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Article Geochemistry 3 Volume 9, Number 9 Geophysics 18 September 2008 Q09002, doi:10.1029/2008GC002037 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Fissural volcanism, polygenetic volcanic fields, and crustal thickness in the Payen Volcanic Complex on the central Andes foreland (Mendoza, Argentina) F. Mazzarini and A. Fornaciai Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, I-56126 Pisa, Italy ([email protected]) A. Bistacchi Dipartimento di Scienze Geologiche e Geotecnologie, Universita` degli Studi di Milano-Bicocca, Piazza della Scienza 4, I-20126 Milan, Italy F. A. Pasquare` Dipartimento di Scienze Chimiche ed Ambientali, Universita` degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy [1] Shield volcanoes, caldera-bearing stratovolcanoes, and monogenetic cones compose the large fissural Payen Volcanic Complex, located in the Andes foreland between latitude 35°S and 38°S. The late Pliocene-Pleistocene and recent volcanic activity along E-W trending eruptive fissures produced basaltic lavas showing a within-plate geochemical signature. The spatial distribution of fractures and monogenetic vents is characterized by self-similar clustering with well defined power law distributions. Vents have average spacing of 1.27 km and fractal exponent D = 1.33 defined in the range 0.7–49.3 km. The fractal exponent of fractures is 1.62 in the range 1.5–48.1 km. The upper cutoffs of fractures and vent fractal distributions (about 48–49 km) scale to the crustal thickness in the area, as derived from geophysical data. This analysis determines fractured media (crust) thickness associated with basaltic retroarc eruptions. We propose that the Payen Volcanic Complex was and is still active under an E-W crustal shortening regime. Components: 9552 words, 8 figures, 1 table. Keywords: volcanic field; crustal shortening; vent distibution; Andes; Argentina. Index Terms: 8178 Tectonophysics: Tectonics and magmatism; 8425 Volcanology: Effusive volcanism; 9360 Geographic Location: South America. Received 27 March 2008; Revised 15 July 2008; Accepted 28 July 2008; Published 18 September 2008. Mazzarini, F., A. Fornaciai, A. Bistacchi, and F. A. Pasquare` (2008), Fissural volcanism, polygenetic volcanic fields, and crustal thickness in the Payen Volcanic Complex on the central Andes foreland (Mendoza, Argentina), Geochem. Geophys. Geosyst., 9, Q09002, doi:10.1029/2008GC002037. 1. Introduction whereas in compressional ones it is assumed by some authors that magma cannot easily reach the [2] Volcanism mainly concentrates at plate margins surface [e.g., Watanabe et al., 1999]. Recent inves- and is quite common in extensional settings, tigations focused the attention on the emplacement Copyright 2008 by the American Geophysical Union 1 of 17 Geochemistry 3 mazzarini et al.: volcanism and tectonics in central andes Geophysics 10.1029/2008GC002037 Geosystems G of shallow crustal-level intrusions [Musumeci et 2006]. Analyses of GPS-derived velocity field al., 2005] and volcanism [Tibaldi, 2005; Galland et highlight that the deformation is localized in the al., 2007a, 2007b] during crustal shortening. Sev- back-arc zone in southern Andes (26°–36°S), eral examples of volcanoes growing in contraction- suggesting a locking of the Nazca oceanic bound- al tectonic settings have been identified in the ary and the possible occurrence of an ‘‘Andes’’ Andes: e.g., the El Reventador and Guaga Pichin- microplate colliding with the back arc [Brooks et cha volcanoes in Ecuador [Tibaldi, 2005; Legrand al., 2003]. Present-day crustal deformation rates in et al., 2002] and the Taapaca and Socompa volca- the central and southern sections of the Andes, noes in Chile [Clavero et al., 2004; van Wyk de derived by the analysis of GPS data, reveal an E-W Vries et al., 2001]. In the Andes Foreland Belt at trending contraction in the order of 10 mm aÀ1 in 37°S latitude, field evidence, along with radiomet- the Andes retro-arc at 36°S–37°S latitude [Klotz et ric 39Ar-40Ar ages, shows that the Quaternary al., 2001; Wang et al., 2007]. In Miocene and basaltic Tromen volcano (Argentina) developed Holocene times, mixed contractional and exten- above active thrust systems coeval with regional sional tectonic regimes affected the eastern slope of crustal shortening [Galland et al., 2007a, and the Andes at 36°–39°S latitude [Folguera et al., references therein]. Alternatively, Folguera et al. 2006]. This complex geodynamic scenario is [2008], on the basis of the analysis of seismic data explained by post-Miocene changes in the subduc- and field observations, associated the Tromen vol- tion zone geometries that affected different seg- canism with crustal collapse and extension during ments of the Andes. Notably, north of 38°S the Quaternary. latitude, the extension is limited to the period between 5 and 3 Ma [Folguera et al., 2006]. [3] A large Pleistocene-recent composite volcanic field, the Payen Volcanic Complex [Inbar and [5] The Payen Volcanic Complex developed in the Risso, 2001; Pasquare`etal., 2008] developed just northern sector of the back-arc Neuque´n Basin, 100 km northeast of Tromen Volcano, above the north of 38°S latitude (Figure 1), and belongs to Neuque´n Basin in the Mendoza province of Argen- the volcanic province of Payenia [Polansky, 1954; tina [Cobbold et al., 1999]. The Payen Volcanic Ramos, 1999; Inbar and Risso, 2001; Pasquare`et Complex comprises basaltic shield volcanoes, ba- al., 2005; Bertotto et al., 2006; Ramos and Kay, saltic scoria cones, scoria and spatter cones, and 2006]. Its western sector reaches the Andean mar- trachyandesitic stratovolcanoes aligned along an gin near the junction between the N-S trending E-W trending eruptive fissure, several tens of Malargue Fold and Thrust Belt, in the north, and kilometers long [Inbar and Risso, 2001; Pasquare` the Agrio Fold Belt, in the south [Cobbold et al., et al., 2005; Bertotto et al., 2006; Risso et al., 2008; 1999; Cobbold and Rossello, 2003; Guzman et al., Pasquare`etal., 2008]. In this paper the spatial 2007]. Analysis of the active stress field in the distribution of monogenetic vents and of the frac- crust of the Neuque´n Basin has been performed by ture network in the Payen Volcanic Complex are studying borehole breakout orientations from nu- described and then discussed in the light of a merous wells [Guzman et al., 2007]. In the basin as possible relationship with the crustal thickness in a whole, the maximum horizontal stress is directed the sector of the Andes foreland between latitude nearly E-W (91°), but there is a small difference in 35°Sand38°S. azimuth between data north of Rio Colorado (100°) and those south of Rio Colorado (82°)[Guzman et 2. Geodynamic and Geologic al., 2007]. In the Payen Volcanic Complex area, the orientation of the maximum horizontal stress, de- Framework and Crustal Structure rived from borehole breakouts [Guzman et al., 2007], ranges from 70° to 127° with an average, [4] The tectonic setting of the studied area is E-W orientation (Figure 1). E-W monogenetic vent dominated by the convergence between the Nazca alignments have also been recognized in the north- and South America plates (Figure 1) at a mean rate ern portion of the volcanic complex [Risso et al., of 80 mm aÀ1, averaged at this latitude over the last 2008]. Moreover, in the Tromen volcano area, 3Ma[Somoza, 1998], or 66 mm aÀ1 with an further south of the Payen Volcanic Complex, late azimuth of 79.5° determined on the basis of GPS Pliocene-Pleistocene dykes trend E-W [Galland et modeling [Kendrick et al., 2003]. At the latitude of al., 2007a]. The Ar39–Ar40 plateau ages of the 38°S, the Nazca-South America convergence is Tromen volcanic products in the work of Galland accommodated in intraarc and back-arc zones by et al. [2007a] depict a complex evolution of the strain partitioning associated with transpression in volcano starting about at 2.3 Ma, characterized by the intraarc [Folguera et al., 2004; Rosenau et al., 2of17 Geochemistry 3 mazzarini et al.: volcanism and tectonics in central andes Geophysics 10.1029/2008GC002037 Geosystems G Figure 1. Geology framework of the Andean Foreland between 35°S and 39°S latitude. The Shaded DEM, illuminated from the NW, is derived from SRTM (courtesy of the University of Maryland Global Land Cover Facility, http://glfc.umiacs.umd.edu/index.shtml) and projected in UTM WGS84 Zone 19. Major structures of the investigated area in the Neuque´n Basin [after Cobbold et al., 1999; Melnick et al., 2006; Folguera et al., 2007; Galland et al., 2007a]; MTB: Malargue Thrust Belt; BCFB: Barrancas Colorado Fold Belt; MTF: Main Thrust Front; AFTB: Agrio Fold and Thrust Belt; GFTB: Guanacos Fold and Thrust Belt; FMT: Filo Morado Thrust; CF: Carbonilla Fault; CL: Cortadera Lineament. Orientation and classification of the maximum horizontal stress are from Guzman et al. [2007]. The black dashed line on the west indicates the boundary between Chile and Argentina, the white line on the east delimits the east margin of the Neuque´n basin. Inset: present-day plate tectonics setting of South America. The blue box locates the investigated area. The Nazca and Antarctic plates converge with South America at rates of 2 to 10 cm aÀ1 (numbered white arrows from Cobbold et al. [1999]). Steep segments of subducted Nazca slab correlate with Quaternary volcanoes of the North Volcanic Zone (NVZ), Central Volcanic Zone (CVZ), and South Volcanic Zone (SVZ). The image of South America is derived from GEOTOPO 30 (courtesy of the University of Maryland Global Land Cover Facility, http://glfc.umiacs.umd.edu/index.shtml). N-S trending pop-up structures and thrusting Folguera et al. [2008] interpreted the folds and coeval with the production of basaltic and silicic inverse faulting affecting the products of Tromen lavas associated with E-W trending eruptive fis- volcano as minor structures accommodating the sures (dome and vent alignments).