STRUCTURAL AND METAMORPHIC OBSERVATIONS ACROSS A CALEDONIAN SHEAR ZONE - IMPLICATIONS FOR GROWTH AND COLLAPSE OF THE EAST GREENLAND CALEDONIDES

The Fjord Region Detachment Zone (FRDZ) is a major E-dipping extensional shear zone in the East Greenland Caledonides. It separates anchimetamorphic to greenschist grade Neoproterozoic to Early Ordovician sedimentary rocks in the hanging wall from migmatitic ortho- and paragneisses (Krummedal Sequence) of Paleozoic and Mesoproterozoic age in the footwall. The appearance of a 20 m thick impure marble unit in "conformable" contact with an augen gneiss over large areas is taken in support of a depositional contact between the Krummedal Sequence and underlying augen-gneiss. Both units have undergone polyphase deformation under upper-amphibolite-facies conditions resulting in the formation of LS-tectonites. The linear and planar structural elements indicate considerable vertical shortening and E-W extension. This flattening strain is superimposed on N-S-trending, isoclinal, recumbent folds that indicate earlier north-directed flow. We interpret all these structures to be related to Caledonian crustal thickening and not to a Grenvillian event as has been suggested. Two phases of contractional deformation are recognized in the greenschist-grade metasedimentary rocks of the upper plate (hanging wall). The first deformation phase (D1) is linked to the development of a cleavage (S1), defined by parallel-oriented white mica, biotite, and locally chloritoid. S1 intersects bedding (S0) at a small angle and is clearly unrelated to the axial surface of large wave-length, open folds (F2) controlling the map-pattern. Locally a conjugate set of kink bands is developed. Their orientation link them to late-orogenic collapse with tops-east movement of the upper plate. Lower-and-upper-plate rocks are juxtaposed along a planar, highly silicified, ca. 0.5 - 1.0 m thick, E-dipping detachment fault. This fault zone grades downward into a 30-50 m thick brecciated mylonite zone which in turn grades into variably mylonitized migmatitic paragneisses. These E-dipping, low-angle mylonite zones consistently show tops-east displacement of the hanging wall. These mylonite zones are locally cut by steep brittle faults.