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Venus Records a Rich Early History

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Venus Records a Rich Early History Venus records a rich early history V.L. Hansen1* and I. López2* 1Department of Geological Sciences, University of Minnesota-Duluth, Duluth, Minnesota 55812, USA 2Área de Geología, Universidad Rey Juan Carlos, Madrid 28933, Spain ABSTRACT (Ghent and Tibuleac, 2000) implies a thermal The distribution and character of Venus’s impact craters led to the widely accepted idea control for the base of the upper strong layer that Venus underwent global catastrophic resurfacing ca. 500 Ma, and thus Venus records corresponding to a regional brittle-ductile tran- only a short history, encompassing surface evolution since postulated catastrophic resurfacing. sition depth (Hansen and Willis, 1998; Hansen, Ribbon tessera terrain (RTT), a structurally distinctive unit, represents some of Venus’s old- 2006; Ruiz, 2007). Calculated heat-fl ow values est surfaces, and is widely accepted as forming prior to postulated global catastrophic resur- for ribbon formation and the estimated depth to facing. We constructed a global geologic map of RTT unit exposures and structural trends the crustal solidus require either extremely thin using National Aeronautics and Space Administration (NASA) Magellan data. Map relations crust, or a crustal magma reservoir across areas illustrate that RTT displays planet-scale patterns that, together with altimetry, record a rich of RTT formation; thus RTT records regional- geologic history that predates proposed global catastrophic resurfacing. scale environmental conditions different than those of contemporary Venus (Ruiz, 2007). INTRODUCTION ≥1 km is required to bury Venus’s large impact About 15 yr ago, the National Aeronautics craters (Stofan et al., 2005). We present results RTT GLOBAL GEOLOGIC MAP: and Space Administration (NASA) Magellan of detailed, but global scale, geologic mapping OBSERVATIONS AND IMPLICATIONS mission revealed that Venus lacks plate tec- of RTT. Geological relations reveal that RTT We constructed a global geologic map of RTT tonic processes (Solomon et al., 1992; Phillips records a rich geologic history that predates using Magellan SAR (synthetic aperture radar) and Hansen, 1994), yet Venus’s evolution and units attributed to global catastrophic resurfac- and altimetry data, and estimated the global dis- operative geodynamic processes remain elu- ing, and could challenge the occurrence of such tribution of shallowly buried RTT (i.e., RTT bur- sive. Two classes of geologic features provide an event. ied <1 km; Fig. DR1 in the GSA Data Reposi- substantial clues to Venus’s evolution: impact tory1). RTT is exposed across 53 × 106 km2, or craters and ribbon tessera terrain (RTT; Hansen RIBBON TESSERA TERRAIN 12% of the surface; shallowly buried RTT con- and Willis, 1996, 1998) representing distinc- Unit RTT (Hansen and Willis, 1998) consti- stitutes an additional 21%–38% of the surface. tive remnants of ancient surfaces. The record tutes Venus’s locally oldest surface unit (Basi- The distribution of RTT and/or shallowly buried of remarkably well preserved craters infl u- levsky and Head, 1998; Head and Basilevsky, RTT together with global topography provide enced thinking about Venus’s history because 1998), although globally not all RTT formed clues for the global-scale evolution of Venus, few craters show substantive embayment by at the same time (Bindschadler et al., 1992; revealed through fi rst-order observations as fol- external fl ows or tectonic disruption (Phillips et Bindschadler, 1995; Hansen and Willis, 1996; lows (Fig. 1B; Videos DR1 and DR2 in the Data al., 1992; Schaber et al., 1992; Herrick et al., Hansen et al., 2000). Unit RTT is marked by Repository). (1) RTT is strongly correlated with 1997). Total craters, crater spatial distribution, a distinctive tectonic fabric and high surface crustal plateaus, as widely recognized. (2) RTT and crater fl ux models indicate a global average roughness, and characterizes crustal plateaus, inliers occur within all volcanic rises, except model surface age (AMSA) of ca. 750 +350/– but also occurs as lowland inliers, widely Imdr and Themis. (3) RTT occurs within most 400 Ma (McKinnon et al., 1997). A wide range interpreted as remnants of collapsed crustal lowland basins. (4) RTT inliers occur indepen- of different surface histories could accommo- plateaus (Phillips and Hansen, 1994; Ivanov dent of basin topography. (5) Groups of RTT date this global AMSA (Hansen and Young, and Head, 1996). The fabric includes short- to inliers describe regional-scale linear to arcuate 2007) that include, but are not limited to, a sin- long-wavelength folds (1–50 km) that record patterns, many of which (6) show no obvious gle global-scale event. The assumption that this layer shortening, and orthogonal structures that correlation with long-wavelength topography; AMSA records a single global event led to the record layer extension, including so-called rib- i.e., some patterns track across lowland basins widely accepted global catastrophic resurfacing bon ridges and troughs, and grabens (Fig. 1A). and highland rises. (7) Few large (>7 × 106 km2) hypothesis: that ca. 500 Ma, ~80% of Venus’s Locally, volcanic deposits fl ood structural lows RTT-poor regions exist. (8) RTT inliers are rare surface was completely renewed in a period of (Bindschadler et al., 1992; Bindschadler, 1995; across one extensive region centered at ~30°S, 10–100 m.y. Postulated surface renewal could Hansen and Willis, 1996, 1998; Ivanov and 180°E, including the area that encompasses have occurred either through (1) extensive deep Head, 1996; Gilmore et al., 1998; Hansen et Artemis, Diana-Dali, Parga, and Hecate Chas- volcanic burial (Strom et al., 1994; Basilevsky al., 2000; Romeo et al., 2005; Hansen, 2006). mata. We focus on observations 1–6 here. and Head, 1998, 2002, 2006; Bullock et al., Although there is debate about whether folds The lack of spatial correlation of RTT out- 1993), or (2) wholesale lithospheric recycling dominantly predate or postdate ribbons, most crop patterns with long-wavelength (>1000 km) (Turcotte, 1993; Herrick, 1994; Turcotte et al., workers agree that (1) fold and ribbon forma- topography that marks lowland basins and vol- 1999). In either case, little record of the pre- tion broadly overlapped in time, (2) grabens canic rises indicates that the processes forming catastrophic surface should be preserved, and formed late, and (3) volcanic activity accom- these features likely operated independent of any preserved pre-catastrophic record would be panied RTT fabric formation. Ribbon character RTT formation. Parallelism of RTT outcrops mostly limited to high-standing crustal plateaus, and spacing, functions of brittle layer thickness which escaped either deep burial or lithospheric (Hansen and Willis, 1998), require a high geo- 1GSA Data Repository item 2010084, Figure DR1 recycling. The proposed thickness of deep burial thermal gradient (minimum heat fl ow similar and Videos DR1 and DR2 (Quicktime movies of global views), is available online at www.geosociety. has evolved, ranging from >3 km to >1 km, but to average terrestrial mid-ocean ridges) during org/pubs/ft2010.htm, or on request from editing@ ribbon formation (Gilmore et al., 1998; Ruiz, geosociety.org or Documents Secretary, GSA, P.O. *E-mails: [email protected]; [email protected]. 2007). Regular ribbon spacing across 106 km2 Box 9140, Boulder, CO 80301, USA. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2010; 2010 v. 38; no. 4; p. 311–314; doi: 10.1130/G30587.1; 1 fi gure; Data Repository item 2010084. 311 Figure 1. A: Magellan synthetic-aperture radar 75°N Ishtar Terra image of ribbon tessera pF terrain (RTT) fabric. Rib- B bons (bright-dark paired 50°N lineaments 20–100 km pTe C long, parallel to r line) D rBl defi ne alternating fl at- rB E 25°N topped ridges and nar- rEw rEe row, steep-sided, fl at- rEc bottomed troughs; folds (gently sloping ridges, 0°E rA Beta/Atla/Be /A/A / Aphrodite Terra 0° parallel to f line) with TThemishemm wavelength <1 km up Ph pO pTh to 50 km; grabens (g, with aspect ratios ~1:3, pA F 25°S and ~60° walls) typi- 210°E cally parallel ribbons. rT 300°E rD B: Mollweide projection 150°E rI of Venus with Magellan 50°S altimetry, RTT (black), LLaLadad shallowly buried RTT TTerrarrr 90°E m (+/- Mean planetary radius) 75°S (gray; see Fig. DR1 -2930 11620 for method [see foot- 25 km 5.5°N m pTe note 1]), and major A f 50°N C E geomorphic features in- V m cluding all regions: iso- r statically supported Lo crustal plateaus (yellow 25°N outlines: pA—Alpha; N c pF—Fortuna; pO—Ovda; c 5°N pTe—Tellus; pTh—The- c 500 km tis); thermally supported N c volcanic rises (red out- 500 km L 0N pO lines: rA—Atla; rB— 25°N 120°E Beta; rBl—Bell; rD—Di- g 30°E 60E one; rEc—central Eistla; 270°E 98°E 98.5°E 50°N rEe—eastern Eistla; D F rEw—western Eistla; rI— G c Imdr; rT—Themis); and 500 km T Phoebe (Ph) with plateau and rise characteristics 25°S (Simons et al., 1997). 500 km Letters C–F indicate pA locations of regional views of global surface; 25°N RTT (dark brown) and shallowly buried RTT rB 300° 30°E (light brown); fold trends (black lines). C: Lowland planitiae: Lowana (Lo), Niobe (N), Vellamo (V), and Llorona (L) preserve temporal relations between two suites of RTT. D: Lowland-highland transition between Beta rise (rB) and Guinevere Planitia (G). E: Montes (m) of Bell and coronae (c) of eastern Eistla disrupt early formed RTT; Tellus (pTe) and Ovda (pO) show different RTT fabric trends. F: Tinatin Planitia (T) and mesoland east of Alpha (pA); coronae (c) locally disrupt RTT. and RTT fabrics within isolated outcrops across lus Regio truncates linear to arcuate patterns interior suggests that RTT underlies this entire lowland basins and highland rises suggests that defi ned by adjacent RTT inliers, suggesting feature.
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