Integrated Rock Mass Characterization of the Lower Continental Crust Along the ICDP-DIVE 5071_1_B Borehole in the Ivrea-Verbano Zone

Abstract

The first borehole 5071_1_B of the ICDP-Drilling the Ivrea-Verbano zonE (DIVE) project in Italy, which intersects the Massone antiform, provides a unique opportunity to integrate downhole geophysical measurements with observations from 100% recovered drill core in rarely drilled lithologies. The objective of this study is to petrophysically and structurally characterize the rock mass and constrain factors influencing the seismic velocity in the lower continental crust. A comprehensive data set, comprising core, well log and vertical seismic profiling data, was collected. The structural analysis indicates that the axial plane of the intersected tightly folded antiform is slightly tilted at the borehole location and thus the borehole intersects the hinge zone at the top and its limb in the lower part of 5071_1_B. Numerous open natural fractures with variable dips and two dominant dip azimuthal orientations are identified along the borehole, which affect the electrical and acoustic properties. The velocities at the core, well log and seismic scale are consistent but lower than intrinsic seismic velocities of the lower continental crust, since they are not only affected by fractures but also by micro cracks at the 5071_1_B in situ conditions. A systematic lithology correlation is not evident for these properties. However, a cluster analysis of gamma ray and magnetic susceptibility logs shows an excellent agreement with the logged core lithologies in the presence of remarkable spatial variability. Furthermore, the main lithologies are grouped into three distinct clusters, suggesting two types of kinzigites with distinct magnetic and radiogenic properties.