Boulder, CO, USA – The March-April 2010 GSA BULLETIN is
online now "ahead of print." Topics span the globe, from the
Greater Caucasus Mountains separating Azerbaijan and Georgia from
Russia; to the Altyn Tagh fault zone, Bohai Bay Basin, Yangtze
craton, and Tian Shan of China; the collision zone between India
and the Himalaya; the Southern Uplands of Scotland; and on to the
western U.S., covering central Idaho, Mammoth Mountain and Long
Valley caldera, California, the King Lear Formation, Nevada, the
Grand Canyon, and the Fountain Formation of Colorado.
Late Cenozoic deformation of the Kura fold-thrust belt,
southern Greater Caucasus
A.M. Forte, Dept. of Geology, University of California, Davis,
California 95616, USA. Pages 465-486.
It has long been speculated that much of the current convergence
between the Arabian and Eurasian plates in the area between Turkey
and Iran is accommodated by the shortening of the crust within the
Greater Caucasus Mountains, which separate Azerbaijan and Georgia
from Russia to the north. Forte et al. used imagery and digital
elevation models from the ASTER satellite along with newly
developed software for pseudo-3-D visualization of this data to
investigate the potential that much of the convergence between
Arabia and Eurasia in Azerbaijan and Georgia has instead been
recently concentrated in a belt of topography at the southern
margin of the Greater Caucasus called the Kura fold-thrust belt.
Paring the remote sensing observations of bedding orientations with
previously published geologic maps for the area allowed them to
estimate that the Kura fold-thrust belt has absorbed 30%-40% of the
convergence between Arabia and Eurasia since five million years
ago. Their results suggest that the Kura fold-thrust belt is an
important and active structure within the Arabia-Eurasia collision
zone and has potential implications for seismic hazard
investigations in Azerbaijan and Georgia.
Age, geochemistry, and tectonic implications of a late
Paleozoic stitching pluton in the North Tian Shan suture zone,
western China
Bao-Fu Han et al., Ministry of Education, Key Laboratory of
Orogenic Belts and Crustal Evolution, School of Earth and Space
Sciences, Peking University, Beijing 100871, People's Republic of
China. Pages 627-640.
In Earth's history, disappearance of an ocean between two
continental blocks is thought to result in collision of the
continental blocks and some remnants of the oceanic crust may be
preserved in the collisional zone. Subsequently, the zone may be
crosscut by granitic intrusions. So the timing of disappearance of
the ocean and the collision of the continental blocks can be
constrained by the youngest remnant of oceanic crust and the oldest
granitic intrusion. Previous work revealed the youngest (325
million-year-old) remnant of the North Tian Shan Ocean between the
Yili and Junggar blocks, North Xinjiang, western China, and in this
study Han et al. report a zircon U-Pb age of 316 million years old
for the Sikeshu granitic intrusion which crosscuts the collisional
zone between the Yili and Junggar blocks and thus places a crucial
upper age bound for the time of disappearance of the North Tian
Shan Ocean and collision between the Yili and Junggar blocks. After
that time, North Xinjiang was in another stage of evolution.
The lower Lesser Himalayan sequence: A Paleoproterozoic arc
on the northern margin of the Indian plate
Matthew J. Kohn et al., Dept. of Geosciences, Boise State
University, Boise, Idaho 83725, USA. Pages 323-335.
The lower Lesser Himalayan Sequence figures heavily in
understanding the Indo-Asian collision and in plate reconstructions
of the about 1800-million-year-old supercontinent Columbia.
Previous studies indicated that lower Lesser Himalayan rocks
reflected deposition on a passive margin. Five lines of evidence
from Kohn et al., however, including textures, ages, chemistry, and
geographic distributions of outcrops, instead indicate these rocks
represent part of a volcanic arc. Lesser Himalayan ages are
distinct from those in continental India, so intrusions do not
represent Indian plate basement. These data help in reconstructing
India's position in the hypothesized supercontinent Columbia, and
in deciphering distributions of rocks prior to the Indo-Asian
collision.
SHRIMP U-Pb dating of recurrent Cryogenian and Late
Cambrian-Early Ordovician alkalic magmatism in central Idaho:
Implications for Rodinian rift tectonics
K. Lund et al., U.S. Geological Survey, MS 973, Denver Federal
Center, Box 25046, Denver, Colorado 80225, USA. Pages 430-453.
Composite alkalic plutonic suites and tuffaceous diamictite,
although discontinuously exposed across central Idaho in roof
pendants and inliers within the Idaho batholith and Challis
volcanic-plutonic complex, define the more than 200-km-long
northwest-aligned Big Creek-Beaverhead belt. Sensitive
high-resolution ion microprobe (SHRIMP) U-Pb zircon dates on these
igneous rocks provide direct evidence for the orientation and
location of the Neoproterozoic-Paleozoic western Laurentian rift
margin in the northern U.S. Cordillera. Dating delimits two
discrete magmatic pulses about 665-650 million years ago and
500-485 million years ago at the western and eastern ends,
respectively, of this belt. Together with the nearby 685 million
year old volcanic rocks of the Edwardsburg Formation, there is a
200 million year history of recurrent extensional magmatic pulses
along the belt. A similar history of recurrent uplift is reflected
in the stratigraphic record of the associated miogeoclinal and
cratonal platform basins, suggesting that the Big Creek-Beaverhead
belt originated as a border fault during continental rift events.
The magmatic belt is paired with the recurrently emergent Lemhi
Arch and narrow miogeoclinal facies belts and it lies inboard of a
northwest-striking narrow zone of thinned continental crust. These
features define a northeast-extending upper-plate extensional
system between southeast Washington and southeast Idaho that formed
a segment of the Neoproterozoic-Paleozoic miogeocline. This segment
was flanked on the north by the St. Mary-Moyie transform zone
(south of a narrow southern Canadian upper-plate margin) and on the
south by the Snake River transfer zone (north of a broad Great
Basin lower-plate margin). These are the central segments of a
zigzag-shaped Cordilleran rift system of alternating
northwest-striking extensional zones offset by northeast-striking
transfers and transforms. The data substantiate polyphase rift and
continental separation events that included (1) pre- and
syn-Windermere rifting, (2) Windermere margin subsidence, (3) late
Ediacaran-Cambrian rifting, and (4) well-developed late
Ediacaran-Devonian passive margin subsidence and deposition. Timing
and geometries support synchronous but opposing divergence along
Cordilleran and Atlantic rifts with a junction in Southern
California-Sonora.
New 40Ar/39Ar ages reveal contemporaneous mafic and silicic
eruptions during the past 160,000 years at Mammoth Mountain and
Long Valley caldera, California
Gail A. Mahood et al., Dept. of Geological and Environmental
Sciences, Building 320, 450 Serra Mall, Stanford University,
Stanford, California 94305-2115, USA;
. Pages 396-407.
Predictions of eruptions at volcanoes in the circum-Pacific
"Ring of Fire" have improved with the experience of the last
several decades at volcanoes such as Mount St. Helens, Pinatubo,
and, most recently, Redoubt, in Alaska. But what about the
eruptions a thousand times more voluminous from rhyolitic
"supervolcanoes," which spread thick ash continent-wide?
Fortunately for humans, "supervolcanoes" erupt infrequently, but as
a result we know less about the triggers and precursors to these
catastrophic events. Long Valley caldera (and adjacent Mammoth
Mountain, a popular ski area in east-central California) and
Yellowstone are the two "restless" calderas in the U.S.,
characterized by earthquake swarms, ground uplift, and gas
emissions indicating shallow intrusion of basaltic magma. A new
40Ar/39Ar study by Mahood et al. of young basaltic and rhyolitic
lavas shows that Mammoth Mountain and lavas in the northwest
quadrant of the Long Valley caldera are considerably younger than
previously thought (younger than or equal to 68,000 years old). As
a result of this high-precision dating they can identify four
eruptive sequences over the past 160,000 years; in each, basaltic
and rhyolitic lavas erupted contemporaneously from spatially
associated vents. This suggests that intrusion of basalt into the
shallow crust triggered eruptions of rhyolitic magma. If the
seismic unrest and deformation of the last three decades is a
result of basalt injected beneath Mammoth Mountain and the western
third of the Long Valley caldera, then there is the possibility of
spatially associated small-volume silicic eruptions, which would
typically be considerably more explosive. The new dating also
demonstrates that in the last 40,000 years, eruptions have occurred
along a N-S linear trend less than 10 km wide, limiting the zone
most subject to volcanic hazards.
The Lower Cretaceous King Lear Formation, northwest Nevada:
Implications for Mesozoic orogenesis in the western U.S.
Cordillera
Aaron J. Martin et al., Dept. of Geology, University of Maryland,
College Park, Maryland 20742, USA;
martinaj@geol.umd.edu. Pages 537-562.
The paper by Martin et al. presents new structural,
sedimentologic, and shallow seismic data that confirm that the
mostly clastic King Lear Formation was deposited in a half-graben,
not a thrust-bounded basin as previously interpreted. Their
interpretation has profound implications for understanding the
tectonic evolution of the western U.S. Cordillera during the
Jurassic and Cretaceous. First, it constrains shortening
deformation to have been completed prior to deposition of the King
Lear Formation at about 125 million years ago. This constraint is
important in the ongoing debate about whether the western U.S.
experienced continuous or punctuated shortening throughout the
Jurassic and Early Cretaceous. Second, they show that north-central
Nevada was probably a regional topographic high in the Early
Cretaceous, an earlier constraint than previously published. Both
new constraints are important for furthering our understanding of
the tectonic evolution of the western U.S. Cordillera during the
Mesozoic.
Structural and anisotropy of magnetic susceptibility (AMS)
evidence for oblique impact on terrestrial basalt flows: Lonar
crater, India
Saumitra Misra et al., Indian Institute of Geomagnetism, Navi
Mumbai 410 218, India. Pages 563-574.
Asteroid impact is a common geological process that shapes the
surfaces of rocky (or icy) planetary bodies in our Solar System.
The hypervelocity (faster than 11 km/s) impacts of asteroids create
some circular to elliptical depressions on the target planetary
bodies called impact craters. Most of the rocky planetary bodies in
our Solar System have basaltic crusts. Out of the two known
terrestrial impact craters that are excavated in basaltic target
rocks, the Lonar crater in India is fully accessible and possibly
the best studied crater. It can be taken as an example to evaluate
planetary impact cratering processes. The present radiometric ages
suggest that the Lonar crater was formed ~50 thousand years ago in
the undeformed and sub-horizontal Deccan Traps that erupted at ~65
million years ago. Two of the important aspects of crater research
are the nature and projectile path of the impactor. Misra et al.
have successfully applied a new technique - the Anisotropy of
Magnetic Susceptibility (AMS) along with satellite imagery and
structural geological studies on the small Lonar crater (~1.8 km
diameter) to evaluate the projectile path of its impactor. Their
studies of satellite images show that the rim of Lonar crater is
almost circular while the continuous ejecta around the crater rim,
estimated to be in its pristine shape, can be best enveloped by an
ellipse having an east-west major axis. The ejecta materials show
greater spread towards the west compared to other directions. When
compared with experimental results, the distribution of ejecta and
the shape of the crater rim suggest that the impactor asteroid hit
the pre-impact target from the east at an angle between 30
and 45 with the horizon. Their AMS data also suggest that
the target basalt about 2 km west of the crater is highly shocked
due to oblique impact from the east compared to the unshocked
target basalt from an equal distance in the east. Oblique impact
from the east also resulted in the vertical or sub-vertical
attitudes of target basalts along the western sector of the Lonar
crater, whereas in other sectors of crater rim they dip gently
outwards.
Numerical modeling of the late Cenozoic geomorphic evolution
of Grand Canyon, Arizona
Jon D. Pelletier, Dept. of Geosciences, University of Arizona,
Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona
85721-0077, USA. Pages 595-608.
Grand Canyon was carved by the Colorado River starting between 6
and 5 million years ago and continuing to today. In this paper,
Pelletier describes a numerical model that quantifies how quickly
erosion of the canyon took place and how downward erosion, lateral
erosion (cliff retreat) and the response of the underlying crust
has occurred over time.
Imbricated ocean-plate stratigraphy and U-Pb zircon ages from
tuff beds in cherts in the Ballantrae complex, SW
Scotland
Yusuke Sawaki et al., Dept. of Earth and Planetary Science, Tokyo
Institute of Technology, O-okayama 2-12-1, Meguro, Tokyo 152-8551,
Japan. Pages 454-464.
Ocean plate stratigraphy (OPS) is the fundamental, first-order
structure of accretionary orogens that are forming today on the
Pacific margins. It consists of cherts that were deposited first
under deep-sea pelagic conditions at or near a mid-oceanic ridge,
later during transport of the ocean floor towards a subduction
zone, and finally the cherts were overlain by clastic infill in the
trench. Movement of oceanic lithosphere, which provides key
information for reconstruction of paleogeography, is recorded in
active margins, and particularly at a subduction zone. One of the
active margins of the Iapetus Ocean was the Southern Uplands of
Scotland that contains a Lower Palaeozoic accretionary prism.
Structures in mélanges and décollement zones in the
Ordovician (northern) belt of the Southern Uplands accretionary
prism resulted from southeast-directed thrusting. The Ballantrae
Complex is located on the north of the Southern Uplands Fault. The
Ballantrae Complex was considered as one of the Ordovician
ophiolites and contains imbricated cherts at Bennane Head in the
west of the Ballantrae Complex. However, in spite of numerous
studies, the Ballantrae Complex has never been interpreted in the
light of a modern understanding of comparative accretionary prism.
Ancient plate motions are usually estimated from geological,
geochronological and paleomagnetic data. Sawaki et al. present
detailed structural relations, new U-Pb ages of zircons from tuffs
interbedded in cherts to the south of Bennane Head, and a
discussion on the movement of the Iapetus oceanic lithosphere in
the Palaeozoic. Ocean plate stratigraphy (basalt, chert, clastics)
of the Balcreuchan Group in the Ballantrae Complex is repeated by
layer-parallel thrusts to form duplex structures south of Bennane
Head. Although mutually distant, five tuff beds, all in chert, have
similar U-Pb zircon ages of about 470 million years ago. The
geometrical polarity of the duplexes and the zircon ages provide
new constraints on the tectonic evolution of the accretionary wedge
of the Ballantrae Complex. Northward thrusting of the duplexes
suggests that subduction was from the northwest to the southeast, a
polarity that is consistent with the northward younging of the
volcanic arcs of the Ballantrae Complex.
Rates and mechanisms of Mesoarchean magmatic arc
construction, eastern Kaapvaal craton, Swaziland
Blair Schoene and Samuel A. Bowring, Dept. of Earth, Atmospheric,
and Planetary Sciences, Massachusetts Institute of Technology, 77
Massachusetts Ave., Cambridge, Massachusetts 02139, USA. Pages
408-429.
Within the oldest core of the Kaapvaal craton (about 3.7-3.1
billion years old) in eastern South Africa and Swaziland, evidence
is preserved for the construction and amalgamation of one of
Earth's oldest continents. Remarkably, some of the sedimentary,
volcanic and plutonic rocks from this region remain undeformed or
metamorphosed, and thus provide unique insight into processes
occurring in the early Earth. Schoene and Bowring provide insight
into the time scales of magmatic intrusion with unprecedented
temporal precision, using new methods of U-Pb zircon dating, and
link this with field mapping and geochemistry to characterize an
episode of Mesoarchean magmatism. Integrating this period of
magmatism and deformation into existing tectonic models for the
region, these new data support the hypothesis that the eastern
Kaapvaal craton formed through subduction and accretion of young
continental fragments by plate tectonic processes similar to those
found today.
Analysis of the Wallowa-Baker terrane boundary: Implications
for tectonic accretion in the Blue Mountains province, northeastern
Oregon
Joshua J. Schwartz et al., Dept. of Geology and Geophysics,
University of Wyoming, Laramie, Wyoming 82071, USA. Pages
517-536.
Schwartz et al. explore the boundary between the Wallowa island
arc terrane and the Baker melange terrane in northeastern Oregon.
This boundary is significant because it records an episode of
intense faulting and folding, which they suggest is related to the
collision of two island arc terranes (Wallowa and Olds Ferry) in
the Middle to Late Jurassic. They propose that this terrane
boundary is an example of a broad zone of imbrication of arc crust
tectonically mixed within an accretionary complex. It may provide
an on-land, ancient analogue to the actualistic arc-arc collisional
zone developed along the margins of the Molucca Sea of the central
equatorial Indo-Pacific region.
Late Paleozoic tectonics and paleogeography of the ancestral
Front Range: Structural, stratigraphic, and sedimentologic evidence
from the Fountain Formation (Manitou Springs, Colorado)
Dustin E. Sweet and Gerilyn S. Soreghan, School of Geology and
Geophysics, University of Oklahoma, 100 E. Boyd Street, Suite 810,
Norman, Oklahoma 73019, USA. Pages 575-594.
Approximately 300 million years ago, during the Late Paleozoic,
Colorado was mountainous. But these so-called "ancestral Rocky
Mountains" were very different from the modern Rockies. For
example, the tectonic forces that created the ancestral Rocky
Mountains are puzzling because the mountains formed in the interior
of the continent, rather than along a plate boundary, and trended
at a high-angle to the nearest such boundary, the well-known
Ouachita-Marathon belt that forms the southwestern continuation of
the Appalachian chain. New data documented by Sweet and Soreghan
show that the ancestral Rockies were marked by sporadic uplift and
a final phase of subsidence. The ancestral Rocky Mountains are
recognized not by topography, because it no longer exists, but
rather by the presence of large piles of sediments that were shed
off the ancestral Rockies and accumulated adjacent to the
highlands. The Fountain Formation exposed along the modern Front
Range of Colorado represents sediments shed off the ancestral Front
Range during the late Paleozoic, and preserve a record of this
ancient mountain-building event. Sweet and Soreghan document three
discrete phases of sedimentation in the Fountain Formation that
record (1) initiation of mountain uplift, (2) reinvigoration of
uplift and (3) cessation of uplift accompanied by an enigmatic
post-tectonic subsidence phase. Comparison of the timing for the
formation of the ancestral Rocky Mountains as recorded in this
study to other regions with similar constraints indicates that
uplift of the ancestral Rockies ceased earlier in eastern Colorado
than western Colorado and New Mexico. This pattern is consistent
with stresses that were imparted onto the continent from the
continent-to-continent collision along the south central margin of
the United States. However, the tenuous age controls on the
sediments also allow for a relatively synchronous end to uplift,
which would require geologists to rethink tectonic models for the
formation of the ancestral Rocky Mountains.
U-Pb (SHRIMP) and 40Ar/39Ar geochronological constraints on
the evolution of the Xingxingxia shear zone, NW China: A Triassic
segment of the Altyn Tagh fault system
The development of structures and their age along the segment of
the Altyn Tagh fault system, and the eastward extension of the
Tianshan orogenic belt, remain speculative. Recent investigations
by Wang et al. on the structural framework, granitic intrusions,
and metamorphic rocks in the eastern Tianshan and adjacent areas
show that the northeast-striking Xingxingxia sinistral ductile
shear zone, northwest China, is sub-parallel to the Altyn Tagh
fault zone and is superposed on the eastern Tianshan orogenic belt.
U-Pb zircon SHRIMP dating, and muscovite, biotite and K-feldspar
40Ar/39Ar thermochronology indicate that sinistral shear along the
Xingxingxia shear zone initiated at about 240-235 million years
ago, broadly at the same time as initial formation of the Altyn
Tagh fault zone, but later than initiation of dextral strike-slip
motion along the ~east-west-trending eastern Tianshan orogenic belt
at about 270-245 million years ago. Formation of the Xingxingxia
ductile shear zone was associated with Gondwanaland convergence
along the southern margin of the Eurasian continent during the late
Permian-early Triassic.
The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay
Basin, China: Constraints from volcanic records and
tectono-stratigraphic sequences
Yuexia Dong et al., PetroChina Jidong Oilfield Company, Tangshan
063004, China). Pages 609-626.
The Bohai Bay Basin, located on the eastern Asian margin, is the
second largest oil production basin in China. It contains numerous
depressions and sags among which the Nanpu Sag has become
particularly important because of significant oil discoveries in
recent years. Geologically and tectonically, however, the rifting
mechanism and geodynamic evolution of the Tertiary Basin remain
uncertain. Based on detailed volcanic and stratigraphic records,
Dong et al. have identified five tectono-stratigraphic sequences
produced by episodic continental rifting, and four basin dynamic
evolutionary phases. A diapiric upper mantle upwelling model is
proposed to explain the dynamics that controlled the multiple
rifting processes, the cyclic volcanism, and the periodic tectonic
evolution of the Sag and the greater Bohai Bay Basin.
Geologic correlation of the Himalayan orogen and Indian
craton: Part 1. Structural geology, U-Pb zircon geochronology, and
tectonic evolution of the Shillong Plateau and its neighboring
regions in NE India
An Yin et al., Dept. of Earth and Space Sciences and Institute of
Geophysics and Planetary Physics, University of California, Los
Angeles, California 90095, USA. Pages 336-359.
Despite being the highest mountain range in the world, the
geologic origin of rocks that make up the Himalayan Range is not
clear. Yin et al. present new results on the ages and types of
rocks that constitute the northern Indian subcontinent. This work
suggests that the Himalayan rocks are most likely composed of
Indian rocks rather than Tibetan rocks from the north as some
researchers have suggested.
Geologic correlation of the Himalayan orogen and Indian
craton: Part 2. Structural geology, geochronology and tectonic
evolution of the Eastern Himalaya
An Yin et al., Dept. of Earth and Space Sciences and Institute of
Geophysics and Planetary Physics, University of California, Los
Angeles, California 90095-1567, USA. Pages 360-395.
The classic view for the development of the Himalayan Range is
that the India-Asia collision has caused the uppermost layer of
sedimentary rocks overlying the Indian continent to be piled up by
faults. However, Yin et al. suggest that the mountain range was
constructed by faults that cut deep into the basement of the Indian
continent and brought the deeply buried rocks to the surface. This
new discovery fundamentally changes the way we think about the
forces that created the most active mountain range in the
world.
Evolution of the Hongzhen metamorphic core complex: Evidence
for Early Cretaceous extension in the eastern Yangtze craton,
eastern China
Guang Zhu et al., School of Resource and Environmental Engineering,
Hefei University of Technology, Hefei 230009, China. Pages
506-516.
The Hongzhen metamorphic Core Complex in the eastern Yangtze
craton, eastern China was first recognized through structural
studies. Brittle normal faulting and basin rifting in the hanging
wall were developed during the formation of the Core Complex. The
exposed Paleoproterozoic Dongling Complex in the footwall is widely
overprinted by a detachment ductile shear zone, which has
consistent southwest-plunging mineral elongation lineations and
top-to-southwest shear indicators. Structural analysis indicates
that they formed within a low-angle, southwest-dipping, extensional
shear zone at mid-crustal levels. Four muscovite grains separated
from mylonites within the shear zone yielded 40Ar/39Ar plateau ages
ranging from 128.5-126.1 million years ago. They are interpreted as
cooling ages of the shear zone associated with the Core Complex. It
is proposed that the Core Complex was initiated as a mid-crustal,
low-angle extensional shear zone with top-to-southwest shear sense
at about 145 million years ago, and the shear zone was then warped
and uplifted by the emplacement of the Hongzhen Granite at 122
million years ago. Zhu et al. have demonstrated that the eastern
Yangtze Craton was also involved in the Early Cretaceous extension
widely occurring in the eastern China continent. A
northeast-southwest extensional direction during the Early
Cretaceous is indicated by the Core Complex.
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