Must magmatic intrusion in the lower crust produce reflectivity? |
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| Authors: | JH McBride RS White JR Smallwood RW England |
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| Institution: | aDepartment of Geology, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA;bBullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, UK;cAmerada Hess Ltd., 33 Grosvenor Place, London, SW1 7HY, UK;dDepartment of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK |
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| Abstract: | The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin. |
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| Keywords: | Oceanic crust Seismic reflection Mantle volcanism Mantle plumes |
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