relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

the time averaged paleomagnetic field we review indications of persistent deviations from the geocentric axial dipole model of the time averaged geomagnetic field and present a zonal harmonic model derived from 185 deep sea sediment piston cores taken from low to middle latitudes to approximately 45 analysis of the paleomagnetic inclination recorded in these cores for the brunhes normal polarity 0 73 ma and matuyama reverse polarity 0 73 2 47 ma chrons after plate motion correction gives well constrained estimates of the dominant long term nondipole contributions the axial quadrupole and axial octupole and shows no significant deviation from axial symmetry the amplitude of the axial quadrupole is found to vary with polarity 2 6 of the geocentric axial dipole for normal 4 6 for reverse while the axial octupole does not show appreciable change 2 9 for normal 2 1 for reverse these estimates of the quadrupole contribution agree well with prior determinations for the plio pleistocene 0 5 ma however the octupole contribution we find is opposite in sign to previous estimates we suggest that a negative octupole is representative of the actual time averaged paleomagnetic field while prior positive octupole estimates probably reflect spurious inclination shallowing the lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

a detailed record of paleomagnetic field change from searles lake california 2 the gauss matuyama polarity reversal this new study of the gauss matuyama transition from searles lake california conjoined with other records from the western united states provides interesting insights into the structure of the reversing magnetic field the present study employs improved measurement and data reduction techniques multiple parallel strings of samples and a finer sampling interval than was used in the original study by liddicoat 1982 particularly crucial to this investigation was the use of overprint directions to reconstruct declinations required because the core was rotary drilled the results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric the fabric facilitated the alignment of core segments the new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study in addition it brings out several small scale variations that were absent in the old record some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes this alternating steady and rapid field change is similar to behavior observed in volcanic records which argues that such behavior is not merely an artifact of episodic volcanism the searles lake record is strongly nonzonal and is defined in the northern hemisphere by a swath of virtual geomagnetic poles vgps stretching from northern eurasia to west africa and to the northwest pacific glen et al 1994 showed that a collection spanning 15 myr of western north american transition and excursion records displays this same pattern indicating that the vgp swath is a persistent feature of the transitional field in addition the compilation reveals that the swath extends into the southern hemisphere outlining a region marked by an absence of poles that is centered on the indian ocean the fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component seven additional records are now available making the western north america data set perhaps the finest regional set of high resolution records consisting of both igneous and sedimentary records the new records which provide an important test of the existence of the vgp pattern strongly support the findings that reveal the presence of persistent long term 15 myr nondipolar transitional fields

palaeomagnetic excursions recorded in mid weichselian cave sediments from skjonghelleren valderoy w norway abstract anomalous single component palaeomagnetic directions in laminated clay silt sediments in the non karstic skjonghelleren cave w norway are interpreted to record two high resolution but incomplete records of geomagnetic field excursions a depositional non distorted origin of magnetization carried by ultra fine magnetite maghemite grains has been inferred from field evidence redeposition experiments and magnetic fabric properties the sediments deposited sub glacially between 56 ka to 12 ka determined by consistent isotope ages obtained from bones and travertine the youngest excursion

the blake geomagnetic polarity episode recorded in chinese loess a geomagnetic anomaly consisting of three clearly defined periods of reverse polarity separated by two short intervals of normal polarity has been documented in a loess section near xining western china on the basis of previous studies of chinese loess stratigraphy we correlate this geomagnetic polarity episode with the blake event while more research is needed to improve our understanding of the exact process of remanence acquisition of loess deposits this study shows that loess may provide records of fast geomagnetic events and lends further support to the global nature of the blake geomagnetic event

evidence for a geomagnetic excursion recorded in the sediments of lac st front france a link with the laschamp excursion anomalous paleomagnetic directions accompanied by low intensities of natural remanent magnetization are recorded at 17 m depth in the sediments cored from lac st front massif central france the rock magnetic properties and the parameters characterizing anisotropy of susceptibility of samples taken from the sediment horizons carrying these anomalous directions spanning some 30 40 cm are identical to those of samples taken from the other stratigraphic intervals carrying a record of normal geomagnetic secular variation therefore the sequence of anomalous directions is interpreted as a geomagnetic excursion furthermore it is concluded that this excursion occurred at a time when the paleointensity field was low the excursion is dated at about 33 14c 37 ka b p by correlating the susceptibility and paleomagnetic logs from lac st front with the corresponding logs from the nearby lac du bouchet sedimentary sequence the excursional directions in lac st front can be averaged out with a triangular window of 20 cm these smoothed excursional directions resemble the directional variations observed in the corresponding lac du bouchet interval this finding indicates that the absence of this excursion in lac du bouchet may be due to a larger smoothing of the geomagnetic signal

seismic structure of the central us crust and shallow upper mantle uniqueness of the reelfoot rift abstract using seismic surface waves recorded with earthscope s transportable array we apply surface wave imaging to determine 3d seismic velocity in the crust and uppermost mantle our images span several proterozoic and early cambrian rift zones mid continent rift rough creek graben rome trough birmingham trough southern oklahoma aulacogen and reelfoot rift while ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences the reelfoot rift is unique in its association with low mantle seismic velocity its mantle low velocity zone lvz is exceptionally pronounced and extends down to at least 200 km depth this lvz is of variable width being relatively narrow 50 km wide within the northern reelfoot rift which hosts the new madrid seismic zone nmsz we hypothesize that this mantle volume is weaker than its surroundings and that the reelfoot rift consequently has relatively low elastic plate thickness which would tend to concentrate tectonic stress within this zone no other intraplate ancient rift zone is known to be associated with such a deep mantle low velocity anomaly which suggests that the nmsz is more susceptible to external stress perturbations than other ancient rift zones

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

age and correlation of a paleomagnetic episode in the western united states by 40ar 39ar dating and tephrochronology the jamaica blake or a new polarity episode high resolution paleomagnetic records from two sites near pringle falls oregon are compared with similar records from summer lake oregon 170 km to the southeast paoha island in mono lake 660 km to the southeast and benton crossing in long valley approximately 700 km to the southeast in east central california the sequences at pringle falls contain a distinctive coarse pumice lapilli tephra layer which we have dated as 218 10 ka by 40ar 39ar step heating of plagioclase feldspar stratigraphically this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards the pumice layer dated at pringle falls is correlated with layers at three of the other localities using all the tephra layers we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities additional age control is obtained from a fifth locality at tulelake in northern california where the stratigraphic interval of interest is bracketed between 171 43 and approximately 140 ka characteristics of the paleomagnetic records indicate virtually identical paleofield variation particularly the geometry of a normal to normal n n geomagnetic polarity episode the observed paleofield behavior resembles the blake geomagnetic polarity episode but is significantly older than the generally accepted age of the blake episode either the age of the blake episode is significantly underestimated or the polarity episode documented here is older perhaps the jamaica episode or is an as yet unreported episode a corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths a conclusion implying that a common mechanism is involved

paleohydrologic regimes in the southwestern great basin 0 3 2 my ago compared with other long records of gobal climate abstract nine distinct paleohydrologic regimes in the southwestern great basin over the last 3 2 my are recorded by the lacustrine deposits in km 3 a 930 m core from searles lake california these are characterized as being wet intermediate or dry like today excepting the present incomplete regime each lasted 0 12 to 0 76 my major regime changes 0 01 0 13 0 6 and 2 5 my ago appear to coincide with recognized changes in global ice sheet histories as represented by 18 o and other records from marine sediments but comparable changes 0 3 1 0 1 3 and 2 0 my ago do not appear to coincide closely with comparable perturbations in ice sheet histories however all regime boundaries during the last 1 75 my coincide closely in time with changes in sea surface temperatures in the tropical atlantic and many coincide with other deep sea and continental paleoclimatic boundaries the average duration of these paleohydrologic regimes was about 0 4 my standard deviation 0 2 my or less depending on assumptions and it is suggested that the regime boundaries reflect times of change in global sea surface temperatures possibly controlled in part by the earth s 413 000 yr orbital eccentricity cycle during the wettest and driest regimes in the searles lake area lake levels were not sufficiently affected by the 23 000 to 42 000 or 100 000 yr climate cycles related to high latitude ice sheet fluctuations to produce changes in the lacustrine sediment character during intermediate regimes however when lacustrine sedimentation in this area was more sensitive to climate the sediments in km 3 record lake fluctuations with average frequencies near those of the ice sheets this seems to indicate that the high latitude ice sheet fluctuations caused local climatic perturbations but did not dominate the hydrologic component of climate in this area other lacustrine deposits in the southwestern great basin of california and nevada have ages comparable in part to those of the wet to intermediate regimes indicated by km 3 and they may all be products of finite periods when lake expansion alluvial fan growth increased spring discharge and fluvial deposition were promoted in this area by widespread wet climates glacier expansion in the sierra nevada may also have been primarily an expression of and in phase with these wet regimes

a 36cl chronology of lacustrine sedimentation in the pleistocene owens river system we have used 36 cl to date climatically sensitive saline lake sediments from hydrologically closed basins in southeastern california primarily sediments from searles and panamint basins during wet periods of the past 2 0 m y lakes that formed in the closed basins fluctuated in size in direct response to the balance between runoff and evaporation we have compiled a chronology for the km 3 core from searles lake based on ages determined by 36 cl for the evaporites together with those determined by 14 c and u th series for younger sediments and by magnetostratigraphy for older sediments this chronology along with other criteria such as correlations between searles and panamint basins the chloride budget and sedimentology is used to reconstruct the history of lake fluctuations in the paleo owens river system we infer that searles lake desiccated at most twice since 600 ka during the interval around 290 ka and from 10 ka to the present the lake history curve shows that the holocene epoch is anomalously arid major overflows from searles to panamint occurred during the intervals between 1 3 ma and 1 0 ma 750 ka and 600 ka 500 ka and 400 ka and 150 ka and 120 ka comparing the lake fluctuation chronology to the 18 o record of marine foraminifera we infer that the strongest similarity is in the periodicities of the cycles 40 to 50 kyr before the matuyama brunhes magnetic reversal 730 ka and 100 kyr thereafter we find however that at searles lake this fluctuation in the lake chronology is modulated by longer term cycles of aridity and humidity thus although the mid latitude quaternary climate record reflects the mid to high latitude ice volume fluctuations that dominate the marine 18 o record it also contains evidence for climatic forcing of a different type

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

compaction induced inclination shallowing of the post depositional remanent magnetization in a synthetic sediment summary a synthetic sediment comprised of kaolinite distilled water and either equidimensional or acicular magnetite was given a post depositional remanent magnetization pdrm by being stirred in a magnetic field this sediment was compacted under pressures which varied continuously from 0 to 0 14 mpa in a water tank consolidometer and to higher pressure steps 2 53 mpa in a standard soil consolidometer compaction took place in the same magnetic field in which the sample was given its pdrm the compaction caused shallowing of the sample s magnetic inclination this shallowing was found to be a function of the sample s initial magnetic inclination and the degree of sample compaction tan i 1 aav tan lo where ir is the remanent inclination after compaction av is the volume change i is the initial magnetic inclination and a is an empirically derived constant the data show a maximum inclination shallowing of 12 for an initial inclination of 54 in good agreement with the maximum inclination shallowing predicted by the above equation we propose an electrostatic mechanism to be the cause of the inclination shallowing in this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain s surface as the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post compaction nrms implying that the smaller higher coercivity grains are most affected by the compaction process these data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected

chronology and correlation of quaternary magnetostratigraphy and nannofossil biostratigraphy in norwegian greenland sea sediments paleomagnetic analyses of a series of sediment cores from the norwegian greenland sea document the existence of seven short lived geomagnetic events during the last about 500 000 years calcareous nannofossil biostratigraphy correlated to oxygen isotope chronostratigraphy provides a time framework to infer age and duration of these periods of extreme divergence of the earth s magnetic paleofield from the regular normal polarity configuration of the brunhes chron the geomagnetic events typically extend over 5 to 10 kyr two events are observed within the oxygen isotope stage 2 4 interval a short event is recorded at the stage 4 5 boundary and another just below the stage 5 6 boundary additional three geomagnetic events are identified in oxygen isotope stage 6 at the stage 9 10 boundary and near the top of oxygen isotope stage 13 respectively

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

high resolution palaeomagnetic study of late pleistocene sediments from baffin bay first results the detailed palaeomagnetic study of core 85027016 collected in central baffin bay confirms the occurrence of anomalous directions in the depositional remanent magnetization previously observed in the upper part of the pleistocene sequence chronology and sedimentation rates are discussed on the basis of new 14c ages 18o isotopic stratigraphy and palaeoclimatic arguments provided by other works two successions of anomalous inclinations are recorded between 15 000 and 30 000 years bp preceded by long period secular variations the possible occurrence of geomagnetic excursions in this region is discussed in considering contemporaneous high resolution palaeomagnetic records of the northern hemisphere

a detailed record of paleomagnetic field change from searles lake california 1 long term secular variation bounding the gauss matuyama polarity reversal more than 33 m of 2 5 ma sediment from searles lake california was studied in order to construct a record of secular variation sv across the gauss matuyama g m normal to reverse polarity transition the behavior of the field preceding and following the reversal is considered here while in a companion paper glen et al this issue the details of the transition are discussed the record encompasses an interval of roughly 183 000 years beginning 50 kyr 9 m before and extending more than 128 kyr 23 m beyond the transition while the main phase of the transition lasts for nearly 5 kyr 1 m because the core was rotary drilled and declinations lost sv was characterized by the inclination and its angular dispersion inclination only statistics reveal that 1 the record displays overall higher than expected values of angular dispersion normal s 20 reverse s 19 expected s 15 5 suggesting that the field proximal to transitions may be more noisy than the distal field in addition normal data from immediately before the transition display higher s than reverse data immediately following it implying that the postransitional field is more stable than the pretransitional field one of the most prominent features of this record is an excursion of the field occurring roughly 4 kyr prior to the onset of the reversal a record of the g m transition from chinese loess r zhu et al submitted manuscript 1999 displays a similar event also occurring roughly 4 kyr before the transition this and the fact that the event is associated with anomalously low intensities suggest that the disturbance may be global in nature the fact that comparable features are associated with other transitions hartl and tauxe 1996 clement 1992 intimates that the field may commonly show signs of early instability this precursory event is actually one of a sequence of oscillations in inclination and intensity preceding the transition that these fluctuations occur at roughly 4 kyr intervals leading up to the reversal which also appears at this same interval strongly suggests that an oscillatory disturbance in the core active over at least 15 kyr prior to the transition had eventually triggered the reversal in addition that these waveforms are absent from the postransitional record suggests the reversal process actively rejuvenates and stabilizes the field

inclination shallowing in deep sea sediments from the north atlantic paleomagnetic data from miocene to recent deep sea sediments recovered during leg 94 of the deep sea drilling project dsdp in the north atlantic exhibit a systematic shallowing of inclinations with depth at some sites this shallowing is coincident with a downhole decrease in water content and is observed only in sediments with a carbonate content consistently greater than 80 compaction due to overburden pressure is believed to be responsible for the shallow inclinations observed in these unconsolidated sediments at sites where the carbonate content of the sediments is consistently below about 80 no significant downcore decreasing trends in the water content or inclination record are observed

effects of clay mineralogy and the electrical conductivity of water on the acquisition of depositional remanent magnetization in sediments new experiments on the interrelationship of clay minerals solution properties and postdepositional remanent magnetization pdrm provide interesting insights into pdrm acquisition we have found that clay mineral content composition and solution conductivity affect pdrm strongly with increasing clay concentration or electrolyte concentration pdrm intensity decreases clay minerals may also be responsible for the inclination shallowing of pdrm different clay minerals behave differently for example montmorillonite diminishes pdrm intensity more rapidly than kaolin anson and kodama 1987 have suggested an electrostatic model to explain their results of a similar study however we cannot explain all of our results by using their electrostatic model to us it seems that colloid coagulation is the key to explaining the results two kinds of forces exist in a colloidal solution electric repulsion and van der waals attraction in low electrolyte concentration solutions repulsion is larger than attraction strong brownian motion is therefore required to enable the particles to cross over the repulsive energy barrier in order to stick to each other but in high electrolyte concentration solutions repulsion becomes relatively small and particles stick together much more easily thus the effective mass of the grain becomes larger clay magnetite but the magnetic moment remains the same and hence the magnetic torque is relatively less effective as a result the magnetic carriers do not align effectively in the direction of the ambient field making the pdrm intensity weaker

experimental and theoretical studies of the stabilities of talc antigorite and phase a at high pressures with applications to subduction processes we have experimentally determined the equilibrium talc enstatite quartz coesite h2o to 40 kbar in the system mgosio2h2o msh using both synthetic and nearly pure mg end member natural talc and other synthetic starting materials for the other solid phases at 40 kbar the equilibrium dehydration boundary lies 150 c higher than that calculated using data from the existing internally consistent thermochemical data bases the reason for this discrepancy lies in the erroneous compressibility data of talc in the data bases we have retrieved the compressibility of talc from the experimental phase equilibrium data and have also calculated sereral other equilibria in the msh system involving talc antigorite and the dense hydrous magnesium silicate dhms commonly referred to as phase a comparison of these equilibria with selected thermal profiles at the leading edge of young and old subducting oceanic slabs along with the dehydration condition of basaltic amphibole and solidus of mantle peridotite provides an explanation for the observed heights of the volcanic fronts above subducting oceanic lithosphere further it is found that in cold oceanic slabs 50 ma with subduction velocity of 10 cm y antigorite will transform to the dhms phase a through a vapor conserved reaction at a depth of 200 km phase a will then serve as a carrier of water into the deeper mantle

decay rate dependence of anhysteretic remanence fundamental origin and paleomagnetic applications 1 we have measured the intensity of anhysteretic remanent magnetization arm as a function of alternating field af decay rate for synthetic and natural single domain sd and pseudo single domain psd magnetites arm intensity increases as decay rate decreases multidomain md magnetites have the opposite response arm increasing as the decay rate increases these are identical to the sd psd and md dependences of thermoremanent magnetization on cooling rate for all grain sizes and domain structures arm intensity increases as the af decay rate used to achieve an initial demagnetized state decreases decay rate differences in arm intensity are a property of low and medium coercivity grains as shown by annealing and by stepwise af demagnetizing samples we interpret the sd results to mean that increased af exposure time permits a closer approach to equilibrium magnetization an approximate thermal activation theory based on neel 1949 and an exact theory by egli and lowrie 2002 predict 6 11 increases in arm for an order of magnitude decrease in decay rate in reasonable accord with the observed 12 increase for 0 065 m sd grains for md grains we hypothesize that increased exposure time slower decay permits more efficient self demagnetization reducing arm low coercivity grains experience the largest self demagnetizing fields and therefore have the largest decay rate response initial state decay rate response is attributed to longer exposure times leaving domain walls more strongly pinned in deeper potential wells the net self demagnetizing field is zero in the demagnetized state acquisition decay rate annealing and initial state responses of psd grains are a blend of sd and md responses because arm is the most frequently used normalizer in relative paleointensity determination it is important either to use a standard decay rate or else to remove the decay rate dependence by demagnetizing the arm to 30 of its initial value arm0 3 a standard demagnetization level for the normalizing arm is particularly important when comparing paleointensity records from different laboratories

research note mono lake excursion in the lahontan basin nevada summary in the lahontan basin of north western nevada the mono lake excursion is recorded in a wave cut bank at pelican point on the west side of pyramid lake the record is almost identical to one across the lake near pyramid island 10 km away and in the carson sink 60 km further to the east at each locality the older half of the excursion is present only as a reduction in the relative field intensity while the younger half shows the expected easterly declination and steep positive inclination that result in a small counterclockwise loop of the virtual geomagnetic poles the younger half occurs during high relative field intensity and the entire intensity pattern is similar to the one for the excursion in the mono basin california 250 km to the south the presence of only part of the mono lake excursion in the lahontan basin might have resulted from the slow sedimentation rate in pyramid lake estimated to be about 12 cm kyr 1 with the field behaviour being more extensively integrated in the subsamples used in an alternate explanation the absence of the older part of the excursion can be attributed to overprinting when magnetic grains that were free to rotate in unconsolidated sediment were realigned during the high relative field intensity in the younger part of the excursion

record of the younger part of the pringle falls excursion at long valley california summary the younger of two closely spaced palaeomagnetic excursions at pringle falls oregon is recorded in lacustrine silts that crop out in long valley california assigned an age of about 220 000 years the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 s latitude east of south america before returning to the northern hemisphere in the pacific ocean west of central america the poles then form a narrow band across north america while moving to high northern latitudes this record matches extremely well feature b of the original excursion record from pringle falls reported by herrero bervera et al 1994 and is similar to this excursion at summer lake oregon negrini et al 1994 in that the pole path is confined primarily to the east central pacific ocean on the basis of an assumed sedimentation rate of 30 cm per thousand years the younger excursion feature b at pringle falls spans an estimated 1200 years and followed by about 1000 years a larger excursion feature a at pringle falls previously discovered at the same long valley site at a second long valley site 30 m away the younger excursion feature b is only partially recorded because of a presumed small hiatus in the sedimentary section

low temperature magnetic properties of siderite and magnetite in marine sediments low temperature magnetic techniques provide useful tools to detect the presence of magnetite and pyrrhotite in sediments through identification of their low temperature transitions to determine the amount of ultrafine grained superparamagnetic material in sediments and can potentially detect the presence of certain types of magnetotactic bacteria application of these types of experiments to nannofossil chalks from beneath the barbados accretionary prism led to some unusual results which are attributed to the presence of siderite thermal demagnetization of low temperature remanence after cooling in zero field and in a 2 5 t field both displayed large remanence losses from 20 k to 40 k below 40 k the magnetization of the chalks was much higher in the field cooled experiments than in the zero field cooled experiments low temperature hysteresis experiments made after cooling in a 2 5 t field displayed offsets in magnetization parallel to the direction of the initial applied field when measured below 40 k the offset loops can be due to either an exchange anisotropy between siderite and magnetite phases in the sediments a defect moment in the siderites or a canted moment in the siderites apparent similarity between the low temperature thermal demagnetization results from these siderite bearing sediments pure siderite and pure rhodochrosite samples and the well known 34 k transition in pyrrhotite should lead to caution in identification of pyrrhotite in marine sediments based on low temperature remanence studies alone

a comparison of holocene paleomagnetic secular variation records from north america the comparison of nine holocene 0 12 000 years b p records of paleomagnetic field secular variation psv obtained from across north america indicates that distinctive field features in inclination and declination often can be traced for more than 4000 km without significant change in pattern independent age determinations corroborate these correlations while also suggesting that westward or eastward drift is not significant on the other hand northward drift may be a significant process at least between 1500 and 5500 years b p several methods of analyzing the overall psv character point to circularity vector looping as a preferred means of defining coherent space time vector variations which are termed waveforms a few distinctive waveforms each discernible for about 1400 years dominate psv across north america during the holocene 4 5 loops and 2 loops being associated with clockwise and counterclockwise circularity respectively the clockwise loops appear to be periodic occurring every 2400 years and most likely result from the recurrence of a distinctive regional dynamo source in the outer core the pattern of circularity noted in north america during the last 3000 years correlates in time with similar circularity changes seen globally this may indicate that there is some degree of coupling between different regional dynamo sources within the outer core the fact that some north american waveforms are almost identical to waveforms observed elsewhere around the world at different times may indicate that similar distinctive regional dynamo sources recur randomly throughout the outer core

the blake magnetic polarity episode in cores from the mediterranean sea abstract palaeomagnetic and oxygen isotope studies on samples taken from five cores in the mediterranean area are described directions of the stable remanent magnetisation show the presence of an anomalous reversed zone during the transition between the oxygen isotope substages 5e and 5d this position corresponds to the age of about 117 kyr b p and thus to the blake polarity episode the duration of this episode is estimated to be about 6000 years the study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections the blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

inclination anomalies from indian ocean sediments and the possibility of a standing nondipole field we report magnetic inclinations measured in deep sea sediments of the equatorial indian ocean which record the behavior of a nondipole component of the time averaged geomagnetic field during the plio pleistocene 0 5 ma the magnitude of the nondipole effect recorded in these sediments appears to depend on polarity state with inclinations showing departures from geocentric axial dipole directions which are small 2 o during normal polarity and larger 5 o during reverse polarity times the overall nondipole effect observed here is consistent with prior spherical harmonic estimates of the paleomagnetic field the polarity bias found agrees in both sense and magnitude with earlier reports of polarity asymmetry in the low degree zonal harmonic fields the presence of this asymmetry supports previous suggestions of the existence of a standing component of the nondipole field which does not invert during reversals of the main field we explore whether the standing field so indicated may have influenced paleomagnetic directions recorded during polarity transitions at other equatorial sites

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

extension of the lac du bouchet palaeomagnetic record over the last 120 000 years a high resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the lac du bouchet sedimentary sequence has been reconstructed from several parallel mackereth and livingstone type cores inter core correlations were established by comparing individual core profiles of magnetic susceptibility individual records of af cleaned directions were then stretched to a common depth scale and merged the mean record was then filtered and smoothed in order to reduce the background noise accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial glacial cycle back to ca 120 kyr b p and allow to perform a depth to time transformation there is a noted absence of widely divergent directions back to 90 kyr b p it is estimated that the recorded signal a pdrm signal represents the geomagnetic input signal attenuated by about 19 it follows that the excursions reported for the laschamp and olby lavas if they existed at all must have been of rather short duration two durable excursions appear in narrow diameter sections at the base of three independent cores b c and d while a reverse to normal transition is possibly documented in the lowermost meter core d this part of the sequence is dated from the recognition of major climatic events related to the 18 o isotopic stages defined in the oceanic record between 116 and 95 kyr this introduces the possibility of new excursions at ca 104 and 95 kyr i e immediately after the recovery of normal polarity documenting the end of the blake event

high resolution magnetostratigraphy of four sediment cores from the greenland sea i identification of the mono lake excursion laschamp and biwa i jamaica geomagnetic polarity events summary high resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount vesteris banken in the greenland basin and one core from the jan mayen fracture zone revealed records of three pronounced geomagnetic events within the last 200 ka dating by stable carbon and oxygen isotope analysis ams14c measurements and biostratigraphic data foraminifera abundances yielded ages of 28 27 ka for the mono lake excursion 37 33 ka for the laschamp event and 189179 ka for the biwa i event in at least one of the cores the laschamp event exhibits a full reversal of the local geomagnetic field vector the same is true of the biwa i event documented in one of the cores

paleointensity of the earth s magnetic field and k ar dating of the louchadiere volcanic flow central france new evidence for the laschamp excursion we report paleointensity results of the earth s magnetic field from a late pleistocene lava flow louchadiere central france which recorded an intermediate geomagnetic field direction 5 sites mean declination 114 1 inclination 58 2 k 130 95 6 7 new k ar age determinations confirm that this flow is contemporaneous with the laschamp and olby flows and that this excursion occurred around 45ka ago using the thellier double heating method reliable paleointensities have been obtained for ten samples from three different sites providing an average field strength of 12 9 t 3 3 this low value and previous results of the laschamp excursion from france and iceland confirm that the earth s magnetic field was in an intermediate state during the laschamp excursion

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

evidence for dipolar fields during the cobb mountain geomagnetic polarity reversals recent palaeomagnetic studies1 4 of geomagnetic polarity reversals have presented conflicting interpretations of the behaviour of the earth s magnetic field during reversals some have argued that the grouping of transitional virtual geomagnetic poles vgps suggests the presence of large scale perhaps even dipolar symmetries during some reversals1 3 whereas others have claimed that the available data do not support the interpretation of simple transitional field geometries4 an earlier comparison5of two north atlantic sedimentary records of reversals bounding the cobb mountain subchron 1 1 myr ago with a volcanic record from tahiti6 revealed striking similarities in the sequences of transitional vgps suggesting the presence of large scale sym metries in these fields here i augment this comparison with two new records of the cobb mountain subchron from the western pacific7 which greatly extend the geographical and temporal coverage these records exhibit sequences of vgp positions that are very similar to those observed in the north atlantic and tahiti providing evidence of dipolar transitional fields during the cobb mountain reversals

mono lake geomagnetic excursion found at summer lake oregon a 25 cm zone of anomalous paleomagnetic directions from lacustrine sediments near summer lake oregon has the same age and the same paleomagnetic signature as the younger part of the proposed mono lake geomagnetic excursion both field observations and rock magnetic studies support the hypothesis that the anomalous directions represent actual geomagnetic field behavior thereby confirming the existence of the mono lake excursion at the oregon site although confirmation of its existence at this site does not make the mono lake excursion any more suitable as a chronostratigraphic horizon this excursion may now be used as an additional constraint on models of geomagnetic field behavior including those that predict the morphology of transitional geomagnetic fields

reconciling short recurrence intervals with minor deformation in the new madrid seismic zone at least three great earthquakes occurred in the new madrid seismic zone in 1811 and 1812 estimates of present day strain rates suggest that such events may have a repeat time of 1000 years or less paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years however pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed this suggests that the seismic zone is a young feature possibly as young as several tens of thousands of years old and no more than a few million years old

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals

stressing of the new madrid seismic zone by a lower crust detachment fault a new mechanical model for the cause of the new madrid seismic zone in the central united states is analyzed the model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust rift pillow regional horizontal compression induces slip on the fault and the slip creates a stress concentration in the upper crust above the rift pillow dome in the coseismic stage of the model earthquake cycle where the three largest magnitude 7 8 earthquakes in 1811 1812 are represented by a single model mainshock on a vertical northeast trending fault the model mainshock has a moment equivalent to a magnitude 8 event during the interseismic stage corresponding to the present time slip on the detachment fault exerts a right lateral shear stress on the locked vertical fault whose failure produces the model mainshock the sense of shear is generally consistent with the overall sense of slip of 1811 1812 and later earthquakes predicted rates of horizontal strain at the ground surface are about 10 7 year 1 and are comparable to some observed rates the model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude

illumination of rheological mantle heterogeneity by the m7 2 2010 el mayor cucapah earthquake 1 major intracontinental strike slip faults tend to mark boundaries between lithospheric blocks of contrasting mechanical properties along much of their length both crustal and mantle heterogeneities can form such boundaries but the role of crustal versus mantle strength contrasts for localizing strain sufficiently to generate major faults remains unclear using the crustal velocity field observed through the global positioning system gps in the epicentral area of the m7 2 2010 el mayor cucapah earthquake baja california we find that transient deformation observed after the event is anomalously small in areas of relatively high seismic velocity in the shallow upper mantle 50 km depth this pattern is best explained with a laterally heterogeneous viscoelastic structure that mimics the seismic structure the mantle of the southern colorado river desert scrd and peninsular ranges pr which bound the fault system to its east and west respectively have anomalously high viscosity and seismic velocity we hypothesize that compared with the rest of the san andreas fault saf system to its north the strike slip fault system in northern baja california is narrow because of the presence of the pr and scrd high viscosity regions which bound it