assessing the importance of drilling predation over the palaeozoic and mesozoic abstract plausible predatory palaeozoic and mesozoic drillholes have been reported in the literature from 62 localities mainly from north america and western europe most of which were discovered in the last 20 years twenty nine of these reports include sufficient data to be statistically valid analyses of these data suggest that there was a highly significant increase in drillhole and hence predator size in the early devonian which may reflect a change in predatory taxa as part of the mid palaeozoic marine revolution in this study frequency of drillholes in particular species has been examined examination of drilling frequency suffered by individual taxa from a range of palaeozoic and mesozoic sites showing that these are highly variable just as they are between different holocene localities nevertheless levels of up to 50 in the palaeozoic and 40 in the mesozoic suggest that at least locally drillers did exert an important selection pressure analysis of the currently available data set reveals no significant change in drilling frequency over the palaeozoic and mesozoic there are fewer records of drilling predation from the palaeozoic and mesozoic than from the cenozoic at present however it is difficult to assess whether drilling was truly less widespread than in the cenozoic or whether at least part of the problem is taphonomic loss of information in particular in the late palaeozoic and mesozoic it is striking that relatively few reports involve molluscan prey despite their attractiveness as profitable prey with high flesh yield when compared with the apparently more frequently drilled brachiopods however study of the style of preservation of drilled molluscs that were mostly originally aragonitic suggests that drillholes are only preserved in faunas which are exceptionally preserved such that the original microstructure is still observable by contrast brachiopods which were either originally phosphatic or calcitic have much better preservation potentials and therefore may be more likely to show drillholes an understanding of the preservation biases involved in the recognition of drillholes may suggest suitable localities for further research more data from a suite of localities world wide are required for a more detailed analysis of patterns of drilling predation in the palaeozoic and mesozoic

quaternary landscape evolution in the san jacinto fault zone peninsular ranges of southern california transient response to strike slip fault initiation well constrained case studies of transient landscape response to external forcing are needed to improve our understanding of erosion processes in tectonically active mountain belts the peninsular ranges portion of the san jacinto fault zone sjfz is an excellent location for such a study because it displays pronounced geomorphic disequilibrium resulting from initiation of a major strike slip fault in the past 1 0 to 2 5 million years we recognize two geomorphic domains in this region 1 a relict low relief upland domain consisting of broad flat valleys and low gradient streams and 2 very steep rough topography with deeply incised canyons and retreating erosional knickpoints pleistocene sediments exposed along and near the sjfz include fluvial conglomerate sandstone and mudstone with weak paleosols and west to nw directed paleocurrents these sediments accumulated in a lowgradient stream system represented by domain 1 during an early phase of slip in the sjfz prior to the modern phase of erosion and degradation domain 2 late pliocene or early pleistocene initiation of the sjfz triggered a wave of headward erosion and stream capture that is still migrating nw along the fault zone using the total distance that capture points have migrated along the fault zone and a range of possible ages for fault initiation the rate of knickpoint retreat is estimated at 12 to 44 km my to explore the signal of transient geomorphic response to fault initiation we analyzed 23 tributaries along an 20 km portion of the main fault valley within domain 2 the analysis reveals three zones with distinctive morphologies 1 strongly convex longitudinal profiles in the nw 2 a large ca 5 6 km 2 landslide in the central zone and 3 concave tributaries in the se with profile complexity decreasing and catchment area increasing from nw to se the distribution of these zones suggests close spatial and temporal association of active fault slip bedrock incision deep seated landslides and erosional modification the fundamental driving force behind these processes is profound geomorphic disequilibrium resulting from initiation of the sjfz we suggest that landslides may have played a significant role in shaping the morphology of this fault zone and that the influence of landslides may be underestimated in areas where characteristic landforms and deposits are obscured by later erosion and faulting d 2005 elsevier b v all rights reserved

edge drilling on the brachiopod perditocardinia cf p dubia from the mississippian of missouri usa examination of a bulk sample of the mississippian orthide brachiopod perditocardinia cf p dubia revealed borings in 25 8 of the 259 specimens the drillholes are small 0 13 0 80 mm in diameter round and each brachiopod has been drilled only once normally paleozoic brachiopods are drilled at a low frequency 5 and the placement of the drillhole is directly over the viscera of the animal the majority of the specimens of p cf p dubia in this sample however are drilled directly on the anterior margin this is the first documentation of paleozoic edge drilling on brachiopods the high frequency of boring and especially the location of drillholes near the margin suggest either a different predatory parasitic organism or a different feeding behavior than is commonly associated with paleozoic brachiopods these drillholes may represent a behavior analogous to modern capulid gastropods that parasitize bivalves by positioning themselves along the host s commissure and sometimes drilling the shell it could also represent a behavior similar to modern naticid gastropods which drill along the commissure of its bivalve prey regardless this example further illustrates that the paleozoic was characterized by a broader repertoire of drilling behaviors than has been previously recognized

predatory drill holes and partial mortality in devonian colonial metazoans elliptical holes 0 2 0 3 mm in diameter with beveled edges have been found penetrating the zooids of encrusting hederellids colonial metazoans of uncertain affinity from several localities in the middle devonian givetian of north america these are the first known predatory drill holes in pre cretaceous colonial animals and a key addition to the record of paleozoic predation some drill holes were subsequently patched from within by new skeletal material proving that the drilling occurred during the life of the colony these drill holes are analogous to predatory drill holes in some modern cheilostome bryozoans which can be similarly patched in this case by the intramural budding of a new zooid into the empty chamber of the old zooid a drilling predator of unknown affinity evidently consumed hederellid zooids one at a time inflicting partial mortality on the colonies the mode of drilling suggests that the predator specialized in this type of colonial prey and the repaired drill holes show that the hederellids had a response to such damage reports of small circular holes interpreted as predatory drill holes are becoming more common in noncolonial shelled invertebrates from the paleozoic notably brachiopods bivalves and crinoids these drill holes are far less frequent than in post paleozoic shells where most are the work of gastropods and many are of questionable origin with some representing the traces of parasites and others postmortem domichnia

on the mobility of pyroclastic currents in light of deposit thickness and clast size trends abstract pyroclastic currents pcs represent the most hazardous manifestations of explosive volcanic activity the evaluation of their mobility as related to the eruptive source parameters and the interactions with the surroundings along their path is crucial for hazard assessment at active volcanoes in order to define the areas exposed to their impacts here we report on two case studies from the latera volcanic complex vulsini volcanic district central italy that may approach the two end members of the classical spectrum of pc dynamics in terms of particle concentration and dominant flow regime i e dilute turbulent vs concentrated laminar pcs for the two examples we document specific and contrasting variation trends of deposit thickness and maximum lithic and pumice clast sizes with distance from vent which provide a further characterization in terms of forced vs inertial flow behavior in the first case thickness and maximum lithic and pumice sizes show a mild linear decrease with distance implying a sort of balance between forced and inertial regimes flow pressure and mass progressively drop via deposition along with flow competence for given source conditions i e mass eruption rate fountaining height erupting mixture etc and slope angle of topography the pc runout depends primarily on the waning density contrast between the flow and the surrounding atmosphere density current ending with the loft of the current the second case representative of moderate to large volume dense pcs derived by continuous feeding with high mass discharge rate i e from collapsing plinian columns or caldera forming events shows an increase of deposit thickness with distance which is accompanied by opposite lithic and pumice clast size distance trends i e decreasing lithic sizes and increasing pumice sizes downcurrent compatible with high particle concentration and forced non turbulent granular flow regime by analogy with the mobility of dry debris flows we suggest that the sliding component of transport which prevails in proximal to intermediate settings may account for the prevailing tendency of the pc to transport than to deposit which leaves behind relatively thin sheets with coarse lithics while the dominant spreading component downcurrent would result in increasing accumulation and the stacking of coarse pumice rich flow lobes in more distal settings the spreading component of transport would become more and more important with increasing material supply at pc source resulting in additional runout these findings cast new light on the first order controlling factors of flow mobility for the two pc end members laying the groundwork for determining the most relevant pc hazard parameters i e the maximum runout and the flow properties upon impact dynamic pressure burial potential temperature

a diode laser system for heating minerals for u th he chronometry we have developed a diode laser 25 w 808 nm system for he extraction from minerals for u th he chronometry the laser beam is delivered via a 600 m fiber cable and focused using a binocular microscope temperatures necessary for he release from apatite 500 600 c and zircon 1100 1300 c encapsulated in pt foil tubes are attained by heating to 0 5 w for 30 s and 1 25 2 5 w for 20 min respectively using a defocused beam heating at these powers does not result in measurable u and or th loss from apatite as noted by the preservation of the distinct th u in multiple splits of two different durango apatite crystals analyses of durango and the california institute of technology internal standard apatite 97mr22 yield u th he ages of 32 8 1 8 ma 1 n 11 and 4 6 0 5 1 n 5 respectively well within accepted ages the u th he age and th u of five fish canyon tuff zircon aliquots yield 29 3 2 2 ma 1 and 0 6 0 03 respectively and are indistinguishable from ages produced by resistance furnace he extraction heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals e g hornblende biotite muscovite garnet and basalt groundmass suggesting that diode lasers offer a cheap small low maintenance alternative to nd yag and ar ion lasers for 40ar 39ar cosmogenic noble gas and stable isotope studies

statistical analysis of mixed motive shell borings in ordovician silurian and devonian brachiopods from northern and eastern canada hundreds of shell borings of different origin and displaying variable patterns were found in strophomenide pen tameride and orthide brachiopods of five paleozoic localities in northern and eastern canada the borings were analyzed using simple statistics as well as cluster and nonmetric multidimensional scaling analyses at the ordovician anticosti island locality all borings are parasitic or post mortem in origin while at the wenlock ludlow baillie hamilton island almost all borings are predatory at the remaining three localities borings represent a mix of predatory parasitic and post mortem domichnial borings in all three brachiopod taxa the proportions of which were controlled largely by brachio pod shell morphology and paleoecology for the strophomenides predatory borings can be segregated from parasitic and post mortem domichnial using simple and multivariate statistical analyses sowerbyella type strophomenides have a higher proportion of predatory borings at the lochkovian localities than at the ordovician localities while the reverse is true for the strophomena type strophomenides in pentamerides and orthides very few predatory borings are identified most bor ings were emplaced by parasitic or post mortem domichnial borers in pentamerides this is due to the internal structure of the shells which elevated the muscles of the organism above the shell floor rendering them inaccessible to boring preda tors in orthides more deliberate defense mechanisms such as toxins or external ornamentation may have deterred preda tion this study indicates that throughout the paleozoic interactions between borers and brachiopods were complex but can be elucidated using a large sample size and statistical analyses resume des centaines de perforations coquillieres d origines et de formes differentes ont eteobservees dans des brachio podes strophomenides pentamerides et orthides de cinq localites paleozoo ques du nord et de l est canadiens ces perfora tions ont eteanalysees al aide de methodes statistiques simples ainsi que d analyses typologiques et d analyses multidimensionnelles non metriques des ordres de grandeur ala localiteordovicienne de l o le d anticosti toutes les per forations sont d origine parasitique ou post mortem alors qu ala localitewenlockienne ludlowienne de l o le baillie hamilton elles sont presque toutes le resultat de la predation aux trois autres localites les trois taxons de brachiopodes presentent un melange de perforations d origines parasitique predatrice et domichniale post mortem dont les proportions respectives sont en bonne partie determinees par la morphologie des coquilles et le milieu paleoecologique en ce qui con cerne les strophomenides des analyses statistiques simples et multivariees permettent de distinguer les perforations resul tant de la predation des perforations parasitiques et domichniales post mortem les strophomenides de type sowerbyella presentent une plus grande proportion de perforations resultant de la predation aux localites lochkoviennes qu aux localites ordoviciennes alors que c est plutot l inverse pour les strophomenides de type strophomena tres peu de perforations re sultant de la predation sont recensees chez les pentamerides et les orthides la plupart etant le fait de foreurs parasitiques ou domichniaux post mortem en ce qui concerne les pentamerides ceci est dua la structure coquilliere interne dans la quelle les muscles de l organisme etaient au dessus du plancher de la coquille de sorte qu ils etaient inaccessibles aux pre dateurs foreurs chez les orthides des mecanismes de defense plus deliberes tels que des toxines ou l ornementation externe pourraient avoir empechela predation l etude indique que durant tout le paleozoo que les interactions entre les foreurs et les brachiopodes etaient complexes mais que ces dernieres peuvent etre elucidees al aide d echantillons de grande taille et d analyses statistiques traduit par la redaction

geomorphic interpretation of low temperature thermochronologic data insights from two dimensional thermal modeling 1 low temperature thermochronologic data are useful for estimating mountain erosion rates because late stage cooling in mountain ranges often reflects exhumation via geomorphic erosion processes traditional interpretations of thermochronologic data assume steady uniform erosion conditions often violated in real mountain ranges here i examine whether unsteady or nonuniform erosion histories are detectable in patterns of erosion rate estimates made by traditional means to do so i numerically model the subsurface temperature field in a two dimensional section of earth s crust under various illustrative geomorphic scenarios i generate simulated cooling ages within two isotopic systems apatite fission track ft and u th he he at two landscape locations ridge tops and valley bottoms from these ages i make field estimates of erosion rates using the commonly employed elevation dependence and mineral pair methods calculations show that if errors on erosion rate estimates are kept small by replicate or transect sampling many unsteady or nonuniform erosional histories do produce distinctive patterns of erosion rate estimates for example temporally increasing erosion rates produce poor agreement between erosion rate estimates made with a single method while decreasing erosion rate produces good agreement between estimates made with a single method nonuniform erosion that decreases relief can produce erroneously high erosion rate estimates based on the elevation dependence method but also accurate and relatively precise mineral pair based estimates a history of decreasing relief can be distinguished from one of steady uniform erosion by 1 close agreement between mineral pair estimates and poor agreement between elevation dependence estimates and 2 the magnitude of the difference between ft and he ages the insights gleaned from these analyses can guide forward modeling of topographic and thermal evolution

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

two phase exhumation of the santa rosa mountains low and high angle normal faulting during initiation and evolution of the southern san andreas fault system low angle detachment fault systems are important elements of oblique divergent plate boundaries yet the role detachment faulting plays in development of such boundaries is poorly understood the west salton detachment fault wsdf is a major low angle normal fault that formed coeval with localization of the pacific north america plate boundary in the northern salton trough ca apatite u th he thermochronometry ahe n 29 samples and thermal history modeling of samples from the santa rosa mountains srm reveal that initial exhumation along the wsdf began at ca 8 ma exhuming footwall material from depths of 2 to 3 km an uplifted fossil miocene helium partial retention zone is present in the eastern srm while a deeper crustal section has been exhumed along the pleistocene high angle santa rosa fault sfr to much higher elevations in the southwest srm detachment related vertical exhumation rates in the srm were 0 15 0 36 km myr with maximum fault slip rates of 1 2 3 0 km myr miocene ahe isochrons across the srm are consistent with northeast crustal tilting of the srm block and suggest the post wsdf vertical exhumation rate along the srf was 1 3 km myr the timing of extension initiation in the salton trough suggests that clockwise rotation of relative plate motions that began at 8 ma is associated with initiation of the southern san andreas system pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low to high angle faulting and indicates local fault geometry may at times exert a fundamental control on rock uplift rates along strike slip fault systems

mystery of naticid predation history solved evidence from a living fossil species observations of living cernina fluctuata sowerby the sole extant species of the mesozoic and cenozoic prosobranch gastropod family ampullospiridae show that this species is an algal grazer and is not a shell drilling predatory naticid gastropod all jurassic and early cretaceous gastropods previously assigned to naticidae belong to ampullo spiridae and are now inferred to be herbivores not shell drillers this finding refutes the triassic origin and favors a cretaceous origin for naticid drilling predation a preliminary revision of cenozoic and cretaceous species previously assigned to naticidae suggests that naticids were absent prior to the cretaceous campanian and diversified at a modern level in eocene time this pattern of naticid diversification from the late cretaceous onward coincides roughly with the evidence of naticid predation the fossil record of naticid drill holes once of anomalously short duration now accurately reflects the geologic history of naticid predation activity

a comparison of pyroclastic flow and debris avalanche mobility many pyroclastic flows have runout distances longer than expected from basic frictional arguments in these cases the ratio of the height descended h and the runout l are as small as 0 2 a feature which has been attributed to fluidization however significant fluidization by gases is now considered unlikely during emplacement of debris avalanches where h l and volume are inversely related we determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout h l and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits as well as 15 pyroclastic flow deposits systematic differences in the volume versus h l plots were explored by statistical analysis results show an inverse relationship between h l and volume for both nonvolcanic and volcanic debris avalanche deposits as expected and also for pyroclastic flow deposits regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non volcanic debris avalanche line is parallel to the other two we argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable h l versus volume it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism based on the data in hand we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large volume debris avalanches

regional crustal thickness variations of the peninsular ranges southern california we used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern peninsular ranges california depth to the moho varies from 37 km beneath the western peninsular ranges batholith to 27 km at the western edge of the salton trough an average apparent dip of 10 to the west over a lateral distance of 60 km we previously obtained a similar result for a profile 100 km to the south a moho dip of 20 over 30 km lateral distance in both cases the moho depth variations do not correlate with topography of the eastern batholith but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes these observations suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that compensation is through lateral variations in crustal or upper mantle density rather than through an airy root

apatite u th sm he thermochronology on the wrong side of the tracks abstract despite the increasing application of apatite u th sm he thermochronology in a range of settings the technique suffers from two major unresolved problems which preclude reliable thermal history interpretations on a routine basis one problem is the common but unexplained over dispersion of single grain ages compared to predictions based on accepted models of diffusion systematics including enhanced he retentivity due to the buildup of radiation damage in the apatite crystal lattice a second related problem is that the widely adopted rdaam model does not provide an accurate representation of the degree to which helium retentivity is enhanced as a result of the accumulation of radiation damage until these problems are resolved and eliminated thermal history interpretations derived from this method cannot be regarded as reliable routine application of the technique requires a more rigorous quantitative understanding of these factors further experimental studies are required to identify all significant sources of variation in measured ages and to develop more accurate models of the thermal response of the system combined with rigorous calibration and validation against independent methods in well constrained natural settings only once this is achieved will the technique be capable of producing robust and reliable interpretations

can dispersive pressure cause inverse grading in grain flows grain flows in the collisional regime are self regulated in such a way that dispersive pressure at every level tends to balance static pressure and so cannot promote the upward segregation of large particles in a steady flow le roux adds new insights to this analysis by pointing out that dispersive and static pressures would not balance in unsteady flows when shear strain rate increases in a grain flow dispersive pressure which is positively correlated to shear strain rate increases too and the gradient of dispersive pressure becomes higher than the gradient of static pressure as a consequence the grain flow expands thus lowering the dispersive pressure until the two gradients reequilibrate during the

new example of devonian predatory boreholes and the influence of brachiopod spines on predator success three abundant devonian frasnian brachiopod species from the lime creek fm of iowa usa douvillina arcuata devonoproductus walcotti and pseudoatrypa devoniana contain boreholes pooled drilling frequency is 10 6 the boreholes are small usually 1 0 mm diameter cylindrical and drilled perpendicular to the shell surface the borings appear to be type a boreholes as described by ausich and gurrola 1979 j paleontol 53 335 344 based on multiple criteria including statistically significant stereotypy with respect to valve preference site on valve and size class of the prey the boreholes are inferred to be predatory or parasitic in origin for each species the predator preferred to attack the valve closest to the substrate regardless of whether that was the pedicle or brachial valve boreholes were concentrated over the muscle field on each of the three species the success percentage percent of complete holes was statistically lower 2 p 0 04 for the spinose devonoproductus walcotti 33 33 than for the non spinose but otherwise morphologically similar douvillina arcuata 54 22 in spite of the fact that d walcotti has a thinner shell this is the first quantitative evidence for an anti predatory function for brachiopod spines

self organization in the rapid flow of granular material evidence for a major flow mechanism based on the assumption that high speed high concentration sediment mass flows are primarily granular flows their dynamic properties were studied such highly sheared granular flows are characterized by interparticle collisions this so called rapid granular flow regime has been analysed using two dimensional computer simulations it is shown that granular flows at the microstructural level are governed by deterministic chaos the bulk behaviour is characterized by self organization and an attractor controlling the energy dissipation of the flow the existence of this rapid granular flow attractor easily explains the linear relation between drop height and travel distance of debris flows a compelling consequence of the attractor is that rapid granular flow is the major flow regime in debris flows

drill hole convergence and a quantitative analysis of drill holes in mollusks and brachiopods from the triassic of italy and poland abstract the history of predation is recorded primarily from drilling in cenozoic invertebrates quantitative data are uncommon from the triassic a period before the appearance and radiation of many known drill hole producers such as various gastropod families and octopods we present quantitative evidence of drilling from the late triassic carnian cassian formation of italy and the middle triassic anisian lower muschelkalk of poland documenting the first drill holes in triassic brachiopods a single brachiopod with a cylindrical complete drill hole was found in a brachiopod sample from of poland drilling percentage 0 3 n 365 the cassian formation yielded drill holes in gastropods bivalves and brachiopods indicating that more species are drilled than was known previously the minimum drilling percentage exclusive of incomplete drill holes of a sample from the stuores wiesen cassian formation is 1 7 n 116 5 prey selectivity is evident complete drill holes are primarily present in one gastropod species polygyrina lommeli 11 8 of specimens with a complete drill hole whereas other common species were not drilled single drill holes in brachiopods are cylindrical and complete and may be predatory in origin multiple drill holes in mollusks are common and drill holes are parabolic and often incomplete with a central boss resembling the shape of drill holes produced by extant naticid gastropods a survey of the paleozoic literature showed that such drill holes are also present in devonian and carboniferous brachiopods however naticids did not evolve until the cretaceous so we propose the term drill hole convergence for similar shaped drill holes produced by different organisms the triassic parabolic drill holes are not caused by domicile seeking or boring organisms instead we favor a predatory origin of these drill holes but we cannot entirely rule out parasitism surveying other triassic invertebrate assemblages should yield more evidence of drilling

simply pyroclastic currents gravity driven ground hugging gas pyroclast mixtures produced during explosive volcanic eruptions define a full spectrum of particle concentration flow regime and particle support mechanisms to describe these phenomena the term pyroclastic density current pdc has become increasingly popular in the last few tens of years here i question the general application of the term pdc to the whole flow spectrum and instead i propose the simpler term pyroclastic current

predation in the paleozoic gastropod like drillholes in devonian brachiopods middle devonian articulate brachiopods ludlowville and moscow formations hamilton group new york have external tapered holes with a central boss that are indistinguishable from drillholes of naticid gastropods that are known from the triassic and later drillholes are specific to prey ribbed shells were avoided and specific to sites on prey healed drillholes suggest penetration of live prey as many as 44 percent of the preferred prey are drilled indicating a level of predation that has been reported only from post paleozoic strata

fragmentation transport and deposition of a low grade ignimbrite the citlaltepetl ignimbrite eastern mexico the citlaltepetl ignimbrite records one of the largest explosive events during the holocene activity of citlaltepetl volcano pico de orizaba multiple pyroclastic flow units a fall deposit and some lahar units were emplaced between 8500 9000 y b p as a result of repetitive but discrete explosive events the whole ignimbrite resulted from discrete fluctuations in eruptive intensity that decreased with time the initial pyroclastic flow pulse was by far the most violent and widespread event and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms of transport and emplacement subpopulation sequential fragmentation transport sft analyses were carried out in order to determine the physical mechanisms that selectively concentrate or remove particles in the moving flows we suggest that lateral and temporal changes in the flow rheology in which fluidization yield strength entrainment of atmospheric air and sedimentation played a dominant role in flow propagation and emplacement may imprint a unique signature in the grain size spectra the lowermost unit of the citlaltepetl ignimbrite can be envisaged by a model in which progressive aggradation near the vent became replaced by en masse emplacement farther outward

boreholes in mississippian spiriferide brachiopods and their implications for paleozoic gastropod drilling abstract bored specimens of the spiriferide brachiopod genera brachythyris and spirifer from several localities of the mississippian fort payne formation kentucky usa represent 7 0 of the total sampled population the boreholes are typically single complete circular in plan view and with their long axes perpendicular to the valve these traits and stereotypy with respect to the pedicle valve and the fold and sulcus are consistent with a predatory parasitic origin of the holes the presence of platyceratid gastropods in the fort payne localities with bored brachiopods the previously documented drilling abilities of these gastropods and the occurrence of a platyceratid snail attached to specimen of a bored devonian brachiopod atrypa argue for platyceratids as the most probable culprits

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

complexities of the san andreas fault near san gorgonio pass implications for large earthquakes 1 geologic relationships and patterns of crustal seismicity constrain the three dimensional geometry of the active portions of san andreas fault zone near san gorgonio pass southern california within a 20 km wide contractional stepover between two segments of the fault zone the san bernardino and coachella valley segments folds and dextral reverse and dextral normal faults form an east west belt of active structures the dominant active structure within the stepover is the san gorgonio pass garnet hill faults a dextral reverse fault system that dips moderately northward within the hanging wall block of the san gorgonio pass garnet hill fault system are subsidiary active dextral and dextral normal faults these faults relate in complex but understandable ways to the strike slip faults that bound the stepover the pattern of crustal seismicity beneath these structures includes a 5 8 km high east west striking step in the base of crustal seismicity which corresponds to the downdip limit of rupture of the 1986 north palm springs earthquake we infer that this step has been produced by slip on the linked san gorgonio pass garnet hill coachella valley banning sgp gh cvb fault this association enables us to construct a structure contour map of the fault plane the large step in the base of seismicity downdip from the sgp gh cvb fault system probably reflects a several kilometers offset of the midcrustal brittle plastic transition u th he thermochronometry supports our interpretation that this south under north thickening of the crust has created the region s 3 km of topographic relief we conclude that future large earthquakes generated along the san andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1 20 km two tasks in seismic hazard evaluation may now be attempted with greater confidence first the construction of synthetic seismograms that make useful predictions of ground shaking and second theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures

four dimensional analysis of the inversion of a half graben to form the whittier fold fault system of the los angeles basin abstract analysis of structural and stratigraphic data from hundreds of oil wells surface geologic maps and earthquake focal mechanisms reveals a three phase structural evolution of the whittier fold fault system that evolution progressed from an extensional phase 16 ma accommodated by the formation of a half graben through an episode of graben filling 14 8 ma to lithospheric shortening which began ca 8 ma with contractional reactivation of the graben forming fault as a blind reverse fault and accompanying anticlinal uplift about 1 ma the blind reverse fault broke through onto the sea floor initiating the activity of the present whittier fault system horizontal contraction estimated from reconstructed cross sections ranges from 1 4 to 3 2 km and has been at an average rate of 0 3 mm year in the p direction n s of the 1987 whittier narrows earthquake for the past ca 8 my that rate may have accelerated after breakthrough of the blind reverse fault to as much as 1 0 mm year the ratio of strike slip to dip slip displacement on the whittier fault over the past ca 8 my is about 0 4

drill hole analysis reveals evidence of targeted predation on modern brachiopods abstract if drill holes are made in prey either by mistaken or opportunistic predators or parasites rather than those which do so on a more regular basis we might predict that such drill holes will commonly be incomplete or repaired and show no evidence of behavioural familiarity such as size selection or constancy of attack site these predictions were used to examine the patterns of drill holes found in bulk samples of death assemblages of modern rhynchonelliform brachiopods from around new zealand four samples yielding data for five different taxa four terebratulides and one rhynchonellide were examined in each the proportion of incomplete or repaired holes was low samples of terebratella sanguinea fiordland and gyrothyris mawsoni antipodes islands show statistically significant evidence of both size selection and preferential drill hole siting the sample of notosaria nigricans waikawa shows only evidence of size selection examination of drill hole morphology and the co occurring gastropod fauna suggests that the distinctive countersunk holes in t sanguinea and g mawsoni were likely made by the trophonid gastropod xymene ambiguus aquarium experiments demonstrate that the ranellid gastropod argobuccinum tumidum is also capable of feeding on brachiopods but that the resulting hole is not likely to be readily recognisable in death assemblages

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

applications of low temperature thermochronometry to quantification of recent exhumation in mountain belts exhumation is a primary phenomenon used to characterize geomorphic and tectonic histories the motion of rock with respect to the earth s surface is produced by erosion and tectonic denudation unroofing by erosion redistributes crustal mass and is a major accommodator of shortening in orogenic belts brandon et al 1998 zeitler et al 2001 willett et al 2003 tectonic denudation typically associated with crustal extension brings rocks to the surface at rapid rates from significant crustal depths hodges et al 1998 hacker et al 2003 vanderhaeghe et al 2003 quantifying exhumation using thermochronology is thus an important element in investigations of long term crustal scale geomorphic and tectonic processes the thermochronometers most appropriate for estimating exhumation related to landscape or neotectonic history are apatite u th he and fission track dating given their low closure temperatures and the typical geothermal gradient of the continents these cooling ages generally record exhumation from 1 4 km depths farley 2002 ehlers and farley 2003 naeser 1979 gleadow et al 1986 the accuracy of the exhumation history inferred from these data is limited by how well the spatially and temporally variant geothermal gradient is known or modeled mancktelow and grasemann 1997 ehlers and farley 2003 beyond these limitations thermochronology is often hindered by what the rocks in a given study area record cooling ages may be too old and predate the tectonic or erosional history of interest or topography accessibility bedrock lithology and exposure may hinder collection of a robust sample set in many studies experimental design is determined by logistics available funding and time and prior analytical training of the investigator thus some studies may not obtain the optimal results to address a given question although many papers deal with technical aspects of thermochronometry the purpose of this paper is to explore

near field transpressive deformation along the san andreas fault zone in southern california based on exhumation constrained by u th he dating low temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the san andreas fault zone in southern california has experienced recent rapid exhumation eight apatite u th he ages from a 1 km relief section along yucaipa ridge in the san bernardino mountains range from 1 4 to 1 7 ma the minimal change in age with elevation implies exhumation of 5 7 mm yr 1 sustained for at least several hundred thousand years three titanite helium ages from the ridge are much older ranging from 57 to 82 ma these show a steep gradient with elevation representing either an exhumed partial retention zone or slow cooling through much of the tertiary these data imply that a total exhumation of 3 to 6 km has occurred since 1 8 ma it is uncertain whether this exhumation terminated as early as 1 ma or has continued up to the present at a decelerated rate we surmise that this exhumation represents rock uplift in the absence of major surface uplift in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief the record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion such rapid large magnitude exhumation within the strands of the san andreas fault zone is important for models of transpressional tectonics it is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain however it is inconsistent with a stress partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike slip fault in addition a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust these models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far field borderlands rather than concentrated in the near field alternatively the exhumation of yucaipa ridge may have been driven by the nearby restraining bend in the san andreas fault at san gorgonio pass in which case it represents local fault geometry rather than accommodation of far field plate motion

geological constraints on the emplacement mechanism of the parinacota debris avalanche northern chile the holocene parinacota volcanic debris avalanche ca 8 000 years b p is located in the central andes of northern chile the avalanche formed by the sector collapse of a major stratovolcano adjacent to a lake basin in a single catastrophic event the deposit has an estimated volume of ca 6 km3 a run out of over 22 km and covers more than 140 km2 of the surrounding terrain the values of the heim coefficient 0 08 and the ratio a v 2 3 ca 50 where a is the area covered and v the volume of the deposit indicate high mobility of the avalanche debris in transport two avalanche units can be distinguished the lower unit consists mainly of blocks of rhyodacitic lavas and domes and pyroclastic flow deposits and glacial fluvial and lacustrine sediments the upper unit consists of a coarse grained breccia with little matrix largely composed of andesite blocks which are angular with little or no rounding by abrasion the avalanche displays pronounced hummocky topography in which hummock volume and amplitude as well as maximum block size within individual hummocks tends to decrease with transport distance and towards the lateral margins of the avalanche deposit some surfaces of individual breccia blocks are covered by tens to thousands of small scale impact marks indicating that neighbouring blocks were vibrating and colliding without significant shearing motion most of the deformation and shearing was instead accommodated in a basal layer of wet structureless sediments incorporated into the avalanche debris from the inundated lake basin we propose that the ancestral parinacota stratovolcano collapsed because of loading of underlying fluvioglacial and lacustrine sediments the edifice disintegrated during collapse along existing fractures into large rock domains volumes from 10 to greater than 1 106 m3 which were transported with little internal deformation and then fragmented into hummocks of breccia as they were deposited the decrease of hummock volume with distance suggests that material that travelled further broke up and had an initial greater kinetic energy

reconciling pyroclastic flow and surge the multiphase physics of pyroclastic density currents abstract two end member types of pyroclastic density current are commonly recognized pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows we provide scaling relations that unify these end members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the stokes number s t the stability factor t and the dense dilute condition d d we recognize five types of particle behaviors within a fluid eddy as a function of s t and t 1 particles sediment from the eddy 2 particles are preferentially settled out during the downward motion of the eddy but can be carried during its upward motion 3 particles concentrate on the periphery of the eddy 4 particles settling can be delayed or fast tracked as a function of the eddy spatial distribution and 5 particles remain homogeneously distributed within the eddy we extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current this stratification may favor particle interactions in the basal part of the flow and d d determines whether the flow is dense or dilute using only intrinsic characteristics of the current our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

assessment of u th he thermochronometry the low temperature history of the san jacinto mountains california u th he ages have been measured on igneous apatites from the san jacinto mountains a high region at the junction of the peninsular and transverse ranges to investigate the potential of this technique for thermochronometry of slowly cooled rocks helium ages from 79 to 17 ma are younger than ages obtained by other dating techniques including apatite fission track counting and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature helium ages are strongly correlated with elevation and record the latest low temperature thermal evolution of the range they suggest relative tectonic quiescence in the latest cretaceous through mid tertiary and provide no evidence for rapid unroofing of the block during this period nor do they obviously require a large degree of uplift associated with convergence between the transverse and peninsular ranges in the last few million years helium ages document modest westward tilting of the block 7 and a significant vertical component of motion on the block s bounding faults after helium retention began this work suggests that apatite helium ages record low temperature tectonic and thermal histories that are not apparent from other dating techniques

transport and emplacement mechanisms of large volcanic debris avalanches evidence from the northwest sector of cantal volcano france abstract large volcanic debris avalanche vda deposits of miocene age occur within the stratigraphic sequence of cantal stratovolcano central france the vda of cantal volcano were initiated by sector collapse of a cone of high elevation up to 4500 m a s l two distinct superimposed vda deposit units breccias i and ii resulted from at least two successive large scale collapse events during the evolution of the volcanic complex breccia i the oldest one is very thick up to 200 m and heterolithologic it is overlain by breccia ii which is thinner up to 50 m and contains more heterometric clasts ranging from hectometer to millimeter size the vda deposits are poorly sorted clasts are angular and highly fractured at different scales often with jigsaw fit in breccia ii jigsaw megaclasts have usually subspheric shapes with size decreasing distally basal deposits commonly display thin inversely graded layers reorientated clasts along ramp structures and cataclasis of the substratum some units have a distal fabric in the form of a poorly developed clast imbrication the clasts are commonly sub rounded thereby indicating abrasion during viscous flow anisotropy of magnetic susceptibility a m s measurements indicate absence of a preferentially oriented matrix fabric magnetic foliation parallel to the bedding suggests that the depositional process is dominated by particle settling the magnetic fabric is better organized in distal deposits where magnetic carriers are imbricated moreover the fragmented clasts and megaclasts are not abraded rather inflation led to isotropic spherical dispersion of clastic material isotropic dispersive inflation which fed the interclast matrix this is also confirmed by the lack of rotation at the individual crystal level all these data strongly suggest transport of vda by liquified non turbulent granular flows distally imbricated deposits suggest more turbulent flow emplacement of the vda results from progressive upward aggradation of the particles by volume reduction and deflation loss the non turbulent liquified stage of the vda disappears the vda behaves as a sliding rigid mass plug flow that exerts strong friction on the basement leading to gouge formation

helium diffusion in rutile and titanite and consideration of the origin and implications of diffusional anisotropy abstract diffusion of helium has been characterized in natural sphene titanite and synthetic rutile polished slabs of sphene and rutile were implanted with 100 kev 3he at a dose of 5 1015 3he cm2 and annealed in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in rutile we obtain the following arrhenius relations d a 2 26 1 0 1 0 exp 126 11 kj mol 1 rt m 2 sec 1 d c 1 75 1 0 8 exp 12 0 7 kj mol 1 rt m 2 sec 1 although activation energies for diffusion parallel to the c and a axes are comparable there is marked diffusional anisotropy with diffusion parallel to the c axis about 2 orders of magnitude faster than transport parallel to the a axis these diffusivities bracket the values determined for he diffusion in rutile in bulk release experiments stockli et al 2005 stockli et al 2007 wolfe 2009 although the role of anisotropy could not be directly evaluated in those measurements in titanite the following arrhenius relation was obtained over the temperature range 252 550 c for diffusion parallel to the a axis d 2 14 1 0 6 exp 148 8 kj mol 1 rt m 2 sec 1 in contrast to rutile and zircon cherniak et al 2009 titanite shows little evidence of anisotropy as diffusivities parallel to the a and c axes are similar and diffusivities for titanites from two different localities are similar he diffusion coefficients obtained in this study are similar to those measured through bulk release of he by step heating shuster et al 2003 over the investigated temperature range diffusion of he in titanite is similar to that of he diffusion in rutile parallel to the c axis but much faster than diffusion parallel to the a axis since the diffusion of he in rutile exhibits such pronounced anisotropy we model diffusional loss of he with a recently developed finite element code watson et al 2010 created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present example applications to evaluate helium losses from rutile grains as a function of grain size and length to diameter ratios in efforts to better understand the occurrence of pronounced anisotropy for he diffusion in some crystals e g rutile zircon but not in others e g apatite titanite we consider the density and distribution of interstitial apertures in the crystal structure that might permit he migration these determinations an extension of the concept of ionic porosity are consistent with observations of relative diffusivities and the existence or absence of significant anisotropy for the minerals rutile zircon apatite and titanite

uplift and erosion of the san bernardino mountains associated with transpression along the san andreas fault california as constrained by radiogenic helium thermochronometry apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the san andreas fault by documenting aspects of the low temperature 40 100 c thermal history of the tectonic blocks of the san bernardino mountains in southern california we have placed new constraints on the magnitude and timing of uplift old helium ages 64 21 ma from the large big bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the late cretaceous period these ages imply that the surface of the plateau may have been exposed in the late miocene and was uplifted only 1 km above the mojave desert in the last few myr by thrusting on the north and south a similar range in helium ages 56 14 ma from the higher san gorgonio block to the south suggests that its crest was once contiguous with that of the big bear block and that its greater elevation represents a localized uplift that the big bear plateau did not experience the structure of the san gorgonio block appears to be a gentle antiform based on the geometry of helium isochrons and geologic constraints young ages 0 7 1 6 ma from crustal slices within the san andreas fault zone indicate uplift of a greater magnitude than blocks to the north these smaller blocks probably experienced 3 4 km of uplift at rates 1 5 mm yr in the past few myr and would stand 2 5 km higher than the big bear plateau if erosion had not occurred the greater uplift of tectonic blocks adjacent to and within the san andreas fault zone is more likely the result of oblique displacement along high angle faults than motion along the thrust fault that bounds the north side of the range we speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike slip system transpression thus appears to have been accommodated by both vertical displacement within the san andreas fault zone and thrusting on adjacent structures

diffusion of helium in zircon and apatite abstract diffusion of helium has been characterized in natural zircon and apatite polished slabs of zircon and apatite oriented either normal or parallel to c were implanted with 100 kev 3he at a dose of 5 1015 3 he cm2 diffusion experiments on implanted zircon and apatite were run in pt capsules in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in zircon we obtain the following arrhenius relations d c 2 3 10 7 exp 146 11 kj mol 1 r t m 2 s 1 d c 1 7 10 5 exp 148 17 kj mol 1 r t m 2 s 1 although activation energies for diffusion normal and parallel to c are comparable there is marked diffusional anisotropy with diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c these diffusivities bracket the range of values determined for he diffusion in zircon in bulk release experiments although the role of anisotropy could not be directly evaluated in those measurements in apatite the following arrhenius relation was obtained over the temperature range of 148 449 c for diffusion normal to c d 2 10 10 6 exp 117 6 kj mol 1 r t m 2 s 1 in contrast to zircon apatite shows little evidence of anisotropy he diffusivities obtained in this study fall about an order of magnitude lower than diffusivities measured through bulk release of he through step heating and within an order of magnitude of determinations where ion implantation was used to introduce helium and he distributions measured with elastic recoil detection since the diffusion of he in zircon exhibits such pronounced anisotropy helium diffusional loss and closure cannot be modeled with simple spherical geometries and the assumption of isotropic diffusion a finite element code cylmod has recently been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present some applications of the code in evaluating helium lost from zircon grains as a function of grain size and length to diameter ratios and consider the effects of shape anisotropy where diffusion is isotropic as in the case of apatite but shapes of crystal grains or fragments may depart significantly from spherical geometry

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

appraisal of u th he apatite thermochronology as a thermal history tool for hydrocarbon exploration an example from the taranaki basin new zealand the taranaki basin contains most of new zealand s commercial hydrocarbon reserves the southern parts of the taranaki basin underwent inversion during the late miocene with published afta apatite fission track analysis and vitrinite reflectance data suggesting approximately 2 5 3 km of section eroded from the structure on which the fresne 1 well was drilled consistent with estimates from reconstructions based on seismic sections the well constrained tectonic and thermal history framework for this well section established by earlier studies provides an ideal test bed for the thermochronological technique of apatite u th he dating helium ages determined in apatites from the earlier afta study decrease from about 6 ma at about 400 m depth to about 1 ma at 2465 m although these results are qualitatively consistent with late miocene cooling quantitative modeling of u th he ages based on best fit thermal histories derived from new afta analyses using improved methods suggests that cooling was protracted involving accelerated cooling during the pliocene possibly indicating a discrete pliocene inversion phase integration of afta and u th he dating can provide improved thermal history constraints in sedimentary basins particularly on cooling rates and or resolution of multiple thermal events by independently defining the timing of peak maturation in relation to formation of potential trapping structures this information offers the potential for improved assessment of hydrocarbon prospectivity and reduced exploration risk

regional scale mid jurassic to late cretaceous unroofing from the adirondack mountains through central new england based on apatite fission track and u th he thermochronology abstract apatite fission track aft ages of 178 to 70 ma for 108 samples from the adirondack mountains and eastern new york state vermont western massachusetts and connecticut and western new hampshire indicate that widespread unroofing occurred throughout the region during the middle jurassic to late cretaceous additionally u th he ages of 167 to 85 ma were determined on nine of those samples and support cooling rates of 1 8 2 2 c m yr from 100 to 65 c during the middle jurassic to late cretaceous assuming a geothermal gradient of 25 c km unroofing rates for the region were estimated to be on average 0 07 0 08 km m yr the aft age discontinuities between the high peaks region and the fault dissected southeastern adirondack mountains and along the ammonoosuc fault in the connecticut river valley between vermont and new hampshire suggest differential unroofing that may have been accommodated by fault reactivation syn post late cretaceous time after the youngest aft ages 82 and 70 ma resp

signatures of divergence and self organization in soils and weathering profiles abstract complex system behaviors such as self organization are difficult to address in geology system evolution often cannot be directly observed and in geology models and theory must be reconciled with field evidence however self organization can be addressed within the historical interpretive paradigm by applying a measure of the degree of self organization of geologic features using standard interpretive methods to determine the nature of changes and determining whether those changes result in an increase or decrease in organization in this way stable non self organizing convergent development can be distinguished from unstable chaotic self organizing divergent development kolmogorov entropy k entropy was used as a measure of the self organization of soil profiles in eastern north carolina in general the profiles are low in k entropy indicating a generally high level of predictability and information in the vertical arrangement of pedogenetic horizons as a broad generality the study p

three types of pyroclastic currents and their deposits examples from the vulsini volcanoes italy abstract this paper deals with ground hugging gas pyroclast currents from explosive volcanic eruptions and their deposits key field observations and laboratory determinations are proposed to relate specific deposit types with flow regimes and particle concentration in the transport and depositional systems three relevant flow scenarios and corresponding deposit types have been recognized from a survey of pyroclastic successions of the vulsini volcanic district central italy 1 dilute turbulent pyroclastic currents producing normally or multiply graded beds by direct suspension sedimentation 2 concentrated bedload regions beneath suspension currents depositing inversely graded beds by traction carpet sedimentation 3 self sustained high particle concentration laminar mass flows developing massive poorly sorted bodies with opposite grading of coarse lithic and pumice clasts overlying fine grained inversely graded basal layers main distinguishing criteria include the occurrence and pattern of clast grading clast thickness relationships grain size ash matrix componentry and pyroclast size density relationships downcurrent and temporal transitions among identified flow scenarios are likely to occur for changing energy conditions and gas pyroclast ratio both on regional and local scales the nature and efficiency of magma fragmentation volatile content conduit geometry which determine the characteristics of the erupted mixture and possible lateral blast component at the vent and the angle of incidence of the column collapse are suggested as the main factors controlling the generation of one type over the other at flow inception dilute fine grained overpressured eruption clouds are thought to favor the formation of low particle concentration turbulent currents column collapse over slightly inclined volcano slopes causing a high degree of compression of the collapsing mixture and of gas expulsion would favor the generation of high particle concentration pyroclastic currents

mid paleozoic latitudinal predation gradient distribution of brachiopod ornamentation reflects shifting carboniferous climate shell ornamentation in modern oceans increases toward the tropics in conjunction with an equatorward increase in shell breaking predation latitudinal gradients in antipredatory ornamentation were previously documented for devonian brachiopods we examined the latitudinal distribution of shell ornamentation in tournaisian visean and namurian articulate brachiopods to test the hypothesis that the latitudinal gradient in antipredatory ornamentation was present in the carboniferous we found a statistically significant latitudinal ornamentation gradient which was most pronounced in the tournaisian when the latitudinal temperature gradient was most steep these results support the hypothesis that a latitudinal gradient in defensive morphology occurred as a result of the mid paleozoic increase in predation although the mid paleozoic and late mesozoic intervals of predator prey escalation may have differed in dynamics and intensity both episodes produced adaptations in prey morphology that varied along a latitudinal gradient

thermochronological constraints on the timing and magnitude of miocene and pliocene extension in the central wassuk range western nevada apatite fission track and u th he thermochronological data provide new constraints on the timing of faulting and exhumation of the wassuk range western nevada where east dipping normal faults have accommodated large magnitude ene wsw oriented extension extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the grey hills and central wassuk range these fault blocks display westward tilts of 60 and expose preextensional paleodepths of up to 8 5 km based on the structural reconstruction of tilted preextensional tertiary andesite flows that unconformably overlie mesozoic basement rocks apatite fission track and u th he thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at 15 ma fission track modeling results indicate rapid fault block exhumation occurred between 15 and 12 ma which is in agreement with miocene volcanic rocks that bracket the tilting history in addition fission track and u th he data suggest reduced rates of cooling following major extension as well as renewed cooling related to active high angle faulting along the present day range front starting at 4 ma thermochronological data from structurally restored fault blocks indicate a preextensional miocene geothermal gradient of 27 5 c km the thermochronological constraints on the timing of extensional faulting and the eruptive history in the wassuk range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension and where synextensional magmatism is suppressed during the highest rates of extension

thermal effects of scanning electron microscopy on he diffusion in apatite implications for uth he dating abstract in order to investigate potential diffusive loss of he from apatites during sem analysis we performed 1 single grain u th he dating for 47 durango apatite fragments from th he ages are internally consistent within their errors and indistinguishable from the reported 40ar 39ar ages of 31 44 0 18 2 ma and the apatite u th he ages of 31 02 1 01 ma standard deviation mcdowell et al 2005 the results from the electron matter interaction simulation suggest that temperature rise t temperature increase during electron bombardment peaks within a very thin layer at the outermost of the hypothetical apatite grain and falls below 50 k within a depth of 0 3 m from the surface based on the simulated t profile combined with available he diffusion parameters the fractional loss of he fhe was calculated for different apatite grain dimensions the numerical simulation supports that the he loss from apatite grains of typical physical dimensions is negligible th he ages for sem treated apatites as well as diffusion simulation using the electron matter interaction model indicate that sem spot analysis or extensive chemical mapping prior to apatite u th he dating does not cause any meaningful diffusive he loss for most of the apatite samples

numerical investigations of apatite 4he 3he thermochronometry apatite 4he 3he thermochronometry has the potential to constrain cooling histories for individual samples provided that several presently untested assumptions are valid here we simulate the sensitivity of 4he 3he spectra to assumptions regarding geometric model crystallographic anisotropy broken grain terminations parent nuclide zonation and the accuracy of results obtained from analyses of aggregates of multiple crystals we find that 4he 3he spectra obtained from a cylinder with isotropic diffusion are almost indistinguishable from those obtained from an equivalent sphere with an equivalent initial 4he distribution under similar conditions anisotropic diffusion from the cylinder can greatly bias 4he 3he spectra but only if diffusion is 10 times faster in the axial than the radial direction existing data argue against anisotropy of this magnitude we find that analysis of apatites with broken terminations will also bias 4he 3he spectra but not greatly so in contrast we find that zonation of a factor of 3 in parent nuclide concentration produces 4he 3he spectra that deviate substantially from the homogeneous model when parent nuclides are highly concentrated near the grain rim and or cooling is fast the resulting 4he 3he spectra will be readily identified as aberrant however more subtle zonation higher concentrations in the grain interior or samples that have cooled slowly regardless of zonation style can yield 4he 3he spectra that look acceptable but will lead to inaccurate thermochronometric interpretation if parent homogeneity is assumed finally we find that analysis of an aggregate of crystals with identical 4he distributions can yield 4he 3he spectra and diffusion arrhenius arrays that are very different from those that would be obtained on the individual crystals if even small variations in he diffusion exist among the grains overall our observations suggest that modeling tools that assume spherical geometry and isotropic diffusion are appropriate for interpreting apatite 4he 3he spectra however it is essential to analyze only individual crystals and to assess the degree of parent nuclide zonation in those crystals

denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology 1 the denudational history of a 500 km long transect across the drakensberg escarpment on the high elevation passive margin of se africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples supplemented by an additional 10 outcrop samples a minimum of 4 5 km of denudation since formation of the margin 130 myr ago is estimated for the coastal zone with a marked early cretaceous episode of accelerated denudation broadly coincident with continental breakup samples from the swartberg borehole sw 1 67 located 30 km seaward of the present position of the drakensberg escarpment indicate a total depth of denudation of 3 1 1 2 km since 91 myr with a phase of accelerated denudation of 2 1 0 9 km at a mean rate of 95 43 m myr between 91 and 69 myr samples from the ladybrand borehole la 1 68 west of the lesotho highlands indicate 1 7 0 5 km of denudation since 78 myr with a phase of accelerated denudation at 82 43 m myr from 78 to 64 myr average denudation rates declined to about 10 m myr during much of the tertiary although the apatite fission track data do not provide any direct constraints on the denudational history of the lesotho highlands interpolation between the two boreholes constrained by geological evidence and extrapolated in situ produced cosmogenic 36cl derived denudation rate estimates suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide these patterns of denudation are incompatible with constant retreat of the drakensberg escarpment from an initial position near the present coast we suggest that the drakensberg escarpment formed by rapid post breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100 200 m myr

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

erosion transport and segregation of pumice and lithic clasts in pyroclastic flows inferred from ignimbrite at lascar volcano chile abstract investigations have been made on the distribution of pumice and lithic clasts in the lithic rich soncor ignimbrite 26 5 ka and the 1993 pumice flow deposits of lascar volcano chile the soncor ignimbrite shows three main lithofacies which grade into one another coarse lithic breccias range from matrix poor stratified varieties irregular shaped sheets and elongate hummocks in proximal environments to breccia lenses with pumiceous ignimbrite matrix massive lithic rich facies comprise the bulk of the ignimbrite pumice rich facies are bimodal with abundant large pumice clasts often with reverse grading rare lithic clasts and occur distally and on high ground adjacent to deep proximal valleys in the 1993 pyroclastic flow deposits lithic rich facies are deposited on slopes up to 14 whereas pumice rich facies are deposited only on slopes

particle fabric in a small type 2 ignimbrite flow unit laacher see germany and implications for emplacement dynamics ignimbrites of the 13 ka upper laacher see tephra were deposited from small highly concentrated moderately fluidized pyroclastic flows their unconsolidated nature and the prominence of accidental devonian slate fragments make these ignimbrites ideal for clast fabric studies the upper flow unit of ignimbrite m14 has characteristics typical of a type 2 ignimbrite layer 2a and the lower part of layer 2b of the flow unit have strong upstream inclined a p fabrics a p means long particle axes parallel to flow direction only clasts with a b axial ratios of 2 5 or greater preserve good a p fabrics whereas the a b planes of flat fragments dip upstream irrespective of axial ratio the a axis fabric becomes weaker flatter and more girdle like in the upper half of layer 2b at one locality the a axis fabric appears to rotate 40 up through the flow unit suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base the existence of similarly strong a p fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non shearing material layer 2b on a thin shear layer layer 2a and that the entire flow freezes en masse to form the deposit the data suggest that if the flow froze en masse it was shearing pervasively through at least half its thickness another possibility is that the flow unit aggraded progressively from the base up and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition

u th he thermochronology of metallic ore deposits in the liaodong peninsula implications for orefield evolution in northeast china abstract the liaodong peninsula is an important region where polymetallic ores are mined in northeast china but precise thermochronological data needed to effectively constrain the evolution of this orefield are presently lacking for this study fifteen samples of granitoid rocks were collected from the qingchengzi and wulong orefields located in the liaodong peninsula to undertake u th he study of zircon and apatite single grain zhe and ahe ages from the qingchengzi orefield range from 73 to 166 ma and from 22 to 48 ma respectively whereas single grain zhe and ahe ages from the wulong orefield range from 42 to 88 ma and from 21 to 56 ma respectively some samples yield dispersed ages comprehensive analyses indicate that the main causes of the dispersion in the zhe ages are heterogeneous distribution of u and th within the crystals radiation damage and cooling rates whereas the dispersion in the ahe ages is primarily due to the first two factors based on the helium ages we calculate the average exhumation and cooling rates of the two orefields during different periods and the associated denudation which are 4 3 km for the qingchengzi orefield and 4 7 km for the wulong orefield the mineralization depths are estimated using homogenization temperature and the salinity of fluid inclusions and are calculated to be 1 5 3 2 km and 1 0 2 2 km for the qingchengzi pb zn and au ag deposits respectively and 2 1 3 1 km for the wulong au deposit these results are much less than the total denudation suggesting that the orebodies have been heavily eroded in conjunction with the distribution of the helium ages the modeled low t thermal histories of the two orefields reveal that the liaodong peninsula experienced three stages of relatively rapid cooling during the early cretaceous ca 135 115 ma from the end of the late cretaceous to the early eocene ca 70 50 ma and from the late eocene to the oligocene ca 38 26 ma the last stage of cooling affected both orefields and represents a period of relatively intense tectonic activity finally these results are compared with those of previous studies from the adjacent jiaodong peninsula this discussion shows that the cooling experienced by the orefields in the liaodong peninsula is largely consistent with that experienced by the jiaodong peninsula these data provide valuable information for further studies of the evolution of orefields in northeast china

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

the occurrence and origin of prominent massive pumice rich ignimbrite lobes within the late pleistocene abrigo ignimbrite tenerife canary islands abstract the 0 196 ma lithic rich abrigo ignimbrite on tenerife canary islands contains localised massive coarse pumice rich ignimbrite lobes mprils they typically form low ridges up to 2 m high with axes parallel to the flow direction and in cross section they range from symmetrical to asymmetrical and highly skewed lobate bedforms generally with flat bases the major components are rounded pebble to cobble sized phonolitic pumice clasts within an ignimbritic matrix of ash fine lithics and minor crystals which varies from lithic rich to lithic poor commonly there is a vertical increase in pumice concentration from matrix supported texture at the base to clast supported at the top accompanied by an increase in pumice clast size mprils often thin and grade laterally perpendicular to current flow into planar pumice concentration zones they occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a laterally complex stratified pumice rich ignimbrite facies lcspis near palaeotopographic highs mprils are original depositional features not erosional in origin and are derived from a larger pyroclastic flow it is likely that pumice was segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic rich zone the more pumice rich parts are interpreted to have detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as mobile derivative pyroclastic flows the flow parallel ridge shape of mprils may be a result of fingering within these flows or concentration of pumice within the intermediary clefts deposition occurred en masse at the termination of the flow front the resultant lobate deposits were then overridden and mantled by normal ignimbrite facies from either a later flow pulse or the following main part of the parent flow

conflicting u th he and fission track ages in apatite enhanced he retention not anomalous annealing behaviour while initial studies showed a high degree of consistency between afta and apatite u th he dating an increasing number of studies are reporting apatite u th he ages which are older than expected on the basis of apatite fission track data from the same region we present data from a range of geological settings to document a systematic discrepancy between the two systems which becomes more pronounced in samples with older fission track ages except for samples with very low uranium contents results from a granite pebble and enclosing volcanogenic sandstone provide a well controlled test case in which independent constraints are available on the underlying thermal history these demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour as has been suggested but as a result of a change in the he retention properties of apatite this change appears to be linked to the degree of accumulated radiation damage within the crystal lattice although underlying mechanisms remain unclear we suggest detailed experiments should be performed to delineate and quantify the processes responsible until this is achieved we suggest that apatite u th he studies should also incorporate apatite fission track data as well as other low temperature indicators in order to monitor the u th he system response and to guard against the anomalous behaviour described here

assessing the role of orogen parallel faulting in post orogenic exhumation low temperature thermochronology across the norumbega fault system maine as a part of a regional effort to determine the extent of low temperature thermochronological discontinuities across major orogen parallel faults in northern new england 41 apatite fission track

pseudotachylyte frictionite at the base of the arequipa volcanic landslide deposit peru implications for emplacement mechanisms abstract the arequipa volcanic landslide deposit peru displays a pseudotachylyte or frictionite at its basal contact with an older ignimbrite the pseudotachylyte consists of a dark 1 cm thick vitreous layer that shows striations on its upper surface it contains small mineral clasts from both the landslide rocks and the underlying ignimbrite embedded in an optically isotropic matrix tem observations revealed a clast melt suspension with a lower black obsidian like vein made of chemically heterogeneous glass the pseudotachylyte is interpreted as the result of partial melting due to frictional heating during landslide emplacement this is the third pseudotachylyte reported at the base of a landslide deposit and the first in a volcanic environment calculations show that basal melting only accounts for a small proportion of the total energy released during landslide emplacement indicating that landslide transport was dominated by dissipative stresses affecting the whole landslide body and not conce

boreholes in the middle devonian blastoid heteroschisma and their implications for gastropod drilling abstract boreholes in over 100 specimens of the devonian blastoid heteroschisma and the presence of the gastropod platyceras orthonychia sp attached to a specimen of heteroschisma subtruncatus hall 1858 suggest that platyceratid gastropods were capable of drilling their hosts at least as early as the middle devonian and thus represent the first known gastropods to have evolved this ability the presence of similar holes in numerous other devonian and mississippian blastoids and the occurrence of several additional examples of platyceratids attached to blastoids indicate that the interaction between gastropods and blastoids was not infrequent as in the case of the gastropod crinoid interaction this relationship is interpreted as having been parasitic rather than predatory the parasitic drilling platyceratids may have been a factor in the paleozoic precursor of the mesozoic marine revolution some of the anti predatory defenses of crinoids described from the late paleozoic may have been strategies of avoiding platyceratid infestation

helium diffusion and u th he thermochronometry of titanite we have explored the diffusivity characteristics of radiogenic he in titanite sphene and have developed analytical techniques for u th he dating of this mineral results of incremental he outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44 6 3 4 2 kcal mol and a frequency factor of 60 cm 2 s diffusivity is highly linearly correlated with the inverse square of the grain size indicating that the he diffusion domain in titanite is the crystal itself for typical titanite grains of 200 to 800 m minimum dimension the he closure temperature is in the range 191 to 218 c for a cooling rate of 10 c myr there is no indication in the titanites we studied that radiation damage plays a major role in he diffusion u th he ages of titanites from quickly cooled rocks yield ages with 5 8 1 reproducibility that are concordant with known ages and u th he ages of titanites from slowly cooled rocks are consistent with independently established cooling paths supporting the closure temperature estimates these experiments suggest that titanite u th he ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40 ar 39 ar dating but higher than apatite fission track or u th he dating

u th he dating of phosphates apatite monazite and xenotime the common phosphate minerals apatite ca 5 po 4 3 f oh cl monazite ce la th po 4 and xenotime ypo 4 have found widespread use in geochronology because they incorporate u and th into their structures for example apatite usually has a few tens of ppm of both u and th while monazite and xenotime usually have hundreds of ppm to weight percent levels of these elements as a consequence these phosphates can be dated using several fundamentally different isotopic techniques elsewhere in this volume harrison et al describe phosphate dating using ingrowth of radiogenic pb the final daughter of u and th series decay and gleadow et al describe dating based on damage tracks from the spontaneous fission of 238 u the most recently developed dating technique applied to phosphates described in this chapter uses the accumulation of particles from u and th series decay u th he dating while phosphate u th pb dating is usually used to date high temperature events such as crystallization of igneous rocks and the timing of prograde metamorphism fission track and u th he dating are more commonly used to establish cooling histories through low temperatures for example in the range 110 40 c in the case of apatite dating of minerals using radiogenic he was first explored shortly after the discovery of radioactivity strutt 1908 and was investigated extensively in the 1950s and 1960s mostly on very u and th rich minerals such as zircon and titanite hurley 1952 1954 damon and kulp 1957 damon and green 1963 apatite he dating was first investigated by zeitler et al 1987 who studied the diffusion rate of he from apatite and proposed that apatite he dating might provide a useful thermochronometer recording cooling through about 100 c further studies both in the laboratory lippolt et al 1994 wolf et al 1996b warnock et al 1997 farley 2000 and in the natural setting house et al 1999 stockli et al 2000 have confirmed this idea and the technique has now been applied to a range of tectonic geologic and geomorphologic problems e g house et al 1997 1998 spotila et al 1997 farley et al 2001 stockli et al 2000 monazite and xenotime have only recently come under scrutiny for he geochronology e g pik and marty 1999 and little is yet known of their potential for routine geo or thermochronometry

runout lengths of sturzstroms the control of initial conditions and of fragment dynamics sturzstroms are giant landslides that travel kilometers within minutes data from terrestrial examples of 0 001 10 km3 suggest that their runout lengths increase in proportion to the square root of their volume this trend is investigated assuming that runout is controlled by fragmental flow the results indicate that runout lengths depend on the potential energy available for motion after initial collapse itself a function of the dimensions of the collapse zone on the degree of rock fragmentation and on the rate of momentum loss by fragment collisions in a basal boundary layer assumed to thicken with time by diffusion a dependence on initial conditions during collapse would explain claims that a minimum volume is required for sturzstroms to form beneath a critical volume insufficient energy is available to initiate fragmental flow and so the unstable mass slumps downslope the drop height h is often similar to the vertical extent of the collapse zone and so is linked by geometry to sturzstrom volume the ratio of h to l the horizontal distance of effective transport normally interpreted as a measure of frictional resistance is thus reinterpreted as an inverse measure of the energy available for runout after collapse by providing a physical basis for observed trends the analysis justifies use of empirical limits for forecasting the runout lengths of major landslides

geophysical evidence for wedging in the san gorgonio pass structural knot southern san andreas fault zone southern california geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the san gorgonio pass region this picture serves as the basis for inferring past fault movements within the san andreas system which are fundamental to understanding the tectonic evolution of the san gorgonio pass region interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of san gorgonio pass beneath basement rocks of the southeast san bernardino mountains subtle long wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of san gorgonio pass and south under peninsular ranges basement and has a southern edge that is roughly parallel to but 5 6 km south of the surface trace of the banning fault this deep magnetic body is composed either of upper plate rocks of san gabriel mountains basement or rocks of san bernardino mountains basement or both we suggest that transpression across the san gorgonio pass region drove a wedge of peninsular ranges basement and its overlying sedimentary cover northward into the san bernardino mountains during the neogene offsetting the banning fault at shallow depth average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data seismicity suggests a deeper detachment surface beneath the deep magnetic body this interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross sectional view given the multilayered nature of deformation it is unlikely that the san andreas fault will rupture cleanly through the complex structures in san gorgonio pass

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

cosmogenic 3he in igneous and fossil tooth enamel fluorapatite igneous fluorapatite samples from a suite of six granitic rocks from the transantarctic mountains have high 3he concentrations to 5 10 9 atoms g 1 and high 3he 4he ratios to 9 10 7 these values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on 6li this 3he is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation several samples of fossil tooth enamel fluorapatite from the turkana basin of kenya are similarly rich in 3he with up to 1 10 7 atoms 3he g 1 and 3he 4he ratios up to 4 10 6 again this 3he is most logically attributed to cosmic ray reactions provided that cosmogenic 3he like radiogenic 4he is quantitatively retained in fluorapatite under earth surface conditions routine 3he exposure dating of this common phase may be possible based on its chemical composition the 3he production rate in fluorapatite is about 100 atoms g 1 yr 1 at sea level and high latitude using this rate the apatites from the transantarctic mountains have apparent exposure ages of 0 5 6 2 myr in agreement with values elsewhere in the range the fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the earth s surface 3he exposure ages can provide insights to the depositional and reworking history of enamel bearing fossils at present the major limitations to 3he exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of 3he in the presence of large quantities of 4he in addition further work is necessary to establish the nucleogenic 3he background in fluorapatite

helium and argon thermochronometry of the gold butte block south virgin mountains nevada one of the largest exposures of precambrian crystalline rock in the basin and range province of the southwestern usa is the gold butte block of the south virgin mountains about 15 km west of the colorado plateau it has been interpreted as a largely continuous crustal cross section about 15 20 km thick that was exhumed by a deeply penetrating normal fault during miocene extension to test this interpretation as well as the use of the newly developed titanite u th he thermochronometer we examined the low temperature thermal history of the gold butte block with the apatite and titanite u th he and muscovite 40 ar 39 ar thermochronometers apatite he ages average 15 2 1 0 2 ma throughout the block indicating that the entire section was warmer than 70 c prior to miocene exhumation titanite he ages increase from 18 6 1 5 ma near the paleobottom west end of the block to 195 15 ma near the paleotop east end a rapid change from mid tertiary to increasingly older titanite he ages to the east is observed at about 9 3 km paleodepth and is interpreted as a fossil he partial retention zone for titanite assuming a pre exhumation geotherm of 20 c km consistent with earlier apatite fission track work this depth would have corresponded to 196 c prior to exhumation indicating that laboratory derived he diffusion characteristics for titanite that yield a closure temperature of about 200 c are applicable and correct muscovite 40 ar 39 ar ages are 1 0 1 4 ga near the paleotop of the block and 90 ma near the paleobottom together with 207 pb 206 pb ages on apatite and titanite and an earlier apatite fission track transect across the gold butte block our data indicate that the continental crust at the western edge of the colorado plateau resided at moderate geothermal gradients and slowly declined in temperature from 1 4 ga to about 100 200 ma a 90 ma cooling event clearly affected the mid crust deepest portions of gold butte which may reflect accelerated cooling or a brief heating and cooling cycle at this time after which gradients returned to about 20 c km prior to rapid exhumation in the miocene this work thus supports previous structural and thermochronologic studies that suggest that the gold butte block is the thickest largely continuous cross section of crust exposed in the southwestern usa

perturbations of the earth s rotation and their implications for the present day mass balance of both polar ice caps summary the present day perturbations of the earth s rotation are sensitive to the glacial isostatic adjustment gia arising from the late pleistocene glacial cycles and also to the recent mass balance of polar ice caps in this study we evaluate the polar wander and the change of degreetwo harmonic of the earth s geopotential j 2 proportional to the rotation rate for four late pleistocene ice models we examine these perturbations as a function of lower and uppermantle viscosities and lithospheric thickness and rheology elastic or viscoelastic in which a compressible earth model with elasticity and density given by the seismological model prem is used by considering the observations and predictions including the gia process arising from the late pleistocene ice and recent mass balance of polar ice caps we discuss the recent mass balance of the antarctic and greenland ice sheets we also examine the effects of internal processes and the melting of mountain glaciers although this work is only preliminary the results shown below seem to be supported even if these effects are included two solutions are obtained for source areas of the recent antarctic melting we denote an equivalent sea level esl rise mm yr 1 from the greenland and antarctic ice sheets as gl and

origin and composition of carbonate buildups and associated facies in the fort payne formation lower mississippian south central kentucky an integrated sedimentologic and paleoecologic analysis an integrated sedimentologic paleoecologic and taphonomic analysis is used to test competing hypotheses for the origin of the fort payne formation of south central kentucky characteristics of eight distinct facies both carbonate and siliciclastic support the hypothesis that the lower mississippian fort payne formation is a progradational shallowing upward basin filling sequence five facies are considered in detail including the autochthonous fossitiferous green shale facies two autochthonous carbonate facies wackestone buildups and crinoidal packstone buildups and two transported carbonate facies channelform packstone facies and sheetlike packstone facies the two types of carbonate buildups are independent features but both developed on mounds of green fossiliferous shale the fort payne wackestone buildups are interpreted to be waulsortian buildups the fort payne waulsortian and associated facies were deposited in the lower portion of the photic zone which is phase d the shallowest phase of the lees and others 1985 waulsortian model the source of carbonate mud in the buildups must have been autochthonous all autochthonous facies had distinct faunas dominated by crinoids and bryozoans the channelform and sheetlike packstone facies are interpreted to be proximal and distal portions respectively of carbonate apron deposits both are composed principally of crinoid packstones and represent deeper water portions of the fort payne basin filling sequence the siltstone facies records background sedimentation