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Current issue articles for Geosphere posted online in February


Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics for articles posted for Geosphere this month include “a tale of five enclaves”; evidence
for mantle and Moho in the Baltimore Mafic Complex (Maryland, USA); and the
after effects of the 1964 Mw 9.2 megathrust rupture, Alaska.

From Ordovician nascent to early Permian mature arc in the southern
Altaids: Insights from the Kalatage inlier in the Eastern Tianshan, NW

Qigui Mao; Jingbin Wang; Wenjiao Xiao; Brian F. Windley; Karel Schulmann

The Kalatage inlier in the Dananhu-Haerlik arc is one of the most important
arcs in the Eastern Tianshan, southern Altaids (or Central Asian orogenic
belt). Based on outcrop maps and core logs, we report 16 new U-Pb dates in
order to reconstruct the stratigraphic framework of the Dananhu-Haerlik
arc. The new U-Pb ages reveal that the volcanic and intrusive rocks formed
in the interval from the Ordovician to early Permian (445-299 Ma), with the
oldest diorite dike at 445 ± 3 Ma and the youngest rhyolite at 299 ± 2 Ma.
These results constrain the ages of the oldest basaltic and volcaniclastic
rocks of the Ordovician Huangchaopo Group, which were intruded by granite-
granodiorite-diorite plutons in the Late Ordovician to middle Silurian
(445-426 Ma). The second oldest components are intermediate volcanic and
volcaniclastic rocks of the early Silurian Hongliuxia Formation (S 1h), which lies unconformably on the Huangchaopo Group
and is unconformably overlain by Early Devonian volcanic rocks (416 Ma).
From the mid- to late Silurian (S2-3), all the rocks were
exhumed, eroded, and overlain by polymictic pyroclastic deposits. Following
subaerial to shallow subaqueous burial at 416-300 Ma by intermediate to
felsic volcanic and volcaniclastics rocks, the succession was intruded by
diorites, granodiorites, and granites (390-314 Ma). The arc volcanic and
intrusive rocks are characterized by potassium enrichment, when they
evolved from mafic to felsic and from tholeiitic via transitional and
calc-alkaline to final high-K calc- alkaline compositions with relatively
low initial Sr values, (87Sr/86Sr)i =
0.70391-0.70567, and positive ?Nd(t) values, +4.1 to
+9.2. These new data suggest that the Dananhu-Haerlik arc is a long-lived
arc that consequently requires a new evolutionary model. It began as a
nascent (immature) intra-oceanic arc in the Ordovician to early Silurian,
and it evolved into a mature island arc in the middle Silurian to early
Permian. The results suggest that the construction of a juvenile-to-mature
arc, in combination with its lateral attachment to an incoming arc or
continent, was an important crustal growth mechanism in the southern

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A tale of five enclaves: Mineral perspectives on origins of mafic
enclaves in the Tuolumne Intrusive Complex

C.G. Barnes; K. Werts; V. Memeti; S.R. Paterson; R. Bremer

The widespread occurrence of mafic magmatic enclaves (mme) in arc volcanic
rocks attests to hybridization of mafic-intermediate magmas with felsic
ones. Typically, mme and their hosts differ in mineral assemblage and the
compositions of phenocrysts and matrix glass. In contrast, in many arc
plutons, the mineral assemblages in mme are the same as in their host
granitic rocks, and major-element mineral compositions are similar or
identical. These similarities lead to difficulties in identifying mixing
end members except through the use of bulk-rock compositions, which
themselves may reflect various degrees of hybridization and potentially
melt loss. This work describes the variety of enclave types and occurrences
in the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex
and then focuses on textural and mineral composition data on five
porphyritic mme from the eHD. Specifically, major- and trace-element
compositions and zoning patterns of plagioclase and hornblende were
measured in the mme and their adjacent host granitic rocks. In each case,
the majority of plagioclase phenocrysts in the mme (i.e., large crystals)
were derived from a rhyolitic end member. The trace-element compositions
and zoning patterns in these plagioclase phenocrysts indicate that each mme
formed by hybridization with a distinct rhyolitic magma. In some cases,
hybridization involved a single mixing event, whereas in others, evidence
for at least two mixing events is preserved. In contrast, some hornblende
phenocrysts grew from the enclave magma, and others were derived from the
rhyolitic end member. Moreover, the composition of hornblende in the
immediately adjacent host rock is distinct from hornblende typically
observed in the eHD. Although primary basaltic magmas are thought to be
parental to the mme, little or no evidence of such parents is preserved in
the enclaves. Instead, the data indicate that hybridization of already
hybrid andesitic enclave magmas with rhyolitic magmas in the eHD involved
multiple andesitic and rhyolitic end members, which in turn is consistent
with the eHD representing an amalgamation of numerous, compositionally
distinct magma reservoirs. This conclusion applies to enclaves sampled

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Suprasubduction zone ophiolite fragments in the central Appalachian
orogen: Evidence for mantle and Moho in the Baltimore Mafic Complex
(Maryland, USA)

George L. Guice; Michael R. Ackerson; Robert M. Holder; Freya R. George;
Joseph F. Browning-Hanson …

Suprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern
North America) have provided key constraints on the fundamental tectonic
processes responsible for the evolution of the Appalachian orogen. The
central and southern Appalachians, which extend from southern New York to
Alabama (USA), also contain numerous ultra- mafic-mafic bodies that have
been interpreted as ophiolite fragments; however, this interpretation is a
matter of debate, with the origin(s) of such occurrences also attributed to
layered intrusions. These disparate proposed origins, alongside the range
of possible magmatic affinities, have varied potential implications for the
magmatic and tectonic evolution of the central and southern Appalachian
orogen and its relationship with the northern Appalachian orogen. We
present the results of field observations, petrography, bulk-rock
geochemistry, and spinel mineral chemistry for ultramafic portions of the
Baltimore Mafic Complex, which refers to a series of ultramafic-mafic
bodies that are discontinuously exposed in Maryland and southern
Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex
comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite
displays geochemical characteristics–including highly depleted bulk-rock
trace element patterns and high Cr# of spinel–characteristic of
subduction-related mantle peridotites and serpentinites. The Hollofield
Ultramafite likely represents the “layered ultramafics” that form the Moho.
Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean-derived
SSZ ophiolite in the central Appalachian orogen raises the possibility that
a broadly coeval suite of ophiolites is preserved along thousands of
kilometers of orogenic strike.

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Detrital zircon petrochronology of central Australia, and implications
for the secular record of zircon trace element composition

Charles Verdel; Matthew J. Campbell; Charlotte M. Allen

Hafnium (Hf) isotope composition of zircon has been integrated with U-Pb
age to form a long-term (4 b.y.) record of the evolution of the crust.
In contrast, trace element compositions of zircon are most commonly
utilized in local- or regional-scale petrological studies, and the most
noteworthy applications of trace element studies of detrital zircon have
been in “fingerprinting” potential source lithologies. The extent to which
zircon trace element compositions varied globally over geological time
scales (as, for example, zircon U-Pb age abundance, O isotope composition,
and Hf isotope composition seem to have varied) has been little explored,
and it is a topic that is well suited to the large data sets produced by
detrital zircon studies. In this study we present new detrital zircon U-Pb
ages and trace element compositions from a continent-scale basin system in
Australia (the Centralian Superbasin) that bear directly on the Proterozoic
history of Australia and which may be applicable to broader interpretations
of plate-tectonic processes in other regions. U-Pb ages of detrital zircon
in the Centralian Superbasin are dominated by populations of ca. 1800,
1600, 1200, and 600 Ma, and secular variations of zircon Hf isotope ratios
are correlated with some trace element parameters between these major age
populations. In particular, elevated ?Hf(i) (i.e., radiogenic
“juvenile” Hf isotope composition) of detrital zircon in the Centralian
Superbasin tends to correspond with relatively high values of Yb/U, Ce
anomaly, and Lu/Nd (i.e., depletion of light rare earth elements). These
correlations seem to be fundamentally governed by three related factors:
elemental compatibility in the continental crust versus mantle, the
thickness of continental crust, and the contributions of sediment to
magmas. Similar trace element versus ?Hf(i) patterns among a
global zircon data set suggest broad applicability. One particularly
intriguing aspect of the global zircon data set is a late Neoproterozoic to
Cambrian period during which both zircon ?Hf(i) and Yb/U reached
minima, marking an era of anomalous zircon geochemistry that was related to
significant contributions from old continental crust.

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Along-strike variations in protothrust zone characteristics at the
Nankai Trough subduction margin

Hannah L. Tilley; Gregory F. Moore; Mikiya Yamashita; Shuichi Kodaira

Significant along-strike changes in the protothrust zone at the toe of the
Nankai Trough accretionary prism were imaged in new high-resolution seismic
reflection data. The width of the protothrust zone varies greatly along
strike; two spatially discrete segments have a wide protothrust zone
(?3.3-7.8 km, ?50-110 protothrusts), and two segments have almost no
protothrust zone (?0.5-2.8 km,
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The Permian Monos Formation: Stratigraphic and detrital zircon evidence
for Permian Cordilleran arc development along the southwestern margin
of Laurentia (northwestern Sonora, Mexico)

Stephen C. Dobbs; Nancy R. Riggs; Kathleen M. Marsaglia; Carlos M.
González-León; M. Robinson Cecil …

The southwestern margin of Laurentia transitioned from a left-lateral
transform margin to a convergent margin by middle Permian time, which
initiated the development of a subduction zone and subsequent Cordilleran
arc along western Laurentia. The displaced Caborca block was translated
several hundred kilometers from southern California, USA, to modern Sonora,
Mexico, beginning in Pennsylvanian time (ca. 305 Ma). The Monos Formation,
a ~600-m-thick assemblage of mixed bioclastic and volcaniclastic units
exposed in northwestern Sonora, provides lithostratigraphic, petrographic,
and geochronologic evidence for magmatic arc development associated with
subduction by middle Permian time (ca. 275 Ma). The Monos Formation was
deposited in a forearc basin adjacent to a magmatic arc forming along the
southwestern Laurentian margin. Detrital zircon U-Pb geochronology suggests
that Permian volcanic centers were the primary source for the Monos
Formation. These grains mixed with far-traveled zircons from both Laurentia
and Gondwana. Zircon age spectra in the Monos Formation are dominated by a
ca. 274 Ma population that makes up 65% of all analyzed grains. The
remaining 35% of grains range from 3.3 Ga to 0.3 Ma, similar to age spectra
from Permian strata deposited in the Paleozoic sequences in the western
continental interior. An abundance of Paleozoic through early
Neoproterozoic ages suggests that marginal Gondwanan sources from Mexico
and Central America also supplied material to the basin. The Monos
Formation was deposited within tropical to subtropical latitudes, yet
faunal assemblages are biosiliceous and heterotrophic. The lack of
photozoan assemblages suggests that cold-water coastal upwelling combined
with sedimentation from the Cordilleran arc and Laurentian continent
promoted conditions more suitable for fauna resilient to biogeochemically
stressed environments. We propose that transform faulting and displacement
of the Caborca block ceased by middle Permian time and a subduction zone
developed along the southwestern margin of Laurentia as early as early
Permian time. The Monos basin developed along the leading edge of the
continent as a magmatic arc developed, and facies indicate a consistent
shoaling trend over the span of deposition.

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Detrital sanidine 40Ar/39Ar dating confirms

Matthew T. Heizler; Karl E. Karlstrom; Micael Albonico; Richard Hereford;
L. Sue Beard …

Crooked Ridge and White Mesa in northeastern Arizona (southwestern United
States) preserve, as inverted topography, a 57-km-long abandoned alluvial
system near the present drainage divide between the Colorado, San Juan, and
Little Colorado Rivers. The pathway of this paleoriver, flowing southwest
toward eastern Grand Canyon, has led to provocative alternative models for
its potential importance in carving Grand Canyon. The ~50-m-thick White
Mesa alluvium is the only datable record of this paleoriver system. We
present new 40Ar/39Ar sanidine dating that confirms a
ca. 2 Ma maximum depositional age for White Mesa alluvium, supported by a
large mode (n = 42) of dates from 2.06 to 1.76 Ma. Older grain
modes show abundant 37-23 Ma grains mostly derived ultimately from the San
Juan Mountains, as is also documented by rare volcanic and basement pebbles
in the White Mesa alluvium. A tuff with an age of 1.07 ± 0.05 Ma is inset
below, and hence provides a younger age bracket for the White Mesa
alluvium. Newly dated remnant deposits on Black Mesa contain similar 37-23
Ma grains and exotic pebbles, plus a large mode (n = 71) of 9.052
± 0.003 Ma sanidine. These deposits could be part of the White Mesa
alluvium without any Pleistocene grains, but new detrital sanidine data
from the upper Bidahochi Formation near Ganado, Arizona, have similar
maximum depositional ages of 11.0-6.1 Ma and show similar 40-20 Ma San Juan
Mountains-derived sanidine. Thus, we tentatively interpret the
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Reconstructing drainage pathways in the North Atlantic during the
Triassic utilizing heavy minerals, mineral chemistry, and detrital
zircon geochronology

Steven D. Andrews; Andrew Morton; Audrey Decou; Dirk Frei

In this study, single-grain mineral geochemistry, detrital zircon
geochronology, and conventional heavy-mineral analysis are used to
elucidate sediment transport pathways that existed in the North Atlantic
region during the Triassic. The presence of lateral and axial drainage
systems is identified and their source regions are constrained. Axial
systems are suggested to have likely delivered sediment sourced in East
Greenland (Milne Land-Renland) as far south as the south Viking Graben
(800 km). Furthermore, the data highlight the existence of lateral
systems issuing from Western Norway and the Shetland Platform as well as a
major east-west-aligned drainage divide positioned adjacent to the Milne
Land- Renland region. This divide separated the catchments that flowed
north to the Boreal Ocean from those that flowed south into a series of
endoreic basins and, ultimately, the Tethys Sea. A further potential
drainage divide is identified to the west of Shetland. The data presented
and the conclusions reached have major implications for reservoir
prediction, as well as correlation, throughout the region. Furthermore,
understanding the drainage networks that existed during the Triassic can
help constrain paleogeographic reconstructions and provides an important
framework for the construction of facies models in the region.

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Subducting oceanic basement roughness impacts on upper-plate tectonic
structure and a backstop splay fault zone activated in the southern
Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

Anne Krabbenhoeft; Roland von Huene; John J. Miller; Dirk Klaeschen

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust
earthquake, the second largest during worldwide instrumental recording. The
coseismic slip and aftershock region offshore Kodiak Island was surveyed in
1977-1981 to understand the region’s tectonics. We re-processed
multichannel seismic (MCS) field data using current standard Kirchhoff
depth migration and/or MCS traveltime tomography. Additional surveys in
1994 added P-wave velocity structure from wide-angle seismic lines and
multibeam bathymetry. Published regional gravity, backscatter, and
earthquake compilations also became available at this time. Beneath the
trench, rough oceanic crust is covered by ~3-5-km-thick sediment. Sediment
on the subducting plate modulates the plate interface relief. The imbricate
thrust faults of the accreted prism have a complex P-wave velocity
structure. Landward, an accelerated increase in P-wave velocities is marked
by a backstop splay fault zone (BSFZ) that marks a transition from the
prism to the higher rigidity rock beneath the middle and upper slope.
Structures associated with this feature may indicate fluid flow. Farther
upslope, another fault extends 100 km along strike across the middle
slope. Erosion from subducting seamounts leaves embayments in the frontal
prism. Plate interface roughness varies along the subduction zone. Beneath
the lower and middle slope, 2.5D plate interface images show modest relief,
whereas the oceanic basement image is rougher. The 1964 earthquake slip
maximum coincides with the leading and/or landward flank of a subducting
seamount and the BSFZ. The BSFZ is a potentially active structure and
should be considered in tsunami hazard assessments.

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