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.
