oceanic transverse ridges a flexural response to fracture zone normal extension one of the most puzzling characteristics of sea floor morphology is the occurrence of anomalously shallow fracture zone parallel oceanic transverse ridges a model is proposed for the formation of transverse ridges near lat 21 and 24 n on the mid atlantic ridge in which the differential responses of large offset and small offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off axis traces of fracture zones numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension 5 km along low angle faults 45 are responsible for the transverse ridges

gravitational sliding on the mid atlantic ridge at the kane transform implications for submarine basin slope degradation and deformation deep tow side scan sonar imagery along with submersible and camera surveys provides a synoptic view of the sea floor geology leading to critical observations and new interpretations for previously unknown gravitational sliding features on the mid atlantic ridge at the kane transform 2338n the sliding on the rift valley wall occurred mostly as massive slumps of slope forming gabbroic bed rocks possibly along mechanically weak low angle 30 detachment faults creating frontal cuspate ridges and downslope trending lineaments the sliding on the transform valley wall occurred mostly as debris avalanches producing distributed surficial sediment and bedrock clasts in an apron of hummocky and chaotic terrain on the lower slope steeply dipping 70 amphitheaterlike escarpments on the upper slope and downslope trending ridges and lineaments on the middle slope local and global comparative analysis suggests that distinct geologic settings of submarine slopes exert fundamental influence on the nature style and scale of gravitational sliding the rift valley wall differs from the transform valley wall primarily because of their different structural grain lithological complexity and sediment thickness the steeply dipping 25 sediment free rift valley wall on the mid atlantic ridge contrasts strongly with the gently dipping 5 passive continental margins loaded with thousands of meters of petroliferous sedimentary rocks results from comparative analysis not only add to the knowledge of complexities of submarine gravitational sliding structures but also contribute to a better understanding of the differences in sea floor degradation and deformation processes in contrasting submarine geologic settings represented by mid ocean ridges and passive continental margins

methodology for obtaining constraints on ground motion from precariously balanced rocks precariously balanced rocks provide constraints on the level of ground motion that could have occurred during the time the rocks have been in their current positions field measurements of the quasi static toppling accelerations of precariously balanced rocks provide information for computer modeling of the response of the rocks to given ground motion histories the quasi static toppling acceleration is determined by the ratio of the quasi static force through the center of mass whose moment about the rocking point counterbalances that of gravity and the mass of the rock for an estimate of dynamic toppling peak ground accelerations we use a finite difference numerical code to model the dynamic response of rocks to arbitrarily complex acceleration time histories we test our computational methodology and results using the university of nevada shake table the shake table tests of both idealized rectangular shapes and actual rocks confirm the accuracy of our methodology for estimating dynamic toppling accelerations of precarious rocks for different waveforms representing earthquake ground motion time histories a study of statistical variation of the dynamic toppling acceleration for a suite of seismograms provides constraints on the peak ground acceleration on hard rock sites in mojave desert about 15 km from the san andreas fault of about 0 5 g for synthetic seismograms with relatively high frequency content or about 0 4 g for seismograms with spectra similar to the recent izmit turkey earthquake a major source of uncertainty is the uncertainty in the spectrum of ground motion for large earthquakes waveforms with relatively more low frequencies can topple the rocks with lower peak ground accelerations we describe methods for reducing the uncertainties with further study and more data

extension along the australian pacific transpressional transform plate boundary near macquarie island the australian pacific transform plate boundary fault zone along the macquarie and mcdougall segments of the macquarie ridge complex mrc south of new zealand is characterized by dominantly normal faults and pull apart basins in apparent conflict with the regional transpressional tectonic setting we propose that present day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on macquarie island result from local stresses related to right lateral right stepping en echelon plate boundary faults and not to the current transpressional setting transpression along the australian pacific transform plate boundary has resulted in uplift along the 1500 km long macquarie ridge complex macquarie island the only subaerial exposure of the complex sits atop a 5 km high 50 km wide submarine ridge of oceanic crust and lies 4 5 km east of the major active plate boundary fault zone thus macquarie island and the surrounding seafloor provide a unique opportunity to study an active oceanic transform fault using complementary marine geophysical and land based geological data mapping of recent faults affecting the topography of macquarie island shows that the island is extensively cut by high angle normal faults forming pull apart basins furthermore evidence for reverse motion is rare using marine geophysical data including swath bathymetry reflectivity and seismic reflection data collected along the australian pacific plate boundary north and south of the island we have defined a 5 15 km wide plate boundary zone a series of right stepping en echelon faults within this zone lies along the main plate boundary trace at the right stepping fault terminations elongate depressions 10 km wide and 1 2 km deep parallel the plate boundary which we interpret as extensional relay zones or pull apart basins we propose that transpression is partitioned into en echelon strike slip faults at the plate boundary and a convergent component that flexes the crust causing the anomalous bathymetric ridge and trough morphology of the mcdougall and macquarie segments of the mrc

the oligocene miocene pacific australia plate boundary south of new zealand evolution from oceanic spreading to strike slip faulting abstract since the eocene the pacific australia plate boundary south of new zealand has evolved from a spreading system into a transform boundary swath data acquired in the southeast tasman oceanic crust between the macquarie ridge complex and the resolution ridge system show that the spreading fabric changes orientation southwards along the puysegur trench striking successively n60 e n85 e and n120 e this reflects the reorganisation of the plate boundary in response to changes in relative plate motion a comparison of these orientations with the positions of the pacific australia relative poles of rotation enables us to estimate the age of stoc where there are no identified magnetic anomalies the youngest age of the oceanic crust is ca 12 ma at the south end of the puysegur trench this age is consistent with spreading rates and the amount of crust generated since 31 ma curved fracture zones on either side of the macquarie ridge complex suggest a continuous reorientation of transform faults between 31 ma and ca 15 ma small scale seafloor morphology shows a 13 change of orientation in the l atalante fracture zone that indicates incremental rather than continuous changes in azimuth of the transform faults patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations using swath data and plate reconstruction models we infer that between 31 and 12 ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments along the whole plate boundary this indicates that since 14 15 ma the plate boundary has become progressively predominantly transcurrent allowing strike slip motion to develop along a line of merging transform faults that connected to the intracontinental alpine fault

consistency of precariously balanced rocks with probabilistic seismic hazard estimates in southern california abstract the overturning fragility of a freestanding block such as a precariously balanced rock pbr has been parameterized as a function of a vector of ground motion intensity measures methodologies are outlined to estimate the failure probabilities of such objects given their residence times for deterministic seismic hazard analyses dshas a pbr is exposed to the scenario earthquakes that occur during its exposure time providing an estimate of the probability that the pbr survives the ensemble of events for probabilistic seismic hazard analyses pshas the pbr overturning fragility is multiplied by the ground motion occurrence rate from a vector valued probabilistic seismic hazard analysis vpsha yielding the marginal overturning rate for each ground motion bin summing the marginal rates over all ground motion bins produces the total overturning rate for time independent poisson based psha estimates the probability of block failure can be easily calculated as a function of exposure time this latter method is used to test vpsha estimates similar to the 2002 u s geological survey usgs national seismic hazard maps via pbr residence times pbr overturning fragilities are estimated at sites in southern california near the san andreas fault between the san jacinto and elsinore faults and near the white wolf fault the resulting failure probabilities for several of the pbrs are very high suggesting that they are inconsistent with the 2002 usgs ground motions an investigation of the hazard calculated with zero aleatory variability in the ground motion prediction equations gmpes suggests that the median ground motions or the earthquake rupture rates are too high at certain pbr sites

reconstructing the regolith from erosionally exhumed corestone and saprock derived from the cretaceous val verde tonalite peninsular ranges southern california usa a case study abstract article history received 13 june 2013received in revised form 27 september 2013accepted 10 october 2013available online xxxxkeywords tonalite corestone and saprocklinear compositional weathering trendsweathering intensity factorselemental mass changesbulk mass changesregolith weathering zones in temperate zones an idealized pro le of the regolith would consist of in descending order and decreasingweathering intensity zones i ii iii and iv eachzone ischaracterized bya distinctive mineralogy and chemistrythat re ects reactionsbetweenrocks and downward percolating uids utilizing these principles we attemptedto reconstruct the original position within the regolith of erosionally exhumed corestone and saprock derivedfrom the cretaceous val verde tonalite at yucca perris and motte rimrock southern california chemical index of alteration cia values derived from saprocks at both sites range from 0 49 to 0 50 at yucca perris average weathering intensity values t are 0 03 0 12 and at motte rimrock are 0 03 0 02 clayminerals at yucca perris are smectite kaolinite and illite and at motte rimrock vermiculite kaolinite and illite nostatisticallysigni cantlossofelementalmassorchangesinbulkmassandloi i e water volatiles occurredat yucca perris while at motte rimrock a signi cant increase in loi is evident we interpret the signi cantincrease in loi to re ect the stronger intensity of weathering at motte rimrock where incongruent dissolutionwas responsible for modest leaching and removal of na ba and sr masses from plagioclase the mass of la atyucca perris and the masses of er tm and yb at motte rimrock were increased such increases imply thatprior to removal of the overlying soil and upper parts of the regolith acidic solutions mobilized the rare earthelements and then transported them downward into the lower regolith where increasing ph resulted in theirprecipitation exchange and adsorption onto mineral surfaces the above data suggest that at motte rimrockthe sampled corestone and saprock formed initially at the transition between the upper and lower parts ofzone ii and at yucca perris at the transition between the upper and lower parts of zone iii 2013 elsevier b v all rights reserved

a modified ground motion attenuation relationship for southern california that accounts for detailed site classification and a basin depth effect the attenuation relationship presented by boore et al 1997 has been evaluated and customized with respect to southern california strong motion data for peak ground acceleration pga and 0 3 1 0 and 3 0 sec period spectral acceleration this study was motivated by the recent availability of a new site classification map by wills et al 2000 which distinguishes seven different site categories for california based on the 1994 nehrp classification with few exceptions each of the five site types represented in the southern california strong motion database exhibit distinct amplification factors supporting use of the wills et al 2000 map for microzonation purposes following other studies a basin depth term was also found to be significant and therefore added to the relationship sites near the center of the la basin exhibit shaking levels up to a factor of 2 greater on average than otherwise equivalent sites near the edge relative to boore et al 1997 the other primary difference here is that pga exhibits less variation among the wills et al 2000 site types in fact the pga amplification implied by the basin depth effect is greater than that implied by site classification the model does not explicitly account for nonlinear sediment effects which if important will most likely influence rock site pga predictions the most evidence for a magnitude dependent variability or prediction uncertainty is also found and included as an option

late quaternary changes in bat palaeobiodiversity and palaeobiogeography under climatic and anthropogenic pressure new insights from marie galante lesser antilles data on lesser antillean late quaternary fossil bat assemblages remains limited leading to their general exclusion from studies focusing on caribbean bat palaeobiodiversity and palaeobiogeography additionally the role of climatic versus human pressure driving changes in faunal communities remains poorly understood here we describe a fossil bat assemblage from blanchard cave on marie galante in the lesser antilles which produced numerous bat remains from a well dated stratified context our study reveals the occurrence of at least 12 bat species during the late pleistocene and early holocene on marie galante whereas only eight species are currently known on the island among these 12 species six are extirpated and one is extinct faunal changes within the blanchard sequence indicate variations in pleistocene bat species representation in the lesser antilles to have been influenced by climatic conditions with northern species greater antilles favored during glacial conditions and southern species southern lesser antilles during interglacial events however few species disappeared at the end of the late pleistocene with most of the extinction extirpation events occurring during the holocene this pattern suggests human activities in the lesser antilles to have played a major role in bat turnover during the late holocene

tectonic implications of fault scarp derived volcaniclastic deposits on macquarie island sedimentation at a fossil ridge transform intersection upper miocene to lower pliocene sedimentary rocks on macquarie island are dominantly volcaniclastic breccia sandstone and siltstone produced by the physical disintegration and tectonic abrasion of oceanic crust in fault zones and mass wasting of these tectonic features they represent small debris fans and small scale turbidites deposited at the base of active fault scarps related to late miocene to early pliocene seafloor spreading most of the sediment is derived from basalts but diabase and gabbro clasts in some sedimentary rocks indicate that middle and lower oceanic crust was exposed to erosion on the sea floor a lack of exotic clasts and a low degree of clast roundness are consistent with a local source for the sediment and no input from continental rocks spatial relationships between sedimentary rocks and major faults associated with seafloor spreading on the island and correlation between sedimentary clast and adjacent up thrown block compositions allow us to infer paleotectonic relief for macquarie island crust during deposition our data support a model involving the deposition of these rocks at the inside corner of a ridge transform intersection furthermore a tectonic reconstruction of the australian pacific plate boundary for the approximate time that macquarie island crust formed 10 9 ma also shows that macquarie island crust most likely formed near a ridge transform intersection this paper describes sedimentation associated with active faulting at a ridge transform intersection that has been uplifted in situ above sea level along with the surrounding oceanic crust and demonstrates that high angle faults have the most pronounced influence compared with low angle faults on sedimentation in this tectonic environment

long term denudation of ocean crust in the central north atlantic ocean near bottom sidescan bathymetric and visual observations of fault scarps in 4 24 ma sea floor show that basaltic ocean crust undergoes extensive denudation that creates wide complex scarp zones most denudation is caused by mass wasting probably accompanied by abrasion by disaggregated debris in 11 24 ma crust trellis drainage patterns are formed by deep cross scarp canyons and intersecting scarp parallel gullies that appear to follow weak zones of fracturing and intracrustal weathering until stabilized e g by sediment burial fault scarps will continue to degrade with time yielding increasingly distorted representations of apparent fault geometry in remotely sensed morphological data

pre wisconsinan mammals from jamaica and models of late quaternary extinction in the greater antilles abstract the vertebrate fauna recovered from indurated conglomerates at wallingford roadside cave central jamaica is shown to be in excess of 100 000 yr old according to uranium series and electron spin resonance dating the wallingford local fauna is therefore pre wisconsinan in age and roadside cave is now the oldest radiometrically dated locality in the west indies containing identifiable species of land mammals in the absence of a good radiometric record for quaternary paleontological sites in the caribbean there is no satisfactory basis for determining whether most extinct antillean mammals died out in a blitzkrieg like event immediately following initial human colonization in the mid holocene fossils of clidomys heptaxodontidae caviomorpha the giant wallingford rodent have never been found in situ in sediments of demonstrably holocene age and its extinction may antedate the middle holocene this is also a possibility for the primate xenothrix mcgregori although its remains have been found in loose cave earth a major climate driven bout of terrestrial vertebrate extinction at about 14 000 12 000 yr b p has been hypothesized for the west indies by g pregill and s l olson annual review of ecology and systematics 12 75 98 1981 but at present there is nothing to connect the disappearance of clidomys with this event either quaternary extinctions in the caribbean may prove to be of critical significance for evaluating the reality of new world blitzkrieg but not until an effort is mounted to constrain them rigorously using modern radiometric approaches

a detrital record of lower oceanic crust exhumation within a miocene slow spreading ridge macquarie island southern ocean uplift exhumation and denudation of the lower oceanic crust are recorded by sedimentary rocks of macquarie island 54 30 s 158 54 e which were deposited within the slow spreading proto macquarie spreading ridge between ca 9 and 12 ma measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid ocean ridge basalt e morb with thin intercalations of gabbroic sedimentary lithofacies basaltic detritus has zeolite to lower greenschist metamorphic grades typical of the upper oceanic crust and gabbroic detritus has upper greenschist to amphibolite metamorphic grades typical of the lower oceanic crust breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading related faults sedimentary structures detrital clinopyroxene major and trace element geochemistry and 206 pb 238 u zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a paleogene aged tholeiitic morb source detrital zircon populations of ca 27 and ca 33 ma correspond to oceanic magnetic anomalies 8o and 13o respectively and exclude ca 8 5 ma gabbroic rocks of macquarie island as a potential source geodynamic reconstructions show that anomaly 8o crust from the southeast indian ridge was juxtaposed against the active proto macquarie spreading ridge when sedimentary rocks of macquarie island were deposited and was a likely source for the gabbroic lithofacies the proto macquarie spreading ridge and southeast indian ridge were connected by the jurru long offset transform which has undergone significant transpression since 27 ma this transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged paleogene source crust which provided the source for macquarie island9s gabbroic sedimentary lithofacies

influence of cumulative convergence on lithospheric thrust fault development and topography along the australian pacific plate boundary south of new zealand the development of faulting and topography resulting from initial convergence within oceanic lithosphere is largely unknown we explore relationships among convergence structural development and topography along 1500 km of the submarine australian pacific plate boundary south of new zealand the macquarie ridge complex mrc due to the variable orientation of the boundary and close proximity of the australian pacific poles of rotation individual segments of the plate boundary have experienced different convergence histories since 10 95 ma chron 5o because interaction along the oceanic extent of the boundary involves oceanic lithosphere of broadly the same age and therefore thermal structure structural and morphologic differences can be attributed primarily to variations in angles and rates of convergence with respect to the plate boundary orientation since 10 95 ma we relate plate boundary normal convergence determined from stage pole rotations to structural development focusing on transitions from purely strike slip faulting to partitioned thrust and strike slip faulting between 47 s and 60 s since 10 95 ma our results indicate that boundary normal convergence of 100 km marks the transition from strike slip dominated faulting to partitioned underthrusting and strike slip faulting incipient subduction establishment of subduction at the puysegur trench and incipient subduction at the hjort trench corresponds to convergence at rates between 2 and 4 cm yr angles 30 and with durations of at least 10 m y resulting in 100 km of boundary normal convergence anomalous topographic volumes resulting from tectonic deformation are quantified from swath bathymetry and compared to convergence history results from analysis of the central mrc suggest that prior to lithospheric scale thrust faulting 100 km of boundary normal convergence can be accommodated by crustal deformation 6 km relief from ridge crest to adjacent trough and strike slip faulting our research supports induced forced intraoceanic initiation of subduction

distribution of precarious rocks at the nevada test site comparison with ground motion predictions from nuclear tests 1 precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years thereby providing important constraints on seismic hazard estimates numerous precarious rocks exist in the region of large nuclear explosions at nevada test site nts ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks very near large nts explosions there are no precarious rocks or semiprecarious rocks cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche proof that the rockfalls have been caused by the explosions as the distance from the explosions increases and the contours for predicted ground motion decrease fewer and fewer rockfalls are observed some semiprecarious rocks begin to appear and finally numerous precarious rocks are found with no evidence of recent rockfalls to quantify the calibration of precarious rocks and ground motion we 1 do field tests on the rocks to determine the quasi static and dynamic toppling accelerations 2 carry out computer tests using estimates of waveforms from nts explosions and 3 compare the results with ground motions predicted by empirical regressions for nuclear explosions we conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations

thermal effects of exhumation of a metamorphic core complex on hanging wall syn rift sediments an example from the rechnitz window eastern alps the removal of overburden by tectonic processes displaces the rock masses and their temperature with respect to the surface consequently high cooling rates and increased heat flow have been reported from rapidly exhuming terrains previous studies were mainly focused on the greenschist and amphibolite facies tectono thermal evolution of the footwall a two dimensional numerical thermal model presented in this study concentrates on the thermal history of the hanging wall and the footwall blocks and evaluates the variation of the near surface heat flow above an evolving metamorphic core complex based on structural thermobarometric and geochronological constraints from the rechnitz core complex of the eastern alps the modelling suggests that rapid movement along the extensional fault took place between 22 and 17 ma during this period the heat flow reached values of 140 150 mw m 2 in the hanging wall in a 10 15 km wide zone parallel to the fault trace the increased heat flow caused the high coal rank in the miocene syn rift sediments deposited on the hanging wall apatite fission track geochronology of the thermally overprinted sedimentary deposits gives the time at which the bottom of the syn rift sediment pile passed through 110oc 13 6 0 9 ma the vitrinite reflectance data rr 0 67 1 04 suggest significant burial of the margin of the hanging wall at the time of core complex formation considering the coal rank data and the thermal conductivity of the sedimentary deposits by modelling of the organic maturation we calculate syn rift early middle miocene burial of the hanging wall margin of the rechnitz core complex to be 1100 1600 m this value shows the importance of the post middle miocene erosion of the region which was related to the inversion of the pannonian basin 1998 elsevier science b v all rights reserved

a high resolution temporal record of environmental changes in the eastern caribbean guadeloupe from 40 to 10 ka bp abstract in neotropical regions fossil bat guano accumulated over time as laminated layers in caves hence providing a high resolution temporal record of terrestrial environmental changes additionally cave settings have the property to preserve such organic sediments from processes triggered by winds deflation abrasion and sandblasting and intense rainfall leaching away this study reports both stable carbon and nitrogen isotope compositions of frugivorous bat guano deposited in a well preserved stratigraphic succession of blanchard cave on marie galante guadeloupe these isotopic data are discussed with regard to climate changes and its specific impact on eastern caribbean vegetation during the late pleistocene from 40 to 10 ka cal bp guano 13 c values are higher than modern ones suggesting noticeable vegetation changes this provides also evidence for overall drier environmental conditions during the pleistocene compared to today meanwhile within this generally drier climate shifts between wetter and drier conditions can be observed large temporal amplitudes in both 13 c and 15 n variations reaching up to 5 9 and 16 8 respectively also indicate these oceanic tropical environments have been highly sensitive to regional or global climatic forcing stable isotope compositions of bat guano deposited from 40 to 35 ka bp the last glacial maximum and the younger dryas reveal relatively wet environmental conditions whereas at least from the end of the heinrich event 1 and the bolling period the region experienced drier environmental conditions nevertheless when considering uncertainties in the model age the isotopic record of blanchard cave show relatively similar variations with known proxy records from the northern south america and central america suggesting thus that the blanchard cave record is a robust proxy of past itcz migration teleconnections through global atmospheric pattern suggest that islands of the eastern caribbean basin could be also under the influence of a bipolar temperature gradients that impact the mean location of the itcz with a southern hemisphere imprint during the glacial period and a more significant role of northern hemisphere during the last deglaciation

eigenoscillations and stability of rocking stones the case study of the hus pulpit in the central bohemian pluton stability of precariously balanced rocks is employed in seismic hazard estimates standard toppling models of these rocks are based on a rigid body motion i e this approximation assumes that their eigenfrequencies are infinite in any position of a rock this assumption is however questionable in the case of rocking stones oscillating with lowest eigenfrequencies of only several hertz when resonance effects facilitating rolling wobbling due to a seismic signal cannot be a priori excluded an example demonstrating existence of such low frequencies is the hus pulpit rocking stone located in the central bohemian pluton 49 568n 14 363e the observed lowest frequencies recorded by a guralp cmg 40t seismometer located on the top of this rocking stone are 3 7 and 9 hz with the quality factors of 60 55 and 77 respectively using the comsol multiphysics software package we model the oscillations of this rocking stone numerically and demonstrate that these observed low frequencies and their dampings can be explained by a force interaction between the stone base and the bedrock that can formally be interpreted as the damped elastic response of the bedrock to its surface deformation by the stone we also numerically study stability of the stone under the presence of such an interaction and conclude that beginning of toppling requires very high horizontal pgv pga for input seismic frequencies even at the resonance frequencies however different numerical models where the seismic displacements are considered as the direct input at the rock base can result in rock instabilities under a presence of an hbox m 7 earthquake at a distance of about 150 km

western caribbean sea surface temperatures during the late quaternary 1 mg ca ratios in the planktonic foraminifera globigerinoides ruber from colombian basin core odp 999a suggest that caribbean sea surface temperatures ssts were from 2 1 to 2 7 c colder than the present during the last three glacial maximums in comparison faunal derived ssts simmax method show that august ssts in the caribbean varied 2 c over the past 360 kyr whereas february ssts varied between 21 0 c and 26 5 c changes in the mg ca sst record contain a strong 23 kyr periodicity suggesting the mg ca sst record reflects a warm season weighted sst average rather than an annual mean sst combining several dissolution indices we identify brief periods of decreased carbonate preservation in our record and show that mis 11 stands out as the most intensive dissolution cycle in the caribbean over the last 460 kyr comparison of caribbean sst change with a similar estimate of tropical sst variability in the western pacific over the past 360 kyr reveals shifts in the east west tropical sst gradient that are coeval with glacial interglacial climate change and consistent both with a southward migration of the glacial itcz and with a glacial el nino like mode of tropical circulation

coupled basin evolution and late stage metamorphic core complex exhumation in the southern basin and range province southeastern arizona abstract records of lithospheric extension and mountain range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations in southeastern arizona metamorphic core complexes form mountain ranges with the highest elevations in the region and supposedly much less extended terranes lie at lower elevations adjacent to the santa catalina rincon metamorphic core complex within the tucson basin stratigraphic sequence geometries evident in a large suite of 2 d seismic reflection data suggest a two phase basin evolution model controlled by the emplacement and subsequent uplift of the core complex in its earliest stage phase i of basin formation was characterized by extensive faults forming relatively small scale proto basins which coalesced with the larger basin bounding detachment fault system synextensional sedimentation within the enlarging basin is evidenced by sediment growth packages derived from adjacent footwall material fanning into brittle hanging wall faults during this phase volcanism was widespread and growth packages contain interbedded sediments and volcanic products but paradoxically no mylonitic clasts from the adjacent metamorphic core complex phase ii of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill this is coupled with the activation of a series of high angle normal faults ringing the core complex these observations suggest a two phase model for metamorphic core complex evolution with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression this was followed after a significant tectonic hiatus by late stage exhumation and flexural uplift of the santa catalina rincon metamorphic core complex through younger high angle faulting moreover the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid crustal rocks from beneath the subsiding tucson basin to the uplifting mountain range

precarious rock constraints on ground motion from historic and recent earthquakes in southern california new examples of precarious rocks are presented these rocks provide constraints on ground motion for historical and recent large earthquakes an approximate field calibration of the precarious rock methodology was provided by the m 7 1 hector mine earthquake of 16 october 1999 previously documented rocks at granite pass near kelso california brune 1996 were overturned by the earthquake and a nearby strong motion station indicated ground motions of about 0 2 g consistent with the toppling accelerations estimated in the published article the time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka consistent with previously estimated ages for precarious rocks this first example of an actual earthquake field test lends strong support to the precarious rock methodology the other rocks discussed here give constraints on ground motions for historic earthquakes such as the 1812 and 1857 san andreas fault earthquakes the 1899 and 1912 san jacinto fault earthquakes the 1952 kern county earthquake and possibly recent but prehistoric earthquakes on the banning and garlock faults the ground motion constraints for these earthquakes are lower than predicted by some recent ground motion attenuation curves which are extrapolations to near fault distances from a data set dominated by data from larger distances but are generally consistent with peak ground accelerations observed from the recent large turkey and taiwan earthquakes and provide important additional information for seismic hazard analysis on the other hand there are areas where precarious rocks would be expected on the basis of previous studies but are apparently not found this suggests possible earthquakes on previously unrecognized or only recently recognized faults one such area is in northwestern san diego and southwestern orange county between the elsinore and newport inglewood faults the lack of precarious rocks in this area might be attributed to recent earthquakes on the blind thrust faults proposed in the area by grant et al 1999 2002 and rivero et al 2000

discrimination of ophiolitic from nonophiolitic ultramafic mafic allochthons in orogenic belts by the al ti ratio in clinopyroxene in mafic igneous suites typical of anorogenic igneous provinces mid ocean ridges back arc basins continental rifts and hotspots clinopyroxenes of tholeiitic and silica undersaturated rocks incorporate al in clinopyroxene chiefly via the charge coupled substitution vimgivsi2 vitiival2 as the catial2o6 molecule this substitution is promoted by high titania activity relative to silica in the melt and hence is particularly characteristic of silica undersaturated mafic rocks mafic igneous suites of convergent plate margins i e arc tholeiites and calc alkalic gabbros and basalts characteristically incorporate al in clinopyroxene largely as the cafe3 alsio6 molecule the vimgivsi vife3 ival substitution is promoted by relatively high fugacities of h2o and o2 a characteristic feature of arc axis magmas recognition of cumulates that are complementary to compositions of eruptive rocks in arcs is crucial to development of valid petrogenetic models for arc magmas plots of tetrahedrally coordinated al vs octahedral ti in augite of gabbroic and ultramafic cumulates show that augite data arrays for arc related cumulates have a trend of substantially higher al ti in clinopyroxene than do those from rift related tholeiites the al ti ratio in clinopyroxene may be utilized to discriminate ophiolitic from nonophiolitic ultramafic mafic allochthons in orogenic belts

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

rocking and overturning of precariously balanced rocks by earthquakes abstract precariously balanced rocks in seismically active regions may be used to provide constraints on probabilistic estimates of strong ground motion a computer program is developed to study the rocking response of models of precariously balanced rocks when subjected to various levels of ground accelerations the computer code validated by comparing the numerical and experimental results for simple shaped rigid blocks is used to study the rocking response of rectangular blocks with various aspect ratios to the el centro strong ground motion recording of the 18 may 1940 imperial valley earthquake results show that during earthquakes the horizontal components of ground motion play a dominant role in overturning rocking objects the rederived and corrected housner9s equation 1963 could be used to estimate the minimum overturning acceleration of precarious rocks at different frequencies by measuring the quasi static toppling acceleration in the field

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

neotectonics of the macquarie ridge complex australia pacific plate boundary new marine geophysical data along the macquarie ridge complex the australia pacific plate boundary south of new zealand illuminate regional neotectonics we identify tectonic spreading fabric and fracture zones and precisely locate the australia pacific plate boundary along the macquarie ridge complex we interpret a 5 10 km wide macquarie fault zone between the two plates along a bathymetrie high that extends nearly the entire length of the australia pacific plate boundary south of new zealand we conclude that this is the active australia pacific strike slip plate boundary arcuate fracture zones become asymptotic as they approach the plate boundary a broad zone of less intense deformation associated with the plate boundary extends 50 km on either side of the macquarie fault zone marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike slip deformation although teleseismic evidence overwhelmingly indicates strike slip motion along the entire surveyed boundary today the mcdougall and southernmost puysegur segments show no evidence for past underthrusting whereas data from the macquarie and hjort segments strongly suggest past convergence the present day strike slip plate boundary along the macquarie ridge complex coincides with the relict spreading center responsible for australia pacific crust in the region our conceptual model for the transition from seafloor spreading to strike slip motion along the macquarie ridge complex addresses the decreasing length of spreading center segments and spacing between fracture zones as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary

precariously balanced rocks and ground motion maps for southern california groups of precariously balanced rocks are effectively low resolution strong motion seismoscopes that have been operating on solid rock outcrops for thousands of years and once the methodology has been developed can provide important information about seismic risk in one zone near victorville only 30 km from the nearest point on the san andreas fault more than 50 precarious rocks have been documented widespread rock varnish suggests that many of these rocks have been in their current unstable positions for thousands of years we have established the mechanical basis for rough estimates of the horizontal accelerations necessary to topple these rocks using field observations and numerical and physical modeling to verify that zones of precarious rocks do not occur near historic earthquakes searches using binoculars were made along roads with occasional foot surveys near large earthquakes based on these reconnaissance searches we conclude that no precarious rock zones are found within 15 km of zones of high energy release of historic large earthquakes to document the occurrence of precarious rocks in southern california road surveys were carried out along major roads four zones of precarious rocks and seven other zones of somewhat less precarious rocks have been documented published probabilistic ground motion maps for southern california are compared with the occurrence of zones of precarious and semi precarious rocks the results are encouraging and suggest that eventually studies of precarious rocks will provide important constraints on the assumptions on which the maps are based results from studies of precarious rocks may eventually provide important information for siting and design of sensitive structures such as hospitals and power plants precarious rocks give a direct indication of past ground shaking in contrast to the indirect inference provided by fault trenching studies which may be subject to uncertainties in the actual time history of slip due to the fault e g fault creep slow earthquakes or unknown dynamic stress drop it is concluded that precarious rocks warrant further study and quantitative analysis

morphology and distribution of lava flows on mid ocean ridges a review abstract both pillow lavas and sheet flows are important features of oceanic volcanism at mid ocean ridges submersible surveys of the neovolcanic zone and off axis areas of these complex seafloor volcanic terrains conducted during the past two decades have uncovered a wide array of basaltic surface forms with pillow basalts displaying bulbous spherical and elongate tubular patterns and sheet flows exhibiting smooth lobate rippled wrinkled ropy whorly hackly and jumbled configurations the genesis of pillow lavas and sheet flows appears to be coupled to variations of spreading rates within the plate boundary zone while pillow lavas predominate at slow spreading centers i e mid atlantic ridge sheet flows prevail along the active volcanic zones at intermediate i e galapagos rift and juan de fuca ridge fast and superfast i e east pacific rise spreading centers yet both types of flows are present at each setting the distribution of pillow lavas and sheet flows along a ridge segment is related to the topographic gradient of the ridge axis hence the formation of sheet flows tends to be favored by the development of topographic highs between ridge segment boundaries where the crustal lid is thinnest and rifting directly taps the underlying magma chamber fissure eruptions here release copious amounts of surface fed fluid lavas which decrease in volume down the topographic gradient away from the bathymetric high of the ridge segment lava flows become more distal and channelized thereby fostering greater concentrations of tube fed pillow flows toward the segment boundaries the type of lava flow morphologies encountered at any specific site on the seafloor is contingent upon the effusion rate cooling rate and slope as demonstrated by laboratory simulations

seismic reflection imaging of an oceanic detachment fault atlantis megamullion mid atlantic ridge 30 10 n abstract we present multichannel seismic reflection data collected over the atlantis megamullion at the eastern ridge transform intersection of atlantis fracture zone on the northern mid atlantic ridge and over its conjugate crust these data image for the first time the internal structure of a young well developed megamullion dome formed by tectonic extension across a long lived oceanic detachment fault the exposed corrugated detachment fault surface exhibits a sharp coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor at the termination of the megamullion the fault is imaged 13 km along strike beneath a volcanic hanging wall block at a sub seafloor depth of 0 2 0 5 s two way travel time reaching north as far as 30 19 n the eastward dipping of the fault beneath the hanging wall block is estimated to be 6 14 the corrugated fault surface is underlain by a continuous strong and relatively smooth reflection d at 0 2 0 25 s sub bottom below the central axis of the dome this reflection deepens up to 0 6 s sub bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope we suggest that atlantis massif formed by sequential slip on two different detachment faults that merged at depth with breakaways as little as 2 km apart the initial detachment is represented by reflection d and the second corresponds to the presently exposed fault surface in this interpretation much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity it lies in contact with less altered higher velocity mantle below the first detachment resulting in the strong smooth character of reflection d mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism in conjugate crust corresponding to termination of the megamullion observed sub bottom reflections are interpreted as base of seismic layer 2a this layer is as thick as or thicker 570 900 m than layer 2a in normal atlantic crust and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming

a case study of a precariously balanced rock its partially exhumed corestone platform and encasing saprock and soil abstract lying between the elsinore and san jacinto faults 0 5 km se of roundtop is a tonalitic corestone perched precariously on an exhumed corestone base such features are referred to as precariously balanced rocks pbrs cosmogenic 10be data listed in an online abstract suggest a tentative exhumation age of the pbr near roundtop at 35 000 years ago the absence of pbrs adjacent to the elsinore and san jacinto faults implies that they were shaken free from their perches during ground shaking alternatively their absence may reflect a more highly fractured and intensely weathered terrane because the weathering histories of pbrs have not been studied previously we analyzed the texture clay mineralogy and geochemistry of the roundtop pbr to answer several basic questions can the characteristics of the regolith that once encased the precariously balanced rock be reconstructed what chemical processes operated in the regolith prior to exhumation and are these processes still occurring following exhumation did new fracture development modify the original regolith to answer these questions we extracted three 1 m long cores from the pbr near roundtop and collected 23 saprock and 9 pebbly sandy loam samples from a trench excavated near the base of the corestone platform plagioclase and biotite alteration along with the concomitant loss of ca na sr ba k mg mn nb and p mass are the primary evidence of weathering intensity within the saprock additionally the presence of mostly kaolinite and lesser quantities of mixed layer biotite smectite in the saprock along with the above observations and data point to significant fluid rock interations during saprock development subsequent formation of pebbly sandy loam appears to have been accomplished primarily through pedoturbation within the uppermost portions of the underlying already weathered saprock and produced little additional chemical weathering hence our data support the general idea that roundtop is an area of weak ground shaking and that since current exhumation little weathering has affected this pbr locality

metabasalts from the mid atlantic ridge new insights into hydrothermal systems in slow spreading crust an extensive suite of hydrothermally altered rocks were recovered byalvin and dredging along the mark mid atlantic ridge south of the kane fracture zone 23 24 n where detachment faulting has provided a window into the crustal component of hydrothermal systems rocks of basaltic composition are altered to two assemblages with these characteristics i type i albitic plagioclase an02 10 mixed layer smectite chlorite or chlorite actinolite quartz sphene 20 of the clinopyroxene is altered and cu and zn are leached the geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals and the degree of alteration of primary phases and does not show simple predictive relationships element mobilities indicate that both alteration types formed at low water rock ratios the mark assemblages are typical of the greenschist and transition to the amphibolite facies and represent two distinct albeit overlapping temperature regimes type i 180 to 300 c and type ii 250 to 450 c by analogy with dsdp odp hole 504b and many ophiolites the mark metabasalts were altered within the downwelling limb of a hydrothermal cell and type i and ii samples formed in the upper and lower portions of the sheeted like complex respectively episodic magmatic and hydrothermal events at slow spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event groundmass and vein assemblages in the mark metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event lack of retrograde reactions or overprinting of lower temperature assemblages e g zeolites suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment the type ii samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed this prediction necessitates interaction between hydrothermal fluids and intersertal glass and or mafic phases in addition to plagioclase in order to produce the observed range in vented fluid ph

differential uplift and tilt of pleistocene reef platforms and quaternary slip rate on the morne piton normal fault guadeloupe french west indies 1 the guadeloupe islands are cut by normal faults that accommodate oblique convergence between the north american and caribbean plates such faults are responsible for part of the shallow seismicity and have produced m 5 damaging earthquakes to better assess the seismic hazard in guadeloupe we quantify the slip rate on one of the largest fault morne piton this roughly e w fault crosses the island of marie galante and uplifts a flight of reef terraces from geomorphic analysis we mapped three main terraces new u th datings show that they formed during the latest interglacials 120 and 240 kyr ago correlation with specmap isotopic records implies that the marie galante plateau emerged during the 330 ka highstand topographic profiles show that the terraces and the plateau are deformed by faulting elastic modeling of their shape constrains the geometry of the fault 70 80 dip 5 km depth and its slip rate 0 5 0 2 mm yr given its length 50 km depth and slip rate this fault might produce maximum m 6 5 earthquakes with a recurrence time of 1400 to 3300 years or more likely smaller events such as the m 5 5 16 may 1851 and 3 august 1992 shocks that might recur every 400 to 1000 years we also show that all the islands and terraces are tilted westward perpendicularly to the trench la desirade closest to the trench is uplifted by 276 m whereas subsidence 70 m is observed 10 km east of the volcanic arc this tilt probably resulted from a transient deformation episode at the subduction interface that predated the late pleistocene

new data on pleistocene and holocene herpetofauna of marie galante blanchard cave guadeloupe islands french west indies insular faunal turnover and human impact abstract this work presents the herpetofaunal remains collected from blanchard cave marie galante guadeloupe archipelago this site has yielded the oldest stratigraphic layers around 40 000 bp of the island along with data concerning the herpetofaunal biodiversity of the island from the late pleistocene to pre columbian and modern times the study of these fossil remains reveals the presence of at least 11 amphibian and squamata taxa eleutherodactylus cf martinicensis iguana sp anolis ferreus leiocephalus cf cuneus thecadactylus cf rapicauda cf capitellum mariagalantae ameiva sp cf antillotyphlops boa sp alsophis sp and colubridae sp 2 during the late pleistocene and holocene on marie galante island and provides new evidence concerning extinction times and the introduced or native status of taxa this study also reveals that this bone assemblage is the result of diverse accumulation processes and provides new morphological data on the past herpetofauna of marie galante

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

underthrusting at the hjort trench australian pacific plate boundary incipient subduction 1 the southernmost portion of the macquarie ridge complex mrc comprises the hjort ridge trench and plateau the topographic expression of the australian pacific plate boundary south of new zealand on the basis of marine geophysical swath bathymetry reflectivity seismic reflection gravity magnetic data teleseismic data and gravity modeling we argue that the australian plate is actively underthrusting the pacific plate along the hjort trench but self sustaining subduction does not yet appear to have commenced we interpret a crustal discontinuity in the trench as a shallowly dipping thrust splay off a sub vertical transform fault in the crest of the hjort ridge hjort fault the hjort fault separates oceanic crust generated at the southeast indian ridge seir crust located to the south from oceanic crust generated at the extinct australian pacific spreading center located to the north mrc crust oblique convergence on the transform has initiated a significant plate boundary decollement in the trench but our estimate of 50 km of underthrusting does not support the existence of an eastwardly dipping australian slab below 20 km depth for the length of the hjort ridge and trench system oblique convergence is partitioned between the decollement in the trench and the hjort fault south of 57 5 s the trench decollement accommodates thrusting north of 57 5 s the boundary parallel component of convergence becomes dominant the trench gradually shallows and the trench decollement evolves into a low angle oblique slip fault the trench decollement and strike slip system in the hjort ridge are structural boundaries that contain tectonic slivers of inferred seir crust of the western flank of the hjort ridge that currently belong to neither the australian nor the pacific plates

assessment of the site conditions of precariously balanced rocks in the mojave desert southern california we examine whether groups of precariously balanced rocks in the mo jave desert southern california may have remained standing for thousands of years because anomalous site conditions prevent the rocks from toppling during large earth quakes measurements of peak ground velocity at bandwidths of 0 5 1 1 2 2 4 4 8 and 8 16 hz for 56 earthquakes ml 3 5 4 8 at two sites of precarious rocks are compared with velocities recorded for the same earthquakes by three tri net stations located on engineering rock national earthquake hazard reduction program nehrp site class b we find that the residuals the logarithm of the ratio of the amplitudes recorded at the precarious rock sites to the trinet amplitudes for the same earthquake and epicentral distance at frequencies less than about 4 hz are negative i e deamplification of 50 250 whereas the residuals are slightly positive at the higher frequencies i e amplification of up to 25 50 high frequency ground motions e g peak ground acceleration may therefore be slightly amplified at the precarious rock sites which means that site conditions do not appear to explain the existence of the precarious rocks in areas where high peak ground accelerations are predicted in recent probabilistic seismic hazard psh models this discrepancy between the precarious rocks and the psh models should be urgently resolved

an unusual occurrence of probable pleistocene corestone within a cretaceous dioritic enclave peninsular ranges california abstract we here document an unusual occurrence of probable pleistocene corestone within an 5 m by 5 m dioritic enclave contained within a cretaceous tonalitic pluton santa margarita ecological reserve sw california the enclave lies within 3 5 km of the seismically active 1 5 2 ma elsinore fault zone and may have been subjected to 70 90 ground shaking events since 22 000 18 000 years ago the studied corestone is elliptical with major axis measuring 55 cm and minor axis 26 cm in length it is surrounded by a discontinuous 7 cm thick rind that breaks apart under slightly greater finger pressure than does the surrounding saprolite in order to assess changes in the physical and chemical properties of the corestone rind and saprolite we collected along an 66 5 cm long traverse 4 samples from the corestone 1 from the rind and 5 from the saprolite for bulk and grain density porosity and major and trace element analyses textural and clay mineral data along with the redistribution of elemental mass indicate that the weathering of biotite and to much lesser degrees apatite and the an rich cores of plagioclase played critical roles in the production of saprolite especially in a narrow 20 cm wide zone adjacent to the rind of the corestone within this zone bulk densities reach their minimum values while porosities and positive volume strains reach their maximums in addition the maximum loss of k fe mn and ca mass occurred just inside this region and is paralleled by the highest cia values and highest additions of si and rb mass in contrast the masses of na and sr are progressively increased and decreased respectively along the entire sampling traverse while the loss of mg ti and p mass is episodic with the greatest losses occurring within the narrow zone adjacent to the rind and at the end of the traverse the above observations indicate that the conversion of biotite to expandable mixed layered clay minerals aided by the alteration of the an rich cores of plagioclase produced sufficient stress on grain boundaries that a weakening or loss of intercrystalline cohesion occurred however unlike the well documented isovolumetric development of saprolite in other areas at the study site saprolitization was accompanied by a volume expansion we speculate that repeated ground shaking in response to earthquakes generated in the nearby elsinore fault zone may be responsible for this difference important byproducts of ground shaking would include additional weakening or loss of intercrystalline cohesion and the production and enhancement of new and older fluid pathways respectively the fact that the most intensely altered material at the study site lies adjacent to the boundary between the rind and saprolite suggests that this interface acted to guide fluid flow around the corestone and rind and into the adjacent saprolite where elemental mass was redistributed down various paths leading to the underlying water table most of the leached elemental mass was removed from the area of the sampling traverse but small increases in si rb and na mass suggest redistribution of these elements from elevated areas outside the area sampled during this study the result of the above complex set of processes is a variably porous and chemically altered saprolitic enclave that is still undergoing modification as it adjusts to the vicissitudes of the paralithic environment and continuing ground shaking during earthquakes generated along the elsinore fault

band of precariously balanced rocks between the elsinore and san jacinto california fault zones constraints on ground motion for large earthquakes a spectacular band of precariously and semiprecariously bal anced rocks extends from riverside to near borrego valley cali fornia about halfway between the elsinore and san jacinto fault zones the rocks are distributed in a band a few kilometers wide midway between the san jacinto and elsinore fault zones an in dication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones these rocks have apparently been in place for thousands of years and thus place important constraints on ground motions from earthquakes on the basis of field tests and photographic analysis the estimated quasi static toppling accelerations for the precariously balanced rocks are 0 32 6 0 10 g g 5 gravitational acceleration the dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the izmit and denali earthquakes these constraints are roughly consistent with the median predicted value of ground mo tion for m7 earthquakes but somewhat lower than 11s ground motion curves and much lower than the values from the 2 in 50 yr hazard maps the evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment including median and standard devia tion of ground motion attenuation curves especially for hard rock for which few instrumental data are available the possible exis tence of random background earthquakes and the smoothing dis tance for historical seismicity

mild displacements of boulders during the 2019 ridgecrest earthquakes strong seismic waves from the july 2019 ridgecrest california earthquakes displaced rocks in proximity to the m 7 1 mainshock fault trace at several locations in this report we document large boulders that were displaced at the wagon wheel staging area wwsa approximately 4 5 km southeast of the southern terminus of the large m 6 4 foreshock rupture hereafter the large foreshock and 9 km southwest of the nearest approach of the m 7 1 mainshock surface rupture some boulders appear to have slid along essentially flat surfaces which suggest that dynamic stresses overcame the coefficient of friction other boulders appear to have rocked within their sockets in both cases we use simple mechanical models to estimate total peak dynamic accelerations between 0 5 and 1g commensurate with modified mercalli intensity 9 it is unclear if the strongest shaking at this location occurred during the large foreshock or the m 7 1 mainshock the inferred accelerations are higher than predicted mainshock ground motions at wwsa although local high accelerations could have been generated by path site or source effects gaps between boulders and their sockets are easily visible in the immediate aftermath of earthquakes and provide a quick indication of strong shaking more importantly the gaps quickly fill with surficial organic debris including seeds and leaves of the year that quickly become entombed boulders may thus potentially be extracted to examine gap fillings associated with past earthquakes providing a new datable paleoseismic method

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

structural controls on sulphide deposition at the dyke lava boundary slow spreading ocean crust macquarie island ocean drilling program hole 504b revealed an ocean crust hydrothermal sulphur anomaly on the dyke lava transition with implications for global sulphur sinks here we confirm the presence of the anomaly sporadically along 7 5 km of dyke basalt contact on the macquarie ridge at macquarie island a 39 9 7 ma slow spreading setting background contact zone pyrite s contents average 1845 p p m across 50 m however zones of small scale brittle faulting that commonly occur on and above the dyke basalt contact average between 5000 and 11 000 p p m s 20 30 m widths these consist of steep ridgeparallel faults and fault splays on the contact overlain by up to 50 m of linked pyritic fault trellis the contact zone faults are haloed by disseminated pyrite chlorite cross cut by quartz chlorite sphalerite and epidote cemented breccias containing evidence of turbulent flow the structural control on sulphur deposition is attributed to the active extensional slow spreading setting with increasing extension diffuse mixing across the contact was replaced by channellized flow and dynamic mixing in fault arrays the magnitude of the dyke lava transition sulphur sink must be reassessed to take account of this heterogeneity

direct geological evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n from a detailed survey and sampling study of corrugated massifs north of the fifteen twenty fracture zone on the mid atlantic ridge we demonstrate that their surfaces are low angle detachment fault planes as proposed but not previously verified spreading direction parallel striations on the massifs occur at wavelengths from kilometers to centimeters oriented drill core samples from the striated surfaces are dominated by fault rocks with low angle shear planes and highly deformed greenschist facies assemblages that include talc chlorite tremolite and serpentine deformation was very localized and occurred in the brittle regime no evidence is seen for ductile deformation of the footwall synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low angle fault surface at very shallow levels directly beneath the ridge axis strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient

observation of sections of oceanic crust and mantle cropping out on the southern wall of kane fz n atlantic the objective of the 20 nautile dives of the recent kanaut cruise was to study the southern wall of the kane fracture zone from its eastern intersection with the mid atlantic ridge mar to 5 myr in age the geological mapping shows four successive massifs wrench faulted and slightly tilted the transform facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites gabbros and basalts the entire crustal exposure is cataclased and metamorphosed to greenschist facies

mechanism for generating the anomalous uplift of oceanic core complexes atlantis bank southwest indian ridge atlantis bank is an anomalously uplifted oceanic core complex adjacent to the atlantis ii transform on the southwest indian ridge that rises 3 km above normal seafloor of the same age models of flexural uplift due to detachment faulting can account for 1 km of this uplift postdetachment normal faults have been observed during submersible dives and on swath bathymetry two transform parallel large offset hundreds of meters normal faults are identified on the eastern flank of atlantis bank with numerous smaller faults tens of meters on the western flank flexural uplift associated with this transform parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of atlantis bank extension normal to the atlantis ii transform may have occurred during a 12 m y period of transtension initiated by a 10 change in spreading direction ca 19 5 ma this extension may have produced the 120 km long transverse ridge of which atlantis bank is a part and is consistent with stress reorientation about a weak transform fault

low footwall accelerations and variable surface rupture behavior on the fort sage mountains fault northeast california the fort sage mountains fault zone is a normal fault in the walker lane of the western basin and range that produced a small surface rupture 20 cm during an m l 5 6 earthquake in 1950 we investigate the paleoseismic history of the fort sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950 rupture of the fort sage fault 5 6 ka resulted in surface displacements of at least 0 8 1 5 m implying earthquake moment magnitudes m w of 6 7 7 1 an older rupture at 20 5 ka displaced the ground at least 1 5 m implying an earthquake of m w 6 8 7 1 a field of precariously balanced rocks pbrs is located less than 1 km from the surface rupture trace of this holocene active normal fault ground motion prediction equations gmpes predict peak ground accelerations pgas of 0 2 0 3 g for the 1950 rupture and 0 3 0 5 g for the 5 6 ka paleoearthquake one kilometer from the fault surface trace yet field tests indicate that the fort sage pbrs will be toppled by pgas between 0 1 0 3 g we discuss the paleoseismic history of the fort sage fault in the context of the nearby pbrs gmpes and probabilistic seismic hazard maps for extensional regimes if the fort sage pbrs are older than the mid holocene rupture on the fort sage fault zone this implies that current gmpes may overestimate near fault footwall ground motions at this site

coseismic landsliding during the mw 7 1 darfield canterbury earthquake implications for paleoseismic studies of landslides abstract the head scarp of the harper hills landslide consists of ground cracks with vertical displacement and extension that opened during the 2010 darfield canterbury mw 7 1 earthquake the geomorphology of the cracks regional geology and ground penetrating radar indicate that the landslide formed by bedding controlled translation and joint controlled toppling and suggest incipient deep seated movement crack depth and displacement along the head scarp vary along the ridge maximum values are located where the head scarp is closest to the local ridge line increased seismic shaking due to topographic and geometric amplification of seismic waves is suggested as an explanation for this relationship an excavation across the head scarp revealed no evidence of prior slip events over a time period that is likely to exceed the return period 1000 2500 years of peak ground accelerations experienced at this location in the darfield earthquake we suggest that specific seismologic attributes of the darfield earthquake may have influenced the location of landsliding in this instance studies of paleo landslides must consider crack preservation potential as well as complex source site effects that may complicate estimates of acceleration return periods from the subsurface investigation of individual landslide head scarps

actualistic ophiolite provenance the cyprus case abstract the island of cyprus represents an excellent site to assess quantitatively petrologic clastic response to actively obducting oceanic sources in order to define an actualistic reference for ophiolite provenance in terms of framework composition and heavy mineral suites an improved methodology an extension of the classic ternary qfl logic to include a wider spectrum of key indexes and ratios provides an accurate synthesis of modal data and allows differentiation of three main petrographic provinces and at least seven subprovinces diagnostic signatures of detritus from various levels of an oceanic lithospheric source and criteria for distinguishing provenance from suprasubduction versus mid oceanic ophiolites are also outlined modern sands derived from the troodos ophiolite contain variable proportions of largely pelagic carbonate to chert boninite to basalt diabase to metabasite plagiogranite to gabbroic and cumulate grains supplied from progressively deeper seated levels of the multilay

ridge reorientation mechanisms macquarie ridge complex australia pacific plate boundary the macquarie ridge complex portion of the australia pacific plate boundary south of new zealand provides a unique complete record of a 60 90 change in spreading direction since 40 ma that resulted in the transition from a spreading center to a transform plate boundary marine geophysical data show that during reorientation most ridge segments completely disappeared and all shortened additionally modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates we propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments rifts and transfer of crust between plates during the gradual reorientation of the spreading axes

dating precariously balanced rocks in seismically active parts of california and nevada precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern california and nevada in this study we used two independent surface exposure dating techniques rock varnish microlamination and cosmogenic 36 cl dating methodologies to estimate minimum and maximum limiting ages respectively of the precarious boulders and by inference the elapsed time since the sites were shaken down the results of the exposure dating indicate that all of the precarious rocks are 10 5 ka and that some may be significantly older at victorville and jacumba california these results show that the precarious rocks have not been knocked down for at least 10 5 k y a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps at yucca mountain nevada the ages of the precarious rocks are 10 5 to 27 0 ka providing an independent measure of the minimum time elapsed since faulting occurred on the solitario canyon fault

seismically damaged regolith as self organized fragile geological feature 1 the s wave velocity in the shallow subsurface within seismically active regions self organizes so that typical strong dynamic shear stresses marginally exceed the coulomb elastic limit the dynamic velocity from major strike slip faults yields simple dimensional relations the near field velocity pulse is essentially a love wave the dynamic shear strain is the ratio of the measured particle velocity over the deep s wave velocity the shallow dynamic shear stress is this quantity times the local shear modulus the dynamic shear traction on fault parallel vertical planes is finite at the free surface coulomb failure occurs on favorably oriented fractures and internally in intact rock i obtain the equilibrium shear modulus by starting a sequence of earthquakes with intact stiff rock extending all the way to the surface the imposed dynamic shear strain in stiff rock causes coulomb failure at shallow depths and leaves cracks in it wake cracked rock is more compliant than the original intact rock cracked rock is also weaker in friction but shear modulus changes have a larger effect each subsequent event causes additional shallow cracking until the rock becomes compliant enough that it just reaches coulomb failure over a shallow depth range of tens to hundreds of meters further events maintain the material at the shear modulus as a function where it just fails the formalism provided in the paper yields reasonable representation of the s wave velocity in exhumed sediments near cajon pass and the san fernando valley of california a general conclusion is that shallow rocks in seismically active areas just become nonlinear during typical shaking this process causes transient changes in s wave velocity but not strong nonlinear attenuation of seismic waves wave amplitudes significantly larger than typical ones would strongly attenuate and strongly damage the rock

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

modern sand from obducted ophiolite belts sultanate of oman and united arab emirates abstract oman and the united arab emirates host the largest ophiolites on earth and are characterized by arid tropical climate and desert conditions they thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction orogen provenance spectacularly exposed in se arabia are two distinct obducted ophiolite sequences sama il and masirah that contrast widely in formation setting spreading rate detachment level thermal state during emplacement and relief masirah beach sand chiefly consists of basaltic diabase to metadiabase plagioclase gabbroic and dunite grains from progressively deeper seated levels of the multilayered oceanic crust dense minerals include diopsidic clinopyroxene and hornblende from gabbros epidote and tremolite actinolite from altered dikes and augite from basaltic lavas in contrast sand from the sama il ophiolite includes cellular serpentinite grains and en

toppling analysis of the echo cliffs precariously balanced rock toppling analysis of a precariously balanced rock pbr can provide insight into the nature of ground motion that has not occurred at that location in the past and by extension can constrain peak ground motions for use in engineering design earlier approaches have targeted 2d models of the rock or modeled the rock pedestal contact using spring damper assemblies that require recalibration for each rock here a method to model pbrs in 3d is presented through a case study of the echo cliffs pbr the 3d model is created from a point cloud of the rock the pedestal and their interface obtained using terrestrial laser scanning the dynamic response of the model under earthquake excitation is simulated using a rigid body dynamics algorithm the veracity of this approach is demonstrated through comparisons against data from shake table experiments fragility maps for toppling probability of the echo cliffs pbr as a function of various ground motion parameters rock pedestal interface friction coefficient and excitation direction are presented these fragility maps indicate that the toppling probability of this rock is low less than 0 2 for peak ground acceleration pga and peak ground velocity pgv lower than 3 m s 2 and 0 75 m s respectively suggesting that the ground motion intensities at this location from earthquakes on nearby faults have most probably not exceeded the above mentioned pga and pgv during the age of the pbr additionally the fragility maps generated from this methodology can also be directly coupled with existing probabilistic frameworks to obtain direct constraints on unexceeded ground motion at a pbr s location

formation of amphibole after clinopyroxene by dehydration reactions implications for pseudomorphic replacement and mass fluxes the replacement of anhydrous by hydrous minerals is commonly used to infer infiltration of a rock by h 2 o however our work indicates that the formation of pseudomorphic amphibole after clinopyroxene uralitization in low grade metavolcanic rocks is a net dehydration process in the ophiolitic slate creek complex northern california clinopyroxene exhibits four textural stages of alteration 1 clinopyroxene 2 clinopyroxene chlorite amphibole 3 amphibole chlorite and 4 amphibole this transformation occurs in subgreenschist to greenschist facies rocks with a common matrix assemblage quartz albite chlorite epidote titanite amphibole white mica rare pumpellyite a simple reaction space model based on these observations indicates that consumption of chlorite in the rock matrix releases more water than required to produce amphibole pseudomorphs of clinopyroxene thus the net flux of h 2 o during uralitization of greenschist facies metavolcanic rocks is outward these results imply that uralitization in metavolcanic rocks results from heating rather than whole rock hydration and that natural mass fluxes may be counter to fluxes inferred from textural evidence alone