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At least 217 records · Page 12Linked to original sources

USGS2014

A conceptual framework for clutch size evolution in songbirds

Causes of evolved differences in clutch size among songbird species remain debated. I propose a new conceptual framework that integrates aspects of traditional life history theory, while including novel elements, to explain evolution of clutch size among songbirds. I review evidence that selection by nest predation on length of time that offspring develop in the nest creates a gradient in offspring characteristics at nest-leaving (fledging), including flight mobility, spatial dispersion, and self-feeding rate. I postulate that this gradient has consequences for offspring mortality rates and parental energy expenditure per offspring. These consequences then determine how reproductive effort is partitioned among offspring, while reproductive effort evolves from age-specific mortality effects. Using data from a long-term site in Arizona, as well as from the literature, I provide support for hypothesized relationships. Nestling development period consistently explains fledgling mortality, energy expenditure per offspring, and clutch size while accounting for reproductive effort (i.e., total energy expenditure) to thereby support the framework. Tests in this paper are not definitive, but they document previously unrecognized relationships and address diverse traits (developmental strategies, parental care strategies, energy requirements per offspring, evolution of reproductive effort, clutch size) that justify further investigations of hypotheses proposed here.

American Naturalist↗

USGS2017

Soils as relative-age dating tools

Soils develop at the earth's surface via multiple processes that act through time. Precluding burial or disturbance, soil genetic horizons form progressively and reflect the balance among formation processes, surface age, and original substrate composition. Soil morphology provides a key link between process and time (soil age), enabling soils to serve as both relative and numerical dating tools for geomorphic studies and landscape evolution. Five major factors define the contemporary state of all soils: climate, organisms, topography, parent material, and time. Soils developed on similar landforms and parent materials within a given landscape comprise what we term a soil/landform/substrate complex. Soils on such complexes that differ in development as a function of time represent a soil chronosequence. In a soil chronosequence, time constitutes the only independent formation factor; the other factors act through time. Time dictates the variations in soil development or properties (field or laboratory measured) on a soil/landform/substrate complex. Using a dataset within the chronosequence model, we can also formulate various soil development indices based upon one or a combination of soil properties, either for individual soil horizons or for an entire profile. When we evaluate soil data or soil indices mathematically, the resulting equation creates a chronofunction. Chronofunctions help quantify processes and mechanisms involved in soil development, and relate them mathematically to time. These rigorous kinds of comparisons among and within soil/landform complexes constitute an important tool for relative-age dating. After determining one or more absolute ages for a soil/landform complex, we can calculate quantitative soil formation, and or landform-development rates. Multiple dates for several complexes allow rate calculations for soil/landform-chronosequence development and soil-chronofunction calibration.

Book chapter↗

USGS1997

Chemical weathering of a soil chronosequence on granitoid alluvium: II. Mineralogic and isotopic constraints on the behavior of strontium

The use of strontium isotopes to evaluate mineral weathering and identify sources of base cations in catchment waters requires an understanding of the behavior of Sr in the soil environment as a function of time. Our approach is to model the temporal evolution of 87 Sr/ 86 Sr of the cation exchange pool in a soil chronosequence developed on alluvium derived from central Sierra Nevada granitoids during the past 3 Ma. With increasing soil age, 87 Sr/ 86 Sr of ammonium-acetate extractable Sr initially decreases from values typical of K-feldspar to those of plagioclase and hornblende and then remains constant, even though plagioclase and hornblende are absent from the soils after approximately 1 Ma of weathering. The temporal variation of 87 Sr/ 86 Sr of exchangeable Sr is modeled by progressively equilibrating Sr derived from mineral weathering and atmospheric deposition with Sr on exchange sites as waters infiltrate a soil column. Observed decreases in quartz-normalized modal abundances of plagioclase, hornblende, and K-feldspar with time, and the distinct 87 Sr/ 86 Sr values of these minerals can be used to calculate Sr flux from weathering reactions. Hydrobiotites in the soils have nearly constant modal abundances, chemistry, and 87 Sr/ 86 Sr over the chronosequence and provide negligible Sr input to weathering solutions. The model requires time and soil horizon-dependent changes in the amount of exchangeable Sr and the efficiency of Sr exchange, as well as a biologic cycling term. The model predicts that exchangeable Sr initially has 87 Sr/ 86 Sr identical to that of K-feldspar, and thus could be dominated by Sr leached from K-feldspar following deposition of the alluvium. The maximum value of 87 Sr/ 86 Sr observed in dilute stream waters associated with granitoids of the Yosemite region is likewise similar to that of the K-feldspars, suggesting that K-feldspar and not biotite may be the dominant source of radiogenic Sr in the streams. This study reveals that, when attempting to use Strontium isotopes to identify sources of base cations in catchment waters and biomass, both preferential leaching of Sr from minerals during incipient soil development and changing Sr exchange efficiency must be considered along with chemical contributions due to mineral dissolution.

Geochimica et Cosmochimica Acta↗

USGS2020

Integrated geophysical analysis provides an alternate interpretation of the northern margin of the North American Midcontinent Rift System, Central Lake Superior

The Midcontinent Rift System (MRS) is a 1.1 Ga sequence of voluminous basaltic eruptions and multiple intrusions followed by widespread sedimentation that extends across the Midcontinent and northern Great Lakes region of North America. Previous workers have commonly used seismic-reflection data (Great Lakes International Multidisciplinary Program on Crustal Evolution [GLIMPCE] line A) to demonstrate that the northern rift margin in central Lake Superior developed as a normal growth fault that was structurally inverted to a reverse fault during a compressional event after rifting had ended. A prominent, curvilinear aeromagnetic anomaly that extends from Isle Royale, Michigan, to Superior Shoal in central Lake Superior, Ontario (the IR-SS anomaly), is commonly presented as a manifestation of this reverse fault. We have integrated multidisciplinary geophysical analyses (seismic-reflection, seismic-refraction, aeromagnetic, and gravity), physical-property information (density, magnetic susceptibility and remanence, and compressional-wave velocity), and geologic concepts to develop an alternate interpretation of the rift margin along GLIMPCE line A, where it intersects the IR-SS anomaly. Our new model indicates that a normal fault is the dominant structure at the northern rift margin along line A, contrary to the original rift-margin paradigm, which asserts that compressional structures are the dominant features preserved today. Integral to this alternate model is a newly interpreted, prerift sedimentary basin intruded by sills in northern Lake Superior. Our alternate model of the northern rift margin has implications for interpreting the style, scale, and timing of extension, rift-related intrusion, and compression during development of the MRS.

Lake Superior↗

USGS2019

Slough evolution and legacy mercury remobilization induced by wetland restoration in South San Francisco Bay

Coastal wetlands have a long history of degradation and destruction due to human development. Now recognized as one of the most productive ecosystems in the world, substantial efforts are being made to restore this critical habitat. While wetland restoration efforts are generally viewed as beneficial in terms of providing wildlife habitat and flood control, they are often accompanied by dramatic physical and chemical changes that may result in unintended consequences, which are rarely studied. Alviso Slough, a tidal slough in South San Francisco Bay, California, is the site of an ongoing effort to restore former salt-production ponds to intertidal marsh habitat. Restoration is complicated by the fact that (1) the ponds undergoing restoration are severely subsided and (2) subsurface sediments within the slough and surrounding ponds are contaminated with legacy mercury deposits. Due to concerns regarding mercury remobilization, restoration has proceeded in a cautious, methodical manner. To assess the amount of legacy mercury remobilized since restoration began, we developed a technique of combining high-resolution, biannual measurements of bathymetric scour with mercury concentration measurements from sediment cores. We estimate that 52 kg (±3) of mercury was remobilized in the 6 years since restoration began. Net bathymetric change analyses revealed seasonal trends of peak erosion during the winter months and little to no net change during summer months. Our analyses provide crucial insight on the spatial and temporal scales of geomorphic evolution within a tidal slough resulting from both natural (seasonal) variability and restoration actions. The technique presented here could be applied to other study sites and various sediment-associated contaminants of concern to aid in the design and management of restoration projects aiming to minimize negative impacts from legacy contaminants.

California↗

USGS2021

Landscape evolution in eastern Chuckwalla Valley, Riverside County, California

This study investigates sedimentary and geomorphic processes in eastern Chuckwalla Valley, Riverside County, California, a region of arid, basin-and-range terrain where extensive solar-energy development is planned. The objectives of this study were to (1) measure local weather parameters and use them to model aeolian sediment-transport potential; (2) identify surface sedimentary characteristics in representative localities; and (3) evaluate long-term landscape evolution rates and processes by analyzing stratigraphy in combination with luminescence geochronology. The new stratigraphic and geochronologic data presented in this report demonstrate the varying local significance of aeolian, alluvial fan, lacustrine (playa), and possibly Colorado River influence over a range of time scales. The dominant sand-transport direction in eastern Chuckwalla Valley is toward the northeast, consistent with the recognized regional west-to-east wind direction. However, occasional strong wind events from the north can transport large quantities of sand southward and temporarily reshape local geomorphic features. Influence of a northwest wind direction is also locally dominant around mountain ranges and controls the modern morphology of the Palen dune field. Modeled sand fluxes are on the order of 10 5 kilograms per meter width per year at the site of weather monitoring, 5 kilometers northwest of the Mule Mountains. Aeolian dunes are locally well developed and actively migrating. Their location and activity are determined largely by sediment supply from playa surfaces and ephemeral stream channels, which also control the dunes’ spatial extent and migration potential; stream channels act as both source and sink for aeolian sediment in this environment. Excavations at five sites along a northwest-to-southeast transect reveal that playa deposits formed around 266–226 thousand years ago south of the McCoy Mountains and immediately north of the present location of Interstate 10. The playa material is overlain by late Pleistocene to Holocene alluvial fan deposits. To the southeast (south of Interstate 10, but north of the Mule Mountains), we identified rapid accumulation of alluvial sediment around the time of the Last Glacial Maximum (23–20 thousand years ago), unconformably overlain by a locally varying assemblage of recent aeolian material or Holocene alluvial fan sediment. We have used stratigraphic characteristics and luminescence ages to calculate accumulation rates for sites in eastern Chuckwalla Valley, and thereby to identify spatial variation in landscape stability over decadal and longer time scales. If future solar-energy development plans are to include natural sand-transport corridors, plans would entail retaining the ability for sand to be transported eastward from the ephemeral stream channels and playas that supply sediment to the dunes, sand sheets, and sand ramps of Chuckwalla Valley, and also to allow for southward transport during episodic strong weather events several times per year. The aeolian sediment-transport corridors are dynamic spatially and temporally, reorganizing on the basis of seasonal changes to wind drift potential. Future landscape stability also will be determined by climate-driven changes to vegetation and thereby to aeolian sediment availability. In a warmer, drier climate, aeolian sediment activity is expected to increase, owing to a decrease in stabilizing vegetation cover and more extreme rain that supplies sediment to ephemeral stream channels and playas from which it is remobilized by wind.

California↗

USGS1999

Climatic-eustatic control of Holocene nearshore parasequence development, southeastern Texas coast

Sediment cores, seismic profiles, radiocarbon dates, and faunal assemblages were used to interpret the depositional setting and geological evolution of the southeastern Texas coast during the last glacio-eustatic cycle. Discrete lithofacies and biofacies zones in the ebb-dominated Sabine Lake estuary and adjacent chenier plain record alternating periods of rapid marine flooding and gradual shoaling related to linked climatic/eustatic fluctuations. Monospecific zones of the mollusks Rangia cuneata and Crassostrea virginica, respectively, indicate high fresh water outflow followed by invasion of marine water, whereas intervening organic-rich zones record bayhead delta deposition. High-frequency parasequence stacking patterns within the valley fill and across the adjacent interfluve reflect an initial rapid rise in sea level about 9 ka that flooded abandoned alluvial terraces and caused onlap of Holocene marsh in the incised valley. The rapid rise was followed by slowly rising and oscillating sea level that filled the deepest portions of the incised valleys with fluvially dominated estuarine deposits, and then a maximum highstand (+1 m msl) about 5 ka that flooded the former subaerial coastal plain between the incised valleys and constructed the highest beach ridges. Between 3.5 and 1.5 ka, sea level oscillated and gradually fell, causing a forced regression and rapid progradation of both the chenier plain and accretionary barrier islands. The only significant sands in the valley fill are (1) falling-stage and lowstand-fluvial sediments between the basal sequence boundary and transgressive surface unconformity, and (2) highstand beach-ridge sediments of the chenier plain.

Texas↗

USGS2005

Evolution of a late Cenozoic supradetachment basin above a flat-on-flat detachment with a folded lateral ramp, SE Idaho

Uplift and exposure of the Bannock detachment system and the synextensional basin fill deposits of the Salt Lake Formation provide a unique exposure of the three-dimensional geometries of a low-angle normal fault system and the stratal architecture of the overlying supradetachment basin. Within this system, structural and stratigraphic analyses, outcrop patterns, tephra geochronology, and geological cross sections document several important relationships: (1) the Bannock detachment system developed above the Sevierage Cache-Pocatello culmination and resembles the Sevier Desert detachment in its geometry, structural setting, and kinematic evolution; (2) the Bannock detachment system initiated and slipped at low angles; (3) flat-on-flat, ramp-flat, and lateral ramp geometries, as well as excision, can significantly affect the hanging wall deformation style due to the shallow depth (~2–4 km) of the Bannock detachment fault during late stages of slip; (4) late Miocene–Pliocene tuffaceous synrift deposits of the Salt Lake Formation record deposition in a supradetachment basin, display an unroofing sequence, and a three-stage evolution that includes pre-translation, translation, and breakup phases. Recycled pre-translation and translation phase deposits are diagnostic of this evolution; and (5) beginning in mid- to late Pliocene time, high-angle, north-striking Basin and Range faults disrupted and dismembered the Bannock detachment system.

Idaho↗

USGS2018

The GFDL global atmosphere and land model AM4.0/LM4.0: 2. Model description, sensitivity studies, and tuning strategies

In Part 2 of this two‐part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part 1. Part 2 provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.

Journal of Advances in Modeling Earth Systems↗

USGS2015

Nest predation research: Recent findings and future perspectives

Nest predation is a key source of selection for birds that has attracted increasing attention from ornithologists. The inclusion of new concepts applicable to nest predation that stem from social information, eavesdropping or physiology has expanded our knowledge considerably. Recent methodological advancements now allow focus on all three players within nest predation interactions: adults, offspring and predators. Indeed, the study of nest predation now forms a vital part of avian research in several fields, including animal behaviour, population ecology, evolution and conservation biology. However, within nest predation research there are important aspects that require further development, such as the comparison between ecological and evolutionary antipredator responses, and the role of anthropogenic change. We hope this review of recent findings and the presentation of new research avenues will encourage researchers to study this important and interesting selective pressure, and ultimately will help us to better understand the biology of birds.

Journal of Ornithology↗

USGS2014

Gully annealing by aeolian sediment: field and remote-sensing investigation of aeolian-hillslope-fluvial interactions, Colorado River corridor, Arizona, USA

Processes contributing to development of ephemeral gully channels are of great importance to landscapes worldwide, and particularly in dryland regions where soil loss and land degradation from gully erosion pose long-term land-management problems. Whereas gully formation has been relatively well studied, much less is known of the processes that anneal gullies and impede their growth. This study of gully annealing by aeolian sediment, spanning 95 km along the Colorado River corridor in Glen, Marble, and Grand Canyon, Arizona, USA, employed field and remote sensing observations, including digital topographic modelling. Results indicate that aeolian sediment activity can be locally effective at counteracting gully erosion. Gullies are less prevalent in areas where surficial sediment undergoes active aeolian transport, and have a greater tendency to terminate in active aeolian sand. Although not common, examples exist in the record of historical imagery of gullies that underwent infilling by aeolian sediment in past decades and evidently were effectively annealed. We thus provide new evidence for a potentially important interaction of aeolian–hillslope–fluvial processes, which could affect dryland regions substantially in ways not widely recognized. Moreover, because the biologic soil crust plays an important role in determining aeolian sand activity, and so in turn the extent of gully development, this study highlights a critical role of geomorphic–ecologic interactions in determining arid-landscape evolution.

Arizona↗

USGS2002

History of the recognition of organic geochemistry in geoscience

The discipline of organic geochemistry is an outgrowth of the application of the principles and methods of organic chemistry to sedimentary geology. Its origin goes back to the last part of the nineteenth century and the first part of the twentieth century concurrent with the evolution of the applied discipline of petroleum geochemistry. In fact, organic geochemistry was strongly influenced by developments in petroleum geochemistry. Now, however, organic geochemistry is considered an umbrella geoscience discipline of which petroleum geochemistry is an important component.

Organic Geochemistry↗

USGS1997

Selection of the Mars Pathfinder landing site

The Mars Pathfinder spacecraft will land on a depositional fan near the mouth of the catastrophic outflow channel, Ares Vallis (19.5°N, 32.8°W). This site offers the prospect of analyzing a variety of rock types from the ancient cratered highlands, intermediate-age ridged plains, and reworked channel deposits. Analyses of these rocks by Pathfinder instruments will enable first-order scientific questions to be addressed, such as differentiation of the crust, the development of weathering products, and the nature of the early environment, as well as their subsequent evolution on Mars. Constraints imposed by (1) spacecraft and rover designs (which are robust), (2) entry, descent, and landing, (3) scientific potential at various sites, and (4) safety were important considerations in site selection. Engineering constraints require a 70 km by 200 km smooth, flat (low slopes) area located between 10° and 20°N that is below 0 km elevation, with average radar reflectivity, little dust, and moderate rock abundance. Three regions on Mars are between 10° and 20°N and below 0 km elevation: Chryse, Amazonis, and Isidis-Elysium. Science considerations favor sites at the mouths of outflow channels (grab bag sites offer an assay of rock types on Mars), highland sites (early crustal differentiation and climate), and sites covered with dark (unoxidized) material. Sites are considered safe if they are clearly below 0 km elevation, appear acceptably free of hazards in high-resolution (<50 m/pixel) Viking orbiter images and have acceptable reflectivity and roughness at radar wavelengths, thermal inertia, rock abundance, red to violet ratio, and albedo. Recent 3.5-cm wavelength radar observations were used to verify elevation, reflectivity, and roughness within the landing ellipses. Three sites meet all of these criteria: Ares Vallis, Tritonis Lacus, and Isidis. Although Isidis appears to be safer than Tritonis and Ares, the greater scientific potential at Ares Vallis resulted in its selection. Comparisons of the Grand Coulee (channel) and the depositional Ephrata Fan of the Channeled Scabland in eastern Washington, with Ares Vallis and its depositional fan also suggest the Ares Vallis landing site is safe and scientifically interesting.

Journal of Geophysical Research Planets↗

USGS2013

The age of scarplike landforms from diffusion-equation analysis

The purpose of this paper is to review developments in the quantitative modeling of fault-scarp geomorphology, principally those since 1980. These developments utilize diffusionequation mathematics, in several different forms, as the basic model of fault-scarp evolution. Because solutions to the general diffusion equation evolve with time, as we expect faultscarp morphology to evolve with time, the model solutions carry information about the age of the structure and thus its time of formation; hence the inclusion of this paper in this volume. The evolution of fault-scarp morphology holds a small but special place in the much larger class of problems in landform evolution. In general, landform evolution means the evolution of topography as a function of both space and time. It is the outcome of the competition among those tectonic processes that make topography, erosive processes that destroy topography, and depositional processes that redistribute topography. Deposition and erosion can always be coupled through conservation-of-mass relations, but in general deposition occurs at great distance from the source region of detritus. Moreover, erosion is an inherently rough process whereas deposition is inherently smooth, as is evident from even casual inspection of shaded-relief, digital-elevation maps (e.g., Thelin and Pike, 1990; Simpson and Anders, 1992) and the current fascination with fractal representations oferoding terrains (e.g., Huang and Turcotte, 1989; Newman and Turcotte, 1990). Nevertheless, large-scale landform-evolution modeling, now a computationally intensive, advanced numerical exercise, is generating ever more realistic landforms (e.g., Willgoose and others, 1991a,b; Kooi and Beaumont, 1994; Tucker and Slingerland, 1994), although many of the rate coefficients remain poorly prescribed

Book chapter↗

USGS2010

Sampling in ecology and evolution - bridging the gap between theory and practice

Sampling is a key issue for answering most ecological and evolutionary questions. The importance of developing a rigorous sampling design tailored to specific questions has already been discussed in the ecological and sampling literature and has provided useful tools and recommendations to sample and analyse ecological data. However, sampling issues are often difficult to overcome in ecological studies due to apparent inconsistencies between theory and practice, often leading to the implementation of simplified sampling designs that suffer from unknown biases. Moreover, we believe that classical sampling principles which are based on estimation of means and variances are insufficient to fully address many ecological questions that rely on estimating relationships between a response and a set of predictor variables over time and space. Our objective is thus to highlight the importance of selecting an appropriate sampling space and an appropriate sampling design. We also emphasize the importance of using prior knowledge of the study system to estimate models or complex parameters and thus better understand ecological patterns and processes generating these patterns. Using a semi-virtual simulation study as an illustration we reveal how the selection of the space (e.g. geographic, climatic), in which the sampling is designed, influences the patterns that can be ultimately detected. We also demonstrate the inefficiency of common sampling designs to reveal response curves between ecological variables and climatic gradients. Further, we show that response-surface methodology, which has rarely been used in ecology, is much more efficient than more traditional methods. Finally, we discuss the use of prior knowledge, simulation studies and model-based designs in defining appropriate sampling designs. We conclude by a call for development of methods to unbiasedly estimate nonlinear ecologically relevant parameters, in order to make inferences while fulfilling requirements of both sampling theory and field work logistics. ?? 2010 The Authors.

Ecography↗

USGS2021

Movement of sediment through a burned landscape: Sediment volume observations and model comparisons in the San Gabriel Mountains, California, USA

Post-wildfire changes to hydrologic and geomorphic systems can lead to widespread sediment redistribution. Understanding how sediment moves through a watershed is crucial for assessing hazards, developing debris flow inundation models, engineering sediment retention solutions, and quantifying the role that disturbances play in landscape evolution. In this study, we used terrestrial and airborne lidar to measure sediment redistribution in the 2016 Fish Fire, in the San Gabriel Mountains in southern California, USA. The lidar areas are in two adjacent watersheds, at spatial scales of 900 m 2 to 4 km 2 , respectively. Terrestrial lidar data were acquired prior to rainfall, and two subsequent surveys show erosional change after rainstorms. Two airborne lidar flights occurred (1) 7 months before, and (2) 14 months after the fire ignition, capturing the erosional effects after rainfall. We found hillslope erosion dominated the overall sediment budget in the first rainy season after wildfire. Only 7% of the total erosion came from the active channel bed and channel banks, and the remaining 93% of eroded sediment was derived from hillslopes. Within the channelized portion of the watershed erosion/deposition could be generally described with topographic metrics used in a stream power equation. Observed sediment volumes were compared with four empirical models and one process-based model. We found that the best predictions of sediment volume were obtained from an empirical model developed in the same physiographic region. Moreover, this study showed that post-wildfire erosion rates in the San Gabriel Mountains attain the same magnitude as millennial time scale bedrock erosion rates.

California↗

USGS2020

Compositional layering in Io driven by magmatic segregation and volcanism

The compositional evolution of volcanic bodies like Io is not well understood. Magmatic segregation and volcanic eruptions transport tidal heat from Io's interior to its surface. Several observed eruptions appear to be extremely high temperature (≥ 1600 K), suggesting either very high degrees of melting, refractory source regions, or intensive viscous heating on ascent. To address this ambiguity, we develop a model that couples crust and mantle dynamics to a simple compositional system. We analyse the model to investigate chemical structure and evolution. We demonstrate that magmatic segregation and volcanic eruptions lead to stratification of the mantle, the extent of which depends on how easily high temperature melts from the more refractory lower mantle can migrate upwards. We propose that Io's highest temperature eruptions originate from this lower mantle region, and that such eruptions act to limit the degree of compositional stratification.

Journal of Geophysical Research↗

USGS2021

Basalt geochemistry and mantle flow during early backarc basin evolution: Havre Trough and Kermadec Arc, southwest Pacific

The Havre Trough (HT) backarc basin in the southwest Pacific is in the rifting stage of development. We distinguish five types of basalt there based on their amount and kind of slab component: backarc basalts (BAB) with little or no slab component, modified BAB with slight amounts, reararc (RA) with more, remnants of the preexisting arc (Colville Ridge horsts), and arc front volcanoes within the HT. Previous subarc mantle is quickly removed and replaced by more fertile mantle with less slab component. The ambient mantle is “Pacific” isotopically, and more enriched in Nb/Yb and Nd and Hf isotope ratios north of the Central Kermadec Discontinuity at 32°S than to the south. The contrast may reflect inheritance in the south of mantle that was depleted during spreading that formed the southern South Fiji Basin and a higher degree of melting because of a wetter slab-derived flux. The slab component also differs along strike, more like a dry melt in the north and a supercritical fluid in the south. The mass fraction of slab component increases southward in the backarc as well as the arc front. RA volcanoes have the most slab component (1%–2%) and form indistinct ridges at high angles to, and <50 km behind, frontal volcanoes. Backarc basalts have less and occur throughout the basin. Slab components are distributed further into the backarc, and more irregularly, during the rifting than spreading stage of backarc basin development. The rifting stage is disorganized geochemically as well as spatially.

Geochemistry Geophysics Geosystems↗