Ground Truth Trekking

Introduction

Importance

In our fieldwork, we explored possible evidence for geologically recent activity on both the Lake Clark and Bruin Bay faults near Lake Iliamna, as well as searching for unexplored faults in the area. We undertook this fieldwork to address some of the large uncertainties about tectonics in this region, which are particularly relevant to the prospects for industrial development in the area, such as the proposed Pebble Mine.

Assessing activity on a fault

Faults are deeply penetrating fractures in the Earth’s crust. Faulting results when stress builds up and causes the rock to shear. A fault is 'active' if that process of building stress is ongoing, and earthquakes are thus likely in the future. This is different from inactive faults, where stress is no longer concentrated and earthquakes no longer occur.

The main way geologists assess whether a fault is active is by attempting to determine whether it has produced earthquakes in the geologically recent past (the past few hundred thousand years). If an earthquake has occurred on a fault recently, then the growing stress that leads to earthquakes is likely still there, and will lead to earthquakes in the future. Given that even very active faults only have an earthquake every few hundred to a few thousand years, and we have only been measuring earthquakes for the last century, it is common that no direct measurements or observations exist of earthquakes on a particular fault. Instead we must rely on evidence that has survived since the most recent earthquake. This geologic evidence becomes part of a detective story - clues that can show an earthquake has occurred.

Many types of evidence can reveal ancient earthquakes. In our fieldwork, we focused primarily on ancient beaches that can provide evidence for vertical motion on a fault.  When a beach forms on a lake or the ocean, it is horizontal, and extends only a few meters above the normal water level.  So if a beach is found high above the water, or a beach varies in elevation along its length, this may be evidence for tectonic activity.

Map 2: Tectonic Overview of Alaska

These major faults, most known to be active, shift as different sections of the crust move relative to each other.

Regional Tectonics (Map 2)

The tectonic deformation of southern Alaska is driven by collision of the Pacific Plate with the North American Plate.  Faults in the area around Lake Iliamna, along with most of Western Alaska, have not been studied in detail. Lake Iliamna straddles the northwest edge of the band of earthquakes and volcanoes that comprises the Pacific 'Ring of Fire'. It's the sort of place where active faults are likely, but few earthquake geologists have worked in the field here, and few instruments have been deployed to measure plate motion or earthquakes.

Geologists do know something about tectonic plates in the larger region surrounding Lake Iliamna and the base of the Alaska Peninsula. There is good evidence that most of southcentral Alaska, called the 'Southern Alaska Block' (Haeussler, 2008) is a section of the earth's crust that is moving westward relative to the rest of North America. The Denali Fault, in the Alaska range, is the main fault that this block moves along, but there are other faults, including the Castle Mountain Fault just north of Anchorage, that allow it to deform and move westward. Additionally, there is evidence that the crust beneath the Bering Sea (called the 'Bering Block', Mackey et al., 1997), is rotating clockwise relative to North America and eastern Russia. And the Pacific Plate, which extends all the way from the Gulf of Alaska down into the South Pacific, is sliding northwards, beneath the continental crust that forms Alaska.

These three pieces of the earth's crust surround our field sites. To the east is the Southern Alaska Block, to the northwest is the Bering Block, and to the south is the Pacific Plate. It is unclear whether the Lake Iliamna region is part of the Bering Block or Southern Alaska Block. It may even be that the Bering Block and Southern Alaska Block have no distinct boundary between them (Redfield et al., 2007).

Also, there are known faults in the area that were once active, and which may or may not currently be active. The Lake Clark Fault (Haeussler et al. 2004), an extension of the Castle Mountain Fault, extends southwest from Lake Clark Pass down through Lake Clark.  Preliminary work on the northeast end of this fault suggest it hasn't had an earthquake that broke the surface of the earth since the end of the last ice age (Koehler, personal communication), but there are no published studies establishing whether this fault is active.  The Bruin Bay Fault branches from the Castle Mountain and Lake Clark faults near Tyonek, and runs south along the Cook Inlet coast into Katmai National Park.  This fault is believed to be inactive because it does not cut an intrusion south of Kamishak Bay (Detterman & Reed, 1980).  Neither of these lines of reasoning rule out fault activity below the surface.

Map 3: Maximum Glacial Extent

Glaciers covered both study areas during the last ice age (blue) and even more so longer ago (green.)

Glacial History (Map 3)

The entire region around the study area has been repeatedly inundated by moving ice (Kaufman & Manley, 2004, and Alaska Paleoglacier Atlas).  During their advance and retreat these glaciers shaped the landscape by eroding and depositing sediment, scouring out the depths of Lake Iliamna, building moraine ridges, flooding areas along their margins with plains of river gravel, and burying ice that melted to leave kettle lakes.  It is against the backdrop of this dramatic reshaping of the landscape that tectonic processes may have had an additional impact.

Glaciers advance and retreat over time, and terminal moraines left at their greatest extent during a given advance provide a widespread indicator of where they were in the past.  The most recent ice age followed an interglacial period similar to, but slightly warmer than, the present that extended from about 130,000 to 110,000 years ago (The Sangamon Interglacial).  During this time temperatures were about 2° C warmer, and sea levels were about 8 m higher than today (Kopp et al., 2009).  The period from about 110,000 years ago to about 12,000 years ago is classified as the Wisconsin glaciation, an "ice age" when glaciers were much more extensive than the present across much of the globe.  The greatest extent of ice in the area of this study likely came late in this period, around 26,000 years ago (Stilwell and Kaufman, 1996).  At this time ice extended south across Kodiak Island and west well beyond Lake Iliamna.  As ice retreated to nearly its current extent sea level rose, reaching approximately its current level about 6000 years ago.