HR: 17:00h
AN: T14B-05    [Abstracts]
TI: Magmatic arc construction: Constraints from the structure of the Coast plutonic complex
AU: * Andronicos, C L
EM: ca98@cornell.edu
AF: Dept. of Earth and Atmospheric Sciences and Institute for the Study of the Continents, Snee Hall Cornell University, Ithaca, NY 14854, United States
AB: The Coast plutonic complex of British Columbia exposes the middle crust of a vigorous magmatic arc formed in the late Cretaceous and early Cenozoic. The arc is divided into two parts by the crustal scale Coast shear zone. West of the Coast shear zone between Prince Rupert and Douglas Channel plutons were emplaced at pressures between 500 and 850 MPa, are elongate with steeply dipping contacts and have tadpole shaped terminations. The steeply dipping sides of the plutons are concordant with country rock foliations that grade into mylonitic foliations in the cores of transpressive shear zones which strike NW parallel to the arc axis. Field relationships demonstrate that pluton emplacement occurred during displacement on the shear zones, folding of adjacent country rocks and pluton enhanced anatexis of host rocks. Pressure differences in single plutons and across shear zones attests to major vertical displacements during pluton construction. The data indicate that the plutons record emplacement during major transpressive deformation and crustal thickening. The tabular shape of the plutons, their steep dips and concordance with country rock fabric show that pluton emplacement and plate scale deformation were synchronous between ~120 and 85 Ma. East of plutons that intrude the Coast shear zone a sequence of upper amphibolite to granulite facies gneiss and migmatite (central gneiss complex) occurs that host gently dipping tonalite and granodiorite sills. Mineral assemblages in country rocks of the sills track uplift and exhumation from pressures of ~800 MPa to 200 MPa during sill emplacement between ~75 and 50 Ma. The gently to moderately dipping fabrics that host the sills are the result of transposition of an earlier steeply dipping foliation. Sill emplacement occurred during both contractional and extensional deformation as indicated by a reversal in shear from reverse to normal sense observed at a regional scale. The sills are concordant with host rock foliations and mutually crosscutting relationships show that sill emplacement occurred during foliation transposition. Geometric analysis of foliations, lineations and kinematic indicators, demonstrate that sill emplacement occurred as the host rocks were flattened. Individual sills range in scale from meter to kilometers in thickness, with the largest sills traceable for tens of kilometers along strike. Host rock gneiss in many regions are dominated by orthogneiss suggesting much of the crustal column within the central gneiss complex has plutonic protoliths, particularly south of Douglas Channel. This indicates that much of the crust was constructed by repeated episodes of sill injection. Overall, the changing patterns of magmatism as tracked by changing pluton geometry and deformation style show that pluton emplacement in the Coast plutonic complex was facilitated and controlled by regional deformation patterns. Heat transported by the plutons and the presence of partially molten rock interacted with regional deformation by enhanced localization of strain during pluton construction. The change in geometry of the plutons from steep sided tabular bodies to gently dipping sills coincides with a switch from transpressional/contractional deformation to transtensional/extensional deformation. Thus, the geometry and kinematics of pluton emplacement can provide constraints on regional deformation processes in arcs.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8104 Continental margins: convergent
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting