Lunar and Planetary Science XXXVII (2006) 2233.pdf

GEOLOGIC MAPPING OF ISABELLA QUADRANGLE (V50), VENUS. Leslie F. Bleamaster III, Planetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, AZ 85719, [email protected].

Introduction. The Isabella Quadrangle (Figure are spatially and temporally independent processes, 1a.) lies south of the eastern extent of the Diana-Dali which preserve local relationships only. Across Chasmata system and Atla Regio, which contains the Venus, highly variable temporal relations between volcanoes Sapas, Maat and Ozza Mons. Isabella shields (sh) and wrinkle ridges (wr) have been Quadrangle is host to numerous coronae and small identified including: sh before wr, wr before sh, volcanic centers (paterae and shield fields) (Figure synchronous formation, and ambiguous relations [4 1b.), focused (Aditi and Sirona Dorsum) and and 5]. These findings argue against a secular distributed (penetrative north-south trending wrinkle transition from shield formation to wrinkle ridge ridges) contractional deformation, and radial and formation and suggest that local processes dominate, linear extensional structures (Figure 1c.); all which leading to locally relevant relationships. More contribute materials to and/or deform the expansive detailed evaluation of these features within V50 will surrounding plains (Nsomeka and Wawalag constrain local relative temporal relations and Planitiae). Specific questions posed for geologic provide additional tests of wrinkle ridge-shield mapping of the V50 quadrangle include: a) what and history. where are the source locations for the regionally Isabella Crater: The most prominent feature, dominant plains materials (e.g., coronae, paterae, and namesake of the V50 quadrangle, is Isabella shield fields, rifts and fissures)? b) what is the Crater! a 176 km diameter crater located at relationship, if any, between focused (deformation 29.9°S/204.2°E (Figure 2); Isabella is the second belts) and distributed (wrinkle ridges) contractional largest recognized impact structure on Venus, having strain and local/regional volcanism? c) what role does at least one, perhaps two, interior ring structures; structural reactivation play in deformation belt and however, Isabella does not preserve a sizable or wrinkle ridge development? d) what are the spatial continuous ejecta blanket. The floor of Isabella is and temporal relationships between craters with radar dark (smooth), which is typically ascribed to outflows and volcanic constructs, and to what extent flooding by lava. In addition to potential interior lava are crater outflows directly related to impact deposits, Isabella displays a considerably large processes versus post-impact modification? outflow deposit to the southeast. Miyamoto and Observations. Preliminary mapping of V50 has Sasaki [6] modeled the outflow and deduced that begun by looking primarily at prominent crater and models with high flow rates, perhaps related to volcanic features and penetrative structural fabrics catastrophic flow induced by impact, best replicated (Figure 1b and 1c). the flow morphology. Such extensive crater outflows, Shields and wrinkle ridges: Initial mapping not seen on the other terrestrial planets, may be the illustrates relationships between the north-south set result of Venusian surface conditions (elevated of wrinkle ridges (1c, purple) and densely populated temperature and pressure), which potentially shield clusters (1b, black). Areas with greater wrinkle facilitate melting and redistribution of the target rock ridge development appear to contain fewer shields. [7 and 8]; large outflows may also be the result of This could be explained in one of several ways: 1) impact spatially coincident with a volcanic feature the shield fields are younger than the wrinkle ridges and active magma reservoir. However, some outflow and materials from the shields are masking the deposits may be a complex combination of impact existing structures; 2) the wrinkle ridges are younger and post-impact modification processes as than the shield clusters and a) the shield fields are not hypothesized for Jhirad Crater (16.8°S/105.6°E) [9], deformed because of local heterogeneities imparted modeled by Miyamoto and Sasaki [6] for Willard and to the crust by shield formation, b) shields are Xantippe craters, and mapped by Rumpf et al. [10] completely destroyed within the interior of the for several large Venusian craters. Further, detailed penetrative wrinkle ridge fabric leaving only those mapping of Isabella Crater will address these shields outboard of the deformation zone, and/or c) hypotheses. shield fields are embayed, and completely buried, by Reactivation: Outboard and to the west of Aditi wrinkle ridge plains (this is most akin with the global Dorsa and the penetrative set of N-S wrinkle ridges stratigraphic model that states that wrinkle ridged (roughly the entire height of the quad between 185° plains material (pwr) are younger than shield fields and 195°E), a second set of wrinkle ridges, oriented [1, 2, 3]; 3) wrinkle ridges and shields are east-northeast, locally deforms the plains. As contemporaneous; or 4) wrinkle ridges and shields illustrated in figure 1c from reconnaissance mapping, Lunar and Planetary Science XXXVII (2006) 2233.pdf

this set of wrinkle ridges mimics the structural 1a. orientation of fractures (red) preserved adjacent and inboard of Aditi Dorsa. Preliminary mapping has not found any continuous lineaments that change from wrinkle ridges to fractures, but the orientations suggest that structural reactivation and topographic inversion of existing fractures may have played a role in the development of the second set of wrinkle ridges [11]. Future: The immediate work plan includes the delineation of map units, writing description of map units (DOMU), and the construction of a sequence of map units (SOMU). The V50 quadrangle will also be incorporated as part of a 1:10M synthesis map of Helen Planitia. References. [1] Basilevsky, A.T., and Head, J.W. (1995) Planet. Space Sci., 43, 1523-1553. [2] Basilevsky, A.T., and Head, J.W. (1998) J. Geophys. Res., 103, 8531- 1b. 8544. [3] Head, J.W., and Basilevsky, A.T. (1998) Geology, 26, 35-38. [4] Addington, E.A. (2001) Icarus, 149, 16-36. [5] Ivanov, M.A., and Head, J.W. (2003) Vernadsky-Brown Microsymposium 38, MSO37. [6] Miyamoto, H., and Sasaki, S. (2000), Icarus, 145, 533-545. [7] Asimov, P.D., and Wood, J.A. (1992), J. Geophys. Res., 97, 13,643-13,665. [8] Chadwick, D.J., and Schaber, G.G. (1993), J. Geophys. Res., 98, 20,891-20,902. [9] Bleamaster, L.F., and Hansen, V.L. (2005), USGS Geol. Inv. Sers. I-2808. [10] Rumpf, M.E., Herrick, R., and Gregg T.K. (2005) EOS Trans, AGU 86(52), Fall abstract P33A-0233. [11] DeShon, H.R., Young, D.A., and Hansen, V.L. (2000) J. Geophys. Res., 105, 6983-6995. " Figure 1a. Synthetic aperture radar (SAR) mosaic of Isabella Quadrangle (V50); 25-50°S, 180-210°E. Preliminary mapping shows spatial shields distribution of large- and small-scale volcanic coronae/paterae centers (1b) and deformation (1c). quasi-circular feature , # Figure 2. Isabella Crater (I), associated outflows not impact crater (O), and nearby coronae: Nott (N) and Epona (E). SAR Left looking image, 31°S, 205°E (center), 1- 1c. degree graticule.

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contractional features extensional features