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Models for the formation of highland regions on Venus

Citation

Kiefer, Walter Scott (1991) Models for the formation of highland regions on Venus. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-12032004-155735

Abstract

The unifying theme of this thesis is an attempt to understand the origin of several major highland units on Venus. Chapters 1 and 2 develop numerical models of mantle plumes. In Chapter 1, I discuss the numerical methods used in calculating the geoid anomalies, topographic uplifts, and heatflow associated with upwelling plumes. Because plumes are a likely cause of at least some terrestrial hotspot swells, Chapter 1 also examines how the numerical models compare with observations of terrestrial hotspots. In Chapter 2, I compare the plume model results with observations of geoid anomalies and topography from the Equatorial Highlands of Venus. Chapter 3 examines another model, in which parts of the Equatorial Highlands are interpreted as spreading centers analogous to terrestrial mid-ocean ridges. Chapter 4 develops a crustal convergence and mantle downwelling model for the Ishtar Terra region of Venus.

Terrestrial hotspot swells are regions of elevated topography and high rates of volcanism. A variety of evidence suggests that at least some hotspots, such as Hawaii, are formed by quasi-cylindrical mantle plumes upwelling from deep in the mantle. I model such plumes using a finite element code in cylindrical, axisymmetric geometry with a depth-dependent Newtonian viscosity. Many previous workers have modeled plumes using a sheet-like, Cartesian geometry, but I find that cylindrical and sheet-like upwellings have significantly different geoid and topography signatures. However, Rayleigh number-Nusselt number systematics in the two geometries are quite similar. Increasing the Rayleigh number or including a low-viscosity asthenosphere decrease the geoid anomaly and the topographic uplift of a plume. For comparison with observations, the models are scaled with the assumptions of whole-mantle convection and a temperature contrast of about 300 °C between the center of a plume and normal mantle. The models are able to explain the amplitudes of the observed geoid anomalies and topographic uplifts at Cape Verde and Hawaii, provided that the Earth's mantle has a low viscosity zone in the asthenosphere and upper mantle similar to that previously inferred by Hager and colleagues on the basis of long-wavelength geoid modeling. However, for aspect ratio 1, the models predict swell widths that are about twice as wide as observed. This discrepancy may be due in part to terrestrial plumes having aspect ratios of less than 1. Alternatively, inclusion of temperature-dependent rheology may lead to narrower swells.

The Equatorial Highlands of Venus consist of four main structures, Atla, Beta, Ovda, and Thetis Regiones. Each of these features has a circular to ovalshaped planform and rises 4 to 6 km above the mean planetary radius. These highland units are also long-wavelength geoid highs, with amplitudes ranging from 35 meters at Ovda to 120 meters at Atla. These features also contain topographic valleys, interpreted as extensional rift zones, and Beta is known to contain shield volcanoes. These characteristics are all consistent with the Equatorial Highlands being formed by upwelling mantle plumes. In order to compare results for Venus and Earth, I assume that the two planets have similar mantle heat flows. With this assumption, I find that in order to satisfy the observed geoid and topography for the Equatorial Highlands, the asthenosphere and upper mantle viscosity must be higher on Venus than on Earth. This conclusion is consistent with modeling of the long-wavelength admittance spectrum of Venus and with the observed differences in the slopes of the geoid spectra of the two planets. One possible explanation for the different viscosity structures of the two planets is that the mantle of Venus is drier than the Earth's mantle.

An alternative model for Ovda and Thetis Regiones, proposed by Crumpler, Head, and colleagues, is that these features are terrestrial-type spreading centers. The strong positive correlation between the geoid and topography observed in Ovda and Thetis is unlike that observed for terrestrial spreading centers. The maximum elevation expected for spreading centers on Venus is 1.5 km, and a cooling plate thermal model predicts a maximum geoid anomaly of 8 meters, both much less than observed. Thus, even if a spreading center is operative in Ovda and Thetis, most of the geoid and topography must be due to other mechanisms. Crumpler et al. also proposed the existence of "cross-strike discontinuities," which they interpreted as transform fault zones, but the evidence for these structures is not conclusive.

The Ishtar Terra region of Venus contains the highest topography known on the planet, over 10 km above the mean planetary radius, as well as abundant tectonic features, many of which are apparently compressional in origin. These characteristics suggest that Ishtar is a crustal convergence zone overlying a region of downwelling mantle. In order to explore quantitatively the implications of this hypothesis for Ishtar's origin, I present models of the viscous crustal flow driven by gradients in lithostatic pressure. For reasonable bounds on the mantle convective velocity, I find that if the crustal convergence hypothesis is correct, then the crustal thickness in the plains surrounding Ishtar can be no more than about 25 km thick. This result is in good agreement with several independent estimates of crustal thickness on Venus based on modeling of the spacing of tectonic features and of impact crater relaxation, but is much less than the estimated crustal thickness derived from an Airy isostasy model of Ishtar's gravity anomaly. Much of the observed gravity anomaly must be due to density anomalies in the mantle beneath Ishtar. Although I treat Ishtar as a crustal convergence zone, the crustal flow model results show that under some circumstances near-surface material may actually flow away from Ishtar, providing a possible explanation for graben-like structures in Fortuna Tessera.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Atla Regio; Beta Regio; Ishtar Terra; mantle plume; Ovda Regio; Venus
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hager, Bradford
Thesis Committee:
  • Unknown, Unknown
Defense Date:16 July 1990
Author Email:kiefer (AT) lpi.usra.edu
Record Number:CaltechETD:etd-12032004-155735
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-12032004-155735
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4744
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:08 Dec 2004
Last Modified:26 Dec 2012 03:11

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