early proterozoic ties between two suspect terranes and the mojave crustal block of the southwestern u s southern california and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust namely the tujunga san gabriel and joshua tree terranes that have been considered part of a larger displaced terrane the santa lucia orocopia allochthon paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and thus an origin extraneous to north america however early proterozoic plutons of the mojave crustal block and the joshua tree and tujunga terranes have strikingly comparable features including 1 crystallization ages of 1 63 to 1 68 ga 2 biotite sphene magnetite pm hornblende pm garnet mineralogy 3 high lil and enriched hfs elemental composition 4 wpg within plate granite trace element chemistry 5 similar and unique oxygen isotopic compositions and 6 distinct pb and nd isotopic signatures these features of the mojave block which clearly originated as part of native north america nevertheless distingui

paleomagnetism of coastal california and baja california alternatives to large scale northward transport paleomagnetic data from the santa lucia orocopia sloa and baja borderland bba allochthons of coastal california and baja california have been interpreted to indicate up to 2500 km of post mid cretaceous northward transport of these regions with respect to interior north america however with neogene strike slip offsets taken into account geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the san andreas and related fault systems we have examined paleomagnetic data from sloa and bba and conclude that apparent discordances can be explained without large scale pre neogene tectonic transport three major observations are fundamental to this analysis 1 paleolatitudes derived from volcanic rocks of the jurassic eugenia formation of bba and coast range ophiolite of sloa are concordant when compared to revised jurassic reference paleomagnetic poles from interior north america 2 isotopic and paleobarometric data from the peninsula ranges batholith in southern california indicate that the batholith has been tilted northeast side up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large scale northward transport 3 literal interpretation of the paleolatitudes determined from paleomagnetic directions in upper cretaceous and paleogene marine sedimentary rocks of sloa and bba requires north then south then north transport and a complex motion history between the two allochthons however concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of sloa and bba are biased towards low paleolatitudes by compaction shallowing

detrital modes and provenance of the stilo capo d orlando formation miocene southern italy the stilo capo d orlando formation sco fm is a miocene clastic unit nonconformably covering the basement of the southern portion of the calabria peloritani arc cpa a small scale terrane which was accreted onto the apennines maghrebides orogenic system during neogene time the sco fm is composed of conglomerate sandstone and mudrock deposited in a marine environment through the action of gravity currents detrital modes of sandstones and conglomerates of the sco fm indicate a local provenance in fact petrological parameters can be matched directly with nearby basement lithologies two petrofacies with distinctive parameters are present a northern petrofacies is predominantly composed of plutonic and low grade metamorphic detritus and was derived from the serre massif which is mostly made of late hercynian plutons intruded into metasedimentary rocks a southern petrofacies is mostly composed of metamorphic detritus and was derived from the aspromonte and the peloritani mountains which are predominantly composed of middle to high grade metamorphic rocks petrological results place important constraints on the geodynamic evolution of the cpa according to several authors the arc is composed of two sectors microterranes with different geological histories and partly composed of contrasting rock types whose timing of juxtaposition is debated early cretaceous against middle late miocene that is much before or soon after deposition of the sco fin the sco fm crops out only in the southern sector of the cpa and does not contain any detritus derived from the northern sector in spite of a favourable sediment palaeodispersal system this fact indicates that the amalgamation of the two sectors probably took place after deposition of the sco fm

remagnetization of the coast range ophiolite and lower part of the great valley sequence in northern california and southwest oregon overprinted magnetizations have been found at four localities in the middle jurassic coast range ophiolite and the overlying upper jurassic and lower cretaceous great valley sequence in northern california and at one locality in the partially correlative lower cretaceous days creek formation in southwest oregon at del puerto canyon on the east side of the diablo range a pilot study of the upper jurassic lotta creek formation gives in situ paleomagnetic directions grouped around the present geomagnetic field suggesting magnetic overprinting long after deposition and folding we suggest that the loss of original magnetization could be the result of long burial at about 7 km depth followed by late tertiary uplift the possibility of chemical remagnetization however can not be excluded paleomagnetic data from sandstones from the lower cretaceous great valley sequence in the wilbur springs area suggest cenozoic remagnetization that could be related to pliocene and pleistocene volcanic and hydrothermal activity in the area the results from calcareous concretions in the wilbur springs area from 21 sites from basalts and overlying sedimentary rocks of the great valley sequence at stonyford and from several sites in various rock types in the coast range ophiolite and great valley sequence near paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the late cretaceous or tertiary chemical remagnetization however can not be excluded the paleomagnetic data from six sites in lower cretaceous sedimentary rocks of the days creek formation in southwest oregon which overlies a dismembered ophiolite fail the fold test in situ paleomagnetic directions group near the expected tertiary field directions these sedimentary rocks may have been remagnetized during a major eocene tectonic event known to have occurred in this region the paleomagnetic results confirm that the coast range ophiolite and the seemingly little deformed great valley sequence in northern california as well as correlative rocks in southwest oregon have had a complex tectonic history once the nature and timing of these events are better understood the timing of remagnetization may be further constrained which in turn could give further insight into the nature of the tectonic events

the sandstone derived provenance record of the gualala basin northern california u s a abstract detrital zircon u pb ages and pb isotope compositions of detrital feldspars from the late cretaceous eocene gualala basin in northern california indicate that the basin received sediments from the southern sierra nevada the mojave block and the salinian block mesozoic detrital zircon age components and pb isotope compositions of detrital feldspars from the basin are similar to u pb zircon ages and pb isotope compositions from previously studied coeval conglomerate clasts the detrital zircons and feldspars however also include additional age and pb isotope components that are unobserved or poorly represented in the conglomerate based provenance record these new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts and strongly imply a connection to the mojave desert provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the mojave desert began during the paleocene when it is inferred that the drainage network expanded eastward from its previous extent the presence of mojave derived late permian zircons as well as feldspars that have high 208pb 204pb ratios in the eocene age german rancho formation indicates that the basin remained in the vicinity of the mojave desert until the eocene this result demonstrates that the gualala basin had undergone very little northward motion at that time and consequently requires that no significant northward translation took place prior to 50 ma the results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

laboratory investigations of marine impact events factors influencing crater formation and projectile survivability given that the earths surface is covered in around two thirds water the majority of impact events should have occurred in marine environments however with the presence of a water layer crater formation may be prohibited indeed formation is greatly controlled by the water depth to projectile diameter ratio as discussed in this paper previous work has shown that the underlying target material also influences crater formation e g gault and sonett 1982 baldwin et al 2007 in addition to the above parameters we also show the influence of impact angle impact velocity and projectile density for a variety of water depths on crater formation and projectile survivability the limiting ratio of water depth to projectile diameter on cratering represents the point at which the projectile is significantly slowed by transit through the water layer to reduce the impact energy to that which prohibits cratering we therefore study the velocity decay produced by a water layer using laboratory analytical and numerical modelling techniques and determine the peak pressures endured by the projectile for an impact into a water depth five times the projectile diameter the velocity of the projectile is found to be reduced to 26 32 its original value for deep water impacts we find that up to 60 of the original mass of the projectile survives in an oblique impact where survivability is defined as the solid or melted mass fraction of the projectile that could be collected after impact

paleomagnetic and 40ar 39ar evidence for remagnetization of mesozoic oceanic rocks on the vizcaino peninsula baja california sur mexico previously published paleomagnetic data for upper jurassic pillow lavas of the vizcaino peninsula indicate that they were deposited near a paleolatitude of 14 n or s whether or not this result implies northward transport with respect to the continental interior has been controversial due to the lack of reliable jurassic reference poles for the north american plate available paleomagnetic data for nearby upper triassic pillow basalts and overlying pelagic sediments at punta san hipolito however fail a fold test indicating that these rocks were remagnetized post folding indistinguishable in situ paleomagnetic directions and perturbed 40ar 39ar age spectra for the triassic and jurassic pillow lavas are consistent with resetting of their magnetic and isotopic systems in the middle cretaceous probably during burial by the overlap valle formation 10 km thick resetting apparently occurred post deformation so the paleolatitude of remagnetization is unknown high coercivity directions from a few samples of the triassic rocks might represent an older magnetization acquired during deposition at paleolatitudes near 6 n or s

holocene hillslope development in glacially formed valley systems in nordfjord western norway abstract large areas of the norwegian fjord landscapes are covered by hillslopes that reflect the influence of glacial inheritance from the last glacial maximum lgm the focus of this paper is two fold 1 analyze the spatio temporal variability of relevant denudational slope processes and process intensities over the holocene and 2 detect holocene modification of the glacial valley morphometry research was performed on hillslope systems within two steep parabolic shaped and glacier fed tributary valleys erdalen and bodalen located on the western side of the jostedalsbreen ice cap in western norway orthophoto delineation high resolution mapping tls detailed geomorphological information and spatial data analysis were combined with dating techniques and geophysical investigations calculated holocene rockwall retreat rates at selected slope test sites range from 0 38 to 0 67 mm yr 1 with a mean value of 0 53 mm yr 1 slightly higher values were found in erdalen with a mean rockwall retreat rate of 0 57 mm yr 1 compared to 0 50 mm yr 1 in bodalen valley wide holocene rockwall retreat rates of 0 38 0 50 mm yr 1 are consistent with other estimates of holocene rockwall retreat rates in cold mountain environments it is shown that the glacial inheritance of topography is the most important factor controlling valley development since the lgm and that sediment storage capacity is primarily conditioned by valley morphometry compared to contemporary rates the results indicate enhanced denudation activity and intensity immediately following deglaciation and during the little ice age the overall tendency of landscape development is postglacial widening of the parabolic shaped valley morphometry through rockwall retreat with associated debris accumulation beneath rockwalls as a result the glacially sculpted topography has not yet adapted to denudational processes acting under holocene environmental conditions

compaction corrected paleomagnetic paleolatitudes for late cretaceous rudists along the cretaceous california margin evidence for less than 1500 km of post late cretaceous offset for baja british columbia the paleolatitudinal distribution of bivalve rudists has important significance for the baja british columbia baja bc hypothesis that western canadian superterranes from british columbia have been displaced 3000 km since cretaceous time rudists are not observed in baja bc sedimentary rocks yet they are common in late cretaceous strata in california and baja california which have the same paleomagnetically determined paleolatitudes approximately 25 n as baja bc rocks of late cretaceous age in order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of baja bc paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the california margin compaction corrected paleomagnetic data from the peninsular ranges and salinia terranes indicate that rudists were restricted to paleolatitudes between 34 and 40 n evidence of coastal upwelling in the latest cretaceous marca shale may explain the northern limit of the rudist distribution these data suggest that baja bc was no farther south than 40 n in the late cretaceous thus limiting its post cretaceous displacement to less than 1500 km and that burial compaction has also affected the paleomagnetism of nanaimo group sedimentary rocks from vancouver island this result also helps resolve the conflict between paleomagnetic results which show 1500 km of post late cretaceous offset between the insular coast plutonic complex superterrane and the intermontane superterrane and geologic observations which can allow only tens of kilometers of offset between these terranes in the methow tyaughton basin

oligocene tectonics and sedimentation california abstract during the oligocene epoch california was marked by extensive nonmarine sedimentation in contrast to its pre oligocene and post oligocene depositional history the oligocene continental deposits are especially widespread in southern california and fill a number of small and generally partly restricted basins fluvial facies in many basins prograded over previously deposited lower tertiary turbidites volcanism from widespread centers was associated with the nonmarine sedimentation however some basins remained marine and a few contain oligocene turbidites and pelagic sediments deposited at bathyal depths the oligocene redbeds of california do not form a post orogenic molasse sequence comparable to the old red sandstone or alpine molasse they are synorogenic and record local uplift of basins and surrounding source areas late cretaceous to contemporary orogenesis in california has been generally characterized by the formation of small restricted basins of variable depth adjacent to small upland areas in response to strike slip faulting deposition of oligocene redbeds was associated with climatic change from warm and humid to cold and semiarid and a global lowering of sea level oligocene tectonism occurred during the transition from subduction of the farallon plate to initiation of the modern san andreas transform system however the major influence that caused uplift formation of fault bounded basins and extensive redbed deposition especially in southern california was the approach of the pacific farallon spreading ridge to the western margin of california

exotic terranes of western california numerous distinct geological terranes compose the north american cordillera1 there may be as many as 50 terranes in california alone2 critical to deciphering the history of cordilleran tectonic assembly is an understanding of the displacement history of individual terranes it is therefore important to know 1 whether a terrane has undergone significant motion with respect to the stable craton that is whether it is allochthonous or exotic 2 if so when relative motion started and stopped 3 from where an individual terrane originated and 4 the nature of interterrane movements we consider here the problem of determining whether the now juxtaposed salinian and stanley mountain terranes of california became amalgamated at or near their present position with respect to cratonic north america or if they collided at a considerable distance from their present positions and were later accreted to north america as a composite package the palaeomagnetic data that we present indicate that the latter was the case

paleomagnetism paleogeographic origins and uplift history of the coast range ophiolite at mount diablo california divergent paleogeographic origins have been proposed for the coast range ophiolite of western california which are testable using paleomagnetic methods paleomagnetic data for middle jurassic pillow lavas and diabase sills of the coast range ophiolite at mount diablo northern california indicate that they contain two components of remanent magnetization the characteristic directions have normal and reversed polarities and apparently are carried by ti poor magnetite this magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge the paleolatitude calculated from its structurally corrected mean direction is 20 n 9 and agrees with the expected direction for stable north america this result is also consistent with the concordant paleolatitude 32 n 8 recently determined for upper jurassic coast range ophiolite at stanley mountain in southern california in addition clockwise vertical axis rotation of mount diablo 143 11 is indicated by the characteristic magnetization direction an overprint component is inferred to have been acquired during uplift of mount diablo since the miocene

paleomagnetic evidence for poleward transport of upper jurassic rocks in the decatur terrane san juan islands washington the san juan islands are underlain by a west vergent late cretaceous thrust and nappe system part of a larger thrust belt exposed in the northwest cascades of washington and the southwestern coast mountains of british columbia published paleomagnetic data from cretaceous plutons for example the coast plutonic complex intruding the thrust belt suggest that it has been displaced northward by as much as 2400 km since 90 100 ma this study focuses on the preintrusive travel history of the decatur terrane one of the units involved in the san juan thrusting the decatur terrane is of special interest because it may be correlative with the coast range ophiolite cro and great valley sequence of the southern california coast ranges we report new paleomagnetic data from the james island formation an upper jurassic preintrusive turbidite unit and from the obstruction formation a terrigenous sandstone unit deposited on the decatur terrane in the late cretaceous nine sites in the obstruction formation yield a single remanence direction grand mean d 90 7 i 67 9 that is clearly a postfolding overprint possibly of late cretaceous age the six site mean remanence directions from the james island formation cluster well only after restoring the beds to horizontal by a scheme that takes account of their complex structure this positive fold test and the presence of two polarities provide evidence that the remanence is primary the mean paleolatitude corresponding to the corrected remanence direction from the james island formation is 0 9 2 the data thus corroborate equatorial paleolatitudes derived from parts of the cro by others and suggest that the decatur terrane and cro represent fragments of a larger terrane originally located near the jurassic equator unlike the cro however the decatur terrane was still near the equator at 145 ma if the plutons intruding the san juan north cascades thrust belt formed 2400 km to the south then the paleomagnetism of the decatur terrane suggests it moved 20 farther poleward in the interval 145 ma to 97 ma than did an equivalent site on north america a much smaller amount of pre late cretaceous motion is allowed by error limits on the paleolatitudes but more is required if tilting is partly responsible for the shallow paleomagnetism of the cretaceous plutons

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

terrane boundary reactivation barriers to lateral fault propagation and reactivated fabrics rifting across the median batholith zone great south basin new zealand prominent pre existing structural heterogeneities within the lithosphere may localise or partition deformation during tectonic events the ne trending great south basin offshore new zealand formed perpendicular to a series of underlying crustal terranes including the dominantly granitic median batholith zone which along with the boundaries between individual terranes exert a strong control on rift physiography and kinematics we find that the crustal to lithospheric scale southern terrane boundary of the median batholith zone is associated with a crustal scale shear zone that was reactivated during late cretaceous extension between zealandia and australia this reactivated terrane boundary is oriented at a high angle to the faults defining the great south basin we identify a large granitic laccolith along the southern margin of the median batholith expressed as sub horizontal packages of reflectivity and acoustically transparent areas on seismic reflection data the presence of this strong granitic body inhibits the lateral south westward propagation of ne trending faults which segment into a series of splays that rotate to align along the margin as they approach further we also identify two e w and ne sw oriented basement fabrics likely corresponding to prominent foliations which are exploited by small scale faults across the basin we show that different mechanisms of structural inheritance are able to operate simultaneously and somewhat independently within rift systems at different scales of observation the presence of structural heterogeneities across all scales need to be incorporated into our understanding of the structural evolution of complex rift systems

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

paleomagnetic evidence for motions of parts of the canadian cordillera abstract paleomagnetic evidence from major terranes of the western cordillera in canada referred to collectively as baja british columbia indicates that they have moved as a whole about 2300 km from the south and rotated 60 clockwise relative to north america in the late cretaceous and earliest tertiary 85 50 ma sometime between the early jurassic and the mid cretaceous 190 100 ma these same terranes moved south over 1000 km and underwent irregular but where well constrained counter clockwise rotations the terranes amongst themselves seem to have shared much the same paleolatitude during and since the late triassic although they may have undergone relative rotations in the later jurassic and early cretaceous these motions are now known to be broadly consistent with the motions of oceanic plates to the west as calculated by independent procedures and indicate that the terranes of the western cordillera were for part of their existence coupled to oceanic plates first to the farallon plate during the late jurassic and early cretaceous and then to the kula plate during the late cretaceous and early tertiary it was this change of allegiance from farallon to kula that caused first the southerly and then the northerly motion the paleomagnetic evidence despite claims to the contrary is consistent with geologically inferred displacements along major faults in the canadian cordillera when due account is taken of the age constraints on those motions

late jurassic tropical radiolaria vallupus and its related forms abstract the vallupinae is a radiolarian subfamily whose range is restricted to the late jurassic and the acme in the latest jurassic tithonian species of the subfamily vallupus group are found in low and middle latitudes of the western pacific east asia north central south america and mediterranean regions available paleomagnetic data suggest that vallupus group bearing rocks in these localities accumulated within 25 of the jurassic paleoequator seawater temperature is considered to be the major factor controlling the distribution of the vallupus group warm seawater temperatures could be favourable to the propagation of the group the vallupus territory is proposed as a water mass inhabited by the vallupus group it occupied tropical surface waters from the low latitude part of the panthalassa to tethys some vallupus group bearing rocks in the circum pacific orogenic belt have been translated from low latitude depositional sites to higher locations by lateral plate movements most upper jurassic strata in japan accumulated outside the vallupus territory this means that the vallupus group cannot be utilized for upper jurassic radiolarian zonal correlations between japan and other regions however the rapid speciation of the vallupus group has a high potential to produce a fine biostratigraphic framework for the upper jurassic within the vallupus territory

cretaceous sinistral strike slip along nacimiento fault in coastal california the san andreas and nacimiento faults of coastal california both separate granitic and metamorphic basement rocks of the salinian block from partly coeval but contrasting mesozoic terranes underlain by the franciscan subduction complex by analogy with neogene dextral strike slip along the san andreas fault cretaceous sinistral strike slip can be inferred along the nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount following restoration of known san andreas and inferred proto san andreas dextral displacements reversal of about 560 km 350 mi of postulated sinistral slip on the nacimiento fault brings four major mesozoic lithotectonic belts of california and baja california into simple al gnment as subparallel terranes related to mesozoic subduction along the continental margin neogene deformation within the san andreas transform system involved a elongation of the salinian block by dextral slip along subsidiary faults that branch from the san andreas fault and b dextral rotation of crustal panels within the transverse ranges latest cretaceous and or earliest paleogene dextral slip along a proto san andreas fault followed the san andreas course in central california but diverged westward in southern california nacimiento sinistral displacements occurred in mid cretaceous to early or medial late cretaceous time after cretaceous emplacement of plutons now within the salinian block but prior to deposition of uppermost cretaceous sedimentary sequences in central california available data on mesozoic relative and absolute plate motions in the mesoameric n region support the likelihood of cretaceous sinistral strike slip subparallel to the california continental margin paleotectonic reconstruction of crustal blocks in california and baja california to their inferred mid cretaceous relative positions shows the salinian block inserted on a bias between the flanking mojave and peninsular ranges blocks salinian granitic rocks thus formed an interior part of the mesozoic batholith belt and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks the similar mesozoic terranes that now lie east and west of the salinian block were then adjacent to one another west of the sierra nevada block available paleomagnetic data neither support nor preclude the reconstruction but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents a correct interpretation of the nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal california

the subduction of the farallon plate beneath north america as derived from relative plate motions high rates of subduction and near normal convergence of the farallon plate correlate with the duration of the laramide orogeny in the western cordillera the motion of the farallon plate relative to north america is reconstructed back to the late mesozoic using relative plate motion data this reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates two models for relative motions are considered the preferred model predicts an increase in the subduction rate of the farallon plate exceeding 150 mm yr with a direction almost due east and perpendicular to the continental margin the duration of this nonoblique normal high rate subduction correlates well with the duration of the laramide orogeny this correlation must be regarded as only suggestive the probable errors when combined and propagated through the relative motion circuit may be of the order of the effect observed however it is encouraging that these reconstructed farallon north american relative motions are similar to the published predictions of others using the alternative more direct approach of hot spot reconstructions

paleomagnetic and geologic data indicating 2500 km of northward displacement for the salinian and related terranes california results of a pair of paleomagnetic studies of sedimentary rocks in the salinian terrane of westernmost california indicate a northward displacement of about 2500 km since cretaceous time stratigraphic relations suggest that the salinian terrane became amalgamated with several others by the late cretaceous and all underwent the same northward translation this composite terrane allochthon became sutured to cratonal north america by early tertiary time one paleomagnetic study involved upper cretaceous turbidites of the pigeon point formation fine grained thin bedded turbidites yielded the most reliable results whereas medium to coarse grained sandstones are not always stably magnetized contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers thereby implying a postdepositional remanence process there is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization paleomagnetic results from four separate locations in the pigeon point formation gave positive fold tests and mean inclination values near 36 a mean geomagnetic latitude calculated from these results is 21 2 5 3 much more southerly than the value of 46 5 2 3 calculated for pigeon point using paleomagnetic pole data from late cretaceous rocks from cratonic north america declination values are not antipodal for normal and reversed polarity sites analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of neogene rotations of small crustal blocks adjacent to the san gregorio fault in a sequence of paleocene turbidites that crop out at point san pedro four sample localities yielded positive fold tests with mean inclination values near 40 declination values however were spread over a range between 230 and 320 comparison with depositional current directions from the same outcrops indicates clockwise rotations related to neogene right slip on the pilarcitos fault the mean geomagnetic latitude calculated for the point san pedro sediments is 24 5 4 0 and is more southerly than the 43 3 2 5 latitude expected for that location for the salinian terrane and the west coast of cratonic north america absolute rates of latitudinal motion were 8 5 cm yr northward and 3 cm yr southward respectively

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

probing the composition of unexposed basement south portuguese zone southern iberia implications for the connections between the appalachian and variscan orogens geochemistry and sm nd and u pb magmatic zircon isotope data from a postcollisional batholith that crosscuts the allochthonous south portuguese zone spz of southern iberia suggest that the base

structures that accommodated differential vertical axis rotation of the western transverse ranges california 1 new paleomagnetic and structural data from the northwestern transverse ranges of california define the location geometry and kinematics of structures that facilitated differential vertical axis rotation paleomagnetic declinations in the southern coast ranges indicate negligible amounts of vertical axis rotation which contrasts sharply with data from rocks of the same age in the western transverse ranges that record approximately 90 of clockwise rotation since early miocene time this change in paleomagnetic declinations occurs across an east west trending zone of reverse faults and folds that includes the western big pine pine mountain fault the santa ynez fault and the structures in between the structures in this zone exhibit increased amounts of shortening to the west such that the zone closed in a fan like fashion resulting in rotation of the southern edge with respect to the northern edge the new data and interpretations refine previous models of vertical axis rotation in southern california by describing the deformation present at rotation boundaries that allowed distinct areas to rotate relative to the surrounding crust

a compaction correction for the paleomagnetism of the cretaceous pigeon point formation of california the paleomagnetism of the cretaceous pigeon point formation turbidites was reexamined to determine whether the 25 of southerly paleolatitudinal offset originally observed champion et al 1984 for these rocks was all or in part due to compaction shallowing of their paleomagnetic inclination the study consisted of two parts 1 a standard paleomagnetic study including detailed thermal and alternating field demagnetization was conducted on oriented cores collected at pigeon point approximately 50 km south of san francisco california the results of this study were combined with the alternating field demagnetized results for samples provided by d champion from the initial pigeon point paleomagnetic study the combined data set has a mean direction for pigeon point i 41 6 d 346 9 similar to that originally obtained by champion et al 1984 2 material from the pigeon point formation was disaggregated given a laboratory analog of a postdepositional remanence and compacted to pressures as high as 0 13 mpa which caused volume losses up to 53 the laboratory compacted samples were alternating field demagnetized and their magnetic inclination and anisotropy of anhysteretic remanence were both measured these data were used to derive correction curves following jackson et al 1991 which describe the specific relationship between remanence anisotropy and inclination shallowing for the pigeon point formation two correction curves were determined one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation these two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification technically caused lineation and a burial compaction foliation the anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the pigeon point formation has been shallowed by burial compaction the compaction corrected pigeon point mean directions assuming either a prolate i 53 1 d 347 2 or an oblate i 49 8 d 346 8 deformation suggest only 13 to 16 of southerly paleolatitudinal offset for the pigeon point formation in the cretaceous not the 25 originally observed champion et al 1984 the resulting paleolatitude for the pigeon point formation could indicate that salinia served as a link between the cratonic sierra nevada arc to the north and the peninsula ranges baja borderlands allochthon to the south alternatively our results suggesting a 10 compaction inclination shallowing for the pigeon point turbidites may indicate that many of the paleomagnetic studies placing the peninsula ranges baja borderlands 15 to the south of north america in the cretaceous and tertiary may have suffered from a similar effect and that the allochthon has been nearly in place since the cretaceous

speculative reconstruction of the mojave snow lake fault implications for paleozoic and mesozoic orogenesis in the western united states available evidence suggests that the mojave snow lake fault a major early cretaceous dextral fault with up to 400 km displacement formerly existed in what is now the axial part of the sierra nevada batholith regional relations suggest that the fault formerly continued in western nevada the mojave desert and in rocks now exposed in the salinian block restoration of 400 km of early cretaceous dextral displacement on the mojave snow lake fault leads to speculative but testable reconstructions of mesozoic and paleozoic tectonic features of california and nevada insights derived from these reconstructions include the following 1 two regionally extensive jurassic thrust belts of opposed vergence appear to have intersected in the northern sierra nevada the eastern east vergent thrust belt which includes the fencemaker and liming thrusts in western nevada and several east vergent thrusts in the northern sierra may be a contractional belt along which an early jurassic interarc or marginal basin was closed in middle to late jurassic time the western west vergent thrust belt is part of the classic nevadan orogen of the western klamath mountains and the western sierra metamorphic belt in which collisions and or thrusting of various island arc terranes occurred during middle and late jurassic time 2 the early mesozoic continental arc included paired belts of triassic jurassic blueschist and melange and igneous rocks including late triassic and younger plutons and volcanic rocks this conclusion is not new but the reconstruction provides a slightly different geometry than previously thought 3 sonomia the late paleozoic island arc terrane of the western u s may have formed a relatively narrow belt extending from northwestern nevada to the latitude of the western mojave desert the geologic development of the paleozoic basement complex of sonomia was very different from that of the cordilleran miogeocline in death valley and the mojave desert requiring that a continuation of the sonoma suture belt or golconda thrust must have extended southward through the sierra nevada batholith to the mojave region 4 a heterogeneous belt of permian to triassic or triassic deformation in the mojave and eastern sierra nevada together with the golconda allochthon in nevada may represent the protracted response to the collision of sonomia with the western margin of north america the contractional deformation continued during the evolution of the early mesozoic continental arc 5 several predictions can be made about the nature of concealed basement within the sierra nevada batholith and within western nevada the reconstruction suggests that elements of the antler and sonoma orogenic belts should underlie areas of the batholith and western nevada as a result of northward displacement along the mojave snow lake fault from the western mojave and southern sierra nevada

where are the missing faults in translated terranes abstract a central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers while the faults along which the terranes moved these great distances are not found relations long known from geology and from recent studies of modern transform plate boundaries suggest that preserving strike slip faults that record 1000 km scale translations is unlikely and therefore in many cases paleomagnetism may be our only reliable indicator of how far terranes translated four reasons are explored here for why large offset strike slip faults are likely to be missing in translated terranes 1 strike slip faults in continents commonly are partially or wholly destroyed or reactivated only 400 500 km of over 1000 km of offset on cretaceous early tertiary strike slip faults in northern british columbia are accounted for in southern british columbia the missing strike slip faults probably continued south into the hinterland of the rocky mountain thrust belt as oblique thrust faults much like the modern alpine marlborough fault system of new zealand the major strike slip faults in southeastern british columbia were obliterated by eocene extensional faulting uplift and erosion 2 offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale many of which are left behind and not found in the translated terrane the greater san andreas plate boundary has accumulated 1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern california there is a narrow terrane in the offshore borderland of california that has moved 1200 km relative to north america but no single fault has nearly that much offset 3 major faults in narrow oceans formed by oblique divergent plate boundaries have a low potential for preservation for example the deep dense oceanic crust of the gulf of california where the major transform faults of the oblique rift lie has a low potential for preservation in contrast baja california is currently a continental terrane that has translated 300 km relative to north america along the plate boundary in the gulf of california because of the relative buoyancy of continental crust baja california is likely to be preserved in any future accretion event 4 faults in the margins of oblique divergent plate boundaries are dominantly normal faults and not strike slip faults there is little strike slip faulting in central baja california even though the plate boundary has a rift angle of only 20 recent field data and modeling show that at 0 20 strike slip faults will dominate the secondary faults surprisingly at 20 to 35 there are both strike slip and normal faults and at 35 45 there are few or no strike slip faults even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion

equatorial origin for lower jurassic radiolarian chert in the franciscan complex san rafael mountains southern california lower jurassic radiolarian chert sampled at two localities in the san rafael mountains of southern california 20 km north of santa barbara contains four components of remanent magnetization components a b and b are inferred to represent uplift miocene volcanism and subduction accretion overprint magnetizations respectively the fourth component c isolated between 580 and 680 c shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during or soon after deposition both sequences are late pliensbachian to middle toarcian in age and an average paleolatitude calculated from all tilt corrected c components is 1 3 north or south this result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern california and mexico this result supports our model in which deep water franciscan type cherts were deposited on the farallon plate as it moved eastward beneath the equatorial productivity high were accreted to the continental margin at low paleolatitudes and were subsequently distributed northward by strike slip faulting associated with movements of the kula farallon and pacific plates upper cretaceous turbidites of the cachuma formation were sampled at agua caliente canyon to determine a constraining paleolatitude for accretion of the jurassic chert sequences these apparently unaltered rocks however were found to be completely overprinted by the a component of magnetization similar in situ directions and demagnetization behaviors observed in samples of other upper cretaceous turbidite sequences in southern and baja california imply that these rocks might also give unreliable results

lower crustal deformation during terrane dispersion along strike slip faults abstract after accretion terranes caught between obliquely converging plates are often displaced several hundreds of kilometers how this motion is accommodated at depth is not well known spurred by our trans alaskan crustal transect tact seismic studies of the yukon tanana terrane alaska i propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin parallel translation the locations of these ductile regions are dependent on temperature composition and strain rate for geotherms based on moderate surface heat flow 60 90 mwm 2 and reasonable geological strain rates 100 in 3 ma typical lower crustal rocks can deform ductilely the resultant subhorizontal layered fabric should be detectable with seismic techniques two terranes that have experienced hundreds of kilometers of margin parallel translation the yukon tanana terrane sampled by our seismic refraction wide angle reflection profiles and the salinian block california sampled by both seismic refraction and reflection have reflective lower crust both of these terranes have moderate to high surface heat flows have relatively thin crust and are interpreted seismically to be underlain by a quartzo feldspathic lower crust conditions favorable for ductile deformation in the lower crust interpretations of lower crustal layering for these two terranes is based on an increase in lower crustal reflectivity this layering is attributed to tectonic banding consistent with ductile deformation

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

the dynamic history of 220 million years of subduction below mexico a correlation between slab geometry and overriding plate deformation based on geology paleomagnetism and seismic tomography global tectonic reconstructions of pre cenozoic plate motions rely primarily on paleomagnetic and geological data from the continents and uncertainties increase significantly with deepening geological time in attempting to improve such deep time plate kinematic reconstructions restoring lost oceanic plates through the use of geological and seismic tomographical evidence for past subduction is key the north american cordillera holds a record of subduction of oceanic plates that composed the northeastern panthalassa ocean the large oceanic realm surrounding pangea in mesozoic times here we present new paleomagnetic data from subduction related rock assemblages of the vizcaino peninsula of baja california mexico which yield a paleolatitudinal plate motion history equal to that of the north american continent since late triassic time this indicates that the basement rocks of the vizcaino peninsula formed in the forearc of the north american plate adjacent to long lived eastward dipping subduction at the southern part of the western north american continental margin tomographic images confirm long lived uninterrupted eastward subduction we correlate episodes of overriding plate shortening and extension to flat and steep segments of the imaged slab by integrating paleomagnetic geological and tomographic evidence we provide a first order model that reconciles absolute north american plate motion and the deformation history of mexico since late triassic time with modern slab structure

paleomagnetism of cretaceous and eocene strata san miguel island california borderland and the northward translation of baja california the history of poleward motion of the baja california peninsula is a subject of continuing geologic investigations including paleomagnetic studies during the past decade geologic correlations seem not to necessitate northward movement while paleomagnetic data suggest movement of 1000 km or more we have made paleomagnetic measurements on cretaceous and eocene sedimentary rocks of san miguel island california which are western lithofacies of rocks cropping out on the baja peninsula a total of 97 drill cores taken from upper cretaceous turbidites though weakly magnetized were found to clean best in response to thermal demagnetization revealing a characteristic remanence the characteristic magnetization has an inclination 43 9 declination of 24 0 and an 95 of 3 9 this direction is 19 shallower and rotated 50 clockwise from the expected direction for the present site position and corresponds to a paleolatitude of 26 n a total of 117 drill cores of middle eocene rocks revealed characteristic magnetizations in response to either alternating field or thermal demagnetizations some samples suffer from a secondary thermal magnetization from early miocene volcanic activity requiring a thermal cleaning that incompletely removes the overprint a mean direction of magnetization can be derived from the intersection of planes fitted to the paths of demagnetization the characteristic magnetization for the eocene rocks has an inclination of 31 9 a declination of 71 0 and an 95 of 3 8 this direction is 19 shallower and rotated 79 clockwise from the expected eocene direction for this site and corresponds to a latitude of 17 n these paleomagnetic data in combination with recently published data from the baja peninsula and peninsular ranges of southern california indicate motion trajectories that mimic the north american craton for the period cretaceous to late eocene time during the oligocene the baja peninsula and adjoining regions to the west moved northward relative to the craton arriving at a pre gulf of california position not later than earliest miocene time a comparison of the northward motion of the baja peninsula with that of the santa lucia orocopia allochthon shows that both areas underwent rapid poleward translations but these motions occurred at different times