The crustal block of Borgarfj¿r¿ur in western Iceland was caught between the propagating Reykjanes-Langj¿kull Rift Zone (RLRZ) and the receding Sn¿fellsnes Rift Zone (SRZ) during a Tertiary rift jump at ~7 Ma. This Tertiary rift jump configuration presents an analogy with the presently unstable plate boundary in south and southwest Iceland. Field examination of the Glj¿fur¿ fault in the Tertiary lavas of Borgarfj¿r¿ur reveals a complicated history of movements and magmatic activity. During its evolution since Tertiary time, this N-S fault acted as follows: (1) The fault acted as a possible dextral strike-slip fault, as judged from Riedel fractures in the oldest associated fault breccia. (2) Then the fault acted as a normal fault, resulting in vertical displacement, new brecciation of part of the oldest breccia, hydrothermal activity, and mineralization dominantly in N-S mode I veins. (3) The fault acted again as a dextral strike-slip fault, when the fault was injected laterally by dikes. Magma was injected mainly into the N-S strike-slip fault and secondarily into adjacent NNE normal faults. Evidence of this stage are the en ¿chelon geometry of the northerly dikes, cooling cracks oblique to dike edges, flow lineation, elongated vesicles, and soft striations on dike edges, as well as bending and displacement of preexisting dikes. (4) After the emplacement and partial erosion of the northerly dikes, the fault acted dominantly as a strike-slip fault, with possible reactivation as an open fissure. The Glj¿fur¿ fault strikes obliquely to the NNE trending rifting structures active during crustal formation in this area. Its activity was initially related to a transform zone or an oblique rift connected to the SRZ. Then a shift in the plate boundary occurred around 7--6 Ma when the RLRZ became active. During this shift the Glj¿fur¿ fault probably played a role similar to the N-S strike-slip faults of the presently active plate boundary within the South Iceland Seismic Zone and the oblique rift of the Reykjanes Peninsula. These faults may, similar to the Glj¿fur¿ fault, change mode and be injected by dikes if they are sufficiently close to a magma source.