Megalineament in southeastern Alaska marks southwest edge of Coast Range batholithic complex

Abstract

The Coast Range megalineament is a prominent, nearly continuous topographic and structural feature that extends southeastward about 550 km (330 mi) from its junction with the Chatham Strait – Lynn Canal fault at Point Sherman to Tongass Passage near the mouth of Pearse Canal where it leaves southeastern Alaska. It probably extends still further southeastward into British Columbia along Work Channel and Chatham Sound – Grenville Channel.The megalineament is a zone a few hundred metres to 10 km (6 mi) wide in which closely spaced joints, foliation, compositional layering, and small faults define the megalineament trend. The zone usually coincides with topographic depressions apparently caused by selective fluvial and glacial erosion of the less resistant rocks of the zone.Studies in the Juneau, Endicott Arm, and Behm Canal areas indicate that the megalineament (1) is locally the site of lateral and (or) vertical separations of no greater than several kilometres; (2) does not mark a major structural or metamorphic discontinuity in the near-surface rocks; (3) may be located near a pre-metamorphic and pre-intrusive discontinuity; (4) is consistently associated with and parallel to steep gradients in both the gravity and aeromagnetic fields; and (5) probably is the surface expression of the western contact, at depth, of the intrusive rocks and gneisses of the Coast Range batholithic complex with the schists to the southwest.

Geological Survey research 1980

“…27) (Brew and others, 1976;Brew and Ford, 1978). This zone essentially coincides with the Juneau Gold Belt of Spencer (1907).…”

“…The belt varies from 2 to 25 km in width, with the narrower segments consisting of a single pluton and the wider ones of as many as three plutons separated by 1-to 5-km-wide screens of migmatitic gneiss. The existence of the sill as an important regional feature was first noted by Brew and others (1976) and the relation of the sill to the parallel Coast Range megalineament is described in Brew and Ford (1978).…”

Reconnaissance geologic map of the Bradfield Canal quadrangle, southeastern Alaska

“…This unit forms a number of separate bodies that are part of a nearly continuous belt of elongate quartz diorite and lesser granodiorite plutons referred to collectively as the tonalite sill belt (Brew and Ford, 1981;Brew andothers, 1976, 1984), or the Great Tonalite Sill (Brew, 1988). The belt extends along the east side of the Coast Range megalineament (Brew and Ford, 1978), and is traceable for more than 1,500 km, throughout the length of southeastern Alaska and into British Columbia (see, for example, Quottoon pluton, j_n Woodsworth and others, 1983).…”

“…Except for the highest peaks, the study area was covered by glacial ice in Pleistocene time, and many remnants of those glaciers remain, mainly at higher elevations. Several faults are mapped in the study area, but the most prominent structural feature that was mapped is a complex shear zone, called the Coast Range megalineament (Brew and Ford, 1978) or the Coast shear zone (M.L. Crawford, oral commun., 1996).…”

“…The zone is marked by a northwest-southeast linear topographic depression, mainly along stream valleys, from near Berg Mountain to beyond Eagle Lake, and which extends beyond the boundaries of the study area (Brew and others, 1984;Berg and others, 1988). The nearly vertical shear zone, which originated about 55 Ma, records intense ductile deformation, and forms the western boundary of the piutonic-metamorphic complex (Brew and Ford, 1978).…”