Exploration of S-wave velocity profiles at strong motion stations in Eskisehir,Turkey, using microtremor phase velocity and S-wave amplification

We have explored 1D S-wave velocity profiles of shallow and deep soil layers over a basement at strong motion stations in Eskisehir Province, Turkey. Microtremor array explorations were conducted at eight strong motion stations in the area to know shallow 1D S-wave velocity models. Rayleigh wave phase velocity at a frequency range from 3 to 30 Hz was estimated with the spatial autocorrelation analysis of array records of vertical microtremors at each station. Individual phase velocity was inverted to a shallow S-wave velocity profile. Low-velocity layers were identified at the stations in the basin. Site amplification factors from S-wave parts of earthquake records that had been estimated at the strong motion stations by Yamanaka et al. (2017) were inverted to the S-wave velocities and Q-values of the sedimentary layers. The depths to the basement with an S-wave velocity of 2.2 km/s are about 1 km in the central part of the basin, while the basement becomes shallow as 0.3 km in the marginal part of the basin. We finally discussed the effects of the shallow and deep sedimentary layers on the 1D S-wave amplification characteristics using the revealed profiles. It is found that the shallow soil layers have no significant effects in the amplification at a frequency range lower than 3 Hz in the area.     

Hydrogeology of karst aquifer systems in SW Turkey and an assessment of water quality and contamination problems

The aim of this research is to determine the relationship between groundwater flow and water quality of different ground and surface water basins in the southwest Turkey. In addition, groundwater vulnerability is assessed taking into consideration groundwater flow and quality. The autochthonous Beydaglari limestone is the major karstic aquifer in the region. According to the groundwater level map of alluvium aquifers in the basins, groundwater discharge toward the carbonate aquifer is direct and indirect. The hydrogeological connection between ground and surface water basins occurs via the karstic aquifer located at the bottom of the alluvium bottom. In Egirdir lake, water also discharges in the karstic aquifer via karstic sinkholes at the western border of the lake. In the research area, general groundwater discharge is toward the Mediterranean Sea by means of autochthonous carbonate system, according to hydrogeological investigations, research of lineament and hydraulic conductivities. This result is supported by the locations of lineaments and shore springs discharging from the limestone. In addition, spreading of contaminants via karstic aquifer to great distance has been clearly identified.     

Seismic response assessment of a stiff structure

Seismic response of a lightly reinforced stiff shear‐wall structure subjected to ground motions grouped as near‐ or far‐field according to their distance to causative faults is investigated. A model structure that had earlier been studied both experimentally and analytically in the context of a co‐ordinated research project is re‐examined. The structure is a five‐storey lightly reinforced shear‐wall model subjected analytically to 55 ground motion records from firm soil sites. Several response parameters are obtained by linear and non‐linear analyses. Additional analyses are performed to ascertain the validity and range of applicability of current approximate displacement‐based analysis procedures. The procedures considered in this study are found to be inadequate in representing the anticipated response of the structure employed here regardless of the type of excitation, so modifications are suggested for improved results. Copyright © 2006 John Wiley & Sons, Ltd.     

The Hadean upper mantle conundrum: evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Pb isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7-3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial εNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69-3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average εNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9-3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high ²⁰⁷Pb/²⁰⁴Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga.Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.     

The origin and compositions of Mesoarchean oceanic crust: Evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills.

The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ε_Nd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (ε_Nd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source.

In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial ε_Nd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.     

Origin of Archean tonalite–trondhjemite–granodiorite (TTG) suites and granites in the Fiskenæsset region,southern West Greenland: Implications for continental growth

Mesoarchean to Neoarchean orthogneisses (2.95–2.79 Ga) in the Fiskenæsset region, southern West Greenland, are composed of an older suite of metamorphosed tonalites, trondhjemites, and granodiorites (TTGs), and a younger suite of high-K granites. The TTGs are characterized by high Al₂O₃ (14.2–18.6 wt.%), Na₂O (3.4–5.13 wt.%), and Sr (205–777 ppm), and low Y (0.7–17.4 ppm) contents. On chondrite- and N-MORB-normalized trace element diagrams, the TTGs have the following geochemical characteristics: (1) highly fractionated REE patterns (La/Yb_cn = 14–664; La/Sm_cn = 4.3–11.0; Gd/Yb_cn = 1.5–19.7); (2) strong positive anomalies of Sr (Sr/Sr* = 1.0–15.9) and Pb (Pb/Pb* = 1.4–34.9); and (3) large negative anomalies of Nb (Nb/Nb* = 0.01–0.34) and Ti (Ti/Ti* = 0.1–0.6). The geochemical characteristics of the TTGs and trace element modeling suggest that they were generated by partial melting of hydrous basalts (amphibolites) at the base of a thickened magmatic arc, leaving a rutile-bearing eclogite residue. Field observations suggest that spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns (La/Yb_cn = 3.8–506; La/Sm_cn = 2.7–18.9; Gd/Yb_cn = 0.92–12.1) and display variably negative Eu anomalies (Eu/Eu* = 0.37–0.96) and moderate Sr (84–539 ppm) contents. Four outlier granite samples have variably positive Eu (Eu/Eu* = 1.0–12) anomalies. Given that the granodiorites have higher K₂O/Na₂O than the tonalites and trondhjemites, it is suggested that the granites were derived from partial melting of the granodiorites. It is speculated that the dense eclogitic residues, left after TTG melt extraction, were foundered into the sub-arc mantle, leading to basaltic underplating beneath the lower rust. Melting of the granodiorites in response to the basaltic underplating resulted in the production of high-K granitic melts. Formation of the Fiskenæsset TTGs, the foundering of the eclogitic residues into the mantle, and the emplacement of the high-K granites led to the growth of Archean continental crust in the Fiskenæsset region.     

Assessment of groundwater vulnerability based on a modified DRASTIC model, GIS and an analytic hierarchy process (AHP) method: the case of Egirdir Lake basin (Isparta, Turkey)

A DRASTIC-model method based on a geographic information system (GIS) was used to study groundwater vulnerability in Egirdir Lake basin (Isparta, Turkey), an alluvial area that has suffered agricultural pollution. ‘Lineament’ and ‘land use’ were added to the DRASTIC parameters, and an analytic hierarchy process (AHP) method determined the rating coefficients of each parameter. The effect of lineament and land-use parameters on the resulting vulnerability maps was determined with a single-parameter sensitivity analysis. Of the DRASTIC parameters, land use affects the aquifer vulnerability map most and lineament affects it least, after topography. A simple linear regression analysis assessed the statistical relation between groundwater nitrate concentration and the aquifer vulnerability areas; the highest R ² value was obtained with the modified-DRASTIC-AHP method. The DRASTIC vulnerability map shows that only the shoreline of Egirdir Lake and the alluvium units have high contamination potential. In this respect, the modified DRASTIC vulnerability map is quite similar. According to the modified-DRASTIC-AHP method, the lakeshore areas of Senirkent-Uluborlu and Hoyran plains, and all of the Yalvaç-Gelendost plain, have high contamination potential. Analyses confirm that groundwater nitrate content is high in these areas. By comparison, the modified-DRASTIC-AHP method has provided more valid results.