Authored by Markus Rast.

The past summer, SGT group members spent another field season in southern Alaska. While my colleagues Prof. Dr. Whitney Behr and Lonnie Hufford are almost real Alaskans, it was the first time for me in the US state that is also called the ‘Last Frontier’. When I arrived, they already spent one week to get familiar with the newest equipment acquisition – a former fishing boat named Ocean Devotion that is used to reach remote places along the Kachemak Bay and to drone non-accessible outcrops of the Chugach accretionary complex directly from the boat (more about 3D models based on drone images you can find here). Starting the drone from the boat is relatively easy, however, it turned out that landing onto a moving area that is surrounded by the boats cabin and railing is somewhat trickier.

Captain Whitney and the boating instructor Janel Harris in front of the Ocean Devotion
Ocean Devotion floating in Halibut Cove
Lonnie is looking for dirt on the paint of our brand new car in Seldovia

After my arrival, we allowed the Ocean Devotion some free time in the harbour and left our base in Homer to spend a week in Seldovia. There, our mobility was assured by an old, rusty GMC provided by the local community (thank you!) and brand new pack rafts that brought us to the outcrops we wanted to study. Simply stated, our research goal for the field work was to gain a better understanding of deformation processes taking place along subduction zones. In Seldovia we focused in particular on mechanisms that are leading to a repetition of typical oceanic crustal rocks such as basalts and cherts. And indeed, we discovered a lot of spectacular pillow basalts (named after their pillow-shaped texture) and nicely folded cherts (sedimentary rocks consisting mainly of fossil silicious shell remains). However, in contrary to our expectations we did not find major fault zones that are responsible for a repetition of the ocean floor rocks (thrusts that bring pillow basalts on top of deep marine sediments). And as so often in fieldwork, we had to correct our hypothesis – nature always seems to be a bit more complicated than we think.

Two pack rafts and one pack rafter – Jakolof Bay
Seldovia Village at low tide
From left to right: Markus, Lonnie, and Whitney – serious people during serious times

After my first week, our colleague Dr. Vénice Akker joined the group. And at the same time, unfortunately, the unstable weather also arrived, something that always has to be expected in Alaska. Nevertheless, Whitney, Vénice, and I prepared our gear to spend three days at the Grewingk Glacier Lake. In addition to the usual field gear like hammer, chisel, and ipad (fieldbook 2.0), we needed tons of other stuff. Tents, sleeping pads, sleeping bags, food, cooking gear, pack rafts, paddles, drone, drone batteries, and some other stuff was packed in three backpacks (or as in my case, attached to the outside of the backpack using every strep that was available). After crossing the Kachemak Bay with the Ocean Devotion and a hike to the lake, we used the pack rafts on the glacier lake to reach the outcrop of our interest. The highly deformed rocks we encountered there are a nice example of a sediment-rich shear zone consisting of a mélange of argillites, cherts, graywackes, and basalts. The style of deformation we can observe in this shear zone is representative for parts of subduction zone megathrusts (major shear zone between subducting and overriding plate) and will improve our understanding of what is going on in active subduction zones.

Lonnie in front of pillow basalts (Seldovia Bay)
Isoclinal fold in cherts (Seldovia Bay)
Markus before the hike to the Grewingk Glacier Lake

However, before we can tell you more about this, we had to bring the collected samples (and ourselves) back across the glacier lake, while the wind blowing down the glacier got stronger and stronger and the waves on the lake got higher and higher. This was the last day on the field for me before I left, while the others explored other places of the Chugach accretionary complex (and met the bears, which really exist, at least on the photographs I received).

Grewingk Glacier Lake
From left to right: Markus, Vénice, and Whitney – happy to have survived the crossing of the lake
Mélange with argillite matrix and chert, graywacke, and basalt blocks

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