Characterization of the Lower Continental Crust in the Ivrea-Verbano Zone from the Well Logging and Core Data of ICDP-DIVE Boreholes 5071_1_B and 5071_1_A

One of the aims of the ICDP Drilling the Ivrea-Verbano zonE project (DIVE) is to unravel lithological drivers of geophysical observations in the lower continental crust. During Phase I of the project, two boreholes, 5071_1_B and 5071_1_A, have been completed at depths of 578.5 and 909.5 m in December 2022 and March 2024, respectively. Borehole 5071_1_B is drilled into the upper part of the lower continental crust, while borehole 5071_1_A extends deeper into the lower crust. Core descriptions identified the lithologies in borehole 5071_1_B as kinzigite, amphibolite, calcsilicate and leucosome. In contrast, borehole 5071_1_A encountered a variety of lithologies including stronalite, anorthosite, gabbro, garnetite, gabbronorite, garnet granulite and pyroxenite. Together, these two boreholes represent a comprehensive cross-section of the lower continental crust in the Ivrea-Verbano zone. To understand the geophysical characteristics and their correlation to lithologies, a comprehensive set of geophysical borehole logs was acquired, including among others spectral gamma ray, magnetic susceptibility, sonic, acoustic and optical televiewer data. To complement the downhole data set, core density and magnetic susceptibility measurements were conducted using a multi-sensor core logger. 

In our previous study, fuzzy c-means clustering of magnetic susceptibility and natural gamma logs from borehole 5071_1_B demonstrated an excellent agreement with the lithological core description, despite notable spatial variability. In this study, we integrate the petrophysical data from both boreholes revealing significant contrasts in petrophysical properties between them. Preliminary results indicate that the rocks in 5071_1_A generally exhibit lower gamma radiation, higher densities and higher velocities compared to those in 5071_1_B, with the exception of some amphibolite intervals in 5071_1_B. With respect to their magnetic susceptibilities the lithologies of both boreholes partially overlap; however, gabbros, gabbronorites and garnet granulites exhibit significantly higher average susceptibilities with values up to 10^-1 SI. Most of the stronalites in borehole 5071_1_A exhibit gamma ray values comparable to the lower range observed in 5071_1_B, whereas gamma ray values for all other lithologies in 5071_1_A are generally lower than those in 5071_1_B. These findings suggest that gamma ray and magnetic susceptibility data may also act as good lithological indicators when analysing the combined data set. Core density measurements further complement this analysis, with values ranging between 2.8 and 3.4 g/cm³ in 5071_1_A, compared to 2.5 to 2.8 g/cm³ in 5071_1_B. The P-wave velocity of 5071_1_A predominantly ranges from 6000 to 7000 m/s, exceeding those observed in borehole 5071_1_B where velocities are strongly influenced by brittle deformation rather than lithological factors. Although numerous fractures are encountered in 5071_1_A, an initial analysis suggests correlations with lithological variations, as evidenced by high P-wave velocities exceeding 7000 m/s in a pyroxenite section. This implies that seismic reflections in 5071_1_A may be attributable to lithological velocity contrasts. To further investigate the origins of seismic reflectivity in these rocks, an acoustic impedance profile for both boreholes is required. This will help evaluate the influence of brittle deformation and lithological variations on seismic reflectivity.