a possible earthquake triggered mega boulder slide in a chilean mio pliocene marine sequence evidence for rapid uplift and bonebed genesis the type area of the bahia inglesa formation north central chile is structurally complex as a result of active margin subduction at the peru chile trench inliers of subaerially exposed mesozoic igneous basement are unconformably overlain by a mid miocene to late pliocene marine siliciclastic sequence which has become known for its abundance of fossil vertebrates found concentrated in a phosphatite on an omission surface mega boulders derived from one of the largest inliers occur exclusively within this bonebed which appears to have formed after major localized uplift caused removal of a significant thickness of unconsolidated sediment the mega boulders were probably dislodged by a high magnitude earthquake event that accompanied tilting of the sea floor and their emplacement was an integral part of the processes involved in genesis of the bonebed

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

the amphibolite granulite facies transition in west uusimaa s w finland a fluid inclusion study this work presents the results of a fluid inclusion study of an amphibolite granulite facies transition in west uusimaa s w finland early fluid inclusions in the granulite facies area are characteristically carbonic co2 in contrast to predominantly aqueous early inclusions in the amphibolite facies area these early inclusions can be related to peak metamorphic conditions 750 820 c and 3 5 kbar for peak granulite facies metamorphism relatively young co2 inclusions with low densities 0 8g cm3 indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift n2 ch4 inclusions with compositions ranging between pure ch4 and pure n2 raman spectral analysis were found in the whole area they are probably syn or even pre early inclusions only nearly critical homogenizing inclusions have been found low density pressure estimates based on densities of early fluid inclusions show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric granulite facies metamorphism in west uusimaa is a thermal event probably induced by the influx of hot co2 bearing fluids

fluid inclusions in high grade anatectic metamorphites fluid inclusions have been studied by microthermometry in metapelites and metabasites which have undergone partial melting in the transition zone from amphibolite to granulite facies mobilizates contain far more abundant and larger fluid inclusions than non mobilized or restite layers and these inclusions occur as several generations of well defined and chemically contrasted fluids carbonic co 2 ch 4 and hydrocarbon aqueous h 2 o nacl and n 2 a garnet bearing mobilizate in amphibolite contains 4 generations of relatively pure fluids most of them trapped after peak metamorphic conditions in the following p t domains co 2 and n 2 p 2 8 kbar t 550 850 c ch 4 p 1 2 kbar t 400 550 c h 2 o p t 120 400 c

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

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting recent thrust faulting models both physical and numerical have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults in this article recent field evidence supporting this strong asymmetry is described earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1 g and 100 cm sec new shattered rock evidence on the hanging wall of thrust faults in southern california supports the occurrence of intense ground motions on the other hand the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in southern california indicate relatively low ground motions thus indicating strong asymmetry in ground motions near the fault trace for thrust faults some current ground motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall this could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the los angeles basin and elsewhere in the world the new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models if such detachment occurs it could also have important implications for understanding thrust fault rupture mechanics for example the paradox of large overthrusts and the heat flow paradox for thrusts

evidence for fluid pathways through archaean crust and the generation of the closepet granite karnataka south india abstract in the archaean amphibolite granulite facies transition zone of southern karnataka anatexis of amphibolite facies peninsular gneiss occurred based on cross cutting relationships this melting event which produced the closepet granite was contemporaneous with the formation of charnockitic rocks this formation of charnockite took place initially due to channelized co 2 rich volatiles moving along an anastamosing network of shears and micro fissures these fluid pathways are now represented by brown veins of charnockite later the fluid became more pervasive and resulted in areas of total inversion to charnockite a lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening can be traced through the country rocks these different effects along the same laterally continuous vein are interpreted to represent the lateral change of volatile composition which in front of the co 2 rich fluids were increasingly h 2 o dominated

origin of the plio pleistocene vrica laminites organic geochemical evidence meyers p a 1993 origin of the plio pleistocene vrica laminites organic geochemical evidence mar geol 115 117 127 organic geochemical properties of samples from lower pleistocene laminite layer h at vrica calabria have been examined for evidence of the origin of these dark colored sediments the laminite samples are geochemically distinct from the surrounding gray marly clays they contain less caco 3 and more toc have higher c n ratios and lower 613c values and their hydrocarbon contents indicate larger proportions of land derived organic matter the vrica taminites appear to have originated principally as a consequence of enhanced runoff of land derived organic matter and clastic sediments from nearby land areas during episodes of wetter regional climate freshening of surface waters by the runoff created salinity stratification of the water column diminished availability of oxygen in near bottom waters and improved preservation of organic matter marine productivity was probably also enhanced but not as much as is evident for pelagic sapropels in the eastern mediterranean

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

cosmogenic chlorine 36 production rates in terrestrial rocks abstract chlorine 36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35cl spallation of 39k and 40ca and slow negative moun capture by 40ca we have measured the 36cl content of 14c dated glacial boulders from the white mountains in eastern california and in a 14c dated basalt flow from utah effective time intergrated production parameters were calculated by simultaneous solution of the 36cl production equations the production rates due to spallation are 4160 310 and 3050 210 atoms 36cl yr 1 mol 139k and 40ca respectively the thermal neutron capture rate was calculated to be 3 07 0 24 105 neutrons kg of rock 1 yr 1 the reported values are normalized to sea level and high geomagnetic latitudes production of 36cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates chlorine 36 dating was performed on carbonate ejecta from meteor crater arizona and late pleistocene morainal boulders from the sierra nevada california calculated 36cl ages are in good agreement with previously reported ages obtained using independent methods

chronology of rock falls and slides in a desert mountain range case study from the sonoran desert in south central arizona abstract in order to respond to the general paucity of information on the chronology of ubiquitous small rock falls and slides that litter the slopes of desert mountain ranges a case study in the sonoran desert reveals new insight into the desert geomorphology of mountain slopes rock falls and rock slides in the mcdowell mountains that abut metropolitan phoenix usa fall in three chronometric groupings dated by conventional radiocarbon and rock varnish microlamination methods first the oldest events are 74 ka and take the form of stable colluvial boulder fields positive relief features that are tens of meters long and a few meters wide second randomly sampled slides and falls of various sizes and positions wasted during wetter periods of the terminal pleistocene and holocene third an anomalous clustering of slides and falls occurred during the late medieval warm period medieval climatic anomaly when an extreme storm was a possible but unlikely trigger one speculative hypothesis for the cluster of medieval warm period events is that a small to moderate sized earthquake shook heavily shattered bedrock close to failure just enough to cause a spate of rock falls and slides a second speculative hypothesis is that this dry period enhanced physical weathering processes such as dirt cracking however the reasons for the recent clustering of rock falls remain enigmatic while the temporal distribution of slides and falls suggests a minimal hazard potential for homes and roads on the margins of the mcdowell mountains this finding may not necessary match other desert ranges in metropolitan phoenix or mountains with different rock types and structures that abut other arid urban centers

the role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments northwest adirondacks field and petrologic studies along the adirondack lowlands highlands boundary near harrisville ny indicate that heat from the synmetamorphic intrusion of the diana syenite complex intrusion temperature of 1 050 c played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units major paragneiss of engel and engel metamorphic temperatures inferred from two feldspar and spinel quartz assemblages decrease from 850 950 c along the diana metasediment contact to 650 700 c 2 3 km away from the contact metamorphic pressures are 7 0 5 kb as determined from coexisting plagioclase garnet sillimanite quartz kyanite sillimanite and garnet rutile ilmenite sillimanite quartz grail in the paragneiss migmatites consisting of quartz microcline perthite sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite sillimanite garnet spinel plagioclase corundum cordierite qualitatively the amount of partial melt and occurrences of corundum bearing assemblages decrease away from the diana contact activity of h2o inferred from coexisting biotite sillimanite quartz garnet k feldspar ranges from 0 01 to 0 17 and is five to ten times lower in corundum bearing rocks

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

residual regolith derived from the biotite controlled weathering of cretaceous tonalite quartz diorite peninsular ranges southern california usa a case study abstract though k feldspar free tonalite and quartz diorite are common plutonic rocks in exhumed continental margin magmatic arc rocks around the globe how they weather has not been extensively studied we therefore undertook an investigation aimed at understanding how an approximately 6 m thick section of residual regolith saprock and overlying alfisol developed from cretaceous k feldspar free tonalite and quartz diorite at santa margarita ecological reserve southern california usa we analyzed three samples of soil 10 of saprock and 10 of corestone for their major trace and rare earth element compositions thin sections of each major textural group also were studied and point counted common xrd techniques were used to determine clay mineralogy three samples of soil and five of saprock were analyzed for their ph within the saprock an extensive network of cracks was likely produced by the volumetric expansion that accompanied the conversion of biotite to vermiculite migrating near neutral fluids responsible for this conversion resulted in a loss of k mass and affected several trace elements that are commonly found in biotite but with the exception of an interval between 177 and 270 cm left most other major trace and rare earth elements ree little affected within the 177 to 270 cm interval at a depth of 208 cm relative to the immobile framework incongruent leaching of plagioclase by acidic solutions resulted in the losses of si al ca na pb sr and ga mass in contrast more neutral solutions added heavy ree hree and y mass at 177 to 237 cm depths while acidic solutions removed ree and y mass at 270 cm depth in the soil acidic solutions attacked plagioclase biotite and apatite leaching and removing relative to the immobile framework ca na mg k and p mass in addition the dissolution of apatite resulted in depletions in hree and y mass none of the elemental mass removed from the soil accumulated in the directly underlying saprock and must have been removed from the near vertical plane of sampling within the saprock the above changes are mostly the result of near neutral ph water rock interactions occurring within a warm and dry holocene climate identified changes in elemental mass other than those associated with the conversion of biotite to vermiculite are localized and are likely the result of the paths taken by fluids with differing ph moving through the complex network of cracks any model purporting to describe the evolution of residual regolith must explain such natural complexities

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

post metamorphic co2 rich fluid inclusions in granulites in granulite facies samples from the adirondack mountains ny estimates of peak metamorphic co2 fugacities based on mineral equilibria are not consistent with estimates based on data for high density co2 rich fluid inclusions of the 21 adirondack samples investigated for this study all contain co2 rich inclusions inclusions occur in quartz apatite and garnet they range in size from 3 to 50 m and are without visible h2o in a few of the inclusions freezing point determinations and preliminary laser raman spectroscopy show the presence of small amounts 3 of other fluids n2 and h2s co2 liquid vapor homogenization temperatures are between 46 and 31 c corresponding to densities between 1 14 and 0 5 gm cc some of these densities are consistent with peak metamorphic entrapment 1 06 to 1 1 gm cc

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

methodology for using precarious rocks in nevada to test seismic hazard models abstract fields of precariously balanced rocks indicate that strong earthquake motions have not occurred at that site since the precarious rocks developed these fields can be characterized with an estimate of the peak acceleration that would be sufficient to topple the rocks and an estimate of how long the rocks have been precarious this article uses this information to test the input to probabilistic seismic hazard analysis the fundamental assumption is that the probability of exceeding a ground motion capable of toppling a precarious rock during a time period equal to the age of the rock is equal to the confidence level at which the inputs to the probabilistic seismic hazard analysis can be rejected we performed a probabilistic seismic hazard analysis for 26 sites of observed precarious rocks in nevada using preliminary estimates of the toppling acceleration and the age of the features two standard models are rejected with over 95 confidence by most of the precarious rock observations we consider several possible explanations for the inconsistency a probabilistic seismic hazard analysis that eliminates the area sources and only includes faults is consistent with the precarious rock observations at most of the sites spatial clustering of small magnitude events could maintain this effect without contradicting the precarious rock observations however it may not be necessary to completely reject the area sources from our probabilistic seismic hazard analysis the physics of rock stability may allow increasing the minimum magnitude to 6 0 in the area sources because the short duration of high frequency accelerations in smaller events may not topple all precarious rocks the precarious rocks could generally have small site effects attenuation models may overestimate ground motions on the foot wall of normal faults and a different attenuation model in general might decrease the ground motion predictions the ergodic assumption that is made in estimating the uncertainty in attenuation models may be contributing to overestimate the hazard individually each of these effects might allow more of the precarious rock sites to be consistent with the area source zones

future trends in paleoseismology integrated study of the seismic landscape as a vital tool in seismic hazard analyses this paper forms the introduction to this special issue of tectonophysics devoted to selected scientific research presented during events sponsored by the inqua subcommission on paleoseismicity in the past few years in this note we summarize the contents of the contributed papers and use the issues they raise to review the state of the art in paleoseismology from a quaternary geology perspective in our opinion the evolution of paleoseismological studies in the past decade clearly demonstrates that in order to properly understand the seismic potential of a region and to assess the associated hazards broad based multidisciplinary studies are necessary to take full advantage from the geological evidence of past earthquakes a major challenge in future paleoseismic research is to build detailed empirical relations between various categories of coseismic effects in the natural environment and earthquake magnitude intensity these relations should be compiled in a way that is fully representative of the wide variety of natural environments on earth in terms of climatic settings quaternary tectonic evolution rheological parameters of the seismogenic crust and stress environment for instance available data indicate that between earthquake magnitude and surface faulting parameters different scaling laws exist and they are a function of the local geodynamic setting including style of faulting typical focal depths heat flow in this regard we discuss in some detail the concept of seismic landscape which provides the necessary background for developing paleoseismological research strategies the large amount of paleoseismological data collected in recent years shows that each earthquake source creates a signature on the geology and the geomorphology of an area that is unequivocally related with the order of magnitude of its earthquake potential this signature is defined as the seismic landscape of the area e g serva l vittori e ferreli l michetti a m 1997 geology and seismic hazard in grellet b mohammadioun b hays w eds proceedings of the second france united states workshop on earthquake hazard assessment in intraplate regions central and eastern united states and western europe october 16 1995 nice france 20 24 ouest editions nantes france michetti a m hancock p l 1997 paleoseismology understanding past earthquakes using quaternary geology

dating offset fans along the mojave section of the san andreas fault using cosmogenic 26al and 10be analysis of cosmogenic 1 0 be and 2 6 al in samples collected from exposed boulders n 20 and from buried sediment n 3 from offset fans along the san andreas fault near little rock california yielded ages ranging from 16 to 413 ka which increase with dis lance from their source at the mouth of little rock creek in order to determine the age of the relatively younger fans the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established cosmogenic nuclide inheritance values that range between 8 5 x 10 3 and 196 10 3 atoms 1 0 be g 1 quartz were determined by measuring the concentrations and ratios of 1 0 be and 2 6 al in boulders n 10 and fine sediment n 7 at the outlet of the present active stream boulder erosion rate ranging between 17 and 160 mm k y 1 was estimated by measuring 1 0 be and 2 6 al concentrations in nearby bedrock outcrops n 8 since the boulders on the fans represent the most resistant rocks in this environment we used the lowest rate for the age calculations monte carlo simulations were used to determine ages of 16 5 and 29 7 ka for the two younger fan surfaces older fans older than 100 ka were dated by analyzing 1 0 be and 2 6 al concentrations in buried sand samples the ages of the three oldest fans range between 227 242 and 413 185 ka although fan age determinations are accompanied by large uncertainties the results of this study show a clear trend of increasing fan ages with increasing distance from the source near little rock creek and provide a long term slip rate along this section of the san andreas fault slip rate along the mojave section of the san andreas fault for the past 413 k y can be determined in several ways the average slip rate calculated from the individual fan ages is 4 2 0 9 cm yr 1 a linear regression through the data points implies a slip rate of 3 7 1 0 cm yr 1 a most probable slip rate of 3 0 1 0 cm yr 1 is determined by using a 2 test these rates suggest that the average slip along the mojave section of the san andreas fault has been relatively constant over this time period the slip rate along the mojave section of the san andreas fault determined in this study agrees well with previous slip rate calculations for the quaternary

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

metamorphic fluids in the deep crust evidence from the adirondacks unusual mineral assemblages of tremolitic hornblende diopside enstatite quartz have been examined to estimate the temperature t and water fugacity at the time of the granulite fades metamorphism in the adirondacks the pyroxenes are closely represented by the system cao mgo sio2 allowing application of experimental data1 with only small corrections for reduced values of activities tremolite equilibria2 buffered log 3 0 0 1 equivalent to 0 9 kbar of water pressure at 7 kbar 710 c nearby marbles contain wollastonite calcite quartz and buffered 1 1 kbar consistent with 5 9 kbar inferred local gradients in fluid compositions suggest that deep crustal fluid conditions are complex and heterogeneous indicating that fluid movements whether of mantle3 or deep crustal origin are channelized rather than pervasive thus some granulite facies rocks were wholly or partly closed systems with respect to externally derived fluids permitting localized buffering of fluid compositions recognition of restricted fluid mixing limits theories that call for massive amounts of pervasive fluid flow to facilitate melting cause metasomatism or to stabilize granulite facies mineralogy

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

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

studies on precarious rocks in the epicentral area of the ad 1356 basle earthquake switzerland summary for the first time precarious rocks have been analysed in the epicentral area of the ad 1356 basle earthquake in northern switzerland several cliff sites in flat lying thickly bedded upper jurassic coral limestones in the jura mountains were investigated seven blocks are regarded as precarious with respect to earthquake strong ground motions the age of these precarious rocks could not be determined directly as for instance by radiometric dating methods however based on slope degradation processes it can be concluded that the formation of these blocks predates the ad 1356 basle earthquake the acceleration required to topple a precarious rock from its pedestal is estimated using geometrical data for individual block sections and earthquake strong motion records from stations on rock sites in the european strong motion database as input data for the computer program rocking v1 0 from the seismological laboratory university of nevada reno the calculations indicate that toppling of a precarious rock largely depends on earthquake strength but also on the frequency spectrum of the signal although most investigated precarious rocks are surprisingly stable for ground motions similar to those expected to have occurred during the ad 1356 basle earthquake at least two blocks are clearly precariously balanced with peak toppling accelerations lower than 0 3 g possible reasons why these blocks did not topple during the ad 1356 basle earthquake include incomplete separation from their base sliding of precarious rocks their size lower than assumed ground accelerations and or duration of shaking

rock varnish microlamination dating of late quaternary geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts in this study we establish a generalized late quaternary i e 0 300 ka varnish layering sequence for the drylands of western usa and tentatively correlate it with the specmap oxygen isotope record we then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late quaternary geomorphic features in the study region vml dating of alluvial fan deposits in death valley of eastern california indicates that during the mid to late pleistocene 5 15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation during the holocene interglacial time however 0 5 1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate vml dating of alluvial desert pavements in death valley and the mojave desert reveals that pavements can be developed rapidly

response of desert pavement to seismic shaking hector mine earthquake california 1999 1 the october 1999 mw 7 1 hector mine earthquake in the mojave desert california generated characteristic surface disturbances on nearby desert pavements these disturbances included 1 zones of wholesale gravel displacement interspersed with zones of intact pavement 2 displaced and rotated cobbles 3 moats around loosened embedded boulders 4 filling of abandoned cobble sockets boulder moats and other depressions with gravel and 5 formation of narrow subparallel linear strips of exposed fine grained subpavement matrix matrix lineations clasts displaced from matrix lineations and from cobble sockets tended to move downslope sharp boundaries of matrix lineations and slope controlled displacement directions on slopes of only a few degrees indicated that clasts remained close to the pavement surface during shaking the regular few decimeter spacing of matrix lineations suggests the presence of standing waves during seismic shaking boulder moats probably have good preservation potential and at some desert pavement locations might provide information on paleoseismic shaking although readily produced by coseismic shaking displaced cobbles are unreliable indicators of past earthquake activity because of potential multiple origins for an assumed earthquake recurrence interval of 10 ka seismically driven sediment fluxes similar to those generated by the hector mine earthquake at the lavic siding pavement study site may be marginally competitive with aseismic smoothing mechanisms driven by bioturbation rainbeat and wash for a 1 ka recurrence interval seismic smoothing is likely to play a significant role in pavement evolution

constraints on probabilistic seismic hazard models from unstable landform features in new zealand we undertake the first new zealand based pilot study to investigate the use of ancient precariously balanced rocks rocks that are unstably balanced on top of a pedestal as a criteria for testing estimates of earthquake shaking from probabilistic seismic hazard models for long return periods to date research to test seismic hazard models in new zealand has been restricted to the short historical record of earthquakes our survey of five sites in the south island of new zealand has yielded a total of 28 precariously balanced rocks which on the basis of established methodology are used to provide estimates of the maximum ground motions that could have occurred at the sites since the rocks became precarious age estimates for the precariously balanced rocks 40 400 to 55 300 years for central otago schist rocks and 77 300 years for northwest nelson granitic rocks are made from a limited number of cosmogenic dates obtained from bedrock removed from the pedestals of the rocks comparisons of the maximum peak ground accelerations and ages of the precariously balanced rocks with seismic hazard curves derived from the new zealand national seismic hazard model show that the rocks indicate considerably lower hazard than the seismic hazard model at sites located within 5 km of active faults whereas the agreement is favorable for sites located away from active faults the variability about the median estimates of peak ground acceleration for the fault sources and or the median accelerations for the fault sources assumed in the seismic hazard model may therefore be overestimated for the sites near active faults

precarious rock evidence for low ground shaking on the footwall of major normal faults preliminary interpretations of precarious rock observations suggest relatively low near fault 3 km footwall ground motions for normal faults relative to strike slip faults as reported for a dynamic foam rubber normal fault model by brune and anooshehpoor 1999 since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace precarious rocks may provide important data for estimating the ground motion for these earthquakes use of empirical curves in the literature based mainly on strike slip data may lead to overestimates of seismic hazard for the near fault footwall of normal faults these preliminary results may warrant more comprehensive and more quantitative followup studies

precarious rock evidence for low near source accelerations for trans tensional strike slip earthquakes abstract this paper describes precarious rock evidence for low ground motions associated with extensional sections of strike slip faults recent evidence from physical and numerical models and data regressions has indicated that ground motion for extensional strike slip regions may be lower than for strike slip faults with a large fault normal tectonic stress component and for thrust faults in general data from compressional strike slip and thrust earthquakes dominates the database used in most regression curves for ground acceleration and in the calculation of current probabilistic seismic hazard maps therefore estimates of ground accelerations on these seismic hazard maps may be too high for sites near extensional sections of strike slip faults this paper discusses precariously balanced rock data from three areas near extensional sections of strike slip faulting 1 the region of the honey fault california with an active holocene fault 2 the red rock canyon region of the garlock fault near a dilatational step over and 3 the region just south of beaumont california near the hemet dilatational step over in the san jacinto fault these are all active strike slip faults with at least a few large earthquakes in the holocene and in the case of the san jacinto example historic large earthquakes m 7 thus the precarious rocks at these sites are evidence of relatively low ground motions associated with extensional strike slip faulting the results of this study could be very important in developing more detailed seismic hazard maps in the future

precarious rock and overturned transformer evidence for ground shaking in the ms 7 7 kern county earthquake an analog for disastrous shaking from a major thrust fault in the los angeles basin precariously balanced rocks and overturned transformers in the vicinity of the white wolf fault provide constraints on ground motion during the 1952 m s 7 7 kern county earthquake a possible analog for an anticipated large earthquake in the los angeles basin shaw et al 2002 dolan et al 2003 on the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves on the other hand on the hanging wall there is evidence of intense ground shattering and lack of precarious rocks consistent with the intense hanging wall accelerations suggested by foam rubber modeling numerical modeling and observations from previous thrust fault earthquakes there is clear evidence of the effects of rupture directivity in ground motions on the hanging wall side of the fault from both precarious rocks and numerical simulations on the southwest part of the fault which is covered by sediments the thrust fault did not reach the surface blind thrust overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall an effect that may not be as effective when the rupture reaches the surface is not blind transformers near the up dip projection of the fault tip have been damaged or overturned on both the hanging wall and footwall sides of the fault the transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in los angeles we suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the los angeles basin specially given the fact that there is so little instrumental data from large thrust fault earthquakes

fractional changes in elemental mass ω and relative variation curves unravelling elemental mass redistribution patterns within the regolith abstract within the arid climate of southern california usa we uncovered through trenching a granodioritic tonalitic corestone a partially encircling fractured rind and an overlying ah2 horizon composed of sandy soil using the geometric mean of chemical data derived from an adjacent erosionally exhumed corestone as a proxy for unweathered bedrock the compositional linear trend derived from weathering was calculated average weathering intensities t values and 95 confidence intervals are as follows uncovered corestone 0 56 0 13 inner fractured rind 0 93 0 08 outer fractured rind 1 5 0 05 and ah2 1 7 0 09 ti was used as a reference frame for calculating the modeled fractional change in a given elements mass resulting relative variation curves plotted on 10 bivariate graphs where the x axis represents t values and the y axis is show that with the exception of p 2 o 5 samples from each of the above textural groups cluster and plot closely about the 10 relative variation curves the clustering and ordering of samples analyzed from the uncovered corestone inner rind outer rind and ah2 horizon about the relative variation curves suggests that weathering of the section of regolith uncovered by trenching occurred in an orderly and sequential manner as the result of increasing ph and decreasing surface area for fluid rock reactions mass balance calculations are consistent with the modeled relative variation curves and the weathering of plagioclase to smectite and kaolinite and biotite and chlorite to vermiculite

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

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

holocene rock varnish microstratigraphy and its chronometric application in the drylands of western usa abstract analyses of hundreds of rock varnish samples from latest pleistocene and holocene geomorphic features in the drylands of western usa reveal a regionally replicable holocene microlamination sequence this sequence consists of 12 approximately evenly spaced weak dark layers intercalated with 13 orange yellow layers preliminary radiometric age calibration indicates that six dark layers in the upper portion of the sequence were deposited during the last 6000 yr diagnostic of the little ice age and late holocene wet events five dark layers in the lower portion of the sequence were deposited after the termination of the pleistocene but slightly before 7000 ybp indicative of the early holocene wet events and one dark layer in the middle portion of the sequence was deposited around 6500 ybp suggestive of the middle holocene wet phase our age calibration further indicates that the holocene wet events represented by the dark layers largely correlate in time with the millennial scale holocene cooling events in the north atlantic region this radiometrically calibrated and climatically correlated holocene microlamination sequence was then used as a unique correlative dating tool to determine surface exposure ages of geomorphic and geoarchaeological features in western usa deserts the varnish microlamination vml dating of debris flow fan deposits in death valley california yields minimum ages of 12 500 12 500 11 100 11 100 10 300 9400 and 2800 ybp for six debris flow fan building events suggesting that such events were more likely to have occurred during relatively wet periods of the holocene the vml dating of a prehistoric grinding stone from chili of northern new mexico yields a minimum age of 900 1100 ybp for the abandonment of this occupation site by the anasazi indians the vml dating of a prehistoric flaked stone a primary core from ocotillo southern california yields a minimum age of 12 500 ybp for the flaking of this stone artifact suggesting at least a paleo indian human occupation at ocotillo during the terminal pleistocene these results indicate that when properly applied the vml dating technique has the great potential to yield numerical age assignments for surface stone tools petroglyphs and geoglyphs of prehistoric age in the drylands of western usa

blind testing of rock varnish microstratigraphy as a chronometric indicator results on late quaternary lava flows in the mojave desert california rock varnish is a manganiferous dark coating ubiquitous in desert landscapes to test the validity of varnish microstratigraphy as a chronometric indicator varnish samples were collected from radiometrically dated and undated late quaternary lava flows in amboy cima and pisgah volcanic fields avf cvf pvf in the mojave desert of california western united states varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the younger dryas and heinrich events in the north atlantic region microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western us sites with available radiometric age constraints based on this regional chronology varnishes from the a flow h flow and a stone pavement surface in the cima volcanic field were estimated to be 16 5 24 74 85 and 74 85 ka respectively these ages are consistent with previously published cosmogenic 3 he ages of 18 20 72 74 and 80 85 ka for these geomorphic surfaces varnishes from the i flow at cima yielded a puzzling age estimate of 39 ka which is consistent with an older 3 he age of 37f6 ka reported for the i flow but inconsistent with a younger 3 he age of 31f7 ka and a cosmogenic 36 cl age of 27f1 3 ka for the same flow reinterpretation of the original varnish age data with knowledge of then available field mapping results of the i flow suggests that the i cone is polycyclic and different flow units were probably unintentionally sampled in the field the revised varnish ages of 30 and 39 ka for the i flow thus may be in good agreement with their corresponding 3 he and 36 cl ages in a blind test of the method varnishes from the phase 1 flow at pisgah an unnamed flow called here the ivflow at cima and the amboy flow were estimated to be 24 30 46 60 and 74 85 ka respectively these ages agree well with 36 cl ages of 22 5f1 3 46f2 and 79f5 ka reported for the same flows by phillips geomorphology 2002 these test results provide convincing evidence that varnish microstratigraphy once radiometrically calibrated can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western us drylands d 2002 elsevier science b v all rights reserved

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

age of unstable bedrock landforms southwest of yucca mountain nevada and implications for past ground motions we determine minimum exposure ages for unstable outcrops at three sites in amargosa desert southwestern nevada including a site at the southern end of yucca mountain varnish microlamination dating techniques provide minimum exposure ages of 12 5 36 k y for the unstable outcrops of welded tuff including a 24 k y age for the south yucca mountain site the youngest exposure age 12 5 k y is found at the site located only 10 km from the death valley furnace creek fault suggesting outcrops may be more frequently modified when close to major active earthquake sources a simplistic stability assessment of the south yucca mountain outcrops suggests peak ground accelerations pgas may not have exceeded about 1 g uncertainty bounds 0 5 2 g in at least 24 k y a pga of 1 g is consistent with the predicted 24 k y return period pgas from the near decade old yucca mountain probabilistic seismic hazard psh model except for the ninety fifth percentile and above we gain confidence in our interpretations by additionally observing 1 minimal damage to the south yucca mountain outcrops from a recent moderate earthquake that is estimated to have produced a pga of less than 0 1 g there i e motions less than 0 5 g do not significantly damage the outcrops and 2 severe damage to similar volcanic outcrops associated with pgas of the order 0 5 1 g near a nuclear blast site from the 1960s these observations support our suggestion that pgas greater than 0 5 1 g have not occurred at the south yucca mountain site for a time period of at least 24 k y significant seismic events that substantially modify the outcrops and produce associated rubble fields may therefore occur on longer time scales

millennial scale varnish microlamination dating of late pleistocene geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a climate based correlative age determination technique used to correlate and date various geomorphic features in deserts in this study we establish a generalized late pleistocene 18 74 ka millennial scale microlamination sequence in fine grained fast accumulating rock varnish for the drylands of western usa radiometrically calibrate the sequence and correlate it with the 18 o record in the gisp2 greenland ice core we then use this climate correlated varnish microstratigraphy to estimate surface exposure ages for radiometrically dated late pleistocene geomorphic features in the study region the vml dating of debris flow deposits on the sehoo recessional shorelines of lake lahontan at the jessup embayment of central nevada yields a minimum limiting age of 14 95 15 95 ka in good agreement with a calibrated 14 c age of 15 22 0 12 ka for the timing of the lake recession the vml dating of a giant ejecta block on the rim of meteor crater in northern arizona yields a minimum limiting age of 49 15 ka closely matching a thermoluminescence tl age of 49 3 ka and slightly younger than a recently updated cosmogenic 36 cl age of 56 0 2 4 ka for the meteor impact event the vml dating of distal q2c fan surfaces on hanaupah canyon alluvial fan in death valley california yields a minimum limiting age of 73 55 ka in accord with cosmogenic 36 cl depth profile ages of 66 22 14 ka and 72 24 20 ka for the same fan deposits the close agreement between the vml age estimates and the independently derived radiometric ages for these geomorphic features attests to the validity and reliability of millennial scale vml dating to further assess its potential in desert geomorphological research we use the vml method to study alluvial fan responses to millennial scale climatic changes the vml dating of a small tributary fan in death valley reveals two episodes of fan aggradation one ceasing at 73 55 86 75 ka during the dry period of the last interglacial mis 5a and the other finishing at 66 15 ka during the wet period of the last glacial mis 4 the vml and 36 cl dating of the distal q2c fan surfaces on hanaupah canyon fan reveal two episodes of large scale fan aggradation ended at 72 24 20 ka and 73 55 ka during the wet period of mis 4 fanhead incision and associated within channel or fantoe aggradation are found to take place during the relatively dry period of the glacial to interglacial climatic transition 12 24 ka and the holocene interglacial dry period 0 12 ka these data indicate that on the millennial to sub milankovitch timescale 10 3 10 4 years fan aggradation is a discrete sedimentational process under various climatic conditions because fan aggradation is ultimately controlled by the intensity and frequency of precipitation events which in turn are modulated by major climatic oscillations such as heinrich events dansgaard oeschger do events and glacial interglacial shifts these major climatic changes could be the pacemaker of regionally contemporaneous large area fan segmentation