Caledonian terrane assembly on Svalbard; new evidence from 40 Ar/ 39 Ar dating in Ny Friesland

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Abstract
Ar-40/Ar-39 dating of hornblendes and muscovites from the Atomfjella Complex of northern Ny Friesland provides new evidence of Caledonian high grade metamorphism, with uplift and cooling occurring during the Mid to Late Silurian, prior to Old Red Sandstone deposition. Hornblende crystallized during transpressive orogeny, the latter characterized by lateral shortening and extensive axial elongation. Ny Friesland is a part of Svalbard's Eastern Terrane. The Atomfjella metamorphic complex is overlain by Late Proterozoic and Carnbro-Ordovician successions closely similar to those of the Laurentian margin of East Greenland. This similarity, suggesting pre-Caledonian juxtaposition, contrasts with the markedly different tectono-stratigraphic evolution of terranes farther west on Svalbard. The assembly of Svalbardian terranes by large transcurrent displacements on major north trending faults, first postulated by W.B. Harland in the mid 1960s, finds support in the new investigations. On Svalbard, these transpressive movements were largely of Silurian age, simultaneous with Scandian collisional convergence between Baltica and Laurentia farther south in the orogen. The transcurrent displacements of Svalbard Terranes may have been a direct response (''escape'') to this Scandian collision. In northeastern Greenland, apparently similar sinistral transpression occurred during the Devonian, suggesting that deformation along the Laurentian margin progressed from the hinterland toward the foreland during the Mid Paleozoic.

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