The Moho – defined as the boundary that separates the Earth's crust from the underlying upper mantle at depths of about 5 to 7 km beneath the oceans and 30 to 40 km below continents – has been difficult to pin down with observations. Different methods have yielded a variety of results, suggesting that this simple definition may be misleading. Here, we present high-resolution and deep penetrating seismic reflection data that image the whole margin basement of the mid-Norwegian rift system, including the lower basement, Moho and upper mantle, from the proximal to the outer margin domains.
Named after the Croatian seismologist Andrija Mohorovičić who first identified it in 1909 based on seismograms, the Moho is defined as a seismic discontinuity - a marked change in propagation velocity of compressional P waves (Mohorovičić 1910). The velocity of a seismic wave being related to the material density, the observed acceleration is interpreted to be caused by a higher density material present at depth and explained it as corresponding to the crust - mantle boundary. After decades of research and different observations, Steinhart (1967) defined the seismic Moho as the depth at which the P-wave velocity first shows an increase to a value between 7.6 and 8.6 km/s, rapidly or discontinuously. Ideally, this seismological definition is supposed to match the crust - mantle boundary (Griffin & O’Reilly 1987) which marks the petrological boundary between the crustal felsic rocks and the underlying ultramafic mantle.
For rifted margin studies, the imaging of the Moho is an important constraint in every interpretation protocol. The identification of the base of the continental crust is a very instructive characteristic for evaluating the overall rifting evolution. It allows assessment of the amount of basement thinning and study of the uplift-subsidence evolution of the sedimentary basins. However, the imaging of the Moho appears non-unique and dependent on the method used for the investigation. The concept of the Moho as a sharp boundary is misleading in its simplicity, for two main reasons: first, as observed in some onshore outcrops, the Moho probably often corresponds to a complex interface rather than to a sharp boundary; and second, the signature of the Moho is to a large degree dependent on the methodology which has been used to obtain the image (Carbonell et al. 2013). Each geophysical method - seismic reflection, seismic refraction, passive seismology, potential fields - being sensitive to specific physical parameters, covering the petrologic-, lithologic-, seismological-, geophysical-… characteristics of the boundary.
For this contribution, focus is set on the seismic reflection Moho. Within the context of the modern Mid-Norwegian rifted margin, based on the new GeoexMCG RDI19 seismic reflection dataset (Peron-Pinvidic et al. 2022), the facies, geometrical variations and reflectivity pattern of the Moho are described and discussed in conjunction with the rifted margin architecture.
The observations support the proposed concept that each margin structural domain is characterized by a specific seismic reflection Moho type, with particular reflectivity patterns at the transition between the lower crust and the upper mantle. The Moho facies range from diffuse in the proximal domain, convergent in the necking domain, focused in the hyperextension subdomain, transparent in the exhumation subdomain, and corrugated and discontinuous in the outer domain. The sharp vs. diffuse seismic facies are discussed in terms of tectonic overprint, and the different reflectivity patterns of the seismic reflection Moho are interpreted to reflect the deformation pattern of each margin domain in the overall rifting evolution.