Fault systems and paleostresses in the Vestfirdir Peninsula. Relationships with the tertiary paleo-rifts of Skagi and Snaefells (Northwest Iceland)

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8-14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a change in trend of the extinct Skagi-Snaefells rift, a structure that cannot be reconstructed directly through geological mapping. The average trends of extension in the southern Vestfirdir Peninsula, N150 degrees E, suggest a N60 degrees E trend for the ancient Snaefells rift segment. Likewise, the N100-110 degrees E trends of extension in northern Vestfirdir suggest a N10-20 degrees E trend for the Skagi paleo-rift. (C) Elsevier, Paris.