the early paleozoic evolution of the argentine recordillera as a laurentian rifted drifted and collided terrane a geodynamic model discussion and reply

rupture directivity of the 2011 mw 5 2 lorca earthquake spain 1 on may 11th 2011 a rather small earthquake caused nine fatalities in the city of lorca se spain we analyze seismograms from a dense network to characterize the source of this earthquake we estimate an oblique reverse faulting mechanism moment magnitude of 5 2 and a shallow hypocenter 4 6 km at only 5 5 km epicentral distance from the city center double difference relocations yield a 5 km long ne sw trending distribution of aftershocks sw of the mainshock suggesting a sw propagating rupture along the alhama de murcia fault we use the mw 4 6 foreshock and an mw 3 9 aftershock as empirical greens functions to estimate apparent source time functions observing a clear directivity effect we model apparent durations with a unilateral and asymmetric bilateral rupture in both cases obtaining rupture directivity of n220 e towards lorca in addition to the near epicenter and shallow depth directivity may have contributed to the significant impact

foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

surface features of winter monsoon surges over south china abstract the surface features associated with two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns surface meteorological parameters over the region 15 50 n 90 130 e for the surges that occurred in the three winters october march from 1988 to 1991 are analyzed for the northerly surge the surface features found are 1 an abrupt temperature drop and wind direction turning from easterly to northerly which can be related to the passage of a cold front 2 an increase in the dewpoint depression and 3 a large north south pressure gradient on the other hand the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h 1 little temperature or pressure change and a southeastward motion of a high pressure center from dahinggangling to the yellow sea together with a sharp pressure ridge along the east china coast furthermore an es and an ns are associated with different perturbations anomalies in

zircon and whole rock nd pb isotopic evidence for a grenville age and a laurentian origin for the basement of the precordillera in argentina suggestions that the precordillera of argentina has an early paleozoic connection to the appalachian ouachita rifted margin of the southern u s have been based on cambro ordovician stratigraphy and faunal assemblages further support comes from comparing precambrian grenville appalachian basement with xenoliths brought up in precordil leran miocene volcanic rocks u pb zircon ages and whole rock pb and nd isotopic data from the xenoliths show that precordilleran basement along with some part of the westernmost sierras pampeanas pie de palo range to the east is essentially unique in south america and is best interpreted as being derived from laurentia a grenville age is substantiated by u pb zircon upper intersect ages of 1102 pm 6 ma for a mafic xenolith and near 1100 ma for acidic xenoliths both xenolith types contain pink doubly terminated prismatic igneous zircons with relatively high u and pb concentrations rounded colorless low u and pb zircons with overgrowths produced in a later pre

rupture process of the 2001 may 7 mw 4 3 ekofisk induced earthquake summary on 2001 may 7 following unintentional water injection a moderate size induced earthquake struck the ekofisk oil field north sea despite of its relatively moderate magnitude clear low frequency waveforms could be recorded up to more than 2000 km epicentral distance suggesting a slow rupture at very shallow depth and wave propagation through low velocity shallow structures the event poses a rare opportunity to constrain rupture velocity duration and rise time of a superficial m 4 event occurring on a horizontal plane in soft water saturated sediments two previous studies discussed the earthquake point source finding vertical dip slip focal mechanisms with opposite senses of p and t axes a further investigation was thus required to provide a basis for a deeper discussion of the failure dynamics we significantly improve the used data set test different earth models and derive a point source as well as a kinematic rupture model we carefully discuss parameter uncertainties and effects related to shallow sources and wave propagation through different crustal structures to resolve the previous controversy we additionally provide a kinematic rupture model based on apparent source times derived from rayleigh and love waves the waveforms resolve a predominant unilateral rupture along a horizontal plane at about 2 km depth we derive an unusually slow rupture consequence of a slow rupture velocity of about 500 m s 1 and a long rise time of about 7 s an independent modelling of gps based static displacements allows to confirm the focal mechanism polarity and to locate the centroid at the eastern side of the field resulting in a much larger seismic moment in comparison with dynamic seismic moment the rupture directivity is confirmed by the relative location of the centroid with respect to the epicentre which is set at the site of water injection

rupture directivity of small earthquakes at parkfield 1 theoretical modeling of strike slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction we test this model with 450 small earthquakes 2 3 exhibiting southeast rupture characteristics some spatial and temporal variability in rupture directivity is also apparent we observe a higher proportion of northwest directivity ruptures following the 2004 m 6 parkfield earthquake which ruptured toward the northwest our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors

source analysis of the february 12th 2007 mw 6 0 horseshoe earthquake implications for the 1755 lisbon earthquake 1 while very large earthquakes are generally confined to subduction zones the sw iberian margin setting of the famous mw 8 5 8 7 1755 lisbon tsunami earthquake may be an exception to this rule evidence for active subduction is not conclusive here but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate length faults we estimate the source parameters of the february 12th 2007 horseshoe earthquake regional moment tensor inversion yields an mw 6 0 reverse to strike slip faulting source in the upper mantle modelling teleseismic surface reflected body waves pp pwp sp indicates a source depth of 40 km beneath the seafloor analysing apparent source time functions allows identifying the preferred fault plane strike n245 e dip 55 rake 50 and estimating rupture area 53 km2 and average slip 0 27 m scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230 315 km being compatible with the length of mapped faults in the area

far field pressurization likely caused one of the largest injection induced earthquakes by reactivating a large preexisting basement fault structure the mw 5 1 fairview oklahoma earthquake on 13 february 2016 and its associated seismicity produced the largest moment release in the central and eastern united states since the 2011 mw 5 7 prague oklahoma earthquake sequence and is one of the largest earthquakes potentially linked to wastewater injection this energetic sequence has produced five earthquakes with mw 4 4 or larger almost all of these earthquakes occur in precambrian basement on a partially unmapped 14 km long fault regional injection into the arbuckle group increased approximately sevenfold in the 36 months prior to the start of the sequence january 2015 we suggest far field pressurization from clustered high rate wells greater than 12 km from this sequence induced these earthquakes as compared to the fairview sequence seismicity is diffuse near high rate wells where pressure changes are expected to be largest this points to the critical role that preexisting faults play in the occurrence of large induced earthquakes

discrimination between induced triggered and natural earthquakes close to hydrocarbon reservoirs a probabilistic approach based on the modeling of depletion induced stress changes and seismological source parameters earthquakes occurring close to hydrocarbon fields under production are often under critical view of being induced or triggered however clear and testable rules to discriminate the different events have rarely been developed and tested the unresolved scientific problem may lead to lengthy public disputes with unpredictable impact on the local acceptance of the exploitation and field operations we propose a quantitative approach to discriminate induced triggered and natural earthquakes which is based on testable input parameters maxima of occurrence probabilities are compared for the cases under question and a single probability of being triggered or induced is reported the uncertainties of earthquake location and other input parameters are considered in terms of the integration over probability density functions the probability that events have been human triggered induced is derived from the modeling of coulomb stress changes and a rate and state dependent seismicity model in our case a 3 d boundary element method has been adapted for the nuclei of strain approach to estimate the stress changes outside the reservoir which are related to pore pressure changes in the field formation the predicted rate of natural earthquakes is either derived from the background seismicity or in case of rare events from an estimate of the tectonic stress rate instrumentally derived seismological information on the event location source mechanism and the size of the rupture plane is of advantage for the method if the rupture plane has been estimated the discrimination between induced or only triggered events is theoretically possible if probability functions are convolved with a rupture fault filter we apply the approach to three recent main shock events 1 the m w 4 3 ekofisk 2001 north sea earthquake close to the ekofisk oil field 2 the m w 4 4 rotenburg 2004 northern germany earthquake in the vicinity of the sohlingen gas field and 3 the m w 6 1 emilia 2012 northern italy earthquake in the vicinity of a hydrocarbon reservoir the three test cases cover the complete range of possible causes clearly human induced not even human triggered and a third case in between both extremes

the 2016 mw5 1 fairview oklahoma earthquakes evidence for long range poroelastic triggering at 40 km from fluid disposal wells abstract wastewater disposal in the central u s is likely responsible for an unprecedented surge in earthquake activity much of this activity is thought to be driven by induced pore pressure changes and slip on pre stressed faults which requires a hydraulic connection between faults and injection wells however direct pressure effects and hydraulic connectivity are questionable for earthquakes located at large distances and depths from the injectors here we examine triggering mechanisms of induced earthquakes which occurred at more than 40 km from wastewater disposal wells in the greater fairview region northwest oklahoma employing numerical and semi analytical poroelastic models the region exhibited few earthquakes before 2013 when background seismicity started to accelerate rapidly culminating in the mw5 1 fairview earthquake in february 2016 injection rates in the 2 2 5 km deep arbuckle formation started to increase rapidly in 2012 about two years before the start of seismicity increase most of the injection activity was concentrated toward the northeast of the study region generating a relatively cohesive zone of pressure perturbations between 0 1 and 1 mpa much of the near injection seismicity was likely triggered by pressure effects and fault assisted pressure diffusion to seismogenic depth outside of the high pressure zone we observed two remarkably detached linear seismicity clusters which occurred at 20 to 50 km distance from the initial seismicity and 10 to 40 km from the nearest high rate injector semi analytical models reveal that poroelastically induced coulomb stress changes surpass pore pressure changes at these distances providing a plausible triggering mechanism in the far field of injection wells these results indicate that both pore pressures and poroelastic stresses can play a significant role in triggering deep and distant earthquakes by fluid injection and should be considered for seismic hazard assessment beyond the targeted reservoir

finite fault slip model of the 2011 mw 5 6 prague oklahoma earthquake from regional waveforms the slip model for the 2011 mw 5 6 prague oklahoma earthquake is inferred using a linear least squares methodology waveforms of six aftershocks recorded at 21 regional stations are used as empirical green s functions egfs the solution indicates two large slip patches one located around the hypocenter with a depth range of 3 5 5 km the other located to the southwest of the epicenter with a depth range from 7 5 to 9 5 km the total moment of the solution is estimated at 3 37 1024 dyne cm mw 5 65 the peak slip and average stress drop for the source at the hypocenter are 70 cm and 90 bars respectively approximately one half the values for the mw 5 8 2011 mineral virginia earthquake the stress drop averaged over all areas of slip is 16 bars the relatively low peak slip and stress drop may indicate an induced component in the origin of the prague earthquake from deep fluid injection

the 2004 mw 4 4 rotenburg northern germany earthquake and its possible relationship with gas recovery we study the 20 october 2004 m w 4 4 rotenburg wumme neuenkirchen earthquake located in a previously aseismic region in the northern german sedimentary basin we constrain the source parameter by using different techniques a possible relationship between this event the regional tectonic setting and local gas recovery is investigated different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km the source mechanism was oblique normal faulting on planes striking roughly north south an inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north consistent with the higher macroseismic intensities found toward the north in the region of hamburg compared with those at a similar distance toward the south in the region of hannover relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals comparison with seismicity triggered in the northern netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the m w 4 4 rotenburg event may be related to gas recovery online material focal mechanism and waveform fit

source analysis of the mw 6 3 2004 al hoceima earthquake morocco using regional apparent source time functions 1 we use small and moderate aftershocks of the 24 february 2004 al hoceima earthquake as empirical greens functions egfs to retrieve the rupture history of the main event the magnitude depth and geometry of faulting were estimated for the main shock and 20 small and moderate aftershocks mw between 3 9 and 5 2 computing moment tensor solutions for the main shock we obtain a moment magnitude of mw 6 3 and a nearly pure double couple source 4 compensated linear vector dipole with predominately strike slip faulting the preferred fault plane has a strike of n11 e a dip of 72 and a rake of 17 fourteen aftershocks having depths 4 km and faulting geometries 11 for the strike of the preferred plane similar to the ones found for the main event were used as egfs apparent source time functions astfs of the al hoceima event were retrieved at twelve regional stations using an iterative time domain deconvolution on the p waveforms at each station time functions are enhanced by stacking of the deconvolved waveforms for different egfs astfs show total durations of 5 to 6 s two consecutive subevents separated by about 3 s and only a weak effect of directivity we further investigate the source history by inverting the set of astfs for the distribution of fault slip and rupture propagation two different source models were selected for presentation one involving one single fault plane and the other involving two parallel fault planes the two fault model fits more accurately the observations and leads to a more plausible slip distribution we estimate rupture dimensions of about 10 10 km2 and 8 8 km2 and moment magnitudes of mw 6 2 and 6 0 for the first and second subevent respectively

the role of coulomb stress changes for injection induced seismicity the basel enhanced geothermal system 1 we estimate coulomb stress variations cfs by considering interactions among 163 earthquakes ml 3 4 that occurred during the hydraulic stimulation of the enhanced geothermal system in basel switzerland in 2006 we observe that overall 75 of event locations are consistent with positive cfs the performance of the model increases with time and distance from injection accordingly with the presumed less dominant role of the pore pressure further from the injection well and after shut in we analyze the sensitivity of results to location and focal mechanism uncertainties the fault plane ambiguity and the friction coefficient we find that cfs are highly sensitive to location accuracy but robust with regard to uncertainties of the other parameters our results suggest that i the coulomb model may complement conventional pore pressure triggering models and ii implementing it for time dependent seismic hazard assessment during fluid injection may improve the forecasting ability but will require highly accurate hypocenter estimates

a global search for stress shadows 1 debate continues regarding the relative proportion of earthquakes triggered by passing seismic waves versus static stress changes from a main shock static stress changes are expected to have long term effects on earthquake probabilities whereas dynamic stress changes due to the passing of seismic waves should not both mechanisms are expected to raise seismicity rates in some areas but only static stress change calculations predict rate decrease shadows thus identification of post main shock earthquake suppression is diagnostic of a static stress change process we note that in principle static stress change theory predicts suppression of particular earthquake mechanisms in a shadow zone rather than an overall rate reduction a stress shadow can therefore be characterized by a change in the average earthquake focal mechanism before and after a main shock that results from suppression of a given mechanism type we examined average mechanisms from 2 radii and 5 year periods before and after 119 ms 7 main shock earthquakes drawn from the harvard centroid moment tensor cmt catalog significant average mechanism changes caused by earthquake suppression were found in only two cases however by stacking the data we were able to resolve statistically significant suppression of particular post main shock focal mechanisms this indicates that while static stress shadows are subtle they are indeed present in the global catalog

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

rupture directivity of microearthquake sequences near parkfield california 1 the direction of propagation is an important factor that affects the pattern of ground motion generated by an earthquake characterizing factors favoring a potential rupture propagation direction is thus an important task here we analyze the earthquake directivity of repeating earthquake sequences located on the san andreas fault near parkfield california all earthquakes of a sequence have very similar waveforms and have overlapping surface ruptures we show that subtle variations of the transfer function between earthquakes of a common sequence can be interpreted as a change of apparent rupture duration relative apparent rupture durations are computed for all pairs of events at all available stations and for each sequence we invert these measurements to obtain an estimation of the apparent rupture duration for each individual event of the sequence relative to a reference event variation of apparent rupture duration with azimuth attests for the rupture directivity we show that the majority of analyzed microearthquakes presents a rupture in the south east direction we also show that on a given repeating sequence most earthquakes tend to show the same rupture direction

extended fault inversion with random slipmaps a resolution test for the 2012 mw 7 6 nicoya costa rica earthquake inversions for the full slip distribution of earthquakes provide detailed models of earthquake sources but stability and non uniqueness of the inversions is a major concern the problem is underdetermined in any realistic setting and significantly different slip distributions may translate to fairly similar seismograms in such circumstances inverting for a single best model may become overly dependent on the details of the procedure instead we propose to perform extended fault inversion trough falsification we generate a representative set of heterogeneous slipmaps compute their forward predictions and falsify inappropriate trial models that do not reproduce the data within a reasonable level of mismodelling the remainder of surviving trial models forms our set of coequal solutions the solution set may contain only members with similar slip distributions or else uncover some fundamental ambiguity such as for example different patterns of main slip patches for a feasibility study we use teleseismic body wave recordings from the 2012 september 5 nicoya costa rica earthquake although the inversion strategy can be applied to any type of seismic geodetic or tsunami data for which we can handle the forward problem we generate 10 000 pseudo random heterogeneous slip distributions assuming a von karman autocorrelation function keeping the rake angle rupture velocity and slip velocity function fixed the slip distribution of the 2012 nicoya earthquake turns out to be relatively well constrained from 50 teleseismic waveforms two hundred fifty two slip models with normalized l1 fit within 5 per cent from the global minimum from our solution set they consistently show a single dominant slip patch around the hypocentre uncertainties are related to the details of the slip maximum including the amount of peak slip 2 3 5 m as well as the characteristics of peripheral slip below 1 m synthetic tests suggest that slip patterns such as nicoya may be a fortunate case while it may be more difficult to unambiguously reconstruct more distributed slip from teleseismic data

oklahoma experiences largest earthquake during ongoing regional wastewater injection hazard mitigation efforts the 3 september 2016 mw 5 8 pawnee earthquake was the largest recorded earthquake in the state of oklahoma seismic and geodetic observations of the pawnee sequence including precise hypocenter locations and moment tensor modeling shows that the pawnee earthquake occurred on a previously unknown left lateral strike slip basement fault that intersects the mapped right lateral labette fault zone the pawnee earthquake is part of an unprecedented increase in the earthquake rate in oklahoma that is largely considered the result of the deep injection of waste fluids from oil and gas production if this is indeed the case for the m5 8 pawnee earthquake then this would be the largest event to have been induced by fluid injection since 2015 oklahoma has undergone wide scale mitigation efforts primarily aimed at reducing injection volumes thus far in 2016 the rate of m3 and greater earthquakes has decreased as compared to 2015 while the cumulative moment or energy released from earthquakes has increased this highlights the difficulty in earthquake hazard mitigation efforts given the poorly understood long term diffusive effects of wastewater injection and their connection to seismicity

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

reliability of coulomb stress changes inferred from correlated uncertainties of finite fault source models 1 static stress transfer is one physical mechanism to explain triggered seismicity coseismic stress change calculations strongly depend on the parameterization of the causative finite fault source model these models are uncertain due to uncertainties in input data model assumptions and modeling procedures however fault model uncertainties have usually been ignored in stress triggering studies and have not been propagated to assess the reliability of coulomb failure stress change cfs calculations we show how these uncertainties can be used to provide confidence intervals for co seismic cfs values we demonstrate this for the mw 5 9 june 2000 kleifarvatn earthquake in southwest iceland and systematically map these uncertainties a set of 2500 candidate source models from the full posterior fault parameter distribution was used to compute 2500 cfs maps we assess the reliability of the cfs values from the coefficient of variation cv and deem cfs values to be reliable where they are at least twice as large as the standard deviation cv 0 5 unreliable cfs values are found near the causative fault and between lobes of positive and negative stress change where a small change in fault strike causes cfs values to change sign the most reliable cfs values are found away from the source fault in the middle of positive and negative cfs lobes a likely general pattern using the reliability criterion our results support the static stress triggering hypothesis nevertheless our analysis also suggests that results from previous stress triggering studies not considering source model uncertainties may have lead to a biased interpretation of the importance of static stress triggering

resolution of rupture directivity in weak events 1 d versus 2 d source parameterizations for the 2011 mw 4 6 and 5 2 lorca earthquakes spain resolving robustly source parameters of small moderate magnitude earthquakes is still a challenge in seismology we infer directivity from apparent source time functions astfs at regional distance and quantify the associated uncertainties astfs are used for i modeling a propagating 1d line source from the duration data and ii inverting the 2d slip distribution from the full signals slip inversion is performed through a popperian scheme where random trial models are either falsified on account of large misfit or else become members of the solution set of the inverse problem we assess the resolution of rupture directivity representing centroid shifts from the solution set in a rose diagram using as example an event with well studied rupture directivity the 2011 mw 5 2 lorca spain earthquake 1d and 2d parameterizations yield similar estimates for direction n213 e and n220 e respectively and asymmetry 67 33 65 35 of rupture propagation as well as rupture length 2 1 km 2 7 km and speed 3 5 km s 3 25 km s the high rupture velocity 90 vs may be held primarily responsible for the strong directivity effect of this earthquake we show that inversion of apparent source durations is intrinsically unable to resolve highly asymmetric bilateral ruptures while inversion of full astfs misses part of the signal s complexity suggesting the presence of deconvolution artifacts we extend the analysis to the mw 4 6 foreshock of the lorca earthquake inferring similar directivity parameters and slip pattern as for the mainshock the rupture towards sw of both earthquakes suggests that this direction could be inherent to the fault segment

paleomagnetic confirmation of the laurentian origin of the argentine precordillera abstract several recent tectonic models have portrayed the argentine precordillera has been recently portrayed as an early paleozoic laurentian derived exotic terrane now found in southwestern south america these models have primarily been based on strong biogeographic and stratigraphic evidence however no paleomagnetic data have previously been available to independently test them a paleomagnetic study was therefore carried out on the early cambrian cerro totora formation exposed in the northern reaches of the argentine precordillera after stepwise thermal demagnetization a pre folding remanence was identified in ten sites of this formation yielding a paleomagnetic pole ct at 37 0 n 314 1 e a95 5 8 this pole is not consistent with the latest proterozoic early paleozoic apparent polar wander path for gondwana but it agrees with the early cambrian section of the laurentian path if the argentine precordillera is positioned as the conjugate margin of the ouachita embayment in southeast laurentia this result confirms that the precordillera is an allochthonous terrane derived from laurentia in cambrian times that was later accreted to gondwana probably in middle ordovician times