Abstract
All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp < 6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.
Highlights
All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust
The ongoing Indo-Asian continental collision has created some of the thickest crust on Earth, which is conventionally assumed to include a thick mafic lower crust with high seismic velocity (Vp > 7.0 km/s)[11,12,13,14,15,16,17], the lowest crustal velocity remains relatively unconstrained by the hitherto available data
The data were interpreted by phase correlation and traveltime picking of the main seismic phases from the crust and uppermost mantle, followed by tomographic inversion and ray tracing traveltime modeling of the crustal velocity structure as well as extensive tests of uncertainties in the model (“Methods” section, Supplementary Figs. 1, 5, 6 and 7)
Summary
All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. It has been suggested that the lower crust does not need to be basaltic[9], but until now all seismic observations show high P-wave velocity, which requires that the bulk composition of the lower crust must include at least 20–40% of mafic rocks[10]. The ongoing Indo-Asian continental collision has created some of the thickest crust on Earth, which is conventionally assumed to include a thick mafic lower crust with high seismic velocity (Vp > 7.0 km/s)[11,12,13,14,15,16,17], the lowest crustal velocity remains relatively unconstrained by the hitherto available data. The resulting crustal doubling[11,12,13,14,15,16,17] implies that the crust should contain large amounts of mafic material
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