PUBLICATIONS Tectonics RESEARCH ARTICLE Tectonic role of margin-parallel and margin-transverse faults 10.1002/2016TC004226 during oblique subduction in the Southern Volcanic Zone Key Points: of the Andes: Insights from Boundary Element Modeling • Transverse-to-the-orogen faults may also participate in slip partitioning A. Stanton-Yonge1,2, W. A. Griffith3, J. Cembrano1,2, R. St. Julien3, and P. Iturrieta1,2 resulting from oblique subduction • We implement a forward 3-D model of 1Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, 2Andean Andean subduction to simulate 3 interseismic and coseismic Geothermal Center of Excellence (CEGA-FONDAP 15090013), Santiago, Chile, Department of Earth and Environmental deformation Sciences, University of Texas at Arlington, Arlington, Texas, USA • We constrain kinematics, slip rate, and seismic hazard associated with differently oriented regional faults Abstract Obliquely convergent subduction margins develop trench-parallel faults shaping the regional architecture of orogenic belts and partitioning intraplate deformation. However, transverse faults also are Supporting Information: common along most orogenic belts and have been largely neglected in slip partitioning analysis. Here we • Supporting Information S1 constrain the sense of slip and slip rates of differently oriented faults to assess whether and how transverse • Table S1 faults accommodate plate-margin slip arising from oblique subduction. We implement a forward 3-D Correspondence to: boundary element method model of subduction at the Chilean margin evaluating the elastic response of A. Stanton-Yonge, intra-arc faults during different stages of the Andean subduction seismic cycle (SSC). Our model results show [email protected] that the margin-parallel, NNE striking Liquiñe-Ofqui Fault System accommodates dextral-reverse slip during the interseismic period of the SSC, with oblique slip rates ranging between 1 and 7 mm/yr. NW striking faults Citation: exhibit sinistral-reverse slip during the interseismic phase of the SSC, displaying a maximum oblique slip of Stanton-Yonge, A., W. A. Griffith, 1.4 mm/yr. ENE striking faults display dextral strike slip, with a slip rate of 0.85 mm/yr. During the SSC J. Cembrano, R. St. Julien, and P. Iturrieta (2016), Tectonic role of margin-parallel coseismic phase, all modeled faults switch their kinematics: NE striking fault become sinistral, whereas NW and margin-transverse faults during striking faults are normal dextral. Because coseismic tensile stress changes on NW faults reach 0.6 MPa at oblique subduction in the Southern 10–15 km depth, it is likely that they can serve as transient magma pathways during this phase of the SSC. Our Volcanic Zone of the Andes: Insights from Boundary Element Modeling, model challenges the existing paradigm wherein only margin-parallel faults account for slip partitioning: Tectonics, 35, 1990–2013, doi:10.1002/ transverse faults are also capable of accommodating a significant amount of plate-boundary slip arising from 2016TC004226. oblique convergence. Received 5 MAY 2016 Accepted 10 AUG 2016 1. Introduction Accepted article online 13 AUG 2016 Published online 5 SEP 2016 Major crustal faults and fold systems at convergent margins are commonly organized into margin-parallel, high-strain domains that shape the first-order architecture of orogenic belts. However, second-order, oblique-to-the-orogen structures can also be found within the overall margin-parallel structural grain that characterize convergent margins. Regional-scale transverse structures have been documented along the length of the Andes [e.g., Katz, 1971; Melnick and Echtler, 2006; Glodny et al., 2008] which cut the well- organized N-S architecture of the Central and Southern Andean orogen. These transverse structures have been recognized as (1) NW striking major magnetic and gravimetric anoma- lies [Yañez et al., 1998], (2) NW and NE striking long-lived faults spatially and genetically associated with por- phyry copper deposits [Mpodozis and Cornejo, 2012; Piquer et al., 2015], (3) ENE and NW trending alignments of major stratovolcanoes and minor eruptive vents within the intra-arc [e.g., Cembrano and Moreno, 1994; López-Escobar et al., 1995], and (4) NW striking faults in the fore arc that recent structural and seismological data have associated with neotectonic activity [e.g., Haberland et al., 2006; Farías et al., 2011]. Geophysical and geological evidence suggests that some of these Andean Transverse Faults (ATF) are long-lived lithospheric-scale features [Yañez et al., 1998; Yañez and Cembrano, 2004] associated with seismic segmentation of the plate interface [Melnick et al., 2009], enhanced rock damage, past and present fluid flow, and ore deposit clustering [e.g., Chernicoff, 2001; Sánchez et al., 2013]. Within the volcanic arc, the ATF exert a fundamental structural control on Quaternary volcanism in the Central and Southern Andes [Tibaldi et al., 2005; Lara et al., 2006; Melnick et al., 2006; Acocella and Funiciello, 2009; Cembrano and Lara, 2009]. Despite the widely acknowledged relevance of ATF in controlling subduction-related processes, their tectonic role ©2016. American Geophysical Union. in terms of whether and how they participate in the accommodation of crustal deformation is still All Rights Reserved. poorly understood. STANTON-YONGE ET AL. SLIP PARTITIONING SOUTHERN ANDES 1990 Tectonics 10.1002/2016TC004226 Within obliquely convergent subduction margins, the trench-parallel component of convergence can be accommodated within the upper plate by the partitioning of intraplate slip [Fitch, 1972; Jarrard, 1986a, 1986b; Beck, 1991; Tikoff and Teyssier, 1994]. Slip partitioning arising from oblique subduction is typically attributed to margin-parallel strike-slip crustal faults, e.g., Sumatra [Fitch, 1972], Eastern Turkey [Jackson, 1992], and New Zealand [Cashman et al., 1992]. Margin-transverse faults, referred to as faults striking 30° to 60° away from the trend of the margin, have been largely neglected, hampering a full understanding of continental margin slip partitioning. Crustal discontinuities, or inherited fault zones, favorably oriented with respect to the long-term, secular stress field (i.e., the interseismic period of the subduction seismic cycle (SSC)) and/or a short-term stress field (coseismic period) may be activated and slip in response to plate motion, accommodating part of the slip arising from oblique subduction, typically associated with margin-parallel structures. Therefore, constraining the slip sense and estimated slip rates of differently oriented crustal faults could greatly improve the under- standing of how deformation due to oblique subduction is partitioned at crustal faults. The Boundary Element Method (BEM) using displacement discontinuity elements [Crouch and Starfield, 1983] is one of the most versatile and widely used numerical techniques to solve problems involving fractures at an elastic medium because it allows modeling complex geometries of fault surfaces without gaps or singularities and at a relatively low computational cost. We implement a forward 3-D BEM model of subduction at the Chilean margin using Poly3d [Thomas, 1993] to simulate the deformation field in the upper plate during different stages of the SSC. Our goal is to evaluate the elastic response of crustal faults to constrain their kine- matics and potential slip rates during the individual phases of the SSC. To accomplish this, we select key case studies of margin-parallel and margin-transverse structures within a particular region of the Andes, the Southern Volcanic Zone (SVZ), which constitutes the intra-arc region of the Southern Andes (Figure 1). We hypothesize that crustal faults with various orientations play a significant role in the partitioning of intra- plate slip arising from oblique subduction. This finding challenges the paradigm that only margin-parallel faults accommodate the trench-parallel slip of convergence. We also explore the idea that the tectonic role of margin-parallel and margin-transverse faults can be strongly influenced by the temporally variable stress regimes associated with individual phases of the SSC. The forward numerical model implemented here to study the relationship between plate motion at the subduction zone and slip along crustal faults constitutes a useful methodology that can be further used to evaluate first-order response of previously constrained fault geometries within any region of the Andes. Additionally, results presented in this work may have a great impact on the estimation of geologic hazard arising from intraplate earthquakes on margin-parallel and margin-transverse faults. 2. Tectonic Setting The Southern Volcanic Zone (SVZ) (Figure 1) makes up the volcanic arc of the Southern Andes between lati- tudes 33°S and 47°S. This region has undergone major megathrust earthquakes (e.g., Mw 9.5, Valdivia 1960; Mw 8.8, Maule 2010) resulting from the oblique subduction of the Nazca plate underneath the South American plate at a rate of 66 mm/yr [Angermann et al., 1999]. Surface velocity vectors measured by GPS [Wang et al., 2007; Ruegg et al., 2009; Moreno et al., 2011; Lin et al., 2013] are displayed in Figure 1, represent- ing the short-term interseismic displacement field within the SVZ. The SVZ displays remarkable along-strike structural variations. The northern portion (33°S–38°S) is character- ized by well-developed margin-parallel fold and thrust belts, and a high (elevations up to 6.9