Crustal plateaus on Venus constitute one of that planet's most intriguing tectonic features. They host so-called "tessera" terrain, or deformed crust exhibiting two or more sets of intersecting tectonic lineaments, which comprises ~8% of Venus' surface. Major questions surrounding crustal plateaus and tessera terrain include: (1) What is the nature of crustal plateau deformation? What structures are present, and what are their spatial and temporal relations? (2) What is the global distribution of tessera terrain? Does it form a global layer under the regional plains, cropping out in crustal plateaus, or is it confined to crustal plateaus? and (3) What are the implications of (1) and (2) for understanding the processes responsible for formation and deformation of crustal plateaus? Structural and kinematic analyses reveal that deformation at Ovda Regio, Venus largest crustal plateau, is characterized by linear shallow steep-sided troughs (ribbons), long arcuate long-wavelength folds, short mid-wavelength folds, and wide complex graben. Temporal relations between structures lead to a three-phase regional strain history consisting of (1) layer-normal flattening of a thin competent membrane resulting in radially oriented, distributed extension (ribbon formation); (2) modest contraction of a thicker layer (concentric folding at plateau margins, interference folding in the plateau interior); and (3) limited extension localized at fold crests (graben formation). Wavelength analysis of ribbons and folds places limits on the thickness of the competent layer during each stage of deformation and reflects an early shallow brittle-ductile transition (BDT) that deepened with time. Ovda's deformation, topography, and gravity are all spatially correlated, strongly suggesting a genetic relationship between surface deformation and processes causing crustal thickening and plateau uplift. Thus tessera terrain at Ovda Regio is likely confined to the topographically high plateau and does not extend beneath the surrounding regional plains. Similar patterns persist at other crustal plateaus, indicating that tessera terrain is in general confined to crustal plateaus and does not form a globally continuous layer. Ovda's structures, their spatial and temporal relations, the resulting strain history, and the behavior of the BDT through time support a model of mantle plume upwelling in a thin lithospheric regime for crustal plateau formation and contradict previous downwelling models. Similarities between structural and kinematic trends at Ovda Regio and other crustal plateaus lend support to this idea.
Bibliographical noteFunding Information:
This work was supported by National Aeronautics and Space Administration Grant NAGW-2915 to VLH and by an Institute for the Study of Earth and Man at Southern Methodist University SEED grant to R.R.G. Discussions with Roger Phillips were helpful. Reviews by M. S. Gilmore, F. Bilotti, and C. D. Brown called our attention to areas which needed clarification and thus helped to improve the quality of the manuscript.
- Crustal plateaus
- Ovda Regio