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 o E, suggest a N60 o E trend for the ancient Snaefells rift segment. Likewise, the N100-110 o E trends of extension in northern Vestfirdir suggest a N10-20 o E trend for the Skagi paleo-rift.La peninsule de Vestfirdir, dans le Nord-Ouest de l'Islande, est composee d'un empilement de laves essentiellement tholeitiques agees de 8 a 14 Ma, plongeant faiblement vers le Sud-Est. Une etude detaillee des decrochements et des failles normales permet de caracteriser les principaux paleo-etats de contrainte. Deux populations de failles normales peuvent etre distinguees : l'une, principale, est de direction ENE-WSW a NNE-SSW ; l'autre est de direction NW-SE a WNW-ESE. La reconstruction des trajectoires de contraintes montre pour l'extension principale le passage graduel d'une direction proche de N-S au Nord a une direction proche de E-W au Sud. Pour l'extension secondaire, le passage se fait d'une direction NNE-SSW au Nord a une direction ENE-WSW au Sud. La quasi-perpendicularite des extensions majeure et mineure peut etre expliquee par des permutations d'axes de contrainte. Le champ de contrainte majeur reflete l'extension liee a l'axe de rift ancien Snaefells-Skagi et temoigne des changements de direction de celui-ci. Les directions d'extension σ3 calculees dans le Sud de la peninsule indiqueraient ainsi, pour le segment du Snaefells, une direction proche de N 60 o E, celles calculees dans le Nord montreraient une direction de N 10 o -20 o E pour le paleo-rift de Skagi.