Research Paper GEOSPHERE Controls on the expression of igneous intrusions in seismic reflection data GEOSPHERE; v. 11, no. 4 Craig Magee, Shivani M. Maharaj, Thilo Wrona, and Christopher A.-L. Jackson Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, 39 Prince Consort Road, London SW7 2BP, UK doi:10.1130/GES01150.1 14 figures; 2 tables ABSTRACT geometries in the field is, however, hampered by a lack of high-quality, fully CORRESPONDENCE: [email protected] three-dimensional (3-D) exposures and the 2-D nature of the Earth’s surface The architecture of subsurface magma plumbing systems influences a va- (Fig. 1). Geophysical techniques such as magnetotellurics, InSAR (interfero- CITATION: Magee, C., Maharaj, S.M., Wrona, T., riety of igneous processes, including the physiochemical evolution of magma metric synthetic aperture radar), and reflection seismology have therefore and Jackson, C.A.-L., 2015, Controls on the expres- sion of igneous intrusions in seismic reflection data: and extrusion sites. Seismic reflection data provides a unique opportunity to been employed to either constrain subsurface intrusions or track real-time Geosphere, v. 11, no. 4, p. 1024–1041, doi: 10 .1130 image and analyze these subvolcanic systems in three dimensions and has magma migration (e.g., Smallwood and Maresh, 2002; Wright et al., 2006; /GES01150.1. arguably revolutionized our understanding of magma emplacement. In par- Biggs et al., 2011; Pagli et al., 2012). Of these techniques, reflection seismol- ticular, the observation of (1) interconnected sills, (2) transgressive sill limbs, ogy arguably provides the most complete and detailed imaging of individual Received 11 November 2014 and (3) magma flow indicators in seismic data suggest that sill complexes intrusions and intrusion systems. In particular, intrusions within sedimentary Revision received 10 April 2015 Accepted 11 May 2015 can facilitate significant lateral (tens to hundreds of kilometers) and vertical basins can be easily identified and mapped in 2-D and 3-D seismic reflection Published online 10 June 2015 (<5 km) magma transport. However, it is often difficult to determine the va- data due to the large acoustic impedance contrast between igneous rocks and lidity of seismic interpretations of igneous features because they are rarely encasing strata (Smallwood and Maresh, 2002). Seismic studies have thus drilled, and our ability to compare seismically imaged features to potential revolutionized our understanding of intrusion systems in sedimentary basins, field analogues is hampered by the limited resolution of seismic data. Here we providing spectacular images of vertically and laterally extensive complexes of use field observations to constrain a series of novel seismic forward models strata-concordant and/or saucer-shaped sills (e.g., Fig. 1) (e.g., Symonds et al., that examine how different sill morphologies may be expressed in seismic 1998; Smallwood and Maresh, 2002; Thomson and Hutton, 2004; Planke et al., data. By varying the geologic architecture (e.g., host-rock lithology and intru- 2005; Polteau et al., 2008; Magee et al., 2013b, 2014a; Sun et al., 2014). Map- sion thickness) and seismic properties (e.g., frequency), the models demon- ping of magma flow indicators in these data has led to an emerging consensus strate that seismic amplitude variations and reflection configurations can be that magma can be transported over significant lateral (to hundreds of kilome- used to constrain intrusion geometry. However, our results also highlight that ters) and vertical (to several kilometers) distances via interconnected sills stratigraphic reflections can interfere with reflections generated at the intru- and transgressive inclined sheets (e.g., Cartwright and Hansen, 2006; Magee sive contacts, and may thus produce seismic artifacts that could be misinter- et al., 2014a). Detailed analyses of these intrusion systems has also shown that preted as real features. This study emphasizes the value of seismic data to (1) the architecture of magma networks is influenced by the host-rock struc- understanding magmatic systems and demonstrates the role that synthetic ture, in particular bedding discontinuities and fractures, and lithology (Scho- seismic forward modeling can play in bridging the gap between seismic data field et al., 2012a; Jackson et al., 2013; Magee et al., 2013c); (2) igneous activ- and field observations. ity may be protracted (e.g., incremental intrusion over 15 m.y.; Magee et al., 2014a); and (3) sill-complex construction can affect the distribution and style of host-rock deformation (Magee et al., 2014a) and volcanism (Magee et al., INTRODUCTION 2013d). Constraining the validity of these observations is, however, difficult to accomplish because of the limited vertical and horizontal resolution of seismic Subsurface networks of igneous intrusions compose a series of intercon- reflection data ≥( 20 m for igneous rocks) and the lack of boreholes intersecting nected conduits and reservoirs. The architecture of these systems influences igneous intrusions. the physiochemical evolution of magma (e.g., Holness and Humphreys, 2003; To help provide a better understanding of the general seismic expression Magee et al., 2013a), extrusion location (e.g., Gaffney et al., 2007), and the of intrusions, we conduct seismic forward modeling to examine how sill accumulation of economic resources (e.g., Bedard et al., 2012; Holford et al., geometries observed in the field are manifested in seismic reflection data. For permission to copy, contact Copyright 2012). Establishing the geometry of individual intrusions and their connectivity By creating simple geometric geologic models and using real host-rock Permissions, GSA, or [email protected]. is thus crucial to understanding igneous processes. Resolving entire intrusion mechanical properties (e.g., Fig. 1), we examine (1) whether seismic data can © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 4 Magee et al. | Imaging intrusions in seismic data Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1024/3332838/1024.pdf 1024 by guest on 30 September 2021 Research Paper Seismic reflection data Field examples 3-D/Map view Cross-section –2.2 SW NE A me (s) –2.4 Section Ti ) Sill Sill Figure 1. Field analogues to intrusion mor- N phologies interpreted from seismic reflec- tion data. (A) Strata-concordant (Strat-con.) VE ≈ 5 Strat-con. sill sills observed in the Bight Basin offshore VE ≈ 5 ~1.5 km 0.5 s (TWT 2 km ~0.1 km Time (s) southern Australia (seismic example) and W E the Theron Mountains in Antarctica (field B – + example; photo courtesy of Donny Hutton). Section (B) Seismic images of a saucer-shaped sill in three dimensions (3-D) and cross section –2 (modified from Magee et al., 2013c) and an oblique view of the Golden Valley Sill –3 Sill exposed in the Karoo Basin, South Africa ) ( image from Google Earth). (C) A 3-D view me (s) Ti Sill of intrusive steps with long axes oriented N N parallel to the dip direction of the inclined sill limb (see also Magee et al., 2013b) and VE ≈ 2 an orthogonal cross section from a sill lo- Saucer-shaped sill VE ≈ 5 ~0.5 km 0.2 s (TWT cated in the Exmouth Subbasin offshore -2.73 1 km 2 km northwestern Australia (seismic example). ~0.5 km SW NE Step C Step Step The field photo is of a sill exposed on Axel N Heiberg Island in the Sverdrup Basin of Arc- ) s tic Canada (courtesy of Martin Jackson). –3.0 (D) Magma fingers from a sill in the Rockall Sill Trough (modified from Thomson and Hutton, Step Step long axes 2004) and the Golden Valley Sill. In the three me (s) VE ≈ 5 seismic sections, VE is vertical exaggeration Ti –3.5 VE ≈ 4 0.2 s (TWT 1 km ~0.1 km and the measured time is in seconds, two- Overall sill Finger way traveltime (TWT). D Lobe Flow direction s No cross-section ~0.5 km examples reported N Magma finger 1 km ~0.5 km be used to determine the connectivity within sill complexes, i.e., whether SYNTHETIC SEISMIC FORWARD MODELING OF IGNEOUS magma can migrate to the surface through a network of sills (Cartwright and INTRUSIONS Hansen, 2006); (2) what inclined sill limbs tell us about magma propagation and emplacement mechanisms; and (3) the utility of subtle geometric fea- Magmatic bodies are traditionally mapped in seismic data by picking high- tures interpreted in seismic data to constraining magma flow directions. Our ampli tude reflections that are considered to correspond to the upper contact results demonstrate that intrusion geometries observed in the field can be between an intrusion and the encasing host rock (Smallwood and Maresh, distinguished in (synthetic) seismic data. Interference between intrusions and 2002; Thomson, 2005). Occasional underlying high-amplitude reflections are the encasing host-rock reflections can, however, generate seismic artifacts observed that may correlate to the lower intrusive contact (e.g., Hansen and that may be misinterpreted. Cartwright, 2006; Jackson et al., 2013). Where both contacts are discernable, GEOSPHERE | Volume 11 | Number 4 Magee et al. | Imaging intrusions in seismic data Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1024/3332838/1024.pdf 1025 by guest on 30 September 2021 Research Paper the mapped intrusions resemble, at least geometrically, those observed in the A Sill 1 Fig. 7 40° field (e.g., Jackson et al., 2013). Most intrusions are, however, expressed as Fig. 8 15° 100 m tuned reflection packages (e.g., Fig. 1) (Smallwood and Maresh, 2002). This Sill 2 tuning effect occurs when the vertical intrusion thickness is between the limit of 25° 100 m separability and the limit of visibility of the seismic data (sensu Brown, 2004). In this scenario, the reflections emanating from the upper and lower intrusion B Sill 2 map-view contact interfere and cannot be distinguished (Widess, 1973; Smallwood and C Sill Maresh, 2002; Brown, 2004; Hansen et al., 2008).