Styles of Deformation in Ishtar Terra and Their Implications

Home , Cleopatra (crater), Ishtar Terra, Maat Mons, Maxwell Montes, Sedna Planitia

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 mostevident for 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 theimagery 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 Montes, MIDRP.jkNImn, wherejk is the northlatitude and Iron is the east whosefaults and folds appearto ariselargely from contemporary longitudeof the centerpoint to the nearestinteger degree. Their tectonics. KAULA •-T AL.' STYLESOF DEFORMATIONIN ISHTARTERRA 16,087

310E 320E 330E 340E 350E 0 10E 20E 30E I Itzpapalotl Tessera o I Q

Freyja E• • Western ß Fortuna Tessera MoranaI Varz North Scarp \Chas•Iß •,asma 70N 70N & Basin

Colette Maxwell Hina PateraSacajawea MontesCleopatra I Patera I •)Chasrna I LAKSHMI PLANUM I I RangridFossae I

Siddon,,

60N 60N / SouthSc Basin 10E Magnani Clotho Kartini Tessera Ut Rupes 320E 330E 340E 350E

Fig.2. Sketchmap of westem Ishtar Terra. A degreeof latitude is 105.6 km long; in the latitudes ofIshtar Terra, a degreeof longitude variesfrom 61 km to 22 km. The dashedline indicatesthe eastemlimit of Figure 1.

2.1. Lakshmi 7). Thesestructures are 1-3 km wide andspaced 3-10 km apart. A LakshmiPlanumis abroad, smooth area rising from a heightof 2.5 volcanicshield 11 km in diameter(63.95øN, 339.5øE) is situatedalong km abovemean planetary radius (MPR), just north of DanuMontes, the trendof one of thesefeatures. The associationof the ridgesand around60øN, 334øE, to almost 5.0 km alongthe edges of Freyja (73øN, scarpswith thecaldera and volcanic shield led Robertsand Head 330øE)and Maxwell Montes(66øN, 355øE). Correlated with eleva- [1990a] andHead et aL [ 1991] to suggestthat they represent a zone tion is the frequencyof appearanceof ridgedterrain. Lava flows of dikeintrusion along which magma has been injected and erupted dominatethe entire plain, with relatively little to be inferred in theway at the surface. of structuralrelationships. Hence, in LakshmiPlanurn a description Sacajaweaappears to lack the distinctive apron of lobateflows that by geomorphicunits, as given in Table1, is appropriate;see Figures surroundsColette, a large shieldand calderacomplex located in 4 and 5. westernLakshmi [Stofan et al. 1987;Roberts and Head, 1990a]). A Sacajawea(Figures 6 and 7: F-MIDRPs 65N330, 65N342). faintregion of mottled,lobate flows (PI) is associated with the ridges Sacajawea(64.5øN, 336.5øE) is a volcaniccaldera previously •e- andscarps trending SE fromthe caldera.An additionalregion of scribedfrom the Arecibo and Venera imagery [Barsukov et al., 1986; mottled,lobate flows (PI andPsd) islocated to the SW of thecaldera Stofanet al., 1987; Roberts and Head, 1990a] and most recently from (63.5øN,333øE). This region w• identifiedas a "ventcomplex" by theMagellan hnagery [Head et all., 1991, this issue]. It is a 260 x 175 Robertsand Head [1990a,b]from the Venera data, since it appeared km depression2 km deep.The depression is enclosed by a zoneof to consist of a cluster of small volcanic shields. These shields are not concentrictroughs that extends 60-130 km fromthe calderafloor. all asclearly observed in theMagellan images, perhaps because of a Individualtroughs are 1-2 km wide,are 4-100 km long,and spaced higherincidence angle (22 ø) thanthat of the yeneraorbiters (10 ø) about3 km apart.These features are thought to berelated to broad [Pettengillet al., this issue], and thus are less sensitive toslight slopes. saggingand downwarping ofthe plateau above apartially drained and Theseregions appear to contain the only lava flows associated with solidifiedshallow crustal magma reservoir [Head et all, 1991].The Sacajaweathat are distinct from the general plain• of Lakshnii. floorof thecaldera is coveredby smooth,mottled plains (Sp). The Ridgedterrain (Tr) (Figures8 and9; F-MIDRPs65N330, 65N342, brightestdeposits are observed about the periphery and in the center 65N354). The ridgedterrain east of Sacajeweahas an extremely of thecaldera floor, where a ponded, leveed flow appears to have been complexstructural fabric, with extensive embayments. It is hypoth- oneof the latest eruptions within the caldera. Linear to sinuous ridges esizedto be theexposed portions of tesseralikebedrock underlying andscarps (centered at 64øN,340øE) extend up to 250km fromthe theplains of Lakshmi[Roberts and Head, 1990a]. Exposuresare easternedge of thecaldera, trending approximately N60øW (Figure foundprimarily at higher elevations than most of Lakshmi, in thearea 16,088 KAULA ET AL.' STYLESOF DEFORMATION• ISHTARTERRA

.o Troughsand scarps oriented N50ø-60øW are observed in thewestern largeexposure of Tr. Thesestructures are not as distinctiveas thosein the easternexposure, possibly due to lower amountsof volcanicinf'fil that highlights the grabenlike troughs to theeast. This is consistentwith the observation that this exposure of ridgedterrain (Tr) is slightlyhigher, on the average, than the eastern exposure. The typicalspacing and width of thesestructures are so narrow that they forma fineNW-trending linear fabric of scarpsand lineaments, some of themclearly troughs. Their widths are from less than 1 km to 10 km, with spacingsof 1 to 5 km. Theirlengths are up to 180km, but themaximum length is obscuredby flooding.The wider troughs are often filled by lavas,suggesting that the minimumwidth is also obscuredby flooding.Some of thesestructures extend into the region e 70 of fracturedterrain (Tf) to the NW. 2. The secondis NE trendinglineaments. Ridges, scarps, and troughsoriented N20-30E are observedin the easternexposure of ridgedterrain. These structures are 1-4 km in widthand spacing. They appearto be highly degradedand floodednarrow troughs. Faint 65 ridgeswith similarorientations and morphologies are observed par- tiallyburied in thesurrounding plains and may represent more heavily floodedregions of the unit. Troughsoriented N45ø60øE are observed in thewestern exposure of ridgedterrain (Tr). Thesestructures are similar in morphologyto the NW-trendingtroughs and scarpsbut are not as pervasively distributed.They are concentrated in thewestern end of theexposure, near65.6øN, 341øE, where they are partially flooded by lavas. Widths andspacings are approximately 1-5 km.

50 3. The thirdis approximatelyN-S trendinglineaments. Broad 330 340 350 0 10 20 30 troughs(about 3 km wide, spaced20-40 km, morethan 100 km Longitude longand oriented N0o-10oW to N0o-10oE)are observed through- Fig. 3. Topographicaltitudes in kilometers(fine lines, 0.5-km intental) and outthe easternexposure of ridgedterrain. Thesetroughs are geoidheights in meters(heavy lines, 10-m inter,,al). The altitudesare from heavilydegraded, and appear to be crosscutby mostof theother Magellanaltimetry with trackspacing of 13km or less (Seealso Figures 16 structuresin thisexposure. To thewest, highly degraded troughs, and 17 of Solomonet al. [thisissue]). The geoidis obtainedby analysisof PioneerVenus tracking [Smith and Netera, 1992]. The spacecraftaltitude scarpsand ridges oriented N0-15W areobserved. Many of them was700 to 2200 km, southto north,over the latitudesof thismap. The appearto be floodedand crosscut or offsetalong strike (e.g., analysistechnique ("a priorisigmas") tends to showany extremum closer to 65.8øN,341øE). Thesestructures are clusteredin threelocations pericenter(i.e., more south)than its actuallocation. (63.8øN,346øE; 65.6øN, 344.5øE; 66øN, 339.5øE); Their spacings are24 km; grabenliketroughs are less than about 4 km wide, 63ø-66øN,330ø-357øE. Magellan imagery shows more heavily whilethe ridges and scarps are 1-3 km wide.Lengths exceed 100 floodedportions of ridged terrain than seen in the Venera data, km. Someof thesestructures extend into theplains to thenorth consistentwith thehypothesis that this unit forms the basement of the andinto the regionof fracturedterrain to theNW. entireplateau. The two largestexposures of ridgedterrain (Tr) are Orientationsof structureswithin the smallerexposures of ridged centeredat 65øN,343øE (Figure 8), and65øN, 35 IøE (Figure 9). These terrain(Tr) aredifficult to measureprecisely, but they appear to be exposurescontain three major lineament orientations, also observed similarto thoseof thestructures in thelarger exposures. in the Veneraimagery [Roberts and Head, 1990a]. Volcanismhas flooded not only the margins but also the interior of 1. The farstis NW trendingtroughs. Troughs oriented N45ø-50øW ridgedterrain. A few smallpits and a chainof pitsare observed that of variablewidth and spacing are observed in theeastern exposure maybe localvolcanic sources (e.g., 65.15øN,349.7øE). (Figure9). Broadtroughs with grabenlikemorphology are observed FracturedTerrain (TjO(Figure 8; F-MIDRPs 65N330,65N342, throughoutthe exposure,many of them extensivelyflooded by 70N339).This terrain was originally mapped from Venera imagery volcanism.They are 2-3 km wide,5-25 km apart,and up to 130km as a "groovedplains" unit by Robertsand Head [1990a]). It was in length.The maximumlengths are obscured by flooding.Several interpretedby Pronin[1986] to be a systemof faultsformed under of thesetroughs extend beyond the Tr unit boundaryinto the sur- tension:see Figure 8. The fracturedterrain, concentrated 250 km roundingplains. Near 64.2øN,351øE a troughis associatedwith a northof Sacajawea(67øN, 338.5øE), consists of linearto curvilinear Seriesof rounded"levees," similar in appearanceto spatterramparts fracturesand troughstrending along three dominant orientations: observedalong terrestrial basaltic fissure eruptions. This association N25ø-30øW,N25ø-30øE, and N0ø-5øE. These structures range in width suggeststhat some of thevolcanic resurfacing on Lakshmi may have fromless than 1 km to3 kin; 1-1.5km is typical.Their spacing varies beenfissure-fed, with fissuresemplaced along preexisting zones of from1 to 25 km,but usually it is lessthan 5 kin. Theirlengths extend weaknessin thebedrock tessera. Narrower troughs or fracturesless at least150-225 km. The boundarybetween exposures of fractured than1 km in widthare also observed. These closely spaced structures terrainand ridged terrain is transitional.There is a gradualchange are lessthan 1 km apartand form a pervasivefabric in the eastern fromthe rugged relief of thewestern large exposure of ridgedterrain exposure.East of 350øEmany of thesefractures extend into the andthe lower relief of fracturedterrain, which is essentiallycharac- surroundingplains, as do the broadertroughs mentioned above. terizedby plainscut by fracturesand troughs. KAULA ET AL.: STYLESOF DEFORMATIONIN ISI4TARTERRA 16,089

TABLE 1. G•Mom,mc Urn'rs

Name& Symbol Definition Interpretation Archetype LatitudeLongitude

Chasma (Ch) canyon,10-60 km wide,>1 km deep,> 150kin large scaleextension, sometimes 64N o, 21øE long, steepwalls andusually a roughfloor, volcanicallymodified with anastamosinglineations

hnpactcrater (Ira) circulardepression, raised hummocky rim, bolide infall 66oN, 7oE oftenmultiple if radius>50 kin; surroundedby irregularbrightness to aboutthree radii

Mountainbelt (Mb) paralleland subparallel ridges 10-30 km apart, large-scalecompression with 6z•,oN,2oE Subtypes:With veryradar-bright altemating with less bright; intensefolding & overthrusting; Deposits(MM) somepartly covered with deposit (Mbd), some chemicalenhancement of brightness 65oN, 7oE Troughs(Mbg) with rectilineartroughs (Mbg) somegravitational modification 67oN, OøE

Fracturedplains (Pf) smooth-appearingplains with, bright lineations, volcanic flows with low strain 53øN, 336øE < 0.5 km wide,up to 100km longor more fracturesor joints

Lobateplains (PI) plainswith lobatedeposits; ridges much less volcanic flows 62.5oN, 332øE abundant than in Pr

Ridgedplains (Pr) smoothplains with manyridges that have broad, mainlyvolcanic flows, followed 59øN, 350øE Subtype: domicalcross sections and crenulations along by compressionor fracturing Anastamosing(Pra) one side 6•N, 336øE

Smoothplains (Ps) smoothplains, in whichindividual flows are low-viscosity,high-volume 70øN, 334øE indistinguishable lava flows ' Subtype:Dark (Psd) 59øN, 331øE

Ridgebelts (Rb) Systemsof roughlyparallel ridges, sometimes hter compressionof pre-existing 73øN, 11øE secting,usually 20-40 km wide,50-300 km, up to terrain,,usually plains 1000 km long: radarbright against dark background

Plumoseridges (Rp) narrowlineations at 2-3 km intervalsagainst a moderateareal compression 62øN, 18oE •&rkerbackground

Calderafloor (Sp) plainswithin a caldera lava flows 64.5øN, 336øE

Chevrontessera (To) tesseracontaining ridges 5-20 km spacing,but mainlycompression, with a changing 66oN, 25øE (=Tch) containingtwo distinctdirections, intersecting stress orientation at 50-100km intervals,to makea chevronpattern

Disruptedridged manysymmetric ridges, parallel to anastomosing; ridgesby compression,troughs 58øN, 336øE terrain (Td) (=Tds) width10-20 km, length < 4 timeswidth; plus by extension. usuallysmall (<6 km) troughs,intersecting ridges at manyangles.

Fracturedterrain (Tf) complexridged terrain dominated by two or more troughsby extension;scarps en- 53øN, 336øE setsof steep-sidedlinear troughs or scarps tail strike-slip,because of offsets

Linearridged terrain manysymmetric ridges, parallel to anastomosing; compression,sometimes followed 65oN, 17oE (TI) (=Tlr=Tsr) width 10-20km, lengthwell over 100 km. by surf'fficialmodification, by Subtype:With Usuallyparalleled by smallerridges and troughs volcanicflow or impactejecta 66oN, 1 IøE deposits(Tic)

Ridgedterrain (Tr) complexstructural fabricwith two or more varyingtectonic stresses, both 65oN, 3500E directionsof lineaments;sonie with many extensionaland compressional; embaymentsby smoothermaterial. somefollowed by lava flooding.

Someof thetroughs in thefractured terrain (Tf) appearto continue Plains (Ps) (Figures 7, 8, 10, and 14; C1-MIDRPs 60N347, up to 200 km beyondthe mapped border of theunit intothe northern 75N338). The plainsof Lakshmiare remarkablysmooth and of plains,where they parallel the troughs at theNE borderof theplateau generaflyuniform moderate radar brightness. Most of themlack (Figure22). This extensionsuggests that fxactured terrain may have distinctivelobate flows. Some local regions'of brighter, lobate plains once formed a more pervasivefabric on the plateau. The part (PI) are observedto the SE and SW of Sacajawea.In the central preservedfrom flooding is a localtopographic high, like theunits of portionof the plateau(68øN, 330øE) these lobate plains perhaps are ridgedterrain. Alternatively,these unusually long troughsmay be flows associatedwith ColettePatera. Near theedges of theplateau relatedto thestresses associated with theunbuttressed plateau border (e.g.,68øN, 341øE) they are associated with fracturesthat parallel the andhence are not part of an underlyingpervasive strctural fabric. boundary.Regions of radar-dark,diffuse plains (Psd) are observed in 16,090 KAULA • AL.: STYLF.,SOF DEFORMATIONIN ISHTARTERRA

330 340 350 0 associationwith the patch of lobateplains to the SW of Sacajaweaand in thesouthern portion of theplateau, near the base of Danumontes (59.5øN,330ø336øE, Figure 14). The southernregion of radar-dark plainsappears to contain wind streaks trending NE-SW. These wind streaksmay be soils from broken-downlavas blown into lows 70 betweenridges observed in thisregion. Effusivevolcanic eruptions are thought to producethe extensive regionsof plainson Lakshmi. Recent analyses by Head and Wilson [1992]suggest that lavas erupted at thehigh elevations of Lakshmi maybe highly vesicular and porous. Such flows may weather rapidly,

- 65 producingrelatively featureless (Ps) and radar-dark (Psd) plains as seen on Lakshmi. But there is no evidence in the radar reflection and emissivityof porosityfrom suchvesiculation or from aeoliantrans- --• port[Pettengill et al., thisissue]. Hence the smoothness indicates low- viscosityflows.

t Plainson Lakshmihave been deformed locally into ridges and troughsnot only in thearea of fracturedterrain (TO, whichhas been embayedand flooded by subsequentvolcanic plains, but also along 330 340 350 0 theplateau margins. Ridged plains were identified at the periphery of theplateau near the borders of Freyja,Maxwell, andDanu montes Fig. 4. Geomorphicmap of LakshmiPlanurn east of longitude330 ø. Locationsof Figures6-11 are outlined. See Table 1 for der'tuitionsof fromthe Venera data [Roberts and Head, 1990a]. Magellandata geomorphicunits.

330 340 350 360

KEY TO STRUCTURAL SYMBOLS

RIDGE sC. ARP

TROUGH

70 BROADTROUGH•>o S•S•n 70

AND]MPACr F_JEC•ACRATER

65 65

60 60 0 300 km

I I I • t 330 34O 35O 36O Fig.5. Structuralmap of LakshmiPlanurn east of longitude330'. KAULA L• AL.' STYLES OF DEFORMATION IN ISHT•R TERR• 16,091

100km I . ...

Fig. 6. SacajeweaCaldera. Latitude 63.1ø-65.6øN, longitude 332.4ø-338.3øE-- 264 x 275 km. From F-MIDRPs65N330 and65N344. revealdetails of thesefeatures and the additional presence of troughs relaxationof the plateauedge where it is unsupportedby the North aboutthe marginsof Lakshmi (Figures10 and 11). Ridges and Basin,whereas ridges are probably related to compressionalstresses troughsare observedsouth and west of Freyja Montes, west of associated with the formation of the mountain belt. Maxwell Montes,and north of DanuMontes and the southern edge of FaintEW trendinglineaments similar to wrinkleridges in mor- theplateau phologyare observed in a zoneextending south from Freyja to about Broadarches and sinuous ridges parallel the mountainsof Freyja 69øN,between longitudes 330ø-337øE. These lineaments are charac- from wherethe range trends eastwest (e.g., 73øN, 333øE) to whereit terizedby broadarches up to 10km in widthand often have crenulated curvesmore northsouth (71.4øN, 337øE). Widths range from less than ridgecrests about 600 m wide.They are spaced about 10 km apartand 1 km to approximately3 km. Spacingvaries from 1-4km to up to 10- havelengths ranging from 50 to200 km. Similarstructures with more 20 km, decreasingtoward the mountain belt. Ridgesare up to 140km stronglydeveloped ridge crests are observed south of 69.5øN,trend- in length.The mostprominent ridges are observed where the altitude ingN30ø-45øW. The origin of thesestructures isunclear but is thought increases0.5-1.0 km towardFreyja Montes. Troughs,many with to be relatedto somedegree of crustalshortening. grabenlikemorphology, also parallel eastern Freyja and the border Faint lineamentstrending generally northsouth are observedin betweenLakshmi and the North Basin and Scarp Province (e.g., 68øN, northcentral Lakshmi (71.2øN, 33 IøE). Thesestructures are less than 341øE). These structuresare 1-2 km wide, 40-200 km long, and 1 km wide, up to 100 km long and spaced1-5 km apart.The most spaced3-10 km apart.They do not parallel northern Freyja, where the prominentlineaments appear to be troughs.Shorter segments are mountainrange trends eastwest. There is a generaltransition from oftenpartially aligned, giving the appearance of enechelon fractures. troughsnearest the edge of the plateau-NorthBasin boundaryto Theorigin of thesestructures is alsounclear. Sinuousridges parallel ridges inboardof the plateau.The troughsmay be the result of the trend of Maxwell Montes at the western base of the mountain 16,092 KAULA Er AL.' STYLES OF DEFORMATXONIN ISHTAR TERRA

Fig.7. Mottledlobate plains (P1) and ridges and scarps trending SE of SacajeweaCaldera. Latitude 63.6ø-65.2øN, longitude 337.5 ø- 340.1øE = 170 x 119 km. From F-MIDRP.65N342. KAULA Er AL.: STYLES OF DEFORMAT[ON ]N IS•'TAR TERRA 16,093

100 km

Fig. 8. Ridgedterrain (Tr) in centralLakshmi Hanum. Latitude64.9ø-67.5øN, longitude 336.O'-344.2øE = 275 x 352 km. FromF- MIDRP.65N342. range(Figure 10). Theseridges are up to 4 km wide,150-300 km long, in lengthand taper in widthto thenorth. Widthsrange from 8 km to and spaced3-10 km apart. Their spacingdecreases toward the 600 m. Most of the troughscurve to the eastnear the edgeof the mountainbelt. These ridges are generally characterized by westfacing plateau,although some curve to thewest to forma complex,intersect- frontsthat appear to be steeperthan the eastfacing fronts, suggesting ing patternof structures(e.g., 61.5øN,343øE, Figure 11). These a possiblethrust-fault origin. Ridgesoriented radial to themountain structuresare best developed within local topographic highs along the belt areobserved at 64.5øN,0øE. Theseridges are similar in appear- southeasternedge of Lakshmi. Their spacingvaries 1-10 km andis anceto theridges that parallel Maxwell; thereason for theirpeculiar the mostdense, with the narrowestwidths, between longitudes 341' orientation is not known. and 352øE (Figures 11 and 27). Troughsare widest and most Narrow, sinuousridges and lineamentstrending approximately distinctivewithin RangridFossae (62.5øN, 358øE; Figure 10), and N55øE parallel the ridgeswithin Danu Montes northwestof the were previouslyobserved in Venera imagery.The origin of these mountainbelt (Figure 14). Thesefeatures are less than 1 km wideand troughsmay be relatedto relaxationof the unsupportedplateau up to about150 km long.Spacing varies from less than 2 to approxi- marginor deformationoccurring at theplateau edge, either at Maxø mately 12 km, decreasingtoward Danu. Extremelyfaint, closely well and/or south of Lakshmi within Danu Montes or the South Basin spacedlineaments that parallel the trends of theseridges extend from andScarp Province [Ronca and Basilevsky, 1986; Vorder Bruegge et near the base of Danu Montes 200 km into Lakshmi. These features al., 1990]. Someof thesefeatures resemble lava channels and sinuous aredifficult to classifyas ridges or troughs. rillesor arecomposed of chainsof pits(e.g., 61.5øN, 340øE), suggest- Closelyspaced, linear to curvilineartroughs and lineamentsare ing an originrelated to dike intrusionand eruption of lavas. Dike mappedalong the border between Lakshmi and the South Scarp and intrusionand surface fracturing also may be thecause of someof the BasinProvince (Figure 10). Thesestructures are oriented at large othertroughs, although this possibility seems inconsistent with the anglesto theplateau edge, trending N30ø-65øW (in contrastto the directionof taper;the troughs widen to thesouth, away from Colette troughswhich parallel the northernborder of Lakshmi). These andSacajawea. troughsoccur in a regionextending from 337øEto 2øEand south of Two sizablecalderas occur along the South Scarp, the southeastern about64øN to theedge of Lakshmi(Figure 10). They are75-270 km borderof LakshmiPlanum. Siddons,at 61.5øN,341.5øE, is a nearly 16,094 KAULA ET AL.' STYLF.• OF DEFORMATION IN ISIt'rAR TERRA

Fig.9. Ridgedterrain (TO andplains (Ps) in eastemLakshmi Planum. Latitude 63.5 ø -66.1øN, longitude 347.0ø-354.5øE = 270x 337 km. From F-MIDRP.65N354. circulardepression 65 km in diameter(Figure 11). Thenortheast rim 2.2. Danu is well definedby multiplearcuate lineaments interpreted to be fault scarps;the southwestrim is notas well defined.Siddons has a dark Danu Montes lies to the south of Lakshmi Planum and is the interior from which radiate a seriesof very narrow lineaments. smallestmountain belt of IshtarTerra. The scarpof Vesta Rupes Siddonsis notassociated with anyobvious flow features.The second boundsthe westernmostedge of LakshmiPlanum as well assouth- caldera,at 61.4øN,359øE, is an ellipse40 x 20 km, with multiple western Danu Montes. Clotho Tessera is located to the southeast of concentricscarps and fractures (Figure 10). A numberof smallerpits DanuMontes. Below we describeand interpret the volcanismand are distributedabout the perimeterof this caldera,but thereare no deformationvisible in radarimages of southernLakshmi Planum, obviousflow features.Immediately to the southof thiscaldera is a Danu Montes,Clotho Tessera,and northern SednaPlanitia. Geomor- quitefeatureless drop of about2 km. phicunits for thisprovince are given in Figure13. KinematicSequence (Figure 12). Therelative ages of unitsmapped LakshmiPlanurn (Figure 14; F-MIDRP60N334). As discussed in withinLakshmi Planurn have been discussed by Robertsand Head section2.1, the portion ofLakshmi Planumjust north of Danu Montes [ 1990a].Ridged terrain (Tr) unitsappear to be the oldest and form the around60øN, 334øE is remarkablysmooth but not entirely featureless underlyingbedrock of the plateau.The formationof Sacajawea, (Figure14). Relevantto thissubsection, there are flows that both Colette,and extensive plains units and the deformation of theseplains coverand are cutby very narrow,anastomosing lineaments which unitsinto fracturedterrain and ridgedplains appear to have over- roughlyparallel ridges in westernDanu Montes. These lineaments lappedin time. Thereappear to havebeen multiple stages of plains areup to 200 km in lengthand are spaced at 3-10 km (upperright of formation,deformation, and burial. Plains were emplacedand then Figure14). deformedinto ridges, fractures and troughs. Subsequent volcanism Danu Montes(Figure 14; F-MIDRPs 60N334, 60N344). Danu andplains formation partially embayed and flooded these structures Montesconsists of a seriesof peakswhich rise up to 2 km above (such as Tf). Continueddeformation about the plateaumargins LakshmiPlanum and drop steeplyto the south,grading into Vesta producedadditional ridges and troughs. Rupesand Clotho Tessera. The peaksare very radar-bright, probably KAULA ET AL.: STYLES OF DEFORMATION IN ISHTAR TERRA 16,095

Fig.10. RangridFossae, troughs in southeasternLakshmi Planum, and smooth plains (P1); also the South Scarp. Latitude 60.7ø-64.0øN, longitude354.0ø-3.0øE = 350 x 450 km. FromC1-MIDRP.60N347. due to a combinationof roughnessand altitude-relatedchanges in paired,forming narrow graben. Several longer scarps (about 100 km) dielectricconstant[Ty ler et al., 1991;Klose et al.,1992 ]. Thepeaks with elevationsof approximatelyseveral hundred meters parallel the consistof broad,curved, linear to lense-shapedridges. Lensesare smallerfaults. Onelarger graben, 2 km wideand about 100 km long definedby either disruptedzones (bright, diffuse lineaments)or is centeredat 58.6øN, 330øE.The trendof thesefaults is parallelto narrow,dark lineaments. The trendof theridges curves from WNW- and,in onecase, collinear with severalpit chainswhich cut across ESE in westernDanu Montesto NE-SW to the east. Lens-shaped Clotho Tessera and Danu Montes. A local., smaller set of NE-SW ridgesare foundat the intersectionof thetwo trends,around 58.5øN, trendingfaults cut across the primary fault set. Thissecondary set of 334øE,with moreelongated ridges to eitherside (Figure 14). These faultsis similar in dimensionto thesmall, numerous NW-SE trending lensesrange in sizefrom 5 by 15 km to 10 by 30 km. Thereare also features. In addition, near 58.2øN, 333.2øE there are two NNE-SSW elongatedridges typically 5 km in width andup to 100 km in length trendinggraben, about 3 by 20 km. A degraded[Schaber et al., this e.g.,at 60.5øN, 337øE.To thenortheast, the ridges are cut by both issue]5 km impactcrater, Kartini (57.8øN,333øE; Figure 14), is EW-trendingpit chainsup to 100 km long and NW-SE trending locatedat the intersectionof Vesta Rupesand ClothoTessera: a troughs,with outflow channels,interpreted to be lava channels, remarkablecoincidence with a changein tectonictrends. approximately50 km long[Solomon et al., 1991]. Severalpits, 1-5 ClothoTessera (Figures 14 and15; F-MIDRPs 60N334,60N344, km in diameter,are foundin the range. Thesepits are probably 55N337). ClothoTessera is southeastof Danu Montesand is broader volcanicin origin:they do nothave ejecta deposits and often occur in andgentler in slopethan Vesta Rupes. It containstwo regionswith a line. Possiblythey are magmadrains back into a dike below distinctmorphologies. In southernClotho Tessera, one linear trend [Gundmanson,1987; Burt and Head, 1991]. is morepronounced than any other trend. This region is classifiedas VestaRupes (Figure 14; F-MIDRP 60N334). VestaRupes is the ridgedterrain (Tr, Figure 13). Justto the southof the lens-shaped steepslope southwest of DanuMontes. Within easternVesta Rupes, ridgesin Danu Montes,Clotho Tessera is highlydisrupted, with no thedominant feature is a setof NWW-SEE trendingfaults spaced at dominanttrend. This area is mappedas disrupted terrain (Td, Figure 0.5-2km withlengths of tensof kilometers.Several of thesefaults are 13). In thisregion, there is somecontinuation of theNW-SE andNE- !6,096 KAULA E'F AL.' STYLES OF DEFORMATIONIN ISHTAR TERRA

Fig. 11. SiddonsPaters and lineaments in southernLakshmi Planurn. Latitude 60.6ø.62.5øN, longitude 338.8ø-345.0øE = 200 x :312 kin. From F-MIDRP.60N344.

328.5E 345.2E Sequence of Events: Lakshmi Planurn 62.1N

Ridged Plains Fractured SacaJawea Ridgedand Terrain Terrain Caldera FracturedPlains Pra youngest eee©eeeeee• Td

Mb/ [m 328.0E

[m oldest T Tr / I Fig. 12. Relativeages of featuresin LakshmiPlanum.

Pi Fig. 13. Geomorphicmap of DanuProvince east of 3300E. Locationsof figures14-15 are outlined. See Table 1 for definitionsof geomorphicunits. The symbolsaround 65.7øN, 339.6øE indicate troughs 0.6-6 km wide;the dottedline at 60.1øN,328.2ø-335.5øE is the northernboundary of the very 52.3N radar-darkplains; the arrows indicate lineament directions. 332.3E 345 5E KAULA ET AL.' STYLES OF DEFORMATION IN ISHTAR TERRA 16,097

Fig. 14. CentralDanu Montes and Vesta Rupes. Portionsof unitsPs, Pra, Mb, andIm arevisible. Latitude 57.5ø-59.7øN, longitude 330.0ø-335.4øE = 232 x 297 km. From F-MIDRP.60N334.

SW trends found in Danu Montes, but lineaments also occur in a embayedby smoothmaterial., probably lava flows. The ridges varietyof otherorientations. Lineaments are fairly short(<25 km) themselvesform a gridin whichNS-trending ridges are largely cut by andtend to be curvedand diffuse in theirradar brightness. The only EW fractures,and the EW ridgesare largely cut by NE-SW fractures. undisruptedfeatures are several long (up to 100km),NW-SE trending The widthof theseridges are commonly 20-40 km, andthe spacing pit chains,e.g., at 58.4øN,335øE (Figure 14). Numerousdark patches betweenthem is approximately25 km. The natureof theNS andNE- suggestthat local volcanism has flooded parts of theregion. The 20- SW trendsis difficultto determinebecause their interaction disrupts km impactcrater Magnani, 58.6øN, 337.2øE (Figures 1 and 2), hasan the pattern. In somecases these lineaments are narrowand fairly interiorcovered with smooth,bright deposits. Its ejectablanket is straight,occasionally occurring in pairs, suggestingthat they are disrupted,more likely by emplacementon rough terrain than by simplefractures or normalfaults; in otherregions they appear to be subsequentdeformation, since the floor andwalls are undeformed. curvedand domed, suggesting that they result from shortening. In southernClotho Tessera, the fine-scale fractureswhich cut the Northern SednaPlanitia (Cl-MIDRPs 60N319, 60N347). The plainsto thesouth (see plains) continue into the tessera and cut all of plainsimmediately south of ClothoTessera have a densepattern of the older lineaments. In the tessera,their orientation is WNW-ESE. radar-brightlineaments which appear to be fractures.The average In manyplaces, lows parallel to thesefractures seem to be filledwith trendof the lineamentsis EW, althoughsets thereof are slightly volcanicflows. Numerouspits are also visible. A NS trendthat also curved. The lineamentsare long (60-100 km) andnarrow (0.1-0.5 consistsof a seriesof closelyspaced fractures underlies the EW trend. km). In a few cases,they occur in pairs,spaced at 0-3 km. In a few Near the transitionbetween the ridgedterrain and the plains,NE locations(e.g., 53.5øN, 336øE)NS to NE-SW trendsare alsovisible. trendingfractures also occur (Figures 13 and 15). In southwestern Theselineaments are clearly cross cut by theEW lineaments.Smooth Clotho Tessera, an elevated area of tessera,about 1 km above the plains,looking like volcanicflows, are abundantin the region,and surroundings,centered at 54.1øNand 332.5øE contains primarily NS bothembay and are cut by the lineaments. fractures,along with other fractureswhich roughlyparallel the Interpretation.In additionto the mantle upwelling anddownwelling topographiccontours. hypothesessuggested for theentire Ishtar Terra region [Pronin, 1986; Withinlongitudes 337ø-343øE, southern Clotho Tessera consists of Bindschadleret al., 1990; Grimmand Phillips, 1990, 1991],Danu a seriesof ridgescut by NS andNE-SW trendingfractures which are Monteswas suggestedto have formedin responseto northward 16,098 KAULA EF AL.' STYLES OF DEFORMATIONIN ISHTAR TERRA

''.•' •: ...• .Z•:-,•.•- ':?.... ::F'"5' .... ,,•., -"•-5.:•.•..•."••i'•-;•,•-•.,....•..•...%.,:,•

I 100 km

Fig. 15. CentralClotho Tessera. UnitsTr andPf areillustrated. Latitude 54.5ø-57.3øN, longitude 334.4ø-339.9øE - 296 x 326 km. From F-MIDRP.55N337.

subductionof thecrust and lithosphere below Laksh•ni Planurn [Janle ridgesin westernDanu Montes (near 60.5øN, 339øE) are cut by several and Jannsen,1984; Head et al., 1990]. NW-SEtrending pit chains,certainly volcanic, implying minor exten- The relativelylow relief of DanuMontes suggests that either this sion within the convergentregime [Burt and Head, 1991]. The areahas undergone much less crustal shortening than other mountain southernend of thelarge graben located at 60øN,338øE is overprinted belts in Ishtar Terra, or it is more extensionallyrelaxed. The by NE-SW trendingridges, implying (but not requiting) that exten- curved,diffuse,and shorterridges in Danu Montes appearmore sionnear the peaks occurred before the stresses building the mountain similarto thoseof FreyjaMontes than to thesharply linear ridges of beltscompletely ceased. Such extension could be the result of release Maxwell Montes. There are severalpossible explanations for the of stressdue to topographicrelief [Solomonet aL, 1991]. Severalpit curvedmorphology of the ridges. The curvilinearridges may be a chainsthat strike EW are collinearwith someof the scarpsin Vesta resultof the interactionof a regionalstress and either a preexisting Rupes,suggesting that thesescarps are mosteasily interpreted as tectonicfabric or simplyvariations in localtopographic relief in the normalfaults. The narrowspacing of faultsand grabenin Vesta form of theedge of theplateau. Alternatively,the ridges may have Rupesand their orientationperpendicular to the steeptopographic beenoriginally linear and were subsequentlydeformed into their slopesuggest that they may be a resultof extensionabove a shallow currentshape. As in Maxwell,Freyja, and Akna montes [Solomon et crustaldetachment [Solomon et al., 1991]. The presenceof exten- al.,thisissue], deformation at Danu Montesseems to be movingout sionalfeatures at fight anglesto thetopographic slope in areasof high onto the plains ofLakshmi Planum. Several ofthe long, domed ridges reliefsuggests gravitational spreading [Solomonet al., 1991;Srnrekar whichborder the plains appear to be thrustsverging NW, asthey are and Solomon,this issue;Burt and Head, 1991]. brighteron theNW sideand have ends which curve back to the SE. In the subparallelridge terrain (T1) of Clotho Tessera,the Thenarrow, NE-SW trending,anastomosing ridges in theplains to the youngesttectonic trend appears to be the approximatelyEW setof northof DanuMontes are approximately parallel to the ridges in Danu lineamentswhich cut the plains and southeastern Clotho Tessera. The Montesand may be a resultof the sametectonic event. narrow width, great length, and apparentlack of relief of these Numerous,low-strain extensional features appear to be approxi- lineamentsimply thatthey are fxactures.In a few cases,such as at matelycontemporaneous with shorteningin Danu Montes. The 53øN,336øE, the fractures appear to be graben sets, suggesting that all KAULA ET AL.: STYLES OF DEFORMATION IN Ish•rAR TERRA 16,099

Sequence of Events: Danu Montes

Plains Ridgesin Lens-shaped NE Pits and NWtroughs, Plains E-W to NE N-S deposits, LakshmiPI. ridges,Danu ridges, channels, VestaR. deposits, lineations, lineations, Lakshmi PI. M. Danu M. Danu M. Sedna PI. Sedna PI. and Clotho Ts. youngest Clotho Ts.

I I I oldest

Fig.16. Relativeages of featuresin DanuProvince. Question marks separate upland events (in Lakshmi Planum and Danu Montes) fromlowland events (in ClothoTessera and Sedna Planitia), because of theuncertainty in relatingevents between these two areas. ofthefracture may have formed in a tensionalregime. These fxactures 330 340 35o o lO cutthe NS trend which extends into northeastern Clotho Tessera. The 80 I I I I 8o nature of the NS trend is uncertain. Kinematicsequence. Age relationshipsare inferredfxom the describedsuperpositions of setsof features;see Figure 16. Local Pf extensionaldeformation, in theform ofpit chainsand graben, in Danu Montes and the disruptedterrain portion of Clotho Tesseraare I interpretedas occurring concurrently with the shortening or asinter- spersedbetween successive episodes of shortening. 75-- ...... '7,5 An obviousquestion is whether or notthe trends observed in Clotho Tessera are related to those in Danu Montes. In western Clotho Tessera,the EW fractureset is orientedmore WNW-ESE, approxi- , it • TfPf matelyparallel to faults in VestaRupes.In easternClotho Tessera, the NE-SW trendparallels ridges in DanuMontes, as do ridges in a small regionof ridgebelts to thesouth. Unfortunately, the disrupted terrain F' anda regionof volcanicplains separate trends in thenorthern part of 7O- 7O theregion fxom those to thesouth, making it impossibleto seea clear relationship.As a result,it is impossibleto determinethe relative age of Danu Monteswith respectto ClothoTessera. In summary,none of thepreviously proposed hypotheses for the originof DanuMontes predicts neatly the observed pattern of deformation. The apparentthrusting of Danu Montes over Lakshmi ?lanum seems more consistent with northward subduction, downwelling,or a combinationof the two thanwith upwelling. 65 I i I However,other tectonic patterns in theregion do notappear to be 340 350 o simplyrelated to theseprocesses. It is notstraightforward to predict Fig.17. Geomorphicmap of Freyja and North Scarp and Basin Provinces east the surfacedeformation at a subductionzone where there is no water, of 330•E. Locationsof Figures19-20 and 22-24 are outlined. See Table 1 for sediments,or erosion.Further study of thetectonics and kinematics definitionsof geomorphicunits. onboth a regionaland local scale combined with modeling may help clarify theorigin of Danu Montes. Montes,which trends EW and(2) EasternFreyja Montes, which trendsNS. Althoughthe high topography ofthe eastern portion of the 2.3. Freyja mountainbelt dies out south of 72øN,NS-striking structures continue severalhundred kilometers. This structuralextension of Freyja Includedin this physiographicprovince are the mountainbelt includesboth part ofLakshmi Planum and lower-lying regions to the FreyjaMontes and its outboardplateau Iztpapalotl Tessera. Rising east,in theNorth Scarp and Basin Province. Similarly, EW ridgeson more than 6 km above MPR and 3 km above Lakshmi Planurn and LakshmiPlanurn south of centralFreyja Montes also indicate a ItzpapalotlTessera, Freyja Montes is a structurallycomplex moun- structuralwidth to the Freyjadeformation zone greater than the tainbelt bounded by slopesas steep as 10 ø over a fewkilometers. We breadth of the mountains. Therefore both central and eastern arms divide Freyja Montes into two major belts on the basisof the definestructural zones about 300 km in width,in contrastto the about orientationof structuralfabric and topography: (1) CentralFreyja 200-kmwidth of themountain belt itself. Figure 17 is a geomorphic 16,100 KAULA ET AL.' STYLESOF DEFORMATIONIN ISHTARTERRA

20 40

Plains

ß 80N :;i';' "

Fortuna

Maxwell

, ,, !'?.?.... : •,,, ',; 75N penetrativefabrics

(ridges?) ...... \ ! :...... ,,; ,, ', ',,,'

if i1. ' '.• . .' , %. throughgoing .'"":i".:'." ' '•':""'"' "":• '•',L "•'•;'"i""', "" ' lineaments ....,..•. •,\ ' ,, . ",, ...... • ..,:"..,.".i:!•!:.i...... ::...... :.... -" ,' ',; ,, ß,,,,-,,,,,, %,,,-..-,-,,, ; ',',, ". troughs :....'::.':'?if:::;::'"" '"",,,,,,, ridges /,. ,:,:;.;,,'.; ,, , $ • 0 VLH1991

Fig.18. StructuralMap of EastemFreyja Montes, ltzpapalotl Tessera, and North Scarp and Basin. unitmap, and Figure 18 is a structuralmap of thisnorth side of Ishtar lentto the south, projecting out of the mountains and on and just above Terra 330øE-5øE. thescarp bounding Lakshmi Planurn (71øN, 339øE to 67øN,342øE). CentralFreyja Montes(Figure 19; F-MIDRP.75N332). This Ridgesare found on both sides of thistrough zone: widely spaced regionis dominatedby EW-trending,crosscutting bright and dark ridgesoccur farther westward into the plateau (e.g, 69øN, 338øE) and lineaments:the"banded terrain" originally seen by Earth-based radar numeroussharp asymmetric ridges dominate to theeast at thelower [Campbellet al., 1983;Solomon and Head, 1984]. Oftenbands can elevationsof theNorth Basin (70øN, 341øE; Figure 22). Theselatter be correlatedwith relativelybroad wavelength (10-20 km) anasto- ridgesin particularappear westward vergent, in contrastto the mosingridges trendEW and dominatethe EW structuralfabric. symmetricappearance ofridges in central Freyja. Note, however, that Troughs(25-100 km long, 1-5+ km wide) parallel the ridges; they are thiseffect could be due solelyto radarforeshortening; apparent bestdeveloped in themiddle of centralFreyja (73.5øN, 333øE) and displacementswould then indicate ridge heights of severalhundred correspondwith the highest region of themountain range. A second meters.As in central Freyja, a secondaryset of troughsis also present setof troughsis developedin theNW partof centralFreyja (74øN, in easternFreyja. Two setsof troughs,WNW- andNNE trending, 332øE)which are thinner, shorter, and trend NW, roughlyparallel to intersectin a NS-elongatelow oval rise at 72øN, 343øE. The region troughswhich comprise amajor fabric element in the inside (Lakshmi) of intersectingtroughs correlates, atleast in part, with a topographic cornerof thecentral-eastern Freyja junction. The troughs in central high;its southernhalf is boundedby slopesof 5ø-10% Freyjaappear locally to crosscut, and be crosscut by, the EW ridges. FreyjaMontes Bend (Figure 19; F-MIDRP.75N332).The central They alsoshow numerous small offsets and curvatures, but we are andeastern arms of FreyjaMontes are separated by a 100-kmwide unableto distinguishwhether these latter features are due to structure transitionzone centeredaround 74øN, 336øE. This subdomainlies or artifacts due to local relief. acrossa topographicsaddle (up EW anddown NS) andappears to be EasternFreyja Montes (F-MIDRPs 75N332, 75N351, 70N339). a zoneof structuraloverprinting or culmination(i.e., the point away EasternFreyja is similarly dominated by broad anastomosing ridges; fromwhich folds plunge). In thenorthern part of thiszone, west- herethe ridge orientation is NS. This fabricis truncatedto the north vergentapexes of cone-shapedridges may represent refolded folds byItzpapalotl Tessera, near 75øN. NW-trending troughs cross cut the resultingfrom the interferenceof EW andNS trendingridges in ridges(e.g., 72.5øN, 339øE). Ridge-parallel troughs are more preva- centraland eastern Freyja, respectively. The lengths (tens ofkilome- KAULA Er AL.' STYLES OF DF20•TiON IN ISHTAR TER]• 16,101

L ß 100 km

Fig. 19. CentralFreyja Montes. Latitude 72.8ø-75.0•N, longitude 330.0•-340.1øE = 232 x 296 km. The elevationchanges from 4.5 km in LakshmiPlanum at the southern edge of thepicture, to over6.0 km aboveMPR atthe crest, latitude 74øN, and then down to 4.0 km at thenorthern edge of thepicture. From F.MIDRP.75N332. ters)of the individuallineaments rule out layoveras the principal floodedlows trending WNW (e.g.,75.5øN, 340øE); these features are causeof the apex-to-the-westappearance. separatedby severaltens of kilometers.The penetrativeridge fabric The structuralfabrics of bothcentral and eastern Freyja fade out is apparentlyrotated into parallelismwith the throughgoinglinea- towardLakshmi Planum. The southernpart of thetransition zone is ments(75.5øN, 343øE); see Figure 21. The spatialdevelopment of smoothand gradational:both ridgesand troughsshow a gradual thesetwo families of structuresdefines a regionallyconsistent changein orientationfrom EW to NS, parallelto the trendsof their asymmetricfabric geometry which defines the Itzpapalotl domain. A respectivebelts and the boundary with LakshmiPlanum. third set of structures,locally developedtroughs, completesthe Westernltzpapalotl Tessera(Figure 20; F-MIDRPs 75N332, structuralfabric of thisdomain. These troughs are 5-20 km long and 75N351; C1-MIDRP.75N338). Formingthe northernmostgeomor- about2 km wide. They developedperpendicular to thepenetrative phic unit of westernIshtar Terra, ItzpapalotlTessera is topographi- ridgefabric and are best observed in westernmostItzpapalotl (76øN, cally and structurallydivided into two regions;the boundaryis 333øE;Figure 20). transitionalbut centeredat about345øE. The westernportion is Thetransition zone to easternItzpapalotl begins near 340øE, where boundedto the northby the steepestpart of UorsarRupes, where thepenetrative NNW fabricis obscuredby a rougherNS fabricthat slopesare 10ø-20ø. Interiorslopes, however, are small, less than 2 ø . is part of the northernmoststructural expression of FreyjaMontes. The scarpitseft showsa numberof Z-shapedoffsets, near 77øN, The WNW throughgoinglineaments generally truncate the NNW 340øE,that become more closely spaced to the west,and individual fabric, however. Across the width of the transition zone, these structuresalong the scarp(such as the basalwrinkle ridges) show lineamentsare continuously displaced 50-100 km to the south.The similaroffsets (e.g., 77øN,339øE). A singlestructural fabric com- transitionzone is alsomarked by a changein topographyas well as prisedof twoprincipal sets of structuresdominates western Itzpapalotl. structuralfabric. The northernmostextension of Freyja,marked by a A highly penetrativefabric, perhapstightly spacedridges, trends broadrise at 344øEabove elevation 3.0 km, falls off to theeast by 1 NNW; individualelements are spacedless than 1 km apart. This km on a 2ø-5ø slope. To the north, Uorsar Rupes is no longer fabricoccurs mainly in theinterior of radar-brightblocks (e.g., 76øN, distinguishableby 353øEas its slopehas fallen to lessthan 2 ø. 339øE). Separatingthese blocks are throughgoinglineaments or Eastern Itzpapalotl Tessera (F-MIDRP 75N351; C1-MIDRP 16,102 KAULA ET AL.' STYLESOF DEFORMATIONIN IShtAR TERRA

100 km

Fig. 20. WesternItzpapalotl Tessera. Latitude 75.1ø-77.5øN, longitude 330.6ø-343.6øE = 253 x 325 km. FromF-MIDRP.75N332.

75N338). Thisregion is characterized by larger(100x300 km) tessera blocksstanding higher than the surroundingnorthern basin; parts of theseblocks lie at heightssimilar (2.5-3.0 km) to thoseof the more uniformlyelevated western subdomain. Structural trends here are variable; NNW orientationsare visible in the north and southcenter, but northto NNE structuresoccur in the$W andan orthogonal ENE! NNW patternis seenin the SE. This orthogonalpattern is similarto structuresobserved elsewhere on Venus(e.g., Laima Tessera)and appearsto havebeen overprinted by NNW-trendingstructures. Interpretation.Freyja Montes,together with the othermountain belts surroundingLakshmi Planum,represents some of the most severecrustal shortening observed on Venus [Head, 1990]. The principalmanifestations of thisshortening are the high topography of the mountainranges and the ridgeswithin them. Ridge-parallel troughsmay have formed as a result of high-levelextension of shortenedcrust. Troughs south of FreyjaMontes are probably related to steepslopes of theNorth Scarp.In southcentralFreyja and eastern Fig.21. Kinematicinterpretation of westemItzapaplotl stmctures illustrating Freyja, troughsgenerally parallel the outlineof LakshmiPlanum, sinistralnoncoaxial deformation. Curved lines representthe traceof the suggestinga structuralcontrol due to lithosphericheterogeneity. penetrativeridge fabric, heavy lines mark throughgoing lineaments, and sho•t Althoughnumerous crosscutting relationships exist, we areunable to grey lines mark the trend of troughs. Strainellipses with principalaxes ilustratethe average strain associated with the structures.Small arrows show assigna consistentoffset or senseof shearwithin Freyja [cf. Solomon displacementalong throughgoing lineaments, large arrowsshow bulk dis- et al. 1991]. If ridge-paralleltroughs formed as a resultof earlier placementdirection. The family of structuresmay be kinematically akin toS- crustalshortening, then ridge development,topography, and cross- C structures[Berthe eta!., 1979; Hansen, 1992]. KAULA Er AL.: STYLES OF DEFORMATION IN ISHTAR TERRA 16,103 cuttingrelations may indicate temporal relations for differentparts of 65N342,65N354). The Central North Basin contains a nearlycircular Freyja. Southeasternand eastern Freyja may have formed early with topographic depression 500 km in diametercentered near 68øN, 348øE ridge and boundaryparallel troughs reflecting advanced states of standingabout 2.8 km aboveMPR and 1.5 km lowerthan adjacent postshorteningcollapse. The comerof Freyja(transition zone) may LakshmiPlanum. It hassome of thefeatures of a corona[Stofan et al., have formednext with ridge formationearlier than NW-striking this issue]. The depressionis structurallybounded to the eastby ridge-paralleltroughs. Locally, NW troughsappear to truncateNS discontinuousgrooved units of intermediateradar backscatter. The structuresof East Freyja. Eastwestboundary-parallel ridges of groovesappear to be roughly radial and concentricto the basin centralFreyja may have formed later, folding previously formed NW- outline; however, some NW-SE and NE-SW structural control is trendingtroughs of the transitionzone. The relativedearth of EW- evidentin the southernand northern parts of thebasin, respectively. ridgeparallel troughs in centralFreyja may reflect a lessmature stage A secondsmaller partial arc, about 150 km across,lies at thecenter of in thecrustal shortening in thisregion. Magellani•nages of western thebasin (68.5øN, 348øE). Although individual grooved units appear Freyja may be usedto test this hypothesisof progressiveFreyja to have low positiverelief, emphasizedby plainsembayment, the deformationfrom eastto central. We expectto seea lack of NW- centersof unitsto the northand NW arelower than the basinproper trendingtroughs and good d.evelopment of EW-trendingridges in and slopeshere are 20-5ø. The larger,northern depression contains westernFreyja. arcuatetroughs. The southernmostgrooved unit (67.5øN, 353øE) lies Thefamily of structuresin westernItzpapalotl Tessera, consisting on the northwestslope of Maxwell Montes. of penetrativeridges (?), throughgoing lineaments (apparently mark- EasternNorth Basin and Maxwell Slope(Figure 24; F-MIDRP ing local topographiclows), and the perpendiculartroughs, are 70N353). The easternmostcomer of thisregion grades into the north interpretedas the result of distributednoncoaxial sinistral shear across slopeof Maxwell Montes,described in section2.6. Here thediscus- Itzpapalofl[e.g., Berthe et al., 1979;Corsini et al., 1991;Shirnarnoto, sion is confined to elevations below 5.0 km. 1989]. The undulationsin UorsarRupes, if notaltimeter artifacts, are A NW-slopingzone of severedistortion runs from 67øN,350øE to also consistent with left-lateral shear with a westward increase in 69øN,2øE, widening to >100 km around68øN, 358øE. It is predomi- intensity.North-trending structures of Freyjato thesouth appear to nanflyradar-dark, with its mostmarked lineaments running SW-NE. be overprintedby, and locally rotatedinto parallelismwith, the Theselineaments are spaced 3-5 km apartand have a rough,ropey Itzpapalotlfabric, and hencedeformation of northernFreyja struc- appearance.In a few places,they are interrupted by moreNS trends, turesis viewedas dominantlyolder thanItzpapalotl deformation, particularlyat 68.2øN, 358øE, where the predominant lineaments bend althoughthe structural domains of thesetwo regionsare mostly well 90ø arounda brighterapex. North of thiszone, there are embayed separated. Furthermore,the superpositionof compositesinistral tesserae,with a finer NW-SE trendcutting across the SW-NE. The structureson the orthogonal fabric of easternItzpapalotl suggests two regionof lowestelevation, 2.5-3.0 km, from 70.2øN,355øE to 71øN, deformationevents, the former responsible for theoriginal formation 2øE,is verysmooth and radar-dark. The higherterrain north of 71øN of the tessera and the latter a left-lateral shear across the northern has a much more marked tessera structure. boundaryof westernIshtar Terra. Interpretation.The throughgoinglineaments and ridges trending In summary,preorogenic deformation included the formationof NE-SW couldbe thrustfaults, either due to gravitysliding or to tessera,relict structures of whichcan be foundin easternItzpapalotl, "extrusion"perpendicular to the principalaxis of Maxwell shorten- andthe Corona region (69øN, 349øE; Figure 23). Freyjaand Maxwell ing. Alternatively,these lineaments could be shearzones; the orien- Montesformed in majororogenies, perhaps contemporaneously, in a tationsof subordinatestructures, if not artifactsof radar foreshorten- bulk strainregime with principalaxes of shorteningprobably con- ingon the steep slope, then indicate right-lateral displacement [Vorder trolled by preexistinglithospheric heterogeneity associated with Brueggeet al., 1990;Hansen, 1992]. LakshmiPlanum. Central Freyja appears to postdateEastern Freyja. The partialradial and concentric groove pattern suggests a corona Left-lateralshear dominates West Itzpapalotland appears to be the [Stofanet al., this issue]formed contemporaneously with plains youngeststructural style in theregion. volcanism(embayments appear to do•ninateto the south,whereas An agesequence joint with theNorth Scarp and Basin Province is fracturesform on theplains to thenorth). The coronaappears to be givenat theend of section2.4. older than Maxwell Montes,because it is embayedby smooth materialoverthrust by Maxwell, but clear indicators of itsage relative 2.4. North Scarpand Basin to Freyjaare lacking. Greater age is alsoconsistent with itsapparent lack of dynamicalrelation to the structuraldevelopment of either This provinceis lessmarked in topographicvariation thanthe Maxwell or Freyja. mountainbelts to thewest or east,but nonetheless has strong struc- Kinematic Sequence (Figure 25). Thestructural trends of Itzpapalofl turaltrends. We divideit intowestern, central., and eastern regions. clearly postdatethose associated with centraland easternFreyja WesternNorth Scarp and Basin (Figure 22; F-MIDRP.70N339). Montes(Figure 18). The trendsof easternFreyja Montes appear to The400-1cm portion of theNorth Scarp running roughly NS at340øE, parallelthose of Maxwell Montes(Figure 1), suggestingcontempo- from71øN to 67øN,is remarkably smooth, despite a topographicdrop rary formation,as mentionedabove. Thusfrom the imageryof the of morethan 1.0 km. All alongthis boundary are fine troughs,less areanorth of LakshmiPlanum alone, the sequence given in Figure25 than2 km wide,with lengthsof 50-100km andspacings of 5-20 kin. is indicated.However, eastern Freyja Montes and the NNW trending In places,there are broader down slope troughs, most notably around regionof Maxwell Montesare morethan 600 km apart,and other 68.5øN,342øE. Downhill from thisscarp, centered at 70øN,341øE, factorsdiscussed below suggest that Maxwell Montes is more recent. thereis a complexof bright,anastomosing lines. These lines become straighterto thesouth. The mostrecent, crosscutting other features, 2.5. SouthScarp and Basin runs280 km, straightas a tautcable, from 70. IøN, 340.2øEto 68.IøN, 344.5øE,where it endsabruptly in a T-shapedintersection with a The SouthScarp and Basin Province has largely been ignored in lineament15 km long. Most of thislength it is hardly1 km wide. examinationsof IshtarTerra based upon Arecibo and Venera 15/16 CentralNorth Basin and Coronaa (Figure 23; F-MIDRPs70N353, data,and thusthe major topographicand structural features of the 16,104 KAULA ET AL.' STYLES OF DEFORMATION IN ISHTAR TERRA

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Fig.22. WestemNorth Scarp and Basin. Latitude 68.1ø-71.2øN, longitude 338.0ø-344.5øE = 327 x 250km. FromF-MIDRP.70N339. region are unnamed.The most topographicallydistinct features eastwesttrending South Scarp and their surfacesare dominatedby (Figures3 and26) consistof the following: plains.A third,less topographically distinct basin is centeredat 6 IøN, 1. The SouthScarp which separatesLakshmi Planum from the 359øEin theplains south of SouthScarp near a setof NE-SW trending lowlandsto thesouth. This scarpcan be consideredcontinuous with ridgesin theplains and is referredto asthe East Basin. Vesta Rupes;however, because of the interruptionby the SWNE 3. A pairof topographichighs extend south from South Scarp. The trendingportion of DanuMontes and Clotho Tessera, 335ø-342øE, we WestRidge (343øE)separates Clotho Tessera from the West Basin adoptthe separatedesignation of SouthScarp (until an appropriate andthe Central Ridge (350øE) separates the West and Central basins. goddessof hearthand home is found). Bothare characterized by thepresence of complexridged terrain. 2. Two basinsare clearly visible in both the topographyand 4. A discontinuousset of topographichighs run approximately imagery. They are referredto as the West and Central basins, parallelto SouthScarp, approximately 500 km to the south.These respectively.They appearin imagesas two scallopsin an otherwise constitutethe Outer Rise (58øN) and are located in approximatelythe KAULA Er AL.: STYLES OF DEFORMATION IN ISHTAR T• 16 105

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Fig.23. CentralNorth Basin and Corona. Latitude 67.6ø-70.42øN,longitude 346.2ø-353.4øE = 296 x 271km. FromF-MIDRP.70N353. sameposition relative to SouthScarp as Ut Rupes(55øN, 322øE) with 10ø),then widensby more than a factor of 2 until it approaches respectto VestaRupes. Maxwell Montes.From the West Basin through the Central Basin, the The geomorphicunits used in thisdiscussion are given in Table 1. topographicscarp coincides with disruptedterrain (Td). As manyas The linear ridgedterrain designated TI thereinis the sameas the threedistinct structural trends are present within the Td alongSouth subparallelridged terrain Tsr of Bindschadlerand Head [ 1991]. Scarp.The first is a setof gentlysloping ridges which parallel the SouthScarp (Figures 10,11 and27; F-MIDRPs65N342, 65N354, scarp. They are mostclearly visible along the northwestern(near 60N344,60N355, 60N005). The scarpseparating Lakshmi Planum 61øN,344øE, Figure 11) and northeasternedges of the West Basin from the lowlandsto the southis 2-3 km high.It is steepestin the (63øN,349øE, Figure 27). The ridgesappear approximately symmet- vicinityof theWest Basin (Figures 11 and 27), with slopesapproach- ric and tend to exhibit cuspateshapes. They are interpretedas ing 10ø over a horizontaldistance of 25 km, widenssomewhat near the compressionalstructures and may be thrustfaults. The secondand CentralRidge, narrows again near the Central Basin (slopes more than third setsof structuresconsist of steep-sidedvalleys which either 16,106 KAULAET AL.' STYLF_.•OF DEFORMATIONIN ISHTARTERRA

Fig. 24. EastemNorth Basin. Latitude 67.4ø-70.5øN, longitude 353.8ø-0.4øE = 327 x 250 km. FromF-MIDRP.70N353. trendparallel or perpendicular tothe strike of SouthScarp (Figures 10 NE-SW orientationbrings the scarp into alignment with a setof steep- and11). In somecases, these steep-sided troughs are collinear with or walled troughsobserved on LakshmiPlanurn (Figures 10 and 27), lie parallelto steep-sided,flat-floored troughs with roundedends and which includeRangrid Fossae. Most scarp-perpendiculartroughs pit chains.These latter features are interpretedas collapsefeatures alsoalign with this regional trend. Thus, most troughs appear to reflect associatedwith movementof subsurfacemagma. regionalNE-SW extension,while the lesscommon scarp-parallel Structuresare most prominent in theportion of SouthScarp that lies troughsgenerally reflect a localdeformation associated with thesteep above the West Basin. Here, broad ridgesare clearly the oldest slopesof SouthScarp. Such a hypothesisexplains the complex pattern structuresalong the scarp and are extensively crosscut by extensional of curvilineartroughs near 6 IøN, 343øE(Figure 11). In this area,the featureswhich bothparallel and trendat high anglesto the scarp. horthwesternmostscarp-parallel troughs curve away from the scarp Troughsparallel the scarptend to be prominentwhere slopesare until they are approximatelyaligned with the moreregional set of •teepestor where the trend of thescarp is approximately NW-SE. This features.Crosscutting relationships between sets of troughsare com- KAULA ET AL.' STYLES OF DEFORMATION IN ISHTAR TERRA 16,107

Sequence of Events: Freyja Montes & N. Scarp and Basin / I i I ! • 65 ø Tr Tr Tr

East Corona in Maxwell Eastern Central Western Itzpapalotl N. Basin Montes FreyJa Freyja Itzpapalotl

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Fig.25. Relativeages of features in FreyjaProvince and the North Scarp and 55øN Basin. plex, althoughin general,NW-SE trendingtroughs tend to crosscut slope-parallel(typically NE-.SW trending) troughs. This is the steepestpart of theSouth Scarp: 10 ø over 17 km near60.9øN, 343.6øE. 340 ø 345ø 350ø 355ø OoE From the Central Basin eastward, the structuralcharacter of South Fig. 26. Geomorphicmap of SouthScarp and Basin. Dashed lines represent Scarpchanges significantly. Both scarp-paralleland NW-SE trend- uncertainortransitional contacts. Locations of Figures27-29 areoutlined. See ing structuresbecome less common and less pronounced. The scarp Table 1 for definitionsof geomorphicunits. widensand has more smooth terrain and fiat-floored troughs are more common.The 50x30 km calderalocated near the top of SouthScarp, at approximately61.5øN, 359.5øE (described in section2.1) maybe Central Basin (F-MIDRP.60N355). The Central Basin sits at a the sourceof muchof the plains depositsthat have embayedthe nearlyuniform elevation, approximately 3.5 km belowadjacent parts fractured terrain to the west. of LakshmiPlanurn. It exhibitsfewer ridgesthan the West Basin. WestRidge (Figure 28; F-MIDœP.60N344). The broadWest Ridgestrend EW and,as in the West Basin,climb upward toward Ridgerises 1.0-1.5 km abovethe plains to the eastand consists of a LakshmiPlanum in theNE cornerof thebasin. Some ridges curve so systemof roughlyNS-trending ridges. The ridgesare approximately asto parallel South Scarp in thenorthern and northeastern portions of continuousbetween 57.5øN and 58.5øN, where they are interrupted by the basin. an EW-trendingzone of diffusedeformation that extendswestward Central Ridge (Figure 28; FMIDRP.60N355). The West and intoClotho Tessera. Ridges resume approximately 50 km tothe north Centralbasins are separated by abroad topographic ridge that extends with a more westerlytrend, and 100 km farthernorth they become southfrom a 0.5-1.0km highalong South Scarp and wraps around to progressivelymore disruptedover a 75-km-wide zone until their the eastto nearlyenclose the CentralBasin. The ridgeconsists of NNW-SSE trendis supplantedby a NE-SW trendwhich parallels disruptedterrain in thenorth and fractured terrain in thesouth. As in Danu Montes. Southof 57.5ø. ridge structuresappear to become SouthScarp, ridges are crosscut by steep-sidedtroughs. Within the progressivelymore embayedby plains until they are completely fracturedterrain, a setof NS trendingsteep-sided troughs are crosscut buriedsouth of 55øN.Ridges within the Pr to the eastof the West by scarpstrending approximately ENE andWNW overan area 70 km Ridge predominantlytrend EW but near the boundarywith the x 70 km centeredon 59.5øN,353øE (Figure 28). Walls of the NS complex-ridgedfractured terrain (Tf), theycurve into alignment with troughsare offsetin a right-lateralsense along the ENE-trending theplains-Tfboundary. No structureswithin the plains can be clearly scarpsand in a left-lateralsense along the WNW-trending scarps. This traced into the fractured terrain of Clotho Tessera. setof relationshipsis consistentwith conjugatestrike-slip faulting WestBasin (F-MIDœP60N344). The WestBasin sits at an eleva- related to EW directed contraction and/or NS-directed extension. tion between2.5 and 3.5 km below the level of adjacentLakshmi Neitherstyle of deformationis evidentin any of the surrounding Planum.Its surfaceis primarilycharacterized by a networkof radar- terrainand both sets of features(scarps and troughs) truncate abruptly bright lineamentsthat appearto be marelikeridges set in smooth, at the plains-fracturedterrain contact. radar-darkplains. Most ridges in thebasin trend EW toESE-WNW, OuterRise (Figure 29; FMIDRPs 60N344,60N355,55N346). A as do mostridges throughout the plains southof SouthScarp. A discontinuoustopographic high rises 0.5 to 1.5 km abovethe plains secondaryset of suchridges trend NS to NNW-SSE andare found in andruns roughly EW along58øN. Along this rise lies a regionof Td the northernand southwesternparts of the basin.In the northwest locatedsouth of theWest Basin and consisting of anL-shaped region portionof thebasin, near the scarp,both sets of ridgestend to curve of deformedterrain. Most of thenorthern ann of thisregion of Td lies sothat they parallel the scarp. Within thehorizontal resolution of the in a gapin theMagellan imagery but can be seenclearly in Venera15/ altimetrydata (---10 km), thesescarp-parallel ridges appear to lie at the 16 imagesof theregion. Along the southern border with the plains, a topographicbase of the scarp.In the NE portionof thebasin, ENE- set of linear, anastomosingridges, about 5 km wide, trend EW, trendingridges can be tracedup to 1.5 km abovethe basinitself. parallelto marelikeridges within the adjacentplains. The northern Numeroussimilarly trending ridges are visible on LakshmiPlanum boundaryis characterizedby indistinctlineaments which parallel the directlyto theNE of theWest Basin, although none can be tracedas boundaryand a few narrow(less than 5 km) ridgesand troughs which continuousfeatures into thebasin. The southernquarter of the basin trendapproximately NS. Thesesmall ridges parallel a setof three is dominatedby a groupof ridgesthat trend EW to ESE andextend as large(about 15 km) ridgeswithin the interior of theregion. The large far as 355E. ridgesare truncated by boundary-parallelstructures both to thenorth 16,108 KAULA ET AL.' STYLES OF DEFORMATION • ISHTAR TERRA

Fig.27. SouthScarp: northeast side of WestBasin. Latitude 61.9ø-63.0 • N, longitude347.7ø-350.0 • E = 116x 112km. FromF-MIDRPs 60N347 and65N342. A singlebroad ridge (20 km wide)runs NW-SE to NS throughthe center of theimage and is extensivelycrosscut by bothscarp parallel and perpendicular troughs and fractures. Lakshmi Planum lies to the NE (upperrighO and the West Basin to the SW 0ower left). Note alsothe colinearityof NE-SW trendingridges on eitherside of the broadridge. andsouth and thus appear relatively old. Approximately250 km to turnto a moreNW trendalong the western slope of Maxwell Montes, the east,near 57.7øN,356.8øE (Figure 29), portionsof a few small near357øE (Figures 10 and 29). Small regionsdominated by EW- ridgesand troughs, oriented NS, appearnear a localtopographic high. trendingfractures are observedjust northof the Td portionof the Thesestructures may represent an extensiveregion of Td whichhas OuterRise. Discontinuities in thefractures and fingerlike extensions beenburied by plainsdeposits. of the zonesof fracturessuggest that they have beenembayed by Ridgedplains (Figure 29; FMIDRPs 60N344,60N355). A region plainsdeposits. ofridgedplains extends south and east of theWest Basin, between the Interpretation.Magellan images contain extensive information on Central Basin and the Outer Rise, andto the south of Maxwell Montes. the relative ages of featureswithin the South Scarp and Basin The strikeof ridgesin thisregion is predominantlyEW to $SE, but Province.Below arelisted some of themore important observations near 355øE,some turn to the NE, while othersturn to a more SE trend. on relativeage; a suggestedsequence of eventsis listedbelow. The northeasttrending ridges occupy part of the East Basin,then 1. All ridge-formstructures within unitsTd and T1 are abruptly proceedNE toclimb approximately 4.5 km ontoLakshmiPlanum and truncatedat boundarieswith plains (both lowland and highland) KAULA L• AL.' $• OF DEFORMATIONIN ISI•AR TERRA 16,109

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Fig.28. WestRidge (Tf). Latitude59.2ø-59.8øN, longitude 351.9ø-353.2øE = 63 x 70 km. FromF-MIDRP.60N344. Small portion of fracturedterrain (Tf) in WestRidge showing apparent offset of NS-trendingtroughs and NW-trending fractures.

unlessthey run parallel to theplains-Tfboundary. In thiscase, relative parallelridges along the base of SouthScarp in somecases appear to agesmust be determinedby othermeans or are indeterminate. truncateNW-trending fractures (e.g., 6 IøN, 344øE). 2. Steep-sidedtroughs and scarps in mappedregions of unitsTd or 7. Crosscuttingtrends in ClothoTessera suggest that scarp-parallel Tf are abruptlytruncated or embayedat boundarieswith lowland ridgespostdate NS-trending ridges (in T1) plains. Kinematicsequence. A proposedsequence of eventsis givenin 3. Relationsbetween troughs and ridges within unit Td indicatethat Figure30. The observedstructural features, their crosscutting rela- troughspostdate ridge formation. According to aboveinterpretations tionships,and the interpretedmodes of origin suggesta scenario of thesefeatures, this indicates that extension followed shortening in (describedbelow) for the formationof the SouthScarp and Basin theseregions. Province.It is importantto notethat although individual "events" are 4. Steep-sidedtroughs often crosscutplains-Tf boundarieson described,the formationof the regionmost likely proceededas a Lakshmi. seriesof interleavingtectonic and volcanic events. 5. NW-trendingfractures on Lakshmi crosscut (and thus postdate) Initially, EW-directedshortening is manifestedin theformation of plainsridges there. shorteningfeatures oriented NS in ClothoTessera and in the forma- 6. Wherecrosscutting relationships can be determined at all, scarp- tionof a setof conjugatestrike-slip faults in theTfsouth of theCentral 16,110 IqJ%ULAETAL.' STYLESOF DEFORMAT[ON IN Is•rFAR TERRA

.%.

Fig.29. Ridged Plains (Pr). 57.7ø-60. IøN, longitude 354.0øO. 1øE= 253x 332kin. From F-MIDRP.60N355. Many plains ridges turn froman EW trend (]eft) to a NE-SW trend near the upper center of the ridge. NE -trending ridges form a belt that merges with the western boundaryof MaxwellMontes to thenorth.

Sequenceof Events:South Scarp and Basin i i i i i i i i

N-Sridges NW/NE Ridges Plains Plainsridges Troughs NWtroughs Scarp& inTds, Tir scarps along formationinLakshmi along S. inLakshmi Maxwell inTf S.Scarp andlowlands Scarp relatedridges youngest

! !

! ! ! '! 1 ! i ! !

! . ]_ ! ! '! _1_ I i ! ! ! T! ! ! ! oldest T T .

Fig.30. Relativeages of featuresin theSouth Scarp and Basin. KAULA ET AL.: $T• OF DF•ORMAT•ON n• ISHTAR TERRA 16,111

the Provinceinto four regionsaccording to their charactersin the

0OE imageryand altimetry: (1) NorthwestArm andSlope; (2) WestSlope 75øN 30øE andCrest; (3) Cleopatra-Influenced;and (4) SoutheastSlope (Figure + + 75øN 31). NorthwestArm and Slope (Figure 32; F-MIDRPs 65N354, 70N353,70NO07).The boundaryof thisregion on threesides is the 5.0-km elevation contour. On the fourth, east,side, there is a marked sinuousline that runs from 65øN, 0øEto 66.5øN, 359øE and then turns northeastwardto run to 69øN,4øE, beyond which it continuesin the lowlandsin a northerlydirection to beyond72øN, 3øE. Within Maxwell Montes, east of this sinuousline there are virtually no indicationsof gravitationalmodification, but to the west of it, thereare appreciableindications. h Mostof thewestern extremity of MaxwellMontes is verybright, exceptfor a belt 20-60 km widefrom 67øN,356øE to 66øN,0øE. This

I darkerbelt hasno correlationwith the topography.The southwest TI I Tc \ slopeof the arm is markedby a seriesof ridgesNW-SE of variable spacing,predominantly 3 to 10 km. On this slope,there are some \ narrowgrabenlike features at a shallowangle to themain ridges (e.g., 65.9øN,356øE) similar to thosein VestaRupes, though much less \ marked.A sinuousfeature about 0.5 km wideruns through this slope at about66.3øN for 230 km. It is definedmainly by offsetsof the 60øN ridges,but for somesegments it lookslike a terrestrialfiver valley. + + 60øN 0øE On thesteep north slope of Maxwell Montes,within 100km of the 30øE NorthScarp and Basin, there are troughs 1-2 km widewith an average spacingof about15 km which trendSE-NW. In somecases, their upperends are truncated by theridge pattern; in others,they fade. At the 4.5-6.0 km elevationlevel thesetroughs cut acrossa seriesof ridgesat 2-4 km intervalsthat parallel the contours,to createan Fig. 31. Geomorphicmap of Maxwell Montesand Western Fortuna Tessera. orthogonal.,blocky terrain. This pattern fades out in thewest at about Locationsof Figures32-35 are outlined. See Table 1 for definition of 353øE and in the east at about 359.5øE. On the western arm of Maxwell geomorplficunits. (67øN,350øE), the fractures are wider, more broadly spaced, and less orthogonal;the overall pattern is fan shaped,closing downhill to the Basin.(The NS-trending troughs would predate this event and are left north. unexplainedunder this scenario.)One possibilityis that the more The higherregions, above 7.5 km in elevation,are a seriesof ridges northwesterlytrending portions of proto-SouthScarp were formed 10-20km apart.Their sawtooth pattern, with the steeper slopes on the duringthis time. Next, shorteningdirected approximately NW-SE westside, suggests that they arethrust faults. led to theformation of proto-SouthScarp and the scarp-parallel rides West Slope and Crest (Figure 33; F-MIDRP.65NO06). The observedthere. Proto-South-Scarp may havebeen a mountainbelt boundaryof thisregion is themarked sinuous lineament on thewest similarto Danu. This phaseof deformationappears to havebeen and northand approximatelythe 8.0 km elevationin the east(near followedby extensive plains volcanism, forming the bulk of Lakshmi 4øE) andsouth (near 62.5øN). Planumand the lowland plains. Unspecified modificational processes To the west of a line from about 67øN, IøE to 63øN, 6øE, there are (gravitationalrelaxation or sliding)and volcanism combine to alter markedridges at right angles to a SW-NEline. Themost prominent proto-SouthScarp into a form similarto that seentoday. Approxi- ridgesare 15-25 km apart,but in placesthe ridge pattern breaks down matelyNW-SE shorteningcontinued, leading to formationof plains to a spacingas small as 3 km. Thissmaller spacing is particularlytrue ridgesin Lakshmiand the lowland plains. Variation in orientationsof of a belt40 km wideall alongthe western boundary of theregion. This ridgesin lowlandsmay be due to localvariations in stressescaused by belt is characterizedby the steepestslopes: an averageof 5ø , or an the topographyof SouthScarp. Either at roughlythe sametime or elevationchange of 3 km in 40 km. The belt extendsfrom 66øN,356øE slightlylater, gravitational stresses led to formationof scarp-parallel to 62øN, 2øE. The elevationcontours (Figure 3) do not coincide troughsin SouthScarp NW of the West Basin. As indicatedby markedlywith theridge pattern, and in placesthere is an appreciable crosscuttingrelationships along South Scarp, this was followedby the anglebetween the two: as muchas 45 ø near 64.5øN, 0øE. formationof NW-trendingtroughs form within Lakshmi.Orienta- Thereare no apparentindications of eithergravitational or impact tions of these features are consistent with continued NW-SE shorten- modificationthroughout the region. In muchof the higherparts, ing. The fact thatthey areonly observed at highelevations suggests above9.5 km elevation,there are bright sinuous lineaments, less than thatgravitational stresses play a role in their formation.The final 0.5 km wide and2-4 km apart,running NS, at about45 ø to the main distinguishableevent consists of continuedNW-directed shortening, features,most marked near 64øN,4øE, which mightbe causedby leadingto the formation of someof theplains ridges running along the gravitationallyinfluenced stress. They alsocould be interpretedas baseof SouthScarp and of theplains ridges SW of Maxwell Montes. continuationsof lineaments to the east of the crest, overlain on the westslope. 2.6. Maxwell Theeastern boundary of theregion is ill defined.Centered on 65øN, 4øEis a bright, featurelessregion about 30 km x 90 km, but to the We define the Province of Maxwell as the terrain above 5.0 km in northeastof it are more markedridges at 15-20 km spacing. The elevation, between 345øE in the west and 15øEin the east. We divide lineamentson the eastslope of Maxwell Montesare moreNS than 16,112 KAULA E-r AL.' STYLES OF DEFORMATION IN ISHTAR TERRA

....

100 km

Fig. 32. NorthwestAnn andSlope of MaxwellMontes. 65.3ø-67.5øN, 355.0'-0.5øE = 232 x 251 km. Theelevation change from the northwestcomer of the figureto the easternedge at 67.0' N is about4.7 km. FromF-MIDRP.65N354. thoseon thewest slope, thus aligning more with thefaint lineaments the bright material at 66.2øN,8.2øE. [Kirk et al., manuscriptin describedin the precedingparagraph. preparation].There are no other impact features in the500,000 km 2 Cleopatra-influenced(Figure 34; F-MIDRP.65NO06).At 130km of Maxwell Montes. (mainlyEW) to 210 km (mainlyNS) fromthe center of Cleopatrathe SoutheastSlope (F-MIDRPs 65N006, 60N005). This crescent- uplandsof Maxwellhave a smoothappearance, as though covered by shapedregion is consistentlybright but lacksthe alternatingpattern a layersome hundreds of metersthick. Thissmoothed upland region of the westslope and crest. This blandbrightness seems not to be a differsfrom the lower region to theeast mainly in brightness.It is as resultof a differingdirection of slopewith respectto theradar, since reflective as the more distorted regions to the west and south, it looksthe samein Areciboand Venera imagery as well, indicative suggestingthat the sourceof thebrightness is a compositionaleffect of a pervasivemultioriented roughness [Vorder Bruegge et al., 1990]. associatedwith altitude[Klose et al., this issue],rather than rough- The boundaryin thenorth with respectto theCleopatra-influenced ness. Embaymentsof the ridge pattern(not as obliteratingas in region, near 64øN, is indistinct. But souththereof are subtlebut WesternFortuna Tessera) suggest that the "Cleopatra influence" was abundantlineaments, with 3-10 km spacing.North of 63øNthese lines subsequentto the tectonic deformationsthat createdthe ridges, run mainlyNS, but souththereof there are NW-SE trendingfeatures probablyassociated with the uplift of Maxwell Montes.There is no thatin placescut across, and in otherplaces are cut by, the NS features. absoluteevidence that the smoothmaterial is volcanic;it may be Thereis no evidenceof gravitationalmodification in thisarea on the ejecta,similar to somehighland regions on the moon. Magellanimagery, but it couldexist in a localized,interlaced manner. Cleopatraitself has two markedrings: a centralpit 45 km in The southernpart of the slope,on F-MIDRP.60N005 westof 9øE, diameterthat is smooth,1.5-2.0 km belowthe uplands to thewest, and doesindicate appreciable gravitational modification (as earlier in- a surroundingshelf of a lightershade, 15-35 km wide. The border ferredby VorderBruegge et al [ 1990]and Vorder Bruegge and Head betweenthe centralpit and the shelf appearsto be much more [ 1991]),though not as great as in theNorthwest Arm and Slope. There hummockythan that between the shelf and the surrounding uplands, aretroughs running NW-SE, asmuch as 6 km wideat 30 km spacing. perhapsbecause the darker inner part affordsmore contrast. A In thewest, these troughs blend in thenorth to thelineaments of the pronouncedsinuous channel, about 2 km wide, extendsfrom the West Slope and Crest. There also are subtleEW trendsin the easternedge of thepit at 66øN,7.5øE, to thenortheast, cutting through SoutheastSlope that appear to underlythe troughs. KAULA • AL.: STYLES OF DEFORMATION •q ISHTAR TERRA 16,113

.="='-==i.

100km

Fig.33. WestSlope and Crest of MaxwellMontes. Latitude 62.4ø-65.4øN, longitude 359.6ø-5.0øE = 317 x 251km. Theelevation change from the southwestto the northeastcomer of the figure is about4.5 kin. From F-MIDRP.65N006.

Interpretation. Compensationof the great height of Maxwell The evidentinterpretation is thatMaxwell Montesis undergoing solelyby a stablecrustal root is quiteimplausible. Even on Earth it severecontemporary compression, in a SWW-NEE direction.This wouldrequire temperature gradients well under10ø/km [Bird, 1991]. compression(and associated downflow) must extend rather deep to The only way to obtainsuch a low gradientis a rapid strainrate accountfor the geoidhigh in the vicinity.We hopea muchmore [VorderBruegge and Head, 1991]: i.e., dynamicsupport. But low detailedgravity field will beobtained to helpexamine this hypothesis. temperaturesare inconsistentwith the interpretationof the banded Acrossthe main edifice of Maxwellthis compression ismanifested by structureas arising from low viscosityat depthsof a few kilometers overthrusting,faulting, and folding, with negligibleindications of [Solomonand Head, 1984; Zuber, 1987]. Crustalcompensation shearor gravitationalmodification. This needfor contemporary wouldalso require a fourfoldthickening of crustbeneath a 500-km compressiondoes not rule out a long historyof convergenceand width,and thence horizontal displacements of 2000 km [Solomonet thickeningof crustalblocks which contribute to compensationof the aL, this issue],for which there is little evidence. topographichigh, involvingboth imbricationnear the surfaceand 16,114 KAULA ET AL.: STYLES OF DEFORMATION IN ISHTAR TERRA

, .

....

Fig. 34. CleopatraCrater, plus areas of MaxwellMontes and Fortuna Tessera affected by it. Latitude64.8ø-67.1øN, longitude 3.7 ø- 12.0•E = 243 x 357 km. From F-MIDRP.65N006. plasticdeformation at depth,as hypothesized by VorderBruegge and However, the high geoid (Figure 3) requiresthat appreciable Head [ 1990]. However, we find no evidencethat the imbricationis supportbe fromthe mantle:a downflowof convection.Supporting sequential. thisconjecture is the depressionrunning along the westside of the An importantincrement from Magellandata is firmerevidence of great sinuousline, resemblingtrenches at terrestrialsubduction extensionalaccommodation on the flanks:pronounced in the north, zones,though shallower: see Figures 3 andFigure 17 of Solomonet above67øN, but slightin the south. In the lowerRidged Belt region al [thisissue]. The likely stiffnessof Venus'supper mantle [Kaula, to the northof Maxwell Montesthere are similar NS ridgepatterns. 1990] would seem to make Earthlike subductionof lithosphere But in theeastern part of SouthBasin south of Maxwell Montesthere unlikely, but finite element modelling indicatesthat lithospheric is an almosttotal lack of any indicationof structurerelated to sinkingcan occuras a relativelyshortlived event [Lenardic et al., Maxwell Montes, other than a moderate continuationof the sinuous 1992]. A questionis the horizontalscale of tectonicmotions on frontaround longitude 0 ø, suggesting very recent volcanism since the Venus. The westernslope of Maxwell Montesis consistentwith a uplift of Maxwell Montes. uniformflow overa spanof 400 km NS, butthe topography to the west Magellanhas settled that Cleopatra is an impactcrater [Kirk et al., indicatesthat the horizontalscale of uppermantle kinematics is not manuscriptin preparation].The superpositionof its effectson what muchlonger. The finite elementmodelling indicates that a plume appearto be tectonicfolds requires that this major infall belate in the underLakshmi would stimulatesteep lithospheric underthrusting sequenceof eventsshaping Maxwell, asearlier inferred by Vorder beneathMaxwell Montes [Lenardic et al., 1992]. Brueggeand Head [ 1990]. The amountof smoothmaterial produced While MaxwellMontes probably has a longhistory, like terrestrial by Cleopatrais muchmore than normal impact melt. If it is such,it mountainbelts, we did notfind persuasiveevidence of pastdiffering suggeststhat much crust under Maxwell Montesis closeto melting. kinematicpatterns, such as the cross-strikediscontinuities inferred But such a heated state is inconsistentwith the great height of fromArecibo and Venera imagery by VorderBruegge et al [ 1990]. Maxwell, whichrequires a high strainrate if crustalthickening is a A joint kinematicsequence of Maxwell andWestern Fortuna is majorcontributor thereto [Vorder Bruegge and Head, 1991]. givenin Figure36 at the end of section2.7. KAUI• ET AL.: ST•ES OF DEFORMATION IN ISHTAR TERRA 16,115

)----• lOOk m •

Fig. 35. RidgedTessera in WesternFortuna Tessera. Latitude64.7ø-67.2øN, longitude 11.9ø'19.9øE = 264 x 344 km. From F- MIDRP.65N018.

2.7. Western Fortuna The regionis characterizedmainly by subparallelarcuate ridges We definethe provinceof WesternFortuna as the areaof com- whichroughly parallel the Maxwell front,with spacingsof 15 to 25 plexly deformedterrain between Maxwell Montesin the westand km. They occuracross the entire 950 km in longitudeof the region. Varz Chasma in the east, 11E to 30E, and in latitude 76øN to 56øN. We Not so marked,but still clear,are somecrosscutting features. Most divideWestern Fortuna Tessera into (1) CentralRidged Tessera, (2) prominentis a curvedpair of lineamentsthat extendsabout 400 km RidgedBelt, (3) SouthernRidged Tessera, (4) ChevronTessera, and from 65øN, 14øE to 66.8øN, 21øE. Less prominentare straight (5) PlumoseRidges. These regions follow VorderBruegge andHead lineaments that trend in a NW-SE direction, less than 5 km wide and [1989], with the exceptionthat their "ArcuateRidged Tessera"is spacedat intervalsof about60 km. The mostmarked are centered at dividedinto centraland southernparts. 66.4øN, 17.4øE and 65.7øN, 17.9øE and extend about 160 km. The CentralRidged Tessera (Figure 35; F-MIDRPs 65N018,70N007, formerof thesecrosses the curvedlineaments and appearsto offset 65N006,60N016). We discussthis region first because it relatesmost themby about5 km. Offsetsof the arcuateridges by the NW-SE closelyto MaxwellMontes, discussed in theprevious section. It also lineamentsoccur but are smallerand unsystematic. abutson all theother regions of FortunaTessera that we havedefined Within about 300 km of the centerof Cleopatrathere is much for thisstudy. The westernboundary is definedby the5.0-km level embaymentof thearcuate ridges by smoothmaterial. Around 66.4øN, alongMaxwell Montes,the line between bright and dark terrain. In 11øE,segments of the ridgesdisappear under this cover. An evident the north,the region fades into theRidge Belt, whichhas a greater sourceof embayingmaterial is the channelthat comesout of the varietyof extensionalas well as shortening features. South of 61.5øN uplandsat 66.4øN,8.5øE. However, as in theMaxwell uplandssouth thereis a transitionfrom steep ridges parallel to Maxwell'seast front of Cleopatra,it cannotbe ruledout thatmuch of thiscover is ejecta toamore heterogenous terrain. In theeast, the arcuate ridges give way from the crater. at about20øE to the lesspronounced chevron tessera and plumose North of 68.2øN,the 20-km-spacedarcuate ridges become less ridges.The northern,eastern, and southern boundaries all coincide marked,and thereare both finer intertwiningfeatures in aboutthe with dropsin elevationbelow 4.5 km. sameorientation plus some straighter troughs in a SW-NE direction, 16,116 KAULA ET AD: STYLES OF DEFOPO,4ATION IN ISHTAR TERRA roughlyradial from Maxwell Montes. There is no clearboundary the chevronpattern in the tessera,but it is not marked,and south of with theRidged Belt region. 65øNit fadesout. East of 26øE,where the elevation drops below 4.0 In thesouth, there is a moremarked boundary around 61.5øN. km, thetessera fade out completelyinto a smoothplain. RidgeBelt (F-MIDRP. 70NO07to 72.5N;C1-MIDRP.75N029 north PlurnoseRidges (F-MIDRPs 65N018,60NO16, 60N026). The area thereof).This region extends off thenortheastern corner of Maxwell southof 65øNand between 16øE and 20.5øE is nottypical tessera, but Montes in a NNE direction,sloping over 750 km from 5.0 km ratheris marked by a welterof finelineaments. These fine NNW-SSE elevationto 3.0 km. Its westernedge is the sinuouslineament striationsat 2-3 km intervalsbecome more prominent with distance describedin section2.6 on MaxwellMontes. Its easternboundary is from Maxwell Montes. East of 20.5øE there are some more arcuate indistinct:a fadinginto the CentralRidged Tessera region. ridges,roughly NS with spacingsof 5-15 km. A smoothbay 25 km North of 69øN, the sinuouslineament discussed in connectionwith x 90 km, containinga 30-km-longrille, existsaround 58.8øN, 21.4øE. Maxwell Montesbecomes Semuni Dorsa: a bundleof bright lines Eastof 25øEand south of 63øEthere appears another sinuous linea- againsta darkerbackground, about 30 km in width andundulating mentbundle, about 15 km wideand extending about 500 km. Butthe EW about50 km, with a NS wavelengthof about200 kin. Eastof the wholeregion is cutoff sharplyto the southeastby a smoothplains sinuouslineament the main trends are SSE-NNW, fading into smooth depositalong a $SW-NNE line from 56øN,21øE to 61øN,27øE. areasabout 25 km wide in places.There also are fainter lineaments Interpretation.The Magellandata appear consistent with themain in a SW-NE direction, suggestiveof earlier differently directed conclusionsof Bindschadlerand Head [1991] andVorder Bruegge tectonics. and Head [1990], i.e., horizontalconvergence, followed by some SouthernRidged Tessera(F-MIDRP60NO16). As the terrain gravitationalrelaxation, particularly on the flanks. This is not slopesdown gradually south of 61.5øN,the arcuateridges are much surprising,since the dominant arcuate ridge features strongly indicate lessmarked. In aband between 13øE and 16øE, the ridges are embayed EW convergencein theCentral Ridged Tessera, an outboard plateau by smoothmaterial., and southof 60øN there appearsto be no of Maxwell Montes.Also identified are lineaments at angles of about preferredorientation. In an area57ø-60øN, 11ø-13øE there appears a 45ø to the ridges,west of Hina Chasmamore likely indicativeof patternof bright,fine NS lineaments:as many as 30 of themover a contemporarystress patterns than past convergence directions. spanof 105km. A prominentEW graben5 km widecuts across them More questionableis where this EW convergenceends; it is at 59.7øN. Startingat 12øE, NNE-SSW trendingtroughs become irksomethat the solarconjunction gap occurs over Central Fortuna apparent,with one markedsinuous rille extendingNNW around Tessera,where Vorder Brueggeand Head [1990] hypothesizeda 60øN, 10.5øE. differentcontemporary direction. ChevronTessera (F-MIDRP.65NO18 and C1-MIDRP.60NO14). Kinematicsequence (Figure 36). As discussedabove, the topo- East of 20E, as the terrainstarts to slope,there is a changein the graphicand geoid highs of centralIshtar strongly suggest that it is a imageryfrom the arcuatetessera: much less contrast and systematic regionof vigorouscontemporary convergent tectonics. This contem- orientationin thebright-dark alternations. From 62øN, 20øE to 66øN, poraryvigor is mostevident in the west slopeof Maxwell Montes, 22øEHina Chasmais a stronglydelineated band about 60 km wide, whichhas a steepslope; marked parallel folding; and few indications withinwhich there is a smooth,ropey texture. From 67øN to 70øNat of eitherearlier tectonic patterns or gravitationalrelaxation (except to 25øE Morana Chasma is a darker line, not more than 10 km wide, theextreme flanks). The northwestarm and the east slope of Maxwell exceptfor its northernmost50 km. EastofHina Chasmathere occurs Montesalso are much deformed, but theyhave more variety, sugges-

Sequence of Events: Maxwell Montes and Western Fortuna Tessera

Fine Lineaments:E. LakshmiN-S Ridges:NW-SE Hina SinuousBelt: NW- SE Plains Slumping: Cleopatra N.W.M b, R b, Plains E.IVlb, 'ri, Ridges:Chasma W.Rb, Ridges:Formation: E. N, S Mb youngest Tc, Rp FormationR b NWMb W.M b W.Mb SouthBasin Slopes

oldest

*Note: N.W. M b= NorthwestArm andSlope region; E. Mb= MaxwellEast Slope region; W. Mb= MaxwellWest Slope region; W. Rb= SamtaxiDorsi

Fig. 36. Relativeages of featuresin Maxwelland Western Fortuna Tessera provinces. Abbreviations in theheadings: N.W.Mb = NorthwestAnn andSlope Region; E.Mb = MaxwellEast Slope Region; W. Mb- MaxwellWest Slope Region; W.Rb= SamuniDorsal. KAULA ET AL.: STYLES OF DEFORMATION IN ISHTAR TERRA 16,117

tiveof greaterage. The eastslope is markedby thegreat impact crater 3.2. Mountain Belts Cleopatra,of a sizethat occurs at intervalsof morethan 100 m.y. It is thusambiguous as to whetherthe physiographicmodification by All the mountainbelts thusappear to be thrustover the plains, Cleopatrapredates or is essentiallycontemporaneous with thetecton- indicatingcompressive shortenings later than the lava flows. But the ics givingthe observedform to the westslope. altimetryindicates that only Maxwell and Freyj a Monteshave trenches Sharingthe kinematicpatterns of the Maxwell West Slope and [Solomonetal., thisissue], less than a kilometerdeep, mild compared Crestare two regionswithin FortunaTessera: the CentralRidged to islandarcs on Earth. A betterterrestrial analogue might be the Tesseraand the Ridge Belt. SemuniDorsa is also shapedby the Gangestrough associated with the continent-continentcollision of sinuouswest margin, and the dominantridges in the centralregion India and Asia, exceptthat Maxwell Montesis associatedwith a parallelMaxwel!'s folds. But boththese regions have underlying markedgeoid high, ratherthan a low, as is the Himalayan-Tibetan patternsindicative of differenttectonic patterns in thepast. Finally, complex. the lower regionsof WesternFortuna Tessera are lessmarked by Of thethree belts, only Danu Monteshas lava channels extending contemporarytectonics, and have more evidence of volcanismand down slopeinto the plains (exceptpossibly the sinuouslineament gravitationalrelaxation. runningEW throughMaxwell Montesnear 66øN). Danu Montes (includingVesta Rupes) also has the most extensionalfeatures, 3. Svt,nmmsis ov OBSERVATIONSA_ND INTERPRETATIONS suggestiveof gravitationalmodification, among the threebelts. All mountain belts have more indications of volcanism on their The Magellan imagery enablesa more accurateand detailed outboardslopes, although in the caseof Maxwell Montesit appears geomorphicmapping, settling debates generated by theVenera data confinedto flows triggeredby the CleopatraImpact. Magnani (e.g.,Cleopatra is clearlyan impact feature, not a caldera[Kirk et al., (58.6øN,337.1øE), 25 km in diameter(thus created by a bolidemuch manuscriptin preparation]).This geomorphicmapping also contrib- lessmassive than Cleopatra's), hasno indicationof impact-associ- utessignificantly to the broaderscale interpretation of the major atedmelt: even its central floor is radar-bright, thus rough. Both Danu features: the relative roles of constructional and modificational tec- Montesand Freyja Montes have pits on theiroutboard slopes, some tonic processes,volcanism, and impacts. But it is desirableto of them aligned,indicative of volcanismfacilitated by extensional synthesizethe contemporary kinematics and, beyond that, a historyof tectonics. Ishtar.Such a synthesismust draw together the indications of current The threebelts all have two armsforming shapes concave to the and past deformationsfrom different provinces. The ability to uplandplain. At the bendsbetween the arms there appearto be reconstructthe past varies greatly amongthe provinces,probably appreciablygreater deformations than in the armsin all threecases, beingleast in themost intensely folded mountain belts and greatest in althoughgenerally the folding andfaulting in Danu Montesis less therelatively smooth lowland basins. intensethan in Freyjaor Maxwell montes.Within eachof the three In this section,we collect the variousparts into terrain types, belts,one arm appearsto havemore intense folding: the eastern,in identify commonfeatures of a type, plus variationswithin them. Danuand Maxwell montes;the western, in FreyjaMontes, suggesting Thesetypes are (1) uplandplains; (2) mountainbelts; (3) outboard thatit is morerecent. Overlaps confirming this suggestion are hard to plateaus;(4)smooth scarps; and (5) basins.The orderof presentation definein Danu andMaxwell but existin Freyja. is from the centeroutward. The one greatupland plains feature, In the moreintensely folded parts of all belts,the ridge spacing is LakshmiP lanum, does have a quitecentral character and thus a central quitevariable. Thesespacings are mainly 3-10 km but in placesare role, while the mountainbelts and smoothscarps are peripheral asclose as 1 km or aswide as20 km. Thesespacings are consistent theretoand the outboard plateaus and basins in turnare peripheral to with a shallowdepth, a few kilometers,to a weaklayer of accommo- thebelts and smooth scarps. Furthermore, the belts and plateaus are dation [Zuber, 1987]. appropriatelyconsidered before the scarpsand basins because they All mountainbelts have very bright crests. In Maxwell Montes,the aremore closely related to contemporarytectonics. brightnessextends over a wide area,about 500 x 500 km, coveringa Placingthe uplandplain first givesan orientationfor the most fairly widerange of imagetextures. One thingin commonover this difficult task, the inferenceof structuralpatterns cutting across areais anelevation greater than 5.0 km, suggestingthat the brightness provinces:to first order,radial in-or-out,tangential clockwise-or- is a chemical effect. But the crest of Danu is inferred to have counter,but not neglectingeither secondaryinternal patternsor appreciableintrinsic reflectivity, as well asroughness, even though it broaderpatterns reflecting processes influencing areas even larger doesnot reach this elevation. In general.,the correlation of brightness than Ishtar. with elevationis not as systematicas it appearsto be for mostof Maxwell; the causeof intrinsicbrightness is still not understood 3.1. UplandPlains [Klose et al., this issue].

As discussedin section2.1, thereappears to be a well-definedage 3.3. Outboard Plateaus sequencefrom the ridged terrains (centering near 65øN, 345øE) to the smoothplains to caldera-associatedand margin-associated deforma- Maxwell Montes, Freyja Montes, and the easternarm of Danu tions. Montesall have highly deformedregions on the side away from The mostpronounced of the margin-associateddeformations are LakshmiPlanum. Theseregions are appreciably lower in elevation thoseparalleling the most prominentfeatures, most notably, the than the mountain belt crests and extend 100-1000 km before a final RangridFossae paralleling the western front of Maxwell Montesnear descentto thelowland plain. The widthsof theseplateaus vary with 62øN,357øE. Those paralleling the North and South scarps are narrow the heightsof the mountainbelts, from 100-200 km for Clotho but distinct;paralleling Danu Montes,rather faint. Tessera,southeast of Danu Montes, to almost 1000 km for Western Markedlyabsent are (1) majorrifts, comparableto thosein Beta Fortuna Tessera, east of Maxwell Montes. Regioand Aria Regio;and (2) any embaymentby the uplandplains Alsocorrelated with mountainbelt characteristics are the intensity materialof thethree mountain belts, Maxwell Montes, Freyja Montes, of deformationand degree of modificationin theseoutboard plateaus. and Danu Montes. Most intenseand monotonousamong the plateausis the Central 16,118 KAULA ET AL.: STYLES OF DEFORMATION IN ISHTAR TERRA

RidgedTessera of WesternFortuna. Although there are many finer changesin trendsfrom one epoch to another.In bothcases, sequences lineamentsin directionsother than the NS arcuateridges 15-25 km of eventscan be inferred. In the north basin, the more recent events apart, few cut acrossthe ridges. ItzpapalotlTessera, north and appearassociated with eitherItzpapalotl to thenorthwest or Maxwell northeastof FreyjaMontes, has more variety: within200 km of the to thesoutheast, but in the southbasin they seem more local and not crest, there exist radar-dark, hence smooth, areas 30 km in width, sodirectly related to whateverforces raise the major features of Ishtar. includinga markedsinuous rille about1 km wide andmore than 100 In particular,there is a strikingabsence of any continuationof the km long (76.5øN,336ø-340øE) [see Baker et al., this issue]. But structuralpatterns in theimagery of MaxwellMontes to theplains to beyondthis smooth embayment there is yet morerugged tessera, until its south. the abruptdrop of 2 km at UosarRupes (near 80øN). See Figures 35 and 20. 4. Stmm•¾ •r• •c•s In all the outboardplateaus there are lineamentsat appreciable anglesto the main trends. In somecases,these crosscutting trends can IshtarTerra is oneof five regionson Venusthat contain elevations be tracedinto the adjacent lowlands. They aremuch less marked than morethan 4.0 km aboveMPR. The otherfour regions are all at low the strike-sliptrends seen in imagingradar of Asia north of the latitudesand appear to fall intotwo pairs on the basis of imageryand Himalayanorogeny on Earth,and they are not asreadily relatable to gravimetry,as well as altimetry. featureslikely to be due to contemporarytectonics, such as the One pair of highlandregions appear to be mantleupflows sur- dominant banded terrain of Maxwell Montes. Hence, in view of the mountedby volcanicconstructs: Atla Regio (0øN, 195øE)and Beta lack of erosion,they are more likely relicts of earlier kinematic Regio (30øN, 185øE)are characterizedby extensiveareas of lava patterns. flows, with little thrustfaulting or folding, the main deformations beingrifts [Head et al., thisissue]. Atla andBeta arethe two greatest 3.4. SmoothScarps geoidhighs in thegravity field of Venus,over 120 in thesolution by Smithand Nerern [ 1992], correlatingvery closelywith thetopogra- The marginsofLakshmi Planum 338ø-353øE in thenorth and 340 ø- phy. Thesegeoid: topography ratios yield apparentcompensation 5øEin thesouth are strikingly smooth compared to themountain belts, depthsover 200 km deep[Srnrekar andPhillips, 1991 ], suggestiveof despitebeing topographic drops of 2-3 km within 40-70 km. The deepmantle plumes. smoothscarps also contrastwith the mountainbelts in that their Theother two highlands appear to bebelts of convergence,leading horizontalshapes are convextoward the uplandplain, seeFigures 1 to topographicuplift and downflow: Ovda Regio (5øS,90øE) and and 3. ThetisRegio (10øS, 130øE) are characterized by extensivedistorted In thenorth, the convexityis onegreat sweep over about 900 km, mountainbelts and tesserae,with little or no extensionaldeformation makinga smoothbend of slightlymore than 90 ø, centered near 67øN, andonly occasionalpatches suggestive of volcanism[Bindschadler 344øE. The northsoutharm of this sweep is the smoother,but et al., this issue]. Ovda and Thetis are also geoid highs,but nonethelessmarked throughout by f'metroughs less than 1 km wide, appreciablymilder, with peaksof 40 and75 m, respectively.These overa widthof 40-80 km, contrastingwith the radar dark plains of the geoidhighs have greaterlateral extentthan thoseof the volcanic plateau and the anastomosinglineaments of the basin. The only constructs,and do not correlate as closely with thetopography. These prominentcrosscut of thesetrends is an irregulartrough 5 km wide geoidsignatures suggest shallower, more complicated density varia- 68ø-69øN,343øE. But in theeastwest arm, east of 345øE,the downslope tionsin theconvergent belts than in thevolcanic uplifts [Bindschadler of the scarpis cut by many more rilles 1-3 km wide at 4-12 km et al., this issue]. intervals. IshtarTerra appearsto havecharacteristics in commonwith both In thesouth, the convexity is brokenup into three scallops, varying the volcanicuplifts and convergentbelts: the imagerycontains from a 180 ø semicircle 180 km in diameter in the west to a smooth extensive areas of both volcanic flows and tectonic deformations. The bend of only 20ø over 270 km in the east. There also is a marked volcanicflows of Lakshmicould be several100 m.y.old. But even decreasefrom west to eastin the numberof troughsparallel to the with a stiff uppermantle, a majorgeoid high could not be maintained scarp.But the topographicdrop remains the stone,and near 359øE, withoutcontemporary mantle convection. Unfortunately, Ishtar is just southof a caldera,the imagery is uniformlydark. The numberof toofar northto obtainan accurategeoid from thePioneer Venus data: uplandtroughs at appreciableangles to thescarps increases apprecia- the36th degreefield yieldsa highof about65 meters500 km south bly from west to east. of thecrest of Maxwell Montes,with a blandpositive area over all of The scarpsare not entirely without areas of disruptedterrain. In the Lakshmi,Freyja, and Westem Fortuna provinces, as well asMaxwell: north,there occur a seriesof troughsat theeastem extreme, 345.5 ø- seeFigure 3. The true geoidhigh couldbe appreciablygreater, 348øE,at the apexof the northwestann of Maxwell Montes. In the perhapsover 100 m, andcentered farther north (the eighth degree field south,there occur severe lineaments and two largetroughs in thecusp doescenter over Maxwell Montes), but this is not necessarily so. The between the Central and West basins. In both north and south, these regionalsolutions of Grimmand Phillips [ 1991]get apparent depths distorted terrains are of limited extent, less than 150 km, and do not of compensationof 130-180km, more easily explained by upwelling. appearto be relatedto trendsin thebasins below. However,those in Theproblem is toreconcile this finding with theabsence of largerifts thesouth scarp are among the features paralleling the Maxwell front, andstrongly convergent appearance of the mountainbelts [Roberts 500 km to the east. and Head, 1990a,b; Bindschadleret al., 1990, this issue;Grimm et al., 1992]. Two- dimensionalfinite element modelling of a conver- 3.5. Basins gentflow in a crustand stiff mantlereproduces the topographyin a rathercomplex evolution taking several100 m.y. [Lenardicet al., The basinsbelow the scarpsare similar in appearanceto the 1991, 1992]. lowlandplains that dominate the surface of Venus. However,they do It istempting to suggestthat Ishtar contains a combinationof flows: haveappreciable elevation: about 2 km aboveMPR in the southand not only convergence,leading to downflow,but alsoan upflow. 3.5 km in thenorth. Like mostof the lowlandplains, they contain a Ishtaris probably similar to theEarth in thatthe recent-to-contempo- variety of patterns,suggestive of long geologicalhistories with rary dynamics,which havethe dominantinfluence on topographic KAULA ET AL.: STYLES OF DEFORMATION IN ISHTAR TERRA 16,119

andgeoidal elevations, are stronglyinfluenced by the past,mainly conditionshidden at depthon Venushave greater effect on surface throughold relativelystable blocks. tectonicsbecause of the lackof an asthenosphere:there is a tighter The Magellanimagery contributes by constraininglikely kine- couplingof thelayer immediately below the crust, which determines maticpatterns and their sequence.The volcanicflows of Lakshmi theregional patterns seen in imagery,to the interior. Planumare more recent than the ridged terrain they embay, suggest- Ishtar'sstructure evidences that the patternof flow can vary ing thatthe latter is relictof a cratonwhose resistance to deformation appreciablyover a few hundredkilometers within Venus: a contrast led to the transfer of deformation elsewhere. All three mountain belts to Earth (certainlyto Earth'soceans), where patterns are controlled appearto havehad tectonism continuing since the volcanic flows in overthousands of kilometersby the tectonicplates. Becauseof the theadjacent parts of LakshmiPlanum, since all of themappear to be highupper mantle viscosity required by thehigh geoid: topography thrustover the upland plain. Amongthe three mountain belts, there ratios,Venus probably gets rid of its heat in a style that is more appearsto be a sequenceyoungest-to-oldest of Maxwell-Freyja- regional,i.e., shorterwavelength, than doesEarth [Kaula, 1992]. Danu. This sequenceis basedmainly on the abundance,least-to- Probablyboth the material velocities and rates of changein thepattern most,of indicationsof gravitationalmodification in thesemountain are somewhatslower than Earth's. The resultfor a complexof belts. But thismay be anoversimplification; details in thestructural heterogeneitiesgenerated in earliertimes, such as IshtarTerra, is a patternsof the mountainbelts and their outboard plateaus suggest marked variation of characteristics over distances of a few hundred shiftsin the magnitudeand directionof thrustingaffecting all the kilometers. belts:north-to-south in Maxwell, with perhaps aclbckwise rotation in Doubtlessthere is vigorouscontemporary convergence contribut- the main direction;east-to-west in Freyja,with a counterclockwise ing a major shareto the uplift of Maxwell Montes. Aspectsof rotation;and, most speculative, west-to-east in Danu. But structural contemporarydynamics that are morein doubtare whetherthere is patternsin theoutboard plateaus and lowlands appear to be younger continuedstrong underthrusting under Freyja and whether there is an thanthe deformations forming Freyja and Danu. In thesouth basin, upflowunder LakshmiPlanum. Ifthere is, then there can exist marked thesepatterns seem related to Maxwell Montes,but in the north,in variationsin theflow patternof Venus'supper mantle over distances centralItzpapalotl and beyond, they seem unrelated to IshtarTerra. of a few hundredkilometers, as in someconvergent zones on Earth, The smoothscarps are enigmatic; Venus lacks Earthlike erosion to despitethe high viscosity. It seemsclear that over the long past of removethe irregularitiesexpected with suchmarked topographic IshtarTerra there has been a shiftingaround of tectonicand volcanic jumps.Smoothing by flow in the weaklayer of thelower crust seems activity,much influenced by structuresrelict from earlier phases. The mostplausible, since it wouldhave obliterated small features much timescale ofthese shifts is unsure, but it islikely, from the complexity fasterthan large [Bird, 1991]. Even so,the scarpsare surprisingly of Ishtarstructures as well as crater density, that they have gone on for steep.Also thismechanism would have left a fracturedterrain, like morethan 1.0 b.y., as well asgenerating structures that are relatively thehill at 72øN,342øE. Hence it seemscovering up by lavaflows is new. Butwe cannothope to resolveconclusively the contemporary required.Corroborative of thissuggestion is thatthe areas on each densitystructure, and thencethe flow patterns,without a more scarppointed out as more deformed locally (347øE in thenorth; 35ere detailedgravity field, whichrequires circularization of the orbit of in the south)both have higher elevationsthan the rest of their Magellan. respectivescarps. There also is no constraintthat the volcanismof Acknowledgments.We arevery gratefulto RichardW. Vorder LakshmiPlanum all occurredin oneepoch; indeed, there are several evidencesof morethan one episode, such as the cutoff of thefractured Brueggefor performing an extremely thorough review, which greatly terrain(Tf) near 68øN,337øE. influencedrevision of the paper. This work wassupported by the NASA MagellanProject through JPL contracts957088 (Brown), The scarps,as a well the secondarystructural patterns in the 958497 (UCLA), 975072 (SMU), and957070 (MIT). outboardplateaus, emphasize that Ishtar is definitelynot uniformly young.The only availableestimator of ageis craterdensity [Phillips et al., thisissue]. In the5 x 10•km2 bounded by 57øN,75øN, 330•E, 30•E,there are 10 impact craters identified, close to the global average density,which corresponds to a meansurface age of about500 My. Baker,V.R., G. Komatsu, TJ. Parker,V.C. Gulick, J.S. Kargel, and J.S. Lewis, Channelsand valleys on Venus: Preliminary analysis of Magellandata, J. More germaneto thequestion raised here is the distributionof crater Geophys.Res., 97, 13,421-13,444,1992. agesfor anarea of extentmuch larger than the average crater spacing. Barsukov,V.L, etal., The geology and geomorphology ofthe Venus surfance The Ishtararea studied in thispaper is about36 timesthe maximum asrevealed by the radar images obtained by Venera15 and16, Proc. Lunar volcanicpatch area in therandom volcanism steady-state model of Planet.Sci. Conf. 16th, Part 2, J. Geophys.Res. 91, suppL,D378-398, 1986. Phillipset a/[this issue].From this model, the median likely age for Berthe, D., P. Choukroune,and P. JegOuzo,Orthogneiss, mylonite and theoldest crater in a 5 x 106km2 area with ten cratersis about1.4 Ga, noncoaxialdeformation of granite:The exampleof the SouthAmerican with a 50% probabilityrange of 1.0-1.9Ga anda 90% rangeof 0.7- shearzone, J. $truct. Geol., 1, 31-42, 1979. 2.3 Ga [Kaula,manuscript in preparation].Thus Ishtar has a high Bindschadler,D.L, and J.W. Head, Tesseraterrain, Venus: Characterization andmodels for originand evolution,J. Geophys.Res., 96, 5889-5907, probabilityof containingrelicts going back more than 1.0 Ga in age. 1991. In this time, there could have been considerableshift of the tectonic Bindschadler,D.L, G. Schubert,and W.M. Kaula, Mantle flow tectonicsand andvolcanic forces affecting Ishtar. Justhow, andwhy, theseshifts theorigin of IshtarTerra, Venus, Geophys. Res. Lett., 17,1345-1348,1990. occuris obscure,compared to Earth, because Venus lacks an astheno- Bindschadler,D.L, G. Schubert,and W.M• Kaula, Coldspotsand hotspots: sphere[Kiefer et al., 1986;Phillips, 1990]. This lack leads to changes Globaltectonics and manfie dynamics of Venus, J. Geophys.Res., 97, 13,495-13,532, 1992 beinggenerated much more by conditionswithin the uppermantle Bird,G.P., Lateral extrusion of lowercrust from under high topography in the thatcan be quiteregional, rather than by conditionsassociated with isostaticlimit, J. Geophys.Res., 96, 10,275-10,286,1991. largetecton• plates generated byglobal scale flow. In other words, Burt,J.D., and •.W. Head,Types and occurrences ofvolcanic features and their tectonicson Venuscannot be expectedto occurin clearlydefined relationsto tectonicsof Freyjaand Danu Montes, Lunar Planet. $ci. Conf., 22, 159-160, 1991. belts and zones as it is on most of Earth, becausethere is not a well- Campbell,D.B., J.W. Head, •.K. Harmon,and A.A. Hine,Venus: Identifica- definedpattern of plates.Possibly similar on Earthare continental tion of bandedterrain in the mountainsof IshtarTerra, Science,221,644- areas of thick crust [Solomonet al., this issue]. But even there 647, 1983. 16,120 KAULA ET AL: STYLES OF DEFORMATION IN ISHTAR TERRA

Corsini, M., A. Vauchez,C. Archanjo,and E.F.L de Sa, Straintransfer at Venus:Models of formationand evolution, Geophys. Res. Lett., 17,1341- continentalscale from a transcurrentshear zone to a transpressionalfold 1344, 1990b. belt:The Patos-Seridosystem, northeastem Brazil, Geology,19, 586-589, Ronca,DB., andA.T. Basilevsky,Maxwell Montesand Tessera Fortuna: A 1991. studyof Venera15 and 16 radarimages, Earth MoonPlanets, 36, 23-39, Grimm,R.E., andR.J. Phillips, Tectonics of LakslmaiPlanum, Venus: Tests 1986. for Magellan, Geophys.Res. Lett., 17, 1349-1352,1990. Schaber,G.G., R.G. Strom, H.J. Moore, DA. Soderblom,R.L. Kirk, D.J. Grimm,R.E., andR.J. Phillips,Gravity anomalies, compensation mecha- Chadwick,D.D. Dawson, DR. Gaddis,J.M. Boyce, and J. Russell, nisms,and the geodynamics of westernIshtar Terra, Venus, J. Geophys. Geologyand distribution of impactcraters on Venus: What are they telling Res., 96, 8305-8324, 1991. us? J. Geophys.Res., 97, 13,252-13,301,1992. Grimm,R.E., R.H. Herrick,and RJ. Phillips,Highlands, IIl, Onthe origin and Shimamoto,T., The originof S-C mylonitesand a newfault zonemodel, J. evolqtionof largeuplands on Venus,Lunar. Planet. Sci. Conf., 23, 453- Struct.Geol., 11, 51.64, 1989. 454, 1992. Smith,D. E., andR. S. Nerem,Gravity field modelingof Marsand Venus at Gundmundsson,A., Geometry,formation, and developmentof tectonic NASA/GSFC,in GravityFieM Determinationfrom Spaceand Airborne fractureson the Reykjanes Peninsula, southwest Iceland, Tectonophysics, Measurements,edited by O. D Colombo,Springer-Verlag, New York,in 139, 295-308, 1987. press, 1992. Hansen,V. L., Regionalnon-coaxial deformation of Venus:Evidence from Smrekar,S. E. and R. J. Phillips,Geoid to topographyratios and their westernItzpapalotl Tessera, l.,to•ar. P lanet.$ci. Conf.,23,479-480,1992. implications,Earth Planet. Sci. Len., 107, 582-597,1991. Head,J.W., The formationof mountainbelts on Venus:Evidence for large- Smrekar,S.E., and S.C. Solomon, Gravitational spreading of highterrain in scaleconvergence, underthrusting, andcrustal formation in FreyjaMon- IshtarTerra, Venus, J. Geophys.Res., this issue. tes,Ishtar Terra, Geology, 18, 99-102, 1990. Solomon,S.C and J.W. Head,Venus banded terrain: Tectonic models for band Head,J.W., and L. Wilson,Magma reservoirs and neutral buoyancy zones on formationand their relationship tolithosphere thermal structure; J.Geophys. Ven•: Implicationsfor theformation and evolution of volcanicland- Res.,,84, 6885-6987, 1984. forms,,J. Geophys.Res., 97, 3877-3904,1992. Solomon,S.C., J.W. Head, W.M. Kaula, D. McKenzie, B. Parsons,R.J. Head, J.W.• R.W. Vorder Bmegge,and L.S. Crumpier,Venus orogenie Phillips,G. Schubert,and M. Talwani,Venus tectonics: Initial analysis environments:architecture and origin, Geophys. Res. Len., 17,1337-1340, from Magellan, Science,252,297-312, 1991. 1990. Solomon,S.C., S.M. Smrekar,D.L. Bindschadler,R.E. Grimm,W.M. Kaula, Head, J.W., D.B. Campbell,C. Elachi,J.E. Guest,D.P. McKenzie,R.S. G.E.McGill, R.J. Phillips, R.S. Saunders, G. Schubert,S.W. Squyres, and Saunders,G.G. Schaber,and G. Schubert,Venus volcanism: Initial analy- E.R.Stofan,Venus tectonics: An overviewof Magellanobservations, J. sisfrom Magellandata, Science,252,276-288, 1991. Geophys.Res., 97, 13,199-13,255,1992. Head,J. W., DS. Cramplet,J.C. Aubele, J.E. Guest, and R.S. Saunders, Venus Stofan,E. R., J.W. Head,and D. B. Campbell,Geology of thesouthem Ishtar volcanism!Classification of volcanicfeatures and stmctures, associations, Terra/Guinevereand Sedna Planitia region of Venus,Earth Moon Planets, andglobal distribution from Magellan data, J. Geophys.Res., 97, 13,153- 38, 183-207, 1987. -13,197, 1992. Stofan,E.R., V.L. Sharpton,G. Schubert,G. Baer,D.L. Bindschadler,D. M. Jarde,P., and D. Janrisen,Tectonics of thesouthern escarpment ofIshtar Terra Janes,and S.W. Squyres, Global distribution and characteristics ofcoronae on Venusfrom obse•'ationsof morphologyand gravity, Earth Moon and relatedfeatures on Venus:Implications for originand relationto Planets,31, 141-155, 1984. mantleprocesses, J. Geophys.Res., 97, 13,347-13,578,1992. Kaula, W.M., Venus:A contrastin evolutionto Earth, Science,247, 1191- Tyler,G.L., P.G.Ford, D.B. Campbell,C. Elachi,G.H. Pettengill,and R.A. 1196, 1990. Simpson,Magellan: Electrical and physical properties of Venus'ssurface, Kaula,W.M., Compositionalevolution of Venus,in ChemicalEvolution of the Science,252,265-270, 1991. Earth andPlanets, Geophys. Monogr. Scr., edited by E. Takahashi,E. VorderBmegge, R.W., andJ.W. Head,Fortuna Tessera, Venus: evidence of Jeanloz,and D. Ruble,AGU, Washington,D.C.,in press,1992. horizontalconvergence and cmstal thickening, Geophys. Res. Lett., 16, Kaula,W.M., Implicationsof craterdistribution for surfaceages on Venus, 699-702, 1989. Geophys.Res. Lett., manuscript in preparation. VorderBmegge, R.W., andJ.W. Head,Tectonic evolution of eastemIshtar Kiefer,W.S., M.A. Richards,B.H. Hagcr,and B.G. Bills,A dynamicmodel Terra, Venus. Earth Moon Planets,50/51,251-304, 1990. of Venus'sgravity field, Geophys.Res. Lett. 13, 14-17, 1986. VorderBruegge, R.W., J.W. Head,and D.B. Campbell,Orogeny and strike- Kirk,R.I.,, G.G.Schabcr, O.V. Nikolayeva,B.A. Ivanov,A.V. Potapov,and slipfaulting on Venus: Tectonic evolution of MaxwellMontes,J. G eophys. A.T. Basilevsky,Cleopatra Crater, Venus: The Magellaa view, J. Geophys. Res., 95, 8357-8391, 1990. Res.,manuscript in preparation. VorderBmegge, R.W., andJ.W. Head,Processes of formationand evolution Klose,K.B., J.A. Wood,and A. Hashimoto,Mineral equilibria and the high of mountainbelts on Venus,Geology, 19, 885-888,1991. radarreflectivity of Venusmountaintops; J. Geophys. Res., this issue. Zuber,M. T., Constraintsonthe lithospheric stmctu re of Venus from mechani- Lenardic,A., W.M. Kaula,and D.L. Bindschadler,The tectonicevolution of cal modelsand tectonic surface features, Proc. Lunar Planet. Sci. Conf., westernIshtar Terra, Venus,Geophys. Res. Lett., 18, 2209-2212, 1991. Part2, J. Geophys.Res., 92, suppl.,E541-551, 1987. Lenardic,A., W.M. Kaula, and D.I• Bindschadler,Maxwell andthe Andes: analogousstructures?, Lunar. Planet. Sci. Conf.,23, 773-774, 1992. D. L. Bindschadlerand W. M. Kaula, Departmentof Earth and Masursky,H., E. Eliason, P.G. Ford, G.E. McGill, G.H. Pettengill,G.G. SpaceSciences, University of California, Los Angeles, CA 90024- Schaber,and G. Schubert,Pioneer Venus radar results:Geology from 1567. imagesand altimetry,J. Geophys.Res., 85, 8232-8260, 1980. R. E. Grimm, Departmentof Geology, Arizona StateUniversity, Pettengill,G.H., P.G. Ford, and RJ. Wilt, Venussurface radiothermal crnis- Tempe, AZ 85287. sionasobservedby Magellan, J. Geophys.Res., 97, 13,091-13,102,1992. V. L. Hansen, Department of Geological Sciences, Southern Phillips,R.J., Convection-driven tectonics on Venus,J. Geophys.Res., 95, MethodistUniversity, Dallas, TX 75275. 1301-1316, 1990. K. M. Roberts, Department of Geological Sciences, Brown Phillips,R.J., R.F. Raubertas,R.E. Arvidson,I.C. Sadcar,R.R. Herrick, N. University, Providence, RI 02912. Izenberg,and R.E. Grimm, Impactcrater distribution on Venus: Implica- S. E. Smrekar, Departmentof Earth, Atmospheric,and Planetary tionsfor planetaryresurfacing history, J. Geophys.Res., this issue. Sciences,Massachusetts Institute of Technology,Cambridge, MA Pronin, A. A., The structureof latkshmi Plateau,an indicationof astheno- 02139-4307. spherehorizontal flows on Venus,Geotectonics, 20, 271-280,1986. Roberts,K.M., andJ.W. Head, Laks•i Planum,Venus: Characteristicsand (Received October 16, 1991; modelsof origin, Earth Moon Planets,50/51,193-249, 1990a. revisedJuly 13, 1992; Roberts,K.M. and J.W. Head, Westem Ishtar Terra and Lakshmi Planum, acceptedJuly 13, 1992.)