Understanding orogen dynamics requires quantifying the relative contributions of heat sources that drive metamorphism in the continental crust. The importance of viscous heating in the orogen-scale heat budget remains debated, largely because it is difficult to identify in the field. Short metamorphic durations at near-peak conditions have been proposed as a potential diagnostic of viscous heating, but other processes such as rapid thrust advection or magmatic intrusions can also produce brief thermal events. To better constrain metamorphic time scales and discriminate between heating mechanisms, we develop a paired albite-apatite Sr-diffusion chronometry approach and apply it to a gneiss (peak temperature [T] ∼530 °C) from the Carboniferous Variscan nappe stack of the French Massif Central. We characterize the mineral textures, constrain their crystallization along the prograde path, and document sharp Sr growth zoning. Forward diffusion modeling shows that the observed profiles, especially the preservation of Sr-poor moats in albite, are reproduced if the rock resided for ∼0.1−0.2 m.y. at ΔT ≤40 °C relative to peak, with near-peak heating and cooling rates of ∼500 °C/m.y. These rates exceed those predicted by thermo-kinematic thrust advection models and, given the lack of Variscan magmatism in the study area, are best explained by a viscous heating pulse. Paired albite-apatite Sr-diffusion chronometry can therefore resolve rapid thermal histories in garnet-free rocks and provide a mineral-scale criterion to identify viscous heating in metamorphic belts.
Earthquake surface rupture results from fracturing of the shallow crust, yet it is rarely studied within a fracture mechanics framework. Here, we combine mobile laser scanning and optical image correlation displacement data to analyze surface rupture fractures in the maximum slip zone of the 2019 M 7.1 Ridgecrest, California (USA), earthquake. The study area comprises a right-bending principal rupture trace with right-lateral and vertical slip of 1−5 m and 0−1 m, respectively, with decreased horizontal and increased vertical slip in the releasing bend. Fracture length tends to vary inversely with right-lateral slip. Segmented, echelon fractures up to ∼5 m long dominate outside the bend, whereas continuous fractures up to ∼20 m long occur within the bend. We estimate the fault-local 3-D displacement gradient tensor as a proxy for near-fault-tip loading and fracture propagation mode. We find that mode III fracturing dominates outside the bend, whereas mode I, mode II, and mixed-mode (I-II-III) fracturing characterize the bend interior. This transition may explain the fracture length distribution. During mode III propagation, fractures twist from the parent tipline into discontinuous segments, whereas for pure modes I and II, the propagating tipline remains intact, favoring longer fractures. Spatial variations in fracture mode may reflect parent fault geometry, neighboring fault strand interactions, rheological conditions, and dynamic effects. Our results show that local fracture mode, rather than slip magnitude alone, influences surface rupture patterns, with implications for fault displacement hazard analysis.
Southern Tibet is marked by an ∼15−20-km-thick high-velocity layer in the lowermost crust at depths of 50−70 km, forming a distinct seismic Moho doublet. However, the origin of this geophysical anomaly remains elusive. Here, we use pressure-sensitive geochemical proxies and isotopic fingerprints to show that 60 Ma and 49 Ma granitoids, collected from the same outcrop (<5 m apart) in the southwestern Gangdese batholith, formed through anatexis of juvenile and ancient continental crust at depths of ∼40−50 km and 55−65 km. However, their mantle-like zircon δ18O values argue against the prevailing hypothesis linking the isotopic shift in Gangdese granitoids to input from Indian or Tibetan supracrustal components. Given the minimal post-collisional upper-crustal deformation in southern Tibet, we propose that the Indian lower crust likely decoupled from its upper crust and underthrust beneath Tibet shortly after the onset of India-Asia collision, leading to the development of the Moho doublet much earlier than previously thought.
Competing hypotheses for the origin of iron oxide-apatite (IOA) deposits include magnetite flotation, immiscible iron-rich melts, salt melts, hydrothermal fluids, or metasomatism. However, no individual hypothesis explains all observations in natural IOA deposits. IOA deposits are spatially associated with evaporites and iron oxide−phosphate-rich (FeP-rich) sediments, but no clear genetic link exists. We ran piston-cylinder experiments at 0.4 GPa and 1000−1200 °C to react silicate magmas with sulfate evaporites and FeP-rich analogues of red beds and phosphorites. Quenched products show chlorine-rich silicate glass, dendritic magnetite, iron oxide-phosphate−silicate (FeP−Si) liquid immiscibility, Fe-bearing salt melts, and fluid bubble adherence to magnetite and apatite. These reproduce key features of all proposed competing hypotheses and are similarly observed in natural IOA deposits. Our experiments reveal that these distinct genetic models are not mutually exclusive but could instead work together to form IOA deposits. When silicate magmas intrude a ferrous sedimentary sequence with evaporites and phosphates, immiscibility, magnetite-fluid interaction, and salt melts can all form.
Red beryl is a rare gem found in vugs or fissures in specific topaz rhyolite layers in the southwestern United States. Unlike other beryl, it contains high U (tens to thousands of parts per million). This study provides U-Pb dates for red beryl crystallization from five localities in three ranges, yielding robust laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) dates at higher precision than coeval sanidine K-Ar and zircon fission-track dates. U-Pb dates for specimens from the Ruby Violet Claim (20.96 ± 0.23 Ma) of the Wah Wah Mountains, Utah, Snowflake Claim (24.96 ± 0.3 Ma) of the Black Range, New Mexico, and Searle Canyon (6.69 ± 0.12 Ma) and Maynard’s Claim (6.87 ± 0.11 Ma) in the Thomas Range, Utah, are compatible with prior regional age constraints. However, red beryl dates from Wildhorse Spring, Utah (22.34 ± 0.20 Ma), are older than the expected age of ca. 6 Ma. These data reassign the red beryl−bearing layer to Spor Mountain volcanism, the host to the world’s largest Be deposit, indicating that a wider region should be explored for Be mineralization. Red beryl provides direct age limits on outgassing, capturing very late-stage volcanic processes not recorded by common geochronometers, which typically form within magma chambers or during eruptions. Our results show that if red beryl, a gem, or other unusual minerals have elevated U, they should be targeted as possible geochronometers.
