Open Session - SWISS GEOSCIENCE MEETINGs

22
Symposium 1: Structural Geology, Tectonics and Geodynamics
Tectonic units not involved in the early LT-HP cycle, notably those derived from the European margin, experienced a much
less spectacular start, in that they never were subducted to depths >20-25 km. Frontal units (e.g. the cover of the Gotthard-
Lucomagno nappe) show prograde LP-MT conditions of metamorphism. There is a clear age progression across the Lepontine
from N to S: Near Lucomagno Pass, for instance, 450 °C were attained 30-29 Ma ago, and T max (560 °C at ≈6 kbar) was reached
at 20-18 Ma (Janots et al. 2008). In the central portion of the belt T max was attained at subsequently earlier stages (Köppel et
al., 1978, 1981), greater depth and higher temperature (Engi et al., 1995); e.g. near Biasca: ≈600 °C, 7-8 kbar, at ≈23 Ma. Further
south, the Barrovian overprint reached T max earlier yet, but at lower pressures; e.g. near Castione: ≈650 °C, 5-6 kbar, at ≈26
Ma (Todd & Engi, 1997). Within the SSB (Southern Steep Belt), partial melting occurred in suitable lithotypes during decompression
in this period of upper amphibolite facies conditions (Burri et al., 2005; Berger et al. 2008). Heating rates for the
Barrovian metamorphism are well constrained in the north (8-10 °C, Janots et al., 2008), but not in the south of the Lepontine
belt. The regional distribution of robust ages across the entire belt shows that the thermal peak was diachronic from the
southern to the northern part of the Central Alps, with T max being reached at 30-27 Ma in the SSB, but only 20-18 Ma in the
northern Lepontine.
Ages documenting the cooling path above 500 °C remain difficult to interpret with certainty, bit fission track data for zircon
(reviewed by Vernon et al., 2008) yield constraints for cooling below ≈250 °C. For the interval from T max to ≈250 °C the age
data result in cooling rates of 31-53 °/My in the northern Lepontine, and of 44-58 °/My in the SSB. Exhumation rates are far
mor difficult to obtain, notably in the south. Cooling here was associated not only with tectonic unroofing and erosion, but
with lateral heat flow into the Southern Alps, owing to dextral transpression along the Insubric Line. The timing of this
movement is poorly known, exhumation rates cannot be obtained from cooling rates.
REFERENCES
Berger, A., Burri, T., Alt-Epping, P. & Engi, M., 2008. Tectonically controlled fluid flow and water-assisted melting in the
middle crust: An example from the Central Alps. Lithos, 102, 598-615.
Berger, A., Mercolli, I. & Engi, M., 2005. The central Lepontine Alps: Notes accompanying the tectonic and petrographic map
sheet Sopra Ceneri (1:100’000). Schweiz. Mineral. Petrogr. Mitt., 85(2/3), 109-146.
Bousquet, R., Goffé, B., Vidal, O., Oberhänsli, R. & Patriat, M., 2002. The tectono-metamorphic history of the Valaisan domain
from the Western to the Central Alps: New constraints on the evolution of the Alps. GSA Bulletin, 114(2).
Brouwer, F. M., Burri, T., Engi, M. & Berger, A., 2005. Eclogite relics in the Central Alps: PT-evolution, Lu-Hf ages and
implications for formation of tectonic mélange zones. Schweiz. Mineral. Petrogr. Mitt., 85(2/3), 147-174.
Burri, T., Berger, A. & Engi, M., 2005. Tertiary migmatites in the Central Alps: Regional distribution, field relations,
conditions of formation, and tectonic implications. Schweiz. Mineral. Petrogr. Mitt., 85(2/3), 215-232.
Engi, M., Todd, C. S. & Schmatz, D. R., 1995. Tertiary metamorphic conditions in the eastern Lepontine Alps. Schweiz.
Mineral. Petrogr. Mitt., 75(3), 347-369.
Janots, E., Engi, M., Rubatto, D., Berger, A. & Gregory, C., 2008. In-situ determination of heating rates in collisional orogeny.
Geology, accepted.
Köppel, V., Günthert, A. & Grünenfelder, M., 1981. Patterns of U-Pb zircon and monazite ages in polymetamorphic units of
the Swiss Central Alps. Schweiz. Mineral. Petrogr. Mitt., 61, 97-120.
Oberli, F., Meier, M., Berger, A., Rosenberg, C. L. & Gieré, R., 2004. U-Th-Pb and 230Th/238U disequilibrium isotope
systematics: precise accessory mineral chronology and melt evolution tracing in the Alpine Bergell intrusion. Geochim.
Cosmochim. Acta, 68(11), 2543-2560.
Todd, C. S. & Engi, M., 1997. Metamorphic field gradients in the Central Alps. Jour. Metam. Geol., 15, 513-530.
Vernon, A.J., van der Beek, P.A., Sinclair, H.D., Rahn, M.K. 2008. Increase in late Neogene denudation of the European Alps
confirmed by analysis of a fission-track thermochronology database, Earth Planet. Sci. Lett. 270, 316-329.