Volcanic Arc Region Research Articles

Zangalou Manto‐type deposit in the Sabzevar zone, northeast Iran: Evidence of mineralogy, geochemistry, U–Pb dating, fluid inclusion, and stable isotopes

The Zangalou Cu deposit lies in the Sabzevar volcanic‐plutonic zone, northeastern Iran. The deposit is hosted by the middle Eocene volcanic‐sedimentary sequences and it has been affected by propylitic, carbonate, sericitic, and minor argillic alterations. The volcanics have features typical of calc‐alkaline and metaluminous magmas and are plotted in the continental volcanic arc region. Zircon U–Pb dating of the andesite porphyry yield ages of 41.2 and 38.4 Ma (Bartonian). Ore mineralization occurs as stratabound with open‐space filling, dissemination, veinlet, and replacement textures in mineralized conglomerate, andesite porphyry, and trachyandesite host rocks. Cu content in Zangalou deposit vary from 1,234 g/t to 6.24% which mostly occurred as chalcocite mineral. Fluid inclusion data of mineralization‐related calcites indicate medium salinities (12.8–16.6 wt.% NaCl equivalents) and a wide range of temperatures (154–295°C) and show evidence of fluid cooling trend during the ore formation. The δ13C (between 19.3 and −2.8‰) and δ18OSMOW (between 24.8 and 25.34‰) values of ore‐related calcites suggests a contribution of sedimentary organic matter and marine carbonates as the source of carbon in the ore‐forming fluid. High positive δ34S values (27.8–33.47‰) suggest that the source of sulphur is related to sedimentary country rocks. Mineralization is lithologically and structurally controlled and it is epigenetic. According to geological, petrological, alteration, mineralization textures and geometry of deposit, fluid inclusion, and isotopic studies, the Zangalou deposit is similar to manto‐type deposits.

Contrasting Roles of DOP as a Source of Phosphorus and Energy for Marine Diazotrophs

The oceanic dissolved organic phosphorus (DOP) pool is mainly composed of P-esters and, to a lesser extent, equally abundant phosphonate and P-anhydride molecules. In phosphate-limited ocean regions, diazotrophs are thought to rely on DOP compounds as an alternative source of phosphorus (P). While both P-esters and phosphonates effectively promote dinitrogen (N2) fixation, the role of P-anhydrides for diazotrophs is unknown. Here we explore the effect of P-anhydrides on N2 fixation at two stations with contrasting biogeochemical conditions: one located in the Tonga trench volcanic arc region (“volcano,” with low phosphate and high iron concentrations), and the other in the South Pacific Gyre (“gyre,” with moderate phosphate and low iron). We incubated surface seawater with AMP (P-ester), ATP (P-ester and P-anhydride), or 3polyP (P-anhydride) and determined cell-specific N2 fixation rates, nifH gene abundance, and transcription in Crocosphaera and Trichodesmium. Trichodesmium did not respond to any DOP compounds added, suggesting that they were not P-limited at the volcano station and were outcompeted by the low iron conditions at the gyre station. Conversely, Crocosphaera were numerous at both stations and their specific N2 fixation rates were stimulated by AMP at the volcano station and slightly by 3polyP at both stations. Heterotrophic bacteria responded to ATP and 3polyP additions similarly at both stations, despite the contrasting phosphate and iron availability. The use of 3polyP by Crocosphaera and heterotrophic bacteria at both low and moderate phosphate concentrations suggests that this compound, in addition to being a source of P, can be used to acquire energy for which both groups compete. P-anhydrides may thus leverage energy restrictions to diazotrophs in the future stratified and nutrient-impoverished ocean.

Tonian and Cryogenian – Early Cambrian sedimentation in NW India: Implications on the transition from Rodinia to Gondwana

The Tonian Punagarh-Sindreth and Cryogenian – early Cambrian Marwar sequences in NW India encompass a geological record that could provide vital information for working out the tectonic evolution of this region and a better understanding of the Rodinia to Gondwana transition. The Tonian Punagarh and Sindreth sub-basins received a dominantly felsic detritus with negligible sorting of heavy minerals and mild to weak chemical weathering conditions. The clastic compositions and geochemical characteristics of the Punagarh and Sindreth groups suggest a back-arc basin setting and the detritus sourced from a volcanic arc and cratonic interior region. The Punagarh basin was proximal to the craton while the Sindreth basin was close to an arc system. In contrast, the Cryogenian – early Cambrian Marwar Supergroup clastic sequence contains mature quartz arenite whose high CIA (77.8–94.8) and low ICV (0.18–1.00) values suggest a strong chemical weathering in the source region and noteworthy physical sorting during transportation and sedimentation. An abundance of quartz grains (87–94%), enrichment of Zr, and detritus predominantly derived from the cratonic basement indicate deposition of Marwar sediments in a tectonically stable basin, temporally coinciding with the Gondwana assembly. Collectively, the ∼ 760 Ma active continental margin in NW India, documented by the provenance and tectonic setting of the Punagarh and Sindreth basins, is in agreement with the subduction of the peripherical Rodinia supercontinent during its breakup. On the other hand, the passive continental margin sedimentation in Marwar Supergroup points toward an open sea between NW India and western Gondwana during the Gondwana assembly.

Arc and forearc rifting in the Tyrrhenian subduction system

The evolution of forearc and backarc domains is usually treated separately, as they are separated by a volcanic arc. We analyse their spatial and temporal relationships in the Tyrrhenian subduction system, using seismic profiles and numerical modelling. A volcanic arc, which included the Marsili volcano, was involved in arc-rifting during the Pliocene. This process led to the formation of an oceanic backarc basin (~ 1.8 Ma) to the west of the Marsili volcano. The eastern region corresponded to the forearc domain, floored by serpentinised mantle. Here, a new volcanic arc formed at ~ 1 Ma, marking the onset of the forearc-rifting. This work highlights that fluids and melts induce weakening of the volcanic arc region and drive the arc-rifting that led to the backarc basin formation. Later, the slab rollback causes the trench-ward migration of volcanism that led to the forearc- rifting under the control of fluids released from the downgoing plate.

Geochemical characterization of mineralized albitite from Paleoproterozoic Bhukia IOCG‐IOA deposit of Aravalli‐Delhi Fold Belt, Rajasthan, western India: Genetic linkage to the gold (±Cu ± U) mineralization

Paleoproterozoic Bhukia gold deposit is located within the Debari Group of Aravalli Supergroup, Rajasthan, northwestern India. The major lithounits that are associated with gold–graphite (±Cu ± U) mineralization exposed in Bhukia are albitite, carbonates, and quartzite which have undergone upper greenschist to amphibolite facies metamorphism. Ore mineralization viz. pyrrhotite, arsenopyrite, chalcopyrite, pyrite, graphite, magnetite, and goethite in the study area are associated with NNW–SSE trending shear zone. Nuggets of gold are identified in the thick gossan zones that occurs parallel to the shear zone. The gold–sulfide lodes of the Bhukia deposits are largely confined either to marble or albitized marble and pure monomineralic albitite. The mineralization that is associated with the albitite, follows the boundaries of albite grains which confirms the role of these weak zones for the mobilization of mineralizing fluid and concentration of metal ions. Hence, geochemical studies of albitite would help to evaluate the petrogenesis, likely magmatic affinity, tectonic setting, controls of the mineralizing fluid, source, and their linkage to the gold (±Cu ± U) metallogeny. Bhukia albitite is characterized by Na–Al–rich and K‐poor compositions along with metaluminous nature. Geochemical analysis of various key elements suggests that albitite is the product of low‐K calc‐alkaline magma series with variation in Nb/Ta ratio indicating their fractionation during the melt–rock interaction within the shear zone. Geochemical signatures suggest the role of upper continental crust in channelizing the mineralized fluids that generated albitite with felsic to intermediate composition in and around Bhukia deciphered from their low Nb/Y ratios. Lower concentration of compatible elements (Ni and Cr) and higher concentration of incompatible elements (Th, La, and Nd) with SiO2 suggest that evolution of albitite rock is related to the fractional crystallization process. Zr/Hf ratios of 9–35 with the average of 18 suggest that albitite formed under magmatic–hydrothermal transitional environment. Rare Earth Elements (REE) patterns of albitite samples are moderately fractionated showing Light Rare Earth Elements (LREE)‐enrichment with a rather steep slope and exhibit distinct negative Eu anomalies (0.42–0.71, average 0.57). The enrichment of LREE in comparison to the Heavy Rare Earth Elements (HREE) suggests a more evolved source, i.e., process of melting at shallow environments. Tectonic setting of albitite samples suggest their origin in a volcanic arc region that underwent rifting. Detailed geochemical characterization unravelled complex evolutionary history of albitite in the Bhukia area, which attests derivation of the parental melt possibly from an enriched SCLM (Sub‐continental lithospheric mantle) source similar to most of the IOCG‐IOA deposits worldwide.

Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: Pore fluid constraints

We present sediment pore fluid and sediment solid phase results obtained during IODP Expedition 340 from seven sites located within the Grenada Basin of the southern Lesser Antilles Volcanic Arc region. These sites are generally characterized as being low in organic carbon content and rich in calcium carbonate and volcanogenic material. In addition to the typical reactions related to organic matter diagenesis, pore fluid chemistry indicates that the diagenetic reactions fall within two broad categories; (1) reactions related to chemical exchange with volcanogenic material and (2) reactions related to carbonate dissolution, precipitation, or recrystallization. For locations dominated by reaction with volcanogenic material, these sites exhibit increases in dissolved Ca with coeval decreases in Mg. We interpret this behavior as being driven by sediment-water exchange reactions from the alteration of volcanic material that is dispersed throughout the sediment package, which likely result in formation of Mg-rich secondary authigenic clays. In contrast to this behavior, sediment sequences that exhibit decreases in Ca, Mg, Mn, and Sr with depth suggest that carbonate precipitation is an active diagenetic process affecting solute distributions. The distributions of pore fluid 87Sr/86Sr reflect these competitive diagenetic reactions between volcanic material and carbonate, which are inferred by the major cation distributions. From one site where we have solid phase 87Sr/86Sr (site U1396), the carbonate fraction is found to be generally consistent with the contemporaneous seawater isotope values. However, the 87Sr/86Sr of the non-carbonate fraction ranges from 0.7074 to 0.7052, and these values likely represent a mixture of local arc volcanic sources and trans-Atlantic eolian sources. Even at this site where there is clear evidence for diagenesis of volcanogenic material, carbonate diagenesis appears to buffer pore fluid 87Sr/86Sr from the larger changes that might be expected given the high abundance of tephra in these sediments. Part of this carbonate buffering, at this site as well as throughout the region, derives from the fact that the Sr concentration in the non-carbonate fraction is generally low (<200 ppm), whereas the carbonate fraction has Sr concentrations approaching ∼1000 ppm.

Three-Dimensional Electrical Resistivity Image of the Volcanic Arc in Northern Chile—An Appraisal of Early Magnetotelluric Data

Magnetotelluric investigations were carried out in the late 1980s across all morphological units of the South American subduction zone with the aim to observe lithosphere structures and subduction-induced processes in northern Chile, southwestern Bolivia, and northwestern Argentina at ~ 22°S. Earlier two-dimensional forward modeling yielded a complex picture of the lower crust and upper mantle, with strong variations between the individual morphological units as well as between forearc and backarc. The principal result was a highly conductive zone beneath the volcanic arc of the Western Cordillera starting at ~ 25 km depth. Goal of this work is to extend the existing 2-D results using three-dimensional modeling techniques at least for the volcanic arc and forearc region between 22°S and 23°S in Northern Chile. Dimensionality analysis indicates strong 3-D effects along the volcanic arc at the transition zone to the Altiplano, in the Preandean Depression and around the Precordillera Fault System at ~ 22°S. In general, the new 3-D models corroborate previous findings, but also enable a clearer image of lateral resistivity variations. The magmatic arc conductor emerges now as a trench-parallel, N–S elongated structure slightly shifted to the east of the volcanic front. The forearc appears highly resistive except of some conductive structures associated with younger sedimentary infill or young magmatic record beneath the Precordillera and Preandean Depression. The most prominent conductor in the whole Central Andes beneath the Altiplano and Puna is also modeled here; it is, however, outside the station array and thus poorly resolved in this study.

Geochemistry of coastal sands of Eastern Mediterranean: The case of Nisyros volcanic materials

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920mg/kg) and Nb (245mg/kg), in the coastal sand. The low REE concentration (ΣREE+Y=240mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE.The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions.

Temporal variation of the stress field during the construction of the central Andes: Constrains from the volcanic arc region (22-26°S), Western Cordillera, Chile, during the last 20 Ma

In order to understand the response of the stress field state to intrinsic processes during the construction of the Andes, such as thickening of the continental crust, lithospheric delamination, and/or thermal weakening, we investigate the stress field evolution of the arc region since the last 20 my, in the Central Andes (22°-26.5°S). The 43 reduced paleostress tensors derived from inversion of 682 fault-slip data reveal a complex pattern of stress states during the last episode of orogenic construction and topographic uplift. We identify two geodynamic stages; the first stage corresponds to the construction of the Altiplano/Puna plateau and the second one to its gravitational collapse. Four stress states that have prevailed in the Altiplano/Puna plateau since middle Miocene times, characterize the transition from one stage to the other. Along the study latitudes, a spatiotemporal change in stress state is clearly observed, which led to an understanding that a change in the stress field may be related not only to the boundary conditions but also to intrinsic factors associated with the construction of the Andean orogeny. Our results suggest that ca. 13-10 Ma and ca. 8-5 Ma, in the southern Altiplano and northern Puna, and in the southern Puna, respectively, regional elevation and crustal thicknesses reached threshold values necessary to generate the orogenic collapse.

Limitations in the Use of Archived Vent Mussel Samples to Assess Genetic Connectivity Among Seafloor Massive Sulfide Deposits: A Case Study with Implications for Environmental Management

Genetic connectivity studies can inform the design of mitigation strategies used in environmental management. However, the expense of developing species-specific molecular markers and collecting samples at appropriate spatial and temporal scales can be prohibitive. Using archived material and existing molecular markers may provide a cost-effective way to assess population connectivity. Genetic connectivity studies are increasingly in demand in the deep sea in response to mounting anthropogenic pressures, including seafloor massive sulfide (SMS) mining. The feasibility of using archived material was assessed using the New Zealand-endemic vent mussel Gigantidas gladius, which inhabits areas licensed for the prospecting phase of SMS mining. Four molecular markers were tested, but only one (mitochondrial COI) provided suitable sequences. Of 942 specimens, only 150 individuals were informative, largely due to poor tissue quality of archived samples. Seven populations spanning the distributional range of G. gladius were assessed. The results indicate that G. gladius has high levels of gene flow among sites 10s to 100s km apart and limited genetic structure. Haplotypic diversity was not equally distributed among populations, with lower diversity for the Macauley Volcano population at the northern extent of the species distribution and greater diversity within central populations. Migrant exchange was also greatest between central populations, with one population at Rumble V Seamount appearing important in terms of maintaining genetic diversity within the Kermadec Volcanic Arc region. However, interpretation of the results should be viewed with caution as small sample sizes may have limited the ability to detect genetic structure. Despite these limitations, mitigation strategies that protect areas of seabed from mining activities should consider the genetic vulnerability of the population at the northern edge of the species’ distribution and the significance of certain central populations.

Constraints on the timing of Quaternary volcanism and duration of magma residence at Ciomadul volcano, east–central Europe, from combined U–Th/He and U–Th zircon geochronology

High-spatial resolution zircon geochronology was applied to constrain the timescales of volcanic eruptions of the youngest, mostly explosive volcanic phase of Ciomadul volcano (Carpathian–Pannonian region, Romania). Combined U–Th and (U–Th)/He zircon dating demonstrates that intermittent volcanic eruptions occurred in a time range of 56–32ka. The reliability of the eruption dates is supported by concordant ages obtained from different dating techniques, such as zircon geochronology, radiocarbon analysis, and infrared stimulated luminescence dating for the same deposits. The new geochronological data suggest that volcanism at Ciomadul is much younger (<ca. 200ka) than previously thought (up to 600ka). A dominantly explosive volcanic phase occurred after an apparent lull in volcanism that lasted for several 10's of ka, after a period of lava dome extrusion that defines the onset of the known volcanism at Ciomadul. At least four major eruptive episodes can be distinguished within the 56–32ka period. Among them, relatively large (sub-plinian to plinian) explosive eruptions produced distal tephra covering extended areas mostly southeast from the volcano. The 38.9ka tephra overlaps the age of the Campanian Ignimbrite eruption and has an overlapping dispersion axis towards the Black Sea region. The wide range of U–Th model ages of the studied zircons indicates prolonged existence of a low-temperature (<800°C) silicic crystal mush beneath Ciomadul. The main zircon crystallization period was between ca. 100 and 200ka, coeval with the older, mostly extrusive lava dome building stage of volcanism. Even the youngest U–Th model ages obtained for the outermost 4μm rim of individual zircon crystals predate the eruption by several 10's of ka. The zircon age distributions suggest re-heating above zircon saturation temperatures via injection of hot mafic magmas prior to eruption. Intermittent intrusions of fresh magma could play a significant role in keeping the intrusive silicic magmatic reservoir in a partially melted for prolonged period. The previous history of Ciomadul suggests that melt-bearing crystal mush resided beneath the volcano, and was rapidly remobilized after a protracted (several 10's of ka) lull in volcanism to trigger several eruptions in a comparatively short time window. This classifies Ciomadul as a volcano with ‘Potentially Active Magma Storage’ (PAMS) which we propose to be common among the seemingly inactive volcanoes in volcanic arc regions. The potential for reactivation of these systems should be included into volcanic hazard assessments.

Deep electrical resistivity structure of the northwestern U.S. derived from 3-D inversion of USArray magnetotelluric data

Long period (10–20,000 s) magnetotelluric (MT) data are being acquired across the continental USA on a quasi-regular grid of ∼70 km spacing as an electromagnetic component of the National Science Foundation EarthScope/USArray Program. These data are sensitive to fluids, melts, and other orogenic indicators, and thus provide a valuable complement to other components of EarthScope. We present and interpret results of 3-D MT data inversion from 325 sites acquired from 2006–2011 to provide a regional scale view of electrical resistivity from the middle crust to nearly the mantle transition zone, covering an area from NW Washington to NW Colorado. Beneath the active extensional subprovinces in the south-central region, on average we see a resistive upper crust, and then extensive areas of low resistivity in the lower crust and uppermost mantle. Further below, much of the upper half of the upper mantle appears moderately resistive, then subsequently the lower upper mantle becomes moderately conductive. This column suggests a dynamic process of moderately hydrated and fertile deeper upper mantle upwelling during extension, intersection of that material with the damp solidus causing dehydration and melting, and upward exodus of generated mafic melts to pond and exsolve saline fluids near Moho levels. Lithosphere here is very thin. To the east and northeast, thick sections of resistive lithosphere are imaged under the Wyoming and Medicine Hat Cratons. These are punctuated with numerous electrically conductive sutures presumably containing graphitic or sulfide-bearing meta-sediments deeply underthrust and emplaced during ancient collisions. Below Cascadia, the subducting Juan de Fuca and Gorda lithosphere appears highly resistive. Suspected oceanic lithosphere relicts in the central NW part of the model domain also are resistive, including the accreted “Siletzia” terrane beneath the Coast Ranges and Columbia Embayment, and the seismically fast “slab curtain” beneath eastern Idaho interpreted by others as stranded Farallon plate. Upwelling of deep fluid or melt in the Cascade volcanic arc region manifests as conductive features at several scales. These include quasi-horizontal conductive patches under the arc and fore-arc, likely denoting fluids evolved via breakdown of hydrous minerals in the current down-going slab. In the backarc, low resistivities concentrate in “plumes” connecting into a deeper aesthenospheric layer to the east, consistent with subduction-driven upwelling of hot, hydrated or melted, aesthenospheric mantle. Low resistivities (<10 Ωm) deep beneath the stable cratons suggest higher levels of hydration there, and/or influence of poorly resolved structures outside the array.

Structure of the crust and the lithosphere beneath the southern Puna plateau from teleseismic receiver functions

We present a teleseismic P and S receiver function study using data from a temporary passive-source seismic array in the southernmost Puna plateau and adjacent regions. The P receiver function images show the distribution of crustal thickness and Vp/Vs ratio for this area. Over much of the southern Puna plateau, the crustal thickness is 50–55 km, whereas to the west a thicker crust (∼60 to 75 km) is observed beneath much of the Andean volcanic arc region. From the Puna southward, there is little obvious change in the crustal thickness across the border of the plateau (south of 28°S). The crust is seen to progressively thin towards the east in the Pampean Ranges where it is 35–40 km thick. The southern Puna plateau is characterized overall by a low crustal Vp/Vs ratio (less than 1.70), implying a felsic crustal composition. An anomalously high Vp/Vs ratio of 1.87 is observed beneath the Cerro Galan volcanic center, in the region where a prominent crustal low-velocity zone identified below ∼10 km depth probably extends into the lower crust. The crustal thickness determined under the Cerro Galan area (59 km) is close to that of the rest of the southern Puna. The prominent high Vp/Vs ratio and low-velocity zone beneath the Galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the Galan caldera. A widespread crustal low-velocity layer observed beneath much of the southern Puna, correlates well with crustal low-velocity anomalies observed by teleseismic tomography. The lithosphere–asthenosphere boundary beneath the array can be clearly observed by both P and S receiver functions at depths of 70–90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau. The mantle transition zone discontinuities appear at expected depths.

Using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of New Zealand

Recently developed analytical techniques to determine the abundances of noble gases in sediment pore water allow noble-gas concentrations and isotope ratios to be measured easily and routinely in lacustrine sediments. We applied these techniques for the first time to ocean sediments to investigate an active cold methane seepage system located in the South Pacific off the coast of the North Island of New Zealand using 3He/4He ratios determined in the sediment pore water. The results show that more 3He-rich fluids are released in the vicinity of the Pacific–Australian subduction zone than at the forearc stations located closer to the New Zealand coast. However, the He isotope signature in the sediment column indicates that only a minor part of the He emanating from deeper strata originates from a depleted mantle source. Hence, most He in the pore water is produced locally by the radioactive decay of U and Th in the sediment minerals or in the underlying crustal rocks. Such an occurrence of isotopically heavy crustal He also suggests that the source of the largest fraction of methane is a near-surface geochemical reservoir. This finding is in line with a previous δ13C study in the water column which concluded that the emanating methane is most likely of biological origin and is formed in the upper few meters of the sediment column. Moreover, the prevalence of isotopically heavy He agrees well with the outcome of other previous studies on island arc systems which indicate that the forearc regions are characterized by crustal He emission, whereas the volcanic arc region is characterized by the presence of mantle He associated with rising magma.

Forearc versus Backarc Attenuation of Earthquake Ground Motion

Understanding of the attenuation of seismic wave amplitudes with distance is a crucial issue for ground motion prediction equations and seismic hazard analysis. In this study, we empirically evaluate the influence of regional geologic structure, in particular the attenuation effects of traveling through a volcanic arc region (forearc versus backarc attenuation) in northern Japan. We perform regression analysis of Fourier amplitude spectra (FAS) of five well-recorded Japanese events that occurred very close to the line formed (in map view) by the volcanic arc. This provides an approximately symmetrical distribution of stations for each event relative to the volcanic front; forearc stations lie to the east of the front, while backarc stations lie to the west. To compare the characteristics of shallow and deep events, we have included both crustal and in-slab events in our dataset. Based on inspection of the data, we assume a hinged bilinear geometric spreading model with a fixed slope of -1 out to a distance of 50 km, with slope of -0.5 thereafter (for all events). Our results show, with a very high level of confidence, that high-frequency attenuation is greater in the backarc direction. The Q values for forearc regions ( Q ∼240 at 1 Hz, increasing to Q ∼3890 at 10 Hz) are about a factor of 2 larger than those for backarc regions ( Q ∼196 at 1 Hz, increasing to Q ∼1669 at 10 Hz) at high frequencies, implying much stronger attenuation as waves travel through the volcanic crustal structure to the backarc stations. The separation of forearc and backarc travel paths results in a significant reduction in the standard deviation ( σ ) of ground motion predictions (by as much as 0.05 log10 units), which has important implications for hazard analyses in subduction zone regions.

Landslide hazard and community-based risk reduction effort in Karanganyar and the surrounding area, central Java, Indonesia

Karanganyar and the surrounding area are situated in a dynamic volcanic arc region, where landslide frequently occurs during the rainy season. The rain-induced landslide disasters have been resulting in 65 fatalities and a substantial socioeconomical loss in last December 2007. Again, in early February 2009, 6 more people died, hundreds of people temporary evacuated and tens of houses damaged due to the rain-induced landslide. Accordingly, inter-disciplinary approach for geological, geotechnical and social investigations were undertaken with the goal for improving community resilience in the landslide vulnerable villages. Landslide hazard mapping and community-based landslide mitigation were conducted to reduce the risk of landslides. The hazard mapping was carried out based on the susceptibility assessment with respect to the conditions of slope inclination, types and engineering properties of lithology/soil as well as the types of landuse. All of those parameters were analyzed by applying weighing and scoring system which were calculated by semi qualitative approach (Analytical Hierarchical Process). It was found that the weathered andesitic-steep slope (steeper than 30o) was identified as the highest susceptible slope for rapid landslide, whilst the gentle colluvial slope with inter-stratification of tuffaceous clay-silt was found to be the susceptible slope for creeping. Finally, a programme for landslide risk reduction and control were developed with special emphasize on community-based landslide mitigation and early warning system. It should be highlighted that the social approach needs to be properly addressed in order to guarantee the effectiveness of landslide risk reduction.

Rivers from Volcanic Island Arcs: The subduction weathering factory

Here the focus is on the geochemistry of rivers draining volcanic arc regions. Although data are rather sparse, first order estimates show that volcanic arc settings are amongst the faster eroding regions of the Earth because they combine high precipitation regimes with high hydrothermal contributions in which increased temperature and increased acidity favor water–rock interaction. The hydrothermal contribution tends to obliterate the effect of climate and shows the importance of better evaluating the hydrothermal input from subduction zones, and the parameters that control it.

Curie Point Depth Estimates and Correlation with Subduction in Mexico

We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available “Magnetic Anomaly Map of North America”. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2° × 2°. The CPD estimates range between 10 and 40 km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20–30 km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD = 12–16 km) and Oaxaca (CPD = ~16 km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O’Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16–32 km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16–20 km) to east (10–12 km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust’s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico.

Poroelastic stress-triggering of the 2005 M8.7 Nias earthquake by the 2004 M9.2 Sumatra–Andaman earthquake

The M9.2 Sumatra–Andaman earthquake (SAE) occurred three months prior to the M8.7 Nias earthquake (NE). We propose that the NE was mechanically triggered by the SAE, and that poroelastic effects were a major component of this triggering. This study uses 3D finite element models (FEMs) of the Sumatra–Andaman subduction zone (SASZ) to predict the deformation, stress, and pore pressure fields of the SAE. The coseismic slip distribution for the SAE is calibrated to near-field GPS data using FEM-generated Green's Functions and linear inverse methods. The calibrated FEM is then used to predict the postseismic poroelastic contribution to stress-triggering along the rupture surface of the NE, which is adjacent to the southern margin of the SAE. The coseismic deformation of the SAE, combined with the rheologic configuration of the SASZ produces two transient fluid flow regimes having separate time constants. SAE coseismic pore pressures in the relatively shallow forearc and volcanic arc regions (within a few km depth) dissipate within one month after the SAE. However, pore pressures in the oceanic crust of the down-going slab persist several months after the SAE. Predictions suggest that the SAE initially induced MPa-scale negative pore pressure near the hypocenter of the NE. This pore pressure slowly recovered (increased) during the three-month interval separating the SAE and NE due to lateral migration of pore fluids, driven by coseismic pressure gradients, within the subducting oceanic crust. Because pore pressure is a fundamental component of Coulomb stress, the MPa-scale increase in pore pressure significantly decreased stability of the NE fault during the three-month interval after the SAE and prior to rupture of the NE. A complete analysis of stress-triggering due to the SAE must include a poroelastic component. Failure to include poroelastic mechanics will lead to an incomplete model that cannot account for the time interval between the SAE and NE. Our transient poroelastic model explains both the spatial and temporal characteristics of triggering of the NE by the SAE.

Nature of crustal terranes and the Moho in northern Costa Rica from receiver function analysis

The Central American subduction zone in northern Costa Rica shows along‐strike variations in both the incoming and overriding plates. By analyzing the subducting oceanic Moho (M1) and the upper plate Moho (M2) with receiver functions, we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of crustal terranes within the overriding Caribbean Plate. We calculate high‐quality P and PP wave receiver functions using broadband data of the Global Seismology Network; Geoscope Project; and the CRSEIZE, Pocosol, and Corisubmod experiments. In addition, we estimate the depth (H) and vertically averaged Vp/Vs (κ) to Moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the H and κ estimates. Our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the Nicoya Peninsula, as compared to a serpentinized oceanic mantle subducting beneath the northern part. In the Caribbean Plate, we describe the nature of the Mesquito, Nicoya, and Chorotega terranes by integrating new and published Vp/Vs estimates. Both the Nicoya and Chorotega terranes display high Vp/Vs (1.80–1.92) consistent with their oceanic character. In contrast, the oceanic Mesquito Terrane mostly displays lower Vp/Vs (1.62–1.80) more compatible with continental crust, which may indicate that subduction zone magmatism is modifying the crust to display continental character. Our estimates show that the deepest M2 (∼42 km) is observed in the volcanic arc region whereas the shallowest M2 (∼27–33 km) is observed in parts of the fore‐arc and back‐arc regions.