STYLE AND SEQUENCE OF EXTENSIONAL STRUCTURES IN TESSERA TERRAIN, VENUS

Recent studies have focused on the question of the stratigraphic seque nce and thus the stages of tessera formation, specifically, if tessera are formed by contractional deformation followed by extensional defor mation or vice versa. A major question centers on the interpretation o f specific lineaments within tesserae as graben (bounded by faults sim ilar to 60 degrees) or, alternatively, open tension fractures (dipping similar to 90 degrees). We document and assess the origin of extensio nal structures in tesserae at several locations on Venus, noting the m orphology, continuity along strike, parallelism of walls, stratigraphi c position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrat e that (1) the extensional structures have variable widths, interior s ubparallel lineaments, and ramp terminations; (2) ridges and lineament s are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; an d (3) intratessera plains are seen to embay ridges and an impact crate r is superposed on a ridge and in both cases these features are subseq uently deformed by the extensional structures. We conclude that the mo rphology 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 s izes found in our survey can be formed when the brittle crust is of th e order of 1 to 10 km thick. To further test the tension fracture mode l, we examine the conditions of a Venus that could produce tension fra ctures of the dimension (similar to 1 km width) of extensional structu res found in tessera terrain and find that thermal gradients of a mini mum of 400 to 1500 K km(-1) (heat flows of 800 to 3000 mW m(-2)) are r equired for a range of diabase rheologies and strain rates thought typ ical of Venus during tessera formation. Such a thermal structure would favor partial melting at depths < 1 km. Dike propagation from this re gion of shallow melting within the tensile stress field would produce vast quantities of volcanism, mitigating against the preservation of t he closely associated tension fractures; this volcanism is not observe d. Both the amplitude and sign of changes in surface temperature induc ed by atmospheric warming due to massive outpourings of lava are not c onsistent with the hotspot model. On the basis of our analysis of tess erae, we conclude that the ridges formed first in response to large-sc ale contraction of the crust and that the graben formed contemporaneou sly and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on Ear th such as Tibet and the Altiplano.