Lunar and Planetary Science XXXVIII (2007) 1936.pdf

GEOLOGIC MAPPING OF THE HESTIA RUPES QUADRANGLE (V-22), NORTHERN OVDA REGIO, VENUS. M. S. Gilmore1 and R. S. Saunders2, 1Dept. of Earth and Environmental Sciences, Wesleyan University, 265 Church St., Middletown CT 06459, [email protected], 2NASA Headquarters, ste- [email protected].

Introduction: Approximately 8% of the surface of three ridge belts intersect at a triple junction marking a Venus comprises tessera terrain, defined as have two topographic low. or more sets of intersecting lineaments that contribute Regional plains, pr. Widespread plains that com- to high radar backscatter [1]. Most tessera occurrences prise the majority of the quadrangle. This unit is sub- lie are in elevated plateaus (~2-4 km above mean divided into two units. Regional plains unit 1, pr1, planetary radius), although tessera inliers can be found includes plains of low to moderate backscatter with at the elevation of the plains. Regional and global high concentration of lineaments and wrinkle ridges at mapping of tessera terrain show that tessera terrain is various orientations. May be smooth or mottled. Nu- embayed by plains materials [2,3,4, see 5 for a possible merous small domes, shields and cones are present exception), leading to the interpretation that tessera singly or as fields. Lineated plain occur throughout terrain is older than the plains and results from a for- the quadrangle and appear preserved in regions of high mation event of finite duration prior to plains em- topography within the plains. Regional plains unit 2, placement. Although several models exist for the for- pr2. Low radar backscatter, homogenous unit. Wrin- mation of tessera terrain [e.g., 6, 7] each include a kle ridges are common. Unit 2 embays unit 1 where transition from high surface strain (tessera formation) they are in contact. to lower strain prior to the production of plains lavas. Smooth plains, ps. Plains of moderate to low back- The history of this transition is recorded at plains- scatter, may contain small shields, domes or fractures. tessera boundaries. Largely confined to localized intratessera deposits. The Hestia Rupes quadrangle (V-22; 0°-25°N, 60°- Flow materials, fl. Radar dark or bright lobate, 90°E; Figure 1) includes the northern portion of Ovda sinuous, digitate flows, typically arranged radially Regio and the plains between Ovda and Tellus Regio. from a central edifice. At the northern boundary of Ovda, plains embay Corona materials, co. Moderate backscatter plains tessera structures and lie at the altitude of the plateau with fractures and small shields. Localized to corona boundary. A primary objective of mapping this quad- summits. rangle is to document this transition between tessera Crater materials,c. Impact crater and crater out- and plains formation. flow (cf) deposits. Eighteen named and 1 unnamed Material units: Mapping is performed on a Ma- crater occur in the quadrangle. Bright and dark pa- gellan SAR 1:5M basemap in Mercator projection pro- rabolas associated with the crater Adivar dominate the vided by the USGS; it includes Cycle 1 and some Cy- lower center of the map area. The crater Mead lies to cle 2 data. Eight major units have been defined in the the west of the map area, and its moderate backscatter quadrangle. Tectonic structures have been mapped deposits and aeolian reworking of these deposits cover independently of units, however, in some cases, tec- the western ¼ of the quadrangle. tonic deformation is an integral part of the unit and is Edifices: Four coronae (Ereshkigal, Kaltash, included in the definition. H’uraru, Habonde) and two large volcanoes (Uti Hiata, Tessera terrain, t. Radar bright material comprising Kunhild – which is a named corona, but is actually a intersecting folds, graben and lineaments. Variations volcano) are located in V22. H’uraru and Kaltash in the type, density and orientation of structures may coronae are clearly embayed by regional plains unit 2 occur over scales of 100s of km resulting in regions of and Kaltash fractures crosscut unit prb. Flows of Er- distinctive structural fabrics. Tessera terrain is limited eshkigal contribute to the regional plains materials. to the Ovda Regio highland, no inliers are observed in Flows emanating from the two volcanoes are younger the quadrangle. than regional plains unit 2, although fractures radial to Ridge belt plains material, prb. Plains of moderate Kunhild deform the regional plains. Kaltash and backscatter that have been deformed into moderate to Habonde coronae erupted within tessera terrain. bright anastomosing ridges typically 1-5 km across and Geologic history: The tessera terrain of Ovda Re- 10s to 100 kms in length. Unit prb occurs throughout gio is the oldest material unit in the map area. This the quadrangle. Ridges generally trend in an WNW to terrain and the plateau topography formed and appear NW orientation. In the western portion of the quad- to cease prior to the eruption of the oldest plains unit. rangle, ridges have multiple orientations; in one case The oldest plains embay tesserae and were subse- quently deformed into ridge belts throughout the map Lunar and Planetary Science XXXVIII (2007) 1936.pdf

unit. This compressional deformation elevated the deformed by pervasive wrinkle ridges that are subse- ridge belts to 1-2 km above the regional plains imme- quently embayed by flows of the two large volcanoes. diately surrounding them, however the actual elevation Subsequent to plains formation, volcanic activity per- of the ridge belts is higher in the eastern portion of the sisted at and was largely confined to these volcanoes. quadrangle than the western portion where the ridge Conclusion: The general history of the quadrangle belts enter the lowlands of Akhtamar Planitia. The is one of waning deformation of tessera and plains general WNW trend of ridge belts throughout the units, early corona formation, some of which contrib- quadrangle records the primary regional direction of uted to regional plains materials. Initial regional plains compressional stress that broadly mimics the shape of with abundant small edifices yield to plains that lack the northern boundary of Ovda Regio and the folds them. Late volcanic activity was localized at two large therein, but whether this imposes a common genesis is volcanoes. unknown. References: [1] Barsukov V. L. et al. (1986) JGR Three and perhaps four coronae erupted after the 91 D378; Sukhanov A. L. (1992) in Venus Geology, formation of the ridge belts and were active prior to Geochemistry and Geophysics, Univ. Arizona Press, p. and, in the case of Ereshkigal, during the emplacement 82. [2] Ivanov M. A, and Head J. W. (1996). JGR 101 of two regional plains units. The first regional plains 591. [3] Basilevsky A. T. and Head J. W. (1995) PSS unit records a history of ubiquitous fracturing and the 43, 1523; ibid. EMP 66 285. [4] Hansen V. L. and widespread eruption of small shield volcanoes. This Willis J. J. (1986) Icarus 123, 296. [5] Guest J. E. and deformation and shield volcanism ceased prior to the Stofan E. R. (1999) Icarus 139, 55. [6] Bindschadler eruption of the second regional plains unit, seemly D. L. et al., (1992) JGR 97, 13495. [7] Hansen et al. indicating a change to more effusive volcanism; the (1999) Geology 27, 1071. sources of this unit are not apparent. Both units are

Figure 1. Topographic/SAR composite of the Hestia Rupes quadrangle (V-22). Elevations scale from approximately – 1 to +5 km relative to MPR of 6051 km.