The Main Recent Fault (MRF) is a major strike-slip fault system within the Zagros collision zone, having generated several destructive earthquakes in western Iran. However, the subsurface architecture of the Silakhur region remains poorly constrained. This study applies local earthquake tomography to investigate the three-dimensional crustal velocity structure beneath the Silakhur region, northwestern Zagros. The analysis is based on aftershock data recorded by two temporary seismic networks deployed following the 2006 Mw 6.1 Silakhur earthquake. A total of 335 well-constrained aftershocks were inverted using the LOTOS algorithm to derive P-wave and S-wave velocity models of the upper crust. The tomographic results reveal significant lateral velocity heterogeneity associated with major tectonic structures. A major velocity boundary coincides with the mapped trace of the Doroud Fault, indicating strong structural control on crustal velocity distribution. A prominent low-velocity zone is imaged beneath the central Silakhur Plain, broadly following the Abi-Diz River and overlapping with areas previously interpreted as concealed fault-related structures. Additional velocity discontinuities along the northern margin of the plain correspond to a previously proposed concealed fault zone. Comparison with regional geological observations indicates a close relationship between seismic velocity variations and geological structure. The spatial correspondence between velocity anomalies and mapped or inferred fault systems suggests that the reconstructed velocity model captures the principal tectonic features of the Silakhur region. The results provide independent geophysical constraints on fault-controlled deformation and support previous interpretations suggesting the development of a pull-apart basin within this segment of the Main Recent Fault system.
Many Arctic sedimentary sequences exhibit complicated magnetizations, including long-lasting reversed intervals during expected normal polarity chronozones and frequent apparent geomagnetic excursions, although the underlying reasons vary. This study investigates the reliability of Pleistocene sediments as paleomagnetic recorders, focusing on u-channel samples from Site U1624 (IODP Expedition 403, Isfjorden drift), drilled in the Fram Strait between Greenland and Svalbard.We used Arai plots constructed from stepwise anhysteretic remanent magnetization (ARM) acquisition and alternating field (AF) demagnetization up to 100 mT, to assess the reliability of paleomagnetic data. The paleomagnetic data show that the analyzed samples from Site U1624 exhibit substantial gyroremanent magnetization (GRM) during AF demagnetization of the natural remanent magnetization (NRM). This GRM acquisition significantly distorts the ARM-based Arai plots, rendering them unreliable for paleointensity estimation and detailed geomagnetic reconstructions within affected intervals. We identify correlations between GRM intensity and specific rock magnetic parameters, allowing us to (i) detect stratigraphic intervals dominated by single-domain ferrimagnetic minerals—such as clusters of diagenetic greigite grains—that induce GRM and bias ARM measurements, and (ii) evaluate which parameters serve as effective proxies for the concentration of GRM carriers.In intervals where GRM acquisition is negligible, Arai plots based on ARM remain a robust tool for assessing the fidelity of the paleomagnetic signal and for constraining geomagnetic field variations. The proposed set of rock magnetic criteria therefore provides a framework to support the distinction between primary and diagenetically overprinted magnetizations in Arctic sedimentary records, improving their use for high-resolution geomagnetic and stratigraphic studies.
Ultrasonic measurements combined with synchrotron X-ray diffraction and radiography in a multi-anvil apparatus were conducted to determine the elastic wave velocities of sodium silicate melts with compositions Na2Si2O5 (NS2) and Na2Si4O9 (NS4) at pressures up to 7 GPa and temperatures to 2100 K. The two melts differ markedly in their degree of polymerization, with NS4 being substantially more polymerized than NS2. The compressional wave velocity (VP) of the NS2 melt increases monotonically with pressure, whereas the NS4 melt exhibits a pronounced non-monotonic pressure dependence, with a local velocity minimum in the low- to intermediate-pressure range. This contrasting behavior reflects fundamental differences in their structural response to compression. The relatively depolymerized NS2 melt undergoes simple densification, while the more polymerized NS4 melt experiences pressure-induced reorganization of its intermediate range structure, including the collapse of large silicate rings at low to intermediate pressures. These results indicate that the compositional boundary separating melts with anomalous, non-monotonic pressure dependence of elastic wave velocities from those showing crystal-like monotonic behavior lies between NS2 and NS4. The velocity minimum in the NS4 melt is therefore attributed to structural modifications associated with its higher degree of polymerization.
In this study, we investigate the spatial and temporal characteristics of geomagnetic secular acceleration (SA) and its temporal changes (Delta SA) at the core-mantle boundary (CMB) over the past 125 years. For the period 2000-2025, we use the CHAOS-8.5 geomagnetic field model to show that SA activity is strongly localized within a limited number of low-latitude regions organized in paired structures of opposite polarity, mainly beneath the Atlantic, Pacific, Caribbean, and Indian sectors. These regions exhibit persistent anti-correlated temporal behaviour and concentrate most rapid geomagnetic field changes associated with recent geomagnetic jerks. A regional principal component analysis (PCA) reveals that the variability in the Atlantic, Pacific, and Caribbean sectors is largely dominated by a single regional dipolar mode, whereas the Indian sector requires two modes of comparable importance. The temporal evolution of these modes highlights the contribution of each region to the sequence of 21st-century jerk events. We extend the analysis to the 20th century using the Kalmag and COV-OBS-x2 geomagnetic field models. Despite larger uncertainties prior to the modern dedicated satellite era, long-term averages and additional regional PCA reveal persistent recurrent SA structures throughout the century. In particular, a dominant Caribbean-North America paired structure characterizes much of the first half of the 20th century, while Pacific and Indian sectors becomes increasingly important after the 1970s, anticipating the spatial patterns observed during the 21st century. Our results suggest that SA variability at the CMB is organized around a small number of long-lived regional dipolar structures whose relative amplitudes evolve through time.
