New Articles for Geosphere Posted Online First
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied include mud
volcanism; the Ponderosa fault zone; Coast Mountains, British Columbia; and
degassing of helium in central Mexico. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent.
Revised age and regional correlations of Cenozoic strata on Bat
Mountain, Death Valley region, California, USA, from zircon U-Pb
geochronology of sandstones and ash-fall tuffs
Theresa M. Schwartz; A. Kate Souders; Jens-Erik Lundstern; Amy K. Gilmer;
Ren A. Thompson
Basin analysis and tectonic reconstructions of the Cenozoic history of the
Death Valley region, California, USA, are hindered by a lack of volcanic
(tuff) age control in many stratigraphic successions exposed in the
Grapevine and Funeral Mountains of California, USA. Although maximum
depositional ages (MDAs) interpreted from detrital zircon U-Pb data may be
a promising alternative to volcanic ages, arguments remain regarding the
calculation of MDAs including, but not limited to, the number of “young”
grains to consider (i.e., the spectrum of dates used to calculate the MDA);
which grains, if any, should be ignored; which approaches yield results
that are statistically rigorous; and ultimately, which approaches result in
ages that are geologically reasonable. We compare commonly used metrics of
detrital zircon MDA for five sandstone samples from the Cenozoic strata
exposed on Bat Mountain in the southern Funeral Mountains of
California—i.e., the youngest single grain (YSG), the weighted mean of the
youngest grain cluster of two or more grains at 1σ uncertainty (YC1σ(2+))
and of three or more grains at 2σ uncertainty (YC2σ(3+)), the youngest
graphical peak (YPP), and the maximum likelihood age (MLA). Every sandstone
sample yielded abundant Cenozoic zircon U-Pb dates that formed unimodal,
near-normal age distributions that were clearly distinguishable from the
next-oldest grains in each sample and showed an apparent up-section
decrease in peak age. Benchmarked against published K/Ar and 40
Ar/39Ar ages and five new zircon U-Pb ages of ash-fall tuffs,
our analysis parallels prior studies and demonstrates that many MDA
metrics—YSG, YC1σ(2+), YC2σ(3+), and YPP—drift toward unreasonably young or
old values. In contrast, the maximum likelihood estimation approach and the
resulting MLA metric consistently produce geologically appropriate
estimates of MDA without arbitrary omission of any young (or old) zircon
dates. Using the MLAs of sandstones and zircon U-Pb ages of interbedded
ash-fall tuffs, we develop a new age model for the Oligocene–Miocene
Amargosa Valley Formation (deposited ca. 28.5–18.5 Ma) and the Miocene Bat
Mountain Formation (deposited ca. 15.5–13.5 Ma) and revise correlations to
Cenozoic strata across the eastern Death Valley region.
Stratigraphy of the Eocene–Oligocene Titus Canyon Formation, Death
Valley, California (USA), and Eocene extensional tectonism in the
Basin and Range
Nikolas Midttun; Nathan A. Niemi; Bianca Gallina
Geologic mapping, measured sections, and geochronologic data elucidate the
tectono-stratigraphic development of the Titus Canyon extensional basin in
Death Valley, California (USA), and provide new constraints on the age of
the Titus Canyon Formation, one of the earliest syn-extensional deposits in
the central Basin and Range. Detrital zircon maximum depositional ages
(MDAs) and compiled 40Ar/39Ar ages indicate that the
Titus Canyon Formation spans 40(?)–30 Ma, consistent with an inferred
Duchesnean age for a unique assemblage of mammalian fossils in the lower
part of the formation. The Titus Canyon Formation preserves a shift in
depositional environment from fluvial to lacustrine at ca. 35 Ma, which
along with a change in detrital zircon provenance may reflect both the
onset of local extensional tectonism and climatic changes at the
Eocene–Oligocene boundary. Our data establish the Titus Canyon basin as the
southernmost basin in a system of late Eocene extensional basins that
formed along the axis of the Sevier orogenic belt. The distribution of
lacustrine deposits in these Eocene basins defines the extent of a
low-relief orogenic plateau (Nevadaplano) that occupied eastern Nevada at
least through Eocene time. As such, the age and character of Titus Canyon
Formation implies that the Nevadaplano may have extended into the central
Basin and Range, ~200 km farther south than previously recognized.
Development of the Titus Canyon extensional basin precedes local Farallon
slab removal by ~20 m.y., implying that other mechanisms, such as plate
boundary stress changes due to decreased convergence rates in Eocene time,
are a more likely trigger for early extension in the central Basin and
Range.
Mantellic degassing of helium in an extensional active tectonic
setting at the front of a magmatic arc (central Mexico)
Andrea Billarent-Cedillo; Eliseo Hernandez-Pérez; Gilles Levresse; Claudio
Inguaggiato; Luca Ferrari ...
The physicochemical and isotopic characteristics of groundwater and
dissolved gas of central Mexico provide valuable information about the
geologic and tectonic context of the area. Low–high-enthalpy manifestations
(up to 98 °C in springs and more than 100 °C in geothermal wells) are
distributed within the San Juan del Río, Querétaro, and Celaya hydrologic
basins, located at the boundary between the current Mexican magmatic arc
and an extensional continental area with intraplate volcanism called Mesa
Central Province. Groundwaters in the study area represent a mixture
between the cold water end-member with a Ca2+-Mg2+
-HCO3- composition and a hydrothermal end-member enriched in Na +, K+, SO2−, and Cl-. Cold and hot
groundwaters δ2H and δ18O plot along the same
evaporation lines and do not exhibit a magmatic input. Dissolved and free
gas do not show a typical volcanic composition signature. He and Ne isotope
composition provide evidence of an important contribution of
non-atmospheric noble gases. Although helium composition mainly has a
crustal origin (21–83%), the mantellic contribution (1–39%) is higher than
expected for an area lacking recent volcanism. A volatile-rich magma aging
at depth was discarded as the source of this mantellic helium signature but
points out a recent mantellic contribution. Thus, we propose that mantellic
helium comes from the sublithospheric mantle into the shallow crust through
the highly permeable tectonic boundaries between the geologic provinces,
namely the N−S Taxco−San Miguel de Allende and Chapala-Tula fault systems.
Mantellic helium flow rates through these fault systems were estimated to
have values ranging from 0.1 m/yr to 2.9 m/yr. This He flux range implies
that aside from subduction, mantle volatile degassing enhanced by crustal
fault systems is the main degassing process in the region studied.
Evidence for regionally continuous Early Cretaceous sinistral shear
zones along the western flank of the Coast Mountains, coastal
British Columbia, Canada
Jordan W. Wang; George Gehrels; Paul Kapp; Kurt Sundell
The plate-boundary conditions of the Mesozoic North American Cordillera
remain poorly constrained, but most studies support large (>800 km)
southward motion of the Insular and Intermontane superterranes during
Jurassic–Cretaceous time. An implicit feature in these models of large
coastwise displacements is the presence of one or more continental-scale
sinistral strike-slip faults that could have dismembered and displaced
terrane fragments southward along the western margin of North America prior
to the onset of mid-Cretaceous shortening and dextral strike-slip faulting.
In this study, we documented a system of sinistral intra-arc shear zones
within the Insular superterrane that may have accommodated large southward
motion. Employment of a new large-n igneous zircon U-Pb method
more than doubled the precision of measurements obtained by laser
ablation–inductively coupled plasma–mass spectrometry (from ~1% to 0.5%)
and allowed us to demonstrate the close temporal-spatial relationship
between magmatism and deformation by dating comagmatic crosscutting phases.
Crystallization ages of pre-, syn-, and postkinematic intrusions show that
the intra-arc shear zones record an Early Cretaceous phase of sinistral
oblique convergence that terminated between 107 and 101 Ma. Shear zone
cessation coincided with: (1) collapse of the Gravina basin, (2)
development of a single voluminous arc that stitched the Insular and
Intermontane superterranes together, and (3) initiation of east-west
contractional deformation throughout the Coast Mountains. We interpret
these concurrent tectono-magmatic events to mark a shift in plate
kinematics from a sinistral-oblique system involving separate terranes and
intervening ocean basins to a strongly convergent two-plate margin
involving a single oceanic plate and the newly assembled western margin of
North America.
The Pondosa fault zone: A distributed dextral-normal-oblique fault
system in northeastern California, USA
Jessica Thompson Jobe; Richard Briggs; Ryan Gold; Stephen DeLong; Madeline
Hille ...
The tectonic domains of Basin and Range extension, Cascadia subduction zone
contraction, and Walker Lane dextral transtension converge in the Mushroom
Rock region of northeastern California, USA. We combined analysis of
high-resolution topographic data, bedrock mapping, 40Ar/ 39Ar geochronology, low-temperature thermochronology, and
existing geologic and fault mapping to characterize an extensive
dextral-normal-oblique fault system called the Pondosa fault zone. This
fault zone extends north-northwest from the Pit River east of Soldier
Mountain, California, into moderately high-relief volcanic topography as
far north as the Bartle (California) townsite with normal and dextral
offset apparent in geomorphology and fault exposures. New and existing 40Ar/39Ar and radiocarbon dating of offset lava flows
provides ages of 12.4 ka to 9.6 Ma for late Cenozoic stratigraphic units.
Scarp morphology and geomorphic expression indicate that the fault system
was active in the late Pleistocene. The Pondosa fault zone may represent a
dextral-oblique accommodation zone between north-south–oriented Basin and
Range extensional fault systems and/or part of the Sierra Nevada–Oregon
Coast block microplate boundary.
A major Miocene deepwater mud canopy system: The North Sabah–Pagasa
Wedge, northwestern Borneo
C.K. Morley; W. Promrak; W. Apuanram; P. Chaiyo; S. Chantraprasert ...
Three-dimensional seismic reflection data, well data, and analogues from
areas with extensive shale tectonics indicate that the enigmatic deepwater
“shale nappe or thrust sheet” region of northern offshore Sabah, Malaysia,
now referred to as the North Sabah–Pagasa Wedge (NSPW), is actually a
region of major mobile shale activity characterized by mini-basins and mud
pipes, chambers, and volcanoes. A short burst of extensive mud volcano
activity produced a submarine mud canopy complex composed of ~50 mud
volcano centers (each probably composed of multiple mud volcanoes) that
cover individual areas of between 4 and 80 km2. The total area
of dense mud canopy development is ~1900 km2. During the middle
Miocene, the post-collisional NSPW was composed predominantly of
overpressured shales that were loaded by as much as 4 km thickness of
clastics in a series of mini-basins. Following mini-basin development,
there was a very important phase of mud volcanism, which built extensive
mud canopies (coalesced mud flows) and vent complexes. The mud canopies
affected deposition of the overlying and interfingering deposits, including
late middle to early late Miocene deepwater turbidite sandstones, which are
reservoirs in some fields (e.g., Rotan field). The presence of the
extensive mud volcanoes indicates very large volumes of gas had to be
generated within the NSPW to drive the mud volcanism. The Sabah example is
only the second mud canopy system to be described in the literature and is
the largest and most complex.
Tectonic setting of metamorphism and exhumation of eclogite-facies
rocks in the South Beishan orogen, northwestern China
Jie Li; Chen Wu; Xuanhua Chen; An Yin; Andrew V. Zuza ...
High-pressure metamorphic rocks occur as distinct belts along subduction
zones and collisional orogens or as isolated blocks within orogens or
mélanges and represent continental materials that were subducted to deep
depths and subsequently exhumed to the shallow crust. Understanding the
burial and exhumation processes and the sizes and shapes of the
high-pressure blocks is important for providing insight into global
geodynamics and plate tectonic processes. The South Beishan orogen of
northwestern China is notable for the exposure of early Paleozoic
high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism
associated with the HP metamorphism and mechanism of exhumation are poorly
understood despite being key to understanding the tectonic evolution of the
larger Central Asian Orogenic System. To address this issue, we examined
the geometries, kinematics, and overprinting relationships of structures
and determined the temperatures and timings of deformation and metamorphism
of the HP rocks of the South Beishan orogen. Geochronological results show
that the South Beishan orogen contains ca. 1.55–1.35 Ga basement
metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional
tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP
metamorphism may have been related to regional contraction in the South
Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies
metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These
HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths
in the footwall of a regional detachment fault during
southeast-northwest–oriented crustal extension, possibly as the result of
roll-back of a subducted oceanic slab. Prior to ca. 438 Ma,
north-south–oriented contraction resulted in isoclinal folding of the
detachment fault and HP rocks. Following this contractional phase in the
middle Mesozoic, the South Beishan orogen experienced thrusting interpreted
to be the response to the closure of the Tethyan and Paleo-Asian Ocean
domains. This contractional phase was followed by late Mesozoic extension
and subsequent surface erosion that controlled exhumation of the HP rocks.
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
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