News Release

New Geology articles published online ahead of print

Peer-Reviewed Publication

Geological Society of America

Boulder, Colo., USA: Article topics and locations include detrital glass in a Bering Sea sediment core; evidence for microbial mediation of silicification in trilobites; sponge-rich sediment recycling; pre-agricultural soil erosion rates in the midwestern United States; and pacing of the East Asian summer monsoon over the past five glacial cycles inferred from land snails. These Geology articles are online at .

Paleozoic vegetation increased fine sediment in fluvial and tidal channels: Evidence from secular changes to the mudrock content of ancient point bars
William J. McMahon; Neil S. Davies; Maarten G. Kleinhans; Ria L. Mitchell
The amount of mudrock preserved globally in alluvium increased in stratigraphic synchrony with the Paleozoic evolution of land plants. This observation has been explained by vegetation promoting both the retention of mud through baffling, stabilization, and flocculation, and the production of mud through chemical weathering. However, the latter explanation has been challenged on the basis that it is perceived to require imbalance in the long-term global carbon cycle. We present a compendium of empirical evidence that is supportive of increased global fine sediment supply, and thus the contention that land plants did, in fact, promote the production of mud on the continents. We refine previous broad-brush analyses of Paleozoic mudrock content by specifically tracking shifts in the mudrock content of regions of alluvial and tidal landscapes that remained locally unvegetated even after the greening of the continents, namely inclined heterolithic stratification (IHS) that records submerged in-channel bars. We show that the Paleozoic mudrock increase was pronounced even within these areas, away from any biomechanical binding and baffling effects of plants. Precambrian and Cambrian IHS are composed almost exclusively of sandstone, whereas Silurian through to Carboniferous examples show a steady increase in total mudrock content. This progressive rise in the mudrock component of channel bars cannot alone be explained by physical retention of mud by vegetation and requires heightened fine sediment concentrations from the hinterland, which suggests that plants increased the volume of mud available at source. The muddying of Earth’s preserved IHS serves as a proxy that suggests evolving Paleozoic land plants triggered a global increase in the production and supply of fine-grained sediment.

Late Miocene to recent tectonic evolution of the Macquarie Triple Junction
Luca Gasperini; Marco Ligi; Daniela Accettella; Alessandro Bosman; Marco Cuffaro ...
The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of Macquarie Island. Morphobathymetric, magnetic, and gravity data have been used to understand the evolution of the three accretionary/transform boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault triple junction. The long life (>6 m.y.) of this configuration can be attributed to a rapid increase in spreading asymmetry along the Southeast Indian Ridge segment as it approaches the MTJ, and to transtension along the southernmost strand of the Macquarie–Pacific transform boundary. A major change in plate motion triggered the development of the Macquarie plate at ca. 6 Ma and makes clear the recent evolution of the MTJ, including (1) shortening of the Southeast Indian Ridge segment; (2) formation of the westernmost Pacific-Antarctic Ridge, which increased its length over time; and (3) lengthening of the two transform boundaries converging in the MTJ. The clockwise change of the Pacific-Antarctic motion (ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to the east of the triple junction, with fragmentation of the long-offset Emerald transform fault and its replacement over a short time interval (1–2 m.y.) with closely spaced, highly variable transform offsets that were joined by short ridge segments with time-varying asymmetries in the spreading rates.

Deep magma mobilization years before the 2021 CE Fagradalsfjall eruption, Iceland
M. Kahl; E.J.F. Mutch; J. Maclennan; D.J. Morgan; F. Couperthwaite ...
The deep roots of volcanic systems play a key role in the priming, initiation, and duration of eruptions. Causative links between initial magmatic unrest at depth and eruption triggering remain poorly constrained. The 2021 CE eruption at Fagradalsfjall in southwestern Iceland, the first deep-sourced eruption on a spreading-ridge system monitored with modern instrumentation, presents an ideal opportunity for comparing geophysical and petrological data sets to explore processes of deep magma mobilization. We used diffusion chronometry to show that deep magmatic unrest in the roots of volcanic systems can precede apparent geophysical eruption precursors by years, suggesting that early phases of magma accumulation and reorganization can occur in the absence of significant increases in shallow seismicity (<7 km depth) or rapid geodetic changes. Closer correlation between geophysical and diffusion age records in the months and days prior to eruption signals the transition from a state of priming to full-scale mobilization in which magma begins to traverse the crust. Our findings provide new insights into the dynamics of near-Moho magma storage and mobilization. Monitoring approaches optimized to detect early phases of magmatic unrest in the lower crust, such as identification and location of deep seismicity, could improve our response to future eruptive crises.

Shale mobility: From salt-like shale flow to fluid mobilization in gravity-driven deformation, the late Albian–Turonian White Pointer Delta (Ceduna Subbasin, Great Bight, Australia)
Gulce Dinc; Jean-Paul Callot; Jean-Claude Ringenbach
Large offshore depocenters above a weak detachment level (either salt or shale) can undergo gravity spreading and/or gliding. The gravitational systems (e.g., gliding deltas) are classically composed of an updip domain affected by extensional listric normal faults and a downdip domain affected by toe thrusts. While the role of salt in such systems is a classic tectonic process, the role and mechanical behavior of mobile shale levels in shale-prone gravity-driven systems are increasingly questioned. A three-dimensional seismic data set in the Ceduna Subbasin (Australia) displays the late Albian–Turonian White Pointer Delta (WPD) as having an unusual diversity of shale-cored structures. The early flow of shale resulted in depocenters showing wedges, internal unconformities, and shale diapirs and ridges, while fluidization of shales underneath a significant burial resulted in mud volcanism, secondary radial fault sets, and collapse features beneath the Campanian–Maastrichtian Hammerhead Delta, which lies above the WPD. Massive shale mobilization, together with downdip shortening and distal margin uplift, localized a major thrust in the core of the basin, ending the downward-propagating failure of the WPD. Mobilization of thick shale intervals, either as salt-like flow or mud volcanism, appears to have been a key process in the deformation, which should be considered at large scale for worldwide gravity-driven deformation systems.

Subsurface heat and salts cause exceptionally limited methane hydrate stability in the Mediterranean Basin
A. Camerlenghi; C. Corradin; U. Tinivella; M. Giustiniani; C. Bertoni
Knowledge of the global reservoir of submarine gas hydrates is of great relevance for understanding global climate dynamics, submarine geohazards, and unconventional hydrocarbon energy resources. Despite the expected presence of gas hydrates from modeling studies, the land-locked Mediterranean Basin displays a lack of evidence of extensive gas hydrate presence from samples and seismic data. We modeled the theoretical Mediterranean distribution of methane hydrate below the seafloor and in the water column using available geological information provided by 44 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) boreholes, measured geothermal gradients, and thermohaline characteristics of the water masses. We find that the pervasive presence of high-salinity waters in sediments, coupled with the unique warm and salty water column, limit the thickness of the theoretical methane hydrate stability zone in the subsurface and deepen its top surface to 1163–1391 m water depth. The theoretical distribution of methane hydrates coincides well with the distribution of shallow, low-permeability Messinian salt deposits, further limiting the formation of pervasive gas hydrate fronts and controlling their or distribution due to the prevention of upward hydrocarbon gas migration. We conclude that the Mediterranean Basin, hosting the youngest salt giant on Earth, is not prone to the widespread formation and preservation of gas hydrates in the subsurface and that the gas hydrate potential of salt-bearing rifted continental margins may be considerably decreased by the presence of subsurface brines.

100 k.y. pacing of the East Asian summer monsoon over the past five glacial cycles inferred from land snails

Rui Bao; Xuefen Sheng; Xianqiang Meng; Tao Li; Chenglong Li ...
The effects of orbital forcing on the East Asian summer monsoon (EASM) after the mid-Pleistocene transition are controversial. Chinese cave δ 18O records only show low-latitude ~20 k.y. cycles, while pedogenic proxy records from Chinese loess are dominated by high-latitude 100 k.y. cycles. This discrepancy may result from the multicomponent origin of proxies, particularly for pedogenic signals in loess deposits, where the primary climatic signals are modified by pedogenic smoothing, leaching, and changes in sedimentation rate, and the latter are also being forced by 100 k.y. cycles. We present an EASM record spanning the past 470 k.y. from the central Chinese Loess Plateau based on the δ13C values of land snail shells (δ13Cshell), which eliminates the influence of the above processes and exclusively records the local past EASM precipitation. The δ13Cshell record is dominated by the 100 k.y. cycle, with more depleted values during interglacials compared to glacials. At the end of marine isotope stage (MIS) 11, δ 13Cshell-based precipitation remained at an interglacial level following the MIS 11 super-interglacial climate in the Northern Hemisphere, although a glacial period had commenced on a global basis. Overall, our δ13Cshell record is highly coupled with high-northern-latitude ice-volume variations, possibly supporting the high-latitude forcing of the EASM.

Origin of the Sierras Pampeanas, Argentina: Flat-slab subduction and inherited structures
Xiaowen Liu; Claire A. Currie
The Sierras Pampeanas (27°–33°S) in South America are characterized by basement-cored uplifts and shortening that occurs >500 km from the nearest convergent margin. The deformation correlates spatially and temporally with an area of flat-slab subduction of the Nazca plate in the last 10 m.y. We use two-dimensional thermal-mechanical models to study the dynamics of Pampean flat-slab subduction and the origin of the Sierras Pampeanas. Models examine a geological time from ca. 12 Ma to present day, during which time the Juan Fernández Ridge subducted beneath South America. Models show that the buoyant ridge triggers slab flattening, resulting in regional continental compression through end loading at the plate margin. Deformation in the continental interior depends on the inherited structure of the continent, where surface uplifts and shortening are concentrated at preexisting weak zones. The inboard migration of deformation is controlled by surface topography caused by the buoyant ridge rather than basal shear from the growing flat slab. Deformation occurs prior to the passage of the ridge and is inhibited when the ridge is beneath the region owing to dynamic uplift.

Eustatic change modulates exhumation in the Japanese Alps
Georgina E. King; Floriane Ahadi; Shigeru Sueoka; Frédéric Herman; Leif Anderson ...
The exhumation of bedrock is controlled by the interplay between tectonics, surface processes, and climate. The highest exhumation rates of centimeters per year are recorded in zones of highly active tectonic convergence such as the Southern Alps of New Zealand or the Himalayan syntaxes, where high rock uplift rates combine with very active surface processes. Using a combination of different thermochronometric systems including trapped-charge thermochronometry, we show that such rates also occur in the Hida Mountain Range, Japanese Alps. Our results imply that centimeter per year rates of exhumation are more common than previously thought. Our thermochronometry data allow the development of time series of exhumation rate changes at the time scale of glacial-interglacial cycles, which show a four-fold increase in baseline rates to rates of ~10 mm/yr within the past ~65 k.y. This increase in exhumation rate is likely explained by knickpoint propagation due to a combination of very high precipitation rates, climatic change, sea-level fall, range-front faulting, and moderate rock uplift. Our data resolve centimeter-scale sub-Quaternary exhumation rate changes, which show that in regions with horizontal convergence, coupling between climate, surface processes, and tectonics can exert a significant and rapid effect on rates of exhumation.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga
A. Triantafyllou; M.N. Ducea; G. Jepson; J.D. Hernández-Montenegro; A. Bisch ...
Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Fault permeability from stochastic modeling of clay smears
Lluís Saló-Salgado; J. Steven Davis; Ruben Juanes
In normally consolidated, shallow (depth <~3 km) siliciclastic sequences, faults develop clay smears. Existing models include the dependence of permeability on the clay fraction, but improved predictions of fault permeability should account for uncertainty and anisotropy. We introduce PREDICT, a methodology that computes probability distributions for the directional components (dip-normal, strike-parallel, and dip-parallel) of the fault permeability tensor from statistical samples for a set of geological variables. These variables, which include geometrical, compositional, and mechanical properties, allow multiple discretizations of the fault core to be populated with sand and clay smears, which can be used to upscale the permeability to a coarser scale (e.g., suitable for reservoir modeling). We validated our implementation with experimental data and applied PREDICT to several stratigraphic sequences. We show that fault permeability is controlled by the clay smear configuration and, crucially, that it typically exhibits multimodal probability distributions due to the existence of holes. The latter is a unique feature of our algorithm, which can be used to build fault permeability scenarios to manage and mitigate risk in subsurface applications.

Trends and rhythms in carbonatites and kimberlites reflect thermo-tectonic evolution of Earth
Shuang-Liang Liu; Lin Ma; Xinyu Zou; Linru Fang; Ben Qin ...
Earth’s thermo-tectonic evolution determines the way the planet’s interior and surface interact and shows temporal changes in both trends and periodic rhythms. By sampling the subcontinental lithospheric mantle that represents the interface between the convecting mantle and the crust, carbonatite and kimberlite should be ideal rock types for documenting this evolution. The first-order secular rise of kimberlites over time has been noted by researchers, but there is much debate over how to interpret this trend, and their second-order variability has received less attention. We compiled a comprehensive global carbonatite database and compared it with an existing kimberlite one. We find that the numbers of carbonatites and kimberlites have similar increasing secular trends, with accelerated growth after ca. 1 Ga, and show the same periodic rhythms that have been synchronized to the supercontinent cycle since ca. 2.1 Ga. We link these trends and rhythms to the long-term change of Earth and the supercontinent cycle, both of which have altered the temperature of, and the subduction-recycled volatile flux into, the subcontinental lithosphere. Such consistent records in carbonatite and kimberlite behavior provide critical evidence for the synchronous thermo-tectonic evolution of the entire subcontinental lithosphere.

Diagenetic priming of submarine landslides in ooze-rich substrates
Nan Wu; Christopher A.-L. Jackson; Michael A. Clare; David M. Hodgson; Harya D. Nugraha ...
Oozes are the most widespread deep-sea sediment in the global ocean, but very little is known about how changes in their physical properties during burial impact slope stability and related geohazards. We used three-dimensional seismic reflection, geochemical, and petrophysical data acquired both within and adjacent to 13 large (in total ~6330 km 2) submarine slides on the Exmouth Plateau, North West Shelf, Australia, to investigate how the pre-slide physical properties of oozes control slope failure and emplacement processes. Our integrated data set allows potential slide surfaces to be detected within ooze successions, a crucial advance for improved submarine geohazard assessment. Moreover, we demonstrate that the interplay of tectonics, ocean current activity, and silica diagenesis can prime multiple slides on very low-gradient slopes in tropical oceanic basins. Therefore, the diagenetic state of silica-rich sediments should be considered in future studies to improve slope stability assessments.

Fluid inclusion evidence for overpressure-induced shale oil accumulation
Yingqi Wang; Jian Cao; Wenxuan Hu; Dongming Zhi; Yong Tang ...
Shale oil is becoming increasingly important in the global energy market, but its accumulation mechanism is not fully understood. We present novel and direct fluid inclusion data from the Lower Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwest China. Shortite veins in this source rock contain abundant two-phase gas-liquid hydrocarbon inclusions and coeval aqueous inclusions. The inclusions have highly variable degrees of bubble filling (5–80 vol% vapor) and homogenization temperature differences between oil and aqueous inclusions (~50 °C), which demonstrate that fluid (oil-gas-water) immiscibility occurred at high pressures. The hydrocarbon inclusions record different levels of fluid overpressure (32.9–43.0 MPa), with a paleopressure coefficient of 1.3–1.7. Episodic fluid overpressure release resulted in shale oil accumulation, with faults/fractures acting as important migration pathways. Oil from deeper and more mature source rocks within the Fengcheng Formation was expelled upward to the shale oil reservoir. These processes are common and important in shale oil systems. These results show that the accumulation of unconventional hydrocarbons occurs pervasively within the reservoirs, and fluid displacement is critical in exploration and exploitation.

Post-subduction porphyry Cu magmas in the Sanjiang region of southwestern China formed by fractionation of lithospheric mantle–derived mafic magmas
Jia Chang; Andreas Audétat
For porphyry Cu deposits that formed during oceanic slab subduction, there is a general consensus that the ore-forming magmas evolved through fractionation of mafic magmas that were generated by slab fluid (±melt)–fluxed melting of the asthenospheric mantle wedge. This model, however, is not applicable to post-subduction porphyry Cu deposits because they formed distinctly after cessation of oceanic slab subduction. A popular model proposes that post-subduction porphyry Cu magmas were produced by partial melting of lower-crustal, sulfide-rich arc cumulates, with or without minor contributions from potassic mafic magmas. To reappraise the crustal melting model, we focused on one of the largest post-subduction porphyry Cu belts on Earth, which formed during the India-Asia collision in the Sanjiang region of southwestern China. Detailed petrographic studies and new Nd-Sr isotopic data from non-metasomatized versus metasomatized lower-crustal xenoliths suggest that previous models based on crustal melting rest upon wrong radiogenic isotope constraints due to pervasive metasomatism of the xenoliths. Based on trace-element quantitative modeling and regional-scale geochemical trends of magmatic rocks, we demonstrate that the Sanjiang post-subduction porphyry Cu magmas were produced by fractionation of potassic mafic magmas derived from lithospheric mantle. This study highlights that post-subduction porphyry Cu magmas attain their K-rich composition, and all the ore-forming ingredients, from subduction-modified lithospheric mantle, the existence of which may be a prerequisite for the formation of porphyry Cu deposits in post-subduction settings.

Detrital glass in a Bering Sea sediment core yields a ca. 160 ka Marine Isotope Stage 6 age for Old Crow tephra
Alberto V. Reyes; Britta J.L. Jensen; Shaun H. Woudstra; Matthew S.M. Bolton; Serhiy D. Buryak ...
For decades, the Old Crow tephra has been a prominent stratigraphic marker for the onset of Marine Isotope Stage (MIS) 5e, the last interglaciation, in subarctic northwest North America. However, new zircon U-Pb dates for the tephra suggest that the tephra was deposited ca. 207 ka during MIS 7, with wide-ranging implications for chronologies of glaciation, paleoclimate, relict permafrost, and phylogeography. We analyzed ~1900 detrital glass shards from 28 samples collected at Integrated Ocean Drilling Program Site U1345 in the Bering Sea, which has a well-constrained age model from benthic foraminiferal δ18O. Except for one possibly contaminant shard dated at 165 ka, Old Crow tephra was absent from all samples spanning 220–160 ka. Old Crow tephra appeared abruptly at 157 ka, comprising >40% of detrital shards between 157 and 142 ka. This abrupt increase in the concentration of detrital Old Crow tephra, its absence in earlier intervals, and its presence at low concentrations in all samples between 134 and 15 ka collectively indicate that the tephra was deposited during the middle of MIS 6 with a likely age of 159 ± 8 ka. As a result, the late Quaternary chronostratigraphic framework for unglaciated northwest North America remains intact, and the timing of key events in the region (e.g., bison entry into North America; interglacial paleoclimate; permafrost history; the penultimate glaciation) does not require wholesale revision.

"Excess Ar" by laboratory alteration of biotite
Igor M. Villa; Giulia Bosio
Many biotite phenocrysts from marine tephra layers have substoichiometric potassium concentrations and alkali occupation <<2.0 atoms per formula unit. Diagenetic alteration is an expected effect of exposure of fresh magmatic minerals to interstitial water and brine intrusions after the deposition and burial of sediments. To test the effect of diagenetic alteration on potassium-argon ages, we irradiated and step heated untreated Fish Canyon biotite (t = 28.2 Ma) and several aliquots leached to various extents in strong and weak acids. Laboratory alteration caused loss of K, age spectrum discordance, high step ages and total gas ages, Ar release at lower furnace temperature, higher Cl/K and Ca/K, and a slight decrease in 36Ar concentration. Potassium loss was always higher than 40Ar* loss. Electron microprobe element maps document that acids preferentially penetrated in phyllosilicate interlayers, removing K (and Na). Because Ar* is removed to a lesser extent than K, we propose that natural 40K decay partly implants radiogenic Ar* into the tetrahedral-octahedral-tetrahedral (T-O-T) phyllosilicate layer, where Ar is shielded from interlayer leaching. The recoiled 39Ar, which was produced by irradiation after the leaching, also partitioned between T-O-T and the interlayer; age spectrum discordance was probably enhanced by the heterogeneous partition of 39Ar and 40Ar* in leached samples.

Chromium isotopes track redox fluctuations in Proterozoic successions of the Chapada Diamantina, São Francisco craton, Brazil
Fabrício A. Caxito; Robert Frei; Alcides N. Sial; Gabriel J. Uhlein; Willian Alexandre Lima Moura ...
The Chapada Diamantina region in the São Francisco craton of eastern Brazil is composed of sedimentary successions containing both Mesoproterozoic and Neoproterozoic carbonate levels, making it a key natural laboratory for understanding the fluctuations of Earth’s biogeochemical cycles during its middle age. The ca. 1.4–1.2 Ga Caboclo Formation stromatolites yielded unfractionated δ53Crauth (authigenic) (~–0.54‰ to +0.08‰). Ediacaran cap carbonates and phosphatic stromatolites of the Salitre Formation, on the other hand, yielded fractionated δ53Crauth reaching as high as +0.51‰, suggesting the input of 53Cr-rich Cr(VI), first delivered through meltwater-induced post–snowball Earth fluctuating redox conditions and then through weathering and mobilization under a fully oxygenated environment. The acquired data set highlights the very distinct redox conditions throughout the Proterozoic and reinforces the suggestion that after the Cryogenian global glaciations, Earth’s atmosphere and hydrosphere became progressively oxygenated during the Ediacaran-Cambrian transition.

Silicification of trilobites and biofilm from the Cambrian Weeks Formation, Utah: Evidence for microbial mediation of silicification
Leslie A. Melim; Sebastien R. Mure-Ravaud; Thomas A. Hegna; Brian J. Bellott; Rudy Lerosey-Aubril
We report on silicified trilobite sclerites with associated silicified biofilm from the Cambrian Weeks Formation, Utah (USA), that support a role for microbial biofilms in silicification. Silicified sclerites are typically small (<3 mm) and incompletely preserved. All studied specimens are partly coated in 5–20 μm (rarely >500 μm) of silica-cemented matrix. High-resolution scanning electron microscope (SEM) study reveals the presence of two different forms of carbon-rich threads, ribbons and mats, coating both sclerites and silica-cemented matrix. Crystalline-looking biofilm threads and ribbons composed of Si, O, and C are interpreted as silicified biofilm associated with the trilobite silicification. Rippled to smooth biofilm mats composed of more C, less Si and O, and a trace of N are post-silicification. Embedded in the silica of the sclerites and matrix are molds of framboids that we interpret as originally framboidal pyrite that was engulfed by silica. These data indicate that silica precipitation continued into the surrounding matrix following the propagation of sulfate-reducing bacteria feeding on the organic matter present in the sclerite and the neighboring sediment. This strongly supports the model that bacterially mediated decay is key to the silicification of carbonate bioclasts and provides the first direct evidence of a microbial community (biofilm). A literature review reveals that silica extends past the fossil more frequently than is recognized, suggesting that silicified biofilm might be common but overlooked.

Eocene magmatism in the Himalaya: Response to lithospheric flexure during early Indian collision?
Lin Ma; Qiang Wang; Andrew C. Kerr; Zheng-Xiang Li; Wei Dan ...
Eocene mafic magmatism in the Himalaya provides a crucial window for probing the evolution of crustal anatexis processes within the lower plate in a collisional orogen. We report geochemical data from the earliest postcollision ocean-island basalt–like mafic dikes intruding the Tethyan Himalaya near the northern edge of the colliding Indian plate. These dikes occurred coeval, and spatially overlap, with Eocene granitoids in the cores of gneiss domes and were likely derived from interaction between melts from the lithosphere-asthenosphere boundary and the Indian continental lithosphere. We propose that these mafic magmas were emplaced along lithospheric fractures in response to lithospheric flexure during initial subduction of the Indian continent and that the underplating of such mafic magmas resulted in orogen-parallel crustal anatexis within the Indian continent. This mechanism can explain the formation of coeval magmatism and the geologic evolution of a collisional orogen on both sides of the suture zone.

Sponge-rich sediment recycling in a Paleozoic continental arc driven by mélange melting
Huichuan Liu; Sune G. Nielsen; Guangyou Zhu
Slab material transfer processes in continental arcs can be challenging to decipher because magmas are often characterized by significant contributions from continental material. In this study, we identified a Prototethyan continental arc (419–418 Ma) that is now located in the Dazhonghe area of the southeast Tibetan Plateau, which, based on Sr-Nd-Hf-O-Si isotope relationships, implies no detectable continental material contributions. The Dazhonghe arc rocks display much lower δ 30Si values than modern arc rocks and average mantle; this is best explained by subduction of sponge-rich marine sediments, which are thought to have been the dominant marine organisms during the Neoproterozoic and early Paleozoic. Our mixing calculations reveal that only bulk mixing among sponge-rich sediments, altered oceanic crust (AOC), and the depleted mantle would be capable of accounting for all the Sr-Nd-Hf-O-Si isotope compositions. This finding implies that the Dazhonghe arc magmas were generated by melting of a mélange that formed at the slab-mantle interface. The Dazhonghe continental arc is the first for which mélange melting has been confirmed.

Preferential preservation of low-elevation biotas in the nonmarine fossil record
Steven M. Holland; Katharine M. Loughney; Marjean Cone
Modern coastal sedimentary basins typically lie at low elevations (<600 m), whereas inland basins commonly occupy elevations as high as 4000–5000 m. Individual basins of all types typically preserve a narrower span of elevations, generally <1000 m, and typically near 200–300 m in coastal basins. As a result, the nonmarine fossil record is expected to preserve mainly low-elevation habitats and a relatively narrow range of elevations. Because many of the basins that preserve high elevations are likely to undergo subsequent destruction via erosion or continental collision, the dominance of low-elevation habitats is likely to become stronger into deep time. This selective preservation of nonmarine communities from sedimentary basins, and specifically from low elevations, suggests that much or even most of ancient nonmarine biodiversity is not preserved. Given the occurrence of many modern biodiversity hotspots in regions of high elevation, long ghost lineages are likely common in the nonmarine fossil record, and divergence times estimated from the nonmarine fossil record may be systematically far too short. The spans of elevations that are preserved in sedimentary basins suggest that the fossil record may preserve gradients in community composition that are correlated with elevation, yet they have been largely undetected.

Pre-agricultural soil erosion rates in the midwestern United States
Caroline L. Quarrier; Jeffrey S. Kwang; Brendon J. Quirk; Evan A. Thaler; Isaac J. Larsen
Erosion degrades soils and undermines agricultural productivity. For agriculture to be sustainable, soil erosion rates must be low enough to maintain fertile soil. Hence, quantifying both pre-agricultural and agricultural erosion rates is vital for determining whether farming practices are sustainable. However, there have been few measurements of pre-agricultural erosion rates in major farming areas where soils form from Pleistocene deposits. We quantified pre-agricultural erosion rates in the midwestern United States, one of the world’s most productive agricultural regions. We sampled soil profiles from 14 native prairies and used in situ–produced 10Be and geochemical mass balance to calculate physical erosion rates. The median pre-agricultural erosion rate of 0.04 mm yr–1 is orders of magnitude lower than agricultural values previously measured in adjacent fields, as is a site-averaged diffusion coefficient (0.005 m2 yr–1) calculated from erosion rate and topographic curvature data. The long-term erosion rates are also one to four orders of magnitude lower than the assumed 1 mm yr–1 soil loss tolerance value assigned to these locations by the U.S. Department of Agriculture. Hence, quantifying long-term erosion rates using cosmogenic nuclides provides a means for more robustly defining rates of tolerable erosion and for developing management guidelines that promote soil sustainability.

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