Dynamics and thermal evolution during orogenic processes

Schott, B. and H. Schmeling, 1998: Delamination and detachment of a lithospheric root. Tectonophysics, 296, 225 - 247.

Schott, B., D.A. Yuen and H. Schmeling, 1999: Viscous heating in heterogeneous media as applied to thermal interaction between the crust and mantle. Geophys. Res. Lett., 26, 513-516.
 

Movie of a delaminating mantle lithosphere (generated by B. Schott)

The following movie shows the delamination of the mantle lithosphere form the crust.Note the additional information about the maximum temperature in degree centegrate(top), the model-time in Ma (middle) and the maximum of the base-10 logarithm of the non-dimensional dissipation function (bottom).

Top: temperature
field and some isotherms.
 

Middle: composition
by tracers,
blue = upper crust,
yellow = lower crust,
red = mantle;
in the mantle lithosphere
the initially vertical tracer columns
are almost not deformed
due to the high viscosity.

Bottom: viscous dissipation function
(non-dimensionallog10),
zoomed-in into the upper half of the model.
 
 

Thermal evolution of a stacked continental crust after delamination of the mantle lithosphere
 
Often the late stage of an orogeny is associated with extensive heating of the crust and granitic pluton emplacement. For example, the collisional stage of the Variscan orogeny occured at the end of the Devonian (345 Ma), while extensive high temperature - high/low pressure metamorphism, granitic intrusions, volcanic activities and extension took place only 5 to 20 Ma after the end of this stage.   Here we propose delamination of the mantle lithosphere and asthenospheric ascent up to a level immediate beneath the thickend crust together with enhanced radiogenic heat generation within the thickened crust as a mechanism of  crustal heating. Because our dynamical models show (Schott and Schmeling, 1998), that the process of delamination and asthenospheric rise can be very fast (< 2 Ma), we choose a stacked crust as initial condition , but introduce the emplacement of a shallow asthenosphere by increasing the initial temperature to 1300°C everywhere below 65 km. This is shown in the Figure (clicke on the Figure to enlarge it):

THis Figure 4 shows the initial condition of a thermo-mechanical model, consisting of  a 2-layer overthrusted crust (black and dark green markers), a 2-layer underthrusted crust (light green and orange markers) and a lithospheric - asthenospheric mantle (red markers) beneath. Radiogenic heating is applied to the crustal and mantle units. No lateral shortening or widening is applied. Mimicking a situation immediately after delamination of the mantle lithosphere, a hot shallow asthenosphere is introduced by choosing 1300°C for all depths greater than 65 km.

As shown in the next Figure, the 2D model heats up fast, and melting temperatures are exceeded in the lowermost part of the thickened crust after 25.8 Ma. After 36 Ma the degree of melting has reached 14 %, and the thickness of the molten region is 11 km.  Due to buoyancy forces of deep crustal material and melt, the crustal root begins to flatten by ductile deformation. As can be seen from the marker field  the deepest part of the root has already risen by 10 km. This deformation is possible, because the chosen rheologies lead to relatively low effective viscosities: 1020 - 21 Pa s in the mantle and 1021 - 22 Pa s in the crustal root. Other runs with stronger crustal and mantle rheologies (e.g. amphibolite rheology for the lower crusts or dry olivine rheology for the mantle) did not allow for noticable deformation within the crustal root even after significant heating. Click on the figure to enlarge it.

 

As the next Figure shows, at later times (71 Ma), further radiogenic and advective heating due to a convecting asthenosphere have increased the degree of melting up to 24% , however, further thinning of the lower crust still confines melting to a 10 km thick layer of the underthrusted crust. The total crustal thickness has reached approximately the average value of 45 km (this rather large value is a consequence of not allowing crustal material to escape to the sides of the model). Most of the crustal deformation has taken place in the underthrusted crust, the overthrusted crust is almost undeformed due to the strong rheology at low temperatures. Within the underthrusted crust the marker field shows characteristic parabola shaped features at both sides of the crustal root. These features indicate a channel type of lower crustal flow.

   Although buoyancy forces are present in the partially molten lower crust, the viscosity of the overburden is too high to allow for diapiric ascent.  External tectonic forces (not acting in the present models) might lead to weakening of the crust e.g. along fault zones, thereby enhancing the potential of diapiric ascent.
   The crustal models show, that  extensive melting is possible within a rather short time period comparable to the times proposed for the late Variscan orogeny. However, radiogenic heat sources alone seem to be insufficient, additional heating from a shallow asthenosphere is needed. Another possible heat source has been suggested by Schott et al. (1999), who showed that during delamination of the mantle lithosphere frictional heating within the lower crust may locally increase the temperature by several hundred degrees. (See the movie above). While the latest stage of the Variscan orogeny is characterized by HT-LP-metmorphism, fast decompression or ascent of lower crustal high-temperature rocks or molten material could not be obtained in the present models. This is due to the fact that a) no extension has been allowed for in the present models, and b) no erosion has been included.