Geologic units on Venus: evidence for their global correlation

Detailed geologic mapping of approximately 30% of the surface of Venus has revealed a stratigraphic sequence that appears to be repeated in widely separated areas on the planet. This sequence shows a transition from oldest highly deformed terrain units, the tessera, to a series of widespread volcanic plains units, each with recognizable morphologic characteristics and interpreted eruption style, and ®nally to individual eruptive centers associated with edi®ces and rift zones. This sequence of events is accompanied by characteristic and repetitive patterns of tectonic deformation. In order to test the validity and broader applicability of this sequence, we compare it with local, regional, and global mapping published in 17 analyses by other authors. This comparison shows that the sequence of units, and thus the relative time sequence of the corresponding geologic events, is generally the same in virtually all of the widely distributed areas that have been studied, comprising about one-half the surface of Venus. We then address several alternative explanations for the observed sequence: (1) that the similar units and sequences correspond to events that occurred in di€erent areas at di€erent times (repetitive in di€erent places but not time correlative); (2) that the sequences occurred generally synchronously in di€erent places (repetitive and time correlative); or (3) that the situation is intermediate between the two cases. In order to distinguish among these possibilities, we analyze evidence for the lateral global continuity of units, the chronology of emplacement on the basis of the density of superposed craters, and the consistency of stratigraphic relationships based on these data. We ®nd that this analysis supports the hypothesis that the individual sequences are repetitive in di€erent parts of the planet, and generally time correlative between these locations. As a further test of this stratigraphy, we outline a global model for the geological evolution of Venus and assess the duration of emplacement of units, the timing and style of tectonic phases, and the volumes and implied ̄uxes of volcanic deposits. We ®nd that the morphologically recognizable part of the history of Venus comprises only the last 10±20% of its total history. Emplacement of tessera-forming material and its deformation into tessera terrain are the major geologic events of the initial Fortunian time. Several stages of areally extensive volcanism occurred subsequently, burying vast areas of tessera and forming regional plains during Sigrunian, Lavinian, and Rusalkian times. The average global rate of volcanism was a few km yÿ1, and the emplacement of plains was accompanied by alternating episodes of contraction and extension. The last global-scale distributed tectonic episode, the formation of an extensive network of wrinkle-ridges, happened near the end of the Rusalkian Period, and marked the transition to the present stage of the history of Venus, the Atlian Period. Atlian times are characterized by a predominance of regional rifting focused at several broad rises, and localized rift-associated volcanism in the form of shield volcanic constructions and lobate volcanic plains-forming units, and is 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. Comparison of this sequence of events with those revealed by continued geologic mapping of Venus will permit further testing and assessment so that a consistent and comprehensive documentation of the geological record can be presented and used to test models of the geodynamic evolution of Venus. # 1999 Elsevier Science Ltd. All rights reserved.