B25606 page 1 of 15 Venus’s shield terrain Vicki L. Hansen† Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA ABSTRACT strong throughout shield-terrain formation. resurfacing. The fl ood-lava hypothesis has been Shield terrain may extend across much of accepted because of a lack of obvious volcanic Plains, planitiae, or lowlands—expanses of Venus’s surface. fl ows or edifi ces, or because alternative viable gentle, long-wavelength (~1000 km) basins— models exist. Thus, it is important to determine cover ~80% of Venus’s surface. These regions Keywords: Venus, shields, resurfacing, fl ood the range of sources and the types of processes are widely accepted as covered by volcanic lava, mud volcano, partial melt. that contributed to lowland resurfacing. fl ows, although the mechanism(s) responsible Locally, coronae, circular to quasi-circular for resurfacing remains elusive; in addition, a INTRODUCTION tectonomagmatic features that range in size from volcanic origin for the lowland surface may ~60 to 1000 km in diameter (200-km mean; Sto- be open to question. Lowland resurfacing is Understanding processes that affect Venus’s fan et al., 1992), expelled lava fl ows that extend typically attributed to catastrophic emplace- lowlands is critical to Venus evolution models. over 500 km and locally contribute lava to adja- ment (10–100 m.y.) of globally extensive, Lowlands—expanses of gentle, long-wave- cent lowlands (e.g., Chapman, 1999; Rosenberg thick (1–3 km) fl ood-type lava. This model of length topography at or below 1.5 km mean and McGill, 2001; Campbell and Rogers, 2002; resurfacing has been postulated on the basis planetary radius—cover ~80% of Venus (Masur- Hansen and DeShon, 2002; Young and Hansen, of impact crater distribution, taken together sky et al., 1980). Although lowland materials 2003). However, many lowlands, including those with a lack of obvious volcanic fl ows or edi- are widely considered volcanic (Banerdt et al., in the Niobe-Greenaway (0–25°N/90–150°E) fi ces, and a lack of viable alternative models. 1997), the mechanism(s) responsible for low- area, lack adjacent coronae or coronae-source Ongoing geologic mapping of ~15,000,000 km2 land volcanism remain mostly elusive. Lowland lava fl ows. This contribution examines the exten- (0–25N/90–150E) using 225 m/pixel and 75 m/ resurfacing is typically attributed to emplace- sive tract of Venus’s lowland in the Niobe-Green- pixel NASA Magellan SAR (synthetic aperture ment of extensive fl ood lava (e.g., Banerdt et away area (Figs. 1 and 2) in order to expressly radar) data indicates that small edifi ces, called al., 1997). This supposition forms the basis for evaluate lowland surface processes. shields (1–15-km-diameter edifi ces, <<1 km two related hypotheses: catastrophic resurfac- The results reveal no evidence for widespread high), play a major role in lowland resurfac- ing (Strom et al., 1994) and global stratigraphy fl ood-lava fl ows; rather, tens to hundreds of ing. Individual shields are radar-smooth or - (Basilevsky and Head, 1996, 1998, 2002; Head thousands of small eruptive centers dominate the rough, quasi-circular to circular features with and Basilevsky, 1998). The related hypotheses lowland across over 10,000,000 km2. Individual or without a central pit. Shield shapes, which imply that Venus’s lowland was covered very eruptive centers (shields) and associated deposits have been previously documented, range from quickly (<10–100 m.y.) by thick (1–3 km) that coalesce into an extensive, thin, mechani- shield, dome, or cone, to fl at-topped or fl at. fl ood-type lava ca. 750 +350/–400 Ma (global cally strong layer (shield paint) together form Shield deposits typically coalesce, forming average model surface age [McKinnon et al., shield terrain. Shield terrain formed in a time- a thin, regionally extensive but discontinu- 1997]). Such thick layers of lava are postulated transgressive manner and is cut by distributed ous, mechanically strong layer, herein called in order to account for complete burial of preex- low-strain deformation marked by extension shield paint. Shield paint conforms to delicate isting impact craters that are presumed to have fractures, wrinkle ridges, and inversion struc- local topography, providing evidence of its formed on tessera terrain and fracture terrain tures. Shield-terrain evolution seems inconsistent thin character and indicating generally low that, within the context of the hypothesis, form with the catastrophic fl ood-lava resurfacing viscosity during emplacement. Shield ter- globally extensive basement units. Impact crater hypothesis. Possible models for shield-terrain rain (shields and shield paint) covers more rim-to-trough heights range from 0.2 to 1.5 km formation include widespread sedimentation than 10,000,000 km2 within the study area. (Herrick and Sharpton, 2000), thus requiring a and mud volcano formation, and shallow in situ Detailed mapping of fi ve 2° × 2° regions using fl ood-lava thickness greater than 1.5 km to cover point-source partial-melt formation resulting in coregistered normal and inverted right- and early-formed craters. The global stratigraphy tertiary melt that subsequently rises to the surface left-illumination SAR imagery indicates shield hypothesis implies that “wrinkle-ridge plains” along preexisting fractures. Both of these hypoth- densities of 3500–33,500 shields/106 km2; thus, or “regional plains” represent the catastrophi- eses present quite different pictures of ancient the map area hosts more than 35,000–335,000 cally emplaced, globally extensive fl ood lava Venus compared with contemporary Venus. shields. Shield terrain generally postdates, that buried all preexisting impact craters across but is also locally deformed by, fractures and Venus’s lowland. But currently no robust tem- BACKGROUND wrinkle ridges, indicating time-transgressive poral constraints are available with regard to the formation relative to local deformation and/ emplacement rate of Venus surfaces, whether Venus’s Geology or reactivation. The regional scale crust was punctuated, catastrophic, or continuous (Camp- bell, 1999). In addition, no studies explicitly Venus exhibits three global topographic prov- †E-mail: [email protected]. evaluate a fl ood-lava mechanism for lowland inces—lowlands, mesolands, and highlands— GSA Bulletin; May/June 2005; v. 117; no. 5/6; p. 000–000; doi: 10.1130/B256060.1; 9 fi gures; 1 table. For permission to copy, contact [email protected] © 2005 Geological Society of America 1 B25606 page 2 of 15 V.L. HANSEN and several types of geomorphic features (Phil- Fortuna Ishtar Terra lips and Hansen, 1994). The lowlands (~80% of 60° the surface at or below mean planetary radius, V3 V4 ~6052 km) are relatively smooth, with widely V11 V12 V13 Tellus distributed low-strain deformation and belts of 30° Bell V10 concentrated deformation (Banerdt et al., 1997). Beta W. Eistla V22 V23 V24 V25 Highlands (<10% of the surface, ~2–11 km E. Eistla above mean planetary radius) include: (1) vol- C. Eistla 0° Atla E. Ovda canic rises—huge (~1100–1800 km diameter) Phoebe W. Ovda Thetis domical rises dominated by volcanic features and Alpha analogous to terrestrial hot spot rises (Smrekar et 30° al., 1997); (2) crustal plateaus—huge (~1400– Dione Artemis Themis 2300 km diameter) quasi-circular plateaus V52 Imdr characterized by deformed crust and supported by thicker than average crust; and (3) Ishtar 60° Terra, a globally unique feature in the northern 300° 0° 60° 120° 180° hemisphere (Hansen et al., 1997, and references Figure 1. Distribution of Venusian features. Fine dots, volcanic rises; dark areas, large therein). About 500 coronae (~60–1000-km- ribbon terrain; light gray areas, crustal plateaus (modifi ed from Hansen et al., 1999). The diameter quasi-circular tectonomagmatic fea- map area (Fig. 2) includes V23 and V24; the V labels show the locations of other map quad- tures) and spatially associated chasmata (troughs) rangles discussed in the text. are concentrated in the mesoland, although coro- nae clusters associate with some volcanic rises, and isolated coronae occur in the lowlands (Sto- fan et al., 1992, 1997, 2001). About 970 essentially pristine impact craters 1997; Donahue, 1999; Hunten, 2002). In addi- Venus preserves a range of volcanic features, (2–270 km diameter) display a near spatially tion, a comparison of carbon, nitrogen, and water many dwarfi ng similar terrestrial features. Large random distribution (Phillips et al., 1992; Scha- abundances at the surface of Earth and Venus volcanic edifi ces range up to hundreds of kilo- ber at al., 1992; Herrick et al., 1997; Hauck et indicates that both planets likely had similar meters in diameter, and lava fl ows can extend al., 1998), refl ecting a global average model primitive atmospheres that reacted differently for hundreds of kilometers (Head et al., 1992; surface age of ca. 750 +350/–400 Ma (McKin- with their surfaces. For Venus, the rate of water Crumpler et al., 1997). Venus also preserves non et al., 1997). Interpretation of global average loss would affect deuterium/hydrogen ratios very small features (<1–15 km diameter) called model surface age is nonunique. Many plausible (Donahue and Russell, 1997). In addition, a shields, interpreted as volcanic in origin (Aubele surface histories are possible, including: (1) cata- combination of crustal hydration and hydrogen- and Slyuta, 1990; Guest et al., 1992; Head et al., strophic resurfacing at t = average model surface escape processes could explain the present-day 1992; Crumpler et al., 1997). Shields are divided age; (2) 50% of surface formed at 0.5t and 50% low amount and high deuterium/hydrogen ratio into two morphological terrains on the basis of at 1.5t; or (3) 20% of surface formed at 2t and of water in the Venusian atmosphere (Lecuyer regional extent: shield clusters and shield plains 80% at 0.75t). Average model surface age also et al., 2000).