LPSC SSII 1169

GEOLOGY OF OVDA REGIO, APHRODITE TERRA, VENUS: PRELIh4INAR-Y RESULTS FROM MAGELLAN DATA, RS. Saundersl, J.W. Head m2,RJ. Phillips3, S.C. Solomon4, R. Herricl?, R. Grimm3, and E.R. Stofan1 Jet Propulsion Laboratory, Calif. Institute of Technology, Pasadena, CA 91109;%epartment of Geological Sciences, Brown University, Providence, RI 02912; 3Southern Methodist University, Dallas, TX 75275;'Massachusetts Institute of Technology, Cambridge, MA, 02139

Ovda Regio is located in Aphrodite Terra, a major highland region on Venus which straddles the equator between 75 and 100 degrees east longitude. Ovda is located in western Aphrodite Terra, and has now been mapped by the Magellan spacecraft. The morphology of highland regions on Venus varies from a high plateau surrounded by mountain belts in Ishtar Terra [1,2], highly deformed tessera terrain in Tellus and Alpha Regiones [3, 41 to volcanism and rifting in Beta Regio [S-71. Aphrodite Terra, previously imaged only by low resolution Pioneer Venus radar, has been the subject of much speculation, and is expected to reveal important information on the formation and evolution of highland terrains on Venus. Pioneer Venus topography indicates that Ovda is a 2000 x 3500 km highland region rising over 4 km above the surroundink plains. Its interior is relatively plateau-like, with steep outer margins. Pioneer Venus roughness and reflectivity data indicated that the surface may be composed of tessera-like terrain [8], while PVO gravity data gave a relatively shallow depth of compensation at Ovda [9]. Several theories have been proposed for Ovda Regio based on Pioneer Venus data. Aphrodite was initially interpreted to be composed of ancient crustal material, representing a continent-like region [lo]. This hypothesis would suggest that Aphrodite should be rather uniformly cratered and ancient. Head and Crumpler [ll] proposed that Ovda was the site of new crust formation and spreading analogous to a terrestrial spreading center, identifying a central trough and cross- strike discontinuities interpreted as transform faults. This theory implies that the age of the surface should increase away from the E-W axis of spreading. A number of workers [12-141 have proposed that Aphrodite formed by hotspots, with individual regions within Aphrodite representing hotspots of varying ages. Depending on the model for hot spot evolution, and how the variations in apparent depth of compensation are interpreted, one model would predict that Ovda has the oldest surface in Aphrodite [I21 and the age of the highlands increases from west to east while another interpretation would have the youngest surfaces in Ovda [I31 These hypotheses are testable with Magellan data. In fact the latter model called "blob tectonics" may best fit some of the new observations of Ovda in the Magellan images. Based on the modest free-air gravity signal and the plateau-like nature of the topography, the blob tectonics model (1) predicts that Ovda Regio is undergoing or has recently undergone massive resurfacing, creating a several Hometer thick crustal cap. Possible sources of tectonic deformation at this stage in the model are 1) those associated with emplacement of massive amounts of lava, such as slumping or ductile detachment and thermoelastic effects, 2) thrust faulting in the interior of the underlying dynamic support has begun to subside (membrane stress effect), and 3) isostatic spreading of the crustal block. These deformational mechanisms are, of course, overprinted on any preexsisting tectonic structures. Overall, the blob tectonics model predicts that Ovda should be geologically young, implying a low crater density. Preliminary Magellan data reveal that Ovda Regio is characterized by highly deformed

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LPSC SXII -. OVDA REG10 GEOLOGY, Saunders, R. S. et al. terrain. Linear ridges spaced approximately 10-20 km apart characterize the interior of Ovda. The interior is characterized by very high reflectivity. Some of the ridges are cut by troughs. While the interior is complexly deformed, it does not resemble other regions identified as tessera on Venus. Some of the ridges resemble terrestrial fold belts, and are interpreted to be compressional in origin. The outer slopes of Ovda are characterized by blocks of tessera-like complexly deformed regions surrounded by smooth deposits interpreted to be volcanic in origin. Thus, the region includes extensive lava plains that embay qedeformed uplands. The latest stage of volcanism, also extensive, appears to have formed on rather steep regional slopes and flowed down off the flanks of the present highlands. FinaUy, Ovda appears to have a low density of impact craters.

References [I] L Cnunpler et al., Geology, 14, 1301, 1986; [2] VL Barsukov et al. J. Geophys Res., 91, 378, 1986; 131 D.L Bindschadler and J.W. Head, in press, J. Geophys. Res., 1991; [4] D.B. Campbell et al., in press, Science, 1990; [5] G.E. McGill et al., Geophys Res. Lett.,S, 737, 1981; [6] D.B. Campbell et al., Science, 226, 167, 1984; [q ER.Stofan et al., GSA Bull.,lOl, 143, 1989; [8] D.L.Bindschadler et al., Geophys. Res. Lett., 17, 171 1989; [9] W. Sjogren et al., J. Geophys. Res., 85, 8295, 1980; [lo] H. Masursky et al., J. Geophys. Res., 85, 8232, 1980; [Ill J.W. Head and L Crurnpler, Science, 238, 1380, 1987; [12] R.S. Saunders et aI., Eos, Trans. AGU, 69, 1295, 1988; [13] R. Hemck and RJ. Phillips, Geophys. Res. Lett., 17, 2129, 1990; 114) E.R. Stofan and R.S. Saunders, Geophys. Res. Lett., 9, 1377, 1990

Acknowledgements: Thie work was performed, in part, at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System