Multi-scale kinking in northeast Tasmania: crustal shortening at shallow crustal levels

Abstract

Analysis of S1 cleavage trends in the Ordovician-Devonian Mathinna Group of northeast Tasmania has identified mega-scale (up to 9 km axial surface spacing) kink-bands and kink folds. These formed after the regional steep slaty cleavage and are contemporaneous with but post-date small-scale (cm-scale) vertical kink-bands. Small-scale sinistral kink-bands strike approximately 032° and predominate in number over the approximately E-W-striking dextral kink-bands. The geometry of the small-scale kink-bands indicates a 166° ± 10°-trending principal shortening during kinking. Kink-bands overprint minerals in the contact aureole of the Middle to Upper Devonian granite-granodiorite Scottsdale Batholith and pre-date deposition of the Parmeener Supergroup of Upper Carboniferous age. Megakinking in NE Tasmania is correlated with megakinking in the Lachlan Fold Belt (New South Wales) which occurred under the same bulk shortening direction and also during the Middle Carboniferous.Prior to kinking, the regional slaty cleavage in the Mathinna Group was near vertical and trended 157°, which is subparallel to the principal shortening direction during kinking (166° ± 10°). Because of this close alignment and the presence of a pre-existing planar anisostropy throughout the region, megakinks developed despite only 4–5% bulk shortening. Kinking in NE Tasmania occurred at 120 ± 45 MPa (approximately 4.2 km depth). The brittle and ‘instantaneous’ nature of kinking is consistent with deformation at such shallow crustal levels. Megakinking may be a typical expression of crustal tectonics of minor bulk shortening at shallow crustal levels, only in the case of regions with a pre-existing steep planar structural grain subparallel to the principal shortening direction.