 |
| Titel |
Transitions in tectonic mode based on calculations of self-consistent plate tectonics in a 3D spherical shell |
| VerfasserIn |
Hein van Heck, Paul Tackley |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043505
|
|
|
|
|
|
| Zusammenfassung |
| In the past decade, several studies have documented the effectiveness of plastic yielding in
causing a basic approximation of plate tectonic behavior in mantle convection models with
strongly temperature dependent viscosity, strong enough to form a rigid lid in the absence of
yielding. The vast majority of such research to date has been in either two-dimensional, or
three-dimensional cartesian geometry. Also, scalings for mixed internally and bottom heated
convection are not well established.
In our previous study (van Heck and Tackley, 2008), mantle convection calculations were
performed to investigate the planforms of self consistent tectonic plates in three-dimensional
spherical geometry. We found, for internally heated convection and fixed Rayleigh number,
that when the yield stress of the lithosphere is low a "great circle"-subduction zone
forms. At low-intermediate yield stresses plates, spreading centers and subduction
zones formed and were destroyed over time. At high-intermediate yield stresses two
plates form, separated by a great circle boundary that is a spreading centre on one
side and a subduction zone on the other side. At high yield stresses a rigid lid was
observed.
Here, the planforms found by van Heck and Tackley (2008) are investigated further,
leading to a more general understanding of how different parameters determine which
planform prevails. In particular the transition from the mobile to the rigid lid is investigated.
New calculations are performed to investigate the effect of varying Rayleigh number and
different internal/bottom heating ratios. The results are compared to newly developed
analytical scalings for boundary regimes.
These scalings can form a basis to study the tectonic evolution of a cooling planet. As
radioactive heat production decreases over time the tectonic mode (e.g. changes in plate size,
rigid lid convection to tectonic plates, smoothly evolving plates to more episodic, time
dependent, tectonics) is likely to change. The results show that with decreasing internal
heating rate (H) the transition from rigid to mobile lid occurs at higher lithospheric yield
stress, meaning that for a cooling planet it becomes more likely to have active plate tectonics
over time. Also, plate tectonics is more likely at lower Ra. In more detail, provided the
dimensionalisation of stress is done via the viscosity, the (dimensional) critical
yield stress scales as Ra-13. A constant critical yield stress scales as H-1 while
yield stress increasing linearly with depth (friction coefficient) is independent of
H.
In principle, this allows us to apply the scalings to the present day Earth as well as to its
history. Our scalings can form a basis to describe the thermal-tectonic evolution of Earth,
Mars and Venus as well as terrestrial extra-solar planets. This however, would need to include
scalings for heat flux as well. Grigné et al. (2005) showed that surface heat flux depends on
the wavelength of convection so the scalings for heat flux will not only depend on the
internal/bottom heating ratio but also on the tectonic planform since the wavelength of
convection (i.e. plate size) changes for different planforms (van Heck and Tackley, 2008). |
|
|
|
|
|
|