Impact craters on geologic units of northern Venus: Implications for the duration of the transition from tessera to regional plains

Using Magellan SAR images and the Schaber el al, [1998] crater data base we examined impact craters in the area north of 35 degrees N and determined the geologic units on which they are superposed. The crater density of the regional plains with wrinkle ridges (Pwr) was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet (T). About 80 to 97% of the craters superposed on a composite unit that includes materials of Tessera terrain (Tt), Densely fractured plains (Pdf), Fractured and ridged plains (Pfr), and Fracture Belts (FB), also postdate the regional plains. Thus, the time interval between the formation of these older units and emplacement of the regional plains (Delta T) should be geologically short, from a few percent to about 20% of T, or approximately 40 to 150 m.y. This means that in the area under study, volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of Venus (about the last 750 m.y.) was much more active than in the subsequent time.