Paper No. 1
Presentation Time: 8:00 AM
LATE-STAGE CALEDONIAN COLLISIONAL TECTONICS OF NORTHERN SCANDINAVIA – LINKED CONTRACTIONAL AND EXTENSIONAL EXHUMATION OF THE BALTOSCANDIAN MARGIN
ANDERSON, Mark W., School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom, STELTENPOHL, Mark G., Auburn Univ, 210 Petrie Hall, Auburn, AL 36849-5305, KEY, Thomas B., Department of Geology and Geography, Auburn University, Petrie Hall, Auburn, AL AL36849, ANDRESEN, Arild, Department of Geology, Univ of Oslo, 1047 Blindern, 0316 Oslo, Oslo, Norway and HAMES, Willis E., Department of Geology and Geography, Auburn University, 210 Petrie Hall, Auburn, AL 36849, manderson@plymouth.ac.uk
The Caledonides of north-central Scandinavia (Torneträsk-Troms-Ofoten) are dominated by a series of crystalline thrust sheets (“nappes”) that structurally overlie autochthonous and internally imbricated pre-Caledonian crystalline basement and cover rocks of the Baltic Shield. The allochthons were emplaced during ESE-directed translation of previously assembled thrust sheets across the margin of Baltica, mainly during the Devonian. This occurred in a largely passive, piggy-back style although major late-stage, out-of-sequence thrusts at several structural levels within the allochthon caused regional scale repetition and truncation of tectonostratigraphy. Hanging-wall and footwall geometries and radiometric dating of shear zone fabrics indicate that the development of these late thrusts closely relates to reactivation of pre-Caledonian Baltic basement during the Devonian (400-370 Ma). East-west contraction during the upper end of this time frame is peculiar considering that this was the period of large magnitude and rapid tops-west extension in western Norway.
40Ar/39Ar thermochronology of muscovite from mylonites of the Gullesfjord-Austerfjord shear zone (GASZ) records cooling from 420-380 Ma (Scandian) tops-east thrusting, whereas muscovites from the lower plate of the GASZ record cooling from 380-370 Ma. Synchronous tops-west extensional movement along the Eidsfjord-Fiskefjord detachment directly west of Gullesfjord implies that contraction was followed closely by extensional movement. These major shear zones thus developed synchronously with the tops-east Øse thrust that cuts and possibly duplicates the nappe stack lying directly to the east. Late-to-post-Caledonian extension in Lofoten-Ofoten thus is interpreted as having been accommodated by gravity-driven, foreland- and hinterland-directed movement away from a thermal dome created beneath over-thickened Caledonian crust.
