DURATION OF TESSERA DEFORMATION ON VENUS

The density and distribution of impact craters superposed on the highl y deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphica lly younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of Craters; fractured by late-stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, of approximately 30 - 60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the t ransition from the terminal stages of tessera compressional deformatio n to the eruption of the global volcanic plains.