Boulder, Colo., USA: Article topics include the Great Unconformity of the
Rocky Mountain region; new Ediacara-type fossils; the southern Cascade arc
(California, USA); the European Alps and the Late Pleistocene glacial
maximum; Permian-Triassic ammonoid mass extinction; permafrost thaw; the
southern Rocky Mountains of Colorado (USA); “gargle dynamics”; invisible
gold; and alluvial fan deposits in Valles Marineris, Mars. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
A new kind of invisible gold in pyrite hosted in deformation-related
dislocations
Denis Fougerouse; Steven M. Reddy; Mark Aylmore; Lin Yang; Paul Guagliardo
...
Abstract:
Mining of “invisible gold” associated with sulfides in gold ores represents
a significant proportion of gold production worldwide. Gold hosted in
sulfide minerals has been proposed to be structurally bound in the crystal
lattice as a sulfide-gold alloy and/or to occur as discrete metallic
nanoparticles. Using a combination of microstructural quantification and
nanoscale geochemical analyses on a pyrite crystal from an orogenic gold
deposit, we show that dislocations hosted in a deformation low-angle
boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative
trace-element enrichment in the dislocations is 3.2 at% higher compared to
the bulk crystal. We propose that trace elements were segregated during the
migration of the dislocation following the dislocation-impurity pair model.
The gold hosted in nanoscale dislocations represents a new style of
invisible gold.
A quantification of the effect of diagenesis on the paleoredox record
in mid-Proterozoic sedimentary rocks
Alec M. Hutchings; Alexandra V. Turchyn
Abstract:
Iron speciation in ancient sedimentary rocks is widely used to reconstruct
oceanic redox conditions over geological time, specifically to assess the
extent of oxic, euxinic (anoxic containing sulfide), and ferruginous
(anoxic containing iron) conditions. We explore how post-depositional
sedimentary processes can skew particular geochemical signals in the rock
record. One such process is when aqueous sulfide—including that produced in
the sediment column—reacts with sedimentary iron, converting non-sulfide,
highly reactive iron minerals to iron-sulfide minerals; this can lead to
increased preservation of iron as pyrite and an overestimation of seafloor
euxinia. We show that sedimentary rocks with higher (>5 wt%) total iron
content are more buffered to this effect and thus are a more reliable
indicator of true water-column euxinia. When considering this effect in the
geological past, we estimate that true euxinia in the mid-Proterozoic may
have been as much as fourfold less than previously thought—more in line
with other recent paleoredox proxies not based on iron minerals. Marine
iron and sulfate concentrations were more equivalent in
Proterozoic–Neoproterozoic oceans, suggesting this time period was
particularly susceptible to this post-depositional alteration, explaining
the extent of euxinia suggested for this geological interval.
How fast do submarine fans grow? Insights from the Quaternary Golo
fans, offshore Corsica
Michael L. Sweet; Gwladys T. Gaillot; Tammy M. Rittenour; Kathrine Love;
Michael D. Blum
Abstract:
High-resolution seismic, core, and chronological data from the Quaternary
Golo deep-sea fans, offshore Corsica, France, give new insights into rates
of submarine fan growth. Average vertical deposition rates for units that
represent the Late Pleistocene glacial periods are 0.1–0.5 m/k.y.
Glacial-age deposits are sand rich; in contrast, post-glacial deposits lack
a significant sand fraction and are dominated by carbonate-rich mud. As a
result, seismically constrained volumetric rates of deposition for glacial
periods with low sea level and a subaerially exposed shelf are ~0.23 km 3/k.y., 2×–5× higher than rates during interglacials when sea
level is high, the shelf is submerged, and sand is trapped in
shallow-marine environments. At millennial time scales, variations in
deposition rate reflect climate-driven sea-level changes, autogenic
avulsion of river channels that extend across the shelf during low sea
level, and autogenic avulsion of submarine channels that shift the locus of
deposition laterally. Short-term deposition rates range from 8.6 m/k.y. at
proximal portions of submarine fans to 0.4 m/k.y. along the distal fringe.
Our data show that submarine fans can be dynamic environments with
formation and evolution of levee-confined channels and lobe complexes in 10 3–104 yr, comparable to the time scales needed to
form fluvial channel belts and delta lobes.
Contrasting northern and southern European winter climate trends during
the Last Interglacial
J. Sakari Salonen; Maria Fernanda Sánchez-Goñi; Hans Renssen; Anna Plikk
Abstract:
The Last Interglacial (LIG; 130–115 ka) is an important test bed for
climate science as an instance of significantly warmer than preindustrial
global temperatures. However, LIG climate patterns remain poorly resolved,
especially for winter, affected by a suite of strong feedbacks such as
changes in sea-ice cover in the high latitudes. We present a synthesis of
winter temperature and precipitation proxy data from the Atlantic seaboard
of Europe, spanning from southern Iberia to the Arctic. Our data reveal
distinct, opposite latitudinal climate trends, including warming winters
seen in the European Arctic while cooling and drying occurred in southwest
Europe over the LIG. Climate model simulations for 130 and 120 ka suggest
these contrasting climate patterns were affected by a shift toward an
atmospheric circulation regime with an enhanced meridional pressure
gradient and strengthened midlatitude westerlies, leading to a strong
reduction in precipitation across southern Europe.
A record of syn-tectonic sedimentation revealed by perched
alluvial fan deposits in Valles Marineris, Mars
J.M. Davis; P.M. Grindrod; S.G. Banham; N.H. Warner; S.J. Conway ...
Abstract:
On Mars, basins formed by tectonic processes are rare and mostly have
unconstrained subsidence histories. One method for understanding this
record of subsidence is through associated alluvial fans, which are sourced
from uplifted areas and accumulate in downthrown basins. The source,
morphology, and superposition of fan deposits can be used to reconstruct
fault kinematics, the relative timing of accommodation space formation,
and, in turn, the influence tectonic processes had on Martian fan
formation. Here we use high-resolution orbital data sets to characterize
sediment fan deposits associated with syn-tectonic sedimentation in two
regions of the Valles Marineris canyons: Coprates Chasma and Juventae
Chasma. These deposits comprise sediment fans on the current canyon floor
and low-gradient surfaces perched several kilometers above the canyon
floor. We interpret the low-gradient surfaces as remnant sediment fan
deposits, which originally formed at the former canyon floor and have since
been offset due to normal faulting. The preservation of vertically offset
generations of sediment fan deposits supports a progressive, basinward
migration of fault activity into the original hanging wall or repeat
activity along a fault zone. Each episode of faulting was followed by a
basinward shift in drainages, which led to fault-scarp degradation and
formation of a new generation of fans. Multiple episodes of syn-tectonic
sedimentation occurred during the evolution of the basins, with fluvial
activity sporadically active. Our results demonstrate, for the first time
on Mars, that depositional cyclicity was linked to tectonic deformation,
possibly representative of regional processes throughout Valles Marineris.
Seismic fault weakening via CO2 pressurization enhanced by
mechanical deformation of dolomite fault gouges
Hyun Na Kim; Byung-Dal So; Min Sik Kim; Kee Sung Han; Sol Bi Oh
Abstract:
Carbon dioxide emissions from dolomite decarbonation play an essential role
in the weakening of carbonate faults by lowering the effective normal
stress, which is thermally activated at temperatures above 600–700 °C.
However, the mechanochemical effect of low-crystalline ultrafine fault
gouge on the decarbonation and slip behavior of dolomite-bearing faults
remains unclear. In this study, we obtained a series of artificial dolomite
fault gouges with systematically varying particle sizes and dolomite
crystallinities using a high-energy ball mill. The laboratory-scale
pulverization of dolomite yielded MgO at temperatures below 50 °C,
indicating that mechanical decarbonation without significant heating
occurred due to the collapse of the crystalline structure, as revealed by
X-ray diffraction and solid-state nuclear magnetic resonance results.
Furthermore, the onset temperature of thermal decarbonation decreased to
~400 °C. Numerical modeling reproduced this two-stage decarbonation, where
the pore pressure increased due to low-temperature thermal decarbonation,
leading to slip weakening on the fault plane even at 400–500 °C; i.e.,
200–300 °C lower than previously reported temperatures. Thus, the presence
of small amounts of low-crystalline dolomite in a fault plane may lead to a
severely reduced shear strength due to thermal decomposition at ~400 °C
with a small slip weakening distance.
Alpine relief limited by glacial occupation time
Bernhard Salcher; Günther Prasicek; Sebastian Baumann; Florian Kober
Abstract:
Glaciers exert a major control on the shape of mountain topography. They
tend to reduce relief above and scour troughs below the equilibrium line
altitude (ELA). While many studies report this dichotomy, relief-limiting
effects are controversial due to difficulties in quantifying key factors
such as the initial topography, the timing of glacial occupancy, or rock
uplift counteracting glacial erosion. Consequently, effectivity and degree
of glacial erosion remain ambiguous. In geologically and climatically
well-investigated parts of the European Central Alps, our calculation of
glacial occupation time (GOT) from Quaternary ELA variations allows the
quantification of gradual topographic modifications generated by the
cumulative impact of cirque erosion over the Quaternary. We show that under
low uplift, relief is effectively limited by glacial and periglacial
headwall retreat, leading to a decline in topographic relief as GOT
increases. Conversely, higher uplift rates seem to induce more persistent
valley glaciation, triggering a positive feedback loop in which steep
slopes are protected against erosion and relief increases.
Spatially variable provenance of the Chinese Loess Plateau
Haobo Zhang; Junsheng Nie; Xiangjun Liu; Alex Pullen; Guoqiang Li ...
Abstract:
Loess sequences of the Chinese Loess Plateau (CLP) compose one of the most
complete Neogene–Quaternary terrestrial paleoclimatic archives.
Understanding the CLP’s sediment sources is critical to tracing Asian
aridification, atmospheric circulation patterns, and Asian monsoon
evolution. Commonly, the sediments that compose the Quaternary strata of
the CLP are considered largely homogeneous, and thus numerous studies have
applied a uniform source model when attempting to use CLP-derived proxies
as paleoclimate indicators. Here we present large-n detrital
zircon U-Pb geochronology data from the Quaternary CLP. These data support
spatial variability in sediment provenance across the CLP. At least three
distinct provenance zones are recognized for Quaternary loess strata:
central western, eastern, and northeastern. These zones received sediment
primarily from their neighboring river systems. This finding conflicts with
the classic views that attribute the Quaternary loess principally to the
deserts north and west of the CLP. We conclude that fluvial processes, and
thus precipitation, played an important and previously underemphasized role
in Quaternary dust production in northern China. Furthermore, nonuniformity
in CLP Quaternary sediment provenance raises questions about the validity
of using paleoclimate information archived in the CLP to make sweeping
regional interpretations.
Igneous rock area and age in continental crust
Shanan E. Peters; Craig R. Walton; Jon M. Husson; Daven P. Quinn; Oliver
Shorttle ...
Abstract:
Rock quantity and age are fundamental features of Earth’s crust that
pertain to many problems in geoscience. Here we combine new estimates of
igneous rock area in continental crust from the Macrostrat database
(https://macrostrat.org/) with a compilation of detrital zircon ages in
order to investigate rock cycling and crustal growth. We find that there is
little or no decrease in igneous rock area with increasing rock age.
Instead, igneous rock area in North America exhibits four distinct
Precambrian peaks, remains low through the Neoproterozoic, and then
increases only modestly toward the recent. Peaks in Precambrian detrital
zircon age frequency distributions align broadly with peaks in igneous rock
area, regardless of grain depositional age. However, detrital zircon ages
do underrepresent a Neoarchean peak in igneous rock area; young grains and
ca. 1.1 Ga grains are also overrepresented relative to igneous area.
Together, these results suggest that detrital zircon age distributions
contain signatures of continental denudation and sedimentary cycling that
are decoupled from the cycling of igneous source rocks. Models of
continental crustal evolution that incorporate significant early increase
in volume and increased sedimentation in the Phanerozoic are well supported
by these data.
Explosive caldera-forming eruptions and debris-filled vents: Gargle
dynamics
Greg A. Valentine; Meredith A. Cole
Abstract:
Large explosive volcanic eruptions are commonly associated with caldera
subsidence and ignimbrites deposited by pyroclastic currents. Volumes and
thicknesses of intracaldera and outflow ignimbrites at 76 explosive
calderas around the world indicate that subsidence is commonly simultaneous
with eruption, such that large proportions of the pyroclastic currents are
trapped within the developing basins. As a result, much of an eruption must
penetrate its own deposits, a process that also occurs in large,
debris-filled vent structures even in the absence of caldera formation and
that has been termed “gargling eruption.” Numerical modeling of the
resulting dynamics shows that the interaction of preexisting deposits
(fill) with an erupting (juvenile) mixture causes a dense sheath of fill
material to be lifted along the margins of the erupting jet. This can cause
an eruption that would otherwise produce a buoyant plume and fallout
deposits to instead form pyroclastic currents as the dense sheath drives
pulsing jet behavior. Increasing thickness of fill amplifies the time
variation in jet height. Increasing the fill grain size relative to that of
the juvenile particles can result in a much higher jet due to poorer mixing
between the dense sheath and the dilute jet core. In all cases, material
collapses along the entire height of the dense sheath rather than from the
top of a simple fountain. These gargle dynamics provide strong backing for
processes that have been inferred to result in intraplinian ignimbrites and
simultaneous deposition from high- and low-energy pyroclastic currents.
Prior oil and gas production can limit the occurrence of
injection-induced seismicity: A case study in the Delaware Basin of
western Texas and southeastern New Mexico, USA
Noam Z. Dvory; Mark D. Zoback
Abstract:
We demonstrate that pore pressure and stress changes resulting from several
decades of oil and gas production significantly affect the likelihood of
injection-related induced seismicity. We illustrate this process in the
Delaware Basin (western Texas and southeastern New Mexico, USA), in which
hydraulic fracturing and waste-water injection have been inducing numerous
earthquakes in the southernmost part of the basin where there has been no
prior oil and gas production from the formations in which the earthquakes
are now occurring. In the seismically quiescent part of the basin, we show
that pore-pressure and poroelastic-stress changes associated with prior oil
and gas production make induced seismicity less likely. The findings of
this study have important implications for the feasibility of large-scale
carbon storage in depleted oil and gas reservoirs.
Excess ice loads in the Indian Ocean sector of East Antarctica during
the last glacial period
Takeshige Ishiwa; Jun’ichi Okuno; Yusuke Suganuma
Abstract:
An accurate reconstruction of the Antarctic Ice Sheet is essential in order
to develop an understanding of ice-sheet responses to global climate
changes. However, the erosive nature of ice-sheet expansion and the
difficulty of accessing much of Antarctica make it challenging to obtain
field-based evidence of ice-sheet and sea-level changes before the Last
Glacial Maximum. Limited sedimentary records from Lützow-Holm and Prydz
Bays in East Antarctica demonstrate that the sea level during Marine
Isotope Stage 3 was close to the present level despite the global sea-level
drop lower than –40 m. We demonstrate glacial isostatic adjustment modeling
with refined Antarctic Ice Sheet loading histories. Our experiments reveal
that the Indian Ocean sector of the Antarctic Ice Sheet would have been
required to experience excess ice loads before the Last Glacial Maximum in
order to explain the observed sea-level highstands during Marine Isotope
Stage 3. As such, we suggest that the Antarctic Ice Sheet partly reached
its maximum thickness before the global Last Glacial Maximum.
Across-arc variations in Mo isotopes and implications for subducted
oceanic crust in the source of back-arc basin volcanic rocks
Xiaohui Li; Quanshu Yan; Zhigang Zeng; Jingjing Fan; Sanzhong Li ...
Abstract:
Molybdenum (Mo) isotope ratios provide a potential means of tracing
material recycling involved in subduction zone processes. However, the
geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We
analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana
subduction zone (western Pacific Ocean), combining newly obtained element
and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical
processes involving Mo. The Mo isotope ratios (δ98/95Mo NIST3134; U.S. National Institute of Standards and Technology
[NIST] Mo standard) of the volcanic rocks showed clear across-arc
variations, decreasing with increasing depth to the Wadati-Benioff zone.
The high δ98/95Mo values in the Mariana Islands (–0.18‰ to
+0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios,
suggesting that altered upper oceanic crust played an important role in the
magma source. The low δ98/95Mo values in the Central Mariana
Trough (–0.65‰ to –0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ 7Li values provide direct evidence for the contribution of deep
recycled oceanic crust to the magma source of the back-arc basin lavas. The
isotopically light Mo magmas originated by partial melting of a residual
subducted slab (eclogite) after high degrees of dehydration and then
penetrated into the back-arc mantle. This interpretation provides a new
perspective with which to investigate the deep recycling of subducted
oceanic lithosphere and associated magma petrogenesis.
Tectonic controls on basement exhumation in the southern Rocky
Mountains (United States): The power of combined zircon (U-Th)/He and
K-feldspar 40Ar/39Ar thermochronology
Jason W. Ricketts; Jacoup Roiz; Karl E. Karlstrom; Matthew T. Heizler;
William R. Guenthner ...
Abstract:
The Great Unconformity of the Rocky Mountain region (western North
America), where Precambrian crystalline basement is nonconformably overlain
by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of
rock record during 10-km-scale basement exhumation. We evaluate the timing
of exhumation of basement rocks at five locations by combining geologic
data with multiple thermochronometers. 40Ar/39Ar
K-feldspar multi-diffusion domain (MDD) modeling indicates regional
multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma
and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains
range from 20 to 864 Ma, and independent forward modeling of ZHe data is
also most consistent with multi-stage cooling. ZHe inverse models at five
locations, combined with K-feldspar MDD and sample-specific geochronologic
and/or thermochronologic constraints, document multiple pulses of basement
cooling from 250 °C to surface temperatures with a major regional basement
exhumation event 1300–900 Ma, limited cooling in some samples during the
770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca.
300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic
control on basement exhumation leading up to formation of the
Precambrian-Cambrian Great Unconformity and document the formation of
composite erosional surfaces developed by faulting and differential uplift.
Reorienting the West African craton in Paleoproterozoic–Mesoproterozoic
supercontinent Nuna
Zheng Gong; David A.D. Evans; Nasrrddine Youbi; Abdelhak Ait Lahna; Ulf
Söderlund ...
Abstract:
The location of the West African craton (WAC) has been poorly constrained
in the Paleoproterozoic–Mesoproterozoic supercontinent Nuna (also known as
Columbia). Previous Nuna reconstruction models suggested that the WAC was
connected to Amazonia in a way similar to their relative position in
Gondwana. By an integrated paleomagnetic and geochronological study of the
Proterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide two
reliable paleomagnetic poles to test this connection. Incorporating our new
poles with quality-filtered poles from the neighboring cratons of the WAC,
we propose an inverted WAC-Amazonia connection, with the northern WAC
attached to northeastern Amazonia, as well as a refined configuration of
Nuna. Global large igneous province records also conform to our new
reconstruction. The inverted WAC-Amazonia connection suggests a substantial
change in their relative orientation from Nuna to Gondwana, providing an
additional example of large-magnitude cumulative azimuthal rotations
between adjacent continental blocks over supercontinental cycles.
New Ediacara-type fossils and late Ediacaran stratigraphy from the
northern Qaidam Basin (China): Paleogeographic implications
Ke Pang; Chengxi Wu; Yunpeng Sun; Qing Ouyang; Xunlai Yuan ...
Abstract:
Ediacara-type macrofossils characterize the late Ediacaran Period and are
pivotal in understanding the early evolution of animals on the eve of the
Cambrian explosion and useful in late Ediacaran biostratigraphy. They have
been discovered on almost all major paleocontinents, except the North China
and Tarim blocks, as well as on a series of northwestwest–oriented cratonic
fragments between the two blocks, including the Olongbuluke terrane of the
Qaidam block, where the terminal Ediacaran successions developed. We report
a newly discovered terminal Ediacaran biotic assemblage, the Quanjishan
assemblage, containing Ediacara-type fossils from the Zhoujieshan Formation
of the Quanji Group in the Olongbuluke terrane, Qaidam Basin, northwestern
China. The Quanjishan assemblage is dominated by the non-biomineralized
tubular taxon Shaanxilithes, which has the potential to be a
terminal Ediacaran index fossil, and by the iconic frondose rangeomorph Charnia, which represents the only unambiguous Ediacara-type
fossil discovered in northwestern China. The co-occurrence of Charnia and Shaanxilithes from the Quanjishan assemblage
likely constrains the depositional age of the Zhoujieshan Formation to be
terminal Ediacaran (ca. 550–539 Ma) and the immediately underlying
Hongtiegou diamictites to be late Ediacaran, probably representing
post-Gaskiers glacial deposition. The occurrence of post-Gaskiers Ediacaran
glaciation and similarities between the late Ediacaran–early Paleozoic
lithostratigraphic and biostratigraphic sequences in the Olongbuluke
terrane of the Qaidam block and the North China block suggest that these
two blocks may have been located close to each other during this time
period, and situated in the middle to high latitudes instead of the
equatorial region.
Slab-derived sulfate generates oxidized basaltic magmas in the southern
Cascade arc (California, USA)
Michelle J. Muth; Paul J. Wallace
Abstract:
Whether and how subduction increases the oxidation state of Earth’s mantle
are two of the most important unresolved questions in solid Earth
geochemistry. Using data from the southern Cascade arc (California, USA),
we show quantitatively for the first time that increases in arc magma
oxidation state are fundamentally linked to mass transfer of isotopically
heavy sulfate from the subducted plate into the mantle wedge. We
investigate multiple hypotheses related to plate dehydration and melting
and the rise and reaction of slab melts with mantle peridotite in the
wedge, focusing on electron balance between redox-sensitive iron and sulfur
during these processes. These results show that unless slab-derived silicic
melts contain much higher dissolved sulfur than is indicated by currently
available experimental data, arc magma generation by mantle wedge melting
must involve multiple stages of mantle metasomatism by slab-derived
oxidized and sulfur-bearing hydrous components.
Long-term Aptian marine osmium isotopic record of Ontong Java Nui
activity
Hironao Matsumoto; Rodolfo Coccioni; Fabrizio Frontalini; Kotaro Shirai;
Luigi Jovane ...
Abstract:
The early to mid-Aptian was punctuated by episodic phases of organic-carbon
burial in various oceanographic settings, which are possibly related to
massive volcanism associated with the emplacement of the Ontong Java,
Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean,
inferred to have formed a single plateau called Ontong Java Nui.
Sedimentary osmium (Os) isotopic compositions are one of the best proxies
for determining the timing of voluminous submarine volcanic episodes.
However, available Os isotopic records during the age are limited to a
narrow interval in the earliest Aptian, which is insufficient for the
reconstruction of long-term hydrothermal activity. We document the early to
mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in
the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain
the timing of massive volcanic episodes and to assess their impact on the
marine environment. Our new Os isotopic data reveal three shifts to
unradiogenic values, two of which correspond to black shale horizons in the
lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels.
These Os isotopic excursions are ascribed to massive inputs of unradiogenic
Os to the ocean through hydrothermal activity. Combining the new Os
isotopic record with published data from the lowermost Aptian organic-rich
interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can
be inferred that Ontong Java Nui volcanic eruptions persisted for ~5 m.y.
during the early to mid-Aptian.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48863.1/600857/Long-term-Aptian-marine-osmium-isotopic-record-of
Anisotropy-revealed change in hydration along the Alaska subduction
zone
Colton Lynner
Abstract:
Megathrust earthquake behavior in subduction zones is controlled by a
variety of factors including the hydration state of the subducting slab.
Increased hydration reduces the occurrence of great, damaging earthquakes
by diminishing the strength of the material along the interface between
tectonic plates. Understanding variations in hydration in subductions zones
is necessary for properly assessing the overall hazard posed by each
region. Fortunately, seismic anisotropy is strongly dependent upon
hydration of the subducting crust and lithosphere. I present shear-wave
splitting measurements that illuminate changes in anisotropy, and therefore
hydration, of the subducting Pacific plate beneath the Alaska subduction
zone (northern Pacific Ocean). Variations in shear-wave splitting directly
correlate to changes in the behavior of great, megathrust earthquakes. My
measurements show that the Shumagin seismic gap is characterized by a
hydrated subducting slab, explaining the long-term lack of great
earthquakes. Observations in the immediately adjacent Semidi segment, which
experiences great events regularly, indicate a far less hydrated slab.
These results are driven by the preferential alignment of paleo-spreading
fabrics of the Pacific plate. Where fabrics are more closely aligned with
the orientation of the trench, outer-rise faulting and plate hydration is
enhanced. These results highlight the importance of changes in preexisting
slab structures and subsequent hydration in the production of great,
damaging earthquakes.
Out-of-phase Late Pleistocene glacial maxima in the Western Alps
reflect past changes in North Atlantic atmospheric circulation
Natacha Gribenski; Pierre G. Valla; Frank Preusser; Thibault Roattino;
Christian Crouzet ...
Abstract:
Paleoglacier reconstructions in the northern and southern forelands of the
European Alps indicate a synchronous Late Pleistocene glacial maximum
during Marine Isotope Stage (MIS) 2, in phase with global ice volume
records. However, strong controversy remains for the western foreland,
where scarce and indirect dating as well as modeling studies suggest
glacial maxima out of phase with the rest of the Alps. New luminescence
dating brings the first direct Late Pleistocene glacial chronology for the
western Alpine foreland and reveals two major glacier advances of similar
maximum extent, at ca. 75–60 and ca. 40–30 ka, coinciding with MIS 4 and
late MIS 3. We propose that asynchrony in glacial maxima between the
western and the northern and southern Alpine forelands results from a
progressive spatial reorganization of the atmospheric circulation over the
North Atlantic in response to Northern Hemisphere ice-sheet fluctuations.
While such a feedback mechanism has emerged from general circulation
models, our Late Pleistocene paleoglacial reconstruction permits tracking
of the spatiotemporal evolution of moisture advection patterns over Western
Europe.
Highly localized upper mantle deformation during plate boundary
initiation near the Alpine fault, New Zealand
Steven Kidder; David J. Prior; James M. Scott; Hamid Soleymani; Yilun Shao
Abstract:
Peridotite xenoliths entrained in magmas near the Alpine fault (New
Zealand) provide the first direct evidence of deformation associated with
the propagation of the Australian-Pacific plate boundary through the region
at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained
(40–150 mm) rocks, indicating that deformation in the upper mantle was
focused in highly sheared zones. To constrain the nature and conditions of
deformation, we combine a flow law with a model linking recrystallized
fraction to strain. Temperatures calculated from this new approach (625–970
°C) indicate that the observed deformation occurred at depths of 25–50 km.
Calculated shear strains were between 1 and 100, which, given known plate
offset rates (10–20 mm/yr) and an estimated interval during which
deformation likely occurred (<1.8 m.y.), translate to a total shear zone
width in the range 0.2–32 km. This narrow width and the position of
mylonite-bearing localities amid mylonite-free sites suggest that early
plate boundary deformation was distributed across at least ~60 km but
localized in multiple fault strands. Such upper mantle deformation is best
described by relatively rigid, plate-like domains separated by rapidly
formed, narrow mylonite zones.
Morphological selectivity of the Permian-Triassic ammonoid mass
extinction
Xu Dai; Dieter Korn; Haijun Song
Abstract:
Ammonoids suffered a diversity bottleneck during the Permian-Triassic mass
extinction (PTME) and experienced a rapid diversification in the Early
Triassic. However, the kinds of ammonoids that were more likely to survive
the PTME and that fueled subsequent diversification are still poorly known.
We compiled a comprehensive morphological data set and used the nonmetric
multidimensional scaling method to reveal the impact of the PTME on the
morphological selectivity of ammonoids. Our results show that
postextinction taxa occupied a quite different morphospace when compared
with the pre-extinction assemblages. The survivors were mainly smooth and
weakly ornamented forms, while the late Permian species were dominated by
coarsely ornamented forms. Contrary to previously recognized nonselective
patterns, these results suggest a morphological selectivity of the
Permian-Triassic crisis. Newcomers in the Griesbachian were mainly
compressed and smooth forms. This morphological shift from the coarsely
ornamented ammonoids dominating the Changhsingian to the smooth ammonoids
dominating the Griesbachian possibly suggests an ecological turnover of
ammonoids during the PTME.
Permafrost thaw induced abrupt changes in hydrology and carbon cycling
in Lake Wudalianchi, northeastern China
Yuan Yao; Yongsong Huang; Jiaju Zhao; Li Wang; Youhua Ran ...
Abstract:
Lakes in the permafrost zone have been proposed to serve as key outlets for
methane and carbon dioxide emissions. However, there has been no geological
record of the hydrological and biogeochemical responses of lakes throughout
the thawing of surrounding permafrost. We use multiple biomarker and
isotopic proxies to reconstruct hydrological and biogeochemical changes in
Lake Wudalianchi in northeastern China during regional thawing of the
permafrost. We show permafrost thawing, as indicated by lignin degradation,
initiated rapid lake water freshening as a result of the opening of
groundwater conduits, and negative organic δ13C excursion due to
increased inorganic and organic carbon fluxes. These hydrological changes
were followed, with an ~5–7 yr delay, by abrupt and persistent increases in
microbial lake methanotrophy and methanogenesis, indicating enhanced
anaerobic organic decomposition and methane emissions from lakes as
permafrost thaws. Our data provide a detailed assessment of the processes
involved during permafrost thaw, and highlight the importance of lakes in
ventilating greenhouse gases to the atmosphere.
Erosion of the Himalaya-Karakoram recorded by Indus Fan deposits since
the Oligocene
Han Feng; Huayu Lu; Barbara Carrapa; Hanzhi Zhang; Jun Chen ...
Abstract:
The Cenozoic erosion history of the Himalaya-Karakoram, which is a function
of tectonically driven uplift and monsoon climatic evolution in South Asia,
remains elusive, especially prior to the Miocene. Here, we present a
multiproxy geochemical and thermochronological analysis of the oldest
samples available from the Arabian Sea, which we used to investigate the
erosion history of the Himalayan and Karakoram orogenic system. The Indus
Fan records rapid and sustained erosion of the Himalayan-Karakoram
mountains from before 24 Ma (ca. 30) to ca. 16 Ma concurrent with changing
provenance from the Indian (Himalayan) and Eurasian plates. Our data,
combined with previous studies of younger Indus Fan deposits, indicate that
the mid-to-late Cenozoic erosion history of the Himalayan-Karakoram
mountains is overall consistent with a vigorous monsoonal climate from the
late Oligocene to middle Miocene and with changes in global climate in the
late Miocene, whereas erosion and deposition are relatively insensitive to
changes in sources and rock erodibility. Although tectonic processes were
active throughout, we suggest that the erosional signatures of the
Himalayan-Karakoram mountains from the Indus Fan largely preserve a record
of climate changes since the Oligocene.
Raising the West: Mid-Cenozoic Colorado-plano related to subvolcanic
batholith assembly in the Southern Rocky Mountains (USA)?
Peter W. Lipman
Abstract:
The Southern Rocky Mountains of Colorado, United States, have the highest
regional elevation in North America, but present-day crustal thickness
(~42–47 km) is no greater than for the adjacent, topographically lower High
Plains and Colorado Plateau. The chemistry of continental-arc rocks of the
mid-Cenozoic Southern Rocky Mountain volcanic field, calibrated to
compositions and Moho depths at young arcs, suggests that paleocrustal
thickness may have been 20%–35% greater than at present and elevations
accordingly higher. Thick mid-Cenozoic Rocky Mountain crust and high
paleo-elevations, comparable to those inferred for the Nevadaplano farther
west in the United States from analogous volcanic chemistry, could be
consistent with otherwise-perplexing evidence for widespread rapid erosion
during volcanism. Variable mid-Cenozoic crustal thickening and uplift could
have resulted from composite batholith growth during volcanism,
superimposed on prior crustal thickening during early Cenozoic (Laramide)
compression. Alternatively, the arc–crustal thickness calibration may be
inappropriate for high-potassium continental arcs, in which case other
published interpretations using similar methods may also be unreliable.
Labyrinth patterns in Magadi (Kenya) cherts: Evidence for early
formation from siliceous gels
Kennie Leet; Tim K. Lowenstein; Robin W. Renaut; R. Bernhart Owen; Andrew
Cohen
Abstract:
Sedimentary cherts, with well-preserved microfossils, are known from the
Archean to the present, yet their origins remain poorly understood. Lake
Magadi, Kenya, has been used as a modern analog system for understanding
the origins of nonbiogenic chert. We present evidence for synsedimentary
formation of Magadi cherts directly from siliceous gels. Petrographic
thin-section analysis and field-emission scanning electron microscopy of
cherts from cores drilled in Lake Magadi during the Hominin Sites and
Paleolakes Drilling Project in 2014 led to the discovery of two-dimensional
branching "labyrinth patterns" in chert, which are a type of fractal
"squeeze" pattern formed at air-liquid interfaces. Labyrinth patterns
preserved in chert from Lake Magadi cores indicate invasion of air along
planes in dewatering gels. These patterns support the precipitation of
silica gels in the saline-alkaline Lake Magadi system and syndepositional
drying of gels in contact with air as part of chert formation. Recognizing
cherts as syndepositional has been critical for our use of them for U-Th
dating. Identification of labyrinth patterns in ancient cherts can provide
a better understanding of paleoenvironmental and geochemical conditions in
the past.
Serpentinized peridotite versus thick mafic crust at the Romanche
oceanic transform fault
Emma P.M. Gregory; Satish C. Singh; Milena Marjanović; Zhikai Wang
Abstract:
The crust beneath transform faults at slow-spreading ridges has been
considered to be thin, comprising a thin mafic layer overlying
serpentinized peridotite. Using wide-angle seismic data, we report the
presence of a Moho at ~6 km depth and a low-velocity anomaly extending down
to 9 km beneath the 20-km-wide Romanche transform valley floor in the
equatorial Atlantic Ocean. The low crustal velocities above the Moho could
be due to either highly serpentinized mantle peridotite or fractured mafic
rocks. The existence of clear Moho reflections and the occurrence of a
large crustal-depth rupture during the 2016 magnitude 7.1 earthquake
suggest that the crust likely consists of fractured mafic material.
Furthermore, the presence of low velocities below the Moho advocates for
extensive serpentinization of the mantle, indicating that the Moho
reflection is unlikely to be produced by a serpentinization front. The
crust to the north of the transform fault likely consists of mafic
material, but that in the south appears to be more amagmatic, possibly
containing serpentinized peridotite. Our results imply that the transform
fault structure is complex and highly heterogeneous, and thus would have
significant influence on earthquake rupture and alteration processes.