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GSA Bulletin; July 2002; v. 114; no. 7; p. 839–848; 7 figures.

Artemis: Surface expression of a deep mantle plume on Venus

Vicki L. Hansen*Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA

ABSTRACT

Artemis, a unique 2600-km-diameter cir-cular feature on Venus, defies geomorphicclassification as a corona, crustal plateau,or volcanic rise. Artemis is similar in sizeto plateaus and rises, yet topographicallyresembles a corona. Geologic mapping us-ing correlated digital remote data sets in-cluding NASA Magellan C1-, C2-, and F-scale synthetic aperture radar (SAR)imagery, altimetry, and synthetic stereo hasled to a determination of the geologic his-tory of Artemis. Artemis comprises a largetopographic welt that includes a paired cir-cular ;1-km-deep trough (Artemis Chas-ma) and ;1-km-high outer rise; thus, Ar-temis is divisible into chasma (trough),interior region, and exterior region. Thechasma hosts trough-normal faults andfolds. The interior includes five ;350-km-diameter coronae (quasi-circular featuresmarked by radial and/or concentric frac-tures) that record rich tectono-volcanic his-tories; radial extension fractures andtrough-concentric wrinkle ridges dominatethe exterior tectonic fabric. Artemis formedas a coherent entity; the coronae, chasma,chasma structures, radial fractures, andwrinkle ridges are all consistent with a deepplume model for Artemis formation. Risingand flattening of the plume head led to ear-ly uplift, doming, and radial fracturing asthe plume head collapsed vertically andspread laterally, likely causing outward mi-gration of the trough, as well as fracturingand wrinkle-ridge formation outboard ofthe trough. Within the trough, material waspulled downward, forming normal faultsand folds. The plume continued to spreadlaterally outboard of the trough, resultingin continued radial fracturing and wrinkle-ridge formation. Small-scale interior con-vection cells or compositional diapirs re-sulted in coronae with radial fractures,

*E-mail: [email protected].

and/or concentric fractures and/or folds,and associated volcanism. Artemis is akinin its formation to crustal plateaus and vol-canic rises and likely formed during thetransition from globally thin to thicklithosphere.

Keywords: coronae, diapirs, plateaus,plumes, synthetic aperture radar (SAR),Venus.

INTRODUCTION

Understanding Artemis, a unique 2600-km-diameter circular feature located in Venus’sSouthern Hemisphere, represents a challeng-ing puzzle in the tectonics of Venus becauseArtemis differs from all other features onEarth’s sister planet. Artemis’s planform shapeand size resemble quasi-circular crustal pla-teaus and volcanic rises (diameter ranges of;1600–2500 km), but differ from these fea-tures topographically. Crustal plateaus displaysteep-sided flat-topped regions, and volcanicrises describe broad domical forms, whereasArtemis is defined by a 1–2-km-deep, ;2100-km-diameter circular trough (chasma) that en-compasses a moderately high interior. By con-trast, coronae—small- to moderate-sizedcircular to quasi-circular features that displayradial to concentric fractures or faults and var-iable evidence of volcanism—commonly hostcircular troughs that are topographically sim-ilar to Artemis Chasma. However, Artemis isapproximately twice the size of the largest co-rona, Hengo Corona (1060 km diameter) andmore than an order of magnitude larger thanmedian coronae (200 km diameter). In addi-tion, Artemis’s interior is structurally quitedifferent from that of coronae. Artemis is notobviously classified as a volcanic rise, crustalplateau, or corona (Hansen et al., 1997; Stofanet al., 1997). Thus, Artemis defies classifica-tion as, or correlation with, other geomorphicfeatures on Venus.

With the exception of Artemis, all of Ve-nus’s major geomorphic features can be ac-

commodated in a model in which the Venu-sian lithosphere increased in thickness overtime (Phillips and Hansen, 1998). Crustal pla-teaus and volcanic rises represent the surfacesignatures of deep mantle plumes on ancientthin and contemporary thick lithosphere, re-spectively. Widespread plains volcanism re-sulted from pressure-release melting duringthe thin-lithosphere time, whereas large topo-graphic basins on the plains record contem-porary broad convective downwellings (Her-rick and Phillips, 1992; Rosenblatt and Pinet,1994; Sandwell et al., 1997). Ishtar Terraformed during the thin-lithosphere time andrepresents a crustal plateau welt (expressed atFortuna Tessera) and other crust capturedwithin an ancient broad convective down-welling and preserved by mantle-melt residu-um at depth (Hansen and Phillips, 1995; Han-sen et al., 1997). Coronae represent surfaceexpressions of mantle diapirs (e.g., Squyres etal., 1992; Stofan et al., 1992, 1997) probablyformed during both thin- and thick-lithospheretime. Many coronae contribute to relatively re-cent volcanic resurfacing in some plains ba-sins (Guest and Stofan, 1999; Young et al.,2000; Rosenberg and McGill, 2001; Hansenand DeShon, 2002, Hansen et al., 2002). Al-though some workers have suggested that Ar-temis represents a region of lithospheric un-derthrusting (Schubert et al., 1994; Brown andGrimm, 1995, 1996, 1999), such an interpre-tation is not easily reconciled with the localtectonic setting (Brown and Grimm, 1995),with the evolutionary picture just described or,as shown here, with Artemis’s geologic rela-tionships. Spencer (2001) interpreted a part ofArtemis’s interior as a region of extensivecrustal extension similar to a terrestrial meta-morphic core complex; he did not place thestudy within a regional or global context.

This paper examines the geology of Arte-mis and the surrounding region in order to de-termine the surface evolution of Artemis. Inthis paper, the term ‘‘Artemis’’ describes thelarge circular geomorphic feature centered atabout 338S, 1338E, including Artemis Chas-

840 Geological Society of America Bulletin, July 2002

V.L. HANSEN

Figure 1. Negative SAR image of Artemis.

ma, the raised region interior to the chasma,and the adjacent exterior region (Fig. 1). Thisbroad definition of Artemis frees one from as-sumptions of the spatial or temporal relation-ships of regions on either side of ArtemisChasma. The major tool of exploration usedhere is geologic mapping. The map shownherein is generally consistent with mapping ofBrown and Grimm (1995), who had access tothe same data, although the resulting interpre-tation differs from theirs. Artemis embodiesthree major features that collectively recorddeformation related to Artemis formation in-cluding Artemis Chasma and the regions in-terior and exterior to the chasma. Artemishosts six coronae or corona-like features—fivewithin its interior and one within the chasma.The data indicate that chasma, interior, and ex-terior all formed broadly contemporaneouslyand that together they record Artemis’s evo-lution as the surface signature of a deep man-tle plume; the coronae record either small-scale convection or compositional diapirsspawned from the hot thermal plume. Arte-mis’s unique morphology, geologic character,and geologic history may indicate that Arte-mis formed during Venus’s transition fromthin to thick lithosphere.

DATA AND METHODOLOGY

The data resulted from the NASA MagellanMission (1991–1994; Ford et al., 1993); cor-related digital data sets used here include Ma-gellan C1-, C2-, and F-scale synthetic apertureradar (SAR) imagery (;225, 675, and 110 mresolution, respectively), altimetry, and syn-

thetic stereo imagery (Kirk et al., 1992). Thisanalysis follows general Magellan data anal-ysis and feature identification outlined by Fordet al. (1993) and mapping criteria describedby Tanaka et al. (1994), with caveats high-lighted by Hansen (2000). Structural-elementand geologic-facies interpretations followmethods outlined previously (Keep and Han-sen, 1994; Ghent and Hansen, 1999; Hansenet al., 2000). SAR images are either left-look(similar to left illumination) or right-look. Asa general rule, radar brightness is a functionof surface roughness and orientation; radar-bright regions are rough and/or oriented to-ward incident radar, whereas radar-dark re-gions are smooth or oriented away from radar(Ford et al., 1993); the opposite is true fornegative SAR images shown herein. Structuralfacies typically show up better in negative im-ages. Artemis’s circular nature lends itself toan analogue clock reference frame with northat 12:00, east at 3:00, south at 6:00, and westat 9:00. Elevation on Venus is typically citedin relationship to mean planetary radius(MPR), 6051.84 km; this convention is fol-lowed herein.

This paper builds on the work of Brown andGrimm (1995) that focused on Artemis Chas-ma. Although Brown and Grimm (1995) usedthe same data set used here, my processingmethods differ from theirs; Brown and Grimm(1995) used an airbrush shaded-relief ap-proach to view three dimensions, whereassynthetic stereo images are used herein).Readers should refer to Brown and Grimm(1995) for regional overviews of Artemistopography and SAR imagery and for nu-

merous detailed SAR and shaded-relief im-ages that are extremely instructive in featureidentification.

ARTEMIS GEOLOGY

The Venusian surface is dominantly com-posed of volcanic rocks; lack of surface waterand the paucity of eolian erosion on Venus(e.g., Kaula, 1990) rules out major sedimen-tary packages, and the lack of widespread ero-sion makes exposure of crustal metamorphicor intrusive igneous rocks unlikely. Impactcrater-related, landslide, or eolian deposits arepresent, but areally restricted. This analysisemploys basic geologic- and structural-map-ping methods in order to construct a geologicmap of the Artemis’s surface, aimed at under-standing the nature of the spatial and temporalevolution of tectonism and volcanism acrossArtemis. Artemis consists of an ;2600-km-diameter topographic welt near the termina-tions of Diana and Dali chasmata (Fig. 1;Brown and Grimm, 1995, Figs. 1 and 2). Thewelt includes an ;2100-km-diameter, ;1-km-deep circular trough (Artemis Chasma) pairedwith an ;1-km-high outer rise (Sandwell andSchubert, 1992). Inward from the trough aninner rise reaches up to 4 km, but typical in-terior highs stand at 1.5–2 km; interior north-east- and northwest-trending high and lowsare also preserved locally. A reduced ver-sion of the geologic map (Fig. 2) compiledat 1:5 000 000 scale agrees with the charac-terization and location of most structural fea-tures defined by Brown and Grimm (1995,Fig. 16); therefore, detailed justification ofgeologic-element identification is not dis-cussed. The current map covers a slightly larg-er region and examines the interior geology ingreater detail than that of Brown and Grimm(1995) and includes additional primary andsecondary structures as well as additional geo-logic and radar units.

At Artemis, radar brightness is locally afunction of both tectonic and volcanic fabricsthat can be difficult to delineate at the scaleof the data (e.g., Fig. 3A). A radar-bright fa-cies is defined herein that includes Brown andGrimm’s (1995) ‘‘fine-scale fabric’’ as a radarunit, but not as a geologic unit owing to itscontaining mixed genetic facies. The radar-bright facies occurs in the interior, exterior,and within the chasma (Fig. 2). Major trend(s)of penetratively developed tectonic fabrics(i.e., spaced at or below data resolution)marked by lineaments or suites of lineamentsare highlighted. Where the character of thetectonic fabric is not clear, only the trends arerepresented. This radar facies does not repre-

Geological Society of America Bulletin, July 2002 841

ARTEMIS: SURFACE EXPRESSION OF A DEEP MANTLE PLUME ON VENUS

sent a distinct geologic unit formed at a spe-cific time; in both the interior and the exteriorregions, the boundaries between radar-brightand -dark facies preserve a range of temporalrelationships. Some flow units postdate tec-tonic patterns as evidenced by embayment re-lationships (e.g., Fig. 3A, C, and F), whereasin other cases, tectonism clearly postdates theradar-dark facies as evidenced by the feather-ing or jagged trace of tectonic fractures orfaults (e.g. Fig. 3B and C). Earlier-formed tec-tonic fabrics are locally preserved as kipukasrepresenting local highs protected by their el-evation from low-viscosity lava flows (e.g.,Fig. 3E and G). Examination of SAR and syn-thetic stereo imagery, discussed in detail in thefollowing sections (Artemis Chasms, InteriorFeatures, and Exterior Features), indicates thattectonism and volcanism were spatially andtemporally intimately related throughout for-mation of the radar-bright and -dark facies andthat the development of both the radar-brightand -dark facies was diachronous.

Artemis includes three major elements (Fig.2): (1) Artemis Chasma, an almost complete(;3008) circular topographic trough that con-tains trough-parallel normal faults and folds;(2) an interior region that hosts five coronae(C1–C5) or corona-like features (e.g., Stofanet al., 1992)—variably defined by radial frac-tures (or faults), concentric fractures (orfaults), and/or folds—that display contempo-raneous volcanism and tectonism includingnumerous volcanic shields; and (3) an out-board region cut by trough-parallel wrinkleridges and trough-normal fractures, the latterof which are locally overprinted by troughstructures.

Artemis Chasma

Artemis Chasma lies 1–2 km below meanplanet radius and ranges in width from ;25–150 km. The trough, which is well definedexcept from ;10:00 to 11:30, hosts a linearfabric that is penetratively developed at thescale of observation and that everywhere par-allels the trough axis. The fabric consists ofnormal faults and folds as noted by Brown andGrimm (1995). Normal faults dominate thefabric from 11:30 to 2:00, whereas folds dom-inate from 6:00 to 10:30. Corona C6, definedby radial lineaments interpreted as extensionfractures or extensional faults, interrupts thechasma at 6:00. From 2:00 to 6:00, the troughdisplays an asymmetric slope and morpholo-gy; normal faults are developed along the in-ner steep slope, and folds have formed alongthe outer slope (Figs. 2 and 4). Chasma foldsdecrease in wavelength from the outer trough

toward the trough axis. Along and within thetrough axis, folds and faults are indistinguish-able as individual lineaments and fall belowdata resolution, as noted by Brown andGrimm (1995). Fault-bend folds developedalong the outer trough margin (Suppe andConnors, 1992) display left-stepping andright-stepping patterns at ;2:00 and 5:00, re-spectively, indicating that shortening was notexactly normal to the trough axis at these lo-cations (Brown and Grimm, 1995). Normalfaults typically step downward into the troughinterior. Locally normal faults, which strikeparallel to fold axes, likely cut folds as notedby Brown and Grimm (1995), although inmany cases, data resolution does not allow ro-bust feature identification or interpretation oftemporal relationships.

Within parts of the chasma, extensionalfractures or normal faults (graben of Brownand Grimm, 1995) trend perpendicular to thechasma folds. For brevity, the structures arereferred to herein as ‘‘fractures,’’ although itis recognized that they comprise fractures,normal faults, and locally grabens. The frac-tures extend into the chasma from the exteriorand locally to the interior (e.g., 5:00). In gen-eral, these structures, which are radial to thechasma, predate the folds (Brown and Grimm,1995), although locally they could have beenreactivated during or after fold formation. Thefractures reflect topographic lows as indicatedby local filling by younger surface flows (e.g.,408S, 1438E), evidence that they record minorextension normal to their trend. AlthoughBrown and Grimm (1995, their Fig. 12) sug-gested that these structures were distortedwithin the chasma by ;60% shortening per-pendicular to fold crests, the region east of thestrain measurements hosts at least two suitesof fractures oriented parallel to their undis-torted and ‘‘distorted’’ equivalents (Fig. 5).The apparent distortion results from combin-ing two orientations of fractures (Fig. 5);thus, suggestions of 60% shortening areunsupported.

The western chasma fades at ;10:30 and,from there, widens and deepens in a counter-clockwise fashion. At 9:00, three distinctsuites of lineaments occur: Closely spacednortheast-trending fractures; more widelyspaced northwest-trending paired fractures orgrabens; and north-northwest–trending folds(Fig. 3B). Northeast-trending fractures formedprior to northwest-trending fractures and foldsas evidenced by their spacing and distributionon either side of the fold belt. Northwest-trending fractures dominantly predate thefolds, although some may have been reacti-vated after or during fold formation. North-

east-trending fractures occur on either side ofthe narrow chasma fold belt and are discern-ible at the fold crests with little (,58) changein orientation, indicating that although the sur-face is downwarped at this location, the sur-face material has not been underthrust orremoved.

At ;1:00, Brown and Grimm (1995) inter-preted a region of 50–250 km of left-lateralshear within the chasma. They based this in-terpretation mainly on a correlation of a fine-scale penetrative (at the scale of observation)‘‘lattice’’ fabric purported to be preserved inboth the interior and exterior (their Fig. 14).At ;12:30 within the chasma, just north oftheir lattice fabric, an early-formed penetrativelinear fabric traverses the trough with no clearevidence of significant left-lateral shear (Fig.2). As suggested by Brown and Grimm(1995), much or all of the apparent left-lateraldisplacement could have resulted from exten-sion associated with normal faults that strikeparallel to the trough axis. At the location ofthe proposed displacement, the trough is ;80km wide and hosts numerous trough-parallelnormal faults (Brown and Grimm, 1995, theirFig. 14B). If zero extension is assumed acrossthis 80-km-wide zone (contrary to the mappednormal faults), the maximum left-lateral dis-placement is ;100 km; given the presence ofnormal faults parallel to the trough, this esti-mate is a maximum value. Thus, 100% exten-sion across the zone would yield a maximumleft-lateral displacement of 50 km; 150% ex-tension would result in the total apparent 100km displacement, with no true left-lateral dis-placement. Additionally, if this region actuallywas the site of significant left-lateral shear,then the normal faults should strike ;308from the trough axis (interpreted as parallel tothe shear zone) rather than parallel to thetrough axis as documented by Brown andGrimm (1995) and confirmed by mappinghere. Thus, this part of the chasma may (ormay not) record moderate (;50 km) left-lat-eral shear, but 250 km of displacement isunlikely.

The chasma is poorly defined from ;10:00to 11:00. Normal faults and scarps associatedwith Quilla Chasma to the west and radial andconcentric fractures associated with small co-ronae merge with Artemis-related structures.A relatively late local lava flow that emergedfrom Quilla Chasma structures flowed south-ward, locally covering radial flows from co-rona C5 within the interior of Artemis (dis-cussed subsequently; Fig. 2). This flow is cut,in turn, by east-trending fractures that couldbe related to C5 or could represent reactivatedstructures.


V.L. HANSEN

Figure 2. Geologic map of Artemis region (simplified from a compilation at 1:5 000 000 scale). Locations of Figure 3 images shown byletters; locations of Figures 4 and 5 shown by numbers; coronae C1–C11 labeled.

Brown and Grimm (1995) divided ArtemisChasma into five distinct regions—the north-east, east, southeast, south, southwest margins.Although differences in structural fabrics arepreserved along the chasma, as has alreadybeen noted, the current analysis did not un-cover any clear boundaries along the chasma(Fig. 2), nor do discontinuities appear inBrown and Grimm’s tectonic map (1995, theirFig. 16). Most specifically there seems to beno evidence to suggest that the southern andsouthwestern parts of the chasma are distinctfrom the rest of the chasma. Chasma foldsfade at ;10:00 and, from there, increase in in-tensity in a counterclockwise direction, and thewidth of the fold belt and fold spacing increasecontinuously along the chasma. At 7:00–8:00,the chasma fold belt narrows at a region spa-tially correlative with a northeast-trending ex-terior fracture zone. Temporal relationshipsbetween the chasma folds and the fracture

zone are poorly determined. However, crustalanisotropy represented by the fracture zonecould have influenced coherent formation ofthe chasma, and therefore the presence of thefracture zone should not be used to define fun-damentally discrete regions of the chasma.Similarly, at 6:00, corona C6—marked by ra-dial fractures—deflects chasma fold crests andfaults, indicating that C6 was at least in partpresent while the chasma, folds, and faultsformed. The strain partitioning that resultsfrom C6 is consistent with a mechanically het-erogeneous crust, as would be expected for atectonically active terrestrial planet. The dif-ference in the chasma structure at the juncturewith either the fracture zone or C6 does notprovide robust evidence that different parts ofthe chasma formed at different times or rep-resent fundamentally different geologic eventsas previously suggested.

Interior Features

The interior preserves five coronae or co-rona-like features defined by radial fracturesand/or by concentric fractures or folds and ra-dial lava flows, informally labeled C1–C5(Fig. 2). The coronae, individually and collec-tively, record a rich history of spatially andtemporally overlapping deformation (frac-tures, faults, folds) and volcanism. Small butnumerous shields (e.g., Head et al., 1992; Ad-dington, 2001) associated with each coronaappear to have formed relatively late, but itmay be that earlier-formed shields are simplynot observable with current data resolution.

C1 is defined by several independent ele-ments that describe a circular feature: (1) ra-dial fractures centered about (2) a radar-darkregion that hosts numerous shields and that issurrounded in turn by (3) a region of radar-bright facies and (4) outboard radial lava flows



Figure 3. Negative SAR F-tile (full resolution), right-look (R) images of selected areasacross Artemis. Tile-center location in each image labeled. Tile long dimension is ;200km. Tile locations are shown by letters A–H in Figure 2; analogue clock–type directionsrefer to the whole structure shown in Figure 2. (A) Artemis radar brightness is locally afunction of both tectonic and volcanic fabrics that can be difficult to delineate at the scaleof the data. (B) Northeast-trending closely spaced fractures and younger, northwest-trend-ing, more widely spaced fractures, deformed by north-northwest–trending fold crests at9:00 within Artemis chasma. (C) Volcanic flows, locally with or as shields, form a thinlayer that locally veils early-formed tectonic fabric. (D) At ;8:00 in Artemis, interior flowsabut the chasma folds and are locally deformed by them. (E) Penetrative tectonic fabrics(horizontal features) are preserved as kipukas; the boundaries with younger lava surfaces(smooth areas) preserve details to the scale of data resolution. (F) Radar-bright and -darkflows from corona C3 that fill local structural topography resulting from penetrative tec-tonic fabric are cut by locally north-trending fractures concentric to C3. (G) Along theeast side of corona C4, northeast-trending grabens cut earlier flows, which in turn variablycover an early-formed penetratively developed tectonic fabric. (H) An early-formed pen-etrative northeast-trending tectonic fabric (upper right) is cut by northwest-trending gra-bens (lower right), which are in turn cut by spaced, northeast-trending scarps or ridges(upper left).

Figure 4. SAR image of part of southeastArtemis Chasma with topographic profilealong transect shown (i.e., the MPR line).Normal faults are developed along thesteep in-trench slope; folds are developedalong the more gently sloping outer-trenchslope. Topographic profile after Brownand Grimm (1996) with 253 verticalexaggeration.

Figure 5. Negative right-look SAR F-tilecentered at 358S, 1458E shows at least twoorientations of fractures west of Artemischasma. The apparent deflection of frac-tures within the chasma by chasma foldsresults from the (incorrect) association ofdifferent parts of separate fracture suites;individual fractures are not deflected byfolds along their trace, as shown by theblack lines, which parallel, but are offsetfrom, the fracture trace, allowing the read-er to follow the continuity of individualfractures.

that flow away from C1 toward C3/C4 andC5. Radial fractures are also preserved withinand cut the radar-bright facies. Three smallvolcanic structures lie along the north side ofC1. Volcanic flows form a thin layer that lo-cally veils early-formed tectonic fabrics (Fig.3C). Shields, likely related to C1, are cut byinboard normal faults at ;328S, 1428E. At C2,located south of C1, radial fractures variablycut both the radar-bright facies and parts ofthe flooded and/or shield-bearing interior. C2also displays northwest-trending fractures andscarps.

C3 and C4 might be considered a doublecorona with C3 subsidiary to C4. Radial frac-tures, concentric fractures and scarps, and ra-dial lava flows define the center of C4, an el-liptical corona with C3 forming a focusmarked by faint radial fractures, and concen-tric fractures and radial flows developed along

its eastern side (Fig. 2). Radial flows that em-anate from C4 flow to the southwest andnorthwest, locally covering earlier-formed ra-dial fractures, yet also cut by radial fractures(Fig. 6). In addition, the flows are cut bynortheast-trending fractures and pit chains(Okubo and Martel, 1998) that parallel thetrend of the tectonic fabric in the underlyingradar-bright facies. The northeast boundary ofthese flows clearly indicates that the flowspostdate the formation of the radar-bright fa-cies at this location, but contact relationshipsalong the southwest margin of the flow areless obvious. At ;9:00–8:00, the interiorflows appear to abut the chasma folds and tobe locally deformed by them (Figs. 3D, 6).Fractures radial to C4 apparently cross, andare cut by, the chasma fold belt at 9:00–10:00;locally flows cover the radial fractures, al-though other radial fractures cut the flows.


V.L. HANSEN

Figure 6. C1-negative SAR image of coronaC4 in the southwest interior of Artemis.Fractures radial to C4 are cut by chasmafolds and locally covered by flows radial toC4; yet lava flows from C4 are locally but-tressed by chasmata folds (see also Fig. 3D),and late radial fractures cut C4 flows. Lo-cations of B, D, E, and G in Figure 3 areshown by letters; sz indicates location of the‘‘right-lateral shear zone’’ of Brown andGrimm (1995); this zone can be interpretedas part of the annulus to C4/C3. Circularfeature in the middle of the image is an im-pact crater.

Tectonic fabrics, penetrative at the scale of ob-servation, are preserved locally; the boundar-ies with younger lava surfaces preserve detailsof the scale of data resolution (Fig. 3E). Ra-dar-bright and -dark flows from C3 that filllocal structural topography resulting frompenetrative tectonic fabric are cut by locallynorth-trending fractures concentric to C3 (Fig.3F). Taken together, these features reflect acomplex tectono-volcanic history of C4/C3and suggest that corona evolution overlappedin time with formation of Artemis Chasmafrom 6:30–10:00. Similar interrelated tectono-volcanic relationships are preserved on theeast side of C4 where northeast-trending gra-bens cut earlier flows, which in turn variablycover an early-formed penetratively developedtectonic fabric (Fig. 3G). Each of the coronaealso hosts numerous small volcanic shields.Shields generally range from 2 to 10 kmacross and are probably ,0.5 km high (Kres-lavsky and Head, 1999, Addington, 2001).The shields occur extensively in places across

the radar-bright facies, as well as within andacross the radar-dark facies. Individual shields,which can be either radar-dark or -bright,overprint and are cut by local tectonic struc-tures; thus, tectonism and volcanism over-lapped in time. The most obvious shields are,of course, those that represent the youngestlocal features, but this observation indicatesmore about preservation than about strict rel-ative temporal relationships. Pit chains, whichprobably represent magma at depth with localmagma erosion to the surface (Okubo andMartel, 1998), generally parallel the trends oflocal tectonic fabric and provide evidence ofrelatively late magmatic activity.

The interior of C4/C3 comprises Spencer’s(2001) southern interior deformation belt (hisFig. 2), and the southwestern scarp of C4/C3represents the interior north-northwest–trend-ing deformation belt of Brown and Grimm(1995, their Fig. 9). The combination of con-centric folds, ridges, faults, and scarps definesan asymmetric annulus to coronae C4/C3(Figs. 2 and 6). Brown and Grimm (1995, p.233) postulated right-lateral shear along thesouthwest part of the annulus on the basis ofthe ‘‘discrete fault (scarp) and en echelon gra-ben[s].’’ The ‘‘en echelon graben[s]’’ couldsimply represent deflection of radial fracturesalong the asymmetric C4/C3 corona annulus(Figs. 2 and 6). Spencer (2001) interpreted thenortheast part of the annulus as a right-lateralshear zone and stated that the penetrative tec-tonic fabric ends abruptly at this zone and ex-ists only to the north. However, the penetra-tively developed northeast-trending tectonicfabric extends for some distance on either sideof both the southwest and northeast scarps, in-dicating a general continuity of the surfaceacross the annulus and scarps and that the pen-etrative tectonic fabric predated annulus andscarp formation (Figs 2 and 6; Brown andGrimm, 1995, their Fig. 9; Spencer, 2001, hisFigs. 2–4).

The northernmost corona feature, C5, is theleast corona-like. The radar-bright facies dom-inates C5, although local radial fractures, con-centric fractures, and scarps and radial flowsdeveloped along the northwest margin of C5lead me to classify it as a corona. C5 encom-passes much of the northeastern interior de-formation belt of Spencer (2001) and includesnortheast-trending ridges, scarps, and lava-flow-filled valleys. C5 probably records themost complex history of the interior coronae;some of its history may predate formation ofthe radial flows and concentric fractures andscarps, attributed to corona formation. Frac-tures radial to C5 extend across the chasma at;11:00–1:00, although it is difficult to deter-

mine whether these structures represent a ge-netically related structure suite. Parts of C5record many episodes of deformation; for ex-ample, at location H (Fig. 2), an early-formedpenetrative northeast-trending tectonic fabricis cut by northwest-trending graben, which arein turn cut by spaced northeast-trending scarpsor ridges (Fig. 3H). Locally, lava flows fill thevalleys, interpreted as rift-related graben(Brown and Grimm, 1995; Spencer, 2001).The variable muted to sharp preservation ofthe tectonic fabric across much of C5 mightbe due to local volcanism similar to that pre-served elsewhere (Fig. 3, A, C, G).

In summary, five coronae, which individu-ally and collectively record intimately relatedtectonism and volcanism, dominate the inte-rior of Artemis. Although Brown and Grimm(1995) defined the interior lava flows as ‘‘in-terior volcanic plains’’ and subsequently in-ferred a temporal correlation to ‘‘exteriorplains,’’ these flows are likely corona-derivedflows, and no direct evidence correlates themwith exterior ‘‘plains.’’ Although there may beremnants of early-formed ‘‘plains,’’ one can-not point to any specific region that cannot beattributed to either the multiphase radar-brightfacies or to coronae.

Exterior Features

The region outside Artemis Chasma is quitevaried. The region to the north (outside themap area) includes part of the crustal plateauThetis Regio, which is locally overprinted bycoronae, and fractures associated with Kuanja,Virava, Quilla, and Diana chasmata. Severalsmall coronae may mark the termination ofDali and Juno chasmata, to the east and westof Artemis, respectively. South from ;4:00 to9:00, the exterior is notably free of coronae orfractures, except for the ;400-km-wide frac-ture zone at ;7:00 and C6 within the chasmaat 6:00. Trough-normal fractures or grabensand trough-parallel wrinkle ridges are variablydeveloped outboard of the chasma. Fracturesare preserved over ;3208 (or more) from;11:00 clockwise to ;9:30. Chasma andchasma folds locally overprint the radial frac-tures as illustrated by Brown and Grimm(1995) and confirmed by mapping presentedhere. Overprinting relationships are document-ed from 2:00 to 10:00 (and possibly to 1:00)within the chasma (Fig. 2). However, as pre-viously noted, there is no clear evidence thatchasma folds distort the fractures. Trough-concentric wrinkle ridges generally lie fartheroutboard of the radial fractures and are pre-served at ;2:00, and from 4:00 to 9:00, ex-cept within the fracture zone at 7:00.



Figure 7. Map illustrating spatial correla-tion of the centers of the circles defined byvarious topographic and structural ele-ments associated with Artemis.

The radial fractures, wrinkle ridges, or bothare locally either not preserved or neverformed. For example, at 4:30 just outboard ofthe chasma (408S, 1438E), radial fractures arenot obvious (Brown and Grimm, 1995). How-ever, close examination of SAR imagery in-dicates that postfracturing lava flows locallycover the fractures, providing evidence that ra-dial fractures originally formed at this loca-tion, that the fractures are topographic valleys,and that local exterior volcanism postdatessome radial fracture formation. At 2:00–4:00,wrinkle ridges are not present outboard of thechasma. However, within this region, smallcoronae, C7–C9, dominate the tectonic fabric(Fig. 2). Flows from the coronae locally coverArtemis-related radial fractures, but elsewhereArtemis-related radial fractures may cut flowsrelated to these coronae. Because coronae re-cord complex tectono-volcanic histories (e.g.,Baer et al., 1994; Copp et al., 1998; Guest andStofan, 1999; Rosenberg and McGill, 2001;Hansen and DeShon, 2002) and because struc-tures can be reactivated, one should not inter-pret local lava-flow and fracture relationshipsas evidence of robust temporal relationshipsbetween outboard coronae and Artemis. Sev-eral interpretations are valid: (1) If the coronaepredated Artemis, then wrinkle ridges mightnot have formed along this region of aniso-tropic crust. (2) If coronae postdated Artemis,then earlier-formed wrinkle ridges could havebeen buried or destroyed. (3) If the coronaeformed broadly contemporaneously with Ar-temis, then the local crustal environment orstress regime might not have favored wrinkle-ridge formation. Thus, the lack of wrinkleridges along this region does not differentiatebetween models of Artemis’s formation, nordoes it set limits on temporal relationships be-tween Artemis and outboard coronae.

Wrinkle ridges are also absent outboard ofthe chasma at ;7:00 where the northeast-trending fracture zone intersects the chasma.Relative temporal relationships between thefracture zone and chasma are so far undefined.Although Brown and Grimm (1995, p. 242)indicated that crosscutting relationships sup-port an interpretation that the fracture zone(‘‘rift’’ in their terminology) is younger, theydid not elaborate (furthermore, the image citedby Brown and Grimm does not include thefracture zone or the region southwest of thechasma). Numerous small volcanic shields oc-cur along and within the fracture zone. On thebasis of reasoning similar to that already pre-sented herein, it is not clear (1) whether theseshields cover earlier-formed wrinkle ridges,(2) whether wrinkle ridges never formed atthis location because of an earlier-imposed

fracture-zone anisotropy, or (3) whether thefracture zone and Artemis formed contempo-raneously. Given that fracture zones displayinterrelated tectonic and volcanic histories(Bleamaster and Hansen, 2001; Hansen andDeShon, 2002) and given that Artemis recordsa complex tectonic history, it is dangerous touse a single temporal relationship, or even asmall suite of locally preserved temporal re-lationships, to extrapolate local temporal con-straints to regional scales.

GEOHISTORY INTERPRETATION

Although the geologic map presented hereis generally consistent with that of Brown andGrimm (1995), I propose a different geologichistory related to three fundamental differenc-es in interpretation. (1) Artemis Chasmaformed as a coherent entity rather than as sep-arate features, as evidenced by the continuityof the chasma and chasma structures illustrat-ed both in the map herein (Fig. 2) and that ofBrown and Grimm (1995, their Fig. 16). (2)Related to item 1, the radial fractures and con-centric wrinkle ridges outboard of the troughformed broadly contemporaneously with thechasma and chasma structures and are genet-ically related to trough formation. (3) Part (ifnot all) of the interior tectonic fabric and lavaflows are related to interior corona formation,and these coronae formed during Artemis evo-lution and do not record widespread, early‘‘plains’’ volcanism or ‘‘captured real estate’’(e.g., Brown and Grimm, 1995; Hansen et al.,1997).

Chasma folds and normal faults, chasma-normal fractures, and outboard chasma-con-centric wrinkle ridges can all be interpretedwithin a coherent tectonic framework consis-tent with temporal and spatial relationshipsgleaned from available Magellan data sets.These structures describe coherent patternsrelative to the chasma along the length of thechasma; specifically, (1) folds or normal faultseverywhere parallel the trend of the chasma;normal faults generally occur inboard of foldswhere both occur along the same length of thechasma; (2) fractures strike perpendicular tothe chasma at all points along it; (3) exteriorwrinkle ridges trend parallel to (or concentricto) the trend of the local chasma; and (4) eachof these elements—chasma, folds, normalfaults, fractures, and wrinkle ridges—indepen-dently define circular features whose centersall lie within the same general region in thecenter of Artemis (Fig. 7). Such spatial cor-relation is consistent with a genetic relation-ship among these diverse geomorphic andstructural features. Local relationships can ex-

plain local deviations from these general pat-terns, as already discussed. Thus, it is viableto conclude that Artemis Chasma and its foldsand normal faults represent a coherent tectonicfeature that formed contemporaneously withradial fractures and concentric wrinkle ridges.

In contrast, Brown and Grimm (1995) pro-posed that the southeastern, eastern (2:00–6:00),and northeastern (12:00–2:00) parts of Arte-mis Chasma formed as a result of northwest-directed underthrusting and related left-lateralshear, respectively, and that the southern,southwestern and western parts of ArtemisChasma (6:00–10:00) formed during earlierunrelated event(s).

The current study indicates that much of theradar-dark, and hence smooth, regions previ-ously interpreted as ancient plains (Brown andGrimm, 1995; Hansen et al., 1997), representregions of relatively recent volcanism (al-though there are also numerous locationswhere local tectonism outlasted volcanism);thus, these surfaces are relatively young ratherthan old. Furthermore, interior volcanic flowsrelate to corona formation rather than ancientvolcanic plains. Although some of the de-formed material within the radar-bright faciescould in part represent ancient ‘‘plains,’’ thereis no direct evidence to support this interpre-tation. Even if a part of the interior could beshown to preserve robust evidence of ancientplains, such preservation could be accommo-dated in a wide range of models of Artemisevolution (including the model proposedherein).

Additionally, relationships noted herein in-dicate broadly contemporaneous development


V.L. HANSEN

of the interior and the chasma. Fractures radialto C5 and C4/C3 are locally cut by folds inthe western chasma, yet volcanic flows asso-ciated with C4/C3 cover these radial fracturesand are locally buttressed by chasma folds. Fi-nally, fractures radial to C4 cut these flows.Similar arguments can be made for each in-terior corona; thus, broadly contemporaneousformation of the Artemis interior and the chas-ma are permissible by the current data, where-as proposals that the interior formed prior tothe chasma are unsupported.

Spencer (2001) postulated that the center ofArtemis represents the grooved surface of a300 by 300 km molded footwall of an exten-sive extensional system; he argued that north-west-trending grooves formed as a result ofplastic molding of the footwall by irregulari-ties on the underside of the hanging wall.However, the penetrative fabric trends north-east across this region (his Fig. 3). AlthoughSpencer stated that the northeast-trending fab-ric terminates at his ‘‘en echelon zone’’ andthus formed after the northwest-trending‘‘grooves,’’ this statement is not supported bySAR imagery (his Figs. 3 and 4), my mapping(Fig. 2), or the mapping of Brown and Grimm(1995). The northeast-trending fabric extendsacross the en echelon zone to the south and isalso contiguous with a similar fabric to thenorth (Fig. 2). The regional development andthe tight-spacing and along-strike character ofthe northeast-trending fabric are more easilyreconciled with early formation of this fabric.

Collectively, I interpret that exterior radialfractures—locally overprinted by chasma-par-allel folds—formed owing to broad radial ex-tension centered on Artemis. Early (and lo-cally continuing) radial extension wasfollowed by formation of local interior coro-nae and the entire Artemis Chasma. Withinthis context the chasma, folds, normal faults,radial fractures, and concentric wrinkle ridgesdisplay a kinematically consistent patternacross Artemis; there is no need to infer dis-tinct geologic events for any of these elementsor for different parts of the chasma. The cir-cular nature of the chasma and the continuityand similarity of structures along the chasmaargue against a two-phase and serendipitousalignment of detailed tectonic structures.

ARTEMIS EVOLUTION

The major features of Artemis—broad to-pographic signature, evidence of interior ex-tension, circular topographic trough, concen-tric contractional and extensional troughstructures, radial fractures, and exterior con-centric wrinkle ridges—can be accommodated

within the spirit of the mantle-plume modelsof Artemis formation proposed previously(Herrick and Phillips, 1990; Griffiths andCampbell, 1991; Sandwell and Schubert,1992). Although the models may require somemodification, the general proposal that Arte-mis represents the surface signature of a largemantle diapir on the lithosphere not only ispermissible, it is strongly supported by geo-logic relationships. Herrick and Phillips(1990) first suggested a diapiric (so-called‘‘blob’’) interpretation for Artemis (and Ovda,Thetis, and Beta Regiones) prior to the acqui-sition of Magellan data. Although I favor thespirit of their model, the original postulatedstages of evolution are not supported by Ma-gellan data (see discussion in Phillips andHansen [1994] and Hansen et al., [1997]), butneither are such stages required by a diapirmodel. The size and circular shape of Artemisare consistent with formation by the interac-tion of a deep mantle plume on the litho-sphere. Gravity-topography analysis is consis-tent with at least partial dynamical support forArtemis (Simons et al., 1997). As a deep man-tle plume rises toward a lithosphere, the lith-osphere will be uplifted, and, if the strengthof the lithosphere is exceeded, radial fracturescould form above the plume head. A circulartrough could also form, as illustrated in lab-oratory experiments aimed at modeling the in-teraction of thermal plumes with the litho-sphere (Griffiths and Campbell, 1991). Inthese experiments, as a plume head approach-es a rigid horizontal boundary, it collapses andspreads laterally; a layer of surrounding man-tle is squeezed out from between the plumeand the surface, resulting in a gravitationallytrapped asymmetric instability and formationof an axisymmetric trough. In addition, the in-terior squeeze layer can lead to convection ona scale much smaller than that of the originalplume. These smaller-scale instabilities couldinteract with the lithosphere inside the axi-symmetric trough. It was on the basis of Grif-fiths and Campbell’s experiments that a plumemodel for Artemis formation was proposedfollowing release of Magellan data (Griffithsand Campbell, 1991). Finite-element modelsof the interaction of a thermal plume with alithosphere, aimed at modeling corona topog-raphy, also show development of an axisym-metric trough above large thermal mantleplumes (Smrekar and Stofan, 1997). Smrekarand Stofan (1997) applied their model to co-ronae; however, their model provides a betteranalogue for Artemis because their resultingtrough-to-trough diameter is .1200 km—much larger than median coronae (200 km di-ameter) (Stofan et al., 1992).

The chasma structures could also form dur-ing the plume-lithosphere interaction. Withinthe trough, material is pulled downward, re-sulting in the formation of folds and normalfaults at the surface (Smrekar and Stofan,1997). One can envision a range of deforma-tion that could be preserved within such atrough, including (1) minor contraction withpreservation of the earlier-formed surface fea-tures such as at 9:00 within the trough (Fig.3B); (2) dominant extension of the trough sur-face marked by trough-parallel normal faults(e.g., at 12:30–1:30); and (3) shortening andextension within the trough, perhaps resultingfrom inward or outward movement of thetrough during plume evolution. Outwardmovement of the trough might be expected toresult in normal faults developed along a steepinner slope and folds developed along the out-er slope (and leading edge of the trough) ofthe chasma as documented from 2:00 to 6:00(Figs. 2, 4). The trough could also show a to-pographic asymmetry as developed in the fi-nite-element model of Smrekar and Stofan(1997). The asymmetry of the axisymmetrictrough (i.e., steep slope along the innertrough) as well as the angle of downwellingbeneath the plume head formed within Smre-kar and Stofan’s (1997) model is consistentwith both the topographic asymmetry of Ar-temis Chasma from 3:00 to 5:00 and the dis-tribution of normal faults and folds within thechasma.

Thus, each of the major elements that definecircular features associated with Artemis canbe accommodated within a model involving adeep mantle plume. Rising and flattening ofthe plume head led to early uplift and domingof the surface and the formation of radial frac-tures. As the plume head collapsed verticallyand spread laterally, a squeeze layer wastrapped, and an axisymmetric trough formed.Both the trough (chasma) and the troughstructures could have deformed early-formedradial fractures. Spreading of the plume lat-erally outboard of the trough might have re-sulted in continued radial-fracture formationand development of contractional structures(wrinkle ridges) concentric to the trough andplume head. The region interior to the topo-graphic trough could have been affected bysmall-scale convection cells or compositionaldiapirs resulting in local corona-like structureswith radial fractures, and/or concentric frac-tures or folds, and associated volcanism.

In arguing against a mantle-diapir interpre-tation for Artemis, Brown and Grimm (1995,p. 245) stated that a diapiric model for Arte-mis formation would require ‘‘interior radiallysymmetric shortening.’’ However, although



both empirical and finite-element modeling ofdiapiric structures show interior radial short-ening, neither requires interior radially sym-metric shortening even if a plume interactswith a homogeneous lithosphere (e.g., With-jack and Scheiner, 1982; Griffiths et al., 1989;Griffiths and Campbell, 1991; Moore et al.,1999). A heterogeneous lithosphere wouldlead to less symmetry. Although the interiorof Artemis indeed lacks evidence of radial axi-symmetric shortening, the distribution ofmostly exterior radial fractures and concentricwrinkle ridges display evidence of regionallydeveloped quasi-axisymmetric shortening(Figs. 2 and 7).

Brown and Grimm (1995, p. 245) furtherstated that Artemis ‘‘chasma has mainly ex-perienced convergence and horizontal shearrather than extension.’’ Two points are rele-vant to consider (Brown and Grimm, 1995,their Fig. 16) (Fig. 2). (1) Contraction withinthe chasma, preserved along ;2008, is con-centric to the chasma—not limited to thesoutheast margin as emphasized by Brownand Grimm (1995). (2) The chasma preservesevidence of both extension and shortening;both could occur in an axisymmetric troughformed as a result of the interaction of a largediapir with a lithosphere (e.g., Griffiths andCampbell, 1991; Smrekar and Stofan, 1997).Therefore, a diapir model for Artemis cannot,and should not, be dismissed. Emphasized byBrown and Grimm (1995) also argued againstcoherent formation of the chasma and itsstructures because they could not accommo-date all of the chasma deformation into a mod-el of 250 km of northwest-directed under-thrusting beneath Artemis, which theyproposed for the eastern part of the chasma.Such reasoning is circular. In addition, the un-derthrusting model encounters problems inlight of recent mapping, as follows.

1. ‘‘Upper-plate’’ volcanism—representedby the interior volcanic flows, shields, and pitchains—would not be expected in the under-thrusting model because of a depressed ther-mal gradient. Terrestrial arc volcanism resultsfrom water fluxing and the resulting melting-point depression, but Venus’s anhydrous rocklacks water for fluxing (Kaula, 1990).

2. The underthrusting model does not ad-dress the remarkable continuity of chasmastructures and structural patterns relative tochasma location. Few geologic features defineremarkably circular features as a result of ser-endipitous alignment of unrelated events.

3. The locations of chasma folds and nor-mal faults are not entirely consistent, and arearguably inconsistent, with the underthrustingmodel. Assuming that the chasma is analo-

gous to a terrestrial subduction trench, thechasma normal faults and folds from 3:00 to6:00 are located opposite from where onewould predict. Normal faults occur on theleading edge of the postulated ‘‘upper-plate,’’whereas chasma folds occur along the bend ofthe downgoing ‘‘plate’’ (Fig. 4). Furthermore,the overall width of the deformed belt (;150km) would seem to suggest steep underthrust-ing, yet such an inference is contrary to thedevelopment of a trough over an ;1008 arcwith a diameter of ;2100 km, which wouldimply shallow underthrusting.

4. If the northeast margin (12:00–2:00) re-corded the postulated significant left-lateralshear, then normal faults should strike at ;308from the trough axis (interpreted as a parallelto the shear zone), rather than trend parallel tothe trough axis.

5. Finally, as noted (Brown and Grimm,1995), the exterior fractures would be expect-ed to form concentric to the outer high, ratherthan perpendicular as documented.

Following Brown and Grimm (1995), Spen-cer (2001) postulated that the center of Arte-mis represents the grooved surface of a 300by 300 km molded footwall of an extensiveextensional system. He argued that northwest-trending grooves formed as a result of plasticmolding of the footwall by irregularities onthe underside of the hanging wall. However,the penetrative fabric trends northeast acrossthis region and likely formed prior to Spen-cer’s ‘‘grooves’’ and ‘‘en echelon zone’’; thusthis fabric is difficult to reconcile with mold-ing of a ductile footwall.

IMPLICATIONS AND CONCLUSIONS

Venusian crustal plateaus and volcanic ris-es, which define quasi-circular structures sim-ilar in size to Artemis, also represent the sur-face signature of large mantle plumes (Hansenand Willis, 1998; Phillips and Hansen, 1998;Hansen et al., 1999, 2000). Differences in geo-physical signature, topographic profile, sur-face structural patterns, and geologic historiesof plateaus and rises reflect differences inglobal lithospheric thickness during the timeof plateau and rise formation; crustal plateausformed in ancient thin lithosphere, whereasvolcanic rises formed in contemporary thicklithosphere. Phoebe Regio, a unique featurethat has some plateau and some rise charac-teristics, is proposed to have formed duringthe transition from thin to thick lithosphere.Artemis also appears to represent a transition-al form of a plume signature, probably formedin late thin-lithosphere time, prior to forma-tion of Phoebe Regio. The very short wave-

length or closely spaced penetrative tectonicfabric across Artemis might be consistent withdeformation of an extremely thin layer andthus perhaps indicate a very shallow depth tothe local brittle-ductile transition during thedevelopment of these fabrics. In contrast,Phoebe Regio preserves a much less penetra-tively developed fabric (Hansen and Willis,1996), consistent with a thicker brittle (strong)layer and hence perhaps a deeper regional brit-tle-ductile transition. If the thermal energytransported by all Venusian deep mantleplumes is grossly similar and if the Venusianlithospheric composition is approximatelyuniform, then differences in the depth to thebrittle-ductile transition across a region af-fected by a plume would be a function of lith-ospheric thickness; possible temporal relation-ships between Phoebe Regio and Artemisrequire further study. On the basis of data pre-sented herein, Artemis likely represents thesurface signature of a deep mantle plume onthe surface of Venus, and Artemis probablyformed at, and thus records, a relativelyunique time in the evolution of Venus’slithosphere.

ACKNOWLEDGMENTS

This work was supported in part by U.S. NationalAeronautics and Space Administration grantsNAG5-4562 and NAG5-10586. Discussions with,and comments by, V. Bennett, I. Campbell, G. Da-vies, D. DePaolo, J. Goodge, R. Griffiths, C. Mer-iaux, and S. Turner were extremely helpful, but anyomissions or oversights are mine. This work grewout of a research sabbatical at the Australia NationalUniversity (ANU), funded by Southern MethodistUniversity and hosted by ANU. I thank the staff atboth institutions for making my visit possible, andJ. Stamatakos and D. Ferrill for helpful reviews.

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