crustal thickening

crustal thickening

1. introduction

   • metamorphism
     • dynamic process involving changes in temperature plus/minus pressure through time.
     • path of a metamorphic rock is the set of all P-T conditions experienced by a rock during its metamorphic history.
   • thermal budget
     • strongly depends on the understanding of P-T-t path diagrams
   • thermal regimes
     • Geochron, structural and heat flow datum enhance the determination of thermal regimes
     • Information gathered through theoretical models show that thermal regimes relate to large wavelengths perturbations in the thermal structure of continents that lasts 10s millions of years.
   • Alpine model
     • not all this data will be exactly accurate with all continental collision zones around the works as the models have all been derived from the Alpine model
     • Until more data is required, this is a universal model. (High P, Low T
     • To create these models, there must be an orogenic event that includes; a rapid phase of crustal thickening and a lengthier episode of erosion and uplift
       • where the majority of temperature change takes place.
       • Heat and mass transfer during development of an orogenic belt may involve the thickening of continental crust by thrusting or magma addition.
         • heat may be supplied to the crust by partial melting of the mantle, or it may be transferred within the crust by conduction or motion of fluids and is generated to a significant extent by the decay of radioactive isotopes within the continental crust
         • If there is no temperature difference between the two rocks, then there will be no transfer of heat.
         • Relaxation time or delay time between orogenic events and erosion depends on the thermal diffusivity of the material and the depth at which it lies.

2. heat sources

   • Major sources of heat in regional metamorphism are from the decay of radioactive elements in the continental crust and the heat transferred to the base of the crust from the upper mantle.
   • Most of the heat is produced during the exhumation proportion of the cycle P-T-t cycle
     • This means that rocks do not begin to cool until they have reached 20-40% of total exhumation.
   • The rate of erosion / velocity of exhumation determines how long it takes for the metamorphosed rocks to reach the surface.

3. We assume that the continental lithosphere is always close to isostatic equilibrium and therefore at some time after the thickening, the crust will have returned close to its original thickens due to erosion action on its elevated surfaces.

4. Effect of erosion / uplift on geotherms

   • - With erosion, pressure is decreased and heat flow increases, making the geotherms regrade back to the initial geotherm as it is exhumed and temperature is decrease.
   • - With no erosion, the rock is continually heated at a constant depth/pressure and the material will eventually enter the sillimanite facies.

5. Effect of erosion/uplift on P-T-t paths

   • - With no erosion, the pressure/depth remains the constant and the temperature gradually increases. These conditions cook the rock into the sillimanite facies
   • - With erosion, after a 20Mya delay, we can see that pressure begins to decrease and the maximum temperature experienced is seen to only reach the kyanite facies before it regrades and cools as exhumed through erosion.

6. Erosion/uplift delay time

   • - the first diagram shows that there will be an onset delay of erosion that lasts 10ma. This means that after 10 Ma, erosion beings and it then takes a further 60 Ma before it reaches the surface.
     • - The amount of time taken for erosion to being depends of the thermal diffusivity of the material and the depth at which it lies.
   • In the second diagram it can be seen that there is a delay on 30Ma before erosion beings. This means pressure /depth stays constant for 30 Ma and then decreases once erosion begins.

7. Erosion/uplift velocity

   • The first diagram has a fast uplift rate which means that after a 20 Ma delay, it is eroded at 0.35mm/yr and is exposed at the surface after a further 20 Ma
   • The second diagram shows that after 50Ma the rocks have not reached maximum temperature and have begun to regrade back. It’s celocity is only 0.1mm/yr

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