deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

late quaternary paleohydrological conditions in the drylands of northern mexico a summer precipitation proxy record of the last 80 cal ka bp abstract we present a late quaternary multi proxy record from the paleolake babicora in order to understand the influence of north american monsoon nam and westerly winter storms on the paleohydrological budget of the drylands of northern mexico stratigraphy 14 c chronology mineral magnetism and geochemistry of the 976 cm long sediment core were used to reconstruct the hydrological conditions over the last 80 cal ka bp the inverse relationship between the proxy records of runoff into babicora and winter precipitation in southwestern usa indicate the minimal influence of westerly winter storms at 29 n during the last glacial period westerly winds transported minimally chemically altered sediments from the dry watershed during the cold stadials and the basin received more than average runoff as a result of a stronger nam during the warm interstadials the highest pluvial discharge occurred prior to 58 cal ka bp and terrestrial plants became the major contributor to organic matter deposited between 71 and 53 cal ka bp over the last 40 cal ka bp the high amplitude fluctuations in runoff and lake water salinity mirrored an unstable summer rainfall regime

a 16 000 14c yr b p packrat midden series from the usa mexico borderlands abstract a new packrat midden chronology from playas valley southwestern new mexico is the first installment of an ongoing effort to reconstruct paleovegetation and paleoclimate in the u s a mexico borderlands playas valley and neighboring basins supported pluvial lakes during full and or late glacial times plant macrofossil and pollen assemblages from nine middens in the playas valley allow comparisons of two time intervals 16 000 10 000 and 4000 0 14c yr b p vegetation along pluvial lake margins consisted of open pinyon juniper communities dominated by pinus edulis juniperus scopulorum juniperus cf coahuilensis and a rich understory of c4 annuals and grasses this summer flowering understory is also characteristic of modern desert grassland in the borderlands and indicates at least moderate summer precipitation p edulis and j scopulorum disappeared or were rare in the midden record by 10 670 14c yr b p the late holocene is marked by the arrival of chihuahuan desert scrub elements and few departures as the vegetation gradually became modern in character larrea tridentata appears as late as 2190 14c yr b p based on macrofossils but may have been present as early as 4095 14c yr b p based on pollen fouquieria splendens one of the dominant desert species present at the site today makes its first appearance only in the last millennium the midden pollen assemblages are difficult to interpret they lack modern analogs in surface pollen assemblages from stock tanks at different elevations in the borderlands

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

structural and metamorphic evolution of the orocopia schist and related rocks southern california evidence for late movement on the orocopia fault the pelona orocopia and rand schists por schists of southern california and southwesternmost arizona are late mesozoic or early tertiary subduction complexes that underlie precambrian to mesozoic continental basement along the low angle vincent chocolate mountains vcm fault system the vcm faults are often considered to be remnants of the original subduction zone but recent work indicates that many have undergone substantial postsubduction reactivation in the orocopia mountains for example the orocopia schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the orocopia fault structures in the schist include isoclinal folds with axial planar schistosity open to tight folds that fold schistosity penetrative stretching lineations and crenulation lineations all of which show a nearly 360 range in trend folds and lineations that trend approximately ne sw occur throughout the schist and are thought to be part of an early phase of deformation related to subduction folds of this orientation show no consistent vergence folds and lineations that trend approximately nw se are concentrated near the orocopia fault and are interpreted to have formed during exhumation of the schist the nw se trending folds and shear indicators in late stage mylonite at the top of the schist consistently verge ne the exhumation event culminated in emplacement of the schist against brittlely deformed upper plate exhumation of the orocopia schist was accompanied by retrograde replacement of garnet biotite epidote and calcic amphibole by chlorite calcite and sericite matrix amphibole has a lower na al ratio than amphibole inclusions in albite consistent with a late stage decrease in pressure as ne vergence in the orocopia mountains is associated with exhumation of the schist the ne movement along other segments of the vcm fault may also be late and therefore have no bearing on the facing direction of the vcm subduction zone contrary to past interpretations

hydrologic variability and the onset of modern el nino southern oscillation a 19 250 year record from lake elsinore southern california there are very few terrestrial palaeoclimate archives spanning the last glacial maximum through the holocene from coastal southern california yet knowledge of past climate dynamics is critical for assessing present and future constraints on the region s dwindling freshwater resources we present initial results from two drill cores extracted from the present day edge of lake elsinore southern california s largest natural lake using a multiproxy approach including lithologic description mass magnetic susceptibility loi 550 c total organic matter and loi 950 c total carbonate we infer first order long term climate change over the past 19 250 calendar years furthermore we suggest possible first order forcing mechanisms that drive this change which include presence absence of continental ice sheets insolation and complex ocean atmosphere interactions our results indicate four distinct millennial scale climate modes over the past 19 250 calendar years these modes include a wet last glacial maximum 19 250 17 120 cal yr bp a relatively dry late glacial holocene transition 17 120 9450 cal yr bp a wet early holocene 9450 7670 cal yr bp and a highly variable mid to late holocene climate i e alternating wet dry cycles 7670 cal yr bp present we attribute the mid to late holocene climate interval to the onset of el nino southern oscillation ca 7000 cal yr bp and a more vigorous hydrologic system these results are supported by a variety of regional terrestrial and marine palaeoclimate archives copyright 2005 john wiley sons ltd

thrusting in the lower crust evidence from the oygarden islands kemp land east antarctica layered orthogneisses of the oygarden islands preserve evidence for four high grade deformation events d 1 to d 4 archaean d 1 and d 2 structures are only patchily preserved due to extensive recrystallization during d 3 and d 4 which represent effects of the c 1000 ma rayner structural episode ductile thrusting at middle to lower crustal levels occurred during d 3 which is separated into two mutually cross cutting phases based on structural geometry the two phases represent changes in finite strain that developed during progressive deformation east directed transport during d 3a developed subhorizontal thrusts that contain co axial east trending f 3a folds and l 3a lineations buckling as a consequence of constriction in thrust duplexes developed upright f 3b folds coaxial to f 3a folds and steeply south dipping d 3b shear zones garnet clinopyroxene and garnet orthopyroxene bearing assemblages in mafic lithologies and garnet sillimanite bearing assemblages in pelitic lithologies reflect d 3 conditions of p 9 kbar and t 800 850 c the well exposed d 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre scale thrust stacking and ductile flow

shear heating not a cause of inverted metamorphism an archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the pelona schist of southern california united states recent studies demonstrate that the pelona schist was subducted and accreted at the onset of laramide fl at subduction under thermal and kinematic conditions not considered in earlier numerical models to test the shear heating hypothesis under these conditions we constructed a thermokinematic model of fl subduction initiation involving continuous accretion of the schist a neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the sierra pelona mountains are satisfi ed only if accretion rates were 0 2 3 6 km m y and shear heating was minimal shear stress 0 19 mpa minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the east fork of the san gabriel river exposure of the schist shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation

geochemistry and origin of mafic rocks from the pelona orocopia and rand schists southern california abstract metabasites from the pelona orocopia rand por schist a presumed subduction complex in southern california have been analyzed for major minor and trace element concentrations to investigate the nature origin and tectonic implications of these rock bodies element abundances indicate that these rocks are tholeiitic and alkalic basalts generated from several sources most samples group 1 resemble normal to transitional mid ocean ridge basalts n type to t type morb variation of morb like samples can be accounted for by low pressure crystal fractionation a second group group 2 which includes five samples from the rand mountains and one from the san gabriel mountains has elemental abundances similar to those of tholeiitic ocean island basalts or e type morb although phosphorus is unusually low several samples of group 2 have high mgo ni and cr characteristics which are commonly associated with crystal accumulation a final group group 3 consists of two samples from the orocopia mountains that resemble alkalic ocean island basalts the analyzed samples were collected from six different bodies of por schist there seem to be no major differences in basalt type which would imply a unique and separate source for any one area various sites of accumulation have been suggested for the protoliths of the por schist open ocean gulf of california type basin trapped back arc basin and rifted back arc basin the first three settings are entirely consistent with the basalt geochemistry the last option is less likely but not impossible

changes in planktic foraminiferal faunas temperature and salinity in the gulf stream during the last 30 000 years influence of meltwater via the mississippi river abstract variations in the distribution of planktic foraminiferal faunas temperature and salinity in the surface water of the gulf stream during the last 30 000 years have been reconstructed based on a sediment core from the blake ridge subtropical western atlantic ocean temperatures and salinities were calculated using planktic foraminiferal transfer functions and previously published oxygen isotope values productivity was estimated using the 106 m size fraction of planktic foraminifera the reconstructed values show that temperature variations have been small during the investigated period with a total range of only 3 c the highest temperatures about 1 5 c above the present temperature in the area occurred during the late glacial heinrich event 1 and the mid holocene time periods the lowest temperatures about 1 5 c below the present temperatures occurred during the bolling interstadial the early holocene period was relatively cool the low temperatures during the bolling interstadial and during the early holocene are in contrast to the northeastern atlantic where the bolling interstadial was the warmest period of the deglaciation and the early holocene the warmest during the holocene we attribute the lower temperatures during the bolling and early holocene periods to colder meltwater from the laurentide ice sheet flowing into the gulf of mexico through the mississippi river system and carried to the blake ridge via the gulf stream

paleohydrology of the santiaguillo basin mexico since late last glacial and climate variation in southern part of western subtropical north america abstract stratigraphy geochemistry and radiocarbon dating of a succession of sediment in the santiaguillo basin central northern mexico help reconstruct the millennial scale dynamics of hydrological variability that occurred in the southern part of western subtropical north america since the late last glacial runoff was generally above average during the late last glacial from 27 to 18 ka following this interval runoff decreased and deposition of authigenic carbonate and aeolian transported sediment increased until 4 ka heinrich 1 and 2 and younger dryas were intervals of reduced runoff and increased aeolian activity the wetter climate of central northern mexico and arid conditions in north northwestern mexico during the late last glacial were probably related to formation of tropical cyclones in the eastern north pacific during the autumn with restricted rainfall swaths and an absent weaker north american monsoon enhanced north american monsoon and tropical cyclones with expanded rainfall swaths brought more summer and autumn precipitation to a broader region extending from the central northern mexico to the continental interiors of southwestern us during the early holocene

mid wisconsin sediment record from baldwin lake reveals hemispheric climate dynamics southern ca usa abstract recently acquired cores from baldwin lake southern california document the first regional terrestrial based evidence for glacial period millennial to orbital scale climate variability eight ams 14c dates on a 14 m core from the lake s present day depocenter provide initial age control over the interval from 20 000 to 48 000 years before present bp a linear age model extends the record to a minimum age of 65 000 years bp a combination of lithologic description and one cm contiguous sedimentological analyses indicates significant changes in the dominant type of depositional environment over the length of the record at millennial to orbital time scales as a first order interpretation the dominant sedimentary environments alternate between permanent and ephemeral lake systems orbital scale forcing includes long term winter summer insolation variability and its modulation of winter storm tracks and the north american monsoon respectively millennial scale variability is attributed to extra tropical ocean atmosphere dynamics akin to historical interdecadal pacific climate variability in addition there is notable correspondence to the independently dated greenland ice core record gisp 2 at millennial to orbital time scales suggesting that baldwin lake contains in addition to its local regional record a hemispheric record of climate change similar to owens pyramid and summer lake of central and northwestern north america an initial comparison to gisp2 18o ice indicates that north atlantic interstadials correlate to permanent lake environments in coastal southwestern north america these results present a notable conflict with the proposed super enso phenomenon in the western tropical pacific these results also highlight the necessity for acquiring additional high resolution glacial period records for resolving questions of spatial temporal phasing of late quaternary climates

last glacial hydrological variations at the southern margin of sub tropical north america and a regional comparison geochemical and magnetic characteristics of sediments from paleolake la salada provide information about the last glacial hydrological conditions at the southern margin of sub tropical north america proxy records of runoff lake water salinity brine composition and provenance of organic matter are based on multi element concentration magnetic susceptibility mineralogy carbonate content and c n relation runoff was higher than average during ca 43 36k cal a bp and at ca 33k cal a bp a shift in mineralogical assemblage from calcite protodolomite halite to protodolomite gaylussite suggests that the brine composition changed from ca mg na hco3 cl to mg na ca hco3 because of more runoff after the desiccation event at ca 28k cal a bp in general organic matter deposited during the intervals of more runoff and deeper lake conditions were sourced from lacustrine phytoplankton the contribution of terrestrial vegetation dominates during the intervals of reduced runoff comparison with other records provides a regional perspective of moisture variability humid conditions at ca 33k cal a bp could be due to more summer precipitation in northern mexico as well as increased winter rainfall in the south western usa

amphibole compositions and metamorphic history of the rand schist and the greenschist unit of the catalina schist southern california the catalina schist and rand schist are two high p t terranes in southern california the catalina schist is correlated with the franciscan complex and occurs in the continental borderland it consists of a blueschist facies melange tectonically overlain by a greenschist unit which in turn is overthrust by an amphibolite unit the greenschist unit itself is inversely zoned from epidote amphibolite fades at the top through greenschist facies in the center to transitional blueschist greenschist facies at the base the rand schist is part of the eugeoclinal pelona orocopia schist terrane which lies interior to the present continental margin structurally beneath precambrian to mesozoic sialic basement the rand schist is inversely zoned from epidote amphibolite facies to transitional blueschist greenschist facies similar to the greenschist unit of the catalina schist

latest pleistocene to holocene hydroclimates from lake elsinore california abstract the hydroclimate of the southwestern united states us region changed abruptly during the latest pleistocene as the continental ice sheets over north america retreated from their most southerly extent to investigate the nature of this change we present a new record from lake elsinore located 36 km inland from the pacific ocean in southern california and evaluate it in the context of records across the coastal and interior southwest united states including northwest mexico the sediment core recovered from lake elsinore provides a continuous sequence with multi decadal resolution spanning 19 9 ka bp sedimentological and geochemical analyses reveal hydrologic variability in particular sand and carbonate components indicate abrupt changes at the oldest dryas od bolling allerod ba and younger dryas yd transitions consistent with the timing in greenland hydrogen isotope analyses of the c28 n alkanoic acids from plant leaf waxes dwax reveal a long term trend toward less negative values across 19 9 ka bp dwax values during the od suggest a north pacific moisture source for precipitation consistent with the dipping westerlies hypothesis we find no isotopic evidence for the north american monsoon reaching as far west as lake elsinore therefore we infer that wet dry changes in the coastal southwest were expressed through winter season precipitation consistent with modern climatology comparing lake elsinore to other southwest records notably cave of bells and fort stanton we find coincident timing of the major transitions od to ba ba to yd and hydrologic responses during the od and ba the hydrologic response however varied during the yd consistent with a dipole between the coastal and interior southwest the coherent pattern of hydrologic responses across the interior southwest us and northwest mexico during the od wet the ba drier and yd wet follows changes in the atlantic meridional overturning circulation presumably via its combined influence on north pacific winter storm tracks and the extent magnitude of the north american monsoon in contrast lake elsinore and the coastal southwest experiences a deglacial drying trend punctuated by abrupt change at the od to ba and ba to yd transitions this trend tracks rising greenhouse gases through the deglacial with an apparent southward shift in westerly moisture sources adjusting to the retreating ice sheet

ductile thrusting within subduction complex rocks on signy island south orkney islands abstract a model involving repeated nnw directed ductile thrusting in subduction complex rocks on signy island south orkney islands accounts for the consistent nnw trending alignment of fold axes stretching and shearing directions in the absence of major discrete thrust planes stratigraphic control or younging criteria the evidence for consistent ductile thrusting is more cryptic and is found in folding stretching relations porphyroblast rotations and mylonite fabrics early fold axes parallel to stretching syn tectonic garnet rotations augen lineations and sheath folds all support nnw directed thrusting as do a common late shear band foliation and minor shear zones development of late open folds with axes parallel to stretching may be due to constrictional deformation a lateral ramp or differential movement during thrusting the structures described from signy island may characterize the type of deformation in deep level ductile thrust zones at the base of an accretionary wedge or in a zone of subcretion the thrusting direction on signy island is inferred to be parallel to the direction of subduction in the south orkney islands in the early jurassic plate reconstructions suggest that this direction was at a high angle to the arcuate trend of the antarctic peninsula continental margin

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

changing amounts and sources of moisture in the u s southwest since the last glacial maximum in response to global climate change abstract the u s southwest has a limited water supply and is predicted to become drier in the 21st century an improved understanding of factors controlling moisture sources and availability is aided by reconstruction of past responses to global climate change new stable isotope and growth rate records for a central texas speleothem indicate a strong influence of gulf of mexico gom moisture and increased precipitation from 15 5 to 13 5 ka which includes the majority of the bolling allerod warming ba 14 7 12 9 ka coeval speleothem records from 900 and 1200 km to the west allow reconstruction of regional moisture sources and atmospheric circulation the combined isotope and growth rate time series indicates 1 increased gom moisture input during the majority of the ba producing greater precipitation in texas and new mexico and 2 a retreat of gom moisture during younger dryas cooling 12 9 11 5 ka reducing precipitation these results portray how late pleistocene atmospheric circulation and moisture distribution in this region responded to global changes providing information to improve models of future climate

structural geology of the pelona schist and vincent thrust san gabriel mountains california the vincent thrust of the san gabriel mountains southern california separates an upper plate that contains abundant granitic rocks from a lower plate composed of metagraywackes and metabasites pelona schist the vincent thrust is part of a regional thrust system of late cretaceous early tertiary age vincent chocolate mountain thrust several earlier workers have concluded that the upper plate was derived from the southwest implying the former presence of continental rocks separated from north america by a marginal oceanic basin other workers consider that regional correlations indicate thrusting of the opposite sense the pelona schist of the san gabriel mountains has undergone a complex structural history the earliest recognized structures are tight synmetamorphic isoclinal folds style 1 folds these folds are present throughout the entire 4 km of presently exposed section and do not become more appressed or more abundant near the thrust isoclinally refolded folds and sheath folds were observed at several localities textural observations indicate that the present schistosity of the pelona schist formed by the extreme flattening of crenulations developed in an earlier schistosity this is evidence that the isoclinally refolded folds although only infrequently noticed in the field are actually widespread the isoclinal folding was followed by the development of a macroscopic synform narrows synform and associated minor folds in the 700 m of pelona schist closest to the upper plate these folds style 2 folds are more open than the style 1 folds and do not have a well developed axial planar schistosity they are generally parallel in orientation to the style 1 folds both style 1 and style 2 folds are in turn parallel to tight folds in mylonitic rocks present at the base of the upper plate it is believed that the three groups of folds are cogenetic and possibly were produced in a subduction zone the sheath folds and other evidence of stretching parallel to fold axes might indicate that fold axes were rotated into the direction of thrusting on the other hand the parallelism of the early isoclinal folds and the later relatively open style 2 folds is evidence against rotation thus the orientation of fold axes relative to the thrusting direction is not clear subsequent to metamorphism the pelona schist and the mylonites were folded into a broad arch style 3 folding uncertainty as to the orientation of the axes of style 1 and 2 folds relative to the thrusting direction the presence of the style 3 folding and recently described paleomagnetic evidence of large scale rotation of crustal blocks in southern california make determination of thrusting direction impossible at the present time thus no evidence was found either to support or to invalidate the marginal basin model for the origin of the pelona schist

petrological evidence for the development of refolded folds during a single deformational event abstract the pelona schist which forms the lower plate of the vincent thrust in the san gabriel mountains of southern california has undergone a complex history of folding the youngest folds in the schist style 2 folds range in shape from open to tight and fold both compositional layering and schistosity these are superposed upon isoclinal folds with axial plane schistosity style 1 folds that in turn overprint older isoclinal folds also called style 1 folds samples from the hinges of style 2 folds contain two generations of muscovite muscovites of the older generation are parallel to the folded style 1 schistosity the newer muscovites recrystallized during and or after style 2 folding microprobe analysis indicates that the two generations of muscovite are very similar in composition although the new muscovites tend to have slightly higher paragonite and celadonite contents than the old muscovites from the gross similarity of the two groups of muscovite it is concluded that the style 1 and style 2 folds were produced during a single progressive deformation the slightly higher paragonite and celadonite contents of the new muscovites are thought to indicate that both pressure and temperature were increasing during the deformation this is consistent with the deformation being due to underthrusting of the pelona schist beneath the upper plate of the vincent thrust

solar forcing of holocene climate new insights from a speleothem record southwestern united states holocene climate change has likely had a profound infl uence on ecosystems and culture a link between solar forcing and holocene climate such as the asian monsoon has been shown for some regions although no mechanism for this relationship has been suggested here we present the fi rst high resolution complete holocene climate record for the north american monsoon region of the southwestern united states southwest in order to address the nature and causes of holocene climate change we show that periods of increased solar radiation correlate with decreased rainfall the opposite to that observed in the asian monsoon and suggest that a solar link to holocene climate is through changes in the walker circulation and the pacifi c decadal oscillation and el nino southern oscillation systems of the tropical pacifi c ocean given the link between increased warming and aridity in the southwest additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions similar to those seen in our record during periods of high solar activity

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

evidence of holocene climatic changes from aeolian deposits in baja california sur mexico abstract well defined late pleistocene and holocene dune fields are present on the surface of the purisima iray magdalena sedimentary basin and on top of barrier islands of the magdalena lagoonal complex in baja california sur mexico thermoluminescence amino acid epimerization and radiocarbon techniques were used for dating dune sands and shell samples respectively during late pleistocene to early holocene 14 to 8 ka bp mega barchans and linear dunes were formed during early holocene through middle holocene warm dry climate dune fields were still active at about 5 to 6 ka bp the stabilization of sea level developed the present lagoonal system in the area sandy barrier islands capped by foredunes were developed at this time coastal dunes were eroded and in part inundated forming large flooding flats and sabkha deposits in the old inter dune areas during late holocene after the lagoonal system was fully developed dunes were emplaced along the lagoonal coasts the presence of some younger dates in older deposits seems to represent dune reactivation

atlantic ocean modulated hydroclimate of the subtropical northeastern mexico since the last glacial maximum and comparison with the southern us abstract proxy records generated from chemical composition and radiocarbon dating of a sediment sequence collected from the el potosi basin provide information about variable hydroclimate of the subtropical northeastern mexico since the last glacial maximum lgm minimal amounts of runoff and more carbonate deposition during 19 ka represent the lgm the deglaciation was wetter compared to the lgm and enhanced runoff during 19 12 ka transported more ti bearing clastics from a relatively humid watershed over the deglaciation the basin received more runoff during the bolling allerod compared to the heinrich stadial 1 and younger dryas reduction in the amount of runoff during 10 1 ka and deposition of unweathered si bearing clastics during 4 1 ka suggest that the holocene was drier compared to the deglaciation hydroclimates of the northeastern mexico and southern us were synchronous during the lgm as well as deglaciation change from the drier lgm to wetter bolling allerod occurred as the gulf of mexico became warmer and the region received more summer precipitation however the dissimilarities observed in regional hydroclimates of the holocene need further evaluation

extensional reactivation of the chocolate mountains subduction thrust in the gavilan hills of southeastern california the ne vergent chocolate mountains fault of south eastern california has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic orocopia schist or as a normal fault involved in the exhumation of the schist our detailed structural analysis in the gavilan hills area provides new evidence to confirm the latter view a zone of deformation is present at the top of the orocopia schist in which lineations are parallel to those in the upper plate of the chocolate mountains fault but oblique to ones at relatively deep levels in the schist both the orocopia schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures these structures formed during retrograde metamorphism and are considered to record the exhumation of the orocopia schist during early tertiary time as a result of subduction return flow the gatuna fault which places low grade supracrustal metasediments of the winterhaven formation above the gneisses of the upper plate also seems to have been active at this time final unroofing of the orocopia schist occurred during early to middle miocene regional extension and may have involved a second phase of movement on the gatuna fault formation of the chocolate mountains fault during exhumation indicates that its top to the ne sense of movement provides no constraint on the polarity of the orocopia schist subduction zone this weakens the case for a previous model involving sw dipping subduction while providing support for the view that the orocopia schist is a correlative of the franciscan complex

leading tropical modes associated with interannual and multidecadal fluctuations in north atlantic hurricane activity abstract interannual and multidecadal extremes in atlantic hurricane activity are shown to result from a coherent and interrelated set of atmospheric and oceanic conditions associated with three leading modes of climate variability in the tropics all three modes are related to fluctuations in tropical convection with two representing the leading multidecadal modes of convective rainfall variability and one representing the leading interannual mode enso the tropical multidecadal modes are shown to link known fluctuations in atlantic hurricane activity west african monsoon rainfall and atlantic sea surface temperatures to the tropics wide climate variability these modes also capture an east west seesaw in anomalous convection between the west african monsoon region and the amazon basin which helps to account for the interhemispheric symmetry of the 200 hpa streamfunction anomalies across the atlantic ocean and africa the 200 hpa divergent wind anomalies and both the structure and spatial scale

ostracode mg ca paleothermometry in the north atlantic and arctic oceans evaluation of a carbonate ion effect 1 the reconstruction of deep sea bottom water temperature bwt is important to assess the ocean s response to and role in orbital and millennial scale climate change deep sea paleothermometry employs magnesium to calcium mg ca ratios in calcitic benthic microfaunas foraminifera ostracodes as a primary proxy method mg ca paleothermometry may however be complicated by bottom water carbonate ion chemistry which might affect mg ca ratios in shells to address temperature and carbonate ion influence on mg ca ratios we studied mg ca ratios in the benthic ostracode genus krithe in the north atlantic and arctic oceans using a 686 specimen core top collection including 412 previously unpublished analyses mg ca ratios are positively correlated to temperature in multiple species from the north atlantic bwt 0 885 mg ca 5 69 r2 0 73 and for k glacialis in the arctic ocean and nordic seas bwt 0 439 mg ca 5 14 r2 0 50 consistent with previously published calibrations we found no evidence for a relationship between krithe mg ca and carbonate ion saturation in the north atlantic ocean nordic seas and arctic ocean supporting the use of krithe mg ca for reconstructing past bwt

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

a holocene record from a former coastal lagoon in bahia kino gulf of california mexico a 10 m long core was recovered from the coastal flood plain on the eastern margin of the gulf of california bahia kino sonora sediments were analysed for loss on ignition magnetic susceptibility pollen diatoms ostracodes and foraminifera three levels were dated by 1 4 c the combined data suggest that the site was flooded during the middle holocene as sea level increased at about 6600 yr bp the area was a freshwater to brackish marsh dominated by typha domingensis and cyperaceae freshwater to brackish conditions suggest the presence of a local source of freshwater during this time and therefore a climate wetter than present continued rise in sea level was associated with an increase in depth and salinity by c 6350 6250 yr bp the coring site was an inner lagoon by c 6125 yr bp a central lagoon and by c 5900 yr bp the site was close to the mouth of the lagoon lower lagoon lower lagoon conditions persisted until the top of the record showing sea level stabilization which was followed by the formation of the dune ridge that presently separates the coring site from the sea

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

out of the tropics the pacific great basin lakes and late pleistocene water cycle in the western united states the water cycle in the western united states changed dramatically over glacial cycles in the past 20 000 years higher precipitation caused desert lakes to form which have since dried out higher glacial precipitation has been hypothesized to result from a southward shift of pacific winter storm tracks we compared pacific ocean data to lake levels from the interior west and found that great basin lake high stands are older than coastal wet periods at the same latitude westerly storms were not the source of high precipitation instead air masses from the tropical pacific were transported northward bringing more precipitation into the great basin when coastal california was still dry the changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle

microstructural evidence for northeastward movement on the chocolate mountains fault zone southeastern california microstructural analysis of rocks from the chocolate mountains fault zone gavilan hills area southeastern california shows unequivocal evidence for northeast directed transport of the upper plate gneisses over lower plate orocopia schists samples were taken from transects through the fault zone prefaulting fabrics in upper plate gneisses show a strong component of northeast directed rotational deformation under lower amphibolite facies conditions in contrast prefaulting lower plate orocopia schists show strongly coaxial fabrics minimum stretch value of 2 2 formed at greenschist grade mylonitic fabrics associated with the chocolate mountains fault are predominantly northeast directed shear bands that are unidirectional northeastward in the gneisses but bi directional in the schists suggesting a significant component of nonrotational deformation occurred in the orocopia schists during and after emplacement of the upper plate the kinematic findings are in agreement with dillon et al 1989 who found that the vergence of asymmetrical folds within the fault zone indicates overthrusting to the northeast toward the craton in this region the available evidence favors a single protracted northeastward movement on the chocolate mountains fault zone with temperatures waning as deformation proceeded

volcano deformation and eruption forecasting recent advances in global positioning system gps tilt and interferometric synthetic aperture radar insar have greatly increased the availability of volcano deformation data these measurements combined with appropriate source models can be used to estimate magma chamber depth and to provide information on chamber shape and volume change however kinematic models cannot constrain magma chamber volume and provide no predictive

interpretation of crustal seismic velocities in the san gabriel mojave region southern california the increased concern over seismic hazards in the los angeles region has created interest in the crustal structure and mechanical properties associated with faults like the sierra madre and san andreas this study presents the seismic properties of major lithologies found in the vicinity of the san andreas fault zone in order to correlate with the los angeles region seismic experiment lars e results scientists from the larse group have recently acquired high quality seismic data along three transects thirty nine rock samples were collected along the larse line i which trends ne sw across the central mojave desert and the san gabriel mountains these rock samples represent the following lithologies precambrian gneisses san gabriel anorthosite mesozoic granitic intrusives pelona schist mendenhall granulite gneisses and san andreas fault zone cataclasites the seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure average compressional and shear wave velocities at 150 mpa show a wide range of values the mendenhall gneiss and the san gabriel anorthosite complex have high velocities 6 5 km s and 6 4 km s respectively the pelona schist the mojave granitic intrusives and the san andreas cataclasitic rocks have average velocities of 5 8 km s 5 2 km s and 5 1 km s respectively average densities range from 2320 kg m3 for fault zone cataclasites to 3100 kg m3 for a mendenhall granulite gneiss gneisses and schists show significant seismic anisotropy 5 20 due to preferred mineral orientation while most other rocks are nearly isotropic poisson s ratios range from a high of 0 29 in the san gabriel anorthosite to a low of 0 22 in the san gabriel gneiss comparison of the measured seismic properties with the larse velocity model allows interpretation of crustal structure in terms of rock compositions the upper crust southwest of the san andreas fault contains granitic intrusives gneisses and schists while under the mojave desert the upper crust is dominated by gneisses high velocities under the san gabriel mountains represent either lower crustal granulites or anorthosite the velocity decrease beneath the san andreas fault zone originates from minor amounts of cataclasite or the presence of low velocity lithologies such as pelona schist 1998 elsevier science b v all rights reserved

early history and reactivation of the rand thrust southern california abstract the rand thrust of the rand mountains in the northwestern mojave desert separates an upper plate of quartz monzonite and quartzofeldspathic to amphibolitic gneiss from a lower plate of metagraywacke and mafic schist rand schist the rand thrust is considered part of the regionally extensive vincent chocolate mountain thrust system which is commonly believed to represent a late cretaceous subduction zone the initial direction of dip and sense of movement along the vincent chocolate mountain thrust are controversial microfabrics of mylonites and quartzites from the rand mountains were analyzed in an attempt to determine transport direction for this region but the results are ambiguous in addition the southwestern portion of the rand thrust was found to have been reactivated as a low angle normal fault after subduction reactivation might have occurred shortly after subduction in which case it could account for the preservation of high pressure mineral assemblages in the rand schist or it could be related to mid tertiary extension in the western united states in either event the reactivation might be responsible for the complicated nature of the microfabrics the rand schist exhibits an inverted metamorphic zonation isograds in the schist are not significantly truncated by the reactivated segment of the rand thrust this indicates that other segments of the vincent chocolate mountain thrust should be re evaluated for the possibility of late movement even if they show an apparently undisturbed inverted metamorphic zonation

phasing of deglacial warming and laurentide ice sheet meltwater in the gulf of mexico evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation however existing studies show that deglacial warming was asyn chronous across the tropical band complicating the identification of causal mechanisms the orca basin in the northern gulf of mexico is ideally located to record subtropical atlantic sea surface temperature sst warming in relation to meltwater input from the laurentide ice sheet paired d 18 o and mg ca data on the planktonic foraminifer globigerinoides ruber from core en32 pc6 are used to separate deglacial changes in sst and d 18 o of sea water sst as calculated from mg ca data increased by 3 8c from ca 17 2 to 15 5 ka in association with heinrich event 1 and was not in phase with greenland air temperature subtracting tem perature effects from d 18 o values in g ruber reveals two excur sions representing laurentide meltwater input to the gulf of mex ico one of 1 5 from ca 16 1 to 15 6 ka and a second major spike of 2 5 from ca 15 2 to 13 0 ka that encompassed melt water pulse 1a and peaked ca 13 8 ka during the bolling allerod conversion to salinity through the use of a laurentide meltwater end member of 225 indicates that near surface salinity de creased by 2 4 during these spikes these results suggest that gulf of mexico sst warming preceded peak laurentide ice sheet decay and the bolling allerod interval by 2 k y and that heat was retained in the subtropical atlantic during heinrich event 1 consistent with modulation of deglacial climate by thermohaline circulation

response of the north american monsoon to regional changes in ocean surface temperature 1 the north american monsoon nam an onshore wind shift occurring between july and september has evolved in character during the holocene largely due to changes in northern hemisphere insolation published paleoproxy and modeling studies suggest that prior to 8000 cal years bp the nam affected a broader region than today extending westward into the mojave desert of california holocene proxy sst records from the gulf of california goc and the adjacent pacific provide constraints for this changing nam climatology prior to 8000 cal years bp lower goc ssts would not have fueled northward surges of tropical moisture up the goc which presently contribute most of the monsoon precipitation to the western nam region during the early holocene the north pacific high was further north and ssts in the california current off baja california were warmer allowing monsoonal moisture flow from the subtropical pacific to take a more direct northwesterly trajectory into an expanded area of the southwestern u s west of 114 w a new upwelling record off southwest baja california reveals that enhanced upwelling in the california current beginning at 7500 cal year bp may have triggered a change in nam climatology focusing the geographic expression of nam in the southwest usa into its modern core region east of 114 w in arizona and new mexico holocene proxy precipitation records from the southwestern u s and northwestern mexico including lakes vegetation pollen and caves are reviewed and found to be largely supportive of this hypothesis of changing holocene nam climatology

topographic gradients and lode gold sourcing recorded by placer gold morphology geochemistry and mineral inclusions in the east fork san gabriel river california u s a abstract topography plays a major role in the morphology geochemistry and mineral inclusion profiles of placer gold from the east fork of the san gabriel river these trends are consistent with a significant change in topographic gradient over a short distance and coincident with the break in slope along the range front of the san gabriel mountains placer gold from above this topographic transition is more rounded and 3 dimensional with elevated silver in the alloy of the native gold little evidence of gold enrichment rims and abundant inclusions of brittle and reactive minerals below the topographic transition placer gold assumes a distinctly flattened form with lower silver alloy content thin gold enrichment rims and rare inclusions of relatively inert minerals this relationship is observed for placer gold shed from two inferred lode sources within the river catchment with placer gold from each of these sources being traceable as a unique population over at least 8 km of the river significantly these placer gold properties are distinctive enough to fingerprint gold from specific portions of the placer deposit these identifying characteristics with virtually identical morphology and chemical trends are observed in the large nuggets of the mel fisher collection of purported san gabriel gold these similarities appear to confirm the provenance of the collection including identification of the specific locations where each nugget originated

paleoecology of a middle wisconsin deposit from southern california analysis of a buried deposit in the diamond valley of southern california has revealed well preserved pollen wood and diatom remains accelerator mass spectrometry dates of 41 200 2100 and 41 490 1380 14 c yr b p place this deposit in marine isotope stage 3 diatoms suggest a shallow lacustrine environment pollen data suggest that several plant communities were present near the site with grassland scrub chaparral forest and riparian communities represented comparison with modern pollen suggests similarities with montane forests in the nearby san bernardino and san jacinto ranges indicating vegetation lowering by at least 900 m elevation and temperatures 4 5 c cooler than today an increase in highelevation conifer pollen documents climatic cooling near the profile top early profile diatoms are typical of warm water with high alkalinity and conductivity whereas later diatoms suggest a higher flow regime and input of cooler water into the system we suggest that the sequence is part of the cooling phase of an interstadial dansgaard oeschger cycle records of the middle wisconsin period are rare in southern california but the diamond valley site is similar to records from tulare lake in the san joaquin valley and the odp site 893a record from santa barbara basin it is probable that the diamond valley assemblage is a local expression of a vegetation type widespread in the ranges and basins of southwestern california during the middle wisconsin c 2002 university of washington

role of extrusion of the rand and sierra de salinas schists in late cretaceous extension and rotation of the southern sierra nevada and vicinity the rand and sierra de salinas schists of southern california were underplated beneath the southern sierra nevada batholith and adjacent mojave salinia region along a shallow segment of the subducting farallon plate in late cretaceous time various mechanisms including return flow isostatically driven uplift upper plate normal faulting erosion or some combination thereof have been proposed for the exhumation of the schist we supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the rand and sierra de salinas schists these data indicate that the schist was transported to the ssw from deep to shallow crustal levels along a mylonitic contact the rand fault and salinas shear zone with upper plate assemblages crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary the resulting flow type within the channel is that of general shear extrusion integration of these observations with published geochronologic thermochronometric thermobarometric and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation we present a model whereby trench directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

qualitative thermobarometry of inverted metamorphism in the pelona and rand schists southern california using calciferous amphibole in mafic schist abstract the rand and pelona schists consist of eugeoclinal rock types overlain by continental basement along the vincent chocolate mountains vcm faults both schists display inverted metamorphic zonation defined in part by a systematic variation in composition of calcic to sodic calcic amphibole in mafic schist structurally upward the compositional progressions include increase of total a1 a1iv and ti but decrease in the ratios of na na ca to a1 a1 si and nam4 to a1vi fe3 ti these variations imply that structurally high rocks belong to a lower pressure metamorphic fades series than those at depth this result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences amphibole composition is dependent not only on structural position i e p t but also upon bulk rock composition the important controls are whole rock mg mg fe2 mn and fe3 fe2 the greatest impact of these factors however is on the absolute values of na and al rather than their ratio thus interpretation of facies series is not seriously hindered by compositional variability sodic amphibole in epidote blueschists from the rand schist is extensively replaced by sodic calcic amphibole sodic calcic amphibole in the rand schist and pelona schist is itself rimmed by actinolitic amphibole similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation in the rand and pelona schists the sequence probably formed during burial

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

terrigenous input response to glacial interglacial climatic variations over southern baja california a rock magnetic approach abstract the sediments deposited off south western baja california have recorded millennial scale fluctuations in biogenic compounds and trace metals accumulation during the last glacial period ortiz j d o connell s b delviscio j dean w e carriquiry j d marchitto t zheng y van geen a 2004 enhanced marine productivity off western north america during warm climate intervals of the past 52 ky geology 32 521 524 dean w e zheng y ortiz j d van geen a 2006 sediment cd and mo accumulation in the oxygen minimum zone off western baja california linked to global climate over the past 52 ka paleoceanography 21 pa4209 since the variations in trace metals concentration appear to result from dilution with nonbiogenic matter the variability of the terrigenous sedimentation needs to be addressed therefore we performed rock magnetic and geochemical analysis on a 38 m long sediment core collected from the slope off baja california the temporal framework provided by 14c dating and identification of palaeoclimatic transitions allows assigning the sequence to the last glacial interglacial cycle last 120 ka the comparison of magnetic and geochemical properties led to retain the hypothesis of a primary modulation of iron oxides quantity and quality by terrigenous input variations with a secondary diagenetic amplification two magnetic mineral input regimes are distinguished i magnetic susceptibility variations reveal changes in titano magnetite concentration related to fluvial transport of the terrigenous fraction ii coercivity changes allow detecting variations of hematite or goethite concentrations minerals generally issued from aeolian deflation of weathered rock surfaces these two regimes are paced by two distinct climatic forcing the millennial scale changes in titano magnetite input are related to the northern hemisphere climatic variability whilst the record of wind blown magnetic mineral input contains its major power in the precessional frequency band with higher input during low insolation periods and conversely this record highlights the great sensibility of this region to high and low latitudes climatic regimes

impact of the atlantic warm pool on united states landfalling hurricanes 1 the 2010 atlantic hurricane season was extremely active but no hurricanes made landfall in the united states raising a question of what dictated the hurricane track here we use observations from 1970 2010 also extending back to 1950 and numerical model experiments to show that the atlantic warm pool awp a large body of warm water comprised of the gulf of mexico the caribbean sea and the western tropical north atlantic plays an important role in the hurricane track an eastward expansion of the awp shifts the hurricane genesis location eastward decreasing the possibility for a hurricane to make landfall a large awp also induces barotropic stationary wave patterns that weaken the north atlantic subtropical high and produce the eastward steering flow anomalies along the eastern seaboard of the united states due to these two mechanisms hurricanes are steered toward the northeast without making landfall in the united states although the la nina event in the pacific may be associated with the increased number of atlantic hurricanes its relationship with landfalling activity has been offset in 2010 by the effect of the extremely large awp

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

mid to late wisconsin pollen record of san felipe basin baja california abstract a lacustrine sequence from laguna seca san felipe baja california in the southwestern sonoran desert provides the first record of mid to late wisconsin vegetation in one of the driest regions of north america pollen analysis was performed in the intermediate part of the sequence from ca 44 000 to ca 13 000 14 c yr b p according to six 14 c radiocarbon dates sedimentation of eolian sands prior to 44 000 14 c yr b p indicates dry conditions high pollen concentration of montane and chaparral species in the pollen assemblages contrast with the modern desert vegetation sixty four pollen types were identified the source of mid to late wisconsin sedimentation must have been the plant communities in the surrounding highlands of the basin mid wisconsin pollen assemblages dominated by pines junipers and artemisia reflect humid conditions in the area by late wisconsin time a significant increment in junipers indicates a lowering in the altitudinal ranges of woodlands chaparral pollen from wind pollinated species is abundant while desert plants specifically animal or insect pollinated species are poorly represented the san felipe record does not identify the last glacial maximum as the time of greatest effective moisture as suggested by pluvial lake levels and other paleoclimatic evidence in the soutwestern united states

petrogenesis of metamorphic rocks within a subduction accretion terrane signy island south orkney islands whole rock and mineral analyses of polydeformed mica schist quartzite marble and amphibolite are presented from signy island south orkney islands part of the scotia metamorphic complex whole rock chemistry suggests that the amphibolites are the metamorphosed equivalent of enriched tholeiitic and alkali basalts of an oceanic intraplate basalt series these together with limestones and mn rich cherts of an oceanic island assemblage were tectonically mixed with trench or trench inner slope basin sediments in a subduction zone environment variation in mineral chemistry indicates an increase in temperature and decrease in pressure during metamorphism pressures of 8 kbar and temperatures of approximately 545 c were reached during amphibolite facies metamorphism in the latter stages of deformation these new data provide good evidence to support the previous interpretation of the scotia metamorphic complex as a subduction complex

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

large holocene lakes and climate change in the chihuahuan desert lake level variations preserved as beach ridges in the laguna el fresnal and laguna santa maria subbasins northern mexico record millennially spaced episodes of increased precipitation during the holocene epoch we find that the early middle and late holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the chihuahuan desert the largest dated pluvial lake covered 5650 km2 during the early holocene constructional beach ridges in these subbasins are 221 33 14c yr b p little ice age equivalent 3815 52 to 4251 59 14c yr b p early neoglacial 6110 80 to 6721 68 14c yr b p mid holocene and 8269 64 to 8456 97 14c yr b p early holocene dates that correlate with other millennially spaced wet or cold events in the northern hemisphere we attribute these wet episodes to increased precipitation cooler temperatures and reduced evaporation following southward shifts in winter storm tracks which are related to long term el nino southern oscillation variability during the holocene

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

persistent summer expansion of the atlantic warm pool during glacial abrupt cold events palaeoclimate records and numerical model simulations indicate that changes in tropical and subtropical sea surface temperatures and in the annual average position of the intertropical convergence zone are linked to high latitude climate changes on millennial to glacial interglacial timescales it has recently been suggested that cooling in the high latitudes associated with abrupt climate change events is evident primarily during the northern hemisphere winter implying increased seasonality at these times8 however it is unclear whether such a seasonal bias also exists for the low latitudes here we analyse the mg ca ratios of surface dwelling foraminifera to reconstruct sea surface temperatures in the northeastern gulf of mexico for the past 300 000 years we suggest that sea surface temperatures are controlled by the migration of the northern boundary of the atlantic warm pool and hence the position of the intertropical convergence zone during boreal summer and are relatively insensitive to winter conditions our results suggest that summer atlantic warm pool expansion is primarily affected by glacial interglacial variability and low latitude summer insolation because a clear signature of rapid climate change events such as the younger dryas cold event is lacking in our record we conclude that high latitude events seem to influence only the winter caribbean climate conditions consistent with the hypothesis of extreme northern hemisphere seasonality during abrupt cooling events