TECTONICS ON THE TERRESTRIAL PLANETS

Each of the terrestrial planets has seen a distinct tectonic history, but for each the nature and timing of deformation provide important clues on interior and surface evolution. On the Moon, near-surface deformation was dominated by local lithospheric loading by mare volcanic deposits. Global cooling and contraction nonetheless influenced the relative timing of graben and wrinkle ridge formation. Moonquakes define modern patterns of ongoing deformation. Shallow moonquakes reflect some combination of cooling-generated thermal stress and non-hydrostatic stress beneath mascon maria, whereas deep moonquakes are triggered by tidal stress. Mercury's distinctive lobate scarps are the surficial expression of thrust faults, whose distribution and apparently random orientation have been interpreted as signatures of global contraction resulting from interior cooling and solidification of an inner core. Extensional and contractional features near and within the Caloris basin point to loading and basin relaxation as important local sources of stress that may have influenced the formation of lobate scarps elsewhere on the planet. On Venus, mountain belts and chasm systems are likely products of large-scale deformation in response to mantle convective stresses. An important source of near-surface stress on Venus is climate change induced by volcanic outgassing of quantities of SO₂ and H₂O sufficient to alter the structure of the Venus cloud decks and the radiative balance of the lower atmosphere. Thermal stress induced by climate change may have been an important contributor to the formation of wrinkle ridges on the ridged plains. The relative roles of climate change, interior dynamics, and magmatism in creating the pervasive deformation within the tessera terrain, stratigraphically the oldest terrain type on the planet, remain uncertain. Tectonic features on Mars are a mix of local- to regional-scale features associated with loading at basins and volcanic centers and larger-scale distributions of deformational features. Whether the comparatively brief episode of contractional deformation preserved in the Hesperian ridged plains, which occupy some 30% of the surface area, was primarily the result of internal (e.g., global cooling) or external (e.g., atmospheric warming) sources of stress is an open question.