Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber

Transcription

1 Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber Steffi Burchardt and Valentin R. Troll, Department of Earth Sciences, CEMPEG, Uppsala University, Villavägen 16, Uppsala, Sweden Lucie Mathieu, CONSOREM, University of Québec à Chicoutimi, 555 Boulevard de l'université Chicoutimi, Québec, G7H2B1, Canada Henry C. Emeleus, Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, United Kingdom Colin H. Donaldson, Department of Earth and Environmental Science, University of St Andrews, KY16 9AL, Scotland Corresponding author: Steffi Burchardt, Department of Earth Sciences, CEMPEG, Uppsala University, Villavägen 16, Uppsala, Sweden. (Steffi.Burchardt@geo.uu.se ) 1

2 Supplementary information 1: Data table Table 1 lists the location, length, and attitude of all cone-sheet traces shown in Figure 1. An x marks which of the cone sheets have been projected in the 3D model. Supplementary information 2: 3D modelling of cone sheets a comparison between traditional and a new state-of-the-art approaches using the Ardnamurchan example SI2.1 2D vs. 3D modelling of cone-sheets When it comes to the projection of cone sheets into the subsurface, the traditional method as applied by e.g. Richey and Tomas (1930) 3 and Schirnick et al. (1999) 13 implies the projection of a surface measurement down-dip as a vector or trajectory for an arbitrary distance. The surface measurement is transposed onto one or more vertical sections (Fig. 1), translating the geometrical data for hundreds to thousands of metres. The intersection between projected cone sheets from opposite sides of the same swarm, the so-called focal point, is then interpreted as the location of the magmatic source reservoir that fed the cone sheets. Fig. 1. Illustration of the traditional approach of cone-sheet projection. Surface attitude data of conesheets are transposed onto one or more cross-sections and projected to depth. Ideally, the trajectories of projected cone-sheets converge to a single point, the focal point (white star at depth). The main shortcoming of this method the fact that both cone sheets and magma reservoirs are complex 3D structures (cf. e.g. 17, 46). The traditional 2D projection method does not do this complexity justice and may even lead to erroneous results due to the misconceptions and implied errors. In particular, these include: The transposition of the data onto one or two vertical sections eliminates the location of the measurement from the data, introducing a large error to the section from e.g. topographic differences and then artificially emphasises the convergence of sheets. A limited number of vertical sections reduces the 3D swarm geometry. The choice of section location has a large influence on the derived location of the focal point. The overall geometry of cone sheets is that of an inverted cone. This geometry results in a subcircular trace of the sheet at the Earth s surface or in any horizontal section. In a 2D projection 2

3 method, this, sometimes complex, trace is reduced to a single point. Admittedly, even in the best exposed cone-sheet swarms, individual sheets are usually exposed as segments of a limited length. Magma chambers are complex 3D structures that evolve in time. In contrast, the concept of a focal point is static. It is unlikely that all cone sheets of a swarm converge to a single point as the concept of a focal point implies that the magma chamber has no shape or volume. Alternatively, the focal-point concept indicates that cone sheets originate from the centre of a chamber. However, field evidence (e.g. the Karoo sills), analogue modelling (e.g. 12), theoretical modelling (18, 19) and numerical modelling (e.g. 47) all show that cone sheets usually nucleate at the upper lateral ends of flat-roofed magma chambers (Figs. 2, 3). Fig. 2. Illustration of a segment of a central volcanic complex with a flat-roofed magma reservoir. Tensile stress concentrations at the upper lateral ends of the reservoir mark the most likely areas of tensile failure and magmatic sheet injection (e.g. 47). The black dashed lines indicate the trajectories of sigma 3, which represent likely paths for cone-sheet propagation (Anderson, 1936) 4. To overcome these shortcomings, projection of cone-sheets in three-dimensional space appears to be a promising first-order approach to contribute to a more realistic understanding of subvolcanic structures (cf. 7). The apparent advantages of 3D cone-sheet modelling can be summarised as follows: A 3D model includes the location data in addition to the attitude of the cone sheet. If outcrop conditions are favourable, the exposed traces of cone sheets at the surface can be recorded and allow to include along-strike variations in the projection. Topographical effects and variations can be accounted for. By not translating the data onto a 2D section, artificial conversion effects are avoided, which may reveal a more realistic quantity and location of intersections of projected sheets. This results in intersection volumes rather than focal points. Consequently, 3D modelling should be the preferred method to project data sets of subvolcanic sheet intrusions, particularly since the availability of software and computational resources are no longer an obstacle. 3

4 SI2.2 An attempt to reproduce Richey and Thomas (1930) 3 results using a 3D model of the Ardnamurchan cone-sheets In order to reconstruct the three centres of magmatic activity derived by Richey and Thomas (1930) 3 from a 2D projection of the Ardnamurchan cone-sheets, we used the following approach: In our 3D model, we created a series of E-W (n = 12) and N-S striking (n = 18) cross sections with a spacing of 1 km. In each section, we marked all intersections of projected cone-sheets from different sides of the swarm (intersections with a high angle). These intersections ideally formed clouds of higher density, which were manually highlighted by a freehand polygon (Fig. 3a). The distribution and size of all the intersection clusters in 3D space does not show a systematic pattern, neither in size nor in location. They are all located below 1500 m, and most of them lie between 2000 and 4000 m depth beneath the present-day surface. We therefore failed to reconstruct Richey and Thomas (1930) 3 three focal points or areas, because cone-sheet intersections do not cluster systematically as individual focal points, but as many individual clusters of different sizes. The locations of the intersection clusters do not coincide with the locations of the focal points of Richey and Thomas (1930) 3 Centres 1, 2 and 3. This leads us to assume that the construction of the three centres was an effect of the shortcomings implicit in the 2D cone-sheet projection method. All rights reserved.). Fig. 3. a) An example of a N-S striking vertical cross-section through the 3D model. Intersections of projected cone-sheets from different sides of the swarm are marked by red crosses. Clusters of these intersections are manually highlighted with grey freehand polygons. b) Perspective view of all the cross sections. Green and blue lines mark the traces of the 3D cone-sheet projections on the sections. The grey polygons highlight clusters of cone-sheet intersections. Background map based upon Emeleus (2009) by permission of the British Geological Survey (CP13/091 British Geological Survey NERC. 4

5 SI2.3 Towards an improved method to construct the source of cone-sheets in 3D modelling A more realistic method to construct the source of projected cone sheets should ideally be based on the concept of cone-sheet initiation at the upper lateral ends of magma reservoirs (see Section SI2.1; Fig. 2). We propose that the areas of tensile stress concentration where cone-sheets are initiated should manifest in cross-section as zones where the projected cone-sheets from the same side of the swarm cluster and/or intersect at low angles (Fig. 4a). We marked all the low-angle intersections of projected cone-sheets at depth in the cross-sections and highlighted clusters manually with polygons (Fig. 4b). In comparison to the intersections from opposite sides of the swarm, these low-angle intersections systematically cluster at shallower depths. In the cross-section with a sufficient amount and spatial distribution of cone-sheets, there is at least one pair of clusters at similar depth and quite often, the pair is located at the upper lateral ends of the polygon marking a cluster of high-angle intersections (Fig. 4b). Since the low-angle intersections should mark the upper lateral ends of the source reservoir(s), and high-angle intersections should be located within or below the reservoir, we constructed ellipses that contain the high-angle intersections and have the low-angle intersections at their upper sides in each section. This step resulted in 14 ellipses that together should outline the location, size, and shape of the source reservoir(s) of the Ardnamurchan cone sheets (Fig. 4c). Note that neither the 2D, nor our approach contains any information about the depth of the bottom of the source reservoir. Consequently, the bottom of the derived source in our approach is defined by the ellipses. In order to construct a three-dimensional body from the ellipses, we first increased the density of the cross-sections in the relevant area by producing additional 16 N-S striking cross sections and 14 E-W striking cross-sections so that the final spacing of cross-section was about 330 m. The resulting 43 ellipses were then used to construct a so-called tetra-volume that consists of individual three-dimensional tetrahedra and has a total volume of ca km 3 (Fig. 5). 5

6 Fig. 4. a) Example of part of a N-S striking cross-section through the 3D model with a zone where the projected cone-sheets from one side of the swarm intersect at low angles (marked by a red ellipse). b) An example of a N-S striking vertical sections with low-angle (yellow crosses) and high-angle (red crosses) intersections of projected cone-sheets are marked. Clusters of these intersections are manually highlighted with blue (low-angle) and grey (high-angle intersections) freehand polygons. c) Perspective view of all the cross sections with ellipses encircling interpreted source locations (in pink). Green and blue lines mark the traces of the 3D cone-sheet projections on the sections. Background map based upon Emeleus (2009) by permission of the British Geological Survey (CP13/091 British Geological Survey NERC. All rights reserved.). This tetra-volume presents our interpretation of the source reservoir of the Ardnamurchan cone sheets. It is a complex, roughly ellipsoidal body with two protrusions pointing towards the E and the S. The main body is ellipsoidal and flat-topped, extending from 2 to 4 km depth. The protrusions extend to slightly shallower depths. The eastern one extends from approximately 3.5 to 1.5 km, and the southern one from approximately 3 to 1.5 km depth. 6

7 Fig. 5. The constructed source volume (purple). Traces of cone-sheets at the surface displayed as white lines. Projected cone-sheets shown as semi-transparent green and blue surfaces. a) Map view. b) The model seen from below. c) Perspective view N. d) & e) Perspective view E. f) Perspective view W. Background map based upon Emeleus (2009) by permission of the British Geological Survey (CP13/091 British Geological Survey NERC. All rights reserved.). 7

8 Supplementary information 3: Detailed sequence of local events The major of events in the development of the Ardnamurchan Central Complex included: (1) early deformation of the Mesozoic sedimentary rocks and Paleocene basalt lavas into an NE- SW elongate ridge/dome which shed debris flows off its NW and SE flanks, leading to the accumulation of the Ben Hiant and Achateny breccia members 11, 14. (2) These were then intruded by the earliest exposed members of the complex, i.e. the sparse major intrusions assigned to Centre 1 3 and likely also the areas of gabbro, dolerite and granophyre that are cut by abundant cone-sheets on the inner side of the Beinn an Ord Eucrite Ring Dyke, but hitherto assigned to Centre 2 as units 2a-d 11 (Fig. A1). Significantly, these include the majority of the granitic and more evolved intrusions in the complex. (3) The majority of cone-sheets were then intruded, their overall elongation being in the same direction as the NE-SW dome. Thus, the structural (NE-SW domed) character of the central complex was established at an early stage in the development of Ardnamurchan. The role of the major quartz gabbros/dolerite and olivine gabbro intrusions does, however, require separate consideration. Historically these substantial intrusions have been assigned to Centre 2 (Hypersthene Gabbro, etc.) or Centre 3 (Great Eucrite etc.) 3 (cf. Fig. A1). They post-date the vast majority of the cone-sheets and it is only in a limited area, on and near Beinn na Seilg, that cone-sheets intrude the Hypersthene Gabbro in any quantity. Elsewhere, they are decidedly sparse, and only a very limited number intrude the gabbros of Centre 3 (e.g. ca. 500 m NE of the Sonachan Hotel). Although it is difficult to locate convincing contacts between gabbros assigned to Centres 2 and 3 (e.g. at Sanna Bay), the Hypersthene Gabbro is truncated by Centre 3 intrusions (near Rubha Carrach and north of Kilchoan 3,11 ; cf. Fig. A1), which points to two separately evolved bodies. An explanation could be that as the initial, magmatic activity along the length of the ridge waned, the rising magma was concentrated into restricted areas along the ridge axis, initially towards the SW, with emplacement of the Hypersthene Gabbro and, finally, somewhat further to the NE with the formation of gabbro ring-dykes and a lopolithic intrusion 9 assigned to Centre 3 3,11. 8

9 Fig. 6. Simplified geological map of Ardnamurchan peninsula after Emeleus (2009) 11 with the main boundaries of units and subunits, highlighting the local and unit names referred to in the appendix. Based upon Emeleus (2009) by permission of the British Geological Survey (CP13/091 British Geological Survey NERC. All rights reserved.). Supplementary information 4: Movie of the 3D model Based on field measurements and geological maps like this one by Emeleus (2009) 11, we digitalised the traces of the cone-sheets of Ardnamurchan, which are visible as white lines. Then we projected the extension of the cone sheets down-dip for 5000 m (the green and blue surfaces). Below the surface, the projected cone-sheets are seen to converge towards an area below the central part of the peninsula. This focal area is elongated in ENE direction, and located between 1.5 and 4 km below the surface. It may be illustrated with a roughly saucer-shaped body with two protrusions that represents the magmatic source chamber that fed the cone sheets. References see main manuscript 9