Crustal Extension Within the Coast Plutonic Complex

ANDRONICOS, C. L., Dept. of Geological Sciences, University of Texas at El Paso, El Paso Texas, 79968, chris@geo.utep.edu;  RUSSMORE, M., Dept. of Geology, Occidental College, Los Angeles, CA 90041; CHARDON, D. H., CEREGE-CNRS Universite Aix-Marseille III Europole Mediterraneen de l'Arbois BP 80 13545 Aix-en-Provence Cedex 4 FRANCE; HOLLISTER, L. S., Dept. of Geosciences, Princeton University, Princeton New Jersey 08544.

Abstract:
Between 55 and 48 Ma extensional deformation affected large portions of the Coast Plutonic Complex (CPC). A broadly north-northwest trending belt of northeast dipping ductile normal faults is present along the eastern side of the CPC between Douglas Channel and Portland Inlet. These normal faults locally define the contact between the metamorphic core of the orogen and
rocks of the Stikine Terrane. Later folding, normal faulting and late kinematic plutonism complicate the original pattern of normal faulting. These normal shear zones are up to 2 km thick and display spectacular mylonitic fabrics.  C/S fabrics, shear bands, tension vein arrays and dike networks are all consistent with top to the east-northeast normal displacement within the shear zones.

Metamorphic mineral assemblages and quantitative thermobarometry indicate peak metamorphic conditions adjacent to the Eocene Kasiks pluton of 5-6 kbar and temperatures as high as 800 C. Kyanite and sillimanite bearing assembalges found along Douglas channel indicate similar levels of burial. In contrast, andalusite bearing schists that sit structurally above the ductile normal faults record pressures of 2-3 kbar and temperatures of 700 C during peak contact metamorphism. This indicates between 5 and 9 km of
vertical displacement between the two crustal blocks. Shear strains of 8-10 are estimated from late kinematic dike and vein networks and are consistent with kilometer scale displacements on the shear zones.

Although large-scale pluton emplacement affected parts of the CPC during extensional shearing, Eocene plutons locally crosscut extensional structures although their emplacement may be controlled by the normal faults they intrude.  These relationships suggest large-scale plate interactions are likely to drive extensional deformation not thermal weakening and body forces associated with pluton emplacement.


 
 

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