A Stratigraphic Study of Small Volcano Clusters on Venus

Abstract

Recently proposed global stratigraphy and geologic history models have led to two end member volcanic evolution models for Venus: a “directional” model that proposes Venus has progressed through a series of stages, each characterized by a particular style of volcanic activity (A. T. Basilevsky and J. W. Head 1997, J. Geophys. Res. 103, 8531–8544), and a [nondirectional] model that proposes a more Earth-like type of volcanic activity with different eruption styles occurring globally on a local scale throughout the recent history of the planet (J. E. Guest and E. R. Stofan 1999, Icarus 139, 55–66). The former infers that most small volcanoes ( D<20 km) define a global unit, referred to as the “shield plains,” that predates emplacement of the widespread plains with wrinkle ridges, or “regional plains.” To test this hypothesis, cross-cutting and superposition relations were used to determine the stratigraphic positions of small volcano clusters in seven quadrangles (11% of the surface). The seven quadrangles included 179 clusters containing over 22,000 constructs. Three materials were defined for each cluster: (1) the constructs, (2) the material immediately surrounding the constructs (called the cluster material), and (3) the regional plains. Where possible, relative ages were established between the constructs, cluster material, and the regional plains. There are six possible temporal relations between the constructs and regional plains emplacement: (1) the constructs postdate regional plains emplacement, (2) some constructs postdate regional plains emplacement, and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous, (3) the constructs predate regional plains emplacement, (4) some constructs predate regional plains emplacement, and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous, (5) the constructs both predate and postdate regional plains emplacement, and (6) the ages of the constructs relative to regional plains emplacement are ambiguous. An age relative to regional plains emplacement could not be determined for almost half of the clusters studied. Of the clusters for which a relative age could be determined, almost half postdate regional plains emplacement, and an additional third contain at least some constructs that postdate regional plains emplacement. Less than a tenth of the clusters predate regional plains emplacement. These observations indicate that many small constructs postdate the emplacement of the regional plains and therefore are consistent with the nondirectional volcanic evolution model in which small constructs represent a global process occurring locally throughout the recent history of Venus.