4.4.1. Assumptions

A simple model with one listric fault and one faulting event has been used in this study. The layers end horizontally in the fault zone and most of the tested scenarios assume a homogeneous lithology. Deposited sediments are modeled to reach the surface in every timestep, meaning that there are no constraints on the sediment supply and sea level is not accounted for. The magmatic sill intrusions are modeled to be horizontal and concordant to the layering with a constant sill thickness. Natural basins commonly consist of several faults, several faulting events, and different fault dips possibly dipping in various directions. Furthermore, layers often show a bending downwards towards the fault zone of a listric fault and they commonly show a large lithological heterogeneity, which might lead to a variation in physical properties. All these factors may result in variations in the estimated transient thermal effects and the time needed for the basin to regain steady state after fault slip, and contribute to contrasting host-rock temperatures. The occurrence of salt in the vicinity of sills will in particular influence the thermal situation in basins. Including all the above mentioned factors is possible and will turn the model into a more realistic example. However, doing so will limit the general conclusions that can be drawn from our study. By keeping the model simple the study is focused on fundamental questions related to transient thermal effects in basins with normal faults and magmatic sills. Revealed trends in this study are therefore applicable for basins in general with such features, as the relative differences between the tested parameters will remain more or less the same.

Sills vary normally in thickness and shape, e.g., saucer-shaped, v-shaped, and transgressive sills (cf., [67,79,80]). Sill thickness influences the temperature effects in the surrounding area; a thick magmatic sill has a wider thermal influence on the surroundings than a thin sill. It influences the size of the thermal aureole surrounding it (e.g., [8,25]) and an uneven sill thickness could possibly lead to an uneven thermal aureole. In a 2D model, the intrusion is extended infinitely in the 3rd direction. This is a good approximation as sills commonly have much larger horizontal dimensions compared to the vertical dimension. The sills are often several km in lengths and widths, while possibly only 50–100 m thick. For typical sill dimensions, the 2D modeling is sufficient for realistic temperature calculations. Fjeldskaar et al. [7] studied the 3D effect of magmatic sill intrusions and investigated what length/width ratio of the sill that requires a 3D calculation to give realistic temperature results.

In the temperature modeling the initial temperature of the intruding magma is set to 1000 ◦C and latent crystallization heat is not accounted for. Most magmas have temperatures ranging from 700 to 1300 ◦C (cf., [81] and references therein) making the chosen temperature in this study a mean value. Higher magma temperatures will increase the thermal e ffects of the sills, whilst a lower magma temperature will have the opposite e ffect. However, the relative di fferences between the compared models in this study will stay the same, as the intruding magma initial temperature is constant in all models.

Magmatic sills are often fed by dykes in magmatic upwelling zones. These near vertical features will contribute with additional heat to the thermal history of basins with sills. However, magmatic sills can also be fed by other sills over relatively large distances which has been observed in the field (e.g., [52]).

Latent heat of crystallization is thermal energy released when the sill undergoes a phase transition from liquid to solid. The composition of the magma may di ffer, which may lead to di fferent timing of the emissions of latent heat as components have di fferent crystallization temperatures. Peace et al. [62] have estimated the additional latent heat to be 488 ◦C, which is a substantial thermal contribution. Heat due to solidification of the magma will be released as long as the sill contains liquids and until the sill has solidified completely. However, introducing this additional heat as one pulse, either by increasing the starting temperature of the magma or introducing it as an additional heat when the sills start to cool, may result in an overestimation of the e ffect of sills [20].
