Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Area of Science:

• Geology
• Tectonics
• Metamorphic Petrology

Background:

• Reconstructing geodynamic evolution of mountain ranges relies on pressure-temperature-time paths from metamorphic rocks.
• The lithostatic pressure assumption converts metamorphic pressure to burial depth, enabling burial and exhumation history quantification.
• Adjacent rocks in the Monte Rosa tectonic unit, Western Alps, show considerable metamorphic pressure differences.

Purpose of the Study:

• To investigate the cause of significant metamorphic pressure variations in adjacent rocks within the Monte Rosa unit.
• To challenge the prevailing lithostatic pressure paradigm in understanding metamorphic rock evolution.
• To propose and support an alternative model involving outcrop-scale pressure gradients.

Main Methods:

• Field and microstructural observations.
• Phase petrology and geochemistry analyses.
• Analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock.

Main Results:

• Pressure differences in adjacent rocks could not be explained by tectonic mixing, retrogression, or lack of equilibration.
• A significant pressure difference of 0.8 ± 0.3 GPa was determined between adjacent rock samples.
• Analytical models demonstrated the mechanical feasibility of significant outcrop-scale pressure gradients in heterogeneous rock.

Conclusions:

• The observed pressure variations indicate a deviation from the lithostatic pressure assumption.
• Significant outcrop-scale pressure gradients are mechanically feasible and likely influence metamorphic pressure interpretations.
• Rethinking the lithostatic pressure paradigm is crucial for accurate geodynamic reconstructions of mountain ranges.