Basalt Underplating and Crustal Extension During the Formation of the Coast Mountains Batholity, British Columbia and Alaska

L.S. Hollister (1), C.L. Andronicos (2), S.B. Smithson (3), and I.B. Morozov  (3)
(1) Dept. of Geosciences, Princeton University, Princeton, NJ 08544, linc@princeton.edu
(2) Dept. of Geological Sciences, University of Texas, El Paso, TX 79968
(3) Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071.

Abstract:
The Coast Mountains batholith (CMB) is one of the largest batholith complexes of the world and was a target for study by the ACCRETE multidisciplinary project.  It is dominated by granodiorite, tonalite, and diorite plutons that intruded between 85 and 50 Ma, with the largest pulse of magmatism occurring between 55 and 50 Ma.  Between 55 and 50 Ma, the CMB was also exhumed by 15 - 20 km.  The CMB is separated from the terrane to the east (Stikinia) by east-dipping normal shear zones between the migmatites of the footwall and the greenschist to amphibolite facies rocks of the hanging wall; these faults were active during the same 5 Ma time interval. The seismic images show that the normal shear zones can be traced at least to mid-crustal depths. The extension that produced these shear zones resulted in an asymmetric core complex with steep fabrics on its west side and gently east dipping fabrics on the east side.  The exhumation of the CMB was due in large part to unroofing during extension.  The Vp and Vp/Vs model across the CMB shows that the present CMB crust is comparable in thickness to the average of extended terranes and that the lowest 10 km has seismic properties corresponding to restite and the metamorphic equivalent of gabbro. A model for crustal melting driven by underplated basalt to form the plutons of the batholith is supported by our data; crustal scale extension helped provide space for pluton intrusion into the middle crust.


 
 

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