Boulder, CO, USA - The September-October 2010 issue of GSA
Bulletin is now available online. Research collected from
around the globe addresses fast erosion during floods; improving
ancient Earth's geological time scale fidelity; geologically recent
tectonic plate movements; potential effects of volcanic activity on
a California community; and an effort to identify the source of
earthquakes in southern Alaska, among other topics.
Highlights are provided below. View abstracts at http://gsabulletin.gsapubs.org/content/current.
Representatives of the media may obtain complimentary copies of
GSA Bulletin articles by contacting Christa Stratton at the
address above. Please discuss articles of interest with the authors
before publishing stories on their work, and please make reference
to GSA Bulletin in articles published. Contact Christa
Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and
Service,
gsaservice@geosociety.org.
230Th/U dating of a late Pleistocene alluvial fan along the
southern San Andreas fault
Kathryn E.K. Fletcher et al., Dept. of Earth and Planetary Science,
University of California, Berkeley, California 94720-4767, USA.
Pages 1347-1359.
In this study, U-series dating of soil carbonate accumulated on
gravel clasts provides a reliable, precise minimum age of 45.1 +/-
0.6 thousand years ago (2 sigma) for the Biskra Palms alluvial fan,
Coachella Valley, California. Concordant ages for multiple
sub-samples from individual carbonate coatings provide evidence
that the U-238-U-234-Th-230 system has remained closed since
carbonate formation. The U-series minimum age is used to assess
published Be-10 exposure ages of cobbles and boulders (see Behr et
al., companion paper in this issue, pages 1360-1377). All but one
cobble age and some boulder Be-10 ages are younger than the
U-series minimum age, indicating that surface cobbles and some
boulders were partially shielded after deposition of the fan and
have been subsequently exhumed by erosion of fine-grained matrix to
expose them on the present fan-surface. Comparison of U-series and
Be-10 ages indicates that the interval between final alluvial
deposition on the T2 fan-surface and accumulation of dateable
carbonate is not well resolved at Biskra Palms, however, the "time
lag" inherent to dating via U-series on pedogenic carbonate can be
no larger than about 10 thousand years old, the uncertainty of the
Be-10-derived age of the T2 fan-surface. Dating of the T2
fan-surface via U-series on pedogenic carbonate (minimum age, 45.1
+/- 0.6 thousand years ago) and Be-10 on boulder-top samples using
forward modeling (preferred age, 50 +/- 5 ka) provides broadly
consistent constraints on the age of the fan-surface and helps to
elucidate its post-depositional development.
Uncertainties in slip-rate estimates for the Mission Creek
strand of the southern San Andreas fault at Biskra Palms Oasis,
southern California
W.M. Behr et al., Dept. of Earth Sciences, University of Southern
California, Los Angeles, California 90089, USA. Pages
1360-1377.
The rate of motion (slip rate) of the San Andreas fault over
long time scales can be estimated using landforms that have been
progressively displaced or offset along the fault. This requires
measurement of both the amount of offset of the feature, and the
time at which the feature formed. Behr and colleagues revisit a
well-known slip rate site on the southern San Andreas and focus on
the uncertainties in estimates of the geologic slip rate where it
offsets an alluvial fan in southern California. They provide new
estimates of the amount of offset of this fan aided by trench
excavations, and new cosmogenic beryllium-10 age determinations
from the tops of 12 boulders on the fan surface. Their results
point to significantly larger geologic uncertainties than
previously reported. The approach of rigorously addressing
uncertainties is relevant to the current climate in the active
tectonics community, which is focused on discrepancies between
geologic and geodetic slip rates, i.e., whether these are real, and
if so, what implications they have for temporal changes in plate
boundary configurations, for loading during the earthquake cycle,
and for seismic hazard. Behr et al. also address issues of
cosmogenic sampling strategies, which have remained controversial
within the literature. They compare boulder-top ages to those
obtained from cobble-sized clasts and find that the boulder-tops
yield much older ages that are consistent with uranium-series data
from pedogenic carbonate (described in a companion paper in this
issue by Fletcher et al., pages 1347-1359). This work emphasizes
the value and necessity of applying independent geochronologic
systems to landforms that clearly have complex geomorphic
histories. The revised age and offset models from this study yield
a range of slip-rates for the past 50,000 years of between 12 and
22 mm/yr, with the preferred range being 14-17 mm/yr; these rates
are entirely compatible with short-term slip-rate estimates from
geodetic models for this region, themselves subject to large
uncertainties.
The West Mariana Ridge, western Pacific Ocean: Geomorphology
and processes from new multibeam data
James V. Gardner, University of New Hampshire, Center for Coastal
& Ocean Mapping, 24 Colovos Road, Durham, New Hampshire 03824,
USA. Pages 1378-1388.
In this study, the West Mariana Ridge and eastern Parece Vela
Basin, a large region of a relict volcanic arc and adjacent
deep-sea basin in the western Pacific Ocean, were mapped with a
multibeam echosounder system. Gardner's new data reveal a landscape
dotted with extinct volcanoes, some aligned along the ridge crest
but others trending at angles away from the ridge. These volcanoes
should have subsided more than a kilometer because the ridge has
been dormant for about 7 million years and therefore should have
subsided as it cooled. However, the summits of several of the ridge
volcanoes are at water depths of 500 m or shallower. This
observation suggests the ridge may have been dormant for a long
time, or may actually still be active. A surprising finding of the
mapping is a series of very young channels that trend west, but not
east, away for the ridge out onto the eastern-most Parece Vela
Basin. The channels cut huge aprons of landslide debris, and the
floors of the channels have been swept clean of the normal pelagic
drape that is seen on the seafloor immediately adjacent to the
channels. These observations suggest the channels are very young or
even active sediment-transport paths, and that the landscape of the
West Mariana Ridge has continued to be modified even though the
ridge is thought to been inactive for a very long time.
Linking river-flood dynamics to hyperpycnal-plume deposits:
Experiments, theory, and geological implications
Michael P. Lamb et al., Jackson School of Geosciences, University
of Texas, 1 University Station C1100, Austin, Texas 78712-0254,
USA. Pages 1389-1400.
River plumes charged with sediment can plunge underneath
seawater and run out along the ocean floor. The deposits of these
plumes provide an important record of river dynamics through
different climatic and tectonic settings. Lamb et al. present one
of the first experimental flume studies aimed at testing whether
river flood discharge can be unraveled from marine deposits.
Counter to dominant theories, their results indicate that plume
velocities can be uncorrelated or even anti-correlated with river
discharge at certain seabed locations because of translation of the
zone of plunging. An advection length scale of settling sediment is
found to be an important control on hyperpycnal-plume deposits,
where fine sediment (mud) is most likely to preserve rising and
falling river discharge.
Eruption chronology and petrologic reconstruction of the ca.
8500 B.P. eruption of Red Cones, southern Inyo chain,
California
Brandon Browne et al., Dept. of Geological Sciences, California
State University, Fullerton, California 92834, USA. Pages
1401-1422.
The 60-km-long Mono-Inyo volcanic chain of eastern California,
stretching from Mono Lake in the north to a pair of basaltic scoria
cones, known as Red Cones, located 5 km south of Mammoth Mountain,
has produced ~20 explosive volcanic eruptions over the past 5000
years, the most recent of which occurred <500 years ago from
vents in the central and northern portions of the chain. Although
no eruptions have occurred in the southern portion of the chain for
several thousand years, recent findings from geophysical studies
suggest the presence of volatile-rich basaltic magma at depths of
10 to 20 km beneath Mammoth Mountain's southern flank near
Horseshoe Lake and the southwest flank near Devil's Postpile
National Monument. If this magma were to erupt at the surface, what
would it be like in terms of eruption style? How much magma might
be erupted? How long would the eruption last? And how might an
eruption of this magma threaten the residents, infrastructure, and
economy of the nearby community of Mammoth Lakes? One way to
address these important questions is to investigate the products of
the 8500-year-old eruption Red Cones, located only 5 km southwest
of Mammoth Mountain, and use the results as a proxy for what may
happen in the future if an eruption near Horseshoe Lake or Devils
Postpile National Monument indeed occurs. Results from this study
indicate that the Red Cones eruption lasted a minimum of 28 days,
varied in eruption style from Hawaiian to Strombolian, and produced
more than 10 million cubic meters of basaltic magma at temperatures
of ~1200 degrees Celsius.
Linking sedimentation in the northern Andes to basement
configuration, Mesozoic extension, and Cenozoic shortening:
Evidence from detrital zircon U-Pb ages, Eastern Cordillera,
Colombia
Brian K. Horton et al., Dept. of Geological Sciences and Institute
for Geophysics, Jackson School of Geosciences, University of Texas
at Austin, Austin, Texas 78712, USA. Pages 1423-1442.
This study analyzes U-Pb isotopes in zircon mineral grains from
26 sandstone samples. These samples, from the northern Andes of
Colombia, reveal sedimentation patterns during Paleozoic
subsidence, Jurassic-Early Cretaceous extension, Late Cretaceous
post-rift subsidence, and Cenozoic shortening and foreland-basin
evolution. Additional U-Pb geochronological results for three
basement samples indicate that the presumed Precambrian basement in
the Eastern Cordillera is actually a product of early Paleozoic
magmatism. Collectively, the new isotopic ages help address
critical unknowns for tectonic reconstructions of the northern
Andes, including (1) the age and nature of crystalline basement,
(2) the role of possible Paleozoic orogenesis, (3) the timing and
extent of Mesozoic rifting, and (4) the onset and tempo of Cenozoic
shortening and surface uplift during the Andean orogeny.
Oligocene-Miocene basin evolution in the northern Altiplano,
Bolivia: Implications for evolution of the central Andean
backthrust belt and high plateau
Bryan P. Murray et al., Dept. of Earth Science, University of
California, Webb Hall, Santa Barbara, California 93106-9630, USA.
Pages 1443-1462.
Murray and colleagues link deposition of upper Oligocene-lower
Miocene basin fill in the central Andean backthrust belt of Bolivia
to surface uplift of the Eastern Cordillera relative to the
Altiplano. Nonmarine sedimentary lithofacies, provenance data, and
growth stratal relationships indicate alluvial fan to braided
fluvial deposition during coeval shortening and propagation of the
backthrust belt toward the Penas-Aranjuez hinterland basin in the
Altiplano. Such a progression of late Oligocene-early Miocene
shortening along the Altiplano-Eastern Cordillera boundary likely
reflects significant crustal thickening, potential isostatic
uplift, and initial topographic expression of the eastern margin of
the central Andean plateau.
Rapid incremental assembly of the Monte Capanne pluton (Elba
Island, Tuscany) by downward stacking of magma sheets
Federico Farina et al., Dipartimento di Scienze della Terra,
Universita di Pisa, Via Santa Maria 53, 56126 Pisa, Italy. Pages
1463-1479.
Granite magmatism is one of the most important mechanisms
contributing to the growth and geochemical differentiation of
Earth's crust. Many granitic intrusions are texturally and
compositionally zoned and provide natural laboratories for the
understanding the construction processes of continental crust. The
Monte Capanne pluton (Elba Island, Italy) is characterized by the
widespread occurrence of large crystals of K-feldspar (megacrysts),
whose variation in size and abundance have been determined at 392
stations all over the pluton. Megacryst abundance variation allows
the recognition of three zones (intrusive facies) characterized by
low, intermediate, and high megacryst content. In this study a new
geological map, based on K-feldspar megacryst distribution, reveals
the composite structure of the pluton. Textural and compositional
differences between facies suggest that they formed at depths as
distinct magma pulses and rose through the crust to the emplacement
level as three discrete injections (magma batches). The composite
structure of the pluton reveals that it built up incrementally by
downward stacking of three slightly different batches. Field
evidence together with 3-D thermal modeling indicate that the three
magma batches were emplaced in a short time sequence.
High-precision U-Pb calibration of Carboniferous glaciation
and climate history, Paganzo Group, NW Argentina
E.L. Gulbranson et al., Dept. of Geology, University of California,
One Shields Avenue, Davis, California 95616, USA. Pages
1480-1498.
The ancient glacial history of a large continental landmass
known as Gondwana has been considered sparse and fragmented.
Nevertheless, icehouse climate stasis has long been inferred for
the late Paleozoic (about 359 to 251 million-years-ago) from
sedimentary records in the ancient tropics. Gulbranson et al.
explore this history of glaciation and climatic change by
high-precision U-Pb zircon calibration of existing and new field
evidence for the presence or absence of ice and major climate
transitions. Their results indicate at least three, rapid (1-8
million year), glacial events occurred on the southwestern margin
of Gondwana that were separated by non-glacial intervals, thus
indicating that the late Paleozoic ice age was more dynamic than
previously considered. This study also documents substantial
aridification of the study area as much as ten million years
earlier than previously reported, coincident with the end of
Carboniferous glaciation in this region.
East-west extension in the NW Indian Himalaya
Esther Hintersberger et al., Institut fur Geowissenschaften,
Universitat Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam,
Germany. Pages 1499-1515.
The northward movement of the Indian tectonic plate toward
Eurasian landmasses leads to shortening which creates the high
mountains of the Himalaya and Tibet. However, there is growing
evidence for east-west extension in the Himalaya and the Tibetan
Plateau, perpendicular to the orientation of the main mountain
chain in the central part of the Himalaya. Hintersberger and
colleagues provide multiple field evidence and interpretation of
high-resolution satellite imagery for ongoing east-west extension
in the northwest Indian Himalaya. This is new and important
information, because it is generally assumed that extension in the
Himalaya and in the Tibetan Plateau is either limited to local
graben or domal structures in the Central Himalaya or to the
plateau region. However, investigations by Hintersberger et al.
demonstrate that extensional tectonics is a pervasive, long-lasting
phenomenon that has been active in the northwest Indian Himalaya
for at least ~16 million years. Their observations are used as an
independent data set to evaluate existing theories about extension
in the Himalaya, since most of those models are based on data from
the Central Himalaya. The conclusions are also relevant for the
ongoing discussion about the role of the Karakorum fault as a
decoupling structure between the Himalaya and the Tibetan
Plateau.
Suprasubduction-zone ophiolite generation, emplacement, and
initiation of subduction: A perspective from geochemistry,
metamorphism, geochronology, and regional geology
John Wakabayashi et al., Dept. of Earth and Environmental Sciences,
California State University, Fresno, California 93740, USA. Pages
1548-1568.
This study presents geochemical data giving insight into the
process of subduction initiation and formation of rock suites known
as ophiolites. Ophiolites are on-land remnants of oceanic
lithosphere, and most well-known ophiolites apparently formed above
a subduction zone; this is called a suprasubduction-zone (SSZ)
setting. Thin sheets of high-temperature metamorphic rocks, known
as metamorphic soles, crop out structurally beneath many SSZ
ophiolites. Such rocks may have formed during subduction initiation
beneath young, hot, oceanic lithosphere. High-temperature
metamorphic rocks from the Franciscan Complex of California may
represent a metamorphic sole beneath the SSZ Coast Range ophiolite
(CRO). Geochemical data indicate the parent rocks (protoliths) of
the high-temperature rocks formed in an SSZ environment, requiring
the existence of a pre-Franciscan subduction zone, whereas
later-subducted, lower-temperature oceanic rocks apparently formed
at mid-ocean ridges or seamounts. The CRO and Franciscan
high-temperature rock protoliths apparently formed over a
pre-Franciscan subduction zone that dipped westward, away from
North America. After west-dipping subduction ceased,
high-temperature Franciscan rocks were metamorphosed as
east-dipping subduction began beneath the CRO, emplacing the
ophiolite and eventually leading to its exposure. Other metamorphic
soles apparently have SSZ protoliths, suggesting that this model
may be globally applicable.
Processes, rates, and time scales of fluvial response in an
ancient postglacial landscape of the northwest Indian
Himalaya
Daniel E.J. Hobley et al., School of GeoSciences, University of
Edinburgh, Edinburgh, Scotland EH9 3JW, UK. Pages 1569-1584.
Rates of landscape change and sediment production are known to
have increased during the Late Cenozoic (the last few million
years). This change is also known to have happened at the same time
as major climatic change on Earth, as glaciers grew for the first
time over large areas of the northern hemisphere. However, how the
presence of glaciers might affect the growth and evolution of
mountain ranges is presently poorly understood, particularly in
terms of how fast change will happen as the landscapes respond to
the retreat of the ice, and how rivers deal with the large
quantities of glacial debris left behind. This study discusses what
happens when rivers reoccupy a mountain landscape that has
relatively recently been abandoned by glaciers, looking in detail
at the Ladakh area of the western Himalayas. They show that the
rivers are beginning to reshape the landscape toward an unglaciated
form and to move the loose material left by the glaciers
downstream, as would be expected from what we already know, but
that the style in which the river moves this material is not what
would be expected based on preexisting work. In particular, they
show that the rivers will take well over one million years to
reshape the valleys and free all the sediment left by the glaciers,
much slower than previous estimates might suggest, and many times
longer than the expected time taken for glaciers to go through
cycles of advance and retreat. This new information means that we
can understand better the geological records of past ice ages, as
well as the way modern high-mountain landscapes are likely to
respond to current glacial melting.
Origin of sackung uphill-facing scarps in the Saint Elias
orogen, Alaska: LIDAR data visualization and stress
modeling
Zhiyong Li et al., Faculty of Earth Sciences, China University of
Geosciences, Wuhan, 430074, People's Republic of China. Pages
1585-1599.
The coastal mountains of southern Alaska and the Yukon rise
dramatically from the sea because of on-going collision between the
North American continental plate and a small microplate. This
region creates some of the world's greatest earthquakes and has the
potential to create destructive tsunamis that may travel throughout
the Pacific Ocean basin. Identifying the sources of earthquakes
within the mountains is of interest both for basic research into
the origin of mountainous landscapes as well as better
documentation of natural hazards. This study investigates the
origin of numerous young fault scarps that have previously been
described as the results of active tectonics on the one hand, and
gravitational collapse of mountain blocks on the other. In the
first case, the scarps would reflect folding above large,
earthquake-generating faults at depth, but in the latter case the
scarps would be superficial features in the landscape. Scarps in
one mountain block were investigated using high-resolution
topographic data collected by airborne laser mapping, field
observations, and computer modeling of stresses acting as the
result of gravity and tectonic force fields. The results
demonstrate that the scarps formed by outward rotation and toppling
of sedimentary layering in the upper 100 meters of the mountain,
and are therefore not directly related to active folding and buried
earthquake faults. However, strong ground shaking caused by large
earthquakes may enhance the toppling process which is primarily
caused by gravitational stress in the steep-sided, previously
glaciated mountains.
Contrasting bedrock incision rates from snowmelt and flash
floods in the Henry Mountains, Utah
Joel P.L. Johnson et al., The University of Texas at Austin, Dept.
of Geological Sciences, 1 University Station C9000, Austin, Texas
78712, USA. Pages 1600-1615.
How fast can rivers erode into bedrock, how erosive are flash
floods, and how do slot canyons form? A new study addresses these
questions through environmental monitoring of floods and bedrock
erosion along a canyon near the Henry Mountains in southeast Utah.
They found that local erosion rates during an individual flood can
be fast: Up to about 0.5 m of vertical incision along a short and
steep channel reach was caused by 23 days of flow and sediment
transport from upstream mountain snowmelt. This rate is much faster
than the well-constrained landscape erosion rate in this region of
~0.4 mm/year, when averaged over millennial timescales. The rapid
short-term erosion rate was caused by a prolonged but moderate
discharge flow event which was interpreted to transport relatively
low concentrations of coarse sediment. In contrast, a flash flood
with a peak discharge nearly 10 times larger than the peak snowmelt
flow caused negligible bedrock erosion but extensive alluvial
deposition. The shape of the bedrock channel that eroded is very
similar to larger slot canyons nearby along the Escalante River,
suggesting similar mechanisms for their development.
What happens when the ocean is overheated? The foraminiferal
response across the Paleocene-Eocene Thermal Maximum at the
Alamedilla section (Spain)
Laia Alegret et al., Departamento de Ciencias de la Tierra e
Instituto Universitario de Investigacion de Ciencias Ambientales de
Aragón, Universidad de Zaragoza, 50009 Zaragoza, Spain.
Pages 1616-1624.
Global warming and a major perturbation of the carbon cycle
occurred during the Paleocene-Eocene transition, about 55 million
years ago, triggering acidification of the oceans and rapid changes
in terrestrial and marine organisms. Alegret et al. investigate
this period of Earth's history in the Alamedilla section (Spain),
where changes in microfossil assemblages and in their test size
have been related to the effects of carbon input and warming, as
well as to interspecific competition, suggesting a high degree of
complexity in the ocean structure.
A late Miocene–early Pliocene chain of lakes fed by the
Colorado River: Evidence from Sr, C, and O isotopes of the Bouse
Formation and related units between Grand Canyon and the Gulf of
California
Jennifer A. Roskowski et al., Dept. of Geosciences, University of
Arizona, Tucson, Arizona 85721, USA. Pages 1625-1636.
The Colorado River has great importance for water and other
resources in the U.S. Southwest. In recent years, it has become
clear that the Colorado River has a near-unique origin among
studied world rivers: It came into existence as a single
through-flowing stream about five and a half million years ago, but
the river propagated downward from the Colorado Plateau to the Gulf
of California, into a preexisting landscape of desert basins. It
formed large lakes in these basins, which became saline from
evaporation in the warm climate, and in some cases supported
organisms that live in salty water. The authors show that
strontium, oxygen, and carbon isotopes in the sediments deposited
in the lake basins can be used to document the geographical extent
of a chain of large Colorado River-fed saline, freshwater lakes
from Las Vegas south to the U.S.-Mexico border. They also use the
isotopic studies to characterize the environments in the lakes, and
demonstrate that no seawater was involved in the system. The lakes
disappeared over the succeeding period of 0.1 to 1 million years as
the river filled the basins with sediment and cut through rock
barriers that originally separated the basins.
The sedimentary architecture of outburst flood eskers: A
comparison of ground-penetrating radar data from Bering Glacier,
Alaska, and Skeioararjokull, Iceland
Matthew J. Burke et al., School of Applied Sciences, Northumbria
University, Newcastle upon Tyne, NE1 8ST, UK. Pages 1637-1645.
Eskers are ridges of sand and gravel that record the infilling
of ice-walled river channels and have been used to infer the
dynamics and hydrology of former ice sheets. Hypotheses regarding
their origin have focused on analysis of ancient eskers that
understandably lack rigorous constraints on depositional timescale.
As such, the conditions under which ancient eskers formed remains a
matter of debate. Burke and colleagues used ground-penetrating
radar to investigate the controls on the formation of eskers that
were generated during relatively well constrained glacial outburst
floods at Bering Glacier, Alaska and Skeioararjokull, Iceland.
These eskers are currently the only known examples that can be
unequivocally linked to rapid deposition during high-magnitude
glacial outburst floods. The similar form and sedimentary
architecture of the eskers suggests glacial outburst floods
generate distinct depositional signatures in eskers. Identifying
these signatures in ancient eskers will help assess the nature of
meltwater drainage systems during retreat of the North American and
European ice sheets following the Last Glacial Maximum.
Whole-rock Pb and Sm-Nd isotopic constraints on the growth of
southeastern Laurentia during Grenvillian orogenesis
Christopher M. Fisher et al., Dept. of Earth and Environmental
Sciences, Vanderbilt University, Nashville, Tennessee 37235, USA.
Pages 1646-1659.
The configuration of continents on Earth's surface has changed
continuously throughout its history, a result of processes
geologists refer to as plate tectonics. Reconstructing the more
recent history of these plate movements, including the formation of
"supercontinents," relies on a combination of paleomagnetic and
fossil correlation studies. However, these techniques are
difficult, if not impossible, to apply to reconstructions of
earlier parts of Earth's history. In these situations geologists
must rely on geochemical data to establish a set of "fingerprints"
for different pieces of crust around the globe. These fingerprints
allow geologists to identify pieces of crust that did not form in
their current position and determine the likely "parent" continent
of an out-of-place piece of crust. These continent-fragment
connections can help in the reconstruction of plate movements over
time. In this study, researchers applied such a fingerprinting
approach (using Pb and Nd isotopes and U/Pb zircon ages) to ancient
rocks within the Appalachian Mountains of southeastern North
America to address a long-standing debate about the origin of these
rocks. Their results suggest that a large fragment of another
continent was added to North America about one billion years ago,
stretching from Georgia to New York, and as far west as the
Tennessee-North Carolina border. This piece of crust was likely a
piece of South America, transferred to North America when it
collided with the western edge of the ancient core of South
America, during the formation of a supercontinent that geologists
refer to as Rodinia.
Extraordinary transport and mixing of sediment across
Himalayan central Gondwana during the Cambrian-Ordovician
Paul M. Myrow et al., Dept. of Geology, Colorado College, Colorado
Springs, Colorado 80903, USA. Pages 1660-1670.
Grains of the mineral zircon were separated from sandstone
samples taken from Cambrian and Ordovician rocks from along the
entire length of the Himalaya and dated for this study. Their ages
represent a census of the sorts of the rocks exposed on Earth's
surface at the time. The specific ages indicate river transport of
sediment for thousands of kilometers within the ancient
supercontinent at this time, Gondwanaland. Possible sources for the
sediment include mountain belts along the eastern part of Africa,
parts of the present-day Arabian Peninsula, and both Antarctica and
Australia. The spectra of ages for these samples are remarkably
similar, which suggests that very large river systems, operating in
the absence of land plants, mixed grains from many sources and
deposited them over great areas of the continental margin.
Quaternary reactivation of the Kern Canyon fault system,
southern Sierra Nevada, California
Elisabeth S. Nadin and Jason B. Saleeby, Division of Geological and
Planetary Sciences, California Institute of Technology, MS 100-23,
Pasadena, California 91125, USA. Pages 1671-1685.
The Kern Canyon fault of the southern Sierra Nevada, California,
has a storied history spanning 100 million years. Since the 1930s,
this fault has been considered long dead. Recent investigations
include groundwork and aerial surveys of the fault line, and reveal
that the fault has ruptured within the past few thousand years.
This discovery, paired with an abundance of (low-magnitude)
earthquakes along the fault, means that the fault is active. There
is a potential for future large earthquakes along the Kern Canyon
fault, posing a flood threat to the city of Bakersfield, with a
population of 800,000, which lies downstream of a dam that crosses
the fault. The study also shows that faults can lie quiet and can
reactivate under different stress conditions of the crust over a
100-million-year span of time.
Maximum depositional age and provenance of the Uinta Mountain
Group and Big Cottonwood Formation, northern Utah: Paleogeography
of rifting western Laurentia
Carol M. Dehler et al., Dept. of Geology, Utah State University,
Logan, Utah 84322, USA. Pages 1686-1699.
Precambrian sedimentary rocks record information about ancient
landscapes and their tectonic settings (>542 million years ago).
However, few of these rocks have been dated numerically, and
therefore it is difficult to understand how these and other
Precambrian sedimentary rocks related to one another in space and
time. In this study two sandstone units located east of Salt Lake
City, Utah, have recently been determined to be about 770-740
million years old and record the presence of a large
transcontinental river system flowing westward to a west-facing
ocean margin at this time. Knowing the age of these strata allows
correlation with other strata of this age range in other parts of
the world, and, together, they record the initial separation of a
supercontinent know as Rodinia.
Testing the limits of Paleozoic chronostratigraphic
correlation via high-resolution (<500 k.y.) integrated conodont,
graptolite, and carbon isotope (delta-13C) biochemostratigraphy
across the Llandovery-Wenlock (Silurian) boundary: Is a unified
Phanerozoic timescale achievable?
Bradley D. Cramer et al., Division of Earth History, School of
Earth Sciences, The Ohio State University, 125 S. Oval Mall,
Columbus, Ohio 43210, USA. Pages 1700-1716.
The ability to tell time in Earth history is a function of the
time period under investigation. Whereas the relative order and
duration of comparatively recent geologic events can be determined
to a level of plus or minus a few days to months to years,
investigating the deep past is more difficult. The order of events
in older parts of Earth history typically can be deciphered only to
within plus or minus a million years or worse. The work of Cramer
et al. has begun to demonstrate that resolving the deep time record
of Earth history to a level approaching more recent intervals is
possible. This order-of-magnitude improvement in the ability to
tell relative time (i.e., order of events) in the distant past was
made possible through the integration of multiple lines of
investigation and approaching the ancient geologic record in a
manner similar to researchers of more recent intervals of time. As
a result, their work has begun to demonstrate that it may be
possible to produce a geologic time scale of equal resolution and
fidelity for perhaps as much as the past 500 million years.
Coupling volcanism and tectonics along divergent plate
boundaries: Collapsed rifts from central Afar, Ethiopia
Valerio Acocella, Dipartimento Scienze Geologiche Universita Roma
Tre Largo San Leonardo Murialdo, 1, Roma, 00146, Italy. Pages
1717-1728.
Divergent plate boundaries consist of rift zones characterized
by active extension and volcanism. On Earth, these processes can be
best appreciated along the Ethiopian Rift extending continental
crust, the Central Afar extending transitional crust, and the
Iceland extending oceanic crust. Here, volcanic activity usually
has a limited impact on rift architecture and evolution, being
associated with moderate eruptions from fissures or collapse
calderas. However, Central Afar is characterized by the emplacement
of a km-thick volcanic sequence (the "Stratoids"), the largest ever
recognized along divergent plate boundaries so far, responsible for
the emission of ~7000 km cubes of magma in ~1.5 million years. This
study considers the effect of such an eruption on the rift in
Central Afar. The rift sides show distinctive features, as steep
topographic gradients coinciding with inward tilted crustal blocks,
resulting from the collapse induced by magma withdrawal during the
emplacement of the Stratoids. This portion of Afar shows how the
entire rift architecture is shaped by voluminous fissure eruptions.
The result is a collapsed rift that is a portion of the rift
undergoing collapse, similarly to calderas. A collapsed rift
represents an end-member type of volcano-tectonic activity, where
the width of the erupting reservoir balances that of the active
rift zone.
SOURCE