deep structure of the central lesser antilles island arc relevance for the formation of continental crust oceanic island arcs are sites of high magma production and contribute to the formation of continental crust geophysical studies may provide information on the configuration and composition of island arc crust however to date only few seismic profiles exist across active island arcs limiting our knowledge on the deep structure and processes related to the production of arc crust we acquired active source wide angle seismic data crossing the central lesser antilles island arc north of dominica where the oceanic tiburon ridge subducts obliquely beneath the forearc a combined analysis of wide angle seismics and pre stack depth migrated reflection data images the complex structure of the backstop and its segmentation into two individual ridges suggesting an intricate relation between subducted basement relief and forearc deformation tomographic imaging reveals three distinct layers composing the island arc crust a three kilometer thick upper crust of volcanogenic sedimentary rocks and volcaniclastics is underlain by intermediate to felsic middle crust and plutonic lower crust the island arc crust may comprise inherited elements of oceanic plateau material contributing to the observed crustal thickness a high density ultramafic cumulates layer is not detected which is an important observation for models of continental crust formation the upper plate moho is found at a depth of 24 km below the sea floor upper mantle velocities are close to the global average our study provides important information on the composition of the island arc crust and its deep structure ranging from intermediate to felsic and mafic conditions in this study we model the deep structure of the lesser antilles island arc we use a hybrid analysis of refraction and reflection seismic data we image the complex structure of two ridges forming the backstop island arc crust composition ranges from intermediate to felsic to mafic conditions we discuss the formation of island arc and continental crust

segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

links between long term and short term rheology of the lithosphere insights from strike slip fault modelling the classical interseismic mechanical models of the lithosphere assume that the earth s lithosphere is a horizontally layered medium recent more sophisticated models include lateral variations of physical parameters and are shown to be as effective and sometimes more at reproducing the geodetic data during the seismic cycle than the stratified models these new models have in common to require a finite width low viscosity zone underneath the fault zone proposed explanations for the formation of these localised low viscosity zones include grain size reduction shear heating and localised presence of fluids all these mechanisms are valid in order to locally weaken an initially strong lower crust however they are not efficient to localise strain across strike slip faults in post orogenic settings when the lower crust is already weak a previous numerical modelling study showed that metamorphic core complexes are exhumed underneath transtensional step over in post orogenic setting here we designed a parametric study of the impact of the initial thickness of the weak crust and the width of the step over on the intensity of strain localisation we find that the smaller the width of the step over and the thicker the lower crustal low viscosity channel is the most intense is the localisation in a second round of experiments we do not impose the step over kinematic and find that transtensional bound ary conditions are sufficient to trigger long lasting weakening of the crust through the exhumation of lower crustal material using viscoelastic models intended to mimic the interseismic behaviour of a strike slip fault we find that the modified geometry of the brittle ductile transition is sufficient to explain the interseismic strain localisation across a strike slip fault zone we conclude that a long time after an earthquake has occurred the interseismic geodetic signal is more prone to reflect the rheology of the crust than postseismic relaxation

tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

an accreted continental terrane in northwestern peru abstract a paleomagnetic study of over 250 cores from 26 sites sampled in early to late cretaceous and paleogene volcanic plutonic and sedimentary formations of the lancones basin in the piura province of northern peru indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable south america a clockwise rotation ranging from 90 for the lowermost units to 35 for the uppermost ones has been documented although the lack of precise chronology has not allowed a detailed temporal description four sites from late carboniferous pennsylvanian formations in the amotape tahuin range also show a 110 clockwise rotation and yield evidence for a northward displacement when considered together with previous geological studies these data are consistent with the hypothesis of the accretion of an amotape tahuin continental terrane to the peruvian margin in neocomian times the accretion was followed by in situ rotation suggesting a dextral shear regime these results indicate that the geodynamical evolution of northern peru is more closely related to the processes observed in ecuador than to those classically assumed for the central andes of peru

significant effect of grain size distribution on compaction rates in granular aggregates we investigate the role of pressure solution in deformation of upper to mid crustal rocks using aggregates of halite as a room temperature analog for fluid assisted deformation processes in the earth s crust experiments evaluate the effects of initial grain size distribution on macroscopic pressure solution rate of the aggregate and compare the results to theoretical models for pressure solution we find that the grain size exponent deviates significantly from the theoretical value of 3 for diffusion controlled pressure solution models typically assume mono dispersed spherical particles in pseudo regular packing we infer that the discrepancy between experimentally determined grain size exponents and the theoretical values are a result of deviation of experimental and natural samples from regular packs of mono dispersed spherical particles moreover we find that compaction rates can vary by up to one order of magnitude as a function of the width of the grain size distribution for a given mean grain size wider size distributions allow for higher initial compaction rates increasing the macroscopic compaction rate with respect to more narrow grain size distributions grain sizes in rocks fault gouges and hydrocarbon reservoirs are typically log normal or power law distributed and therefore pressure solution rates may significantly exceed theoretical predictions spatiotemporal variations in pressure solution rates due to variations in grain size may cause the formation of low porosity zones which could potentially focus deformation in these zones and produce pockets of high pore pressures promoting nucleation of frictional instability and earthquake rupture

mesozoic oceanic terrane in the central andes of colombia a suspect terrane exists along the western flank of the central cordillera in the colombian andes it consists of sea floor of late jurassic or early cretaceous age that was accreted onto the continental edge at about 125 130 ma along the romeral fault zone during a pause between the jurassic and cretaceous cycles of plutonism

the north cycladic detachment system and associated mineralization mykonos greece insights on the evolution of the aegean domain 1 in the aegean back arc domain some 30 35 ma ago an increase of the rate of slab retreat led to the initiation of post orogenic extension largely accommodated by large scale structures such as the north cycladic detachment system ncds although this extension is still active nowadays an e w compressional regime developed in the late miocene with the propagation of the north anatolian fault on mykonos island cyclades the ne sw back arc extension is particularly well expressed with the livada and mykonos detachments that belong to the ncds and that are associated with nw se barite veins emplaced during the synkinematic cooling of the mykonos intrusion this study shows that the formation of the mineralization occurred when the pluton crossed the ductile to brittle transition during its exhumation below the ncds at 11 10 ma in addition the kinematics of mineralized structures evolved with time 1 most of the displacement was accommodated by the top to the ne livada and mykonos detachments accompanied by the formation of mineralized normal faults that were 2 reworked in a strike slip regime with an e w direction of shortening and a persistent ne sw stretching and 3 a late post mineralization e w compressional stage with a minor reworking of shallow dipping faults locally including the detachments themselves we interpret this increase of the e w shortening component recorded during the mineral deposition as a consequence of the initiation of the westward motion of anatolia from 10 ma thus 4 ma before the propagation of the north anatolian fault in the dardanelles strait and the localization of the strain on the aegean sea margins

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

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

from ductile to brittle evolution and localization of deformation below a crustal detachment tinos cyclades greece 1 the cycladic oligo miocene detachment of tinos island is an example of a flat lying extensional shear zone evolving into a low angle brittle detachment a clear continuum of extensional strain from ductile to brittle regime is observed in the footwall the main brittle structures marking extension are shallow and steeply dipping normal faults associated with subvertical extensional joints and veins the earliest brittle structures are low angle normal faults which commonly superimpose on and reactivate earlier precursory ductile shear bands but newly formed low angle normal faults could also be observed low angle normal faults are cut by late steeply dipping normal faults the inversion of fault slip data collected within and away from the main detachment zone shows that the direction of the minimum stress axis is strictly parallel to the ne sw stretching lineation and that the maximum principal stress axis remained subvertical during the whole brittle evolution in agreement with the subvertical attitude of veins throughout the island the high angle of 1 to the main detachment suggests that the detachment was weak this observation together with the presence of a thick layer of cataclasites below the main detachment and the kinematic continuum from ductile to brittle leads us to propose a kinematic model for the formation of the detachment boudinage at the crustal scale induces formation near the brittle ductile transition of ductile shear zones near the edges of boudins shear zones are progressively exhumed and replaced by shallow dipping cataclastic shear zones when they reached the brittle field most of the displacement is achieved through cataclastic flow in the upper crust and only the last increment of strain gives rise to the formation of brittle faults the formation of the low angle brittle detachment is thus prepared by the ductile shear zone and the cataclasites and favored by the circulation of surface derived fluids in the shear zone

using footwall structures to constrain the evolution of low angle normal faults controversy exists over the presence of normal faults that initiate and slip at low angles c 15 east on the island of elba italy indicates that the early stages of low angle faulting were synchronous with development of subsidiary high angle footwall faults these footwall structures controlled fault rock distribution and caused the development of flow folds similar to those produced in analogue studies of synchronous high and low angle faulting the current configuration of footwall structures indicates that the zuccale fault cannot have originally dipped greater than c 20 east and represents a primary low angle normal fault

the north cycladic detachment system low angle normal faults accommodate a large part of continental post orogenic extension besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow dipping shear zones at the brittle ductile transition the role of pre existing low angle structures such as large thrusts has been proposed by several authors we explore this question with the example of the north cycladic detachment system ncds that is composed of a series of distinct detachments cropping out on the islands of andros tinos and mykonos separating the cycladic blueschists in the footwall from the upper cycladic nappe in the hanging wall we show that these extensional structures are part of a single large scale structure more than 200 km along strike that reactivates the vardar suture zone it extends eastward on ikaria and westward offshore evia and thessalia where it probably connects to recent shallow dipping normal faults evidenced on published seismic reflection profiles the ncds started its activity in the oligocene concommitantly with the aegean extension and was still active in the late miocene it has exhumed a series of metamorphic domes from southern evia to mykonos below low angle detachment systems made of low angle normal faults and low angle ductile shear zones the ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation we identify three main detachments that are part of a continuum of extension on the ncds tinos detachment livada detachment and mykonos detachment a fourth detachment vari detachment is the reactivation of an eocene exhumation related structure deformation in the footwall is characterized by intense stretching and flattening using the spatial evolution observed along strike from andros to mykonos we construct a history of formation of the ncds starting with the reactivation of former thrusts leading to the exhumation of high temperature metamorphic domes the aegean example shows that reactivation of earlier shallow dipping discontinuities can play a fundamental role in continental post orogenic extension

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

aegean tectonics strain localisation slab tearing and trench retreat abstract we review the geodynamic evolution of the aegean anatolia region and discuss strain localisation there over geological times from late eocene to present crustal deformation in the aegean backarc has localised progressively during slab retreat extension started with the formation of the rhodope metamorphic core complex eocene and migrated to the cyclades and the northern menderes massif oligocene and miocene accommodated by crustal scale detachments and a first series of core complexes mccs extension then localised in western turkey the corinth rift and the external hellenic arc after messinian times while the north anatolian fault penetrated the aegean sea through time the direction and style of extension have not changed significantly except in terms of localisation the contributions of progressive slab retreat and tearing basal drag extrusion tectonics and tectonic inheritance are discussed and we favour a model 1 where slab retreat is the main driving engine 2 successive slab tearing episodes are the main causes of this stepwise strain localisation and 3 the inherited heterogeneity of the crust is a major factor for localising detachments the continental crust has an inherited strong heterogeneity and crustal scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation the dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate successive slab ruptures from the middle miocene to the late miocene have isolated a narrow strip of lithosphere still attached to the african lithosphere below crete the formation of the north anatolian fault is partly a consequence of this evolution the extrusion of anatolia and the aegean extension are partly driven from below asthenospheric flow and from above extrusion of a lid of rigid crust

andean forearc dynamics as recorded by detrital zircon from the eocene talara basin northwest peru abstract eocene strata of the talara forearc basin of northwest peru contain detrital zircon dz populations that when integrated with other provenance indicators record paleogeographic shifts in the andean forearc that can be linked to larger scale tectonic drivers such as subduction erosion plate convergence rate and regional accretion events because forearc basins along the western margin of south america are particularly difficult to preserve the new dz results provide rare insight into the trench arc dynamics associated with the andean orogenic cycle the talara basin tapped into both the andean volcanic arc and older basement and the dz populations subsequently reflect long lived plate interactions and progressive crustal growth along the northwestern edge of amazonia gondwana and south america dominant u pb zircon age populations include 36 110 ma andes convergence 215 285 ma western gondwana extension 460 625 ma rodinia extension and western gondwana convergence 950 1250 ma grenville orogeny and 1250 ma amazonia assembly the data corroborate previous geochronology on pre andean basement and support the existence of a famatinian arc 480 ma and carboniferous triassic magmatism along northwestern gondwana pre andean and amazonian dz likely was recycled from paleozoic metasedimentary rocks of the andean cordillera and amotape terrane although the data do not preclude delivery of amazonian detritus directly into the forearc via a large scale westward drainage the andean dz population constrains the timing of arc activity and quiescence with two peaks at 75 ma and 50 ma and a gap at 70 ma linked to accretion of the caribbean oceanic plateau against northwest south america arc volcanism was concurrent with eocene forearc deposition and syndepositional dz is used to revise maximum depositional age for the upper eocene helico member and verdun formation progressive subsidence and arc shut down in the late eocene coincide with arc collision to the north uplift of the amotape block subduction erosion and slab shallowing

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

first paleomagnetic results from neocene formations in evia skyros and the volos region and the deformation of central aegea paleomagnetic results from neogene volcanic and sedimentary formations in central aegea indicate that evia and skyros have undergone a 48 and 26 clockwise rotation respectively the volos region does not show any significant rotation in the last 3 ma the rotation of evia is about twice as large as the one measured from coeval formations in the external hellenic arc the results are interpreted in terms of a published model of distributed deformation by faulting which also provides an explanation for the significant differential rotation of evia and skyros

weakness and mechanical anisotropy of phyllosilicate rich cataclasites developed after mylonites of a low angle normal fault simplon line western alps abstract the simplon fault zone is a late collisional low angle normal fault lanf of the western alps the hanging wall shows evidence of brittle deformation only while the footwall is characterized by a c 1 km thick shear zone the simplon fault zone which continuously evolved during exhumation and cooling from amphibolite facies conditions to brittle cataclastic deformations due to progressive localization of the active section of the shear zone the thermal rheological evolution of the footwall resulted in a layered structure with higher temperature mylonites preserved at the periphery of the shear zone and cataclasites occurring at the core indicated as the simplon line in order to investigate the weakness of the simplon line we studied the evolution of brittle cataclastic fault rocks from nucleation to the most mature ones cataclasites are superposed on greenschist facies mylonites and their nucleation can be studied at the periphery of the brittle fault zone this is characterized by fractures micro faults and foliated ultracataclasite seams that develop along the mylonitic scc fabric exploiting the weak phases mainly represented by muscovite and chlorite approaching the fault core both the thickness and frequency of cataclasite horizons increase and as their thickness increases they become less and less foliated the fault core itself is represented by a thicker non foliated cataclasite horizon no andersonian faults or fractures can be found in the footwall damage zone and core zone whilst they are present in the hanging wall and in the footwall further from the fault applying a stress model based on slip tendency we have been able to calculate that the friction coefficient of the simplon line cataclasites was

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

reappraisal of paleomagnetic data from gargano south italy vandenberg j 1983 reappraisal of paleomagnetic data from gargano south italy in e mcclelland brown and j vandenberg editors palaeomagnetism of orogenic belts tectonophysics 98 29 41 the platform limestones of apulia italy outcropping in the gargano peninsula have been restudied paleomagnetic research has been carried out on upper cretaceous lower cretaceous and jurassic rocks despite the low intensities of the nrm 10 100 pa m all samples 268 could be cleaned by stepwise a f and or thermal demagnetization treatments nrm directions could be determined accurately and reproducibly for 85 of the samples using a sct cryogenic magnetometer and double precision measuring procedures nrm of the jurassic limestone is carried by secondary haematite and the results are therefore rejected from further consideration the upper and lower cretaceous limestones have an nrm carried by magnetite minor bedding tilt corrections improve the grouping of the site mean results the upper cretaceous scaglia limestone turonian senonian reveals a characteristic mean direction of decl 327 7 incl 38 2o a ss 4 3 21 sites while the lower cretaceous maiolica limestone neocomian aptian albian reveals a characteristic mean direction of decl 303 1 o incl 35 1 as s 8 7o 8 sites the cretaceous results show a post aptian albian counterclockwise rotation of about 25 which is expressed by the smeared distribution of the late cretaceous site mean results and a post senonian i e tertiary counterclockwise rotation of the same amount with respect to the pole these results are in excellent agreement with contemporaneous paleomagnetic results from other peri adriatic regions a tertiary counterclockwise rotation of all the stable adriatic block is strongly supported by the new results

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

a low angle normal fault in the umbria region central italy a mechanical model for the related microseismicity abstract in the northern apennines the upper crust is thinned by a set of east dipping low angle normal faults lanfs the easternmost and more recent of these lanfs is the altotiberina fault atf located in northern umbria the geometry of the atf has been reconstructed matching surface geology with seismic reflection profiles and borehole data the fault whose average dip is 20 borders the upper pliocene quaternary tiber basin and has a displacement of about 8 km the deeper portion of the atf is located below the axial zone of the northern apennines where the strongest instrumental and historical seismicity is recorded the microseismicity of the region m 1 to evaluate the boundary conditions for the brittle activity of the atf the fault can be reactivated for low values of differential stress 1 3 t 10 mpa and tensile fluid overpressure p f 3 e g v 0 93 in the peculiar situation of the northern apennines the deep emissions of large amounts of co 2 documented in the area can be entrapped in their ascent by structural seals e g atf favouring localised fluid overpressures the impossibility of sustaining p f 3 for wide fault portions counteracted by hydraulic fracturing increased permeability under low effective stress and load weakening behaviour for normal faulting would prevent the nucleation of moderate ruptures along the fault the short lived attainment of p f 3 along small fault portions can account for the microseismic activity located along the atf which occurs on rupture surfaces in the range of 10 1 10 3 km 2

post oligocene rotations in southern ecuador and northern peru and the formation of the huancabamba deflection in the andean cordillera abstract a paleomagnetic study of more than 410 samples from 41 sites from paleocene to early oligocene volcanic and intrusive formations in southern ecuador and northern peru reveals a pattern of rotations in opposite senses on both sides of the huancabamba deflection the amplitude of the rotations is 25 clockwise north of the deflection and 20 anticlockwise south of it north of the deflection the results do not allow us to ascertain whether the rotation arises from oroclinal bending or from block rotation in a distributed shear south of it the absence of geological evidence for widespread strike slip faulting suggests that the results reflect a rotation of the peruvian margin which could be related to the shortening documented in southern peru and to the uplift of the central andes

a continuum mechanics approach to quantify brittle strain on weak faults application to the extensional reactivation of shallow dipping discontinuities a number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field the existence of active low angle normal faults is much debated because 1 the classical theory of fault mechanics implies that faults are locked when the dip is less than 30 and 2 shallow dipping fault planes do not produce large earthquakes m 5 5 to reconcile observations and theory we propose a new model for fault reactivation by introducing an elasto plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties contrary to the classical model which implies that the dilation angle equals the friction angle our model accounts for and permits 0 in the fault gouge as deduced from laboratory and field observations whilst the predicted locking angles differ in most cases by less than 10 from the classical model a significant amount of plastic strain strain occurring in elasto plastic regime is predicted to occur on badly oriented faults prior to locking in a strain hardening regime we describe four modes of reactivation which include complete partial reactivation with without tensile failure in the surrounding medium this paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking we conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

allochthonous terranes in the andes of ecuador and northwestern peru in ecuador and northwestern peru the andes and adjacent country particularly on the pacific side are composed of at least five distinctive geologic terranes the terranes are distinguished from one another and from cratonic south america to the east by dissimilar basements cover rocks intrusive rocks and bouguer gravity anomaly fields the pinon terrane occupying most of coastal ecuador has a basaltic basement characterized by the largest known on land positive bouguer anomalies in the western hemisphere the tahuin terrane occupies most of northwestern peru and the southwestern corner of ecuador the terrane has an especially complex basement and is the site of generally positive bouguer anomalies the small biron terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent late cretaceous k ar mineral ages the wedge shaped chaucha terrane lies in part on the western andean slope between the oceanic pinon terrane on the north and the continental bir

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

structural evolution of andros cyclades greece a key to the behaviour of a flat detachment within an extending continental crust the continental crust extends in a brittle manner in its upper part and in more distributed ductile manner in its lower part during exhumation of hp metamorphic rocks brittle features superimpose on earlier ductile ones as a result of the progressive localization of deformation the islands of tinos and andros are part of the numerous metamorphic core complexes exhumed in the aegean domain they illustrate two steps of a gradient of finite extension along a transect between mt olympos and naxos this study confirms the main role of boudinage as an initial localizing factor at the brittle ductile transition and emphasizes the continuum of strain from ductile to brittle during exhumation early low angle semi brittle shear planes superimpose onto precursory ductile shear bands whereas steeply dipping late brittle planes develop by progressive steepening of structures or sliding across en echelon arrays of veins the comparison between tinos and andros allows us to propose a complete dynamic section of the aegean extending continental crust and emphasizes that the strain localization process depends on both its rheological stratification and its compositional heterogeneity

cenozoic stages of forearc evolution following the accretion of a sliver from the late cretaceous caribbean large igneous province sw ecuador nw peru

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

the seismic cycle at subduction thrusts 2 dynamic implications of geodynamic simulations validated with laboratory models 1 the physics governing the seismic cycle at seismically active subduction zones remains poorly understood due to restricted direct observations in time and space to investigate subduction zone dynamics and associated interplate seismicity we validate a continuum visco elasto plastic numerical model with a new laboratory approach paper 1 the analogous laboratory setup includes a visco elastic gelatin wedge underthrusted by a rigid plate with defined velocity weakening and strengthening regions our geodynamic simulation approach includes velocity weakening friction to spontaneously generate a series of fast frictional instabilities that correspond to analog earthquakes a match between numerical and laboratory source parameters is obtained when velocity strengthening is applied in the aseismic regions to stabilize the rupture spontaneous evolution of absolute stresses leads to nucleation by coalescence of neighboring patches mainly occurring at evolving asperities near the seismogenic zone limits consequently a crack or occasionally even pulse like rupture propagates toward the opposite side of the seismogenic zone by increasing stresses ahead of its rupture front until it arrests on a barrier the resulting surface displacements qualitatively agree with geodetic observations and show landward and from near the downdip limit upward interseismic motions these are rebound and reversed coseismically this slip increases adjacent stresses which are relaxed postseismically by afterslip and thereby produce persistent seaward motions the wide range of observed physical phenomena including back propagation and repeated slip and the agreement with laboratory results demonstrate that visco elasto plastic geodynamic models with rate dependent friction form a new tool that can greatly contribute to our understanding of the seismic cycle at subduction zones

paleomagnetic study of cretaceous rocks of peru south america evidence for rotation of the andes abstract paleomagnetic data for the cretaceous volcanic and sedimentary rocks in the andean region of peru are given reliable paleomagnetic field directions were obtained for three cretaceous albian to cenomanian formations from calcareous sediments in northern peru stable remanent magnetization directions were also derived from twelve cretaceous lava flows and dikes in coastal peru paleomagnetic data of the same age from the stable areas of south america such as brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole but peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts this suggests counterclockwise tectonic rotation of an extensive block which includes the whole of andean peru

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

volcano tectonic segmentation of the northern andes data from landsat images published geologic and geophysical reports and field observations are used to document eight major transverse boundaries that divide the northern andes into segments about 125 km long most of the segment boundaries have no apparent relation to structure on the nazca plate being subducted and may instead represent fundamental and long lasting structures of the continent subduction of the carnegie ridge results in increased uplift of the andes widespread and chemically diverse volcanism and major faulting and seismicity subduction of a highly fractured zone in southern ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style from andesitic stratovolcanoes to rhyolitic ash flow sheets recognition of the segmented structure of the northern andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to subduction

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

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

evolution of hydrothermal regime along a crustal shear zone tinos island greece 1 structural cathodoluminescence and sims 18o analyses of quartz calcite veins are combined to constrain the evolution in space and time of fluid infiltration in an exhuming detachment shear zone tinos island cyclades greece careful description of vein arrays shows that the plumbing system evolved into an interconnected network just beneath the ductile to brittle transition microtextures of quartz and calcite infilling veins display deformed relics and newly precipitated grains high resolution 18o mapping in relics yields a steady quartz calcite fractionation of 2 at 400 c whereas fractionation increases in neograins from 2 to 5 in 190 m toward the fault or a 150 c temperature fall cooling is associated with a shift of fluid stable isotope signature from 15 to 0 beneath the detachment results are interpreted as advective removal of heat by massive infiltration of surface derived fluids over depths of 10 15 km fluid penetration was promoted by the coalescence of late ductile early brittle veins as the exhuming footwall crossed the ductile brittle transition only small amounts of fluids penetrated the ductile crust beneath the transitional rheology

sacha oil field of ecuadorian oriente the sacha oil field in the ecuadorian oriente was discovered in early 1969 production began in july 1972 and at the end of 1980 had exceeded 190 million bbl drilling through 1980 had resulted in 91 oil wells and 2 dry holes estimated original primary recoverable reserves surpass 632 million bbl the field is on a very low relief anticline about 17 5 mi 28 km long and averaging 4 mi 6 5 km wide vertical closure amounts to 200 ft 60 m and there are 41 000 acres 16 600 ha of areal closure on top of the principal reservoir the cretaceous sandstones at drilled depths between 9 300 and 10 100 ft 2 835 and 3 080 m provide excellent reservoirs the hollin formation the basal cretaceous sandstone is the principal reservoir having produced 80 of the oil through 1980 and containing about 68 of the original reserves

rifting through a stack of inhomogeneous thrusts the dipping pie concept 1 orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages while the thermal state of the region is reequilibrating strong lateral variations of the depth to the brittle ductile transition develop as a consequence of these preexisting heterogeneities we use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension the competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments the results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation the presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge in this case the migration of the basin is slow continuous and limited by gravity driven processes that lead to the rise of hot weak material under the subsiding basin in case of a high competence contrast overall crustal strength is reduced by a dipping pie effect assuming overall high crustal strength the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults crustal scale planar faults rooting on the brittle ductile transition of the crust and thin skinned listric faults rooting on the nappe itself this bimodality results in a jump like migration of the basin downward along the dipping weak nappe applying this model to the case of the gulf of corinth greece allows us to explain in the case of assumed high competence contrast the observed microseismicity patterns the asymmetry of the gulf and the kinematics of fault migration within the basin

paleomagnetism of the puente piedra formation central peru abstract paleomagnetic samples were collected from 15 sites in the early cretaceous puente piedra formation near lima peru this formation consists of interbedded volcanic flows and marine sediments and represents the oldest known rocks of the andean coastal province in this region the puente piedra formation is interpreted as a submarine volcanic arc assemblage which along with an overlying sequence of early cretaceous clastic and carbonate rocks represents a terrane whose paleogeographic relationship with respect to the peruvian miogeocline in pre albian time is unknown moderate to high coercivities blocking temperatures below 320 c and diagnostic strong field thermomagnetic behavior indicate that pyrrhotite is the dominant magnetic phase in the puente piedra formation this pyrrhotite carries a stable crm acquired during an event of copper mineralization associated with the intrusion of the santa rosa super unit of the coastal batholith at about 90 5 m y b p the tectonically uncorrected formation mean direction of d 343 2 i 28 6 95 3 4 is statistically concordant in inclination but discordant in declination with respect to the expected direction calculated from the 90 m y reference pole for cratonic south america the observed declination indicates approximately 20 of counterclockwise rotation of the puente piedra rocks since about 90 m y this is consistent with other paleomagnetic data from a larger crustal block which may indicate modest counterclockwise rotation during the cenozoic associated with crustal shortening and thickening in the region of the peru chile deflection

lithospheric convective instability could induce creep along part of the san andreas fault along the western border of the sierra nevada microplate the san andreas fault california united states is comprised of three segments two north and south segments are locked and support large earthquakes e g the m 7 7 1906 san francisco and the m 7 8 1857 fort tejon earthquakes while the central segment from parkfield to san juan bautista is creeping based on mechanical models we show that the late pliocene quaternary convective removal delamination of the southern sierra nevada mantle lithosphere and associated uplift of the sierra nevada mountains causes the great valley upper crust to deform by flexure and buckling additional three dimensional flexural models imply that the local flexural bulge overlaps with the creeping segment of the fault system while geological observations indicate that the local weakening of the san andreas fault started at the same time that the sierra nevada started its recent phase of uplift we argue that bending stresses promote lithostatic pore pressure to occur in the depth range of 7 15 km causing the effective strength of the fault to vanish and locally favoring creep our results suggest for the first time that earthquake cycles along a major plate boundary may be influenced by convective instabilities in the adjacent upper mantle

synextensional granitoids and detachment systems within cycladic metamorphic core complexes aegean sea greece toward a regional tectonomagmatic model within deforming continental regions where metamorphic core complexes mccs and synextensional granitoids are closely associated deciphering the link between detachment faulting and magmatism often remains complex as 1 the rheological weakness of magma may stimulate mechanisms of strain localization and conversely 2 tectonic processes may open close drains where magmas can intrude here we tackle this issue by focusing on the cyclades with the comparison of five granitoid cored mccs tinos mykonos ikaria naxos and serifos and their flanking detachment systems in this region granitoids were emplaced into the middle upper crust over a relatively short time period 15 9 ma while metamorphic domes were largely exhumed after more than 10 myr of extension none of those intrusions thereby proves to be a real candidate for the genesis of mccs but would rather be a consequence of a warmer regime during lithospheric thinning however all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated indeed late evolution stages of mccs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred interfering with the sequential development of detachments during their tectonically controlled emplacement magmatic products intruded already formed detachments at depth locally inhibiting their activity associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities the newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions followed by cataclasis along detachments

paleomagnetic study of an arcuate fold belt developed on a marginal orogen the cajamarca deflection northern peru abstract a paleomagnetic study of over 500 cores from 48 sites sampled in albian to turonian sedimentary formations along the cajamarca deflection in northern peru indicates that these lithologies carry a stable remanent magnetization in the two major segments of the deflection positive fold tests establish that the magnetization predates the folding except for five sites in the western part which appear to be remagnetized by the intrusion of paleocene granites the eastern part of the deflection does not appear to have undergone any significant rotation whilst a post late eocene counterclockwise rotation of about 25 is recorded in the central and western parts this angular difference only accounts for about half of the difference in the strikes of structures in the two sections of the deflection when considered with the available tectonic sedimentological and geodynamic data these results indicate that the arcuate shape of the cajamarca fold belt results from complex processes early multi phased compressional events of paleocene eocene age created the folds and built an arcuate structure with non rotational mechanisms then in post late eocene times the structure reached its present form after rotation of its central and western parts the bearing of these results on the geodynamic evolution of the andean cordillera is discussed

deciphering the late cretaceous cenozoic structural evolution of the north peruvian forearc system the link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm this paper aims at deciphering the structural architecture and uplift history of the north peruvian forearc system to better understand the evolution and the mechanics that govern the late cretaceous cenozoic building of this active margin in this study we report surface structural geology data interpretation of seismic reflection profiles apatite fission track data and the construction of two offshore onshore crustal scale balanced cross sections the structure of the north peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust related structural highs involving continental oceanic basement rocks and off scrapped sediments the thrust systems bound thick thrust top forearc depocenters mainly deformed by crustal normal to strike slip faults and thin skinned gravitational instabilities the sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift shortening and plate convergence velocity during late cretaceous and miocene pliocene quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease the development of crustal normal to strike slip faulting and subsidence zones might be the consequence of slab flexure local basal erosion along subduction fault and or oblique subduction associated with sediment loading control we conclude that the evolution of the north peruvian forearc system was controlled by subduction dynamics strong sediment accumulation and recent ridge subduction and it recorded the orogenic loading evolution of the andes over the cenozoic

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

mechanical basis for slip along low angle normal faults 1 the existence of active low angle normal faults is much debated because 1 the classical theory of fault mechanics implies that normal faults are locked when the dip is less than 30 and 2 shallow dipping extensional fault planes do not produce large earthquakes m 5 5 however a number of field observations suggest that brittle deformation occurs on low angle normal faults at very shallow dip to reconcile observations and theory we use an alternative model of fault reactivation including a thick elasto plastic frictional fault gouge and test it at large strain by the mean of 2d mechanical modeling we show that plastic compaction allows reducing the effective friction of faults sufficiently for low angle normal faults to be active at dip of 20 as the model predicts that these faults must be active in a slip hardening regime it prevents the occurrence of large earthquakes however we also evidence the neoformation of riedel type shear bands within thick fault zone which we believe may be responsible for repeated small earthquakes and we apply the model to the gulf of corinth greece

geochemical window into subduction and accretion processes raspas metamorphic complex ecuador the high pressure low temperature p 1 3 2 gpa t is 600 c raspas metamorphic complex is an exhumed fragment of the partially accreted partially subducted amotape chaucha terrane in southwest ecuador comparative analysis of major and trace elements plus sr nd and pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments subduction and exhumation of oceanic lithosphere resulted in selective bulk trace element geochemical changes rb ba and sr have been lost in amounts from approximately 85 50 from the high p low t metamorphosed pelites and basalts whereas pb is enriched in mafic rocks during formation of the eclogite u pb and rare earth elements rees were immobile high p low t metamorphosed terranes form the basement of active ecuadorian arc volcanoes partial melting of this basement by mantle wedge derived basalt is a likely source of adakitic components

late cretaceous early tertiary paleomagnetism of aruba and bonaire netherlands leeward antilles for a paleomagnetic study of the upper cretaceous of the netherlands leeward antilles we have analyzed 187 oriented samples from the complex norite tonalite batholith and the diabase schist tuff formation on aruba and 44 oriented samples from various igneous units of the washikemba formation on bonaire both alternating field and thermal demagnetization procedures were used to demagnetize the samples most of the cores yielded univectorial zijderveld diagrams above 10 20 mt or 400 c virtual geomagnetic poles calculated from site means were then compared with equivalently aged poles for cratonic south america and other caribbean sites the washikemba formation of bonaire and the diabase schist tuff formation of aruba yield poles that are similar to those of guajira colombia and the caribbean mountains of northern venezuela whereas they are rotated approximately 90 with respect to the pole for stable south america the batholith on aruba yields a pole which is rotated clockwise over approximately 20 with respect to the south american pole thus our data strongly support the hypothesis that aruba and bonaire have been rotated clockwise by as much as 90 since the late cretaceous in addition paleolatitudes calculated from the inclinations of the paleomagnetic vectors suggest that there was a northward drift with respect to south america of perhaps as much as 10 of the islands during the rotation although this can hardly be called a statistically significant amount

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

chapter 22 paleomagnetism of continental north america implications for displacement of crustal blocks within the western cordillera baja california to british columbia

rotated oceanic blocks in western ecuador a paleomagnetic study of over 250 cores from 21 sites sampled over a distance exceeding 250 km in the early cretaceous pinon formation in the costa of western ecuador indicates that the sampled lithologies essentially morb type basalts and dolerites carry a stable remanent magnetization whose direction significantly deviates from that of coeval formations of stable south america although both normal and reverse directions have been recorded the two polarities cannot easily be distinguished because of the low latitude of the ocean floor at the time of formation the most likely interpretation is that the entire area has undergone 70 clockwise rotation since its genesis these results thus yield new evidence for an allochtonous origin of this suspect terrane

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

active aseismic creep on the alto tiberina low angle normal fault italy the existence of active low angle normal faults has been questioned because the standard theory of fault mechanics precludes normal faults from slipping at low

geochemistry and sr nd isotopic composition of eocene lamphrophyre dykes southeastern british columbia mineral compositions geochemical analyses and sr nd isotopic compositions are reported for alkaline and calc alkaline lamprophyres collected along the southern margin of the valhalla complex southeastern british columbia the lamprophyres were emplaced during eocene extension and lithospheric thinning associated with tectonic denudation of the valhalla complex sio2 contents range from 44 4 51 6 wt k2o from 1 3 3 7 wt and volatile contents h2o co2 so3 from 0 8 4 6 wt mgo and cr contents are 9 5 7 6 wt and 540 130 ppm respectively for samples with mg s between 0 69 and 0 65 chrondrite normalized rare earth element patterns are strongly fractionated with cen 120 375 and ybn 8 4 12 7 alkaline lamprophyres contain biotite kaersutite calcic plagioclase and exhibit a limited range in initial 87sr 86sr 0 7051 0 7057 initial end 3 7 to 4 3 and tdm 766 796 ma calc alkaline lamprophyres contain f rich phlogopite and sodic plagioclase and exhibit a wider range in initial 8