Geologic Map of the Lakshmi Planum Quadrangle (V–7), Venus

Prepared for the National Aeronautics and Space Administration Geologic Map of the Lakshmi Planum Quadrangle (V–7), Venus By Mikhail A. Ivanov and James W. Head, III Pamphlet to accompany Scientific Investigations Map 3116 75° 75° V–1 V–6 V–3 50° 50° V–7 V–2 V–17 V–10 V–18 V–9 V–19 V–8 25° 25° V–29 V–22 V–30 V–21 V–31 V–20 270° 300° 330° 0° 30° 60° 90° 0° 0° V–43 V–32 V–42 V–33 V–41 V–34 –25° –25° V–55 V–44 V–54 V–45 V–53 V–46 V–61 V–56 –50° –50° V–60 V–57 2010 V–62 –75° –75° U.S. Department of the Interior QUADRANGLE LOCATION U.S. Geological Survey Photomosaic showing location of map area. An outline of 1:5,000,000-scale quadrangles is provided for reference. Contents The Magellan Mission ..................................................................................................................................1 Magellan Radar Data ..........................................................................................................................1 Introduction ....................................................................................................................................................1 Magellan Sar and Related Data ........................................................................................................2 General Geology ............................................................................................................................................2 Stratigraphy ...................................................................................................................................................4 Structures .......................................................................................................................................................6 Stratigraphic Relations of Units .................................................................................................................7 Geologic History ............................................................................................................................................9 Acknowledgements ....................................................................................................................................14 References Cited .........................................................................................................................................14 Tables Table 1. List of volcanic and volcano-tectonic centers in the Lakshmi Planum quadrangle (V–7), Venus .............................................................................................................................................17 Table 2. List of impact craters within the Lakshmi Planum quadrangle (V–7), Venus ....................18 Table 3. Area of units mapped in the Laksmi Planum quadrangle (V–7), Venus .............................18 i The Magellan Mission cal elevation, above which high-dielectric minerals or coatings are thought to be present. This leads to very bright SAR echoes from virtually all areas above that critical elevation. The Magellan spacecraft orbited Venus from August 10, The measurements of passive thermal emission from 1990, until it plunged into the Venusian atmosphere on October Venus, though of much lower spatial resolution than the SAR 12, 1994. Magellan Mission objectives included: (1) improving data, are more sensitive to changes in the dielectric constant of knowledge of the geological processes, surface properties, and the surface than to roughness. As such, they can be used to aug- geologic history of Venus by analysis of surface radar charac- ment studies of the surface and to discriminate between rough- teristics, topography, and morphology, and (2) improving the ness and reflectivity effects. Observations of the near-nadir knowledge of the geophysics of Venus by analysis of Venusian backscatter power, collected using a separate smaller antenna gravity. on the spacecraft, were modeled using the Hagfors expression The Magellan spacecraft carried a 12.6-cm radar system to for echoes from gently undulating surfaces to yield estimates map the surface of Venus. The transmitter and receiver systems of planetary radius, Fresnel reflectivity, and root-mean-square were used to collect three data sets: (1) synthetic aperture radar (rms) slope. The topographic data produced by this technique (SAR) images of the surface, (2) passive microwave thermal have horizontal footprint sizes of about 10 km near periapsis, emission observations, and (3) measurements of the backscat- and a vertical resolution on the order of 100 m. The Fresnel tered power at small angles of incidence, which were processed reflectivity data provide a comparison to the emissivity maps, to yield altimetric data. Radar imaging, altimetric, and radio- and the rms slope parameter is an indicator of the surface tilts metric mapping of the Venusian surface was done in mission which contribute to the quasi-specular scattering component. cycles 1, 2, and 3 from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with radar resolution on the order of 120 meters. The SAR observations were projected to a 75-m nominal horizontal resolution, and these Introduction full-resolution data compose the image base used in geologic mapping. The primary polarization mode was horizontal-trans- The Lakshmi Planum quadrangle (V–7, fig. 1) is in the mit, horizontal-receive (HH), but additional data for selected northern hemisphere of Venus and extends from lat 50° to areas were collected for the vertical polarization sense. Inci- 75° N., and from long 300° to 360° E. The elevated volcanic dence angles varied between about 20° and 45°. plateau of Lakshmi Planum, which represents a very specific High resolution Doppler tracking of the spacecraft took and unique class of highlands on Venus, dominates the northern place from September 1992 through October 1994 (mission half of the quadrangle (fig. 1). The surface of the planum stands cycles 4,5,6). Approximately 950 orbits of high-resolution grav- 3–4 km above mean planetary radius (fig. A2 ) and the plateau is ity observations were obtained between September 1992 and surrounded by the highest Venusian mountain ranges, 7–10 km May 1993 while Magellan was in an elliptical orbit with a peri- high (figs. 2A and 3). apsis near 175 km and an apoapsis near 8,000 km. An additional Before the Magellan mission, the geology of the Lakshmi 1,500 orbits were obtained following orbit-circularization in Planum quadrangle was known on the basis of topographic mid-1993. These data exist as a 75°by 75°harmonic field. data acquired by the Pioneer-Venus and Venera-15/16 altim- eter (Masursky and others, 1980; Pettengill and others, 1980; Magellan Radar Data Barsukov and others, 1986) and radar images received by the Arecibo telescope (Campbell and others, 1991) and Venera- Radar backscatter power is determined by (1) the morphol- 15/16 spacecraft (Basilevsky and others, 1986; Barsukov and ogy of the surface at a broad range of scales and (2) the intrinsic others, 1986; Sukanov and others, 1989). These data showed reflectivity, or dielectric constant, of the material. Topography at unique topographic and morphologic structures of the moun- scales of several meters and larger can produce quasi-specular tain belts, which have no counterparts elsewhere on Venus, and echoes and the strength of the return is greatest when the local the interior volcanic plateau with two large and low volcanic surface is perpendicular to the incident beam. This type of centers (Colette and Sacajawea Paterae) and large blocks of scattering is most important at very small angles of incidence tessera-like terrain (Barsukov and others, 1986; Sukanov and because natural surfaces generally have few large tilted facets others, 1986; Pronin, 1986). From the outside, Lakshmi Planum at high angles. The exception is in areas of steep slopes, such as is outlined by a zone of complexly deformed terrains that occur ridges or rift zones, where favorably tilted terrain can produce on the regional outer slope of Lakshmi. Vast low-lying plains very bright signatures in the radar image. For most other areas, surround this zone. After acquisition of the Venera-15/16 data, diffuse echoes from roughness at scales comparable to the radar two classes of hypotheses were formulated to explain the unique wavelength are responsible for variations in the SAR return. In structure of Lakshmi Planum and its surrounding. The first either case, the echo strength is also modulated by the reflectiv- proposed that the western portion of Ishtar Terra, dominated ity of the surface material. The density of the upper few wave- by Lakshmi Planum, was a site of large-scale upwelling (for lengths of the surface can have a significant effect. Low-density example, Pronin, 1986, 1992; Grimm and Phillips, 1990) while layers, such as crater ejecta or volcanic ash, can absorb the the alternative hypothesis considered this region as a site of incident energy and produce a lower observed echo. On Venus, large-scale downwelling and underthrusting (for example, Head, there also exists a rapid increase in reflectivity at a certain criti- 1986; Bindschadler and others, 1990). 1 Early Magellan results (Kaula and others, 1992) showed our unit definition and characterization is described in Tanaka important details of the general geology of this area displayed (1994), Basilevsky and Head (1995a,b), Basilevsky and others in the Venera-15/16 images. Swarms of extensional structures (1997), and Ivanov and Head (1998). and massifs

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