Throughout geological history, changes in metamorphic conditions plays a crucial role in reflecting the evolution of the tectono-thermal conditions of Earth. The determination of temperature and pressure conditions for evaluating metamorphism typically relies on a limited number of samples, and the testing and analysis process requires significant time and work for thin section preparation, thin section analysis, geochemical analysis, as well as pseudosection calculations based on thin section and geochemical constraints. Following these procedures, it is commonly assumed that localized results can represent thermobaric conditions at regional or even orogenic belt scales. This assumption holds true when metamorphism reflects rock burial depth (i.e., lithostatic pressure). However, on small spatial scales, deviational stresses resulting from tectonic extension or compression can induce structural underpressure or overpressure, thereby rendering this assumption invalid. Therefore, we introduce magnetic susceptibility (k) as a measure of material magnetization in an applied magnetic field. It exhibits two key characteristics: (i) Fe-Ti oxides in mafic rocks display a mineral phase sequence largely associated with changes in pressure, with approximate pressure ranges for ilmenite + magnetite (<6 kbar), ilmenite (6–12 kbar), rutile + ilmenite (~12 kbar), and rutile (>12 kbar); (ii) Magnetic susceptibility can be quickly and extensively measured to reflect variations in Fe-Ti oxide content within mafic rocks. Our findings demonstrate that Fe content decreases while Ti content increases with increasing pressure. Thus, as pressure increases during prograde metamorphism, Ti generally replaces Fe, and this process can be detected as systematic decreases in magnetic susceptibility. This pressure–susceptibility correlation is further corroborated by retrograde metamorphism, where k inversely increases with decreasing pressure during exhumation. Based on our comprehensive petrological and geophysical analysis of mafic rock samples collected from the Paleoproterozoic (ca. 1.8 Ga) Jiao–Liao–Ji Belt of the North China craton, including coupled thin section, geochemical, and thermal susceptibility (kT) fingerprinting of Fe-Ti oxides, we posit that magnetic susceptibility can serve as a reliable indicator for discerning metamorphic pressure. Furthermore, owing to its ease of rapid and extensive data acquisition in fieldwork settings, we contend that it can play a pivotal role in identifying potential deviations from lithostatic stress due to tectonic stress on either small and/or large spatial scales.

Abstract In a deforming partially molten rock, melt concentrates into a grain‐scale melt pocket aligned at a preferred orientation (melt‐preferred orientation, or MPO). However, observing this texture alone provides limited information on the 3D orientation and geometry of these melt pockets, which are critical parameters for estimating permeability. Here, we modeled the MPO of experimentally deformed peridotites by simulating melt streaks arising from melt pockets of various shapes and 3D orientations. The model aims to identify 3D distribution and characteristics of melt pockets that could account for the observed length, thickness, and the probability of melt streaks. Results show that melt pockets at preferred orientation exhibit greater length, thickness, and number density compared to those perpendicular. These results can be incorporated into the simulation of melt flow through individual melt pockets, which allows us to estimate the permeability corresponding to the observed MPO. We found that the permeability of vertically compressed peridotites increases with increasing compressive strain and a more elongated and thickened shape for melt pocket aligned at preferred orientation. The vertical permeability in the sample with 30% compressive strain is at least 40 times larger than that of an undeformed sample. For peridotites deformed under simple shear, the permeability exhibits an anisotropy of at least three. Such anisotropic permeability, coupled with the formation of melt‐rich bands and other melt channels, is believed to cause lateral melt focusing beneath mid‐ocean ridges.

The Great Unconformity, a profound gap in Earth’s stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a century. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terrestrial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of three to five vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard.