Quantitative structural analyses and numerical modelling of ...

HEAT SOURCES AND EXHUMATION MECHANISMS 89Table 2. Mechanical properties (density, thermal expansivity and coefficients of temperature-dependent viscosity) of individuallithologies used for numerical simulations.Material Description Reference density Thermal expansion coefficient Temperature-dependent viscosity range andcoefficientsRadioactive heat productionq0 (kg m )3 ) a Effect. viscosity (Pa s )1 ) C1 C2 Hr (lW m )3 )UC Upper crust 2700 0 10 22 Viscosity const. 2 · 10 )6MCs Middle crust (schists) 2800 2 · 10 )5 1.5 · 10 20 to 2.5 · 10 19 10 16 6000 0MCg Middle crust (gneisses) 2800 2 · 10 )5 2.5 · 10 20 to 3.5 · 10 19 10 17 6000 0MLC Mafic lower crust 2950 0 2 · 10 21 to 5 · 10 20 10 18 7000 0FLC Felsic lower crust 2750 3 · 10 )5 1.5 · 10 19 to 6 · 10 18 10 17 5000 2 · 10 )6 to 8 · 10 )6M Lithospheric mantle 3300 0 5 · 10 21 Viscosity const. 0 (2 · 10 )7 )c. 20 Myr for the two radioactive heat productionvalues, respectively. Keeping in mind the time-scalesof magmatic and metamorphic events related toPalaeotethys subduction discussed above, the time of5–15 Myr available between the arrival of continentalcrust into the subduction zone (354–346 Ma) and themetamorphic climax (c. 340 Ma) corresponds to thethermal incubation time estimated for radiogenic heatproduction of 4 lW m )3 .Results of 2D modellingThe distribution of densities and viscosities for allmodels generally resulted in the development of adiapiric structure located at the introduced perturbation.The P–T of selected samples (samples 1–3 arewithin lower crust, sample 4 in mafic layer and sample5 in middle crust; for locations, see Fig. 6), trackedduring the model evolution are plotted on Figs 8–10.Similar to the static 1D models, the simulations show aclear relation between the initial temperature increase(20–150 °C) and radioactive heat production withinlower crust.Two types of evolution have been calculated: (i) adiapiric structure formed because of mantle heatsource (heat production 0.2 lW m )3 ); and (ii) a diapiricstructure caused by radioactive heat productionwithin the FLC for radioactive heat productivitiesranging from 1 to 6 lW m )3 . The results of thenumerical simulations for the case of mantle heatproduction are shown in Fig. 8. For the case ofradioactive heat production within the FLC, it wasterminated at 900 and 1000 °C. These results are presentedtogether with a simulation in which the radioactiveheat production was not switched off (Fig. 9)allowing a direct comparison of differences in P–Tevolution (Fig. 10).Several major conclusions can be drawn from theseresults. The diapir reflecting the mantle heat source(Fig. 8) exhibits a typical bell shape during the first30 Myr and most importantly shows contrasting P–Tevolution for samples located in different units andinitial depths. Whereas the upper part of the mantle(Fig. 7d) is almost isobarically heated, the sampleslocated in the felsic crust, mafic lower crustal layersand the middle crust reveal relatively slow exhumationand moderate heating associated with the diapirgrowth.In contrast, experiments assuming high radiogenicheat production show typical bollard type diapirs.After 10–20 Myr imposed gravitational instability andvariable radioactive heat production within lowercrust, the viscosity of the overlying mafic layer is significantlyreduced allowing relatively fast exchange ofmaterial and development of the central diapir. Thereare differences in rates of vertical exchange betweenindividual simulations as increased heat productioncause increase of buoyancy forces and decrease ofviscosity of the diapir surroundings (samples 4 and 5).The growth of bollard type diapirs is associated witheither isothermal decompression or important coolinglinked to diapir growth for samples located in deepfelsic lower crustal layer. These are indeed the P–Tevolutions retrieved from for Bohemian Massif granulites(Sˇtı´pska´ et al., 2004; Racek et al., 2006;Tajcˇmanova´ et al., 2006). The samples located originallyhigher in the column and at the middle crustallevels reveal important heating associated with exhumation,which is also in accord with recent petrologicalstudies (Racek et al., 2006; Sˇtı´pska´ et al., 2008). Theother important consequence is that rocks fromany original position show convergence of P–T conditionsafter exhumation to mid-crustal depths. Thetime-scales of heating (10–20 Myr) and exhumation(5–10 Myr) calculated in this model are also in agreementwith petrological and geochronological datareported by Sˇtı´pska´ et al. (2004) and Tajcˇmanova´ et al.(2006, 2010). It should be noted that time-scales of ourmodels are directly controlled by the rheology of thematerials, which in our simulations is significantlysimplified (Eqn A.3, Table 2).Closer inspection of Fig. 10 confirms that the best fitof modern petrological and geochronological data withcalculated P–T paths is with an initial radioactive heatproduction of 4 lW m )3 . Here, the maximum temperaturesattained at the bottom of thickened crust are950 °C, while samples located in the central part ofthe felsic crustal column reach a maximum 800 °C atthe thermal peak. These values may reconcile modernTHERMOCALCmodelling data of Sˇtı´pska´ & Powell(2005b), Tajcˇmanova´ et al. (2006) and Racek et al.(2008) who reported peak temperatures 800 °C withÓ 2010 Blackwell Publishing Ltd191