role of subduction erosion in the generation of andean magmas subduction erosion or scraping off of the continental crust by subducting oceanic lithosphere is an important tectonic process along parts of the andean convergent plate boundary estimated rates of subduction erosion increase northward north of the southem andes due to decreasing subduction angle and decreasing sediment supply to the trench west of the central andes subduction erosion may remove as much as 535 km 3 of crust per 1 km of trench per 1 m y or 7 vol of the subducting oceanic lithosphere the higher 87 sr 86 sr ratios of mafic magmas erupted north of the southern andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub arc mantle magma source

cenozoic evolution of the northwestern salar de atacama basin northern chile 1 since 90 ma the nonmarine salar de atacama basin has been the largest deepest and most persistent sedimentary basin of northern chile integration of 200 km of two dimensional seismic reflection data with surface geological data clarifies oligocene and neogene evolution of the northern part of the basin a normal fault with 6 1 km of vertical separation controlled the western boundary of the basin during the accumulation of the oligocene lower miocene paciencia group the combination of this structure a similar one in the calama basin and regional structural data suggests that localized extension played an important role within a tectonic environment dominated by margin perpendicular compression and margin parallel strike slip deformation seismic data substantiate the surface interpretation that much of the cordillera de la sal ridge resulted from diapiric flow of the paciencia group diapiric flow initiated during the late early miocene or middle miocene associated with a deep reverse fault

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

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

late mesozoic to paleogene stratigraphy of the salar de atacama basin antofagasta northern chile implications for the tectonic evolution of the central andes abstract the salar de atacama basin the largest pre andean basin in northern chile was formed in the early late cretaceous as a consequence of the tectonic closure and inversion of the jurassic early cretaceous tarapaca back arc basin inversion led to uplift of the cordillera de domeyko cd a thick skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike the almost 6000 m thick upper cretaceous to lower paleocene sequences purilactis group infilling the salar de atacama basin reflects rapid local subsidence to the east of the cd its oldest outcropping unit tonel formation comprises more than 1000 m of continental red sandstones and evaporites which began to accumulate as syntectonic growth strata during the initial stages of cd uplift tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance representing the sedimentary response to renewed pulses of tectonic shortening which were deposited in alluvial fan fluvial and eolian settings together with minor lacustrine mudstone purilactis formation these are covered by 500 m of coarse proximal alluvial fan conglomerates barros arana formation the top of the purilactis group consists of maastrichtian danian alkaline lava and minor welded tuffs and red beds cerro totola formation 70 64 ma k ar deposited during an interval of tectonic quiescence when the el molino yacoraite late cretaceous sea covered large tracts of the nearby altiplano puna domain limestones interbedded with the totola volcanics indicate that this marine incursion advanced westwards to reach the eastern cd slope cd shortening in the late cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north chilean magmatic arc to the west of the cd domain indicating that oblique plate convergence prevailed during the late cretaceous oblique convergence seems to have been resolved into a highly partitioned strain system where margin parallel displacements along the thermally weakened arc coexisted with margin orthogonal shortening associated with syntectonic sedimentation in the salar de atacama basin a regionally important early paleocene compressional event is echoed in the salar de atacama basin by a distinctive angular unconformity which separates paleocene continental sediments from purilactis group strata the basin also records the eocene early oligocene incaic transpressional episode which produced renewed uplift in the cordillera de domeyko and triggered the accumulation of a thick blanket of syntectonic gravels loma amarilla formation

crustal regime of deformation and ascent of granitic magma abstract structural studies and gravity data demonstrate the magma flow pattern of granitic plutons and their morphology at depth the deepest zones calculated from gravity data provided they show vertical lineations in outcrop are interpreted as the feeder channels which fed the pluton their morphology at depth as well as the position and number of root zones vary according to the type of deformation suggesting that regional deformation determines the emplacement of granitic plutons therefore deformation must also influence the segregation and the ascent of acid magmas a thermal model is constructed which demonstates the structure and heat generation of the hercynian crust according to that model the thermal gradient changes the conditions for the deformation of the crust obviously a large amount of heat must be supplied to induce melting of the lower crust as a result magma segregation and ascent occur entirely within the ductile field when upwelling magma reaches the brittle ductile transition zone the hydrodynamic force buoyancy is insufficient to fracture the upper brittle crust parameters which control diapirism wavelength viscosity thickness do not match those related to the volume of erupted magma as a consequence diapiric upwelling appears to be limited under favourable conditions to the lower ductile crust and does not apply to intrusions in a brittle crust shearing helps to segregate magma and to feed conduits toward the surface since it creates the required zones of local extension for the magma to infiltrate

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

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

paleogene clockwise tectonic rotations in the forearc of central andes antofagasta region northern chile 1 for the central valley of northern chile antofagasta region a paleomagnetic analysis of data from 108 sites mainly in mesozoic and paleogene volcanic rocks has yielded stable remanent magnetization directions for 86 sites from these data we infer clockwise tectonic rotations of up to 65 within the forearc domain of the central andes the apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the incaic orogenic event of eocene early oligocene age a few paleomagnetic results obtained in neogene rocks do not show evidence of clockwise rotations hence the development of the bolivian orocline during late neogene time cannot be explained by simple bending of the whole margin these results demonstrate that tectonic rotations within the forearc and pre cordillera are key elements of early andean deformation which should be taken into account by kinematic models of mountain building in the central andes

tectonic styles and crustal shortening of the central andes pampean flat slab segment in northern chile 27 29 s abstract the andean orogenic belt located in the central andes pampean flat slab segment in northern chile 27 29 s is composed of two major tectonic regions the coastal cordillera and the frontal cordillera to understand their internal tectonic styles history of growth and the shortening absorbed by the upper crustal structure of this segment we combined regional geological mapping data new ages obtained from radiometric u pb dating and a semibalanced and restored cross section 225 18 km in length the results as shown in the previous mesozoic extensional fault systems established in northern chile by the gondwana breakup have played a fundamental role in the orogenic buildup the central structure is characterized by an asymmetric basin upper cretaceous paleocene confined by a doubly vergent fault system composed of inverted faults related to the edges of the mesozoic chanarcillo and lautaro basins the u pb geochronological data obtained from synorogenic volcano sedimentary deposits and the angular unconformities recorded between the cenozoic geological units have revealed that the compressive deformation in this segment started at around 80 ma by tectonic inversion in the eastern coastal cordillera and western frontal cordillera however the presence of paleocene and miocene synorogenic successions at the footwall of the basement reverse faults of the frontal cordillera suggests a migration of andean deformation from the west to the east during the paleocene miocene by propagation of ramps involving inherited basement highs the pre compression restoration makes it possible to estimate 40 94 km of minimum shortening concentrated by inversion anticlines and fault controlled basement highs across the frontal cordillera

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

a tectonostratigraphic synthesis of the sub andean basins inferences on the position of south american intraplate accommodation zones and their control on south atlantic opening it has been shown in the accompanying paper that the sub andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains to test the hypothesis that changes in palaeo depositional setting rely on the presence of a series of transverse zones of structural accommodation data have been digitally compiled from across the south american continent spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries structural accommodation zones and patterns of subsidence and intraplate deformation the results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which on a regional scale interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization their strong spatial relationship with mesozoic intraplate alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during gondwana breakup these localized zones of high heat flow have important implications for source rock maturity in the interior phanerozoic intracratonic basins of south america on the south atlantic margin the majority of these crustal lineaments correlate with failed arms of triple junction rifts and define the boundaries to tectonostratigraphic domains recognized along the south atlantic rift system

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

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

two ages of porphyry intrusion resolved for the super giant chuquicamata copper deposit of northern chile by ela icp ms and shrimp zircon u pb ages measured in situ by excimer laser ablation inductively coupled plasma mass spectrometry ela icp ms and verified by sensitive high resolution ion microprobe shrimp on ore bearing felsic porphyries from the chuquicamata porphyry copper molybdenum deposit northern chile identify two discrete igneous events the volumetrically dominant east porphyry has an age of 34 6 0 2 ma whereas the bench and west porphyries yield ages of 33 3 0 3 ma and 33 5 0 2 ma respectively the age of the east porphyry is indistinguishable from a re os age for early molybdenite mineralization 35 ma and the oldest reported 40 ar 39 ar ages for hydrothermal alteration confirming a genetic link with mineralization previous geological studies and 40 ar 39 ar and re os geochronology identify two main hydrothermal events high temperature potassic alteration with chalcopyrite at 33 4 0 3 ma followed by lower temperature quartz sericite alteration with pyrite at 31 1 0 3 ma the ages of the west and bench porphyries match the ages for potassic alteration younger quartz sericite alteration may reflect an additional fourth intrusion concealed at depth the anomalously large size of chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic hydrothermal systems

importance of small block rotations in damage zones along transcurrent faults evidence from the chuquicamata open pit northern chile chuquicamata in northern chile is one of the largest porphyry copper deposits in the world the western side of its orebody is bounded by a major longitudinal fault the west fault we report paleomagnetic results from surface sites and drill cores from different geological units at chuquicamata especially within the late eocene fiesta granodiorite of the western block of the west fault characteristic remanent magnetizations chrm were determined after detailed thermal or alternating field demagnetization soft components carried by multidomain magnetite crystals in the fiesta granodiorite were removed by af demagnetization at 10 20 mt the chrms not demagnetized by alternating fields up to 100 mt have unblocking temperatures above 580 c with 75 of the magnetization removed in the temperature range of 580 590 c optical and sem mineralogical observations and microprobe data indicate the occurrence of multidomain magnetite formed during a late magmatic stage of alteration coeval with strong oxidation of primary titanomagnetite and formation of ilmenite hematite pseudobrookite and rutile the characteristic directions have negative inclinations and declinations 330 to 230 strongly deflected from the expected eocene direction anisotropy of magnetic susceptibility ams with degree up to 1 4 is carried by multidomain magnetite ams ellipsoids have subvertical foliations with azimuth varying strongly from n280 to n20 we show that both the chrms and the ams fabrics record the same apparent relative rotations between sites although the ams anisotropy is high there is no evidence for a solid state deformation and the apparent rotation of the magnetic fabric is interpreted to be the consequence of small block rotation the apparent large 100 counterclockwise rotations of small blocks within the fiesta granodiorite suggest a wide damaged zone related to sinistral displacement along the west fault this interpretation is consistent with previous models indicating that the fiesta granodiorite was sinistrally translated and brought in front of the early oligocene porphyry copper deposit during the oligocene early miocene this study shows that paleomagnetic markers are useful for improving the quantification and understanding of small scale deformation within plutons adjacent to major fault zones

middle eocene deformation and sedimentation in the puna eastern cordillera transition 23 26 s control by preexisting heterogeneities on the pattern of initial andean shortening the quebrada de los colorados formation at the north end of calchaqui valley in salta province northwest argentina preserves evidence of syndepositional deformation since the middle eocene ca 40 ma that includes 1 an angular unconformity with the underlying salta group paleogene and 2 internal unconformities and changes in vertical facies succession and provenance its fossil record mammalian notoungulates middle eocene is correlatable to the casa grande formation which also unconformably overlies the salta group both units record middle eocene deformation along the eastern border of the puna plateau and outline previous fi rst order mechanical heterogeneities related to the cretaceous rift basin border along the western margin of the puna eocene deformation coincides with thermal magmatic arc and mechanical basin inversion heterogeneities thus the distribution of eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities

thick skinned and thin skinned thrusting in the la ramada fold and thrust belt crustal evolution of the high andes of san juan argentina 32 sl abstract the cenozoic structural evolution of the la ramada fold and thrust belt in the high andes of argentina between 32 00 s and 32 30 s shows in time and space the interaction of thin skinned and thick skinned deformations based on geometric and kinematic constraints three different deformation stages have been recognized the first began with thin skinned nnw trending structures detached in upper jurassic gypsum in the second stage the basement was involved by tectonic inversion of triassic normal faults producing a thick skinned fold and thrust belt this inversion combined with the ductile rheology of the jurassic gypsum resulted in refolding of older structures the uplift of the basement by high angle reverse faults at the ramada espinacito massif produced a sticking point in the foreland propagation of the thrust belt this was responsible for the third stage of deformation characterized by thin skinned nnw out of sequence thrusts developed in the westernmost sector the total andean shortening calculated in the area was 21 24 km near 30 the structure of the la ramada fold and thrust belt was integrated into a crustal scale cross section at 32 s latitude based on gravimetric models and surface geology a crustal model is proposed for the andes of chile and argentina a shortening of 180 km was calculated between the chile trench and the sierra de pie de palo this implies a mean deformation rate of 2 8 mm year 1 during cenozoic times an initial crustal thickness of ca 33 km was computed a two layer model with a middle detachment level is proposed to explain the spatial decoupling between maximum shortening at the surface and maximum crustal thickening the upper layer was balanced based on surface structural information the lower layer is interpreted to have been constructed by an imbricate thrust system controlled by the mercedario rift master shear and the precordillera chilenia suture

fault controlled pluton emplacement in the sevier fold and thrust belt of southwest montana usa abstract problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening not extension occurs on a regional scale new structural data from the pioneer and boulder batholiths of southwest montana usa suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km frontal thrust facilitated pluton emplacement was accommodated by 1 a magma feeder zone created along the ramp interface 2 providing releasing steps at ramp tops that serve as initial points of emplacement and subsequent pluton growth and 3 localizing antithetic back thrusts that assist in pluton ascent a model of magma emplacement is proposed that involves these elements this model for syntectonic ramp top emplacement of plutons helps explain how space is made for plutons within fold and thrust belts

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

lithological and structural controls on the genesis of the candelaria punta del cobre iron oxide copper gold district northern chile abstract the candelaria punta del cobre district is the largest iron oxide cu and au iocg district in the chilean iocg belt with more than 13 mt of contained copper candelaria is the most important deposit in the district which includes seven other smaller producers carola punta del cobre mantos de cobre candelaria norte granate alcaparrosa atacama kozan las pintadas the district which is characterized by an early cretaceous volcanic sedimentary arc sequence 135 132 ma punta del cobre formation overlain by the marine sedimentary chanarcillo group formed in an extensional back arc basin 132 130 ma the copiapo batholith which occupies the western side of the district was emplaced between 118 and 110 ma during the change from extensional to transpressional tectonics mineralization is hosted predominately in the upper part of the lower andesite member and the overlying volcano sedimentary and dacite members all within the punta del cobre formation mineralization is hosted by fault zones breccias and specific lithologies north northwest faults are the dominant host for vertically extensive orebodies stratigraphically controlled mineralization forms extensive stratabound ore bodies mantos textural evidence suggest that the hydrothermal system evolved and advanced upwards over time the earliest event was dominated by magnetite actinolite in stratigraphically controlled mantos and extensive zones of disseminated magnetite actinolite below candelaria which were subsequently overprinted by chalcopyrite dominant mineralization with magnetite actinolite biotite k feldspar alteration in addition to magnetite iron oxides include widespread mushketovite and hematite in the upper part of some deposits geochronological data suggest that the main phase of mineralization occurred between 122 ma and 115 ma u pb in zircon overlapping in age with the two major early phases of the copiapo batholith there is no field evidence to indicate that exposed phases of the batholith were the source of mineralizing fluids alteration and mineralization in the earliest phase of the batholith la brea occurs in structures and is minor compared to mineralization in the punta del cobre formation later phases of the batholith cut the main stage of mineralization the extent of iocg mineralization in the candelaria punta del cobre district reflects a combination of factors including changing regional kinematics and related fault architecture multiple intrusive events fluid access to permeable and reactive lithologies vertical changes in redox conditions and the potential role of multiple fluids

strain in metacherts as determined from deformed shapes of radiolaria and fry s method abstract strain magnitudes of plastically deformed metacherts were inferred in the same sample by means of fry s method using the center positions of deformed radiolaria and from deformed shapes of the initially spherical radiolaria the magnitudes and orientations of the strains deduced from the two strain markers are very similar accordingly strains in the pure metacherts are reasonably homogeneous i e the radiolaria and the matrix deformed uniformly

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

fast forward modeling of compressional arrival slowness logs in high angle and horizontal wells abstractcompressional p and shear s sonic propagation modes are routinely measured with modern borehole acoustic logging instruments slownesses inverse of velocity of p and s modes provide information on compressional and shear slownesses of rock formations surrounding the wellbore respectively when a high angle well penetrates horizontal thin layers the layers function as waveguides for the propagation of sonic waves layer trapped and borehole guided waves interfere with each other giving rise to abnormal amplitude variations and phase discontinuity in the time waveforms acquired at receivers consequently waveforms measured across the receiver array often exhibit low semblance which renders conventional waveform semblance based processing techniques inadequate for reliable estimation of formation slowness instead of calculating waveform semblance we invoke compressional first arrival times to describe the propagation properties of the p mode in high angle and horizontal ha hz wells a

the role of inversion tectonics in the structure of the cordillera oriental nw argentinean andes the salta basin of cretaceous age in the andes of nw argentina was inverted during the development of the cordillera oriental between the neogene and recent the different orientations of the extensional faults of the salta rift system and the subsequent w e shortening resulted in a great variety of inversion structures extensional faults dipping ne to ese were preferentially reactivated as contractional faults the initial stepped geometry of these faults gave rise to a distinctive sigmoidal pattern of basement involved thrusts and related folds these contractional structures tended to run parallel to the extensional faults and acquired a n s trend across the previous accommodation zones resulting in folding interference patterns other interference patterns are attributed to the superposition of contractional folds on previous rollovers and to the constrictional deformation imposed by the reactivation of adjacent extensional faults with different orientations the nnw to wsw dipping extensional faults show no evidence of reactivation despite the fact that some of them are folded the inversion at crustal scale is limited owing to the presence of the cretaceous salta rift across the different andean structural units and foreland

paleomagnetism and tectonics of the southern atacama desert 25 28 s northern chile we report paleomagnetic results for 131 sites from the modern forearc of northern chile 25 s and 28 s remanent magnetization in volcanic and intrusive rocks is mostly primary while a secondary magnetization is observed in sedimentary rocks comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 7 3 21 6 counterclockwise to 52 7 17 clockwise jurassic to early cretaceous rocks from the coastal cordillera and cretaceous to paleocene rocks from the central depression show similar magnitude 30 clockwise rotations while more variable rotations occur in mesozoic to eocene rocks of the precordillera clockwise rotations in mesozoic and paleogene rocks occur in the chilean frontal cordillera south of 27 30 s paleomagnetic results in three large miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the precordillera and pre andean depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 ma early miocene and likely during and after the incaic tectonic event which affected large tracts of the central andes the postulated onset of rotations in the north chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the eastern cordillera in bolivia and northwestern argentina rotation of the chilean forearc enhancement of the curvature of the central andes and the formation of the bolivian orocline seem to be for the most part closely linked to the evolution of the eocene oligocene tectonics of the eastern cordillera

geometry and late mesozoic cenozoic evolution of the salar de atacama basin 22 30 24 30 s in the northern central andes new constraints from geophysical geochronological and field data abstract the late mesozoic cenozoic evolution of the central andes of northern chile consists of a series of events related to changing conditions along the plate margin leading to different episodes of compression and extension a long lasting basin which records these changes corresponds to the salar de atacama basin its tectonic history has been blurred by several interpretations of seismic reflection lines and multiple chronostratigraphic correlations between subsurface surface units as such many aspects of its tectonic evolution are still unclear in this study we performed an e w gravimetric profile across the barros arana syncline and the basin we analyzed interval velocities obtained from seismic reflection studies and reviewed the profiles closest to the gravity survey four u pb samples for detrital zircon analysis were obtained from late cretaceous paleogene units geophysical data show a segmentation of physical properties within the basin with denser units and faster interval velocities around the late mesozoic early cenozoic units and lighter slower units in the actual salt pan the main contrast in physical properties is observed in the evaporitic and fine grained continental deposits of the san pedro formation geophysical and geological constraints show that its thickness varies between 800 and 1200 m and that it was deformed during later events the u pb data show that the rocks within the barros arana syncline reach well into the paleogene and that they were mainly deformed during the late eocene incaic event where an east verging thrust system also involving permo triassic units was developed establishing the actual structural configuration the late oligocene early miocene units were then deposited in a post orogenic setting with minor compression a stronger phase around the late miocene reactivated previous structures and folded these deposits this continuous compressive history of the basin which is mostly a result of the incaic event is in agreement with the regional evolution of the central andes

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

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

the inverted triassic rift of the marrakech high atlas a reappraisal of basin geometries and faulting histories the high atlas of morocco is an aborted rift developed during the triassic jurassic and moderately inverted during the cenozoic the marrakech high atlas with large exposures of basement and triassic early syn rift deposits is ideal to investigate the geometries of the deepest parts of a rift constituting a good analogue for pre salt domains it allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another a detailed structural study of the main triassic basins and basin margin faults of the marrakech high atlas shows that only a few rift faults were reactivated during the cenozoic compressional stage in contrast to previous interpretations and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems preserved extensional features demonstrate a dominant dip slip opening kinematics with strike slip playing a minor role at variance to models proposing a major strike slip component along the main basin bounding faults including faults belonging to the tizi n test fault zone a new middle triassic paleogeographic reconstruction shows that the marrakech high atlas was a narrow and segmented orthogonal rift sub perpendicular to the main regional extension direction which was nw se in contrast to the central and eastern segments of the atlas rift which developed obliquely this difference in orientation is attributed to the indented ouzellarh precambrian salient part of the west african craton which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution as for the cenozoic inversion total orogenic shortening is moderate 16 and appears accommodated by basement involved large scale folding and by newly formed shortcut and by pass thrusting with rare left lateral strike slip indicators triassic faults commonly acted as buttresses

the transition from thick skinned to thin skinned tectonics in the basque cantabrian pyrenees the burgalesa platform and surroundings interpretation of seismic data in the margins of the burgalesa platform in the basque cantabrian pyrenees has allowed proposition of a new structural model that combines different modes of deformation during oblique tectonic inversion conditioned by the distribution of triassic salts deformation was decoupled by the presence of the salt horizon between basement involved thrusts inverting formerly triassic and late jurassic early cretaceous extensional faults and a detached thrust system involving the upper triassic to neogene sedimentary package structural units experiencing different styles of deformation are not only stacked vertically above and below the salt but most importantly they change from one to the other along strike across the transversal edges of the triassic salts the burgalesa platform detached thrust system was confined between the basement involved structures of the cantabrian mountains westward and the nw tip of the iberian basement involved structures san pedro southward this together with the obliquity between the pyrenean shortening direction and the strike of the previous extensional faults mostly during the late stages of deformation determined the strike slip reactivation of the basement involved inverted faults and the lateral extrusion of the burgalesa platform detached mesozoic successions above the salt towards the se to form a prominent thrust salient oblique to the main pyrenean trend the proposed model combines thick skinned with thin skinned structural styles during oblique tectonic inversion and is consistent with the surface data including the fracture system the available subsurface data and the mechanical stratigraphy

the interaction of extensional and contractional deformations in the outer zones of the central apennines italy abstract the relationships among normal faults and thrusts in the apennines of italy are often unclear and the local absence of syn tectonic stratigraphic controls have led to contrasting interpretations on the relative chronology for both classes of structures the activity of normal faults has been variously regarded as due to pre syn or post orogenic extension and the contrasting evidence from different sites has produced an ongoing debate on the normal fault thrust interaction the results of a kinematic analysis on selected composite structures of the outer zones of the central apennines make it possible to unequivocally establish a relative chronology of extensional and contractional deformations detailed mapping outcrop scale observations and structural overprinting relationships support a positive inversion tectonic history where normal faults and fault controlled escarpments formed first and were later deformed by thrusts and related folds all normal faults control the distribution of foredeep deposits thus indicating that the recognised episode of positive inversion is related to the incipient stages of construction of the apennine thrust belt the systematic collection of structural data may help to unravel the evolution of adjacent sectors of the apennine chain as well as of other belt foredeep foreland systems whose extension contraction relationships are poorly constrained

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

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing

the control of pre existing extensional structures on the evolution of the southern sector of the aconcagua fold and thrust belt southern andes abstract the aconcagua fold and thrust belt located in the andean mountains at 32 30 to 34 s has been described as a classic model of a thin skinned thrust belt however new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple mesozoic extensional phases overprinted by structural inversion as well as thin and thick skinned tectonics two major superimposed extensional structural styles have been identified for the mesozoic characterized by distinctly oriented stress fields a key role in the evolution of this part of the fold and thrust belt was played by a late triassic to early jurassic depocentre and by late jurassic block faulting shortening was accommodated by a combination of inversion of pre existing normal faults development of footwall short cuts and both thin and thick skinned thrusting synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults with further shortening along newly formed thin skinned thrust faults the geometry of thin skinned fault systems is controlled by the architecture of the rift basin competent footwalls forming barriers to the lateral propagation of detachments

sources sinks and long term cycling of iodine in the hyperarid atacama continental margin abstract the atacama region in northern chile hosts the driest desert on earth and is the world s premier iodine production province the origin of iodine enrichment in atacama is controversial and fundamentally different processes have been invoked over the years that involve marine eolian and more recently deep sedimentary fluid and groundwater sources as a result of the very limited geochemical iodine data in atacama and the western south american margin the origin of iodine enrichment in this region still remains elusive in this study we present a comprehensive survey of iodine concentrations and isotopic ratios 129i i of different reservoirs in the atacama desert of northern chile including nitrate soils supergene copper deposits marine sedimentary rocks geothermal fluids groundwater and meteoric water nitrate soils along the eastern slope of the coastal cordillera are found to have mean iodine concentrations of at least three orders of magnitude higher than the mean crustal abundances of 0 12 ppm with a mean concentration of 700 ppm soils above giant copper deposits in the central depression are also highly enriched in iodine 100 s of ppm range and cu iodide and iodate minerals occur in the supergene enrichment zones of some of these deposits further east in the precordillera jurassic sedimentary shales and limestones show above background iodine concentrations the latter averaging 50 ppm in the southern portion of the study area the highest iodine concentrations in fluids were measured in groundwater below nitrate soils in the coastal range 3 5 10 ppm and in geothermal waters 1 3 ppm along the volcanic arc although highly variable the iodine isotopic ratios 129i i of jurassic marine sedimentary rocks 300 600 10 15 nitrate soils 150 1500 10 15 and waters 215 10 15 are consistently low

episodic out of sequence deformation promoted by cenozoic fault reactivation in nw argentina abstract the central andes are the product of contractional deformation related to subduction of the nazca plate beneath the south american continent in this geodynamic setting thrust belt propagation in the central andean retroarc in nw argentina and southern bolivia has been linked with the critical wedge model implying progressive eastward migration of the deformation front during the cenozoic however eocene unconformities and growth strata within the orogenic hinterland of the eastern cordillera and puna plateau suggest selective reactivation of pre andean and andean faults revealing a pattern of diachronous and spatially disparate range uplifts the calchaqui valley in nw argentina is characterized by basin and range morphology which is intimately linked with the heterogeneous neoproterozoic basement cretaceous rift structures and the cenozoic foreland in this study we analyze this complex deformation by reconstructing cenozoic fault reactivation and sedimentation patterns in the eastern cordillera detailed structural mapping along the eastern calchaqui border tonco area reveals five cenozoic unconformities in the former foreland deposits related to multiple episodes of fault reactivation during the eocene to pleistocene a refined chronostratigraphy u pb zircon new ahe thermochronological data apatite u th sm he and existing evidence for deformation indicate episodic out of sequence deformation and increased eo oligocene puna plateau to miocene eastern cordillera fault displacement apparently controlled by pre cenozoic heterogeneities the coeval eastward propagation of deformation and continuing deformation in the west generally challenge the applicability of the critical wedge model