On the relationship between isostatic elevation and the wavelengths of tectonic surface features on Venus

Abstract

Regional isostatic elevation ( d) and the widths and spacings (wavelengths, λ) of tectonic features formed due to horizontal extension and compression are observed quantities on Venus that are sensitive to the thermal and compositional structure of the lithosphere. To investigate the relationships between these parameters and the implications for Venus' lithosphere structure, we formulate models in which d and λ are functions of surface thermal gradient ( dT/ dz 0) and crustal thickness ( c). In models that relate λ to dT/ dz 0 and c, we impose a requirement that for tectonic features with multiple wavelengths of deformation to develop, the growth rates of instabilities that control the dominant wavelengths must be “similar” in magnitude. This constrains the maximum values of c and dT/ dz 0 that can result in multiple tectonic wavelengths on Venus to be less than or equal to those determined in our previous study, c ⩽ 30 km and dT/ dz 0 ⩽ 25°K km − (M. T. Zuber, 1987, J. Geophys. Res. 92, E541–E551) . We also estimate the strength of Venus' upper crust in areas with multiple wavelengths and obtain limits that are most consistent with a compressional origin for ridge belts, which are enigmatic features that have been purported to be of both extensional and compressional origin. In models that relate d, λ, dT/ dz 0, and c, we show that in areas of Venus where the upper mantle is stronger than the upper crust, the spacings of short-wavelength ( λ ≲ 30 km) features should increase with increasing d if the change in d is due to increasing c, but should decrease with increasing d if the change in d is due to increasing dT/ dz 0. Long-wavelength ( λ ≳ 30 km) features are not strongly correlated to variations in d due to the effect of crustal thickness on the depth distribution of lithospheric strength, which influences wavelength selection in a complex manner. In areas of Venus where the upper mantle is weaker than the upper crust, the spacings of short-wavelength features should remain constant with increasing d if the change in d is due to a change in c, but should decrease with increasing d if the change in d is due to increasing dT/ dz 0. The results demonstrate that in shallowly compensated areas of Venus that contain regularly spaced tectonic features, it may be possible to distinguish whether isostatic elevation differences are a consequence of variations in crustal thickness or thermal gradient. We interpret the observed relationship of λ with d for short-wavelength tectonic features in several tessera regions to reflect crustal thickness variations.