Arc construction during high-temperature transpression and orogen parallel extension

M.L. Crawford, Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010 and K.A. Klepeis, Dept. of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia

In the central Coast Mountains orogen of northern coastal BC and SE Alaska a late Cretaceous to Paleogene (~75 to 50 Ma) batholith was emplaced into upper amphibolite facies metamorphic rocks. Field observations suggest the early plutons of the batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension followed by exhumation. Pluton emplacement was controlled in large part by the deformation of the country rocks (both prior to and during magma intrusion). Many of the older (60 Ma) plutons are tabular and range from 100's of meters to a few kilometers in thickness. The oldest are concordant to regional foliation and are folded into kilometer scale upright and recumbent folds with N-plunging axes parallel to a regional north-plunging stretching lineation. These are followed by sills concordant to the foliation that dip gently to moderately northward and steeply dipping to vertical dikes that strike north, parallel to the axial planes of the regional folds and to the stretching lineation. The latest group of intrusions (60-50 Ma) comprises two large tonalite plutons: the Khyex sill and the Quottoon pluton and a number of thin sheet-like bodies oriented perpendicular to the regional stretching lineation. The ~60 Ma Quottoon pluton is steeply dipping, trends NW and is 10-15 km wide and 100 km long. The shape of this pluton, its relations to several generations of north side down extensional shear zones in the adjacent Central Gneiss Complex, its steeply dipping magmatic fabrics and concordant margins, and the lack of evidence of tectonic strain until late in its crystallization history suggest that space for the pluton was created by orogen parallel extension. Accumulation of large volumes of igneous rock within discrete volumes of the crust in turn apparently played a role in modifying crustal ductility thereby producing regional scale shear zones that separate those zones of enhanced ductility from crust with few or no coeval igneous rocks. One is a major crustal detachment zone in the batholith core that separates a complex of sheeted intrusive rocks and migmatitic gneiss from overlying amphibolitic and metasedimentary rocks. Rocks above the detachment zone experienced a minimum of intrusive activity, little to no melting, and display a strong structural discordance with the detachment zone and with the fabric of the underlying complex. The other is the Coast shear zone on the west side of the Quottoon pluton. This shear zone is 2-3 km wide where the Quottoon is widest and narrows to a few tens of meters where the Quottoon is absent. The Coast shear zone separates the Paleogene batholith from rocks to the west that show little evidence of deformation or igneous activity younger than ~90 Ma. Both of these shear zones can be related to uplift and eventual exhumation of the Paleogene batholith.