Processes of crustal formation and evolution on Venus: An analysis of topography, hypsometry, and crustal thickness variations

Abstract

On Venus, present evidence indicates a crust of predominantly basaltic composition and a relatively young average age for the surface (several hundreds of millions of years). Estimates of crustal thickness from several approaches suggest an average crustal thickness of lo-20 km for much of the lowlands and rolling plains and a total volume of crust of about 1 x 10" km3, approximately comparable to the present crustal volume of the Earth (1.02 x 10" km3). The Earth's oceanic crust is thought to have been recycled at least lo-20 times over Earth history. The near-coincidence in present crustal volumes for the Earth and Venus suggests that either: (1) the presently observed crust of Venus represents the total volume that has accumulated over the history of the planet and that crustal production rates are thus very low, or (2) that crustal production rates are higher and that there is a large volume of "missing crust" unaccounted for on Venus which may have been lost by processes of crustal recycling.Known processes of crustal formation and thickening (impact-related magma ocean, vertical differentiation, and crustal spreading) are reviewed and are used as a guide to assess regional geologic evidence for the importance of these processes on Venus. Geologic evidence for variations in crustal thickness on Venus (range and frequency distribution of topography, regional slopes, etc.) are outlined. The hypothesis that the topography of Venus could result solely from crustal thickness variations is assessed and tested as an end-member hypothesis. A map of crustal thickness distribution is compiled on the basis of a simple model of Airy isostasy and global Venus topography. An assessment is then made of the significance of crustal thickness variations in explaining the topography of Venus. It is found that the distinctive unimodal hypsometric curve could be explained by: (1) a crust of relatively uniform thickness (most likely lo-20 km thick) comprising over 75% of the surface, (2) local plateaus (tessera) of thickened crust (about 20-30 km) forming less than 15% of the surface, (3) regions of apparent crustal thicknesses of 30-50 km (Beta, Ovda, Thetis, Atla Regiones and Western Ishtar Terra) forming less than 10% of the surface and showing some geologic evidence of crustal thickening processes (these areas can be explained on the basis of geologic observations and gravity data as combinations of thermal effects and crustal thickening), and (4) areas in which Airy isostasy predicts crustal thicknesses in excess of 50 km (the linear erogenic belts of Western Ishtar Terra, less than 1% of the surface).It is concluded that Venus hypsometry can be reasonably explained by a global crust of generally similar thickness with variations in topography being related to (1) crustal thickening processes (orogenic belts and plateau formation) and (2) local variations in the thermal structure (spatially varying thermal expansion in response to spatially varying heat flow). The most likely candidates for the formation and evolution of the...

Constraints on the Venusian crustal thickness variations in the isostatic stagnant lid approximation

“…For Model B, where a crust of constant thickness is assumed to float on top of the lithosphere we limit the maximum lithospheric thinning to 25, 50 and 75% of the global average thickness through the addition of regions of thickened crust. We vary the global average crustal thickness from 20 to 80 km to include both thin crust cases [e.g., Head , 1990; Nimmo and McKenzie , 1998] as well as thick crust cases where the thickened crust would be deeper than the gabbro to eclogite phase transition. The global average lithospheric thickness is varied between 300 and 600 km to include a range of suggested thicknesses [ Smrekar and Phillips , 1991; Turcotte , 1993; Kucinskas and Turcotte , 1994; Phillips , 1994; Smrekar , 1994; Moore and Schubert , 1995; Nimmo and McKenzie , 1996; Solomatov and Moresi , 1996; Vezolainen et al , 2004; Orth and Solomatov , 2011].…”

“…It is interesting to note that there are some similarities between Airy models of crustal thickness variations proposed in the past [ Head , 1990] and our models. Airy models produced variations about a 50 km average ranging from 40 to 145 km with the majority of the topography corresponding to crustal thicknesses between 45 and 55 km [ Head , 1990]. This roughly agrees with our models.…”

Geology and structure of Beta Regio, Venus: Results from Arecibo Radar Imaging

“…Analyses from Pioneer Venus image data suggest that material to the west of Beta, in the area not imaged with the Arecibo system, is tessera-like [Senske, 1990]. In the initial step, we suggest the presence of a region of tessera, interpreted as a thick crust [Smrekar and Phillips, 1990;Head, 1990b] within the east flanking region of tessera is parallel to Devana Chasma, but it may be related to deformation and rift formation along a preexisting zone of weakness. In the second model, Mantle Plume/Active Crust (Figure 3b), an upwelling plume causes uplift and underplating to account for the high topography and results in volcanism producing the edifices and plains on the topographic high.…”

Western Ishtar Terra and Lakshmi Planum, Venus: Models of formation and evolotion

“…In terms of height and area, Lakshmi Planum as a whole falls well within the range of values reported for tessera (elevation 2.5-5.0 km; area ~20 x 105 km2). The first-order elevation differences between the tessera and surrounding plains are attributed primarily to initial tessera crustal thickness (Head, 1990b) ) results in volcanism on the plateau, and regions of ridged terrain represent exposed islands of the underlying tessera block which have not been buried by volcanic deposits. The presence of ridged plains indicates that deformation and volcanism were at least partly contemporaneous (Roberts and Head, 1990a).…”

“…Any density differences (Masursky et al 1980) could add to the elevation. If crustal thickening by underplating of underthrust material has caused uplift of the tessera, then the additional elevation could be accounted for by Airy isostasy and increased crustal thicknesses in the 30-50 km range (Head, 1990b;Roberts and Head, 1990a,b). Evidence favoring this process is the preferential location of much of the high topography on Lakshmi in bands adjacent to the mountain belts (Fig.…”