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Many igneous dikes do not reach the surface, instead triggering normal faulting and graben formation in overlying rock. The surface expression of these dike-induced faults provides important records of active and ancient diking. For example, surface measurements of graben half-widths have been used to estimate dike upper-tip depths by projecting faults straight downdip, whereas extension measured at the surface across dike-induced fault pairs (i.e., their cumulative heave) is considered a proxy for dike thickness. We use three-dimensional seismic reflection data to test how the surface expression of two buried dike-induced faults relates to dike geometry. The dike-induced faults are nonplanar, suggesting fault dips should not be assumed constant when using graben half-widths to estimate dike depth. Multiple displacement maxima occur across the dike-induced faults, but rarely at their lower or upper tips, suggesting they formed through linkage of isolated faults that nucleated between the dike and free surface. Fault heave is greatest where these subsurface displacement maxima occur, meaning the cumulative heave of the dike-induced fault pair measured at the synfaulting free surface underestimates their total extension and poorly reflects dike thickness. Our results imply that at-surface analyses of dike-induced fault geometry cannot be used to estimate key dike parameters without a priori knowledge of fault structure and kinematics or host rock lithological variations. [ABSTRACT FROM AUTHOR]

Dike swarm emplacement accommodates extension during rifting and large igneous province (LIP) formation, with ancient dike swarms serving to localize strain during later tectonic events. Deciphering three-dimensional (3-D) dike swarm geometry is critical to accurately calculating magma volumes and magma-assisted crustal extension, allowing syn-emplacement mantle and tectonic processes to be interrogated. It is also important for quantifying the influence of ancient dike swarms on post-emplacement faulting. However, the essentially 2-D nature of Earth's surface, combined with the difficulties in imaging subvertical dikes in seismic reflection data and the relatively low resolution of geophysical data in areas of active diking, means our understanding of dike swarm geometry at depth is limited. We examine an ~25-km-wide, >100-km-long, west-southwest--trending dike swarm imaged, due to post-emplacement rotation to shallower dips, in high-quality 2-D and 3-D seismic reflection data offshore southern Norway. Tuned reflection packages correspond to thin (<75 m thick), closely spaced dikes. These data provide a unique opportunity to image and map an ancient dike swarm at variable structural levels. Crosscutting relationships indicate emplacement occurred in the Late Carboniferous--Early Permian, and was linked to the formation of the ca. 300 Ma Skagerrak-centered LIP. Dike swarm width increases with depth, suggesting that magma volume and crustal extension calculations based on surface exposures are dependent on the level of erosion. During the Mesozoic, rift-related faults localized above and exploited mechanical anisotropies within the dike swarm. We demonstrate that seismic reflection data are a powerful tool in understanding dike swarm geometry and the control of dikes on subsequent faulting. [ABSTRACT FROM AUTHOR]

Magma flow within the subsurface is heavily influenced by the pre-existing structure of the upper crust. During continental rifting, normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma. However, the way in which magma intrudes into fault planes is poorly understood. Here, we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three-dimensional seismic reflection data from the Exmouth Sub-basin, offshore northwest Australia. Inclined segments of saucer-shaped sills intrude several faults along convex-into-the-hang-ingwall fault-plane corrugations. We suggest that stress field perturbations associated with the fault-plane corrugations provide suitable conditions for fault reactivation as magma conduits. Pre-existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded, potentially resulting in the formation of a new sill or the development of minor intrusive steps. This work emphasizes the importance of the pre-existing structural template in controlling the growth and final geometry of intrusive networks. [ABSTRACT FROM AUTHOR]

Space for intruding magma is created by elastic, viscous, and/or plastic deformation of host rocks. Such deformation impacts the geometries of igneous intrusions, particularly sills and dikes. For example, tapered intrusion tips indicate linear-elastic fracturing during emplacement, whereas fluidization of host rocks has been linked to development of elongate magma fingers with rounded tips. Although host rock fluidization has only been observed at the lateral tips of magma fingers, it is assumed to occur at their leading edges (frontal tips) and thereby control their propagation and geometry. Here, we present macro- and microstructural evidence of fluidized sedimentary host rock at the lateral tips of magma fingers emanating from the Shonkin Sag laccolith (Montana, western United States), and we explore whether fluidization could have occurred at their frontal tips. Specifically, we combine heat diffusion modeling and fracture tip velocity estimates to show that: (1) low intrusion tip velocities (=10-5 m s-1) allow pore fluids ahead of the intrusion to reach temperatures sufficient to cause fluidization, but (2) when tip velocities are high (~0.01-1 m s-1), which is typical for many sheet intrusions, fluidization ahead of propagating tips is inhibited. Our results suggest that intrusion tip velocity (i.e., strain rate) is a first-order control on how rocks accommodate magma. Spatially and temporally varying velocities of lateral and frontal tips suggest that deformation mechanisms at these sites may be decoupled, meaning magma finger formation may not require host rock fluidization. It is thus critical to consider strain rate and threedimensional intrusion geometry when inferring dominant magma emplacement mechanisms. [ABSTRACT FROM AUTHOR]