Apr
11

Abstract: Eclogitization and Shear Zone Kinematics in the Lofoten Islands, Norway

Although a plethora of studies have been done regarding shear zones and eclogites, the true characteristics of the eclogite-facies shear zones in the Lofoten Islands, northern Norway remain enigmatic. Eclogites, high-pressure and high-temperature metamorphic rocks, can form up to 50 km beneath the Earth’s surface, and are found to be one of the only rock types that are stable at this lower crust-mantle boundary. Typical eclogite mineral assemblages include omphacite, garnet, and plagioclase as the primary constituents.

In the case of the eclogites found in the Lofoten Islands, particularly on the island of Flakstadøy, they experienced a retrograde metamorphic event of amphibolite-facies caliber sometime after their formation. Some retrograded eclogite samples showed a converted mineral assemblage of sparse omphacite, the occasional relic garnet, and transformed pyroxene, amphibole, and plagioclase. Additionally, many of the omphacite grains got replaced with a plagioclase-clinopyroxene symplectite, and under more severely retrograded conditions, amphibole further replaces clinopyronexe grains. Thus, by examining the mineral assemblage transitions and chemical compositions of those minerals, it can speak to the physical conditions endured by the eclogites found in the Lofoten Island shear zones.

Shear zones are essentially fault systems that form deep underground. There are three types of shearing: pure, simple, and general shear. Pure shear means the rock enjoyed flattening, or squishing from one direction. Simple shear implies that the rock was pushed in opposing directions from the top and the bottom, to the left and the right (or vice versa) respectively. General shear is a mix of pure and simple shear. We can further characterize shear zones into three types, and they all record the histories of the rocks in slightly different fashions. Type I shear zones have margins that are easier to deform than their interiors, which causes them to thicken over time and create “dead” zones in their centers where there is no longer any active deformation. Type II shear zones are in many ways the opposite of Type I; the actively deforming region thins over time towards the interior of the zone, which causes the marginal zones to become “dead.” Finally, the most-actively deforming regions of Type III shear zones do not exhibit thickening or thinning.

Shear zones kinematics, when combined with the information garnered from the eclogite metamorphic conditions, can give us clues to the mechanics of the eclogites’ extremely deep burial and exhumation, the mechanics of collisional tectonic events, and the deformational processes they underwent during these journies. Not only will this advance the geologic community’s knowledge of eclogite-facies shear zones, but it will also help describe the dynamic processes that happen deep beneath the Earth’s surface, which have yet to be fully understood. Thus, the purpose of my research is to quantify the kinematics of eclogite-facies shear zones in the Lofoten Islands, northern Norway and to determine the pressure and temperature at which the eclogites may have formed.

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