Sequence of tectonic deformation in the history of Venus: Evidence from global stratigraphic relationships

Analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of Venus, spanning the past several hundreds of millions of years. We find that shortening characteristic of intensely deformed tessera terrain gave way to widespread distributed fracturing and extension within the tessera and early post-tessera volcanic plains. This phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following—and sometimes simultaneous—extension to form fracture belts. Emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains’ surfaces. Focused extensional deformation (localized, linear rift systems) dominated the latest stages. These major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of Venus. Figure 1. Venus stratigraphic units and global correlations (Basilevsky and Head, 1995). T = average age of surface. tively broad 5–10-km-wide ridges tens of kilometers long and associated fractures. Relatively radar-dark plains peppered with small shields of apparent volcanic origin (Psh, Rusalka Group) embay fractured and ridged plains and older units. The most abundant unit exposed on the surface is a regional plains unit that shows variations in radar backscatter and that has been deformed by wrinkle ridges subsequent to its emplacement (Pwr, Rusalka Group). Undeformed, smooth, flow like units commonly with lobate margins (Ps, smooth plains; Pl, lobate plains; Atla Group) are superposed on plains with wrinkle ridges and older units. Finally, craters of impact origin characterized by dark parabolas (Cdp; Aurelia Group) are superposed on virtually all older units. ORIGIN AND DISTRIBUTION OF STRUCTURES By using this stratigraphic sequence (Fig. 1) as a basis, we can now examine the origin of the observed structures (Fig. 2) and assess whether the deformation is local, regional, or global. Tessera terrain is interpreted to involve an early phase of compression and crustal shortening, followed by a later phase of crustal extension (Fig. 2A), the latter related to gravitational relaxation of the initially thickened crust (Solomon et al., 1992; Ivanov and Head, 1996). The alternative interpretation of Hansen and Willis (1996) suggests that tessera deformation changed from extension to compression. Tessera terrain, embayed by all other units, is currently exposed over 8% of the surface, primarily in large contiguous highlands and widespread smaller patches (Ivanov and Head, 1996). Tessera deformation is thought to involve high strain rates (Grimm, 1994) and to have occurred over a relatively short period of time (Ivanov and Basilevsky, 1993; Gilmore