deformation of the oceanic crust between the north american and south american plates fracture zone trends and magnetic anomalies in the atlantic ocean indicate that the north american plate must have moved with respect to the south american plate during the opening of the atlantic a comparison of plate tectonic flow lines with fracture zones identified from geosat and seasat altimeter data suggests that the north american south american plate boundary migrated northward from die guinea demarara shear margin to the vema fracture zone before chron 34 84 ma to north of the doldrums fracture zone before chron 22 51 9 ma and to north of the mercurius fracture zone between chron 32 72 5 ma and chron 13 35 5 ma the paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area indicate that the triple junction may have been located between the mercurius and the fifteen twenty fracture zones after 67 ma chron 30 plate reconstructions indicate a late cretaceous phase of transtension followed by transpression in the tertiary for the tiburon barracuda ridge area south of the fifteen twenty fracture zone the ocean floor in this area is characterized by a series of ridges and troughs with large bouguer gravity anomalies up to 135 mgal we use smoothing spline estimation to invert bouguer anomalies for crustal layer structure our model results suggest that the moho is uplifted 2 4 km over short wavelengths 70 km at the barracuda and tiburon ridges and imply large anelastic strains the severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted we propose that the north south american plate boundary migrated to the latitude of the tiburon ridge bounded by the vema and marathon fracture zones before chron 34 84 ma post chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the vema marathon mercurius and fifteen twenty fracture zone troughs but reaching the fifteen twenty fracture zone and future barracuda ridge area only after chron 32 72 5 ma this interpretation concurs with our crustal structural model which shows stronger crustal thinning underneath the tiburon ridge than at the barracuda ridge subsequent transpression may have continued along the existing zones of weakness in the tertiary creating the presently observed crustal deformation and uplift of the moho accompanied by anelastic failure of the crust middle eocene upper oligocene turbidites on the slope of the tiburon ridge now located 800 m above the abyssal plain suggest that most of its uplift occurred at post oligocene times the unusually shallow moho underneath the tiburon and barracuda ridges represents an unstable density distribution which may indicate that compressive stresses are still present to maintain these anomalies and that the north american south american plate boundary may still be located in this area

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

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

paleomagnetically defined rotations from the precordillera of northern chile evidence of localized in situ fault controlled rotations a paleomagnetic study of paleocene cretaceous and jurassic units in the precordillera of northern chile 26 s to 27 s demonstrate a complex pattern of crustal rotations the region was selected to investigate the pattern of deformation associated with the sierra castillo agua amarga fault and its associated structural subdomains which form part of the domeyko fault system the fault system that controls much of the structure of the chilean precordillera paleocene lavas from the center of the study area pass a fold test indicating the primary nature of the remanence in these rocks a second group of lavas from the south has a similar formation mean inclination after tilt correction for a uniform dip which coupled with the presence of reversals suggests that these lavas also carry a primary remanence the data indicate clockwise rotations of 35 and 42 respectively for the two sampling localities three geographically separate areas of lower cretaceous red beds also reveal primary remanences based on the presence of sites with reversed polarity at two of the localities and an inclination only fold test of the three locality mean directions for this formation these sampling areas yield remanence directions that reveal 30 of counterclockwise and 31 of clockwise rotation as well as an area without statistically significant rotation these data include the first well constrained counterclockwise rotation to be recovered from rocks in the southern central andes although mapping suggests the block involved is of limited geographic extent jurassic strata failed to yield any reliable results overall the data indicate a greater variability in the rotation pattern than has previously been observed in most comparable areas south of the arica deflection the variation cannot be explained by large scale rotation mechanisms such as oroclinal bending or domino style block rotation by widely separated transandean faults instead the rotations are consistent spatially and temporally and in their sense and magnitude with the eocene transpressional deformation associated with the domeyko fault system the deformational event that generated most of the important structures in the studied area the data emphasize the significance of local tectonics in controlling rotations in this part of the south central andes and the importance of understanding the local structure of regions sampled for paleomagnetic study

paleomagnetism in the precordillera of northern chile 22 30 s implications for the history of tectonic rotations in the central andes abstract widespread clockwise rotations in mesozoic and lower tertiary rocks of northern chile have been interpreted as the sum of two rotational events separated in time an early rotation related to local deformation plus a late rotation related to wholesale rotation of northern chile linked to late cenozoic oroclinal bending in the central andes in this paper we report new paleomagnetic data from cretaceous upper oligocene and miocene sedimentary rocks in the precordillera of northern chile the results suggest that all these rocks acquired their remanence at or close to the time of deposition the lack of rotation in undeformed lower miocene strata clearly indicates that clockwise rotations found in underlying faulted and folded cretaceous rocks were completed before the late cenozoic results from nearby localities in deformed upper oligocene strata would argue for little 5 rotation since the late oligocene data from widely separated miocene localities covering an area of about 5000 km2 in the calama basin strongly suggest that northern chile did not undergo significant wholesale rotation during the late cenozoic this together with previous paleomagnetic evidence against neogene rigid body like rotation of the southern peruvian forearc suggests that the curved shape of the central andean forearc was not significantly enhanced during the late cenozoic by inference all of the rotation in most mesozoic and lower tertiary rocks of northern chile was accomplished in the cretaceous and or early cenozoic when the locus of deformation in the central andes was localized in the present forearc region

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

paleomagnetism of jurassic and cretaceous rocks in central patagonia a key to constrain the timing of rotations during the breakup of southwestern gondwana abstract a paleomagnetic study in jurassic and cretaceous rocks from the canadon asfalto basin central patagonia indicates the occurrence of about 25 30 clockwise rotation in upper jurassic lowermost cretaceous rocks whereas the overlying mid cretaceous rocks do not show evidence of rotation this constrains the tectonic rotation to be related to a major regional unconformity in patagonia which in turn seems to be close in time with the early opening of the south atlantic ocean the sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern gondwana suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent on the other hand the mid cretaceous rocks in the region yield a paleopole located at lat 87 south long 159 east a95 3 8 this pole position is consistent with coeval high quality paleopoles of other plates when transferred to south american coordinates implying it is an accurate determination of the aptian circa 116 ma geomagnetic field in south america

clockwise block rotations along the eastern border of the cordillera de domeyko northern chile 22 45 23 30 s abstract we report new paleomagnetic results from a study of 38 sites in cretaceous to early tertiary red beds and volcanic rocks belonging to the purilactis group which outcrop along the western border of the salar de atacama basin after detailed thermal demagnetization characteristic directions were determined for 32 units in most cases red bed sediments from the lower members of the purilactis group have a well defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement a large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre tectonic magnetization we interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the cretaceous normal polarity superchron large deviations of the paleomagnetic declinations from the expected ones for stable south america provides new evidence for clockwise tectonic rotations associated with tertiary deformation in the cordillera de domeyko these data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north chilean andes this study however indicates spatial variation in the magnitude of the rotation with rotations 60 in the cerro totola area these rotations have occurred in conjunction with transpressional deformation that affected large tracts of the cordillera de domeyko during eocene deformation the systematic observation of clockwise rotations contemporaneous with sinistral displacements in the cordillera de domeyko can be explained by shear traction at the base of the brittle crust

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 cretaceous alkaline complexes southeastern brazil paleomagnetism and geochronology late cretaceous alkaline rocks from pocos de caldas itatiaia passa quatro and sao sebastiao island were studied with the main purpose of improving the south american apparent polar wander path apwp rbsr age determinations for passa quatro and alkaline stocks from sao sebastiao island are around 70 and 81 ma respectively the magnetic carriers in the rocks studied were maghemite and highly oxidized titanomagnetites the paleomagnetic poles obtained in this work are 320 1 e 83 2 s n 47 a95 2 7 ksc 2609 8 for pocos de caldas 360 0 e 79 5 s n 18 a95 5 7 ksc 607 4 for passa quatro and itatiaia and 331 9 e 79 4 s n 18 a95 4 9 ksc 862 4 for sao sebastiao island the island set

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

on the mechanism of crustal block rotations in the central andes comment in this paper beck 1998 reviews the database of palaeomagnetically observed rotations in the andean margin of south america in particular with respect to the well known change in sense of rotation linked to the arica deflection bolivian orocline in the orogen the paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation deformation is time dependent while we essentially agree with the former conclusion we would define it more tightly in that the over arching control on the pattern of rotations is a combination of differential shortening as opposed to oroclinal bending along the orogen coupled with distributed shear across the orogen our main concern with this paper is that it fails to provide a convincing demonstration that there is a time dependent element to the magnitude of rotations primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline at least not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time as postulated we also make specific points about aspects of the paper dealing with the atacama fault zone and its relation to the observed rotations

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

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

effect of a sedimentary basin on estimations of qc and qlg abstract the effect which a sedimentary basin granada basin southern spain exercises on the estimated q c and q lg attenuation values within a determined frequency range are described it has been shown that for the values of q c there is a decrease in the number of fits with a correlation coefficient 0 8 in the 1 5 hz frequency band in comparison with the other frequency bands on the other hand an overestimation of the values of the coda q is observed between a station located on the sediment in the interior of the basin and the stations situated on the borders of the basin the values of the attenuation for lg waves in the paths which cross the granada basin show a series of peaks centred in the 1 3 hz frequency band which overestimate the values of q lg with respect to other frequency bands all these phenomena indicate that the presence of a low velocity sedimentary basin modifies the processes of generation of coda waves and the propagation of lg waves

palaeomagnetism of the continental sector of the cameroon volcanic line west africa summary measurement of samples from 154 sites in the continental sector of the cameroon volcanic line yielded six palaeomagnetic poles at 243 6 e 84 6 n a 95 6 8 224 3 e 81 2 n a 95 8 4 176 1 e 82 0 n a 95 8 5 164 3 e 86 4 n a 95 3 4 169 4 e 82 6 n a 95 4 6 and 174 7 e 72 8 n a 95 9 5 belonging to rocks which have been dated by the k ar method at 0 4 0 9 ma 2 6 ma 6 5 11 ma 12 17 ma 20 24 ma and 28 31 ma respectively the results are in general agreement with other palaeomagnetic poles from oligocene to recent formations in africa the first three poles for rocks formed between 0 4 and 11 ma are not significantly diverent from the present geographical pole together with other african poles for the same period this suggests that the african continent has moved very little relative to the pole since 11 ma the other three poles for rocks dated between 12 and 31 ma are significantly diverent from the present geographical pole showing a 5 polar deviation from the present pole in the miocene and 13 in the middle oligocene

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

tectonic signatures in arid continental basins an example from the upper miocene pleistocene calama basin andean forearc northern chile abstract the calama basin a transtensional basin situated within the andean forearc and its extension to the pampa del tamarugal central depression is filled with 700 m of sediments detailed sedimentary logging and 40ar 39ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick upper miocene pleistocene succession five facies associations fluvio lacustrine diatomite palustrine carbonate massive anhydrite fluvial and alluvial fan have been identified within this part of the basin fill the following events are recognised in the basin evolution 1 deposition of diatomaceous lacustrine sediments in two separate depocentres in the pampa del tamarugal and the central calama basin with alluvial fan accumulation along the basin margins at 8 ma 2 localised deformation within the central calama basin between 7 and 6 ma but continuous sedimentation elsewhere 3 palustrine carbonate deposition across the entire area from 6 to 3 ma except in the pampa del tamarugal where diatomaceous sedimentation continued until 5 ma before being replaced by palustrine carbonate deposition 4 widespread folding after 3 ma and entrenchment as the drainage system cut down to reach a new base level caused by the rio loa breaching the coastal cordillera and reaching the sea localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period comparison with similar age facies elsewhere in northern chile has shown changes such as these to be localised and diachronous indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting the lacustrine deposits do not appear to be a reliable recorder of low frequency climatic change this is exemplified by the change from lacustrine to palustrine deposition at 6 5 ma which would appear to record a drying of the climate however a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run off

a tale of two continents some tectonic contrasts between the central andes and the north american cordillera as illustrated by their paleomagnetic signatures comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large scale tectonic processes comparison of the paleomagnetic signatures of the western cordillera of north america and the central andes shows that the western edges of these belts have had profoundly different mesozoic and cenozoic histories specifically the north american cordilleran pattern shows strong evidence of post middle cretaceous relative northward displacement of outboard crustal blocks but there is almost no comparable evidence of margin parallel displacement in the andes we speculate that this may largely be a consequence of a simple difference in shape the convex westward western margin of north america facilitates margin parallel displacement as a response to oblique subduction whereas the concave westward margin of south america inhibits it the patterns of block rotations found along the western edge of the two orogens also are quite different nearly everywhere within the western north american cordillera crustal blocks have rotated clockwise since mid cretaceous time reflecting a pervading state of dextral shear within the western andes of peru and northern chile however mesozoic and cenozoic rocks in peru and northernmost chile are rotated strongly counterclockwise whereas rocks of the same age in the remainder of chile to about latitude 48 s are rotated clockwise a model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations

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

paleomagnetic study of upper miocene rocks from northern chile implications for the origin of late miocene recent tectonic rotations in the southern central andes paleomagnetic studies in the southern central andes have shown the widespread presence of clockwise vertical axis rotations rock units sampled in these studies however are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern chile being from mesozoic and lower tertiary rocks whereas most data in the southern altiplano puna and cordillera oriental are from upper tertiary rocks in this paper we present the results of a paleomagnetic study on upper miocene sedimentary rocks and ignimbrites from the precordillera of northern chile at 22 s these rocks are coeval with the initiation of crustal shortening in the eastern foreland fold thrust belt which some tectonic models relate to oroclinal rotation of northern chile primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites suggesting the study area has acted as a single coherent block with respect to vertical axis rotational deformation although minor inadequate sampling of paleosecular variation can affect our data set its time averaged paleomagnetic direction is indistinguishable from the expected late miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 ma this suggests that late miocene recent oroclinal rotation of the northern chilean forearc if present must be either very low or nonuniform a direct implication of this result is that much of the unquestionable tectonic rotations detected from upper miocene rocks in the southern central andes is of local origin available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the andes

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

magmatic arc tectonics in the central andes between 21 and 25 s abstract in northern chile four magmatic arc systems each displaced towards the east in relation to the previous one developed in sequence since the lias this migration of the magmatic arc from the coastal cordillera to the western cordillera probably was a consequence of tectonic erosion of the continental margin within each of these systems tectonic deformation took place contemporaneously to magmatic activity as the continental crust of the upper plate has been weakened in the area of the magmatic arc by the igneous processes it is relatively sensitive to the stresses introduced by plate convergence a very important factor for the deformation in the magmatic arc is the angle of convergence obliqueness which may cause shortening if obliqueness is small

development of a continental forearc a cenozoic example from the central andes northern chile in order to understand the response of a continental forearc to changes in subduction zone geodynamics we constructed a high resolution chronostratigraphic cross section across the central andean forearc of northern chile 21 24 s the tectono stratigraphic development of the forearc differs from established models no relationship was found between changes in rate of relative plate convergence and amount and style of deformation forearc response to continual compression since the oligocene has been uplift and segmentation into discrete tectono stratigraphic zones from west to east these zones are the extensional coastal cordillera the extensional and or transtensional central depression and the transpressional and or compressional precordillera preandean depression each area has recorded almost continuous sedimentation from oligocene eocene time to the present day accommodation space has been generated by basin margin uplift rather than active subsidence we propose a model in which uplift of the leading edge of the south american plate is driven by subcrustal accretion of material removed at the trench by subduction erosion uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge the tectono stratigraphic complexity exhibited within the central andean forearc is likely to be representative of cordilleran type margins and would be difficult to recognize in an ancient continental forearc

paleomagnetic results from the mogollon datil volcanic field southwestern new mexico and a refined mid tertiary reference pole for north america miocene oligocene rocks of the mogollon datil volcanic field in southwestern new mexico have yielded paleomagnetic data giving 61 site mean unit vgps and a paleomagnetic pole located at 81 9 n 143 6 e a95 4 5 k 17 6 this pole is indistinguishable from other mid tertiary reference poles for north america combining the mogollon datil data with vgp data from other studies of mid tertiary 20 40 ma rocks yields a paleomagnetic pole at 81 5 n 147 3 e n 239 a95 2 4 k 15 4 when divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95 confidence level mcfadden and lowes 1981 the paleopole determined from reversely magnetized units located at 78 6 n 138 5 e n 125 a95 3 4 k 14 5 is 6 8 on the far side of the paleopole determined from normally magnetized units positioned at 84 2 n 166 4 e n 114 a95 3 3 k 17 0 when viewed from a common site location i e the western united states similar pole positions an differences are also seen in 10 ma subsets of this data set incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results rather we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term merrill and mcelhinny 1977 merrill et al 1979

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

paleomagnetic evidence for rotation in the precordillera of northern chile structural constraints and implications for the evolution of the andean forearc abstract palaeomagnetic results are reported for two late mesozoic to early cenozoic continental sedimentary sequences from the precordillera cordillera de domeyko of northern chile comparison of isolated components of magnetisation from the late cretaceous palaeocene purilactis formation and the unconformably overlying oligo miocene paciencia group with reference poles for south america indicate that significant post palaeocene and post lower miocene clockwise rotation has taken place a structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the cenozoic stratigraphie structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the purilactis formation lower eocene incaic orogeny and the paciencia group lowest miocene peheunche orogeny and resulted in the folding uplift and clockwise rotation of the two sequences differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets 12 of rotation took place prior to deposition of the paciencia group followed by 17 of post lower miocene clockwise rotation this study demonstrates that palaeomagnetically detected rotation in the precordillera of northern chile 250 km inland of the peru chile trench is related to the periodic movement of an easterly propagating thin skinned thrust front between 22 30 and 23 30 s the north chilean forearc contains areas of non rotation thin skinned compressional related rotations and large block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the central andes should be modified in particular evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event

a topographic fingerprint to distinguish alluvial fan formative processes we demonstrate here a topographic fingerprint derived from a 1 m high resolution elevation model dem developed from airborne laser swath mapping alsm that distinguishes between formative processes from surface form in alluvial fans generated through relatively dry debris flows df and those surfaces formed primarily by fluvial sediment transport mf we selected alluvial fans with primarily holocene age surficial deposits along the eastern side of death valley california fans were initially classified in the field into df and mf types largely on the basis of sedimentological characteristics and surface morphometry trend surface analyses were performed on dems of the individual fans in order to remove the long wavelength trends both profile and planimetric from the residual data we found that high resolution surface data distinguishes process information in two ways 1 local relief increases as a function of length scale especially at length scales less than 40 60 m and 2 local relief is significantly higher for debris flow dominated fans than for fluvially dominated fans this fingerprint which is directly related to the dominant surface processes is unavailable from standard 30 m resolution data providing significant support for the acquisition of high quality high resolution data this fingerprint can be used to help calibrate numerical simulations of alluvial fan development and has the potential for similar applications to distinguish between processes of similar landforms on other planetary bodies within our solar system