The tessera terrain on Venus, comprised of areas of high radar backscatter, complex deformation patterns relative to other units, and topography standing higher than surrounding plains, covers ∼35.33 × 106 km2, about 8% of the surface of Venus, and is nonrandomly distributed, being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30°S. Individual tessera occurrences range in area from the lower limits of our measurements (about 200 km2) up to the largest tessera, Ovda, with an area of about 8.6 × 106 km2, or about 2% of the surface area of Venus. The size‐frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes, reflecting the great abundance of small tessera fragments. Modes of occurrence include (1) large clusters (e.g., Aphrodite Terra and Ishtar Terra); (2) arc‐like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it; (3) areas where tesserae are rare or absent which occur both as low‐lying plains (e.g., Guinevere Planitia), and as elevated regions (e.g., Atla Regio). Tessera terrain has a bimodal elevation‐frequency distribution, with the main peak at about 0–1 km and an additional peak at about 3 km above mean planetary radius. In terms of number of occurrences, however, tesserae do not display a correlation with elevation at the global scale, since small tessera patches commonly occupy low‐lying regions. Although tessera exhibit a range of gravity signatures, many occurrences are interpreted to represent relatively shallow (crustal) levels of compensation. Tessera boundaries include Type I (sinuous/embayed, dominated by adjacent lava plains embaying tessera massifs; 73% of total tesserae boundaries) and Type II (linear/tectonic). Only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation. Occurrence of broad slopes of post‐tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places. Several lines of evidence suggest the possibility that a widespread tessera‐like basement, comprising at least 55% of the surface of Venus, is buried under a cover of lava plains a few hundred meters to as much as 2–4 km thick. A wide variety of deformational structures and patterns is observed within the tessera including those representing extension, compression, shear, and transpression; in some cases the apparently complex patterns can be resolved into single‐event kinematic interpretations involving noncoaxial deformation (e.g., Itzpapalotl), while in other cases, polyphase deformation is more likely (e.g., central Ovda and Thetis). Where relations can be determined stratigraphically, earliest deformation within the tessera is primarily related to crustal shortening and compression (Phase I), followed by pervasive extensional deformation commonly oriented normal to the strike of Phase I features, generally along the same principal stress direction (Phase II). Evidence also exists for the contemporaneous formation of these distinctive deformation patterns. Lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with Phase II extensional deformation and with regional tilting. Tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed, an observation consistent with many other mapping studies. We see no evidence for transitional stages between tessera and volcanic rises and/or lowlands, that might represent a long‐term sequence of upwelling or downwelling followed by crustal deformation and tessera formation. No impact craters deformed by Phase I deformation have yet been observed on tessera, suggesting that Phase I tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before ∼300–500 m.y. ago; however, the starting time, and thus duration of tessera formation, is unknown. On the basis of the very small number of on‐tessera craters deformed by Phase II extensional deformation, this period probably did not last more than several tens of millions of years after the cessation of Phase I. Little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years, during which time the vast volcanic plains were emplaced, although tilting of early plains along some tessera margins is observed. Building on the global synthesis presented here, future analyses of individual tessera occurrences will provide the detailed descriptions, kinematic interpretations, and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation.
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Sep 1, 2006
D Kacy Cullen + 1
Open Access