Reverse-slip structures at oceanic diverging plate boundaries and their kinematic origin: data from Tertiary crust of west and south Iceland

Normal faulting predominates within rift zones, but reverse fault plane solutions are occasionally found for earthquakes along the Mid-Atlantic Ridge. Furthermore, field observations of striae and in situ measurements in places show off-rift compression and reverse faulting. We analyse eight cases of reverse-slip motion observed in the eroded Tertiary crust of the Borgarfjِrًur and Hreppar rift-jump blocks, respectively, in west and south Iceland. These structures were the only ones observed where both striae and marker horizons could be used to deduce the sense of reverse-slip motion. The vertical displacements are less than 10 m, and the motions are of local origin. We believe these structures are formed independently from regional compression, as they have variable kinematic origins. Some occur in association with dykes, sills, or cone-sheet, without evidence of horizontal shortening across intrusions; others are due to local bends of regional steeply-dipping normal faults or secondary fractures (Riedel or secondary fractures). Reverse-slip striations are found at the irregularities of steeply-dipping normal faults, or at the tips of steeply-dipping normal faults uplifted by an underlying propagating dyke. Striations, even conspicuous ones, do not necessarily reflect significant amounts of displacement. If horizontal compression acts in Iceland, its magnitude is not sufficient to shorten the rocks and to close the fractures of various tectonic settings. The observed examples are explained as local kinematic adjustments to companion structures, without regional compressional stress field or shortening, and can lead to misinterpretation if not put into the correct context.