Magnitude of crustal extension across the northern Basin and Range province: constraints from paleomagnetism

The magnitude of crustal extension across the northern Basin and Range province is a matter of longstanding controversy; estimates range from 10 to 300%. Recently published estimates of extension across the southern Basin and Range province (36[deg]N) are in the range of 80-100%. Thus, the larger values suggested for the northern part of the province (40[deg]N) seem to require substantial counterclockwise rotation of the Sierra Nevada during Tertiary extension. Paleomagnetic data from the range, however, limit rotation to 4 +/- 10[deg] at the 95% confidence level. These limits, combined with estimates of extension near the Garlock fault, allow severe constraints to be placed on the magnitude of extension across more northerly parts of the province. We conclude that the maximum extension at 40[deg]N is about 50% and that values of 39 +/- 12% (188 +/- 43 km) are likely.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25578/1/0000122.pd

Evidence of Uppermost Proterozoic to Lower Cambrian miogeoclinal rocks and the Mojave-Snow Lake Fault: Snow Lake Pendant, central Sierra Nevada, California

This is the published version. Copyright 2010 American Geophysical Union. All Rights Reserved.Displaced uppermost Precambrian to Lower Cambrian miogeoclinal strata occur within Snow Lake pendant in the central Sierra Nevada. These rocks have been correlated with the Stirling Quartzite, the Wood Canyon Formation, the Zabriskie Quartzite, and the Carrara Formation in the western Mojave Desert and the San Bernardino Mountains (Lahren and Schweickert, 1989; Lahren, 1989). This correlation is based on new, updated, and previously reported data including (1) lithologic similarities, (2) overall stratigraphic sequence, (3) vertical sequence within individual formations, (4) approximate stratigraphic thicknesses, (5) Skolithos in the correct stratigraphie position, (6) depositional environments, and (7) petrographic character and provenance of quartz arenites. The correlation is strengthened by the fact that Snow Lake pendant and the western Mojave share many other close similarities including (1) initial 87Sr/86Sr ratios of associated granitic rocks >0.706, (2) passive margin tectonic setting of Precambrian to Cambrian miogeoclinal rocks, (3) dikes of the Independence dike swarm, (4) possible Lower Triassic overlap sequence, the Fairview Valley Formation, (5) petrographically similar gabbroic complexes of the same age, (6) associated eugeoclinal rocks, and (7) identical(?) pre-Tertiary structural configuration. New U/Pb zircon geochronology unequivocally shows that dikes at Snow Lake pendant are coeval with the Independence dike swarm of the eastern Sierra and the western Mojave desert and that associated gabbroic complexes in both the Mojave and Snow Lake pendant are the same age. Correlation of Snow Lake pendant with the western Mojave requires about 400 km of dextral displacement of the rocks of Snow Lake pendant, together with associated rocks (Snow Lake block), from the western Mojave Desert along the Mojave-Snow Lake fault. Displacement most likely occurred after 150 Ma, the age of the Independence dike swarm, and before about 110 Ma, the age of major plutons within the Sierra Nevada batholith. This interpretation, if correct, holds major implications for allochthonous terranes west of Snow Lake pendant, which were probably attached to the Snow Lake block before its northward transport. In addition, a number of Paleozoic and Mesozoic tectonic features in western Nevada and eastern California may have been offset dextrally along the proposed Mojave-Snow Lake fault

Paleomagnetism of Jurassic Rocks in the Western Sierra Nevada Metamorphic Belt and its Bearing on the Structural Evolution of the Sierra Nevada Block

The western metamorphic belt of the Sierra Nevada consists of two eugeosynclinal terranes separated by the Melones and Sonora faults. Subvertical, bedded Mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west, whereas Paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east. In order to study the structural development of the faults, 121 samples of basalt and diabase were collected for paleomagnetic analysis from three Jurassic formations, the Logtown Ridge and Penon Blanco formations west of the Melones fault and the Sonora dike swarm to the east of the Sonora fault. A northwesterly, downward directed magnetization occurs in each unit. Three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary, postdating Nevadan (Late Jurassic) folding and is probably coeval with peak metamorphism. An average of five paleomagnetic poles from the Sierra Nevada, three derived from the secondary magnetizations given herein and two previously published, all of probable Kimmeridgian age, yields λ′=67.2°N, ϕ′=161.2°E, and α95 =6.5°. Southeasterly magnetizations also occur in the Logtown Ridge Formation and Sonora dike swarm. Directions from the Sonora dikes are approximately antipodal to the secondary directions and are reversed; magnetizations from the Logtown Ridge Formation yield similar results only if corrected for the tilt of bedding. The Logtown Ridge magnetizations (tilt-corrected) yield a pole position near to that expected for North America. The data from the Sonora dikes require a tilt correction of 25°-30° toward the south-southwest about a horizontal axis parallel to the regional structure in order to yield a North American pole position. We conclude that the eastern wall rocks of the Melones and Sonora faults have been rotated 25°-30° in response to Nevadan deformation in contrast to the western wall rocks, which have been rotated about 90°

Paleotectonic and paleogeographic significance of the Calaveras Complex, western Sierra Nevada, California

The Calaveras Complex of the western Sierra Nevada, as defined here, consists of a 375 km long, 35 km wide belt of metasedimentary and metavolcanic rocks, bounded on the west by the Melones fault zone and Kings-Kaweah suture, and on the east by the Sierra Nevada batholith. The Calaveras Complex forms a continuous northwest-trending belt between the Placerville area and the Merced River area. South of the Merced River the belt extends in numerous roof pendants at least as far south as the Tule River. A sequence of four lithologic units is recognized, each of which is thousands of meters thick. Precise original stratigraphic thicknesses cannot be measured because of intense soft-sediment and post-consolidation deformation. The lowest unit consists of mafic pillow lava, breccia, tuff, and argilllte, and may represent layer 2 of oceanic crust. This basal unit is overlain by a predominantly chaotic unit of argillite with variable amounts of chert and siltstone often occurring as clasts in a diamictite. Olistoliths of shallow water limestone are locally an important component of this argillite unit. The overlying chert unit contains abundant large olistoliths of rhythmically bedded chert and locally important limestone olistoliths in a matrix of streaky argillite and diamictite. The highest unit included within the Calaveras Complex contains abundant, well-bedded quartzite with abundant interbedded olistostromes containing quartzite clasts and limes tone olistoliths. Fossils from limestone olistoliths reported here indicate a maximum Permo-Carboniferous age for the upper part of the argillite unit, and a maximum late Permian age for the over lying chert unit. Published fossil data indicate the upper parts of the quartzite unit are late Triassic to early Jurassic. The argillite and chert units apparently comprise numerous olistostromes that accumulated on oceanic crust in a marginal basin that was broad enough to have been relatively free of elastic detritus derived from the basin margins. Olistostromes apparently were shed from tectonically elevated areas within the marginal basin that were denuded of their pelagic and hemi pelagic cover. The quartzite unit may represent an early Mesozoic northwestward progradation of mature continent-derived sand across the western end of the late Paleozoic marginal basin. The marginal basin is considered to have been situated between the Cordilleran miogeocline to the southeast and a volcanic arc terrane to the northwest. The late Paleozoic Havallah sequence of north-central Nevada is believed to have accumulated in the same marginal basin. The Melones fault zone and Kings-Kaweah suture represent a zone of early Mesozoic tectonic truncation a long which the Calaveras Complex is juxtaposed against upper Paleozoic ophiolitic rocks and Jurassic volcanic and epiclastic rocks. Thus, we infer that the Calaveras Complex represents the westernmost exposure of the late Paleozoic marginal basin

Surviving Sepsis Campaign: international guidelines for management of severe sepsis and septic shock, 2012

OBJECTIVE: To provide an update to the "Surviving Sepsis Campaign Guidelines for Management of Severe Sepsis and Septic Shock," last published in 2008. DESIGN: A consensus committee of 68 international experts representing 30 international organizations was convened. Nominal groups were assembled at key international meetings (for those committee members attending the conference). A formal conflict of interest policy was developed at the onset of the process and enforced throughout. The entire guidelines process was conducted independent of any industry funding. A stand-alone meeting was held for all subgroup heads, co- and vice-chairs, and selected individuals. Teleconferences and electronic-based discussion among subgroups and among the entire committee served as an integral part of the development. METHODS: The authors were advised to follow the principles of the Grading of Recommendations Assessment, Development and Evaluation (GRADE) system to guide assessment of quality of evidence from high (A) to very low (D) and to determine the strength of recommendations as strong (1) or weak (2). The potential drawbacks of making strong recommendations in the presence of low-quality evidence were emphasized. Recommendations were classified into three groups: (1) those directly targeting severe sepsis; (2) those targeting general care of the critically ill patient and considered high priority in severe sepsis; and (3) pediatric considerations. RESULTS: Key recommendations and suggestions, listed by category, include: early quantitative resuscitation of the septic patient during the first 6 h after recognition (1C); blood cultures before antibiotic therapy (1C); imaging studies performed promptly to confirm a potential source of infection (UG); administration of broad-spectrum antimicrobials therapy within 1 h of the recognition of septic shock (1B) and severe sepsis without septic shock (1C) as the goal of therapy; reassessment of antimicrobial therapy daily for de-escalation, when appropriate (1B); infection source control with attention to the balance of risks and benefits of the chosen method within 12 h of diagnosis (1C); initial fluid resuscitation with crystalloid (1B) and consideration of the addition of albumin in patients who continue to require substantial amounts of crystalloid to maintain adequate mean arterial pressure (2C) and the avoidance of hetastarch formulations (1B); initial fluid challenge in patients with sepsis-induced tissue hypoperfusion and suspicion of hypovolemia to achieve a minimum of 30 mL/kg of crystalloids (more rapid administration and greater amounts of fluid may be needed in some patients (1C); fluid challenge technique continued as long as hemodynamic improvement is based on either dynamic or static variables (UG); norepinephrine as the first-choice vasopressor to maintain mean arterial pressure ≥65 mmHg (1B); epinephrine when an additional agent is needed to maintain adequate blood pressure (2B); vasopressin (0.03 U/min) can be added to norepinephrine to either raise mean arterial pressure to target or to decrease norepinephrine dose but should not be used as the initial vasopressor (UG); dopamine is not recommended except in highly selected circumstances (2C); dobutamine infusion administered or added to vasopressor in the presence of (a) myocardial dysfunction as suggested by elevated cardiac filling pressures and low cardiac output, or (b) ongoing signs of hypoperfusion despite achieving adequate intravascular volume and adequate mean arterial pressure (1C); avoiding use of intravenous hydrocortisone in adult septic shock patients if adequate fluid resuscitation and vasopressor therapy are able to restore hemodynamic stability (2C); hemoglobin target of 7-9 g/dL in the absence of tissue hypoperfusion, ischemic coronary artery disease, or acute hemorrhage (1B); low tidal volume (1A) and limitation of inspiratory plateau pressure (1B) for acute respiratory distress syndrome (ARDS); application of at least a minimal amount of positive end-expiratory pressure (PEEP) in ARDS (1B); higher rather than lower level of PEEP for patients with sepsis-induced moderate or severe ARDS (2C); recruitment maneuvers in sepsis patients with severe refractory hypoxemia due to ARDS (2C); prone positioning in sepsis-induced ARDS patients with a PaO (2)/FiO (2) ratio of ≤100 mm Hg in facilities that have experience with such practices (2C); head-of-bed elevation in mechanically ventilated patients unless contraindicated (1B); a conservative fluid strategy for patients with established ARDS who do not have evidence of tissue hypoperfusion (1C); protocols for weaning and sedation (1A); minimizing use of either intermittent bolus sedation or continuous infusion sedation targeting specific titration endpoints (1B); avoidance of neuromuscular blockers if possible in the septic patient without ARDS (1C); a short course of neuromuscular blocker (no longer than 48 h) for patients with early ARDS and a PaO (2)/FI O (2) 180 mg/dL, targeting an upper blood glucose ≤180 mg/dL (1A); equivalency of continuous veno-venous hemofiltration or intermittent hemodialysis (2B); prophylaxis for deep vein thrombosis (1B); use of stress ulcer prophylaxis to prevent upper gastrointestinal bleeding in patients with bleeding risk factors (1B); oral or enteral (if necessary) feedings, as tolerated, rather than either complete fasting or provision of only intravenous glucose within the first 48 h after a diagnosis of severe sepsis/septic shock (2C); and addressing goals of care, including treatment plans and end-of-life planning (as appropriate) (1B), as early as feasible, but within 72 h of intensive care unit admission (2C). Recommendations specific to pediatric severe sepsis include: therapy with face mask oxygen, high flow nasal cannula oxygen, or nasopharyngeal continuous PEEP in the presence of respiratory distress and hypoxemia (2C), use of physical examination therapeutic endpoints such as capillary refill (2C); for septic shock associated with hypovolemia, the use of crystalloids or albumin to deliver a bolus of 20 mL/kg of crystalloids (or albumin equivalent) over 5-10 min (2C); more common use of inotropes and vasodilators for low cardiac output septic shock associated with elevated systemic vascular resistance (2C); and use of hydrocortisone only in children with suspected or proven "absolute"' adrenal insufficiency (2C). CONCLUSIONS: Strong agreement existed among a large cohort of international experts regarding many level 1 recommendations for the best care of patients with severe sepsis. Although a significant number of aspects of care have relatively weak support, evidence-based recommendations regarding the acute management of sepsis and septic shock are the foundation of improved outcomes for this important group of critically ill patients

Composite Devonian island-arc batholith in the northern Sierra Nevada, California

The Bowman Lake batholith intrudes the pre-Upper Devonian Shoo Fly Complex in the northern Sierra Nevada. The eastern margin of the batholith extends to within 1.5 km of the base of a thick sequence of Paleozoic island-arc rocks that rests unconformably on the Shoo Fly Complex. Hypabyssal silicic intrusions associated with the batholith penetrate the Upper Devonian Sierra Buttes Formation, the lowest volcanic unit of the arc sequence, but do not extend to higher stratigraphic levels. The intrusions in the Sierra Buttes Formation possess marginal peperites and show other evidence of their injection into wet, unconsolidated sediments. On the basis of these field relations, the batholith is interpreted to represent a subvolcanic magma chamber emplaced concurrently with deposition of Sierra Buttes arc rocks. The batholith is composite and consists of trondhjemite, granodiorite, biotite granite, and hornblende tonalite, formed from discrete batches of magma injected into a common plutonic chamber. Mingling of tonalitic and trondhjemitic magmas produced abundant rounded inclusions of tonalite dispersed within trondhjemite and led to hybridization of the two magmas by fine-scale intermixing. Zircons from the trondhjemitic, granodioritic, and granitic phases of the batholith and an associated Sierra Buttes rhyolite sill show complex U-Pb isotopic systematics. A multistage history is suggested, involving the incorporation of earliest Paleozoic and/or Proterozoic zircons during magma generation or ascent in Devonian time and one or more stages of disturbance in Mesozoic-Cenozoic(?) time. Consideration of these different discordance mechanisms results in a model U-Pb igneous age for the batholith of 364 to 385 Ma (Middle to Late Devonian); on the basis of the field relations, a Late Devonian age for the batholith is adopted. Zircon characteristics and isotopic systematics suggest that petrogenesis of the granite involved mixing of trondhjemitic and/or granodioritic magmas with partial melt from a separate, large-ion-lithophile-enriched source

Franciscan Complex limestone deposited at 17° South paleolatitude

At Laytonville, California, about 230 km north-northwest of San Francisco, three blocks of pelagic limestone, each a few tens of metres long, are incorporated in the Franciscan melange. Bedding is well defined, but top indicators are lacking. Paleomagnetic study of two of the blocks (16 samples, 52 specimens) yielded directions of D: 183.9°, I: −22.6°, α_95: 21.6° (block 1), and D: 229.5°, I: −31.8°, α_95: 10.7° (block 2) relative to the present orientation of bedding. Inclinations were not significantly different, but the difference in declinations shows that magnetization preceded emplacement of the blocks in the melange. Study of foraminifera showed that the blocks are of Albian and Cenomanian age and were deposited during the Cretaceous Long Normal Polarity Interval. Details of the foraminiferal zonation show that both blocks are right side up. The observed inclinations imply deposition on the Farallon plate at 17° ± 7° South paleolatitude at about 96 m.y. B.P. (block 2; block 1 data agree but are less definitive). Consideration of reasonable paleolongitudes and possible times of incorporation of the blocks in the melange indicates a Farallon–North America convergence rate significantly higher than any observed today. The most commonly accepted emplacement time (latest Cretaceous) indicates convergence at about 38 cm/yr (range: 24 to 60 cm/yr). An alternate interpretation, that emplacement occurred at 30 ± 15 m.y. B.P., would indicate convergence at about 15 cm/yr (range: 9 to 24 cm/yr). The convergence rates obtained in the first case are astonishingly high, and they suggest that perhaps current ideas on the Franciscan should be re-evaluated, with consideration given to the possibility that tectonic mixing continued until the middle or late Tertiary. Even so, the Farallon–North American convergence rate was apparently higher than any convergence rate observed today. This rapid convergence coincides with the Cretaceous rapid spreading pulse, and it may be responsible for such features as the large volumes of Late Cretaceous batholiths and subduction complexes in western North America