Structures in tessera terrain, Venus: Issues and answers

Abstract

Abstract. Many workers assume that tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that tessera terrain does not form a global onionskin on Venus, although ribbon-bearing tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon-bearing tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600-2500 km diameter regions above hot mantle plumes. Plume-related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. [ 1998] summarized post-Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. [ 1998] did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon-fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre-Magellan and post-Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming [Phillips and Hansen, 1998], lead to serious errors in Gilmore et al. 's [1998] criticism. Furthermore, we show that the data of Gilmore et al. [ 1998] are actually more consistent with upwelling than do...

Radar Albedo Feature

“…Ribbon tessera constitutes Venus' locally oldest surface unit Head and Basilevsky 1998), although globally not all ribbon tessera formed at the same time (Bindschadler et al 1992b;Bindschadler 1995;Hansen and Willis 1996;Hansen et al 2000;Hansen and Young 2007). Ribbon tessera is marked by a distinctive tectonic fabric and high surface roughness and characterizes crustal plateaus but also occurs as lowland inliers, widely interpreted as remnants of collapsed crustal plateaus (Phillips and Hansen 1994; Ribbon Tessera, Fig.…”

“…2 and 3). Locally, volcanic deposits flood structural lows (Bindschadler et al 1992a;Bindschadler 1995;Hansen andWillis 1996, 1998;Ivanov and Head 1996;Gilmore et al 1998;Hansen et al 2000;Romeo et al 2005;Hansen 2006). Although there is debate about whether folds dominantly predate or postdate ribbons, most workers agree that (1) fold and ribbon formation broadly overlapped in time (or at least the formation of short-and intermediate-wavelength folds and ribbons structures), (2) graben formed late relative to folds and ribbons, and (3) volcanic activity accompanied ribbon tessera fabric formation -that is, the formation of ribbons, folds, and graben.…”

“…Ribbon tessera (Hansen and Willis 1996;Pritchard et al 1997;Hansen and Willis 1998) comprised of narrow ($1-3 km), long (10s to over a 100 km), linear alternating troughs and ridges; short (<2 km) to long ($30 km) wavelength folds, generally perpendicular to ribbon troughs, occur in spatial association with ribbons; short (tens of kilometers), wide ($10-25 km) graben can also occur (e.g., Bindschadler and Parmentier 1990;Bindschadler and Head 1991;Bindschadler et al 1992a, b;Ivanov and Head 1996;Hansen and Willis 1996;Ghent and Hansen 1999;Brown and Grimm 1999;Banks and Hansen 2000;Hansen et al 2000).…”

Surface evolution of Venus

“…The characteristics of the most deformed and most ancient tessera terrain have yet to provide conclusive keys to the geodynamical origin of this most distinctive terrain, and several different models exist for the formation of tessera and the geodynamical evolution of the planet as a whole [e.g., Sukhanov, 1992;Bindschadler et al, 1992;Ivanov and Head, 1996;Hansen et al, 1997Hansen et al, , 2000. Future studies need to concentrate on this problem by acquiring significantly higher resolution imaging of the surface of Venus on a global or at least regional scale.…”

Volcanism and tectonism across the inner solar system: an overview

“…This has been taken as further evidence for the lack of plate tectonics on Venus, where contractional deformation is widely distributed throughout the entire lithosphere rather than at discrete sites (Solomon et al 1992;Solomon 1993). Indeed, folds and faults are particularly pervasive in the tesserated terrains, which are expressions of multiple deformations on highrelief crustal plateaux and crustal remnants within plains Hansen & Willis 1996;Hansen et al 1999Hansen et al , 2000Romeo & Turcotte 2008). Folds and faults are also common in the planitiae, either concentrated in narrow contractional ridge belts or in widely distributed wrinkle ridges (Tanaka et al 1997;McGill et al 2010).…”