Basalt Underplating and Eocene Extension, Coast Mountains Batholith, British Columbia

HOLLISTER, Lincoln S., Department of Geosciences, Princeton University, Princeton, NJ 08544, linc@princeton.edu; MOROZOV, Igor, and SMITHSON, Scott, Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071;
CHRISTENSEN, Nikolas, Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI 53706; ANDRONICOS, Christopher, Dept. of Geological Sciences, University of Texas, El Paso, TX 79968

Abstract:
The present-day crustal thickness (31-32 km) under the Coast Mountains batholith (CMB), near Prince Rupert, BC, is similar to that typical of extended terranes.  The batholith is dominated by granodiorite, tonalite, and diorite plutons that intruded between 55 and 50 Ma.  During the same time interval, the CMB was exhumed by 15 - 20 km.  The crustal velocities of the upper part of the CMB can be correlated directly with laboratory compressional and shear wave velocity measurements of tonalite to diorite. The lower 10 km of the crust under the CMB has seismic velocities that correspond to mafic garnet granulite, which is what basalt/gabbro would be at the granulite facies of metamorphism.  Because the temperature at the Moho is now about 850oC
(based on Pn velocity), the lower crust under the CMB is now in the granulite facies of metamorphism.  Mafic garnet granulite in the lower crust is not found to either side of the CMB.  Based on these observations, we infer that the lower 10 km under the CMB consists mostly of what was initially underplated basalt.

Given the need for a heat source to produce the granodiorite to diorite plutons by melting of the crust, we propose that underplated basalt provided that heat at the base of the crust. The thermal event also softened the crust. During the Eocene extension, the CMB was ductily thinned, from the present level of exposure to the Moho, concurrently with the exhumation and intrusion of granodiorite to diorite plutons.


 
 

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