LEFT-LATERAL TRANSPRESSIVE DEFORMATION AND ITS TECTONIC IMPLICATIONS,SVECONONWEGIAN OROGEN, BALTIC SHIELD, SOUTHWESTERN SWEDEN

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sve conorwegian orogen (Baltic Shield) of southwestern Sweden and southeas tern Norway which has a strike length of over 400 km and an across-str ike width which often exceeds 5 km. It is an orogen-parallel deformati on zone which formed under retrogressive metamorphic conditions relati ve to the higher-grade structures in the surrounding crustal units. Th e MZ marks a conspicuous metamorphic break in the area south of lake V anern and a distinct lithological break in the area north of this lake . Regional metamorphic considerations suggest that its surface exposur e represents an oblique section through the crust with deeper levels e xposed along the southern parts of the zone and shallower levels expos ed farther north. Structural studies in three areas north of lake Vane rn (Varmlandsnas, Kil and Torsby-Charlottenberg) suggest that the MZ d isplays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active und er upper greenschist-facies conditions. In the northernmost Torsby-Cha rlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defin es the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within t his thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorw egian Frontal Deformation Zone (SFDZ) in the frontal part of the oroge n. The MZ is inferred to have formed in an extrusive (dispersive) tect onic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and s hortening was absorbed, at this stage, by crustal thickening. Late, po ssibly out-of-sequence thrusting with a right-lateral, strike-slip sen se of shear along the SFDZ marked the waning stages of crustal shorten ing in the Sveconorwegian orogen.