late jurassic bimodal magmatism in the northern sea floor remnant of the rocas verdes basin southern patagonian andes magmatic and detrital zircon ages from the rocas verdes basin a tectonically juxtaposed remnant of sea floor in the magallanes fold and thrust belt southern patagonia south america indicates that a rifting phase of the rocas verdes basin occurred between 152 and 142 ma and was accompanied by bimodal magmatism a dacite dyke cross cutting pillow basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal sarmiento ophiolite complex contain 150 ma zircon crystals indicating that mafic submarine volcanism had started prior to or during the late jurassic 10 15 ma earlier than previously thought the silicic pyroclastic rocks of the tobifera formation with two samples dated at 148 and 142 ma were heralded by synrift sedimentation along fault bounded grabens within palaeozoic metasediments no evidence for an active volcanic arc during the early formation of the rocas verdes basin was detected in detrital zircon grains of the lower sedimentary member of the tobifera formation a minimum of 25 ma of continuous sedimentation in the rocas verdes basin is suggested by detrital zircon grains in the upper member of the zapata formation the rocas verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution

rates of shortening propagation underthrusting and flexural wave migration in continental orogenic systems the rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen this rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface the rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening this rate is controlled by the rates of mass accretion to the orogenic wedge and erosion the orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction or convergence or shortening in orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge in addition orogenic wedges may accelerate solely in response to a reduction in taper

segmentation and configuration of subducted lithosphere in italy an important control on thrust belt and foredeep basin evolution the apennine foredeep basin system located on continental crust consists of four roughly parallel basin segments each containing a flexural outer rise and an inner trough that deepens continuously toward the thrust belt flexural modeling of subsidence data across the basin indicates that this foredeep basin results mainly from deep subsurface loads acting on the subducted slab from within the subduction zone rather than from the topographic load of the apennine mountains the geometry of the four basin segments observed within the apennine foredeep is thus inferred to reflect subduction of segmented lithosphere where each lithospheric segment corresponds to a segment of the foredeep basin beneath the basin and in the foreland the lithosphere is continuous segmentation occurs only at depth beneath the thrust belt this pattern olf segmentation of the subducted lithosphere at depth is reflected in the large scale geometry of the outer part of the apennine thrust belt and appears to control it similarly the pattern of segmentation also appears to control the position of the eastern limit of back arc type extension in western italy these relations strongly suggest a genetic relation between subduction zone processes foredeep basin geometry and thrust belt evolution in which the configuration of the subducted lithosphere largely controls the large scale surface deformation observed in the apennine system

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

consequences of foreland basin development on thinned continental lithosphere application to the aquitaine basin sw france abstract we present an analysis of the consequences of foreland basin development on thinned continental lithosphere inherited from pre orogenic phases of extension bathymetry at the transition from pre orogenic extensional basin to foreland basin and compaction of pre orogenic sediments contribute to the accommodation space for foreland basin sediments and thrust loads in addition the extension induced transient thermal state of the lithosphere results in ongoing thermal subsidence and a flexural rigidity which changes through time quantitative modelling of the phase of extension and the foreland basin stage of the aquitaine basin southern france shows that the inherited transient thermal state of the lithosphere contributes significantly to 1 the total foreland basin depth and width 2 the post compressional subsidence history and 3 the cratonward onlap pattern accounting for the thermo mechanical effects of pre orogenic extension significantly reduces the estimates of both the flexural rigidity 30 43 for the aquitaine basin and the required topographic or thrust load 40 for the aquitaine basin at foreland basins emplacement of thrust loads below sea level as expected in a pre orogenic extensional basin setting further reduces the required topographic load this sheds light on the wide range of flexural rigidity values reported for continental lithosphere from foreland basin modelling studies and explains in many instances the inferred hidden load or subsurface load in flexural modelling studies at foreland basins the present study has shown that pre orogenic extension phases significantly affect the record of vertical motion and the stratigraphy of the aquitaine basin and is probably important for foreland basin evolution in general

elastic thickness structure of south america estimated using wavelets and satellite derived gravity data abstract we used a wavelet formulation of the classical spectral isostatic analysis to invert satellite derived gravity and topography bathymetry for elastic thickness te over south america and its surrounding plates to provide a homogeneous representation of the gravity field for this vast region we corrected free air anomalies derived from a combination of terrestrial marine gravity data with data from the grace and champ satellite missions model eigen cg03c by a simple bouguer slab using a smoothed representation of surface relief wavelengths 125 km the resulting bouguer anomaly compares well with terrestrial data acquired in the central andes and allows te to be confidently estimated for values greater than 10 km the te map resolves regional scale features that are well correlated with known surface structures and shows maximum values of 100 15 km over the archean neoproterozoic core of the continent decreasing to less than 30 km around continental margins several regions of the oceanic plates and continental margins have an elastic thickness less than 10 km we performed a quantitative analysis by comparing the elastic thickness with the thermal structure predicted from the age of oceanic crust and igneous metamorphic rocks this demonstrates that oceanic plates have been weakened by thermal interaction with hotspots and locally by fracturing and hydration near the trench we observe that only the nucleus of the continent has resisted the thermomechanical weakening induced by the rifting of africa and south america along the passive margin and the andean orogeny along the active margin this latter region shows along strike variations in te that correlate with the geotectonic segmentation of the margin and with the pattern of crustal seismicity our results reveal that the rigidity structure follows the segmentation of the seismogenic zone along the subduction fault suggesting a causal relationship that should be investigated in order to improve the understanding and predictability of great earthquakes and tsunamis

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

late cenozoic tectonism and landscape development in the foreland of the andes northern sierras pampeanas 26 28 s argentina the argentine sierras pampeanas are reverse fault bounded mountain blocks of precambrian to paleozoic basement rocks in the foreland of the central andes uplift in the northernmost sierras pampeanas fault blocks of sierra de quilmes sierra cumbres calchaquies and sierra aconquija started about 7 ma and became pronounced between 4 and 3 4 ma the movements culminated after 2 9 ma when the conformable mio pliocene santa maria group was overthrusted faulted and folded in the course of principal basement uplift these movements created climate and base level conditions that resulted in the formation of five pediment levels in the deformed basin strata between 2 5 and 0 3 ma tectonically induced base level changes in the piedmont resulted in three tectonism related pediment surfaces between 2 5 and 0 6 ma which attest to the neotectonic activity in the andean foreland the chronology of young tectonic uplift contradicts models in which early uplift between 20 and 13 ma is due to the interaction between high topography on the subducting nazca plate and the south american craton in conjunction with the principal uplift of the adjacent puna plateau the northernmost sierras pampeanas are better explained by e w compressional stresses superposed on a thinned lithosphere and inherited zones of structural weakness that facilitated uplift

simulation of foreland basin stratigraphy using a diffusion model of mountain belt uplift and erosion an example from the central alps switzerland foreland basin stratigraphy can be considered as the result of three interacting processes thrust deformation which builds the tectonic load sedimentary and erosional processes which redistribute that load and the flexural response of the lithosphere the resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities to understand the development of these unconformities we present a simple model of these three processes coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere the model describes the thrust wedge foreland basin system in terms of four parameters 1 the effective elastic thickness of the foreland plate te 2 the sediment transport coefficient k 3 the thrust wedge advance rate 4 the surface slope of the thrust wedge the model is applied to the oligocene miocene north alpine foreland basin nafb of eastern switzerland the stratigraphy of the nafb can be simplified into two large scale shallowing upward cycles separated by an unconformity at the base of the burdigalian 22 ma geological information is taken from the nafb to estimate suitable values for the parameters listed above assuming a linear elastic lithospheric rheology the te value is estimated at 10 5 km from decompacted sediment columns data to constrain the sequential development of the thrust wedge come from structural geology the early stages 40 24 ma of compression involved a relatively low angle thrust wedge with an advance rate of approximately 2 4 mm yr at about 24 ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate the value for the transport coefficient has been estimated from previous studies prior to attempting to simulate the broad scale geometry of the nafb the role of each parameter was assessed individually the values for te and k are held constant throughout the simulation of the nafb at 7 5 km and 500 m yr respectively the geometry of the base burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge the surface slope angle of the wedge is increased from 2 5 to 4 over 0 2 m y and the rate of thrust advance is decreased from 2 mm yr to 0 2 mm yr during the thickening event between 24 and 23 8 ma the rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and after a time lag erosion of the distal stratigraphy over the forebulge so simulating the base burdigalian unconformity

an integrated analysis of an orogen sedimentary basin pair latest cretaceous cenozoic evolution of the linked eastern cordillera orogen and the llanos foreland basin of colombia the integration of restored basin geometry and internal features of syntectonic units e g stratal architecture thickness sandstone composition with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth spatial temporal loading confi guration using this approach we determined the maastrichtian cenozoic polyphase growth of the eastern cordillera of colombia an inverted mesozoic extensional basin the record of this growth occurs in an andean post middle miocene thrust belt the eastern cordillera and in adjacent foreland basins such as the llanos basin to the east this approach permitted the identifi cation of fitectono stratigraphic sequences in the foreland basin and fi ve phases of shortening for the eastern cordillera thermochronological and geochronological data support the spatial and temporal evolution of the orogen foreland basin pair tectono stratigraphic sequences were identifi ed in two restored cross sections one located at a salient and the other in a recess on the eastern fl ank of the eastern cordillera the lower two sequences correspond to late maastrichtian paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the llanos basin these sequences consist of amalgamated quartzarenites that abruptly grade upward to organic rich fi ne grained beds and to the top light colored mudstones interbedded with litharenites in isolated channels amalgamated conglomeratic quartzose sandstones of the third sequence record 15 m y of slow subsidence in the llanos basin and llanos foothills during early to middle eocene time while shortening was taking place farther west in the magdalena valley the fourth sequence of late eocene middle miocene age records a new episode of eastward migration of tectonic loads and depositional zero in the llanos basin this sequence begins with deposition of thick fi ne grained strata to the west whereas to the east in the llanos basin amalgamated quartzarenites unconformably overlie cretaceous and older rocks former forebulge apatite fi ssion tracks in the axial zone of the eastern cordillera growth strata in the

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

tectonic control on coarse grained foreland basin sequences an example from the cordilleran foreland basin utah newly released reflection seismic and borehole data combined with sedimentological provenance and biostratigraphic data from upper cretaceous paleocene strata in the proximal part of the cordilleran foreland basin system in utah establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin during campanian time coarse grained sand and gravel were derived from the internally shortening charleston nebo salient of the sevier thrust belt a rapid regional campanian progradational event in the distal foreland basin 200 km from the thrust belt in 8 m y can be tied directly to active thrust generated growth structures and an influx of quartzose detritus derived from the charleston nebo salient eustatic sea level variation exerted a minimal role in sequence progradation

geoconservation and geotourism in arxan chaihe volcano area inner mongolia china abstract in east inner mongolia the arxan chaihe volcano area acva constitutes an important component part of the cenozoic volcanic eruption area in northeast china with great scientific and aesthetic value the area has received much attention since the 1980s the arxan national geopark was established in 2004 and the zalantun autonomous geopark in 2010 and volcanic geoconservation and geotourism developed to a certain degree along with some common problems in geopark development this paper analyzes the characteristics geoconservation and geotourism of volcanic geoheritage in the area the main problems identified are the inefficient conservation management uncoordinated development and the limited interpretation system we provide some suggestions on the problems in order to promote the local development such as strengthening legislation training tour guides as volcanologists with a background in geology who can interpret volcano geoheritage correctly building a web site and combining the two geoparks to apply for designation as a global geopark

geodynamic models of convergent margin tectonics transition from rifted margin to overthrust belt and consequences for foreland basin development a quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads the lithosphere and the associated foreland basin the model treats the lithosphere as a two dimensional nonuniform elastic plate whose strength is controlled thermally the thermal and flexural evolution of a margin is followed from initial rifting and passive margin development through overthrusting and foreland basin deposition to postdeformational erosion our models indicate the following 1 foreland basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting sensitivity decreases the farther the overthrust load is empl ced onto the craton 2 foreland basin depth is strongly influenced by the size of the overthrust load maximum depths in excess of 10 km 6 mi can occur given overthrust topography similar to the present day himalayas 3 a few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained very little of this eroded material is deposited in the foreland basin most must bypass the basin because it also is subjected to erosion and associated flexural uplift 4 thick overthrusts as interpreted from cocorp profiling are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere

evolution of deep water stratigraphic architecture magallanes basin chile abstract the 4000 m thick and 20 myr deep water sedimentary fill of the upper cretaceous magallanes basin was deposited in three major phases each with contrasting stratigraphic architecture 1 the oldest deep water formation punta barrosa formation comprises tabular to slightly lenticular packages of interbedded sandy turbidites slurry flow deposits and siltstone that are interpreted to record lobe deposition in an unconfined to weakly ponded setting 2 the overlying 2500 m thick and shale dominated cerro toro formation includes a succession of stacked conglomeratic and sandstone channel fill deposits with associated finer grained overbank deposits interpreted to record deposition in a foredeep axial channel levee system 3 the final phase of deep water sedimentation is characterized by sandstone rich successions of highly variable thickness and cross sectional geometry and mudstone rich mass transport deposits mtds that are interpreted to record deposition at the base of slope and lower slope segments of a prograding delta fed slope system the deep water formations are capped by shallow marine and deltaic deposits of the dorotea formation these architectural changes are associated with the combined influences of tectonically driven changes and intrinsic evolution including 1 the variability of amount and type of source material 2 variations in basin shape through time and 3 evolution of the fill as a function of prograding systems filling the deep water accommodation while the expression of these controls in the stratigraphic architecture of other deep water successions might differ in detail the controls themselves are common to all deep water basins information about source material and basin shape is contained within the detrital record and when integrated and analyzed within the context of stratigraphic patterns attains a more robust linkage of processes to products than stratigraphic characterization alone

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

flexural versus dynamic processes of subsidence in the north american cordillera foreland basin 1 dynamic subsidence related to subduction is an important process in retroarc foreland basin systems dynamic subsidence in the north american retroarc foreland has been proposed as dominant in the late cretaceous however questions remain about the nature of the subcrustal load and the basin response to such processes we present new isopach data using 130 data points covering a large portion of the u s and a revised flexural analysis of the sevier foreland higher rigidity associated with the wyoming craton can best model flexural subsidence directly prior to 81 ma we constrain the transition from flexural to nonflexural subsidence to 81 ma and correlate this with large late cretaceous progradation the locus of subsidence and the location of the subducted shatsky oceanic plateau suggest that dynamic loading from viscous flow above the subducting plate is the mechanism driving dynamic subsidence in the retroarc foreland basin of the north american cordillera

erosional control of the kinematics and geometry of fold and thrust belts imaged in a physical and numerical sandbox 1 we investigate the effect of systematically varying erosion intensity k on the geometry and kinematics of fold and thrust belts modeled using equivalent conditions in physical and numerical experiments similar material properties and boundary conditions were used to compare numerical experiments to those modeled in the sandbox using an erosion rule that removes mass from the scaled sand wedges according to rates expected from bedrock fluvial incision the geometry of both modeling approaches is quantitatively compared with that expected from analytical theory the fold and thrust belt growth rate is inversely proportional to k and is well predicted by theory except for the high erosion case in which both the numerical and the experimental sandbox grows supercritically a direct relationship exists between the erosion intensity and the number of fore shear bands and shear strain magnitude the number of fore shear bands decreases and their strain increases with erosion intensity the results indicate that realistic and mechanistic erosion rules can be applied to physical experiments to model mass removal and this approach opens up the possibility of calibrating strain history to erosion intensities for predictions in natural settings quantitative comparisons between the physical and numerical sandbox experiments indicate that deformation style and geometry of the thrust and fold belts are similar when identical rheologies and boundary conditions are used this benchmarking suggests that numerical experiments realistically model conditions observed in the simple cohesionless rheologies commonly employed in sandbox experiments this suggests that numerical simulation may reliably model experimental deformation in geological situations that are difficult to represent in scaled experiments because of the different scales and rheologies of natural orogenic wedges

structural evolution of the pyrenees tectonic inheritance and flexural behaviour in the continental crust abstract the ecors deep seismic reflection profile and additional geological data are used to understand deformation processes in the pyrenees the middle crust shows two major sets of seismic reflectors interpreted as hercynian structures during permian and the late jurassic early cretaceous rifting events these structures were extensionally reactivated mainly within the european plate during the pyrenean orogeny tectonic inversion of the early cretaceous basins took place located mainly on the european plate and hercynian southward directed verging thrusts were also reactivated in the iberian foreland a regional model is proposed which takes into account the lithospheric flexure and the evolution of the sedimentary load this study shows the different rigidities of the european and iberian plates difference related to differential thinning events as well as an east west diachronism of the flexure for the european plate and unflexing behaviour after the orogeny

tectonic implications of a paleomagnetic study of the sarmiento ophiolitic complex southern chile abstract a paleomagnetic study was carried out on the late jurassic sarmiento ophiolitic complex soc exposed in the magallanes fold and thrust belt in the southern patagonian andes southern chile this complex mainly consisting of a thick succession of pillow lavas sheeted dikes and gabbros is a seafloor remnant of the late jurassic to early cretaceous rocas verdes basin that developed along the south western margin of south america stepwise thermal and alternating field demagnetization permitted the isolation of a post folding characteristic remanence apparently carried by fine grain sd magnetite both in the pillow lavas and dikes the mean in situ direction for the soc is dec 286 9 inc 58 5 95 6 9 n 11 sites rock magnetic properties petrography and whole rock k ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the late cretaceous the mean remanence direction is anomalous relative to the expected late cretaceous direction from stable south america particularly a declination anomaly over 50 is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the jurassic el quemado fm located over 100 km north of the study area in argentina nevertheless a significant ccw rotation of the whole soc is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back arc basin sub parallel to the continental margin a rigid body 30 westward tilting of the soc block around a horizontal axis trending nnw is considered a much simpler explanation being consistent with geologic evidence this may have occurred as a consequence of inverse reactivation of old normal faults which limit both the soc exposures and the cordillera sarmiento to the east the age of tilting is unknown but it must postdate remanence acquisition in the late cretaceous two major orogenic events of the southern patagonian andes in the eocene ca 42 ma and middle miocene ca 12 ma respectively could have caused the proposed tilting

recognition of forebulge unconformities associated with early stage foreland basin development example from the north alpine foreland basin the peripheral upwarp forebulge associated with early foreland basin evolution may be recorded as an unconformity at the base of the foreland basin fill a forebulge unconformity represents a distinctive type of megasequence boundary forward modeling of foreland flexure illustrates the expected geometry of an unconformity cut by erosion of the forebulge during the early stages of foreland basin development internal conformity a zone of rapid increase in the depth of erosion and an external zone of steady state stratigraphic gap this geometry is modified by the preexisting bathymetry and by eustatic sea level changes which result in the formation of a composite unconformity however the geometry of the unconformity and the onlap of the overlying sediments are fund mentally different from those associated with a major eustatic sea level fall and represent a distinctive assemblage of petroleum play components producing hydrocarbon accumulations are associated with both forebulge arching and truncational unconformity traps modeling achieves an accurate simulation of the geometry and stratigraphic gap of the basal unconformity of the north alpine foreland basin switzerland further support for the flexural forebulge interpretation is provided by facies patterns in the overlying sediments suppression and smothering of forebulges during the late molasse stage of foreland basin development coupled with the cannibalization of the early flysch stage stratigraphy by the advancing orogenic wedge may limit the formation and recognition of forebulge unconformities model results may be helpful in predicting the timing duration and amount of uplift associated with flexural forebulges and thereby aid in assessing petroleum play potential particularly the formation of traps and the development of enhanced porosity

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

chronology of motion in a complete thrust belt the precordillera 30 31 s andes mountains in the precordillera thrust belt of west central argentina faulted rotated and onlapping relations of piggyback basin units and foreland basin units yield an accurate but imprecise chronology of thrust motion addition of data from provenance history strata accumulation rates and presence of thick conglomerate units in the bermejo foreland basin results in a thrust chronology that is more precise but not entirely independent of assumptions about foreland basin dynamics seven new radiometric dates allow substantial revision of the interpreted age of first deformation in the precordillera at about 30 south units previously assigned to the paleozoic and mesozoic are of oligocene and miocene ages a redbed unit that began before 21 6 pm 0 8 ma appears to have pre dated thrusting deposits in the early bermejo foreland basin were dominated by eolian sandstones unlike the younger fluvial and alluvial units an andesitic volcanic center within the miocene foreland basin had long term effects on sub

the chronology of the eocene tectonic and stratigraphic development of the eastern pyrenean foreland basin northeast spain abundant and clearly exposed relationships between structures and syntectonic sedimentary rocks in the southern pyrenees make this area particularly suitable for detailed studies of the sequential development of a collisional orogenic belt during the pyrenean orogeny major shortening occurred in the eastern pyrenees in eocene and early oligocene times until now individual tectonic and depositional events have been only loosely defined temporally by biostratigraphic data in order to provide a more precise chronological framework four new magnetostratigraphic sections spanning 8 km of eocene strata and encompassing 18 m y have been developed in the eastern pyrenees of northern spain the ages of nine major depositional sequences in the eastern pyrenean foreland have been specified within the context of these chronologic data beginning with the early eocene transgression that commenced at 58 ma the timing of numerous tectonic events that occurred during the subsequent 16 m y can also be delineated including the initial emplacement of the pedraforca thrust sheet 58 54 ma the development of the pedraforca breakback thrusts 47 5 40 ma at a mean shortening rate of 2 4 mm yr rotation of the pedraforca footwall three intervals of motion along the vallfogona thrust 42 5 44 0 40 5 41 5 and

cenozoic foreland basin system in the central andes of northwestern argentina implications for andean geodynamics and modes of deformation 1 cenozoic strata in the central andes of northwestern argentina record the development and migration of a regional foreland basin system analogous to the modern chaco parana alluvial plain paleocene lower eocene fluvial and lacustrine deposits are overlain by middle upper eocene hypermature paleosols or an erosional disconformity representing 10 15 myr this supersol disconformity zone is traceable over a 200 000 km2 area in the andean thrust belt and is overlain by 2 6 km of upward coarsening eastward thinning upper eocene through lower miocene fluvial and eolian deposits middle miocene pliocene fluvial lacustrine and alluvial fan deposits occupy local depocenters with contractional growth structures paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments detrital zircon ages from cenozoic sandstones cluster at 470 491 522 544 555 994 and 1024 1096 ma proterozoic mesozoic clastic and igneous rocks in the puna and cordillera oriental yield similar age clusters and served as sources of the zircons in the cenozoic deposits arc derived zircons become prominent in oligo miocene deposits and provide new chronostratigraphic constraints sediment accumulation rate increased from 20 m myr during paleocene eocene time to 200 600 m myr during the middle to late miocene the new data suggest that a flexural foreland basin formed during paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front despite differences in deformation style between bolivia and northwestern argentina lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late paleocene time

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

variations in the degree of crustal extension during formation of a back arc basin abstract ophiolite complexes in southern chile represent the remnants of the mafic portion of the floor of a cretaceous back arc basin which widened markedly from north to south over a length of 600 km detailed field and geochemical studies of ophiolites in the northern sarmiento complex and southern tortuga complex extremities of the originally wedge shaped back arc basin floor indicate significant north south differences in the mode of emplacement of basaltic magmas into the pre existing continental crust during the formation of the basin in the northern narrow extremity of the original basin mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust in the southern wider part of the original basin mafic magmas appear to have been emplaced at a localized oceanic type spreading centre the observed north south variations resulted in formation of back arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic these variations are interpreted as representing different stages of evolution of a back arc basin which formed due to a subtle interplay between subduction induced back arc mantle convection and the release of stress across the convergent plate boundary possibly related to ridge subduction prior to the release of stress heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in south america with the release of stress mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism initially basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic type spreading centre observations in southern chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back arc convection and hence the mechanism of emplacement into the crust of mafic mantle melts the various lithologies observed in southern chile could also be expected to form during the opening phase of major ocean basins and to currently underlie atlantic type continental margins

stratigraphic modeling of foreland basins interpreting thrust deformation and lithosphere rheology the authors incorporate the processes of erosion and deposition in a numerical model to predict the stratal geometries and facies patterns produced during episodic thrusting in a nonmarine foreland basin the resultant stratigraphic record is characterized by a stairstepped facies package in which each retrogradation of facies toward the thrust marks the onset of a thrusting event the retrogradation of facies coincides with the migration of the forebulge toward the thrust and the generation of an erosional unconformity in the past changes in basin wavelength during basin evolution have been interpreted to record viscoelastic relaxation of the lithosphere this model suggests that changes in basin wavelength are a natural consequence of the interplay between thrust and sediment loading on an elastic lithosphere

combined receiver function and surface wave phase velocity inversion using a niching genetic algorithm application to patagonia a combined inversion of body wave receiver functions and rayleigh wave phase velocities using a niching genetic algorithm nga increases the unique ness of the solution over separate inversions and also facilitates explicit parameteri zation of layer thickness in the model space this parameterization requires fewer layers and a priori constraints for modeling more complex structures than traditional linearized inversion ngas solve for a suite of locally optimal solutions in addition to isolating the globally optimal solution by using an evolutionary paradigm this nonlinear examination of the error space provides an opportunity to examine trade off within the model space the method when applied to synthetic data locates interfaces within 2 km of the test structure and identifies appropriate structure and velocity applying this method to data from a deployment of five broadband stations in chilean patagonia yields a regional model of crustal structure that is consistent with the geological history of the region sediment thickness is inversely proportional to crustal thickness with sediment thicknesses reaching more than 4 km where the crustal thickness thins to 28 km this is consistent with previous geological studies which suggests that the rocas verdes basin in western patagonia formed by crustal thinning isostatic compensation and subsequent sedimentation

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

structural and stratigraphic evolution of the western malvinas basin argentina the malvinas basin is one of the few basins on the argentine continental shelf that contains a proven petroleum system however uneconomical oil discoveries keep the basin at a frontier exploration status the malvinas basin evolved through three main tectonic phases rift sag and foredeep the sedimentary fill of the basin is closely related to its tectonic history middle jurassic rifting resulted in north northwest oriented grabens that filled with volcanic and pyroclastic continental rocks diminished faulting and generalized subsidence during the late jurassic neocomian early sag phase were accompanied by deposition of a basal transgressive marine wedge the aptian maastrichtian interval was characterized by tectonic quiescence and deposition of offshore mud prone sediments southerly localized early paleogene transtensional tectonism accompanied the early development of a foredeep trough outer shelf glauconite rich sandstones basinal claystones and localized carbonate buildups partially filled the basin by the middle eocene oligocene a strong deepening event marked the initiation of the foredeep sensu stricto phase this phase resulted in the full development of the malvinas foredeep and the formation of compressional structures in the foreland the foredeep basin was replenished by a westerly derived offlapping siliciclastic wedge of oligocene miocene age noncommercial hydrocarbon discoveries in 5 of the 17 wells drilled in the basin suggest the presence of an undercharged lower inoceramus springhill petroleum system

modified gulf of california model for south georgia north scotia ridge and implications for the rocas verdes back arc basin southern andes new field mapping and an integrated trace element and nd isotopic study have been undertaken on the jurassic cretaceous larsen harbour complex on the island of south georgia recognized as a displaced part of the southern andes ensialic autochthonous rocas verdes marginal basin five basalt groups groups i v are identified from trace element data initial nd isotopic compositions suggest that they are derived from only two sources magmas produced during early stages of continental lithospheric attenuation groups i iii were derived by varying degrees of partial melting and fractional crystallization from a large ion lithophile element lile enriched low epsilon sub nd mantle source relative to normal n type mid ocean ridge basalt morb magmas produced during later stages of rifting groups iv and v were however derived from a high epsilon sub nd asthenospheric mantle source similar to n type morb unaffected by earlier lile enrichment our data do not occur with previous suggestions that the larsen harbour complex and by implication the rocas verdes formed in a supra subduction zone setting on the basis of geologic and geochemical evidence we favor basin formation along an oblique slip margin akin to that of the gulf of california

two phase stratigraphic model of foreland basin sequences application of flexural models to nonmarine foreland basin evolution indicates that two different stratigraphic styles of basin fill may develop over time basin subsidence is most rapid during times of thrust load emplacement associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine grained deposits that cover most of the basin the distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin during this postorogenic phase of adjustment a regional unconformity develops in the proximal part of the foreland basin proximal deposits along with thrust derived sediment are redeposited in the distal foreland basin and beyond this two phase model of foreland sedimentation predicts that coarsening upward sequences in the proximal and distal parts of the basin have reciprocal significance the proximal sequence represents thrust belt advance whereas the distal sequence represents thrust belt cessation

stratigraphic record across a retroarc basin inversion rocas verdes magallanes basin patagonian andes chile the mesozoic evolution of the andean cordillera of southern patagonia is recorded in two formations that are now part of the fold thrust belt the zapata formation and the punta barrosa formation extension in the ultima esperanza region began in the late jurassic with deposition of the marine volcaniclastic tobifera formation and eventually resulted in the full fledged oceanic rocas verdes basin the zapata formation was deposited over a time span of 50 m y in an irregular basin ultimately bordered by an early cretaceous arc to the west it is characterized by interbedded shale and siltstone mostly deposited in shallow water and in areas of oceanic crust emplacement also as deep water hemipelagic deposits sediments of the zapata formation were derived initially from local oceanic upwarps and tobifera highs and later from the ande sitic cover of the juvenile arc and or exhumed oceanic crust the paucity of sandstone in the zapata formation in the ultima esperanza region indicates a highly irregular basin partitioned by prominent horsts with sand deposition confined to sub basins closer to the arc changes in depositional regimes and sediment dispersal patterns related to the onset of andean contraction and formation of the magallanes foreland basin are recorded by sediments of the overlying punta barrosa formation this formation records the evolution of a fold thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale the presence of an arc is indicated but nearby andean metamorphic terranes are more significantly represented in punta barrosa sediments stratigraphic details across the zapata and punta barrosa formations indicate that deformation and development of a proto andean cordillera in southern south america was initiated in the late mesozoic and involved conspicuous crustal shortening

history of the sevier orogenic wedge in terms of critical taper models northeast utah and southwest wyoming problems with applying critical taper models to ancient orogenic wedges are overcome in the late cretaceous late paleocene sevier orogenic wedge in utah and wyoming by a symptomatic approach in which wedge behavior is inferred from the time space distribution of thrust faulting erosion and synorogenic sediment accumulation in association with the orogenic wedge in turn the causes of wedge behavior are interpreted in terms of features that are known about the sevier wedge such as the locations of major decollements and the lithologic constituents of major thrust sheets from coniacian through late paleocene time 35 m y basement and cover rocks in the sevier wedge were shortened by 100 km in three major and one minor events an overall eastward progression of thrusting was punctuated by several episodes of out of sequence and hinterland vergent thrusting over the long term wedge taper was controlled by internal wedge strength initial taper and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge while the basal decollement was mainly in basement rocks the wedge was highly tapered and thrust spacing was relatively close 15 km beginning ca 75 ma the basal decollement ramped upward and eastward into weak cambrian shale and jurassic salt thrust spacing increased to 25 45 km and wedge taper decreased drastically the highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements whereas the frontal lower taper part of the wedge carried sedimentary rocks above extremely weak salt and shale decollements through time internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time both by strain softening beneath the rear of the wedge and by propagating along extremely weak rocks beneath the front of the wedge provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long term history of the sevier wedge as an increasing proportion of the erosional surface became occupied by resistant precambrian quartzite and basement rocks this would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope which would have helped the wedge to remain at critical to supercritical taper the sevier wedge also exhibits several shorter term 10 km and 3 was subjected to regional erosion marked by major unconformities that ultimately caught up with tectonic thickening and caused the wedge to stall this cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag time between wedge thickening and uplift and wedge erosion erosion eventually catches up to the rate of uplift the wedge stalls and the locus of deformation shifts to the rear of the wedge in order to rebuild taper this analysis suggests that orogenic wedges continually deform and find a way to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists the actual taper value may change substantially through time as lithostratigraphy internal strength upper surface durability and presence absence of weak potential stress guides conspire to maintain a critical taper

kinematic evolution of the patagonian retroarc fold and thrust belt and magallanes foreland basin chile and argentina 51 30 s the kinematic evolution of the patagonian fold and thrust belt and cogenetic magallanes retroarc foreland basin is reconstructed using new geologic mapping two dimensional 2 d seismic reflection data and zircon u pb geochronology results span an 160 km wide transect of the thrust belt and magallanes basin near 51 30 s and highlight the influence of inherited extensional structures on basin paleogeography syntectonic sedimentation and late cretaceous neogene foreland shortening south of 50 s the patagonian fold and thrust belt developed on oceanic and attenuated crust of the predecessor late jurassic rocas verdes rift basin resulting in a collisional nature to early fold and thrust belt development and foreland sedimentation atop rifted south american crust we identify six principal stages of development between late cretaceous and neogene time a palinspastic restoration indicates 32 40 km 19 23 of retroarc shortening following closure of the rocas verdes basin and incipient growth of the thrust belt more than half of the estimated crustal shortening occurred synchronously with the deep water phase of late cretaceous foreland basin sedimentation subsequent deformation migrated into the foreland accounting for 12 km of shortening across the cretaceous early miocene basin fill thick skinned thrust faulting along multiple detachment levels in paleozoic metamorphic basement resulted in 5 km of foreland uplift and exposure of preforeland basin deposits the final phase of early miocene deformation ca 21 18 ma may reflect enhanced coupling between the continental and oceanic lithospheres causing foreland basement uplifts as the chile ridge spreading ridge approached the trench we speculate that neogene foreland shortening was accommodated by reactivation of mesozoic normal faults zones and accounts for broad uplift of the patagonian fold and thrust belt

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

crustal and upper mantle structure of southernmost south america inferred from regional waveform inversion 1 we determine the crustal and upper mantle structure of southern south america by inverting regional seismograms recorded by the seismic experiment in patagonia and antarctica we present a waveform inversion method that utilizes a niching genetic algorithm the niching genetic algorithm differs from the traditional genetic algorithm in that the inversion is performed in multiple subpopulations thus generating a broader search of the model space and enabling us to examine alternative local error minima the vertical and transverse waveforms were used extending from the p arrival to the surface waves and the inversion was performed at either 0 005 0 06 hz larger events or 0 02 0 06 hz smaller events the inversion included anisotropy by solving for separate sv and sh structures in the upper mantle results indicate that crustal thickness varies from 26 to 36 km with thicker values toward the northeast suggesting that there is little crustal thickening beneath the austral andes the average upper mantle velocities are similar to the preliminary reference earth model prem except that the southernmost region shows velocities of 5 slower than prem the upper mantle has up to 5 polarization anisotropy between the moho and 120 km depth the strongest anisotropy is localized in a lithospheric lid shallower than 65 km depth which overlies a pronounced low velocity zone this shallow limit to anisotropy is consistent with the relatively small shear wave splitting values found in this region these results suggest that the anisotropy is limited to lithospheric depths and may imply the absence of a strong mantle flow pattern in the asthenosphere

constraints on deformation of the southern andes since the cretaceous from anisotropy of magnetic susceptibility the southernmost segment of the andean cordillera underwent a complex deformation history characterized by alternation of contractional extensional and strike slip tectonics key elements of southern andean deformation that remain poorly constrained include the origin of the orogenic bend known as the patagonian orocline here renamed as patagonian arc and the exhumation mechanism of an upper amphibolite facies metamorphic complex currently exposed in cordillera darwin here we present results of anisotropy of magnetic susceptibility ams from 22 sites in upper cretaceous to upper eocene sedimentary rocks within the internal structural domain of the magallanes fold and thrust belt in tierra del fuego argentina ams parameters from most sites reveal a weak tectonic overprint of the original magnetic fabric which was likely acquired upon layer parallel shortening soon after sedimentation magnetic lineation from 17 sites is interpreted to have formed during compressive tectonic phases associated to a continuous n s contraction our data combined with the existing ams database from adjacent areas show that the early cretaceous late oligocene tectonic phases in the southern andes yielded continuous contraction variable from e w in the patagonian andes to n s in the fuegian andes which defined a radial strain field a direct implication is that the exhumation of the cordillera darwin metamorphic complex occurred under compressive rather than extensional or strike slip tectonics as alternatively proposed if we agree with recent works considering the curved magallanes fold and thrust belt as a primary arc i e no relative vertical axis rotation of the limbs occurs during its formation then other mechanisms different from oroclinal bending should be invoked to explain the documented radial strain field we tentatively propose a kinematic model in which reactivation of variably oriented jurassic faults at the south american continental margin controlled the late cretaceous to cenozoic evolution of the magallanes fold and thrust belt yielding the observed deformation pattern

weddell sea anomalies excitation propagation and possible consequences 1 antarctic marginal seas are susceptible to significant decadal variability as revealed by the analysis of a 200 year integration of a regional ice ocean model forced with the atmospheric output of the ipcc climate model echam5 mpiom the strongest signal occurs on the southern and western weddell sea continental shelf where changes in bottom salinity are initiated by a variable sea ice cover and modification of surface waters near the greenwich meridian related zonal shifts of the western rim current guide deep waters with different temperature out of the weddell sea with a deep boundary current the temperature signal propagates westward through southern drake passage and along the upper continental rise in the southeast pacific thereby influencing the hydrographic conditions on the continental shelf of bellingshausen amundsen and ross seas

the modern foreland basin system adjacent to the central andes regional variations in sediment thickness internal structures average elevation and bouguer gravity define a four component foreland basin system adjacent to the central andes in the most proximal part of the foreland basin system the eastern subandean zone and westernmost chaco plain 1 3 km of cenozoic deposits overlies active folds and thrusts of the frontal andean orogenic wedge these wedge top deposits pass cratonward into a foredeep depozone containing a 3 4 km thick sedimentary prism that tapers toward and locally pinches out against a broad wavelength forebulge in the central eastern chaco plain the forebulge is underlain by precambrian mesozoic rocks and is largely covered by a thin veneer of quaternary alluvium east of the forebulge a thin 0 5 km saucer shaped accumulation of sediment beneath the pantanal wetland represents a back bulge depozone ancient counterparts of these four depozones can be identified in the central andes suggesting that modern basin architecture is the result of continuous eastward migration of the coupled orogenic wedge and foreland basin system since the late cretaceous paleocene

flexure of lithosphere beneath apennine and carpathian foredeep basins evidence for an insufficient topographic load the pliocene foredeep basin external to the apennine thrust belt and the late miocene foredeep basin external to the outer east carpathians are considered typical molassic foredeeps that formed contemporaneously with thrusting in the adjacent mountain belts a simple mechanical model in which the response of the lithosphere to loading was approximately by a semi infinite broken elastic plate overlying a weak fluid was used to analyze the deflection of the foreland lithosphere beneath the apennine and outer east carpathian foredeep basins along three transects perpendicular to the thrust belts comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the foreland lithosphere during mountain building failed to pr duce a sufficient load to create the observed foredeep basins and observed topography regardless of the position of the slab end or the flexural rigidity assumed for the foreland lithosphere we infer that an additional subsurface load must be present and that its magnitude must be roughly equal to the load created by that part of the thrust belt that is above sea level for the two apennine transects an acceptable fit to the shape and size of the observed foredeep basin was generated by assuming effective elastic plate thicknesses of 8 and 15 km 4 9 and 9 3 mi and applied vertical forces of 2 1015 dynes cm and 1 3 1015 dynes cm at the slab end for the carpathian transect an effective plate thickness of 30 km 18 6 mi and an applied force of 1 5 215 1015 dynes cm generated an acceptable fit this study suggests a method whereby an estimate of the depth to autochthonous basement rocks beneath overthrust belts can be made in some areas where this information is not available from seismic or drilling data such an approach may enable exploration geologists to determine if crystalline rocks encountered in drilling overthrust terranes are a part of the autochthonous basement or if they are part of an overthrust sheet in the latter situation oil bearing sequences of sedimentary rocks might lie below crystalline rocks contained in the allochthonous sheet