MDF: The structure and evolution of near-surface massively dilatant faults
Final Report Abstract
The DFG research project “MDF: The structure and evolution of near-surface massively dilatant faults” was – in our opinion - very successful and lead to several peer-reviewed publication and contributed also to the qualification of our junior scientific stuff. Furthermore, Dr. Ch. Weismüller received his PhD from RWTH Aachen University (magna cum laude) and PI von Hagke was appointed tenured professor (Chair of Geology at Univ. Salzburg/Austria). We achieved new data and models for testing our hypotheses. We acknowledge financiation of our project by DFG and are thankful for the support. The working plan was even enlarged with respect to the original proposal and added topics regarding the fracture networks at Lilstock. Significant time delay due to complications after the outlined changes (see 11 personnel) resulted in a prolongation of the entire project, and lastly in the compilation of the final report. Summarizing the main results of the project, our main scientific findings and conclusions are: 1) Remote sensing and Iceland field work after a) Kettermann et al. 2019, Large near-surface block rotations at normal faults of the Iceland rift: Evolution of tectonic caves and dilatancy, and b) Weismüller et al. 2019, Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data. a) Kettermann et al. 2019 • Near-surface tilted blocks (TB) are common near-surface fault-relay structures in basaltic rocks. Three end-member types of TBs can form, defined by differences in mechanical heterogeneity. Each end member implies different subsurface fault structures. • A method was established to estimate depth of hinges, basement fault dip, and errors for surface apertures. • Field observations of TBs at the Iceland plate boundary show mean errors of 16%. TBs are closely similar to structures in physical and numerical models, and based on those, we predict large open fractures and tectonic caves formed by the rotation of the rigid blocks. • The main dilatant fault can remain hidden underneath the TB, whereas lateral fluid pathways form in deeper parts of growth faults at buried TBs and in dilatational jogs of hybrid failure zones. b) Weismüller et al. 2019 • Measurements of vertical offset follow the trend of an elliptical fault without much local variation, whereas opening width is more prone to local variations when measured along strike. • The local variations in opening width can be caused by formation of tilted blocks or by erosion, corroborating earlier studies. Erosional processes such as collapse of the fracture walls or disintegrated relays of the fracture walls may lead to overestimation of opening width; when fractures are filled and/or covered by sediment or vegetation, opening width may be underestimated. • Underestimation of the vertical offset of the master fault at depth occurs when measurements are taken on the slope of a tilted block. • Structures that appear as non-dilatant normal faults on the surface can consistently be shown to have a blind opening hidden by vegetation or sedimentation. Fig. 8 Conceptual model of fault zones in Iceland with • Tilted blocks are common features observed four distinct structural regimes. To retain readability we along all faults. They may be present along entire indicate columnar basalts only where necessary, however faults (Thingvellir) or exist only locally (Vogar most of the rock mass shows vertical polygonal joints. southeastern fault). 2) Lab experiments after von Hagke et al., 2019. The effect of obliquity of slip in normal faults on distribution of open fractures. • Dilatancy changes progressively with increasing obliquity. Low fault obliquities are characterized by long open fractures with large apertures close to the surface and smaller dilatant fractures at depth, often filled with rubble. 12 • High fault obliquity results in dilatancy localized close to the surface, but more, deeper reaching vertical open conduits in releasing bends as compared to lower obliquities. • Deepest open fractures occur at antithetic faults, as absence of tilted blocks and smaller opening width and displacement reduces the amount of available debris. • Dilatancy determined at the surface is higher than horizontal displacement of the basement fault. 3) Lilstock area after Weismüller et al. 2020. Mapping the fracture network in the Lilstock pavement, Bristol Channel, UK: • The five inferred fracture generations are not equally distributed throughout our five selected areas, the spatial variation can be significant in the same layer. • The connectivity of the fracture network increased over time while different generations have a different impact on the overall connectivity based on their fracture numbers and orientation to pre-existing fractures. • Later fracture generations are influenced by preceding fracture generations. • The network topology and connectivity in this area is strongly influenced by the last generation. • Nodes with more than four intersecting branches are possible in this fracture network. • When mapping within a 2D boundary, the selected size of the mapping area can impact the measurements, e.g., when the largest fractures are longer than the outlines of the mapping boundary. • While manual mapping is superior to the automatic mapping procedure, the required time can be reduced, and results of comparable quality produced. • The automatic interpretation of fracture generations is not yet fully possible and requires manual input/corrections.
Publications
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2016. Dilatant normal faulting in jointed cohesive rocks: a physical model study. Solid Earth 7 (3), 843-856
Kettermann, M., von Hagke, C., Gent, H.W., Grützner, C., Urai, J.L.
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2019. Cutting-Edge Analogue Modeling Techniques Applied to Study Earth Systems. Frontiers in Earth Science 7, 265
von Hagke, C., Reber, J.E., Philippon, M.
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2019. Large near-surface block rotations at normal faults of the Iceland rift: Evolution of tectonic caves and dilatancy. Geology
Kettermann, M., Weismüller, C., von Hagke, C., Reicherter, K., Urai, J.L.
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2019. Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data. Solid Earth 10, 1757–1784
Weismüller, C., Urai, J.L., Kettermann, M., von Hagke, C., Reicherter, K.
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2019. The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures. Frontiers in Earth Science 7
von Hagke, C., Kettermann, M., Bitsch, N., Bücken, D., Weismüller, C., Urai, J.L.
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2019. The effect of obliquity of slip in normal faults on distribution of open fractures. Frontiers in Earth Science 7, 18
von Hagke, C., Kettermann, M., Bitsch, N., Bücken, D., Weismüller, C., Urai, J.L.
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2020. Mapping the fracture network in the Lilstock pavement, Bristol Channel, UK: manual vs. automatic. Solid Earth, 11, 1773-180
Weismüller, C., Prabhakaran, R., Passchier, M., Urai, J.L., Bertotti, G., Reicherter, K.
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2021. Large-scale natural fracture network patterns: Insights from automated mapping in the Lilstock (Bristol Channel) limestone outcrops. J. Struct. Geol., 147, 104405
Prabhakaran, R., Urai, J.L., Bertotti, G., Weismüller, C., Smeulders, D.M.J.
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2021. The join sets on the Lilstock Benches, UK. Observations based on mapping a full resolution UAV-based image. J. Struct. Geol., 147, 104332
Passchier, M., Passchier, C., Weismüller, C., Urai, J.L.
