The Interplay of Eclogitization and Deformation During Deep Burial of the Lower Continental Crust—A Case Study From the Bergen Arcs (Western Norway)

Abstract

The island of Holsnøy in the Bergen Arcs, which belong to the Caledonides of western Norway, represents an excellent example of how fluid-induced eclogitization modifies material deeply buried by subduction and continental collision. We produced a new detailed map of the northwestern part of Holsnøy, differentiating not only the magnitude of eclogitization but also the strain intensity at different spatial scales: from the outcrop to the entire massif. Using structural data from eclogite-facies shear zones and eclogitized low-strain domains, we show that fluid-mediated eclogitization not only progresses via the development of shear zones (dynamic eclogitization) but occurs over large areas of the island without associated deformation, creating a characteristic static eclogite-facies overprint. Static eclogitization preserves the structural features of the granulitic protolith while the rock body is transformed from a granulite-to an eclogite-facies mineral assemblage. The extent of static eclogitization was underestimated strongly in the past, a finding also relevant for the interpretation of seismological images in currently active orogens, where presumably similar processes are currently occurring. In addition, we find that the general structure of the eclogite-facies shear zones is scale-independent over several orders of magnitude. Although crustal-scale eclogite-facies complexes are rarely preserved without significant modification during exhumation, this implies that similar geometrical configurations are likely produced at the scale of the whole lower crust during subduction or continental collision and therefore shape the crustal geophysical signature.

Modification of the Seismic Properties of Subducting Continental Crust by Eclogitization and Deformation Processes

“…Only in rare cases a sharp contrast is retained. This is not because the transition zone in those areas is missing but because the thickness of this zone is below the scale mapped by Zertani et al (). This is consistent with results from a mid to upper crustal shear zone studied by Rey et al (), where an increased thickness of such a transition zone results in decreased reflection coefficients.…”

“…When using Holsnøy as an analog for deeply buried lower crust, it has to be noted that not all eclogites on Holsnøy are formed in shear zones. It was pointed out recently that large (up to 2 km in diameter) eclogitized domains on Holsnøy are not related to deformation processes but that in some areas metamorphism is dominated by static, fluid‐induced eclogite‐facies reequilibration (Jamtveit et al, ; Zertani et al, ). In combination with the low seismic anisotropy of the granulites, this suggests that in low‐strain domains P wave anisotropy will be low (likely <5%).…”

“…The solid line in each diagram shows the variation assuming a fixed value for the transitional layers. Those were set to 70% eclogite (eclogite breccia) and 35% eclogite (granulite with minor shear zones and small‐scale static eclogite features; see Zertani et al, , for detailed descriptions). The dashed lines show the variation when assuming linear changes between the different rock types.…”

“…Hence, the variation of seismic properties is also gradual. For this reason, we introduced simple linear gradients throughout the shear zones that reflect the gradually decreasing abundance of eclogite away from the shear zone center (Figure ), by defining eclogite abundances in the center and the boundaries of each of the layers in agreement with the classification discussed in Zertani et al (). The resulting P and S wave velocities are calculated as Voigt‐Reuss‐Hill averages (Hill, ).…”

“…The inset (a) shows the location of the map in western Norway near the city of Bergen. The map (b) was compiled and modified from Jolivet et al () and Zertani et al ().…”