New Articles for Geosphere Posted Online in September
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied this month
include the western Mexican arc and northern Walker Lane. You can find
these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Heterogenous late Miocene extension in the northern Walker Lane
(California-Nevada, USA) demonstrates vertically decoupled crustal
extension
Michael C. Say; Andrew V. Zuza
Abstract:
The spatial distribution and kinematics of intracontinental deformation
provide insight into the dominant mode of continental tectonics: rigid-body
motion versus continuum flow. The discrete San Andreas fault defines the
western North America plate boundary, but transtensional deformation is
distributed hundreds of kilometers eastward across the Walker Lane–Basin
and Range provinces. In particular, distributed Basin and Range extension
has been encroaching westward onto the relatively stable Sierra Nevada
block since the Miocene, but the timing and style of distributed
deformation overprinting the stable Sierra Nevada crust remains poorly
resolved. Here we bracket the timing, magnitude, and kinematics of
overprinting Walker Lane and Basin and Range deformation in the Pine Nut
Mountains, Nevada (USA), which are the westernmost structural and
topographic expression of the Basin and Range, with new geologic mapping
and 40Ar/39Ar geochronology. Structural mapping
suggests that north-striking normal faults developed during the initiation
of Basin and Range extension and were later reactivated as
northeast-striking oblique-slip faults following the onset of Walker Lane
transtensional deformation. Conformable volcanic and sedimentary rocks,
with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest
by east-dipping normal faults. This relationship demonstrates that dip-slip
deformation initiated after ca. 6.8 Ma. A retrodeformed cross section
across the range suggests that the range experienced 14% extension.
Subsequently, Walker Lane transtension initiated, and clockwise rotation of
the Carson domain may have been accommodated by northeast-striking
left-slip faults. Our work better defines strain patterns at the western
extent of the Basin and Range province across an approximately 150-km-long
east-west transect that reveals domains of low strain (~15%) in the Carson
Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude
extension (~150%–180%) in the Singatse and Wassuk Ranges. There is no
evidence for irregular crustal thickness variations across this same
transect—either in the Mesozoic, prior to extension, or today—which
suggests that strain must be accommodated differently at decoupled crustal
levels to result in smooth, homogenous crustal thickness values despite the
significantly heterogeneous extensional evolution. This example across an
~150 km transect demonstrates that the use of upper-crust extension
estimates to constrain pre-extension crustal thickness, assuming pure shear
as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be
reliable.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02409.1/607909/Heterogenous-late-Miocene-extension-in-the
Origin of alkali olivine basalts and hawaiites in the western Mexican
arc: Evidence of rapid phenocryst growth and magma mixing during ascent
along fractures
Juliana Mesa; Rebecca A. Lange
Abstract:
A detailed petrological study is presented to constrain the origin of a
suite of alkali olivine basalt and hawaiite (>5 wt% MgO) lavas that were
erupted in a rift zone within the western Mexican arc (Trans-Mexican
Volcanic Belt), adjacent to the Sangangüey andesitic stratovolcano,
together with more evolved lavas (mugearites and benmoreites; <5 wt%
MgO). As previously documented in the literature, the Sangangüey mafic
lavas are devoid of any arc geochemical signature, despite their location
within an arc. In this study, a new olivine-melt thermometer/hygrometer,
based on the partitioning behavior of Ni2+ and Mg2+,
was applied to the Sangangüey basalts (SB). The results show that the
high-MgO (>9 wt%) SB crystallized at higher temperatures and lower
melt-water contents (0–1.3 wt%) compared to high-MgO arc basalts (≤5.7 wt%
H2O) erupted in the west-central Mexican arc. The Sangangüey
lavas with 5–8 wt% MgO display evidence of mixing between high-MgO alkali
olivine basalts and low-MgO mugearites. It is proposed that the unique
composition of the mugearites (i.e., low SiO2 contents and
elevated FeO and TiO2 contents) is the result of partial melting
of mafic lower crust driven by the influx of high-MgO intraplate basalts
under relatively hot, dry, and reduced conditions. On the basis of crystal
textures and compositional zoning patterns, it is shown that both
phenocryst growth and magma mixing occurred rapidly, most likely during
ascent along fractures, and not slowly during prolonged storage in a
crustal magma chamber.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02365.1/607532/Origin-of-alkali-olivine-basalts-and-hawaiites-in
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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