Ring Dike Research Articles

A volcanic inventory of the Moon

The volcanic and magmatic activity of the Moon is intimately tied to its internal thermal and geodynamic evolution through time. While the extrusive nearside maria dominate the volcanic record, little is known regarding their underlying structure and the details of their emplacement. Intrusive activity is even more enigmatic, with most intrusions expressing little to no surface signature. Although prior studies have provided insights into the local igneous activity, no global compilation has been conducted. Here, we present a volumetric inventory of extrusive and intrusive activity. Gravity and topography data are inverted using a two-layer loading model to constrain mare and cryptomare thickness. The mean thickness of mare units is found to be 2.8 km, though with substantial lateral variations, with average values of 7.9 km within large mare basins compared to 1.6 km outside of these basins. This substantial variation in mare thickness associated with minimal change in the surface topography may be explained by some combination of long-distance transport of low viscosity mare and/or a buoyancy control limiting mare eruptions to a constant level surface. Our preferred volumes of mare and cryptomare are 18.2×106 km3 and 2.2×106 km3, respectively. Crustal intrusions associated with linear gravity anomalies, floor-fractured craters, ring dikes, graben, and beneath volcanic constructs, are investigated and yield a total volume of 9.1×106 km3. Our inventory reveals that intrusive activity dominates in the farside (intrusive:extrusive ratio of 5:2), whereas extrusive volcanism is more pronounced in the nearside (1:5). The combined volume of intrusives and extrusives is found to be 3 times greater in the nearside than in the farside. Both are related to the lunar asymmetry in which the thinner crust and warmer subsurface beneath the Procellarum KREEP terrane enables enhanced melting and magma ascent. These observations may have implications for the interpretation of the thermal and geodynamic history of other celestial bodies, where intrusive volcanism remains poorly constrained.

Mineralogical and Geochemical Evidence of Dissolution-Reprecipitation Controlled Hydrothermal Rare Earth Element Mineralization in the Amba Dongar Carbonatite Complex, Gujarat, Western India

Abstract The Amba Dongar carbonatite complex in western India comprises an inner ring of carbonatite breccia surrounded by a sövite ring dike. The various carbonatite units in the body include calcite carbonatite, alvikite, dolomite carbonatite, and ankerite carbonatite. The carbonate phases (calcite and ankerite) occur as phenocrysts, groundmass phases, fresh primary grains, and partially altered grains and/or pseudomorphs when hydrothermally overprinted. Rare earth element (REE) enrichment in the groundmass/altered calcite grains compared to the magmatic ones is ascribed to the presence of micron-sized REE phases. Fluorapatite and pyrochlore constitute important accessory phases that are altered to variable extents. Higher concentrations of Sr, Si, and REEs in fluorapatite are suggestive of a magmatic origin. Fresh pyrochlore preserves its magmatic composition, characterized by low A-site vacancy and high F in the Y-site, which on alteration becomes poorer in Na, Ca, and F and displays an increase in vacancy. The C-O isotope compositions of the carbonates also corroborate the extensive low-temperature hydrothermal alteration of the carbonatites. The REE mineralization is the result of interaction of the carbonatite with a sulfur-bearing, F-rich hydrothermal fluid that exsolved from late-stage carbonatitic magmas. The hydrothermal fluids caused dissolution of the primary carbonates and simultaneous precipitation of REEs and other high field strength element (HFSE)-bearing minerals. Complex spatial associations of the magmatic minerals with the REE fluorocarbonates, [synchysite-(Ce), parisite-(Ce), bastnäsite-(Ce)] and florencite-(Ce) point to the formation of these REE phases as a consequence of postmagmatic hydrothermal dissolution of the REEs from fluorapatite, pyrochlore, and carbonates. Ubiquitous association of fluorite and barite with REE minerals indicates transport of REEs as sulfate complexes in F-rich fluids. Precipitation of REE fluorocarbonates/florencite resulted from fluid-carbonate interaction, concomitant increase in pH, and decrease in temperature. Additionally, REE precipitation was aided and abetted by the removal of sulfur from the fluid by the precipitation of barite, which destabilized the REE sulfate complexes.

Paraná quartz andesite rings and arcs formed by distal imprint of dune design from the Botucatu paleoerg

Rings and arcs in a Paraná Volcanic Province quartz andesite were formed by distal imprint of design of the top of barchan and linear dunes. Hydrothermal jetting occurred in the Lower Cretaceous from the dunes onto the upper amygdaloidal crust of the lava flow in Fronteira Oeste of southernmost Brazil. In one occurrence, hot water overflowed the quartz andesite in a hot spring and deposited sinter quartz. This unique register of the process in the continents was investigated with varied techniques, including satellite images, geological field survey, field gammaspectrometry, petrography and whole-rock geochemistry. We characterized the rings and arcs in the upper crust of the Catalán quartz andesite, design defined by hydrothermal processes, no ring dikes observed. We interpret the evidence as hydrothermal injection of sand and alteration of the upper crust of the quartz andesite, including silicification and deposition of epithermal quartz. The reproduction of the design of the dune tops was generated by focused jetting of hot water and vapor (plus sand) through the top of the dunes. The vertical channel maintained the design of the dune tops distally in the quartz andesite. The quartz andesite crusts now display sand dune design above the massive core in the pampas as a result of a special process in geology.

Diversity of weed species composition of Cyperaceae and Poaceae in paddy rice field in An Giang province

This study was conducted to provide data on the weed species composition of the Cyperaceae and Poaceae in the rice fields in An Giang province. The study results show that the weed species composition of Cyperaceae and Poaceae families is very diverse with 38 species belonging to 25 genera, of which 27 are usable. The largest number of species is in crop 1 and in the fields inside the ring dike. Although the species composition is diverse, only a few species are very high frequencies, and most of the remaining species have very low frequencies. Chau Phu district has the highest weed density, averaging 28.42 trees/m2. Weed density in crop 1 is highest in the 3 crops (15.79 trees/m2). Fields outside the ring dike have a higher weed density than paddy fields inside the ring dike (15.37 trees/m2). Although crop 1 has more species abundance than crop 3, the species diversity and the species equality in the weed community in crop 1 are lower than crop 3. Rice fields inside the ring dike have higher species abundance than those outside the ring dike, but the indicators H 'and 1 – λ are lower.

ROSEN, BULGARIA: A NEWLY RECOGNIZED IRON OXIDE-COPPER-GOLD DISTRICT

AbstractThe Rosen copper veins in southeastern Bulgaria are recognized for the first time as an iron oxide-copper-gold (IOCG) district. The veins are located in the East Srednogorie segment of the Carpathian-Balkan calc-alkaline volcano-plutonic arc and were formed during an end-stage interval of extreme slab rollback and intra-arc rifting, which gave rise farther east to seafloor spreading in the Western Black Sea basin. The resulting submarine volcano-sedimentary rift basin is dominated by intermediate to mafic shoshonitic to ultrapotassic volcanism and subsidiary gabbro to syenite intrusion. The E- to NE-striking veins define a NW-striking alignment along the western contact of the syenite-dominated Rosen pluton, inferred to be part of a large ring dike. More than 40 veins, the most important formerly mined to depths as great as 1,000 m, contain an early, pegmatoidal, calcic-potassic assemblage followed by predominant magnetite (including the mushketovite variety), chlorite, and carbonates but also quartz, chalcopyrite, pyrite, and numerous other metallic minerals, which combine to give an unusual Fe-Cu-Au-Mo-Co-Ni-U-light rare earth element (LREE)-W-Bi-Zn-Pb geochemical signature. The close correlation between Fe, Cu, U, and LREEs is evident even in the flotation tailings. Vein molybdenite was dated during this study at 80.6 ± 0.4 Ma, which is similar to a U-Pb zircon age for monzosyenite from the Rosen pluton. The mineralogic and compositional features of the Rosen district are comparable to those of well-known IOCG deposits worldwide and geometrically similar to the vertically extensive IOCG veins in the Coastal Cordillera province of northern Chile. The subsidiary granitophile signature that accompanies the characteristic siderophile IOCG suite was also recognized recently at the giant Olympic Dam deposit in South Australia and elsewhere. Although no exposed intrusion is definitively implicated in the genesis of the Rosen veins, coexisting gabbro and syenite fluid sources may be hypothesized at depth in or beneath the coeval ring dike.

Host-rock deformation during the emplacement of the Mourne Mountains granite pluton: Insights from the regional fracture pattern

Abstract The Mourne Mountains magmatic center in Northern Ireland consists of five successively intruded granites emplaced in the upper crust. The Mourne granite pluton has classically been viewed as a type locality of a magma body emplaced by cauldron subsidence. Cauldron subsidence makes space for magma through the emplacement of ring dikes and floor subsidence. However, the Mourne granites were more recently re-interpreted as laccoliths and bysmaliths. Laccolith intrusions form by inflation and dome their host rock. Here we perform a detailed study of the deformation in the host rock to the Mourne granite pluton in order to test its emplacement mechanism. We use the host-rock fracture pattern as a passive marker and microstructures in the contact-metamorphic aureole to constrain large-scale magma emplacement-related deformation. The dip and azimuth of the fractures are very consistent on the roof of the intrusion and can be separated into four steeply inclined sets dominantly striking SE, S, NE, and E, which rules out pluton-wide doming. In contrast, fracture orientations in the northeastern wall to the granites suggest shear parallel to the contact. Additionally, contact-metamorphic segregations along the northeastern contact are brecciated. Based on the host-rock fracture pattern, the contact aureole deformation, and the north-eastward–inclined granite-granite contacts, we propose that mechanisms involving either asymmetric “trap-door” floor subsidence or laccolith and bysmalith intrusion along an inclined or curved floor accommodated the emplacement of the granites and led to deflection of the northeastern wall of the intrusion.

Impacts of Dam Break at Upstream of Lo River on Red River and Thai Binh River Delta

The Red River – Thai Binh River Delta is a large economic area, densely populated, especially Hanoi, the capital of Vietnam. This is the key area protected by the dike system that has been built for nearly 1000 years. Recently, along with the dike system, the Red River – Thai Binh River Delta is also protected by a large reservoir system at upstream. However, this raises concerns about dam safety, and requires assessments about the impact of dam break on the delta to prepare appropriate response measures. This paper applies 2D mathematical simulation model of flood propagation due to dam break at Thac Ba and Tuyen Quang reservoirs. The results clearly show the characteristics of the flood caused by dam break (such as water level, depth, velocity, lag time) in affected areas. Furthermore, overflow incident of main dikes and ring dikes at the downstream areas is also determined for both cases dam break with flood and without flood.

Nb–V-enriched sövites of the northeastern and eastern part of the Amba Dongar carbonatite ring dike, India – A reflection of post-emplacement hydrothermal overprint?

Wakefieldite-(Ce,La) and vanadinite in coarse-grained calciocarbonatites (sövites) are for the first time reported from the northeastern part of the worldwide largest fluorite deposit at the Amba Dongar carbonatite ring dike, India. Sövite in this part of the carbonatite ring dike is rich in pyrochlore, calcite and magnetite. Pyrochlore makes up almost 50% of some sövite samples and shows core-to-rim compositional changes. The core of pyrochlore consists of primary fluorcalciopyrochlore with high F and Na contents while the margins gained elevated amounts of Pb, La and Ce with the associated loss of F and Na due to circulation of hydrothermal solutions. The presence of wakefieldite-(Ce,La) and vanadinite points to an exceptionally high V abundance in hydrothermal solutions formed towards the end of the carbonatite magma activity. This investigation thus opens new promising areas for Nb and REE prospection in the eastern part of the Amba Dongar carbonatite body.

Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger

Remote sensing investigations of West Africa reveal a roughly 10 km in diameter, circular feature in north-central Niger. The circular feature is located within Early Devonian fluvio-marine sedimentary strata about 100 km to the north of a suite of igneous ring dikes located in the Aïr Massif. Whether the circular feature is related to the ring dikes, structural doming, or an impact crater can be evaluated from earlier geologic mapping (Black, 1967) and image analyses. Previous mapping indicates that the circular feature is defined by outcrops of Devonian Idekel sandstone and the Late Proterozoic, weakly-metamorphosed molasse of the Proche-Ténéré Formation. Absent in published lithologic descriptions of these sedimentary and metasedimentary rocks are diapiric evaporite deposits that could produce the observed circular morphologies. Lithologies that define the circular feature are confirmed by classification techniques using the ENVI software platform and band ratios/math from Landsat 8 and Sentinel 2A data. Spectral profile comparisons between the igneous intrusive rocks exposed in the Aïr Massif, outcrops of sedimentary rock that are proximal to the circular feature, and desert sand confirm that the sedimentary rocks mapped by Black (1967) define the circular feature. The shallow subsurface structure, revealed through Spaceborne Imaging Radar-C data, and 3-D modeling using Sentinel 1A data, is also not consistent with nearby ring dike structures. These data suggest that the circular feature did not likely form as a magmatic intrusion close to the time of sedimentary rock deposition. We propose that the circular feature formed from either a meteorite impact after deposition of the host sedimentary units or by some other, but less likely, geologic process. Confirmation of an impact explanation for the circular feature would require sample analysis and inspection for shock-metamorphosed materials.

Ferrocarbonatites in the Amba Dongar Diatreme, Gujarat, India

In the Amba Dongar diatreme, “ferrocarbonatite” is not a single unit of late differentiate of calciocarbonatite magma but it is a family with variation on field occurrence, mineralogy and chemistry of each unit. The family includes dikes of ankeritic carbonatites (phase I and II), plugs of ankeritic carbonatite within sovite ring dike, dikes of sideritic carbonatite in ankeritic carbonatite plug and rodberg veins. Their intrusive relations are very clear in the field and each phase has characteristic mineralogy and trace and REE geochemistry. According to the nomenclature suggested by Harmer and Gittins (1997) majority of “ferrocarbonatites” of Amba Dongar plot in field of “ferruginous calciocarbonatite” and only siderite and rodberg plot in the field of “ferrocarbonatite”. Within these family members, their trace and REE show clear increase from early phase to last phase of sideritic carbonatite. The present short communication discusses various aspects of “ferrocarbonatites”.

Ring faults and ring dikes around the Orientale basin on the Moon

The Orientale basin is the youngest and best-preserved multiring impact basin on the Moon, having experienced only modest modification by subsequent impacts and volcanism. Orientale is often treated as the type example of a multiring basin, with three prominent rings outside of the inner depression: the Inner Rook Montes, the Outer Rook Montes, and the Cordillera. Here we use gravity data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission to reveal the subsurface structure of Orientale and its ring system. Gradients of the gravity data reveal a continuous ring dike intruded into the Outer Rook along the plane of the fault associated with the ring scarp. The volume of this ring dike is ∼18 times greater than the volume of all extrusive mare deposits associated with the basin. The gravity gradient signature of the Cordillera ring indicates an offset along the fault across a shallow density interface, interpreted to be the base of the low-density ejecta blanket. Both gravity gradients and crustal thickness models indicate that the edge of the central cavity is shifted inward relative to the equivalent Inner Rook ring at the surface. Models of the deep basin structure show inflections along the crust–mantle interface at both the Outer Rook and Cordillera rings, indicating that the basin ring faults extend from the surface to at least the base of the crust. Fault dips range from 13° to 22° for the Cordillera fault in the northeastern quadrant, to 90° for the Outer Rook in the northwestern quadrant. The fault dips for both outer rings are lowest in the northeast, possibly due to the effects of either the direction of projectile motion or regional gradients in pre-impact crustal thickness. Similar ring dikes and ring faults are observed around the majority of lunar basins.

Timescale of magma chamber processes revealed by U–Pb ages, trace element contents and morphology of zircons from the Ishizuchi caldera, Southwest Japan Arc

AbstractU–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.

Pikes Peak batholith (Colorado, USA) revisited: A SIMS and LA-ICP-MS study of zircon U–Pb ages combined with solution Hf isotopic compositions

The Lake George ring complex (LGRC) is an epizonal intrusion that is part of seven alkalic stocks distributed throughout the Mesoproterozoic Pikes Peak batholith (PPB) in central Colorado. Despite having been studied for more than a century, details about the emplacement history of the PPB lithologies have remained elusive. To parse out the relative sequence of intrusive events and magma petrogenesis, we analyzed zircons from different LGRC intrusive phases for their U–Pb ages and Lu–Hf isotopic compositions. We found that the LGRC is composed of a 1428±8Ma syenomonzonite at its center, which has a Hf isotopic composition indistinguishable from that of coeval wall rocks related to the Berthoud plutonic suite. We hence interpret it as a roof pendant. The three ring dikes that surround the LGRC center have ages of 1085±5Ma, 1115±12Ma, and 1078±11Ma from innermost to outermost, respectively. The local phase of Pikes Peak granite that surrounds LGRC was emplaced at 1066±10Ma. Therefore, LGRC represents an early stage in PPB emplacement, and evidently formed over a protracted period of time. The LGRC zircon εHf values (+4.8±0.2 at 1115Ma, +0.4±0.9 at 1078Ma, and −0.8±0.2 at 1066Ma) indicate input of shallow mantle-derived melts and direct involvement of the Proterozoic Colorado Front Range (CFR) crust with the latter gradually prevailing over time. The results presented here provide new insights into the petrogenesis of the PPB and call for a general reassessment of emplacement ages and source characteristics of the other alkalic intrusive centers in the batholith.

Petrology and mineralogy of the La Peña igneous complex, Mendoza, Argentina: An alkaline occurrence in the Miocene magmatism of the Southern Central Andes

The La Peña alkaline igneous complex (LPC) is located in the Precordillera (32°41′34″ S - 68°59′48″ W) of Mendoza province, Argentina, above the southern boundary of the present-day flat-slab segment. It is a 19 km2 and 5 km diameter subcircular massif emplaced during the Miocene (19 Ma) in the Silurian-Devonian Villavicencio Fm. The LPC is composed of several plutonic and subvolcanic intrusions represented by: a cumulate of clinopyroxenite intruded by mafic dikes and pegmatitic gabbroic dikes, isolated bodies of malignite, a central intrusive syenite that develops a wide magmatic breccia in the contact with clinopyroxenite, syenitic and trachytic porphyries, a system of radial and ring dikes of different compositions (trachyte, syenite, phonolite, alkaline lamprophyre, tephrite), and late mafic breccias. The main minerals that form the LPC, ordered according to their abundance, are: pyroxene (diopside, hedenbergite), calcium amphibole (pargasite, ferro-pargasite, potassic-ferro-pargasite, potassic-hastingsite, magnesio-hastingsite, hastingsite, potassic-ferro-ferri-sadanagaite), trioctahedral micas (annite-phlogopite series), plagioclase (bytownite to oligoclase), K-feldspar (sanidine and orthoclase), nepheline, sodalite, apatite group minerals (fluorapatite, hydroxylapatite), andradite, titanite, magnetite, spinel, ilmenite, and several Cu–Fe sulfides. Late hydrothermal minerals are represented by zeolites (scolecite, thomsonite-Ca), epidote, calcite and chlorite.The trace element patterns, coupled with published data on Sr-Nd-Pb isotopes, suggest that the primary magma of the LPC was generated in an initially depleted but later enriched lithospheric mantle formed mainly by a metasomatized spinel lherzolite, and that this magmatism has a subduction-related signature. The trace elements pattern of these alkaline rocks is similar to other Miocene calc-alkaline occurrences from the magmatic arc of the Southern Central Andes.Mineral and whole-rock chemical compositions support the interpretation that a first batch of a tephritic magma produced a cumulate of clinopyroxenite (clinopyroxene + magnetite + apatite) and a residual melt that crystallized as malignite at a shallow emplacement level (<5 km). Fractional crystallization in a deep chamber, coupled with rock assimilation, produced successive magma pulses that gave the composite central syenite, the syenitic and trachitic porphyries, and trachytic dikes. The latest rocks (phonolites) reflect an extreme fractionation with Ca, Al and K removal by feldspars. Mingling relationships between tephrites-basanites and trachytes-phonolites, plus compositional variations linked to the reabsorption surface observed in the K-feldspar from the phonolitic dikes, suggest recharge events with local mixing in the late-stages of the LPC evolution.The emplacement of the LPC melts at a shallow crustal level was favored by the aperture of extensional NNW-SSE fractures genetically linked to a local brittle shear zone, active at 18–19 Ma during a strong compressional event, that initiated the regime of flat-slab subduction in this part of the Andes.

The 1997 flood event in the Red River basin: Causes, assessment and damages

In May 1997, the Red River Valley of Manitoba, North Dakota and Minnesota experienced a record flood which came to be known as the “Flood of the Century”. Prospects for only moderate flooding were completely transformed by a major snowstorm in the first week of April. Record snow and rain over the entire basin upstream of Winnipeg produced record discharges all along the river. At Winnipeg, the peak discharge was about 50% greater than the previous highest measured flow and was probably the second largest event in two centuries of documented flood history. At its peak, about 1840 km2 in Manitoba were inundated in a water body 100 km long with a maximum width of about 40 km. In Manitoba, 27,400 people were evacuated and damages and costs exceeded CAD $500 million (and were possibly closer to $1 billion). Nevertheless existing flood control structures, flood proofing and damage reduction programs, and emergency measures prevented losses which would have been truly catastrophic in Winnipeg. Furthermore, the peak stage might have been even greater except for manageable flows on the Assiniboine River and favourable weather during the flood itself. In the United States where flood control/flood proofing measures were less well developed, damages were many times greater and almost three times as many people were evacuated. The legacies of the flood in Manitoba include a CAD $665-million program to increase flood control and flood proofing for Winnipeg to the 700-year flood level. In the Red River valley outside Winnipeg, ring dikes were constructed around eight previously unprotected communities, individual properties were diked or elevated on pads, and flood proofing standards throughout the valley were raised to 0.6 m above the 1997 peak stage.

Caldera faults capture and deflect inclined sheets: an alternative mechanism of ring dike formation

The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet deflection into the subvertical ring fault. Our findings provide an alternative mechanical explanation for magma channeling along caldera ring faults, which is a process likely to be fundamental in controlling the location of post-caldera volcanism.

The “eye of Africa” (Richat dome, Mauritania): An isolated Cretaceous alkaline–hydrothermal complex

The Richat dome is a spectacular circular structure located in the Mauritanian part of the Sahara Desert. The current erosion level of this igneous complex presents a wide variety of contrasting extrusive and intrusive rocks from shallow to deep source regions providing insight into the magmatic process at the origin of the complex.The Richat is the superposition of a bimodal tholeiitic suite crosscut by carbonatitic and kimberlitic magmatic rocks. The bimodal series is characterized by two concentric gabbroic ring dikes and two extrusive rhyolitic centers representing the remnant of two maar systems. Silica undersaturated magmas occur as carbonatite dikes, a kimberlite plug, and kimberlite sills extruded along the old regional anisotropies filling NNE–SSW dextral strike-slip faults and en-echelon tension gashes. An intense low-temperature hydrothermal event affected the Richat area. It is responsible, notably, for the karst-collapse central mega-breccia, the alteration of the rhyolites, the potassic alteration of the gabbros and the stable isotope enrichment in the carbonatites. A piston-like collapse is proposed to explain the contrast existing between the central and outer part of the Richat.Structural inheritance played an important role in the history of the Richat complex. Pre-existing anisotropies acted as a pathway for the ascent of asthenospheric and sub-continental melts and allowed the coexistence of alkaline and tholeiitic magmas within the same igneous complex.

The role of aeromagnetic data analysis (using 3D Euler deconvolution) in delineating active subsurface structures in the west central Arabian shield and the central Red Sea, Saudi Arabia

In this paper, we present a case study of structural mapping by applying the 3D Euler method to the high-resolution aeromagnetic data that was collected in the west central Arabian Shield region and the coastal region of the central Red Sea in Saudi Arabia. We show the 3D Euler deconvolution algorithm and apply it to magnetic potential field data from the west Central Arabian Shield and the Central Red Sea. The solution obtained with 3D Euler deconvolution gives better-focused depth estimates, which are closer to the real position of sources; the results presented here can be used to constrain depth to active crustal structures (volcanisms) for the study area. The results indicated that the area was affected by sets of fault systems, which primarily trended in the NNW–SSE, NW–SE, EW, and NE–SW directions. Moreover, estimated Euler solution map from aeromagnetic data delineated also the boundaries of shallow, small, and confined magnetic bodies on the offshore section of the study area. These nearly exposed basement intrusions are most likely related to the Red Sea Rift and may be associated with structures higher up in the sedimentary section. These volcanic bodies are extended to the continental part (onshore) of the west central Arabian Shield, particularly beneath both sides of the Ad Damm fault zone. This extension verifies that the fault was largely contemporaneous with a major period during the extension of the Red Sea Basin. Moreover, according to the distribution of circular magmatic-source bodies (circular-shaped ring dikes) that resulted from this study, we can state that the clustering of most earthquakes along this fault may most likely be attributed to the active mantle upwelling (volcanic earthquakes), which are ultimately related to volcanic processes. Furthermore, the oceanic crustal structures near and in the Red Sea offshore regions were also estimated and discussed according to the ophiolite occurrences and further opening of the Red Sea. Our results are largely comparable with studies of previous crustal sections, which were performed along the Red Sea Rift and the Arabian Shield. As a result, the areas above these anomalies are highly recommended for further geothermal study. This example illustrates that high-resolution aeromagnetic surveys can greatly help delineating the subsurface active structures in the west central Arabian Shield and the middle coastal region of the Red Sea of Saudi Arabia.

Sedimentation in the floodplains of the Mekong Delta, Vietnam Part II: deposition and erosion

AbstractDeposition and erosion play a key role in the determination of the sediment budget of a river basin, as well as for floodplain sedimentation. Floodplain sedimentation, in turn, is a relevant factor for the design of flood protection measures, productivity of agro‐ecosystems, and for ecological rehabilitation plans. In the Mekong Delta, erosion and deposition are important factors for geomorphological processes like the compensation of deltaic subsidence as well as for agricultural productivity. Floodplain deposition is also counteracting the increasing climate change induced hazard by sea level rise in the delta. Despite this importance, a sediment database of the Mekong Delta is lacking, and the knowledge about erosion and deposition processes is limited. In the Vietnamese part of the Delta, the annually flooded natural floodplains have been replaced by a dense system of channels, dikes, paddy fields, and aquaculture ponds, resulting in floodplain compartments protected by ring dikes. The agricultural productivity depends on the sediment and associated nutrient input to the floodplains by the annual floods. However, no quantitative information regarding their sediment trapping efficiency has been reported yet. The present study investigates deposition and erosion based on intensive field measurements in three consecutive years (2008, 2009, and 2010). Optical backscatter sensors are used in combination with sediment traps for interpreting deposition and erosion processes in different locations. In our study area, the mean calculated deposition rate is 6.86 kg/m2 (≈ 6 mm/year). The key parameters for calculating erosion and deposition are estimated, i.e. the critical bed shear stress for deposition and erosion and the surface constant erosion rate. The bulk of the floodplain sediment deposition is found to occur during the initial stage of floodplain inundation. This finding has direct implications on the operation of sluice gates in order to optimize sediment input and distribution in the floodplains. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Geochemical and isotopic constraints on the petrogenesis of the Puesto La Peña undersaturated potassic complex, Mendoza province, Argentina: Geodynamic implications

Major and trace element and radiogenic and stable isotope data are reported for the Miocene Puesto La Peña undersaturated potassic complex, located in the Argentine Precordillera. The complex is composed of a pyroxenite core surrounded by a malignite–borolanite association, followed by radial and ring dikes composed of alkaline lamprophyres, tephrite to phonolite and intermediate varieties, porphyritic microledmorite, benmoreite, alkaline trachyte and a swarm of ultrabasic ouachitite dikes. Late trachytic to phonolitic volcanic necks and associated dikes crop out surrounding the malignitic–borolanitic massif.Major and trace element data are consistent with their derivation by partial melting from an enriched, probably lithospheric, metasomatized heterogeneous mantle involving spinel- and garnet-facies mantle sources. The trace element and isotope data indicate that all rock types are derived from a common parental magma and are thus cogenetic.Sr, Nd and Pb variations in the different studied lithologies are restricted, except for the evidence of Sr decoupling in the ouachitite samples. Their Sr–Nd isotope compositions follow the “mantle array” defined by oceanic basalts, within the OIB field, consistent with an intraplate depleted source. The genesis of the ouachitite is interpreted to be related to mixing between asthenospheric magma and melts from the lower lithosphere consisting of K-rich metasomatic layers. The Dupal like Pb isotopes signature suggests a mantle modification by introduction of continental crust material in the upper mantle; this is consistent with the Sr–Nd isotope data from the late silica-undersaturated felsic dikes and volcanic necks.Multistage mantle extraction would have occurred by the end of Neoproterozoic to lower Paleozoic times as indicated by the isotopic data. Partial melting was initiated by mantle upwelling decompression during lithosphere extension. Back-arc extensional conditions during the latest Early Miocene (19Ma), related to the geometry of the Pacific subducted plate beneath the South American plate, favored the tectonic conditions for magma ascent from a magmatic reservoir located at around 30km depth in the crust. A fractional crystallization process, primarily of clinopynoxene, plagioclase, alkali feldspar, nepheline and lesser biotite, magnetite, and apatite, first yielded the cumulate clinopyroxenite, followed by the malignite–borolanite association and the late dikes and volcanic necks. The morphology of the complex, as well as the relationship between their different facies, points to a cauldron subsidence model of emplacement.