EVOLUTIONOF VOLCANISM ON VENUS

M.A. Ivanov1,2 and J.W. Head2

1- Vernadsky Inst., Moscow, RAS

2- Brown Univ., Providence

• Tectonics and volcanism: form the geological record of Venus.

• Volcanic landforms: permit tracing of the history of volcanism.

• Volcanism: plays a key role in understanding of Venus' evolution.

Introduction

Global geological map [Ivanov and Head, 2011]:

1) portrays the spatial and temporal distributions of units,

2) permits global assessment of the history and style of volcanism.

Introduction

Tectonized vs. Volcanic units

• Mildly- or non-deformed.

• Preserved volcanic nature.

• Mostly are in local to regional lows.

• Multiple sets of tectonic structures.

• Distorted original morphology.

• Form local to regional highs.

Tectonized: Volcanic:

Main volcanic units: 1) Shield plains

• Numerous small shields (~500,000 exposed).

• Make up ~18.5% of Venus.

• Associated with SSD (arrow).

• Predate regional plains.

• Mildly deformed by wrinkle ridges.

Main volcanic units: 2) Regional plains

• Smooth, sourceless plains.

• Make up ~42.8% of Venus.

• Associated with long channels (arrow).

• Mildly deformed by wrinkle ridges.

Main volcanic units: 3) Lobate plains

• Numerous lava flows.

• Make up ~8.8% of Venus.

• Emanate from large, distinct source areas.

• Occasionally deformed by graben (arrow).

Age relationships among volcanic units

Globally observed age relationships:

• Regional plains embay shield plains; both are deformed by wrinkle ridges.

• Unit rp2 embays unit rp1; both are deformed by wrinkle ridges.

• Lobate plains embay both rp1 and rp2 and are free of wrinkle ridges.

Age relationships with tectonized units

Volcanic units:

• Contrastingly lighter deformed.

• Embay/overlay structures of tectonized units.

• Shield and regional plains occur at relatively low elevations.

Topographic distribution:

Volcanic units:

• psh is higher than rp.

• In lowlands (Atalanta) psh is at lower elevations.

• pl is at higher elevations.

Tectonized units:

• Local to regional highs.

• Broadly embayed by volcanic units.

Topographic distribution:

Correlation of topography and stratigraphy at global scale:

1) Older units (from tectonized units to regional plains): progressive lowering of topography.

2) Younger units (lobate plains): preferentially associated with isolated rises.

Conclusions:

• The main volcanic units embay the majority of tectonized units. This indicates a major change from a tectonically dominated regime to a volcanically dominated regime. The transition appears to occur near the beginning of visible geologic history of Venus.

• Systematic and globally consistent age relationships among the main volcanic units indicate progressive changes of both the style of eruption and characteristics of the source regions:

psh: small eruptions from globally distributed shallow sources,

rp: massive, short-lived eruptions similar to terrestrial flood basalts,

pl: persistent, localized sources associated with rifts and shield volcanoes.

Conclusions:

• The global-scale correlation of topography and stratigraphy of the tectonized units and shield and regional plains imply that the major features of the global topographic pattern (most of the uplands and the lowlands) were established prior to emplacement of regional plains.

• Lobate plains (and rift zones) are concentrated in isolated rises. This suggests that the plains/rifts and rises manifest late volcano-tectonic episodes related to interaction of deep mantle plumes with thicker lithosphere.

Conclusions:

The interpretation of the observable geological record of Venus:

1) suggests that the level of interior activity of the planet changed as a function of time,

2) favors evolutionary geodynamic models characterized by non-linear heat loss mechanisms.