The geologic history of Venus: A stratigraphic view


  • Alexander T. Basilevsky,

  • James W. Head III


On the basis of regional and global stratigraphic analyses, we outline the major events in the geologic history of Venus determined by photogeological study of surface features. Because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved, the stratigraphic record presented comprises only the last 10–20% of the total history of Venus. The estimated range of the mean crater retention age of the surface (from ∼200 to 1600 million years) leads us to describe the timing and duration of different events in terms of fractions of the mean surface age T. The beginning of the observed history of Venus was characterized by intensive tectonic deformation of global or semi-global scale which formed the tessera terrain. Termination of the compressional stage is estimated to have occurred at about 1.4T while the tensional stage lasted for another 0.1–0.2T. After tessera formation, several stages of extensive volcanism occurred, burying vast areas of tessera and forming what are now observed as regional plains. The combined duration of the emplacement of these plains is estimated to be about 0.2–0.3T, with an implied average global rate of volcanism of a few cubic kilometers per year. Regional plains-forming materials can be subdivided and are separated from each other, and from underlying and overlying units, by unconformities. These unconformities are formed, from oldest to youngest, by tessera-forming deformation, dense fracturing, broad ridging, and, finally, wrinkle ridging. These tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression, then tension, then again compression, and, finally, tension. The last global-scale tectonic episode, extensive wrinkle ridging, happened at about time T, which was very close in time to the emplacement of the most areally abundant plains unit. This marked the transition to the present stage of the history of Venus, which is characterized by a predominance of regional rifting and related volcanism. This stage appears to have lasted from about time T to the present, making it the longest time duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10–20% of the surface of Venus. These observations mean that the general intensity of tectonics and the flux of volcanism (a few tenths of a cubic kilometer per year) in this latest period were much lower than those in earlier times. In summary, the morphologically observable part of the history of Venus was characterized by two key characteristics that stand in contrast to the comparable period of Earth history (approximately the Phanerozoic) when global geodynamic processes were dominated by plate tectonics: (1) Venus shows no signature of plate tectonics; instead, its global tectonic environment passed from an initial dominance of compression, through tension, then again compression, and finally tension, with the density of deformational structures and the strain rate declining with time. (2) In the beginning of this period of time on Venus, plains-forming volcanism occurred at a rate comparable to volcanism at mid-ocean ridges but was emplaced in an entirely different style. The predominant component of volcanism on Earth during this time was the extrusive volcanism at mid-oceanic ridges. For the last few hundred million years, Venus has been dominated primarily by rift-associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on Earth.