T143 Cordilleran Arc Magmatism, BATHOLITHS and Continental Crustal Genesis I

Tuesday, 9 November 2004
8:00 AM - 12:00 PM

STRUCTURE OF THE JURA-CRETACEOUS COAST BELT AND ADJACENT STIKINIA NEAR BELLA COOLA, COAST MOUNTAINS, B.C.: PRELIMINARY RESULTS

Paper number: T143-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

New mapping within the central Coast Mountains of British Columbia reveals that Jura-Cretaceous magmatic arc rocks are cut by ductile shear zones, and confirms earlier conclusions that the Coast shear zone (CSZ) juxtaposes markedly different lithologic assemblages. NE of the CSZ subgreenschist-facies volcanoclastic strata and tonalitic plutons, presumably arc-related and part of Stikinia, are exposed in upper Dean Channel. Comparison with new mapping by Haggart et al. (2004) suggest a Late Jurassic to Early Cretaceous age for these units. In general, the rocks are only weakly deformed; sparse bedding/cleavage relations suggest the presence of large upright to overturned folds. Four NW-striking ductile shear zones disrupt the units. Shear zones are marked by 50 m to 800 m of mylonites derived from tonalite and metavolcanic rock. All shear zones dip steeply NE. Lineations are down dip in two of the shear zones, and field observations suggest NE-side-up, reverse motion. Moderately to gently plunging lineations characterize the other shear zones; both show dextral shear in outcrop. The shear zones strike SW toward a 68 Ma unfaulted pluton, suggesting they are Late Cretaceous in age and formed during the later stages of arc construction. Together, the shear zones may represent strain partitioning driven by oblique subduction.

West of the CSZ, amphibolite facies metamorphic rocks form screens and pendants in mid-Cretaceous plutons. Previous work (Gehrels and Boghossian, 2000) shows that many of the pendants have affinities to the Yukon-Tanana and Alexander terranes. Our field observations show sillimanite-garnet-biotite assemblages in several screens. Sillimanite is psuedomorphic after kyanite in a few samples. The notably higher metamorphic grade west of the CSZ requires significant exhumation of mid-crustal rocks prior to NE-side up reverse motion on the CSZ at 65-50 Ma.