CONTINENTAL-INTERIOR DEFORMATION DEEPER DOWN: HINTS OF CRUSTAL BUCKLING AND TRANS-CRUSTAL SHEAR ZONES IN THE CRATONIC PLATFORM, MIDCONTINENT USA (Invited Presentation)

Basement of the midcontinent cratonic platform was assembled between ~1.8 and 1.4 Ga. This region, which was possibly cratonized by ~1.3 Ga, was subjected to two phases of Proterozoic continental rifting, and to the ~1.1 Ga Grenville orogeny. Did rifting episodes and the orogeny produce structures in the midcontinent's deep crust? We applied two techniques to address this issue. Receiver-function analysis, incorporating corrections for dipping surfaces, using the new H-k-c technique, provides a higher-resolution map of Moho topography in the northern midcontinent (Xiao et al., 2022; and in prep.). This map reveals systematic north-south-trending undulations in Moho depth in the cratonic platform west of the Grenville Province. These undulations, which have a wavelength of ~160 km and an amplitude of ~15 km, may represent variations in rift-related crustal delamination and magmatic underplating. Alternatively, they are crustal buckles. If so, their development implies that thermal weakening during development of the Midcontinent Rift system weakened the craton just prior to the terminal Grenville collision, making the craton susceptible to crustal buckling. Tectonism in the midcontinent, prior to the Grenville orogeny, may also have left a signature in cratonic-platform crust by producing trans-crustal shear zones. Magnetotelluric studies suggest that northwest-trending high-conductivity zones occur in the cratonic platform in the region west of the Grenville front (DeLucia et al., 2019; Murphy et al., in prep.). Some of these zones extend at a steep dip to, or perhaps even across, the Moho. Their steepness suggests that they initiated as transform or transcurrent faults. High conductivity in these zones may be due to graphite precipitated from mantle CO₂ that had leaked through damaged rock during rift-related shear-zone movement. Reestablishment of cratonic stability, after the Grenville orogeny, made Moho undulations and trans-crustal shear zones a permanent feature of the deep crust. Notably, since the shear zones lie within the craton and have not been annealed, they remain weak and have influenced the evolution of Phanerozoic fault-and-fold zones (some of which remain seismically active), as well as epeirogenic basins and arches, in the upper crust.