CHAKACHAMNA FAULT – AN OLDER EXTENSION OF THE CASTLE MOUNTAIN FAULT SPLITTING THE ALASKA PENINSULA TERRANE

Examination of new compilations of integrated digital geologic maps and isotopic ages suggest that the Alaska Peninsula Terrane was separated 160-200 km dextrally, along what we call the Castle Mountain-Chakachamna Fault (CM-CF), during the Late Jurassic or Cretaceous. Evidence for this offset is:

1. Middle Jurassic plutonic rocks of the Alaska Peninsula terrane, in the Alaska-Aleutian Range, trend northeast and are truncated on the north near the Chakachamna River. Comparable rocks show up 160-180 km to the east, north of the Castle Mountain Fault (CMF), and continue trending northeast.

2. The associated arc volcanic rocks (Talkeetna Formation) and later sedimentary rocks (Tuxedni Group, Chinitna and Naknek Formations) lie on the southeast side of the plutonic rocks. These rocks strike predominantly northeast in both regions, but are offset by 160-190 km along the proposed CM-CF.

3. 200 km restoration along the CM-CF brings the Cottonwood Bay Greenstone in-line with Triassic greenstone in the Talkeetna Mountains; both lying west of the Middle Jurassic plutonic rocks.

Displacement occurred after the deposition of the Middle Jurassic Chinitna Formation but timing is poorly constrained. With uncertainty we suggest the minimum age for this offset is Cretaceous; based primarily on the lack of an equivalent to the Matanuska Formation in the Alaska-Aleutian Range and evidence for mid-Cretaceous faulting along the Border Ranges Fault System (BRFS).

The CMF is commonly linked with the Lake Clark Fault (LCF) but the proposed offset could not have taken place along the LCF. Previous workers have matched older metamorphic rocks across the LCF. Thus, an older extension of the CMF must lie north of the LCF. The Chakachamna Lake area is a prime candidate because it connects with the Mulchatna Lineament on the west.

Recognition of the CM-CF has been elusive because of younger structural, plutonic, volcanic, and sedimentary events complicating the geology of the region. Pavlis and others (1988) suggested a comparable offset but only mentioned it as a side note to a study of the BRFS. The Telaquana Fault, proposed by Haeussler and Saltus (2006), with 26km of Neogene offset may be related to the proposed fault, but is much younger. We will present geologic, geochronologic, structural, aeromagnetic, and gravity data supporting this model.