JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B9, PAGES 12,103-12,131, SEPTEMBER 10, 1989 Tests of Crustal Divergence Models for Aphrodite Terra, Venus ROBERT E. GRIMM 1 AND SEAN C. SOLOMON Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge Aphrodite Terra, the largest highland region of Venus, is a likely site of mantle upwelling, active volcanism, and extensional tectonics. We examine three alternative kinematic models for the interaction of mantle convection and the surface: one in which little horizontal surface displacement results from mantle flow ("vertical tectonics"), one in which shear strain from large horizontal displacements is accommodatedonly in narrow zones of deformation ("plate divergence"), and one in which strain from large horizontal motions is broadly accommodated ("distributed deformation"). In support of a divergent plate model for Aphrodite, Head and Crumpler have cited the presence of an organized system of lineaments held to be analoguesto oceanic fracture zones; small-scale bilateral symmetry of topographic elements, hypothesized to be rifted and separatedrelief analogousto some terrestrial oceanic plateaus; and subsidence of topography proportional to the square root of distance, consistentwith that expected for a divergent thermal boundary layer. We undertake quantitative tests of each of these assertions, and we compare the results with those from similar tests of the Mid-Atlantic Ridge. We find that apart from the long-wavelength symmetry of broadly elevated regions, there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet. The fit of the topographyof Aphrodite to that of a thermal boundary layer is in general much poorer than for the Earth, and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography. The broad saddle-shaped region between Thetis and Atla Regiones shows the best fit to the root-distance relation, yielding apparent spreading half rates of a few centimeters per year, but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed. Calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two-plate model, regardlessof past pole motions. A multiple-plate geometry is therefore called for, and one or more of the lineaments must act as a plate boundary along nearly its full length, if the plate divergence hypothesis is correct. Such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming Magellan radar images. In the absenceof such evidence, as long as the existence and distribution of the lineaments are verified, then broad disruption of a thin lithosphere is favored. In such a model, lineaments may be surface manifestations of mantle convective flow. A model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by Magellan. INTRODUCTION Aphrodite Terra is the largest highland region on Venus, stretchingnearly 15,000 km in length [Pettengill et al., 1980]. Aphrodite Terra together with Beta Regio and its extensions constitutethe equatorial highlands, •which are marked by elevated topography, large free-air gravity anomalies, and numerous large-scale extensional and volcanic structures [Masursky et al., 1980; Phillips et al., 1981]. Because of the strong correlation of long-wavelength gravity and topogra- phy and the large apparent depth of isostatic compensation for several equatorial highlands regions [Esposito et al., 1982; Sjogren et al., 1983], it is likely that these character- istics are the result of upwelling mantle convective flow [Phillips and Malin, 1983]. The planform of this convection and its interaction with the surface, however, are matters of dispute. Several work- ers [Phillips and Malin, 1983; Morgan and Phillips, 1983; Banerdt, 1986; Kiefer et al., 1986] have proposed mantle plume or hotspot models for the equatorial highlands, with the implication that regional tectonics involves largely ver- tical motions. It has alsobeen suggested [Kaula and Phillips, 1Now atDepartment of Geological Sciences, Southern Methodist University, Dallas, Texas. Copyright 1989 by the American Geophysical Union. Paper number 89JB01020. 0148-0227/89/89JB-01020505.00 1981] that Aphrodite Terra may resemble a terrestrial diver- gent plate boundary. Head and Crumpler [1987] have sharp- ened this hypothesisby presenting evidence for an organized pattern of lineaments [Crumpier et al., 1987] and elements of bilateral topographic symmetry [Crumpier and Head, 1988a] in a direction parallel to these lineaments. The mechanical lithosphere of Venus, however, may be too weak to behave as a rigid plate, so a third scenario is that upwelling and divergence of mantle flow are accommodated at the surface by large-scale horizontal motions manifested as broadly distributed deformation. In this paper, we first review the characteristics of Aph- rodite Terra and their relation to these kinematic tectonic models. We undertake several quantitative tests of evidence proposed to support the rigid plate model, namely, that the lineaments, inferred to be fracture zone traces, are organized in a geometrical pattern describable by plate kinematics, that the topography is consistentwith a diverging, cooling ther- mal boundary layer, and that the rise displays bilateral topographic symmetry. Since terrestrial analogy is an impor- tant part of the plate divergence hypothesis, we perform the same tests on the Mid-Atlantic Ridge, restricting our infor- mation to a level comparable to that for Venus [cf. Kaula and Phillips, 1981]. We examine the implications of these tests for the other models as well, and we suggestobserva- tions from the upcoming Magellan mission that will lead to a discrimination among these competing hypotheses. 12,103