Structural and kinematic analysis of eastern Ovda Regio, Venus: Implications for crustal plateau formation

Crustal plateaus on Venus constitute one of that planet's most intriguing t ectonic features. They host so-called "tessera" terrain, or deformed crust exhibiting two or more sets of intersecting tectonic lineaments, which comp rises similar to 8% Of Venus' surface. Major questions surrounding crustal plateaus and tessera terrain include: (1) What is the nature of crustal pla teau deformation? What structures are present, and what are their spatial a nd temporal relations? (2) What is the global distribution of tessera terra in? 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 t he 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 plat eau, is characterized by linear shallow steep-sided troughs (ribbons), long arcuate long-wavelength folds, short mid-wavelength folds, and wide comple x graben. Temporal relations between structures lead to a three-phase regio nal strain history consisting of (1) layer-normal flattening of a thin comp etent membrane resulting in radially oriented, distributed extension (ribbo n 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 compete nt layer during each stage of deformation and reflects an early shallow bri ttle-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 cru stal thickening and plateau uplift. Thus tessera terrain at Ovda Regio is l ikely confined to the topographically high plateau and does not extend bene ath the surrounding regional plains. Similar patterns persist at other crus tal plateaus, indicating that tessera terrain is in general confined to cru stal plateaus and does not form a globally continuous layer. Ovda's structu res, their spatial and temporal relations, the resulting strain history, an d the behavior of the BDT through time support a model of mantle plume upwe lling in a thin lithospheric regime for crustal plateau formation and contr adict previous downwelling models. Similarities between structural and kine matic trends at Ovda Regio and other crustal plateaus lend support to this idea. (C) 1999 Academic Press.