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Surface ramps in normal fault zones of the Iceland plate boundary have been described in many studies, but their structure and evolution are not well understood. We show that surface ramps are manifestations of large tilted blocks (TBs) formed in dip relays of normal faults. Based on existing modeling studies, we propose three classes of TBs defined by kinematics and location of the hinge of the TB. TBs are considered a member of the family of fault relay structures that form near the surface, commonly, but not exclusively, in columnar basalts with orthotropic strength. We present high-resolution aerial vehicle–based observations of a representative set of normal faults in Iceland and compare these with geometric models we derived from modeling studies. We predict extensive tectonic cave (fluid conduit) systems under the TB, which interact with magma and groundwater flow. The general fault structure is dominated by large, subvertical open fractures reactivating cooling joints that are locally filled by basalt rubble. We propose the existence of a hybrid failure zone at larger depths before the effective vertical stress is sufficient to initiate shear fractures in intact basalt.

Close to surface, cohesive rocks fail in extension, which results in open fractures that can be several tens of meters wide, so-called massively dilatant faults. These open fractures make fault slip analysis in rifts challenging, as kinematic markers are absent. Faults in rifts often have oblique slip kinematics; however, how the amount of obliquity is expressed in the surface structure of massively dilatant faults remains enigmatic. Furthermore, the structures of oblique dilatant faults at depth is largely unconstrained. To understand the subsurface structures we need to understand how different obliquities of slip influence the surface structures and the corresponding structures at depth. We present analog models of oblique massively dilatant faults using different cohesive materials in a sandbox with adjustable basement fault slip obliquity from 0 • to 90 •. Experiments with different mean stress and material cohesion were run. Using photogrammetric 3D models, we document the final stage of the experiments and investigate selected faults by excavation. We show that fault geometry and dilatancy changes systematically with angle of obliquity. Connected open fractures occur along the entire fault to a depth of 6-8 cm, and as isolated patches down to the base of the experiments. Using the scaling relationship of our models implies that transition from mode-1 to shear fracturing occurs at depths of 250-450 m in nature. Our experiments show the failure mode transition is a complex zone and open voids may still exist at depths of at least 1 km. We apply our results to the dilatant faults in Iceland. We show that the relationship between angle of obliquity and average graben width determined on faults on Iceland matches experimental results. Similarly, fracture orientation with respect to fault obliquity as observed on Iceland and in our experiments is quantitatively comparable. Our results allow evaluation of the structure of massively dilatant faults at depth, where these are not accessible for direct study. Our finding of a complex failure mode transition zone has consequences for our understanding of fracture formation, but also influences our interpretation of fluid flow in rift systems such as magma ascent or flux of hydrothermal waters.

[1] We use 3-D Discrete Element Method simulations to model the evolution of boudin structures in a layered material under nonplane strain conditions. As the models are shortened perpendicular to the layer orientation, they are extended at different rates in the two layer-parallel directions. The particular emphasis of the study is on the orientation of fractures between the boudin blocks. The results show that the fracture orientation distribution is closely connected to the ratio of the two layer-parallel extension rates. The anisotropy of the fracture orientation distribution increases systematically from no anisotropy at isotropic layer-parallel extension to a highly anisotropic distribution in case of uniaxial extension. We also observe an evolution of the anisotropy of fracture orientation distribution with increasing deformation in each individual model from a high-initial anisotropy towards a value characteristic for the ratio of the layer-parallel extension rates. The observations about the relation between the strain ratios and the fracture patterns do have the potential to serve as the basis for a new method to analyze strains in naturally boudinaged rocks.Citation: Abe, S., J. L. Urai, and M. Kettermann (2013), Fracture patterns in nonplane strain boudinage-insights from 3-D discrete element models,

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