79 Recent Advances in Tectonics

Monday, 1 November 2010
8:00 AM - 12:00 PM
Room 405

LATE-STAGE CALEDONIAN COLLISIONAL TECTONICS OF NORTHERN SCANDINAVIA – LINKED CONTRACTIONAL AND EXTENSIONAL EXHUMATION OF THE BALTOSCANDIAN MARGIN

Paper number: 79-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

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.