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

testing the occurrence of thick skinned triangle zones in the central andes forearc example from the salar de punta negra basin in northern chile abstract the salar de punta negra basin in the forearc of the central andes serves as a natural laboratory to understand the evolution of thick skinned triangle zones dominated by opposite verging contractional systems we combined field data and w e 2d seismic profiles along parts of the basin where stratigraphic units are well constrained the seismic data show the occurrence of thick skinned triangle zones under the basin with along strike variation these are composed of west and east verging basement involved faults and thrust ramps or partially inverted normal faults the plan view structural configuration of the basin usually coincides with the sub surface position of the triangle zones the interplay between purely reverse and inverted faults in the region indicates that the thick skinned triangle zones could be kinematically related to the superposition of two contractional episodes consisting of i partial tectonic inversion of half graben structures and ii basement deformation dominated by reverse faults and thrust and anticline ramps the post extensional tectono sequences interpreted on the seismic profiles generally correlate with the upper cretaceous to paleocene synorogenic successions exposed on surface which allow us propose an age of deformation correlated with the peruvian and k t andean orogeny the triangle zone could still be an active structure

circum pacific mid cretaceous deformation and uplift a superplume related event evidence of short lived episodes of mid cretaceous deformation metamorphism uplift and hiatus in sedimentation is widespread in the lower cretaceous rocks that bordered the cretaceous pacific basin i present a model in which these coeval but widely spaced events are linked to increased ridge push force at subduction zones triggered by large scale upwelling of mantle associated with the mid cretaceous superplume this model can account for mid cretaceous terrane accretion and ophiolite obduction events and can explain brief compressional phases in predominantly extensional orogenies as recognized in alaska british columbia california western south america and west antarctica

the toarcian oceanic anoxic event in southwestern gondwana an example from the andean basin northern chile palaeoenvironmental and palaeoclimatic changes during the toarcian oceanic anoxic event t oae are reported from two successions deposited in the andean basin chile based on a high resolution biostratigraphic calcareous nannofossils sedimentological mineralogical and geochemical analysis a large negative carbon isotope excursion was identified in a lower toarcian storm dominated marl limestone alternation underlining the global extent of the carbon cycle perturbation and the occurrence of episodic high energy conditions during the t oae low total organic carbon content and absence of redox sensitive trace element enrichment indicate that the dynamic depositional conditions did not favour the development of oxygen depleted conditions and the preservation of organic matter the decrease in kaolinite reflects a shift to more arid conditions whereas the increase in the chemical index of alteration at the onset of the t oae is probably related to a change in the sediment source less hydrolytic activity probably promoted a decrease in nutrient input in the andean basin mercury anomalies support the link between the onset of the karoo ferrar large igneous province activity the carbon cycle perturbation and the generated environmental change this paper presents one of the most complete t oae studies from the southern hemisphere and reveals that regional conditions strongly modulated the expression of this global event in the andean basin supplementary material the complete dataset the coefficient of correlation r for major and trace elements and the nannofossil assemblages are available at https doi org 10 6084 m9 figshare c 4133687

two dimensional viscosity structure of the northeastern japan islands arc trench system 1 two dimensional viscosity profiles were constructed for the northeastern japan islands arc trench system covering the source area of the 2011 tohoku oki earthquake from seismologically determined models of lithospheric structure experimentally derived constitutive laws of various rocks and densely measured geothermal gradient data we have predicted the steady state effective viscosity across the subduction zone the profile reveals strong lateral viscosity gradients both parallel and normal to the trench axis the detailed viscosity structures presented here contribute to accurate evaluation of viscoelastic relaxation components when modeling geodetically measured postseismic deformation at high spatial and temporal resolution

the mesozoic marginal basin of central peru the west peruvian trough1 2 extends some 1 300 km from north of trujillo to about 150 km short of arequipa fig 1 it received upwards of 9 000 m of material during the mesozoic and within the cretaceous rocks there is a well defined west to east facies change whereby thick volcaniclastic turbidites and pillow lavas the eugeosyncline of cobbing3 thin eastward to be replaced by a mixed clastic shelf sequence the miogeosyncline we report here chemical analyses of the volcanic rocks and data on the association of cherts volcaniclastic turbidites dyke and sill complexes and large gabbro masses as well as geophysical evidence which suggests that the eugeosyncline is an extensional marginal basin similar to that seen in rocks of the same age in southern chile4 the coastal batholith is thought to have been emplaced within this marginal basin during the upper cretaceous5 and the evolution of this ensialic new crust therefore has important implications for the growth of the continental margin of south america as well as for batholith genesis

the chloride source for atacamite mineralization at the radomiro tomic porphyry copper deposit northern chile supergene mineralization at the radomiro tomic porphyry copper deposit involves the copper chloride atacamite cu4cl2 oh 6 as the principal ore mineral distributed in a thick oxide zone chloride rich aqueous fluids were introduced during supergene mineralization causing this atypical variation in copper oxide mineralogy the source of this chloride has been investigated using several geochemical techniques including chlorine isotopes and bromide chloride geochemistry these methods have proven to be the most useful in evaluating the potential chloride sources and their likely influence on copper mineralization unmineralized biotite bearing whole rock protore samples from the radomiro tomic deposit have typical igneous values of 2 ppm br and 37cl values of 0 1 to 2 5 per mil in contrast elevated bromine contents 22 180 ppm br and lower 37cl values near 0 0 per mil for the atacamite mineralization at radomiro tomic are clearly indicative of a nonmagmatic origin for the chloride moreover atacamite mineralization at chuquicamata 48 75 ppm br 37cl 0 1 to 0 2 and mina sur 52 48 ppm br 37cl 0 0 0 1 also has elevated bromine contents and lower 37cl values indicating a similar source for the introduction of chloride upper jurassic sediments 22 24 ppm br 37cl 0 8 to 0 5 have geochemical characteristics indicating that they were the source of chloride for the oxide zone atacamite mineralization at radomiro tomic a later mineralization event where chloride was derived from marine evaporites was responsible for the atacamite mineralization in the overlying gravel at radomiro tomic 63 180 ppm br 37cl 0 6 small distal deposits containing atacamite with 20 to 100 ppm br and 37cl values of 0 1 to 3 2 per mil occur to the west and south of radomiro tomic and have geochemical signatures which are indicative of lower jurassic sediment derived chlorine 9 23 ppm br 37cl 0 3 to 2 6 bromide chloride geochemistry of regional atacamite samples is quite variable and therefore argues against a meteoric chloride source which would lead to regionally homogeneous values similarly seawater was ruled out as the primary source of chloride at radomiro tomic due to the distance from the pacific ocean

strigoceras septicarinatum jurassic ammonoidea in the precordillera of north chile paleobiogeographical and paleoenvironmental implications abstract the first circum pacific record of the bajocian dimorphic ammonite strigoceras septicarinatum from the quebrada san pedro area antofagasta northern chile is described and illustrated morphological features of two specimens display macroconch and microconch characters of a single biospecies usually assigned to separate morphospecies of the west tethyan genera strigoceras macroconch and cadomoceras microconch preservational features of these two specimens found in situ in the same stratigraphic level at the top of the torcazas formation correspond to resedimented elements of the uppermost bajocian dimorphinites biohorizon contemporaneous with the sedimentary matrix these specimens represent dimorphic shells produced by extremely scarce individuals inhabiting the tarapaca basin without evidence of lasting biostratinomic modifications such as sorting by necroplanktic drift or biostratinomic encrusting in the tarapaca basin the distribution of ammonite shells was taphonomically and ecologically driven by regional changes of relative sea level taphonomic paleoecological and paleobiogeographical observations in the areas of quebrada san pedro and caracoles corroborate the development of an advanced deepening phase with episodes of sedimentary starvation during the uppermost bajocian parkinsoni zone this phase characterizes the last episode within a deepening half cycle of a third order and the maximum deepening of a second order transgressive regressive facies cycle in the marine back arc tarapaca basin during the bajocian bathonian interval these paleontological and paleoenvironmental results represent new criteria to characterize the dimorphinites biohorizon in the tarapaca basin allowing the identification the upper bajocian parkinsoni zone in the east pacific subrealm as well as chronocorrelation between the west tethyan and east pacific basins at the bajocian bathonian boundary

composition of the crust and upper mantle in the central andes 19 30 s inferred from p wave velocity and poisson s ratio the present day crustal structure of the andean chain results from deformation processes and magmatic activity since the onset of subduction one line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper mantle by imaging the structures in velocity and poisson s ratio or vp vs a high density seismic network was operated during six months in 1994 across the whole andean chain the p and s travel times of the best located local earthquakes mainly situated in the subduction zone have been inverted for vp and vp vs using thurber s 3d iterative simultaneous inversion method the resulting models giving short scale information about the crustal and upper mantle composition provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the andean crust beneath the western cordillera the seismic properties are well explained by typical active volcanic arc processes and the thickening of the crust is explained by magmatic addition conversely beneath the central altiplano basin and the eastern cordillera our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process moreover cratonic lower crust and subcontinental lithospheric upper mantle are observed under the eastern cordillera up to 67 w evidence of the underthrusting of the brazilian craton which drives the crustal shortening beneath the central altiplano basin a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the eastern cordillera thus partial melt is observed in the shallow mantle west of the western cordillera up to the brazilian craton

thermal and exhumation history of the coastal cordillera arc of northern chile revealed by thermochronological dating abstract the thermal and erosional history of convergent plate boundaries is important for understanding the links between subduction arc magmatism genesis of ore deposits topography and climate of orogenic belts unlike the continent continent collision that formed many of the largest orogenic belts known today the central andes of south america is a unique case where an oceanic continent collision has given rise to the earth s longest and second tallest orogenic belt over the last thirty years a plethora of models have been suggested in an attempt to explain how a plateau type orogen formed at the leading edge of western south america in the central andes most research have focussed attention on the study of the evolution of the arc and backarc since continuous subduction erosion of the forearc has left little trace of the interplate dynamics that initiated the orogenic belt in this article we present a new insight into the thermal and exhumation history of the forearc along the coastal cordillera of northern chile based on biotite k ar apatite fission track and apatite zircon u th he dating we collected diorite samples in a 2 km thick crustal section at the coastal cliff 22 s and a sea level isoelevation profile between 21 and 27 s results from all three dating methods show that the cooling of coastal cordillera took place shortly after emplacement during a period of rifting in jurassic times cooling took place in two episodes mainly in late jurassic early cretaceous 118 152 ma but also during late cretaceous 60 80 ma due to the resumption of compression rift closure arc uplift exhumation eastward migration of magmatic arc activity and thermal relaxation the youngest apatite u th he ages reveal a cooling event never reported previously between 40 and 50 ma eocene this thermal event affected a 500 km long and 1 km thick section of the coastal cordillera in northern chile rock cooling recorded in the eocene cannot be explained by the thermal effect of the magmatic arc we explore two scenarios that can provide an explanation for the observed cooling 1 forearc uplift and exhumation and 2 changes in plate subduction dynamics erosion rates of 0 24 to 0 36 km myr for a period of 10 myr are necessary to explain the cooling event alternatively the subduction of a ridge could explain significant cooling by flat slab dewatering based on recent sea floor spreading reconstruction we suggest the subduction of the farallon phoenix ridge as a possible candidate the subduction of this ridge can account for the cooling event at 40 50 ma but also for some important aspects of the eocene development of the central andes notably the onset of uplift an abrupt decrease in magmatic activity and porphyry copper mineralization

the emplacement of the granitic las tazas complex northern chile the relationship between local and regional strain abstract the coastal batholith of northern chile grew under transtensional extensional conditions that prevailed along the andean margin during the mesozoic the batholith hosts the atacama fault zone a major arc parallel fault system which was characterised by sinistral transtensional shearing during the early cretaceous the las tazas complex is a composite granitoid intrusion that was emplaced syntectonically along the atacama fault zone at 130 ma syntectonic emplacement is indicated by a consistent kinematic history between the complex and its wall rocks together with synchronous crystallisation and shearing ages in contrast to regional patterns the las tazas complex was emplaced during a local change from vertical east side down to dextral transcurrent displacement along the fault zone during intrusion strain was partitioned between non coaxial simple shearing within country rock mylonites and a flattening strain across the crystallising complex this combination indicates that the pluton was emplaced under temporary transpressive conditions that were localised around the pluton probably induced by magma emplacement such a difference between local and regional strain suggests that emplacement related structures should only be related to regional strain states with great care

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

early jurassic mass extinction a global long term event the end pliensbachian extinction event 187 ma has been interpreted either as one of 10 global periodically recurring mass extinctions of the past 250 m y or as a minor localized european event elevated levels of family extinction spanned five ammonite zones during the late pliensbachian and the early toarcian an interval of 7 5 m y and were distributed unequally in the boreal tethyan and austral realms detailed sampling of invertebrate macrofaunas through complete expanded sequences in northwest europe shows that most species extinctions occurred in the early toarcian following a regional anoxic event the early jurassic mass extinction event took place over a long time scale and it was global in extent

geology of the escondida porphyry copper deposit antofagasta region chile the escondida porphyry copper deposit located in northern chile was one of the two largest copper producers of the world in the 1990s the hydrothermal evolution of this deposit is associated with the emplacement of a late eocene oligocene quartz monzonitic to granodioritic intrusive stock complex composed of at least three intrusive phases and hosted by paleocene andesite this paleocene andesite is underlain by mesozoic and paleozoic sedimentary and volcanic rocks which are characteristic of the basement of the precordillera de domeyko province in northern chile we propose that the intrusive stock associated with the mineralization at escondida was emplaced in a tensional gash formed between sinistral strike slip faults of the regional domeyko fault system the complete evolution of the porphyry system is characterized by overprinting of pervasive and vein associated alteration mineralization styles grouped in three main hydrothermal stages the early stage includes a zone of pervasive biotitization of andesite and development of a silicification shell around the intrusive complex propylitic alteration around the biotitic zone and a vein associated orthoclase quartz anhydrite biotite alteration that mark the end of stage one this early alteration contains magnetite chalcopyrite and bornite with less than 0 5 vol percent of sulfides and a hypogene copper grade of 0 2 wt percent the second hydrothermal stage is represented by vein and vein selvage associated chlorite sericite quartz and by quartz sericite with sulfides including chalcopyrite pyrite and molybdenite in the chlorite sericite and quartz sericite zones the content of sulfides ranges from less than 0 5 to 2 vol percent with a chalcopyrite to pyrite ratio of 3 to 1 and copper grades that range between 0 4 and 0 6 wt percent the intrusion of a rhyolite dike and a dome are at least 3 m y younger than the first and second hydrothermal stages and separate them in time from the late hydrothermal stage the late hydrothermal stage is represented by an acid sulfate mineral association that includes pyrophyllite alunite and quartz as alteration minerals and a variety of sulfides that include bornite chalcopyrite pyrite chalcocite covellite enargite sphalerite tennantite and galena this acid sulfate event occurred mainly in west northwest striking veins and also along the contact of the rhyolite and its host rock where sulfides from this event overprint previous sulfides the primary copper grades range from 0 6 to higher than 1 0 wt percent in the escondida deposit the highest hypogene and supergene copper grades occur in areas where all three hydrothermal stages are present

extensional tectonics in convergent margin basins an example from the salar de atacama chilean andes the salar de atacama basin of northern chile preserves stratigraphic evidence for the evolution of the andean cycle it has evolved from a non arc related rift through back arc and inter arc stages to a neogene fore arc basin accumulation of the sedimentary succession was mainly due to extensional faulting important but short duration contractional episodes do link to known fast order plate margin changes but their stratigraphic effect appears to be restricted to uplift erosion rather than creation of significant flexural subsidence the salar de atacama basin originated as part of a regional rift system during permian time and contains 2 km of permo triassic continental detritus and volcanic rocks the area remained above depositional base level throughout late triassic to late cretaceous time syn rift continental red beds were deposited to a thickness of at least 2 km on a parallel rift segment in the domeyko area to the west continued triassic jurassic extension in the domeyko basin resulted in a classic continental to marine transition with deposition of a 2 km jurassic mixed carbonate clastic sequence in latest cretaceous eocene time the salar basin was an arc related basin and accommo dated some 4 km of continental detritus purilactis group due to back arc extension sediment being derived from the domeyko cordillera and arc rocks to the west late eocene right lateral strike slip faulting and associated restraining bend uplift were driven by a high rate of oblique convergence between the farallon and south american plates this deformation complicated the stratigraphy of the purilactis group and inverted the western basin margin which formed the dominant provenance area for a 2 km thick oligocene continental basin fill component paciencia group the oligocene basin was an extensional to trans tensional basin the miocene holocene salar basin is a continental fore arc basin this latest segment of the basin fill comprises pyroclastic and continental sedimentary rocks thrust over quaternary gravels in many places the cordillera de la sal is an intrabasinal uplift initiated as a thin skinned contractional feature thus the superposed basin fill components represent responses to distinctly different geodynamic settings

the role of inherited tectono sedimentary architecture in the development of the central andean mountain belt insights from the cordillera de domeyko abstract the structure of the cordillera de domeyko is dominated by a number of elongated n s trending basement ridges these ridges were uplifted by steep reverse n s faults that deformed the mesozoic cenozoic cover the vergence of the fault system varies along strike conferring an apparent doubly vergent pop up geometry to the axial zone two mesozoic pre compressional extensional events were recorded in the area new structural data presented in this paper indicate that most of the generated n s trending thrusts and related folds were controlled by the inversion of the pre existing mesozoic extensional faults thin skin structures in the mesozoic cenozoic cover are genetically linked to major basement upthrusts which could be interpreted as basement short cuts formed during inversion rather than as uplifted blocks associated with major cenozoic strike slip faults growth strata geometries date the beginning of the andean compressional event which generates the chilean precordillera as far back as 90 ma ago the resulting structural architecture is strongly controlled by inherited pre andean extensional structures the association of porphyry intrusives with major reverse faults suggests that the emplacement of the eocene oligocene porphyry cu mo deposits in northern chile can be explained by an oblique inversion tectonics model the upper eocene lower oligocene giant porphyry copper bodies chuquicamata la escondida el salvador located in the cordillera de domeyko show an adakitic affinity this magma affinity together with structural evidence presented in this work indicates that porphyry emplacement occurred at the end of the basement involved contractional stage that generates the anomalous thickened crust needed to generate these magmas this tectonic evolution is coherent with the existence of a flat slab subducting beneath the central andes 22 26 s during early cenozoic that will also produce the eastward migrating of the compressional regime in the upper plate since late cretaceous

azimuthal anisotropy of rayleigh waves in the crust in southern tohoku area japan to reveal seismic anisotropy within the crust in the southern tohoku area japan we analyze the azimuthal anisotropy of rayleigh waves in the frequency range of 40 65 mhz rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014 the frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust 1 north south fast velocity direction around the upper crust and 2 trench normal fast velocity direction in the deeper crust we also conduct a time lapse analysis of rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 tohoku oki earthquake we demonstrate that the azimuthal location of teleseismic events influences the accuracy of time lapse analysis moreover a decrease of azimuthal anisotropic ratio in time lapse results might be related to changes in the velocity structure associated with release of crustal stress during the tohoku oki earthquake

tectonic interaction between mesozoic to cenozoic extensional and contractional structures in the preandean depression 23 25 s geologic implications for the central andes abstract a multidisciplinary study supported by field data and 2 d seismic information was conducted to understand the tectonic interactions between mesozoic to cenozoic extensional and contractional structures along the preandean depression 23 25 s in northern chile which have been largely debated this work gives some new ideas about the control that ancient basement extensional faults exert on the crustal shortening and growth of those orogenic belts located over normal subduction zones similar to the central andes in this work two sedimentary basins were analyzed the salar de atacama and the salar de punta negra both of which correspond to intra montane basins bounded by the domeyko cordillera and the current volcanic arc of the central andes the interpretation of a series of west to east oriented 2 d seismic profiles revealed three types of structural interaction i pure positive reactivation ii decapitation of previous normal faults by reverse faults and iii transport of previous normal faults by thrust ramps inversion anticlines and basement involved contractional structures were the most common observed structures indicating that the geometry and distribution of inherited normal faults of the paleozoic and mesozoic basement played an important role in basin contraction the different modes of tectonic interaction frequently created variations on the geometry and kinematic of the final structural styles present in the region commonly the upper cretaceous to cenozoic synorogenic tectonosequences recorded the ages of contractional deformation that affected the region similar to other regions of northern chile basin inversion occurred during the late cretaceous and subsequent propagation of reverse faults occurred from the upper late cretaceous to the present the basement involved reverse fault structures usually create the most important positive topographic relief in this part of the western central andes using as example our study area we finally conclude that the original position and distribution of ancient basement pre orogenic normal faults in the orogenic belts condition the final vergence and kinematic of the subsequent thrust systems related to shortening episodes

faunal replacement in the early jurassic of northern chile implications for the evolution in mesozoic benthic shelf ecosystems abstract long term within habitat species replacement is either evolutionary or ecological in nature ecological replacement of dominant species by unrelated species which share essentially the same niche requirements is a very characteristic feature of early jurassic benthic shelf faunas from northern chile more than that an examination of the relevant literature reveals that the process was significant in mesozoic benthic shelf ecosystems on the whole it occurred among all important guilds and in a broad range of environmental subdivisions salinity controlled environments coarse grained and fine grained shallow shelf mid to outer shelf and oxygen controlled environments in contrast evolutionary replacement of dominant faunal elements appears to have been limited to restricted basins faunal replacement was the underlying process of several distinctive patterns in the temporal distribution of mesozoic benthic shelf faunas that is the evolutionary stability of dominant morphoptypes the widespread occurrence of parallel associations and the persistence of guild assemblages through time fluctuations of relative sea level are regarded as a substantial controlling factor of within habitat species replacement

stress before and after the 2011 great tohoku oki earthquake and induced earthquakes in inland areas of eastern japan 1 stress fields in inland areas of eastern japan before and after the tohoku oki earthquake were estimated by inverting focal mechanism data before the earthquake s1 axis was oriented ew in tohoku but nw se in kanto chubu the stress fields changed after the earthquake in northern tohoku and in southeastern tohoku near iwaki city where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake this indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 mpa the stress field did not change in central tohoku even though the stresses loaded after the earthquake had nearly reversed orientations which indicates that the differential stress magnitudes there were significantly larger than 1 mpa in kanto chubu stresses having nearly the same orientations as the background stresses were loaded after the earthquake and the stress fields did not change as expected this may have caused very high induced seismicities in kanto chubu citation yoshida k a hasegawa t okada t iinuma y ito and y asano 2012 stress before and after the 2011 great tohoku oki earthquake and induced earthquakes in inland areas of eastern japan geophys res lett 39 l03302

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

forced regressions in a large wave and storm dominated anoxic lake rhaetian sinemurian kap stewart formation east greenland during rhaetian sinemurian time a large wave and storm dominated lake was situated in the jameson land basin east greenland lake deposits consist of alternating black unfossiliferous mudstones and sheet sandstones anoxic conditions dominated at the lake bottom during deposition of the muds and the water column was probably stratified the sandstones were deposited by progradation of wave and storm dominated deltas in a water depth of less than 15 m sequence stratigraphic interpretation suggests that the mudstones were deposited in periods of rising and very high stands of lake level whereas progradation of the deltaic sheet sandstones took place during forced regressions caused by significant falls the lake thus underwent a large number of fairly high amplitude changes in level probably caused by climatic fluctuations the high order cycles can be grouped into several long period cycles that show the same number of major fluctuations as published eustatic sea level curves this similarity suggests a causal link between eustasy and long period variations in the lake the kap stewart formation represents one of the few ancient examples of a large wave and storm dominated lake and it is probably the first documented case of abundant well developed lacustrine forced regressions

continental rift to back arc basin jurassic cretaceous stratigraphical and structural evolution of the larsen basin antarctic peninsula the larsen basin developed in jurassic times as a result of continental rifting during the early stages of gondwana break up lower upper jurassic non marine sedimentary and volcanic rocks constitute a syn rift megasequence recording initial largely amagmatic extension and subsequent widespread extension related silicic volcanism a succeeding kimmeridgian early berriasian transgressive megasequence consisting largely of anoxic dysoxic hemipelagic mudstones is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak jurassic eustatic sea levels served to maximize sediment starvation the fragmentary record for late berriasian barremian times suggests that a regressive megasequence may have developed in the earlier part of this period recording increased sediment yield to the larsen basin from the increasingly emergent antarctic peninsula arc subsequently strata in the southern but not the northern part of the basin underwent relatively intense eastward verging deformation possibly during the formation of a retro arc fold thrust belt where exposed the lower part of the succeeding aptian eocene megasequence consists of a deep marine clastic wedge deposited along the fault bounded western basin margin during a phase of arc uplift and related differential subsidence following partial basin inversion in late cretaceous times regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward

trench parallel crustal anisotropy along the trench in the fore arc region of japan in northeastern japan the pacific plate is descending beneath the north american plate it is generally understood that trench normal principal stress is dominant in the crust along the japanese island arc because the stress field is controlled by the force of the subducting slab observations of shear wave splitting using crustal earthquakes reveal a marked lateral variation in fast polarization direction with distance from the trench trench normal and trench parallel fast polarization directions are observed on the back arc and fore arc sides respectively in this study two dimensional finite element modeling with subducting slab was conducted to investigate the interseismic stress field during earthquake cycles taking into account linear viscoelasticity in the model trench normal compression is found to dominate in the island arc region however an extensional field appears in the shallow upper crust of the fore arc region during earthquake cycles the trench parallel crustal anisotropy can be explained by this extensional field

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

evolution of a callovian oxfordian carbonate margin in the neuquen basin of west central argentina facies architecture depositional sequences and global sea level changes abstract this paper reviews a detailed stratigraphic and sedimentologic study of a carbonate complex developed on the foreland side of the neuquen embayment a protected shallow water epicratonic site behind the active edge of the south american plate superb outcrops at the core of basement involved andean structures expose the shelf to basin transition and reveal with clarity the external and the internal architecture of the depositional sequences and component system tracts platform carbonates are largely represented by ooid and mainly rhodoid grainstones with associated patches of coral framestone the deeper platform and slope facies are composed of oncoidal and skeletal micritic limestones with scattered coral sponge algal build ups the overall composition and facies pattern bears resemblance to other late jurassic carbonate complexes form europe and with the smackover formation from the gulf coast basin of north america analysis based on mapping of the stratal patterns and facies associations in outcrops allowed the recognition of four depositional sequences timing provided by ammonite biochronology suggests that eustatic fluctuations were a major factor influencing the carbonate margin architecture and regulated episodes of condensed sedimentation shifts of the depositional belts and development of stratigraphic discontinuities the onset and the end of carbonate sedimentation were associated with episodes of marine retreat and accumulation of evaporites and eolian fluvial deposits at basin centre locations however most of the marine fluctuations recorded within the carbonate complex were insufficient to expose the shelf break type 2 and accordingly lowstand system tracts are poorly represented on the shelf the transgressive system tracts are represented by thin grain supported carbonate blankets these taper out downslope into omission surfaces or are replaced by patches of small sponge buildups highstand system tract organization changes through time reflecting changes in productivity and accomodation presumably tied to second order sea level changes callovian highstand accumulation featured a catch up carbonate system and produced a thin aggradational ramp configuration whereas conditions during middle late oxfordian allowed a keep up system and produced outbuilding depositional geometries with steeper slopes

extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling a recent extensive seismic wide angle experiment revealed a new image of crustal and uppermantle structure across northern honshu arc japan the western part of the arc recorded the crustal deformation by the miocene back arc spreading of the sea of japan the crust is composed of highly deformed tertiary sedimentary layers a relatively low velocity 5 75 5 9 km s crystalline basement and a 15 km thick lower crust with a velocity of 6 6 7 0 km s clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading the crust attains the maximum thickness 32 35km east of the backbone range for which the magmatic intrusion underplating since 10 15 ma is a predominant factor the eastern part of the arc has a less deformed upper crust and a reflective middle lower crust probably remaining a stable block since the time of the backarc spreading

sea level related architectural trends in coarse grained delta complexes distinct stratigraphical styles of delta development can be related to basically different patterns of relative sea level base level change 1 an overall rise with periods of standstill and without significant fall rs 2 an overall rise with superimposed falls rf 3 an overall fall with superimposed rises fr and 4 an overall fall with periods of standstill and no significant rises fs the related stratigraphical models contain either preserved elements of prototype deltas or show characteristic trends in development from one prototype delta into another each stratigraphical model is characterized by lateral and vertical lithofacies features which can be reasonably well established from outcrop in coarse clastic basin margins where age control is poor sedimentological and stratigraphical delta models as presented here may help refining reconstructions of basin fill patterns and basin subsidence kinematics

sequence stratigraphy of the mesozoic domeyko basin northern chile the domeyko basin of northern chile records late triassic to early cretaceous mixed carbonate and siliciclastic marine deposition along the western margin of gondwana sequence stratigraphical analysis has identified five long term sequences of 11 17 ma duration comparison of the relative sea level fluctuations interpreted from the domeyko basin succession with those documented from other similar aged south american marginal basins and northern hemisphere basins allows the distinction of regional from global events relative sea level fall in the early pliensbachian early aalenian and early valanginian and rises in the early hettangian early and late toarcian early and late bajocian late bathonian and early oxfordian of the domeyko basin appear time equivalent to similar events in other southern and northern hemisphere basins and are thus interpreted to be products of global sea level cycles long term relative sea level falls in the early bathonian late oxfordian early valanginian and rise in the late kimmeridgian are interpreted to be tectonically driven continental scale changes in sediment accommodation space the domeyko basin succession thus appears to have been dominantly controlled by global sea level fluctuations during the early to mid jurassic and by continental scale but not global fluctuations during the mid jurassic to mid cretaceous interpreted to have been driven by the fragmentation of gondwana

gps derived interseismic coupling on the subduction and seismic hazards in the atacama region chile the atacama region between 29 s and 25 s is located in the north central area of chile a tectonically complex transition area between north and central chile deformation in atacama is due mainly to elastic loading on the subduction interface but also to diffuse shortening in the sierras pampeanas argentina the seismicity of the subduction is complex in this region seismic swarms often occur moderate mw 6 to large mw 7 earthquakes occur repeatedly and finally megathrust earthquakes of magnitudes significantly larger than 8 occur once in a while the last one being in 1922 almost a century ago we use new gps data we collected in the atacama region between 2008 and 2012 to complete and densify existing data we acquired since 2004 in north central chile these new data allow to quantify the motion of the andean sliver and assess the kinematic coupling on the subduction interface at these latitudes we find that only 7 per cent of the whole convergence motion is taken up by an eastward rotation of the rigid sliver a large part of the remaining 93 per cent approximately 6 cm yr gives way to accumulation of elastic deformation in the upper plate due to locking on the plate interface this accumulation shows important along strike and along dip variations interpreted in terms of variable coupling which we correlate with seismicity we identify two areas of low coupling near the la serena 30 s and baranquilla 27 5 s bays both are correlated with the subduction of singular bathymetric features and seem to stop the propagation of large seismic ruptures these zones are also seismic swarm prone areas which seem to occur rather on their edges these low coupling areas separate two seismic segments where coupling is high the atacama segment 100 km long between 29 s and 28 s and the cha naral segment 200 km long between 27 s and 25 s should they rupture alone these segments are sufficiently coupled and apparently since long enough to produce mw 8 events however a collective failure of both segments could generate a megathrust earthquake of magnitude close to 8 5 similar to the 1819 and 1922 complex events which produced important tsunamis such giant events may occur in the area once a century

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

crustal anisotropy in the forearc of the northern cascadia subduction zone british columbia summary this paper aims to identify sources and variations of crustal anisotropy from shear wave splitting measurements in the forearc of the northern cascadia subduction zone of southwest british columbia over 20 permanent stations and 15 temporary stations were available for shear wave splitting analysis on 4500 event station pairs for local crustal earthquakes results from 1100 useable shear wave splitting measurements show spatial variations in fast directions with margin parallel fast directions at most stations and margin perpendicular fast directions at stations in the northeast of the region crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress however studies have also shown anisotropy can be complicated by crustal structure southwest british columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults to use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure we determine the source of anisotropy at each station by comparing fast directions from shear wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion most stations show agreement between the fast direction and the maximum horizontal compressive stress this suggests that anisotropy is related to stress aligned fluid filled microcracks based on extensive dilatancy anisotropy these stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress and in the future may be used as a monitoring and mapping tool

evidence from core reflected shear waves for anisotropy in the earth s mantle seismologists have begun to recognize that the upper mantle at depth is anisotropic and that seismological data are not fully explicable if anisotropy is ignored1 5 on entering an anisotropic zone a plane shear wave splits into two orthogonally polarized phases6 an effect observed by several researchers7 notably by ando et al 5 8 9 the near vertical scs wave an s wave reflected once at the core mantle boundary is one of the most suitable phases to study the phenomenon of shear wave birefringence because it is isolated from other phases and because its particle motion is nearly horizontal and thus free from s to p conversion the split scs shear waves travel with different velocities in the anisotropic zone and consequently there is a time delay between their arrival at stations my aim was to determine this time delay and the azimuth of polarization using the scs records of two horizontal components from the japanese islands for a deep shock beneath the sea of okhotsk from the total of 41 stations 34 show a polarization of the maximum velocity phase in the nnw sse direction with an average time advance of 0 8s from the minimum velocity phase the relevant anisotropic zone seems to be deep seated

the early toarcian jurassic anoxic event stratigraphic sedimentary and geochemical evidence detailed paleogeographic studies in the southern alps suggest that an oxygen minimum model is appropriate for interpreting conditions on tethyan continental margins advection of manganese in this low oxygen layer may explain the local occurrence of carbonate ores of this element carbon sulfur and iron sulfur ratios although showing considerable scatter suggest that bottom waters at this time were locally euxinic containing free hydrogen sulfide this oceanic anoxic event was preceded by significant faunal turnover of ammonites in tethys and accompanied by widespread extinction of benthos in northern europe in response to the lateral spread of anoxic bottom waters during transgression similar changes may be recognized in other parts of the world other anoxic events may have taken place during the jurassic but documentation is as yet meagre furthermore models for such phenomena remain largely speculative although upwelling and increased planktonic productivity commencing in pre toarcian time are favored for the falciferum zone event documented here from author

displacement above the hypocenter of the 2011 tohoku oki earthquake the moment magnitude m w 9 0 2011 tohoku oki mega thrust earthquake occurred off the coast of northeastern japan combining global positioning system gps and acoustic data we detected very large sea floor movements associated with this event directly above the focal region an area with more than 20 meters of horizontal displacement that is four times larger than those detected on land stretches several tens of kilometers long along the trench the largest amount reaches about 24 meters toward east southeast just above the hypocenter furthermore nearly 3 meters of vertical uplift occurred contrary to observed terrestrial subsidence

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

what is the structure of the forearc region in the central andes of northern chile an approach from field data and 2 d reflection seismic data abstract the tectonic evolution of the andean forearc in the central andes is understood by the superimposition of deformational events associated with different tectonic regimes which control the main morpho tectonic units of northern chile varied interpretations have opened a great discussion about the deep structure of the andean forearc and its implications in the evolution of the central andes these ideas are commonly accompanied by the lack of regional tectonic models that explain the geometry and kinematics of first order structures the main limitations are due to the unknown origin of the structural styles in the substrate and their relationships with those exposed in the field in order to solve this problem we present a new structural model of the forearc region of northern chile specifically along the central depression 22 23 s supported by field observations and the structural interpretations of 2d seismic data four major tectono sequences have been recognized related to a a paleozoic triassic basement rocks b a jurassic lower cretaceous synrift deposits c upper cretaceous paleocene volcano synorogenic deposits and d a late cenozoic covered gravel the first order structural styles identified correspond to inverted half graben basement structures related to the jurassic tarapaca basin the hanging wall blocks of the master faults are usually composed of large inversion anticlines along which jurassic and lower cretaceous siliciclastic deposits were expelled other basement contractional structures consist of short cut and by pass faults that modified the previous geometry of extensional systems the second order thin skinned fold and thrust exposed in the region accommodated the shortening experienced by the mesozoic synrift deposits during the basin inversion the upper cretaceous paleocene deposits usually unconformably overlie the folded mesozoic synrift deposits this structural feature is first order evidence of the positive reactivation of the mesozoic rift basins and therefore is a regional marker of the onset of the andean uplift in the region

azimuthal seismic anisotropy in the upper crust of the japanese islands induced by the 2011 tohoku oki megathrust earthquake

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

lithospheric evolution of the pre and early andean convergent margin chile abstract the proto andean and early andean evolution of the southwestern gondwana margin comprises three stages that differ in their magmatic evolution and deformational style the gondwana cycle 330 280 ma the pre andean stage 280 210 ma and the early andean cycle 210 100 ma these stages have been traditionally interpreted as the upper crustal response to changes in the tectonic setting which include cordilleran style continental arc gondwana cycle orogenic collapse and possibly slab break off that led to continental rifting and extensive crustal melting pre andean stage and subsequent subduction re initiation in oceanic arc style context early andean cycle the petrological and geochemical characteristics of carboniferous to jurassic igneous rocks from this region however do not support the described model elemental and sr nd pb isotopic data of 86 samples along with a compilation of 1230 samples from the literature suggest that subduction was the most likely process by which the magmatic record was generated sub alkaline affinities lile enrichment over hfse nb ta troughs porphyritic textures and hornblende and biotite bearing lithologies are present in all studied units whereas isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust even though the aforementioned features are common to all igneous rocks some changes point to a decline in the contribution of crustal lithospheric sources to the magmatism with time thus sio2 lan ybn and 87sr 86srinitial exhibit a systematic decrease from 285 to 150 ma whereas the endinitial parameter increases in the same period these changes were accompanied by the shift from dominant compressional carboniferous early permian to transtentional middle permian jurassic stresses in the upper crust suggesting that the margin went from advancing to retreating due to pangea reorganization and break up following a potential flat slab event slab roll back may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

temporal change in shear velocity and polarization anisotropy related to the 2011 m9 0 tohoku oki earthquake examined using kik net vertical array data 1 the m9 0 tohoku oki earthquake took place off ne japan which is covered by a dense seismic network in particular kik net observed the strong motions of the main shock and numerous aftershocks both on the ground and at the bottom of boreholes at a depth of a few hundred meters we applied cross correlation analysis to the coda part of a seismogram observed by kik net and estimated not only the temporal change in shear velocity but also the change in polarization anisotropy between the bottom of the borehole and the ground the shear velocity was suddenly reduced by 5 to 10 after the tohoku oki earthquake over a wide area of ne japan which is consistent with the results of a previous study although some stations also reveal a change in polarization anisotropy the polarization anisotropy change is smaller than the velocity change which may suggest that the static stress change due to the coseismic slip on the pacific plate is not large enough to change the direction of polarization anisotropy and or deviatoric stress in the land area of ne japan

evidence for rapid fault healing derived from temporal changes in s wave splitting 1 tadokoro et al 1999 had made an s wave splitting analysis on the nojima fault zone that ruptured during the 1995 hyogo ken nanbu earthquake for a period 9 12 months after the mainshock the direction of faster shear wave polarization had been parallel to the fault strike suggesting fractures of shear fault origin in this study we made an additional s wave splitting analysis for a period 33 45 months after the mainshock and detected the rotation of that is the direction was parallel to the regional maximum horizontal compressional stress which is oblique to the fault strike this temporal change suggests that all the fractures of shear fault origin had completely healed and cracks of tectonic stress origin recovered between the two periods this is strong evidence for rapid fault healing which completed within 33 months after the mainshock

widespread seismicity excitation throughout central japan following the 2011 m 9 0 tohoku earthquake and its interpretation by coulomb stress transfer 1 we report on a broad and unprecedented increase in seismicity rate following the m 9 0 tohoku mainshock for m 2 earthquakes over inland japan parts of the japan sea and izu islands at distances of up to 425 km from the locus of high 15 m seismic slip on the megathrust such an increase was not seen for the 2004 m 9 1 sumatra or 2010 m 8 8 chile earthquakes but they lacked the seismic networks necessary to detect such small events here we explore the possibility that the rate changes are the product of static coulomb stress transfer to small faults we use the nodal planes of m 3 5 earthquakes as proxies for such small active faults and find that of fifteen regions averaging 80 by 80 km in size 11 show a positive association between calculated stress changes and the observed seismicity rate change 3 show a negative correlation and for one the changes are too small to assess this work demonstrates that seismicity can turn on in the nominal stress shadow of a mainshock as long as small geometrically diverse active faults exist there which is likely quite common