New GSA Bulletin Articles Published Ahead of Print in July
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics include GPS
measurements in Tibet; the Cheechoo stockwork gold deposit; and theGaviota coast, Santa Barbara County, California.You can find
these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Initiation of clockwise rotation and eastward transport of southeastern
Tibet inferred from deflected fault traces and GPS observations
Weijun Gan; Peter Molnar; Peizhen Zhang; Genru Xiao; Shiming Liang ...
Abstract:
Eastward transport and clockwise rotation of crust around the southeastern
margin of the Tibetan Plateau dominates active deformation east of the
Eastern Himalayan Syntaxis. Current crustal movement inferred from GPS
measurements indicates ongoing distortion of the traces of the active Red
River fault and the Mesozoic Yalong-Yulong-Longmen Shan thrust belt. By
extrapolating current rates back in time, we infer that this pattern of
deformation developed since 10.1 ± 1.5 Ma. This date of initiation is
approximately synchronous with a suite of tectonic phenomena, both near and
far, within the wide Eurasia/Indian collision zone, including the
initiation of slip on the Ganzi-Yushu-Xianshuihe fault and crustal thinning
and E-W extension by normal faulting on N-S−trending rifts in the plateau
interior. Accordingly, the eastward movement of eastern Tibet and the
clockwise rotation of that material seem to be local manifestations of a
larger geodynamic event at ca. 10−15 Ma that changed the kinematic style
and reorganized deformation not only on the plateau-wide scale, but across
the entire region affected by the India/Eurasia collision. Convective
removal of some or all of Tibet’s mantle lithosphere seems to offer the
simplest mechanism for these approximately simultaneous changes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36069.1/606437/Initiation-of-Clockwise-Rotation-and-Eastward
High-resolution late Paleozoic cyclostratigraphy and tectonic evolution
of the Keeler Basin, California, southwest Laurentia
Patrick J. Gannon; M. Elliot Smith; Paul J. Umhoefer; Ryan J. Leary
Abstract:
Cyclic strata exposed in the Inyo Mountains of eastern California contain a
continuous 6 m.y. record of deep marine deposition that spans the
Pennsylvanian−Permian boundary. To better understand the geologic evolution
of southwest Laurentia and the role of glacially driven eustasy in upper
Paleozoic stratigraphy, we measured two detailed ∼600 m composite
stratigraphic sections of the Keeler Canyon Formation and collected a
handheld spectral gamma ray log. Post-depositional deformation complicates
field relationships, but 1:5000 scale mapping of faults and folds permits
assembly of two continuous sections. Measured strata alternate at the 5−20
m scale between intervals of fine-grained laminated marlstone and intervals
of mixed carbonate and siliciclastic turbidites and debrites. Based on
facies characteristics and a prominent marker horizon, we reassign the
Pennsylvanian-Permian age upper Salt Tram unit of the upper Keeler Canyon
Formation to a new Estelle Member. We estimate sediment accumulation rates
within the Keeler Canyon Formation using published conodont biostratigraphy
and correlative U-Pb geochronology from Eastern Europe combined with
spectral analysis and timescale optimization using the Astrochron R
package. Evolutive harmonic analysis of gamma ray-derived element
concentrations reveals prominent bundled periodicities that are consistent
with both long and short eccentricity cycles. Average sediment accumulation
rates calculated using the time scale optimization function of Astrochron
suggest a gradual increase from 40−60 m/m.y. to ∼120 m/m.y. during the late
Pennsylvanian and then a minima of ∼50 m/m.y. near the
Pennsylvanian−Permian boundary, which is followed by an increase to ∼175
m/m.y. into the Early Permian. This trend in sediment accumulation rates
and subsequent Permian contractile deformation are compatible with flexural
subsidence in a SW-migrating foreland basin related to the southern part of
the late Antler orogenic system.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36027.1/606416/High-resolution-late-Paleozoic-cyclostratigraphy
Picrite-basalt complex in the Baoshan-Gongshan Block of northern
Sibumasu: Onset of a mantle plume before breakup of Gondwana and
opening of the Neo-Tethys Ocean
Li Su; Shuguang Song; Chao Wang; Mark B. Allen; Hongyu Zhang
Abstract:
Mantle plumes are thought to play key roles in Earth’s geodynamics,
including mantle convection, continental formation, and plate tectonics.
The connection between plume activity and continental dispersion, as
exemplified by the breakup of Gondwana and the generation of the Neo-Tethys
Ocean, is a key question for the geosciences. Here, we present detailed
investigations for the picrite-basalt sequence in the Baoshan-Gongshan
Block of the northern Sibumasu terrane, southwest China. Field relations
and petrological and geochemical data reveal that these volcanic rocks are
continental flood picrites and basalts, consistent with a mantle plume
origin. The estimated mantle potential temperatures range from 1527 ± 86 °C
to 1546 ± 98 °C, and melting depths vary from the spinel to garnet
stability fields (1.1−5.3 GPa), similar to Cenozoic Hawaiian picrites.
Zircon geochronological data show that the mantle plume activity started at
ca. 335 Ma and lasted to 280 Ma; this range is earlier than the breakup of
the Gondwana continent and opening of the Neo-Tethys Ocean (270−260 Ma). We
conclude that the long-lived mantle plume impacted the continental
lithosphere but it did not drive continental breakup and the opening of
Neo-Tethys Ocean, which took place because of the subduction-induced stress
generated by initial subduction of the Paleo-Tethys Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36028.1/606372/Picrite-basalt-complex-in-the-Baoshan-Gongshan
Multi-stage India-Asia collision: Paleomagnetic constraints from
Hazara-Kashmir syntaxis in the western Himalaya
Umar Farooq Jadoon; Baochun Huang; Syed Anjum Shah; Yasin Rahim; Ahsan Ali
Khan ...
Abstract:
The India-Asia collision is the most spectacular, recent, and still active
tectonic event of the Earth’s history, leading to the uplift of the
Himalayan‐Tibetan orogen, which has been explained through several
hypothetical models. Still, controversy remains, such as how and when it
occurred. Here we report a paleomagnetic study of Cretaceous-Tertiary
marine sediments from the Tethyan Himalaya (TH) in the Hazara area, north
Pakistan, which aims to constrain timing for the onset of the India-Asia
collision and to confirm the validity of already proposed models,
particularly in western Himalaya’s perspective. Our results suggest that
the TH was located at a paleolatitude of 8.5°S ± 3.8° and 13.1°N ± 3.8°
during the interval of ca. 84−79 Ma and 59−56 Ma, respectively. A
comparison between paleopoles obtained from the current study and coeval
ones of the India Plate indicates that the TH rifted from Greater India
before the Late Cretaceous, generating the Tethys Himalaya Basin (THB). Our
findings support a model for a multi-stage collision involving at least two
major subduction systems. A collision of the TH with the Trans-Tethyan
subduction system (TTSS) began first in Late Cretaceous-Early Paleocene
times (ca. 65 Ma), followed by a later collision with Asia at 55−52 Ma. The
onset of the collision between the TH (plus TTSS) and Asia could not have
occurred earlier than 59−56 Ma in the western Himalaya. Subsequently, the
India craton collided with the TH, resulting in the diachronous closure of
the THB between ca. 50 and ca. 40 Ma from west to east. These findings are
consistent with geological and geochemical evidence and have a broad
implication for plate reconfigurations, global climate, and biodiversity of
collisional processes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36116.1/606374/Multi-stage-India-Asia-collision-Paleomagnetic
Unraveling the link between mantle upwelling and formation of
Sn-bearing granitic rocks in the world-class Dachang tin district,
South China
Jia Guo; Kai Wu; Reimar Seltmann; Rongqing Zhang; Mingxing Ling ...
Abstract:
Increasing evidence shows that the mantle contributes (directly or
indirectly) to Sn-bearing granites worldwide. However, the specific role of
mantle in the formation of tin granites and related mineralization remains
poorly understood. In the world-class Dachang district, South China, tin
mineralization is related to the Longxianggai equigranular/porphyritic
biotite granites and tin orebodies are cut by granite porphyry dykes
hosting mafic microgranular enclaves (MMEs). A combination of zircon U-Pb
dating and Hf-O isotopes, mineral chemistry, and whole-rock elemental and
Sr-Nd isotopic compositions—for granitic rocks and MMEs, is employed to
constrain the petrogenesis and to unravel the link between tin fertility
and mantle upwelling. Laser ablation−inductively coupled plasma−mass
spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the biotite
granites were emplaced at ca. 93 Ma, and the granite porphyry dykes and
MMEs were formed at ca. 86 Ma. The biotite granites are silica- and
alkali-enriched with A/CNK ratios of 1.04−1.36, and exhibit elevated
concentrations of Li, F, P, Rb, Cs, Ta, Sn, W, and U, showing affinities
with highly fractionated S-type granites. Whole-rock geochemical and Nd
isotopic (εNd(t) = −10.0 to −7.8) data, and in situ zircon Hf-O
(εHf(t) = −9.9 to −3.9, δ18O = 6.2−8.9‰) isotopes
indicate that the biotite granites were formed by partial melting of
metasedimentary rocks at relatively high temperatures (≥782 °C), possibly
with minor input of mantle material. Likewise, the post-ore granite
porphyry dykes have similar chemical and mineralogical characteristics as
fractionated S-type granites. Zircon Hf-O isotopes (εHf(t) =
−9.0 to −4.9, δ18O = 6.5−8.2‰) and whole-rock geochemical data
suggest they were derived from a similar source as the biotite granites,
whereas elevated εNd(t) values of −5.0 to −3.3 for granite
porphyry dykes relative to biotite granites reveal an increasing mantle
input. Distinct εNd(t) (−0.4 and −0.3) and zircon Hf-O (ε Hf(t) = 1.5−5.0, δ18O = 6.5−7.2‰) isotopes of the
MMEs, suggest that the mafic melt could be sourced from the asthenospheric
mantle, contaminated by subcontinental lithospheric mantle/continental
crust during magma ascent, and hybridized by felsic melt at
emplacement-level. The magmatic sequence in the Dachang district is
indicative of an extensional tectonic setting where mantle-derived magmas
are predicted to migrate to shallower crustal levels as the crust
progressively becomes thinner and hotter. High-temperature partial melting
of mature metasedimentary crust triggered by heat input from the upwelled
mantle, may contribute to biotite breakdown, which is important for
concentrating tin in melts. Fractional crystallization of initially Sn-rich
felsic melts under reduced conditions makes further tin enrichment and
produces Sn-bearing granites (the Longxianggai pluton). Prolonged mantle
upwelling results in distinct magma mixing and the formation of granite
porphyry dykes and MMEs. These dykes are highly fractionated with elevated
Sn and W contents, which show great potential to form hydrothermal Sn-W
mineralization.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35492.1/606306/Unraveling-the-link-between-mantle-upwelling-and
Rapid transition from oceanic subduction to postcollisional extension
revealed by Carboniferous magmatism in East Junggar (NW China),
southwestern Central Asian orogenic belt
Jian Wang; Yuping Su; Jianping Zheng; E.A. Belousova; Ming Chen ...
Abstract:
Knowledge of the subduction to postcollision tectonic transition in
response to oceanic closure is crucial for tracking the final stage of
orogenic evolution. Here, we report new geochronology, geochemistry, and
isotopic data for Carboniferous magmatism in East Junggar (NW China),
southwestern Central Asian orogenic belt, which may record such processes
following the closure of the Kalamaili Ocean (a branch of the Paleo-Asian
Ocean). The early Carboniferous calc-alkaline volcanic rocks (dominated by
basalt and basaltic andesite) yielded zircon U-Pb ages of ca. 340−330 Ma
and are characterized by arc-like trace-element patterns showing enrichment
of light rare earth elements (LREEs) and large ion lithophile elements
(LILEs; e.g., Pb) but depletion of high field strength elements (HFSEs;
e.g., Nb, Ta, and Ti). Combined with their variable Ba/Nb (9.80−454) and
low Nb/La (0.21−0.54) and Sm/Yb (1.77−3.08) ratios as well as depleted
mantle−like Sr-Nd-Pb-Hf (whole-rock 87Sr/86Sri = 0.7037−0.7040; ɛNd[t] = +3.5 to +5.9;206Pb/204Pbi = 17.728−17.996; zircon ɛ Hf[t] = +11.8 to +18.8) isotopic values, we suggest
that they were produced by melting of a lithospheric mantle wedge fluxed by
slab-derived fluids under spinel-facies conditions. With whole-rock 40Ar/39Ar dating of ca. 320 Ma, the late
Carboniferous mafic dikes have geochemical features and Sr-Nd-Pb ( 87Sr/86Sri = 0.7039−0.7041; ɛNd
[t] = +6.6 to +6.8; 206Pb/204Pbi
= 17.905−17.933) isotopic compositions similar to those of the early
Carboniferous volcanics, but they show less pronounced Pb anomalies and
negative Nb and Ta anomalies. They are interpreted to have formed by
partial melting of a spinel-bearing lithospheric mantle metasomatized by
limited influx of subduction-related fluids. The late Carboniferous felsic
volcanic rocks (dacite and rhyolite) yielded zircon U-Pb ages of ca. 305 Ma
and are geochemically equivalent to those of A2-type granites in
East Junggar. They have juvenile isotopic compositions (ɛNd[ t] = +4.5 to +6.8; ɛHf[t] = +13.3 to +18.7)
and relatively young Nd and Hf model ages that roughly coincide with the
ages of the ophiolites in the area, suggesting that they could have
originated from melting of a juvenile basaltic lower crust. Whole-rock
geochemistry, assimilation−fractional crystallization (AFC), and isotopic
mixing modeling argue for insignificant crustal contamination for the
Carboniferous magmatism. We suggest that the early Carboniferous lavas
erupted in an island-arc setting related to the northward subduction of the
Kalamaili oceanic crust, whereas the late Carboniferous magmatism formed in
a postcollisional extensional regime in response to slab breakoff or
lithospheric delamination. Combined with regional geological information,
we propose that a rapid tectonic transition from oceanic subduction to
postcollisional extension may have occurred in East Junggar during the
Carboniferous, marking the final closure of the Kalamaili Ocean, which most
likely took place ca. 330−320 Ma. This study provides overall
geochronological and petrogeochemical evidence to better constrain the
amalgamation of the southwestern Central Asian orogenic belt and may be of
great importance for understanding the final stage of orogenic evolution
elsewhere.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36074.1/606273/Rapid-transition-from-oceanic-subduction-to
Laramide structure of southeastern Arizona: Role of basement-cored
uplifts in shallow-angle subduction
Daniel A. Favorito; Eric Seedorff
Abstract:
Laramide reverse faults in southeastern Arizona commonly are obscured by
mid- to late Cenozoic extension and subsequent cover, resulting in debate
about their configuration and origin. A new mid-Cenozoic paleogeologic map
depicts the structural configuration before extension, and new structural
reconstructions characterize Laramide shortening in terms of structural
style, magnitude, evolution, and timing. Reverse faults restore to moderate
to high angles, are associated with fault-propagation folds, and involve
significant basement and thus constitute thick-skinned deformation. The
paleogeologic map suggests several major basement-cored block uplifts, many
of which are newly identified. The largest uplifts may measure 150 km along
strike, similar to those in the classic Laramide province of Wyoming and
Colorado. Estimated shortening across the central study area is 14% or 23
km, whereas it is only 5% (9 km) to the north and 11% (12 km) to the south.
Shortening by this mechanism is inadequate to explain previous estimates of
crustal thickening in the region (∼50−60 km). Therefore, magmatic
underplating, lower-crustal flow, or underplating of trench sediments and
lithospheric material also may have contributed to thickening. Shortening
largely occurred from 86 Ma to 64 Ma and possibly as late as 53 Ma, with
initiation being younger to the northeast or north. Integration with data
from southwestern New Mexico implies complex geometry for the subducting
flat slab. Finally, reverse faults generally do not appear to have
reactivated older faults, as previously suggested, primarily because
reverse faults have associated fault-propagation folds in rocks that
predate supposed reactivated structures.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35894.1/606274/Laramide-structure-of-southeastern-Arizona-Role-of
A crucial geologic test of Late Jurassic exotic collision versus
endemic re-accretion in the Klamath Mountains Province, western United
States, with implications for the assembly of western North America
Todd A. LaMaskin; Jonathan A. Rivas; David L. Barbeau, Jr.; Joshua J.
Schwartz; John A. Russell ...
Abstract:
Differing interpretations of geophysical and geologic data have led to
debate regarding continent-scale plate configuration, subduction polarity,
and timing of collisional events on the western North American plate margin
in pre–mid-Cretaceous time. One set of models involves collision and
accretion of far-traveled “exotic” terranes against the continental margin
along a west-dipping subduction zone, whereas a second set of models
involves long-lived, east-dipping subduction under the continental margin
and a fringing or “endemic” origin for many Mesozoic terranes on the
western North American plate margin. Here, we present new detrital zircon
U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath
terranes in the Klamath Mountains of northern California and southern
Oregon that provide a test of these contrasting models. Our data show that
portions of the Rattlesnake Creek terrane cover sequence (Salt Creek
assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic)
and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone
samples of the Galice Formation are no older than ca. 158–153 Ma (middle
Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based
on a comparison of our data to published magmatic and detrital ages
representing provenance scenarios predicted by the exotic and endemic
models (a crucial geologic test), we show that our samples were likely
sourced from the previously accreted, older terranes of the Klamath
Mountains and Sierra Nevada, as well as active-arc sources, with some
degree of contribution from recycled sources in the continental interior.
Our observations are inconsistent with paleogeographic reconstructions that
are based on exotic, intra-oceanic arcs formed far offshore of North
America. In contrast, the incorporation of recycled detritus from older
terranes of the Klamath Mountains and Sierra Nevada, as well as North
America, into the Rattlesnake Creek and Western Klamath terranes prior to
Late Jurassic deformation adds substantial support to endemic models. Our
results suggest that during long-lived, east-dipping subduction, the
opening and subsequent closing of the marginal Galice/Josephine basin
occurred as a result of in situ extension and subsequent
contraction. Our results show that tectonic models invoking exotic,
intra-oceanic archipelagos composed of Cordilleran arc terranes fail a
crucial geologic test of the terranes’ proposed exotic origin and support
the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the
North American continental margin.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35981.1/606013/A-crucial-geologic-test-of-Late-Jurassic-exotic
Quantifying structural controls on submarine channel architecture and
kinematics
W. Hamish Mitchell; Alexander C. Whittaker; Mike Mayall; Lidia Lonergan;
Marco Pizzi
Abstract:
Over the past two decades, the increased availability of three-dimensional
(3-D) seismic data and their integration with outcrop and numerical
modeling studies have enabled the architectural evolution of submarine
channels to be studied in detail. While tectonic activity is a recognized
control on submarine channel morphology, the temporal and spatial
complexity associated with these systems means submarine channel behavior
over extended time periods, and the ways in which processes scale and
translate into time-integrated sedimentary architecture, remain poorly
understood. For example, tectonically driven changes in slope morphology
may locally enhance or diminish a channel’s ability to incise, aggrade, and
migrate laterally, changing channel kinematics and the distribution of
composite architectures. Here, we combined seismic techniques with the
concept of stratigraphic mobility to quantify how gravity-driven
deformation influenced the stratigraphic architecture of two submarine
channels, from the fundamental architectural unit, a channel element, to
channel complex scale, on the Niger Delta slope. From a 3-D, time-migrated,
seismic-reflection volume, we evaluated the evolution of widths, depths,
sinuosities, curvatures, and stratigraphic mobilities at fixed intervals
downslope as the channel complexes interacted with a range of
gravity-driven structures. At channel element scale, sinuosity and bend
amplitude were consistently elevated over structured reaches of the slope,
displaying a nonlinear increase in length, perpendicular to flow direction.
At channel complex scale, the same locations, updip of structure,
correlated to an increase in channel complex width and aspect ratio.
Normalized complex dimensions and complex-averaged stratigraphic mobilities
showed lateral migration to be the dominant form of stratigraphic
preservation in these locations. Our results explain the intricate
relationship between the planform characteristics of channel elements and
the cross-sectional dimensions of the channel complex. We show how channel
element processes and kinematics translate to form higher-order
stratigraphic bodies, and we demonstrate how tectonically driven changes in
slope develop channel complexes with distinct cross-sectional and planform
architectures.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36001.1/606014/Quantifying-structural-controls-on-submarine
Basement-decoupled hyperextension rifting: The tectono-stratigraphic
record of the salt-rich Pyrenean necking zone (Arzacq Basin, SW France)
M. Ducoux; E. Masini; J. Tugend; J. Gómez-Romeu; S. Calassou
Abstract:
Half grabens and supra-detachment basins correspond to end-member basin
types of magma-poor rift settings, each of them showing a characteristic
stratigraphic architecture. The occurrence of a basement-cover décollement
has been shown to drastically change the stratigraphic architecture of half
graben basins, however, the effect of such basement-cover décollement
remains to be documented in supra-detachment basins formed during
hyper-extension. We investigate the tectono-stratigraphic record of the
Arzacq Basin (SW France) recording the formation of a salt-rich Cretaceous
hyperextended rift system. Combining 2-D and 3-D seismic reflection
calibrated from well data, we show that this basin is an asymmetric
syn-rift extensional syncline growing above a pre-kinematic salt layer. By
mapping the sub-salt basement, we show that the formation of this syncline
is controlled by the South-Arzacq Fault (SAF), soling in the sub-salt
basement. Based on crosscutting relationships and the observed southward
migration of syn-rift depocenters, this N110°-striking, 20°-dipping
structure accommodates >10 km of thick-skinned extension. The overlying
supra-salt cover coherently glided, following the basement geometry. The
3-D segmentation of the SAF and the sub-salt stratigraphic architecture of
the Arzacq Basin suggest a roughly dip-slip kinematic. A post-kinematic
kilometer-scale uplift is documented on the southern side of the Arzacq
Basin. It may result from the increasing lithospheric thinning and thermal
support at the end of asymmetric hyperextension. As salt commonly occurs in
extensional settings, we believe that our description of the
tectono-stratigraphic record of a basement-decoupled supra-detachment basin
has global applicability to unleash the tectono-stratigraphic evolution of
worldwide hyper-extended rifted margins.
View article
:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35974.1/606015/Basement-decoupled-hyperextension-rifting-The
Formation of the crater suevite sequence from the Chicxulub peak ring:
A petrographic, geochemical, and sedimentological characterization
Pim Kaskes; Sietze J. de Graaff; Jean-Guillaume Feignon; Thomas Déhais;
Steven Goderis ...
Abstract:
This study presents a new classification of a ∼100-m-thick crater suevite
sequence in the recent International Ocean Discovery Program
(IODP)-International Continental Scientific Drilling Program (ICDP)
Expedition 364 Hole M0077A drill core to better understand the formation of
suevite on top of the Chicxulub peak ring. We provide an extensive data set
for this succession that consists of whole-rock major and trace element
compositional data (n = 212) and petrographic data supported by digital
image analysis. The suevite sequence is subdivided into three units that
are distinct in their petrography, geochemistry, and sedimentology, from
base to top: the ∼5.6-m-thick non-graded suevite unit, the ∼89-m-thick
graded suevite unit, and the ∼3.5-m-thick bedded suevite unit. All of these
suevite units have isolated Cretaceous planktic foraminifera within their
clastic groundmass, which suggests that marine processes were responsible
for the deposition of the entire M0077A suevite sequence. The most likely
scenario describes that the first ocean water that reached the northern
peak ring region entered through a N-NE gap in the Chicxulub outer rim. We
estimate that this ocean water arrived at Site M0077 within 30 minutes
after the impact and was relatively poor in rock debris. This water caused
intense quench fragmentation when it interacted with the underlying hot
impact melt rock, and this resulted in the emplacement of the ∼5.6-m-thick
hyaloclastite-like, non-graded suevite unit. In the following hours, the
impact structure was flooded by an ocean resurge rich in rock debris, which
caused the phreatomagmatic processes to stop and the ∼89-m-thick graded
suevite unit to be deposited. We interpret that after the energy of the
resurge slowly dissipated, oscillating seiche waves took over the
sedimentary regime and formed the ∼3.5-m-thick bedded suevite unit. The
final stages of the formation of the impactite sequence (estimated to be
<20 years after impact) were dominated by resuspension and slow
atmospheric settling, including the final deposition of Chicxulub impactor
debris. Cumulatively, the Site M0077 suevite sequence from the Chicxulub
impact site preserved a high-resolution record that provides an
unprecedented window for unravelling the dynamics and timing of proximal
marine cratering processes in the direct aftermath of a large impact event.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36020.1/606016/Formation-of-the-crater-suevite-sequence-from-the
Quaternary chronology and rock uplift recorded by marine terraces,
Gaviota coast, Santa Barbara County, California, USA
Daniel L. Morel; Kristin D. Morell; Edward A. Keller; Tammy M. Rittenour
Abstract:
The Transverse Ranges of southern California are a region of active
transpression on the western margin of North America that hosts some of the
world’s highest uplift rates at the Ventura anticline. Yet, the manner in
which rock uplift rates change along strike from Ventura to the westernmost
Transverse Ranges and the structures that may be responsible for this
uplift remain unclear. Here, we quantified rock uplift rates within the
westernmost 60 km of the Transverse Ranges by obtaining new age constraints
from raised beach and shoreface deposits from marine terraces along the
Gaviota coast. Twelve radiocarbon (seven sites) and eight luminescence (six
sites) ages, ranging from ca. 50 to 40 k.y. B.P. and ca. 56 to 43 ka,
respectively, consistently suggest that the first emergent terrace dates to
marine isotope stage (MIS) 3, rather than MIS 5a as previously reported for
the western Gaviota coast. These younger ages yield rock uplift rates
between 0.8 ± 0.3 and 1.8 ± 0.4 m/k.y., i.e., over five times higher than
previous estimates for this region. The spatial distribution of rock uplift
rates and the abrupt along-strike changes in marine terrace elevations
favor a regional tectonic model with a step-wise change in rock uplift
across the south branch of the Santa Ynez fault. The south branch of the
Santa Ynez fault appears to separate two regional tectonic blocks,
characterized by rock uplift rates of ∼1.3−1.6 m/k.y. to the east and
slightly lower rates to the west (∼0.8−1.4 m/k.y.). Our observations
suggest that coastal rock uplift is primarily accommodated by deeply rooted
far-field structures such as the offshore Pitas Point−North Channel fault
system and the Santa Ynez fault, and that smaller through-going structures
impart second-order controls and locally accommodate short-wavelength
(<10-km-long strike length) deformation. These results imply that
although the rates of rock uplift decline westward along strike, the
westernmost portion of the western Transverse Ranges nonetheless
accommodates relatively high (>1 m/k.y.) rock uplift rates at a
significant distance (>50 km) from the rapidly uplifting (6−7 m/k.y.)
Ventura anticline, and >100 km from the prominent restraining bend (“Big
Bend”) in the San Andreas fault. The new constraints on the geometry of
Quaternary-active structures and regional rates of fault-related
deformation have implications for regional earthquake source models and
seismic hazard assessment in the highly populated southern California coast
region.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35609.1/605907/Quaternary-chronology-and-rock-uplift-recorded-by
Vein topology, structures, and distribution during the prograde
formation of an Archean gold stockwork
François Turlin; Stéphane De Souza; Michel Jébrak; Pierre-Arthur Groulier;
Jordi Turcotte
Abstract:
The Archean Cheechoo stockwork gold deposit is
hosted by a felsic intrusion of tonalitic-granodioritic composition and
crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec,
Canada (Archean Superior craton). The evolution of the stockwork is
characterized herein using field relationships, vein density, and
connectivity measurements on drill core and outcrop zones. The statistical
distribution of gold is used to highlight mechanisms of stockwork
emplacement and gold mineralization and remobilization. Two statistical
populations of gold concentration are present. Population A is represented
by gold grades below 1 g/t with a lognormal cumulative frequency. It is
widespread in the hydrothermally altered (albite and quartz) and
mineralized facies of the pluton. It is controlled by the development of
quartz-feldspar-diopside veins as shown by the similar lognormal
distribution of grades and vein density and by the correspondence of grades
with network connectivity. Diopside and actinolite porphyroblasts in
deformed veins within sodic and calcsilicate alteration zones are evidence
for auriferous vein emplacement prior to the amphibolite facies peak of
metamorphism. Population B (>1 g/t) is erratic and exhibits a strong
nugget effect. It is present throughout the mineralized portion of the
pluton and in pegmatites. This population is interpreted as the result of
gold remobilization during prograde metamorphism and pegmatite emplacement
following the metamorphic peak. The pegmatites are interpreted to have
scavenged gold emplaced prior to peak metamorphism. These results show the
isotropic behavior of the investigated stockwork during regional
deformation and its development during the early stages of regional
prograde metamorphism.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36057.1/605908/Vein-topology-structures-and-distribution-during
Sr isotopes in the Tortonian-Messinian Lake Bira and Gesher marshes,
Northern Valleys of Israel: Implications for hydroclimate changes in
East Mediterranean−Levant margins
A.G. Rozenbaum; M. Stein; E. Zilberman; D. Shaked Gelband; A. Starinsky ...
Abstract:
87
Sr/86Sr isotope and Sr/Ca ratios in lacustrine carbonates were
used to reconstruct the hydroclimate conditions in the watershed of Lake
Bira that filled during the Tortonian-Messinian the tectonic depressions of
the Northern Valleys of Israel in the East Mediterranean-Levant region. 87Sr/86Sr ratios of the Tortonian (ca. 10−8 Ma)
carbonates of ∼0.7075 and the great expansion of the lake indicate wet
conditions and enhanced supply of freshwater from the regional Mesozoic
aquifers. Upon the transition to the Messinian period (ca. 7−6 Ma), the 87Sr/86Sr ratios in the carbonates rose to
∼0.7080−0.7085, reflecting the contribution of Sr from Sahara Desert dusts
that came to comprise the regional surface cover. This contribution is also
reflected in the silicate fraction of the lacustrine formations that show
“granitic-crustal” 87Sr/86Sr ratios of ∼0.711. During
the Messinian salinity crisis (5.9−5.6 Ma), the region became arid and Lake
Bira possibly dried. Later, during the Lago Mare stage (ca. 5.5−5.3 Ma),
the rainfall increased and paludal waterbodies scattered the area of the
larger Lake Bira.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35949.1/605909/Sr-isotopes-in-the-Tortonian-Messinian-Lake-Bira
Forced subduction initiation within the Neotethys: An example from the
mid-Cretaceous Wuntho-Popa arc in Myanmar
Liyun Zhang; Weiming Fan; Lin Ding; Alex Pullen; Mihai N. Ducea ...
Abstract:
Despite decades of research, the mechanisms and processes of subduction
initiation remain obscure, including the tectonic settings where subduction
initiation begins and how magmatism responds. The Cretaceous Mawgyi
Volcanics represent the earliest volcanic succession in the Wuntho-Popa arc
of western Myanmar. This volcanic unit consists of an exceptionally diverse
range of contemporaneously magmatic compositions which are spatially
juxtaposed. Our new geochemical data show that the Mawgyi Volcanics
comprise massive mid-oceanic ridge basalt (MORB)-like lavas and dikes, and
subordinate island arc tholeiite and calc-alkaline lavas. The Mawgyi
MORB-like rocks exhibit flat rare earth elements (REEs) patterns and are
depleted in REEs, high field strength elements (except for Th) and TiO 2 concentrations relative to those of MORBs, resembling the
Izu-Bonin-Mariana protoarc basalts. Our geochronological results indicate
that the Mawgyi Volcanics formed between 105 and 93 Ma, coincident with
formation of many Neotethyan supra-subduction zone ophiolites and
intraoceanic arcs along orogenic strike in the eastern Mediterranean,
Middle East, Pakistan, and Southeast Asia. Combined with its
near-equatorial paleo-latitudes constrained by previous paleomagnetic data,
the Wuntho-Popa arc is interpreted as a segment of the north-dipping
trans-Neotethyan subduction system during the mid-Cretaceous. Importantly,
our restoration with available data provides new evidence supporting the
hypothesis of a mid-Cretaceous initiation of this >8000-km-long
subduction system formed by inversion of the ∼E-W−trending Neotethyan
oceanic spreading ridges, and that this was contemporaneous with the final
breakup of Gondwana and an abrupt global plate reorganization.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35818.1/605911/Forced-subduction-initiation-within-the-Neotethys
Evaluating the role of topographic inversion in the formation of the
Stanislaus Table Mountains in the Sierra Nevada (California, USA)
Emmanuel Gabet
Abstract:
The Table Mountains, a flat-topped series of ridges capped by a 10.4 Ma
latite flow in the Stanislaus River watershed, are considered to be
evidence for late Cenozoic uplift-driven landscape rejuvenation in the
northern Sierra Nevada range (California, USA). The commonly accepted
theory for the formation of these mesas posits that the latite flowed and
cooled within a bedrock paleovalley and, since then, the surrounding
landscape has eroded away, leaving behind the volcanic deposit as a ridge.
Although this theory is accepted by many, it has not been thoroughly
tested. In this study, I examine a series of geological cross-sections
extracted along the length of the latite deposit to determine whether the
evidence supports the existence of bedrock valley walls on both sides of
the 10.4 Ma flow. I find that the presence of older Cenozoic deposits
adjacent to the latite flow precludes the possibility that the flow would
have been constrained within a bedrock valley. Moreover, the cross-section
from an 1865 report that has been offered as evidence of topographic
inversion (and subsequently reproduced in numerous publications) does not
accurately represent the topography at that site. I conclude that there is
no evidence that the bedrock topography has been inverted and that instead,
the latite flowed within a channel cut into underlying Cenozoic deposits,
which have since mostly eroded away. This study, therefore, refutes the
hypothesis that the Stanislaus River watershed was rejuvenated in the late
Cenozoic and challenges the claim for recent significant uplift of the
region.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35940.1/605913/Evaluating-the-role-of-topographic-inversion-in
Metamorphism and geochronology of high-pressure mafic granulites
(retrograded eclogites?) in East Cathaysia terrane of South China:
Implications for Mesozoic tectonic evolution
Yanfei Xia; Changqing Yin; Shoufa Lin; Jian Zhang; Jiahui Qian ...
Abstract:
High-pressure mafic granulites (retrograded eclogites?) were discovered as
minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu
Complex of the East Cathaysia terrane in South China. These rocks consist
mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite
with or without omphacite pseudomorphs that are indicated by clinopyroxene
+ sodic plagioclase symplectic intergrowths. Mineral textures and reaction
relationships suggest three metamorphic stages: (1) an eclogite-facies
stage (M1) characterized by the mineral assemblage of garnet +
clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a
high-pressure granulite-facies (M2) stage mainly represented by
garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the
matrix; and (3) an amphibolite retrograde stage (M3) defined by
hornblende + plagioclase + ilmenite + quartz symplectites surrounding
garnet porphyroblasts. Conventional geothermometers and geobarometers in
combination with phase equilibria modeling constrain metamorphic P−T conditions of 15.8−18.2 kbar/625−690 °C (M1),
11.8−14.5 kbar/788−806 °C (M2), and 5.4−6.4 kbar/613−668 °C (M 3), respectively. Two-staged decompression processes are defined
after the peak pressure, which suggests a two-staged exhumation of these
deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb
dating and trace element analysis show that the high-pressure metamorphism
occurred at 240−244 Ma. Complete early Mesozoic orogenic processes
characterized by initial subduction and/or crustal thickening and
subsequent exhumation followed by rapid uplift are reconstructed for this
part of the East Cathaysia terrane, South China.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36025.1/605914/Metamorphism-and-geochronology-of-high-pressure
Early Pleistocene to present paleoclimate archive for the American
Southwest from Stoneman Lake, Arizona, USA
Spencer E. Staley; Peter J. Fawcett; R. Scott Anderson; Gonzalo
Jiménez-Moreno
Abstract:
Long, continuous records of terrestrial paleoclimate offer insights into
natural climate variability and provide context for geomorphological
studies, climate model reconstructions, and predictions of future climate
change. STL14 is an 80 m lacustrine sediment core that archives
paleoenvironmental changes at Stoneman Lake, Coconino County, Arizona, from
the early Pleistocene (ca. 1.3 Ma) to present. Full-core sedimentology was
analyzed using smear slides and core face observations. Lithofacies
strongly correlate with wet bulk density and bulk magnetic susceptibility
(MS), and these data resemble a sawtooth pattern characteristic of
glacial-interglacial climate cycles. A linkage between deep to shallow lake
depth transitions and glacial terminations is supported by an age model
that incorporates accelerator mass spectrometry radiocarbon dates and
tephrochronology of ashes from the Lava Creek B and multiple Long Valley,
California, volcanic eruptions. We correlated middle and late Pleistocene
glacial maxima to deep lake deposits defined by well-preserved bedding,
increased biosilica, boreal pollen taxa (i.e., Picea), and lower
density and MS. Interglacial periods are associated with shallow-water
deposits characterized by banded-to-massive siliciclastic material, some
authigenic calcite, the alga Phacotus, and higher density and MS.
Prior to the marine isotope stage (MIS) 24−22 interval, smaller-amplitude
changes in the lake environment suggest milder glacial conditions compared
to those of the middle and late Pleistocene. Thus, abrupt intensification
of glacial conditions may have occurred ca. 900 ka in the American
Southwest, mirroring a global characteristic of the mid-Pleistocene
transition. The STL14 record suggests that lake environments throughout the
history of this small (3.5 km2), internally drained, basaltic
catchment are sensitive to the regional hydrologic balance, which, at
orbital time scales, is largely influenced by the northern cryosphere and
associated changes in atmospheric circulation. The predominance of quartz
in sediment throughout the record indicates significant eolian inputs. Few
paleoclimate records from this region extend beyond the last glacial cycle,
let alone the middle Pleistocene, making STL14 a valuable resource for
studying environmental responses to a range of natural climate states and
transitions throughout much of the Quaternary.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36038.1/605915/Early-Pleistocene-to-present-paleoclimate-archive
Refining the Paleoproterozoic tectonothermal history of the Penokean
Orogen: New U-Pb age constraints from the Pembine-Wausau terrane,
Wisconsin, USA
Jian-Wei Zi; Stephen Sheppard; Janet R. Muhling; Birger Rasmussen
Abstract:
An enduring problem in the assembly of Laurentia is uncertainty about the
nature and timing of magmatism, deformation, and metamorphism in the
Paleoproterozoic Wisconsin magmatic terranes, which have been variously
interpreted as an intra-oceanic arc, foredeep or continental back-arc.
Resolving these competing models is difficult due in part to a lack of a
robust time-frame for magmatism in the terranes. The northeast part of the
terranes in northern Wisconsin (USA) comprise mafic and felsic volcanic
rocks and syn-volcanic granites thought to have been emplaced and
metamorphosed during the 1890−1830 Ma Penokean orogeny. New in situ U-Pb
geochronology of igneous zircon from the volcanic rocks (Beecher
Formation), and from two tonalitic plutons (the Dunbar Gneiss and Newingham
Tonalite) intruding the volcanic rocks, yielded crystallization ages
ranging from 1847 ± 10 Ma to 1842 ± 7 Ma (95% confidence). Thus, these
rocks record a magmatic episode that is synchronous with bimodal volcanism
in the Wausau domain and Marshfield terrane farther south. Our results,
integrated with published data into a time-space diagram, highlight two
bimodal magmatic cycles, the first at 1890−1860 Ma and the second at
1845−1830 Ma, developed on extended crust of the Superior Craton. The
magmatic episodes are broadly synchronous with volcanogenic massive sulfide
mineralization and deposition of Lake Superior banded iron formations. Our
data and interpretation are consistent with the Penokean orogeny marking
west Pacific-style accretionary orogenesis involving lithospheric extension
of the continental margin, punctuated by transient crustal shortening that
was accommodated by folding and thrusting of the arc-back-arc system. The
model explains the shared magmatic history of the Pembine-Wausau and
Marshfield terranes. Our study also reveals an overprinting metamorphic
event recorded by reset zircon and new monazite growth dated at 1775 ± 10
Ma suggesting that the main metamorphic event in the terranes is related to
the Yavapai-interval accretion rather than the Penokean orogeny.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36114.1/605893/Refining-the-Paleoproterozoic-tectonothermal
GSA BULLETIN articles published ahead of print are online at
https://bulletin.geoscienceworld.org/content/early/recent
. Representatives of the media may obtain complimentary copies of articles
by contacting Kea Giles. Please discuss articles of interest with the
authors before publishing stories on their work, and please make reference
to The Geological Society of America Bulletin in articles published.
Non-media requests for articles may be directed to GSA Sales and Service,
gsaservice@geosociety.org.
https://www.geosociety.org
# # #