Lakshmi Planum, Venus: Characteristics and models of origin

Abstract

Lakshmi Planum is distinctive and unique on the surface of Venus as an expansive (-2 x lo6 km2), relatively smooth, flat plateau containing two large shield volcanoes and abundant volcanic plains in the midst of a region of extreme relief. It rises 3-5 km above the datum and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. The major units mapped on Lakshmi are volcanic edifices, smooth, ridged and grooved plains units, and structural units referred to as ridged terrain. Three styles of volcanism are observed to dominate the surface of Lakshmi. Distributed effusive volcanism is associated with extensive plains deposits and many of the small shields, domes and cones mapped within the plateau. Centralized effusive volcanism is primarily associated with the paterae, Colette and Sacajawea, and their circumferential low-shieldforming deposits. The precise origin and evolution of these unusually large and complex structures is not understood, although a catastrophic, explosive origin is unlikely. Pyroclastic volcanism may be represented by a unit referred to as the "diffuse halo". The origin and evolution of Lakshmi Planum is closely related to its compressional tectonic environment; volcanism on Lakshmi has occurred synchronously with tectonism in the surrounding erogenic belts. A model for the origin and evolution of Lakshmi Planum consisting of a continuous sequence of convergence and horizontal shortening of crustal segments against a preexisting block of tessera seems best able to account for the elevation, plateau shape and irregular polygonal outline of Lakshmi, as well as the presence of ridged terrain and its resemblance to tessera. Volcanism on Lakshmi is proposed to be the result of basal melting of a thickened crustal root. According to this model, the origin and evolution of Lakshmi Planum has consisted of the following sequence of events: (1) formation of a large, elevated block of tessera surrounded by low-lying plains; (2) convergence and underthrusting of crustal segments to produce peripheral mountain ranges, thickening, and uplift of the plateau; and (3) basal melting of the thickened crust and underthrust material and surface volcanism that occurred synchronously with continued edge deformation.

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area, Venus: Results from Arecibo image data

“…A bright plains unit is concentrated in the area of eastern Lavinia where the digitate plains occur and this may be evidence that the unit is relatively young compared to the dark plains elsewhere in the mapped area. Examples of individual radar dark flows superposed on radar bright units in this region and elsewhere (Stofan et al 1987;Roberts and Head, 1990) indicate that there are exceptions to the general radar darkening with time trend .…”

“…They are found associated with linear deformation zones in Phoebe Regio, Themis Regio, the region made up of Ushas, Innini and Hathor Montes and along a linear trend extending to the southeast toward Lada Terra from Eve. The edifices possess a variety of characteristics; Ushas Mons shows a structure similar to the volcano Sif Mons (Campbell et al, 1989;Senske, 1990), while edifices mapped in the vicinity of Tefnut Mons show evidence of large caldera structures and are more similar to Colette and Sacajawea Roberts and Head, 1990).…”

“…The presence of radar-bright flows might then indicate that volcanic activity occurred in the last several hundred million years and that Ushas Mons is the youngest of the three volcanoes. However, radar-dark flows are observed to superpose and thus post-date, bright flows in several places on Venus (Roberts and Head, 1990). Thus the difference in radar brightness of lava flows may be dominated by textural and/or compositional differences rather than age differences.…”

“…volcanic activity on the planet. Although stratigraphic evidence indicates that these bright flows are some of the youngest units in the area, observations elsewhere (Campbell et al, 1989;Roberts and Head, 1990) indicate that radar dark flows are locally found superposed on radar bright flows in data from the same radar system. This, combined with the fact that variations in surface brightness are seen with range along these flows, suggest that it is unlikely that flow-brightness variations are solely the result of aging processes; variations in parameters such as lava rheology and thermal structure also need to be considered.…”

“…The lack of distinctive features and the homogeneous nature of the plains could be attributed to a greater age (lack of recent activity and degradation of older features) but the distribution of craters is not markedly different here than elsewhere. In addition, Lakshmi Planum appears relatively homogeneous in Arecibo images although abundant detail is observed in Venera 15-16 images Roberts and Head, 1990). Thus, viewing geometry and radar system characteristics may also be factors in the interpretation of Helen Planitia.…”