KLEPEIS, K.A., Dept. of Geology, Univ. of Vermont, Burlington, VT 05405, kklepeis@zoo.uvm.edu; CRAWFORD, M.L., Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010; GEHRELS, G.E., Dept. of Geosciences, Univ. of Arizona, Tucson, AZ 85721; DAVIDSON, C., Dept. of Geology, Beloit College, Beloit, WI 53511
Abstract:
Studies of convergent margins show that regional-scale shear zones
form important parts of the internal structure of batholiths. However,
determining the origin and effects of displacements within these shear
zones remain problematic aspects of how batholiths are emplaced and evolve.
In the central coast belt of northern British Columbia, we have mapped
shear zones that record a history of changing kinematic regimes during
emplacement of the Paleogene Coast Mountains batholith. Our data
show that the sense of
displacement on major, arc-parallel shear zones within the batholith
reversed during two periods of tectonic activity. The first reversal
involved the onset of sinistral strike-slip displacements on arc-parallel
faults within an overall dextral transpressional system at 68-63 Ma. The
second reversal coincided with a change from a dominantly transpressional
to a dominantly transtensional regime at 59-52 Ma when reverse faults,
including those in the Coast shear zone, were reactivated as ductile normal
faults. Kinematic
analyses suggest that the onset of sinistral displacements during the
Paleogene resulted from an increase in the rate of motion on arc-parallel
faults inside the batholith. This increase in rate reflected both
increased strain localization, enhanced by the presence of syntectonic
melt, and an increase in the coaxial component of deformation relative
to the strike-slip component. An increase in the efficiency of arc-parallel
extrusion and coaxial deformation also appears to be linked to an increase
in the angle of Kula-North America plate convergence at this time.
The second kinematic reversal may be linked to a sudden decrease in convergence
rate and the onset of orogenic collapse. These data show that construction
of a large part of the central Coast Mountains batholith occurred during
the extensional collapse phase of orogenesis and could be linked to changes
in plate boundary conditions.
