Eighth International Conference on Mars (2014) 1406.pdf

Identifying the Pre-Tharsis Structures Associated with the Terra Sirenum Region, Mars. Robert. C. Andersona, James M. Dohmb, S. Robbinsc, and Jeff Schroedera, aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, bUniversity of Arizona ([email protected]), Department of Hydrology and Water Resources, Tucson, Arizona, USA, cLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado USA, dColorado School of Mines, Department of Geophysics, Goden, Colorado USA

Introduction: The Terra Sirenum region of Mars, detailed spatial and temporal paleotectonic which is located on the southwestern flanks of information. Tharsis, records not only the development of the Tharsis magmatic complex, at least since the Middle Crater statistics have been completed for our Noachian [1-3] up to present-day, but just as stratigraphic map of the Terra Sirenum region using a importantly, contains some of the oldest stratigraphic new global impact crater database [6-7]. In addition, units found in the western hemisphere region. all impact craters with diameters ≥ 3 km were Identification and detailed examination of these manually examined to identify only those superposed ancient structures and units within this region on the most recent resurfaced terrains: those impact provides an excellent window into clarifying the craters that display pristine rims and ejecta blankets tectonic processes that influenced the early geologic and well-defined, bowl-shaped basins with little to no evolution of Mars. Here, we provide a summary from infill that have no visible evidence of volcanic, our mapping effort detailing the earliest geologic fluvial, and tectonic resurfacing. The superposed history of this region. impact craters were additionally verified through ConTeXt camera images where there was coverage [8]. The impact crater retention ages are shown in Figure 2 based on the modeling schemes of [9] and [10].

Fig. 1 Stratigraphic map of the Terra Sirenum region of Mars.

Methodology: We have recently completed a detailed Fig. 2 Crater counts for the units mapped in the 1:5,000,000-scale geologic map for the Terra Terra Sirenum Region. Sirenum region, which included mapping all stratigraphic units and identifying all tectonic (i.e. faults and ridges), erosional (valley forms, Results - Ancient Pre-Tharsis Terrains: Early depressions, and scarps), depositional (ridges and tectonism in the Terra Sirenum region is expressed flows), and impact structures (Figure 1). We by 1) large, north-trending prominent faults (defined followed the procedure for mapping surface units as macrostructures [4] due to their enormous defined by Tanaka et al., [2005]. Stratigraphic units geometric proportions including lengths reaching were differentiated on both stratigraphic thousands of kilometers), 2) structurally-controlled basins (widths vary from kilometers to hundreds of (crosscutting, overlap, and embayment) and contact kilometers) displaying water enrichment in the relations and morphologic characteristics. High substrate, and 3) stratigraphic units displaying resolution MOC, NA, CTX, HiRise, and HRSC data magnetic signatures. was utilized to produce the base map and provided Eighth International Conference on Mars (2014) 1406.pdf

crater counts and are the result of the formation of the Tharsis Rise; (4) Late Noachian-Early Amazonian resurfacing of cratered highlands material, tectonic structures, and basins based on the superposed crater counts, correlative with the significant stages of Tharsis development (Stages 1-4), and (5) the source region of Mangala Valles has no superposed impact craters, indicating an Amazonian resurfacing event.

Fig. 3 MOLA topography map showing ancient structures trending north-south and bounded by large normal faults representing the margins of large basins. M.V. equals Mangala Valles.

Figure 3 displays the ancient, large macrostructures mapped in this region. The white arrows illustrate the bounding normal faults surrounding the large basins. Mangala Valles (M.V.), which is located at the top of the figure, represents one of the large fluvial basins identified in this region. Fig. 4 The Mars Global Surveyor spacecraft obtained a globally-distributed vector magnetic field From Figure 1, the structures identified in Figure 3 measurements that were acquired approximately 400 represent the oldest unit identified and mapped in this km above the surface of Mars. region and belongs to the Highland 1 unit (dark grey). As previously mentioned, this unit is found The identifiecation and mapping of these ancient predominately in the central region of the map region terrains records an early dynamic phase in the and consists of north-south trending massifs. Figure evolution of Mars, and may represents a time when 4 is an overlaid of the MGS magnatic field map onto the Martian crust was mobile. Futther understanding the mapped stratigraphic units shown in Figure 1. of the Terra Sirenum region could help us address From Figure 3, the magnetic data shows predominate, whether Mars records any ancient phase of plate north-south trending underlying ancient basement tectonism, perhaps very different then that of the sturtures. The distribution of these sources is Earth. consistent within this region and because of they contained reversed signatures (e.g. red and blue REFERENCES. [1] Dohm, J.M., et al., 2001. J. region in Figure 3), therefore, they were formed Geophys. Res. 106, 32 943-32 958. [2] Anderson, R.C., et when reversing dynamo was still active before it al., 2001. J. Geophys. Res. 106, 20,563-20,585. [3] Dohm, halted early in Mars evolution (e.g. Noachian). J.M., et al., 2007. In Superplumes: beyond plate tectonics. Although Mangala Valles occupies one of the ancient D.A Yuen, S. Maruyama, S-I Karato, and B.F. Windley basins, it is much younger and originates from one of (eds.). Springer, London, 523-537. [4] Dohm, J.M.et al. the younger fault system associated with the (2002) Lunar Planet. Sci. XXXIII, 1639 (abstract). [5] Scott, formation of the Tharsis Rise. D.H., and Tanaka, K.L. (1986) USGS Misc. Inv. Ser. Map

I-1802-A (1:15,000,000), [6] Robbins, S.J., 2011. Ph.D. Summary: The following is observed through crater Thesis, CU, Boulder. Robbins, S., and Hynek, B., [7] counting in conjunction with the geologic mapping Robbins, S.J.; and Hynek, B.M., 2011. Submitted to JGR- for the Terra Sirenum region: (1) ancient cratered Planets. doi: 10.1029/2011JE003966. [8] Malin, M.C., et highlands basements (Figure 4), including al., 2007. J. Geophys. Res. 112, doi: macrostructures identified in Figure 3, are Early 10.1029/2006JE002808. [9] Hartmann, W.K., 2005. Icarus Noachian-Middle Noachian; (2) basin formation was 174, 294-320. [10] Neukum, G., et al., 2001. In established by the Middle Noachian with possible Chronology and Evolution of Mars (R. Kallenbach, J. Geiss, subsequent growth; (3) lavas on the western flank of and W. K. Hartmann, eds.), Kluwer Academic Publishers, Tharsis were emplaced during the Late Noachian to 55-8. Late Hesperian (Stages 2-4; see [1-3]) and even as late as Middle Amazonian based on the superposed