The Origin of Kinematic Reversals Within the Central Coast Mountains Batholith and Their Relationship to Paleogene Magmatism and Tectonics

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.


 
 

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