JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. El2, PAGES 26,015-26,028,DECEMBER 25, 1994 Structural history of Maxwell Montes, Venus: Implications for Venusian mountain belt formation Myra Keep and Vicki L. Hansen Departmentof GeologicalSciences, Southern Methodist University, Dallas, Texas Abstract. Modelsfor Venusianmountain belt formationare importantfor understanding planetarygeodynamic mechanisms. A rangeof data setsat variousscales must be consideredin geodynamicmodelling. Long wavelengthdata, such as gravity and geoid to topographyratios, needconstraints from smaller-scaleobservations of the surface.Pre-Magellan images of the Venusiansurface were not of highenough resolution to observedetails of surfacedeformation. High-resolutionMagellan images of Maxwell Montesand the otherdeformation belts allow us to determinethe natureof surfacedeformation. With theseimages we canbegin to understand the constraintsthat surfacedeformation places on planetarydynamic models. Maxwell Montes andthree other deformation belts (Akna, Freyja, and Danu montes)surround the highland plateauLakshmi Planum in Venus'northern hemisphere. Maxwell, thehighest of thesebelts, stands11 km abovemean planetary radius. We presenta detailedstructural and kinematic study of Maxwell Montes. Key observationsinclude (1) dominantstructural fabrics are broadly distributedand show little changein spacingrelative to elevationchanges of severalkilometers; (2) the spacing,wavelength and inferred amplitude of mappedstructures are small;(3) interpretedextensional structures occur only in areasof steepslope, with no extensionat the highesttopographic levels; and (4) deformationterminates abruptly at thebase of steepslopes. One implicationof theseobservations is thattopography is independentof thin-skinned,broadly distributed,Maxwell deformation.Maxwell is apparentlystable, with no observedextensional collapse. We proposea "deformation-from-below"model for Maxwell, in which the crust deformspassively over structurallyimbricated and thickenedlower crust. This modelmay have implicationsfor the otherdeformation belts. Introduction beneath; lava from a small rille from the crater floods -64,000 km2 of easternMaxwell andwestern Fortuna Tessera. The Maxwell Montes is one of four deformation belts (Akna, steepnorthern, western and southernslopes of the mountain Freyja,Danu andMaxwell Montes)that surround the highland rangevary in slopefrom 2 ø over-300 km (northand south), to plateau Lakshmi Planum in Venus' northern hemisphere 30 ø over several tens of kilometers (west) [Smrekar and (Figure1). LakshmiPlanum rises 4 km abovemean planetary Solomon,1992]. Deformationterminates abruptly at the base radius(MPR) with the fringingdeformation belts at elevations of thesesteep, deformed slopes. The easternslope of Maxwell of .-.5 km (Danu), 6 km (Akna), 7 km (Freyja), and 11 km gradesinto westernFortuna Tessera. (Maxwell) above MPR. Outboardof Lakshmi Planum, all the Maxwell Montes is enigmaticbecause of its vast elevation. deformedbelts are adjoinedby areasof intenselywrinkled, Any mechanismfor the supportof Maxwell mustalso apply to complexlydeformed crust known as tesserae,which lie at an IshtarTerra as a whole, and thushave significantimpact on averageelevation of 5 km. Together, Lakshmi Planurn,the planetarygeodynamic models. Most workersfavor dynamic deformation belts and the tesserae define a continent-sized supportfor Ishtar Terra [e.g., Pronin, 1986; Basilevsky,1986; regionreferred to asIshtar Terra. Kiefer and Hager, 1989; Bindschadleret al., 1990; Grimm and Maxwell Montes lies at the eastern edge of Lakshmi Phillips, 1990, 1991; Bindschadlerand Parmentier, 1990]. Planum. It is the highest and steepestfeature on Venus, Grimm and Phillips [1991] provide a discussion of the towering4 km aboveFreyja Montes,the next highestof the geophysicalmerits of isostaticversus dynamic models. Venusiandeformation belts. It occupiesan area-850 km long Current dynamic-supportmodels for Ishtar Terra suggest by 700 km wide, and the adjacenttessera, western Fortuna thatit is the siteof eitherlocal mantle upwelling [Pronin, 1986; Tessera,covers an area greater than 1.5 million km 2. MaxwellBasilevsky, 1986; Grimm and Phillips, 1990, 1991], or mantle is characterizedby dominant, northwest-trending, radar-bright downwelling [Bindschadler et al., 1990, 1992; Bindschadler lineaments,and an impactcrater, Cleopatra (the highestcrater and Parmentier, 1990; Lenardic et al., 1991]. In order to on Venus), that lies on the eastern slope. Ejecta from evaluatevariations in crustaldensity, topography and gravity, Cleopatracovers most of centralMaxwell, obscuring structures both models use scales greater than those observedfor the individual deformation belts of Ishtar Terra. As a result, none of these models are capable of predicting the nature of Copyright1995 by theAmerican Geophysical Union. structures, orientations or kinematics within the Ishtar deformedbelts, althoughon a broad scalemantle-upwelling Papernumber 94JE02636. and -downwelling mechanisms provide a means of local 0148-0227/95/94JE-02636505.00 crust/mantlethickening around Ishtar Terra. 26,015 26,016 KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS 78øN 30øE Freyja Akna Lakshmi Planum Maxwell 56ON Danu 300øE Figure 1. (a) Portion of Magellan C2-MIDRP.60N333;2 showingIshtar Terra, includingAkna, Freyja, Danu, and Maxwell Montes, and adjacenttesserae. Black areasare missingdata. (b) Topographiccontours for area representedin Figure la. Contoursare in meters. Dashedline is easternsinuous boundary lineament. The character, wavelength, and spatial distribution of PhysiographicDivisions stamctureson Maxwell providesconstraints for the surfaceand crustal deformation of individual Ishtar mountain belts. Maxwell Montes comprisesfive physiographicprovinces: Geophysicaldata provide global constraintson lithosphere- the northwest arm, the eastern and western ridges, the area and mantle-scale structures. The constraints from each of these associatedwith the impactcrater Cleopatra, and the southern data sets together provide insights into mechanismsof slope(Figure 2). Theseprovinces are discussed in turnbelow. mountain belt formation. Current models for the structural The northwestarm comprisesa triangular-shapedregion that evolution of Maxwell [Vorder Bruegge and Head, 1989; juts westwardfrom the main bodyof the mountainbelt. It is Vorder Brueggeet al., 1990] did not have the benefit of high- separatedfrom the rest of Maxwell by a sinuouslineament that resolutionMagellan images,and somestructures predicted by trends southwest from 69øN, 4øE to 66.5øN, 359øE, and then these models are not seen in Magellan data. This paper trends south to 65øN, 0øE, where it marks the western presentsa structuralanalysis of Maxwell Montes. We useour boundaryof Maxwell, extendingto 61.5øN, 2øE (Figure 2). observationsto proposea mechanismfor the deformationand This lineament is herein referred to as the 0øE lineament. Most support of Maxwell, in which the upper crust deforms of the northwestarm is at lower elevationthan the main body passivelyover a structurallyimbricated and thickenedlower of Maxwell. The highestarea of the arm (-10 km aboveMPR) crust layer that supportsthe short-wavelength(500 km) is centered around 66.4øN, 357.5øE, and elevations decreasein mountain-belt-scaletopography [e.g. Phillips and Hansen, all directionsaway from the 0øE lineament,to a low of-5.5 1994]. The favoredmodel explainsthe observeddeformation km above MPR around 67.3øN, 353.5øE. The northern flank featuresof Maxwell Montes, and it may have implicationsfor of the arm slopes2 ø over -300 km [Smrekar and Solomon, other Ishtar deformation belts. 1992]. KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS 26,017 ..::•.:: ß....,.:•.,.;:,':.: •. ..... ?i'-- •, ..... : ?; ,, :--.: ..... :;'.:'<.•." .., ;.,:.•:: ,. *'•.•., ';::• .. .'.:". ß :........ .:,.......•.....::• ::•,:. -".:.-:...:::;:. ..:•::::- .. •,.,.... ..... :::• : ....•. • <.•,: ß .,.. .....:.................. -.•............-..... ::,............ :. :-- . .:'"• 4".'• ....•:..--.-• .....--......•-.-.: ':•"-' .....½½-." .•::.:•:•' :-'. .... ::.:::...::..........'-.:--.• ,;,..:..•?:,:., ..... :...<•..:•-•:•.•••••,..• .•••4:•::.,.•,•:.. .::•'•':.:4:. '•.. ,, 70.5øN 20øE ) NW1&rm Cleo Eastern Ridges Western Impact Rid s area Southern 60.5øN Slope 350øE Figure 2. (a) Compositeimage of Maxwell Montes. Black stripesare missingdata. Dark circle at centeris the impactcrater Cleopatra. The 0øE lineamentis visible as offsetof radar-darkmaterial at left, just abovecenter. (b) Topographiccontours for Figure2a showingthe physiographicprovinces described in the text, locationsof Figures2a, 4a and5a, andthe locationof the 0øE lineament described in the text, shown as dashed line. The easternand westernridges, crater-modifiedarea, and adjacent plains; the eastern boundary, a sinuous trough, the southernslope are all part of the main body of Maxwell extendingfrom 62.5øN,11 ø to 68øN 10øE,marks the beginning Montes. Transitionsbetween these physiographic provinces of an ~200 km transition between Maxwell and western are gradational;they are distinguishedon elevation changes Fortuna Tessera. and extent of impact ejecta, both of which causevariations in The easternridges encompassthe radar-dark area east of radar-brightness.The northernboundary of the main body of Cleopatra,extending from 63ø to 68øN, and 9 ø to 13øE. They Maxwell is the 0øE lineament. Steepelevation changes mark occupythe topographicallylowest part of Maxwell,