Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review

https://www.geology.cwu.edu/facstaff/huerta/g501/pdf/Chopin2003EPSL.pdf

Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

Exhumation of oceanic blueschists and eclogites in subduction zones :Timing and mechanisms

Using the peri-Mediterranean blueschist belts as a case study we discuss the mechanisms of syn-orogenic exhumation of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The Mediterranean examples, within an overall convergent zone, show variations in the rates of convergence, rates of slab retreat, available space, and various stages of maturation of accretionary complexes. After a compilation of the deformation history and the kinematic boundary conditions and their evolution through time, we discuss P-T-t paths. Most of the structures found in the field today relate to the exhumation stage and often to quite superficial events related to syn-orogenic detachments. However a significant vertical motion occurred from the depth of eclogites (or UHP eclogites) to the depth of the blueschist or greenschist facies along cold P-T paths. Two types of mountain belts are described: those where a single thermal gradient was recorded throughout its’ history, like the Franco-Italian Alps, which suggest a steady-state evolution; and those where the thermal gradient has changed through time (and space), such as the Aegean region, which suggest a non-steady-state evolution. Slab retreat within a subduction complex does not lead to the exhumation of UHP rocks because the open subduction channel allows for fast circulation and detachment of sediments from the subducting basement. Subducted sediments lubricate the subduction channel and the basement is thus not involved in the return flow. Early exhumation is fast and the thermal regime in the subduction channel is partly controlled by the velocity of convergence and the velocity of slab retreat as well as by the nature of the subducted material. Final exhumation occurs within the accretionary complex at a much slower rate below extensional detachments. The removal of the overburden is achieved primarily by extension in the upper part of the accretionary complex. Extensional faults and shear zones are rooted in the brittle-ductile transition of the accretionary complex. Deeper “extensional” shear zones result from shearing along the base of the upper plate. We discuss a model with several levels of circulation of subducted material and compare it with available thermo-mechanical models.

Subduction tectonics and exhumation of high pressure metamorphic rocks in the Mediterranean orogens

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen-scale viscous heat production of 0.1 to >1 muW m(-3), which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 degreesC higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time-span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation-generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.

http://www.geosci.usyd.edu.au/users/prey/Teaching/Geos-3003/geos3003papers/BurgGeryaJMetPetrol05.pdf

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

http://perso.numericable.com/laetitia.le.pourhiet/publications_files/burovetal2001.pdf

A Poliakov

Papers

A thermomechanical model of exhumation of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks in Alpine-type collision belts