Timing of syn-orogenic, high-grade transtensional shear zone formation in the West Uusimaa Complex, Finland

We present zircon and titanite U-Pb ages from granulite-facies transtensional shear zones and a greenschist facies strike-slip shear zone in the Karkkila-Vihti area, West Uusimaa. The zircon rim and titanite ages of c. 1815−1820 Ma from the transtensional shear zones constrain the evolution of the orogenic deformation from compression-dominated to syn-compressional lateral-escape within this part of the Fennoscandian shield while the zircon rim and titanite ages of 1790−1800 Ma give the timing of the late- to post-orogenic low-grade reactivation of the large-scale Somero-Karkkila fault zone. The new age data further constrain the timing of the changes in the crustal deformation style during orogenesis from i) compression-dominated to combined transpression - lateral stretching and escape, to ii) the late- to post-orogenic transition into retrograde conditions.