LEGS AGU Poster

Leinster Granite Seismic Project (LEGS): A Wide-Angle Study of a Late Caledonian Granite

J.A. Hodgson¹, P.W. Readman¹, B.M. O'Reilly¹, P.S. Kennan², S. Harder³, G.R. Keller³ and H.Thybo*

¹ Dublin Institute for Advanced Studies, ² University College Dublin, ³ University of Texas, El Paso, * Unversity of Copenhagen

1. Introduction & Geology 2. Gravity 3. The Experiment 4. Wide-Angle Seismic Data 5. Preliminary Results & Models 6. Conclusions

(Click on images for full size)

1. Introduction and Geology

The Leinster Granite is the most extensive batholith in Ireland. It lies with in the Appalachian/Caledonian orogen with an outcrop covering an area approximately 1600 km². The granite is a late Caledonian post-tectonic intrusion ca. 405 ± 2 Ma emplaced within a system of compressional tectonic structures.

The granite consists of five en-echelon domed units, generally poorly exposed, covered by a blanket of peat and till and sits within folded and thickened low grade metasediments of Palaeozoic age (see Fig. 1).

Geological mapping has delineated the boundaries of the granite and some internal structure, but its overall geometry and subsurface extent are still unknown. The LEGS project (LEinster Granite Seismic) hopes to rectify this.

Fig. 1: Geological map of Ireland.

2. Gravity

A Bouguer anomaly map (Fig. 2) shows a large gravity low centred on the Tullow pluton, trending northeast southwest, and correlating well with the granite outcrop. This negative anomaly also extends further towards the south-west, implying that buried granites continue to extend to the southwest. There is a clearly defined boundary where the negative anomaly associated with the granite is paired against a high positive anomaly produced by dense metasediments. The main tectonic structure relating to the crustal features apart from the granite is the predominantly northeast-southwest trending fold structures relating to the main Caledonian orogen and deformation. This shows dominant transcurrent sinistral movement active during shortening and emplacement of the Caledonian granites, and is seen most clearly in the surrounding Caledonian country rocks.

Fig. 2: Bouguer anomaly map.

3. The Experiment

The LEGS experiment was conducting in March 1999 using 300 REFTEK TEXAN's borrowed from the University of Texas at El Paso and the University of Copenhagen. The TEXAN's are miniature digital seismic recorders and are ideally suited to controlled source work, being flexible and lightweight and hence easy to deploy.

Three seismic lines were deployed within the Leinster region, crossing the granite with 19 shots (14 sea and 5 land) recorded, as shown in Fig. 3.

Line A is the main axial line, 185 km long with a receiver spacing of 1.0 km.
Line B is the northern transverse line, 68 km in length, with a receiver spacing of 1.5 km.
Line C is the southern transverse line 105 km long, with 82 receivers at 1.3 km spacing.

Fig. 3: Experimental layout of seismic lines and shots across the northern and southern (Tullow) plutons.

Wide-Angle Seismic Data

Preliminary modelling concentrated on the upper crust of the 3 lines A, B and C. High quality single component data was recorded. Sections (Figs 4-7) are bandpass filtered at 2-14 Hz and are reduced to 6 km/s, with black lines representing common phases. Results of shots 3 and 10 on line A and shots 13 and 5 on lines B and C respectively are shown as a sample of the data. Lateral velocity variations are apparent along all lines and correlate well with the surface geology.

Shot 3: This was the southern shot at the end of line A. First arrivals Ps and Pg, have strong amplitudes with high signal-to-noise, but disappear after 60 km. Ps increases in velocity from 5.2 km/s to 5.5 km/s, while Pg begins fast at 6.0 km/s before slowing at 30 km, then speeds up after 50 km. Beyond Pg strong, secondary phases generated from interfaces in the upper and mid-crust, are observed, as is PmP, seen from an offset of 95 km to 140 km, although Pn is less clear.

Fig. 4: Shot 3. Sea shot from southern end of line A.

Shot 10: This was fired in Dublin Bay at the northern end of line A. High velocities are seen early with Pg reaching a velocity of 6.0 km/s at an offset of -15 km before slowing slightly at -25 km, then returns back to a velocity of 6.0 km/s, possibly implying a thicken ing then thinning of the granite. Pg ends sharply at -46 km offset, when a step in the phases is seen, with Pg at 6.0 km/s increasing to 6.2 to 6.3 km/s. This suggest a steep gradient, which coincides with the transition from the granite into higher velocity volcanics. At an offset of -56 km phases slow again on re-entry into the granite. Pg arrivals eventually die out at -90 km, where a change in amplitude is also observed. A high amount of energy is returned between -10 and -35 km offsets, corresponding to reflections from the granite at about 4.0-4.5 km depth. There are several strong secondary phases, with a clear PmP phase. A high gradient Pn is seen with some kind of transition zone at the base of the crust.

Fig. 5: Shot 10. Sea shot from northern end of line A.

Shot 13: This was the eastern sea shot on line B, with good signal-to- noise ratio. Pg arrivals begin fast before slowing to ~5.7 km/s at 10 km, then increase to about 6.0 km/s. A sharp step is seen at -25 km offset as velocities increase dramatically, before levelling off at 6.0 km/s, relating to where dense metasediments bound the granite on its eastern edge. Few reflections are observed and the section is too short for PmP or Pn to be recorded.

Fig. 6: Shot 13. Sea shot from eastern end of line B.

Shot 5: This was the eastern sea shot on line C, with good signal-to-noise. Pg begins fast over the first 8 km, then slow to around 5.8 km/s over the next ~25 km. At -36 km offset a slight step in phases is seen, with velocities increasing to ~6.1 km/s, implying a steepish gradient, correlating with the transition from Ordovician metasediments into the granite. A strong amplitude change is witnessed at -68 km offset, perhaps indicating a change from granite to country rock at depth. Faint shallow reflections are seen around -35 to -55 km offset, with PmP recorded from -90 km onwards.

Fig. 7: Shot 5. Sea shot from eastern end of line C.

Preliminary Results and Models

Models were created using the two-dimensional forward modelling program SEIS83. Preliminary crustal models show the top 8 km of the crust along lines A, B and C (Fig. 8, 9 and 10).

Line A is dominated by two bodies interpreted as granite, exhibiting internal layering at about 1.5 and 2.5 km depth, with velocity increasing from 5.3 km/s at the surface to around 5.9 km/s near 5 km depth. The northern granite reaches a depth of 4.0-4.5 km, thinning towards the northern coast, while the southern granite extends to about 5 km, with a degree of subsurface extension to the south. A series of near horizontal sedimentary layers exist to the south of the granites, which tend to rise towards and below the granite bodies, and show a degree of lateral velocity variations.

In the south, a zone of slower material is seen at around 40 km, which quickly changes to a high velocity zone of around 6.0 km/s near to the surface at 70 km. A high velocity zone separates the two granite bodies at about 135 km, with velocities up to 5.7 km/s at the surface.

Fig. 8: Velocity model: line A.

Line B shows a series of layers rising towards the central layered granite body, with a similar velocity structure as line A. A rise in velocity is seen on the eastern edge of the granite body.

Fig. 9: Velocity model: line B.

Line C shows a similar pattern to that of lines A and B. A layered central body with velocities ranging from ~5.3 to 5.9 km/s is wedged between rising horizontal layers. A slight velocity rise is seen at depth at the western edge of the granite body.

Fig. 10: Velocity model: line C.

Conclusions

The TEXAN recorders worked very well with about 98% data recovery, the majority with high data quality.
Clear first arrivals relate to the granite geometry and also correlate with the surface geology.
Clear crustal and upper mantle phases are identified.
Preliminary models for lines A, B and C indicate an approximate 4 km thickness for the northern granite. A slightly thicker body for the Tullow pluton is indicated.
Granite bodies show a layered velocity structure.
A picture of the geometry and subsurface extension of the granite will contribute to the understanding and the history of emplacement of the Leinster Granite.