Ground Truth Trekking

Abstract

Elevated beaches along the shore of Lake Iliamna and Kamishak Bay have the potential to provide constraint on vertical crustal deformation in southwest Alaska (Map 1).  Tectonics in this area are little studied, and seismic hazard is of particular interest because of the possible development of Pebble Mine and its associated perpetual tailings storage structures.

Lake Iliamna 

Vertical shifts in the ancient beaches surrounding lake Iliamna provide evidence of a potential fault running near the north shore of the lake.  We conducted GPS surveys of these beaches, and found that the highest, likely dating to around 26,000 years ago (Stilwell and Kaufman, 1996), drops by about 10 meters over a distance of a few kilometers, though it is nearly flat outside this narrow band.  Given that this beach was originally horizontal, the localized change in elevation of the beach must be related to localized deformation of the earth's crust.  This deformation is most likely related to the Lake Clark Fault or a previously undiscovered fault in the area. We plan to conduct further surveys in 2011 to further test this hypothesis.

Kamishak Bay

UNDER CONSTRUCTION:  Beaches stranded tens of meters above Kamishak Bay show that this coastline has experienced at least gradual uplift over the last 100 millennia, and there may be evidence for more dramatic recent deformation.

Map 1:  Elevated beaches along Lake Iliamna (outlined in white) and along Kamishak Bay (outlined in blue) overlap major faults (black) in this area.  The black rectangle marks an area where our GPS survey appears to show localized deformation (detail in Fig. 2).  We found possible evidence of deformation in both these systems of elevated beaches, suggesting these faults may be active.  Activity on these faults is relevant to potential development of the Pebble Mine (red).  Larger Map