sign in sign up
<<
Home , Guinevere Planitia, Ovda Regio

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 ’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 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) 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 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 / 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 (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. RTT fabrics parallel the elongate form a regional surface decorated by RTT fabric was that Tellus Regio formed after the RTT inliers, of RTT outcrops and topographic trends that downwarped and upwarped during basin and consistent with diachronous, rather than coeval, defi ne the circular feature; RTT fabrics also par- rise formation, respectively. RTT formation (Bindschadler, 1995). allel highs that mark the northeast-trending belt. In contrast, strong spatial correlation of RTT Further clues emerge from a closer look at The circular feature shows both structural and and individual topographically defi ned crustal four large areas. In the fi rst region, an exten- topographic truncation by the northeast-trend- plateaus suggests a genetic relationship between sive lowland across four planitiae hosts two ing belt, suggesting that the two suites of RTT the plateau formation and the RTT they host, con- dominant features: an ~2000-km-diameter cir- represent different events, formed at different sistent with earlier conclusions (Bindschadler et cular feature, and a northeast-trending belt that times. Most of the area is underlain by shallowly al., 1992; Bindschadler, 1995; Ghent and Han- extends for thousands of kilometers beyond buried RTT, consistent with geologic mapping sen, 1999; Hansen et al., 2000). Some plateaus the map area (Fig. 1C). Both topography and indicating that a thin cover of shield terrain apparently lost topographic support, positioning RTT fabrics defi ne each feature. RTT inliers likely overlies RTT (Aubele, 1996, 2009; Han- RTT for possible burial (Phillips and Hansen, typically correspond to local topographic highs, sen, 2005); thus this area is diffi cult to reconcile 1994; Ivanov and Head, 1996) or, alternatively, although several RTT inliers show little topo- with extensive fl ooding (>1 km). Although this some (or all) tracts of lowland RTT were never graphic expression, and some local highs lack region includes several million square kilome- greatly elevated above their surroundings. Tel- RTT; RTT preserved within the circular feature ters of the lowland, geologic relations cannot

312 GEOLOGY, April 2010 be reconciled with interpretations that it hosts ing would be expected to bury intermediate- ers collectively defi ne coherent patterns based extensive thick (>1 km) fl ows, or represents wavelength topography; if not, then RTT should on outcrop exposure, topography, and RTT fab- newly formed lithosphere replacing older recy- be preserved in the local topographic highs that ric trends; these suites describe linear to arcuate cled lithosphere. escape burial. If the surrounding lowland rep- patterns that cover millions of square kilometers. A second region straddles a highland-lowland resents young lithosphere, then the similarity in The spatial extent of suites is not yet defi ned, transition marking the boundary between Beta topographic character of the lowland and RTT and it is likely that many suites of coherent RTT Regio and Guinevere Planitia (Fig. 1D). Frac- inliers would be coincidental. The geologic his- have yet to be recognized. It is unclear whether ture zones and fl ows disrupt coherent RTT fab- tory that emerges includes (1) RTT formation, individual suites of RTT were once topographi- ric within Beta rise, whereas RTT occurs even (2) local burial of RTT that preserves isolated cally elevated, like crustal plateaus, and sub- at low basin elevation within Guinevere Plani- kipukas of RTT yet shallowly buries other por- sequently collapsed, or if they never attained tia. RTT exposed in and Guinevere tions of the RTT, preserving intermediate topog- elevated status, or both. (4) Crosscutting rela- Planitia could record two events, or a single raphy (shield terrain emplacement [Hansen, tions indicate that at least some crustal plateaus event based on current map relations. Shallowly 2005; Stofan et al., 2005)] is one process that are younger than the suites of RTT preserved in buried RTT extends across much of this area, could accommodate these requirements), and adjacent lowlands (e.g., Tellus, Alpha, and Ovda with only isolated regions potentially preserv- (3) regional uplift at Bell and Eistla Regiones Regiones). (5) Suites of RTT formed via mul- ing cover >1 km. Thus, geologic relations are and emplacement of montes and coronae. The tiple events through time and space, and record consistent with local shallow burial of RTT prior relative timing of Bell and Eistla Regiones is a rich history that can be unraveled by further to Beta rise and Guinevere basin formation. The unconstrained based on this study. The elongate detailed mapping. geologic history that emerges includes (1) early inliers and their internal RTT fabrics trend par- Volcanic rises and lowland basins each pre- formation of RTT (by one or more events), allel to, and line up globally with, the northeast- serve RTT inliers. The occurrence of RTT inli- (2) local shallow burial of RTT, (3) upwarping trending RTT belt within Niobe Planitia dis- ers in several rises, thermally supported contem- and downwarping of the RTT and/or shallow cussed here (Fig. 1C). These two suites of RTT porary features (Simons et al., 1997), requires cover surface within Beta Regio and Guinevere could be correlative; however, more mapping is that these areas of RTT inliers either somehow Planitia, respectively, and (4) local fracturing required to test this hypothesis. escaped catastrophic resurfacing (whether and burial of RTT within Beta Regio. Map rela- The fourth region is in the mesoland-lowland through deep burial or lithospheric overturn) tions are inconsistent with interpretations that east of (Fig. 1F); here RTT inli- before rise formation, or resurfacing occurred Guinevere Planitia hosts extensive fl ows >1 km ers preserve several patterns defi ned by inter- after rise formation. The same can be stated thick, or represents a large tract of young litho- mediate-wavelength topography and RTT fab- of lowland basins, although with inverse topo- sphere; thick fl ows are inconsistent with shal- rics. Large tracts of RTT display crosscutting graphic development. Given that lowland basins lowly buried RTT estimates; new young litho- fabrics recording different events or phases of preserve RTT and/or shallowly buried RTT, sphere would lack RTT. fabric development; elsewhere RTT inliers are these regions either did not undergo catastrophic A third region, which includes portions of Bell small (at the scale of data effective resolution; resurfacing, or resurfacing occurred after basin and eastern Eistla Regiones (Fig. 1E), preserves Zimbelman, 2001), and more detailed study is formation. If the basins did not undergo cata- narrow elongate exposures of RTT (hundreds of required to establish the regional extent of vari- strophic resurfacing, then it is diffi cult to justify kilometers long) that collectively extend thou- ous fabric domains and possible temporal rela- global resurfacing; if resurfacing occurred after sand of kilometers in a north-northeast direc- tions. However, the distribution of RTT inliers basin formation, then basin RTT should not tion, and defi ne a strike-normal belt >2000 km provides evidence that (1) RTT is shallowly be preserved. These results provide a serious wide. Individual exposures, separated by hun- buried across the region, (2) cover material challenge to (1) global resurfacing hypotheses, dreds of kilometers, display parallel structural could have numerous sources, and (3) long- whether by the emplacement of extensive fl ows fabrics; fold crests parallel the long axes of wavelength topography formed after RTT, and >1 km thick, lithospheric overturn, or global outcrops and ribbons trend perpendicular to the likely after emplacement of cover deposits. subduction, with or without pulsating continents folds. The RTT inliers, and their internal fabrics, Locally coronae obliterate RTT. RTT fabrics (Romeo and Turcotte, 2008), and (2) the impli- track across and beyond Bell and Eistla rises. within Alpha Regio trend at a high angle to inli- cation that Venus records only a temporally lim- Viewed globally, crustal plateaus Tellus and ers with northeast outcrop and fabric trends, ited geologic history. Ovda Regiones and their respective RTT fabrics indicating that Alpha Regio likely formed later. The picture that emerges is one in which truncate the elongate RTT inliers and their inter- The intermediate-wavelength topographic char- Venus’s surface records a rich and prolonged nal fabrics (Fig. 1B), suggesting that Tellus and acter is similar to that of the lowland adjacent history (e.g., Guest and Stofan, 1999); this study Ovda Regiones formed after this suite of RTT. to Bell and Eistla Regiones. Map relations are serves to highlight the history that awaits discov- Relative timing of Tellus versus Ovda formation inconsistent with interpretations that this region ery. RTT formed during a specifi c geologic era, is unconstrained. Montes and coronae, which hosts extensive fl ows >1 km thick, or represents marked by relatively unique environmental con- characterize Bell and Eistla Regiones, respec- young lithosphere. ditions, conceptually similar to Earth’s Archean. tively, obliterated local RTT, consistent with RTT records a regional-scale history of defor- the history derived from 1:5,000,000-scale geo- SYNTHESIS mation phases or events that vary in space and logic mapping (Campbell and Campbell, 2002; Geologic relations gleaned from global obser- time. At a global scale, patterns within RTT out- Campbell and Clark, 2006). However, the lack vations and region-scale map areas lead to the crops and fabrics extend over millions of square of RTT in the surrounding lowlands is not the following implications for the global to regional kilometers, and individual suites record variable result of montes and coronae. These lowlands scale evolution of Venus’s surface. (1) The unit temporal evolution, which could potentially be are characterized by intermediate-wavelength RTT records a rich geologic history that broadly used to correlate temporally distinct events over topography similar in character to that of RTT predated the formation of long-wavelength rises large regional scales. inliers. Deep, extensive, post-RTT fl ooding of and basins. (2) Crustal plateaus formed individ- Our ability to understand terrestrial planet the adjacent lowland region is diffi cult to recon- ually via a process that formed both RTT fabric evolution relies on the preserved geologic record. cile with this topographic character. Deep fl ood- and plateau topography. (3) Suites of RTT inli- Earth’s limited ancient record is dismembered

GEOLOGY, April 2010 313 due to plate tectonic processes, and eroded and Ghent, R.R., and Hansen, V.L., 1999, Structural and Phillips, R.J., Raubertas, R.F., Arvidson, R.E., buried due to surface processes. The record of kinematic analysis of eastern , Sarkar, I.C., Herrick, R.R., Izenberrg, N., and Mars’ early history has been obscured by bolide Venus: Implications for crustal plateau forma- Grimm, R.E., 1992, Impact craters and Ve- tion: Icarus, v. 139, p. 116–136, doi: 10.1006/ nus resurfacing history: Journal of Geophysi- impact, surface burial, or reworking by volca- icar.1999.6085. cal Research, v. 97, p. 15,923–15,948, doi: nic, hydrologic, or atmospheric processes. In Ghent, R.R., and Tibuleac, I.M., 2000, Ribbon spac- 10.1029/92JE01696. contrast, Venus’s surface might preserve a novel ing in Venusian tessera: Implications for layer Romeo, I., and Turcotte, D.L., 2008, Pulsating con- record of terrestrial planet evolution given its thickness and thermal state: Geophysical Re- tinents on Venus: An explanation for crustal search Letters, v. 29, p. 994–997. plateaus and tessera terrains: Earth and Plan- lack of plate tectonics and a hydrologic cycle, Gilmore, M.S., Collins, G.C., Ivanov, M.A., Mari- etary Science Letters, v. 276, p. 85–97, doi: and its dense atmosphere that protected the sur- nangeli, L., and Head, J.W., 1998, Style and 10.1016/j.epsl.2008.09.009. face from extensive bolide impact. Renewed sequence of extensional structures in tes- Romeo, I., Capote, R., and Anguita, F., 2005, Tec- study of Magellan’s spectacular global data sets sera terrain, Venus: Journal of Geophysi- tonic and kinematic study of a strike-slip zone has the potential to reveal this history. cal Research, v. 103, p. 16,813–16,840, doi: along the southern margin of Central Ovda 10.1029/98JE01322. Regio, Venus: Geodynamical implications Guest, J.E., and Stofan, E.R., 1999, A new view for crustal plateaux formation and evolu- ACKNOWLEDGMENTS of the stratigraphic history of Venus: Icarus, tion: Icarus, v. 175, p. 320–334, doi: 10.1016/ Hansen gratefully acknowledges the Mc Knight v. 139, p. 55–66, doi: 10.1006/icar.1999.6091. j.icarus.2004.11.007. Foundation and the National Aeronautics and Hansen, V.L., 2005, Venus’s shield-terrain: Geologi- Ruiz, J., 2007, The heat fl ow during the formation of Space Administration (grants NNX06AB90G and cal Society of America Bulletin, v. 117, p. 808– ribbon terrains on Venus: Planetary and Space NNG05GM26G) for support of this work. R. Bannis- 822, doi: 10.1130/B256060.1. Science, v. 55, p. 2063–2070, doi: 10.1016/ ter, S. Graham, T. Nordberg, S. Stark, and D. Young Hansen, V.L., 2006, Geologic constraints on crustal j.pss.2007.05.003. assisted in mapping and/or geographic information plateau surface histories, Venus: The Schaber, G.G., Strom, R.G., Moore, H.J., Soderb- system issues, J. Goodge and K. Tanaka reviewed early pond and bolide impact hypotheses: Journal lom, L.A., Kirk, R.L., Chadwick, D.J., Daw- versions of the manuscript. Comments by W. Kiefer of Geophysical Research, v. 111, E11010, doi: son, D.D., Gaddis, L.R., Boyce, J.M., and and two other reviewers improved the manuscript. 10.1029/2006JE002714. Russell, J., 1992, Geology and distribution of Hansen, V.L., and Willis, J.J., 1996, Structural analy- impact craters on Venus: What are they telling REFERENCES CITED sis of a sampling of tesserae: Implications for us?: Journal of Geophysical Research, v. 97, Aubele, J.C., 1996, Akkruva small shield plains: Defi - Venus geodynamics: Icarus, v. 123, p. 296– p. 13,257–13,302, doi: 10.1029/92JE01246. nition of a signifi cant regional plains unit on Ve- 312, doi: 10.1006/icar.1996.0159. Simons, M., Solomon, S.C., and Hager, B.H., 1997, nus [abs.]: Houston, Texas, Lunar and Planetary Hansen, V.L., and Willis, J.J., 1998, Ribbon ter- Localization of gravity and topography: Con- Science Conference XXVII, p. 49–50. rain formation, southwestern Fortuna Tessera, straints on the tectonics and mantle dynam- Aubele, J.C., 2009, Shield fi elds and shield plains Venus: Implications for lithosphere evolu- ics of Venus: Geophysical Journal Interna- on Venus: Contrasting volcanic units example tion: Icarus, v. 132, p. 321–343, doi: 10.1006/ tional, v. 131, p. 24–44, doi: 10.1111/j.1365 in Shimti Tessera (V-11) and Vellamo Planitia icar.1998.5897. -246X.1997.tb00593.x. (V-12) quadrangles [abs.]: Houston, Texas, Lunar Hansen, V.L., and Young, D.A., 2007, Venus evolu- Solomon, S.C., Smrekar, S.E., Bindschadler, D.L., and Planetary Science Conference XL, 2396.pdf. tion: A synthesis, in Cloos, M., et al., eds., Con- Grimm, R.E., Kaula, W.M., McGill, G.E., Basilevsky, A.T., and Head, J.W., 1998, The geologic vergent margin terranes and associated regions: Phillips, R.J., Saunders, R.S., Schubert, G., history of Venus: A stratigraphic view: Journal A tribute to W.G. Ernst: Geological Society of Squyres, S.W., and Stofan, E.R., 1992, Venus of Geophysical Research, v. 103, p. 8531– America Special Paper 419, p. 255–273. tectonics: An overview of Magellan observa- 8544, doi: 10.1029/98JE00487. Hansen, V.L., Phillips, R.J., Willis, J.J., and Gh- tions: Journal of Geophysical Research, v. 97, Basilevsky, A.T., and Head, J.W., 2002, Venus: Tim- ent, R.R., 2000, Structures in tessera terrain, p. 13,199–13,255, doi: 10.1029/92JE01418. ing and rates of geologic activity: Geology, Venus: Issues and answers: Journal of Geo- Stofan, E.R., Brian, A.W., and Guest, J.E., 2005, v. 30, p. 1015–1018, doi: 10.1130/0091-7613 physical Research, v. 105, p. 4135–4152, doi: Resurfacing styles and rates on Venus— (2002)030<1015:VTAROG>2.0.CO;2. 10.1029/1999JE001137. Assessment of 18 Venusian quadrangles: Ica- Basilevsky, A.T., and Head, J.W., 2006, Impact craters Head, J.W., and Basilevsky, A.T., 1998, Sequence of rus, v. 173, p. 312–321, doi: 10.1016/j.icarus on regional plains on Venus: Age relations with tectonic deformation in the history of Venus: .2004.08.004. wrinkle ridges and implications for the geologi- Evidence from global stratigraphic relations: Strom, R.G., Schaber, G.G., and Dawson, D.D., cal evolution of Venus: Journal of Geophysical Geology, v. 26, p. 35–38, doi: 10.1130/0091 1994, The global resurfacing of Venus: Journal Research, v. 111, doi: 10.1029/2006JE002714. -7613(1998)026<0035:SOTDIT>2.3.CO;2. of Geophysical Research, v. 99, p. 10,899– Bindschadler, D.L., 1995, Magellan—A new view Herrick, R.R., 1994, Resurfacing history of Venus: 10,926, doi: 10.1029/94JE00388. of Venus geology and geophysics: Reviews of Geology, v. 22, p. 703–706, doi: 10.1130/0091 Turcotte, D.L., 1993, An episodic hypothesis for Geophysics, v. 33, p. 459–467, doi: 10.1029/ -7613(1994)022<0703:RHOV>2.3.CO;2. venusian tectonics: Journal of Geophysical Re- 95RG00281. Herrick, R.R., Sharpton, V.L., Malin, M.C., Lyons, search, v. 98, p. 17,061–17,068, doi: 10.1029/ Bindschadler, D.L., deCharon, A., Beratan, K.K., S.N., and Feely, K., 1997, Morphology and 93JE01775. and Head, J.W., 1992, Magellan observations morphometry of impact craters, in Bouger, Turcotte, D.L., Morein, G., and Malamud, B.D., of Alpha Regio: Implications for formation S.W., et al., eds., Venus II: Tucson, University 1999, Catastrophic resurfacing and episodic of complex ridged terrains on Venus: Journal of Arizona Press, p. 1015–1046. subduction on Venus: Icarus, v. 139, p. 49–54, of Geophysical Research, v. 97, p. 13,563– Ivanov, M.A., and Head, J.W., 1996, Tessera terrain doi: 10.1006/icar.1999.6084. 13,577, doi: 10.1029/92JE01332. on Venus: A survey of the global distribution, Zimbelman, J.R., 2001, Image resolution and evalu- Bullock, M.A., Grinspoon, D.H., and Head, J.W., characteristics, and relation to surrounding ation of genetic hypotheses for planetary land- 1993, Venus resurfacing rates—Constraints units from Magellan data: Journal of Geophys- scapes: , v. 37, p. 179–199, doi: provided by 3-D Monte-Carlo simulations: ical Research, v. 101, p. 14,861–14,908, doi: 10.1016/S0169-555X(00)00082-9. Geophysical Research Letters, v. 20, p. 2147– 10.1029/96JE01245. 2150, doi: 10.1029/93GL02505. McKinnon, W.B., Zahnle, K.J., Ivanov, B.A., and Campbell, B.A., and Campbell, P.G., 2002, Geologic Melosh, H.J., 1997, Cratering on Venus: Mod- map of the Bell Regio Quadrangle (V-9), Ve- els and observations, in Bouger, S.W., eds., nus: U.S. Geological Survey Geologic Investi- Venus II: Tucson, University of Arizona Press, gations Map I-2743, scale 1:5,000,000. p. 969–1014. Manuscript received 3 August 2009 Campbell, B.A., and Clark, D.A., 2006, Geologic Phillips, R.J., and Hansen, V.L., 1994, Tectonic and Revised manuscript received 26 October 2009 map of the Quadrangle (V-21), Venus: magmatic evolution of Venus: Annual Review of Manuscript accepted 28 October 2009 U.S. Geological Survey Scientifi c Investiga- Earth and Planetary Sciences, v. 22, p. 597–654, tions Map 2897, scale 1:5,000,000. doi: 10.1146/annurev.ea.22.050194.003121. Printed in USA

314 GEOLOGY, April 2010