Failure Modes in Normal Faults - an investigation into structure and permeability evolution in physical models and outcrop studies

This thesis deals with structures and processes related to failure mode transition in normal faults in mechanically layered sediments. Dilatant, hybrid and shear mode faults are common in the upper crust and associated with different deformation processes, fault and fracture geometries and transport properties. Fault systems are of great importance for applications such as hydrocarbon exploration and production, groundwater flow, hydro-thermal systems, CO2 sequestration and natural hazard assessment. However, our knowledge of faults still has room for improvement and here I address some issues more or less directly related to different failure modes and mechanical stratigraphy. To this end a variety of methods, including analogue modeling, field work and numerical flow simulations are used to determine a general structural trend from dilatant to shear failure mode and later on to study details of fault zones in some of these settings. This failure mode transition is the central theme of my thesis. While some experimental studies have been done on failure mode transition in experimentally deformed samples and 2-D discrete element models, we lack a good understanding of the effect of failure mode transition on 3-D structures on normal faults. This is addressed in Chapter 3 of this thesis. Here I present a series of scaled analogue models of normal faulting using cohesive powder embedded in cohesionless sand. By varying the overburden thickness, the failure mode changes from tensile to hybrid and finally to shear. 3-D investigation of the hardened and excavated cohesive layers shows three structural domains: (1) The tensile domain is characterized by steep open fissures and sharp changes in strike at segment boundaries and branch points; (2) the shear domain shows shallower fault dips, fault planes develop striations and map-view fault traces undulate with smaller changes in strike at branches; (3) the hybrid domain shows characteristic features of dilatant and shear domain, alternating both laterally and vertically. These attributes can be recognized in map-view, which is a useful tool in understanding and interpreting further work in this thesis. I study dilatant faults in the graben system of Canyonlands National Park, Utah, USA (Chap. 4). Here, I present results of ground-penetrating radar (GPR) surveys in combination with field observations and analysis of high-resolution airborne imagery. This study shows that Quaternary sediments at the flat graben floors are intensely faulted, implying a more complex fault structure than visible at the surface. The presence of sinkholes gives insights in local dilatancy and shows where water and sediments are transported underground. These field observations are later compared with a series of scaled analogue models using cohesive powder in a half-graben setting (Chap. 5). Pre-formed vertical, cohesionless joints in the upper 5 cm of the model vary in strike direction with respect to a predefined basement fault, to cover a range of joint-fault angles observed in Canyonlands NP. The surface trace of faults follows the joints geometry to a certain extent and therefore, fault geometry and amount of secondary fractures differ strongly from experiments without joints. Map-view analyses show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increases, while the area fraction of massively dilatant fractures shows only a minor increase. The transition from dilatant to hybrid mode faulting in this thesis is explored by a ’proof-ofconcept’ study on salt intrusion into underlying normal faults (Chap. 6). The motivation of this project is the deformation-rate depending lubricating effect of the rock salt on the fault movement and potential effects on induced seismic events in the Groningen gas field. We combine cohesive powder as analog for brittle carbonates with different viscous salt analogs in two experiments. Using resin as salt analog shows the resulting geometry of salt in the faults which form in dilatant mode as result of low overburden stress. Using transparent silicone oil as salt analogue allows dynamic observation of the developing fault geometry, where faults form in hybrid mode due to larger overburden stress. I present a study on hybrid failure in clay smears with 3-D structural investigation and flow measurements (Chap. 7). Despite numerous published outcrop and analogue studies, this structural domain is underrepresented in current research. Water saturated sandbox experiments with large clay smear surfaces (500 cm2) are coupled with across-fault flow measurements to determine the dynamic permeability evolution. This is combined with structural analyses of excavated clay smears. We observe an initial hybrid failure of the clay with early breaching of the clay layer and increased cross-fault flux, followed by subsequent phases of fault back-stepping, shearing and reworking. We find diagnostic relationships between fault structures and flow responses that allow a prediction of fluxes from observed structures or vice versa. In addition to simple single-layer experiments we present experiments with two source clay layers, which show a different characteristic flux response with a generally lower permeability at the same total clay volume in the faulted sequence. Experimental observations are then compared with numerical flow simulations using finite element method (FEM) in 2-D and 3-D, showing a good fit between the measurements and flow simulations. The thesis concludes with an outcrop study on clay smears forming in shear mode in unlithified clay-sand sequences in lignite mines of the Lower Rhine Embayment (Chap. 8). This work focuses on 3-D outcrops in freshly cut normal faults with shale gouge ratios of 0.1 0.3. Vertical profiles show that fault zones are often layered, with sheared sand, sheared clay and tectonically mixed sand–clay gouge. 3-D thickness data derived from sequential slicing in excavated fault zones are approximately log-normal and show heterogeneous thickness distributions. The thinnest parts of clay smears are often located close to source layer cut-offs rather than in the center of the smear as proposed in earlier studies. Profile and 3-D data show, that clay smears are strongly affected by Rand R’-shears, which mostly form at the footwall side. These shears can locally cross and offset clay smears, which we identify as prominent process in the formation of holes. On micro-scale, clay smear samples show that grain-scale mixing can lead to thickening of the low permeability smears and intense mixing and amalgamation of parallel smears may lead to resealing of holes.