Geometry of eclogite‐facies structural features: Implications for production and exhumation of ultrahigh‐pressure and high‐pressure rocks, Western Gneiss Region, Norway

Abstract

Well‐preserved eclogite‐facies structural features in metamorphosed gabbro and diorite gneiss in Baltica crust, formed at a minimum depth of 60–70 km during the Scandian orogeny, allow direct inferences to be made regarding the geometry of folds, kinematics, and original structural orientations related to production and exhumation of high‐pressure rocks. Folds associated with eclogite‐facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation. Strain estimates and L > S or L ≫ S fabrics indicate that these structures were formed in a constrictional strain field. Locally, they are well preserved in eclogite‐facies mylonite zones at least 40 m thick that cut Proterozoic gabbro and adjacent gneiss. Where steeply dipping, they show a north‐side‐up shear sense across a younger, northeast trending, shallowly plunging, amphibolite‐facies anticlinorium formed in a constrictional, noncoaxial strain field. Where original structural facing direction can be inferred, small‐circle rotation of the eclogite‐facies lineation about the anticline axis indicates that the relative motion vector of Baltica with respect to overlying imbricated crust and Laurentia was oriented 320° in a present‐day reference frame. This result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with Late Silurian to Early Devonian oblique sinistral convergence between Baltica and Laurentia. Results of this study indicate that coherent Baltica crust at 60–70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops‐southeast thrusting parallel to plate motion. High‐pressure rocks were progressively imbricated and stacked along eclogite‐facies mylonite zones, with the deforming zone separating coherent Baltica crust from imbricated basement above. The constrictional strains under eclogite‐facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere.