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Fig. 8. Example plots of data collected during the experiments showing the four stages of crack propagation where a vertical propagating crack turned to propagate horizontally into the interface. Stage 1: a vertically propagating crack with constant velocity begins to decelerate as it approaches a more rigid horizontal layer. Stage 2: vertical propagation of the crack is inhibited, the propagation continues along the breadth of the crack. Stage 3: horizontal crack propagation into the interface is initiated, sometimes with contemporary inclined propagation into the upper more rigid layer (characterised by an initial acceleration and then rapid deceleration). The crack propagation accelerates as it intrudes the interface. Stage 4: The propagation of the crack into the interface rapidly decelerates when the horizontal crack dimension becomes comparable to the size of the experimental tank. (a) Distance versus time plot of experimental sill formation in a hydrostatic two-layered system, with E,,/E,=1.10 (Experiment 11). (b) Plot of length velocity versus length of intrusion during experimental sill propagation. propagation velocity along the crack breadth is unaffected (see Fig. 8). Stage two of experimental sill formation is characterised by pure lateral propagation along the breadth of the crack (see Fig. 5). Experimental sill nucleation is initiated at the start of stage three,

Figure 8 Example plots of data collected during the experiments showing the four stages of crack propagation where a vertical propagating crack turned to propagate horizontally into the interface. Stage 1: a vertically propagating crack with constant velocity begins to decelerate as it approaches a more rigid horizontal layer. Stage 2: vertical propagation of the crack is inhibited, the propagation continues along the breadth of the crack. Stage 3: horizontal crack propagation into the interface is initiated, sometimes with contemporary inclined propagation into the upper more rigid layer (characterised by an initial acceleration and then rapid deceleration). The crack propagation accelerates as it intrudes the interface. Stage 4: The propagation of the crack into the interface rapidly decelerates when the horizontal crack dimension becomes comparable to the size of the experimental tank. (a) Distance versus time plot of experimental sill formation in a hydrostatic two-layered system, with E,,/E,=1.10 (Experiment 11). (b) Plot of length velocity versus length of intrusion during experimental sill propagation. propagation velocity along the crack breadth is unaffected (see Fig. 8). Stage two of experimental sill formation is characterised by pure lateral propagation along the breadth of the crack (see Fig. 5). Experimental sill nucleation is initiated at the start of stage three,

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Table | observation that sills often abut a particularly rigid horizontal layer (such as a previously emplaced sill [35] or a particularly rigid sandstone layer [33]) provides an
Table | observation that sills often abut a particularly rigid horizontal layer (such as a previously emplaced sill [35] or a particularly rigid sandstone layer [33]) provides an
Fig. 1. Schematic diagram of a pressure driven fluid-filled crack propagating in an elastic solid (based on [39]).
Fig. 1. Schematic diagram of a pressure driven fluid-filled crack propagating in an elastic solid (based on [39]).
Fig. 2. Apparatus and set-up used during the experimental series. 4.1. Preparation of experiments The experiments were conducted in a square based 0.4 m*0.4 m*0.3 m perspex container with nine equally spaced injection points in its base (see Fig. 2). A forty-litre gelatine solution is poured into the experi- mental tank to cool and solidify overnight. To accelerate the solidification process all experiments were prepared inacold room kept at 7 °C. A thin layer of oil poured on top of the solution inhibits water evaporation during the cooling process [14,16]. A two-layered system com- prises equal volumes of gelatine with contrasting properties. The properties of the gelatine are altered by varying the wt.% of gelatine, enabling a range of rigidities to be obtained. Adding salt increased the density of the gelatine and injecting fluid.
Fig. 2. Apparatus and set-up used during the experimental series. 4.1. Preparation of experiments The experiments were conducted in a square based 0.4 m*0.4 m*0.3 m perspex container with nine equally spaced injection points in its base (see Fig. 2). A forty-litre gelatine solution is poured into the experi- mental tank to cool and solidify overnight. To accelerate the solidification process all experiments were prepared inacold room kept at 7 °C. A thin layer of oil poured on top of the solution inhibits water evaporation during the cooling process [14,16]. A two-layered system com- prises equal volumes of gelatine with contrasting properties. The properties of the gelatine are altered by varying the wt.% of gelatine, enabling a range of rigidities to be obtained. Adding salt increased the density of the gelatine and injecting fluid.
Material properties of each experiment Table 2
Material properties of each experiment Table 2
Fig. 3. Photograph of experimental dyke formation propagated with an overlying load (side view, at 909 s, Experiment 12). The load acts as an attractor to dyke propagation.
Fig. 3. Photograph of experimental dyke formation propagated with an overlying load (side view, at 909 s, Experiment 12). The load acts as an attractor to dyke propagation.
* Py is evaluated at the start of each experiment when the crack length, created by the insertion of the injector, is approximately 1 cm. > Py is evaluated when the experimental crack has propagated to the interface and the crack length is approximately 12.5 cm. © The buoyancy pressure is calculated when the crack length is 12.5 cm. When an experimental sill forms at the interface the buoyancy pressur -ontribution over its thickness is included (see Eq. (11)). Pressure scales calculated during experiments
* Py is evaluated at the start of each experiment when the crack length, created by the insertion of the injector, is approximately 1 cm. > Py is evaluated when the experimental crack has propagated to the interface and the crack length is approximately 12.5 cm. © The buoyancy pressure is calculated when the crack length is 12.5 cm. When an experimental sill forms at the interface the buoyancy pressur -ontribution over its thickness is included (see Eq. (11)). Pressure scales calculated during experiments
Fig. 5. A schematic sketch of the three stages of sill formation under initially hydrostatic conditions and with an upper more rigid gelatine layer. Stage 1: front view, the initial dyke formation is a circular to elliptical disc. Stage 2: front view, the vertical dyke propagation has stalled at the interface between the contrasting solid layers. The propagation is now lateral along the crack breadth and is perpendicular to the interface. Stage 3: side view, the dyke has fractured parallel to the interface separating upper rigid layer from lower layer forming a sill with contemporary dyke protrusion into the upper more rigid layer.
Fig. 5. A schematic sketch of the three stages of sill formation under initially hydrostatic conditions and with an upper more rigid gelatine layer. Stage 1: front view, the initial dyke formation is a circular to elliptical disc. Stage 2: front view, the vertical dyke propagation has stalled at the interface between the contrasting solid layers. The propagation is now lateral along the crack breadth and is perpendicular to the interface. Stage 3: side view, the dyke has fractured parallel to the interface separating upper rigid layer from lower layer forming a sill with contemporary dyke protrusion into the upper more rigid layer.
Fig. 6. A series of photographs from the experiments. (a) A vertically propagating dyke has turned to intrude as a sill along the interface separating upper rigid layer from lower layer (Experiment 15, Ey/E,,=6.6). Note the protrusions from the sill periphery into the lower less rigid layer. (b) Formation of a dyke—sill hybrid (Experiment 11). The rigidity ratio of upper to lower layer is 1.096. Side view, T= 187 s. The dyke has reached the interface and has intruded horizontally as a sill and into the upper rigid layer. The camera is not perpendicular to crack axis. (c) Deformation structures that formed during sill emplacement. Side view, en echelon fractures formed in the lower less rigid layer during sill emplacement (Experiment 10, Ey/ E,,= 11.45). (d) Plan view, inclined fractures formed into the lower less rigid layer during the sill-to-laccolith transition phase. propagation front was lobed as the fluid-filled crack intruded the interface. During the propagation defor- mation structures, such as inclined en echelon tensior structures and faults, formed within the lower les: rigid layer (see Fig. 6c and d). Additionally, once the lateral extent of the experimental sill reached greate1 than ~30 cm diameter upwards doming of the Plotting the dimensionless driving and resistive pressures, (Po/Pry), against the rigidity ratio of upper
Fig. 6. A series of photographs from the experiments. (a) A vertically propagating dyke has turned to intrude as a sill along the interface separating upper rigid layer from lower layer (Experiment 15, Ey/E,,=6.6). Note the protrusions from the sill periphery into the lower less rigid layer. (b) Formation of a dyke—sill hybrid (Experiment 11). The rigidity ratio of upper to lower layer is 1.096. Side view, T= 187 s. The dyke has reached the interface and has intruded horizontally as a sill and into the upper rigid layer. The camera is not perpendicular to crack axis. (c) Deformation structures that formed during sill emplacement. Side view, en echelon fractures formed in the lower less rigid layer during sill emplacement (Experiment 10, Ey/ E,,= 11.45). (d) Plan view, inclined fractures formed into the lower less rigid layer during the sill-to-laccolith transition phase. propagation front was lobed as the fluid-filled crack intruded the interface. During the propagation defor- mation structures, such as inclined en echelon tensior structures and faults, formed within the lower les: rigid layer (see Fig. 6c and d). Additionally, once the lateral extent of the experimental sill reached greate1 than ~30 cm diameter upwards doming of the Plotting the dimensionless driving and resistive pressures, (Po/Pry), against the rigidity ratio of upper
Fig. 7. Dimensionless ratio of driving and resistive pressure scales Po/ Pry (416%) as a function of a dimensionless rigidity ratio Ey/E, (+6.7%), showing different styles of intrusive behaviour. The solid line represents the boundary between experimental dyke formation and sill formation. The dashed line represents the approximate boundary between experimental sill formation and dyke—sill hybrid formation.
Fig. 7. Dimensionless ratio of driving and resistive pressure scales Po/ Pry (416%) as a function of a dimensionless rigidity ratio Ey/E, (+6.7%), showing different styles of intrusive behaviour. The solid line represents the boundary between experimental dyke formation and sill formation. The dashed line represents the approximate boundary between experimental sill formation and dyke—sill hybrid formation.
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