Posted by: Felix Halpaap, Photos by: Stéphane Rondenay
With SWaMMIS coming to an official end soon, we set off for the final project meeting on Sotra on Monday, 16 September. What a great time it’s been since the start nearly four years ago, and the meeting turned out to be a worthy conclusion!
Group photo at the Panorama Hotel
We checked in at the Panorama Hotel all the way at the southern tip of Sotra. Sotra is a narrow, north-south-stretching island made up of pre-Caledonian basement rocks, and separates Bergen from the open North Sea. Here, we were optimally located for our field trip to some world-class outcrops on the third day of the meeting. But first, we spent two days of exciting geoscientific discussions in a cozy conference room – away from any distractions except the view of the sea and rocks. Continue reading Synthesis meeting and field trip report→
Volcanic arcs are chains of volcanoes that form above a subducting slab, for example, the Cascades in the Pacific Northwest, the Japanese Archipelago, the Aleutian Islands, and parts of the Andes. Volcanic arcs are a result of rising magma produced in the mantle wedge, but the exact mechanisms controlling the arc position are debated. This study led by SWaMMIS collaborators at Imperial College London finds that the cold corner of the mantle wedge plays a key role. Results from kinematically driven 2D thermo-mechanical modelling help constrain the influence of various factors, including subduction velocity, slab dip, slab age, overriding plate thickness and the depth of decoupling between the slab and the overriding plate.
Western Greece hosts one of the world’s more intriguing and complex subduction systems. To the north, the subducting slab is buoyant continental crust; to the south, the subducting slab is denser oceanic crust. This study provides a detailed description of the transition from continental to oceanic subduction using state-of-the-art seismic imaging techniques on a large data set of local earthquakes. The results help us to better understand how and why fluid processes and seismicity change across the region.
Writer Ellen Viste joined our first field trip to Holsnøy in 2015, and was struck by how much sampling has been done over the years at one of the main outcrops of high pressure rocks on the island. The visit inspired a thought-provoking article in the Norwegian magazine Frifluftsliv about the legalities — and ethics — of collecting rocks.
Håkon Austrheim leads the SWaMMIS field trips to the world-class outcrops of Holsnøy. Håkon is a professor at the University of Oslo, and a petrologist specializing in fluid-rock interactions and pseudotachylytes.
Geologic terranes of the Bergen area. Holsnøy is the island labelled with the number 5, and is part of the Lindås Nappe. Map from the website of Håkon Fossen; see his excellent page on the Bergen Arcs System for more information.
The outcrops on Holsnøy reveal the complexity of rock reactions in the deep continental crust. During the Caledonian Orogeny (400 to 450 million years ago), these rocks were 60 km down at the base of an ancient mountain belt, where the pressure-temperature conditions would be expected to transform granulite facies rocks into ecologites. However, the outcrops we see at the surface today contain both types of rock — white-grey granulite and dark green ecologite. Why is this? Because the process of eclogitization not only requires high pressures and temperatures, it also needs water (H2O). H2O can come from hydrous minerals in the rock or from fluids percolating through the rock. The granulite of the Holsnøy outcrops is rather dry, and thus ecologitization occurred only where it came in contact with aqueous fluids.
Granulite (white) and ecologite (dark green) with red-brown garnets.
Pseudotachylytes (locally molten rock, quickly cooled as amorphous glass) within some outcrops on Holsnøy are evidence of earthquakes at the base of the Caledonides mountain belt, at depths far deeper than where one would normally expect earthquakes according to the brittle-ductile transition (usually around 15 km). Earthquakes deeper than 40 km depth, termed “intermediate depth earthquakes”, are observed today in active subduction zones and at the base of the Himalaya. The mechanism behind intermediate depth earthquakes remains one of the outstanding questions in seismology.
Håkon was one of the first geologists to recognize the field record of deep earthquakes, which led to a number of high-profile publications about the geology in Bergen’s backyard (see, for example, Pseudotachylytes Generated During Seismic Faulting and Eclogitization of the Deep Crust). He has also shaped current ideas that metamorphism cannot be predicted through pressure-temperature conditions alone, but that fluids are usually required to transform an otherwise metastable rock.
How “vein” are rock-stars? This is perhaps one of the most photographed outcrops in the Bergen area. Fluids circulating through this vein transformed the surrounding granulite (grey) into eclogite (green).Blocks of “dry” garnet granulite (anorthosite gabbro) within an eclogitized matrix.
Subduction zone Water and Metamorphism: a Modelling and Imaging Study
