Artemis, Venus: The largest tectonomagmatic feature in the solar system? Vicki L. Hansen1 and Anthony Olive2 1Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA 2Department of Geological and Environmental Sciences, Youngstown State University, Youngstown, Ohio 44555, USA ABSTRACT wrinkle ridges parallel the chasma, and fracture New geologic mapping reveals that Artemis, a unique 2400-km-diameter feature on Venus, suites include both concentric and radial orien- is much larger than previously recognized, including a wide outer trough (>5000 km diameter), tations relative to the chasma. a radial dike swarm (12,000 km diameter), and a concentric wrinkle ridge suite (13,000 km diameter). Artemis’s evolution included formation of its interior and chasma, accompanied NIOBE-APHRODITE MAPPING by lateral propagation of radial dikes. The escape of dike magma to the surface formed local We are constructing 1:10,000,000 scale geo- cover deposits that buried parts of the remaining radial fracture suite. Cover deposits are logic maps of the Niobe and Aphrodite regions cut, in turn, by wrinkle ridges that likely formed by coupling of convective mantle fl ow with (57°N–57°S; 60–180°E) that straddle Aphrodite the lithosphere. The outer trough formed late relative to radial fractures, cover deposits, and Terra as part of the National Aeronautics and wrinkle ridges. We suggest that Artemis represents the magmatic signature of a deep mantle Space Administration–U.S. Geological Survey plume acting on relatively thin lithosphere. As such, it appears to represent the largest tec- planetary mapping program. We mapped local tonomagmatic feature in the solar system. The newly recognized vast extent of Artemis holds fractures (defi ning radial or concentric patterns implications for the formation of giant radial dike swarms, wrinkle ridge formation, terres- associated with coronae, mons, or volcanoes), trial planet mantle-lithosphere coupling, and Venus’s surface and geodynamic evolution. regional fractures (elements not obviously part of a local suite, and that are outside of the frac- INTRODUCTION composite of several major features, formed ture zone), and wrinkle ridges (Figs. 1B, 1C). Artemis, a unique feature on Venus, is one of by different mechanisms at different times: (1) We refer here collectively to fractures, fi ssures, the largest circular structures on a rocky planet an extensive fracture zone, comprising a region and graben as “fractures.” These features form in our solar system. Artemis includes an inte- marked by extremely closely spaced fractures sharp lineaments in synthetic aperture radar rior topographic high surrounded by Artemis including the Diana-Dali chain (stippled in data; graben and fi ssures show resolvable Chasma (a 2100-km-diameter, 25–100-km- Fig. 1); (2) a series of highland features, includ- troughs, whereas fractures do not. Fractures wide, 1–2-km-deep circular trough) and an ing the Ovda and Thetis plateaus to the west and (including fi ssures and graben) represent sub- outer rise (2400 km diameter) that transitions Atla rise to the east, connected by the fracture surface dikes (Head et al., 1992; McKenzie et outward into the surrounding lowland. The zone; and (3) Artemis, south of Thetis. Crustal al., 1992b; Grosfi ls and Head, 1994a; Ernst et chasma describes a nearly complete circle, plateaus Ovda and Thetis, supported by thick al., 2001). Wrinkle ridges, marked by distinctive lacking defi nition to the north and northwest. crust or low-density upper mantle, represent sinuous ridges, represent modest (<2%–5%) The enigmatic nature of Artemis has perplexed ancient features formed on thin lithosphere; distributed surface-layer contraction (Watters, researchers since acquisition of Pioneer Venus volcanic rise Atla, thermally supported in the 1992; Mége and Reidel, 2001). data in the late 1970s. Hypotheses for Artemis’s mantle, represents a modern feature on a thick Here we discuss patterns defi ned by wrinkle formation include the following. (1) Artemis lithosphere (Simons et al., 1997; Phillips and ridge trajectories and a specifi c regional frac- Chasma records convergence and subduction Hansen, 1998). The thermally supported frac- ture suite that have particular implications for (McKenzie et al., 1992a; Brown and Grimm, ture zone system includes splays that crosscut Artemis. Wrinkle ridge trajectories, consistent 1995, 1996; Schubert and Sandwell, 1995). (2) earlier formed Ovda and Thetis (Head et al., with global-scale map efforts, reveal a uniform Artemis’s interior is analogous to a terrestrial 1992; Bleamaster and Hansen, 2005); fractures concentric pattern south of Aphrodite, and metamorphic core complex (Spencer, 2001). (3) both predate and postdate Artemis formation more complex patterns to the north (Fig. 1A; a Artemis formed due to a huge bolide impact on (Bannister and Hansen, 2010). A portion of the detailed lineament map is available in the GSA cold strong lithosphere before 3.5 Ga (Hamilton, fracture zone is associated with Atla’s evolution, Data Repository1). Wrinkle ridges also deform 2005). (4) Artemis represents the surface expres- as shown by Atla’s triple junction (Smrekar et fi ll of local basins located at high elevations sion of a mantle plume (Griffi ths and Campbell, al., 1997). Collectively these fi rst-order rela- within Ovda and Thetis. Wrinkle ridge trajecto- 1991; Smrekar and Stofan, 1997; Hansen, 2002; tions suggest that Artemis formed after Ovda ries reveal regional-scale patterns, the largest of Bannister and Hansen, 2010). We present evi- and Thetis, but before Atla. which is concentric to Artemis. Regional frac- dence here that Artemis also includes a long- As historically defi ned, Artemis includes an ture suites occur across the map area; of particu- wavelength outer trough (>5000 km diameter), interior region, Artemis Chasma, and an outer lar interest are fractures that trend at high angles a radial dike swarm (12,000 km diameter), and high. Previous mapping defi ned Artemis’s to the concentric wrinkle ridges, referred to here a suite of concentric wrinkle ridges (13,000 km major characteristics, including the interior, as radial fractures. diameter). The vast scale of these newly recog- circular chasma, and outer high, that evolved Concentric wrinkle ridges and radial fractures nized features leads us to reevaluate the origin coevally (Brown and Grimm, 1995, 1996; Spen- together defi ne a coherent pattern centered on of Artemis. cer, 2001; Hansen 2002; Bannister and Hansen, Artemis Chasma (Figs. 1A, 1D, 1E; note that 2010). The interior hosts magmatic centers, a BACKGROUND northeast-trending fracture zone, and a penetra- 1GSA Data Repository item 2010126, a detailed lineament geologic map, is available online at www Aphrodite Terra, Venus’s largest highland, tively developed tectonic fabric. The chasma .geosociety.org/pubs/ft2010.htm, or on request from extends along an equatorial band between displays normal faults along the inner wall and [email protected] or Documents Secretary, ~50°E and ~210°E (Fig. 1). Aphrodite is a fold hinges in the center and outer wall. External GSA, P.O. Box 9140, Boulder, CO 80301, USA. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2010; 2010 v. 38; no. 5; p. 467–470; doi: 10.1130/G30643.1; 1 fi gure; Data Repository item 2010126. 467 0°E 90°E 180°E Wrinkle ridge trajectories m (± Mean planetary radius) A Radial fracture trends Ishtar Terra LP Wrinkle ridges* -2930 11620 pF 0 m geoid 50°N >0 m solid <0 m dashed pTe rB rBl Niobe Planitia 25°N rEw rEe rEc eastern Aphrodite Terra RP Ovda Atla Regio Regio 0°N western Aphrodite Terra Thetis Ph Regio pA 25°S rD rT Artemis rI 50°S D 150°E E rL 0°N 0°N rA pO pTh 75°S pTh 60E60°E 163°E 164°E wr B 50 km C 50 km fr Wrinkle A A 8°S ridges 8°S Fractures Figure 1. A: Mollweide projection of Venus showing Magellan altimetry, fracture zones (from Price and Suppe, 1995) (stipple), wrinkle ridge trajectories (red, and white*; from Price and Suppe, 1995), and radial fracture trends (black). Major geomorphic features: crustal plateaus (pA—Alpha Regio; pF—Fortuna Tessera; pO—Ovda Regio [in D]; pTe—Tellus Regio; pTh—Thetis Regio [in D, E]), volcanic rises (Atla Regio; rB—Beta Regio; rBl—Bell Regio; rD—Dione; rEc—central Eistla Regio; rEe—eastern Eistla Regio; rEw—western Eistla Regio; rL—Lada Terra; rI—Imdr Regio; rT—Themis Regio), Phoebe Regio (Ph), Lakshmi Planum (LP), Rusalka Planitia (RP), Niobe Planitia, Ishtar Terra, west- ern and eastern Aphrodite Terra, and Artemis trough in dark blue. Wrinkle ridge trends in intratessera basins in western Aphrodite are exag- gerated for illustration. Thin black polygon marks map area. Surface strain model features include (Sandwell et al., 1997) 0 m geoid shown between 65°N and 65°S, and predicted surface contraction (blue); according to the global surface strain model, wrinkle ridges should parallel blues lines and not form with geoid >0 m. Thick white line demarks newly defi ned Artemis. B: Example synthetic aperture radar image of fractures (fr) and wrinkle ridges (wr). C: Map of fractures and wrinkle ridges. D: Orthographic view of a virtual globe to the west of Artemis (A) with Magellan gravity (low—black; high—light), wrinkle ridge (red) and radial fracture (yellow) trends, fracture zones (from Price and Suppe, 1995) (green), and Artemis chasma (pink); symbols as in A. E: View of virtual globe to the east of Artemis. rA—Atla Regio. the chasma is not defi ned between the northwest clockwise to the west, wrinkle ridges trend east ture-parallel wrinkle ridges) indicating broadly and north). Directly outside of the chasma from and northeast in Niobe Planitia; there fractures coeval evolution of fractures, cover deposits, the north-northeast clockwise to the west, both defi ne a fan pattern.