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Flexural Ridges, Trenches, and Outer Rises Around Coronae on Venus

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Flexural Ridges, Trenches, and Outer Rises Around Coronae on Venus JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. El0, PAGES 16,069-16,083, OCTOBER 25, 1992 Flexural Ridges, Trenches,and Outer RisesAround Coronaeon Venus DAVID T. SANDWELL GeologicalResearch Division, Scripps Institution of Oceanography,Za Jol• California GERALD SCHUBERT Departmentof Earthand SpaceSciences, Institute of Geophysicsand Planetary Physics, University of California,Los Angeles High-resolutionaltimetry collected by the Magellan spacecraftreveals trench and outerrise topographic signaturesaround major coronae(e.g. Eithinoha,Heng-O, Artemis, and Latona). In addition,Magellan syntheticaperature radar images show circumferential fractures in areaswhere the platesare curveddownward. Both observationssuggest that the lithospherearound coronae is flexed downwardby the weight of the overridingcoronal rim or by the negativebuoyancy of subductedlithosphere. We havemodelled the trenchand outerrise topography as a thin elasticplate subjected to a line load andbending moment beneath the coronarim. The approachwas testedat northernFreyja Montes where the bestfit elasticthickness is 18 km, in agreement with previouslypublished results. The elasticthicknesses determined by modellingnumerous profiles at Eithinoha,Heng-O, Artemis,and Latonaare 15, 40, 37, and 35 km, respectively.At Eithinoha,Artemis, and Latona where the platesappear to be yielding, the maximumbending moments and elasticthicknesses are similar to thosefound at the Middle America,Mariana, and Aleutiantrenches on Earth, respectively.Estimates of effectiveelastic thickness and plate curvature are usedwith a yield strengthenvelope model of the lithosphere to estimatelithospheric temperature gradients. At Heng-O,Artemis, and Latona, temperature gradients are less than 10 K/km, which correspondto conductiveheat losses of lessthan one half the expectedaverage planetary value. We proposetwo scenariosfor the creationof the ridge,trench, and outerrise topography:differential thermalsubsidence and lithospheric subduction. The topographyof Heng-O is well matchedby the differential thermal subsidencemodel. However, at Artemis and Latona the amplitudesof the trench and outer rise signaturesare a factorof 5 too largeto be explainedby thermalsubsidence alone. In thesecases we favor the lithosphericsubduction model wherein the lithosphere outboard of thecorona perimeter subducts (rolls back) and the corona diameter increases. INTRODUCTION severalprominent coronae which display clear trench and outer rise The lithospheresof Earth and Mars are known to have elastic signatures.Using a thin elasticplate flexure model to characterize upper layers that undergoflexural deformation[Walcott, 1970; the shape of the trench and outer rise, we find that Venusian Comeret al., 1985]. These flexures are commonlycaused by flexures are similar in both amplitude and wavelength to largevolcanic loads on the lithospherewhich producea moat and lithospheric flexures seaward of subduction zones on Earth. outer rise. On Earth, flexures are also associatedwith subduction Moreover, we show that circumferential fractures are concentrated zones; the cold subductedplate appliesa bendingmoment to the in areas where the topographyis curved downward in good not yet subductedlithosphere creating a trench and outer rise agreementwith the high tensile stresspredicted by the flexure [Caldwellet al., 1976]. Despiteits high surfacetemperature (730 models. Finally, we presenttwo scenariosfor the developmentof K), Venusmay alsopossess an upperelastic lithospheric layer that the ridge-trench-outerrise flexuraltopography and circumferential could deform flexurally. A possible example of flexural fracturesof coronae. The first scenarioinvolves reheating and deformationon Venus occurs on the North Polar Plains just thermalsubsidence of the lithosphereinterior to the coronawhile northwardof Freyja Monteswhere the topographyis reminiscent the first scenarioinvolves expansion of the coronainterior and roll of the outerrise andforedeep of a plate underthrustinga terrestrial backof the subductinglithosphere exterior to the corona. mountainrange [Solomon and Head, 1990]. The high-resolution radar images and topographicdata of Venus obtained by the FLEXURAL CHARAUFERISTICS OF CORONAE orbitingMagellan spacecrafthave revealed numerous additional examples of possible flexural deformation of the Venusian The peripheriesof manycoronae consist of a relativelynarrow lithosphere.Prominent among these are the downwarpedtrenches annularridge surroundedby a trench or moat [Barsukovet al., and outerrises around some coronae as well as interiorslopes of 1986; Basilevskyet al., 1986; Pronin and Stofan,1990; Stofan large calderalike structures [Sandwell and Schubert, 1991; and Head, 1990;Solomon eta/., 1991;Squyres et al., this issue]. Solomonet al., 1991;Squyres et al., this issue]. Circumferentiallyoriented fractures (indicative of radialextension) In this paper we focus on flexural signaturesoutboard of are oftenfound within the trenchand on its exteriorwall [Squyres coronalrims with the purposeof inferring the thicknessof the et al., this issue]. Two prominentexamples of this ridge/trench Venusianelastic lithosphere.We presenttopographic profiles signatureare Eithinoha and the northernperimeter of Heng-O [Pettengillet al., 1991;Ford andPettengill, this issue] outboard of (Plate 1, top andbottom, respectively). In theseimages, discrete color changesrepresent changes in elevation (200 m per color change)and variations in graylevel represent the intensity of radar Copyright1992 by the AmedcanGeophysical Union. backscatter. This combination enables one to correlate the Papernumber 92JE01274. fractures,apparent in the syntheticaperture radar (SAR) images, 0148-0227/92/92JE-01274505.00 with the elevation,slope, and curvatureof the surface. 16,069 Plate1. Superposition ofSAR image (brightness variations) andtopography (colorchanges each200 m). Eithinoha (top) is a 400- km-diameterplateau surrounded byridges and deep trenches. Heng-O isa 1200-km-diametercorona;its northern perimeter (bottom)consists ofa ridge,a trench,and an outer rise; circumferential fractures occur in the trench. Dashed line marks locations of profilesin Figure1. SANDWELLAND SCHUBERT: RIDGES, TRENCHES, AND OUTER RISES AROUND CORONAE ON VENUS 16,071 3.0- 2.5 2. O Eithinoha 101.5 0.5 0.0 •-0.5 i i -60 -59 -58 -5F -56 -55 -54 -53 -52 3.0 - 2.5 2.0 Heng-O 1.5 1 0 0.0 J -0.5 0.5I • I I • I I •_ 4 5 6 7 8 9 10 11 12 Latitude (deg) Fig. 1. Topographicprofiles across Eithinoha (top) and northern Heng-O (bottom). Track locations are shown in Plate1. The ridge,treaeh, aad outer rise topography at northeraHeng-O is characteristicof lithospheric flexure under a lineload. Theheavy line above each treaeh marks the loeatioa of the circumferential fractures. Eithinoha(Plate 1, top), locatedat 57øS,8øE, is a plateaulike represents400 m of elevationchange. The interiorof Artemisis structureapproximately 400 km in diameter.The highlyfractured elevatedby about1.5 km with respectto the southeasternplains. interiorof this coronais elevated0.6-1.0 km with respectto the A topographicprofile crossingthe southernrim (Figure 2, top, surroundingplains. Lava flowsemanate from theperimeter of the orbit 1191) revealsthe elevatedinterior, a low-broadridge, a 2.5- coronaand cover someof the fractureson the surroundingplains. kin-deeptrench and ~l.O-km-highouter rise. The heightof the A southto north topographicprofile (orbit 519) crossingthe outerrise was measuredwith respectto the regionaltopographic easternside of Eithinoha (Figure 1, top) reveals its peripheral gradientwhich slopes downward to thesouth. The circumferential ridgesand trenches. In thiscase the trenches are very narrow(-50 fractures,which dominatethe SAR images,are confined to the km) andthe ridgessag toward the centerof the corona. The thick trench and do not extend to the crest of the outer rise. The interior horizontal lines above the trenches(Figure 1, top) mark the of Artemiscontains impact craters and complexfracture patterns locationsof circumferentialfractures apparent in the SAR images. [Stofanet 02.,this issue; McKenzie et al., thisissue]. Basedon the height of the coronae,its fracturedinterior, its Finally, one of the most strikingexamples of a ridge-trench- numerouslava flows, andits lack of impactcraters, we speculate outer rise signatureoccurs at Latona which is a semicircular thatEithinoha is in a relativelyearly stage of its thermomechanical structure(-600 km diameter) located on the southern side of and/ortectonic development [Squyres et 02.,this issue]. Aphrodite(center 22øS, 172øE). The southernpart of Latonais Heng-O(-IøN, 355øE)is a muchlarger corona (-1200 km shownin Plate 2 (bottom)where each color change represents 400 diameter)with a pronouncedridge-trench-outer rise signature m of elevationchange. The interiorof Latonais not a flat plateau alongits northernperimeter as shownin Plate 1 (bottom). The but insteadconsists of two or more topographicridges that have interiorof Heng-Olies at approximatelythe sameelevation as the the morphologyof thrust sheets. On average,the interior of surroundingplains. A topographicprofile (orbit 501) crossingits Latonalies -1.5 km abovethe southeasternplains. A topographic northernrim (Figure 1, bottom), reveals a 1.0-kin-tall ridge profilecrossing the southernmostrim towardthe south(Figure 2, followedby a 0.5-km-deeptrench that is about80 km wide anda bottom,orbit 1376) reveals a 1.5-kin-highridge, a 2.5-kin-deep broad outer rise to the north. The circumferential fractures are trench,and
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