There are two major and currently unresolved issues in Alpine geology concerning the metamorphic evolution of the rocks in the internal zones of the Alps. First, rocks showing evidence for geologically young, high-pressure to very high-pressure metamorphism are now exposed at the Earthâs surface, implying exhumation from depths as much as 100 km in a belt less than 250 km wide. Secondly, these high-pressure rocks, in places, show evidence for significant re-heating during their ascent, a phenomenon known in the central Alps as the Lepontine metamorphism.

This thesis addresses geological evidence for these two features in rocks from the Alps, and investigates possible causes. This is done by a combination of detailed field-based studies involving reconstruction of pressure-temperature-time (PTt) trajectories of pertinent rocks, and numerical modelling of processes that may explain a re-heating during exhumation. The PTt paths serve as constraints on numerical modelling results, and allow testing of contending hypotheses regarding possible causes for heating during exhumation of the high-pressure rocks in the Alps.

Three areas in the metamorphic Pennine Zone of the Alps were selected for a detailed study of their metamorphic (PT) histories. These areas, in the Lepontine gneisses near Alpe Arami in the central Alps, the Gran Paradiso basement in the western Alps, and the Voltri massif in the Ligurian Alps, respectively, were chosen at spatial intervals of the order of 150 kilometres measured along the Alpine arc, in order to obtain an orogen-scale insight in similarities and differences in metamorphic evolution of the rocks in question.

The present study of the Lepontine gneisses surrounding the Alpe Arami garnet peridotite body in the central Alps focuses on thermal information, obtained from a sliver of pelitic rocks from the Cima Lunga nappe as well as from metabasic rocks in the surrounding Arbedo zone. The rocks underwent high-pressure metamorphism at peak pressures of up to 21 kbar1, equivalent to depths in excess of 70 km, and temperatures of around 900 ¡C. Recent geochronological studies suggest an age of this stage of metamorphism around 37 to 40 Ma. The rocks subsequently cooled below about 650 ¡C during initial exhumation at rates of at least 7 mm/yr. The rocks were re-heated by about 110 ¡C around 32 Ma ago, leading to medium-pressure, high-temperature metamorphism at pressures of 4 to 6 kbar equivalent to depths around 20 km, and climax temperatures up to 760 ¡C. Final exhumation to the Earthâs surface occurred at slower rates of about 0.8 mm/yr.

The metamorphic history of the Gran Paradiso massif in the Italian western Alps is somewhat similar. High-pressure metamorphism took place at conditions of 525 ¡C and 12 to 14 kbar, equivalent with depths of some 50 km, around 43 million years ago. Initial decompression was fast, at an estimated average rate of 3.3 mm/yr, and was accompanied by cooling. At around 20 kilometres depth the rocks were re-heated by a few tens of degrees, reflected by medium-pressure, medium-temperature metamorphism at 6 kbar and 550 ¡C. On the basis of geochronological studies by other workers this re-heating is inferred to have occurred shortly before 34 Ma. Final exhumation occurred at a slow rate of 0.6 mm/yr.

A tectonic mŽlange in the Voltri massif in the Ligurian Alps of Italy contains blocks of eclogitic metagabbro. Thermobarometric studies of these rocks indicates that metamorphism culminated at pressures of about 18 kbar, equivalent with depths of 65 km, and temperatures of around 525 ¡C. There are as yet no geochronological data available to constrain the age of this high-pressure metamorphism. Peak metamorphic conditions were followed by continuous cooling during decompression, initially at relatively high P/T ratios, i.e., along a low thermal gradient, followed by eventual exhumation along a distinctly higher thermal gradient. The lack of age data, however, precludes quantification of the cooling and exhumation rates involved. Deposition of continental sediments on exhumed high-pressure rocks in the Voltri massif indicates that the Voltri massif was exposed at the Earthâs surface by 34 Ma.

Plausible causes for the re-heating observed in the central and western Alps are explored, and two hypotheses are selected that, at least qualitatively, seem capable to provide the heat explaining the observed thermal evolutions during exhumation: detachment or breakoff of the subducted slab, and heating by accreted radiogenic material (TARM). Thermomechanical numerical modelling of these two hypotheses allows comparison of their thermal implications with the PTt trajectories reconstructed for the metamorphic rocks in the study areas selected. Modelling results show that re-heating in the central Alps may have been caused by slab detachment, whilst the time-scale implied by the geochronological data precludes the observed re-heating to be caused by radiogenic heating due to the presence of a TARM wedge. Due to oblique convergence, block rotations and out-of section transport, conclusions cannot be drawn in a straightforward manner for the case of the western Alps. Again, the available age data preclude re-heating in the western Alps in response to the presence of a TARM wedge, but a possible role of slab detachment is difficult to confirm or discard. On the basis of existing evidence for large-scale transpression, and changes in convergence direction I propose as a possible explanation that the Gran Paradiso rocks experienced their moderate re-heating in a position much closer to the Lepontine gneisses of the central Alps, prior to tectonic transport toward their present-day position in the western Alps.

This study does not provide constraints regarding the precise kinematics or dynamics underlying the exhumation of the high-pressure to very high-pressure rocks. Inasmuch as these rocks reached sub-crustal depths, a tentative scenario involves buoyancy-driven exhumation to the base of the crust, followed by further exhumation via corner flow in the orogenic wedge driven by continuous underplating of subducting material at the base of the wedge (Platt, 1986). Changes in exhumation rates along the pressure-temperature-time trajectories in both the central and western Alps may point to a combination of exhumation mechanisms. Slab detachment, inferred to have taken place in the central Alps, with implications for the thermal evolution of rocks presently exposed in the western Alps, can explain the re-heating observed in these areas, but its role in the exhumation of the high-pressure rocks is, as yet, unclear.

This research was supported by the Dutch Organisation for Scientific Research (NWO-ALW).

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Last modified 18th March, 2001

Thermal evolution of high-pressure metamorphic rocks in the Alps