautín. Regiones de la Araucanía y del Bío Bío. Carta Geológica de Chile, 1: , Servicio Nacional de Geología y Minería de Chile, 71. p. 105: Santiago. Chile. 14. Vergara, M. y Muñoz, J La Formación Cola de Zorro en la alta cordillera Andina Chilena (36-39 Lat. S), sus características petrográficas y petrológicas: una revisión. Revista Geológica de Chile. Vol. 17. p Zapata, T., Brissón I. y Dzelalija F The role of basement in the Andean fold and thrust belt of the Neuquén Basin. in Thrust Tectonics. London. Inglaterra. Abstracts. p

09/06/2017. The orogenic cycle in the Andes: Arc magmatism and delamination as control of the fold and thrust belts

09/06/2017. The orogenic cycle in the Andes: Arc magmatism and delamination as control of the fold and thrust belts The orogenic cycle in the Andes: Arc magmatism and delamination as control of the fold and thrust belts Victor A. Ramos Laboratorio de Tectónica Andina Instituto de Estudios Andinos Don Pablo Groeber Universidad