CONSTRAINTS ON TESSERA FORMATION FROM STRUCTURAL MAPPING AND MECHANICAL MODELING

Venus tessera terrain is characterized by two or more sets of intersecting structures that result in high radar backscatter. Tesserae comprise ~8% of the Venus surface and occur as large elevated plateaus and small outcrops. The tesserae are generally embayed by plains materials, but yield crater ages of 1 – 1.4X the average surface age of the planet, ~800 Ma. Tesserae are thus the oldest materials on Venus and yield unique constraints on pre plains geologic history and the plains forming event.

Structural mapping of one region of tessera terrain, SW Tellus Regio, reveals a fold belt of plains material between classic tessera units. These observations support the hypotheses that 1) tessera fabrics can form under distinct strain histories, and that 2) SW Tellus was formed by the assembly of these preexisting fabrics and intervening plains during a collisional event. Mean fold wavelengths of the tessera units are ~13 km. Smaller mean wavelengths of ~5 km are recorded in the fold belt.

To evaluate the lithospheric properties constrained by fold wavelengths at Tellus Regio we have implemented a linearized perturbation model in Matlab^®. In this model a dominant wavelength instability develops in response to layer-parallel shortening of a relatively stiff brittle surface layer overlying a relatively soft ductile substrate. Appropriate fold wavelengths are generated for a range of likely strain rates, thermal gradients and compositions. Acceptable models fall into two broad classes: 1) dry mafic to ultramafic materials that require high thermal gradients and low strain rates. 2) Wet or felsic materials deformed at lower thermal gradients and/or high strain rates. Dry quartzite is too weak to buckle under reasonable conditions for Venus. These results are predicated on the appropriateness of the buckling model. Observations of fold geometry suggest that some structures may be fault-cored and we are thus pursuing more sophisticated mechanical models to better constrain the evolution of tessera terrain.