geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

distribution and genesis of soils of the northeastern united states abstract the soils of the northeastern united states central and northern appalachians north of the glacial border are spodosols inceptisols alfisols and some entisols and histosols these soils are relatively young 12 18 ka climate frigid soil temperatures parent material acidic sandy and vegetation conifers are the major soil forming factors affecting the genesis of the spodosols while parent material calcareous material is a determining factor affecting the genesis of the alfisols the inceptisols of southern new england are slightly different better developed cambic horizon than those of the appalachian plateau the genesis of the alfisols of the unglaciated appalachian plateau and triassic lowlands are also significantly influenced by carbonates and basic minerals in their parent material ultisols are found only south of the glacial border ultisols on the appalachian plateau and in the northern ridge and valley are young soils and should be identified as parent material ultisols while those of the southern ridge and valley piedmont and coastal plain show significant soil development and should be identified as genetic ultisols the soils south of the glacial border on steep slopes have been affected significantly by erosion during the cold cycles of the pleistocene while soils on gentle slopes have been affected to a lesser degree much of the material eroded during the late pleistocene has accumulated on lower sideslopes as colluvium the erosional and depositional processes have resulted in a complex mosaic of moderate to well developed soils on the more stable landscape surfaces and moderate to weakly developed soils on the erosional and depositional surfaces

the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

bookshelf tectonics rotated crustal blocks within the sovanco fracture zone the 15 kilometer wide sovanco fracture zone is a dextral slip transform fault linking the juan de fuca and explorer ridges in the n e pacific seabeam bathymetry shows that the sovanco comprises several rhomb shaped elevated blocks of oceanic crust bounded by northeast and northwest trending lineaments we hypothesize that the northeast trending lineaments are sinistral strike slip faults that are reactivated ridge parallel normal faults both the sinistral block bounding faults and the blocks themselves have rotated clockwise about 30 degrees in response to dextral shear within the sovanco zone

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

kinematics of the philippine sea plate in this study new instantaneous rotation vectors describing the motion of the philippine sea plate relative to surrounding plates are determined that most nearly satisfy all available geological geophysical and seismological observations along the plate margins pacific philippine and caroline pacific poles are generated from a set of trial eurasia philippine poles of rotation using published determinations of the eurasia pacific and caroline philippine poles the pacific philippine caroline pacific and eurasia philippine poles are then checked for agreement with 1 slip vectors determined from focal mechanisms of earthquakes along the ryukyu trench and the nankai trough 2 convergence at the sagami trough 3 extension in the mariana trough 4 extension behind the bonin trench 5 extension in the western sorol trough and 6 convergence at the mussau trench trial eurasia philippine poles are eliminated if predicted plate motions do not match observed plate motions the locations and rotation rates of the three best fitting poles are eurasia philippine 37 0 n 141 0 e 1 60 ma pacific philippine 3 8 n 130 4 e 1 68 ma caroline pacific 16 9 n 141 4 e 0 54 ma the plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the philippine pacific and caroline plates intersect intermediate and deep focus earthquake hypocenters along the southern mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the mariana trough disappears the absence of volcanism and intermediate depth seismicity along the southernmost mariana yap and palau sections of the plate boundary is attributed to very low convergence rates a new focal mechanism solution along the trench south of guam is the first clear underthrusting solution found along the mariana trench and agrees with the convergence direction implied by the pacific philippine pole of rotation no clear underthrusting focal mechanism solutions were found along the yap and palau trenches although three high angle thrust solutions demonstrate that compression is occurring in the upper plate perpendicular to the plate boundary

constriction in a transpressive regime an example in the iberian branch of the ibero armorican arc abstract detailed strain analyses in the armorican quartzites have been performed in various places within the centro iberian zone of western europe the observed strain pattern shows that in the inner ibero armorican arc as well as near the hinge zone sinistral transpression produces constrictive ellipsoids in the outer arc away from the hinge zone the deformation regime produces plane strain to oblate strain ellipsoids some of these data clearly contradict existent kinematical models which associate constriction with transtension and consider transpression zones to generate oblate strain ellipsoids however newly developed models show that it is possible to obtain a wide range of finite strain ellipsoids in transpression zones if one takes account of lateral escape axial depression and volume change the geodynamic implications of the strain analyses suggest generation of constriction in a transpressive regime caused by lateral expulsion in front of the cantabrian indenter that produced the ibero armorican arc this mechanism was responsible for the variscan wrench component in western europe which was mainly sinistral in iberia and dextral in britanny in iberia the displacement by lateral escape was easily accommodated by subduction of the south portuguese terrane under the ossa morena zone of the iberian terrane

a large normal fault earthquake at the junction of the tonga trench and the louisville ridge the source mechanism of a large m s 7 2 earthquake that occurred in the oceanic plate at the junction of the tonga kermadec trench systems with the aseismic louisville ridge is found by inverting long period vertical component rayleigh waves recorded by the ida network the solution is an almost pure normal fault on a plane striking roughly parallel to the trench axis with seismic moment of 1 7 10 27 dyn cm and thus is among the ten largest documented shallow normal fault earthquakes a point source depth of 20 km for the event is resolved by modeling teleseismic body waves the actual rupture may have extended deeper to 30 or 40 km the earthquake was a multiple event consisting of two sources separated by 16 s a rupture velocity of 3 5 km s 1 is inferred the earthquake can be interpreted as tensional failure in the shallow portion of the downgoing plate caused by the gravitational pull of the slab the louisville ridge may be creating a local degree of decoupling of the oceanic plate from the overriding plate and or a zone of extension within the slab which could enhance the effect of the gravitational forces in the shallower part of the downgoing plate in particular the earthquake could be associated with the break up of the leading seamount of the ridge which is currently right at the trench alternatively the earthquake may have been caused by stresses associated with the bending of the plate prior to subduction

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

domino boudinage under layer parallel simple shear abstract the boudin segments of a torn competent layer experience synthetic rotation in layer parallel simple shear as long as the individual segments in a boudin train are constrained by their neighbors even a highly viscous boudin deforms internally to create the necessary space for rotation the rotation rate is then much smaller compared to the case of an isolated segment hence a small tilt of boudin segments is not indicative of low strain the rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width once the tilted boudins are no longer constrained by their neighbors the rotation rate greatly accelerates in the case of a low viscosity ratio between the boudins and the host the boudin segments develop complex shapes which may give an impression of shear band boudins forming under the opposite shear sense we furthermore investigate the behavior of boudin trains of finite length the terminal segments are displaced out of the shear plane deforming into isoclinal folds and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure natural examples of domino boudinage from a high shear strain detachment zone in the western cyclades greece show many similarities with the modeled structures suggesting that under simple shear deformation the rotation and separation of boudin segments is an indicator for high shear strain

geodynamic evolution of southern costa rica related to low angle subduction of the cocos ridge constraints from thermochronology abstract the late tertiary shallow subduction of the cocos ridge under the caribbean plate controlled the evolution of the cordillera de talamanca in southeast costa rica which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting fission track thermochronology using zircon and apatite as well as 40ar 39ar and rb sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the cordillera de talamanca and the age of onset of subduction of the cocos ridge shallow intrusion of granitoid melts resulted in fast and isobaric cooling a weighted mean zircon fission track age 13 analyses and rb sr biotite ages of about 10 ma suggest rapid cooling and give minimum ages for granitoid emplacement in some cases 40ar 39ar and rb sr apparent ages of amphibole and biotite are younger than the zircon fission track ages which can be attributed to partial resetting by hydrothermal alteration apatite fission track ages range from 4 8 to 1 7 ma but show no correlation with the 3090 m elevation span over which they were sampled the apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes the combination of the apatite fission track ages with subduction parameters of the cocos plate such as subduction angle plate convergence rate and distance of the cordillera de talamanca to the trench implies that the cocos ridge entered the middle america trench between 5 5 and 3 5 ma

forearc structures and tectonic regimes at the oblique subduction zone between the hikurangi plateau and the southern kermadec margin the convergent southern kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic hikurangi plateau swath bathymetric data have been combined with other geophysical data to reveal that the south kermadec inner trench wall is segmented into distinct upper middle and lower margins the lower margin is a nongrowing compressionally unstable accretionary wedge that is dissected by transcurrent faults the wedge has been thinned and narrowed as a result of tectonic erosion the middle margin is stable by comparison and separated from the lower margin by the 220 km long awanui fault this fault is a landward dipping strike slip reverse fault the middle upper margin including east cape ridge ecr forms a deforming backstop with a sharp along strike change in tectonic regime south of 36 40 s transpression characterizes the ecr shear zone north of 36 40 s the ecr is marked by uplift by extension and farther north by subsidence it is suggested that subsidence and subsequent tectonic erosion of the margin north of 35 50 s in the wake of hikurangi plateau subduction have broken the link between once continuous segments of the ecr tonga platform the unstable nature of the southern kermadec wedge under compression together with a comparison of its taper characteristics with those of modern convergent margins implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime strike slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the hikurangi plateau

domino structures evolution in strike slip shear zones the importance of the cataclastic flow abstract the porto tomar ferreira do alentejo dextral shear zone is one of the most important structures of the iberian variscides in its vicinity close to abrantes central portugal a localized heterogeneous strain pattern developed in a decimetric metamorphic siliceous multilayer this complex pattern was induced by the d2 dextral shearing of the early s0 s1 foliation in brittle ductile conditions giving rise to three main shear zone families one of these families with antithetic kinematics delimits blocks with rigid clockwise rotation surrounded by coeval cataclasites generating a local domino structure the proposed geometrical and kinematic analysis coupled with statistical studies highlights the relation between subsidiary shear zones and the main shear zone despite the heterogeneous strain pattern a quantitative approach of finite strain was applied based on the restoration of the initial fracture pattern this approach shows the importance of the cataclastic flow coupled with the translational displacement of the domino domain in solving space problems related to the rigid block rotation such processes are key in allowing the rigid block rotation inside shear zones whenever the simple shear component is a fundamental mechanism

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

effect of aseismic ridge subduction on slab geometry and overriding plate deformation insights from analogue modeling abstract we present analogue models simulating the subduction of a buoyant ridge oriented perpendicularly or obliquely with respect to the trench beneath an advancing overriding plate the convergence velocity is imposed by lateral boundary conditions in this experimental set we analyze the three dimensional geometry of the slab the deformation and topography of the overriding plate experiments suggest that ridge subduction diminishes the dip of the slab eventually leading to the appearance of a horizontal slab segment in case boundary conditions impose a rapid convergence this result contrasts with that obtained in free subduction experiments in which ridge subduction diminishes the convergence velocity which in turn increases the dip of the slab beneath the ridge the slab dip decrease is accompanied by the indentation of the overriding plate by the ridge resulting in arc curvature experiments suggest that indentation is larger for small convergence velocity and large slab dip ridge subduction also uplifts the overriding plate uplift first occurs close to the trench fore arc area and is accompanied by the flexural subsidence of the overriding plate behind the uplifted area back arc subsidence the uplifted area migrates within the overriding plate interiors following the appearance of a horizontal slab segment these results are compared with natural examples of ridge subduction in the circum pacific area they explain why ridge subduction may have contrasted effects on the overriding plate dynamics depending on the global conditions that constrain the converging system

the dilemma of asymmetric porphyroclast systems and sense of shear abstract asymmetric porphyroclast systems have long been used as independent shear sense indicators based on their clast tail asymmetry and or stair stepping tail geometry over the last two decades advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics using examples from naturally deformed rocks combined with results from recent advances our study indicates that 1 in general only porphyroclast systems composed of equant shaped clasts can be treated as independent shear sense indicators 2 special attention should be given to type porphyroclast systems composed of high angle intermediate aspect ratio clasts which may contain meaningful information on bulk flow kinematics 3 bookshelf structures cannot serve as independent shear sense indicators multi criteria and thorough three dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

petrographic and geochemical characterization of coal from santa susana basin portugal abstract the current and future importance of coal as a geological resource is related with the production of coke carbon materials carbon derivatives and other chemical products as well as a promising alternative source of critical trace elements this study aims the comprehensive characterization of coals from the santa susana basin ssb sw of portugal the ssb is a pennsylvanian continental basin located along the santa susana shear zone that separates two tectonostratigraphic zones of iberian massif samples of coal and coaly silt claystone from the main outcrops jongeis and vale de figueira were collected for this study the methodologies used for the petrographic and geochemical characterization included optical microscopy scanning electron microscopy with energy dispersive x ray spectrometry proximate and elemental analysis and inductively coupled plasma mass spectrometry icp ms the results demonstrate that the organic matter of coal from ssb is essentially composed of vitrinite and small amounts of inertinite the mineral matter includes detrital minerals mainly quartz and clay minerals iron oxides and oxidized and non oxidized framboidal pyrite epigenetic carbonates dolomite and ankerite were only observed in samples from jongeis outcrop vitrinite random reflectance of the samples ranges between 0 90 and 1 25 indicating a bituminous coal rank the slightly higher vitrinite reflectance and the occurrence of epigenetic carbonates in samples from jongeis can be related with differentiated burial history of the basin and post depositional processes including fluids circulation that may have promoted the enhancement of the thermal maturity when compared with the geochemical composition of worldwide hard coals samples from vale de figueira are significantly enriched in cs and in while samples from jongeis are significantly enriched in in and mn considering the elements mode of occurrence the majority of elements are preferentially associated with mineral matter the elements s cd mo pb and se have organic affinity the strong positive correlation between ca and mg mn and te indicates their association with carbonate phases which is in accordance with the carbonate composition

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

development of different types of pull apart microstructures in mylonites an experimental investigation abstract intragranular fracturing of feldspar porphyroclasts in granite mylonites of a ductile shear zone has led to development of diverse types of pull apart and bookshelf structures porphyroclasts with centrally located fractures show pull aparts with parallel walls type 1 whereas those with off centered fractures display pull aparts with both parallel type 1 and non parallel type 2 geometry analog model experiments performed by embedding segmented rigid elliptical objects within a viscous matrix indicate that the orientation and location of the fracture and the aspect ratio of the porphyroclast are the principal parameters in controlling the development of bookshelf structures and pull aparts in the case of centrally located fractures porphyroclasts of moderate aspect ratios develop either type 1 pull apart or bookshelf depending upon the fracture orientations whereas those of large aspect ratios 3 form only type 1 pull aparts irrespective of the fracture orientation off centered fracturing of porphyroclasts gives rise to fragments of unequal size which rotate independently at equal or contrasting velocities forming type 1 or type 2 v pull aparts respectively in the latter case depending upon the orientation of fracture the smaller fragment rotates faster or slower than the larger fragment showing relative tilt synthetic type 2a or antithetic type 2b to the bulk shear sense respectively type 2b geometry generally develops when the fracture angle with respect to the short axis of porphyroclasts is low antithetic to the shear direction and the long axis of the porphyroclast is at a high angle to the shear plane

boudinage in multilayered rocks under layer normal compression a theoretical analysis abstract this paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer normal compression based on the mode of fracturing of individual brittle layers boudinage is classified into three types tensile fracture boudinage type 1 shear fracture boudinage type 2a and extensional shear fracture boudinage type 2b the layer thickness ratio t r t b t d and the strength ratio f t 2 e between the brittle and the ductile units are the principal physical factors determining the type of boudinage type 1 boudinage develops rectangular boudins and occurs when t r is low f is high 0 8 in contrast type 2a boudinage takes place when t r is high 8 5 or f is low a r of all the types of boudins is inversely proportional to layer thickness ratio t r however type 1 and type 2 boudins have contrasting aspect ratios which are generally greater and less than 1 respectively

the toa baja drilling project puerto rico scientific drilling into a non volcanic island arc massif the toa baja drilling project was a broad interdisciplinary experiment to document the in situ geology and geophysics of a non volcanic island arc massif this overview provides a brief summary of oil exploration on puerto rico that lead up to the present investigation and summarizes some of the problems addressed by drilling

crustal suture preserved in the paleoproterozoic trans hudson orogen canada lithoprobe seismic reflection data acquired in 1994 within the trans hudson orogen include a 180 km profile that crosses the boundary between a collapsed marginal basin kisseynew domain and an older volcanic plutonic arc terrane lynn lake belt the seismic data image a prominent band of northeast dipping anastomosing reflections that extend to 10 s two way traveltime from the surface location of a geological boundary where interpreted oceanic crust is preserved and separate thick crust 16 s of the kisseynew domain from the abruptly thinner crust 12 s of the lynn lake belt highly reflective lower crust 10 16 s of the kisseynew domain dips gently northward and extends laterally for at least 40 km beneath the adjacent lynn lake belt in a manner reminiscent of the western alps complementary gravity modeling shows that the thickness of metasedimentary rocks within the kisseynew domain is 20 3 km the seismic image is interpreted as portraying a crustal scale collisional suture that formed as the end result of northward subduction of backarc basin crust ancestral kisseynew basin floor and subsequent contraction and thrusting of the remnant kisseynew basin fill under the margin of the lynn lake belt

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

use of boudinaged rigid objects as a strain gauge insights from analogue and numerical models abstract using analogue experiments we investigate how far the extensional strain estimated from an array of rigid boudins tracks the bulk extension of the host rock the quadratic elongations of boudinaged objects b were compared with the corresponding bulk quadratic elongations in physical models m both for pure and simple shear a noticeable difference occurs between the two which depends on the aspect ratios of initial object r o and boudins r b boudin types and the nature of bulk deformation for a given r o the difference is proportional to r b and tends to diminish with increasing r o when r b is constant the difference is normalized with the quadratic elongation measured from boudins b to define a departure factor m b b approaches a stable value with increasing b in pure shear the stable for torn symmetric boudins is larger 10 1 than that for asymmetric boudins among which the planar type shows the departure close to 0 boudins in pure shear show values smaller than that in simple shear in case of simple shear antithetic slip boudinage involves lower values compared to synthetic slip boudinage finite element models with elasto viscous rheology also corroborate the experimental findings

geologic and paleomagnetic evidence for a possible miocene collision in western panay central philippines the palawan metamorphic terrane represented by paleozoic basement on the northwestern tip of panay central philippines collided with the remainder of the island between late oligocene and middle miocene time subduction continued north and south of the collision zone along the manila and negros trenches respectively evidence for the collision is 1 the juxtaposition along a major fault of old melange terrane against highly metamorphosed paleozoic continental sediments similar to those exposed on palawan island itself and 2 miocene thrusting in the melange terrane oriented parallel to the suture paleomagnetic studies show that since the early miocene the island of panay has rotated 20 clockwise declination 20 9 inclination 22 1 for six sites whereas the northern philippines has rotated counterclockwise these contrasting rotations are consistent with a collision of the philippine arc with a northeastward converging palawan block

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

gravity and structure of the continental margins of southwestern mexico and northwestern guatemala geophysical measurements over the eastern end of the tehuantepec ridge and adjacent continental margins of southern mexico and northern guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces a positive free air gravity anomaly which extends northwestward along the outer continental shelf of guatemala curves abruptly landward in the gulf of tehuantepec the positive shelf anomaly is on trend with the positive anomaly of the nicoya peninsula costa rica and suggests that rocks genetically related to cretaceous rocks of the nicoya complex extend northwestward along the continental shelf to the gulf of tehuantepec a crustal and subcrustal cross section of the continental margin of guatemala constrained by gravity magnetic and seismic refraction data indicates that the rock strata causing the outer shelf gravity high dip landward consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin a model crustal cross section of the continental margin of southern mexico north of the tehuantepec ridge shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis

a review of the arcuate structures in the iberian variscides constraints and genetic models abstract in the iberian variscides several first order arcuate structures have been considered in spite of being highly studied their characterization formation mechanisms and even existence is still debatable the main ibero armorican arc iaa is essentially defined by a predominant nw se trend in the iberian branch and an e w trend in the brittany one however in northern spain it presents a 180 rotation sometimes known as the cantabrian arc ca the relation between both arcs is controversial being considered either as a single arc due to one tectonic event or as the result of a polyphasic process according to the last assumption there is a later arcuate structure ca overlapping a previous major one iaa whatever the models they must be able to explain the presence of a variscan sinistral transpression in iberia and a dextral one in armorica and a deformation spanning from the devonian to the upper carboniferous another arcuate structure in continuity with the ca the central iberian arc cia was recently proposed mainly based upon on magnetic anomalies geometry of major folds and ordovician paleocurrents the critical review of the structural stratigraphic and geophysical data supports both the iaa and the ca but as independent structures however the presence of a cia is highly questionable and could not be supported the complex strain pattern of the iaa and the ca could be explained by a devonian carboniferous polyphasic indentation of a gondwana promontory in this model the ca is essentially a thin skinned arc while the iaa has a more complex and longer evolution that has led to a thick skinned first order structure nevertheless both arcs are essentially the result of a lithospheric bending process during the iberian variscides

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

plate rupture by hydraulic fracture resulting in overthrusting lithospheric plates bending into subduction zones may become ruptured by hydraulic fracturing initiated by the injection of mobilized sediments thick poorly indurated water laden sediments of the proximal continental rise are the most obvious potential source for the large volume of material required for this process rupture of the descending plate may result in the subducted oceanic part of the lithosphere becoming detached and sinking independently while the more buoyant continental part rises by isostatic rebound we show here that applying this model to eastern indonesia provides an explanation for the banda arc geometry the apparent absence of a normal forearc and for the position of the thrust sheets relative to the benioff zone1 we also report that it can account for the short phase of deformation of the australian continental margin preceding its uplift of 3 km ref 1

subduction system and flat slab beneath the eastern cordillera of colombia seismicity at the northern terminus of the nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the bucaramanga nest we relocate about 5000 earthquakes recorded by the colombian national seismic network and produce the first 3 d velocity model of the area to define the geometry of the lithosphere subducting below the colombian andes we found lateral velocity heterogeneities and an abrupt offset of the wadati benioff zone at 5 n indicating that the nazca plate is segmented by an e w slab tear that separates a steeper nazca segment to the south from a flat subduction to the north the flat nazca slab extends eastward for about 400 km before dip increases to 50 beneath the eastern cordillera where it yields the bucaramanga nest we explain this puzzling locus of intermediate depth seismicity located beneath the eastern cordillera of colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust we relate the flat subducting geometry to the entrance at the trench at ca 10 ma of a thick buoyant oceanic crust likely a volcanic ridge producing a high coupling with the overriding plate sub horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the andes of south america at latitudes 5 n

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

relationship between polarity of extensional fault arrays and presence of detachments abstract extensional fault arrays are often dominated by a single fault vergence forming domino fault blocks the polarity of faults can be defined as synthetic or antithetic in relation to the overall shear sense across the fault system observations of the geometry of a number of field examples of extensional fault arrays vs the lithologies as a proxy for strength profile show that synthetic extensional arrays from the north sea and bristol channel basin detach on salt layers whereas examples of antithetic arrays from the north sea are found to pin out downwards and no basal detachment is present in the sacramento mountains core complex of the basin and range the major low angle detachment fault post dates antithetic shears that evolved at mid crustal level and are preserved in the granitic footwall these antithetic shears were cut by the main low angle detachment whose hanging wall disintegrated into an array of synthetic faults the kinematics of synthetic vs antithetic arrays dictate that faults within synthetic arrays must branch onto a basal detachment whereas faults within antithetic arrays may die out downwards therefore the results emphasise that fault polarity in domino arrays may be related to the boundary conditions of the fault blocks and therefore may be diagnostic of the strength profile of the faulted stratigraphy

geodynamic effects of subducted seamount at the manila trench insights from numerical modeling abstract we used numerical modeling to investigate the geodynamic effects of subducted seamounts at the manila trench a series of numerical modeling experiments were conducted with variable parameters including the activation volume vact and cohesion c which influence lithospheric rheology the plate convergence velocity and the age of subducting slab modeling results indicate that varying the vact and c within an appropriate range have limited effects on the geodynamic process of subduction a lower vact allows the slab to sink more easily and results in a steeper dip angle a slab break off is more likely to occur under subduction at depths of 100 300 km while the existence of a seamount further promotes the break off process the convergence rate is a key parameter affecting the break off timing and depth in contrast under subduction where subducted oceanic plate move faster upper plate the model results exhibit non break off steady subduction slab age is another factor controlling break off where break off time extends with slab age a subduction without seamount will cause a 2 myr delay in break off timing we suggest that the low velocity zone under the manila trench at 17o n is the result of a break off event due to subduction of the zhenbei huangyan seamount chain further to the north such as the location at 19o n the absence of seamount and an older oceanic crust would favor a delay in break off timing during subduction

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

a new model for the hercynian orogen of gondwanan france and iberia abstract the hercynian orogen of gondwanan france and iberia is a collage of disparate terranes separated by faults plate reconstructions indicate that the orogen developed during a massive dextral transpression as laurentia slid along the boundary of gondwana and as laurentia baltica and east avalonia rotated clockwise to form pangea it is proposed therefore that the collage is the result of this transpression and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the gondwanan shelf with its stable platform ordovician devonian sedimentary sequence and the neighbouring rheic ocean specifically it is proposed that 1 the slice of the gondwanan shelf that now forms north brittany was displaced dextrally from nw africa around iberia by more than 2000 km 2 the ophiolites oceanic material and volcanic arcs of south armorica and the massif central represent slices of the rheic ocean displaced dextrally along the gondwana margin and shuffled in amongst slices of gondwanan shelf 3 the convergent component of the transpression is represented by thrusts throughout the region the leon terrane of france and ophiolite bearing klippe in nw iberia the ibero armorican arc was formed by wrapping the mobile dextral shear belts about a rigid iberian basement block the dextral porto tomar shear zone represents the more important shearing in iberia and the iberian sinistral shears are interpreted as bookshelf type structures subsidiary to the dextral shearing

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

late variscan deformation in the iberian peninsula a late feature in the laurentia gondwana dextral collision the late variscan deformation event in iberia is characterized by an intraplate deformation regime induced by the dextral oblique collision between laurentia and gondwana this episode in iberia is characterized by nne ssw brittle to brittle ductile strike slip faults which are considered by the classic works as sinistral strike slip faults however the absence of mesozoic formations constraining the age of this sinistral kinematics led some authors to consider it as the result of alpine reworking structural studies in almograve and ponta ruiva sectors sw portugal show that nne ssw faults have a sinistral kinematics and are occasionally associated with e w dextral shears moreover this kinematics is related to the late deformation episodes of variscan orogeny in almograve sector the late variscan structures are characterized by nne ssw sinistral kink bands spatially associated with e w dextral faults these structures are contemporaneous and affect the previously deformed carboniferous units the ponta ruiva sector constrains the age of deformation because the e w dextral shears affect the late carboniferous late moscovian units but not the overlying triassic series the new data show that the nne ssw and the e w faults are dynamically associated and result from the late variscan deformation episode the nne ssw sinistral faults could be considered as second order domino structures related to first order e w dextral shears linked to laurentia gondwana collision during late carboniferous permian times

relative timing of transcurrent displacements in northern gondwana u pb laser ablation icp ms zircon and monazite geochronology of gneisses and sheared granites from the western iberian massif portugal abstract the variscan belt of western and central europe was formed by the oblique subduction of the rheic ocean and the collision of laurussia with gondwana during the late palaeozoic we present field relationships and new u pb la icp ms zircon and monazite ages for variscan gneisses and granites from a key section of the western iberian massif the martinchel section records the interplay of two kilometre scale variscan transcurrent shear zones active in the gondwana basement of pangaea the porto tomar fault zone ptfz and the coimbra cordoba shear zone ccsz different kinematic models have been invoked to explain the formation and evolution of these major variscan structures mainly based on assumptions made in the absence of reliable radiometric ages we show that 1 ductile deformation and metamorphism were active in the ccsz during the visean serpukhovian c 335 318 ma and created conditions for amphibolite facies metamorphism and coeval emplacement of granites and 2 later ductile brittle deformation related to dextral movements along the ptfz overprinted the earlier foliation and folds derived from the ccsz deformation and deformed the previously intruded granites u pb dating of zircon and monazites yield c 335 ma ages for the ductile deformation developed under amphibolite facies metamorphic conditions in the martinchel gneisses of the ccsz the gneisses were intruded by granites at c 335 318 ma and both were later deformed under ductile brittle conditions by dextral motion on the ptfz the geometry of the martinchel gneisses typical of the ccsz changed from one of thrusting to one of normal faulting by refolding of the early foliation stretching lineation and asymmetric structures related to the later ptfz dextral shear episode this pattern of interference is not fully considered in previous models and may lead to incorrect tectonic interpretations according to our data and recently published ages we suggest that the ptfz was active after the serpukhovian kasimovian since the c 318 308 ma granites are deformed by north south 170 dextral shear planes these data are critical to the interpretation of large scale carboniferous transcurrent displacements in northern gondwana iberian massif and bear upon global models of crustal deformation that emphasize the importance of long lived dextral movements during the collision between northern gondwana and laurussia following the closure of the rheic ocean

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

a double island arc between taiwan and luzon consequence of ridge subduction analysis of geomorphological geochronological geochemical and geophysical features in the segment of the taiwan luzon arc between taiwan and luzon the bashi segment allows the recognition of a double arc structure the two volcanic chains are separated by 50 km just north of luzon 18 n and converge near 20 n islets in the western chain are older and largely composed of volcanic rocks of miocene to pliocene age they all show low relief lateritic platforms and wave cut terraces and are covered by massive recrystallized limestone in contrast all active volcanoes in this segment of the taiwan luzon arc belong to the eastern chain where most islets are quaternary in age the volcanoes have well developed cone shapes and well preserved deposits of near vent facies magmas of the eastern chain have higher ksi la n la yb n and lowerend than their counterparts at the same latitude in the western chain therefore the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain moreover the available seismological data seem to suggest an abrupt increase of the dip angle from 30 at 18 n to 80 at 20 n thus the double arc structure is located in the region where the benioff zone suddenly changes in analogy with the lesser antilles arc we propose a geodynamic model in which the double arc in the bashi strait is the tectonic manifestation of the subduction of the aseismic scarborough seamount chain the extinct mid ocean ridge of the south china sea before that ridge reached the manila trench the western chain was the volcanic front when the ridge reached the subduction zone at 5 4 ma its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity furthermore this ridge arc collision was probably also responsible for regional uplift causing extensive sub aerial weathering and erosion as well as massive reef formation in the western chain when subduction started again the dip angle became shallower in response to the extra buoyancy of the downgoing ridge if the depth of magma generation remained constant the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain moreover we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest thus imprinting a geochemically more enriched signature on the magmas of the eastern chain

the geometry of planar domino style normal faults above a dipping basal detachment abstract geometrical analysis of planar domino style normal faults rooted into a dipping basal detachment fault allows derivation of equations which relate 1 horizontal extension within the upper plate 2 dip of the detachment 3 final fault dips 4 rotation that faults and beds undergo and 5 net slip on domino style faults past geometrical models have focused on extremely idealized and non unique geometries in which domino style faults are parallel and similar cut off points are all at the same elevation after faulting this corresponds to evenly spaced domino style faults above a horizontal detachment considering non parallel faults and dipping detachments introduces unique geometries which allows calculation for example of permissible depth ranges to the detachment horizontal extension varies significantly 1 with dip of the detachment and 2 for synthetic and antithetic cases in which the domino style faults dip in the same or opposite direction respectively as the detachment when other factors e g rotation and fault dips are held constant horizontal extension greatly increases for moderate detachment dips and moderately decreases for antithetic detachment dips as compared to the horizontal detachment case this is important because the synthetic case has been widely reported

rotation offset and separation of oblique fracture rhombic boudins theory and experiments under layer normal compression abstract boudinage structures are generally produced by extension fracturing of competent rocks normal to layering boudin like structures may also develop by shear fracturing of competent rocks oblique to layering in such structures boudins of rhomboid shape are rotated and offset with respect to each other it is usually considered that shear fracture boudins do not get separated until a critical rotation is attained when they can just touch each other however the experiments under layer normal compression and the theoretical analysis of the present study indicate that the layer segments produced by a set of parallel shear fractures may undergo rotation with separation or rotation with interfacial slip depending upon their geometry the thickness to length ratio g t of layer segments and the orientation of fractures are the parameters that could control the rate of rotation versus rate of displacement of layer segments

deformation within a strike slip fault network at westward ho devon u k domino vs conjugate faulting abstract a system of ne trending left lateral faults and nw trending right lateral faults at westward ho north devon u k cut steeply dipping 60 strata faults were accurately mapped in the field and from aerial photography and lateral separations of marker beds measured along the fault traces these data are used to examine the displacements within the network of interacting faults and to calculate variations in the density and relative proportions of the fault sets the displacements are also used in a tensor analysis of the strain and together with block rotations used to restore the deformation the results show a range of heterogeneity within the fault network both in terms of the fault patterns and strain some sub areas show a dominance of one fault set with regularly spaced larger displacements separating relatively weakly deformed blocks with smaller antithetic faults within these areas up to 20 rotation of the faults and bedding produces a domino style deformation that accommodates up to 15 extension the domino regions are separated by areas of conjugate faulting in which both sets of faults are equally developed and have similar displacement ranges conjugate areas have little or no rotation of the bedding and generally lower strains than domino regions

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

post subduction volcanism in the baja california peninsula mexico the effects of tectonic reconfiguration in volcanic systems abstract late cenozoic volcanism in the baja california peninsula records the effects of cessation of subduction at a previously convergent plate margin prior to 12 5 ma the predominant volcanic activity had a calc alkaline signature however after 12 5 ma during the period of tectonic transition from a convergent to a strike slip boundary the style and composition of the magmatic products changed dramatically it has been proposed that the origin of the post subduction anomalous slab melt related magmas i e adakites niobium enriched basalts magnesian andesites is linked to regional tectonic events such as the subduction of an active ridge the breakoff of the subducted slab or the tearing of the stalled slab here we compile already published data on this volcanism and critically examine the tectonic models proposed to explain its origin an alternative approach based on a physical model of volcanic systems suggests that post subduction volcanism is strongly influenced by more local aspects including the stress field and the tensile strength of the rock overlying the zones where partial melting occurred this interpretation allows us to reconcile the apparent contradictory observations of the temporal and spatial distributions of post subduction activity providing a more adequate explanation of many characteristics of the baja california post subduction volcanism

late cenozoic structural and tectonic development of the western margin of the central andean plateau in southwest peru 1 structural and thermochronologic studies of the western margin of the central andean plateau show changing styles of deformation through time that give insights into tectonic evolution in southwest peru uplift of the plateau proceeded in several distinct phases first nw striking ne dipping reverse faults accommodated uplift prior to 14 16 ma subsequent uplift of the plateau relative to the piedmont between the plateau and the pacific ocean occurred between 14 and 2 2 ma and was accommodated by nw striking sw dipping normal faults and subparallel monoclinal folds the youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block although the uplift magnitude associated with phase 1 is unknown phases 2 and 3 resulted in at least 2 4 3 0 km of uplift up to 1 km of this may have occurred during phase 3 geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

tectonic environments of south american porphyry copper magmatism through time revealed by spatiotemporal data mining porphyry ore deposits are known to be associated with arc magmatism on the overriding plate at subduction zones while general mechanisms for driving magmatism are well established specific subduction related parameters linking episodes of ore deposit formation to specific tectonic environments have only been qualitatively inferred and have not been formally tested we develop a four dimensional approach to reconstruct age dated ore deposits with the aim of isolating the tectono magmatic parameters leading to the formation of copper deposits during subduction we use a plate tectonic model with continuously closing plate boundaries combined with reconstructions of the spatio temporal distribution of the ocean floor including subducted portions of the nazca farallon plates the models compute convergence rates and directions as well as the age of the downgoing plate through time to identify and quantify tectonic parameters that are robust predictors of andean porphyry copper magmatism and ore deposit formation we test two alternative supervised machine learning methods the random forest rf ensemble and support vector machines svm we find that a combination of rapid convergence rates 100 km myr subduction obliquity of 15 a subducting plate age between 25 70 myr old and a location far from the subducting trench boundary 2000 km represent favorable conditions for porphyry magmatism and related ore deposits to occur these parameters are linked to the availability of oceanic sediments the changing small scale convection around the subduction zone and the availability of the partial melt in the mantle wedge when coupled these parameters could influence the genesis and exhumation of porphyry copper deposits

effect of ductile and frictional decollements on style of extension abstract scaled analogue models were used to study the effect of frictional and ductile detachments on thin skinned extension models consisted of two halves one half is the ductile and the other has a frictional detachment extension occurred above two different basal configurations a stretchable rubber sheet and a folded banded sheet intended to produce homogeneous and heterogeneous extension respectively model parameters varied systematically and included the brittle ductile thickness ratio rheologies and bulk strain structures in the two halves are compared in profiles and plan views a series of graben developed above both halves of models extended above a banded sheet although there were differences in style propagation rate and width of the deformation zone between the two halves different rates of propagation of structures in the two halves led to the formation of an accommodation or transfer zone parallel to the extension direction most relay ramps and inflection of normal faults in this zone indicate differential extension between the two halves in contrast in models extended above a stretchable rubber sheet extensional structures such as horst and graben developed only above the ductile detachment model results indicate that heterogeneous mechanical stratigraphy and displacement rate have no effect on extensional structure above a rubber sheet however above 20 bulk extension deformation becomes heterogeneous along multiple sets of conjugate faults oblique to the extension direction

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon

determining the neogene behavior of the nazca plate by geohistory analysis geohistory analysis was performed on neogene deposits south and north of la serena central chile to determine the behavior of the crust since the middle miocene the tectonic history of both study areas was found to be similar but the timing of uplift and downwarp events at the two localities differs by 1 3 and 0 8 m y respectively which we attribute to the approach and passing of the juan fernandez ridge from trigonometric relationships between the ridge trend the plate vector and the trend between the study areas we calculated a plate convergence rate of 6 2 cm yr between 11 8 and 10 5 ma and 10 1 cm yr between 7 7 and 6 9 ma rapid uplift commenced ca 2 3 ma with a delay of 0 3 m y between the two study areas trigonometric constraints suggest the presence of a hitherto unsuspected oceanic plateau trending 014 subducted below the present continental margin flat plate subduction may thus be better developed where it is assisted by the combined buoyancy of ridge clusters instead of single subducting plateaus

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes