Tesserae extension estimation and comparison with crustal plateau thickness, Venus

Most of the Venus’ surface is geologically young¹, while the locally oldest materials are tessera terrains covering around 8% of the surface². Most tesserae feature a penetrative extensional fabric characterized by long narrow graben so called ribbons. Ribbon-bearing tesserae are typically found on Venusian crustal plateaus, elevated, quasi-circular regions with steep edges and flat tops. They show small gravity anomalies, low gravity to topography ratios and shallow apparent compensation depths (ADC), all of which suggest a thickened crust³.

The tectonic patterns of crustal plateaus are highly complex, featuring both extensional and compressional structures across a wide range of wavelengths and spacings. While radial extension is widely accepted as the final evolutionary phase, earlier stages are debated. The plume and lava pond hypotheses suggests a strong brittle layer thickening during cooling causing an increase of the structures wavelength (or spacing) with time^4,5,6. Other works provide evidence of initial compression followed by extension^7,8,9. Moreover, different wavelengths of structures can simultaneously be caused by the tectonic deformation of a layered crust [10]. Recently, dyke swarm emplacement has been linked to ribbon formation¹¹, which provides an elegant explanation to the persistent spacing.

Despite all crustal plateaus feature thick crusts associated to negative gravity anomalies, a wide range of crustal thicknesses, topographic elevations, and Bouguer anomalies is observed¹². At one end, Ovda Regio exhibits a significantly thickened crust, high topography, and a large Bouguer anomaly, while the opposite is true for Alpha Regio. In this study, we conduct a structural analysis of tessera terrain and calculate the total extension produced, with the aim of identifying possible relations between the main geophysical features and ribbon formation.

Geologic mapping and structural analysis of Ovda Regio and Alpha Regio were made using high-resolution SAR images (~75m resolution) and altimetry (10 to 20 km horizontal resolution and 50 to 100 m vertical resolution) from NASA Magellan mission. Data visualization was performed using QGIS. The digitization of regularly-spaced, long narrow graben allowed us to identify and evaluate the presence of different extensional families, as well as their orientations and spatial continuity.

Once regularly-spaced long narrow graben (ribbons) were identified in Ovda Regio and Alpha Regio, the study areas were subdivided into a 200 x 200 km grid. For subsequent analysis, we selected the zones within each grid cell exhibiting the maximum density of normal faults (to evaluate the maximum registered extension). Assuming a pure dip slip kinematics for the normal faults, the maximum observed extension was calculated along lines perpendicular to each fault set. Average fault heave was obtained assuming a fault dip of 60º (typical of normal faults) and a fault throw inferred from partially lava filled grabens at fold limbs. In areas where two graben sets coexist, the resultant extension was obtained by adding the deformation of each set. The estimated maximum extension was obtained by summing the heave of all the normal faults present. A stretching value was calculated for each grid cell, and was subsequently represented by deformation ellipses. Finally, we compared the stretching values with the crustal thickness^12,13in the same 200 x 200 km areas for both regions Ovda and Alpha.

Previous estimates of stretching generated by ribbon within Fortuna Tessera have ranged from 58% to 84%⁵. However, our analysis of Ovda Regio and Alpha Regio revealed maximum stretch values of 12%. We found a general trend where maximum stretch values generated by ribbons are higher in areas of lower crustal thickness, suggesting that ribbon play a role during gravitational collapse rather than being involved in plateau construction.

References

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Acknowledgements

This work was supported by the Spanish Agencia Estatal de Investigación through the research project PID2022-140686NB-I00 (MARVEN), the associated predoctoral grant CT21/24 and grant PR3/23-30839 (GEOMAVE), funded by the Universidad Complutense de Madrid.