Style and sequence of extensional structures in tessera terrain, Venus

Recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation, specifically, if tessera re formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within tesserae as graben (bounded by faults -•60 ø) or, alternatively, open tension fractures (dipping -•90ø). We document and assess the origin of extensional structures in tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (-• 1 km width) of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km -1 (heat flows of 800 to 3000 mW m -2) are required for a range of diabase rheologies and strain rates thought ypical of Venus during tessera formation. Such a thermal structure would favor partial melting at depths < 1 km. Dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism, mitigating against he preservation of the closely associated tension fractures; this volcanism is not observed. Both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model. On the basis of our analysis of tesserae, we conclude that the ridges formed first in response to large-scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on Earth such as Tibet and the Altiplano.