Statistics – Computation
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p41a..07s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P41A-07
Statistics
Computation
[0545] Computational Geophysics / Modeling, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [8120] Tectonophysics / Dynamics Of Lithosphere And Mantle: General, [8121] Tectonophysics / Dynamics: Convection Currents, And Mantle Plumes
Scientific paper
The hemispheric dichotomy and Tharsis volcanic province are dominant planetary scale features on Mars. The formation mechanism of the dichotomy remains unclear and arguments have been made for both exogenic (i.e., giant impact) and endogenic (i.e., related to internal dynamics) origin. A recently proposed model by Zhong (2009) aims to link the formation of the dichotomy and the subsequent emplacement of the Tharsis region. This model requires a large-scale (spherical harmonic degree 1) flow in the mantle and involves a differential rotation of Martian lithosphere with respect to the sublithospheric mantle. It has been shown previously that a long-wavelength pattern of mantle flow naturally arises in convection models when realistic viscosity profiles are used. The lithosphere-mantle differential motion is excited by lateral variations in lithospheric thickness, assumed to represent the melt residue left after dichotomy formation by an endogenic process that involves melting. This motion could explain the inferred past migration of the Tharsis volcanic center from the current southern hemisphere to the dichotomy boundary. We present a suite of 3-D spherical-shell models to investigate the effect of various parameters on crucial ingredients of this model. First we look at the effect of varying asthenospheric channel thickness on the wavelength of convection for uniform lithospheric thickness, thus extending results of previous studies. We find a trade-off between the channel thickness and viscosity reduction in the channel, where a thinner asthenosphere requires a larger decrease in viscosity in order to generate degree-1 flow pattern. We then incorporate into our models a rigid lithospheric keel of hemispheric extent and observe the following. i) The position of the keel controls the orientation of the thermal structure, such that in the early stages of evolution the thermal upwelling is focused below the center of the keel. ii) Differential movement between the stagnant lithospheric shell and the deeper mantle is excited. This motion ceases when the upwelling reaches the edge of the lithospheric keel. iii) The presence of the keel modulates the wavelength of convective flow; in particular, some models that show thermal structure of relatively short wavelengths in cases with uniform lithospheric thickness, exhibit degree-1 flow when the keel is present. We also investigate the effect of varying the lithospheric keel shape and its thickness. We consider an additional case where the high-viscosity keel is replaced with a near-surface layer of decreased thermal conductivity in one hemisphere. This should approximate the situation shortly after the dichotomy formation if the giant impact hypothesis is adopted. The initial behavior of this model is similar to those with high-viscosity keel (i.e., the thermal upwelling forms below the center of the low-conductivity lid). However no migration of the plume occurs in this case, therefore this model cannot easily explain the current position of the Tharsis volcanic center. Further studies that include the effect of melt residue stiffening are in order.
Šrámek Ondřej
Zhong Sijia
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