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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 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.
Results 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).
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 2D-steady-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.
Fig. 3 Temperature difference between model 1
and a model without crustal thickening
Fig. 4 Temperature difference between model 2
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. 6 Degree of partial melt for model 2
Fig. 1 Schematic view of model setup
Fig. 2 Temperatures in 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).
Model 1 Model 2
Crustal conductivity kc 2 W/(mK) 2 W/(mK)
Mantle conductivity km 4 W/(mK) k(T,P)
Contact sandra.schumacher_at_uni-muenster.de
Acknowledgement This work was financially
supported through the European Community's
Improving Human Potential Programme under
contract RTN2-2001-00414, MAGE.