THE GEOLOGIC HISTORY OF VENUS - A STRATIGRAPHIC VIEW

On the basis of regional and global stratigraphic analyses, we outline the major events in the geologic history of Venus determined by photo geological study of surface features. Because the morphological signat ures of terrain emplaced prior to the time of tessera formation are no t preserved, the stratigraphic record presented comprises only the las t 10-20% of the total history of Venus. The estimated range of the mea n crater retention age of the surface (from similar to 200 to 1600 mil lion 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 tecton ic 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 obse rved as regional plains. The combined duration of the emplacement of t hese 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 eac h other, and from underlying and overlying units, by unconformities. T hese unconformities are formed, from oldest to youngest, by tessera-fo rming deformation, dense fracturing, broad ridging, and, finally, wrin kle ridging. These tectonic episodes are interpreted to be generally g lobally 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, exten sive wrinkle ridging, happened at about time T, which was very close i n time to the emplacement of the most areally abundant plains unit. Th is marked the transition to the present stage of the history of Venus, which is characterized by a predominance of regional rifting and rela ted volcanism. This stage appears to have lasted from about time T to the present, making it the longest time duration among the stratigraph ic units considered, although the resulting tectonic and volcanic feat ures and deposits cover only 10-20% of the surface of Venus. These obs ervations 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 m orphologically observable part of the history of Venus was characteriz ed by two key characteristics that stand in contrast to the comparable period of Earth history (approximately the Phanerozoic) when global g eodynamic processes were dominated by plate tectonics: (1) Venus shows no signature of plate tectonics; instead, its global tectonic environ ment passed from an initial dominance of compression, through tension, then again compression, and finally tension, with the density of defo rmational structures and the strain rate declining with time. (2) In t he beginning of this period of time on Venus, plains-forming volcanism occurred at a rate comparable to volcanism at midocean ridges but was emplaced in an entirely different style. The predominant component of volcanism on Earth during this time was the extrusive volcanism at mi d-oceanic ridges. For the last few hundred million years, Venus has be en dominated primarily by rift-associated volcanism emplaced at a prod uction rate comparable to or even lower than present intraplate volcan ism production rates on Earth.