|30 April 2010
GSA Release No. 10-22
Director of Education, Communication, & Outreach
May 2010 Geology Highlights
Boulder, CO, USA - GEOLOGY examines a wide swath of topics, from the minute to the vast — zircon grains and the story of an ancient mountain range; what bubbles and crystals in volcanic rocks tell us about Strombolian eruptions; cyanobacterial blooms and the end-Permian faunal crisis; pumice formation in submarine domes; fossil “snapshots” of Earth's climatic history from Oklahoma cave fills; Mauna Loa's Ninole rift zone; and "the largest tectonomagmatic feature in the solar system" — Artemis, Venus.
Representatives of the media may obtain complimentary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Contact Christa Stratton for additional information or assistance.
View abstracts for the complete issue of GEOLOGY at http://geology.gsapubs.org/. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published.
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Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level
Blair Schoene et al., Earth Sciences, University of Geneva, Rue des Maraîchers 13, CH-1205 Geneva, Switzerland. Pages 387-390.
Mass extinction events set the stage for subsequent biodiversity and its relationship to environment and geochemical cycling. However, the causes, rates, and durations of mass extinction events remain poorly understood. Schoene et al. use uranium-lead geochronology to date volcanic ash beds that were sampled from within sedimentary rocks that contain the fossil and isotope record of the about 200 million-year-old end-Triassic mass extinction event, one of the largest such crises in Earth’s history. They also date the onset of one of the largest volcanic events ever and show that it correlates with the extinction event to about 150,000 years, thus increasing substantially the confidence that the two events were linked. This work also calibrates the rates of isotopic and environmental changes associated with the events, which are key inputs to models exploring the feedbacks between biology, geology, and global geochemical cycling.
Effects of water depth on pumice formation in submarine domes at Sumisu, Izu-Bonin arc, western Pacific
Sharon R. Allen et al., ARC Centre of Excellence in Ore Deposits, and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 391-394.
Allen et al. use sampling and observations collected by remotely operated vehicle of three adjacent submarine lava dome volcanoes in the western Pacific to show that magmatic volatile-driven explosivity is reduced with increasing vent water depth. The shape of pumice clasts (which are formed by expansion of magmatic volatiles as the magma rises to the sea surface), and the deposits they form in, can serve as useful indicators of eruption water depth. Pumice generated from lava domes at water depths of more than 500 meters formed as a thick carapace on dense rhyolite, whereas at water depths shallower than 500 meters, pumice was erupted explosively.
Climatic and vegetation control on sediment dynamics during the last glacial cycle
A. Dosseto et al., GEMOC National Key Centre, Dept. of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia. Pages 395-398.
In a time where climate is rapidly changing under the influence of human activity, it is important to understand how the environment responds to climate variability. In this study, Dosseto et al. look at the past and investigate how soil erosion and sediment transport in rivers have responded to climatic variations over the past 100,000 years in a catchment of south-eastern Australia. Using the composition of uranium isotopes in ancient sediments, they are able to show that erosion and sediment transport are closely linked to climatic variations. Nevertheless, this relationship is not simple and the authors propose that vegetation is an important link between a change in climatic conditions and the response of erosion and sediment transport.
Evidence of former majoritic garnet in Himalayan eclogite points to 200-km-deep subduction of Indian continental crust
Anju Pandey et al., National Oceanography Centre, Southampton SO14 3ZH, UK. Pages 399-402.
Pandey et al. deciphered the depth of subduction of the Indian continental crust beneath the Asian plate using sophisticated techniques of laser-ablation inductively coupled plasma-mass spectrometer (LA-ICP-MS). Using LA-ICP-MS, they reported the occurrence of relict majorite, a variety of mineral garnet, which is stable only under ultra-high pressure conditions. The researchers discovered that as the Indian and Asian tectonic plates collided, the Indian continental crust subducted to a minimum depth of 200 km in the mantle. The subduction of continental crust to this depth has never been reported in the Himalayas and is also extremely rare in the rest of world. The findings are significant because there have not been unequivocal opinions on the depth of subduction of the Indian plate beneath Asia. In fact, the previous depth estimates were considerably different from those obtained by numerical modeling. The new study published here, however, is in tandem with numerical modeling and will radically improve our understanding about the subduction process of Indian continental crust beneath Himalayas. The new discovery is also set to modify several fundamental parameters of the Himalayan tectonics, such as the rate of Himalayan uplift, angle, and subduction of the Indian plate.
Depth to gypsic horizon as a proxy for paleoprecipitation in paleosols of sedimentary environments
Gregory J. Retallack and Chengmin Huang, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 403-406.
In the past, reconstructing ecosystem services, such as carbon sequestration, has been important for predicting the effects of rising atmospheric carbon dioxide. Depth to carbonate in desert soils has long been known to be related to mean annual precipitation and related ecosystem variables, such as biomass and soil respiration. Now, Retallack and Huang have demonstrated a comparable relationship for soil salts such as gypsum. Paleoclimatic information can now be extracted from saline paleosols as well as calcareous ones.
New weathering index improves paleorainfall estimates from Vertisols
L.C. Nordt and S.D. Driese, Dept. of Geology, Baylor University, Waco, Texas 76712, USA. Pages 407-410.
Rainfall, as a driver of the terrestrial hydrological cycle, is arguably the most important component of the climate system. Estimating rainfall amounts in deep time has been elusive and requires a proxy based on modern analogs. Field-based morphological proxy of depth to soluble salts for mean annual precipitation (MAP) is problematic because many fossil soils do not contain these features and because precise and accurate measurements are difficult to make with consistency. More recently, geologists have turned to chemical weathering indexes as a way of estimating MAP. The weathering index in use since 2002 was developed as a universal equation (CIA-K) applicable to all fossil soils, which yielded a large standard error. Nordt and Driese have modified this equation by developing a weathering relationship in the modern for the most commonly occurring fossil soil in the rock record (vertisols), improving the statistical reliability. Further, their new equation generates higher MAP than expected indicating the need to re-evaluate all previous estimates determined from fine-grained fossil soils since 2002. An important implication of this finding is that ancient ecosystems were functioning in response to higher rainfall amounts than previously thought, which should be considered when evaluating ecosystem response to past and future global change.
Deglacial history of the West Antarctic Ice Sheet in the Weddell Sea embayment: Constraints on past ice volume change
Michael J. Bentley et al., Dept. of Geography, University of Durham, South Road, Durham DH1 3LE, UK. Pages 411-414.
The volume of the Antarctic ice sheets during the last ice age is still not well-constrained. Bentley et al. provide new geological field evidence from the Ellsworth Mountains, in the southern Weddell Sea region, coupled to an ice sheet model that shows that the maximum ice sheet in this region was probably smaller than previously suggested. The data also suggest that the ice sheet thinned progressively over the last 15,000 years. The findings have implications for understanding the source of meltwater pulse-1A, a rapid rise in sea level about 14,500 years ago.
Evidence for active El Niño Southern Oscillation variability in the Late Miocene greenhouse climate
Simone Galeotti et al., Dipartimento GeoTeCA, Università degli Studi di Urbino, Campus Scientifico, 61029 Urbino, Italy. Pages 419-422.
Tropical climate dynamics are a subject of intense debate in the past, present, and future climate communities. It has been hypothesized that the current ocean-atmosphere system resides near a threshold, whereby a slight temperature increase may trigger the oscillating tropical ocean-atmosphere system to collapse into a permanent El Niño state. This transition would provide a positive feedback to global warming and a major perturbation to global hydrological cycles with major implications for future climate scenarios. Here, Galeotti et al. provide a paleoclimate test of this theory of ocean-atmosphere interaction. An annually resolved paleoclimate record from the Miocene Epoch (about 5.7 million years ago), when mean global temperature were comparable to those predicted by numerical simulations for year 2050-2100, contains an unequivocal signal of interannual variability that can be attributed to the influence of the El Niño Southern Oscillation (ENSO) over the Mediterranean area. In line with this observation, a numerical simulation of Miocene climate condition and dynamics provides evidence for both the persistence of ENSO and enhanced teleconnections under mean climate conditions warmer than today. This suggests that some theories for tropical atmosphere-ocean interaction may require further work.
Bedrock fracture control of glacial erosion processes and rates
Miriam Dühnforth et al., Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80303, USA. Pages 423-426.
The landscape of Yosemite National Park has a unique glacial signature, from the massive, steep valley walls bounding Yosemite Valley to the smoothly rounded, glacially-polished domes in and around Tuolumne Meadows. The Park’s exceptional landforms motivated Dühnforth et al. to explore the influence of rock fracture spacing on the efficiency of glacial erosion processes. The last glaciation of the park lasted approximately 10,000 years, ending by 13,000 years ago. Dühnforth et al. find that the glaciers eroded more than 3 meters of rock during this time at many locations within the park boundaries. The crystalline rocks in these areas have an average fracture spacing of 1.1 plus or minus 0.03 meters. In contrast, at six of the study sites in the park, the glaciers were less efficient in eroding the bedrock surfaces, removing less than 3 meters during the last glaciation. The fracture spacing at these sites is significantly wider: 3.3 plus or minus 0.1 meters. Based on these results, Dühnforth et al. suggest that glaciers erode more rapidly where the fracture spacing is small, because smaller blocks of rock are more easily carried away. This work demonstrates that the fracturing of rock fundamentally affects the efficiency of glacial erosion and the resulting glacial morphology of the landscape.
Negative correlations between Mg:Ca and total dissolved solids in lakes: False aridity signals and decoupling mechanism for paleohydrologic proxies
Mark D. Shapley et al., Dept. of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA. Pages 427-430.
The systematic evolution of lake chemistry as lake waters precipitate minerals from solution is a much-studied process exploited by paleolimnologists (scientists studying environmental change recorded in lake sediments) to reconstruct past climate conditions. Precipitation of calcium carbonate by lake waters causes the ratio of dissolved-magnesium-to-calcium (Mg:Ca) to increase, a trend often accompanying evaporative concentration of lakes and therefore expected to mark increasing lake salinity. Sedimentary records of changing Mg:Ca in lakes are routinely interpreted as indicative of varying aridity on this basis. Shapley et al. examine complications to this relationship that arise in lakes fed by inflowing groundwater, a common circumstance in semiarid climates. Using limnological and geochemical data from a groundwater-fed lake in Montana, they show how loss of carbon dioxide from inflowing groundwater to the atmosphere initially reverses this expected relationship between Mg:Ca and lake salinity, implying that a more complex paleoclimatic interpretation of sedimentary Mg:Ca may often be called for.
Interplate earthquakes as a driver of shallow subduction erosion
Kelin Wang et al., Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada. Pages 431-434.
At some subduction zones, sediment is scraped off the subducting seafloor and accreted to the leading edge of the upper plate. The situation is the opposite at many other subduction zones, where rock material is eroded from the underside of the upper plate and transferred to greater depth by the subducting plate. Both the accretion and erosion processes are linked to interplate earthquakes that occur along the megathrust fault between the two converging plates. Wang et al. use a mechanical model to explore how interplate earthquakes and subduction erosion affect each other. An outstanding challenge to all models is to reconcile the requirement for a relatively strong megathrust fault by subduction erosion and the requirement for a very weak megathrust fault by the presence of normal faults in the wedge above the fault. They propose that the opposite fault-strength requirements are met at different stages of subduction earthquake cycles. Updip of the megathrust seismogenic zone is a segment of predominantly aseismic slip which has a velocity-strengthening behavior. At the time of an earthquake, this shallow segment strengthens to resist slip, and the brief strengthening can result in subduction erosion. Between earthquakes when the seismogenic zone is locked, the shallow segment relaxes, and the decrease in fault strength can result in normal faulting in the overlying wedge. The aseismic to seismic transition of the megathrust is closely linked to the process of subduction erosion.
Intermediate water formation in the Bering Sea during glacial periods: Evidence from neodymium isotope ratios
Keiji Horikawa et al., Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan. Pages 435-438.
Deep/intermediate water circulation is the one of the controlling factor that can affect and reorganize global climate conditions, since deep/intermediate water ventilation rates and their extent directly influence the distribution and sequestration of gases (such as CO2) and nutrients in the oceans. Presence of intermediate water formation in the Subarctic North Pacific (SANP) has been debated in the past decades. Horikawa et al. reveal a glacial mode of intermediate water circulation in the SANP based on sedimentary records taken from the Bering Sea. These researchers used neodymium (Nd) isotopic compositions extracted from Fe-Mn oxyhydroxides (which are formed at bottom water-sediment interface) to reconstruct past ocean circulation pattern and provenance. Nd isotopes of Fe-Mn oxyhydroxides of the Bering Sea sediments represent radiogenic Nd values during glacial periods (e.g., the period of Last Glacial Maximum, 19,000 to 23,000 years ago). These glacial Nd isotope data observed at the intermediate depth in the Bering Sea are best explained by subduction of the surface water to the intermediate depth due to brine rejection. The Nd data also indicate that the coastal water off northeastern Kamchatka was the provenance of the glacial intermediate water. The new paleoceanogaphic data by Horikawa et al. imply a possibility that extension and retreat of seasonal sea-ice coverage in the Bering Sea played a significant role in the distribution and sequestration of gases and nutrients in the North Pacific.
Basaltic scoria textures from a zoned conduit as precursors to violent Strombolian activity
C. Cimarelli et al., Dipartimento Scienze Geologiche, Università Roma Tre, Largo S.L. Murialdo 1, 00146 Rome, Italy. Pages 439-442.
Volcanic explosive eruptions are a prime natural hazard. Although not the strongest ones, basaltic eruptions are frequent, may last long, and recently have been shown to be more complex and violent than previously thought. Cimarelli et al. illustrate how textures of basaltic pyroclasts (i.e., rock fragments generated by explosive volcanic activity) can be used to monitor the internal dynamics of a volcano during an ongoing eruption. In fact, variable content of bubbles and crystals in the pyroclasts are shown to mirror changes in the flow conditions of magma rising in the Earth’s crust below the volcanic vent. At the Spanish volcano, Cimarelli et al. studied pyroclast textures changed in the course of the eruption, indicating an increase in the rise velocity of magma. Velocity increased right before the eruption shifted from a mild activity, likely ejecting incandescent blocks to an height of several hundred meters, to a paroxysmal phase where a volcanic plume several kilometers high spread ash and lapilli over a large area. They conclude that monitoring pyroclast textures during an eruption can be an effective way to predict the onset of violent eruptive phases, thus helping mitigate their potentially catastrophic effects.
Sedimentation and crustal recycling along an active oblique-rift margin: Salton Trough and northern Gulf of California
Rebecca J. Dorsey, Dept. of Geological Science, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA. Pages 443-446.
In areas of continental rifting and lithospheric rupture, sediment accumulates in deep rift basins where it is buried and metamorphosed to form a new generation of crust. Although this process was recognized over 30 years ago, its relationship to erosion in source areas and significance for crustal recycling has received little attention. This study by Dorsey uses data from recent geophysical surveys to estimate the volume of sediment and metasedimentary rock sequestered in subsurface basins along the oblique-divergent plate boundary in the Salton Trough and northern Gulf of California. The volume is about 2.2 - 3.4 x 105 km3, similar to the volume of rock that has been removed from the Colorado Plateau by fluvial erosion and transport over the past 5 to 6 million years. Lowering of topography due to crustal thinning creates potential energy that enhances erosion of the continent interior and directs sediment into the plate-boundary basins. This mechanism of mass transfer and crustal recycling may help explain the origin of transitional crust at some types of ancient rifted margins, and could be important in other settings where a large river system is captured following tectonic collapse of a pre-rift orogenic highland.
Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis
Shucheng Xie et al., State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China. Pages 447-450.
What caused the most severe and protracted faunal crisis about 252 million years ago is one of the most fascinating enigmas in Earth history. Associated with the faunal crisis is the appearance of episodic cyanobacterial blooms for 5 million years. The sedimentary rocks formed by these microbes, known as microbialites, are the critical record to decipher the environmental stress and thus a linkage with the driver of the faunal crisis. In this study by Xie et al., microbialites in south China show a spatial and temporal relationship with the sediments of volcanic origins, inferring volcanism might induce the episodic cyanobacterial blooms, and thus could be the culprit of the faunal mass extinction. The well-known Siberia flood traps, however, erupted later than the end-Permian faunal mass extinction. Volcanism in South China and elsewhere could be the important driver of both the faunal mass extinction and cyanobacterial blooms.
Tracking exhumation of Andean ranges bounding the Middle Magdalena Valley Basin, Colombia
Junsheng Nie et al., Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Pages 451-454.
The shortening history of the Andes is important to understanding retro-arc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from the Cretaceous to Pliocene eras. Detrital zircon uranium-lead ages from the Middle Magdalena Valley basin in Colombia reveal two provenance shifts during Cenozoic time. The first shift occurs between lower and upper Paleocene strata, and the second shift occurs between middle-upper Eocene and upper Oligocene strata. These two provenance shifts are best attributed to uplift and exhumation in the northern Andes. The results by Nie et al. show that significant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of the Paleogene climate.
Speleothem climate records from deep time? Exploring the potential with an example from the Permian
Jon Woodhead et al., School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. Pages 455-458.
Speleothems (stalagmites, stalactites, and other cave deposits) are increasingly recognized as valuable archives of past climate change. Until recently, however, their utility was limited to the past 600,000 years by an inability to provide accurate ages for materials older than this, the practical limit of uranium-series geochronology. Recent technological advances, using the uranium-lead decay scheme, now offer the prospect of providing radiometric ages for much older speleothems, thus extending their utility back through much of Earth history. Woodhead et al. explore this potential with an example from Permian cave fills in Oklahoma, which also host one of the most important assemblages of terrestrial vertebrate fossils. The ability to provide robust radiometric ages for these deposits, and similar ones elsewhere, will be of fundamental significance in charting the evolution of many fossil assemblages while the provision of precisely dated “snapshots” of our planet’s diverse climatic history provides a valuable new tool in studies of climate and ecosystem change.
Detrital zircon ages from the Chugach terrane, southern Alaska, reveal multiple episodes of accretion and erosion in a subduction complex
Jeffrey M. Amato and Terry L. Pavlis, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Pages 459-462.
Modern subduction zones, such as the Aleutian Trench in Alaska, are where oceanic plates descend into the Earth, resulting in earthquakes and volcanism. As the plate enters the trench, any sediment on top of the plate is typically scraped off and becomes part of the overlying crust, a process called subduction accretion. Sometimes this sediment is subducted with the plate, a process called subduction erosion. Today, this is happening in various locations but below sea level. In southern Alaska near Anchorage, an ancient subduction zone ranging from 200 to 60 million years old is exposed on land, allowing detailed study of these processes. The sediment scraped off forms the Chugach terrane. Zircon minerals in this sediment were collected across a 15-km-long transect and radiometrically dated using uranium-lead isotopes. Amato and Pavlis find that the ages fall into two groups: Some that indicate deposition around 150 million years ago, and others that were much younger, around 90 million years ago. They attribute the gap between these two ages as indicative of a period of subduction erosion. This may have been triggered by the subduction of an oceanic magmatic ridge, or spreading center, around 120 million years ago.
Detrital zircon record of thrust belt unroofing in Lower Cretaceous synorogenic conglomerates, central Utah
Timothy F. Lawton et al., Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Pages 463-466.
In a study by Lawton et al., the determination of ages of individual zircon grains in lithified gravel and sand derived from an ancient mountain range, termed the Sevier Orogenic Belt and which formed during the Cretaceous Period, documents progressive erosion of the mountains during the early part of the Cretaceous. Zircon is a very durable mineral that forms in igneous rocks and incorporates trace quantities of uranium into its crystal structure. When liberated from the original granite, the zircon grains retain their uranium as a signature of the original source, but the zircons may be recycled as sand grains through many episodes of erosion and deposition. The ages of the zircon grains were determined by measuring their uranium and lead concentrations by means of a laser beam connected to an instrument termed a mass spectrometer, which determines the isotope abundances present due to decay of the radioactive uranium. The resulting ages analyzed in the sandy material can be tied back to individual formations that were uplifted in the mountain belt and eroded from it. This technique offers promise to evaluate the uplift histories of ancient mountain belts elsewhere in the world and might prove useful in establishing connections between individual formations having similar zircon age populations.
Artemis, Venus: The largest tectonomagmatic feature in the solar system?
Vicki L. Hansen and Anthony Olive, Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA. Pages 467-470.
New geologic mapping reveals that Artemis, a unique circular feature on Venus, is much larger than previously recognized. Artemis includes a wide outer trough (greater than 5000 km in diameter), a radial dike swarm (12,000 km diameter), and a concentric wrinkle ridge suite (13,000 km diameter). Artemis’ evolution included formation of an interior region surrounded by an ~2,000-km-diameter, 2-km-deep circular trough, an outer broad circular high, and a wide outer trough (greater than 5000 km diameter), a 12,000 km diameter swarm of magma-filled fractures (called dikes) that radiate from the trough and cut upward through the crust, and a 13,000-km-diameter suite of ridges concentric to the deep trough. All of these features are likely genetically related. Escape of dike magma to the surface formed local cover deposits that buried parts of the radial fracture suite. Concentric ridges cut the cover deposits; they were likely formed due to coupling of convective flow within the underlying mantle and the crust. The outer trough formed late relative to radial fractures, cover deposits, and wrinkle ridges. Hansen and Olive suggest that Artemis represents the signature of a deep mantle thermal plume acting on a relatively thin lithosphere. Artemis may represent the largest tectonomagmatic feature in the solar system.
Rift zone abandonment and reconfiguration in Hawaii: Mauna Loa’s Ninole rift zone
Julia K. Morgan et al., Dept. of Earth Science, Rice University, 6100 Main Street, Houston, Texas 77005, USA. Pages 471-474.
Large oceanic volcanoes are commonly characterized by elongate rift zones that transfer magmas from the summits to the distal portions of the edifice. These rift zones are easily recognized on Hawaiian volcanoes, including Mauna Loa’s extensive southwest rift zone and northeast rift zone. A new onshore-offshore seismic velocity study by Morgan et al. of Mauna Loa volcano in Hawaii, however, reveals the presence of a very large south-trending rift zone within this enormous volcano, now buried by younger surface flows and difficult to recognize from the surface geology. This inactive rift zone, referred to as the Ninole Rift Zone, formed early during the history of Mauna Loa and was subsequently abandoned in favor of the active southwest rift zone. Morgan et al. suggest that rift zone abandonment and reconfiguration was caused by large-scale landsliding of Mauna Loa’s western flank, perhaps multiple times. Similar rift zone migration may be underway at Kilauea volcano, and could one-day lead to the abandonment of the east rift zone. Such rift zone reconfiguration is probably much more common than previously assumed, and may enable the growth of very large volcanic edifices such as Mauna Loa.
Evidence for an active shear zone in southern Nevada linking the Wasatch fault to the Eastern California shear zone
Corné Kreemer et al., Nevada Bureau of Mines and Geology, and Nevada Seismological Laboratory, University of Nevada, Reno, Nevada 89557-0178, USA. Pages 475-478.
Seismologists have long known of an enigmatic, east-west-trending seismicity zone in southern Nevada. In this study by Kreemer et al., crustal motions derived from high-precision global positioning system measurements now show that this seismic zone accommodates up to 2 mm/yr of left-lateral shear. The motion across this shear zone (named the Pahranagat shear zone) diminishes from east-to-west, consistent with the shear zone being a boundary between the non-deforming crust to its north and an area of diffuse east-west extension to its south. This diffuse extensional zone extends into the southwestern Colorado Plateau and includes Las Vegas and most of the Mojave Desert. The shear zone makes a natural connection between the Wasatch fault zone in Utah and the Eastern California shear zone. The shear zone is a required component of a southward-broadening extensional zone that is narrow across the Wasatch fault zone and wide in the Mojave Desert and southern Colorado Plateau.