New Articles for Geosphere Posted Online First
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics this month include the Mojave
Desert; Obsidian Dome, California; and the Appalachian Inner Piedmont,
North Carolina. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent.
Using discordant U-Pb zircon data to re-evaluate the El Paso
terrane: Late Paleozoic tectonomagmatic evolution of east-central
California (USA) and intense hydrothermal activity in the Jurassic
Sierra Nevada arc
Diane Clemens-Knott; Michelle Gevedon
Quantitative modeling of discordant detrital zircon U-Pb isotope data from
the northern El Paso terrane reveals metamorphosed Laurentian
passive-margin strata within the Kern Plateau (southeastern Sierra Nevada),
resolving a 40-year-long debate regarding this terrane’s origin. Previous
studies of Kern Plateau pendants identify deep-water metasediments
containing detrital zircon populations similar to the Roberts Mountains
allochthon; yet structural observations seemingly contradict proposed
correlations to the Mississippian Roberts Mountains thrust, which
juxtaposes exotic deep-water rocks over shallow-water, passive-margin
strata in central Nevada. Here, new samples are combined with published
data to identify segments of the thrust within the Kern Plateau,
demonstrating that the El Paso terrane was offset ~350 km by late Paleozoic
sinistral translation along the braided Kern Plateau shear zone, an
abandoned model first proposed more than 20 years ago. New U-Pb-Hf isotope
data from plutons intruding the Kern Plateau shear zone are virtually
identical to published data from the El Paso Mountains, indicating that the
Sierra Nevada–Mojave arc initiated in the late Early Permian (ca. 274 Ma)
along the entire length of the El Paso terrane and was active into the
Middle Triassic (ca. 240 Ma). Previously implicated Late Triassic arc
activity within the Kern Plateau is not corroborated by single-crystal U-Pb
data. Published structural evidence indicating reactivation of the late
Paleozoic Kern Plateau shear zone is reinterpreted as indicating
sinistral-oblique relative plate motion during Permian arc initiation
followed by Middle Jurassic extension in the southeastern Sierra Nevada
arc, which facilitated intense hydrothermal activity and zircon lead loss.
Thermal history modeling techniques and interpretation strategies:
Applications using QTQt
Alyssa L. Abbey; Mark Wildman; Andrea L. Stevens Goddard; Kendra E. Murray
Advances in low-temperature thermochronology have made it applicable to a
plethora of geoscience investigations. The development of modeling programs
(e.g., QTQt and HeFTy) that extract thermal histories from
thermochronologic data has facilitated growth of this field. However, the
increasingly wide range of scientists who apply these tools requires an
accessible entry point to thermal history modeling and how these models
develop our understanding of complex geological processes. This
contribution offers a discussion of modeling strategies, using QTQt,
including making decisions about model design, data input, kinetic
parameters, and other factors that may influence the model output. We
present a suite of synthetic data sets derived from known thermal histories
with accompanying tutorial exercises in the Supplemental Material 1. These data sets illustrate the opportunities and limitations
of thermal history modeling. Examining these synthetic data helps to
develop intuition about which thermochronometric data are most sensitive to
different thermal events and to what extent user decisions on data handling
and model set-up can control the recovery of the true solution. We also use
real data to demonstrate the importance of incorporating sensitivity
testing into thermal history modeling and suggest several best practices
for exploring model sensitivity to factors including, but not limited to,
the model design or inversion algorithm, geologic constraints, data trends,
the spatial relationship between samples, or the choice of kinetics model.
Finally, we provide a detailed and explicit workflow and an applied example
for a method of interrogating vague model results or low
observation-prediction fits that we call the “Path Structure Approach.” Our
explicit examination of thermal history modeling practices is designed to
guide modelers to identify the factors controlling model results and
demonstrate reproducible approaches for the interpretation of thermal
histories.
Identifying sources of non-unique detrital age distributions
through integrated provenance analysis: An example from the
Paleozoic Central Colorado Trough
Tyson M. Smith; Joel E. Saylor; Tom J. Lapen; Kendall Hatfield; Kurt E.
Sundell
To address the longstanding issue of provenance interpretation of
non-unique detrital zircon age populations, we integrated zircon U-Pb, rare
earth element (REE), and εHf(t) data from upper Paleozoic strata in the
northern Central Colorado Trough and Cambrian intrusions with petrography,
paleocurrent data, and structural and stratigraphic observations. This data
set indicates that Cambrian sediment was shed by multiple local sources
instead of distant sources hundreds of kilometers away, and it reveals a
detailed history of tectonic drainage reorganization in the northern
Central Colorado Trough during Ancestral Rocky Mountain deformation. During
the Early–Middle Pennsylvanian, Cambrian detrital zircons were a minor
constituent of northern Central Colorado Trough sediment. However, during
the Late Pennsylvanian–early Permian, westward advancement of the adjacent
Apishapa Uplift deformation front precipitated drainage reorganization,
which resulted in an episode of dominant Cambrian detrital zircon sourcing.
Paleocurrent and petrographic data indicate that the source of Cambrian
detritus was shed by an igneous rock body that was ≤15 km northeast of the
depocenter, which has since been eroded away or mantled by Tertiary
volcanic rocks. The addition of zircon petrochronology to the data set
applied here was critical in confirming this hidden source of detritus and
elucidating the compositional characteristics of that igneous rock. Zircon
εHf(t) provided a regional provenance indicator of a western Laurentian
affinity, and REE composition aided in discriminating possible local
sources of Cambrian zircon. Furthermore, this work serves as a case study
of a dominant Cambrian detrital zircon signature sourced from outside of
the well-known Amarillo-Wichita Uplift, and it has implications for the
interpretation of such detrital spectra in the context of a
direct-from-basement source or the recycling of Cambrian zircon-dominated
rocks. In summary, we demonstrate the utility of this multi-provenance
proxy approach in interpreting a complex structural history of a dynamic
hinterland and concomitant drainage reorganization through an in-depth
investigation into the basin record.
Making sense of brittle deformation in rhyolitic lavas: Insights
from Obsidian Dome, California, USA
Shelby L. Isom; Graham D.M. Andrews; Stuart Kenderes; Alan G. Whittington
The scarcity of observed active extrusive rhyolitic lava flows has skewed
research to extensively focus on prehistoric lavas for information about
their eruptive and emplacement dynamics. The first ever witnessed silicic
lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in
Chile, were illuminating to the volcanology community because they featured
a range of emplacement processes (endogenous versus exogenous), movement
limiting modes, and eruptive behaviors (explosive versus effusive) that
were often regarded as acting independently throughout an eruptive event.
In this study, we documented evidence of a continuum of brittle and
brittle-ductile deformation and fracture-induced outgassing during the
emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California,
USA. This study focused on mapping the textural-structural relationships of
the upper surface of the lava onto high-resolution (<10 cm2
/pixel) orthorectified color base maps. We found that the upper surface is
characterized by small (<1 m) mode 1 tensile fractures that grew and
initiated new cracks, which linked together to form larger tensile
fractures (1–5 m), which in turn penetrated deeper into the lava. We
recorded ornamentations on these fracture surfaces that allow snapshot
views into the rheological and outgassing conditions during the lava’s
effusion. The largest fractures developed during single, large fracture
events in the final stages of the lava’s emplacement. Ornamentations
preserved on the fractured surfaces record degassing and explosive
fragmentation away from the vent throughout the lava’s emplacement,
suggesting explosive activity was occurring during the effusive
emplacement. Field-based cataloguing of the complexities of fracture
surfaces provides qualitative constraints for the future mechanical
modeling of effusive lavas.
Magmatic record of changing Cordilleran plate-boundary
conditions—Insights from Lu-Hf isotopes in the Mojave Desert
Keith A. Howard; Stirling E. Shaw; Charlotte M. Allen
Belts of Cordilleran arc plutons in the eastern part of the Mojave crustal
province, inboard from the southwestern North American plate boundary,
record major magmatic pulses at ca. 180–160 and 75 Ma and smaller pulses at
ca. 100 and 20 Ma. This cyclic magmatism likely reflects evolving
plate-margin processes. Zircon Lu-Hf isotopic characteristics and inherited
zircons for different-age plutons may relate magma sources to evolving
tectonics. Sources similar in age to the bulk of the exposed Mojave crust
(1.6–1.8 Ga) dominated the magmas. Rare zircons having εHf(t) values as low
as −52 indicate that Cretaceous melt sources also included more ancient
crustal components, such as Archean-derived detritus in supracrustal
gneisses of the Vishnu basin. Some rocks signal contributions from mantle
lithosphere (in the Miocene) or asthenosphere (middle Cretaceous). Temporal
shifts in isotopic pattern in this sample of the Cordillera relate to
cyclic pulses of magmatic flux. Hf-isotopic pull-downs suggestive of
dominantly crustal sources characterize the Jurassic and Late Cretaceous
flare-ups. The Late Cretaceous flare-up, occurring near the onset of flat-
slab subduction, produced abundant Proterozoic xenocrystic zircon and Hf
isotopes implicating derivation largely from heterogeneous deep Mojave
crust. Isotopic pull-ups characterize the lower-flux middle Cretaceous and
Miocene magmatic episodes. The middle Cretaceous pulse ca. 105–95 Ma
produced Mojave crust signals but also the isotopically most juvenile
magmatic zircons, ranging upward to barely positive εHf values and
suspected to signal an asthenosphere contribution. This may point toward
transtension or slab retreat causing 105–95 Ma backarc extension in the
Mojave hinterland of the Cordillera. That possibility of backarc extension
raises questions about the tectonic environment of the contemporaneous main
Sierra Nevada high-flux arc closer to the continental margin.
Tectonics, geochronology, and petrology of the Walker Top Granite,
Appalachian Inner Piedmont, North Carolina (USA): Implications for
Acadian and Neoacadian orogenesis
Arthur J. Merschat; Robert D. Hatcher, Jr.; Scott D. Giorgis; Heather E.
Byars; Russell W. Mapes ...
The Walker Top Granite (here formally named) is a peraluminous megacrystic
granite that occurs in the Cat Square terrane, Inner Piedmont, part of the
southern Appalachian Acadian-Neoacadian deformational and metamorphic core.
The granite occurs as disconnected concordant to semi-concordant plutons in
migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet.
Locally garnet-bearing, the Walker Top Granite contains blocky alkali
feldspar megacrysts 1–10 cm long in a groundmass of
muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace
zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies
from granite to granodiorite and contains several xenoliths of biotite
gneiss, amphibolite, quartzite, and in one location encloses charnockite
(here formally named Vale Charnockite). New sensitive high-resolution ion
microprobe U-Pb zircon magmatic crystallization ages obtained from the
plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains;
366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square
area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its
type locality. Major-, trace-element, and isotopic chemistry indicates that
Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or
syn-collisional on tectonic discrimination diagrams and suggests that it
represents deep-seated anatectic magma with S- to I-type affinity. The
alkali calcic, ferroan Vale Charnockite likely formed by deep crustal
melting, and similar geochemical and trace-element compositions suggest a
similar tectonic origin as Walker Top Granite. The discontinuous nature of
the Walker Top Granite plutons precludes it intruded as a volcanic arc.
Instead, the peraluminous nature, common xenoliths of surrounding country
rock, and geochemical and isotopic signatures suggest it formed by partial
melting of Cat Square and Tugaloo terrane rocks. Following emplacement and
crystallization, Walker Top plutons were deformed into elliptical to linear
shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and
quartzofeldspathic rocks. Collectively, Walker Top and other plutons helped
weaken the crust and facilitate lateral crustal flow in a SW-directed,
tectonically driven orogenic channel during the Acadian-Neoacadian event. A
comparison with the northern Appalachians recognizes a similar temporal
magmatic and deformational history during the Acadian and Neoacadian
orogenies, although while the Walker Top Granite intruded the lower plate
during eastward subduction beneath the peri-Gondwanan Carolina
superterrane, the northern Appalachian plutons intruded the upper plate
during subduction of the Avalon superterrane westward beneath Laurentia. We
hypothesize that a transform fault, located near the southern end of the
New York promontory, accommodated oppositely directed lateral plate motion
and different subduction polarity between the Carolina and Avalon
superterranes during the Acadian and Neoacadian orogenies.
Boudinage and the rheology of syntectonic migmatites in the
high-strain Taili deformation zone, NE China
Zhiyong Li; Zuoxun Zeng; Yongjiang Liu
This paper presents a detailed field characterization of boudinage in a
high-strain zone several kilometers wide in Northern China to establish
relationships between boudin types and rheological contrasts between
different parts of migmatites during the migmatization process. This zone
contains nearly all types of boudins: foliation boudins, block-torn
boudins, pinch-and-swell structures, tapering boudins, object boudins, and
modified boudins. These boudinage structures record the different stages of
melt-involved and solid-state deformation. The boudinage of migmatites is
significantly controlled by the evolving rheological contrasts between the
leucosome and melanosome. During the melting stage, the deformation and
boudinage of rocks are controlled by the melt fraction. Migmatite strength
progressively decreases with increasing melt fraction. The occurrence of
melt-filled foliation boudins and melanosome block boudins suggests that
the residuum and melanosome are more competent than the leucosome. During
solid-state deformation after crystallization, the existence of
recrystallized solid-state leucosomes and the intrusion of pegmatites cause
the migmatite strength to increase. The relationship is reversed: the
leucosome is much more competent than the melanosome. The type and geometry
of boudins and pinch-and-swell structures are correlated to the fraction of
leucosome in the migmatites. The mechanical strength and strain
localization of migmatites after crystallization depend on the presence and
volume fraction of the different mineral phases, as well as the mineral
grain size. The type and geometry of boudins suggest that the effective
viscosity of migmatite can be ranked, from high to low, as: quartz veins;
coarse-grained, thick pegmatite; coarse-grained, diatexite migmatite;
medium-grained leucosome; and fine-grained melanosome. While experiencing
partial melting and migmatization, a rheologically homogeneous protolith is
turned into two dominant lithologic domains: a competent diatexite
migmatite domain and an incompetent melanosome migmatite domain. Spatially,
with the increasing leucosome fraction in migmatites, the migmatite
rheology of rock changes from homogeneous to heterogeneous and anisotropic,
and then back to homogeneous. The strain distribution likewise changes from
uniform to partitioned, and then back to uniform. This evolutionary process
of strength and rheological properties of rocks during migmatization may
promote strain localization at mid crustal conditions.
GEOSPHERE articles are available at
https://geosphere.geoscienceworld.org/content/early/recent. Representatives of the media may obtain complimentary copies of GEOSPHERE
articles by contacting Kea Giles at the address above. Please discuss
articles of interest with the authors before publishing stories on their
work, and please refer to GEOSPHERE in articles published. Non-media
requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
https://www.geosociety.org
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