The importance of lateral variations in crustal thickness for
the existence of a partial melt zone on Mars
S. Schumacher, D. Breuer, T. Spohn, G. Neukum and the HRSC Co-Investigator Team
Introduction
Results
New images by the HRSC on Mars Express have shown clear evidence
for recent volcanism in the regions of Tharsis and Elysium. The
explanation most often used for this ongoing volcanic activity is the
existence of mantle plumes which are however difficult to sustain under
recent conditions. We therefore propose an alternative explanation which
concentrates on lateral variations in crustal thickness. As the crust
thickens substantially in the major volcanic regions of Tharsis and
Elysium and also exhibits a low thermal conductivity k of approximately 2
W/(mK), its influence on the temperature distribution can not be
neglected. As a thicker crust causes temperatures in the upper mantle to
be higher than average, the existence of a zone of partial melt
underneath these regions is likely and a possible explanation for recent
volcanism on Mars.
Models
We used a 1D thermal evolution model to determine parameter values
of mantle heat flow qm and thickness of the stagnant lid for recent
conditions. These data were then used as boundary conditions for 2Dsteady-state models of the temperature distribution within the stagnant
lid. We considered an average crustal thickness of 50 km which
increased up to 70 km and also up to 4 km of topography were added
(Fig.1). Moreover, we investigated a model with a constant and one with
a temperature- and pressure-dependent mantle conductivity.
1. Amount and location of temperature increase
The simulations show that temperatures underneath the
thickened crust are considerably higher (up to 120 K)
compared to an average crustal thickness as can be seen
in Fig. 3 and 4 where the temperature difference between
models with thickened crust and models with only an
average crust of 50 km thickness are presented.
It can also be observed that the location of the maximum
temperature increase varies significantly depending on the
thermal conductivity in the mantle. For km = 4 W/(mK) the
maximum is located right at the base of the crust (Fig. 3),
while for km = k(T,P) the maximum is situated at the base
of the stagnant lid (Fig. 4).
Fig. 3: Temperature difference between
model 1 and a model without crustal
thickening
2. Increase in partial melt
In both models an increase in partial melt can be observed,
but the amount of partial melt is dramatically higher for
model 2 than for model 1 (Fig. 5 and 6, please note the
differences in the maximum values). The reason for this is
the average mantle thermal conductivity of about 3 W/(mK)
for model 2 with km = k(T,P) and not 4 W/(mK) as in model
1. Because of this, model 2 even exhibits a global partial
melt zone.
Fig. 5: Degree of partial melt for model 1
Fig. 1: Schematic view of model setup
Fig. 2: Temperatures in model 2
Model 1
Model 2
Crustal conductivity kc
2 W/(mK)
2 W/(mK)
Mantle conductivity km
4 W/(mK)
k(T,P)
Fig. 4: Temperature difference between
model 2 and a model without crustal
thickening
Fig. 6: Degree of partial melt for model 2
Discussion
As regions with increased crustal thickness exhibit a temperature increase and
subsequently an increased degree of partial melt within the stagnant lid they
could act both as source regions for recent volcanism and as pathways for
rising magmatic material. It is therefore possible that the observed recent
volcanism in Tharsis and Elysium is solely due to changes in crustal thickness
and independent of any interactions with the mantle. Although the partial melt
is located relatively deep within the stagnant lid, th ere are nevertheless
possible explanations for its transport to the surface. The higher density of
volcanic edifices compared to the surrounding crust leads to a higher
buoyancy for the partial melt in volcanic regions. Moreover, old volcanic vents
could also act as new pathways for the rising material. These facts, together
with the increased degree of partial melt underneath the thickened crust could,
cause the focussing of recent volcanism to old volcanic centers like Tharsis
and Elysium even if there may be a global partial melt zone as indicated by
model 2 (Fig. 6).
Contact: [email protected]
Acknowledgement: This work was financially supported through the
European Community's Improving Human Potential Programme under
contract RTN2-2001-00414, MAGE.