BIRPS MOBIL LINE 1 (CCSS DATASET 2)
INTERPRETATION AND DISCUSSION
The MOBIL 1 coincident deep seismic reflection/refraction dataset provides a two-dimensional image of the crust and uppermost mantle nature at near-vertical incidence, possibly being a gradational rather than a sharp boundary. The upper crust in this region is also essentially transparent between 2-3 s to about 7 s TWT, except on the westernmost 50 km of the line where it is transparent up to only 5-6 s. This feature is consistent with the wide-angle data indicating almost a homogeneous upper crusta! structure with a nearly constant velocity of 6.1-6.2 km/s
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-160-from 4 km to 19-21 km depth. The lower crusta! structure is depicted by reflectors also reveal laterally variable reflectivity along this line appearing as bands of discontinuous reflector segments. This may be indicative of the variable nature (sharp or gradational) of the PCP and the PMP reflectors. The wide-angle recordings at the land seismic stations 54 and 47 however reveal high amplitudes for both the PCP and PMP reflections in the entire recording range. The coincidence of the deep crusta! and Moho reflectors observed from the near-vertical and wide-angle reflection datasets is well within the error limits of the phase correlations.
Another prominent feature evident from the COP reflection sections (Figures 1 0-12) is the presence of a good number of dipping events (marked as D), both in the lower crust and the uppermost mantle. Their amplitude decreases rapidly with increasing TWT. We interpret these events as diffractions which are generated from small scale heterogeneities in the path of the seismic wavefronts. These heterogeneities may represent regions of reflective segments with good acoustic impedance contrasts but with their dimensions equal to or smaller than the Fresnel-zone diameter. At relatively low frequencies of the order of 15 Hz or lower, corresponding to the lower crustal/uppermost mantle depths, the Fresnel-zone diameter would typically be 4-5 km or even larger. The small scale heterogeneities, of these dimensions may represent regions of extensive mixing of rock types with contrasting acoustic properties and originating the diffractions similar to those observed in the reflection section. According to Freeman et al. (1988), one of many geological possibilities to achieve this is by imbrication of continental crust with remnants of subducted oceanic crust. explained by diffracting elements. The remarkable persistence of the POP event with high amplitudes over about 20 km distance in the near-vertical reflection section, as well as its coincidence on the wide-angle record sections suggest that the source of this reflection should be an anomalous region of considerable dimensions with distinctive velocity/density characteristics.
-161-Following the explanation given by Freeman et al. (1988) for the parallel reflectors P1 and P2 observed on the NEC deep seismic reflection profile in this region, we suggest that the steep reflector (about 17° dip) modelled as the source of the POP event on the MOBIL 1 line is possibly related to the structures of the Caledonian origin rather than being related to later crusta! extension. lt may also be interesting here to refer to a recent study of similar 'diffraction-like' events on the DEKORP 2-S deep seismic reflection profile by Kampfmann (1988) who successfully demonstrated by amplitudes modelling that such strong events can be observed due to curved reflectors with dimensions significantly greater than the Fresnel-zone. Due to the similar travel time curves of such strongly curved reflectors and diffractors, they cannot be distinguished from purely kinematic analyses. However, amplitude computations support the hypothesis of curved reflectors as also the results obtained by us from synthetic seismograms modelling of the POP phase as a subcrustal reflector steeply dipping towards the west on the MOBIL LINE 1. Based on a recent analysis of the common reflection signature within the mantle in the North Sea area on surveys SHET, MOBIL and NSDP, Matthews et al.
(1990) found that on the western margin of the North Sea reflectors within the mantle consistently dip towards the west and the southwest.
Our inference of the west dipping subcrustal reflector on the MOBIL 1 profile is also quite consistent with their findings. We observe that the reflectivity pattern in the area of present study is controlled by the Caledonian structures.
ACKNOWLEDGEMENTS
We are grateful to the Director, National Geophysical Research Institute, for providing necessary facilities to undertake this study and permission to publish the results. We are also grateful to the organizers of this workshop, especially to Ernst R. Flueh, for providing the necessary datasets for this study and a preprint of Blundell et al., on the MOBIL survey. We thank Messers G.S.P. Rao, G.B.K. Shankar and G.Khandekar of NGRI for their help in computations and Mr. M.Shankaraiah, also of NGRI, for neat drafting of the figures.
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162-REFERENCES
Blundell, D.J., Hobbs, R.W., Klemperer, S.L., Scott-Robinson, R., Long, R.E., West, T.E., and Duin, E. 1991: Crustal structure of the central and southern North Sea from BIRPS deep seismic reflection profiling; J.Geoi.Soc.Lond., 148, 445-457.
Bott, M.H.P., Long, R.E., Green, A.S.P., Lewis, A.H.J., Sinha, M.C., and Stevenson, D.L. 1985: Crustal structure south of the lapetus suture beneath northern England; Nature, 314, 724-727.
Cerveny, V. and Psencik, I. 1981: 2-D seismic ray package; Charles University, Prague.
Freeman, B., Klemperer, S.L., and Hobbs, R.W. 1988: The deep structure of northern England and the lapetus Suture zone from BIRPS deep seismic reflection profiles; J.Geoi.Soc.Lond., 145, 727-740.
Kampfmann, W. 1988: A study of diffraction-like events on DEKORP 2-S by Kirchhoff theory; J.Geophys., 62, 163-174.
Klemperer, S.L. and Matthews, D.H. 1987: lapetus suture located beneath the North Sea by BIRPS deep seismic reflection profiling; Geology, 15, 195-198.
Matthews, D.H. and THE BIRPS GROUP (Fiack, C.A., Hobbs, R.W., Klemperer, S.L., Snyder, D.B., Warner, M.R., and White, N.J.) 1990: Progress in BIRPS deep seismic reflection profiling around the British Isles;
Tectonophysics, 173,387-396.