Plains Tkctonics on Venus

Tectonic deformation in the ploins of Venus is pervasive, with virtually every area of the planet showing evidence for faulting or fracturing. l’his dcfonna[ion can bc classified into three general categories, deiined by the intensity and arcal extent of the surface deformation: distribolcd deformation, conccntramJ defon]lation, and local fracture ptittcrns. Each of these styles offers informfltion about the tectonic history of the surface, as well as the physical properties and processes of the crust ~nd upper mantle. Distributed deformation is manifested as individun[ly narrow wrinkle ridges, troughs, and fIaC!UreS which occur in subparallc] sels that commonly extend over hundreds of km. The orientations of these seL$ tend to bc gencrolly constant over large regions, providing a valuable indicator of the regiood stress and implying a basal (e.g., mantle convection) origin to this stress. Goss-cutlirlg relati~nsl~ips with o[her geologic feoturcs (such as craters and volcanic Irmdfonns) and among different sets of frm.tures in the same region coo provide relative timing information about [hc evolution of the surface. Conccntmtcd dcfonmrtion occurs in dcfonnzrtion belts, the nature and origin of which arc problcnmtic. The charfic[eris[ics of ridge belts are generally consistent With an origin related to a regional compressive stress, akhough sornc evidence suggests that their formation is not simply related to a regiomrl stress field. lt is not evident how froc[urc bells formed, wi!h brrd] extensional fcfiturcs and elevated topography; the process by which clcfcrrmation is concentnrted into belts is also unclear. L.oc21 fractorc patterns mngc in choroctcr fronl faint, parallel fracture sets to polygonai fracturing reminiscent of cooling or desiccation cracking (albeit on a much ksrgersca]e). The nature. spacings, rind leng, (hs of these featurrs canolTer insight into the mechanical properties of the uppermost Ifiycrs of the crust ond mechanical lithosphere. New flow lows ovailable for dry diabasc have important implications for modeling tectonic feqturcs ond using them to infer the structure of the crust ond mantle. In p~rticulw, these results call into question the importance, or cveo the existence of o weak lower crustal chnnrrcl.