Boulder, CO, USA - Digging dinosaurs, burrowing worms, weakened
rocks, immobilized uranium, slowstanding sea levels, isolated
nunataks, colliding sediments, retreating ice sheets, westerly
winds, electrically conductive lithosphere, dynamic debris flows,
reactivated landslides, rock incompetence, recrystallized quartz,
and biased sedimentation are some of the topics presented in the
August issue of GEOLOGY. August GSA TODAY science presents new data
and digital elevation models acquired during the February 2000
Shuttle Radar Topography Mission aboard the Space Shuttle
Endeavour.
Prehistoric earthquakes on the Caribbean-South American plate
boundary, Central Range fault, Trinidad
Carol S. Prentice et al., U.S. Geological Survey, 345 Middlefield
Road, MS 977, Menlo Park, California 94025, USA. Pages 675-678.
Carol Prentice of the U.S. Geological Survey and colleagues
describe the first geologic investigation of the Central Range
fault that shows this is an active strike-slip fault capable of
producing large earthquakes on the island of Trinidad. The Central
Range fault is a major part of the Caribbean-South American plate
boundary. Prentice et al. show that the most recent earthquake on
the Central Range fault occurred between 2710 and 550 years before
the present, indicating that the Central Range fault is a major
seismic hazard in Trinidad.
Investigation of the strength contrast at the Moho: A case study
from the Oman Ophiolite
Janelle M. Homburg et al., Dept. of Earth and Environmental
Sciences, Columbia University, Lamont Doherty Earth Observatory,
Palisades, New York 10964, USA. Pages 679-682.
The strength contrast at the Moho (lower crust to upper mantle
transition) controls such processes as the depth of earthquakes,
interactions between the crust and the mantle, and the deformation
and motion of the tectonic plates. While previous work has
concluded that the lower crust is weaker than the mantle at the
Moho, recent work, based on the location of deep earthquakes in
Tibet and other areas around the world, has suggested the lower
crust may be stronger than the mantle. In this paper, scientists
from the Lamont Doherty Earth Observatory and Brown University show
compelling geological evidence that crustal rocks are significantly
weaker than mantle rocks at Moho conditions, using outcrop scale
and microstructural observations. Their observations are unique
because they studied lower crustal rocks and upper mantle rocks
that deformed together at Moho conditions. Their results are in
good agreement with predictions based on extrapolation from
laboratory observations. Homburg et al. conclude that the lower
crust is weaker than the mantle wherever the crustal composition is
similar to the rocks they studied. This study was funded in part by
the U.S. National Science Foundation.
Potassium and uranium in the upper mantle controlled by Archean
oceanic crust recycling
Sune G. Nielsen, Dept. of Earth Science, University of Oxford,
Parks Road, OX1 3PR Oxford, UK. Pages 683-686.
Processes that occurred in the early stages of Earth's history
are difficult to study because few clues remain that bear evidence
of those times. Geoscientist Sune Nielsen of the UK's University of
Oxford shows that the ratio observed today between the elements
potassium (K) and uranium (U) in Earth's mantle was controlled by
the environmental conditions at the surface during the first 2000
million years of Earth's history. Presently, the mantle displays
K/U far higher than the bulk Earth value, but no modern-day
mechanism can account for this disparity. The atmosphere contained
little or no oxygen in the early Earth and this would have rendered
uranium immobile during weathering, as it is only soluble in the
presence of oxygen. Conversely, potassium was delivered amply to
the oceans because its solubility does not depend on surface
oxygenation. Consequently, the ocean crust would have inherited a
high K/U through hydrothermal alteration and sedimentation. This
material was then mixed back into the mantle through subduction at
convergent plate margins, and thus slowly forced K/U toward the
high value observed today. This process was halted when the
atmosphere became oxygenated about 2400 million years ago, and thus
K/U in the mantle records a snapshot of early Earth weathering.
Quantum magmatism: Magmatic compositional gaps generated by
melt-crystal dynamics
Josef Dufek and Olivier Bachmann, School of Earth and Atmospheric
Sciences, Georgia Institute of Technology, 311 Ferst Drive,
Atlanta, Georgia 30332, USA. Pages 687-690.
For more than a century, geologists have observed gaps in the
chemical composition of many erupted magmas from worldwide volcanic
centers. The clustering of these compositions, rather than a
continuous distribution of compositions in the eruptive record,
influences the mechanical and thermal properties of the crust, and
holds clues about how our planet differentiates. Geoscientists
Dufek and Bachmann of the Georgia Institute of Technology have
explored the potential of magma dynamics to generate compositional
gaps using analytical calculations and numerical simulations
coupling crystallization kinetics and multiphase fluid dynamics of
magma reservoirs. They show that gaps are inherent to crystal
fractionation for all compositions, as crystal-liquid separation
takes place most efficiently within a crystallinity window around
50-70 vol% crystals. The probability of melt extraction from a
crystal residue in a cooling magma chamber is highest in this
crystallinity window due to (1) enhanced melt segregation in
absence of chamber-wide convection, (2) buffering by latent heat of
crystallization, and (3) diminished chamber-wall thermal gradients.
This mechanical control of igneous distillation is likely to have
played a dominant role in the formation of the compositionally
layered Earth's crust by allowing multiple and overlapping
intrusive episodes of relatively discrete or quantized composition
that become more silicic upward. This study was funded in part by
the U.S. National Science Foundation.
Mount Etna-Iblean volcanism caused by rollback-induced upper
mantle upwelling around the Ionian slab edge: An alternative to the
plume model
W.P. Schellart, School of Geosciences, Monash University,
Melbourne, VIC 3800, Australia. Pages 691-694.
Mount Etna is the largest volcano in Europe, and its origin has
long been a mystery. Most volcanism on Earth occurs at plate
boundaries, in places where tectonic plates move apart (e.g.,
Iceland) and in places where tectonic plates come together with one
plate diving (subducting) below the other plate into the mantle
(e.g., Pacific "ring of fire"). Some volcanism (named intraplate
volcanism) occurs far from plate boundaries, and its origin is more
controversial. The chemistry of the volcanic rocks from Mount Etna
and the nearby Iblean volcanics in Sicily indicate that they are
intraplate volcanics. Interestingly, the volcanoes are located
close to, but are laterally offset from, the Calabrian subduction
zone plate boundary, where the African plate sinks below the
European plate. This suggests that the volcanics are somehow
related to the Calabrian subduction zone. New modeling of
subduction and mantle flow by W.P. Schellart of Monash University
confirms this, showing that backward sinking of the African plate
at the Calabrian subduction zone induced flow around the southern
edge of the subducted plate and upward below Sicily. The upward
flow induced decompression melting of upper mantle material and
these melts extruded at the surface in Sicily, forming Mount Etna
and the Iblean volcanics.
Drowned coastal deposits with associated archaeological remains
from a sea-level "slowstand": Northwestern Gulf of Maine, USA
Joseph T. Kelley et al., Dept. of Earth Sciences, University of
Maine, Orono, Maine 04469-5790, USA. Pages 695-698.
The rise of sea level since the end of the last Ice Age has
drowned the continental shelves of the world and, along with them,
innumerable archeological sites. In formerly glaciated areas like
the Gulf of Maine, sea-level change has involved more than a simple
rise in water level. Owing to the weight of the ice, the landscape
was initially depressed and flooded as the ice receded. Following
melting of the ice, the land rose back up and exposed extensive
areas of the Gulf of Maine. After the land stabilized, sea level
continued to rise, but at an uneven rate. Between 11,500 and 7,500
years ago, sea level barely rose off the Maine coast, a time this
team from the University of Maine and the University of New
Hampshire have termed the "slowstand." During this time, glacial
deposits were eroded and formed beaches that sheltered lakes and
estuaries. Off Bass Harbor, Maine, scallop draggers recovered stone
tools from this time period on the shores of what their bathymetric
imagery, seismic reflection profiles, and cores suggest was a beach
associated with freshwater wetlands and tidal flats. The time of
the slowstand and the associated depth range, 15-25 m, represent
the most probable settings from which submerged archeological
remains will be recovered in the Gulf of Maine.
Predatory digging behavior by dinosaurs
Edward L. Simpson et al., Department of Physical Sciences, Kutztown
University of Pennsylvania, 424 Boehm, Kutztown, Pennsylvania
19530, USA. Pages 699-702.
This paper by Edward Simpson of Kutztown University of
Pennsylvania and colleagues provides evidence for a dinosaur
hunting strategy for securing fossorial mammals. The predatory
behavior of dinosaurs has commonly been identified through specific
adaptations, jaws, teeth and post-cranial elements, taphonomic
associations, and trace fossil evidence, including bite marks, gut
contents, coprolites, and trackways. Minimal direct evidence exists
in the rock record of dinosaurs and mammals behaving as predators
and prey, respectively. However, a newly discovered Late Cretaceous
trace fossil association of digging traces of maniraptoran theropod
dinosaurs and mammalian den complexes indicates a predator-prey
relationship. Three distinct associated trace fossils occur within
a floodplain siltstone-mudstone bed of the Upper Cretaceous Wahweap
Formation in southern Utah, United States. Simpson et al. show that
one trace records digging by a maniraptoran theropod dinosaur,
possibly a dromeosaurid or troodontid. The other two are
interpreted as mammalian den complexes. The fact that these traces
are so close together suggests that dinosaurs used excavation
techniques to prey on mammals.
Pleistocene dynamics of the interior East Antarctic ice
sheet
Kat Lilly et al., Dept. of Geology, University of Otago, PO Box 56,
Dunedin, New Zealand. Pages 703-706.
Kat Lilly of the University of Otago and colleagues from New
Zealand, Australia, and the UK measure how long nunataks (islands
within a glacier or ice sheet) have been exposed above the ice in
the remote interior of the East Antarctic ice sheet. Their results
provide an understanding of the long-term stability of the ice
sheet over the past 4 million years. The study turns up extremely
old exposure ages, indicating both that erosion rates in the area
are very low and that this part of the East Antarctic ice sheet has
been remarkably stable over the past few million years. Modeling by
Lilly et al. also shows that the ice-sheet surface elevation has
oscillated with the global ice age cycles but has also very slowly
lowered at a rate of about 50 meters per million years.
Source-side shear wave splitting and upper mantle flow in the
Chile Ridge subduction region
R.M. Russo et al., Dept. of Geological Sciences, P.O. Box 112120,
241 Williamson Hall, University of Florida, Gainesville, Florida
32611, USA. Pages 707-710.
Subduction of active spreading ridges is a geodynamic oddity: if
ridges occur at mantle upwellings, then they should not coincide
with regions of downwelling or subduction. Ray Russo of the
University of Florida and colleagues show that, in the region where
the Chile Ridge subducts beneath South America (46.5S), the
upper mantle flows through a gap - a slab window - formed between
the trailing edge of the subducting Nazca Plate and the leading
edge of the subducting Antarctic Plate. The upper mantle flows
horizontally southward, parallel to the Nazca trench, north of the
slab window, but turns eastward into the window where the gap
between the Nazca and Antarctic slabs exist. This upper mantle flow
has been implicated in the unusual pattern of Late Cenozoic
tectonics and volcanism in overriding South America. This study was
funded in part by the U.S. National Science Foundation.
Priapulid worms: Pioneer horizontal burrowers at the
Precambrian-Cambrian boundary
Jean Vannier et al., UMR 5125 PEPS, Universite de Lyon, Universite
Lyon 1, Bat. Geode, 2 rue Raphael Dubois, F-69622 Villeurbanne
Cedex, France. Pages 711-714.
The Precambrian-Cambrian transition (about 550 million years
ago) represents a key period in the evolution of life on Earth,
during which the direct ancestors of most present-day animal phyla
appear in the fossil record. These anatomical novelties generated
an unprecedented behavioral complexity that allowed animals to
colonize new horizons of the marine ecospace. However, major
uncertainties remain concerning the actors, the chronology, and the
amplitude of this colonization. Jean Vannier and a team of
geoscientists from France, Germany, and Poland take an innovative
approach that combines fossil data and laboratory experiments with
recent worms from Sweden that reveals that priapulid worms were of
particular importance in the early colonization of marine sediment.
Priapulid worms are known to be one of the earliest pioneer
burrowers of the sea floor. Their horizontal burrow systems are
preserved in the Cambrian rocks of all continents. These worms were
carnivorous, preying upon a variety of small animals living at the
water-sediment interface. This is attested by exquisitely
well-preserved gut contents from the Burgess Shale Lagerstatte.
Priapulids survived all major extinctions and are still living in
present-day environments, showing a remarkable ability to tolerate
very low oxygen levels.
Equivalence of abrupt grain-size transitions in alluvial rivers
and eolian sand seas: A hypothesis
Douglas J. Jerolmack and Theodore A. Brzinski III, Dept. of Earth
and Environmental Science, University of Pennsylvania, 240 S. 33rd
Street, Philadelphia, Pennsylvania 19104-6316, USA. Pages
719-722.
Rivers and deserts are not so different, according to Douglas
Jerolmack and Theodore Brzinski of the University of Pennsylvania.
Rivers often undergo a rapid transition from gravel to sand
substrate, with a notable absence of intermediate grain sizes.
Desert sand seas show a similar pattern, in which dune fields
composed of sand are fringed by deposits of silt-sized particles
called loess. Jerolmack and Brzinski hypothesize that the collision
of sediments during transport produces a distinct population of
smaller particles by chipping and rounding of corners in both
systems. They calculate the collision dynamics of gravel in rivers
and sand in deserts, and show that they are equivalent. Abrasion
occurs for the fraction of grains that travel in "saltation,"
essentially bouncing along the sediment surface. Chips broken off
by abrasion are small enough to be suspended in the current and
moved long distances. As grains wear down from saltation, their
collision energy decreases until abrasion becomes ineffective. The
lack of intermediate grain sizes - between the smallest abraded
particles and their products - generates an abrupt transition when
sediment is sorted by water or wind.
Large-scale glaciation and deglaciation of Antarctica during the
Late Eocene
Shanan E. Peters et al., Dept. of Geoscience, University of
Wisconsin, Madison, Wisconsin 53706, USA. Pages 723-726.
A coastal, incised river valley complex in the Western Desert of
Egypt documents the timing and magnitude of an ~40-m eustatic
sea-level fall and rise that was driven by the transient growth and
retreat of a large Antarctic ice sheet prior to substantial global
cooling at the Eocene-Oligocene boundary. The sea-level fall and
rise is coincident in time with an oscillation in atmospheric CO2
concentrations of about 750 parts per million. Because many of the
carbon emission scenarios for the coming century predict that
atmospheric CO2 will cross this same 750 parts per million
threshold, results from this study by Shanan Peters and colleagues
from the University of Wisconsin the University of Michigan raise
the possibility that global climate could transition to a state not
unlike the late Eocene, when a large, permanent Antarctic ice sheet
was not sustainable. This study was funded in part by the U.S.
National Science Foundation.
Covariability of the Southern Westerlies and atmospheric CO2
during the Holocene
P.I. Moreno et al., Dept. of Ecological Sciences and Institute of
Ecology and Biodiversity, University of Chile, Santiago, Chile.
Pages 727-730.
Ice core records of atmospheric CO2 variations feature a steady
deglacial rise that reached a maximum value about 11 thousand years
ago, a conspicuous reversal between 11 and 8 thousand years ago,
and a steady multi-millennial increase since then. Although several
attempts to explain these aspects have been proposed, no single
mechanism has been able to account for the timing and structure of
natural atmospheric CO2 changes during the Holocene. Recent studies
have proposed that changes in the latitudinal position and strength
of the Southern Hemisphere Westerly Winds (SWW) can greatly
influence large-scale ocean circulation and degassing of the deep
ocean via changes in wind-driven upwelling in the Southern Ocean
(SO); however, very few paleoclimate records allow testing of this
hypothesis. Based on terrestrial ecosystem proxies from western
Patagonia, P.I. Moreno of the University of Chile and colleagues
reconstruct variations in the intensity of zonal flow over the past
14,000 years. These variations correspond to the timing and
structure of atmospheric CO2 changes, and are consistent with the
modeled magnitude of CO2 changes induced by varying strengths of
the SWW. The close match between data and models supports the view
that the SWW-SO system underpins multi-millennial CO2 variations
during the current interglacial and, possibly, during glacial
cycles.
Deep deformation pattern from electrical anisotropy in an arched
orogen (Betic Cordillera, western Mediterranean)
Ana Ruiz-Constan et al., Departamento de Geodinamica, Universidad
de Granada, Granada, Spain. Pages 731-734.
Our knowledge of lithospheric mantle deformation is poor, due to
the lack of accurate deep geophysical techniques and the scarcity
of peridotite outcrops. The magnetotelluric (MT) method allows for
characterization of the electrical conductivity of the lithosphere,
proving a sensitive tool for determining its structure. The Betic
Cordillera makes up the western end of the Alpine orogen in Europe
and was formed in a general setting of Eurasian-African plate
convergence. Ana Ruiz-Constan and colleagues from Spain, Mexico,
and Portugal present MT data acquired in the Betic Cordillera and
its foreland that provides the first evidence of electrical
anisotropy in the upper mantle of the Mediterranean region. These
data reveal preferred structure orientation related to olivine
elongation in the mantle and an increase in the amount of
deformation toward the axis of the plate boundary. At deep levels,
all the sites show a common north-south geoelectrical strike, which
may represent a low-intensity deformation, possibly related to
"frozen" pre-alpine plate tectonics. For lower crust levels, a
north-south constant strike at the Betic Cordillera sites contrasts
with the east-west strike in the Iberian Massif. The integration of
MT data with previous seismic studies allows discussion of the
recent evolution of the western Mediterranean, which is a matter of
great controversy among earth-science researchers.
Evolution of a natural debris flow: In situ measurements of flow
dynamics, video imagery, and terrestrial laser scanning
Scott W. McCoy et al., CIRES and Dept. of Geological Sciences,
University of Colorado, Boulder, Colorado 80309, USA. Pages
735-738.
Debris flows are rapidly moving mixtures of soil, rock, and
water that pose immediate hazards to down-slope communities and
infrastructure. This team from CIRES, the USGS, and East Carolina
University uses a novel combination of in situ measurements of
debris-flow dynamics, video imagery, and high-resolution laser
scanning to capture a natural debris-flow event at Chalk Cliffs in
central Colorado (United States). The monitored debris flow started
from clear-water flow and rapidly entrained sediment to form
multiple, hazardous boulder-rich fronts. These new data confirm the
importance of pore-fluid pressure and the non-uniform distribution
of flow resistance in controlling hazardous aspects of debris flows
such as travel distance, flow depth, and peak discharge. The
researchers emphasize the fact that dynamic flow properties, such
as pore-fluid pressure, are of equal or greater importance than
channel topographic properties in determining where a particular
debris flow will stop. Though this fact makes hazard assessments
more difficult, incorporation of these additional controls on flow
behavior should increase the accuracy of predictive models. This
study was funded in part by the U.S. National Science
Foundation.
Platy layer silicate minerals for controlling residual strength
in landslide soils of different origins and geology
Shinya Nakamura et al., University of the Ryukyus, Senbaru 1,
Nishihara-cho, Okinawa 903-0213, Japan. Pages 743-746.
"Residual" strength is the final strength that develops on
particle-reoriented surfaces of soil and rock after large
displacements. Therefore, the residual strength is extremely
important for understanding the stability for reactivated
landslides. Shinya Nakamura of the University of the Ryukyus and
colleagues describe the relationship between the residual strength
and clay mineralogy of landslide soils to clarify the role of
lamellar to platy clay minerals in developing the residual
condition of a slip surface and to provide an estimate of the
residual strength from the mineralogical soil composition. They
examine the residual strengths and mineralogical compositions of
soils collected from different landslides and showed that the total
content of smectite, vermiculite, chlorite, and mica in the soil
particle size fraction smaller than 425 micrometers is a
controlling factor of the residual strength. These minerals are
2:1-type layer silicate minerals, which are lamellar to platy in
shape and apt to undergo particle reorientation after
large-displacement shear, leading to a decrease in shear strength
of the residual state. Nakamura et al. believe that the total
content of smectite, vermiculite, chlorite, and mica in the
particle size fraction smaller than 425 micrometers is a suitable
mineralogical parameter for estimating the magnitude of the
residual strength. It is interesting that minute particles such as
clay minerals can be one of the determining factors for the
occurrence of landslides.
Paraná flood basalts: Rapid extrusion hypothesis
confirmed by new 40Ar/39Ar results
David S. Thiede and Paulo M. Vasconcelos, School of Earth Sciences,
The University of Queensland, St Lucia, Queensland 4072, Australia.
Pages 747-750.
The relationship between continental flood basalts (CFB) and
mass extinctions has been a subject of vigorous debate.
Geochronology for four of the largest CFB provinces show both rapid
extrusion rates and temporal overlaps between volcanism and major
mass extinction events, leading to suggestions of a causal
relationship between CFB volcanism and mass extinctions. However,
the comparably sized Paraná CFB in South America appears to
be an exception, as no major extinction event overlaps the proposed
time of volcanism. The age and duration of Paraná volcanism
has been in dispute due to conflicting sets of 40Ar/39Ar
geochronology data. One set of results indicates a short, rapid
eruption similar to other CFBs linked to mass extinction events,
and another set of results supports a longer, slower eruption
sequence that would be consistent with a lack of associated
extinction event. David Thiede and Paulo Vasconcelos of the
University of Queensland resolve the age and duration controversy
of the Paraná CFB by reanalyzing the exact samples that
previously gave the oldest and youngest ages in the protracted
eruption results. Their new ages indicate a short eruption event. A
rapid extrusion for the Paraná CFB and the lack of a major
co-temporal extinction event challenge proposed direct links
between CFB volcanism and mass extinctions.
Mélange rheology and seismic style
Ake Fagereng and Richard H. Sibson, Dept. of Geology, University of
Otago, P.O. Box 56, Dunedin 9054, New Zealand. Pages 751-754.
Deformation of Earth's crust is commonly accommodated by shear
displacement on faults. This deformation occurs in a range of
styles, including fast earthquake ruptures and slow aseismic slip.
There is, however, much uncertainty in understanding which factors
determine whether a fault will slip seismically or aseismically.
Simple fault models treat faults as discrete, planar interfaces.
Ake Fagereng and Richard Sibson of the University of Otago review
observations in fault zones at a range of depths, in light of
observations in faults exposed on Earth's surface, and suggest that
some faults are rather thick layers of deformed rocks of mixed
composition. If this is true, then faults may comprise strong,
competent rock fragments mixed with weaker, incompetent material.
The relatively incompetent rocks likely accommodate deformation by
aseismic flow, while competent rocks break in faster, seismic
events. It follows that the composition of a fault zone, and the
relative proportions of competent and incompetent rocks, must be
considered critical controls on the seismic behavior and earthquake
potential of a fault.
A new perspective on paleopiezometry: Dynamically recrystallized
grain size distributions indicate mechanism changes
Michael Stipp et al., Marine Geodynamics, IFM-GEOMAR,
Wischhofstrasse 1-3, 24148 Kiel, Germany. Pages 759-762.
Deformation of Earth's crust and mantle, ranging from seismic
rupture on discrete faults to plastic flow processes in wide shear
zones, is controlled by tectonic stresses. Direct stress
measurements are possible in boreholes down to shallow depths in
the crust; indirect measurements at greater depths can only be
carried out using microstructural indicators in exhumed rocks. The
most reliable and widely used stress indicator (piezometer) is the
dynamically recrystallized grain size resulting from plastic flow
within the deeper crust and upper mantle. Although measured in many
studies on deformed rocks and also metals and ceramics, global
analyses on the frequency distribution of recrystallized grain
sizes are completely lacking. Michael Stipp of Germany's IFM-GEOMAR
and colleagues present the first systematic investigation of the
recrystallized grain-size distribution for the mineral quartz. The
distribution is strikingly discontinuous, indicating the operation
of three distinct recrystallization mechanisms, which breaks new
ground for theoretical conceptions of recrystallization and its
stress dependence. The findings are not only relevant for quartz
but for any rock-forming mineral (e.g., olivine, feldspar,
calcite), for water ice, and for any crystalline solid used in the
fabrication of ceramics, metals, semiconductors, or other
materials.
Shallow-marine records of pyroclastic surges and fallouts over
water in Jeju Island, Korea, and their stratigraphic
implications
Y.K. Sohn and S-H. Yoon, Dept. of Earth and Environmental Sciences,
Research Institute of Natural Science, Gyeongsang National
University, Jinju 660-701, Republic of Korea. Pages 763-766.
One of fundamental questions in stratigraphy and sedimentary
geology is how faithfully the geological processes are preserved in
sedimentary records. On the basis of their study of a
shallow-marine volcaniclastic deposit in Jeju Island, Korea, Y.K.
Sohn and S-H. Yoon of Gyeongsang National University show that
there can be extreme biases in Earth's stratigraphic records. The
Jeju Island deposit accumulated very rapidly during a brief
volcanic eruption, about a million times faster than the adjacent
sedimentary strata. Because of the unusually high sedimentation
rate, the volcaniclastic deposit could record the "usual"
fair-weather processes in the depositional site at a resolution
that is almost never provided by ordinary sedimentary deposits.
This finding highlights the biases in Earth's stratigraphic records
and tells us that volcanic deposits, commonly regarded as the
products of catastrophic events, can in some cases record more
faithfully the ordinary and usual processes that nonvolcanic
deposits cannot record.
GSA Today Science Article
Structural analysis of three extensional detachment faults with
data from the 2000 Space-Shuttle Radar Topography Mission
Jon E. Spencer, Arizona Geological Survey, 416 W. Congress St.
#100, Tucson, Arizona, 85701, USA
The Shuttle Radar Topography Mission (SRTM), flown in February
of 2000 aboard the Space Shuttle Endeavour, acquired new data used
to produce a new digital elevation model (DEM) of almost all land
areas between 60 degrees N and 56 degrees S latitude. The new DEM
provides very high quality topographic data for 80% of Earth's
land, much of which was not well surveyed before the SRTM. The
mission has given us extraordinary images associated with some
recent faults, revealing domal, planar, and grooved surfaces. Some
of these surfaces form remarkably smooth topography over tens to
hundreds of square kilometers in regions otherwise deeply incised
by rapid erosion. Here, images of three spectacular examples (from
New Guinea, the Himalayas, and Indonesia) are presented, and some
structural, tectonic, and geomorphic processes associated with
these exhumed faults are discussed. While data from the mission
have been a bonanza for geomorphologists studying extensional
tectonic regimes, aspects of the behavior and morphology of the
faulting, as related in particular to the geothermal gradient,
remain areas of significant research opportunities.
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