Magmatic evolution of the Andean Eastern Cordillera of Colombia during the Cretaceous: Influence of previous tectonic processes

The Eastern Cordillera of the Colombian Andes represents an inverted Cretaceous basin where Cretaceous magmatism is characterized by rare mafic dykes and sills. We use ⁴⁰Ar/³⁹Ar, Sr–Nd–Pb isotopes, as well as major and trace elements analyses of Cretaceous intrusions from both flanks of the Eastern Cordillera in combination with structural data to document the complex evolution of the basin. Magmatism, which is diachronous and geochemically diverse, seems to be related to mantle melting beneath the most subsiding segments of each sub-basin during enhanced extensional tectonics. The mafic intrusions display two different compositional series: an alkaline one with OIB-like pattern and a tholeiitic one with MORB-like features. This indicates at least two diverse mantle sources. Trace-element patterns suggest that the intrusions were emplaced in an extensional setting. ⁴⁰Ar/³⁹Ar dating on primary plagioclase and hornblende provides plateau ages between ～136 and ～74 Ma.The geochemical and temporal diversities show that the emplacement of the magmas was tectonically controlled, each sub-basin reflecting an individual subsidence event.     

Tectono‐sedimentary evolution of the northern Iranian Plateau: insights from middle–late Miocene foreland‐basin deposits

Sedimentary basins in the interior of orogenic plateaus can provide unique insights into the early history of plateau evolution and related geodynamic processes. The northern sectors of the Iranian Plateau of the Arabia–Eurasia collision zone offer the unique possibility to study middle–late Miocene terrestrial clastic and volcaniclastic sediments that allow assessing the nascent stages of collisional plateau formation. In particular, these sedimentary archives allow investigating several debated and poorly understood issues associated with the long‐term evolution of the Iranian Plateau, including the regional spatio‐temporal characteristics of sedimentation and deformation and the mechanisms of plateau growth. We document that middle–late Miocene crustal shortening and thickening processes led to the growth of a basement‐cored range (Takab Range Complex) in the interior of the plateau. This triggered the development of a foreland‐basin (Great Pari Basin) to the east between 16.5 and 10.7 Ma. By 10.7 Ma, a fast progradation of conglomerates over the foreland strata occurred, most likely during a decrease in flexural subsidence triggered by rock uplift along an intraforeland basement‐cored range (Mahneshan Range Complex). This was in turn followed by the final incorporation of the foreland deposits into the orogenic system and ensuing compartmentalization of the formerly contiguous foreland into several intermontane basins. Overall, our data suggest that shortening and thickening processes led to the outward and vertical growth of the northern sectors of the Iranian Plateau starting from the middle Miocene. This implies that mantle‐flow processes may have had a limited contribution toward building the Iranian Plateau in NW Iran.     

Altimeter's Capability of Reconstructing Realistic Eddy Fields Using Space-Time Optimum Interpolation

Sea surface height anomaly maps of realistic eddy activity were obtained by applying space-time optimum interpolation to altimeter data. Analysis error and rate of reconstructing eddy signals were investigated by taking account of: 1) dependency on orbit configurations of single and multiple altimeters; 2) dependency on space-time scales of realistic, dominant eddies; and 3) effect of space-time scales of eddy propagation. Large-scale sea surface height anomalies are subtracted from altimeter data by applying an along-track filter to allow easy handling of eddy signals. The space-time scales of the first-guess error in the optimum interpolation are statistically evaluated by fitting a space-time anisotropic Gaussian function to space-time-distributed correlation coefficients of sea surface height using the TOPEX data. The results of the optimum interpolation clarify the followings: 1) ERS has a better capability of reconstructing eddy signals than TOPEX. Comparison of maps from multi-altimeter data shows that TOPEX+ERS has a better capability than Jason−1+TOPEX in lower latitudes and vice versa in higher latitudes, though the differences are small. 2) The small space-time scale yields a low reconstruction rate in marginal seas and alongside the equator. The persistent timescale is large, and westward propagation is dominant in the subtropical and subarctic regions, where the reconstruction rates are high. 3) The optimum interpolation, taking account of eddy propagation, provides higher reconstruction rates than that taking no account of the propagation. The effect of propagation on the optimum interpolation is greater when it is applied to single-altimeter data than to multi-altimeter data. This revised version was published online in July 2006 with corrections to the Cover Date.     

A basic study on the application of LRFD in “the technical standard for port and harbour facilities in Japan”: a case of gravity type quay wall in a persistent design situation

ABSTRACT

The Partial Factor Method (PFM) based on the Level 1 reliability design method was introduced in 2007 to the Technical Standards for Port and Harbour Facilities (TSPHF-2007) in Japan. After nearly 10 years of practical use of TSPHF-2007, the design standard has been revised based on requests from the practitioners who recommend the transition from the PFM to the Load Resistance Factor Design (LRFD). In this paper, we discuss the setting method of the target failure probability to determine the partial factors based on code calibration. Furthermore, we examine the impacts of implementing the LRFD concept to the TSPHF by taking the sliding and overturning of a gravity type quay wall as an example to represent port and harbour facilities. We found no practical difference in caisson width derived using the LRFD and the PFM, whereas the degree of matching of the target failure probability was somewhat more precise for the PFM. This finding indicates that the LRFD is a more reasonable design method than the PFM in terms of the simplicity of the performance function itself and the ease of engineering interpretation during the design procedure.     

Transition of eruptive style in an arc–arc collision zone: K–Ar dating of Quaternary monogenetic and polygenetic volcanoes in the Higashi-Izu region, Izu peninsula, Japan

K-Ar ages were measured on Quaternary polygenetic and monogenetic volcanoes in the Higashi-Izu region, Izu peninsula, central Japan, using the unspiked sensitivity method with mass-fractionation correction procedure to investigate when eruptive style changed, whether a hiatus existed between the two types of eruptive activity, and the effect of tectonics on the change in eruptive style. The K-Ar ages range from 0.3-0.08 Ma for monogenetic volcanoes and from 1.8-0.2 Ma for polygenetic volcanoes; thus, no volcanic hiatus was found between the two types of eruptive styles. The transition from polygenetic to monogenetic volcanism occurred during a time of overlap between 0.3 and 0.2 Ma, after collision of the Izu block (the future Izu peninsula) with central Japan, estimated as 1.0-0.8 Ma by previous researchers. Based on the review of several tectonic models of the area, the measured age of transition in eruptive style is interpreted to correspond to the change in the stress field of the Higashi-Izu region.     

K–Ar geochronology of a Quaternary monogenetic volcano group in Ojika Jima District, Southwest Japan

 An unspiked K–Ar dating method using a mass-fractionation correction procedure was applied to a Quaternary independent group of monogenetic volcanoes, Ojikajima Volcano Group, located in northwestern Kyushu in Southwest Japan, in order to clarify in detail secular variations in eruptive volume, locations of eruptive vents, and magma compositions in a single monogenetic volcano group. The major results were as follows: (a) K–Ar ages of monogenetic volcanoes distribute from 1.08 to 0.30 Ma, with voluminous peaks at approximately 1.0 and 0.6 Ma. (b) The volcanic activity commenced in the central part of the field, expanded to the whole field at approximately 0.6 Ma, and then shrank to the central area. (3) Concentrations of incompatible elements, such as Ba, K, and Nb, increase with decreasing age, whereas P, Y, and Zr concentrations remain constant. These concentrations suggest gradual decrease in the degree of partial melting from an identical mantle source with residual garnet. Received: 15 December 1997 / Accepted: 23 May 1998     

Interannual-decadal variability of wintertime mixed layer depths in the North Pacific detected by an ensemble of ocean syntheses

Climate Dynamics - The interannual-decadal variability of the wintertime mixed layer depths (MLDs) over the North Pacific is investigated from an empirical orthogonal function (EOF) analysis of an...     

New <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age constraints on cooling and unroofing history of the metamorphic host rocks (and igneous intrusion associates) from the Bulfat Complex (Bulfat area), NE-Iraq

The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~?40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~?80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. ⁴⁰Ar/³⁹Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78?±?0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~?400 °C/Ma and ~?3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78?±?0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).     

Salinity in Soils and Tsunami Deposits in Areas Affected by the 2010 Chile and 2011 Japan Tsunamis

The accumulation of data sets of past tsunamis is the most basic but reliable way to prepare for future tsunamis because the frequency of tsunami occurrence and their magnitude can be estimated by historical records of tsunamis. Investigation of tsunami deposits preserved in geological layers is an effective measure to understand ancient tsunamis that occurred before historical records began. However, the areas containing tsunami deposits can be narrower than the area of tsunami inundation, thus resulting in underestimation of the magnitude of past tsunamis. A field survey was conducted after the 2010 Chile tsunami and 2011 Japan tsunami to investigate the chemical properties of the tsunami-inundated soil to examine the applicability of tsunami inundation surveys considering water-soluble salts in soil. The soil and tsunami deposits collected in the tsunami-inundated areas are rich in water-soluble ions (Na⁺, Mg²⁺, Cl^?, Br^? and SO ₄ ^2? ) compared with the samples collected in the non-inundated areas. The analytical result that the ratios of Na⁺, Mg²⁺, Br^? and SO ₄ ^2? to Cl^? are nearly the same in the tsunami deposits and in the tsunami-inundated soil suggests that the deposition of these ions resulting from the tsunami inundation does not depend on whether or not tsunami deposits exist. Discriminant analysis of the tsunami-inundated areas using the ion contents shows the high applicability of these ions to the detection of tsunami inundation during periods when the amount of rainfall is limited. To examine the applicability of this method to palaeotsunamis, the continuous monitoring of water-soluble ions in tsunami-inundated soil is needed as a future study.