Kattenhorn, S.A. (2003)
Upward growth of normal faults in southwest Iceland: Driven by dike magmatism?
Ridge 2000/Nordvulk Tectonic-Magmatic Interaction Workshop Proceedings, Geysir, Iceland, Aug. 31-Sep. 9, 2003.
On the Reykjanes Peninsula, southwest Iceland, vertical normal faults and joints are arranged into discrete curvilinear fracture zones separated by regions of little or no fracturing. The faults have typically been interpreted to have formed at the surface and then subsequently propagated downwards; however, surface geometries and kinematic indicators suggest otherwise. For example, the faults are typically associated with narrow monoclinal folds that parallel the fault traces at the surface. Such folds could not have formed through drag effects because vertical fault surfaces are not in frictional contact. Individual fracture segments along the fault traces are commonly rotated out of the general trend of the fracture cluster, indicating oblique motion along subsurface normal faults and subsequent upward growth of echelon segments. This fault slip behavior implies a rotation of the stress field occurred during fault growth and evolution. These field observations have been combined with the results of fracture mechanics based numerical models to demonstrate that vertical normal faults propagated to the surface from below. In the subsurface, faults have typical normal fault dips (~60°); however, at depths of between 500 and 250 m, the reduction in confining stress allowed tension fractures to form at the upper tips of faults. These fractures subsequently propagated vertically towards the surface, inducing narrow monoclinal folds at the surface projection of the faults. The implication is that fault nucleation occurred at depth in the brittle crust in response to a driving mechanism that concentrated stress in the subsurface rather than at the surface of the Earth. A likely driving mechanism is dike intrusion from below, suggesting a link between magmatic activity and fault nucleation and subsequent growth history. Variations in magmatic activity may have affected local stress fields which thus showed temporal rotations that impacted on fault kinematics.
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