C. Mock et al., Ar-40/Ar-39 thermochronology of the Ethiopian and Yemeni basements: reheating related to the Afar plume?, TECTONOPHYS, 314(4), 1999, pp. 351-372
In order to constrain the timing of mantle plume-related basement uplift an
d to discuss the active or passive mechanisms of the continental rifting in
the Afar triple junction area, a Ar-40/Ar-39 thermochronological study has
been carried out on the Panafrican basement in Ethiopia and Yemen. Muscovi
tes, biotites and K-feldspars from granitoids and high-grade metamorphic sa
mples have been analysed using the Furnace step heating procedure. R-feldsp
ar thermal histories have been numerically modelled, according to the multi
-domain Ar diffusion theory. The cooling ages range from 462 Ma to 679 Ma f
or granitoids and from 495 Ma to 796 Ma for high-grade metamorphic rocks, w
hich suggests that magmatic and metamorphic episodes belong to the same the
rmotectonic event. A gradual younging of cooling ages from north to south i
s consistent with the assumption that Panafrican continental collision bega
n in Southern Ethiopia.
Age gradients recorded by K-feldspars reflect Ar loss and are most likely r
elated to the Cenozoic reheating of the basement. The estimated mean temper
ature over the last 50 Ma is between 138 degrees C and 177 degrees C at 2 k
m below the surface, which suggests a minimum mean thermal gradient of 69-8
8 degrees C/km. The thermal anomaly is expected to reflect heat conduction
from the mantle plume, magma underplating and melting of the lower crust, a
nd heat advection related to the magma transfer through the crust. However,
due to limited exhumation, it was not possible to constrain the timing of
any mantle plume-related basement uplift. This study suggests that the Ar-4
0/Ar-39 thermochronology is generally unsuited to monitor denudation of a c
ontinental crust in an extensional setting. Furthermore, it highlights some
of the thermal effects that continental flood basalts may have on the crus
t, which can lead to inconsistencies between K-feldspar and mica age data.
(C) 1999 Elsevier Science B.V. All rights reserved.