coastwise transport reconsidered lateral displacements in oblique subduction zones and tectonic consequences coastwise transport margin parallel displacement of arc and forearc slivers is a common feature of zones of oblique subduction today but at rates that rarely exceed a few tens of kilometers per million year paleomagnetic studies strongly indicate that coastwise transport in the north american cordillera went on during late cretaceous early tertiary time at rates several times higher than this other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over riding crust the analysis developed here indicates that in the absence of a buttress a physical impediment to sliver motion at its leading edge sliver motion should obey the relationship tan p r tan 1 1 r2 sin2 12 where is the angle of obliquity is the angle of subduction and r rfrs is the ratio of resistance to slip on strike slip faults cutting the over riding plate to resistance to slip on the subduction zone in this equation p is a partitioning factor 0 p 1 where p 1 implies oblique subduction and no sliver motion and p 0 is the case in which the sliver moves at velocity v sin equivalent to the transverse component of relative motion rates of coastwise transport calculated from this relationship are systematically too high presumably indicating that most modern subduction zones are buttressed key factors in the buttress effect are 1 the greater strength of oceanic vs continental lithosphere and 2 the energy required to thicken continental crust large scale tectonic transport in the north american cordillera probably was the result of a combination of favorable factors long continued highly oblique convergence and a plate margin geometry that avoided the buttress problem

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

palaeomagnetism of neogene ocros dyke swarm the peruvian andes implication for the bolivian orocline summary remanent magnetization directions of 32 dykes and lava flows sampled near ayacucho the peruvian andes revealed 14 2 5 5 counterclockwise rotation after the neogene intrusion of this dyke swarm palaeo magnetic results of these rocks and other palaeomagnetic evidences from the central andean mesozoic rocks suggest relatively recent occurrence of the andean oroclinal bending around the axis at the peru chile border

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

paleomagnetism of upper miocene ignimbrites at the puna an analysis of vertical axis rotations in the central andes the origin of the rotations detected paleomagnetically in the central andes is controversial tectonic models proposed to explain it involve late cenozoic oroclinal bending the bolivian orocline and or small block rotations driven by oblique subduction in this paper we report paleomagnetic data from upper miocene ignimbrites from the puna in the presumed southern limb of the orocline these rocks showed high unblocking temperature and high coercivity magnetization the resulting pole position latitude 85 7 s longitude 80 5 e a95 7 9 k 22 n 17 indicates that the study area has not been significantly affected by vertical axis rotation since late miocene onward this implies that the whole puna has not undergone significant regional rigid body rotation during the same time span a farther analysis based on neogene paleomagnetic data from 31 andean localities distributed from 11 s to 31 s shows the presence of rotated and unrotated areas systematic counterclockwise rotations in peru and northern bolivia are observed whereas clockwise rotations are present in southern bolivia northern chile and perhaps northwestern argentina no systematic rotations are found farther south the overall time spatial distributions of these neogene paleomagnetic data suggest that if orogenic bending occurred it must have taken place before the middle miocene thus the oroclinal hypothesis cannot explain the rotations observed in middle miocene rocks or younger ones which also show orocline like declination anomalies we suggest that small block rotations driven by distributed shear may be a single process that can account for the whole rotations in the central andes it is hypothesized that horizontal shear in the andean crust could be controlled by ancient structures

late cenozoic paleomagnetism and chronology of andean basins of bolivia evidence for possible oroclinal bending new paleomagnetic and radioisotopic data are reported from two late tertiary high elevation non marine basins in the eastern cordillera of bolivia 1 quebrada honda located at 22 s lat consists of a 300 m thick section containing abundant santacrucian or friasian middle miocene fossil mammals this locality is constrained by mean 40 k 40 ar ages of 12 83 0 11 ma and 11 96 0 11 ma and the local magnetostratigraphy is correlated to chrons c5aa through c5a on the magnetic polarity time scale mpts the fossil mammals from quebrada honda have an extrapolated age of about 13 0 to 12 7 ma 2 micana located at 17 s lat consists of a 205 m thick section containing late miocene fossil mammals including a megatheriid sloth and the tiny mesothere microtypotherium cf m choquecotense this locality is constrained by a fission track age determination of 6 9 1 1 ma and the local magnetostratigraphy is correlated to chron 7 on the mpts the fossil mammals from this section have an extrapo

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

a seismologically consistent expression for the total area and volume of earthquake triggered landsliding we present a new seismologically consistent expression for the total area and volume of populations of earthquake triggered landslides this model builds on a set of scaling relationships between key parameters such as landslide spatial density seismic ground acceleration fault length earthquake source depth and seismic moment to assess the model we have assembled and normalized a catalog of landslide inventories for 40 shallow continental earthquakes low landscape steepness causes systematic overprediction of the total area and volume of landslides when this effect is accounted for the model predicts the total landslide volume of 63 of 40 cases to within a factor 2 of the volume estimated from observations r2 0 76 the prediction of total landslide area is also sensitive to the landscape steepness but less so than the total volume and it appears to be sensitive to controls on the landslide size frequency distribution and possibly the shaking duration some outliers are likely associated with exceptionally strong rock mass in the epicentral area while others may be related to seismic source complexities ignored by the model however the close match between prediction and estimate for about two thirds of cases in our database suggests that rock mass strength is similar in many cases and that our simple seismic model is often adequate despite the variety of lithologies and tectonic settings covered this makes our expression suitable for integration into landscape evolution models and application to the anticipation or rapid assessment of secondary hazards associated with earthquakes

zircon u pb and galena pb isotope evidence for an approximate 1 0 ga terrane constituting the western margin of the aravalli delhi orogenic belt northwestern india abstract zircon u pb ages of 987 6 4 and 986 3 2 4 ma have been established for rhyolites in the southern and northern parts respectively of the ambaji sendra arc terrane in the western fringe of the aravalli delhi orogenic belt pb isotope data for volcanogenic massive sulphide deposits within this arc terrane define a linear trend which is considered a useful reference palaeoisochron or mixing line at an age of about 990 ma and establish continuity of the terrane between ambaji and sendra these results are in contrast to the traditional and continuing assignment of the ambaji sendra terrane to the 1700 ma delhi supergroup the isotopic composition of galena from an apparently epigenetic occurrence at punagarh hill yields a model age of about 940 ma suggesting that the punagarh group could form part of the same arc sequence the phulad lineament which has been considered by most workers to represent the western boundary of the aravalli delhi orogenic belt appears in terms of this stratigraphic assignment to represent an oblique structure which has not greatly offset the punagarh and ambaji sendra domains within the arc terrane the eastern boundary of the terrane is marked by the sabarmati fault a zircon u pb age of 836 7 5 ma for the siwaya gneissic granite in the southern part of the ambaji sendra belt is in accord with previous age data for felsic erinpura plutons that have intruded the arc sequence a monazite age of 826 5 ma may reflect slow cooling of the siwaya pluton or a younger thermal event a model age of about 820 ma for galena is obtained from the tosham sn cu mineralized zone in haryana state about 280 km north northeast of ajmer which is related to the felsic anorogenic malani volcanism plutonism hence this widespread magmatism found extensively west of the ambaji sendra terrane may have been coeval with erinpura plutonism or followed it very closely the present geochronological data warrant the recognition of the ambaji sendra arc terrane as defined here as a distinct metallogenic province which saw the formation of vms type deposits in the late mesoproterozoic around 1 0 ga the pb isotope compositions of the deposits however do not clearly define the metal sources the lead from danva prospect is most primitive and must have the greatest component from a juvenile mantle source the birantiya khurd deposit on the other hand contains a much greater component of lead from a significantly older crustal source

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

causes of compression and extension behind trenches abstract three factors have hampered attempts to find the causes of compression and extension within overriding plates of subduction zones 1 1 adjacent subduction zones are often similar in many subduction parameters 2 2 proposed causal variables are themselves correlated and 3 3 physical models are limited by uncertainties in physical quantities and complexity of the subduction process nevertheless modern subduction zones provide a rich source of observations that help to distinguish among models we consider ten variables or combinations of variables which may control strain regime behind trenches convergence rate vc slab age as shallow slab dip ds absolute motion voa asthenospheric flow slab pull force as vc as voa vc ds voa and vc ds as ten observations are used to test these models a key prerequisite to these tests is the semiquantitative classification of the strain regime of each modern subduction zone into one of seven strain classes from active back arc spreading class 1 through active folding and foreland thrusting class 7 statistical analysis indicates that the modern variance in strain classes is not adequately accounted for by any single or dual variable model but the combination of vc ds and either asorvoa can account for 77 of this variance r 0 88 rapid changes in strain class both with time and along a subduction zone are similarly more consistent with the 3 variable models the role of vc is also supported by maximum compression orientations parallel to vc the role of ds is also supported by the tendency for oceanic overriding plates to be more extensional than continental overriding plates two factors suggest that the vc ds as model is more likely than vc ds voa 1 1 the dominance of one or more of the variables vc as andds in other subduction relations slab length earthquake moment trench relative depth deep dip and arc trench gap and 2 2 the correlation between strain class and maximum earthquake moment which is affected by vc and as however no confident discrimination between the two 3 variable models is yet possible

post oligocene rotations in southern ecuador and northern peru and the formation of the huancabamba deflection in the andean cordillera abstract a paleomagnetic study of more than 410 samples from 41 sites from paleocene to early oligocene volcanic and intrusive formations in southern ecuador and northern peru reveals a pattern of rotations in opposite senses on both sides of the huancabamba deflection the amplitude of the rotations is 25 clockwise north of the deflection and 20 anticlockwise south of it north of the deflection the results do not allow us to ascertain whether the rotation arises from oroclinal bending or from block rotation in a distributed shear south of it the absence of geological evidence for widespread strike slip faulting suggests that the results reflect a rotation of the peruvian margin which could be related to the shortening documented in southern peru and to the uplift of the central andes

neogene rotations and quasicontinuous deformation of the pacific northwest continental margin paleomagnetically determined rotations about vertical axes of 15 to 12 ma flows of the miocene columbia river basalt group of oregon and washington decrease smoothly with distance from the plate margin consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid the average rate of northward translation of the continental margin since 15 ma calculated from the rotations using this model is about 15 mm year which suggests that much of the tangential motion between the juan de fuca and north american plates since middle miocene time has been taken up by deformation of north america the fluid like character of the large scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

new temporal constraints on the rotation of the peruvian central andes obtained from paleomagnetism new paleomagnetic data from middle eocene to middle miocene volcanic and plutonic formations of the western cordillera of central peru document a counterclockwise rotation of 11 7 4 this rotation similar to previous results reported for the northern peruvian margin is post middle miocene in age it is thus coeval with the acceleration of the andean uplift and supports isacks model

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

on the nature of buttressing in margin parallel strike slip fault systems margin parallel strike slip faults commonly develop along convergent plate margins where an oceanic plate subducts obliquely beneath a continental plate such faults detach a sliver of continental crust from the leading edge of the overriding plate fault motion then causes the sliver to move parallel to the continental margin in the direction of the transverse component of relative plate motion however few slivers move as rapidly as predicted by simple analyses of the forces acting on them this discrepancy must be due principally to the buttress effect resistance to displacement of the sliver arising from a space problem at its leading edge more simply a sliver can move only if it has somewhere to go buttresses can be geometrical as when the continental margin changes trend abruptly or they can arise from changes in the physical properties of the overriding plate or in other factors such as subduction angle and angle of obliquity that help determine whether sliver motion will occur a strongly buttressed sliver will move only insofar as the buttress can be overcome obvious ways in which a buttress can be overcome are by thickening the crust or widening the sliver examples of both are known crustal thickening entails elevation increase sliver motion is opposed by both gravity and friction widening the sliver is a more complex process that results in areas of extension and compression forming within the sliver as well as a characteristic pattern of overlapping curved fault segments this latter response may be the cause of curvature in many strike slip fault systems

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

paleomagnetism of middle tertiary volcanic rocks from the western cascade series northern california the western cascade series wcs is a 3 5 km thick crudely homoclinal east dipping calcalkaline volcanic sequence of mid oligocene to early miocene age that crops out near the southern tip of the cascade range in northern california the mean direction of remanent magnetization in the wcs is d 4 9 i 57 6 n 53 k 14 4 95 5 3 when compared to a reference direction for the north american craton the wcs direction indicates that the southern cascade range has rotated 14 0 9 0 since the wcs accumulated a difference in mean direction between the lower and upper halves of the wcs suggests that much of this rotation occurred during the late oligocene six other paleomagnetic studies of rock units of an age roughly comparable to the wcs also are available from western oregon northwestern california and southwestern washington all show statistically significant clockwise rotation inclinations of all but one are concordant comparison of directions indicates that the pacific northwest did not rotate as a rigid body but neither did it behave as a collection of small independently rotating domains the amount of rotation found throughout the area increases to the west or northwest suggesting a driving force for rotation operating at the continental margin