Deformation, emplacement and tectonic inferences: the great tonalite sill, southeast Alaska, U.S.A.

Abstract

The unique late Cretaceous to early Tertiary Great Tonalite Sill (GTS) of SE Alaska and British Columbia is a very long (c.1000km) and thin (<25km), orogen- parallel, composite batholith, which may separate two major superterranes in the western Cordillera: the Insular superterrane (including the Alexander and Wrangellia terranes) from the Intermontane superterrane (including the Stikine and Cache Creek terranes).The steeply NE dipping, sheet-like plutons of the Great Tonalite Sill are dominated by NW-SE striking concordant fabrics with steep lineations, which formed within a country rock shear zone of similar dimensions - the Great Tonalite Sill shear zone - prior to the complete crystallization of the calc-alkaline tonalitic magmas. The steep, multiple dyke like nature of this composite body and its emplacement during orogenic contraction, imply that ascent and emplacement have been achieved by dyke wedging mechanisms along the deep reaching, probably crustal scale, shear zone. The remarkable narrowness and yet persistence of the Great Tonalite Sill, is probably the result of petrogenesis associated with a very localised zone of crustal thickening, produced by the associated narrow shear zone extending along the orogen length. Deformation in the Great Tonalite Sill shear zone is dominated by NE-SW directed contraction orthogonal to the orogenic strike associated with a component of NE over SW high angle shear. Such a shear zone of late Cretaceous to early Tertiary age, lying along 800 km of the boundary between the Insular and Intermontane superterranes, strongly implies that it represents the actual boundary between them. That being the case, then terrane accretion during this interval was orthogonal and not obliquely dextral as in some current interpretations of paleomagnetic data. NE side up tilting of mid Cretaceous plutons may therefore be responsible for much of the anomalous palaeomagnetic data determined for these intrusions.