| The data returned by the Magellan spacecraft have brought forth the view that Venus experienced a resurfacing event of nearly global proportions roughly 400 million years ago. The "catastrophic" resurfacing hypothesis holds that minimal volcanic and tectonic activity has occurred in the interim, an explanation for the spatially random impact crater distribution on the planet. A parallel argument is that the lithosphere has conductively cooled and thickened in the post-resurfacing period. This dissertation investigates two linked traits of the tectonics and geodynamics of Venus that document such a lithospheric cooling trend.; Contractional deformation on Venus assumes three distinct styles. The plateau highlands tessera is intensely, pervasively deformed crust covering areas with dimensions of {dollar}sim{dollar}1000 km. A characteristic structural spacing of tessera is 15 km. Ridge belts are distributed across thousands of kilometers of the northern plains, but only disrupt isolated bands of crust {dollar}sim{dollar}300 km apart. Artemis Chasma is the site of wholesale lithospheric underthrusting akin to terrestrial plate interactions. Motions were coherent over distances of {dollar}sim{dollar}2500 km; the characteristic tectonic spacing of Artemis is infinite because the strain was accommodated only in a single, narrow zone. The pattern of deformation styles marked by increasing scale and coherence correlates with the ages of the features: tesserae are ancient, ridge belts are of intermediate age, and Artemis is apparently young.; Geodynamic modeling of lithospheric deformation offers additional, quantitative evidence of this trend. The highly regular spacing of tessera ridges implies they formed by a process of instability growth in a strong layer being horizontally shortened. The requisite geotherm is at least {dollar}sim{dollar}20 K km{dollar}sp{lcub}-1{rcub}{dollar}--in agreement with the heat flow predicted for Venus by scaling from Earth. At the other end of the spectrum, lithospheric flexural deformation at Artemis Chasma is characterized by a broad, high-amplitude outer rise much larger than seen at terrestrial subduction zones. Its dimensions can only be explained by a combination of an extremely low thermal gradient--less than {dollar}sim{dollar}4 K km{dollar}sp{lcub}-1{rcub}{dollar}--and an immense compressive in-plane force. Floor modification of large, young impact craters is dominated by thermal subsidence, not isostatic rebound, consistent with a strong, cold lithosphere. The observed transition in tectonic style and contemporary low heat flow are accounted for by the passive, conductive cooling of the lithosphere over hundreds of millions of years. |
