The Origin and Tectonic Setting of Tow Hill, Queen Charlotte Islands

Abstract

<p> Tow Hill is a 109m butte located on the north shore of Graham Island, Queen Charlotte Islands. It is composed of a massive, 105m thick layer of olivine tholeiite, underlain by thin sills of tholeiitic basalt and interbedded sediments of the Skonun Formation. The age of this basalt body has been estimated to be less than 5Ma. Three hypotheses have been proposed as to the origin of Tow Hill: 1. a sill intrusion; 2. hydroclastic material; and 3. a lava flow or flows. Macroscopic features observed on Tow Hill do not indicate an origin and each of the three hypotheses can adequately explain the features. Petrographical studies indicate that the basalt is not fragmental and thus, the second hypothesis can be rejected. Neither of the two remaining hypotheses can be strictly accepted or rejected with petrographical examination. Chemical analyses indicate that Tow Hill is made up of "within plate" basalts which are probably related to activity associated with rifting and/or transtension in a continental environment. This constrains the late Tertiary tectonic history of the Queen Charlotte Islands in that any model of the tectonics of this area must involve rifting and/or transtension.</p>