New Geology articles published online ahead of print in September

Geological Society of America

Boulder, Colo., USA: Article topics and locations include lava dome instability; deep drilling into the Nankai subduction zone, Japan; marine impact craters; and the Pacific’s oldest crust. These Geology articles are online at .

Plate tectonics, mixed heating convection, and the divergence of mantle and plume temperatures

Johnny Seales; Adrian Lenardic; Julian Garrido Tomasini

Abstract: Petrological data indicate that upper mantle and mantle plume temperatures diverged 2.5 b.y. ago. This has been interpreted as plate tectonics initiating at 2.5 Ga, with Earth operating as a single-plate planet before that time. We took an Occam’s razor view and hypothesized that the continuous operation of plate tectonics can explain the divergence. We validated this hypothesis by comparing petrological data to results from mixed heating mantle convection models in a plate-tectonic mode of mantle cooling. The comparison shows that the data are consistent with plate tectonics operating throughout geologic history.

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Across-strike asymmetry of the Andes orogen linked to the age and geometry of the Nazca plate

Pedro Val; Jane K. Willenbring

Abstract: The crest of the Andes—the trace of the highest mountain topography—weaves back and forth, in places near the coastline, in others farther inland. Its position reflects the asymmetric distribution of orogen mass and coincides with asymmetry of orographic precipitation. This coincidence is thought to reflect a primary influence of orographic precipitation on accumulated orogenic mass whereby the more erosive (wetter) side promotes crest migration toward the less erosive (drier) side. However, whether this remains the case after excluding tectonic controls on the size and asymmetry of the wedge is an open question. We assessed relationships between precipitation, erosion rates, and the macromorphology of the Andes. We find that precipitation rates cannot sufficiently explain orogen asymmetry after statistically controlling for the age or dip of the Nazca slab. Slab age and dip are known to primarily affect mountain building in the Andes by impacting stress and strain propagation into the retro-arc region, thus better explaining the position of the mountain chain within the orogenic wedge. Accordingly, using basin-wide erosion rate, topographic, and precipitation data, we find that precipitation possibly influences erosional efficiency in semi-arid Andean landscapes but falls short in explaining the variability of erosion rates in regions of high orographic precipitation. We conclude that the orographic effect cannot change the Andean macromorphology that is set by bottom-up tectonic processes.

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Early accretion and prolonged carbonation of the Pacific Ocean’s oldest crust

Mark A. Kendrick; Jianxin Zhao; Yuexing Feng

Abstract: Newly formed oceanic crust is altered by seawater and carbonated at low temperatures over poorly defined periods of time. We applied in situ U-Pb dating to investigate 28 carbonate veins from Ocean Drilling Program Hole 801C, which is situated in the oldest Jurassic-age oceanic crust preserved in the western Pacific Ocean. Our results indicate that Pacific Ocean crust began accreting at 192 ± 6 Ma, which is ~25 m.y. earlier than previously recognized. Carbonation peaked at 171 ± 5 Ma and continued at a low rate for more than ~65 m.y. after accretion. Jurassic carbonation rates varied over ~10 m.y. timescales but encompassed a range similar to that observed today. These data suggest that carbonation rates are relatively insensitive to changes in atmospheric CO2, but confirm the longevity of seafloor alteration as a critical control in global volatile cycling.

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Cryogenian glacial erosion of the central Canadian Shield: The “late” Great Unconformity on thin ice

Kalin T. McDannell; C. Brenhin Keller

Abstract: The Great Unconformity has been recognized for more than a century, but only recently have its origins become a subject of debate. Hypotheses suggest global Snowball Earth glaciations and tectonic processes associated with the supercontinent Rodinia as drivers of widespread kilometer-scale erosion in the late Neoproterozoic. We present new integrated zircon and apatite (U-Th)/He and fission-track thermochronology from Precambrian basement samples of the central Canadian Shield in northern Manitoba to test these ideas. Bayesian inverse modeling indicates that 150–200 °C of cooling (>3 km of exhumation) occurred simultaneously with Cryogenian glaciations at ca. 690–650 Ma within interior North America. This estimate for the timing of unroofing is more precise than previous appraisals and does not align with any known tectonic or magmatic events (i.e., large igneous province eruptions) potentially associated with the supercontinent cycle that occurred during the late Proterozoic along the Laurentian margins. Based on these results and interpretations, the timing and magnitude of exhumation is best explained by glacial erosion, and further establishes the importance of multiple thermochronometers for resolving detailed deeptime thermal histories.

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Marine impacts: Sedimentologic fingerprint of event magnitude

M.I. Herreros; J. Ormö

Abstract: Previous studies of resurge sediments in drill cores from several marine-target impact craters indicated a relationship among the sedimentology, the target water depth, and the magnitude of the event. This offers a potentially valuable opportunity to obtain one of these variables if the other two are known. However, the mechanisms controlling the relationship have remained enigmatic. During the cratering process, fragments from the solid target are mixed with the seawater, and, after an initial turbulent phase, they commonly get deposited inside the seafloor crater when it is filled up by the resurging water. We present a mathematical model for the aquatic settling process of the resurge material. The starting hypothesis is that the finer particles mixed with water give rise to a fluid mixture in which the density and viscosity will depend on the relative water-to-solid content. This variation of the mixture’s properties will cause a different settling velocity of the coarser particles, and thus different settling patterns will be observed for craters with different relative target water depths.

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Large-scale lava dome fracturing as a result of concealed weakened zones

Claire E. Harnett; Michael J. Heap; Valentin R. Troll; Frances M. Deegan; Thomas R. Walter

Abstract: Mechanically weakened alteration zones in lava domes are thought to jeopardize their stability. Such zones can be hazardous when concealed within the dome, either because they formed by subsurface hydrothermal circulation or because they formed on the surface but were subsequently buried by renewed lava extrusion. We present a new suite of computational models showing how the size and position of a weakened brittle zone within a dome can affect large-scale fracture formation, displacement, and the collapse mechanism. By combining recent laboratory data for the mechanical behavior of dome rocks with discrete element method models, we show (1) the presence of a weak zone increases instability, which is exacerbated when the size of the zone increases or the zone is positioned off-center; (2) the position of the weak zone changes the deformation mechanism from slumping-type slope deformation when the zone is positioned centrally, compared with deep-seated rotational slope failure when the zone is positioned toward the dome flank; and finally, (3) dome-cutting tensile fractures form in the presence of a small weak zone (60 m diameter, ~14% of dome width), whereas large weak zones (120 m diameter, ~27% of dome width) promote the formation of longer and deeper fractures that jeopardize larger dome volumes. Our results corroborate previous field observations at lava domes and indicate that large fracture formation, which greatly influences dome stability and outgassing, can be explained by the presence of concealed alteration zones. This improved understanding of the mechanisms responsible for dome instability enables better hazard assessment at volcanoes worldwide.

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Osmium isotopes fingerprint mantle controls on the genesis of an epithermal gold province

Santiago Tassara; Alan D. Rooney; Jay. J. Ague; Diego Guido; Martin Reich …

Abstract: The formation of crustal blocks enriched with gold (Au) deposits above subduction zones is intimately bound to the genesis and evolution of magmatic-hydrothermal systems. A long-standing question, however, is whether the metal fertility of these systems stems from distinct sources that are anomalously enriched in Au or from subsequent processes occurring during crustal magma emplacement and hydrothermal activity. The Deseado Massif auriferous province in southern Patagonia (Argentina) is a unique place to test these contrasting hypotheses because Au-bearing mantle xenoliths indicate the presence of an underlying Au-rich lithospheric mantle reservoir. However, direct geochemical links between the Au-rich mantle source and the formation of the Deseado Massif auriferous province in the overlying crust remain to be established. To address this prominent gap in knowledge, we used sulfide Re-Os geochronology to identify the source of Au at Cerro Vanguardia, the largest low-sulfidation epithermal Au-Ag deposit in the Deseado Massif. Pyrite from high-grade Au quartz veins yielded an isochron age of 147.4 ± 2.9 Ma (mean square of weighted deviates = 1.04, n = 8) and an initial 187Os/188Os ratio of 0.26 ± 0.01, fingerprinting a dominant mantle control for the source of Os and, by inference, the source of Au. Our data provide a unique geochemical linkage between an Au-rich subcontinental lithospheric mantle source and the genesis of epithermal Au deposits, supporting the hypothesis that pre-enriched mantle domains may be a critical factor underpinning the global-scale localization of Au provinces.

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Don’t judge an orogen by its cover: Kinematics of the Appalachian décollement from seismic anisotropy

Michael G. Frothingham; Vera Schulte-Pelkum; Kevin H. Mahan; Arthur J. Merschat; Makayla Mather …

Abstract: As North America collided with Africa to form Pangea during the Alleghanian orogeny, crystalline and sedimentary rocks in the southeastern United States were thrust forelandward along the Appalachian décollement. We examined Ps receiver functions to better constrain the kinematics of this prominent subsurface structure. From Southeastern Suture of the Appalachian Margin Experiment (SESAME) and other EarthScope stations on the Blue Ridge–Piedmont crystalline megathrust, we find large arrivals from a 5–10-km-deep converter. We argue that a strong contrast in dipping anisotropic foliation occurs at the subhorizontal Appalachian décollement, and propose that such a geometry may be typical for décollement structures. Conversion polarity flips can be explained by an east-dipping foliation, but this orientation is at odds with the overlying northeast-trending surface tectonic grain. We suggest that prior to late Alleghanian northwest-directed head-on collision, the Appalachian décollement accommodated early Alleghanian west-vergence, independent of the overlying Blue Ridge–Piedmont structural inheritance. The geophysical expression of dipping anisotropic foliation provides a powerful tool for investigating subsurface kinematics, especially where they are obscured by overlying fabric, to disentangle the tectonic complexities that embody oblique collisional orogens.

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Orbitally paced global oceanic deoxygenation decoupled from volcanic CO2 emission during the middle Cretaceous Oceanic Anoxic Event 1b (Aptian-Albian transition)

Yi Wang; Stéphane Bodin; Jerzy S. Blusztajn; Clemens Ullmann; Sune G. Nielsen

Abstract: The ongoing oceanic oxygen loss will have a profound impact on the distribution and density of life on Earth. However, drivers of the initiation and termination of global oceanic deoxygenation are poorly understood. Here we present a thallium isotope record that reveals three rapid (50 k.y.) global deoxygenation intervals during the mid-Cretaceous that postdate massive volcanism by at least 1 m.y. New strontium isotope data reveal gradually enhanced continental weathering under a warmer climate following volcanism. However, global deoxygenation occurred only under the combined influences of a long-term increase in weathering rates in a warmer climate and short-term orbital modulation that led to atmospheric-circulation reorganization. Interactions of multiple drivers are consistent with the abrupt termination of each deoxygenation interval. Dynamic oxygenation responses in the mid-Cretaceous highlight the role of these processes for understanding the consequences and potential termination of the current oceanic deoxygenation.

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Hematite geochronology reveals a tectonic trigger for iron ore mineralization during Nuna breakup

Liam Courtney-Davies; Martin Danišik; Erick R. Ramanaidou; Christopher L. Kirkland; Noreen J. Evans …

Abstract: Hematite and goethite deposits hosted in banded iron formations (BIFs) in the Pilbara craton (Western Australia) represent one of Earth’s most significant Fe reserves; however, the timing and tectonic triggers underpinning deposit genesis remain contentious. Uncertainty in ore genesis stems from a lack of direct age measurements, which could aid in correlating periods of BIF mineralization with tectono-thermal events observed elsewhere. Archean–Paleoproterozoic BIFs in the Hamersley Province host extensive martite–microplaty hematite orebodies that formed at 2.2–2.0 Ga, based on indirect constraints. In contrast, combined hematite in situ U-Pb geochronology and (U-Th)/He thermochronology demonstrate that martite–microplaty hematite ores in the Chichester Range crystallized ca. 1.26–1.22 Ga and underwent cratonic denudation between ca. 0.57 and 0.38 Ga. Nanoscale imaging of dated hematite indicates that U-Th-Pb is lattice bound and not hosted in inclusions. New U-Pb hematite ages overlap with other mineral ages reported at the margins of the Pilbara and Yilgarn cratons (1.3–1.1 Ga), where mineral formation was driven by plate reorganization following breakup of the Nuna supercontinent. This age correlation suggests that a combination of increased orogenic (+diagenetic) and heat (+fluid) generative processes resulting from supercontinent reconfiguration was a key trigger for iron ore formation in the Pilbara craton.

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Direct constraints on in situ stress state from deep drilling into the Nankai subduction zone, Japan

Harold J. Tobin; Demian M. Saffer; David A. Castillo; Takehiro Hirose

Abstract: Stress state is a long-sought but poorly known parameter on subduction megathrusts and in overlying accretionary wedges in general. We used direct observations made during drilling of Integrated Ocean Drilling Program (IODP) borehole C0002 to a depth of 3058 m below the seafloor (mbsf) in the Nankai subduction zone of southwestern Japan to constrain in situ pore pressure and stress state in the deep interior of an accretionary wedge for the first time. These data included downhole pressure, active pumping tests, and logging and sample measurements. We found a nearly linear gradient in minimum horizontal principal stress (S hmin) and show that it remained consistently smaller than the vertical stress (Sv), definitively ruling out a thrust-faulting stress regime to at least 3 km depth, and to within ~2 km above the subduction megathrust. At 3000 mbsf, the estimated effective stresses were: Sv = 33 MPa, SHmax = 25–36 MPa, and Shmin = 18.5–21 MPa. We therefore interpret that the stress state throughout the drilled interval, which lies entirely in the hanging wall of the active megathrust, lies in a normal or strike-slip faulting regime (Sv ≥ S Hmax > Shmin). Total differential stresses are below ~18 MPa. We conclude that (1) basal traction along the megathrust must be small in order to permit both locking (and frictional sliding at failure) of the décollement and such low differential stresses deep within the upper plate; and (2) although differential stresses may remain low all the way to the plate boundary at ~5000 mbsf, S Hmax must transition to become greater than the vertical stress—either spatially below the base of the borehole or temporally leading up to megathrust fault rupture—in order to drive thrust motion along the plate boundary as observed in great earthquakes and in recurring very low-frequency earthquakes and slow-slip events.

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Quantifying controls on the occurrence of nonmarine fossils

Anik K. Regan; Raymond R. Rogers; Steven M. Holland

Abstract: Although numerous studies have described differential preservation of nonmarine fossils in channel and floodplain facies, quantitative comparisons are lacking. We present measurements of the probability of occurrence of plant, mollusk, and vertebrate fossils in the Campanian Judith River Formation of north-central Montana, USA. The data reveal little difference in the probability of occurrence among higher taxa in channel and floodplain facies, except for plants, which have a higher probability in floodplain deposits. This surprising result supports more recent models in which many organisms are buried initially in floodplain deposits, but those deposits are subsequently exhumed by migrating rivers, and finally buried in those channels. Comparisons across systems tracts revealed intriguing patterns in which plants have a higher probability of occurrence in high-accommodation systems tracts in channel and floodplain deposits, and vertebrates have a higher probability of occurrence in channels of the low-accommodation systems tract. These results confirm that sequence-stratigraphic architecture should be considered in interpretations of the nonmarine fossil record. This probability of occurrence method has promise for many other comparisons of fossilization potential.

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