slab detachment during continental collision influence of crustal rheology and interaction with lithospheric delamination abstract collision between continents can lead to the subduction of continental material if the crust remains coupled to the downgoing slab a large buoyancy force is generated this force slows down convergence and promotes slab detachment if the crust resists to subduction it may decouple from the downgoing slab and be subjected to buoyant extrusion we employ two dimensional thermo mechanical modelling to study the importance of crustal rheology on the evolution of subduction collision systems we propose simple quantifications of the mechanical decoupling between lithospheric levels and the potential for buoyant extrusion of the crust the modelling results indicate that a variable crustal rheological structure results in slab detachment delamination or the combination of both mechanisms a strong crust provides coupling at the moho low and remains coherent during subduction low it promotes deep subduction of the crust 180 km and slab detachment exhumation occurs in coherent manners via eduction and thrusting slab detachment triggers the development of topography 4 5 km close to the suture a contrasting style of collision occurs using a weak crustal rheology mechanical decoupling at the moho high promotes the extrusion of the crust high disabling slab detachment ongoing shortening leads to buckling of the crust and development of topography on the lower plate collisions involving rheologically layered crust allow decoupling at mid crustal depths this structure favours both the extrusion of upper crust and the subduction of the lower crust such collisions are successively affected by delamination and slab detachment topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment our results suggest that the occurrence of both delamination apennines and slab detachment himalayas in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions

crustal structure at colliding plates boundary from receiver functions analysis a close look beneath the northern apennines italy 1 teleseismic receiver functions from a seismic experiment in a small area of the northern apennines italy reveal strong crustal structure variations across the mountain chain receiver functions imaging and full waveform inversion technique are used to constrain the s wave velocity profile in the crust and to reconstruct the geometry of the main seismic discontinuities at depth we highlight the presence of the main mode converting discontinuities in the study area most importantly we identify the crust mantle transition which is represented almost everywhere in the study area by a sharp s wave velocity increase over 4 km s at depth between 35 and 40 km however farther west the s wave velocity reaches values typical for the sub crustal mantle at about 54 km depth possibly marking the locus where the subducting adriatic plate starts to dip into the mantle here the presence of a shallower discontinuity at about 36 km depth with s velocity values around 4 km s can be interpreted as the moho signature of the overriding tyrrhenian plate

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

lateral variations of thermo mechanical properties in the tyrrhenian northern apennine region abstract we present lithospheric strength profiles calculated for a transect running from the island of corsica in the west to the apennine foredeep in the east strength profiles are constructed at locations selected on the basis of the availability of seismic heat flow gravity data and earthquake foci distribution rheological modelling points to the presence of strong lateral variations in the thermo mechanical state of the lithosphere the tyrrhenian tuscany area has a low rigidity in relation with the intense stretching active from the early miocene and with very high heat flow associated with asthenospheric rise on the contrary hinterland corsica and apenninic foreland areas adriatic area show a higher level of lithospheric strength effective elastic thickness estimates confirm this scenario exhibiting a strong reduction from the northern tyrrhenian to the inner apennines and then an abrupt increase in correspondence of the adriatic plate the distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the tuscany area whereas close to the plate boundary much lower thermal gradients partly induced by the westward plunging adriatic plate enable seismic activity to occur at larger depths

deep structure and tectonics of the northern central apennines as seen by regional scale tomography and 3 d located earthquakes 1 in this study we present new high resolution regional scale vp and vp vs models of the northern central apennines along with accurate 3 d locations of a large set of local earthquakes the main velocity anomalies are consistent with the surface geology in the shallow layers and present evidence for fluids stored within the basement at greater depths beneath the extensional belt the adria and tyrrhenian mantle are defined by positive velocity anomalies below 30 km depth while a low vp high vp vs region in between indicates the existence of a hydrated wedge the results yield new constraints on active processes in the apennines and more generally envisage the evolution of a postcollisional belt velocity anomalies and earthquakes are consistent with a complex system of delamination and sinking of the adria continental lithosphere with the peeling of the crust identified by intermediate depth seismicity change of seismicity and structural patterns along the belt indicates that this tectonic process is diachronous and that fluids released by sunken lithosphere are stored within the crust conditioning the occurrence of seismicity and the onset of extension

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

the development and behaviour of low angle normal faults during cenozoic asymmetric extension in the northern apennines italy abstract movements on low angle normal faults lanf are not predicted by traditional anderson byerlee frictional fault mechanics our investigations centre on three normal fault systems active at distinct times during the regional extension of the northern apennines with each showing different degrees of crustal exhumation these are from e to w the altotiberina fault system umbria the radicofani fault system tuscan mainland and the zuccale fault system isle of elba regional extension has been a continuous process since middle miocene migrating progressively from west to east with deformation accommodated by a set of e to ne dipping lanf and more steeply sw dipping antithetic structures the lanf acted as regional detachments accommodating a majority of the extension in the northern apennines with individual faults exhibiting several kilometres of displacement regionally the stress field has been characterised by a vertical 1 and a ne sw tuscan mainland and umbria to e w elba trending 3 where exposed at the surface the main lanf detachments possess a well developed fault core of foliated fault rocks in which fluids have played a key role in weakening due to reaction softening and the onset of stress induced solution precipitation mechanisms we speculate that the e dipping normal faults initiated with relatively low angles of dip due to differential drag generated by mantle flow following slab retreat and roll back beneath the apennine chain

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

coupled and decoupled regimes of continental collision numerical modeling useful geodynamic distinction of continental collision zones can be based on the degree of rheological coupling of colliding plates coupled active collision zones which can be either retreating or advancing are characterized by a thick crustal wedge and compressive stresses i e himalaya and western alps while decoupled end members which are always retreating are defined by a thin crustal wedge and bi modal distribution of stresses i e compressional in the foreland and extensional in the inner part of the orogen northern apennines in order to understand physical controls defining these different geodynamic regimes we conducted a 2d numerical study based on finite differences and marker in cell techniques in our experiments we systematically varied several major parameters responsible for the degree of rheological coupling between plates during collision such as convergence rate crustal rheology and effective velocity of upward propagation of aqueous fluids and melts in the mantle wedge low convergence rates and fluids melts propagation velocities favor continuous coupling and convergence between the plates coupled collision zones are characterized by continuous accretion of the weak upper continental crust resulting in the development of a thick and broad crustal wedge by hot temperature in the inner parts of the orogen due to radiogenic heating of the thickened crust by compressive orogenic stresses and appearance of a double seismogenic brittle layer involving upper crust and sub moho mantle in contrast high convergence rates and fluid melt percolation velocities produce efficient weakening of the mantle wedge and of the subduction channel triggering complete decoupling of two plates mantle wedging into the crustal wedge and retreating style of collision the evolution of fully decoupled collision zones are characterized by the disruption of the accretionary wedge formation of an extensional basin in the inner part of the orogen and delamination of the weak portion of the continental crust that is first thrusted toward the foreland and subsequently dissected by extensional tectonics transition from coupled to decoupled regime occurs always at the early stages of continental collision indicating that insertion of rheologically weak crustal material in the subduction channel is critical for the subsequent evolution of the collision zone we found good correlations of our numerical results with some of the major collisional orogens in particular the decoupled retreating collision regime reproduces what is observed in the northern apennines

seismic attenuation and mantle wedge temperature in the northern apennines subduction zone italy from teleseismic body wave spectra 1 we analyze p and s wave spectra from moderate to deep focus teleseisms recorded at the retreating trench extension and accretion tectonics retreat temporary broadband seismic network to assess the variations of the earth mantle attenuation in the northern apennines region italy for each earthquake we compute the ratio between the spectrum at each station and the average spectrum in order to estimate t residuals t from the spectral ratio decay the number and distribution of the teleseisms useable for the p wave t calculation allow for a gross azimuthal analysis although the t values at single station display in most cases azimuthal dependent fluctuations their overall distribution shows a partition of the study region into two main areas whose gross features remain almost unchanged over the whole azimuthal range this partition is confirmed by the s wave t mean values computed for each station over the set of useable events we distinguish a relatively high attenuation area on the western tyrrhenian side and a relatively low attenuation area on the eastern adriatic side by correlating our t estimates and the velocity structure derived from the existing tomographic models we compute the ranges of possible p and s wave q values in the mantle wedge above the apennines slab on the tyrrhenian side and in the asthenosphere below the adriatic region furthermore the determined attenuation properties are used to draw some inferences on the thermal state of the uppermost mantle and on the physical properties of the tectonic elements which constitute the subduction system in the region

deep crustal roots beneath the northern apennines inferred from teleseismic receiver functions abstract teleseismic waveforms recorded by a regional array crossing the northern italian peninsula and northern corsica are analyzed using the receiver function technique to determine the first order crustal structure the receiver function approach is used to isolate receiver side ps conversions generated at the crust mantle boundary and any major velocity discontinuity beneath the stations we used the time delay between the direct p wave and the ps wave converted at the moho discontinuity to infer crustal thickness beneath the stations the crust mantle boundary is estimated at 25 1 km of depth in northern corsica 20 2 km beneath the elba island and 20 to 24 2 km beneath tuscany in the eastern portion of the array moho depth increases from 28 2 km beneath the adriatic coast to 49 3 km beneath the apennine chain in a distance of about 100 km a double ps conversion produced beneath the val tiberina graben to the west of the apennines corresponds to interfaces as deep as 20 2 and 52 2 km this observation supports the hypothesis of partial overlapping between the shallow tuscan moho and the deeper adriatic moho the westward deepening of the adriatic moho beneath the northern apennines can be explained by lithosphere delamination that has dragged downward the adriatic lower crust the deep crustal root estimated beneath the northern apennines indicates that this portion of the chain is isostatically overcompensated

carbon dioxide earth degassing and seismogenesis in central and southern italy 1 we present the first regional map of co2 earth degassing from a large area most of central and south italy derived from the carbon of deep provenance dissolved in the main springs of the region the investigation shows that a globally significant amount of deeply derived co2 10 of the estimated global co2 emitted from subaerial volcanoes is released by two large areas located in western italy the anomalous flux of co2 suddenly disappears in the apennine in correspondence to a narrow band where most of seismicity concentrates here at depth the gas accumulates in crustal traps generating co2 overpressurized reservoirs which induce seismicity

rheology of the northern apennines lateral variations of lithospheric strength abstract a thermorheological wsw ene profile along the northern apennines is presented the variations in rheology are estimated using two different models the conventional model in which only frictional sliding and power law creep are taken into account and a new model in which in addition to the previous two mechanisms a high pressure brittle fracture mechanism is included upper and lower bounds for the rheology are considered a hard case where the lower crust is mafic granulite and the lithospheric mantle is dry peridotite and a soft case where the lower crust is felsic granulite and the lithospheric mantle is wet peridotite important differences in lithospheric rheological structure are inferred between the western and the eastern sectors of the chain in the former a creme brulee structure generally applies in the latter a jelly sandwich structure applies these rheological models therefore are not alternative but end members of a continuous spectrum of rheological behaviours

dependence of active normal fault dips on lower crustal flow regimes two key questions are raised by the presence in the basin and range province of subhorizontal normal faults which were active in and around the miocene first how did they develop when their existence seems to be precluded by elementary rock mechanics arguments second if they did slip in the recent geological past why do normal faults now active in the brittle upper crust have steeper dips both in the western usa and elsewhere one may now answer both questions by considering the stress field in regions where the isostatic response during extension involves horizontal flow in the plastic lower crust this stress tensor will have inclined principal axes making it possible for normal faults to form with either very steep or subhorizontal dips depending on the sense of flow this can be controlled by environmental conditions which influence rates of hanging wall sedimentation and footwall erosion and thus determine lateral variations in lithostatic pressure in the lower crust regional scale lateral variations in the geothermal gradient caused by subduction can also influence lower crustal flow regimes in adjacent zones of continental extension enabling extension to occur on low angle normal faults dipping away from the active continental margin

thermal regime and post orogenic extension in collision belts in order to assess the conditions leading to post orogenic extension the temperature and strength of the lithosphere during continental collision are calculated the calculations assume that heat is transported by conduction and either that the lithosphere is homogeneously thickened or that only the crust is thickened during the collision despite increased crustal thickness and heat production the temperature does not increase if the rate of shortening is moderate 5 10 16s 1 leading to 100 increase in crustal thickness in 40 myears the temperature distribution is used to determine a rheological profile that is compared with the stress induced by compensated topography the calculations show that for homogeneous lithospheric thickening the strength of the lithosphere increases and is higher than the tensile stress except in the shallow crust the total strength of stable continental lithosphere at the end of shortening is on the order of 1013 n m 1 it is larger by one order than the force induced by compensated topography 1012 n m 1 it appears that homogeneous lithospheric thickening would not lead to post orogenic extension unless the initial conditions are very special with the lithosphere hotter than normal for crustal thickening only the strength of the lithosphere decreases slightly and is on the same order as the tensile stress extension does not necessarily follow from thickening of the crust only but it could take place for a relatively wide range of initial conditions with initial surface heat flow 60 mw m 2 or higher alternatively an event such as rapid removal of the mantle lithosphere by delamination or small scale convection would increase the tensile stress heat rapidly the lithosphere reduce its strength and always trigger extension

architecture and seismotectonics of a regional low angle normal fault zone in central italy information from surface geology subsurface geology boreholes seismic reflection and refraction profiles and seismicity are used to depict the geometry and the possible seismogenic role of the altotiberina fault af a low angle normal fault in central italy the af extends along the inner umbria region for a length of 70 km with an average dip of 30 and an horizontal displacement up to 5 km it emerges west of the inner border of the tiber basin and deepens beneath the umbria marche carbonate fold and thrust belt to a depth of 12 14 km close to the af surface trace low angle synthetic east dipping normal faults extensively outcrop whereas high angle antithetic west dipping normal faults prevail farther east integrating geological and seismologic information it can be stated that the af behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the umbria marche region the af belongs to a regional ne dipping low angle normal fault system etrurian fault system efs which extends for 350 km from northwestern tuscany to southern umbria early preliminary considerations suggest that the efs may play an important role in controlling active extension and related seismicity in northern central italy

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

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

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

effects of lateral variations of crustal rheology on the occurrence of post orogenic normal faults the alto tiberina fault northern apennines central italy abstract the northern apennines na are characterized by formerly compressive structures partly overprinted by subsequent extensional structures the area of extensional tectonics migrated eastward since the miocene the youngest and easternmost major expression of extension is the alto tiberina fault atf we estimate 2d rheological profiles across the na and conclude that lateral rheological crustal variations have played an important role in the formation of the atf and similar previously active faults to the west lithospheric delamination and mantle degassing resulted in an easterly migrating extension compression boundary coinciding at present with the atf where i the thickness of the upper crust brittle layer reaches a maximum ii the critical stress difference required to initiate faulting at the base of the brittle layer is at a minimum and iii the total strengths of both the brittle layer and the whole lithosphere are at a minimum although the location of the fault is correlated with lithospheric rheological properties the rheology by itself does not account for the low dip 20 of the atf two hypotheses are considered a the low dip of the atf is related to a rotation of the stress tensor at the time of initiation of the fault caused by a basal shear stress 100 mpa possibly related to corner flow associated with delamination or b the low dip is associated to low values of the friction coefficient 0 5 coupled with high pore pressures related to mantle degassing our results establishing the correlation between crustal rheology and the location of the atf are relatively robust as we have examined various possible compositions and rheological parameters they also provide possible general indications on the mechanisms of localized extension in post orogenic extensional setting the hypotheses to account for the low dip of the atf on the other hand are intended simply to suggest possible solutions worthy of further study

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

a geodynamic model for extension in the shuswap core complex southeastern canadian cordillera the shuswap core complex records mesozoic and paleocene crustal shortening with superimposed eocene extension during the shortening phase the crust was thickened and slowly uplifted within at most 20 ma of the climax of compression the crustal welt was rapidly uplifted and tectonically denuded shortening of the continental crust may be balanced in the underlying lithosphere by thickening or underplating forming in either case a lithospheric root beneath the crustal welt we consider a model whereby tertiary crustal extension in the shuswap core complex and possibly also in other segments of the north american cordillera is accounted for by the detachment of this root both the time frame and the stress field resulting from the detachment are compatible with geological and geophysical observations far field stresses played an accommodating role but the potential energy required for extension of the shuswap complex was induced by lithospheric thickening

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

evaluation of stresses in two geodynamically different areas stable foreland and extensional backarc areas which are geodynamically different have different behaviors both in their thermal regime and seismic activity a stable area has a geotherm which can be considered as standard extensional and compressional areas have respectively high and low temperature gradients the italian region includes different geodynamical areas and all such situations are present we consider the apulian platform as an example of a stable area and the tuscany latium as an example of an extensional area for both of them the present geotherms are calculated taking into account for the tuscany latium its thermal history assuming that each region is subject to a constant strain rate the stresses are calculated as functions of depth and time the rheological behavior is assumed to be linear viscoelastic with viscosity dependent on temperature and elastic parameters dependent on lithology the geothermal profile and the rheological structure of the lithosphere remarkably affect the processes of stress accumulation which control the distribution of seismic activity the abrupt decrease of the temperature gradient at the moho produces considerably higher stress values with respect to the case of uniform gradient thus favoring subcrustal seismicity in the case of a standard temperature gradient subcrustal seismicity is predicted and a gap in seismicity indicating a soft intracrustal layer exists if there is a discontinuity in rheology by contrast in the case of a high temperature gradient subcrustal seismicity is not to be expected even in the presence of a discontinuity in rheology since subcrustal temperatures are already too high to permit a sufficient stress accumluation

moho depth and vp vs ratio in peninsular italy from teleseismic receiver functions 1 we present a new data set of moho depth estimates for peninsular italy obtained from receiver functions analysis and a grid search approach teleseismic records from 174 broadband seismic stations were used to compute the receiver function data set in general our results agree well with previous knowledge adding original information in many regions where previous data were lacking or uncertain we find broad regions with consistent moho depth namely 1 the peri tyrrhenian areas bordering the homonymous sea have a 20 to 25 km deep moho 2 the foreland regions adriatic apulia iblean with 30 35 km deep moho 3 the apennines belt where we find westward deepening of the adriatic moho down to 50 km depth that well delineate the westward adria subduction overlayed by the shallow tyrrhenian moho 4 calabria shows a flat 40 km deep moho suddenly dipping at high angle from the tyrrhenian coast westward defining the ionian subduction and 5 a clear separation between northern and southern apennines is marked by a region of almost flat moho possible evidence of a detached slab in between we also find that in the northern apennines the shallow tyrrhenian moho is continuous for more than 200 km from the sea to under the northern apennines with a generally flat geometry and a few undulations suggesting either the boudinage or episodic pulses of extension of the tyrrhenian crust during its pliocene quaternary stretching the eastward extent of the tyrrhenian moho below the apenninic belt approximately corresponds to the eastward extent of the observed extension in the upper crust as mapped by the normal faulting earthquakes in this portion of the apennines we interpret an intracrustal discontinuity sw dipping below the belt parallel to the adria moho as the top of the lower crust being subducted beneath the apennines the subcrustal earthquakes of this region down to 80 km depth seem to be located within this subducted crustal layer the southern apennines show the same general geometry of subduction as the northern arc although the former have a less defined on land extended region and no earthquakes in the lower crust uppermost mantle in between the central apennines present a different moho geometry no clear doubling of the two plates but rather a constant bilateral deepening possibly associated to a crustal root with no evidence of a subduction setting this reinforces the hypothesis of a detached slab in this area we also present the first large scale mapping of average crustal vp vs ratios for peninsular italy the results although representing averages over the whole crust enhance some interesting features the most evident are belts of high vp vs in the thinned crust of the apennines where extension has long been active in the quaternary probably as an effect of highly fractured rocks with abundant fluid circulation strong correlation exists between areas of high vp vs and quaternary volcanoes suggesting the presence of fluids at depth

tectonic evolution of a low angle extensional fault system from restored cross sections in the northern apennines italy 1 broad geological and geophysical documentation is available on regional extensional systems driven by low angle normal faults however little information exists about the three dimensional geometry and the offset distribution of such extensional structures we present a new set of balanced geological sections across the extensional fault system driven by the altotiberina low angle normal fault in the northern apennines of italy we document this extensional system throughout a large set of surface field surveys and geological maps and subsurface data seismic reflection profiles and boreholes the subsurface data allowed us to define the fault deep geometry and to obtain its structural contours the fault geometry is characterized by both along dip and along strike irregularities in cross section the fault displays a staircase trajectory with the shallowest part being dome shaped and flattened to horizontal this bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 mpa the sequential restoration of five geological cross sections yields a maximum extension of about 10 km accumulated over approximately 3 ma the resulting long term slip rate is about 3 mm yr which is of the same order as the present day extensional rate measured by gps 2 5 3 0 mm yr suggesting an almost steady state extension over the last 3 ma the distribution of the extension values along the fault strike is bell shaped as expected for a continuous surface

p and s wave upper mantle seismic velocity structure beneath the northern apennines new evidence for the end of subduction orogeny in the geologically young northern apennines is explained by the eastward retreat of a convergence zone in which adriatic lithosphere subducts to the west it is unclear however whether all of the lithosphere or only the lower portion has subducted since 15 ma when the convergence zone lay near corsica we combine teleseismic p and s wave arrival time data from the retreat seismic network 2003 2006 and surrounding permanent stations to estimate tomographic images of the upper mantle structure beneath the northern apennines we image a vertically oriented slab that that extends to only 300 km depth our slab termination is shallower than previous studies but is confirmed by resolution tests furthermore our images resolve the southern edge of the northern apennines slab at 43 n with no deep continuity with any slab segment to the south as earlier proposed our results suggest that only a 300 400 km strip of lithosphere has subducted since 15 ma beneath the northern tyrrhenian sea a length roughly equivalent to the distance from the present day west coast of corsica to the crest of the northern apennines although not a definitive indicator the largely aseismic vertical slab configuration and its limited extent coupled with other volcanic and geophysical indicators suggests the delamination scenario of bird 1979 in which only the lower portion of the continental lithosphere subducts

anisotropic seismic structure of the lithosphere beneath the adriatic coast of italy constrained with mode converted body waves received 7 may 2002 revised 26 august 2002 accepted 3 september 2002 published 26 november 2002 1 ps converted waves observed near ancona on the adriatic coast of central italy as revealed by teleseismic receiver functions rfs vary with earthquake backazimuth and epicentral distance in a manner consistent with a 1 d anisotropic seismic structure using reflectivity calculations we develop a profile of anisotropic seismic velocity through the adriatic lithosphere at this locality we infer crustal thickness of 45 km anisotropy within the crust appears at 15 km depth suggesting a decollement between the subducting adriatic lithosphere and the overriding crustal wedge lineation of inferred rock fabric is compatible with simple shear in ene wsw direction in the upper mantle we infer an anisotropic layer at 80 90 km depth if caused by olivine crystals alignment the nearly north south lineation of the inferred rock fabric would be consistent with some nearby shear wave splitting observations this anisotropic layer may be related to mantle deformation induced by the rollback of adriatic lithosphere index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8102 tectonophysics continental contractional orogenic belts citation levin v l margheriti j park and a amato anisotropic seismic structure of the lithosphere beneath the adriatic coast of italy constrained with mode converted body waves geophys res let t 29 22 2058 doi 10 1029 2002gl015438 2002

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

crustal shortening and duplication of the moho in the northern apennines a view from seismic refraction data abstract a reappraisal of the dss refraction seismic campaigns of 1978 and 1974 in the northern apennines and the northern tyrrhenian sea after digitization of the original analog data and a new interpretation of crustal structures are presented in this paper the layering of the adria crust consists of a low velocity lower crust topped by a 6 7 km s horizon and an upper crust again formed by a low velocity layer capped by a faster one in the tuscany sector the crust mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the adriatic counterpart these differences are due to the extensional tectonics restricted to tuscany and occurred in a high heat flow regime related to an uplifting astenosphere it has been confirmed that a deep thrust is responsible for moho doubling in correspondence with the zero bouguer anomaly line the tuscan moho overlaps the umbro marchean one for a length of about 30 km it is hypothesized that another shallower thrust involves upper crust in the mid tuscany range mt cetona and perugia massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks no conclusive proof of the existence of a european moho below a tuscan one in the area underneath elba has yet been found revision of dss data excludes the presence of a refractor reflector 60 km deep the same data however are compatible with the existence of a crust mantle boundary at 30 35 km depth

tectonothermal processes and mechanical strength in a recent orogenic belt northern apennines 1 this paper investigates the seismic cutoff temperature at the maximum depth of seismicity across a young orogenic belt the northern apennines by attempting to take account of the different tectonothermal processes which have occurred in the region since the last 20 myr the analysis of seismicity over a period of 20 years 1981 2002 in combination with the thermal regime shows that the western part of the northern apennines peri tyrrhenian zone is characterized by extensional tectonics few earthquakes generally shallow and of low magnitude and high surface heat flux the entire crust of the eastern part of the orogenic belt external zone is affected by seismic activity of moderate magnitude and low heat flux earthquakes increase both in number and energy released in the inner portion of the belt internal zone where events with larger focal depth indicate subduction the cutoff potential temperature at the largest depth of seismicity is 550 c for the internal zone in the external zone of the orogen the temperature at the maximum depth of seismicity is about 550 c this result agrees with recent work on the thermal structure and seismicity of several continental areas the limiting temperature in the peri tyrrhenian zone can instead exceed 750 c a possible explanation is that the deepest earthquakes might be associated to magma movement the overall earthquake spatial distribution across the orogen appears related to the long term rheology which varies roughly in the same direction of tectonic transport of the chain this consistency appears also from the b value of the frequency magnitude relation which can be interpreted in relation to the rheological properties of the medium the b value seems to be inversely correlated to the expected mechanical strength of the crust and increases eastward in the direction of tectonic transport the largest number of shocks and magnitude of the internal zone happen where a sharp lateral change of strength should occur

the main characteristics of the lithosphere asthenosphere system in italy and surrounding regions we present a summary of the available information on rayleigh wave dispersion data for the italian area the dispersion curves can be grouped into two main families according to their shape one with an almost linear trend in the period range from 40 to 150 sec the other with a relatively flat portion in the period range from 40 to 80 sec the inversion of these data shows that there are strong lateral variation in the lithosphere system a very rapid transition in the lithospheric thickness from about 130 km to about 30 km as well as in the s wave velocity of the lithosphere and asthenosphere

mafic granulite rheology implications for a weak continental lower crust abstract the jelly sandwich model of continental rheology long the consensus model has recently been challenged the controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust however because of technical issues in experimental rock deformation there are very few data on this problem here we report the rheology and fabrics of a reconstituted fine grained nominally dry i e no hydrous mineral mafic granulite 57 plagioclase 24 clinopyroxene 14 orthopyroxene 5 opaque minerals 0 16 0 28 wt h 2 o deformed at 1 1 1 2 gpa pressure in a modified griggs type deformation apparatus the rheology of this mafic granulite can be described by the constitutive equation of e a 3 2 0 4 exp 244 35 kj mol r t where e is in s 1 in mpa t in kelvin and a 10 2 0 1 6 mpa 3 2 s 1 our results provide new experimental evidence in support of the jelly sandwich lithosphere strength model and imply that mafic granulite with a moderate amount of water 0 05 0 08 wt h 2 o is likely to be a weak layer in the lithosphere both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation the deformation mechanism is dislocation creep with 100 001 and 001 100 010 100 being the dominant slip systems for pyroxenes and plagioclase respectively the low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase a weak continental lower crust has many important implications for geodynamics of crust mantle interactions such as lower crust channel flow and delamination

convective support of long wavelength topography in the apennines italy two different free air gravity datasets from the apennines have been analysed in the frequency domain to investigate the mechanisms of support of topography at short wavelength 150 km shows that such topography is not supported by elastic stresses in the lithosphere but is dynamically supported by mantle convection the distribution of gravity anomalies and topography suggests that the mantle is rising beneath the main topographic bulge of the apennines and that this flow is maintained by buoyancy forces rather than being driven by movement of the subducted slab

seismotectonic zoning in east central italy deduced from an analysis of the neogene to present deformations and related stress fields this paper concerns the space time evolution of the late miocene to present contractional and extensional deformations within east central italy with an analysis of the pattern and orientation of the related stress field an insight into the cover basement relations and a discussion of the seismotectonic implications the pattern and orientation of the deformation and stress fields have remained unchanged since late miocene time and have been always characterized by the coexistence and parallelism in contiguous areas in a given time of shortening to the east and elongation to the west with time during a multistage deformation history both the contraction and the extension fields have moved eastward basement involved tectonics with a west dipping multiple detachment system would have allowed the progressive eastward shift of crustal slices corresponding at the surface to well defined tectonic domains with consequent crustal thinning in the tuscan zone and crustal thickening beneath the apennine mountain chain by comparing macroseismic epicenters hypocenters and focal mechanisms with crustal structure and independently known active faults it appears evident that the pattern frequency and distribution of the crustal seismicity are tectonically controlled and that a contraction extension pair is active also at the present time in fact along the adriatic coastline contractional deformation prevails with involvement of pleistocene layers whereas beneath the apennine range extensional deformation prevails both t axes in the inner domain and p axes in the outer domain are oriented southwest northeast or west south west east northeast and this coaxiality reflects the situation depicted by the geologic stress field on this basis we have produced a seismotectonic zoning of east central italy with three distinct domains intramountain seismic zone isz foothills seismic zone fsz and coastal seismic zone csz a plausible explanation for the genesis of the active seismic deformations of east central italy is that the locally intense extensional upper crustal seismicity of the intramountain seismic zone is confined to the hanging wall of a major west dipping active extensional discontinuity whereas the less frequent contractional seismic activity of the coastal seismic zone is within the hanging wall of an outer major active west dipping contractional discontinuity an area of overlap between these two zones is represented by the foothills seismic zone where superposed epicentral patterns from different surfaces of weakness at different depths are the predictable result of the asymmetry of the deformation

recent seismicity of italy active tectonics of the central mediterranean region and seismicity rate changes after the mw 6 3 l aquila earthquake abstract in this paper we present a new image of the instrumental seismicity of italy obtained by refining hypocentral determinations for about 100 000 earthquakes that occurred in the period 2005 2012 the improved locations yield new constraints on active tectonics of the central mediterranean area where prolonged interaction between nested plates and continental slivers led to the development of the alpine and apennines systems intermediate depth and deep earthquakes define a lateral heterogeneous process of delamination and sinking of the continental lithosphere active beneath the mountain belts shallow seismicity prevalently occurs beneath elevated topography and correlates with low velocity mantle anomalies suggesting a superposition of gravity related forces to the eurasia africa plate convergence the delamination process drives a paired system of compression and extension that stretches the mountain range while shortening the external side of the belts the updated seismic catalog permits us to resolve a sharp variation of seismic rates that occurred in recent years timely after the 2009 mw 6 3 l aquila earthquake the increase of seismic rates is reasonably due to regional scale perturbation of the stress field induced by fluid flow and pore pressure variations within the crust probably related to deep dehydration processes active beneath the mountain belt

anomalous uplift of the apennines and subsidence of the adriatic the result of active mantle flow 1 we consider the potential for interpreted mantle flow to generate anomalous topography across the north central mediterranean a seismic tomography section was translated into the temperature domain and used as input into a series of 2d thermo mechanical numerical models of crust mantle deformation the generated dynamic topography signal was then compared with a derived residual topography that showed a distinct residual low across the adriatic sea and easternmost apennines and a residual high to the west of the apennine range and into the tyrrhenian sea a good spatial correlation was found between the modeled dynamic effects of inferred underlying mantle structure and the observed residual topography the results of this study suggest that underlying mantle structure may play a role in supporting the dynamic anomalous surface topography along the investigated profile and within the north central mediterranean

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

critical stress difference and orientation of faults in rocks with strength anisotropies the two dimensional case abstract equations are derived for the critical stress difference on thrust normal and strike slip faults with finite cohesive strength both in homogeneous isotropic rock and along pre existing strength anisotropies with different cohesion and coefficient of friction subject to the limitation that the plane of anisotropy contains the intermediate axis of stress the range of orientations for which sliding occurs along pre existing planes of weakness rather than along a new fault is given as a function of material parameters and critical stress difference for the intact rock given the principal stress directions the equations allow the direct calculation of the stress and orientation conditions for the three faulting regimes

linking the alps and apennines subduction systems new constraints revealed by high resolution teleseismic tomography abstract we report a new model of the upper mantle structure beneath italy obtained by means of p wave teleseismic tomography besides the recent and remarkable development of the italian seismic network a high model resolution has been achieved improving the inversion method upon the ach method used in previous investigations and picking high quality arrival times with the multi channel cross correlation technique the finer details of our vp model yield new insights into the heterogeneous structure of the adria continental lithosphere involved in the collision between the africa and europe plates a wide low vp anomaly located in the northern adria mantle facing the alpine high vp slab supports the idea that the adria lithosphere has been hydrated and thinned during the alpine subduction we argue that this mantle softening may have played a key role in favoring the subsequent delamination of the adria lithosphere in the northern apennines we hypothesize that delamination of continental lithosphere previously thinned in a back arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation conversely in the central southern apennines the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle in agreement with the widely accepted geodynamic evolution of the central mediterranean by slab retreat and back arc spreading the oceanic slab is discontinuously detached from the surface plate suggesting a different structure of the adria lithosphere which resists subduction instead of favoring delamination

gravitational potential energy and active deformation in the apennines we use velocity measurements from a network of continuous gps sites spanning the apennines of peninsular italy to test the hypothesis that the active deformation of the region is explained by variations in gravitational potential energy of the lithosphere the simple geometry of the mountain chain allows us to treat the deformation as two dimensional neglecting gradients of velocity along the strike of the chain under this assumption the integral of gravitational potential energy per unit area of the lithosphere gpe in the direction perpendicular to the chain is related by a simple expression to the velocity in the same direction we show that the observed velocities match this expression with an rms misfit of 0 5mm yr this agreement suggests that deformation of the apennines reflects a balance within the mountain chain itself between lateral variations in gpe and the stresses required to deform the lithosphere forces arising from processes external to the belt are not required to explain the observations

from underplating to delamination retreat in the northern apennines abstract recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern apennines and support the scenario of a retreating detachment within the mid crust lithospheric delamination appears crucial to the formation and evolution of the apennines orogen receiver function rf stacks outline a continuous west dipping ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the adria continental lithosphere which is descending into the shallow mantle the correlation of seismicity with two rf profiles across the northern apennines suggests distinct stages of lithospheric delamination active penetration of the detachment into the adria lithosphere seems evident in the south east with induced shallow mantle flow facilitated by slab dehydration penetration of the detachment in the north west seems to have arrested and is possibly marked by crustal underplating this layer atop the apennines slab is visible only down to 80 km depth and suspends above an oppositely dipping paired positive negative ps converted phase in stacked receiver functions the break in the west dipping adria lithosphere conflicts with a westward subduction scenario continuous from the oligocene lateral changes of deep structure and seismicity along the northern apennines suggest that underplating of crustal material and delamination retreat are distinct mechanisms active today in the western and eastern sectors respectively of the northern apennines negative ps pulses at 100 120 km depth help to define a seismic lithosphere asthenosphere boundary lab but cross cut a volume of high velocity mantle rock as inferred from tomographic models we hypothesize that this seismic lab is a rheological discontinuity that affects the frequency band of seismic body waves but not the long term viscous response that governs the evolution and eventual detachment of the continental slab

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

subcrustal earthquakes in the northern apennines italy evidence for a still active subduction previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the northern apennines italy these earthquakes occur beneath a zone of abundant upper crustal seismicity mostly confined to the upper 20 km of the crust although there are relatively few well located subcrustal earthquakes there appears to be a general southwestward deepening of the hypocenters from the adriatic to the tyrrhenian sea consistent with other geophysical data which suggest that the adriatic lithosphere is presently subducting beneath the northern apennines

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

constraints on the origin of adakites and porphyry cu mo mineralization in chongjiang southern gangdese the tibetan plateau chongjiang is a low grade porphyry cu deposit located in the gangdese belt south tibet the petrogenesis and geodynamic settings of the miocene intrusions associated with the deposit remain controversial this study presents new results on in situ zircon hf o isotopic compositions and u pb ages whole rock major and trace elements and sr nd isotopes for the adakitic intrusions from chongjiang deposit the ore bearing biotite monzogranite porphyry has adakitic characteristics with enriched large ion lithophile elements lile and light rare earth elements lree and depleted in high field strength elements hfse p and ti la icp ms zircon u pb dating indicates that the ore bearing and barren adakites were emplaced at 14 9 0 3 ma and 12 9 0 3 ma respectively the porphyry is characterized by relatively high initial sr 87 sr 88 ratios 0 7059 to 0 7066 and negative whole rock epsilon nd t values 3 8 to 2 6 zircon delta 18 0 is slightly higher than mantle values 5 0 to 7 2 parts per thousand with varied epsilon hf t 1 0 to 7 6 most of the in situ zircon hf o isotopic data plot in a binary mixing trend between morb and lower continental crust derived melts these results indicate contributions from mixing of a mantle like source e g slab melts with continental crust interestingly most of the samples plot in the field defined by dabie adakites representing partial melting of the lower continental crust with several samples near in the circum pacific adakite field representing partial melting of subducted oceanic slabs which seemingly indicates that chongjiang adaldtes mostly formed through partial melting of lower continental crust with a small amount derived from oceanic slab melts these may be plausibly explained by plagioclase retention in the thickened tibetan continental crust which lowers sr contents in the magmas during crustal assimilation such a model is supported by other adakite discrimination diagrams which all point towards slab melting crustal contamination can compellingly explain the low grade of the chongjiang deposit considering the temporal spatial distribution of porphyry cu deposits geochemical characteristics and high oxygen fugacity we propose that the subducting ninetyeast ridge probably played a critical role in controlling the formation of miocene adakites and porphyry copper deposits in the eastern gangdese belt c 2017 elsevier b v all rights reserved