Duration of tessera deformation on Venus

Abstract

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undefonned. 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, or approximately 30 -60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.

Geologic units on Venus: evidence for their global correlation

“…Later work by Gilmore et al (1997), which concluded that the average surface age of tessera was about 1.4 T, con®rmed this estimate. The di cult part of the tessera age estimate, besides the fact that the morphologically rough and radar bright surface of this terrain camou¯ages smaller craters from identi®cation (see Basilevsky et al, 1997a;Gilmore et al, 1997 for details), is the large error bars and the problem of boundary craters.…”

“…Later work by Gilmore et al (1997), which concluded that the average surface age of tessera was about 1.4 T, con®rmed this estimate. The di cult part of the tessera age estimate, besides the fact that the morphologically rough and radar bright surface of this terrain camou¯ages smaller craters from identi®cation (see Basilevsky et al, 1997a;Gilmore et al, 1997 for details), is the large error bars and the problem of boundary craters. The ®rst is due to the relatively small number of on-tessera craters which, in turn, is a result of the relatively small percentage of the surface of Venus occupied by tessera, about 8% according to Ivanov and Head (1996), and the previously mentioned rather small total number of impact craters on the planet.…”

“…The morphology of the Psh plains is evidence that their materials are composed of ma®c lavas. Steep-sided volcanic domes a few tens of kilometers in diameter are sometimes observed in association with Psh plains and are thought to be possibly genetically related (Ivanov and Head, 1997,b, 1999b. The distinctive dome morphology is evidence of the high viscosity of lavas and may indicate silicic composition or high abundance of vesicles Pavri et al, 1992).…”

“…The average surface age of tessera, which comprises the predominant part of Fortuna Group, was found to be about 1.4 T (Gilmore et al, 1997) with a possible range from 1.01 to 1.93 T (Ivanov and Basilevsky, 1993). The large error bars are due to the small number of craters superposed on tessera terrain.…”

The geologic history of Venus: A stratigraphic view

“…Because none of about 80 craters observed on tessera show evidence of early tessera-forming compressional deformation, and only seven craters are overprinted by later tessera-forming tensional deformation, one may conclude that the tesseraforming deformation ceased within a relatively short time period: less than 1/80 of the tessera age (about 0.01T) for the compressional stage and within about 7/80 of the tessera age (about 0.1-0.2T) for the tensional stage [Gilmore et al, 1997]. This also has two implications: (1) that the termination of the compressional stage of tessera-forming deformation was the starting point of the accumulation of the on-tessera crater population, and (2) that during the subsequent tensional phase of tessera formation, craters were mostly mildly deformed and not completely wiped out.…”