Abstract
A high-resolution seismic reflection survey has been conducted across the Insubric Line from the Sesia Zone into the Ivrea-Verbano Zone (IVZ), where a remarkably complete cross-section of lower continental crust is exposed. The survey was carried out in preparation for the DIVE (Drilling the Ivrea-Verbano zonE) project, which was recently approved by the International Continental Scientific Drilling Program (ICDP). DIVE aims to gain new insights into the characteristics of the lower continental crust through targeted drilling, sampling, and borehole logging. A key borehole is planned near the Insubric Line at Balmuccia, where the deepest parts of the lower continental crust are exposed. As such, the primary objective of this seismic survey was to explore whether the sub-vertical structures prevailing at the surface can be expected to continue at depth or whether there are any indications for major flattening or fault-related offsets. Correspondingly, the acquisition and processing of the seismic reflection data were geared towards revealing weak backscattered events from local heterogeneities associated with the prevailing sub-vertical structural grain. The migrated sections, contain coherent backscattered events to a depth of ~1 km, which form numerous short lineaments that seem to align sub-vertically. To substantiate this observation, we have generated synthetic seismic reflection surveys for canonical models of sub-vertical structures associated with Gaussian- and binary-distributed heterogeneities. Both the observed and synthetic seismic data were then subjected to energy-based attribute analysis as well as geostatistical estimations of the structural aspect ratios and the associated dips. The results of these quantitative interpretation approaches are indicative of the overall consistency between the synthetic and the observed seismic data and, hence, support the original qualitative interpretation of the latter in that the sub-vertical structural grain evident at the surface seems to prevail throughout the imaged part of the upper crust.
Highlights
The Insubric Line corresponds to the western end of the Periadriatic fault system delineating the boundary between the European and Adriatic plates, which can be followed over more than 700 km from Slovenia to the southwest of Torino (e.g., Schmid et al, 1989; Handy et al, 2015)
We focus on the data acquired along L1, which crosses from the Sesia Zone in the west into the Insubric Line in the east, because it exemplarily illustrates our meth odological approach, quasi-perpendicularly crosses some of the most pertinent structures, and has a significantly higher signal-to-noise ratio (S/N) than L2 and L3
To explore the seismic expressions of a sub-vertical structural grain associated with local heterogeneities, we evaluate and analyse synthetic seismic data for canonical crustal models based on Holliger et al.’s (1993) geostatistical conceptualization of Ivrea-type lower crust (Fig. 8)
Summary
The Insubric Line corresponds to the western end of the Periadriatic fault system delineating the boundary between the European and Adriatic plates, which can be followed over more than 700 km from Slovenia to the southwest of Torino (e.g., Schmid et al, 1989; Handy et al, 2015). G., Berckhemer, 1968; Kissling, 1984; Scarponi et al, 2020) and an associated seismic high-velocity anomaly (e.g., Diehl et al, 2009; Lu et al, 2018), whose origins extend to shallow crustal levels This socalled Ivrea Geophysical Body (IGB) is assumed to represent a sliver of Adriatic lower crust and upper mantle, which was “upwarped” in the course of the continental collision process (e.g., Schmid et al, 2017). This is generally best achieved through seismic reflection surveys, which are widely regarded as a conditio sine qua non for drilling campaigns in general and ICDP projects in particular (e.g., Demirel-Schlueter et al, 2005; Juhlin et al, 2010; Simon and Buske, 2017) To this end, we have carried out high-resolution seismic reflec tion measurements across the Insubric Line from the Sesia Zone into the IVZ near the planned DIVE drilling location at Balmuccia. We complement the conventional qualitative visual assessment of crustal seismic reflection images with numerical simulations as well as with attribute and geostatistical analyses
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