Equilibrium in Southeast Alaska:

A struggle of rock and ice

James E. Toomey IV
ES767 Global Tectonics
Dr. James Aber, Instructor
Spring 2011
Emporia State University

Over the past 200 million years, southeastern Alaska was formed as the result of terrane accretion from the interaction of the Pacific and North American plates. Glaciation in the Miocene would result in the rapid erosion and uplift of southeastern Alaska terranes in the Holocence. Active faulting is present throughout the region and can be found beneath mountain ranges and glaciers as well as the deep waters ubiquitous to southeastern Alaska. Similar attributes along the plate boundary can be observed from the Aleutian Islands to the San Andreas Fault, though, southeastern Alaska provides a unique and rugged platform for which one can observe plate tectonics.

The origins of southeastern Alaska

Southeastern Alaska, also known as the Alexander Archipelago, is a composition of 11 terranes that were accreted to the North America by the Pacific Plate. These terranes originated from oceanic crust, islands, plateaus, ridges and island arcs (Jones, 1982). The oldest rocks date back to the Triassic Period, about 240 million years ago. Carried thousands of kilometers from a Triassic region know as the Tethys, these terranes were accreted by microplate tectonics after colliding with the North American continent. Current models show that the crust of the southeast Alaskan coast is mobile, moving northward relative to North America (Freymueller, 2010)

In the early Jurassic (200 million years ago), the Stikine terrane docked with North America and would later become interior British Columbia. The Wrangellia and Alexander terranes conjoined off the coast of North America in the Middle Jurassic and then collided with the continent. This collision was noted by metamorphism that generated granite rock that infused into the Coast Mountain range.

The collision of the Alexander terrane with the ancient shore of southeastern Alaska resulted in the formation of a series of strike-slip faults that fragmented the terrane into blocks (Shulte, 2002). In the Tertiary Period (65 million years ago), a final terrane arrived to the west of the Wrangellia known as the Chugach.

On the northern borders of southeastern Alaska one can see a powerful example of this block interaction. The Chugach/St. Elias orogen developed as the result of the northwestward translation, collision, and subduction of the Yakutat terrane along the North American plate margin for the last 25 million years (Inteplay, 2003). This motion of the Yakutat terrane along the Queen Charlotte-Fairweather transform fault system is similar to the collision of the Indian subcontinent against Asia. These resulting mountain range towers 18,000 feet above the nearby shores of Glacier Bay.


Terrane Origins and Current Faulting
Photo Credit: USGS Publication
This Dynamic Earth
Note the oceanic origins of Alaskan terranes

Examples of terranes in southeastern Alaska

(Above) Mt. Edgecumbe: This dormant volcano lies to the west of Sitka, AK and is part of the Chugach Terrane.

(Right) North Sawyer Glacier: One of three tide-water glaciers in southeastern Alaska, the north Sawyer Glacier lies at the terminus of the Tracy Arm fjord in the Tracy Arm Terrane.

Terranes of central and southeastern Alaska: Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE)

(Left) Kupreanof Island and Petersburg Creek as seen from the top of Mitkof Island. These two islands are contained within the Alexander Terrane.

(Below) Hanus Bay as seen from Peril Strait, AK. Beneath this small inlet lies the Peril Strait fault line; this fault line has divided the Wrangellia and Alexander terranes since the late Mesozoic (Johnson, 1985)

Retreat and Advance: Glaciation in southeastern Alaska

In the last 30,000 years, southeastern Alaska has been scoured and polished by late Pleistocene ice (Connor). Although it is assumed that the Gulf of Alaska was glaciated in the Miocene, data on ice formation prior to the end of Pleistocene is difficult to obtain. Most evidence relies on drill hole data which has yielded glacial sediment deposits approximately six million years old.

The coastal mountains along the Gulf of Alaska and the Alaska panhandle contain around 70,000 square kilometers of glaciers and icefields, the world’s largest non-polar ice complex. (Larsen, 2005). Following a global temperature rise from the last Ice Age, this ice complex began to melt, and the result was a significant loss in ice mass on the surface. As a consequence of the change in glacier ice mass, the earth’s crust and underlying layers have been adjusting under a process known as “glacial isostatic adjustment (GIA)” (Motkya, 2007).

Southeastern Alaska exhibits the highest isostatic rebounds rates in the world, particularly noted in Glacier Bay. Researcher Christopher Larsen detected uplift rates of 28 mm per year and predicts that another 6-8m of uplift will occur in Glacier Bay over the next 700-800 years before achieving gravitational equilibrium as the result of ice already lost (Larsen, 2005).

A consequence of six millions years of glacial advance and retreat, Southern Alaska is an example of continent-scale denudation (Meigs and Sauber, 2000). Throughout the region, calving glaciers and u-shaped valleys indicative of retreating ice serve as a constant reminder the powerful eroding forces still at work.

Iceberg calved from the LeConte Glacier near Petersburg, AK

Uplift Rates in southeastern Alaska: USGS (measures in 2mm/yr intervals)

Conclusion

Southeastern Alaska is a unique laboratory in which one can observe many geological processes. Comparable to those of the Himalayan orogeny, one is immersed by the presence of massive glacial erosion , active faulting beneath mountains, glaciers and fjords, and continental crust uplift coincident with glacial retreat. A region of active faulting today, inhabitants are under constant threat of earthquake, volcano and tsunami activity.

References

Connor, Cathy L. “Geology of Southeast Alaska: With Special Emphasis on the Last 30,000 Years.” University of Alaska. Juneau, AK.

Freymueller, Jeffrey T. 2010. “Active tectonics of plate boundary zones and the continuity of plate boundary deformation from Asia to North America.” Current Science Vol 99, No. 12.

Johnson, Bruce R and Karl, Susan M. 1985. “Geological Map of Western Chicagof and Yakobi Islands, Southeastern Alaska.” Department of the Interior. U.S. Geological Survey Miscellaneous Investigations 1506, 15 p.

Jones, David L., et al. 1982. “The Growth of Western North America.” Scientific American Vol 247/5, p 70-84.

Larsen, Christopher F. 2005. “Rapid viscoelastic uplift in southeast Alaska caused by post-Little Ice Age glacial retreat.” Earth and Planetary Science Letters 237. p 548-560.

Meigs, Andrew and Sauber, Jeanne. 2000. “Southern Alaska as an example of the long-term consequences of mountain building under the influence of glaciers.” Quatenary Science Reviews 19. p 1543-1562

Motkya, Roman J. 2007. “Post Little Ice Age Glacial Rebound in Glacier Bay National Park and Surrounding Areas.” Alaska Park Science Journal, Vol. 6, Issue 1

Schulte, Priscilla, Ph. D. and Crocker-Bedford, Karalynn, M.S. 2002. “Karst and Caves of Southeast Alaska.” UAS Ketchikan and Cultural Heritage Research

“The Interplay of Collisional Tectonics and Late Cenozoic Glacial Climate in Alaska and the Northeastern Pacific Ocean.” A Continental Dynamics/NSF and JOI/USSSP Sponsored Workshop. May 2003.

All photographs © by the author.