Metamorphic core complex formation by density inversion and lower-crust extrusion.

Area of Science:

• Geology
• Tectonics
• Geophysics

Background:

• Metamorphic core complexes are geological features characterized by domal uplifts of deep crustal rocks.
• Their formation mechanisms are debated, with models including low-angle normal faulting and plutonic intrusions.
• The D'Entrecasteaux islands in Papua New Guinea offer a unique active example within a rift setting.

Purpose of the Study:

• To investigate the formation mechanism of active metamorphic core complexes in the D'Entrecasteaux islands.
• To test the hypothesis of vertical extrusion driven by crustal density inversion.
• To assess the general applicability of this mechanism to other geological settings.

Main Methods:

• Isostatic modeling to understand density variations and buoyancy forces.
• Analysis of geological data, including rock types and structural features.
• Heat-flow measurements to infer thermal regimes and crustal processes.

Main Results:

• Evidence supports the vertical extrusion of ductile lower-crustal material as the primary mechanism.
• A crustal density inversion, with denser ophiolite over less-dense continental crust, accentuates this buoyant extrusion.
• The active rifting in the region (approx. 6 Myr old, 25 mm/yr) facilitates the process.

Conclusions:

• Buoyant vertical extrusion driven by crustal density inversion is a key mechanism for forming metamorphic core complexes.
• This mechanism is particularly effective in regions with specific density contrasts, like the D'Entrecasteaux islands.
• The findings may be applicable to other regions experiencing thermal expansion and density reduction in the crust.