Ph.D. research: Thermal evolution of high-pressure metamorphic rocks in the Alps

From 1996 to 2000 I carried out a 4-year Ph.D. project in the Structural Geology and Tectonics group at Utrecht University, the Netherlands. I investigated the retrograde history of high-pressure metamorphic rocks from three localities in the Central and Western Alps. The project was supervised by Profs Reinoud Vissers and Rinus Wortel.

The three field areas are in the upper Valle Piantonetto (Gran Paradiso National Park, Piemonte, Italy), near Gorduno in Ticino (Switzerland) and in the Voltri massif (Liguria, Italy). The first part of my work consisted of field- and labwork to constrain the retrograde history of the three areas as well as possible.

Tectonic map of the Alps, after: Structural Model of Italy, C.N.R Italia, 1983.

My work in Ticino focussed on the retrograde history of the migmatitic gneisses and pelitic schists surrounding the Alpe Arami garnet peridotite. These rocks have caught a lot of attention in recent years (e.g. Dobrzhinetskaya et al, 1996), because there are indications that these rocks originate from very deep in the earth (possibly deeper than 400 kms). Obviously, the peridotite body and the surrounding rocks went through the later part of their history together. The retrograde history of the country rocks is therefore of great importance.

The Gran Paradiso massif in the Italian western Alps is a composite of metamorphosed sediments, granites and mafic dykes, and it is the deepest part of the stratigraphy exposed in this part of the Alps. In the supposedly equivalent Dora Maira massif to the south coesite has been found, indicating that the rocks went to at least 100 kilometers depth when they underwent high-pressure metamorphism. One of the reasons to work in this area was to see whether any relics of ultra high-pressure metamorphism could be retrieved from Gran Paradiso.

The rocks from the Voltri massif are eclogitic blocks in a serpentinite mŽlange. They are of interest to us, because they represent a slightly different (oceanic) setting within the same orogen.

The Pressure-Temperature-time (PTt) paths of the rocks from all three areas show high-pressure metamorphism, followed by cooling during decompression. Strinkingly, the rocks from Gorduno, as well as those from Gran Paradiso then recorded increasing temperatures during further decompression, followed by cooling during final exhumation. An important task was to explain the cause and consequences of this heat pulse in terms of geodynamics.

The second part of my project was a numerical study of two hypotheses that have been proposed to explain the thermal evolution of high-pressure rocks in a convergent setting (like the Alps). The modeling was carried out in co-operation with Menno van de Zedde of the Tectonophysics group at Utrecht. The reconstructed PTt paths from Ticino and Gran Paradiso were used as constraints.

We compared the thermal implications of slab breakoff with those of the presence of a wedge of radiogenic material. The first mechanism was introduced by Wortel and Spakman (1992) for the Hellenic Arc, and later applied to the Alps by Von Blanckenburg and Davies (1995). After the onset of continental subduction the subducted slab yields, and allows for an influx of hot, asthenospheric material. The second mechanism involves the tectonic accretion of a wedge of highly radiogenic material at the subduction zone, causing additional heating of the collisional orogen (cf. Jamieson et al., 1998). The results of our modeling suggest that the additional radiogenic material cannot account for the observed thermal histories of Gran Paradiso and the rocks from Gorduno. On the basis of our results I conclude that slab breakoff may have occurred in the Alps, and that it can account for the re-heating observed in Gran Paradiso and Ticino.

Acknowledgements

This research was (in part) supported by The Earth and Life Sciences Foundation (ALW) with financial aid from the Netherlands Organisation for Scientific Research (NWO). Microprobe analyses were carried out at the EU Geochemical Facility at Bristol University (UK) with funding from TMR (contract ERBFMGECT980128) and assistance from Dr. S.L. Kearns.

PhD dissertation

"Thermal evolution of high-pressure metamorphic rocks in the Alps." PhD awarded on November 20th, 2000 at Utrecht University. View abstract. Thesis published as: Geologica Ultraiectina 199, 221 pp. ISBN 90-5744-056-3.
I have a clickable pdf-file of my thesis. Please contact me in case you are interested.

Publications

Please check my CV, to see my list of publications.

Presentations

• "Thermal evolution of high-pressure metamorphic rocks in the Alps." Departmental seminar, Universität Bern, 15 April 2002.
• "Thermal evolution of high-pressure rocks from the Alps: The response to slab breakoff?" Oral presentation at the GSA Annual Meeting, Boston, November 2001. View abstract
• "Thermal evolution of high-pressure metamorphic rocks in the Alps." Invited oral presentation at the Department of Geology, University of Maryland, College Park, 16 February 2001.
• "Exhumation and Thermal Evolution of High-Pressure Rocks From the Alps: the Response to Slab Breakoff?" Poster presentation at Fall AGU, San Francisco, December 2000. Eos Trans. AGU, 81 (48), Fall Meet. Suppl., Abstract T51C-13, p. F1071 (2000). View abstract
  
• "Exhumation and Thermal Evolution of High-Pressure Rocks From the Alps: the Response to Slab Breakoff?" Poster presentation at VMSG symposium "Synergy in Earth Sciences". Delft, 24 November 2000. Best Poster Award. View abstract
  
• "Testing geodynamic models for exhumation of high-pressure metamorphic rocks in the Alps." Poster presentation at VMSG symposium "Geodynamics of the Tethys region". Utrecht, 19 November 1999 and NAC V, Veldhoven, April 2000. View abstract
  
• "Metamorphic rocks in the Alps: A thermal history and its geodynamic significance." Invited oral presentation at Vening Meinesz research School for Geodynamics (VMSG) symposium "Geodynamics of the Tethys region". Utrecht, 19 November 1999. View abstract
  
• "Testing geodynamic models for exhumation of high-pressure metamorphic rocks in the Alps." Oral presentation at MSG-meeting "Exhumation of metamorphic terranes", Rennes, France, 31 August - 2 September 1999. View abstract
  
• "PTt histories and exhumation of HP rocks in the Alps." Oral presentation at EGS 1999, The Hague, Netherlands, April 1999. Geoph. Res. Abstr. 1 (1), 64. View abstract
  
• "PTt histories in the Alps: implications for exhumation mechanisms." Oral presentation at EUG 10, Strasbourg, France, March 1999. J. Conf. Abstr. 4 (1), 64. View abstract
  
• "Deformation and metamorphism of the Gran Paradiso nappe, Western Alps, Italy." Poster presented at the GV-meeting, Berne, Switzerland, February 1998 and NAC IV, Veldhoven, May 1998. Terra Nostra 98/1, 41-42. View abstract