Journal of Geodynamics最新文献

New geochemical and U-Pb isotopic data from the Torud volcanic and subvolcanic rocks and their associated dikes exposed along the southern margin of the Sabzevar-Torud zone provide new evidence for Early-Middle Eocene, post-collision magmatism in northeast Iran. The 52–46 Ma (late Ypresian-Lutetian) U-Pb age interval of zircons separated from these rocks confirm the paleontological age of nummulite-bearing limestone interlayers. Inherited zircons separated from these igneous rocks have a much wider range of U-Pb ages that include Archean, Paleoproterozoic, Mesoproterozoic, Carboniferous, Permian, Triassic, Middle Jurassic, and Late Cretaceous. Most volcanics and subvolcanic rocks display high-K calcalkaline and shoshonitic trends. Distinct crystal fractionation patterns of the volcanic and subvolcanic rocks suggest magmatic differentiation in separate magma chambers. The studied rocks, depleted in the HFSE and enriched in the LILE, display nearly homogeneous isotopic Sr (0.7039–0.7055) and Nd (0.5126–0.5129) ratios and positive ɛNd values (0.08–0.56) indicating partial melting from an enriched lithospheric mantle source that was slightly contaminated with crustal material. Fluids, especially those released from the subducted slab, affected the composition of the source for the studied rocks. The magmatism occurred in the Torud area after the Late Cretaceous-Paleocene collision of the Central Iran microcontinent and Binalud-Kopeh-Dagh blocks and the Early-Middle Eocene break-off of the subducted slab.

In this study, we obtained new 3D seismic tomography models of the crust and uppermost mantle beneath the northwestern Himalayas down to a depth of 120 km. The data were provided by the India Meteorological Department (IMD) and complemented by the Global International Seismological Centre (ISC) Catalogue. The distribution of anomalies correlates with the main geological features of the region. Specifically, the mountain ranges of the Greater and Lesser Himalaya stand out as a low-velocity anomaly, and the Indian Plate is visible as a high-velocity anomaly underthrusting the Himalayas. The Indian Plate not only underthrusts northwards below the Himalayas, but also bends westwards as it gets closer to the Hindukush Region. A peculiar feature of the model is a high-velocity anomaly in the Kaurik Chango Rift, interpreted as a remnant of the oceanic crust, left after the Indotethys Ocean’s closure. In the seismically active Delhi-Haridwar Ridge, a low-velocity upper crustal layer is possibly associated with the sediments of the Indo-Gangetic Plain and with a large number of fault structures. The fragmentation of the Delhi-Haridwar Ridge softens the movement of the Indian Plate to the north, so that the Tethyan Himalaya crust in the area of the Kaurik Chango Fault has remained consolidated and manifests itself as a high-velocity anomaly.

Data Availability

The full directory of LOTOS code with data corresponding to this study is available at https://doi.org/10.5281/zenodo.5519210 (accessed on 23 September 2021).

In seismic hazard assessment, subsurface geophysical surveying has gained popularity in recent years towards fault mapping and determining seismic deformation parameters such as fault offset, recurrence, and depth of fault, locating proper trench sites based on the subsurface information. In the present study, electrical resistivity tomography (ERT) was used to (1) locate the trace of the southern segment of the Ulsan fault, (2) to test the applicability of ERT techniques for active fault mapping in a close to the highly urbanized and complex geological environment with a slow tectonic activity. We have applied the ERT technique at five sites. At one place, we used the Wenner array, Schlumberger array, Dipole-Dipole array, and Seismic-profiling techniques to know which method provides a better result in complex geological conditions like Korea. Out of these methods, the Dipole-Dipole array provided high-resolution results and was used for the other two sites. The ERT result shows high and low resistivity zones interpreted as bedrock (mainly Tertiary and Cretaceous formations) and coarse fluvial sediment layer, respectively. The maximum vertical displacement recorded along the fault varies from 6 m to 12 m. Based on the ERT results, two trenches were excavated to directly investigate the deformation pattern associated with the southern segment of the Ulsan fault. The ERT and trench survey results support that the southern Ulsan fault has slipped multiple times since Quaternary. Using this multi-approach, ~5 km long active fault map was prepared for the southern Ulsan Fault. It is found from this study that the integrated approach is highly beneficial where contrasting sub-lithological units exist in terms of their physical properties, even though human activity or the ongoing urbanization process has modified the surface morphology. This study argues for judicious use of ERT techniques to delineate the shallow subsurface geology across various active faults in the Korean peninsula and similar tectonic settings.

On 22 May 2021 (CST), an Mw7.4 earthquake struck Maduo County, Qinghai Province, China, which was the largest seismic event in China since the 2008 Mw7.9 Wenchuan earthquake. Several scientific questions associated with the event could be addressed: (1) what fault slip model can explain the Maduo earthquake? (2) what effects do historical earthquakes impose on the Maduo earthquake? and (3) what implications does the Maduo earthquake have for future rupture potential of adjacent tectonic faults? So we conduct a comprehensive study to answer the three questions by collecting satellite SAR images, GPS data, seismic waveform data, historical earthquakes, and aftershocks associated with the Maduo earthquake. The estimated fault slip model shows that the Maduo earthquake ruptures two faults in a manner of dominant sinistral strike-slip motion, with slip peaks of ~4.8 m occurring near the surface. The minor fault to the east dips to the south accommodating an obvious reverse slip, well consistent with reverse fault scarps, reverse faulting aftershocks, and significant upward surface displacements immediately south of this branch. Such a reverse slip is probably controlled by the motion of nearby major sinistral strike-slip faults (the Eastern Kunlun fault and the Maduo–Gande fault). Among 32 historical Mw≥ 6.0 earthquakes used in this study, we find that the 1937 Mw7.8 Huashixia earthquake may affect the Maduo earthquake most, delaying its occurrence by decreasing the Coulomb failure stress (CFS) at the hypocenter by > 1 bar and on the entire causative fault by an average of 0.68 bar. Besides, the Mw6.1 Yangbi earthquake, which occurred ~4.5 h ahead of the Maduo earthquake, appears to make little influence on the Maduo earthquake because it hardly perturbates the CFS at the hypocenter of the Maduo earthquake. Furthermore, the cumulative CFS change due to both the 32 historical earthquakes and the 2021 Maduo event indicates that the Tuosuo Lake–Maqu segment of the Eastern Kunlun fault may be at high risk of future rupture.

The geochemistry of the Permian–Triassic large-scale igneous rock body of northern Mongolia is a key factor in understanding the subduction-related magmatism at the margin of “Siberian continent.” Several studies have been done in the Permian–Triassic igneous body; however, its detailed magmagenesis and tectonic significance remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen Formation) and the Late Permian–Middle Triassic plutonic rocks (Selenge plutonic rock complex) of the Permian–Triassic igneous body, and intermediate dike intruding into them, and discusses the Late Permian–Middle Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low K₂O/Na₂O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen Formation is calculated to be 37.1° N based on thermal remanent magnetization. Therefore, the Late Permian–Middle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic rock complex, indicating that adakitic igneous activity continued after the Early Triassic.

The Ordos Block in the western part of the North China Craton is enigmatic in having contrasting topographic structure in the northern and southern parts, while previous geophysical studies show little difference in crustal and upper mantle structure across the region. Here we present a new model of upper mantle structure in the Ordos Block region in order to test the importance of mantle heterogeneity for topographic differences. Our model is based on P- and S-wave seismic receiver functions calculated for data from 171 stations. It documents the presence of an uppermost mantle low-velocity zone between the Mid Lithospheric Discontinuity (MLD) and the Lehmann discontinuity. Clear converters at the 410 and 660 km discontinuities show constant Mantle Transition Zone (MTZ) thickness within the Ordos Block region, which indicates that no deep mantle thermal anomaly affects its present dynamics. However, the amplitude of the MTZ-converters is higher in the southern than the northern Ordos Block. In contrast, the conversions from MLD and the Lehmann discontinuity are strongest in Northern Ordos, which we interpret as a block with essentially preserved cratonic lithospheric mantle. We speculate that smaller amplitudes of the MLD and Lehmann converters in Southern than Northern Ordos may be related to either rheological weakening of cratonic lithosphere during the Mesozoic convergence between the North and South (Yangtze) China Cratons, or northeast extrusion of Tibetan lower crust and upper mantle in the Cenozoic caused by the India-Asia collision.

In early March 2021, three shallow earthquakes, two mainshocks with M6.3 and M6.0 and one major aftershock with M5.6 impacted both the mountainous Damasi-Tyrnavos region (northern Thessaly, Greece) and the adjacent Plio-Quaternary basin. Each major event was followed by rich aftershock activity recorded by local and regional seismographs and accelerographs. Herein, we present a comprehensive analysis of the seismic sequence, from its foreshock activity starting on 28 February, 2021 and for a period of two months using new high-resolution catalogues of relocated earthquakes and hundreds of focal mechanisms. The results indicate that the aftershocks form a zone that spans ~50 km NW-SE, while focal depths range between 5 and 15 km. More than 400 focal mechanisms, computed for events with M≥ 2.5, mainly exhibit normal faulting in a NW-SE direction, while WNW-ESE to E-W normal faulting is also evidenced, in particular after the occurrence of the last major event on 12 March. The stress-field was reconstructed on a local and broader scale by inverting focal mechanism data, revealing a rotation of the σ₃ axis trend from NNE-SSW, in the Damasi-broader region, to NW-SE northwards, to the region of Kozani-Grevena that hosted an M_w = 6.6 shallow mainshock in 1995. Subcrustal seismicity, present beneath those areas, implies that large-scale tectonics and plate dynamics are likely involved in the deformation of the upper crust. Coulomb stress transfer after the 3 major events of the 2021 Damasi-Tyrnavos sequence reveals that stress-loaded areas include those where most aftershocks were triggered. The analysis provides implications to the seismic hazard of the activated area, as a major NW-SE active normal fault close to Larissa city became stress-loaded, constituting a possible candidate source for significant future earthquakes.

Water levels in anthropogenic reservoirs are often studied in terms of the influence of their fluctuation to pressure perturbations in the bedrock and possible triggering of nearby seismic activity. In this paper, we examine the possibility of a similar relationship in the West Bohemia/Vogtland region on the border of the Czech Republic and Germany. This area is well known for the occurrence of earthquake swarms that are located mainly around Nový Kostel village with the Horka dam nearby, just 5 km to the E-SE of the seismogenic zone. We are looking for any evidence of a mutual relationship between rainfall and the water level at the Horka dam and the seismic catalog that contains more than 25,000 nearby events for the period 1995 − 2019. For this purpose, we applied the methods of cross-correlation and Singular Spectrum Analysis (SSA). The analysis was performed on both full and declustered seismic catalogs and on the full time series and the series averaged over a single-year period. No significant correlation was found between the hydrologic and seismic data; the seismic activity occurs randomly in time. The SSA method found strong seasonal variations of the water level in the dam with annual periodicity; however, no similar periodicity was found in the rainfall and seismicity data. Our results show that not only the earthquake swarms, but also the background seismic activity have no relationship to the rainfall or water level in the Horka dam. The hypothesis of hydrologic triggering of the seismic activity in the area appears rather unlikely and other mechanisms should be studied in more detail to account for the earthquake swarms’ occurrence.

The Black Sea is a deep marine basin formed by lithosphere extension and active rifting in a back-arc tectonic setting, by general consensus, in the Cretaceous. Its present structural architecture, however, is mainly defined by compressional tectonics during the Cenozoic when large scale “basin inversion” reactivated extensional fault systems formed in the Cretaceous. Rifting during the Cretaceous is usually taken to represent the main process forming the present-day basin (that is, producing crustal thinning and concomitant subsidence prior to its modification during Cenozoic inversion). Rifting at this time took place within continental lithosphere that had been accreted to and, by the Cretaceous, formed part of the Eurasian lithospheric plate. The precise history of how and when pre-Cretaceous aged tectonic domains were accreted to Eurasia forming the continental lithosphere underlying the Black Sea is poorly known. A critical issue to the tectono-thermal evolution of the Black Sea basin with important implications for paleogeography and sedimentary depositional environments is the degree of crust (and lithosphere) thinning during Cretaceous rifting and whether oceanic or “sub-oceanic” crust was formed at that time. The main focus of this paper, in order to illuminate this issue, is on kinematic observations related to the Cretaceous (Albian-Cenomanian) rifting phase, including subsidence analysis, as well as the immediate post-rift sedimentation and stratigraphy. The results suggest that rifting during the Cretaceous was insufficient in its own right to reveal exhumed mantle or to promote ocean crust formation beneath the deep basins of the Black Sea. It is concluded that an important contribution to observed present-day crustal and lithosphere architecture of the Black Sea area are legacy extensional tectonic events affecting the area in pre-Cretaceous times, with implications for the Late Palaeozoic-Mesozoic paleogeography and paleotectonic evolution of this area.