On the evolution of orogens: pressure pulses and deformation mode switches

 

Marco Beltrando 1 , Gordon Lister 1 , Joerg Hermann 1 and Roberto Compagnoni 2

1 Resesarch School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
2 Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso, Universita` di Torino, Italy

The pressure-temperature evolution recorded in rock units found in orogenic belts is generally believed to be characterized by an early stage of high-pressure metamorphism followed by exhumation to surface, during which the early metamorphic assemblage is overprinted in greenschist to amphibolite facies conditions. In contrast to this commonly held view, our study of the Piemonte unit of the Western Alps provided evidence for two burial-exhumation cycles that took place during a single orogenic cycle (Figure 1). An early high-pressure event, which resulted from tectonic burial down to pressures of 1.5 GPa, was followed by exhumation to ca. 0.15-0.25 GPa as a result of extensional deformation. Renewed shortening culminated in a second burial episode down to pressures of 0.48-0.65 GPa, before the final exhumation took place. The extent of the second pressure-temperature cycle has been determined through studies of the compositional vari ations of amphiboles (Figure 2)

Combined structural studies (Figure 3 and 4) revealed that the first exhumation was accomplished as a result of generalized extensional deformation (D2) accommodated by extensional shear zones. Subsequent folding of the extensional structures indicates that a deformation mode switch from extension to shortening affected the study area. Folding was followed by renewed top-to-the-west extension accommodated by west-dipping shear planes. Therefore, shortening deformation seems to be responsible for episodes of tectonic burial, while extensional deformation leads to exhumation to shallower depths.

White micas have been dated by the 40Ar/39Ar step heating method in order to constrain the time scales associated with the observed pressure cycles and deformation mode switches (Figure 5). A strong correlation is observed between measured ages, mica composition and deformation fabrics. Crystallization of phengitic mica in eclogite facies conditions took place at ca. 44 Ma. A major deformation-recrystallization event in greenschist facies conditions (D2) culminated in the formation of muscovitic mica at ca. 42-41 Ma. Muscovites are found along shear fabrics associated with shear zones that accommodated fast exhumation and cooling of the studied units. Folding of the shear-related structures during D3 resulted in the formation of pervasive axial planar cleavage in micaschists after ca. 36.5±0.5 Ma. The ages estimated for the different steps of the tectonometamorphic evolution of the study area compare with those obtained in other parts of the Penninic units with other geochronological techniques. Therefore, ages of deformation/metamorphic events may be preserved in white micas even when complex thermal histories follow their crystallization. Diffusive loss of Ar from white micas may be negligible if the studied minerals escape recrystallization and/or the time scales of the observed Pressure-Temperature evolution are very short.

 

Therefore, we suggest that the evolution of orogens is characterized by multiple short-lived burial-exhumation cycles related to orogen-scale alternance between shortening and extensional deformation. Such complex evolution may be related to internal dynamics of orogenic wedges or to repeated episodes of rollback of the hinge of subduction zones in front of the evolving orogen (Figure 6).