New Geology Articles Published Online Ahead of Print in March

Boulder, Colo., USA: Article topics and locations include faulting in the Imperial Valley, California; stagnation and tearing of the subducting northwest Pacific slab; the vigor of Cenozoic ocean bottom currents; and Greenland tidal water advance during the era of Norse settlement. These Geology articles are online at .

Reconstructing source-to-sink systems from detrital zircon core and rim ages
Li Liu; Daniel F. Stockli; Timothy F. Lawton; Jie Xu; Lisa D. Stockli ...
Abstract: Grenville-age (1.3–0.9 Ga) zircons represent one of the most ubiquitous detrital zircon (DZ) age modes on Earth. In North America, given the widespread occurrence of Grenville basement, Grenville DZs are commonly viewed as nondiagnostic with regard to source region in provenance studies. Systematic recovery of DZ core-rim U-Pb ages makes it possible to identify and differentiate previously indistinguishable basement source terranes by leveraging their multistage tectono-magmatic evolution. Our analysis demonstrates that Grenville DZs exhibit distinct rim ages in different parts of the North American Paleozoic Appalachian-Ouachita-Marathon foreland. Whereas Grenville DZ grains in the eastern foreland, sourced from the southern Appalachian orogen in the eastern United States, exhibit Taconian and Acadian (490–350 Ma) rims, grains in the western foreland, derived from Mexico, mainly show Neoproterozoic (750–500 Ma) rim ages. This difference permits differentiation of nondiagnostic core ages by their distinctive rim ages. Furthermore, core-rim paired ages can illuminate potential genetic relationships among coexisting age components in DZ spectra, thereby indicating whether the DZs are derived from separate sources or from a single source with multistage tectono-magmatic histories. Thus, DZ rim-core ages can provide critical insights into reconstructing global source-to-sink systems and elucidating genetic linkages within multistage orogenic systems.
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Deformation and metasomatism recorded by single-grain apatite petrochronology
Margaret L. Odlum; Drew A. Levy; Daniel F. Stockli; Lisa D. Stockli; Joel W. DesOrmeau
Abstract: The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.
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Groundwater seepage is a key driver of theater-headed valley formation in limestone
Aaron Micallef; Nader Saadatkhah; Jurgen Spiteri; Enzo Rizzo; Luigi Capozzoli ...
Abstract: Groundwater seepage leads to the formation of theater-headed valleys (THVs) in unconsolidated sediments. In bedrock, the role of groundwater in THV development remains disputed. Here, we integrate field and remote-sensing observations from Gnejna Valley (Maltese Islands) with numerical modeling to demonstrate that groundwater seepage can be the main driver of THV formation in jointed limestone overlying clays. The inferred erosion mechanisms entail (1) widening of joints and fractures by fluid pressure and dissolution and (2) creeping of an underlying clay layer, which lead to slope failure at the valley head and its upslope retreat. The latter is slower than the removal of the talus by creep and sliding on the valley bed. The location and width of THVs are controlled by the location of the master fault and the extent of the damage zone, respectively. The variability of seepage across the fault zone determines the shape of the valley head, with an exponential decrease in seepage away from the fault giving rise to a theater-shaped head that best matches that of Gnejna Valley. Our model may explain the formation of THVs by groundwater in jointed, strong-over-weak chemical sedimentary lithologies, particularly in arid terrestrial settings.
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Greenland tidewater glacier advanced rapidly during era of Norse settlement
Danni M. Pearce; James M. Lea; Douglas W.F. Mair; Brice R. Rea; J. Edward Schofield ...
Abstract: Our ability to improve prognostic modeling of the Greenland Ice Sheet relies on understanding the long-term relationships between climate and mass flux (via iceberg calving) from marine-terminating tidewater glaciers (TWGs). Observations of recent TWG behavior are widely available, but long-term records of TWG advance are currently lacking. We present glacial geomorphological, sedimentological, archaeological, and modeling data to reconstruct the ~20 km advance of Kangiata Nunaata Sermia (KNS; the largest tidewater glacier in southwest Greenland) during the first half of the past millennium. The data show that KNS advanced ~15 km during the 12th and 13th centuries CE at a rate of ~115 m a –1, contemporaneous with regional climate cooling toward the Little Ice Age and comparable to rates of TWG retreat witnessed over the past ~200 years. Presence of Norse farmsteads proximal to KNS demonstrates their resilience to climate change, manifest as a rapidly advancing TWG in a cooling climate. The results place limits on the magnitude of ice-margin advance and demonstrate TWG sensitivity to climate cooling as well as warming. These data combined with our grounding-line stability analysis provide a long-term record that validates approaches to numerical modeling aiming to link calving to climate.
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Can deepwater bottom currents generate clinothems? An example of a large, asymmetric mounded drift in Upper Jurassic to Lower Cretaceous sediments from northwestern Australia
O. Mantilla; F.J. Hernández-Molina; N. Scarselli
Abstract: Clinoforms and clinothems are ubiquitous in shallow marine and shelf margin environments, where they show remarkable seaward progradation trends. Consensus holds that these features do not form in deepwater settings. This study describes an example of a large, asymmetric mounded deposit formed in Upper Jurassic to Lower Cretaceous sediments along the Exmouth Plateau (offshore northwestern Australia). Although it formed in deepwater environments, the deposit has previously been interpreted to reflect either a deltaic or shelf margin system based on clinoform and clinothem geometries. We support that this deposit shares similarities with a delta drift that evolved into a large, mounded drift (~180 km in length, ~120 km in width, and up to ~1.7 km in sedimentary thickness) that exhibits two migration trends: one westward and the other northeastward. Three evolutionary phases are proposed: (1) an onset drift stage (ca. 146.5–143.5 Ma); (2) a growth drift stage (ca. 143.5–138.2 Ma); and (3) a burial stage (ca. 138.2 Ma), which marks the completion of the drift and a shift in depositional style. The drift asymmetry and clinoform orientations indicate the influence of a northward-flowing water mass with two main cores. Our analysis thus suggests that bottom currents can create complex deposits with geometries that resemble clinothems in deepwater environments.
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Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet
William F. Hoover; Sarah Penniston-Dorland; Lukas Baumgartner; Anne-Sophie Bouvier; Besim Dragovic ...
Abstract: Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ 7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.
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Magmatism at oceanic core complexes on the ultraslow Southwest Indian Ridge: Insights from near-seafloor magnetics
Fei Zhou; Jérôme Dyment; Chunhui Tao; Tao Wu
Abstract: Oceanic core complexes (OCCs) and detachment faults play a key role in crustal accretion at slow and ultraslow spreading centers. We investigated the effect of different magma supply at three OCCs of the Southwest Indian Ridge using high-resolution deep-sea bathymetric and magnetic data. The average equivalent thickness of extrusive basalt deduced from the magnetic anomalies, a proxy for magma supply, decreases from west to east, from the Yuhuang (49.25°E) to Longqi (49.65°E) to Junhui (51.75°E) OCCs. Conversely, serpentinite outcrops become more abundant, the domal OCC morphology flattens as the footwall rotation (measured by the magnetization vector inclination) increases, and hydrothermal evidence becomes sparse. Combined with results from the amagmatic easternmost Southwest Indian Ridge, our study shows that the magma supply controls the character and evolution of the OCCs and detachment faults on the Southwest Indian Ridge.
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Deep-sea hiatuses track the vigor of Cenozoic ocean bottom currents
Adriana Dutkiewicz; Dietmar Müller
Abstract: The deep-sea stratigraphic record is full of gaps. These hiatuses track changes in ocean circulation and chemistry, but determining their timing and causes has been limited by sparse data and incomplete knowledge of ocean gateway evolution in earlier studies. We combine a significantly expanded, age-calibrated deep-sea stratigraphic database with a global tectonic and paleo–water depth model to investigate the distribution of >400 Cenozoic hiatuses longer than ~0.2 m.y. We find that only a small number of hiatuses are due to carbonate dissolution. The majority of hiatuses were, by implication, caused by mechanical erosion and redistribution of sediments by bottom currents into regions of increased sedimentation such as contourite drifts. We link peaks in regional hiatuses to changes in ocean circulation and intensification of deep-water formation. Widespread hiatuses in the South Atlantic, South Pacific, and southern Indian oceans between ca. 34 Ma and 30 Ma are attributed to the coeval widening and deepening of the Drake Passage and the opening of the deep Tasman Gateway. A peak in hiatuses in the Atlantic in the early Miocene is linked to the initiation of a proto–Atlantic Meridional Overturning Circulation driven by the complete opening of the deep Drake Passage and the progressive closure of the Tethys seaway. A long-term 30% decline in hiatus frequency since ca. 14 Ma is synchronous with post–Miocene Climate Optimum cooling, suggesting the slowing of abyssal circulation. Our synthesis of deep-sea hiatuses could be used to track the fate of deep-sea sediments and to ground-truth deep-ocean circulation models.
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Groundwater age persistence in topography-driven groundwater flow over paleohydrogeologic time scales
Yueqing Xie; Andrew J. Love; Craig T. Simmons; Adrian Costar; Jichun Wu
Abstract: The Pleistocene-Holocene climate transition resulted in a dramatic reduction in groundwater recharge in many aquifers in arid and semiarid regions throughout the world. This study conducted numerical experiments to compare the evolution of groundwater hydraulics and age patterns in arid and semiarid aquifers in response to transient conditions associated with recharge decline from the Pleistocene to the Holocene. Our results show that after a rapid reduction in recharge, the amplitude of water-table undulations and regional groundwater slope both reduced. This resulted in a general, and relatively rapid, contraction of local flow systems and an increase in the extent of intermediate and regional systems. The previous hierarchy of local, intermediate, and regional flow systems was completely replaced by largely horizontal and regional flow patterns after ~10,000 yr. However, in stark contrast, we observed that the original Pleistocene age patterns have remained almost unchanged throughout the 10,000 yr Holocene period. Thus, groundwater age is more likely to be indicative of past rather than current flow systems. Consequently, due to this age persistence, the use of modern groundwater age data to calibrate models or compute recharge with methods that do not account for this potentially significant spatial and temporal mismatch between age and hydraulics will be misleading and erroneous. This has significant implications for hydrogeologic analyses. The findings of this study may also apply to areas that have undergone dramatic changes in land cover or land use that strongly influence transient groundwater recharge processes.
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Nanoscale trace-element zoning in pyrite framboids and implications for paleoproxy applications
Daniel D. Gregory; Libor Kovarik; Sandra D. Taylor; Daniel E. Perea; Jeremy D. Owens ...
Abstract: Pyrite framboids (spherical masses of nanoscale pyrite) are among the earliest textures of pyrite to form in sediments. It has been proposed that their trace-element (TE) contents can be used to track the TE composition of the water column in which they formed. However, it is not clear how these TEs are associated with the framboidal pyrite grains. For instance, it is important to know whether they are incorporated uniformly or are enriched in different regions of the framboid. We used high-resolution scanning transmission electron microscopy to identify chemical zoning within pyrite framboids. We found that initial, nanoscale pyrite euhedral crystals, which make up the volumetric majority of the framboids, are covered/infilled by later pyrite that templates on the earlier pyrite. Further, this later pyrite is enriched in TEs, suggesting that many TEs are incorporated in pyrite relatively late (during early diagenesis; not in the water column). This observation suggests that although chemical analyses of pyrite framboids may provide ocean-water chemistry trends through time, the details are complex. Specifically, the TEs found in pyrite may be linked to adsorption onto organic matter, detrital material, and authigenic minerals such as Fe- and Mn-oxide phases followed by desorption in the sediments or release via dissolution and incorporation into pyrite as overgrowths on the initial nanoscale euhedral crystals that make up framboids. While the use of pyrite chemistry to understand past ocean conditions remains promising, and even diagenetic additions may not preclude the utility of pyrite for reconstructing ancient ocean conditions, care must be taken in interpretations because the end concentration may be influenced by diagenesis.
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How old are the Jack Hills metasediments really?: The case for contamination of bedrock by zircon grains in transported regolith
P.D. Kinny; C. Clark; C.L. Kirkland; M. Hartnady; J. Gillespie ...
Abstract: The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains in some samples of metasedimentary rocks from the Jack Hills, Western Australia, that otherwise appear to have been deposited before 3.0 Ga is explained by contamination with embedded zircon grains sourced from transported regolith. Zircon U-Pb age spectra obtained by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of modern alluvium in creek beds draining the metasedimentary belts of Mount Narryer and Jack Hills are dominated by these exotically sourced younger components, as is the red soil that partly covers and infiltrates fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The consistency of the age patterns found in superficial deposits sampled over some 100 km implies a dominance of colluvium transported from the wider Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather than material produced from the local (Mesoarchean and older) bedrocks. The potential for similar cases of “environmental contamination” of surface rocks by exogenous zircons needs to be considered more widely, particularly in situations where interpreted maximum deposition ages are based on only a small number of outlying youngest analyzed grains.
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Re-Os geochronology for the Cambrian SPICE event: Insights into euxinia and enhanced continental weathering from radiogenic isotopes
Alan D. Rooney; Alexie E.G. Millikin; Per Ahlberg
Abstract: The late Cambrian Steptoean positive carbon isotope excursion (SPICE) represents a major perturbation to the global carbon cycle and was associated with trilobite extinctions and expansion of anoxic and/or euxinic water masses during episodes of eustatic sea-level change. We present a new Re-Os age together with Os and Nd isotope stratigraphy and major- and trace-element data from the Alum Shale Formation (Scania, Sweden). The Re-Os age of 494.6 ± 2.9 Ma is from the interval of peak δ 13Corg values, providing the first radiometric age constraint for this Cambrian carbon isotope excursion, interpreted as a possible pre-Mesozoic ocean anoxia event, and the timing of biomere-level extinctions. The Os isotope chemostratigraphic profile can be explained by an increase in terrigenous weathering prior to the SPICE, potentially driven by sea-level fall, and in agreement with enhanced nutrient supply, primary productivity, and organic matter burial as the driver of the SPICE event. Post-SPICE, the Os isotopes become increasingly unradiogenic; however, invariant εNd(t) values argue against a change in provenance and instead support a decrease in the continental weathering flux, possibly related to eustatic sea-level rise.
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Nanoscale isotopic evidence resolves origins of giant Carlin-type ore deposits
E.A. Holley; A. Fulton; C. Jilly-Rehak; C. Johnson; M. Pribil
Abstract: The western North American Great Basin’s Carlin-type deposits represent the largest accumulation of gold in the Northern Hemisphere. The controversy over their origins echoes the debate between Neptunists and Plutonists at the birth of modern geology: were the causative processes meteoric or magmatic? Sulfur isotopes have long been considered key to decoding metal cycling in the Earth’s crust, but previous studies of Carlin-type pyrite lacked the spatial resolution to quantify differences among the numerous generations of sulfide mineralization. We developed a new dual-method, nanoscale approach to examine the fine-grained ore pyrite. The δ 34S of the ore pyrite varies systematically with Au concentration at the nanoscale, indicating that both magmatic and meteoric fluids contributed during mineralization, but the magmas brought the gold. Repeated oscillations in fluid ratios upgraded the metal content, resulting in high gold endowment. Our results demonstrate that high-spatial-resolution studies are key to elucidate the spatiotemporal evolution of complex hydrothermal systems.
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Calcite U-Pb ages constrain petroleum migration pathways in tectonic complex basins
Fuyun Cong; Jinqiang Tian; Fang Hao; Andrew R.C. Kylander-Clark; Wenqing Pan ...
Abstract: Tracing secondary oil migration pathways is critical for understanding petroleum system evolution histories. Traditional tools (e.g., molecular indicators and numerical modeling) utilized for evaluating oil migration processes either lead to ambiguous interpretations or only provide qualitative estimates. We quantitatively constrain secondary oil migration processes under an absolute time frame by integrating oil-inclusion fluorescence and in situ calcite U-Pb dating on calcite veins and cements hosting primary oil inclusions. Fluorescence spectra of oil inclusions and U-Pb ages were obtained on samples from ultra-deep Ordovician reservoirs along two major faults in the Halahatang oilfield, Tarim Basin (northwestern China). Absolute U-Pb ages suggest two main oil charge events during 475–433 Ma and 294–262 Ma, respectively, and revealed a northward-decreasing trend for oil maturity during single charge events. Vertical migration of oil from in situ source rock through active (or reactivated) faults is believed to be the key process inducing the spatial maturity variation in charged oils and considered as the main mechanism of secondary migration, with brecciated fault zones and dilatant fractures along faults acting as major vertical oil-migration pathways. The successful application of this approach has wider implications for elucidating petroleum migration processes in tectonic complex basins worldwide.
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Significance of U-Pb detrital zircon geochronology for mudstone provenance
Paul J. Sylvester; A. Kate Souders; Rui Liu
Abstract: Detrital zircon U-Pb studies of mudstone provenance are rare but may preferentially fingerprint distal zircon sources. To examine this issue, Pierre Shale and Trinidad Sandstone deposited in a Late Cretaceous deltaic environment in the Raton Basin, Colorado (USA), were measured for detrital zircon U-Pb age by laser ablation–inductively coupled plasma–mass spectrometry. Two major detrital zircon age peaks at ca. 70 and 1690 Ma are found in both Pierre Shale and Trinidad Sandstone but in inversely varying proportions: 68% and 16%, respectively, for the finest zircon fraction (~15–35 μm) in the shale, and 25% and 32%, respectively, for the coarsest zircon fraction (~60–80 μm) in the sandstone. Proximal sources in the Sangre de Cristo Mountains, directly west of the Raton Basin, contain coarse-grained, ca. 1690 Ma zircon, whereas distal sources in Laramide uplifts and basins in Colorado, New Mexico, and Arizona contain fine-grained, ca. 70 Ma zircon. The results indicate that U-Pb zircon provenance of mudstone reflects availability of volcanic and other fine-grained source rocks rather than simply distal sources. U-Pb zircon provenance studies should routinely include mudstone units because these units may identify fine-grained zircon sources more reliably than sandstones alone.
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Cumulate granites: A perspective from new apatite MgO partition coefficients
Qiong-Yao Zhan; Di-Cheng Zhu; Roberto F. Weinberg; Qing Wang; Jin-Cheng Xie ...
Abstract: Apatite is a powerful research tool because it is common in various rocks and incorporates many chemical elements. Understanding how elements partition between apatite and melt and the controls on the partition are critical for applications of apatite. It has been recently proposed that MgO content in magmatic apatite is proportional to that of the melt, highlighting the potential importance of this oxide as a critical parameter. A comprehensive compilation of experimental data in this study shows that the relationship between apatite MgO and melt MgO defines two distinct equilibrium trends, indicating that melt composition or, more specifically, melt polymerization strongly controls the partition of MgO. These findings have great implications for using apatite in the study of magma petrogenesis and sediment provenance. We show that when apatite MgO is plotted against host-rock MgO, volcanic and plutonic systems have different behaviors. The volcanic data plot around the experimental apatite-melt equilibrium trends, while apatites in most plutonic rocks have lower MgO than expected if they were in equilibrium with the host bulk rock. For granites with high apatite saturation temperatures, this indicates that apatites crystallized from low-MgO parent melts but later became part of a cumulate with higher MgO than the parental melt, possibly due to extraction of evolved interstitial melt. The contrast between apatite MgO in volcanic and plutonic rocks therefore can provide a new perspective on the debate about volcanic-plutonic connection.
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Millennial pulses of ore formation and an extra-high Tibetan Plateau
Yang Li; Mark B. Allen; Xian-Hua Li
Abstract: Quantifying the rhythms and rates of magmatic-hydrothermal systems is critical for a better understanding of their controls on ore formation and the dynamics of magmatic reservoirs that feed them. We reconstructed the evolution of ore-forming fluids using hydrothermal quartz from the 17.4 Ma Zhibula skarn, Tibet. Ion probe analysis reveals sharp and dramatic changes in quartz δ18O values between 5‰ and –9.3‰, with fluid δ 18O values varying between 2.8‰ and –18.2‰, which are best explained by transient meteoric water incursion into a hydrothermal system dominated by magmatic fluids. Two pulses of magmatic fluids and a meteoric water incursion event are inferred, which operated at the millennium scale (760–1510 yr) as constrained by the aluminum diffusion chronometer. Our results indicate that magmatic reservoirs are likely water unsaturated for most of their lifetime (>105–106 yr), with transient and episodic fluid exsolutions (~103 yr) being driven by magma replenishment or crystallization-induced water saturation. With focused and efficient metal deposition, multiple pulses of metalliferous fluids favor the formation of giant deposits with high grade. Meteoric water δ18O values (–25.4 ± 2.3‰) derived from Zhibula quartz further suggest a paleo-elevation of 5.9 ± 0.3 km; this transient early Miocene surface uplift plausibly was due to break-off of the oceanic slab attached to the Indian Plate. Our research highlights that ubiquitous hydrothermal quartz in orogenic belts can probe the dynamics of magmatic-hydrothermal systems and also quantify paleo-elevations, which has significant tectonic implications.
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Linking metamorphism and plate boundaries over the past 2 billion years
Yebo Liu; Ross N. Mitchell; Michael Brown; Tim E. Johnson; Sergei Pisarevsky
Abstract: Since the Jurassic, there has been a clear spatiotemporal correlation between different types of metamorphism and active convergent plate margins. However, the extent to which this relationship extends into the past is poorly understood. We compared paleogeographic reconstructions and inferred plate kinematics with the age and thermobaric ratio (temperature/pressure [T/P]) of metamorphism over the past 2 b.y. The null hypothesis—that there is no spatiotemporal link between inferred plate margins and metamorphism—can be rejected. Low- T/P metamorphism is almost exclusively located near plate margins, whereas intermediate- and high-T/P metamorphism skews toward increasingly greater distances from these margins, consistent with three different tectonic settings: the subduction zone, the mountain belt, and the orogenic hinterland, respectively. However, paleogeographic reconstructions suggest that so-called “paired metamorphic belts” are rare and that high- and low-T/P localities more commonly occur along strike from each other. The observation that bimodal metamorphism is largely a function of distance from the trench and that end-member T/P types rarely occur in the same place can be explained if the style of orogenesis has evolved from hotter to colder, consistent with the abrupt emergence of low-T/P metamorphism in the Cryogenian. The widespread development of high-T/P rocks in orogenic hinterlands in the Proterozoic was followed by the production and efficient exhumation of low-T/P rocks in subduction channels in the Phanerozoic.
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Stagnation and tearing of the subducting northwest Pacific slab
Muchen Sun; Youqiang Yu; Stephen S. Gao; Kelly H. Liu
Abstract: Despite numerous observational and geodynamic modeling studies, the presence of the northwest Pacific slab tear and its influence on mantle dynamics remain controversial. By imaging the mantle transition zone (MTZ) discontinuities beneath the Japan Sea and adjacent areas, we demonstrate an ESE-WNW elongated zone with significant MTZ thinning extending from central Honshu, Japan, to the Korean Peninsula, which provides additional supporting evidence for the existence and distribution of a large-scale slab tear. Our results, when combined with other geophysical and geochemical evidence, indicate that the hot mantle material oceanward of the slab may flow through the slab tear and contribute to surface volcanism. Substantial MTZ thickening is widely observed in the region south of the slab tear and suggests the existence of slab stagnation, possibly related to the strong resistance at the bottom of the MTZ.
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Hot atmospheric formation of carbonate accretionary lapilli at the Cretaceous-Paleogene boundary, Brazos River, Texas, from clumped isotope thermometry
David G. Burtt; Gregory A. Henkes; Thomas E. Yancey; Daniel Schrag
Abstract: The Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five major extinctions. Researchers have examined ejecta from the Chicxulub impact, and most recently a drill core from the crater itself, yet the processes and chemical reactions occurring in the impact vapor plume are poorly constrained. Rounded carbonate particles, identified as accretionary lapilli, have been found thousands of kilometers from the impact crater and may be a unique record of plume conditions. We present carbon (δ 13C), oxygen (δ18O), and clumped (Δ47) isotope ratios of lapilli from the Brazos River, Texas (USA), as well as from foraminifera and a mudstone. Unaltered lapilli δ13C and δ 18O values covary, ranging from –9.38‰ to –2.10‰ and from –7.72‰ to –5.36‰, respectively, and they are distinct from mudstones, foraminifera, and secondarily altered lapilli in the same section. Clumped isotope temperatures [T47)] from the lapilli range from 66 °C to 539 °C and average 155 ± 46 °C (1 standard deviation), with sedimentary and fossil carbonates recording clement, shallow ocean–like T47). These data are consistent with petrography and hypothesized vapor plume formation, and we argue that the δ13C and δ18O values result from target rock decarbonation. Atmospheric temperatures >100 °C extending >1800 km from the Chicxulub crater imply an uninhabitable zone within seconds to minutes of the impact that was 10× larger in diameter than the crater itself.
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Shallow distributed faulting in the Imperial Valley, California, USA
Valerie J. Sahakian; Boe J. Derosier; Thomas K. Rockwell; Joann M. Stock
Abstract: In the tectonically complex Imperial Valley, California (USA), the Imperial fault (IF) is often considered to be the primary fault at the U.S.-Mexico border; however, its strain partitioning and interactions with other faults are not well understood. Despite inferred evidence of other major faults (e.g., seismicity), it is difficult to obtain a holistic view of this system due to anthropogenic surface modifications. To better define the structural configuration of the plate-boundary strain in this region, we collected high-resolution shallow seismic imaging data in the All American Canal, crossing the Imperial, Dixieland, and Michoacán faults. These data image shallow (<25 m) structures on and near the mapped trace of the Imperial fault, as well as the Michoacán fault and adjacent stepover. Integration of our data with nearby terrestrial cores provides age constraints on Imperial fault deformation. These data suggest that the Michoacán fault, unmapped in the United States, is active and likely produces dynamic or off-fault deformation within its stepover to the Dixieland fault. Together, these data support more strain partitioning than previously documented in this region.
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A Tonian volcano-sedimentary succession in Newfoundland, eastern North America: A post-Grenvillian link to the Asgard Sea?
Susan Strowbridge; Aphrodite Indares; Greg Dunning; Markus Wälle
Abstract: Early Tonian bimodal volcanic and sedimentary rocks (the Pine Pond succession) were identified in the Appalachians of eastern Laurentia for the first time, with U-Pb ages of igneous zircon at 951.4 ± 9 Ma for a felsic tuff and 952 ± 10 Ma for a crosscutting granite sheet. This Neoproterozoic succession is part of the East Pond metamorphic suite on the Baie Verte Peninsula (Newfoundland, Canada), which also contains 1491 Ma orthogneiss and is consistent with deposition over Mesoproterozoic basement in a pre-Iapetan extensional setting. Collectively, the data suggest a correlation with the southern Labrador Pinware terrane of the Grenville Province, where 1.52–1.46 Ga bedrock is intruded by 975–950 Ma granite. In addition, the age and location of this 952 Ma succession are consistent with deposition on the western margin of the Asgard Sea. Thus, the Pine Pond succession provides a link between key events in the assembly of the supercontinent Rodinia: the end of the Grenville orogeny, which led to the amalgamation of Laurentia and Amazonia, and the early Neoproterozoic evolution of the Asgard Sea, which separated Laurentia and Baltica.
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Quantifying large-scale continental shelf margin growth and dynamics across middle-Cretaceous Arctic Alaska with detrital zircon U-Pb dating
Richard O. Lease; David W. Houseknecht; Andrew R.C. Kylander-Clark
Abstract: Sequence stratigraphy provides a unifying framework for integrating diverse observations to interpret sedimentary basin evolution; however, key time assumptions about stratigraphic elements spanning hundreds of kilometers are rarely quantified. We integrate new detrital zircon U-Pb (DZ) dates from 28 samples with seismic mapping to establish a chronostratigraphic framework across 800 km and ~20 m.y. for the middle-Cretaceous Torok-Nanushuk clinothem of Arctic Alaska (USA). Shelf-margin DZ dates indicate continent-scale sediment routing with Russian Chukotka provenance and provide reliable maximum depositional ages derived from arc volcanism. Shelf-margin advance rates display a clear relationship to toplap trajectories and provide empirical support for long-held inferences linking sediment supply to margin architecture. Two distinct shelf-margin growth regimes are evident: (1) a ca. 115–107 Ma phase of rapid ~50 km/m.y. shelf advance rates with mainly progradational trajectories; and (2) a ca. 107–98 Ma phase of moderate ~13 km/m.y. shelf advance rates with progradational-retrogradational-aggradational trajectories. We established a subsequent shelf–to–deep water correlation by independently dating ca. 98–95 Ma low shelf accommodation and basin-floor deposition as far as 240 km east that indicate lowstand shedding and a change to localized routing with Brooks Range provenance. Finally, we dated a ca. 95 Ma basin-wide transgression at deep-water to shelfal settings across 350 km that exhibits apparent synchroneity consistent with an event-significant surface. In one of the world’s largest foreland-basin clinothems, our work constrains the timing and duration of key depositional elements to test large-scale sequence stratigraphic assumptions, enables reliable correlation and quantification of sediment dynamics across 800 km, and captures the chronology of a giant regressive-transgressive cycle.
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For Immediate Release
31 March 2022
GSA Release No. 22-19

Kea Giles