Global distribution of volcanic centers and mountains on Io: Control by asthenospheric heating and implications for mountain formation Michelle R. Kirchoff a,b, ⁎, William B. McKinnon a,b , Paul M. Schenk c a Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Dr., St. Louis, MO 63130, United States b McDonnell Center for the Space Sciences, Washington University, Campus Box 1105, One Brookings Dr., St. Louis, MO 63130, United States c Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, United States abstract article info Article history: Received 4 June 2010 Received in revised form 3 November 2010 Accepted 9 November 2010 Available online 3 December 2010 Editor: T. Spohn Keywords: Io orogenesis volcanism tectonics thermoelastic stress spherical harmonics Jupiter's moon Io possesses numerous tectonic mountains in addition to its ubiquitous volcanoes and volcanic features. Remarkably, a distinct global anticorrelation exists between the spatial distribution of mountains and volcanic centers on Io. This relationship indicates an explicit connection between volcanism and mountain formation, even though the mountains are tectonic in origin (predominantly upthrusted crustal blocks). Spherical harmonic analysis shows the distribution of mountains and volcanic centers have statistically significant power at degree 2; this result is especially striking for the volcanic center distribution, and directly implicates models of asthenospheric tidal heating. The latter predict enhanced heat flux along the equator in a degree-two pattern that matches observations. Mountain formation on Io appears to be a form of dominantly vertical tectonism unique in the modern Solar System: continual burial by widespread volcanism drives the crust inward, which leads to strong compression, and at discrete locations, mountains. Correlation coefficients between the volcanic and mountain distributions indicate statistically meaningful anticorrelation at low spectral degrees (l = 1, 2, 4, and 6); the anticorrelation is especially significant between the longitudinal (sectorial) l = 2 components when considered on their own. We compare this anticorrelation with published models that link volcano and mountain formation. While consistent in part with l =2 convection models, which predict such an anticorrelation (in principle), such low degree anticorrelations are also (if not more) compatible with mountain formation due to, or influenced by, thermal expansion of Io's crust, and deep compression and thrust faulting in regions of lower than average volcanic heat-piping. Positive correlations between mountain and volcanic center distributions at high spectral degree may reflect structural links between a good fraction of mountain blocks and adjacent volcanic paterae, whereas the anticorrelation at low degree implies that most volcanic features (which are far more numerous overall) form independently of mountains. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The majority of mountains on Io appear to be rugged crustal blocks uplifted along major thrust faults and tilted with respect to the sur- rounding plains (McEwen et al., 2004; Schenk et al., 2001; Turtle et al., 2001, 2006). The exact set of circumstances that generate the thrust faults and form these mountains is yet unknown. Three major tectonic formation hypotheses have been proposed. Each is a modification or variation of the fundamental global subsidence stress hypothesis of Schenk and Bulmer (1998), which was originally advanced to account for the fact that mountains are found as isolated features rather than as part of an organized pattern or patterns (Jaeger et al., 2003; Schenk and Bulmer, 1998; Schenk et al., 2001; Turtle et al., 2001). Global subsidence stresses (proportional to Δz/R, where Δz is subsidence and R is Io's average radius = 1821 km) are created by the lateral squeezing of concentric spherical shells as older layers shrink in radius as they are pushed down into the interior by newer flows distributed evenly (averaged over time) across the globe of Io. These stresses are naturally compressional, and as they do not accumulate in the vertical direction, and in the absence of viscous relaxation, are strongly non-hydrostatic. Such lateral subsidence strains (Δz/R ~1–3% for Δz ~25–50 km) would on Earth be considered minor compared with the enormous strains generated by plate tectonics (≫1, i.e., very large). On Io, however, no such process as plate tectonics is obviously available to break the lithosphere, nor are Io's mountains and more numerous (and well-known) volcanic centers arranged in linear or arcuate patterns. Moreover, subsidence stresses have been quantitatively shown to potentially dominate Io's in-plane, or shell, stresses (Jaeger Earth and Planetary Science Letters 301 (2011) 22–30 ⁎ Corresponding author. Present address: Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, United States. Tel.: + 1 303 546 9670; fax: + 1 303 546 9687. E-mail addresses: kirchoff@boulder.swri.edu (M.R. Kirchoff), mckinnon@levee.wustl.edu (W.B. McKinnon), schenk@lpi.usra.edu (P.M. Schenk). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.11.018 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl