No mafic layer in 80 km thick Tibetan crust.

Area of Science:

• Geophysics
• Tectonics
• Petrology

Background:

• Continental crust formation models predict a mafic lower crust.
• Crustal doubling in Tibet and the Himalayas suggests a high-velocity mafic layer above the Moho.

Purpose of the Study:

• To investigate the composition and structure of the exceptionally thick crust in the middle Lhasa Terrane.
• To challenge existing models of continental crust formation in collision zones.

Main Methods:

• Seismic data acquisition and analysis.
• Seismic tomography to image crustal structure and lithosphere.
• Analysis of seismic P-wave velocity (Vp) and earthquake distribution.

Main Results:

• The 80 km thick crust in the middle Lhasa Terrane exhibits exceptionally low seismic P-wave velocity (Vp < 6.7 km/s) throughout.
• Deep crustal earthquakes and thick lithosphere suggest low crustal temperatures.
• The observed low velocity and inferred low temperature indicate a predominantly felsic crustal composition, contradicting mafic lower crust predictions.

Conclusions:

• The thick continental crust in the middle Lhasa Terrane is predominantly felsic, not mafic.
• Existing models of magmatic and plate tectonic processes may require revision for continental collision zones.
• These findings support alternative models for the formation of extremely thick, felsic juvenile crust.