fluid inclusions and hydrothermal alteration on the dixie valley fault nevada footwall rocks of the 1954 rupture segment of the dixie valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages an early biotite feldspar assemblage is followed by later fe chlorite and epidote both chlorite and epidote are replaced by hydrothermal sericite and cross cut by calcite hematite and quartz calcite veins biotite is partially replaced by prehnite the latest hydrothermal minerals are stilbite laumontite kaolinite alunite smectite illite and pervasive replacement of rock units with fine grained quartz chalcedony and opal secondary fluid inclusions trapped in healed microfractures in igneous quartz include type i inclusions that contain a moderate salinity aqueous liquid and vapor type ii inclusions that contain a moderate salinity aqueous liquid and co2 type iii inclusions that show eutectic melting temperatures below the nacl h2o eutectic and contain substantial cacl2 and type iv inclusions containing halite and other daughter minerals microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures salinities of type i inclusions vary from 0 1 to 12 9 wt nacl with the mode in the interval 0 to 1 salinities of type ii co2 bearing inclusions range from 0 62 to 6 81 wt nacl relative to h2o and salinities of type iii inclusions with low eutectic melting temperatures are 12 9 to 25 3 nacl equivalent wt salinities of halite bearing inclusions are 30 1 to 39 2 wt nacl homogenization temperatures span the range 120 to 400 c the processes of isochemical cooling with upward displacement of the footwall mixing of cool low salinity water with hotter components and mixing of cool evaporite brine with hotter components could be responsible for variable fluid inclusion compositions homogenization temperatures and densities the p t path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics the path includes a lithostatic fluid pressure at 305 c and 1570 bars along with cooling and escape of co2 from fluids the fluid p t path probably approaches hydrostatic pressure conditions at lower temperatures hydrothermal alteration product minerals fluid temperatures pressures and compositions in the footwall of the dixie valley fault constrain minimum fault age to 20 to 25 ma displacement to 6 km with about 3 km of pre 10 to 13 ma and 3 km of post 10 ma uplift fluid compositions and p t data suggest the following mechanism for rupture initiation and arrest ruptures may be initiated as a result of high fluid pressures then opening of dilatant fractures causes drastic decrease in fluid pressure separation of steam and co2 the drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced in areas where fluid pressure reduction is not accompanied by phase separation fractures are arrested by dilatant hardening

regional variability in the frequency and magnitude of large explosive volcanic eruptions quantifying the frequency at which volcanic eruptions of different size occurs is impor tant for hazard assessment volcanic records can be used to estimate the recurrence rate of large magnitude eruptions magnitude 4 but recording biases that impact data complete ness complicate analysis to overcome these biases we conceptualize the volcanic record as a series of individual and unique time series associated by a common behavior thus we approach issues of completeness on a volcano by volcano basis and use a hierarchical bayes ian approach to characterize the common frequency magnitude f m behavior for different groups of volcanoes we identify variations in the f m relationship between different volcano types and between different volcanic arcs by accounting for systematic under recording in the volcanic record we also calculate the global recurrence rates for large magnitude erup tions during the holocene which are similar to previous estimates however higher recur rence rates for smaller magnitude events are observed which is a result of our adjustments for data completeness quantifying how the f m relationship varies between different groups of volcanoes provides an opportunity to understand how the tectonic setting in uences f m behavior which is important to quantify long term regional volcanic hazard

fluid inclusion evidence for minimum 11 km vertical offset on the wasatch fault utah the footwall of the wasatch fault in the corner creek area near salt lake city is hydrothermally altered and deformed quartz monzonite of the oligocene little cottonwood stock secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation thermometric measurements of fluid inclusion characteristics indicate entrapment of a co2 h2o nacl fluid at minimum temperatures of 223 353 c and minimum fluid pressures of 900 2800 bar the 2800 bar fluid pressure is near lithostatic pressure at a depth of 11 km the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates the large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall and by geologic reconstruction of the eroded footwall

normal fault thermal regimes and the interpretation of low temperature thermochronometers abstract exhumation rates inferred from thermochronometers are dependent on spatial and temporal variations in temperature in active extensional mountain belts the temperature field is complicated by tectonic and surface processes including 1 lateral heat flow across large range bounding normal faults due to the juxtaposition of a cool hanging wall and a relatively warmer footwall 2 uplift and erosion of the footwall 3 sedimentation and burial of the hanging wall 4 lateral heat refraction around low thermal conductivity sediments deposited in the hanging wall basin and 5 3d temperature variations due to high relief topography developed on the footwall we explore these mechanisms through a series of 2d conductive thermal models designed to investigate the effect of tectonics and topography on apatite fission track aft and u th he thermochronometer data models were tuned to the geometry and kinematics of the wasatch mountains utah usa the principal parameters in our model are exhumation and burial rates ranging from 0 2 to 5 7 mm per year at the range front and decreasing with distance from the fault surface morphology taken from usgs digital elevation models and basin geometries inferred from seismic and gravity surveys predicted aft and u th he ages were generated using cooling rate dependent annealing and diffusion kinetic models results indicate after 10 million years of exhumation footwall u th he and aft closure temperature isotherms within 10 km of the fault are advected upward 500 and 1000 m respectively from their initial position the upward advection of isotherms and the 2d nature of the thermal regime can result in erroneous exhumation rates calculated from plots of sample elevation versus age using 1d thermal models for simulations with a uniform vertical uplift rate and canyon and ridge topography 1d and 2d exhumation rate differences were 20 70 for u th he and 10 for aft data samples collected perpendicular to fault strike up the range front are sensitive to the exhumation rate and footwall tilt differences between 1d and 2d range front exhumation rates were 10 95 for u th he and 10 40 for aft data furthermore 1d thermal models are incapable of deciphering footwall tilt at simulated exhumation rates of 3 4 and 5 mm per year predicted u th he and aft ages form closely spaced near vertical lines on a plot of elevation versus sample age therefore suggesting a decreased sensitivity of data to topography and surface processes at high exhumation rates despite large errors in previously collected aft data from the wasatch mountains our model suggests a revised fault adjacent exhumation rate of 0 5 0 6 mm per year compared to previous estimates of 0 7 mm per year based on 1d thermal models

paleoclimatic implications of late pleistocene sediment yield rates for the bonneville basin northern utah abstract climate factors such as precipitation and temperature as well as climate driven geomorphic processes such as glaciation affect rates of erosion and the amount of sediment supplied from a drainage basin sediment yield thus sediment yield rates provide paleoclimatic implications although very little work has been published on ancient sediment yield rates late pleistocene lake bonneville utah idaho and nevada with its well documented lake level history and well preserved deltaic sequences provides a unique opportunity to estimate ancient sediment yield rates two end member delta systems coarse grained gravel classic gilbert type deltas and fine grained sand silt and clay deltas are exposed along the eastern margin of paleolake bonneville one example of each end member delta is examined in this study american fork and weber river deltas the established bonneville hydrograph can be used to determine time of deposition for a given package of sediments because these shoreline deltaic sediments were deposited in shallow water the geomorphic expressions of these deltas are well preserved and their sediment volumes can be determined river drainage basin area can be combined with time constraints and delta volumes to estimate sediment yield rates sediment yield rate estimates for the coarse grained american fork and fine grained weber river deltas are 478 m 3 km 2 yr 1 34 and 705 m 3 km 2 yr 1 15 respectively the similarity of these values is somewhat surprising due to the large difference in drainage basin sizes american fork 160 km 2 and weber river 3328 km 2 however this similarity suggests that precipitation may be the dominant control on sediment yield using sediment yield precipitation regression equations developed for similar modern basins in the southern alps of new zealand a first order approximation of paleoprecipitation is 1 6 m yr 14 for american fork delta and 1 9 m yr 7 for weber river delta these paleoprecipitation value are comparable to modern precipitation values of 0 4 1 5 m yr range of mean values over both drainage basins a maximum late pleistocene precipitation increase of 33 over the modern day value is suggested this is determined by the difference between the highest paleoprecipitation rate 1 9 m yr 7 of the weber river delta and the modern value 1 5 m yr previous climate models and this 33 paleoprecipitation increase suggest a maximum decrease in basin floor paleotemperature of approximately 13 c this paleotemperature decrease is consistent with previous estimates of late pleistocene temperatures using amino acid epimerization rates and periglacial wedges in the bonneville basin and adjacent areas these estimates suggest that although late pleistocene precipitation rates may have been slightly higher than present up to 33 higher late pleistocene temperature and evaporation rates were more important in the development of lake bonneville and associated glaciers

facies architecture and sequence stratigraphy of fine grained lacustrine deltas along the eastern margin of late pleistocene lake bonneville northern utah and southern idaho this study documents the effects of changing lake level limnostasy tectonics sediment yield and basin physiography on the facies architecture sequence stratigraphy and reservoir quality of three fine grained deltas deposited along the eastern margin of late pleistocene lake bonneville analysis of facies architecture indicates that the weber and spanish fork deltas were strongly wave modified because they were situated along openly exposed portions of the shoreline the bear river delta nestled in a relatively isolated northeast arm of the lake records both fluvial and wave processes these fine grained deltas were fed by low gradient rivers that drained large regions that were sparsely glaciated whereas other contemporaneous coarse grained gilbert deltas were fed by steep gradient rivers that drained local source areas that were strongly influenced by glaciers limnostasy tectonics and sediment yield were similar for all three fine grained deltas implying that the most influential forcing parameter on sequence stratigraphy is basin physiography basin physiography specifically ramp length and accommodation most strongly controlled the external and internal geometry of the lowstand systems tract of each delta potential hydrocarbon reservoir quality e g grain size and sorting is largely a product of drainage basin size and stream gradient relative differences in ramp lengths seemed to determine thickness and lateral continuity of delta front deposits relative differences in accommodation appeared to determine the internal geometry of the delta front deposits especially in the lowstand systems tract these deltas can serve as analogs for lacustrine exploration and production where many forcing parameters typically are unknown

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

is the wasatch fault footwall utah united states segmented over million year time scales the wasatch fault zone utah is a 370 km long segmented normal fault system with topographic salients depths of footwall exposure geomorphic properties and geophysical anomalies that suggest differential long term footwall uplift and exhumation on segments that are partitioned by long lived structural segment boundaries apatite u th he ages from footwall samples along the range front from the five central footwall segments av erage 5 3 6 1 0 ma coupled two dimensional thermokinematic and helium diffusion mod els suggest average long term 5 m y exhumation rates of 0 2 0 4 mm yr for most of the wasatch front the exception is the southern end of the salt lake city segment where exhumation rates are two times as great as elsewhere along the wasatch front the rel atively invariant he ages and exhumation rates imply that most of the wasatch did not behave kinematically as independent footwall segment blocks with differential exhumation amounts over the past 5 m y the structural boundaries such as salients and intrabasin highs that partially delineate segments may have persisted since the pliocene and con trolled the locations of the surface rupture segments

evaluation of wasatch fault segmentation and slip rates using lake bonneville shorelines 1 analysis of lake bonneville shorelines using lidar digital elevation data challenges accepted models of wasatch fault deformation since the late pleistocene while footwall deformation of the weber segment of the wasatch fault is consistent with back rotation of the footwall block and greatest displacement rate toward the center of the segment shorelines along the footwall of the salt lake city segment decrease in elevation toward the interior and are highest at the segment boundaries an opposite pattern of footwall deformation than predicted for boundaries arresting or strongly inhibiting displacement during earthquakes the spatial pattern of footwall rebound implies that some of the proposed persistent fault segment boundaries do not stop earthquake ruptures that originate on adjacent fault segments nor constrain ruptures initiated within the salt lake city segment net vertical fault displacement at the boundary between the salt lake and provo segments is 16 20 m over the past 16 3 18 5 ka corresponding to a vertical displacement rate of 0 8 1 2 mm yr a net fault slip rate of 2 0 2 8 mm yr and horizontal extension rate of 1 8 2 6 mm yr on the 25 west southwest dipping fault that forms the southern salt lake city segment boundary shoreline analysis suggests isostatic rebound caused by a drop in lake level was concentrated during a relatively short 2000 year time period following the bonneville flood at 16 ka lidar derived topography in conjunction with robust geomorphic datums improves our ability to map deformation associated with lithospheric flexure and faulting while demonstrating the limitation of lacustrine shorelines in this type of analysis

seismic and hydroacoustic effects of the may 29 2010 submarine south sarigan volcanic explosion energy release and interpretation abstract the may 2010 submarine volcanic crisis of the shallow south sarigan seamount marianas arc ended on may 29 by a violent explosion that emitted a 12 km high atmospheric plume and created a crater 350 m in diameter the application of the multiphase localization method to the pg p t phases of this explosion allows us to refine considerably mainly based on the intense and impulsive t phases the location of their source that fits the position of the newly formed south sarigan crater we highlight the numerous similarities of this final explosion with artificial underwater explosions conventional or nuclear tests of known energy especially the explosive nature of the source in the volcanic basement is confirmed by the similarity of the p phases with those of underground nuclear tests and the application of discriminating criteria ms mb the explosive nature of the source in the water is also confirmedby the application of the identification criteria for hydroacoustic sources and the t phases exceptionally impulsive and of short durations these similarities allow us to provide a rough evaluation of the energy released by the south sarigan final explosion using the methods commonly applied to man generated explosions we estimate a minimal released energy of 1 kt 4 2 1012 j for the seismic effects of the shock and ca 1 t of equivalent tnt i e 4 2 109 j for the hydroacoustic effects of the explosive source in the water the processes leading to this final explosion are discussed on the basis of the sequence of the earlier seismic and eruptive events within the comprehensive nuclear test ban treaty framework this explosion that affected concomitantly the solid liquid and subaerial media is the first well documented one having generated intense waves p hydroacoustic t acoustic infrasounds and tsunamis

dependence of active normal fault dips on lower crustal flow regimes two key questions are raised by the presence in the basin and range province of subhorizontal normal faults which were active in and around the miocene first how did they develop when their existence seems to be precluded by elementary rock mechanics arguments second if they did slip in the recent geological past why do normal faults now active in the brittle upper crust have steeper dips both in the western usa and elsewhere one may now answer both questions by considering the stress field in regions where the isostatic response during extension involves horizontal flow in the plastic lower crust this stress tensor will have inclined principal axes making it possible for normal faults to form with either very steep or subhorizontal dips depending on the sense of flow this can be controlled by environmental conditions which influence rates of hanging wall sedimentation and footwall erosion and thus determine lateral variations in lithostatic pressure in the lower crust regional scale lateral variations in the geothermal gradient caused by subduction can also influence lower crustal flow regimes in adjacent zones of continental extension enabling extension to occur on low angle normal faults dipping away from the active continental margin

the global magnitude frequency relationship for large explosive volcanic eruptions abstract for volcanoes as for other natural hazards the frequency of large events diminishes with their magnitude as captured by the magnitude frequency relationship assessing this relationship is valuable both for the insights it provides about volcanism and for the practical challenge of risk management we derive a global magnitude frequency relationship for explosive volcanic eruptions of at least 300 mt of erupted mass or m4 5 our approach is essentially empirical based on the eruptions recorded in the lameve database it differs from previous approaches mainly in our conservative treatment of magnitude rounding and under recording our estimate for the return period of super eruptions 1000 gt or m8 is 17 ka 95 ci 5 2 ka 48 ka which is substantially shorter than previous estimates indicating that volcanoes pose a larger risk to human civilisation than previously thought

exhumation of the central wasatch mountains utah 2 thermokinematic model of exhumation erosion and thermochronometer interpretation 1 the wasatch fault is a 370 km long normal fault in utah that marks the boundary between the stable colorado plateau to the east and the extending basin and range to the west understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones e g east africa rift transantarctic mountains we explore the thermokinematics of footwall exhumation and erosion in the cottonwood intrusive belt of the central wasatch mountains emphasis is placed on using low temperature thermochronometers to quantify 1 the spatial and temporal variability of exhumation and erosion rates 2 the geometry of footwall tilt 3 the fault dip angle and 4 the magnitude and duration of exhumation these processes are investigated using two dimensional 2 d thermal kinematic models coupled with cooling rate dependent kinetic models which predict exhumed apatite fission track aft and u th he ages the range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0 2 and 2 0 mm yr 1 footwall tilt hinge positions between 15 and 40 km distance from the fault a single planar normal fault with dip angles of 45 and 60 and exhumation magnitudes of up to 15 km at the fault simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat producing layers erosion maintains a constant topographic profile the kinematic and exhumation history of the wasatch mountains is investigated by comparing model predicted thermochronometer ages to observed aft zft and u th he ages predicted and observed ages are compared using a reduced chi square analysis to determine a best fit kinematic model for the wasatch mountains the preferred model includes exhumation occurring on either a 45 or 60 dipping fault a footwall hinge located a minimum of 20 25 km from the fault and a step decrease deceleration in the footwall exhumation rate at the fault from 1 2 to 0 8 mm yr 1 at around 5 ma the model also suggests an exhumation duration of 12 myr 2 myr index terms 1035 geochemistry geochronology 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8109 tectonophysics continental tectonics extensional 0905 8130 tectonophysics heat generation and transport keywords exhumation erosion normal faults numerical modeling heat flow thermochronometers

super volcanic investigations multi disciplinary analyses of earth s most destructive volcanic systems show that continuous monitoring and an understanding of each volcano s quirks rather than a single unified model are key to generating accurate hazard assessments

ignimbrite morphology and the effects of erosion a new zealand case study ignimbrite morphology previously generalised using aspect ratios is here quantified as the relationships between the various thicknesses of material forming an ignimbrite and the areas and volumes represented by those thicknesses the morphology can be measured for the deposit in its present day eroded condition or reconstructed for the original deposit the reconstructed morphology of the 22 500 year bp ca 11 500 km2 ca 300 km3 oruanui ignimbrite in new zealand is documented to illustrate the latter approach the oruanui ignimbrite is an intermediate aspect ratio deposit and shows broadly linear relationships between 1 in thickness and the cumulative area occupied by that thickness or less of material and 2 thickness and the volume represented by that thickness or less of material two theoretical morphologies one where thicknesses exponentially decay with distance from a maximum and the other of uniform thickness slab are compared with the oruanui data limited comparative data suggest that low aspect ratio violently emplaced ignimbrites will show upward concave curves at one extreme following the exponential decay model and high aspect ratio gently emplaced ignimbrites downward concave curves with the slab model as an extreme when plotted on diagrams where the oruanui data show linear trends the effects of erosion on oruanui and model ignimbrite morphologies are modelled using two theoretical erosion scenarios 1 material is evenly removed from the land surface and 2 thinner non welded material is preferentially removed for the oruanui ignimbrite data area is lost much more rapidly in the first instance than volume for example 5 m of erosion is sufficient to remove 50 area whereas 40 m scenario 1 or 120 m scenario 2 of erosion is required to remove 50 volume in old ignimbrites volume estimates may be reasonably accurate even after strong erosion provided the original thicknesses of ponded landscape forming material can be inferred but estimates of original area and aspect ratio will be inaccurate an envelope enclosing all known outcrops of an ignimbrite will give a better estimate of original area and aspect ratio than simply summing the areas of known outcrops

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

sources of uncertainty in the mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale this study uses the tephra from the caldera forming eruption of mount mazama crater lake or usa to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude we combine 250 mazama tephra occurrences reported by a range of disciplines including archaeology paleoclimatology and volcanology with new field studies to better understand the extent of the distal tephra we find that the mazama tephra has been remobilised to varying degrees over the past 7000 years so each tephra locality was appraised for the likelihood that it records primary tephra fallout we designated 45 of the distal 100 km from source tephra sites as suitable for use in the production of isopachs using a spline fitting method the new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic mazama eruption to 176 km3 61 km3 dense rock equivalent dre this study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit interpreting remobilised deposits can provide insight into post eruptive processes that could potentially pose secondary hazards following large magnitude eruptions we also show that in some circumstances remobilised deposits preserve important physical properties such as grain size

insights into wasatch fault vertical slip rates using the age of sediments in timpanogos cave utah abstract timpanogos cave located near the wasatch fault is about 357 m above the american fork river fluvial cave sediments and an interbedded carbonate flowstone yield a paleomagnetic and u th depositional age of 350 to 780 ka fault vertical slip rates inferred from calculated river downcutting rates range between 1 02 and 0 46 mm yr 1 these slip rates are in the range of the 0 12 ma wasatch range exhumation rate 0 5 0 7 mm yr 1 suggesting that the long term vertical slip rate remained stable through mid pleistocene time however the late pleistocene 0 250 ka decelerated slip rate 0 2 0 3 mm yr 1 and the accelerated holocene slip rate 1 2 mm yr 1 are consistent with episodic fault activity assuming that the late pleistocene vertical slip rate represents an episodic slowing of fault movement and the long term 0 12 ma average vertical slip rate including the late pleistocene and holocene should be 0 6 mm yr 1 there is a net late pleistocene vertical slip deficit of 50 75 m the holocene and late pleistocene slip rates may be typical for episodes of accelerated and slowed fault movement respectively the calculated late pleistocene slip deficit may mean that the current accelerated wasatch fault slip rate will extend well into the future

earthquake supercycles as part of a spectrum of normal fault slip styles like thrust boundaries in subduction zones some normal faults show evidence of temporally transient slip patterns such as temporally clustered earthquakes or periods of elevated creep rate we create rate and state friction numerical models to assess whether typical distributions of frictional parameters can explain transient slip on normal faults we constrain our models with geometry from the wasatch fault zone where 5 10 kyr timescale earthquake clusters are recorded on fault scarps we use numerical models to test whether these earthquake clusters could be the geologic representation of supercycles which we define as long period stress cycles containing multiple seismic cycles field observations laboratory data and analogue experiments suggest that the ratio of velocity weakening to velocity strengthening material varies with the ratio of brittle to ductile material in the shear zone we therefore employ a linear transition of frictional parameters with depth to simulate bulk frictional properties of the shear zone we find that for generalized normal faults rate and state friction predicts a spectrum of slip behaviors including steady aseismic creep aseismic transients clustered earthquakes and regular seismic cycle earthquakes the dominant control on slip behavior is the degree of weakening material in the potentially seismogenic fault patch earthquake clusters occur with higher proportions of velocity weakening material which correspond to more competent less ductile shear zone assemblages we find that rate and state friction predicts a wider spectrum of slip behavior on active normal faults than has been previously recognized potentially due to geologic preservation and identification issues

a constant slip rate for the western qilian shan frontal thrust during the last 200 ka consistent with gps derived and geological shortening rates abstract active thrust faulting at the front of the qilian shan accommodates the northeastward growth of the tibetan plateau however the lifespan of individual faults and their slip history on different timescales remain largely unknown here we show that the main range bounding thrust fault of the western qilian shan has accrued tectonic slip at an almost constant rate during the last 200 ka and possibly since fault initiation in the mid miocene our finding is based on 10be ages from a flight of five deformed fluvial terraces along the hongshuiba river which constrain the vertical slip rate of the qilian shan frontal thrust to be 1 2 0 1 m ka during the last 200 ka with a fault dip of 30 5 constrained by seismic reflection data we obtain a horizontal shortening rate of 2 0 0 3 m ka this value is consistent with both the short term shortening rate of 1 7 0 3 mm a derived from gps data and the long term shortening rate of 2 1 0 4 m ka which is based on a balanced geological cross section the latter provides a total shortening estimate of 25 3 km since the thrust fault initiated 12 2 ma ago the agreement between the shortening rates on the range of timescales between 100 and 107 years suggests that the western qilian shan frontal thrust has slipped at a steady rate since its initiation and implies that this fault is the main structure responsible for the growth of the western qilian shan

tomography from 26 years of seismicity revealing that the spatial extent of the yellowstone crustal magma reservoir extends well beyond the yellowstone caldera the yellowstone volcanic field has experienced three of earth s most explosive volcanic eruptions in the last 2 1 ma the most recent eruption occurred 0 64 ma forming the 60 km long yellowstone caldera we have compiled earthquake data from the yellowstone seismic network from 1984 to 2011 and tomographically imaged the three dimensional p wave velocity vp structure of the yellowstone volcanic system the resulting model reveals a large low vp body interpreted to be the crustal magma reservoir that has fueled yellowstone s youthful volcanism our imaged magma body is 90 km long 5 17 km deep and 2 5 times larger than previously imaged the magma body extends 15 km ne of the caldera and correlates with the location of the largest negative gravity anomaly a 80 mgal gravity low this new seismic image provides important constraints on the dynamics of the yellowstone magma system and its potential for future volcanic eruptions and earthquakes

palaeoproterozoic oxygenated oceans following the lomagundi jatuli event the approximately 2 220 2 060 million years old lomagundi jatuli event was the longest positive carbon isotope excursion in earth history and is traditionally interpreted to reflect an increased organic carbon burial and a transient rise in atmospheric o2 however it is widely held that o2 levels collapsed for more than a billion years after this here we show that black shales postdating the lomagundi jatuli event from the approximately 2 000 million years old zaonega formation contain the highest redox sensitive trace metal concentrations reported in sediments deposited before the neoproterozoic maximum concentrations of mo 1 009 g g 1 u 238 g g 1 and re 516 ng g 1 this unit also contains the most positive precambrian shale u isotope values measured to date maximum 238u 235u ratio of 0 79 which provides novel evidence that there was a transition to modern like biogeochemical cycling during the palaeoproterozoic although these records do not preclude a return to anoxia during the palaeoproterozoic they uniquely suggest that the oceans remained well oxygenated millions of years after the termination of the lomagundi jatuli event the oceans probably remained well oxygenated for millions of years after the palaeoproterozoic lomagundi jatuli event according to high concentrations and isotope signatures of redox sensitive metals in the 2 billion year old zaonega formation russia

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

stress field rotation and rooted detachment faults a coulomb failure analysis several well known mechanical models have shown that unusual boundary or loading conditions can alter principal stress orientations into configurations consistent with low angle normal faulting such models however have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low angle surfaces we present the results of simple coulomb failure analyses to determine whether and where such models predict frictional slip assuming geologically plausible boundary stresses pore pressures and rock strengths models that invoke a sizable shear traction at the base of the upper crust or spatially varying loads on the upper crust reorient principal stresses and failure planes but do not produce frictional failure on crustal scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures models that reorient stresses by midcrustal dike intrusion produce slip on low angle surfaces at relatively deep crustal levels but only in the area of the dike tip the low angle failure surfaces curve into a high angle orientation a short distance from the dike all of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest that in any system in which stresses are allowed to evolve over time failure and stress release will occur on high angle faults before low angle ones have developed these assertions are true even when near lithostatic pore pressures are assumed unless there is an inhomogeneous extraordinarily fortuitous distribution of pore pressures and rock strengths at the time of initiation of a new detachment fault one model we tested for example required pore pressures exceeding 0 96 times lithostatic in the area of the hypothesized low angle normal fault with lower pore pressures both above and below the detachment to prevent slip and stress release on high angle normal faults in the upper part of the modeled region and on low angle thrust faults in the lower part

a healing reloading feedback control on the growth rate of seismogenic faults spatial and temporal variations in the growth rates of faults are explained in terms of a stress feedback mechanism operating in the seismogenic upper crust it is based on the idea that seismic rupture of a fault perturbs the surrounding stress field advancing the occurrence of future earthquakes on some faults that are optimally oriented while relaxing stress levels on others if post slip healing is geologically rapid then the earthquakes that are thus induced will contribute to reloading along the earlier rupture zone because of the symmetry of the optimal geometry a positive feedback is set up so that even in areas that are undergoing uniform tectonic straining some faults develop higher displacement rates and grow more rapidly while others experience reduced rates or become inactive using a thin plate elastic model for lithospheric scale faulting it is shown that this healing reloading feedback mechanism drives rapid localisation and the formation of major through going faults moving at plate boundary velocities enhanced displacement rates compared to an isolated fault develop shortly after the onset of deformation along those faults which are optimally positioned in the overall fault population thus the formation of a new plate boundary fault zone is predetermined and is a consequence of rather than the precursor of preferentially high displacement rates also fault segments located at points of rupture symmetry e g the central portion of a fault zone are reloaded more frequently and develop higher displacement rates and consequently have longer segment lengths and or larger displacement to length ratios episodic fault movement through time is a general feature of the model these predictions are consistent with available field observations over a wide range of scales thus elastic brittle failure and healing appear to be important rheological components of the lithosphere on long time scales 104 106 y as well as on the time scale of earthquake recurrence

magnetotelluric images of magma distribution beneath volcan uturuncu bolivia implications for magma dynamics the altiplano puna volcanic complex in the central andes records a history of major caldera forming eruptions over the past 10 m y geophysical and geodetic data indicate that magma is still present and perhaps mobile in the crust broadband magnetotelluric data were used to generate two dimensional and three dimensional electrical resistivity models of the altiplano puna magma body apmb with a focus on the zone of inflation around volcan uturuncu in southern bolivia low electrical resistivities 20 the upper surface of the apmb is shallowest beneath uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location the shallower resistivity structure is characterized by discrete electrically conductive bodies oriented east west near sea level depth of 5 km which are interpreted as a combination of partial melt and fluids

the use of fluid inclusions to constrain fault zone pressure temperature and kinematic history an example from the alpi apuane italy the alpi apuane is a tectonic window that exposes ductilely deformed greenschist facies metaigneous and metasedimentary rocks beneath relatively unmetamorphosed brittlely deformed sedimentary rocks of the tuscan nappe the brecciated fault zone the window fault separating the two tectonic units was originally described as a simple thrust fault but has recently been interpreted to have been reactivated as a later extensional detachment although evidence for extensional faulting is seen above and below the window fault the amount of extensional displacement along this fault is unclear fluid inclusions from veins cementing the fault breccia were used to estimate the pressures and temperatures during the last fault movement minimum pressure estimates obtained from these inclusions range from 105 to 240 mpa pressure corrected trapping temperatures for these fluids range from about 300 to 345 c these pressures and temperatures indicate that the fault was last active at a depth of about 10 km assuming a geothermal gradient at the time of 31 c km 1 this rules out complete extensional unroofing of the alpi apuane by movement along the window fault fluid salinities increase abruptly from the footwall into the fault zone this pattern suggests that fluids rose from the footwall entered the fault zone and were channeled within it leaching salt from the overlying evaporite the lack of quartz veins above the fault zone indicates that these fluids did not circulate into the overlying tuscan nappe

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

synchronous ruptures along a major graben forming fault system wasatch and west valley fault zones utah abstract the salt lake city segment slcs of the wasatch fault zone and the antithetic west valley fault zone wvfz form a large holocene active intrabasin graben in northern salt lake valley utah we integrate previous paleoseismic data with new data from recent trench investigations and compare earthquake timing and displacement for both the master and antithetic faults of this major graben forming system to address whether the wvfz ruptures simultaneously with the slcs or is a separate independent source of earthquakes nine slcs surface faulting earthquakes postdate the lake bonneville highstand 18 ka however the record is most complete since 14 ka yielding latest pleistocene and holocene mean recurrence estimates of 1 5 ky and 1 3 1 6 ky respectively six post bonneville highstand wvfz earthquakes yield a mean recurrence of 2 0 3 6 ky however we consider the wvfz earthquake record incomplete because of distributed faulting and limited paleoseismic data five of six wvfz earthquakes have mean and 2 times that are very similar to those of slcs earthquakes wvfz earthquake w5 lacks an apparent temporal correlation with an slcs earthquake but occurred during a period for which the slcs chronology may be incomplete mean wvfz per event vertical displacement 0 5 m is 26 42 of that for the slcs 1 2 1 9 m consistent with that predicted by previous mechanical modeling of antithetic faulting triggered by slip on a listric master fault we conclude that large wvfz earthquakes are likely synchronous with or triggered shortly after slcs surface faulting earthquakes although earthquake timing uncertainties preclude determining an unequivocal coseismic link between the wvfz and slcs structural models suggest a high likelihood for synchronous rupture these results have important implications for forecasting earthquake probabilities in complex normal faulting environments

mass and energy budgets of explosive volcanic eruptions analysis of the frequency size relationship of explosive volcanic eruptions for the past 2000 years reveals that the time averaged eruptive mass flux of tephra from subaerial explosive eruptions is 3 104 kg s 1 this mass flux corresponds to a thermal energy release of 4 1010 w half of the long term 10 year mass and thermal energy release from explosive volcanic eruptions is accounted for by eruptions of 10 km of tephra