47th Lunar and Planetary Science Conference (2016) 1858.pdf

DIKE-RELATED FAULT MORPHOLOGY FROM STRUCTURAL MAPPING OF SIRENUM FOSSAE. Elizabeth A. Carey Boden ([email protected]) and Nancye H. Dawers, Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA, 70118, USA.

Introduction: Tharsis is an extensional volcanic Observations: The Sirenum Fossae graben system province and planetary analog to terrestrial magmatic extends about 3,360 km from [136.2 W, 22.3S] to rifts. An extensive system of narrow closely spaced [172.1 E, 39.4 S]. The majority of graben strike graben, Sirenum Fossae, radiates southwestward from ~080 and based on cross-cutting relationships appear Arsia Mons through a complex and ancient terrain of to postdate graben that strike ~065 (Fig. 1). Fissures the cratered uplands. Cross-sections of the graben re- are inferred where multiple channels originate from a veal distinct geometric properties [1,2] that indicate linear source (Fig. 2). These linear fissures are of the dike-related graben formation [3,4]. On Earth magmat- same orientation as the strike of the older graben. Sin- ic and erosional processes remove evidence of dike- uous valleys, similar to terrestrial fluvial features, orig- related graben formation, which is an important com- inate from these fissures and are interpreted as pale- ponent of rift initiation. Lower rates of erosion and a ochannels. The fissures may therefore have formed rigid Martian cryosphere provide a unique opportunity during the earlier diking episodes, and served as to examine similar features in a relatively pristine state. sources for outflow channels and groundwater sapping. We examine the structural, geomorphic and magmatic Sirenum Fossae normal faults primarily occur as 2 features in an approximately 840,000 km area to char- full-graben boundary faults, in contrast to normal acterize relationships between the graben and other faults in terrestrial rift zones that are typically half- surface features. Previous work [5] describes Sirenum graben. Simple graben are less common than complex Fossae extending to the 180 meridian and not to the or compound graben, indicating that compound graben end of the graben system; here we determine the total may have formed by fault-facilitated segment linkage length of Sirenum Fossae. Our objective is to identify due to repeated episodes of dike injection (Fig. 3). properties of the graben that are diagnostic of dike- Abandoned splays within graben arrays and complex induced graben formation, and place them into a re- graben resemble those observed in terrestrial fault sys- gional context. tems, which imply a long period of tectonic evolution. Data & Methods: Structural mapping was per- Thrust-fault-related landforms (TFRLfs) are orient- formed at 1:100,000 scale using over 320 images (~6 ed roughly perpendicular to the normal faults that bor- m/pixel) from the Mars Reconnaissance Orbiter der the Sirenum graben. Mutual cross-cutting relation- (MRO) Context Camera (CTX). CTX images were ships between normal and thrust faults suggest coeval located using PILOT [6], then projected and down- extensional and compressional tectonics. Elongate, loaded using POW [7]. Digitization was carried out asymmetrical and irregularly shaped depressions, in- manually using ESRI’s ArcGIS v.10.3. terpreted to be collapse features, are frequently

Figure 1: Rotated (342) CTX digital image mosaic showing the different strike directions (green ~065° and pink ~080° ) of normal faults in Sirenum Fossae. Black rectangle (top) shows location of the lower image. 47th Lunar and Planetary Science Conference (2016) 1858.pdf

Figure 2 (Right): Channels emerge from the subsurface through a linear source. Fissure is located at the right of the image and indicated by white arrows.

Figure 3 (Below): Normal fault morphologies. Mapped faults in Sirenum Fossae are primarily complex graben and graben ar- rays. This indicates that the fault system evolved over a long period of time, rather than having formed during a single diking episode.

oriented parallel to the graben or located on graben are consistent with expected stresses produced during floors. Characterized by smooth or scalloped rims with the formation of the Tharsis Montes. convex inward-sloping walls, these negative topo- References: [1] Rubin, A.M. and Pollard, D.D. graphic features lack a raised outer rim or displaced (1988) Geology, 16, 413-417. [2] Rubin, A.M. (1992) ejecta material characteristic of impact craters. Col- J. Geophys. Res., 97, 1839-1958. [3] Schultz, R.A. et lapse features coinciding with dike-related graben like- al. (2004) Geology, 32, 889-892. [4] Goudy, C.L. and ly formed by the collapse of shallow crustal rock, pos- Schultz, R.A. (2005) Geophys. Res. Lett., 32, L05201. sibly due to magma evacuation (either with or without [5] Anderson, R.C. et al. (2001) J. Geophys. Res., 106, explosive volcanism due to volatile release) or due to 20,563-20,585. [6] Bailen, M.S. et al., (2013) LPS outflow of ice melt-water. XLIV, 2248. [7] Hare, T.M. et al., (2013) LPS XLIV, Summary and Conclusions: Sirenum Fossae ex- 2068. [8] Mège, D. and Masson, P. (1996) Planet. tends from the base of Arsia Mons at [136.2 W, 22.3 Space Sci., 44, 1499-1546. [9] Wilson, L. and Head, S] to the the floor of an adjoining pair of impact craters J.W. (2002) J. Geophys. Res., 107, 5057. at [172.1 E, 39.4 S]. The graben system extends over Acknowledgements: This work was partially sup- 3,360 km in length. Mapping deformational and geo- ported by NSF EPSCoR funding administered by the morphic features reveals patterns and relationships that Louisiana Board of Regents (contract LEQSF- support the dike-related graben hypothesis. Repeated EPS(2014)-PFUND-373). Christian Klimczak provid- episodes of dike intrusion can explain the complex cross-cutting relationships exhibited by paleochannels ed helpful feedback and advice about the mapping and graben. There appears to be a relationship between methodology. the graben and source locations for paleochannels, suggesting melting of permafrost. Upwelling mantle plumes during the formation of Tharsis would have been accompanied by laterally propagating dikes [8,9]. Perpendicular and coeval TFRLFs and normal faults