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

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

assessing the age of relief growth in the andes of northern chile magneto polarity chronologies from neogene continental sections the andes of northern chile currently experience a phase of relief rejuvenation as indicated by valleys that are 1000 m dissected into pediplains however it has been unclear when this phase of relief enhancement started this paper discusses the use of palaeomagnetic chronologies from four sections in the taracapa region northern chile to assess this age the sections are located in distal and proximal positions sediment accumulation occurred between c 22 5 and 8 7 5 ma with a hiatus that possibly spans a time interval between c 19 5 and 11 ma the magnetic polarity chronologies suggest a preferred age between 8 0 and 7 5 ma for the time when relief growth started in proximal positions however alternative correlations suggest an age of 8 5 ma in addition the palaeomagnetic data reveal no rotation of the analysed strata suggesting a minimum age of c 22 5 ma for the tectonic block rotation south of the arica deflection

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

complex resistivity measurements of confined rock complex resistivity was measured for samples of westerly granite and berea sandstone saturated with 0 01 m kclaq a four electrode technique was employed to make measurements over a frequency range of 10 3 to 106 hz and a confining pressure range of 0 2 to 200 mpa when the electrical response was analyzed in terms of equivalent conductivity and relative permittivity k both of these quantities varied more with confining pressure for the granite samples than they did for the sandstone in addition both rock types exhibited large low frequency dispersion of permittivity while previous studies have measured the response of near dc conductivity to changes in pressure measurements of pressure dependence of the full complex resistivity provide greater insight into earthquake related phenomena as well as induced polarization measurements and other remote sensing techniques

modeling petroleum generation in the paleozoic of the carnarvon basin western australia implications for prospectivity maturity and petroleum generation modeling of the paleozoic succession in the carnarvon basin shows that most potential source rock intervals reached their maximum generation migration during the carboniferous permian and could have charged traps developed during rifting in the middle carboniferous to early permian except for the peedamullah shelf where generation peaked during the cretaceous however identification of such traps is challenging because it is difficult to differentiate between the deformation associated with middle carboniferous early permian rifting and that of the early cretaceous breakup rifting given the presence of suitable reservoir and sealing units in most of the paleozoic the prime risks for this section are the volume of available source rock trap integrity because of the long period of preservation required and relative timing of generation vs trap formation such that charging of younger traps requires secondary migration the best paleozoic oil prone source beds identified in the carnarvon basin are thin beds in carbonate dominated silurian and devonian units on the gascoyne platform but devonian source beds are restricted to the northern parts of the platform the maturity of these units progressively increases from immature in the south southeast to mature in the north northwest following increasing depth of burial in that direction the best gas prone source beds lie within the lower permian of the merlinleigh subbasin and their maturity ranges from immature along the margins of the subbasin to overmature toward the center within the upper permian the best source beds for oil and gas are in the peedamullah shelf where they range from immature in the southeast to mature in the northwest commercial paleozoic petroleum has yet to be discovered in the paleozoic of the carnarvon basin but the assessment of this part of the basin is limited because few if any of the exploration wells drilled to date were valid tests of these objectives

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

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

sedimentary evolution of an active margin during middle and upper cretaceous times the north peruvian margin from late aptian up to senonian the sedimentary evolution of the north peruvian margin during upper cretaceous can be summarized as follows

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

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

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

paleomagnetic tracking of mountain building in the peruvian andes since 10 ma 1 we report paleomagnetic data from 73 sites 628 samples of upper oligocene to pliocene rocks from central and northern peru the data indicate that the subandean zone has not experienced vertical axis rotation since the upper oligocene whereas the coast and the western cordillera record a coherent pattern of counterclockwise rotations emplaced in the last 10 ma this pattern can best be explained by two competing hypotheses 1 a propagation in rotations from the bolivian orocline toward the north or 2 a more punctual and widespread event linked to subduction of the nazca ridge that caused the rotations on the basis of the time space relationship of paleomagnetic rotations deformation magmatism and nazca south america plate convergence the latter appears more likely moreover because the rotations north of the abancay deflection 15 5 s are synchronous with deformation and exhumation they indicate that major mountain building in the peruvian andes has occurred since 10 ma finally we suggest that a succession of aseismic ridge subduction events played an important role in shaping the modern topography of the andean chain

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

kuroko type deposits in the middle cretaceous marginal basin of central peru barite massive sulfide and siliceous stockwork deposits of kuroko type in the lima region are associated with the casma group a sequence of submarine volcanic rocks of middle cretaceous age these deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill volcanic hosted deposits occur in the entire region sediment hosted deposits are restricted to the eastern casma volcanic facies which intercalate with limestones and shales deposited on a shelf platform adjacent to the marginal basin in most cases mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz serieite alteration the latter have locally been dated at 116 to 106 m y by the k ar method strata bound deposits of bedded barite pyrite sphalerite and pyrrhotite overlie the feeder zones the most important deposits of this kind are leonila graciela and juanita with 4 million tons of produced barite and 2 5 million tons of production plus reserves of massive sulfide ore they are located in a roof pendant of folded strata intruded by two plutons of the coastal batholith contact metamorphism of hornblende hornfels facies affects both ore deposits and wall rocks k ar ages on hornblende biotite pairs from the granitic rocks indicate that they were emplaced 82 and 65 m y ago whole rock ages on postmetamorphic dikes vary between 31 and 39 m y p t conditions for contact metamorphism of hornblende hornfels facies at leonila graciela are estimated at 2 1 to 2 6 kb and 300 degrees to 500 degrees c on the basis of sphalerite geobarometry stratigraphic reconstruction metamorphic mineralogy and interpretation of discordant k ar age patterns mole percent fes in sphalerites increases in a prograde sense from the actinolite zone at juanita to the biotite muscovite zone at gracicla in massive sulfide specimens it varies correspondingly from 15 4 or 0 2 to 17 6 or 0 7 sphalerites from siliceous stockworks show the same trend with 14 7 or 0 4 mole percent fes and 17 6 or 1 1 mole percent fes metamorphic equilibration was reached only in the biotite muscovite zone at graciela this is demonstrated by the homogeneity of high mole percent fes values detected in sphalerite which coexists in mutual contact with pyrite and hexagonal pyrrhotite

vertical axis rotation in the bolivian orocline south america 1 paleomagnetic analysis of cretaceous and cenozoic rocks the results of a paleomagnetic study are reported for 45 new sites or localities in the central andes of bolivia and northern chile 17 s 23 s south america in intrusive volcanic and sedimentary rocks ranging in age between 7 and 80 ma these outcrop across the width of the northern and southern limbs of the bolivian orocline from the sub andean zone in the east to the volcanic arc in the west for most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field a comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable south america reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks hundreds of meters to tens of kilometers across relative to stable south america these data when combined with previous paleomagnetic studies of upper cretaceous and cenozoic rocks show a pattern of vertical axis rotations which defines three regional domains extending across the width of the bolivian andes the differences between the various domains are best seen in rocks in the age range 20 to 80 ma younger rocks acquired a primary magnetization at later stages in the rotation history for rocks older than 20 ma crustal blocks in domain 1 on the northern flank of the bolivian orocline have undergone an anticlockwise rotation about a vertical axis in the range 8 to 33 with a geometric mean of 22 blocks from domain 2 immediately south of the main oroclinal bend have rotated between 19 anticlockwise and 40 clockwise with a geometric mean of 8 clockwise even farther south in domain 3b blocks have rotated between 3 and 57 clockwise with a geometric mean of 37 a localised area domain 3b in which blocks have undergone 32 clockwise rotation in the last 65 myr lies within the southeastern part of domain 1 the northern boundary of domain 1 and the southern boundary of domain 3a are as yet not well defined the amount and timing of cenozoic vertical axis rotation vary across the width of the andes and are interpreted to record both regional bending of both limbs of the bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the cenozoic 1 45 to 25 ma 2 25 to 10 ma 3 10 ma to 0 bending of the bolivian andes during the last 25 myr was accommodated relative to stable south america by gradients of shortening along the eastern margin of the andes particularly in the sub andean zone

paleomagnetic evidence for rapid vertical axis rotation in the peruvian cordillera ca 8 ma paleomagnetic results from 31 neogene sites in the peruvian andes yield primary magnetizations as demonstrated by positive fold and reversal tests strata dated as 18 9 ma record a significant counterclockwise rotation 11 5 whereas unconformably overlying younger strata 7 6 ma are not rotated the age of rotation thus is between 9 and 7 ma a period that coincides with the widespread quechua 2 deformation phase moreover eight independent studies on 107 9 ma rocks from peru between 9 s and 15 s reveal similar and significant rotations 15 6 this suggests that the region rotated during a 2 m y period of deformation ca 8 ma when the andes underwent rapid uplift and important deformation commenced in the subandean zone

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

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

stress fields recorded on large scale strike slip fault systems effects on the tectonic evolution of crustal slivers during oblique subduction abstract in continental margins large scale strike slip fault systems resulted from oblique subduction commonly exhibit a complex pattern of faulting where major faults define the inland boundary of tectonic slivers that can be detached from the margin in turn subsidiary faults bound and define internal tectonic blocks within the sliver which are expected to rotate translate and or internally disrupt in order to accommodate the internal deformation the geometrical and spatial arrangement of faults and tectonic blocks thus determines the evolution of the sliver given a particular stress field regime the paposo segment of the atacama fault system in northern chile displays a series of brittle faults whose orientations are hierarchically arranged low order faults splay off higher order faults forming riedel type and strike slip duplexes geometries at several scales the master 1st and highest order paposo fault defines the inland boundary of a tectonic sliver whereas subsidiary faults bound and disrupt internal tectonic blocks by using newly collected brittle fault slip data we estimated the orientations and regimes of the stress fields that acted upon the entire sliver the different fault orders and the tectonic blocks results indicate that an overall transtensional with nw compressional and ne tensional principal axes strike slip regime affected the sliver and triggered the development of left lateral strike slip structures an incomplete split of the stress field imposed by the subduction process resulted in the generation of a nested pattern of r type faults as well as in a combined strike slip normal faulting disruption of the tectonic blocks within the sliver

lithospheric flexure and bending of the central andes gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere t in the bend region of the central andes values of z increase from nearly zero north and south of the bend to values greater than 50 km at the bend in bolivia there is a close correlation between t and the style and magnitude of the shortening in the central andes since the late miocene in the bend region where the lithosphere is flexurally strong with large t foreland deformation is concentrated into a thin skinned fold and thrust belt above a basal decollement which has absorbed more than 100 km of shortening further north and south where the lithosphere is flexurally weak with low t foreland deformation is more complex involves basement and has absorbed less shortening the along strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the bolivian andes we speculate that the observed variation in t is related to the proximity of the brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation and hence bending of the central andes the flexural properties of the lithosphere may play an important role in determining the large scale evolution of mountain belts