Paper No. 3
Presentation Time: 9:00 AM-6:30 PM


BUCHANAN II, John Wesley1, BOWMAN, Dannena R.2, STELTENPOHL, Mark G.3, HAMES, Willis E.2, AUGLAND, Lars Eivind4 and ANDRESEN, Arild4, (1)Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401, (2)Department of Geology and Geography, Auburn University, 210 Petrie Hall, Auburn, AL 36849, (3)Department of Geosciences, Auburn University, 210 Petrie Hall, Auburn, AL 36849, (4)Dept. of Geosciences, University of Oslo, PO Box 1047, Blindern, Oslo, 0316, Norway,

Southern Liverpool Land (LL; 70.5oN) is located in the central East Greenland Caledonides and is divided by the Gubbedalen Shear Zone (GSZ), a ~500 m thick, N-dipping high-strain zone. The structurally lower part of the GSZ records tops-south amphibolite-facies contraction overprinted by tops-north greenschist-facies extension in the upper quarter. The hanging wall consists of high-grade Mesoproterozoic supracrustal rocks intruded by undeformed Caledonian plutons. The footwall block is a highly deformed Paleo-to-Mesoproterozoic migmatitic orthogneiss basement complex, including garnet-pyroxenite, amphibolite, ultramafics, and Caledonian (~399 Ma) eclogites. Mafic bodies occur as boudinaged pods wrapped by crystal-plastic mylonites.

Through overprinting and cross cutting fabric relationships, a five phase deformational history is established. D0 gneissosity is overprinted by D1 migmatitic foliation, which is retrograded by D2 mylonitic foliations and lineations over mafic pods. The GSZ overprinted aforementioned structures during D3 thrusting and subsequent D4extension. Stereonets of gneissosity (S0), migmatitic foliation (S1), and mylonitic foliation (S2) indicates the GSZ is an E-W structural monocline. Plots of mineral alignments (L0), neosomal “cigars” (L1), and elongation lineations (L2) indicate a cone axis reflecting sheath fold geometries created as fabrics are swept into the GSZ.

Muscovite samples from rocks within different structural levels were dated using 40Ar/39Ar thermochronology by single-grain fusion and incremental heating. Muscovite from a hanging wall pegmatite is 387±1.5 Ma, and muscovite from a shear zone mylonite is 378.5±0.86 Ma. Muscovite from an orthogneiss in the structurally lower part of the footwall is 376.1±2.0 Ma. These dates indicate Middle Devonian regional cooling of the hanging wall prior to extension along the GSZ, which tectonically unroofed the footwall. The LL T-t cooling path is exponential, from initially very rapid rates that are followed by slow cooling during final exhumation and subsequent deposition of late Devonian sediments. We compare T-t paths with other Caledonian lower-plate eclogite terranes on conjugate sides of the orogen and explore palinspastic restorations and tectonic mechanisms that fit their exhumation histories.