Formation, erosion and exposure of Early Amazonian dikes, dike swarms and possible subglacial eruptions in the Elysium Rise/Utopia Basin Region, Mars G.B.M. Pedersen a, ⁎, J.W. Head III b , L. Wilson c a Department of Earth Sciences, University of Aarhus, Hoegh-Guldberggade 2, 8000 Aarhus C, Denmark b Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA c Lancaster Environment Centre, Lancaster University, LA1 4YQ UK abstract article info Article history: Accepted 6 August 2009 Available online xxxx Editor: T. Spohn Keywords: dike dike swarms subglacial volcanism Utopia Basin Elysium Mars Hundreds of narrow, linear ridge segments are found in the transition zone between the Elysium Rise and the Utopia basin, occurring as both single and multiple ridges. The ridges are distinctive because of their very linear, steep-sided nature, their often sharp ridge crest (which sometimes is fractured), their association with stubby flows, their continuity over long distances and their cross-cutting of different terrain. The linear ridges are interpreted to be single dikes and dike swarms, either emplaced as normal dikes or as dikes emplaced subglacially feeding an explosive or effusive eruption. Five dike swarms are identified, having lengths ranging from 10–45 km and being between 1–7 km wide, while single ridges are up to 20 km long and 100–500 m wide. In the areas of dike swarms, crustal dilatation is estimated to vary from 15–60%. Dikes emplaced en echelon suggest that variations in the local stress field caused rotation during dike emplacement and dikes crosscutting flow units imply that dike emplacement can account for some of the observed linear fractures in the area. The ridges both modify and constrain Early Amazonian flows and flood plain deposits suggesting intense dike emplacement in the Early Amazonian. The association with different stages of inverted craters, as well as some features of ice-related origin (possible ice-cauldron and tindar-like features), indicate that the dikes may have been exposed due to eolian erosion and loss of volatile rich units subsequent to their emplacement. © 2009 Elsevier B.V. All rights reserved. 1. Introduction It has been known since the Mariner 9 mission that Mars exhibits the largest volcanic edifices known in the solar system (Carr, 1973), and these and related volcanic units provide very important information on martian thermal and geologic evolution. Dikes and dike swarms feeding and associated with these units are another important part of understanding these processes (Mege and Masson, 1996; Wilson and Head, 1994; 2002a,b). Furthermore, dike emplace- ment and configuration is governed by stress state, and thus their regional setting in relation to source regions is of crucial importance. Field and geodetic observations on Earth show that dike emplacement results in extensional near-surfaces stresses that can produce narrow, deep and v-shaped tension cracks, faulting, and linear, straight- walled, flat-floored grabens (Mastin and Pollard, 1988; Rubin and Pollard, 1988; Wilson and Head, 2002a). Since dikes rarely are exposed on a planet like Mars (with low erosion rates and minor uplift) these morphologies representing the near-surface manifesta- tion of dike emplacement have been the main source for studying dike systems on Mars. However, examples of dike outcrops in the form of positive linear features representing eroded and exposed dikes do exist, including a giant dike system 800 km north of Hellas Basin (Head et al., 2006), and observed dikes in the region of Tharsis (Mege and Masson, 1996; Wilson and Mouginis-Mark, 1999). The exposed dikes are character- ized by being narrow, symmetrical and linear, low-relief ridges with no visible stratigraphy. They usually appear sharp-crested, crosscut different units, and their width and height remain virtually unchanged over long distances, (Mége, 1999; Wilson and Mouginis-Mark, 1999; Head et al., 2006). No observations of eroded and exposed swarms of dikes have been reported to date and thus information on dike swarms is only known through mapping of graben and fractures. Ernst et al. (2001) and Wilson and Head (2002a,b) interpret several graben zones around Tharsis to be the surface manifestation of giant dike swarms reflecting plume-related dike intrusion complexes. These systems extend thousands of kilometers and thus appear to have very significant implications for martian geologic history, particularly in relation to the existing hypothesis regarding a global cryosphere and a sequestered groundwater system (Clifford, 1993; Clifford and Parker, 2001). Dike swarm activity has been proposed in the Elysium volcanic Earth and Planetary Science Letters xxx (2009) xxx–xxx ⁎ Corresponding author. Tel.: +45 89429539; fax: +45 89429406. E-mail address: gro.birkefeldt@geo.au.dk (G.B.M. Pedersen). URL: http://www.marslab.dk/ (G.B.M. Pedersen). EPSL-09966; No of Pages 16 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.08.010 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl ARTICLE IN PRESS Please cite this article as: Pedersen, G.B.M., et al., Formation, erosion and exposure of Early Amazonian dikes, dike swarms and possible subglacial eruptions in the Elysium Rise/Utopia Basin Region, Mars, Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.08.010