地球科学最新文献

The Fujairah basin in the Gulf of Oman experienced a complex tectonic evolution related to Late Cretaceous ophiolite obduction and Oligocene-Miocene Zagros continental collision. The structure of the foreland basin in Oman-UAE is well-known, but the structure and evolution of the hinterland basin behind the obducted ophiolite and underlying thrust sheets are less understood. Therefore, we use a combination of seismic reflection interpretation, gravity and magnetic forward modeling, and backstripping of well data to investigate the spatio-temporal deformation pattern in the basin and its connection with regional tectonics operating at the Central Iran/Arabia Plate boundary. The ophiolite complex in the Gulf of Oman is characterized by high-amplitude Bouguer gravity anomalies (>120 mGal) and short-wavelength (∼10 km) magnetic anomalies with a predominant north-south orientation along the coast. The top of the ophiolite/Cretaceous oceanic crust ranges in depth from 1 to 10 km, and up to 9 km of Upper Cretaceous-Holocene sedimentary successions underlies the shelf of the Gulf of Oman margin. Normal faults are interpreted within the Neogene and Quaternary, while reverse faults are confined to the lower Miocene to Upper Cretaceous. NW-SE to NNW-SSE-oriented reverse faults exhibit dominant dips ranging from 40° to 80°, while normal faults oriented mainly in the N-S direction display dominant dips of 40°–65°. Seismic and backstripping analysis of biostratigraphic data in wells reflects an Oligocene-early Miocene compressional event due to the collision of the Arabian and Central Iran plates and late Miocene-Holocene extensional event. Onshore in the Musandam Peninsula, west-vergent thrusts such as the Hagab thrust were active synchronously with N-S-oriented normal faults along the east coast of Musandam. A more recent extensional event is linked to transtensional movement resulting from the varying convergence rates along the Minab-Zendan strike-slip fault.

To discuss slip behaviors in shallow slow earthquake regions, we investigate source characteristics of shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula in the Nankai subduction zone. Very low frequency earthquakes are a kind of slow earthquake and are clearly observed at frequencies below 0.1 Hz. A non-linear inversion technique for moment rate function estimation and the permanent ocean-bottom seismometer network provided us with precise locations and detailed kinematic source characteristics of shallow VLFEs. The high activity of shallow VLFEs around the western edge of the subducted Paleo-Zenisu ridge is similar to previous studies. A notable trend change in the along-dip dependency of shallow VLFE moment rates was found. Along the profile west side of the Paleo-Zenisu ridge, moment rates of shallow VLFEs increase with reaching the megathrust zone. Small-scale topographic fluctuations of the subducted oceanic plate exist along this profile, but large-scale seamount subduction has not been identified even from dense seismic surveys. Similar tendencies have been reported in tectonic tremors in the Nankai and Cascadia subduction zones. On the other hand, the opposite trend appeared along the profile with the Paleo-Zenisu ridge. Small shallow VLFEs were dominant near the summit of the Paleo-Zenisu ridge. Fracture networks or stress fields due to seamount subduction possibly impede large shallow VLFEs around the subducted seamount. Our results suggest that the large-scale heterogeneity of the upper surface of the subducted oceanic plate could control source characteristics of shallow slow earthquakes.

4-D seismic tomography study was conducted in the northwestern region of Iran using travel time tomography during 1996 to 2016. The study area is situated within the active tectonic and seismic zones of the collision between the Eurasian and Arabian plates. This region has experienced multiple significant and destructive earthquakes in the past. Furthermore, it includes Lake Urmia, which has undergone a substantial decrease in water level by about 8 meters during the study period. To minimize the artifacts caused by uneven distribution of seismic rays, similar data sets including about 700 earthquakes in each period were selected for five periods. Subsequent experiments utilizing synthetic models demonstrated that the selected subsets of data for different time periods have the potential to identify anomalies in different periods. 3-D tomography was performed for each selected data sets, and the results were compared across different time periods. The results, focused on two depths of 8 and 23 kilometers and receiving significant attention, exhibit certain similarities in the observed anomalies across different time periods. These anomalies correspond to the tectonic structures of the region. Some differences are also observed among the various time periods. The trends of most anomalies and the differences in velocity models across different time periods are consistent with the general trends of the active faults in the area. The majority of the differences in velocity models for different time periods are observed at a depth of 8 km, along northwest-southeast-trending faults.

Comprehensive understanding of the effects of alluvial fan geomorphology on the soil and vegetation properties can lead to better conservation and management of vegetation and soils. This study aims to analyse the effects of geomorphological landforms and processes of Davarzan alluvial fan on the properties of soils and vegetation. Aerial images were obtained by a DJI Phantom4 Professional drone to measure vegetation density (VD) and canopy cover (VCC). Subsequently, six topographic profiles from fan surfaces were provided by the 10-cm DEMs, prepared by aerial images. A total of 36 soil samples were gathered from different landforms (bar, swale, channel and interfluve) and positions (toe and apex) of fan surfaces. Soil properties including sand%, silt%, clay%, total organic carbon (TOC), soil hydraulic conductivity (K) and soil water content (W) were measured. Results show that TOC, clay% and silt% are higher at the old surface than the relict and young surfaces. Lower TOC, clay% and silt% of the relict surface (especially at its toe) can be attributed to its entrenched and crenulated surface with relatively steep topography, making it prone to greater soil erosion. Nevertheless, the means of vegetation canopy cover and vegetation density are higher at the relict surface than old and young ones, likely due to the fact that beds of deep channels developed on the relict surface have shaded conditions and are closer to groundwater. Data reveal that VD and VCC are higher at toes than apexes, at channels than interfluves of abandoned surfaces, and at swales than bars of young surface. Overall, the relative age, landforms, channel morphometry, geomorphological processes (i.e., weathering and aggradation) and positions (toe and apex) of alluvial fan are dominant factors affecting soil and vegetation characteristics.

Rainfall and snowmelt runoff are the primary drivers of soil detachment in cold regions. Understanding how hydraulic properties influence the soil detachment rate (D_r) is essential for accurately modelling soil erosion during the spring thaw period. This study aimed to clarify the relationship between hydraulic parameters and D_r at different slope positions. Experiments were conducted under four flow discharge conditions (4.5, 6.5, 8.5 and 10.5 L min⁻¹), two slope gradients (10° and 15°) and four thawing depths (2, 5, 10 and 15 cm). Results indicated Dr could be adequately described as a power function of the flow discharges increasing with shear stress, the resistance coefficient and stream power. Shear stress was identified as the most effective hydrodynamic parameter for predicting D_r at upslope (first slope) and midslope (second slope) positions, whereas stream power best predicted D_r at the downslope (third slope) position. Unit stream power was not a reliable predictor of D_r. These findings enhance the mechanistic understanding of soil erosion processes occurring during the spring thaw period and improve the predictive capabilities of soil erosion models.

The epithermal Au–Ag mineralization at the Pechalnoe deposit is of considerable interest, since it was formed in carbonaceous terrigenous rock sequences making the basement of a volcanic dome structure at a distance of about 200 km from the boundary of the Okhotsk–Chukchi marginal continental volcanic belt. The geological structure of the Pechalnoe deposit consists of two levels, quartz–adularia and quartz Au–Ag veins are localized in keratinized terrigenous rocks of the lower level, while the quartz rhyolites and comendites of the Pechalninskaya rock sequence in the upper level harbor potential industrially valuable REE mineralization. The productive veins make three zones striking nearly east–west; the veins in these zones are 200–300 m long, occasionally reaching 640, 840 m; the mean thickness is 0.1–3 m, rarely reaching 6.2 m; the mean concentrations are Ag 266 g/t, Au 4.4 g/t. The following mineralogical features of the ores have been identified: low sulfides (1‒2%); the productive minerals are native Ag, low fineness Au, polybasite, and high-selenium acanthite. In addition, the ores have abundant enough arseniferous pyrite, arsenopyrite, pyrrhotine, ferruginous sphalerite, chalcopyrite, and marcasite. The geochemical features of the ores are in good agreement with their mineral composition. The ores are enriched in a wide range of trace elements (arranged in decreasing concentration): Ag, Au, As, Sb, Sе, W, Tl, Li, Be, Bi, Cs, and Mo; light lanthanoids prevail over heavy ones; very low Eu/Sm ratios (( ll 1)), little inclined near-chondrite distributions (without distinct europium maxima or minima); the relationship between Ce/Ce* and Eu/Eu* indicates oxidizing conditions during the mineralization; the REE spectra are dominated by light “hydrophile” lanthanoids of the “cerium” group; ΣREE varies in a wide range. The mineralogical and geochemical data obtained enable us to classify the Pechalninskaya mineralization as belonging to the selenium subtype of the low sulfide class of epithermal deposits. According to geological and mineralogic-geochemical evidence, the deposit can be classified as poorly eroded, which suggests future identification of new ore bodies not at the ground surface.