A mountain range can be viewed as a wedge of material that tries to maintain a particular taper – we call it the critical taper – while material enters into the wedge during collision of two tectonic plates. It will attempt to maintain this taper despite any erosion or deposition of sediments that is occurring in the system. So, for example, if the amount of erosion greatly increases in the mountains, such as due to glaciers, the mountain range’s taper is decreased by mass taken from the mountains and dumped as sediments at the toe of the mountains. If erosion and associated deposition are faster than tectonics, the system can become what is called sub-critical and complicated faulting can develop. This scenario is exactly what we think is happening in the St Elias Mountains.

The tectonic driver for this interaction is the underthrusting of the Yakutat Terrane which during STEEP was discovered to be an remarkably thick oceanic plateau (see STEEP 3D figure).  This discovery was possible due to geophysical imaging and modeling geophysical where reflections and refractions of soundwaves through the mountains, shelf and deep sea fan where recorded and processed to make images from surface to mantle.  Integration offshore to onshore allowed mapping of key fault lines that defined the transition from nearly flat underthrusting of this Terrane to collision of this Terrane with North America as the thicker portions ident into southeast Alaska driving mountain building.

Based on our offshore imaging and mapping to drill intervals on the shelf and in the fan, around 1 million years ago climate change that amounted to an intensificaiton of North American glaciation changed the erosional process and erosion began to dominate over tectonics. The changes in faulting that resulted, combined with the erosion, moved a great deal of rock that was deeply buried towards the Earth’s surface, while simultaneously increasing the slope of the surface of the earth locally, creating dramatic relief.  This was possible due to effcient removal of massive quantities of sediment from the mountains and across the shelf via wet-based ice streams during glacial times.  This sediment was added to the deep sea Surveyor Fan which fills much of the Gulf of Alaska and is one of the five largest deep sea sediment bodies in the world.  This Fan in turn then influences earhquake hazards along the northern part of the Alaska-Aleutian Trench as exemplified by the 1964 Good Friday earthquake.  STEEP also added strength to the argument that this pivotal earthquake actually occurred at depth on the interface between the Yakutat Terrane and North American and then ruptured southwards towards where the Pacific Plate and the Surveyor Fan are underthrusting (see USGS Earthquake Rupture Zones figure). 

The clearest impact this project has had is to showcase just how interwoven tectonic and surface processes are. The St Elias region is starting to look like the world’s premier example of how surface processes of erosion can change the landscape so fast that the tectonic system has to adjust to the changes. Since erosion is fundamentally driven by climate changes, this discovery leads to thought-provoking issues of how much these processes have affected the whole global climate system. We suspect that ultimately we will see indications that the changes taking place in the St Elias system were important factors in major climate changes of the past ~1 million years, but we simply do not know the answer to that question yet. Additionally, we think that intensification of the cycle of glaciations had a fundamental role in enhancing mountain building, but the same signal needs to be found in other glaciated mountain ranges to truly know if this is correct.

Excerpts of this text can be found in International Innovation intervi...

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