STRUCTURAL, METAMORPHIC, AND TIMING RELATIONS ACROSS THE FJORD REGION DETACHMENT ZONE: GROWTH AND COLLAPSE OF THE EAST GREENLAND CALEDONIDES

The Fjord Region Detachment Zone (FRDZ) in Kejser Franz Joseph fjord is a major E-dipping extensional shear zone in the East Greenland Caledonides, separating upper plate, greenschist-facies, Neoproterozoic to Early Ordovician, sedimentary rocks from Mesoproterozoic and Paleozoic, migmatitic ortho- and paragneisses (Krummedal Sequence) in the lower plate. Krummedal gneisses have experienced polyphase deformation under upper-amphibolite-facies conditions and strong LS-tectonites indicate considerable vertical shortening and E-W extension. This flattening strain is superimposed on N-S-trending, isoclinal, recumbent folds indicating earlier N-directed flow. We interpret all these structures to be related to Caledonian crustal thickening and not to a Grenvillian orogenic event. Only two phases of contraction are recognized in rocks of the upper plate. 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 has orientations consistent with tops-E directed movement of the upper plate during late-orogenic collapse. 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 top-east displacement of the hanging wall. The mylonite zones locally are cut by steep brittle faults. We have found neither evidence that the extensional shear zones reactivated earlier thrusts nor that a thrust separates the Krummedal supracrustals from the underlying augen-gneiss (basement). U-Pb geochronology on monazite from syn-extensional granites indicate a late Silurian age for the extension. P-T estimates, based on elemental-partitioning thermobarometry, allow for comparison of thermobarometric histories across the FRDZ. The P-T estimates are combined with new 40Ar/39Ar mineral dates to elucidate the P-T-t path followed by the lower plate rocks.