Lower Allochthon Research Articles

Hercynian blueschist metamorphism in North Portugal: tectonothermal implications

Abstract Aegirine–jadeite clinopyroxene (>60 mol% jadeite) locally occurs within blueschists of the ‘Lower Allochthon’exposed in the Trás‐os‐Montes region of northern Portugal. Peak conditions attained during blueschist facies metamorphism are estimated to have been c. 420° C and >11 kbar. Porphyroblastic white mica (paragonite/phengite) within the blueschist assemblage records a 36Ar/40Ar versus 39Ar/40Ar isotope correlation age of 329.4 ± 1.6 Ma. In view of the relatively low‐T nature of the metamorphism, the c. 330‐Ma age is interpreted to date closely the high‐P recrystallization. This tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex (‘Upper Allochthon/Ophiolite Nappe’) onto Iberian protoliths of the Lower Allochthon during terminal stages of the Hercynian orogeny.

Balanced sections from the Lower Allochthon of the Västerbotten Caledonides and implications for the allochthonous nature of basement rocks in the tectonic windows

Balanced sections from the Lower Allochthon of the Västerbotten Caledonides and implications for the allochthonous nature of basement rocks in the tectonic windows

Arenig-Llandovery stratigraphy and faunas across the Scandinavian Caledonides

Summary The nature and development of Arenig-Llandovery sedimentary environments is reviewed with reference to 18 allochthonous-parautochthonous sections in the Scandinavian Caledonides. The faunas suggest that age constraints are imprecise, and direct correlations with sequences on the adjacent platform are difficult. However, it is possible to recognize a northerly transition from near-shore to offshore facies on Hardangervidda and a platform to slope transition in Jämtland and E Jotunheimen during the early Ordovician. There is no direct evidence of a distinct Finnmarkian orogenic event (late Cambrian-early Ordovician) in the lower Allochthon, while the central Scandinavian upper Allochthon, containing N American shelly fossils, has a pre-Caradoc history far removed from that of the Baltic platform. In the Bergen area no fossils older than Ashgill age are known and the youngest marine rocks are early Llandovery. Locally-important features of the platform sequences, including breaks in sedimentation, the deposition of bentonites, basement faulting and the early termination of marine sedimentation (late Llandovery in the N and W), cannot be directly related to discrete tectonic events in the orogen.