Two‐dimensional seismic reflection modeling of the inferred fossil oceanic crust/mantle transition in the Bay of Islands Ophiolite

Abstract

We investigate the origin and character of oceanic Moho reflections by computing two‐dimensional synthetic seismogram profiles of the inferred fossil oceanic crust/mantle transition observed in the Bay of Islands Ophiolite. To simulate a seismic reflection experiment, we calculated near‐vertical‐incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length. In the synthetic profiles the Moho reflection event varies from a single phase to two or more phases of up to 1‐s (two‐way travel time) total duration. Individual phases show lateral variation in amplitude, and their two‐way travel times vary by as much as 0.25 s over horizontal distances as short as 10 km. Lateral discontinuity of phases results in abrupt variations in the travel times of first‐arriving, high‐amplitude Moho phases. The geological structures generating the highest‐amplitude Moho reflections vary from high‐ and low‐velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the Moho Transition Zone. Reflection amplitudes from the residual upper mantle are insignificant, and our modeling suggests that using the first‐arriving, high‐amplitude Moho phase to estimate thickness of magmatic material might result in errors of up to 1‐s two‐way travel time (∼3–4 km). Multichannel seismic data from both the western Pacific and western North Atlantic show Moho travel time variations similar to those observed in the synthetic profiles. The western North Atlantic data also show multiphase Moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data, suggesting that the structures observed in the inferred fossil crust/mantle transition of the ophiolite are characteristic of oceanic lithosphere.