17:15 〜 19:15
[SMP28-P17] Investigating the buoyancy-driven exhumation process in the Dora-Maira UHP unit (western Alps)
キーワード：浮力、ドラマイラ岩体、超高圧変成作用、上昇メカニズム
Ultrahigh-pressure (UHP) crustal rocks have been reported from many localities worldwide, but their exhumation mechanisms remain incompletely understood. Buoyancy is the dominant force driving material transport in the solid Earth, including plate tectonics. It is also thought to be the driving force behind the exhumation of UHP units from within the mantle, where they form up to lower crustal levels. However, the geodynamic implications of this model and how they compare to geological observations have only been incompletely explored.
The Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif in the Western Alps is not only one of the famous sites of UHP rocks—it was the first place UHP rocks were identified—but its geological characteristics are also well known, making it ideally suited to study the geodynamics of exhumation. We present a detailed estimate of the BIU's bulk density at peak metamorphic conditions and evaluate the buoyancy during exhumation. We also calculate the retrograde P-T path through thermal modeling and compare the results with the observed P-T path.
The bulk density of the BIU was estimated using the rock densities of the main lithologies. These rock densities were determined by thermodynamic calculations, considering uncertainties in peak P-T estimates, the activity of H2O, variations in bulk rock compositions, and the extent of the quartz-coesite transition. The resulting bulk density at peak metamorphic conditions (730 ºC, 4 GPa) was 3030–3250 kg/m3, and the density difference relative to the mantle (ρmantle-ρBIU), which controls buoyancy, was estimated to be 120–340 kg/m3. Assuming that all quartz had transformed into coesite, these values could be further constrained to 3110–3250 kg/m3 and 120–260 kg/m3, respectively.
The estimated buoyancy can also be used to evaluate the shear stress acting between the UHP fragment and the subducting plate during the early stage of exhumation (~130 km), assuming force equilibrium. The resulting shear stress was 0.2–1.2 MPa, providing geological constraints on the shear stress distribution along the subduction plate interface.
The retrograde P-T path was calculated using a simple one-dimensional heat conduction model. The effect of shear heating was also assessed based on the estimated shear stress. By comparing these results with the observed P-T path from the BIU, we will discuss the validity of the buoyancy-driven exhumation model and its future implications.
The Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif in the Western Alps is not only one of the famous sites of UHP rocks—it was the first place UHP rocks were identified—but its geological characteristics are also well known, making it ideally suited to study the geodynamics of exhumation. We present a detailed estimate of the BIU's bulk density at peak metamorphic conditions and evaluate the buoyancy during exhumation. We also calculate the retrograde P-T path through thermal modeling and compare the results with the observed P-T path.
The bulk density of the BIU was estimated using the rock densities of the main lithologies. These rock densities were determined by thermodynamic calculations, considering uncertainties in peak P-T estimates, the activity of H2O, variations in bulk rock compositions, and the extent of the quartz-coesite transition. The resulting bulk density at peak metamorphic conditions (730 ºC, 4 GPa) was 3030–3250 kg/m3, and the density difference relative to the mantle (ρmantle-ρBIU), which controls buoyancy, was estimated to be 120–340 kg/m3. Assuming that all quartz had transformed into coesite, these values could be further constrained to 3110–3250 kg/m3 and 120–260 kg/m3, respectively.
The estimated buoyancy can also be used to evaluate the shear stress acting between the UHP fragment and the subducting plate during the early stage of exhumation (~130 km), assuming force equilibrium. The resulting shear stress was 0.2–1.2 MPa, providing geological constraints on the shear stress distribution along the subduction plate interface.
The retrograde P-T path was calculated using a simple one-dimensional heat conduction model. The effect of shear heating was also assessed based on the estimated shear stress. By comparing these results with the observed P-T path from the BIU, we will discuss the validity of the buoyancy-driven exhumation model and its future implications.