Articles for Geosphere Posted Online in April
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied this month include the
Central Anatolian Plateau; the Southern Rocky Mountain Volcanic Field;
petrogenesis in the Grand Canyon; and the evolution of the Portland and
Tualatin forearc basins, Oregon.
A physical and chemical sedimentary record of Laramide tectonic shifts
in the Cretaceous-Paleogene San Juan Basin, New Mexico, USA
Kevin M. Hobbs; Peter J. Fawcett
Abstract:
Fluvial siliciclastic rocks bracketing the Cretaceous-Paleogene (K-Pg)
boundary in the San Juan Basin, New Mexico (USA), provide records of
regional fluvial and tectonic evolution during the Laramide orogeny.
Petrographic analyses of sandstones from the Upper Cretaceous Fruitland
Formation and Kirtland Formation and the Paleocene Ojo Alamo Sandstone and
Nacimiento Formation show that the rivers depositing these sediments were
sourced in areas where unroofing of crystalline basement rocks took place,
introducing an increasing proportion of immature detrital grains into the
fluvial system through time. After the Cretaceous-Paleogene boundary,
rivers deposited an increasing amount of microcline and orthoclase feldspar
relative to plagioclase feldspar, suggesting a growing source in unique
crystalline basement rocks. Geochemical analyses show significant
differences between Al- and K-poor Upper Cretaceous sandstones and Al- and
K-rich lower Paleocene sandstones in the San Juan Basin. The high
proportion of sand-sized material in the Ojo Alamo Sandstone suggests that
it was deposited in a basin with a low ratio of sediment supply to
accommodation. However, magnetostratigraphic age constraints suggest it had
a relatively high sedimentation and/or subsidence rate of as much as 0.38
m/k.y. The sediment supply must have been high in order to deposit a
basin-wide coarse sand-dominated package, suggesting rapid creation of
topographic relief in the San Juan uplift, the proposed source area of the
Ojo Alamo fluvial system. The observed sedimentary architecture and age
constraints of the Ojo Alamo Sandstone, including kilometers-wide sand
bodies and limited overbank mudstones throughout most of the outcrop area,
are difficult to reconcile with accepted models of aggradation and avulsion
in large fluvial systems, but available age and lithologic data make
difficult a complete understanding of Paleocene San Juan Basin fluvial
systems and basin evolution. Here, we present new lithologic, petrographic,
and thickness data from San Juan Basin K-Pg fluvial siliciclastic units and
interpretations of their origins.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02324.1/596304/A-physical-and-chemical-sedimentary-record-of
The spatial and temporal evolution of the Portland and Tualatin forearc
basins, Oregon, USA
Darby P. Scanlon; John Bershaw; Ray E. Wells; Ashley R. Streig
Abstract:
The Portland and Tualatin basins are part of the Salish-Puget-Willamette
Lowland, a 900-km-long, forearc depression lying between the volcanic arc
and the Coast Ranges of the Cascadia convergent margin. Such inland seaways
are characteristic of warm, young slab subduction. We analyzed the basins
to better understand their evolution and relation to Coast Range history
and to provide an improved tectonic framework for the Portland metropolitan
area. We model three key horizons in the basins: (1) the top of the
Columbia River Basalt Group (CRBG), (2) the bottom of the CRBG, and (3) the
top of Eocene basement. Isochore maps constrain basin depocenters during
(1) Pleistocene to mid-Miocene time (0–15 Ma), (2) CRBG (15.5–16.5 Ma), and
(3) early Miocene to late Eocene (ca. 17–35 Ma) time. Results show that the
Portland and Tualatin basins have distinct mid-Miocene to Quaternary
depocenters but were one continuous basin from the Eocene until mid-Miocene
time. A NW-striking gravity low coincident with the NW-striking,
fault-bounded Portland Hills anticline is interpreted as an older graben
coincident with observed thickening of CRBG flows and underlying
sedimentary rocks. Neogene transpression in the forearc structurally
inverted the Sylvan-Oatfield and Portland Hills normal faults as high-angle
dextral-reverse faults, separating the Portland and Tualatin basins. An
eastward shift of the forearc basin depocenter and ten-fold decrease in
accommodation space provide temporal constraints on the emergence of the
Coast Range to the west. Clockwise rotation and northward transport of the
forearc is deforming the basins and producing local earthquakes beneath the
metropolitan area.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02298.1/596305/The-spatial-and-temporal-evolution-of-the-Portland
Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite
or deep-arc fragment?
S.J. Seaman; M.L. Williams; K.E. Karlstrom; P.C. Low
Abstract:
Recognition of fundamental tectonic boundaries has been extremely difficult
in the (>1000-km-wide) Proterozoic accretionary orogen of southwestern
North America, where the main rock types are similar over large areas, and
where the region has experienced multiple postaccretionary deformation
events. Discrete ultramafic bodies are present in a number of areas that
may mark important boundaries, especially if they can be shown to represent
tectonic fragments of ophiolite complexes. However, most ultramafic bodies
are small and intensely altered, precluding petrogenetic analysis. The
91-Mile peridotite in the Grand Canyon is the largest and best preserved
ultramafic body known in the southwest United States. It presents a special
opportunity for tectonic analysis that may illuminate the significance of
ultramafic rocks in other parts of the orogen. The 91-Mile peridotite
exhibits spectacular cumulate layering. Contacts with the surrounding
Vishnu Schist are interpreted to be tectonic, except along one margin,
where intrusive relations have been interpreted. Assemblages include
olivine, clinopyroxene, orthopyroxene, magnetite, and phlogopite, with very
rare plagioclase. Textures suggest that phlogopite is the result of late
intercumulus crystallization. Whole-rock compositions and especially
mineral modes and compositions support derivation from an arc-related mafic
magma. K-enriched subduction-related fluid in the mantle wedge is
interpreted to have given rise to a K-rich, hydrous, high-pressure partial
melt that produced early magnetite, Al-rich diopside, and primary
phlogopite. The modes of silicate minerals, all with high Mg#, the sequence
of crystallization, and the lack of early plagioclase are all consistent
with crystallization at relatively high pressures. Thus, the 91-Mile
peridotite body is not an ophiolite fragment that represents the closure of
a former ocean basin. It does, however, mark a significant tectonic
boundary where lower-crustal arc cumulates have been juxtaposed against
middle-crustal schists and granitoids.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02302.1/596306/Petrogenesis-of-the-91-Mile-peridotite-in-the
Secular variations of magma source compositions in the North Patagonian
batholith from the Jurassic to Tertiary: Was mélange melting involved?
Antonio Castro; Carmen Rodriguez; Carlos Fernández; Eugenio Aragón; Manuel
Francisco Pereira ...
Abstract:
This study of Sr-Nd initial isotopic ratios of plutons from the North
Patagonian batholith (Argentina and Chile) revealed that a secular
evolution spanning 180 m.y., from the Jurassic to Neogene, can be
established in terms of magma sources, which in turn are correlated with
changes in the tectonic regime. The provenance and composition of
end-member components in the source of magmas are represented by the Sr-Nd
initial isotopic ratios (87Sr/86Sr and 143
Nd/144Nd) of the plutonic rocks. Our results support the
interpretation that source composition was determined by incorporation of
varied crustal materials and trench sediments via subduction erosion and
sediment subduction into a subduction channel mélange. Subsequent melting
of subducted mélanges at mantle depths and eventual reaction with the
ultramafic mantle are proposed as the main causes of batholith magma
generation, which was favored during periods of fast convergence and high
obliquity between the involved plates. We propose that a parental diorite
(= andesite) precursor arrived at the lower arc crust, where it underwent
fractionation to yield the silicic melts (granodiorites and granites) that
formed the batholiths. The diorite precursor could have been in turn
fractionated from a more mafic melt of basaltic andesite composition, which
was formed within the mantle by complete reaction of the bulk mélanges and
the peridotite. Our proposal follows model predictions on the formation of
mélange diapirs that carry fertile subducted materials into hot regions of
the suprasubduction mantle wedge, where mafic parental magmas of batholiths
originate. This model not only accounts for the secular geochemical
variations of Andean batholiths, but it also avoids a fundamental paradox
of the classical basalt model: the absence of ultramafic cumulates in the
lower arc crust and in the continental crust in general.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02338.1/596307/Secular-variations-of-magma-source-compositions-in
Fast Pliocene integration of the Central Anatolian Plateau drainage:
Evidence, processes, and driving forces
Gilles Y. Brocard; Maud J.M. Meijers; Michael A. Cosca; Tristan Salles;
Jane Willenbring ...
Abstract:
Continental sedimentation was widespread across the Central Anatolian
Plateau in Miocene–Pliocene time, during the early stages of plateau
uplift. Today, however, most sediment produced on the plateau is dispersed
by a well-integrated drainage and released into surrounding marine
depocenters. Residual long-term (106–107 yr) sediment storage on the
plateau is now restricted to a few closed catchments. Lacustrine
sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it
is also restricted to the residual closed catchments. The present-day
association of closed catchments, long-term sediment storage, and
lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation
also occurred in closed catchments. The termination of sedimentation across
the plateau would therefore mark the opening of these closed catchments,
their integration, and the formation of the present-day drainage. By
combining newly dated volcanic markers with previously dated sedimentary
sequences, we show that this drainage integration occurred remarkably
rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of
stream incision documented by these markers and newly obtained 10Be erosion rates allow us to discriminate the respective
contributions of three potential processes to drainage integration,
namely, the capture of closed catchments by rivers draining the outer
slopes of the plateau, the overflow of closed lakes, and the avulsion of
closed catchments. Along the southern plateau margin, rivers draining the
southern slope of the Central Anatolian Plateau expanded into the plateau
interior; however, only a small amount of drainage integration was achieved
by this process. Instead, avulsion and/or overflow between closed
catchments achieved most of the integration, and these top-down processes
left a distinctive sedimentary signal in the form of terminal lacustrine
limestone sequences. In the absence of substantial regional climate wetting
during the early Pliocene, we propose that two major tectonic events
triggered drainage integration, separately or in tandem: the uplift of the
Central Anatolian Plateau and the tectonic completion of the Anatolian
microplate. Higher surface uplift of the eastern Central Anatolian Plateau
relative to the western Central Anatolian Plateau promoted more positive
water balances in the eastern catchments, higher water discharge, and
larger sediment fluxes. Overflow/avulsion in some of the eastern catchments
triggered a chain of avulsions and/or overflows, sparking sweeping
integration across the plateau. Around 5 Ma, the inception of the full
escape of the Anatolian microplate led to the disruption of the plateau
surface by normal and strike-slip faults. Fault scarps partitioned large
catchments fed by widely averaged sediment and water influxes into smaller
catchments with more contrasted water balances and sediment fluxes. The
evolution of the Central Anatolian Plateau shows that top-down processes of
integration can outcompete erosion of outer plateau slopes to reintegrate
plateau interior drainages, and this is overlooked in current models, in
which drainage evolution is dominated by bottom-up integration. Top-down
integration has the advantage that it can be driven by more subtle changes
in climatic and tectonic boundary conditions than bottom-up integration.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02247.1/595917/Fast-Pliocene-integration-of-the-Central-Anatolian
Postcaldera intrusive magmatism at the Platoro caldera complex,
Southern Rocky Mountain volcanic field, Colorado, USA
Amy K. Gilmer; Ren A. Thompson; Peter W. Lipman; Jorge A. Vazquez; A. Kate
Souders
Abstract:
The Oligocene Platoro caldera complex of the San Juan volcanic locus in
Colorado (USA) features numerous exposed plutons both within the caldera
and outside its margins, enabling investigation of the timing and evolution
of postcaldera magmatism. Intrusion whole-rock geochemistry and phenocryst
and/or mineral trace element compositions coupled with new zircon U-Pb
geochronology and zircon in situ Lu-Hf isotopes document distinct pulses of
magma from beneath the caldera complex. Fourteen intrusions, the Chiquito
Peak Tuff, and the dacite of Fisher Gulch were dated, showing intrusive
magmatism began after the 28.8 Ma eruption of the Chiquito Peak Tuff and
continued to 24 Ma. Additionally, magmatic-hydrothermal mineralization is
associated with the intrusive magmatism within and around the margins of
the Platoro caldera complex. After caldera collapse, three plutons were
emplaced within the subsided block between ca. 28.8 and 28.6 Ma. These have
broadly similar modal mineralogy and whole-rock geochemistry. Despite close
temporal relations between the tuff and the intrusions, mineral textures
and compositions indicate that the larger two intracaldera intrusions are
discrete later pulses of magma. Intrusions outside the caldera are younger,
ca. 28–26.3 Ma, and smaller in exposed area. They contain abundant
glomerocrysts and show evidence of open-system processes such as magma
mixing and crystal entrainment. The protracted magmatic history at the
Platoro caldera complex documents the diversity of the multiple discrete
magma pulses needed to generate large composite volcanic fields.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02242.1/595918/Postcaldera-intrusive-magmatism-at-the-Platoro
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