early mis 3 occupation of mochena borago rockshelter southwest ethiopian highlands implications for late pleistocene archaeology paleoenvironments and modern human dispersals abstract between 70 and 50 ka bp anatomically modern humans dispersed across and out of africa to eventually populate all inhabitable continents knowledge of paleoenvironments and human behavioral patterns in africa prior to and during these dispersals is crucial for understanding how and why hunter gatherers were able to adapt rapidly to the new environments they encountered however few well dated sites from this time period are known from the horn of africa one of the purported staging areas for population movements into southern arabia and asia excavations at mochena borago rockshelter situated on the western slopes of a dormant volcano where the sw ethiopian highlands meet the ethiopian rift have yielded the first securely dated archaeological sequence for later periods of the dispersal three major lithostratigraphic groups incorporating occupational episodes have yielded charcoal radiocarbon ages 53 38 ka calbp deeper deposits have been tested but remain undated archaeological assemblages consist mainly of obsidian flaked stone artifacts manufactured from small minimally prepared single to multi platform flake cores radially prepared cores are rare and blade cores are absent small unifacial to bifacial points from non radial cores dominate the earliest shaped tool assemblages and backed pieces first appear by 45 ka calbp by 43 ka calbp scrapers and backed pieces are predominant rather than points however there is little evidence for technological change other than the appearance of bipolar technology mochena borago s archaeological sequence thus cannot be neatly classified as middle stone age later stone age or transitional and calls into question some of the principles by which archaeologists have attempted to classify african toolmaking traditions

azimuthal variations of teleseismic p wave residuals for northern california teleseismic p wave residuals from 70 events 1971 1979 for seven stations in central and northern california are investigated as a function of azimuth to model upper mantle velocity structure relative residuals are determined for each station using berkeley as the reference station maximum relative residual variation is 0 9 s in the northern coast ranges to 1 7 s in the klamath mountains interpretations of the residual variations represent lateral variations of the p wave velocity in the upper mantle the modeling technique utilizes a simple ray tracing technique to calculate time residual equivalent path lengths in the anomalous velocity regions a model of the paleosubduction zone farallon plate beneath northern california represents a region 40 to 150 km depth of high velocity 8 3 8 5 km s which explains the negative residual pattern for the klamath mountains cascades and modoc plateau region a low velocity zone 7 2 7 4 km s beneath the gorda plate 25 to 155 km depth produces the observed positive residual pattern of the northern coast ranges the low velocity region is related to proposed models of the mendocino triple junction migration and associated upwelling of asthenospheric material

marine derived 14c calibration and activity record for the past 50 000 years updated from the cariaco basin abstract an expanded cariaco basin 14c chronology is tied to 230th dated hulu cave speleothem records in order to provide detailed marine based 14c calibration for the past 50 000 years the revised high resolution cariaco 14c calibration record agrees well with data from 230th dated fossil corals back to 33 ka with continued agreement despite increased scatter back to 50 ka suggesting that the record provides accurate calibration back to the limits of radiocarbon dating the calibration data document highly elevated 14c during the glacial period carbon cycle box model simulations show that the majority of observed 14c change can be explained by increased 14c production however from 45 to 15 ka 14c remains anomalously high indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today additional observations of the magnitude spatial extent and timing of deep ocean 14c shifts are critical for a complete understanding of observed glacial 14c variability

transition from slab to slabless results from the 1993 mendocino triple junction seismic experiment three seismic refraction reflection profiles part of the mendocino triple junction seismicexperiment allowustocompareandcontrastcrustanduppermantleofthenorth american margin before and after it is modified by passage of the mendocino triple junction upper crustal velocity models reveal an asymmetric great valley basin overlying sierran or ophiolitic rocks at the latitude of fort bragg california and overlying sierran or klamath rocks near redding california in addition the upper crustal velocity structure indicates that franciscan rocks underlie the klamath terrane east of eureka california thefranciscancomplexis onaverage laterallyhomogeneousandisthickestinthe triple junction region north of the triple junction the gorda slab can be traced 150 km inboardfromthecascadiasubductionzone southofthetriplejunction strongprecritical reflections indicate partial melt and or metamorphicfluids at the base of the crust or in theuppermantle breaksinthesereflectionsarecorrelatedwiththemaacamaandbartlett springs faults suggesting that these faults extend at least to the mantle we interpret our datatoindicatetectonicthickeningofthefranciscancomplexinresponsetopassageofthe mendocino triple junction and an associated thinning of these rocks south of the triple junction due to assimilation into melt triggered by upwelling asthenosphere the region of thickenedfranciscancomplexoverliesazoneofincreasedscattering intrinsicattenuation or both resulting from mechanical mixing of lithologies and or partial melt beneath the onshore projection of the mendocino fracture zone our data reveal that we have crossed thesouthernedgeofthegordaslabandthatthisedgeand ortheoverlyingnorthamerican crust may have fragmented because of the change in stress presented by the edge

coulomb stress interactions among m 5 9 earthquakes in the gorda deformation zone and on the mendocino fault zone cascadia subduction zone and northern san andreas fault 1 the gorda deformation zone a 50 000 km2 area of diffuse shear and rotation offshore northernmost california has been the site of 20 m 5 9 earthquakes on four different fault orientations since 1976 including four m 7 shocks this is the highest rate of large earthquakes in the contiguous united states we calculate that the source faults of six recent m 5 9 earthquakes had experienced 0 6 bar coulomb stress increases imparted by earthquakes that struck less than 9 months beforehand control tests indicate that 0 6 bar coulomb stress interactions between m 5 9 earthquakes separated by 9 months are unlikely to occur by random chance suggesting that the multiple short term stress interactions observed among the recent gorda zone earthquakes are not an apparent effect in all well constrained 0 2 bar coulomb stress interactions between earthquakes that occurred within 4 years of each other the second earthquake is promoted on longer timescales calculated stress changes imparted by the 1980 mw 7 3 trinidad earthquake are consistent with the locations of m 5 9 earthquakes in the gorda zone until at least 1995 as well as earthquakes on the mendocino fault zone in 1994 and 2000 coulomb stress changes imparted by the 1980 earthquake are also consistent with its distinct elbow shaped aftershock pattern from these observations we derive generalized static stress interactions among right lateral left lateral and thrust faults near triple junctions

irregular geometry of the gorda subduction and deep structure of the eel river basin determined from teleseismic p delays abstract nearly 1000 observations of travel time residuals for teleseismic p arrivals across the humboldt bay seismic network near cape mendocino california are analyzed for a study of 3 d geometry of the gorda plate near mendocino triple junction mtj by the application of a three dimensional velocity inversion procedure the area is underlain by the subducted edge of the gorda plate and the eastward extension of the mendocino fracture zone the observed variation of the travel time gradient with azimuth favors either deepening of the slab southward or southward increase of dilatational velocity across the mendocino fracture zone at depths of more than 50 km the topographically corrected mean travel time delays at individual stations suggest that the eel river basin is deep and tectonically controlled further support for this view is provided by the inversion results which point to the possible extension of the low velocity material through the entire crustal section these results also suggest that from the south the eel river basin may be confined at depth by a steeply northward dipping sliver of high velocity material possibly a detached oceanic slab this feature may be responsible for the intense seismic activity reported beneath the eastern end of the mendocino fracture zone mfz thus the role of the eel river basin in accommodating the relative motion between the adjacent plates may be significant the leading edge of the downgoing slab appears irregular and at some latitudes may not extend beyond the coast line the estimated velocity anomalies vary by as much as 20 in the near surface blocks and nearly half as much at depth the pattern may result from intense tearing and deformation of the slab producing buckling folding and compression ridges

isotope calibrated greenland temperature record over marine isotope stage 3 and its relation to ch4 large temperature variations on millennial time scales in greenland characterised the last ice age abrupt warmings known as dansgaard oeschger do events can be traced in the 18oice record of greenland ice cores however it has been shown that 18oice is not a direct temperature proxy measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer here we present a continuous temperature record based on high resolution 15n measurements and firn model studies it covers a sequence of 9 do events 9 17 during the time period from 38 to 64 kyr bp for which temperature changes of 8 to 15 c were estimated the difference between the modern and the glacial 18oice t relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation a detailed comparison of the temperature evolution with reconstructions of the atmospheric methane ch4 concentration shows that ch4 rises lag temperature increases at the onset of do events by 25 to 70 yr within data resolution the strong correlation between greenland temperature and ch4 on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent

formation of inner gorges abstract under certain conditions a distinctive type of landform the inner gorge evolves in drainage basins undergoing persistent base level lowering inner gorges are typical of basins in north coastal california u s a and one such basin was selected for detailed study based on analysis of along channel distribution of rock types streamside land sliding and stored sediment as well as computation of stream power the following set of conditions appears to accompany the formation of an inner gorge 1 inner gorge parent materials are relatively competent homogenous rock types 2 in inner gorge reaches there is an absence of any base level control that would prevent downcutting 3 continued downcutting occurs due to continued base level fall and this fall is most likely induced by tectonic uplift 4 mass slope failures that form steep straight slopes debris slides are the main denudational hillslope process in the inner gorge 5 sufficient stream power is available to transport landslide derived material out of the inner gorge reach such that alluvial sediment does accumulate in the reach over time and 6 the initial condition of the landscape is a low relief erosionally mature land surface into which the inner gorge is cut this initial condition implies that inner gorges develop in tectonically rejuvenated landscapes the processes of denudation on inner gorge slopes acting over a time scale of 105 years result in a style of landform evolution that approximates parallel slope retreat this type of slope retreat maintains the distinct inner gorge morphology during sustained downcutting though the analysis is confined to one basin in northern california so as to provide a detailed framework to understand inner gorge landform inner gorges appear to be commonplace in areas of moderate to high uplift where lithologic and tectonic conditions are appropriate for their formation a global search for this landform yet to be undertaken is needed to confirm whether it indeed occurs where the above conditions are met

abrupt or gradual change point analysis of the late pleistocene holocene climate record from chew bahir southern ethiopia we used a change point analysis on a late pleistocene holocene lake sediment record from the chew bahir basin in the southern ethiopian rift to determine the amplitude and duration of past climate transitions the most dramatic changes occurred over 240 yr from similar to 15 700 to 15 460 yr during the onset of the african humid period ahp and over 990 yr from similar to 4875 to 3885 yr during its protracted termination the ahp was interrupted by a distinct dry period coinciding with the high latitude younger dryas stadial which had an abrupt onset less than similar to 100 yr at similar to 13 260 yr and lasted until similar to 11 730 yr wet dry wet transitions prior to the ahp may reflect the high latitude dansgaard oeschger cycles as indicated by cross correlation of the potassium record with the northgrip ice core record between similar to 45 20 ka these findings may contribute to the debates regarding the amplitude and duration and mechanisms of past climate transitions and their possible influence on the development of early modern human cultures

structure and seismotectonics of the mendocino triple junction california 1 we analyze and interpret two marine multichannel seismic reflection onshore offshore ocean bottom seismometer obs seismic profiles that cross in the region of the mendocino triple junction over the fault plane of the 1992 petrolia california ms 7 1 earthquake from the offshore reflection data and the wide angle constraints provided by the onshore offshore and obs data we have determined a rough three dimensional view of crustal and uppermost mantle structure beneath the triple junction region this structure is also consistent with gravity data the upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the franciscan terrane a ubiquitous lower crustal layer with velocities of 6 5 7 1 km s 1 is 5 7 km thick throughout most of the region over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick this area is overlain by undisturbed pre 11 ma sediments indicating little deformation since that date the thick lower crust extends beneath the coast suggesting that the deep boundary between pacific and north american terranes lies onshore the northeastern edge of the thick lower crustal layer is marked by concentrated seismicity with the hypocenter of the 1992 petrolia earthquake lying close to the ne corner we speculate that at depth the pacific lower crust acts an asperity acting to concentrate stress from which the petrolia earthquake ruptured

recent large earthquakes near cape mendocino and in the gorda plate broadband source time functions fault orientations and rupture complexities the northward migration of the mendocino triple junction is associated with complex faulting within the gorda plate and in the convergent zone between the gorda and north american plates this region has experienced substantial recent large earthquake activity and quantification of these faulting processes is essential for understanding the evolution of the triple junction using an empirical green function deconvolution method for teleseismic and regional surface waves and body waves we obtain relative source time functions for the april 25 1992 cape mendocino thrust earthquake m w 7 2 its two large strike slip aftershocks on april 26 1992 m w 6 5 6 6 and two large strike slip events in the gorda plate that occurred july 13 1991 m w 6 8 and august 17 1991 m w 7 1 the removal of propagation effects using empirical green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration analyzing the directivity effects observed in the source time functions we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock significant differences in the rupture duration and stress drop are observed between the cape mendocino mainshock and its two largest aftershocks the mainshock which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the gorda and north american plates had a smooth 9 s long rupture with a 0 9 4 6 mpa static stress drop the two largest aftershocks occurred within the uppermost mantle of the gorda plate and have more complex ruptures with total durations of 14 15 s and static stress drops of less than 0 3 mpa perhaps reflecting a difference between mantle and crustal earthquakes in this region the aftershocks appear to have ruptured conjugate strike slip faults accounting for differences in their damage patterns the august 17 1991 gorda plate event ruptured a southwest striking fault while the july 13 1991 event ruptured a southeast striking fault with both events relieving north south compression within the deforming gorda plate the existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating triple junction

influence of the mendocino triple junction on the tectonics of coastal california abstract the migration of the mendocino triple junction through central and northern california over the past 25 30 million years has led to a profound change in plate interactions along coastal california the tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the san andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations the primary driver of this tectonism is a coupling among the subducting juan de fuca gorda north american and pacific plates that migrates with the triple junction this coupling leads to ephemeral thickening of the overlying north american crust associated uplift and subsequent subsidence and a distinctive sequence of fault development and volcanism

long river profiles tectonism and eustasy a guide to interpreting fluvial terraces along three rivers at the mendocino triple junction northern california strath cut and fill terraces have formed in response to tectonic and eustatic processes detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level primarily aggradation and formation of fill terraces during sea level high stands alternating with deep incision during low stands a eustasy driven deposi tional wedge extends tens of kilometers upstream on all rivers tapering to zero thickness this distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere due in part to the large size of these rivers however the change in gradient is nearly identical to other base level rise studies the depositional gradient is about half that of the original channel middle to upper reaches of each fiver are dominated by the effects of long term uplift primarily lateral and vertical erosion and formation of steep unpaired strath terraces exposed only upstream of the depositional wedge vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates strath terraces have steeper gradients than the modem channel bed and do not merge with marine terraces at the river mouth consequently they cannot be used to determine altitudes of sea level high stands strath formation is a continuous process of response to long term uplift and its occurrence varies spatially along a river depending on stream power and hence position upstream strath terraces are found only along certain parts of a coastal stream upstream of the aggradational effects of oscillating sea level and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load for a given size fiver the greater the uplift rate the greater the rate of vertical incision and consequently the less the like lihood of strath terrace formation and preservation

three dimensional geometry of the gorda plate beneath northern california the isostatic residual gravity field over northern california displays a gravity gradient interpreted to reflect the south edge of the gorda plate where it is subducted eastward beneath the north american plate the locus of points of maximum slope defines a line trending s60 e from a coastal point approximately 20 km south of cape mendocino a point where the buried plate boundary is inferred from magnetic and seismicity data southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the blanco fracture zone and the present direction of relative motion between the pacific and northern gorda plates calculations from the form of the anomaly yield depth estimates that fit an east southeast plunge of approximately 9 for the top of the gorda south edge the sense of the anomaly higher gravity to the south supports the hypothesis that a window developed in the subducted slab east of the san andreas fault and south of the gorda plate south of the gorda boundary the base of the north american plate is thus in contact with hot material from the asthenosphere that invaded the window because the overlying north american plate has been moving relatively south across the gorda boundary the north american plate beneath the coast ranges east of the san andreas fault in central california may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the north american plate passed over it

late cenozoic volcanism subduction and extension in the lassen region of california southern cascade range hundreds of short lived small to moderate volume mostly mafic volcanoes occur throughout the lassen region of ne california and surround five longer lived large volume intermediate to silicic volcanic centers younger than 3 ma volcanic rocks older than 7 ma are scarce in the lassen region we identify 537 volcanic vents younger than 7 ma and we classify these into five age intervals and five compositional categories based on sio2 content maps of vents by age and composition illustrate regionally representative volcanic trends by 2 ma the eastern limit of voicanism had contracted westward toward the late quaternary arc late quaternary volcanism is concentrated around and north of the silicic lassen volcanic center the belt of most recent volcanism 25 0 ka has been active since at least 2 ma most mafic volcanism is cakalkaline basalt and basaltic andesite however lesser volume of low potassium olivine tholeiite lkot a geochemically distinctive basalt type found in the northern basin and range province also has erupted throughout the lassen segment of the cascade arc since the pliocene thus models of the mantle source and tectonic control of lkot magmatism should be applicable both within and behind the subduction related arc normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the lassen region nnw alignments of these features indicate nnw orientation of maximum horizontal stress ene extension which is similar to the stress regime in the adjacent northwestern basin and range and northern sierra nevada provinces the large long lived volcanic centers developed just west of a zone of closely spaced nnw trending normal faults within that zone of faulting pervasive ene extension has precluded growth of large long lived crustal magma systems we interpret the western limit of the zone of nnw trending normal faults as the western boundary of the basin and range province where it overlaps the lassen segment of the cascade arc in our view the lassen volcanic region occurs above the subducting gorda north plate but also lies within a broad zone of distributed extension that occurs in the north american lithosphere east and southeast of the present cascadia subduction zone an episode of ene extension that began in the late miocene in the northwestern basin and range province appears to have triggered widespread late miocene to quaternary mafic volcanism in the lassen region the scarcity of volcanic rocks older than 7 ma suggests that a more compressive lithospheric stress regime prior to the late miocene extensional episode may have suppressed volcanism even though subduction probably was occurring beneath the lassen region

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

multiple event relocation of earthquakes near the gorda rise mendocino fracture zone intersection earthquakes along the southern edge of the gorda plate and the mendocino fracture zone that were relocated jointly generally tend to move 25 to 35 km southwest of their pde and isc locations this has the effect of reducing the number of events occurring deep in the plate and increasing the number of events located on the gorda rise as well as revealing a tightly grouped linear zone of activity extending along the mendocino fracture zone the resulting spatial pattern of seismicity is more consistent with recent models of the tectonics of the gorda plate and the mendocino fracture zone

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

geology of the eel river basin and adjacent region implications for late cenozoic tectonics of the southern cascadia subduction zone and mendocino triple junction two upper cenozoic depositional sequences of principally marine strata about 4000 m thick overlie accreted basement terranes of the central and coastal belts of the franciscan complex in the onshore offshore eel river basin of northwestern california the older depositional sequence is early to middle miocene in age and represents slope basin and slope blanket deposition whereas the younger sequence late miocene to middle pleistocene in age consists largely of forearc basin deposits youthful tectonic activity related to gorda north american plate convergence indicates an active cascadia subduction zone and strong partial coupling between these plates structures of the northeastern margin of the eel river basin are principally north northwest trending east northeast dipping thrust and reverse faults that form imbricate thrust fans many faults merge downward into sole thrusts that extend to or near the gorda north american plate interface in the southern part of the region these structures have been modified by northward encroachment of the pacific plate the coastal belt fault the early tertiary accretionary suture between the franciscan central and coastal belts can be traced from arcata bay northward offshore to the southern oregon border it is tentatively extended farther northward based on aeromagnetic data to an offshore position west of cape blanco thereafter it may coincide with the offshore fulmar fault if so the franciscan coastal belt in california is correlative with the offshore fulmar terrane in oregon the cascadia subduction zone csz does not join the mendocino transform fault at the commonly depicted offshore location of the mendocino triple junction mtj instead the csz extends southeastward around the southern eel river basin and shoreward along mendocino canyon to join the petrolia shear zone similarly the mendocino fault may extend shoreward via mattole canyon and join the cooskie shear zone these two shear zones intersect onshore north of the king range and the area of their intersection is the probable location of the mtj

40ar 39ar dating into the historical realm calibration against pliny the younger laser incremental heating of sanidine from the pumice deposited by the plinian eruption of vesuvius in 79 a d yielded a40ar 39ar isochron age of 1925 94 years ago close agreement with the gregorian calendar based age of 1918 years ago demonstrates that the 40ar 39ar method can be reliably extended into the temporal range of recorded history excess 40ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating holocene samples

hemipelagic sedimentation in a region of crustal doming between the mendocino and pioneer fracture zones abstract pacific study area w n lies 300 km west of california south of the gorda ridge between the mendocino and pioneer fracture zones hemipelagic mud covers the seafloor over a broad low rise and grades eastward into distal turbidite deposits of the delgada fan 3 5 khz profiles show three kinds of acoustic character for the seafloor strongly reflecting turbidite deposits depositionally draped hemipelagic sediment and an unusual morphology characterized by low relief closely spaced hills these small hills occur in the same area as the low rise implying a common origin calculations of the plate motions for this small corner of the pacific plate suggest that it is overriding the upper mantle thermal trace of the gorda ridge magma chamber this heat source apparently causes doming offsets in the layered hemipelagic sediment and formation of the closely spaced small hills

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

holocene earthquake records from the cascadia subduction zone and northern san andreas fault based on precise dating of offshore turbidites abstract we present preliminary evidence for a 10 000 year earthquake record from two major fault systems based on sediment cores collected along the continental margins of western north america new stratigraphic evidence from cascadia demonstrates that 13 earthquakes ruptured the entire margin from vancouver island to at least the california border since the eruption of the mazama ash 7700 years ago the 13 events above this prominent stratigraphic marker have an average repeat time of 600 years and the youngest event 300 years ago coincides with the coastal record we also extend the record of past earthquakes to the base of the holocene at least 9800 years ago during which 18 events correlate along the same region the sequence of holocene events in cascadia appears to contain a repeating pattern of events a tantalizing first look at what may be the long term behavior of a major fault system the northern california margin cores show a cyclic record of turbidite beds that may represent holoce

deformation in the lower crust of the san andreas fault system in northern california a continuous seismic velocity and reflectivity cross section of the san andreas fault system in northern california shows offsets in the lower crust and the mohorovicic discontinuity near the san andreas and maacama strike slip faults these faults may cut through the crust to the upper mantle in a zone less than 10 kilometers wide the northern california continental margin to the eastern edge of the coast ranges is underlain by a high velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the gorda plate slab the rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models

fluids in the lower crust following mendocino triple junction migration active basaltic intrusion geodynamic and plate tectonic models for the mendocino triple junction a fault fault trench triple junction in northwestern california predict a slab free zone south of the triple junction in which asthenospheric mantle upwells to the base of the crust a variety of geological and geophysical data support this model although fine scale 20 km details of the lithospheric structure have been unknown previously seismic investigations in the onshore transform regime south of the mendocino triple junction region reveal very strong short offset reflections from the lower crust and at the crust mantle boundary beneath the entire width of the coast range particularly near lake pillsbury california seismic analysis suggests that these reflections are from discrete zones of fluid the reflector geometry implies that the source of the fluid is within the upper mantle in this tectonic context it is likely that the fluids are largely partial melt segregated from asthenospheric mantle upwelling into the slab free zone the tectonic setting and the location of lake pillsbury relative to the estimated position of the southern edge of the gorda slab and the clear lake volcanic field suggest that volcanism may initiate in this region within the next 400 k y

crustal structure of the northwestern vizcaino block and gorda escarpment offshore northern california and implications for postsubduction deformation of a paleoaccretionary margin the vizcaino block is an anomalously shallow region of the western u s continental margin located southwest of the mendocino triple junction it originated as part of the accretionary prism of the north america plate and was transferred to the pacific plate in the miocene as the pacific north america plate boundary migrated 130 km eastward forming the gorda escarpment at its northern boundary we present hybrid crustal models for the northwestern part of the vizcaino block derived from marine seismic and gravity data the velocity and density structure of the northwestern vizcaino block are compatible with paleoaccretionary complex material similar to san simeon patton terrane overlying oceanic crust or a mafic layer the most significant result of our modeling is an abrupt increase in moho dip from 5 to 20 30 beneath the western edge of the oconostota ridge along the northwestern margin of the vizcaino block this moho dip is steeper than observed anywhere along the cascadia subduction zone indicating postsubduction deformation we suggest that the paleotrench was deformed by compression which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone at least part of the deformation predates late pliocene pacific north america plate convergence and may result mainly from north south compression between the pacific juan de fuca plates across the mendocino transform fault north south compression continues today and may dynamically support the uplifted northern margin of the vizcaino block although the primary locus of deformation shifted to the relatively weak gorda plate sometime prior to 3 ma

dating volcanic and related sediments by luminescence methods a review abstract applications of luminescence dating of geological materials have undergone extensive development and refinement since 1974 when wintle first attempted to date volcanic lava using thermoluminescence tl today luminescence techniques are potentially highly useful methods for dating volcanic and related materials and events over timescales ranging from 10 2 to 10 6 years while luminescence approaches generally do not possess the precision afforded by techniques such as 40 ar 39 ar and 14 c methods they offer a useful alternative approach with advantages in terms of age range and scope of material although the application of luminescence methods has in some cases been successfully described a comprehensive study outlining and defining protocols for routine luminescence dating of volcanic materials has not been forthcoming this review surveys previous attempts to develop luminescence techniques to date volcanic products and discusses the state of knowledge of the luminescence behaviour of the four principal materials currently used for luminescence dating of volcanic events particular attention is paid to the red luminescence emissions primarily because of previous successes in using red thermoluminescence rtl of quartz and feldspar for dating volcanic products for which the upper age limit may extend beyond a million years proposals are made for future research directions these include technological developments in the measurement of optically stimulated red luminescence the systematic characterization of rtl and optically stimulated red luminescence as key luminescence parameters in volcanic quartz feldspar glass and polymineral sample fractions and optimization of luminescence dating methods to define a comprehensive protocol for routine dating of volcanic materials

three dimensional crustal structure of the mendocino triple junction region from local earthquake travel times the large scale three dimensional geometry of the mendocino triple junction at cape mendocino california was investigated by inverting nearly 19 000 p wave arrival times from over 1400 local earthquakes to estimate the three dimensional velocity structure and hypocentral parameters a velocity grid 175 km n s by 125 km e w centered near garberville california was constructed with 25 km horizontal and 5 fan vertical node spacing the model was well resolved near cape mendocino where the earthquakes and stations are concentrated at about 40 6 n latitude a high velocity gradient between 6 5 and 7 5 km s dips gently to the south and east from about 15 km depth near the coast relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted gorda plate therefore the depth to the top of the gorda plate near cape mendocino is interpreted to be 15 km the gorda plate appears intact and dipping 8 eastward due to subduction and flexing downward 6 12 to the south both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40 3 n latitude and maintains a linear trend 15 south of east to at least 123 w longitude the top of a large low velocity region at 20 30 km depth extends about 50 km n s and 75 km e w roughly between garberville and covelo and is located above and south of the southern edge of the gorda plate we interpret this low velocity area to be locally thickened crust 8 10 km due to either local compressional forces associated with north south compression caused by the northward impingement of the rigid pacific plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the cascadia subduction zone south of cape mendocino and southwest of the garberville fault high velocities indicative of oceanic crust are detected at 15 km depth we interpret this evidence as tectonic underplating of the pacific oceanic crust beneath the coastal franciscan complex similar to the situation seismically imaged in central coastal california page and brocher 1994 this geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening while deep seated mantle upwelling is confined to an area centered about 75 km north of clear lake benz et al 1992 zandt et al 1992

geometry and evolution of the san andreas fault zone in northern california the evolution of the san andreas fault system is controlled by thermal mechanical processes associated with the development and evolution of a narrow slabless window formed beneath the western edge of north america this fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the mendocino triple junction as a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle the plate boundary separating the north american and pacific plates follows a complex three dimensional geometry which varies through time seismic velocity structure heat flow seismicity surface deformation uplift and fault development are controlled by the evolving thermal structure in the region after triple junction passage thermal mechanical models have been used to evaluate the fault system s time varying three dimensional dynamical behavior simulating the principal processes involved in the thermal mechanical evolution of the san andreas fault system results from this modeling indicate that the fault system has essentially a three stage history 1 in the vicinity of the mendocino triple junction the san andreas fault maps the eastern edge of the pacific plate with a broad 100 km zone of asthenospheric mantle separating the pacific and north american plates in the 25 to 90 km depth range 2 between 37 n and 39 n the plate boundary within the mantle separating the pacific and north american plates has developed approximately 40 60 km east of the surface trace of the san andreas fault and lies beneath the hayward calaveras faults and associated faults the surface trace of the plate boundary appears to be connected to the mantle segment via a lower crust subhorizontal detachment surface this fault zone orientation produces the surface deformations observed geodetically in the region 3 south of 37 n the surface fault again overlies the deeper plate boundary apparently as a result of an eastward jump in the surface fault

regional three dimensional seismic velocity model of the crust and uppermost mantle of northern california 1 we present a three dimensional 3d tomographic model of the p wave velocity vp structure of northern california we employed a regional scale double difference tomography algorithm that incorporates a finite difference travel time calculator and spatial smoothing constraints arrival times from earthquakes and travel times from controlled source explosions recorded at network and or temporary stations were inverted for vp on a 3d grid with horizontal node spacing of 10 to 20 km and vertical node spacing of 3 to 8 km our model provides an unprecedented comprehensive view of the regional scale structure of northern california putting many previously identified features into a broader regional context and improving the resolution of a number of them and revealing a number of new features especially in the middle and lower crust that have never before been reported examples of the former include the complex subducting gorda slab a steep deeply penetrating fault beneath the sacramento river delta crustal low velocity zones beneath geysers clear lake and long valley and the high velocity ophiolite body underlying the great valley examples of the latter include mid crustal low velocity zones beneath mount shasta and north of lake tahoe

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

late neogene geohistory analysis of the humboldt basin and its relationship to convergence of the juan de fuca plate geohistory analysis of neogene humboldt basin strata provides important constraints for hypotheses of the tectonic evolution of the southern cascadia subduction margin leading up to the arrival of the mendocino triple junction this analysis suggests that the tectonic evolution of the humboldt basin area was dominated by coupling between the downgoing juan de fuca plate and the continental margin this coupling is reflected in the timing of major hiatuses within the basin sedimentary sequence and margin uplift and subsidence which occur during periods of tectonic plate adjustment stratigraphic evidence indicates that humboldt basin originated at the base of the continental slope in early miocene time syndepositional uplift of basin strata began in the late pliocene and was both thermal isostatic and tectonic in origin isostatic uplift was a function of an increasingly more buoyant slab being subducted whereas tectonic uplift was due to imbricated thrusting of the accretionary complex and underplating of offscraped sediment during subduction a component of margin uplift is postulated to have been caused by a change in the rate of convergence between the juan de fuca and north american plates coeval with late pliocene uplift documented onshore was a sharp decrease in covergence rate 3 ma a reduction in rate of tectonic uplift observed in the eel river section in early pleistocene time was coeval with a marked increase in relative motion parallel to the continental margin this localized subsidence may have been caused by syndepositional folding

chronology of the later stone age and food production in east africa abstract evidence from several archaeological sites in sub saharan africa suggests that the transition to modern human technology marked by the change from the middle to the later stone age lsa occurred first in east africa enkapune ya muto rockshelter in the central rift valley of kenya contains the oldest known archaeological horizons spanning this transition radiocarbon and obsidian hydration dates from this 5 6 m deep cultural sequence show that the later stone age began substantially earlier than 46 000 years ago ostrich eggshell beads were made 40 000 years ago early dates for the lsa and beads may have implications for the origin and dispersal of modern human behaviour and modern humans out of africa this site also contains the only known occurrences dating to the middle holocene dry phase in highland kenya and tanzania as well as occurrences that span the transition from hunting and gathering to food production and from the neolithic to the iron age the adoption of domestic animals by indigenous eburran hunter gatherers in highland east africa occurred gradually between 4900 and 3300 uncorrected radiocarbon years bp and the neolithic later iron age transition occurred around 1300 bp

density structure of the lithosphere in the southwestern united states and its tectonic significance we calculate a density model of the lithosphere of the southwestern united states through an integrated analysis of gravity seismic refraction drill hole and geological data deviations from the average upper mantle density are as much as 3 a comparison with tomographic images of seismic velocities indicates that a substantial part 50 of these density variations is due to changes in composition rather than temperature pronounced mass deficits are found in the upper mantle under the basin and range province and the northern part of the california coast ranges and adjacent ocean the density structure of the northern and central southern sierra nevada is remarkably different the central southern part is anomalous and is characterized by a relatively light crust underlain by a higher density upper mantle that may be associated with a cold stalled subducted plate high densities are also determined within the uppermost mantle beneath the central transverse ranges and adjoining continental slope the average density of the crystalline crust under the great valley and western sierra nevada is estimated to be up to 200 kg m 3 higher than the regional average consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region

ephemeral crustal thickening at a triple junction the mendocino crustal conveyor as the north american crust interacts with the migrating mendocino triple junction the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers within a time frame of 5 m y or less this process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward migrating gorda slab and the base of north america south of the triple junction a time dependent thermal mechanical finite element model is developed to test this hypothesis of plate boundary tectonics results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area the observed crustal structure and triple junction regional seismicity and localized regions of crustal extension coincident with the position of a hypothesized lower crustal melt zone

late neogene and quaternary tectonics associated with northward growth of the san andreas transform fault northern california strain patterns within the forearc at a convergent margin adjacent to a passing fault fault trench triple junction show a systematic evolution we describe late neogene and quaternary deformation in northern coastal california in the last 3 m y associated with the migration of the mendocino triple junction deformation in the north american forearc near humbolt bay consists of a 30 km wide zone of on land contraction with a narrower zone of translation to the east the zone of subaerial contraction is part of the relatively wide southernmost extent of forearc contraction within the cascadia subduction zone net quaternary northeast southwest contraction across forearc thrust faults is at least 7 9 km and minimum fault slip rates on the six major thrust faults are 0 8 2 3 mm yr net right slip within the zone of translation is a minimum of 3 km south of the forearc translation is predominant in the north american plate and is accommodated along two major right lateral fault zones of the san andreas transform boundary the lake mountain and garberville fault zones discontinuous exposures of late neogene sediments along these faults near covelo and garberville respectively indicate that crustal contraction expressed by thrust faults and associated folding predated translational deformation at each site inception of contraction near covelo was related to inception of internal deformation of the gorda plate at 3 ma this contraction due to coupling of the convergent plates migrated northward in the forearc in advance of and at the rate of migration of the triple junction present day deformation near humboldt bay thus reflects contraction associated with subduction of the gorda plate and translation farther east associated with oblique convergence at the plate boundary as the triple junction migrates the strike slip faults of the san andreas transform boundary will extend to the north northwest into the present forearc and forearc strike slip faults will become faults of the san andreas system contractional structures near humboldt bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south

regional extent of great valley basement west of the great valley california implications for extensive tectonic wedging in the california coast ranges interpretation and modeling of the magnetic field of central california indicate that the magnetic basement of the forearc deposits of the great valley sequence extends westward beneath the coeval subduction related rocks of the franciscan complex the basement surface slopes gently to the west reaching midcrustal depths 15 19 km at distances of 50 100 km west of the great valley this magnetic basement is disrupted by the hayward rodgers creek fault system and is cut by the san andreas fault at the south end of the great valley and possibly throughout much of central california the widespread presence of the great valley basement beneath rocks of the franciscan complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the franciscan complex great valley sequence contact this result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones the eastern boundary fault of the franciscan complex coast range fault is not and never was a subduction zone thrust fault but rather was originally a roof thrust wedge roof fault formed above the eastward wedging mass of franciscan complex intruded along the top of the basement beneath the great valley deposits this tectonic interpretation offers a solution for the question of how high pressure metamorphic rocks of the franciscan complex were juxtaposed at the coast range fault against low pressure metamorphic rocks of the great valley sequence this interpretation also implies an older flat lying thrust fault wedge floor fault that forms the top of magnetic basement between the active san andreas and hayward faults at depths of 15 17 km this older thrust fault may today transfer strain between the two young strike slip faults possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults the former east dipping subduction zone along which the rocks of the franciscan complex accumulated must lie west of the western limit of the great valley magnetic basement

a new local magnitude scale for the finnish seismic network abstract a total of 1259 synthesized wood anderson recordings from 216 local and regional earthquakes were used to establish a new ml scale for finland the hypocentral distances of the events range from 25 to 1940 km and the reported magnitudes from 1 to 4 the attenuation function log a0 in richter s original definition of ml magnitudes and station corrections for the eleven stations used in this study were determined by a multiple linear regression analysis the shape of the attenuation function was modelled by two parametric forms the bilinear function log a0 f was described in terms of a frequency dependent anelastic attenuation and a geometrical spreading with separate rates for sg and lg domains the log a0 function comprised a single geometrical spreading term and a frequency independent anelastic attenuation term the results are as follows loga0 f 1 27log r 0 00041f0 36r 0 36 s for r 350 km loga0 f 0 83log r 0 00041f0 36r 1 48 s for 350 km loga0 f 1 42log r 0 00008r 0 15 s for r 1900 km where r is hypocentral distance in kilometers f is frequency hz and s is station correction the absolute levels of the attenuation functions were adjusted so that at 60 km the log a0 and log a0 10 hz curves equal the revised log a0 for southern california a correction was determined to accommodate the use of vertical seismograms in the ml formula the logarithm of the horizontal h to vertical z shear wave amplitude ratio was found to be distance dependent log h z 0 00009r the difference in magnitudes computed from the above distance correction functions was less than 0 1 units the frequency independent form is preferred as it is simple to apply in routine bulletin work the new ml scale can be written as ml f log a 0 00017r 0 15 s for r 350 km where a is the synthesized wood anderson amplitude mm measured from the vertical component seismogram

kinematics of the pacific north america plate boundary zone northern california we measured motions of 54 sites in an east west transect across northern california at 38 40 north by global positioning system gps observations over a 4 year we estimate the total slip rate on the san andreas fault system to be 39 6 0 6 1 5 mm yr 68 6 upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters our best fitting model fits the fault parallel velocities with a mean square error of 1 04 and the following estimated fault slip rates all in mm yr san andreas 17 4 3 1 2 5 ma acama 13 9 2 8 4 1 and bartlett springs 8 2 1 9 2 1 the data are fit best by models in which the san andreas fault is locked to 14 9 7 1 12 5 km the ma acama fault locked to 13 4 4 8 7 4 km except for shallow creep in the upper 5 km and the bartlett springs fault creeping at all depths the ma acama fault most likely poses a significant seismic hazard as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake we find little evidence for contraction across coast ranges except at western edge of great valley where 1 3 mm yr of shortening is permitted by the data no strain is observed within the great valley or sierra nevada except that associated with right lateral strike slip on the san andreas fault system this is consistent with models of the pacific north america plate boundary zone in which the relative plate motion is partitioned into two domains one strike slip and one dominantly extensional separated by the elastically deforming sierra nevada great valley block

seismotectonics of the april 25 1992 petrolia earthquake and the mendocino triple junction region the april 25 1992 petrolia earthquake m s 7 1 occurred at the southern tip of the cascadia subduction zone this is the largest thrust earthquake ever recorded instrumentally in the cascadia subduction zone the earthquake was followed by two large strike slip aftershocks both ms 6 6 moment release of each of the earthquakes is as follows 4 0 1019 nm in the first 10 s for the mainshock 0 7 1019 nm in the first 8 s for the first aftershock and 0 9 1019 nm in the first 2 s for the second aftershock these indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds the best depth estimates of the mainshock and the two aftershocks are 14 km 18 km and 24 km respectively the slip direction of the mainshock is between n75 e and n80 e this slip direction is not consistent with either the relative motion of the north american and juan de fuca plates n60 e or between the north american plate and the gorda deformation zone n40 e it has been suggested that the north american pacific plate motion is accommodated by right lateral slip on both the san andreas and maacama rodgers creek hayward fault systems the intervening block is the humboldt plate if we modify the relative motion of the southernmost gorda deformation zone to conform with the seismicity trends and allow the humboldt pacific plate motion to be about half the total north american pacific motion then the gorda deformation zone humboldt relative motion matches the direction of the petrolia slip vector also the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the gorda deformation zone and pacific plate and the humboldt and north american plates the gorda deformation zone is subducting beneath the humboldt plate in the cape mendocino area and the petrolia earthquake ruptured the entire subduction segment between the gorda deformation zone and humboldt plate

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

tectonic uplift of a middle wisconsin marine platform near the mendocino triple junction california an uplifted wave cut marine platform eroded across bedrock of the franciscan complex at point delgada northern california is overlain by 0 5 to 5 m of wave worked pea gravel which is in turn directly overlain by fluvial gravel and silt deposited as alluvial fans woody plant debris at the base of the fluvial deposits includes cones of brewer spruce picea breweriana which today are found only at higher elevations and latitudes fossil wood debris from this horizon yields a 14c date of 44 800 1 300 yr the 44 800 yr radiocarbon age for the base of the fluvial deposits establishes an approximate age for the immediately underlying marine gravels and wave cut platform we tentatively correlate this terrace with the middle wisconsin high sea level stand at 37 m dated at about 45 000 yr b p if this age is correct the tectonic uplift since middle wisconsin time has been 44 m and the average rate of uplift has been at least 1 0 m 1 000 yr this relatively high rate is probably related to interaction among the pacific gorda and north american plates at the mendocino triple junction

source parameters of aftershocks of the 1991 costa rica and 1992 cape mendocino california earthquakes from inversion of local amplitude ratios and broadband waveforms abstract source parameters of aftershocks of the 22 april 1991 m w 7 7 costa rica and the 25 april 1992 m w 7 1 cape mendocino california earthquakes are determined using a grid search inversion of p sh and sv amplitude ratios recorded by sparse local networks of three component broadband and short period stations the inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source receiver pairs over a range of source depths calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of p sh and sv waves combining the fundamental fault amplitudes for all possible values of strike dip and rake at 10 increments and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic p sh p sv and sv sh amplitude ratios the ambiguity in the sense of motion on the nodal planes arising due to the use of amplitude ratios is resolved by examining p wave polarities the sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored for events with good station coverage focal mechanism determinations are stable for a wide range of assumed values of crustal structure earthquake location and depth source parameters for many of the largest events m 3 4 are also determined by inversion of broadband displacement waveforms using a similar grid search technique comparable results were obtained using both broadband waveforms and amplitude ratios focal mechanism solutions for 20 aftershocks of the costa rica earthquake reveal a complicated faulting geometry indicating active thrust normal and strike slip faults in the back arc of costa rica the 1992 cape mendocino earthquake occurred at the intersection of the north american gorda and pacific plates while the mainshock was associated with underthrusting of the gorda plate beneath the north american plate fault plane solutions for 70 of the 38 largest aftershocks indicate that these events result from either motion between the gorda and pacific plates or from internal deformation within the gorda plate

the khormusan evidence for an msa east african industry in nubia abstract there is clear evidence of lithic technological variability in middle paleolithic mp assemblages along the nile valley and in adjacent desert areas one of the identified variants is the khormusan the type site of which site 1017 is located north of the nile s second cataract the industry has two distinctive characteristics that set it apart from other mp industries within its vicinity one is the use of a wide variety of raw materials the second is an apparent correlation between raw material and technology used suggesting a cultural aspect to raw material management stratigraphically site 1017 is situated within the dibeira jer formation which represents an aggradation stage of the nile and contains sediments originating from the ethiopian highlands while it has previously been suggested that the site dates to sometime before 42 5 ka the dibeira jer formation can plausibly be correlated with nile alluvial sediments in northern sudan recently dated to 83 24 ka mis 5a this stage coincides with the 81 ka age of sapropel s3 indicating higher nile flow and stronger monsoon rainfall at these times other sites which reflect similar raw material variability and technological traditions are the bns and khs sites in the omo kibish formation ethiopia dated to 100 ka and 190 ka respectively based on a lithic comparative study conducted it is suggested that site 1017 can be seen as representing behavioral patterns which are indicative of east african middle stone age msa technology adding support to the hypothesis that the nile valley was an important dispersal route used by modern humans prior to the long cooling and dry trend beginning with the onset of mis 4 techo typological comparison of the assemblages from the khormusan sites with other middle paleolithic sites from nubia and east africa is used to assess the possibility of tracing the dispersal of technological traits across the landscape and through time

location of the southern edge of the gorda slab and evidence for an adjacent asthenospheric window results from seismic profiling and gravity as the mendocino triple junction migrates northward along the california margin it is widely presumed to leave a slab free or asthenospheric window in its wake a 250 km long south north seismic refraction reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the north american margin before and after it is modified by passage of the mendocino triple junction from the seismic data we have determined that 1 the crust is laterally homogeneous in velocity to a depth of 20 km interpreted by us as franciscan complex 2 below 20 km depth the crust is characterized by velocities of 7 0 km s for the southern half of the profile and by velocities of 3 2 g cm 3 is required on the southern end of the profile we interpret these combined results to indicate that our profile crosses the southern edge of the gorda plate and that directly adjacent to this edge is an asthenospheric window with overlying mafic rocks in the crust these mafic rocks and a reforming lithospheric mantle increase in thickness southward

tectonic history of the north portion of the san andreas fault system california inferred from gravity and magnetic anomalies geologic and geophysical data for the san andreas fault system north of san francisco suggest that the eastern boundary of the pacific plate migrated eastward from its presumed original position at the base of the continental slope to its present position along the san andreas transform fault by means of a series of eastward jumps of the mendocino triple junction these eastward jumps total a distance of about 150 km since 29 ma correlation of right laterally displaced gravity and magnetic anomalies that now have components at san francisco and on the shelf north of point arena indicates that the presently active strand of the san andreas fault north of the san francisco peninsula formed recently at about 5 ma when the triple junction jumped eastward a minimum of 100 km to its present location at the north end of the san andreas fault prior to 5 ma the triple junction was located at the north end of a proposed northwesterly extension of the pilarcitos fault this jump took place at what is now about latitude 38 20 n on the north american plate and may have occurred during part of the time when the relative motion between the north american and pacific plates rotated 20 clockwise resulting in transpression along the earlier transcurrent san andreas fault system the proposed 150 km eastward movement of the triple junction explains the submarine topography near cape mendocino where the continental shelf south of the mendocino fault extends about 130 km farther west than does the shelf directly north of the fault

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

ophiolitic basement to the great valley forearc basin california from seismic and gravity data implications for crustal growth at the north american continental margin the nature of the great valley basement whether oceanic or continental has long been a source of controversy a velocity model derived from a 200 km long east west reflection refraction profile collected south of the mendocino triple junction northern california in 1993 further constrained by density and magnetic models reveals an ophiolite underlying the great valley great valley ophiolite which in turn is underlain by a westward extension of lower density continental crust sierran affinity material we used an integrated modeling philosophy first modeling the seismic refraction data to obtain a final velocity model and then modeling the long wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model the crustal section of great valley ophiolite is 7 8 km thick and the great valley ophiolite relict oceanic moho is at 11 16 km depth the great valley ophiolite does not extend west beneath the coast ranges but only as far as the western margin of the great valley where the 5 7 km thick great valley ophiolite mantle section dips west into the present day mantle there are 16 18 km of lower density sierran affinity material beneath the great valley ophiolite mantle section such that a second deeper present day continental moho is at about 34 km depth at mid crustal depths the boundary between the eastern extent of the great valley ophiolite and the western extent of sierran affinity material is a near vertical velocity and density discontinuity about 80 km east of the western margin of the great valley our model has important implications for crustal growth at the north american continental margin we suggest that a thick ophiolite sequence was obducted onto continental material probably during the jurassic nevadan orogeny so that the great valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle

lithospheric evolution in the wake of the mendocino triple junction structure of the san andreas fault system at 2 ma as the mendocino triple junction mtj moves northwards up the north american margin the tectonic regime changes from subduction to strike slip for the first few million years following triple junction migration the san andreas fault system consists of several strike slip faults distributing deformation over a region 150 km wide this same region is expected to be affected by a slab gap beneath north america created by the northward removal of the subducting gorda plate and into which asthenospheric mantle is thought to rise to crustal depths the onshore and offshore mendocino triple junction seismic experiment mtjse provides a continuous seismic velocity reflectivity cross section across the deforming zone from the pacific ocean basin to the eastern edge of the california coast ranges the accretionary complex rocks that make up most of the crustal thickness are underlain by a 5 10 km thick high velocity 6 4 7 2 km s 1 layer at the base of the crust that extends from the pacific to at least 50 km and probably 90 km east of the san andreas fault the top of the lower crustal layer deepens from 7 km beneath the pacific ocean basin at the west end of the profile to 23 km at the east end by a gentle 5 10 eastward dip punctuated by abrupt offsets at the san andreas and maacama fault zones at each fault the top of the lower crust is offset by up to 4 km down to the east the moho is similarly deformed beneath the faults although by only 2 km such localized deformation of the moho implies that these two strike slip faults penetrate through the entire crust to the upper mantle good agreement between seismic velocity and seismic reflectivity in the vicinity of the faults gives confidence in these results although details of the offset beneath the san andreas fault are better resolved than those under the maacama fault seismic velocities in the upper mantle show only a small change along the profile from 8 1 km s 1 beneath the pacific to about 7 9 km s 1 beneath the coast ranges we infer that upwelling of asthenosphere into the slab gap is limited laterally or a lithospheric lid is present in the slab gap by 2 ma gravity data and crustal density structure show that most of the margin width is in local airy isostasy with the changes in crustal thickness near the strike slip faults corresponding closely to changes in surface topography the crustal blocks defined by the strike slip faults appear to be independently in isostatic equilibrium provided that the mantle beneath the coast ranges has a somewhat lower density than that beneath the pacific plate the densities in the coast range upper mantle are consistent with limited temperature elevation suggesting that the asthenospheric mantle is present beneath the depth of seismic energy penetration from our survey

seismic images of the deep structure of the san andreas fault system central coast ranges california three dimensional inversion of teleseimic p wave travel time residuals recorded at the u s geological survey central california array has resolved small scale approx tens of kilometers crustal and upper mantle heterogeneity down to depths of 90 km beneath the california coast ranges upper crustal lateral velocity variations of 8 correlate closely with surface geology lower than average velocities are associated with thick tertiary sedimentary fill and higher than average velocities with basement exposures lower crustal velocity heterogeneity of 4 appear to reflect crustal thickness variations a thinner crust is indicated southwest of the san andreas fault and northwest of san pablo bay a linear zone of low velocities 0 to 40 subparallel to the san andreas fault was resolved in the upper mantle the preferred interpretation is that the low velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths approx 45 km beneath the coast ranges such an unusual upper mantle structure may have been produced by the northwestward migration along the california coast of a transiently unstable mendocino triple junction the inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle

100 000 years of african monsoon variability recorded in sediments of the nile margin abstract multiproxy analyses were performed on core ms27pt recovered in hemipelagic sediments deposited on the nile margin in order to reconstruct nile river palaeohydrological fluctuations during the last 100 000 years the strontium and neodymium isotope composition of the terrigenous fraction and the major element distribution reveal large and abrupt changes in source oscillating between a dominant aeolian saharan contribution during arid periods and a dominant nile river contribution during pluvial periods iron content shows a strong correlation with strontium and neodymium isotopes this allows the use of a high resolution continuous fe record as a proxy of blue nile sediment input over the last 100 000 years the detailed fe record with approximately 10 years resolution during pluvial periods is consistent with subtropical african records of well dated lake level fluctuations and thus constitutes a first continuous high resolution record of the east african monsoon regime intensity over ethiopia the detailed fe record shows the two main known pluvial periods attributed to strengthening of the african monsoon over ethiopia the nabtian period from 14 to 8 ka cal bp and the saharan period from 98 to 72 ka bp for the first time the last glacial period marine isotope stage mis 2 3 and 4 is documented with a continuous record showing large oscillations between high and low east african palaeo monsoon regimes the end of the nabtian period occurred at 8 ka in core ms27pt i e much earlier than on the east equatorial african region where it ended around 5 5 ka we interpret this as evidence that the southward shift of the rain belt occurred 3000 years earlier over the eastern ethiopian highland and propose that the gradual southward migration of the rain belt was associated with highly variable intensity and longer rainy seasons from 8 to 5 ka during the last glacial period two wet periods are present around 60 50 ka bp and 38 30 ka bp these two humid periods are in phase with the rise of atmospheric ch4 concentrations suggesting that wetland tropical african area was one of the sources of the atmospheric ch4 during the mis 3 during the last glacial maximum and mis 4 high saharan aeolian influxes in phase with records of aeolian dust deposited in east antarctica are documented this study highlights the importance of reconstructions of monsoon rainfall fluctuation at high temporal resolution to better understand the link between low and high latitude climate variability at millennial timescales

pyrolysis combustion 14c dating of soil organic matter abstract radiocarbon 14c dating of total soil organic matter som often yields results inconsistent with the stratigraphic sequence the onerous chemical extractions for som fractions do not always produce satisfactory 14c dates in an effort to develop an alternative method the pyrolysis combustion technique was investigated to partition som into pyrolysis volatile py v and pyrolysis residue py r fractions the py v fractions obtained from a thick glacigenic loess succession in illinois yielded 14c dates much younger but more reasonable than the counterpart py r fractions for the soil residence time carbon isotopic composition 13c was heavier in the py v fractions suggesting a greater abundance of carbohydrate and protein related constituents and 13c was lighter in the py r fractions suggesting more lignin and lipid related constituents the combination of 14c dates and 13c values indicates that the py v fractions are less biodegradation resistant and the py r fractions are more biodegradation resistant the pyrolysis combustion method provides a less cumbersome approach for 14c dating of som fractions with further study this method may become a useful tool for analyzing unlithified terrestrial sediments when macrofossils are absent

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

indian ocean climate and an absolute chronology over dansgaard oeschger events 9 to 13 oxygen isotope ratios of a stalagmite from socotra island in the indian ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present the pattern of precipitation bears a striking resemblance to the oxygen isotope record from greenland ice cores with increased tropical precipitation associated with warm periods in the high northern latitudes the largest change at the onset of interstadial 12 occurred very rapidly in about 25 years the chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period

southward younging apatite u th he ages in the northern california coast ranges due to a northward migrating crustal welt abstract u th he in apatite dating of franciscan formation samples from between 39 47 n and 39 08 n in the northern california coast ranges nccr reveals a southward and downward younging pattern of ages 21 8 0 6 ma to 7 9 0 9 ma these ages are interpreted to represent a northward or northeastward dipping fossil he partial retention zone that was tilted and exhumed by a northward migrating pulse of crustal thickening that begins 150 200 km ahead of the migrating mendocino triple junction mtj k p furlong r govers ephemeral crustal thickening at a triple junction the mendocino crustal conveyor geology 27 1999 127 130 the 8 ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the he partial retention zone when crustal thickening began 5 my before the mtj passed by these rocks have now reached the surface 3 my after passage of the mtj the older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the he partial retention zone prior to crustal thickening ahead of the mtj and which therefore record ages transitional from older subduction related cooling and exhumation to more recent transform related cooling and exhumation in addition to providing support for the model of a migrating crustal welt associated with the mtj the existence of an essentially intact gently tilted fossil partial retention zone in the northern california coast range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region

the great arc of dispersal of modern humans africa to australia abstract during the late pleistocene anatomically modern humans amh dispersed out of africa across the continents their routes obeyed the limitations placed on any large terrestrial mammal dependent on daily drinking water following certain climate permissive corridors amh first spread north with game across the sahara to the levant during the eemian interglacial c 125 ka but failed to continue to europe then occupied by neanderthals the savannah ecosystem in north africa and the middle east then dried up and amh vanished from the levantine fossil record being replaced there by neanderthals later amh successfully left africa as a single group by the southern route to india the added ability to make short but deliberate open water crossings allowed them first to cross the mouth of the red sea from eritrea and subsequently wallace s line to reach the isolated sahul continent at least by 48 000 years ago and possibly by 60 50 000 years ago they only finally arrived in europe from south asia around 45 50 000 years ago probably linked to climatic amelioration during ois 3

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

millennial scale variability during the last glacial the ice core record the oxygen isotope records from greenland ice cores show a very strong reproducible pattern of alternation between warm greenland interstadials gi and cold greenland stadials gs at millennial scale during the last glacial period here we summarise what is known about this variability from ice core records the typical cycle has a sawtooth pattern with a very rapid warming event occurring in a few decades a slow cooling trend and then a final fast cooling 25 such events have been numbered in the last glacial the recent gicc05 age scale provides the best available age scale that can be directly applied to this stratigraphy and we summarise the timing of the warming events and the length and strength of each event the greenland stratigraphy can be transferred to other records if we make assumptions about the contemporaneous nature of rapid events in different archives other parameters such as the snow accumulation rate and the concentration of terrestrial dust and sea salt recorded in the greenland cores also show a strong contrasting pattern between gi and gs methane concentrations are generally high during gi and lower during gs with the increase from gs to gi occurring within a century antarctic ice cores show a different pattern each gi has an antarctic counterpart but antarctica appears to warm while greenland is in a gs and cool during gi these changes are consistent with a mechanism involving ocean heat transport but the rapid nature of warmings poses a challenge for modellers while the rapid methane changes pose questions about the pattern of land biosphere emissions during the glacial that are also relevant for understanding glacial interglacial methane variability

seismotectonics of the san andreas fault system between point arena and cape mendocino in northern california implications for the development and evolution of a young transform the northernmost and relatively youthful segment of the san andreas fault system is situated within a 100 km wide zone of northwest trending strike slip faults that includes from west to east the san andreas maacama and bartlett springs faults although the san andreas fault is the principal strike slip fault in this system it has been virtually aseismic since the 1906 earthquake moderate levels of seismicity locate to the east along the maacama fault and to a lesser extent the bartlett springs fault at focal depths typical of other strike slip faults within the san andreas fault system in central california north of the san andreas fault system within the cape mendocino area earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting gorda slab and slip along the mendocino fracture zone seismicity along the maacama and bartlett springs faults is dominated by right lateral to oblique reverse slip along fault planes that dip 50 75 to the northeast the northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the gorda slab the onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the north american plate as it enters the asthenospheric window the maacama and bartlett springs faults are strike parallel with active reverse faults within the forearc region of the cascadia subduction zone this preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults continuation of the moderately dipping maacama fault to the southeast along the steeply dipping healdsburg and rodgers creek fault zones and the near vertical hayward and calaveras fault zones in the san francisco bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

seismicity of california s north coast at least three moment magnitude m 7 earthquakes occurred along california9s north coast in the second half of the nineteenth century the m 7 3 earthquake on 23 november 1873 occurred near the california oregon coast and likely was located on the cascadia subduction zone or within the north american plate the m 7 0 earthquake on 9 may 1878 was located about 75 km offshore near the mendocino fault the surface wave magnitude m s 7 0 earthquake on 16 april 1899 was located about 150 km offshore within the gorda plate there were at least three m 7 north coast earthquakes in the 35 years before 1906 two m 7 earthquakes in the 20 years after 1906 no m 7 earthquakes from 1923 until 1980 and four m 7 earthquakes since 1980 the relative seismic quiescence after 1906 for m 7 earthquakes along california9s north coast mimics the post 1906 seismic quiescence in the san francisco bay area for m 6 earthquakes the post 1906 relative quiescence did not extend to lower magnitudes in either area the 18 april 1906 earthquake apparently influenced the rate of occurrence of m 7 north coast earthquakes as it apparently influenced the rate of m 6 earthquakes in the san francisco bay area the relative seismic quiescence along the california north coast region after 1906 should be taken into account when evaluating seismic hazards in northwest california

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

late neogene and quaternary landscape evolution of the northern california coast ranges evidence for mendocino triple junction tectonics a landscape records the surface response to tectonics at time scales intermediate between short time scale information derived from seismic imaging and global positioning systems and the long term geologic record we link late neogene and quaternary deposits and landforms in the northern california coast ranges to the tectonics of the mendocino triple junction in the northern california coast ranges the mendocino crustal conveyor geodynamic model describes crustal thickening thinning and dynamic topography that produce a double humped pattern of uplift that migrates northward with the mendocino triple junction the tectonics are manifest in the drainage system and elevation pattern of the coast ranges at long wavelengths the elevation pattern closely matches the predicted double peaked shape of mendocino crustal conveyor topography and the high points of uplift control the location of drainage divides presently the divide between the russian and eel rivers and the divide between the eel and van duzen rivers approximately correspond to the peaks of uplift predicted by the mendocino crustal conveyor model as the triple junction migrates northward the double humped pattern of uplift and subsidence migrates and the coast ranges emerge smaller drainages develop and evolve by stream capture and flow reversal and the two main divides migrate in concert with the triple junction in contrast to the systematic development of the small streams the largest trunk streams can maintain grade through regions of high uplift and coastal river mouths remain stationary despite the uplift moving north before ca 2 ma the majority of the coast range drainage flowed to a southern coastal outlet near the present mouth of the russian river at 2 ma facilitated by headwater stream capture at key locations the drainage direction reversed and the majority of coast range rivers now drain into the north flowing eel river the major drainage reorganization at 2 ma highlights the potential for complexity in geomorphic response to tectonics

soil carbon isotope evidence for holocene habitat change in the kenya rift valley in eastern africa the altitude of the boundary between montane forest and lowland savanna grassland changed substantially in response to climate change during the later holocene but this is not clearly reflected in regional pollen records the carbon 13 to carbon 12 ratios of tropical grasses are higher than those of most other plants and this difference is preserved in soil organic carbon stable isotope ratios soil organic matter 13 c 12 c ratios in profiles along an altitude transect in the central rift valley of kenya suggest that the forest savanna boundary advanced more than 300 meters in altitude this could have implications for understanding the effects of climate change on the configuration of floral zones prehistoric hunter gatherer land use patterns and the timing of the advent of neolithic food production

timing and hydrological conditions of sapropel events in the eastern mediterranean as evident from speleothems soreq cave israel this paper explores the connection between the timing of sapropel events in the eastern mediterranean sea and the time of low 18o events in speleothems of the soreq cave israel a new well dated 18o and 13c profile of the soreq cave speleothems for the last 140 kyr reveals five prominent low 18o events dated at 124 to 119 kyr with the main peak at 122 kyr event v 108 to 100 kyr with peaks at 107 and 102 kyr event iv 85 to 79 kyr with peak at 80 kyr event iii 55 to 52 kyr with a peak at 54 kyr event ii and 8 5 to 7 kyr with peaks at 8 5 and 7 kyr event i these events are characteristic of period of enhanced rainfall in the eastern mediterranean area particularly so for events v and i where the marked decrease in 18o and the correspondingly marked increase in 13c indicate very wet conditions the chronology of the low 18o events v iv iii and i match the ages estimated for the formation of sapropels s5 s4 s3 and s1 which are also associated with high hydrological activity thus it is suggested that the low 18o events in the soreq cave speleothems constrain the maximum duration of the sapropel formation pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that sapropels s5 and s1 were deposited during very wet periods when there were major decrease in the sea surface salinity coupled with frost free winters and drought free summers sapropels s4 and s3 were formed during periods of increased precipitation but ones in which the general climate was mediterranean semi arid

the 1992 cape mendocino earthquake broadband determination of source parameters the april 25 1992 cape mendocino earthquake ms 7 1 has renewed speculation about the nature of subduction along the cascadia subduction zone and the associated seismic hazard this event may represent the first large m 6 thrust event along the entire cascadia subduction zone in historic times last 200 years we analyze long period surface waves and broadband body waves in order to estimate the mainshock source parameters we also examine broadband body waves from the nearby 1991 honeydew earthquake m 6 in order to assess the contributions of both rupture complexity and unmodeled source and receiver structure in the cape mendocino waveforms from both body and surface wave inversions as well as forward modeling of body waves we estimate a best double couple mechanism for the cape mendocino earthquake strike 330 10 dip 12 2 rake 75 15 seismic moment 1 93 1019 n m and mw 6 8 this mechanism contains a significant component of slip in the estimated direction of gorda north america plate convergence although this earthquake had suitable geometry for relieving strain accumulated by gorda north america plate convergence we cannot resolve whether it occurred on the interplate megathrust or on a fault within the overriding accretionary prism we find evidence for southwest offshore rupture toward an azimuth of 240 in addition we find evidence for early aftershocks in both the teleseismically recorded body waves and in locally recorded strong motions we model one aftershock delayed 13 s from the mainshock with a mechanism that is different from the mainshock mechanism but is consistent with the north south trending horizontal compression found offshore within the gorda plate we postulate that this aftershock and two additional large strike slip aftershocks that ruptured the gorda plate within 24 hours of the mainshock were caused by the transfer of stress accumulated across the cascadia subduction zone and accretionary prism far offshore to the gorda plate where it reduced the normal stress across nw se oriented faults triggering failure the complexity of fault interactions near the mendocino triple junction needs to be understood before potential seismic hazards of the southern cascadia subduction zone can be quantified

a kinematic model for the evolution of the gorda plate the magnetic lineation pattern produced by the gorda rise reflects a complex tectonic history marked by propagating ridges and nonrigid deformation of the gorda plate the pacific side lineation pattern is matched using a kinematic model which incorporates northward and southward propagating ridge segments and ridge rotation by superimposing flexural slip style deformation on the resulting gorda side lineations and allowing convergent motion at the mendocino fracture zone the gorda magnetic pattern is also successfully predicted a similar model but one which does not allow convergence at the mendocino fracture zone fails to produce the observed lineation pattern model predictions of the amount of material that would be obducted at the mendocino due to convergence between the gorda and pacific agree with bathymetric and density estimates of the amount of material located along the mendocino ridge

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

reconstructing prehistoric settlement models and land use patterns on mt damota sw ethiopia abstract although high altitude mountain habitats are often regarded as unfavorable for human occupation e g aldenderfer 2014 on the other hand tropical highlands in africa are suggested as potential refugia during times of environmental stress e g basell 2008 brandt et al 2012 archaeological investigations on mount damota 2908 m a s l located on the boundary between the southwest ethiopian highlands to the west and the southern main ethiopian rift valley to the east yielded a large number of archaeological sites from the middle stone age period until historical times in this paper we try to reconstruct settlement models for the late pleistocene and holocene occupation in this area and speculate about potential land use patterns such complex topics demand a landscape archaeological approach that includes open air sites and rock shelters the results from our excavations at mochena borago rock shelter and evidence from open air sites that were recorded during intensive surveys on the slopes and plateau of the mountain allow a first reconstruction of the settlement history of the area

new radiocarbon dating of the transition from the middle to the upper paleolithic in kebara cave israel the middle to upper paleolithic transition mp up transition is considered a major technological and cultural threshold at the time when modern humans spread out of africa expanded from the levant into europe and possibly into central and northern asia the dating of this techno cultural transition has proved to be extremely difficult because it occurred sometime before 40 000 radiocarbon years before present 14c years bp which is close to the end of the effective dating range of radiocarbon other dating methods such as thermoluminescence tl or electron spin resonance esr are not sufficiently precise to date the recorded archaeological mp up transition in the levant here we report a consistent set of stratified radiocarbon ages on freshly excavated charcoal from kebara cave mt carmel israel that span the late middle paleolithic mp and early upper paleolithic eup this study applied novel strategies to improve sample preparation techniques and data analysis to obtain high resolution radiocarbon models from this study it is proposed that the mp up transition for this site can be placed immediately after 45 200 700 14c years bp and before 43 600 600 14c years bp or from 49 48 to 47 46 radiocarbon calibrated years before present years cal bp

a comparison of brittle deformation models for the gorda plate abstract the gorda plate is an actively deforming subplate of the juan de fuca system located off the west coast of north america the site of intense seismicity and unusually curved magnetic lineations the gorda has defied rigorous explanation since it was originally mapped in 1961 a variety of models for its deformation mechanism have been proposed two of which are analyzed in depth here the right lateral shear and flexural slip models are compared using constraints imposed by geometry seismicity stress and basement fabric also examined are the questions of convergent motion between the pacific and gorda plates at the mendocino transform fault and shortening by means of pure shear of the magnetic lineations while both deformation models fit most of the constraints neither fits all but the flexural slip model best fits the most well known of the constraints if in fact flexural slip is the preferred deformation mechanism lineation shortening must be ruled out in favor of convergent motion

a model for large scale plastic yield of the gorda deformation zone the lithosphere east of the gorda ridge often called the gorda plate but referred to here as the gorda deformation zone gorda dz is constricted as it spreads eastward to be subducted beneath the north american plate for the plane strain conditions defined by the seismicity the distortion of the gorda dz is analogous to the ironing of metal cups here the punch boundary is the blanco fracture zone and the die that does the cutting is the mendocino transform fault from the gorda ridge to the mendocino triple junction this analogy is useful in analyzing this problem but is not identical due to differences in boundary constraints the deformation of the gorda dz is determined for an elastoplastic lithosphere with a von mises yield criterion plate motion is constrained to be parallel both to the blanco fracture zone and to the mendocino transform fault west of 1250w the gorda ridge is considered a free boundary unlike the ironing analogy and the lithosphere is pulled through the constriction from the trench any nonlinear feedback between distortion and ridge push forces is not considered traction due to slip along the mendocino transform is varied to fit both seismicity and plate distortion for these conditions the amount of plate movement required before a constant resistance is achieved depends upon the ratio of elastic moduli to plastic yield stress and may vary from hundreds of meters to kilometers the minimum slab pull for steady movement occurs when the transform is weak lubricated and is 60 of the average plate yield stress the yield stress is determined using the temperature structure inferred from plate age and the laboratory constraints on brittle and ductile strength for olivine for an average plate age of 3 m y the yield stress is about 550 mpa so the pull required is 330 mpa when the transform is about as strong as the lithosphere the pull required is twice this value the stress directions in the gorda dz can be inferred from both deformation and seismicity the current seismic data are not precise enough to differentiate between a strong and weak transform but the deformation data are sufficient the constraints placed by distortion of magnetic isochrons on the modeling results indicate a mixed history about 5 ma changes in the position of the pole of rotation of the juan de fuca system began to increase the convergence of the gorda dz with the mendocino transform fault increasing the effective normal stress and brittle shear stress across the fault a strong fault is evident in the deformation data until about 2 ma and this corresponds to the time when convergence stabilized with a stable configuration the analysis indicates that about a kilometer of relative slip reduced the apparent strength of the transform to negligible values compared to the strength of the plate

thermal evolution of the sierra nevada tectonic implications of new heat flow data eight new heat flow measurements in the southern sierra nevada constrain models of thermal evolution and lithospheric structure low reduced heat flows 18 to 21 mw m in the southwest sierra nevada are consistent with previous results from the northwestern and central sierra nevada and extend the known region of linear heat flow heat production correlation an additional 150 km to the south isostatic residual gravity and measured rock densities are also linearly correlated in the central sierra nevada suggesting a general association between the upper crustal distribution of density and heat production the linear residual gravity density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of cenozoic sierra nevada uplift new reduced heat flows measured in the southeast sierra nevada are relatively high 32 to 57 mw m and correlate spatially with a region of high seismicity that includes extensional earthquake swarms this correlation supports the view that basin and range extensional tectonics and associated magmatic processes are encroaching on the eastern sierra nevada in the stable southwest sierra nevada as in the central and northwestern sierra nevada the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the cenozoic uplift of the sierra nevada provided the conductive lithosphere is at least 60 to 90 km thick simple order of magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere advection of basaltic magma and heat into the crust simple thermal expansion and eclogite to basalt phase conversion can account for the timing and amount of cenozoic sierra nevada uplift thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for cenozoic uplift driven by a mesozoic crustal root

late cenozoic basins of northern california the late cenozoic basins of northern california developed in response to both convergent tectonics associated with subduction of the farallon plate and its modern representatives the juan de fuca and gorda plates and transform tectonics associated with northward migration of the mendocino triple junction and formation of the san andreas fault system the modern eel river basin north of the mendocino triple junction is an active forearc basin located between the cascade magmatic arc to the east and the trench at the foot of the continental slope of northern california and oregon to the west five different types of late cenozoic basins or fragments of basins are preserved in onshore northern california south of the mendocino triple junction 1 remnants of a formerly more extensive neogene forearc basin preserved locally in downdropped blocks within the san andreas fault system 2 remnants of slightly older trench slope basins that generally developed west of but which may locally structurally underlie the younger forearc basin 3 younger strike slip related structural basins that have developed along active right lateral faults of the san andreas fault system 4 broad shallow embayments perhaps similar to the modern san francisco bay that were connected to the deeper pacific ocean to the west and 5 structurally emplaced remnants of oceanic crust and its overlying sedimentary cover the king range terrane our preliminary stratigraphic and sedimentologic studies suggest that much of northern california was covered during the neogene by a forearc basin that may have extended as far south as the san francisco bay region and into the southern san joaquin valley as the mendocino triple junction migrated northward during the late cenozoic the southern margin of the forearc basin was uplifted basin deposits were stripped off by erosion and the locus of forearc sedimentation shifted progressively northward through time preserved but isolated fragments of the forearc basin provide a partial record of the original paleogeography and tectonic framework of northern california during the late cenozoic

interpretation of gravity data in a complex volcano tectonic setting southwestern nevada this regional gravity study based on an irregular 2 km data grid was conducted during the past few years at yucca mountain southern nye county nevada as part of a program to locate a suitable repository for high level nuclear waste about 100 surface rock samples three borehole gamma gamma logs and one borehole gravity study provide excellent density control a nearly linear increase in density of 0 26 g cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain isostatic and 2 0 g cm3 bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects respectively the bare mountain gravity high with an isostatic anomaly maximum of 48 mgal is connected with a greater gravity high over the funeral mountains to the southwest together these highs result from a continuous block of dense metamorphosed precambrian and paleozoic rocks that stretches across much of the walker lane from the east edge of death valley to bare mountain the calico hills gravity high appears more likely to originate from a northeast trending buried ridge of paleozoic rocks that extends southwestward beneath busted butte 5 km southeast of the proposed repository where two and three dimensional modeling indicates that the pre cenozoic rocks lie less than 1000 m beneath the surface tuff at least 4000 m thick fills a large steep sided depression in the pretuff rocks beneath yucca mountain and crater flat the gravity low and the thick tuff section lie within a large collapse area that includes the crater flat timber mountain silent canyon caldera complexes gravity lows in crater flat itself are interpreted to coincide with the source areas of the prow pass member the bullfrog member and the tram member of the crater flat tuff these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes southward extension of the broad gravity low associated with crater flat into the amargosa desert is evidence for sector graben type collapse segments related to the formation of the timber mountain caldera and superimposed on the other volcanic and extensional structures within crater flat

cenozoic volcanism in the california coast ranges numerical solutions we present the results of numerical simulations of cenozoic volcanism in the coast ranges of california associated with the northward migration of the mendocino triple junction mtj three aspects of the thermal evolution and magmatism in the wake of the mtj are investigated 1 mantle flow and thermal perturbation in the slabless window 2 pressure release partial melting in the upwelling asthenosphere and 3 thermally induced crustal anatexis numerical results show that upwelling of asthenosphere in the slabless window causes a drastic change in the thermal structure of the lithosphere and results in significant pressure release partial melting in the upwelling asthenosphere up to 4 5 km of basaltic magma may be produced most of this mafic magma is likely stored near the base of the crust thermally induced crustal anatexis mainly occurs in the deep crust below 20 km where a high degree 50 of partial melting may be predicted most of the crustal melt is probably generated within 1 m y after the passage of the mtj this work also suggests that the spatial and temporal distribution of cenozoic volcanism in the california coastal ranges is directly related to the variation of the relative velocity between the north america and pacific plates in the last 30 m y the voluminous volcanic rocks in the northern california coast ranges correlate to the relatively high velocity between these two plates in the last 10 m y the reduced volcanism in the central coast ranges may be related to the slow migration of the mtj between 10 and 20 ma

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

stratigraphic and spatial distribution of ochre and ochre processing tools at porc epic cave dire dawa ethiopia abstract reassessment of the archaeological assemblages recovered by kenneth d williamson in 1975 and 1976 at porc epic cave dire dawa ethiopia demonstrates that the middle stone age msa levels of this site yielded one of the richest known collections of ochre and ochre processing tools from this period in terms of quantity we analyze the vertical and horizontal distribution of all ochre fragments n 4213 and ochre processing tools n 23 of known provenance and compare them with the distribution of other categories of archaeological remains our results suggest that no major post depositional reworking occurred at the site virtually no ochre fragments and processing tools are recorded in the superficial levels of the site 0 60 cm below datum concomitant and statistically significant changes in the location of ochre and ochre processing tool concentrations are observed at various depth intervals in deeper levels interpreted as changes in the location of the area devoted to ochre processing comparison of the vertical distributions of ochre fragments and lithics highlights that in most cases ochre and lithics covary the vertical distribution of gastropod opercula interpreted by other authors as ornaments and of ochre fragments throughout the sequence also follows the same trends however concentrations of ochre and ochre processing tools do not coincide with that of the opercula spatially 14c determinations obtained at porc epic indicate a relatively short chronology for the accumulation of the main archaeological deposits and date to ca 40 ka cal bp the levels containing most of the ochre this is consistent with the fact that no significant typo technological variations in the lithics are observed throughout the sequence and argues in favour of the site reflecting a phase of the transition from the middle stone age to the later stone age lsa in this region of africa