Lunar and Planetary Science XXIX 1920.pdf

GEOLOGIC MAPPING OF THE METIS REGIO (V6) QUADRANGLE OF AT 1:5,000,000 SCALE. J.M. Dohm and K.L. Tanaka, U.S. Geological Survey, Flagstaff, Az, 86001, [email protected]

Introduction. We are currently mapping the area, and 22 impact craters dot the map area. The broad geologic materials and structure of the Metis Regio stratigraphy of the Metis Regio is defined by (V6) quadrangle of Venus at 1:5,000,000 scale to widespread ridged, fractured, and lobate plains units, determine the local stratigraphy and to construct terrain, and ridge belts. These plains units and provisional models for the regional geologic history. structures may have relative-age significance on V6 is ideally suited for the geologic investigation, regional to global scales [9-10]. which includes the relative age determination, of a Preliminary mapping results. Our number of coronae of various types as well as large geologic mapping of the Metis Regio (V6) quadrangle volcanoes and arachnoids because of their volcanic and is partly complete. Thus far, we have identified nearly tectonic associations with regional stratigraphic two dozen geologic units. The map units are markers. Geologic units are distinguished by their categorized into dome and miscellaneous flow material stratigraphic relations, (e.g., smooth (3 types), large volcanic construct material (2 ), vs. lobate), backscatter properties, and relations to coronae material (7), younger and older plains material structural features (including major lineaments and (7), and impact crater material (2). Our preliminary regional wrinkle-ridge sets). In particular, we are mapping indicates that the coronae materials formed assessing (1) whether coronae formed during single or concurrently with the older plains materials but are multiple interval(s) relative to the emplacement of buried in most places by younger plains deposits. This major plains units, (2) how the formation of coronae relation appears consistent with findings for older and other local tectonism interacted with regional coronae elsewhere on Venus [6]. structure patterns, and (3) whether or not coronae Future work. In addition to completing decrease in size and structural complexity as they near the geologic mapping and reconstruction of the rock- . stratigraphic history of V6, we also will resolve the General physiography and geology. structural and tectonic histories. This work will Among the most studied venusian features in terms of include digitizing and scanning the map units and detailed formational sequence and relative ages are the structure for conversion into a GIS database. That coronae, arachnoids, and large volcanoes, which are database will form the basis for quantitative analysis of well represented in the Metis Regio quadrangle (Fig. the interplay between tectonic features and structural 1); they have tectonic, volcanic, and topographic orientations. signatures that indicate complex evolutionary histories [1-6]. Although several of the coronae in the map area REFERENCES have been described in some detail using data [1] Stofan, E.R., and Head, J.W., 1990, Icarus, 83, [1], the greatly improved resolution of the 216-243. [2] Head, J.W. et al., 1992, J. Geophys. SAR and altimetry data enables more accurate and Res., 97, 13,153Ð13,198. [3] Janes, D.M. et al., 1992, detailed interpretations (for example, some large J. Geophys. Res., 97, 16,055-16,068. [4] Sandwell, volcanic shields have since been discriminated and D.T., and Schubert, G., 1992, , J. Geophys. Res., 97, 16,069-16,084. [5] Stofan, E.R., 1995, LPSC Abs. several broad rises can be reinterpreted as structural XXVI, 1361-1362. [6] Chapman, M.G., and features rather than volcanic edifices). Zimbelman, J.R., 1998, this volume. [7] USGS, The Metis Regio quadrangle mostly consists 1989, USGS Misc. Inv. Ser. Map I-2041, scale of terrain within about 1 km above the planetary mean 1:15,000,000. [8] Ford, P.G., and Pettengill, G.H., radius [7-8], except for shallow plains over the 1992, J. Geophys. Res., 97, 13,103-13,114. [9] southeastern part of the map area (northwesternmost Arvidson, R.E. et al., 1992, J. Geophys. Res., 97, ) and broad volcanoes that exceed 2 13,303Ð13,318. [10] Basilevesky, A.T., and Head, km in elevation. Locally, coronae, minor ridge and J.W., 1995, Earth, Moon, and Planets, 66, 285-336. fracture belts, and small volcanoes produce topographic highs and lows hundreds of meters in relief (see topographic profiles in [1]). Four large volcanoes, each a few hundred kilometers across, rise hundreds of meters to more than 1 km above the surrounding plains. Low patches of tessera terrain rise above plains in the northeast corner and western margin of the map Lunar and Planetary Science XXIX 1920.pdf

240 o 300o o o 75 o 75 255o 285 270o Atropos Bachue CO SF Tessera Corona SF Ben C Metis Dorsa Regio Feronia C Corona LV CO AR? C o IV/SSD CO C Otua 65 Corona 65o SFC Okipeta Ale CO C Tholus SF Mnemosyne Dorsa Minera Regio LV C C Fossae AR Rananeida NertusTholus Corona CO AR IV/SSD Virilis CO C Coatlicue AR SF Tessera Corona SF C SF Mokosha o Mons C C 55 C 55o CD? C? C Demeter LV Corona C IV CO SF IV/FD C o Bau o 50 Hemera Corona 50 C Dorsa o Atira Iyele o 240 Mons C 300 CO C CO Dorsa LV o 255 o 285 270o Figure 1. Feature map of the Metis Regio Quadrangle (V6).

LEGEND Impact crater Circular depression Small shield field Corona Intermediate volcano (unspecified) Fluted dome (scalloped dome; "tick") Steep-sided dome ("pancake") Stellate fracture center ("nova) Large volcano