| 12 Sep 2025
Lower Carboniferous igneous intrusions within the crystalline basement of the Baltic Basin (SW edge of the East European Craton, Poland) – insight based on seismic data interpretation and seismic forward modelling
Abstract. The deep, high-resolution regional-scale seismic profiles of the PolandSPAN® survey provided a unique insight into the extensive system of inferred igneous intrusions imaged within the crystalline basement of the Baltic Basin (SW part of the East European Craton). These intrusions, that continue laterally for 100+ km, are located at depth of c. 6–7 – 19–20 km, and are far beyond the reach of the deepest wells. They are represented by packages of strong seismic reflectors, sometimes saucer-shaped, and are often characterized by step-wise geometry, sometimes diverging into several separate branches. Forward seismic modelling was used in order to provide insight – “educated guess” – regarding their lithology, exact lateral extent, and lateral thickness variations. It was concluded that most probably these are doleritic intrusions of thickness in the range of 60 to 200 m. Due to lithological coherence and close spatial relation to shallow sills and massifs recognized in wells, they have been interpreted as Mississippian (early Carboniferous) intrusions belonging to the recently recognized Lublin-Baltic Igneous Province. Developed methodology that combines quantitative assessment of seismic data resolution and estimation of tuning thickness with 2D seismic forward modelling based on geological constrains might be used to better understand, or estimate, parameters characterizing deep intrusions that are beyond reach of deep wells.
“Lower Carboniferous igneous intrusions within the crystalline basement of the Baltic Basin (SW edge of the East European Craton, Poland) – insight based on seismic data interpretation and seismic forward modelling”. Krzywiec et al.
Seismological imaging of igneous intrusions within the continental crust is an area of extensive, global, continuing research. Much of this effort globally has used passive seismic methods applied to teleseismic arrivals. The data from deep seismic reflection profiling, much of it legacy, is an under-used resource and so I welcome this contribution by Krzywiec and co-workers. It is an interesting contribution that interprets and models results of deep seismic reflection profiling in Poland (PolandSPAN), focussing on discontinuous mid-crustal reflection packages. The authors have an opportunity here to make this study far more useful to the global community of earth scientists by drawing out the general application of their methods and making broader comparisons with other examples. It is this recommendation that underpins many of the comments in this review. As written, the paper tends to the parochial – the authors have a great opportunity here to do something more! So while this could be published with only minor modifications, I encourage the authors to reframe the text in this light.
The paper is clearly within the scope of SolidEarth, is generally well-written and illustrated and has high scientific quality.
Given then opportunities noted above, it would be useful to clarify the aims of this contribution. Of course, large igneous intrusions are exposed at outcrop, exhumed from various levels (including deep). The point here is detecting them in the subsurface, potentially linking them to coeval extrusives at outcrop. But why does this have importance for understanding anything other than the evolution of some local continental crust (i.e. beneath Poland)? As noted in the geological setting… the region has igneous rocks at outcrop. Is the point here to establish their extent at depth? If yes, say so! Or is it about establishing the geometry of intrusions? Do sills emplaced in the basement have the same propensity for steps and splays as they do when emplaced in layered sedimentary rocks, nearer the syn-volcanic earth’s surface? The abstract does a better job at setting the scene. Likewise, the general importance of this study, the opportunities provided by deep seismic reflection profiling, the methods used here, and the general interpretations could be developed much more fully in the discussion.
Analysis of the tuning thickness is used to determine the seismic velocity and this inputs to modelling of synthetic seismic responses, the better to characterise the shape and velocity structure of these subsurface bodies. These build from extensive discussion of the impact of tuning thickness on imaging. This reads well generally and is well-paced for non-seismologists. This allows the thickness of sills and their geometry to be made, together with deductions made of their velocity structure (and hence composition). The overall seismology and deductions seem convincing (though I’m not an expert!). I do have issues with the justifications and descriptions of methods and results which could be tightened up by better (and possibly more relevant) referencing. These are outlined below.
At various points in the text, the authors’ approach is described as “educated guessing”. While this is very self-effacing, it is of course common to all interpretation – which naturally always carries uncertainty and assumptions. It would be helpful to be explicit about the limitations imposed by the specific assumptions adopted here – a point that could be returned to in the Discussion.
Section 3.1
It’s good to see the information on the acquisition and processing parameters for the PolandSPAN survey. Presumably it is the quality of these that has allowed the detailed work here (as the authors say, these are unparalleled). But if the authors choose to broaden the vision of this paper to promote the greater use of legacy surveys elsewhere – it would be worth comparing the PolandSPAN parameters with other, historical, onshore surveys (ECORS, FIRE, various Iberian experiments etc). Or address this in the discussion.
Section 3.3.
Introductory paragraph (lines 181-187). The arguments for adopting synthetics rather than ray-tracing methods needs referencing to justify these statements – especially as the only citation (Slonka et al) is to a study of sedimentary rocks rather than as a tool to enhance crustal imaging.
3.3.1
This discusses the selection of modelling parameters. It would be helpful to state explicitly the values (and their ranges) as used in this study (velocities, densities). And be specific in the comparisons made with the geology (end sentences of this section).
Section 4
Line 253 – why was 500 ms used as the wavelet extraction window? What are the implications of changing this?
Line 260-2. Please be explicit that the tuning thickness chart is Fig. 6c (it’s in the caption but it makes the text easier to follow if in there too). Maybe note that tuning thickness charts are also provided for Fig 7 and 8.
2-D seismic modelling
It’s good to contrast acid/intermediate composition (granite-granodiorite) with basic-composition igneous rocks. However, it’s curious to use parameters from both basalt and dolerite. Presumably the basalt ones relate explicitly to lavas while dolerite are for intrusive rocks (the difference not being compositional but textural – as basalt may have porosity). Surely only dolerite is relevant here (e.g. rhyolite hasn’t been modelled along with granite).
But more critically – given the thickness of the interpreted igneous bodies (up to 200m for the wedge approximations) – these are likely to be layered – and could include ultrabasic compositions… (And indeed the basic component is likely gabbro). So over-emphasising specific values on seismic velocity and density is un-necessary – beyond eliminating those for acid/intermediate composition rocks. Simply setting this up as a two-comparison acid/intermediate vs basic (dolerite/gabbro) test – would suffice.
I’d expect to see justification for the host rock parameters (“Precambrian basement”) – as of course it is the contrast with this that is important for investigating the intrusions.
This aside apart – the modelling section reads well. The illustrations are nice (Figs. 10 and 12, 11 not needed if you drop the basalt!).
Section 4.4
(line 430 onwards)
The comparisons here with interpretations of igneous intrusions in the crust revealed by seismic imaging could be enlarged and better-structured – though may be better reserved for the Discussion.
The wedge model and the deductions of the thicknesses of the sill, together with its steps – are great (Figs. 14, 15, 16). Although the authors have chosen to reserve further comparison with outcrop “analogues”, choosing only to compare with seismic examples of Cartwright et al. – I think again that this is a missed opportunity. Stepping and branch sills have been known and illustrated in the geological record for over 200 years (yes, really – Macculloch 1819) … which are well-known to the igneous geology community… Simply citing work from the 2000s onwards isn’t great!
The Discussion focusses on Poland and its geology. As noted above, this would be far more valuable if these discussions were broadened out. The final paragraph hints at this. More discussions should be made about the underpinning assumptions
Seismic Interpretations
The seismic interpretations (figs 3, 4, 5) should include uninterpreted (clean) images.
As presented – the images have very high vertical exaggerations. Is the deep normal fault in Fig 5 really dipping at just 25 degrees (as implied by this interpretation when at ~v=h…)?
Figure 1 – please change the colours between extrusive and intrusive igneous rocks – they’re too similar at the moment.
Rob Butler
Aberdeen
October 2025