JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. El0, PAGES 16,085-16,120, OCTOBER 25, 1992 Stylesof Deformationin IshtarTerra and Their Implications Wn.T.TAMM. KAU•A,• DOAN•L. BINDSCHAD•-R,l ROBERT E. GPaM•,2'3 VICKIL. HANSEN,2KARl M. ROBERTS,4AND SUZANNE E. SMREr,AR s IshtarTerra, the highest region on Venus, appears to havecharacteristics of both plume uplifts and convergent belts.Magellan imagery over longitudes 330ø-30øE indicates a great variety of tectonicand volcanic activity, with largevariations within distances of onlya few 100km. Themost prominent terrain types are the volcanic plains of Lakshmiand the mountain belts of Maxwell,Freyja, and Danu. Thebelts appear to havemarked variations in age. Thereare also extensive regions of tesserain boththe upland and outboard plateaus, some rather featureless smoothscarps, flanking basins of complexextensional tectonics, and regions of gravitationalor impactmodifica- tion.Parts of Ishtarare the locations of contemporaryvigorous tectonics and past extensive volcanism. Ishtar appearsto be the consequence of a history of several100 m.y., in whichthere have been marked changes in kinematic patternsand in whichactivity at any stage has been strongly influenced by the past. Ishtar demonstrates three general propertiesof Venus:(1) erosionaldegradation is absent,leading to preservationof patternsresulting from past activity;(2) manysurface features are the responses ofa competentlayer less than 10 km thick to flowsof 100km orbroaderscale; and (3) thesebroader scale flows are controlled mainly by heterogeneities inthe mantle. Ishtar Terra doesnot appear to bethe result of a compressionconveyed by anEarthlike lithosphere. But there is stilldoubt as to whetherIshtar is predominantlythe consequence of a mantleupflow or downflow.Upflow is favoredby the extensivevolcanic plain of Lakshmiand the high geoid: topography ratio; downflow is favoredby the intense deformationof themountain belts and the absence of majorrifts. Both could be occurring, or have recently occurred, withLakshmi the most likely locus of upflowand Maxwell the main locus of downflow.But doubts about the causes of Ishtarwill probablynever be resolvedwithout circularization of the Magellanorbit to obtaina moredetailed gravityfield. 1. INTRODUCTION superiorconjunction hiatus. If we takethe 3-km elevationcontour as IshtarTerra is of specialinterest as the mostprominent departure the limit, then the latitudinal bounds would be about 55øN to 78øN. fromhydrostatic equilibrium on Venus. The peak of MaxwellMontes However,tectonic patterns associated with IshtarTerra extendto is ! 0 km abovethe mean planetary radius (MPR), whichoutstrips the superiorconjunction hiatus. If we takethe 3-km elevationcontour as secondhighest feature, Maat Mons in Atla Regio,by morethan a the limit, then the latitudinal bounds would be about 55øN to 78øN. kilometer.This promontory is partof a generaluplands more than 3 However, tectonicpatterns associated with Ishtar Terra extendto km high in topographyand morethan 20 m high in geoid,with a lowerelevations in manyplaces. Within these longitude and latitude horizontalextent of about4000 km: about1% of theplanet's surface boundaries,we identify sevenmajor provinces:Lakshmi, Danu, area.These uplands have sharply defined boundaries in mostplaces Freyja,North Scarp and Basin, South Scarp and Basin, Maxwell, and and significantvariations in characteron scalesof 200 to 1000 km. WesternFortuna. Each province is namedafter its dominant feature, Thevariety of terrainsis evidentin theimagery, of whichFigure ! is but the province includesother named featuresthat have close a mosaic. The immediate causes of most of these variations are relationshipto the dominantfeature: e.g., the province"Danu" evident:lava flows for thesmooth plains of LakshmiPlanurn, conver- includesnot only Danu Montes but also VestaRupes, Clotho Tessera, gentmotions for thesurrounding mountain belts of Maxwell,Freyja, andparts of LakshmiPlanurn and Sedna Planitia. Akna, andDanu [Masurskyet al., 1980;Barsukov et al., 1986]. But Importantto interpretation of theimagery are topographic altitudes thereare differences among these physiographic provinces of varying andgravity variations. Figure 3 givesthe Magellan altimetry for the subtlety,and for somefeatures (e.g., the smoothnessof the scarps areadiscussed in theform of contoursat 0.5-kmintervals. Superim- borderingeastern Lakshmi) the proximate cause is not at all evident. posedon it is thegeoid from the 36th degree field of SmithandNerem Nearlyall papersin thisspecial issue are devoted to particular feature [1992],based on PioneerVenus tracking data. The spacecraftwas types;it isour belief that one directed to anarea markedly anomalous, 700-2200km highat theselatitudes, so the signalis bland,and the but widelyranging in character,would be a valuablecomplement. geoidhigh probably shifted toward the pericenter, i.e., southward.A The main physiographicprovinces of westernIshtar Terra are regionalgravity solution by Grimmand Phillips [1991] alsoobtains shownin Figure2. Our discussionwill be confined,however, to the a geoidhigh to thesouth of Ishtarbut somewhat west of thatin Figure partsof IshtarTerra between 330øE and 30øE: i.e., those imaged by the 3. For interpretationof IshtarTerra, circularization of theMagellan Magellanradar between the start of themission and the start of thefirst orbiterto an altitudeof about250 km is required. IshtarTerra' s greatheight, broad extent, and particularly its large geoidanomaly require appreciable support from mantle convection. •DapartmentofEarth and Space Sciences, University of California, Buteven the first-order nature of thisdependence (upflow or downflow, Los Angeles. or combinationsthereof) is debated[Pronin, 1986; Bindschadleret 2Dapartmentof Geological Sciences, Southern Methodist Universi- ty, Dallas, Texas. al., 1990; Grimm and Phillips, 1990, 1991; Roberts and Head, 3Nowa• Department of Geology, Arizona State University, Tempe. 1990a,b;Lenardic et al.., 1991]. Examinationof the Magellan 4Dopartmentof GeologicalSciences, Brown University, Provi- imageryshould help to addressthese questions. Ideally, suchan dence, Rhode Island. SDapartmentof Earth, Atmospheric,and PlanetarySciences, examinationwould result in a kinematicmap of the surface,analo- MassachusettsInstitute of Technology,Cambridge. gousto thoseinferred on Earth from remanentmagnetism, dated offsets of interfaces, and other devices. The clues to motions on Copyright1992 by the American GeophyscialUnion. Venus are much more limited. Furthermore, the immediate causesof Paper number92JE01643. surfacefeatures often are quite indirect in theirrelationships to any 0148-0227/92/92JE-01643505.00 mantleflow pattern.This indirectnessis most evidentfor the lava 16,085 16,086 KAULA ET AL.' STYLES OF DEFORMATION IN ISHTAR TERRA Fig. 1. WesternIshtar Terra. Latitudes55ø-80øN, longitudes 300ø-30øE. From C2-MIDRP.60N333E. flows coveringthe Lakshmi plateau,but it is also true for the projectionis sinusoidal.Twenty-one F-MIDRPs are availablefor variationson a 10-km scalein the mountainbelts, arising from the IshtarTerra. The imagesprinted in thispaper are nearly all segments weaknessof the lower crust,as occurringin many partsof Venus of F-MIDRPs. However,they offer a ratherselective view, so the [Zuber, 1987; Solomonet al., tthis issue]. seriousreader is urgedto consultthe original F-MIDRPs, obtainable The varietyof featuresin IshtarTerra compelsa somewhatseg- on compactdiscs from the NSSDC at NASA GoddardSpace Flight mentedapproach. Section 2 is the majorpart of this paper,which Centeror from a PlanetaryData Center. containsdetailed description and interpretation for eachof theseven 2. Theradar altimetry by Magellan is conveyed through 1:4,500,000 provinces.Section 3 drawson section2 to identifyand interpret contourplots of 500-m interval;see Figure 3. characteristicfeature types. Section4 attemptsto infer the contem- 3. The gravimetryfor Ishtar Terra is of low resolutionbut porarystructure and kinematics of IshtarTerra, plus the evolution nonethelesshas a stronginfluence on our interpretations;see Figure leadingthereto. 3. The main systematicproducts of theseanalyses are geomorphic 2. ANALYSESOF PHYSIOGRAPHICPROVINCES units:characteristic combinations of featuresidentified in theimag- ery and altimetrythat cover sufficientareas to be representedon Theseanalyses are variedin form not only becauseof thediffer- 1:5,000,000-1:20,000,000scale maps. SeeTable 1. encesin characteramong the sevenprovinces (Figure 2) but also For someareas, we presentdelineations of structuralfeatures and becausethe effort was necessarilydivided among coauthors. For histories.But it is difficult to makea systematicmap for asbroad an purposesofdescription, we divide the provinces into 28 regions,some areaas Ishtar Terra, so they have been limited to selectedareas. The of themevident in Figure2, othersdefined on the geomorphicunit ability to infer historiesvaries from oneprovince to another,depen- maps. denton how overwhelminghas been recent activity: tectonic, volca- The followingare principal data sources: nic,or impact.Thus provinces with a varietyof featuresof relatively 1. The radarimagery is conveyedthrough digital images or photo mild relief, suchas SouthScarp and Basinand Danu montes,have printsthat areof theminimum scale at whichthe emulsioncan carry more dues of the past than do LakshmiPlanum, which has been the 75-meter resolution: 1:3,000,000. They are designatedF- largelyflooded by relativelyrecent flows, and Maxwell