The Link Between Volcanism and Tectonics in the Southern Volcanic Zone of the Chilean Andes: a Review

Tectonophysics 471 (2009) 96–113 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto The link between volcanism and tectonics in the southern volcanic zone of the Chilean Andes: A review José Cembrano a,⁎, Luis Lara b a Universidad Católica del Norte Avda. Angamos 0610, Antofagasta, Chile b Servicio Nacional de Geología y Minería, Avda. Santa María 0104, Santiago, Chile article info abstract Article history: The nature of the interplay between tectonics and volcanism is a major question in continental margin Received 7 January 2008 tectonics. The Southern Andes volcanic zone (SVZ), located at the obliquely convergent Nazca–South America Received in revised form 16 January 2009 plate margin between 33°S and 46°S, offers a unique opportunity to address this question because of along- Accepted 26 February 2009 strike changes in crustal thickness, tectonic style and well-constrained long-term and short-term kinematic Available online 13 March 2009 history. The complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems. Main first-order factors accounting for the Keywords: Southern Andes along-strike variations in the nature and composition of volcanism are crustal thickness and the existence of Volcanism a major, intra-arc fault system, the Liquiñe–Ofqui fault zone (LOFZ). Second order factors include the local Arc tectonics nature of the volcanic arc basement. Liquiñe-Ofqui fault zone Two main categories of volcano–tectonic associations have been identified, according to the spatial Dykes distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second-order active/inactive basement faults. Kinematically-coupled associations, those that are directly related to the current dextral transpressional tectonic regime, include NE-trending volcanic alignments of stratovolcanoes and/or monogenetic cones; kinematically uncoupled associations, in which coupling with the current regime is not required, include stratovolcanoes built on top of ancient reverse and strike–slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the LOFZ. Whereas mostly primitive magmas are found in the first category, more evolved terms can occur genetically associated with the second category. We propose a simplified, three-dimensional model for the magma plumbing system along and across the SVZ that integrates kinematically-coupled and kinematically uncoupled volcano–tectonic associations within the framework of first- and second-order geological and structural constraints. © 2009 Elsevier B.V. All rights reserved. 1. Introduction magma migration. More importantly, it may exert a fundamental control on magma differentiation processes that, in turn, can determine the 1.1. Statement of the problem nature and composition of volcanism along and across continental margins (e.g. Cembrano and Moreno, 1994). One fundamental problem in continental margin tectonics is the The Chilean Andes provides one of the best natural laboratories in nature of the interplay between deformation processes and magma the world to assess the link between tectonics and volcanism. Apart transport through the lithosphere (e.g. Hutton, 1988; Vigneresse, 1999; from its well-constrained plate kinematic history, there is a marked Petford et al., 2000). Deformation-induced fault-fracture networks have latitudinal segmentation in crustal thickness, upper plate deformation been regarded as efficient pathways through which magma is trans- and a changing nature of the basement upon which the volcanic arc ported, stored and eventually erupted at the earth surface (e.g. Hill, 1977; has developed (e.g. Jordan et al.,1983; Tassara and Yáñez, 2003). Then, Shaw, 1980; Clemens and Mawer, 1992; Petford et al., 2000). Thus, the the relative importance of present-day kinematics and inherited state of stress of the lithosphere at the time of magmatism should crustal composition and structure in the mechanisms of magma somehow control the spatial distribution of plutons, dikes swarms and transport and in the nature and composition of volcanism can be volcanic centers (e.g. Nakamura, 1977; Nakamura et al., 1978; Delaney successfully examined along the same orogenic belt. In this paper, we et al.,1986; Hutton,1988; Takada,1994; Glazner et al.,1999; Acocella et al., review and discuss all relevant tectonic, field and geochemical data on 2007). However, crustal deformation not only plays a significant role in the Chilean volcanic arc from 33 to 46°S, a region encompassing the whole southern volcanic zone of the Andes. We hypothesize that one ⁎ Corresponding author. fundamental, usually overlooked factor controlling the wide variety E-mail address: [email protected] (J. Cembrano). of volcanic forms and rock compositions present along a single 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.02.038 J. Cembrano, L. Lara / Tectonophysics 471 (2009) 96–113 97 continental magmatic arc, is the contrasting kinematics of the fault- underlying faults and, in some cases, their kinematics. Further work by fracture networks under which they are transported within the same Adiyaman et al. (1998) and Dhont et al. (1998) has restricted the magmatic arc. Thus, magmas that migrate under crustal compression/ kinematic inference of tension cracks only for elongated cones or transpression would be significantly more evolved (differentiated) linear clusters with a major edifice in the middle. than those that have migrated under upper crustal extension/ According to Lister and Kerr (1991) deviatoric stress plays a key transtension. Furthermore, as the Chilean volcanic arc shows a sig- role in the initiation and orientation of dykes. For low viscosity nificant degree of strike–slip deformation, compression and extension magmas, dykes are expected to be oriented perpendicular to the can occur at the same time but in different orientation, both oblique to minimum principal stress axis at all depths within the lithosphere the magmatic arc trend. (Emerman and Marrett, 1990). Hence, spatially associated volcanic alignments would reflect the direction of the maximum horizontal 1.2. The role of tectonics in magma transport through the lithosphere stress, which may be either σ1orσ2. Complications to this general model arise from: (1) gravity forces acting on the volcano in addition Most early work regarding the origin of igneous rocks largely to the regional tectonic stress; according to Nakamura (1977), gravity neglected the active role of deformation in the transport of magma. forces add up to the regional tectonic stress, and thus, may modify the One notable exception is Cloos (1923), who suggested that magmas trajectory of the principal stress axes, producing less well-defined, were squeezed upward by regional shortening. This early interpreta- usually curved, volcanic center alignments, and (2) pre existing tion anticipated more elaborated models of intrusion within trans- anisotropies in the basement rocks which may also serve as channel- pressional settings as recently postulated (e.g., De Saint Blanquat et al., ways for ascending magma (non-Andersonian dikes). In this case, 1998]. Especially for plutonic rocks, the nature and transport of volcanic alignments may not indicate by themselves the orientation of magma was traditionally viewed mostly from a petrological or geo- the long-term stress field (e.g. Lara et al., 2004a,b). chemical point of view, almost completely separated from tectonics. Nakamura's model, which can be extended to alignments of SV, has Magma was assumed to migrate from the mantle to the upper crust as been successfully tested in the Aleutians (Nakamura et al., 1978). For buoyant diapirs, by analogy with salt domes (e.g. Daly, 1903; Grout, this volcanic arc, the expected direction of the σHmax, as obtained by 1945; Ramberg, 1967). The fact that the most common plan view of the inversion of fault–slip data, is consistent with that suggested by plutons is sub-circular to elliptical was taken as evidence to support the systematic orientation of volcanic structures. The same regional- this hypothesis. Later work, however, proved that it is almost scale scenario has been postulated for the southern Andes volcanic impossible for a granitic diapir to reach the upper crust without zone (Cembrano and Moreno, 1994; López-Escobar et al.,1995; Lavenu crystallizing and stalling at deeper levels. This is because the country and Cembrano, 1999). The fact that volcanic arcs occur along active rock is required to flow around the diapir as it rises (Marsh, 1982). convergent margins (‘compressive’ in a loose sense) led Nakamura Over the last two decades, dike propagation, largely controlled by (1977) to state that the overall tectonics of volcanic arcs should be tectonics, has emerged as a more likely mechanism for magma strike–slip (σ1 and σ3 horizontal; σ2 vertical), instead of compres- transport in the lithosphere, mainly because it is consistent with the sional (σ1 and σ2 horizontal; σ 3 vertical). This would allow magma mechanics, nature and rates of geologic processes (e.g. Petford et al., to ascent through vertical dykes having a direction parallel to that of 2000). These studies have emphasized that the final pluton shape is σHmax (Nakamura and Uyeda, 1980). More recent work on the causal not necessarily indicative

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