Journal of Geophysical Research: Planets
Signatures of Lithospheric Flexure and Elevated Heat Flow
in Stereo Topography at Coronae on Venus
Joseph G. O’Rourke
1
and Suzanne E. Smrekar
2
1
School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA,
2
Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA, USA
Abstract
Signatures of lithospheric flexure were previously identified at a dozen or more large coronae
on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of
mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the
low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority
of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural
signatures around 99 coronae over
∼
20% of the surface in Magellan altimetry data and stereo-derived
topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses
of
∼
2 to 30 km (mostly
∼
5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss
the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses
are estimated as
<
19 km, corresponding to thermal gradients
>
24Kkm
−1
. Implied surface heat flows
>
95 mW m
−2
—twice the global average in many thermal evolution models—imply that coronae are major
contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show
that “Type 2” coronae with incomplete fracture annuli are significantly less likely to host flexural signatures
than “Type 1” coronae with largely complete annuli. Stress calculations predict extensional faulting where
nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural
parameters based on type, geologic setting, or morphologic class. Obtaining quality, high-resolution
topography from a planetwide survey is vital to verifying our conclusions.
Plain Language Summary
Hundreds of complex and diverse features called coronae—circular
structures associated with myriad types of volcanic and tectonic activity —have been identified on the
surface of Venus. Coronae have been studied for decades, but the mechanism(s) of their formation remain
hotly debated. We analyzed the topography at
∼
20% of the total population using a relatively new
topographic data set assembled from overlapping radar images taken from different angles. We compared
elevation profiles radiating outward from each corona to models of how a rigid plate (representing the crust,
possibly plus the uppermost mantle) deforms in response to a central load. From our best fit models, we
derived the local plate thicknesses followed by temperature gradients and surface heat flows that prevailed
when these coronae were formed. Uncertainties in our estimates result from ignorance about the strength
of rocks in the crust and mantle. We searched for but failed to find systematic differences in plate thickness
as functions of the geologic setting or extent of tectonic fractures. In general, we calculated heat flows
that are larger than the global average on present-day Earth —roughly twice typical values predicted for
Venus, which may cool slowly or even heat up absent plate tectonics. Therefore, we conclude that coronae
are sites of above average heat flow on Venus and in fact contribute a significant portion of the total heat
loss budget.
1. Introduction
Coronae are enigmatic, quasi-circular structures found with a range of associated volcanic features. Rings of
concentric fractures ranging in diameter from
∼
60 to
>
1,000 km are their primary distinguishing character-
istics (Barsukov et al., 1986). Their diverse range of topographic shapes, including interior domes, plateaus,
and depressions—and exterior troughs and rims—are uniquely observed on Venus (e.g., Smrekar & Stofan,
1997; Stofan et al., 1992). Coronae are preferentially located near chasmata or fracture belts, not randomly
distributed on the surface like impact craters (e.g., Squyres et al., 1993; Stofan et al., 1992). But at least some
coronae are found in every geologic environment including topographic rises, volcanic plains, and tessera
RESEARCH ARTICLE
10.1002/2017JE005358
Key Points:
• Newly available stereo topography
assists search for flexural signatures at
small coronae
• Elastic and mechanical thicknesses
are derived using thin plate models
and simplified rheology
• Low-average values for elastic
thickness confirm elevated heat
flow during coronae formation
Supporting Information:
• Supporting Information S1
• Figure S1
• Figure S2
•DataSetS1
Correspondence to:
J. G. O’Rourke,
jorourke@caltech.edu
Citation:
O’Rourke, J. G., & Smrekar, S. E.
(2018). Signatures of lithospheric
flexure and elevated heat flow
in stereo topography at coronae
on Venus. Journal of Geophysical
Research: Planets, 123, 369–389.
https://doi.org/10.1002/2017JE005358
Received 1 JUN 2017
Accepted 4 JAN 2018
Accepted article online 9 JAN 2018
Published online 8 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
O’ROURKE AND SMREKAR LITHOSPHERIC FLEXURE AT CORONAE ON VENUS
369
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