New Articles for Geosphere Posted Online in April
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations for research include Lago di
Cignana, Italy; the western North China craton; the Kaikōura coast, New
Zealand; and the Hayes Range, Alaska, USA. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Lithium in garnet as a tracer of subduction zone metamorphic reactions:
The record in ultrahigh-pressure metapelites at Lago di Cignana, Italy
Gray E. Bebout; Tsutomu Ota; Takuya Kunihiro; William D. Carlson; Eizo
Nakamura
Abstract:
Lithium is of great interest as a tracer of meta-morphic reactions and
related fluid-mineral interactions because of its potential to isotopically
fractionate during inter- and intracrystalline diffusional processes. Study
of its transfer through subduction zones, based on study of arc volcanic
and metamorphic rocks, can yield insight regarding ocean-to-mantle chemical
cycling. We investigated major- and trace-element concentrations and δ 7Li in garnet in ultrahigh-pressure (UHP) Lago di Cignana
metasedimentary rocks, relating these observations to reconstructed
prograde devolatilization history. In all garnet crystals we studied, heavy
rare earth elements (HREEs), Y, and Li showed strong zoning, with elevated
concentrations in cores (15–50 ppm Li) and marked high-concentration
anomalies (up to 117 ppm Li, 5500 ppm Y; little or no major-element shift)
as growth annuli, in which some crystals showed subtle elevation in δ 7Li greater than analytical error of ~3‰ (2σ). Rutile inclusions
appeared abruptly at annuli and outward toward rims, accompanied by
inclusions of a highly zoned, Ca- and rare earth element–rich phase and
decreased Nb concentrations in garnet. These relationships are interpreted
to reflect prograde garnet-forming reaction(s), in part involving titanite
breakdown to stabilize rutile, which resulted in delivery of more abundant
Y and HREEs at surfaces of growing garnet crystals to produce annuli.
Co-enrichments in Li and Y + REEs are attributed to mutual incorporation
via charge-coupled substitutions; thus, increased Li uptake was a passive
consequence of elevated concentrations of Y + REEs. The small-scale
fluctuations in δ7Li (overall range of ~9‰) observed in some
crystals may correlate with abrupt shifts in major- and trace-element
concentrations, suggesting that changes in reactant phases exerted some
control on the evolution of δ7Li. For one garnet crystal,
late-stage growth following partial resorption produced deviation in major-
and trace-element compositions, including Li concentration, accompanied by
a 10‰–15‰ negative shift in δ7Li, perhaps reflecting a change in
the mechanism of incorporation or source of Li. These results highlight the
value of measuring the major- and trace-element and isotope compositions of
garnets in high-pressure and UHP metamorphic rocks in which matrix mineral
assemblages are extensively overprinted by recrystallization during
exhumation histories. Lithium concentrations and isotope compositions of
the garnets can add valuable information regarding prograde (and
retrograde) reaction history, kinetics of porphyroblast growth,
intracrystalline diffusion, and fluid-rock interactions. This work,
integrated with previous study of devolatilization in the Schistes
Lustrés/Cignana metasedimentary suite, indicates retention of a large
fraction of the initially subducted sedimentary Li budget to depths
approaching those beneath volcanic fronts, despite the redistribution of
this Li among mineral phases during complex mineral reaction histories.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02473.1/613340/Lithium-in-garnet-as-a-tracer-of-subduction-zone
Paleoproterozoic–Paleozoic tectonic evolution of the Longshou Shan,
western North China craton
Chen Wu; Jie Li; Andrew V. Zuza; Peter J. Haproff; An Yin ...
Abstract:
The Longshou Shan of western China is the northern backstop of the Cenozoic
Himalayan-Tibetan orogen and occupies a key linkage between the Tarim
continent and North China craton which separate the pre-Cenozoic Tethyan
orogenic system and Central Asian orogenic system. Therefore, the
Paleoproterozoic–Paleozoic evolution of this region is critical to
understanding the extent of overprinting Cenozoic deformation, construction
of the Eurasian continent, and relationships between the pre-Cenozoic
Tethyan orogenic system and Central Asian orogenic system. Here we present
detailed field observations and results of geochronological and major and
trace element and Sr-Nd isotope geochemical analyses of samples from the
Longshou Shan to decipher its complex Paleoproterozoic–Paleozoic tectonic
history. Our results show that the Paleoproterozoic basement rocks of the
Longshou Shan were part of the North China craton and involved in
Paleoproterozoic northern North China orogeny. A ca. 965 Ma granitoid in
the Longshou Shan provides key evidence for a spatial linkage between
northern Tibetan continents, the North Tarim continent, and the North China
craton in the early Neoproterozoic. The presence of Early Ordovician
granitoids and arc volcanic rocks in the Longshou Shan suggest that
bivergent subduction of Qilian oceanic lithosphere occurred during the
early Paleozoic. Crustal shortening and thickening during
Ordovician–Carboniferous orogenesis are evidenced by the presence of
several unconformities in the Longshou Shan. Late Carboniferous arc
granites exposed in the study area are likely associated with the southward
subduction of the Paleo-Asian Ocean to the north and with Permian
siliciclastic strata sourced from a proximal arc-subduction system, based
on detrital zircon ages. Although the tectonic history of the Long- shou
Shan can be traced back to Neoproterozoic time, most of the recorded
deformation and uplift of the region occurred during the early Paleozoic
Qilian orogeny and late Paleozoic Central Asian orogeny. Furthermore, we
interpret that the several orogenic events recorded in the Longshou Shan
(i.e., northern North China, Qilian, and Central Asian orogenies) are
spatially and temporally correlative along strike with those recorded in
the Tarim and North China cratons.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02491.1/613136/Paleoproterozoic-Paleozoic-tectonic-evolution-of
Groundwater silcrete linked to brine migration in a continental rift:
An alternative to the near-surface model of silcrete
David G. Lowe; E. DeSantis; R.W.C. Arnott; J. Conliffe
Abstract:
Silcretes have long been recognized as modern and ancient duricrusts, but
more recently also as silicified layers precipitated at groundwater tables,
termed groundwater silcrete. However, the silica sources, transport
mechanisms, and precipitation processes of groundwater silcrete are not
well understood, and models are limited to the near-surface groundwater
environment, where silica saturation is low. Here, an example of a
groundwater silcrete from Upper Cambrian strata of the Potsdam Group is
described and interpreted to be formed in a rift where Cambrian fault
reactivation coincided with silcrete formation. Field relationships
strongly support a connection between fault activity and silicification,
including a systematic thickening and development of massive silcrete
horizons above shear zones, brecciated silcrete near where faults intersect
shear zones, and nodules along the margins of shear zones. Petrographic and
cathodoluminescence microscopy of silcrete reveal early pre-compaction
overgrowth cements with abundant primary fluid inclusions. Fluid inclusion
microthermometry indicates that these fluids were high salinity (22.7–25.8
eq. wt% NaCl+CaCl2) brines with homogenization temperatures of
~120.2 °C–151.6 °C, which implies that silica precipitated from a hot,
silica-saturated crustal brine from Grenville Province basement. A
combination of weathering reactions and direct quartz dissolution explains
the chemical evolution of the source fluid, which likely originated as
infiltrated meteoric water that had chemically equilibrated with Grenville
crust at depth. Later, this brine was mobilized upward along reactivated
faults during the Late Cambrian, and ultimately to the water table, where a
combination of reduced pH and temperature promoted quartz supersaturation
and quartz overgrowths on detrital quartz. This case example, therefore,
expands the definition of silcrete to include near-surface silicification
from externally sourced crustal fluids, here termed brine silcrete, and
provides a basis for interpreting silcrete as a feature of deformation and
fluid migration along shear zones in fault-bounded continental basins.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02458.1/612948/Groundwater-silcrete-linked-to-brine-migration-in
Late Holocene coseismic uplift of the Kaikōura coast, New Zealand
Andy Howell; Kate J. Clark
Abstract:
The complex 2016 MW 7.8 Kaikōura earthquake ruptured >20
faults and caused highly variable uplift and subsidence of an ~110 km
stretch of coastline. The earthquake raised questions about fault
interactions in regions of oblique convergence and especially subduction to
strike-slip transition zones like the Kaikōura region. We integrate 2016
coastal vertical deformation observations with new mapping and dating of
Holocene marine terraces to: (1) compare spatial patterns of 2016 coseismic
and longer-term vertical motions, (2) investigate possible past multi-fault
ruptures or temporal clusters of earthquakes around Kaikōura, and (3)
assess the relative contributions of crustal faults and the Hikurangi
subduction interface to late Holocene coastal uplift. We identify possible
multi-fault ruptures or loose clusters of earthquakes at ca. 850–550 yr
B.P. and ca. 350–100 yr B.P. Most (and possibly all) of the Kaikōura coast
has been uplifted over the late Holocene; the 25-km-long Parikawa section
of coast subsided coseismically in 2016 but appears to be uplifted through
reverse slip on an offshore fault. Late Holocene uplift everywhere along
the coastline of interest can be attributed to slip on known upper-plate
faults; slip on a shallow-dipping (<20°) subduction interface cannot be
ruled out but is not required to explain uplift.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02479.1/612949/Late-Holocene-coseismic-uplift-of-the-Kaikoura
Large-scale, crustal-block vertical extrusion between the Hines Creek
and Denali faults coeval with slip localization on the Denali fault
since ca. 45 Ma, Hayes Range, Alaska, USA
Jeff A. Benowitz; Sarah M. Roeske; Sean P. Regan; Trevor S. Waldien; Julie
L. Elliott ...
Abstract:
Oblique convergence along strike-slip faults can lead to both distributed
and localized deformation. How focused transpressive deformation is both
localized and maintained along sub-vertical wrench structures to create
high topography and deep exhumation warrants further investigation. The
high peak region of the Hayes Range, central Alaska, USA, is bound by two
lithospheric scale vertical faults: the Denali fault to the south and Hines
Creek fault to the north. The high topography area has peaks over 4000 m
and locally has experienced more than 14 km of Neogene exhumation, yet the
mountain range is located on the convex side of the Denali fault Mount
Hayes restraining bend, where slip partitioning alone cannot account for
this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and
K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we
test whether these two parallel, reactivated suture zone structures are
working in tandem to vertically extrude the Between the Hines Creek and
Denali faults block on the convex side of the Mount Hayes restraining bend.
We document that since at least 45 Ma, the Denali fault has been bent and
localized in a narrow fault zone (<160 m) with a significant dip-slip
component, the Mount Hayes restraining bend has been fixed to the north
side of the Denali fault, and that the Between the Hines Creek and Denali
faults block has been undergoing vertical extrusion as a relatively
coherent block along the displacement “free faces” of two lithospheric
scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the
long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma.
Southern Alaska is currently converging at ~4 mm/yr to the north against
the Denali fault and driving vertical extrusion of the Between the Hines
Creek and Denali faults block and deformation north of the Hines Creek
fault. We apply insights ascertained from the Between the Hines Creek and
Denali faults block to another region in southern Alaska, the Fairweather
Range, where extreme topography and persistent exhumation is also located
between two sub-parallel faults, and propose that this region has likely
undergone vertical extrusion along the free faces of those faults.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02466.1/612950/Large-scale-crustal-block-vertical-extrusion
GEOSPHERE articles are available at
https://geosphere.geoscienceworld.org/content/early/recent
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