Transcript EVOLUTION OF VOLCANISM ON VENUS

EVOLUTION
OF VOLCANISM ON VENUS
M.A. Ivanov1,2 and J.W. Head2
1- Vernadsky Inst., Moscow, RAS
2- Brown Univ., Providence
Introduction
• 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.
Tectonized vs. Volcanic units
Tectonized:
Volcanic:
• Multiple sets of tectonic structures.
• Mildly- or non-deformed.
• Distorted original morphology.
• Preserved volcanic nature.
• Form local to regional highs.
• Mostly are in local to regional lows.
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.