ACTA GEOLOGICA SINICA (English Edition) Vol. 90 Supp. 1 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags July 2016 Daniel MÈGE and Joanna GURGUREWICZ, 2016. On Mars, Location and Orientation of Dykes Exposed along the Valles Marineris Walls Reveal Expected and Unexpected Stress Fields. Acta Geologica Sinica (English Edition), 90(supp. 1): 177-179. On Mars, Location and Orientation of Dykes Exposed along the Valles Marineris Walls Reveal Expected and Unexpected Stress Fields Daniel MÈGE1, 2, * and Joanna GURGUREWICZ1, 3 1 Space Research Centre, Polish Academy of Sciences, Warsaw, Poland 2 Planetology and Geodynamics Laboratory, UMR CNRS 6112, University of Nantes, France 3 Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland Structural and geomorphological analysis of the Martian surface in the visible spectral range using the NASA/ Viking images in the 90's, complemented by experimental modelling (Mège and Masson, 1996a; Mège et al., 2003) suggested that the Valles Marineris trough (chasma) system is aligned with a mafic dyke swarm, named the Syria Planum Dyke Swarm. Cross-cutting relationships and terrestrial inspiration led to the interpretation that the dykes were injected from the summit of the Tharsis dome, Syria Planum (Figure 1), and may have guided the normal faulting that gave several of the Valles Marineris troughs their graben structure (Mège and Masson, 1996a, 1996b; Ernst et al., 2001) within the framework of a plume tectonics model. At that time, the resolution of images, several tens of meters per pixel, was too coarse to identify dykes when exposed, and only the derived volcanic landscapes at the surface could be identified. Since 2006, the HiRISE telescope orbits Mars in NASA's Mars Reconnaissance Orbiter spacecraft and takes images of Mars at a resolution of 0.25 to 1 m/pixel. Many sites in Valles Marineris have been imaged (Figure 2), some of Fig. 1. Stress field generated by the Syria Planum magma centre inferred from dyke-related landforms (Mège and Masson, 1996a). which show eroded dykes. Flahaut et al. (2011) identified The Valles Marineris grabens were interpreted to form in this mafic dykes in eastern Coprates Chasma of orientation context, similar to continental rifts in Large Igneous Provinces predicted by the plume tectonics model. Huang et al. (2012) (Mège, 2001). identified several dykes exposed on the plateau south of Valles Marineris that also follow this trend. We have been performing a systematic survey of HiRISE images in Valles Marineris in order to identify dykes exposed along the walls and in the troughs, noting their coordinates, orientation, maximum thickness, elevation above the MOLA sphere, and dip angle when not vertical. Segmented dykes are counted as single observations. Seventy-five HiRISE images of Valles Marineris walls have been surveyed to date. They have revealed the presence of more than 300 dykes, of thickness between ca. Fig. 2. Distribution of HiRISE images in Valles Marineris on April 1 m (lower detection limit) and 60 m (Figure 3). A few 12, 2016, of which 75 that cover the wall slopes have been examined from dyke identification. * Corresponding author. E-mail: [email protected] 177 ACTA GEOLOGICA SINICA (English Edition) Vol. 90 Supp. 1 July 2016 bodies up to 200 m thick may be either dykes or other types from the Syria Planum area to the Coprates and Ophir of intrusions, with uncertainty resulting from mantling by chasmata through the western part of Valles Marineris, as slope deposits. implied by Mège and Masson's (1996a) plume tectonics The dykes are mapped from HiRISE images only, which model, but too deeply to be exposed in the western area. implies that their distribution partly reflects HiRISE image Preliminary measurements indicate indeed that most dykes distribution. Several long portions of Valles Marineris on the Valles Marineris walls are located at a depth > 4000- walls and floors are not imaged (Figure 2). Nevertheless, it 5000 m below the surface of the surrounding plateau. is likely, from the current mapping, that dyke concentration Although many chasma floors are deeper than this, huge is higher in some areas, especially eastern Coprates debris slopes mantle the lower part of the walls and in many Chasma and Ophir Chasma (Figure 3), perhaps indicating parts of Valles Marineris bedrock is not exposed at the such that these two areas were preferred sites of local magmatic depths, which may be the explanation for the absence of activity. Alternatively, the dykes may have propagated exposed dykes in these areas. Wilson and Head (1994) Fig. 3. Dyke distribution in Valles Marineris from the 75 HiRISE images examined to date. The numbers are measured dyke maximum thicknesses. The Candor Chasma Dyke Swarm (in the green box) is apparent on CTX images (5 m/pixel) but is currently not imaged at HiRISE resolution. The Ophir Chasma Dyke Swarm (in the yellow box) is made of ~50 thick dykes discussed in another paper (Mège and Gurgurewicz, 2016). The white box indicates the area of Coprates Chasma where dyke direction analysis was performed (Figure 4b). Fig. 4. Dyke orientation has been measured and binned in sectors 22.5° wide. Left: Orientation of all the dykes mapped in Valles Marineris to date. The mean orientation is subparallel (N100, arrow) to the mean Valles Marineris trough orientation (N105). Right: Dyke orientation in eastern Coprates Chasma. The mean orientation (N081, arrow) is parallel to the local orientation of Valles Marineris (N090). Note that in both diagrams, dykes of different orientation (including perpendicular to the main trend) are many. 178 ACTA GEOLOGICA SINICA (English Edition) Vol. 90 Supp. 1 July 2016 calculated that the depth of the level of neutral buoyancy also been significant, making the tectonic evolution of (LNB) of basaltic magma on Mars was estimated at ca. 11 Valles Marineris surprisingly more complex than expected. km, which may be considered as a proxy for mean propagation depth. Due to uncertainty on Valles Marineris References bedrock composition as well as dyke composition (most Banerdt WB, and Golombek MP, 2000. Tectonics of the Tharsis identified dykes are too thin to have their composition region of Mars: insight from MGS topography and gravity. Lunar and Planetary Science Conference XXXI, Houston, inferred from near-infrared spectra from orbit using the Texas, Abstract 2038. MRO/CRISM spectral imaging instrument), the LNB depth Ernst RE, Grosfils EB, and Mège D, 2001. Giant Dyke Swarms inferred by Wilson and Head (1994) only indicates the on Earth, Venus and Mars. Annual Review of Earth and great depth of propagation of the Valles Marineris dykes, Planetary Sciences, 29: 489-534. consistent with the observations. Flahaut J, Mustard JF, Quantin C, Clenet H, Allemand P, and The dyke-related landforms identified by Mège and Thomas P, 2011. Dikes of distinct composition intruded into Noachian-aged crust exposed in the walls of Valles Marineris. Masson (1996a) are generally parallel to Valles Marineris. Geophysical Research Letters, 48: L15202, doi: 10.1029/ So are the dykes identified in the survey reported here 2011GL048109. (Figure 4), which strengthens the plume tectonics model. Huang J, Edwards CS, Horgan BHN, Christensen PR, Kraft MD, Nevertheless, some are not parallel to Valles Marineris, and and Xiao L, 2012. Identification and mapping of dikes with sometimes are additionally parallel to other tectonic relatively primitive compositions in Thaumasia Planum on Mars: Implications for Tharsis volcanism and the opening of structures, such as narrow grabens on the plateau, or the Valles Marineris. Geophysical Research Letters, 39: L17201, Louros Valles sapping channels. In northern Candor doi:10.1029/2012GL052523. Chasma, unfortunately not yet imaged at HiRISE Mège D, 2001, Uniformitarian plume tectonics: the post-Archean resolution, the dykes are perpendicular to the Valles Earth and Mars. In Ernst RE and Buchan KL (Eds.), Mantle Marineris-parallel dykes, and form a true local swarm that plumes: Their identification through time. Geological Society indicates that at one moment in the evolution of Valles of America Special Paper 352, 141-164. Mège D, Cook AC, Lagabrielle Y, Garel E, and Cormier MH, Marineris, dyke dilation and tectonic stretching occurred 2003. Volcanic rifting at Martian grabens. Journal of perpendicular to the tectonic stretching that produced the Geophysical Research-Planets, 108: E5, doi: 10. 1029/ Valles Marineris grabens. Determining the chronology of 2002JE001852. dyke swarms would help reconstruct the evolution of the Mège D, and Gurgurewicz J, 2016. The Ophir Chasma Dyke crustal stress field with time; unfortunately, only two sites Swarm: description and implications for the genesis of the Valles Marineris northern troughs. Int. Dyke Conference 7, where dyke cross-cutting relationships can be determined Abstract. have been found to date, making this chronology out of Mège D, and Masson P, 1996a. A plume tectonics model for the reach. Tharsis province, Mars. Planetary and Space Science, 44: Dyke mapping in Valles Marineris reveals an evolving 1499-1546. stress field that models aiming at solving either the inverse Mège D, and Masson P, 1996b. Amounts of stretching in Valles problem (Mège and Masson, 1996a; Mège, 2001) or the Marineris. Planetary and Space Science, 44: 749-782. Tenzer R, Eshagh M, and Jin S, 2015. Martian sub-crustal stress forward problem (e.g., Banerdt and Golombek, 2000; from gravity and topography models. Earth and Planetary Tenzer et al., 2015) had not anticipated. The regional NNE- Science Letters, 425: 84-92. SSW extension of Valles Marineris predicted by these Wilson, L, and Head J.W. III, 1994. Mars: review and analysis of models is supported by the orientation of many dykes volcanic eruption theory and relationships to observed throughout the region; nevertheless, dyke mapping reveals landforms. Reviews of Geophysics, 32: 221-263. that in the details, extension following other directions has 179 .