November-December 2010 GSA Bulletin highlights

Boulder, CO, USA – Topics in the November-December 2010 GSA Bulletin include earthquake hazard assessment, tectonics, fault ruptures, paleo-earthquakes, magmatism, landslides, climate modeling, and geochronology. The issue also reports the first combined field and geochronological investigation of the Big Creek Gneiss and the first optically stimulated luminescence dating of the Bolson sand sheet, including its affect on cultural resource management at Fort Bliss. The invited review article finds a new absolute timeline for first Cambrian appearances of skeletal animals.

Keywords: Cambrian diversification, Oregon Coast Range, “Barrovian” metamorphism, Wasatch fault, pediments, the Malari leucogranite, Eastern Cordillera, Colombia, North Anatolian Fault, Tuolumne intrusion, zircon dating, Ellesmerian clastic wedge, Big Creek Gneiss, Ionian basin, Bolson sand sheet, meteorite impact, shock microstructures, Vredefort Dome, Sub-Milankovitch solar forcing, radioactive waste disposal, Jurassic Moenave Formation, Cascadia subduction zone, Songpan-Ganzi complex, Tarim and Central Asian Orogenic Belt, blueschist.

Highlights are provided below. 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. Abstracts for the November-December issue of GSA BULLETIN are available at Contact Christa Stratton for additional information or assistance.

Earliest Cambrian record of animals and ocean geochemical change

Adam C. Maloof et al., Dept. of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA. Pages 1731-1774.

The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Stage, 17 million years above the base of the Cambrian. To test this idea, Adam C. Maloof of Princeton University and colleagues match earliest Cambrian (Nemakit-Daldynian through Tommotian) records of carbon isotope (delta-13C) variability from Siberia, Mongolia, and China with a Moroccan delta-13C record constrained by five U/Pb ages from interbedded volcanic ashes. This process avoids the circularity of using biostratigraphy for interpreting patterns in the fossil record. They also present new delta-13C from organic matter, and 87Sr/86Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan carbon isotope curve. The result is a new absolute timeline for first appearances of skeletal animals and for changes in the carbon, strontium and redox chemistry of the ocean during the earliest Cambrian. The timeline suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the stage. Fossil first appearances occurred in three pulses that each are superimposed on long-term increases in sea level, the hydrothermal flux of Sr, and the oxygenation of shallow-water sediments.

Sediment reservoirs at mountain stream confluences: Dynamics and effects of tributaries dominated by debris-flow and fluvial processes

Stephen T. Lancaster et al., Dept. of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA. Pages 1775-1786.

Careful study of sediment deposits at the mouths of two different tributaries with similar-sized mainstem streams in the Oregon Coast Range reveals different dynamics of sediment delivery and storage, as well as different effects on sediment routing through the adjacent mainstems. The smaller (0.14 square kilometers), steeper (10%) tributary has a much larger deposit, a fan composed predominantly of bouldery debris-flow deposits, whereas a tributary that is 10 times larger (1.5 square kilometers) and half as steep (5%) has a deposit only one-tenth the size and composed predominantly of fluvial silt, sand, and gravel. The sediment residence times, calculated from extensive radiocarbon dating, in the two tributary deposits are actually similar, at 1400 and 1700 years, respectively, and in both cases, a significant amount of sediment remains for several millennia before being evacuated. The key difference between the tributaries is that sediment delivery from the smaller one is dominated by debris flows with a recurrence interval of about 100 years, whereas debris flows usually stop upstream of the mouth of the larger. The smaller tributary’s fan-shaped, bouldery deposit traps 60% or more of the sediment eroded from the contributing watershed, but less than 1% of the larger tributary’s deposits are trapped at its mouth. The delivery, or lack of delivery, of debris flows to the mainstems has a substantial, and somewhat counter-intuitive, effect on sediment storage in the adjacent mainstem valleys. Bouldery deposits delivered by the smaller tributary help the adjacent mainstem valley store about 7 times the amount of sediment stored in the mainstem adjacent to the larger tributary, but the residence time of sediment in the latter valley reach is almost 10 greater (4000 years vs. 400 years): debris flows entering the former stream force its channel to move and therefore erode deposits more frequently than in the latter, whereas boulders from infrequent debris flows trap some sediments for much longer times, some exceeding 10,000 years. This study by Stephen T. Lancaster of Oregon State University and colleagues implies that small, steep, debris flow-delivering tributaries have disproportionately large effects on the delivery, storage, and transport of gravels crucial for the spawning of threatened and endangered salmonids.

Timing of ophiolite obduction in the Grampian orogen

David M. Chew et al., Dept. of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland. Pages 1787-1799.

Regional-scale metamorphism at intermediate pressures is a common feature in nearly all collisional orogenic belts. This type of metamorphism (“Barrovian” metamorphism) was first characterized in detail by a British Geological Survey field geologist, George Barrow, working in the Scottish Highlands in the early part of the twentieth century. The Grampian metamorphic belt of Scotland and Ireland is thus the type area for Barrovian metamorphism, but the cause remains enigmatic despite a century of research. It is usually attributed to the collision of the margin of an ancient continent (Laurentia), with a chain of volcanic islands and ophiolites (fragments of oceanic lithosphere). David M. Chew of Trinity College Dublin and colleagues demonstrate that the timing of metamorphism on the continental margin of Laurentia took place 20 million years after the timing of metamorphism in the ophiolites. If Barrovian metamorphism on the Laurentian margin was caused by conductive heating in thickened crust, then collisional thickening on the margin must have started soon after ophiolite obduction 490 million years ago to generate the 470 million year old metamorphic peak seen in the Grampian metamorphic belt of Scotland and Ireland.

Shallow subsurface structure of the Wasatch fault, Provo segment, Utah, from integrated compressional and shear-wave seismic reflection profiles with implications for fault structure and development

John H. McBride et al., Dept. of Geological Sciences, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA. Pages 1800-1814.

The Wasatch fault system cuts through much of Utah, acts as a major tectonic boundary for the Basin and Range Province, and is a source of concern for seismic hazard; yet, it has not been extensively explored using high-resolution geophysical methods. John H. McBride of Brigham Young University and colleagues combine geological and geophysical data from a classic geological site along the Wasatch fault in Provo, Utah, in order to produce a shallow-earth image of the fault zone. Their results reveal a complex pattern of geologically recent faulting that includes deformation extending beyond the rupture zone where previous seismic hazard studies had focused. This study thus suggests that earthquake-hazard assessments made without integrated geological and geophysical information may be biased toward a narrower fault rupture zone by the previous mapping of too few faults.

How do pediments form?: A numerical modeling investigation with comparison to pediments in southern Arizona, USA

Jon D. Pelletier, Dept. of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth St., Tucson, Arizona 85721-0077, USA. Pages 1815-1829.

Pediments are gently sloping bedrock plains at the base of mountain ranges. Geologists have debated for over a century about how they form. In this paper, Jon D. Pelletier of the University of Arizona argues that the warping of the crust following erosion of the adjacent mountain leads to a tilting of the plain that can strip it of sediment and soil. A pediment can then form if the climate is sufficiently arid, so that new soil forms slowly on the tilted surface. This new mechanism requires a particular combination of a dry climate and a weak (fractured) crust that helps explain why pediments in the United States are concentrated in the Sonoran and Mojave deserts of Arizona and California.

A relict paleoseismic record of seven earthquakes between 2000 B.C. and 600 A.D. on the central North Anatolian fault at Elmacik, near Osmancik, Turkey

J. Fraser et al., Seismology Section, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium. Pages 1830-1845.

The North Anatolian Fault is a right-lateral strike-slip fault ~1500-km-long, extending approximately east-west across northern Turkey. This study by J. Fraser of the Royal Observatory of Belgium and colleagues links the deposition of localized soil deposits to significant disturbance of a small (about 2 hectares), steep catchment, which is interpreted to reflect the timing of large-magnitude earthquakes on the central section of the North Anatolian Fault, which last ruptured in 1943. Local stream incision caused sedimentation at the trench site to stop about 1000 years ago, so the record of seven earthquakes, occurring about every 500 years locally, is relict. Subsequently, the incised stream has been offset by the fault yielding a slip rate of ~21.4-25.6 mm/yr, suggesting significant variability in the amount of displacement caused by locally by paleo-earthquakes.

The eastern foothills of the Eastern Cordillera of Colombia: An example of multiple factors controlling structural styles and active tectonics

Andres Mora et al., Institut fur Geowissenschaften. Universitat Potsdam, Karl Liebnecht-Str 24, D14476 Potsdam-Golm, Germany. Pages 1846-1864.

Andres Mora of Universitat Potsdam and colleagues decipher and assess the influence of inherited structural fabrics, changes in basin geometry, erosional denudation, and characteristics of the tectonic stress field in the Eastern Cordillera of Colombia. The study area reveals a complex combination of factors that affect structural style and partitioning of active deformation, and that, over time, the most important factor changes, from the role of inherited structural fabrics to the geometries of basin fills.

The Malari leucogranite, Garhwal Himalaya, northern India: Chemistry, age, and tectonic implications

Himanshu K. Sachan et al., Dept. of Geosciences, Boise State University, Boise, Idaho 83725, USA. Pages 1865-1876.

This study by Himanshu K. Sachan of Boise State University and colleagues describes the juxtaposition of Greater and Tethyan Himalayan sequences by a major ductile normal fault, the South Tibetan detachment system, a large-scale normal fault system at the top of the High Himalaya. In the Garhwal region, India, the Malari leucogranite crosscuts both sequences, and therefore provides an age limits the timing of fault movement. A relatively old age for the granite (about 19 million years old) implies that ductile shear had ceased soon after initiation, estimated to have been about 22-24 million years ago. This short-duration movement of 3-5 million years in turn implies a brief period of flattening of the upper Himalayan crust, probably because of weakening associated with partial melting of the Greater Himalayan sequence. This research was supported in part by U.S. National Science Foundation grant EAR-0803549.

New models for Paleoproterozoic orogenesis in the Cheyenne belt region: Evidence from the geology and U-Pb geochronology of the Big Creek Gneiss, southeastern Wyoming

Daniel S. Jones et al., Dept. of Geology and Geophysics, University of Wyoming, 1000 E. University Ave., Laramie, Wyoming 82071, USA. Pages 1877-1898.

Daniel S. Jones of the University of Wyoming and colleagues report the first combined field and geochronological investigation of the Big Creek Gneiss, a metamorphic complex exposed in the southern Sierra Madre of southeastern Wyoming. This metamorphic complex includes a suite of metasedimentary rocks and metavolcanic rocks intruded by two generations of plutonic rocks. The older intrusive suite is about 1.78 billion years old and likely formed in an island-arc setting. The younger intrusive suite is about 1.76 billion years old and is attributed to rifting within the postulated arc. All of these rocks were deformed and metamorphosed about 1.75 billion years ago as they were accreted to North America. This is a significant revision of earlier models in which the arc rocks were interpreted to have accreted shortly after the cessation of arc magmatism with no intervening extensional event. Jones et al. discuss two broader consequences of their results. First, the intrusion of basaltic magmas during the inferred rifting event implies that some crustal growth in this region occurred due to the addition of magmas of mantle originating about 1.76 billion years ago. Second, the revised timing of events in southeastern Wyoming can be correlated with events documented in central Colorado, and therefore, extension and extensional magmatism documented in the Big Creek Gneiss may reflect regional processes.

Detrital zircon geochronology of the western Ellesmerian clastic wedge, northwestern Canada: Insights on Arctic tectonics and the evolution of the northern Cordilleran miogeocline

Luke P. Beranek et al., Geological Survey of Canada, 625 Robson Street, Vancouver, British Columbia, V6B 5J3, Canada. Pages 1899-1911.

The present-day Arctic margin of northern North America was the site of convergent-style plate tectonics from 360 to 430 million years ago. Mountain building was driven by the collision of crustal fragments against the North American continent. Tectonism, rock uplift, and erosion along the Arctic margin resulted in the deposition of shale, sandstone, and conglomerate across northern and northwestern Canada. Luke P. Beranek of the Geological Survey of Canada and colleagues utilize more than 700 U-Pb detrital zircon ages from sandstone in Yukon and Northwest Territories, Canada, to better constrain the evolution of sedimentation adjacent to the Arctic mountain belt and evaluate the origin of the accreted crustal fragments. Their results confirm that rocks originating near the paleocontinent of Baltica, modern day northern Europe, are responsible for mountain building. Based on these data, Beranek et al. discuss the importance of European-derived detrital zircon in rocks along the western margin of North America and speculate on the position of crustal fragments in the Arctic region prior to Cretaceous opening of the western Arctic Ocean.

Magmatic lobes as “snapshots” of magma chamber growth and evolution in large, composite batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California

Valbone Memeti et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Pages 1912-1931.

Magmatic lobes, fingers of magma extending out from central magma chambers, preserve shorter and simpler magmatic histories than those preserved in large central magma chambers. Thus, studies of lobes provide a powerful means of unraveling the complex processes that occur in large magma chambers. UCLA’s Valbone Memeti and colleagues use precise CA-TIMS U-Pb zircon ages in combination with detailed field mapping, 40Ar/39Ar thermochronology, and finite difference thermal modeling in lobes of the Tuolumne batholiths. Their analyses indicate that lobe-sized magma chambers were present between ~0.2 to 1 million years ago, during which fractionation and some remixing formed a concentric pattern of normal compositional zoning, inward crystallization, and widespread zircon recycling. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary but that the locus of magmatism shifted both inward and northwestward.

Large-scale sedimentary recycling of tectonic melange in a forearc setting: The Ionian basin (Oligocene-Quaternary, southern Italy)

William Cavazza and Mirko Barone, Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy. Pages 1932-1949.

For 11 million years, from ~17 to 6 million years ago, large sediment landslides were generated in the central Mediterranean region along the collision front between the Corsica-Sardinia-Calabria microplate and northern Africa. This study by William Cavazza and Mirko Barone of the University of Bologna traces the largest (up to 1 km thick) landslide over a distance of more than 70 km, and determines a total volume in excess of 100 cubic kilometers.

Geochronology of the Bolson sand sheet, New Mexico and Texas, and its archaeological significance

Stephen A. Hall et al., Red Rock Geological Enterprises, 3 Cagua Road, Santa Fe, New Mexico 87508-8116, USA. Pages 1950-1967.

The Bolson sand sheet in the Tularosa Valley, New Mexico, and the Hueco Bolson, Texas, consists of two principal eolian sand units with a combined thickness of less than 2 meters. The first optically stimulated luminescence (OSL) dating in the region provides a new chronology of the sand sheet that relates well to the formation, preservation, and visibility of the local archaeological record. Stephen A. Hall of Red Rock Geological Enterprises and colleagues analyze multiple OSL ages that indicate a slow net sedimentation rate of 0.06𔂾.09 mm/yr and elevated amounts of airborne silt in the upper sand unit, representing higher amounts of dust in the atmosphere during glacial and late-glacial time. According to Hall et al., the sand sheet experienced slow, continuous growth through mid-Holocene times but has been stable for the past 5,000 years, with the exception of the development of thousands of coppice dunes during the twentieth century. Archaeological sites that postdate 3,000 B.C. are also concentrated, sometimes together, on the surface of the sand sheet. Previous chronologies of the sand sheet were based on radiocarbon dates of soil carbonates and of charcoal from these archaeological sites, as well as on soil-geomorphology correlations with Rio Grande Valley alluvium. Hall et al. find that the new OSL chronology does not support these various correlations and recommend that the alluvial names Isaacks’ Ranch, Fillmore, and Organ no longer be applied to the Bolson sand sheet. The new OSL chronology is also now being applied to cultural resource management at Fort Bliss.

A record of ancient cataclysm in modern sand: Shock microstructures in detrital minerals from the Vaal River, Vredefort Dome, South Africa

Aaron J. Cavosie et al., Dept. of Geology, University of Puerto Rico, P.O. Box 9000, Mayaguez, Puerto Rico 00681, USA. Pages 1968-1980.

Since its formation, Earth has experienced numerous meteorite impacts, some catastrophic enough to cause global climate change and mass extinctions. The geologic record of impacts is fragmentary, however, because most impact structures are eroded or have been buried over time. To explore new ways that evidence of ancient impact events might be preserved, Aaron J. Cavosie of the University of Puerto Rico and colleagues investigate sand grains eroding from the 2-billion-year-old Vredefort Dome meteorite impact structure in South Africa, the largest and oldest so far identified on Earth. The high pressures and temperatures of meteorite impacts cause unique textures (shock microstructures) to form in minerals within rocks that are hit by a meteorite; this study evaluates whether these unusual microstructures survive in sand grains. Cavosie et al. collected sand samples from the Vaal River, a large meandering river actively eroding the Vredefort Dome. They identify shock microstructures in multiple grains of detrital quartz (SiO2), zircon (ZrSiO4), and monazite (CePO4) — the first such discovery of shocked minerals preserved in sand. This study, according to Covosie and his colleagues, is proof of the concept that shocked minerals can be identified in sediments up to 2 billion years after an impact event, thus demonstrating their potential for preserving evidence of ancient impacts. The recognition of a new geological repository for impact evidence provides a means for identifying transported shocked detritus from eroded structures of any age and may be particularly relevant to early Earth studies.

Sub-Milankovitch solar forcing of past climates: Mid and late Holocene perspectives

Samuli Helama et al., Dept. of Geology, P.O. Box 64, 00014 University of Helsinki, Finland. Pages 1981-1988.

Because of climate change, the relationships between solar activity variations and climate on Earth have probably never been more controversial. Only geological records can reveal the Sun-climate relationships on multi-centurial and longer timescales prior to the telescopic era. However, the chronological uncertainties in these records have so far impeded detailed Sun-climate studies, leaving the geological correlations ambiguous. Samuli Helama of the University of Helsinki and colleagues present a comparison of high-resolution reconstructions of temperatures and sunspots. These records were derived both from fossil and modern tree-rings, and thus, are without chronological uncertainties due to dendrochronological dating methods. The analyses exhibit a very detailed picture of Sun-climate linkages over the current interglacial period. Among other findings, the results guide Helama et al. to state that the climate system has not yet fully responded to the current episode of high sunspot numbers (that of the 20th Century) due to a century-long lag that has existed in this response through the Holocene. Consequently, the climate models predicting the 21st Century climate could be underestimating the near-future warming trends due to the compound influence of anthropogenic and natural forces.

Evaluation of halite dissolution at a radioactive waste disposal site, Andrews County, Texas

Robert M. Holt and Dennis W. Powers, Dept. of Geology and Geological Engineering, University of Mississippi, University, Mississippi 38677-1849, USA. Pages 1989-2004.

Waste Control Specialists (WCS) has been granted permits to dispose of radioactive waste at their surface facility in western Andrews County, Texas, USA. The facility is located over Permian-age salt-bearing formations, and the possibility of dissolution and its effects on the long-term performance of the disposal site have to be considered. Robert M. Holt and Dennis W. Powers of the University of Mississippi assess the subsurface geology of the site area and compared it to three conceptual hydrologic models of dissolution processes. They find no evidence of past dissolution of salt from beneath the site. Furthermore, Holt and Powers find that the salt at depth behaves as a ductile material and fluid flow from the salt beds is outward, limiting the potential for future dissolution.

Magnetostratigraphy of the uppermost Triassic and lowermost Jurassic Moenave Formation, western United States: Correlation with strata in the United Kingdom, Morocco, Turkey, Italy, and eastern United States

Linda L. Donohoo-Hurley et al., Dept. of Earth and Planetary Sciences, MSC 03 2040 1, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA. Pages 2005-2019.

The boundary interval between the Triassic and Jurassic periods (at about 200 million years ago) has been associated with global climate change, mass extinction, and considerable magmatic activity associated with the initiation of break-up of the supercontinent Pangea. Understanding of the timing of events at this boundary is limited due to uncertainties in the correlation of marine and terrestrial sedimentary rock sequences across this boundary. In this paper, Linda L. Donohoo-Hurley of the University of New Mexico and colleagues present new, detailed paleomagnetic data obtained from uppermost Triassic to lowermost Jurassic Moenave Formation, Utah and Arizona. They then use magnetostratigraphy to correlate the timing of Moenave deposition to the marine successions exposed at St. Audrie’s Bay, southern United Kingdom, southern Alps, Italy, and Oyuklu, Turkey, with the terrestrial sedimentary sequences of the High Atlas, Morocco (with associated basalt flows), and the Newark Basin, Eastern North America. The results of this study provide a framework for interpreting the timing of events associated with this important period in Earth’s history.

A Permian large igneous province in Tarim and Central Asian Orogenic Belt (CAOB), NW China: Results of a ca. 275 Ma mantle plume?

Chuan-Lin Zhang et al., Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China. Pages 2020-2040.

Chuan-Lin Zhang of the Chinese Academy of Sciences and colleagues present new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon and geochemical data along with a synthesis of existing stratigraphical, geochronological, and geochemical data. Results from the Tarim and Central Asian Orogenic Belt (CAOB) in northwest China suggest the presence of a Permian (about 275 million year old) Large Igneous Province (the Bachu LIP). The LIP consists of dominantly coeval mafic rocks (basalts and mafic-ultramafic intrusions) with an aerial coverage of more than 600,000 square kilometers, and was accompanied by voluminous A-type granites. This LIP, interpreted to be of mantle plume origin, occurred about 15 million years before the ~260 million-year-old Emeishan LIP in southwestern China and about 25 million years before the 251 million-year-old Siberian Trap in Russia. Such a sudden flare-up of plume activities in the Permian may represent the early stage of the dipolar Pangea and southwest Pacific superplumes. The Permian plume event likely played a role in the late Paleozoic continental crust growth in the CAOB. In addition, there appear to be two types of mantle geochemical provinces (domains) in the region — a long-term enriched Tarim Province and a subduction-metasomatized depleted CAOB province.

Detrital zircon provenance from three turbidite depocenters of the Middle-Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production

A.L. Weislogel et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Pages 2041-2062.

In the central part of China lies a broad region approximately the size of Colorado known as the Songpan-Ganzi complex. This region is often referred to as “China’s Bermuda Triangle” given its triangular shape and is mysterious origin. The area is composed of over 2 million cubic kilometers of Triassic sediments deposited in a deep-water basin. Many different hypotheses have been proposed to explain the origin of these deposits. New work by A.L. Weislogel of Stanford University and colleagues studying the age of detrital zircon grains in the Songpan-Ganzi complex has determined that the Songpan-Ganzi complex is actually made up of several smaller deep-water basins that were compressed together during plate tectonic collision. Zircon ages link sediments of each sub-basin to source areas in surrounding mountain belts, marking a period of great erosion as central China formed by collisional tectonic processes.

Subduction complex uplift and exhumation and its influence on Maastrichtian forearc stratigraphy in the Great Valley Basin, northern San Joaquin Valley, California

Christopher Mitchell et al., Stanford University, Geological and Environmental Sciences, Braun Hall, 450 Serra Mall, Stanford, California 94305, USA. Pages 2063-2078.

New seismic reflection data from Christopher Mitchell of Stanford University and colleagues suggest that the blueschist rocks in the coast ranges of California were brought to the surface by erosion. Previous researchers have argued that these blueschist rocks, which are part of the Franciscan geological complex, were exhumed by tectonic processes. However, a seismic examination of the northern San Joaquin Valley shows sediments, from a previously unrecognized landmass, that were deposited during the Late Cretaceous period. This landmass eroded quickly, exhuming the Franciscan blueschist buried more than 20 km below.

Rupture area and displacement of past Cascadia great earthquakes from coastal coseismic subsidence

Lucinda J. Leonard et al., Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada. Pages 2079-2096.

Progressively buried marshes along the coast of northern California, Oregon, Washington, and British Columbia record a 6500-year history of abrupt, vertical displacements in great earthquakes on the Cascadia subduction zone. Lucinda J. Leonard of the Geological Survey of Canada and colleagues compile radiocarbon ages and displacement estimates for past events and use the displacement data to constrain slip on the megathrust fault. They find that the marsh data are consistent with rupture extents defined by the record of deep-sea landslide deposits (turbidites), with an average recurrence that increases from about 230 years in the south to about 480 years in the north. Displacements tend to be approximately constant from event to event, suggesting limited variability among the dominant long-rupture events. At the northern and southern ends of the margin, slip in the last great earthquake (A.D. 1700) was consistent with the expected release of strain accumulated over the 200 years since the previous event. However, in southern Washington and northern Oregon, unusually high slip in A.D. 1700 may indicate catch-up slip making up for a deficit in the region from smaller slip in the preceding event.

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