crustal thinning in the northern tyrrhenian rift insights from multichannel and wide angle seismic data across the basin extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning here we compare two coincident wide angle and multichannel seismic profiles across the northern tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center tomographic inversion reveals that the crust has thinned homogeneously from 24 km to 17 km between the corsica margin and the latium margin implying a factor of 1 3 1 5 on the transect 80 km to the south the crust thinned from 24 km beneath sardinia to a maximum of 11 km in the eastern region near the campania margin factor of 2 2 the increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast we interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting as observed on multichannel seismic images locally basalt flows are imaged intruding sediment in this zone and heat flow values locally exceed 100 mw m2 velocities within the entire crust range 4 0 6 7 km s which are typical for continental rocks and indicate that significant rift related magmatic underplating may not be present the characteristics of the pre tectonic syn tectonic and post tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting in the western part of the southern transect thick postrift sediments were deposited in half grabens that are bounded by large fault blocks fault spacing and block size diminish to the east as crustal thinning increases recent tectonic activity is expressed by faults cutting the seafloor in the east near the mainland of italy the two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south this structural evolution is consistent with w e rift propagation and southward increasing extension rates

early stage rifting of the northern tyrrhenian sea basin results from a combined wide angle and multichannel seismic study extension of the continental lithosphere leads to the formation of rift basins and ultimately may create passive continental margins the mechanisms that operate during the early stage of crustal extension are still intensely debated we present the results from coincident multichannel seismic and wide angle seismic profiles that transect across the northern tyrrhenian sea basin the profiles cross the corsica basin france to the latium margin italy where the early rift stage of the basin is well preserved we found two domains each with a distinct tectonic style heat flow and crustal thickness one domain is the corsica basin in the west that formed before the main rift phase of the northern tyrrhenian sea opening 8 4 ma the second domain is rifted continental crust characterized by tilted blocks and half graben structures in the central region and at the latium margin these two domains are separated by a deep 10 km sedimentary complex of the eastern portion of the corsica basin travel time tomography of wide angle seismic data reveals the crustal architecture and a subhorizontal 15 17 1 km deep moho discontinuity under the basin to estimate the amount of horizontal extension we have identified the pre syn and post tectonic sedimentary units and calculated the relative displacement of faults we found that major faults initiated at angles of 45 50 and that the rifted domain is horizontally stretched by a factor of 1 3 8 10 mm a the crust has been thinned from 24 to 17 km indicating a similar amount of extension 30 the transect represents one of the best imaged early rifts and implies that the formation of crustal scale detachments or long lived low angle normal faults is not a general feature that controls the rift initiation of continental crust other young rift basins like the gulf of corinth the suez rift or lake baikal display features resembling the northern tyrrhenian basin suggesting that half graben formations and distributed homogeneous crustal thinning are a common feature during rift initiation

heterogeneous brittle ductile deformation at shallow crustal levels under high thermal conditions the case of a synkinematic contact aureole in the inner northern apennines southeastern elba island italy abstract we present an example of interaction between magmatism and tectonics at shallow crustal levels in the late miocene the metamorphic units of the eastern elba island northern apennines were intruded at very shallow crustal levels by a large pluton 60 km 2 with the development of an hectometre sized contact aureole defined by growth of low pressure high temperature mineral assemblages p max max 650 c structural data show that the contact aureole is associated with a km sized antiform of the foliation and by several metre to decametre thick high strain domains consisting of strongly foliated rocks containing synkinematic ht lp mineral assemblages and ductile shear zones of variable thickness these shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration associated with the thermal peak of contact metamorphism to subgrain rotation and bulging recrystallization the latter mostly associated with the cataclastic overprint these transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation deformation accompanied the development and cooling of the contact aureole which recorded the switch from high temperature ductile to low temperature brittle conditions the geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks

vein development during folding in the upper brittle crust the case of tourmaline rich veins of eastern elba island northern tyrrhenian sea italy abstract detailed structural analysis of tourmaline rich veins hosted in the contact aureole of the 6 ma porto azzurro granite in southeastern elba island northern tyrrhenian sea is presented using geometric features of the veins the physical conditions at the time of vein formation are estimated namely the stress ratio 2 3 1 3 driving stress ratio r p f 3 1 3 and fluid overpressure p o p f 3 two vein sets a veins and b veins have been recognized based on orientation and thickness distributions and infilling material analysis of vein pole distributions indicates 0 57 and r 0 24 for the a veins and 0 58 and r 0 47 for the b veins and fluid pressures less than the intermediate stress magnitude analysis of geometric features of the veins gives estimated fluid overpressures of between 16 mpa a veins and 32 mpa b veins we propose a model for the tectonic environment of vein development in which formation of secondary permeability in the deforming thermal aureole of the porto azzurro pluton was controlled by ongoing development of fracture systems in the hinge zone of a regional nnw sse trending fold that favored transport and localization of hydrothermal fluids

geological structure of a long living geothermal system larderello italy we have examined the data of 600 geothermal wells and re interpreted approximately 500 km seismic profiles through the field of larderello italy we conclude that the two main seismic reflectors present below the geothermal area host two different fluids 1 superheated steam in the upper h horizon reached by drillholes and 2 supercritical fluid in the deeper k horizon reached by few unproductive or damaged wells the superheated steam has the physical and chemical connotation of the geothermal fluid exploited so far at larderello whereas the supercritical fluid represents a potential unconventional deep seated resource still to be assessed the high temperatures existing in correspondence of the k horizon suggest that the silica rich rocks are close to a plastic state and the fluids should remain confined in a medium sealed to the confining rocks unless occasional fluid overpressure and abrupt high strain rates occur

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

high resolution magnetostratigraphy of four sediment cores from the greenland sea ii rock magnetic and relative palaeointensity data summary high resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount vesteris banken in the greenland basin and one from the jan mayen fracture zone revealed records of three pronounced geomagnetic events the mono lake excursion 28 27 ka the laschamp event 37 33 ka and the biwa i jamaica event 189 179 ka rock magnetic investigations measurements of magnetic susceptibility and arm and irm acquisition demagnetization experiments show that only fine grained titano magnetite 0 5 m is the dominant magnetic carrier mineral in the hemipelagic muds there is no evidence for high amounts of high coercivity minerals like haematite or goethite volcanic basaltic ash layers in sediments around vesteris banken seamount are characterized by extremely high values of magnetic susceptibility reaching 16 000 times 10 6 si as well as by low median destructive fields of the anhysteretic remanent magnetization mdfarm indicating magnetite of larger grain sizes 1 m besides this no significant variation in rock magnetic parameters across intervals of intermediate to reversed chrm inclinations are observed therefore these are interpreted as true records of geomagnetic field variations the recovered sediments characterized by only moderate fluctuations in concentration as well as grain size related rock magnetic parameters provided good conditions for the reconstruction of the relative palaeointensity of the geomagnetic field the recorded geomagnetic events are clearly linked to a drastic reduction in the relative palaeointensity of the field the lowermost values were obtained for the time of the polarity transition with a trend to higher relative intensities in the course of the polarity events event ages and the relative palaeointensity variations based on different methods derived from the greenland sea sediments investigated are in good agreement with data published in the literature

dating of deformation phases using k ar and 40ar 39ar techniques results from the northern apennines abstract paleozoic to oligocene metasedimentary rocks present in the alpi apuane region of the northern apennines italy have been sequentially deformed during a tertiary progressive deformation in an attempt to date the individual deformation episodes over 50 conventional k ar and 1140 ar 39ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known mineralogy x ray diffractometry and thin section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3 4 kbars and 300 400 c during all deformational phases pre existing micas were variably crenulated during each subsequent deformational phase both k ar and 40ar 39ar analyses were carried out on 0 6 2 m 2 6 m and 6 20 m size separates of the phengitic white mica although the k ar apparent ages range from 11 to 27 ma and are consistent with available stratigraphic constraints the 40ar 39ar age spectra display variable internal discordancy these isotopic data indicate that 1 both the k ar and 40ar 39ar total gas ages decrease as the degree of crenulation increases 2 the k ar and 40ar 39ar total gas ages decrease as grain size decreases 3 for each sample characteristics of the 40ar 39ar age spectra depend upon grain size with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and 4 phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures the isotopic results are interpreted to indicate that the major deformation phase d1 occurred at approximately 27 ma with subsequent pulses ending by c 10 ma these results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10 15 and 10 14 s 1 a 27 ma age indicates mid oligocene initiation of plate tectonic activity in the western mediterranean and concomitant deformation in the northern apennines

pliocene crustal shortening on the tyrrhenian side of the northern apennines evidence from the gavorrano antiform southern tuscany italy the northern tyrrhenian sea and the inner northern apennines are classically regarded as a late miocene pleistocene back arc system developed as a consequence of slab rollback along active subduction zones we present new geological and structural data on the gavorrano antiform a key sector of the inner northern apennines lying close to the northern tyrrhenian sea it provides clear evidence of pliocene shortening deformation and magma emplacement the orientation of 1 n50 e n80 e derived by fault slip data inversion is consistent with a general ene wsw shortening direction furthermore this ene wsw trending orientation of 1 is compatible with the compressive deformation recorded in coeval sedimentary basins on this basis we suggest that the inner northern apennines were affected by crustal shortening during the pliocene this scenario matches well geophysical data suggesting that since the late messinian 6 5 ma subduction rollback and back arc extension strongly decreased in the northern tyrrhenian sea whereas they continued as active processes in the southern tyrrhenian sea

paragenetic specularite and adularia elba italy concordant u th he and k ar ages abstract in order to test the usefulness of the helium method for detecting young geological events in hematite u th and 4 he concentrations were analyzed in five specularite samples from rio marina type hematite deposits northeastern elba italy four specularite samples define a u 0 235th 4 he isochron age of 5 39 0 46 ma 2 the isochron intercepts the 4 he axis at 49 88 pl g 2 adularia which is paragenetic with specularite in one sample used for the isochron gives a meank ar age of 5 32 0 11 ma in addition two specularite samples one from rio marina and one from terra nera southeastern elba give helium model ages of 6 4 0 4 and 7 3 0 4 ma respectively depending on the model of genesis the high ages reflect incomplete 4 he degassing and or 4 he excess the study demonstrates that geologically meaningful helium ages as young as pliocene can be obtained from specularite in spite of expected interferences from initial helium if the 4 he axis intercept is a general measure of initial 4 he in hematite its effect on the calculated ages in mesozoic hematites in previous studies is negligible the age concordance implies that similar concordance should exist in samples with a known thermal history much older than pliocene moreover the helium method could also work on specularite with a thermal history younger than pliocene

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

exhumation and reshaping of far travelled allochthonous tectonic units in mountain belts new insights for the relationships between shortening and coeval extension in the western northern apennines italy abstract coeval extensional and contractional tectonics are a common feature in orogenic belts especially in post collisional phases the attempt to constrain their relative timing and the responsible mechanisms led to several models taking into account different geodynamic contexts of various mountain chains in peri mediterranean collisional orogens extension and compression are commonly associated and coeval to the emplacement of far travelled allochthonous units therefore their internal deformation acquired during the translation across collisional belts is likely imprinted by the extensional and contractional tectonics affecting the orogen in the western portion of the northern apennines of italy one of the northernmost peri mediterranean tertiary orogens the far travelled allochthonous units i e the ligurian and subligurian units are widely exposed the present study integrates published and newly acquired surface geological data geological maps cross sections subsurface boreholes data and paleothermal vitrinite reflectance mixed layer illite smectite and kubler index and thermochronological data apatite fission tracks in order to investigate the deformation history and consequently the record of the extension compression relationships within these far travelled allochthonous units the study of modes and timing of reshaping of the far travelled allochthonous units allowed us to strengthen the hypothesis that extensional and contractional tectonics were coeval in the northern apennines and tightly connected by a cause effect relationship in particular the late orogenic low and high angle extensional tectonics represent the response to an overall contractional regime which induced episodes of gravitational instability in the orogenic wedge

periodic floods from glacial lake missoula into the sanpoil arm of glacial lake columbia northeastern washington at least 15 floods ascended the sanpoil arm of glacial lake columbia during a single glaciation varves between 14 of the flood beds indicate one backflooding every 35 to 55 yr this regularity suggests that the floods came from an ice dammed lake that was self dumping probably the self dumping lake was glacial lake missoula montana because the floods accord with inferred emptyings of that lake in frequency and number apparently entered lake columbia from the east and produced beds resembling backflood deposits of lake missoula floods in southern washington

kinematic model for out of sequence thrusting motion of two ramp flat faults and the production of upper plate duplex systems abstract kinematic models developed here suggest a bewildering array of structural styles can be generated during out of sequence thrusting many of these structures would be difficult to distinguish from a normally stacked thrust sequence and the process can produce younger on older faults that could easily be misinterpreted as normal faults this paper considers a small subset of this problem within a large model space by considering structures that develop along a pair of ramp flat faults that are moving simultaneously or sequentially motion on the lower ramp warps the structurally higher fault due to fault bend folding and when the fault ruptures through the warp it transfers a horse to the upper hanging wall continuity of the process generates what is referred to here as an upper plate duplex to distinguish the structure from a conventional duplex kinematic parameters are developed for two models within this general problem 1 a system with a fixed ramp in the lower thrust overridden by an upper thrust and 2 a double duplex system where a conventional duplex develops along the lower fault at the same time as an upper plate duplex is formed along the upper fault the theory is tested with forward models using 2d move software and these tests indicate different families of structural styles form in association with relative scaling of ramp systems slip ratio between faults and aspect ratios of horse blocks formed in the upper plate duplex a first order result of the analysis is that an upper plate duplex can be virtually indistinguishable from a conventional duplex unless the trailing branch lines of the horses are exposed or imaged a condition seldom met in natural exposures restoration of an upper plate duplex produces counterintuitive fault geometry in the restored state and thus restorations of upper plate duplexes that erroneously assume a conventional duplex model would produce restored states that are seriously in error in addition in most of the models some fault segments place younger rocks on older rocks which could be easily misinterpreted as normal fault systems in some models younger on older juxtapositions are significant and if scaled to crustal scale would produce core complex style structures that would be difficult to recognize as contractional features collectively these observations imply that many areas where simultaneous contraction and extension are inferred may be entirely contractional with younger on older relationships generated by out of sequence thrust systems examples where this process may have occurred are in southwestern north america and the moine thrust system and future studies should evaluate these systems in light of these models distinguishing upper plate duplex from conventional duplex is potentially important in economic evaluations of thrust systems because fluid migration paths would be very different in the two alternatives the process may also be important in seismogenic mechanisms particularly in subduction megathrusts because faults warping faults could produce fault irregularities that would form transient asperities along the fault

shallow submarine seep of abiotic methane from serpentinized peridotite off the island of elba italy abstract abiotic methane ch4 is today widely reported in gas seeps and hyperalkaline springs in ophiolites and peridotite massifs characterized by low temperature continental serpentinization origin and distribution of this gas have far reaching implications in microbiology astrobiology and carbon cycle we report an in depth study of a recently described abiotic ch4 seep occurring in shallow seafloor along the western coast of elba island tyrrhenian sea italy the gas is characterized by stable c and h isotopic compositions of ch4 13c 18 2h 141 and a very low co2 content that are typical of abiotic gas in continental ultramafic rock systems based on local geothermal gradients the temperature of methane production is estimated to be below 100 c the isotope signature of methane is similar to that occurring in the liguria region about 200 km north of elba island where the same ophiolite unit exposed a mantle co2 component suggested by relatively high 3he 4he ratios has likely acted as ch4 precursor the reconstruction of the geological structural setting of elba ophiolite sequence highlighted that the seep occurs in correspondence with a faulted reverse limb of the antiform of the ophiolite unit the gas bearing fault forms a contact between mafic and ultramafic serpentinized rocks as typically observed in other continental seeps and springs related to ophiolites magmatic intrusions in the island may have contributed to the c feedstock of methane

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

sericitic alteration at the la crocetta deposit elba island italy interplay between magmatism tectonics and hydrothermal activity the la crocetta mine near porto azzurro elba island tuscany italy is an important producer of raw material for the ceramic industry exploitation focuses on a pervasively sericitized porphyritic aplite of the tuscan magmatic province locally known as eurite which underwent significant potassium enrichment during sericitic alteration eurites are located along the hanging wall of the elba centrale fault a low angle extensional lineament of regional significance a later carbonatization stage apparently associated with high angle extensional tectonics locally overprinted the sericitized facies it is expressed by carbonate pyrite quartz veins with adverse effects on ore quality sericitization was accompanied by addition of potassium and loss of na ca fe rubidium was not enriched along with potassium during sericitization contrary to what would be expected for interaction with late magmatic fluids new 40ar 39ar data from eurites provide an isochron age of about 6 7 ma for the sericitization whereas the age of the unaltered protolith is ca 8 8 ma field evidence indicates the elba centrale fault to be the main channel for the hydrothermal fluids on the other hand the involvement of heat and or fluids contributed by the porto azzurro pluton which crops out in the la crocetta area is ruled out by field geochemical and geochronological data 40ar 39ar age of porto azzurro 5 9 ma i e significantly younger than the sericitization event fluid inclusion studies suggest that sericitization was associated with a low temperature 250 c hydrothermal system fluids were locally boiling of variable salinity 4 17 wt nacl equiv and contained some co2 xco2 0 027 their ultimate source is not unequivocally constrained meteoric and or magmatic contributions may be possible low salinity 2 6 wt nacl equiv low temperature 250 c fluids are associated with the late carbonate veining they are considered to be of dominantly meteoric nature because of their low salinity in summary sericitization at la crocetta is regarded as the product of a detachment fault related low temperature hydrothermal system resulting from the structurally controlled focusing of meteoric and possibly magmatic fluids hence potential targets for exploration for similar resources are represented by aplitic bodies located in the hanging wall of elba centrale fault electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 002 0279 2

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

cover thrust reactivations related to internal basement involvement during neogene quaternary evolution of the northern apennines up to recently the neogene quaternary evolution of the northern apennines italy has been described by the classic model of a migrating eastward compressive external front with an extensional regime in the back areas connected with the tyrrhenian basin formation however in the last few years new structural data have been collected in the internal marine and continental episutural basins and in the external exposed thrust belt a complex structural evolution has now been reconstructed with coeval main tectonic phases that affect both areas with stress field change four main tectonic phases have been identified since late tortonian times these are dated as messinian late pliocene middle and late pleistocene the thrust belt has a complex evolution detected in a wide external area of the chain after the first emplacement of the main thrust sheets with reactivations and out of sequence thrusting these reactivation phases fit very well with the compressive phases affecting the sediments of the internal basins suggesting a direct relationship within a single evolutive model the increased knowledge of the deep structure of the northern apennines through geophysical and subsurface data acquired in the last few years indicates the basement involvement at least for the internal side of the northern apennines this basement involvement also played an important role in the tectonic evolution of the external sector of the apennines in this paper a new model is proposed that integrates field data geophysical evidence and geodynamic constraints the thrust reactivations and the out of sequence structures of the external area are related to internal crustal thrust activity the deformed sediments of the neogene quaternary basins are dating the thrust activity all the evidences point to a late neogene rejuvenation of the tectonic evolution but some inferences can be drawn on the development of the foredeep

40ar 39ar laser dating of tektites from the cheb basin czech republic evidence for coevality with moldavites and influence of the dating standard on the age of the ries impact abstract moldavites central european tektites are genetically related to the impact event that produced the 24 km diameter ries crater in germany representing one of the youngest large impact structures on earth although several geochronological studies have been completed there is still no agreement among 40ar 39ar ages on both moldavites and glasses from ries suevites even recently published data yielded within sample mean ages with a nominal spread of more than 0 6 ma 14 24 14 88 ma this age spread which significantly exceeds current internal errors must be in part ascribed to geological and or analytical causes this study reports the results of a detailed geochronological investigation of moldavites from the cheb area czech republic which have never been dated before and for comparison of two samples from type localities one in southern bohemia and the other in western moravia we used 40ar 39ar laser step heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization in addition with the purpose of ascertaining the influence of the dating standards on the age of the ries impact and making data from this study and literature consistent with the now widely used fish canyon sanidine fcs standard we performed a direct calibration of multi grain splits of the fish canyon biotite fct 3 with fcs the intercalibration factors r fcs fct 3 determined for eight stack positions in one of the three performed irradiations were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1 0086 0 0031 2 in agreement with previous works suggesting that biotite from the fish canyon tuff is somewhat older 0 8 than the coexisting sanidine laser total fusion analysis of milligram to sub milligram splits of five tektite samples from the cheb area yielded mostly concordant intrasample 40ar 39ar ages and within sample weighted mean ages of 14 66 0 08 14 75 0 12 ma 2 internal errors ages relative to fcs that overlap within errors these ages match those obtained for samples from western moravia 14 66 0 08 ma and southern bohemia 14 68 0 11 ma supporting the genetic link between cheb basin tektites and moldavites and consequently between cheb basin tektites and the ries impact in contrast to samples from the cheb area and moravia 40ar 39ar ages from total fusion experiments on the bohemian specimen ranged widely from 14 6 to 17 0 ma older apparent ages however were systematically obtained from fragments characterized by visible surface alteration laser step heating experiments although displaying slightly disturbed age profiles were in line with total fusion analyses and yielded well defined plateau ages of 14 64 0 11 14 71 0 11 ma 2 internal errors ages relative to fcs a thorough comparison of our and previous 40ar 39ar ages on both moldavites and ries suevite glasses recalculated relative to the 40ar 40k ratio recently determined for fcs using intercalibration factors available in or derivable from the literature reveals some inconsistencies which may be ascribed to either geological or analytical causes based on our data decay constants in current use in geochronology and ages calculated relative to fcs we infer that the age of moldavites is 14 68 0 11 ma 2 neglecting uncertainties in the 40k decay constants

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

the zuccale fault elba island italy a new perspective from fault architecture the zuccale fault central eastern elba island has been regarded since the 1990s as a low angle normal fault that records neogene crustal extension in the inner tyrrhenian side portion of the northern apennines the flat lying attitude of the fault zone and the strong excision of thick nappes were the main reasons for this interpretation previous structural and petrographic studies have focused primarily on the fault rocks themselves without map scale investigation of the structural setting and deformation structures in the hanging wall and footwall blocks furthermore despite the complex history proposed for the zuccale fault the timing of deformation has not yet been constrained by radiometric age data we present the findings of recent geological studies on eastern elba island that provide significant new insight on the nature and tectonic significance of the zuccale fault we document in detail the architecture of breccias and cataclasites that comprise the zuccale fault our new observations are consistent with a purely brittle deformation zone that crosscuts older early middle and late miocene regional and local tectonic structures the activity on the fault postdates emplacement of the late miocene porto azzurro pluton and it displaces a previously formed nappe stack 6 km eastward without any footwall exhumation or hanging wall block rotation these new data raise questions about the development of misoriented faults in the upper crust

tertiary extensional tectonics in tuscany northern apennines italy abstract two extensional events have been recognized in the tertiary evolution of tuscany the first event occurred in the period between the late oligocene and late tortonian and caused a stretching of 120 or greater the second event occurred between the late tortonian and quaternary periods and caused a stretching not greater than 10 the first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes the most significant structures of the first event are low angle normal faults which characterize the upper plate of the core complexes and ductile shear zones which characterize the lower plate the low angle normal faults tend to die out in the late triassic evaporite level which constitutes the main detachment of the core complexes the main structures of the second extensional event are high angle normal faults which dissect all the previous tectonic features these faults border tectonic depressions which were the site of clastic sedimentation from late tortonian to quaternary time these normal faults may tend to flatten in a seismic lozenge shaped band explained as an active extensional shear zone which is located at the brittle ductile boundary a similar explanation is given for the detachment located in the late triassic evaporite level during the first extensional event while the first extensional event is in the framework of the northern apennine post collisional evolution the second extensional event because it affects an already stretched crust could be attributed to the southern tyrrhenian opening which began during late tortonian

research note mono lake excursion in the lahontan basin nevada summary in the lahontan basin of north western nevada the mono lake excursion is recorded in a wave cut bank at pelican point on the west side of pyramid lake the record is almost identical to one across the lake near pyramid island 10 km away and in the carson sink 60 km further to the east at each locality the older half of the excursion is present only as a reduction in the relative field intensity while the younger half shows the expected easterly declination and steep positive inclination that result in a small counterclockwise loop of the virtual geomagnetic poles the younger half occurs during high relative field intensity and the entire intensity pattern is similar to the one for the excursion in the mono basin california 250 km to the south the presence of only part of the mono lake excursion in the lahontan basin might have resulted from the slow sedimentation rate in pyramid lake estimated to be about 12 cm kyr 1 with the field behaviour being more extensively integrated in the subsamples used in an alternate explanation the absence of the older part of the excursion can be attributed to overprinting when magnetic grains that were free to rotate in unconsolidated sediment were realigned during the high relative field intensity in the younger part of the excursion

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

extension and compression in the northern apennines italy hinterland evidence from the late miocene pliocene siena radicofani basin and relations with basement structures 1 the northern apennines na hinterland area is characterized by a complex neogene quaternary tectonics where both crustal extension associated with the tyrrhenian basin opening and crustal shortening in the onshore area took part in the deformation analysis of synorogenic deposits preserved in the nnw trending siena radicofani basin srb extending along a large part of the na hinterland documents the evolution of deformation of this sector during the last 9 ma information from subsurface geology deep seismic lines commercial seismic lines and deep wells surface geology mapping and structural analysis and from analogue modeling was integrated and used to infer the tectono sedimentary history of the srb and its relation with structures in the substratum as well as the possible implications for the evolution of the na hinterland the results we obtain indicate that the srb and the adjoining hinterland basins were bounded by thrust anticlines controlling basin development and deformation seismic lines across the srb display various examples of compressional structures affecting the basin fill and the substratum such as thrust anticlines and reverse faults notably these basin scale structures exhibit a good correlation with those observed in the field and are consistent with the kinematics of the outcrop scale compressional structures the thrust anticlines bounding the basins are often cored by triassic evaporites burano formation suggesting that this weak layer decoupling the sedimentary cover from the underlying crystalline basement controlled their evolution in this circumstance three series of scaled brittle ductile physical models have been used to investigate the development of a basement cover system with a decollement ductile layer at the base of the sedimentary cover these models simulate the evolution of the northern apennines hinterland where the basement is involved in the thrusting and the sedimentary cover is shortened above a basal ductile layer given by the burano formation longitudinal models cross sections display similar deformation patterns to those observed in the na hinterland such as characteristic wavelength of both basement and cover structures as well as detachment and fault propagation folding in the cover extensional tectonics is instead found to control sedimentation in the southern part of the srb at 8 ma or representing recent deformation the older extensional event is here related to the forelandward propagation of tyrrhenian related extension that could reactivate suitably oriented basement thrusts transfer zones produced a differential propagation of extension such that along strike sectors of the hinterland were eventually deformed by different stress fields analysis of the srb and of other adjoining basins reveals that the na hinterland experienced alternated periods of forward and backward migration of the compression extension transition in our interpretation the competition between extension and compression in the hinterland is related to the nnw directed africa pelagian indentation responsible for the lateral extrusion of the na and the contemporaneous opening of the tyrrhenian basin in the inner zone increased marginal loading at the foreland promoting reactivation of hinterland thrusts may have influenced this process as well

fault zone weakening and character of slip along low angle normal faults insights from the zuccale fault elba italy a seismically active low angle normal fault is recognized at depth in the northern apennines italy where recent exhumation has also exposed ancient examples at the surface notably the zuccale fault on elba field based and microstructural studies of the zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability promoting influx of co 2 rich hydrous fluids this triggered low grade alteration and the onset of stress induced dissolution precipitation processes e g pressure solution as the dominant grain scale deformation process in the pre existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine grained hydrous mineral phases these rocks exhibit ductile deformation textures very similar to those formed during pressure solution accommodated frictional viscous creep in experimental fault rock analogues the presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core which significantly enhanced the sealing capacity of the fault zone a slip model for low angle normal faults in the apennines is proposed in which aseismic frictional viscous creep occurs on a weak slow moving slip rate 1 fault interspersed with small seismic ruptures caused by cyclic hydrofracturing events our findings are potentially applicable to other examples of low angle normal faults in many tectonic settings

level of lake lahontan during deposition of the trego hot springs tephra about 23 400 years ago abstract the trego hot springs tephra bed is a silicic tephra about 23 400 yr old found at several localities in pluvial lake sediments in northern nevada southern oregon and northeastern california it has been characterized petrographically by the major and minor element chemistry of its glass and by its stratigraphic position with respect to other tephra layers at a newly described locality on squaw creek northwest of gerlach nevada at the north end of the smoke creek desert trego hot springs tephra has been found in sediments of the sehoo and indian lakes formations the depositional environments of these sediments show that when the tephra fell pluvial lake lahontan stood between 1256 and 1260 m and that immediately thereafter the lake rose to at least 1275 m these data corroborate earlier findings by benson quaternary research 9 300 318 from radiometric dating of calcareous tufa however the lake lahontan area has been affected by isostatic subsidence and rebound in response to changing water loads so that caution is required in the use of lakeshore elevations in correlation

evolution of shear zones through the brittle ductile transition the calamita schists elba island italy abstract a network of shear zones that evolved through the brittle ductile transition is exposed in the calamita schists elba island italy the shear zones formed during late miocene contractional deformation coeval with high grade contact metamorphism 650 c related to the emplacement of plutonic rocks at shallow crustal levels 7 10 km an early stage high metamorphic grade foliation was overprinted by mylonitic deformation that progressively localized on low metamorphic grade shear bands producing s c mylonites during cooling of contact aureole localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning between hard high grade relics and soft shear bands softening of shear bands occurred likely due to fluid influx and retrograde growth of fine grained phyllosilicates the interconnection of anastomosing shear bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing mylonites with a composite mylonitic foliation and c shear bands an estimate of the vorticity number wk of the flow of 0 3 0 5 was obtained from the orientation of c shear bands measured at the meso and thin section scale close to the brittle ductile transition the growth of soft phyllosilicates allowed c shear bands to act as precursory structures to brittle deformation localized into an array of low angle faults and shear fractures

age of the mono lake excursion and associated tephra abstract the mono lake excursion mle is an important time marker that has been found in lake and marine sediments across much of the northern hemisphere dating of this event at its type locality the mono basin of california has yielded controversial results with the most recent effort concluding that the mle may actually be the laschamp excursion earth planet sci lett 197 2002 151 we show that a volcanic tephra ash 15 that occurs near the midpoint of the mle has a date not corrected for reservoir effect of 28 620 300 14c yr bp 32 400 gisp2 yr bp in the pyramid lake basin of nevada given the location of ash 15 and the duration of the mle in the mono basin the event occurred between 31 500 and 33 300 gisp2 yr bp an age range consistent with the position and age of the uppermost of two paleointensity minima in the napis 75 stack that has been associated with the mle philos trans r soc london ser a 358 2000 1009 the lower paleointensity minimum in the napis 75 stack is considered to be the laschamp excursion philos trans r soc london ser a 358 2000 1009

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

holocene multidecadal and multicentennial droughts affecting northern california and nevada abstract continuous high resolution 18 o records from cored sediments of pyramid lake nevada indicate that oscillations in the hydrologic balance occurred on average about every 150 years yr during the past 7630 calendar years cal yr the records are not stationary during the past 2740 yr drought durations ranged from 20 to 100 yr and intervals between droughts ranged from 80 to 230 yr comparison of tree ring based reconstructions of climate change for the past 1200 yr from the sierra nevada and the el malpais region of northwest new mexico indicates that severe droughts associated with anasazi withdrawal from chaco canyon at 820 cal yr bp calendar years before present and final abandonment of chaco canyon mesa verde and the kayenta area at 650 cal yr bp may have impacted much of the western united states during the middle holocene informally defined in this paper as extending from 8000 to 3000 cal yr bp magnetic susceptibility values of sediments deposited in pyramid lake s deep basin were much larger than late holocene 3000 0 cal yr bp values indicating the presence of a shallow lake in addition the mean 18 o value of caco 3 precipitated between 6500 and 3430 cal yr bp was 1 6 less than the mean value of caco 3 precipitated after 2740 cal yr bp numerical calculations indicate that the shift in the 18 o baseline probably resulted from a transition to a wetter 30 and cooler 3 5 c climate the existence of a relatively dry and warm middle holocene climate in the truckee river pyramid lake system is generally consistent with archeological sedimentological chemical physical and biological records from various sites within the great basin of the western united states two high resolution holocene climate records are now available from the pyramid and owens lake basins which suggest that the holocene was characterized by five climatic intervals tic and 18 o records from owens lake indicate that the first interval in the early holocene 11 600 10 000 cal yr bp was characterized by a drying trend that was interrupted by a brief 200 yr wet oscillation centered at 10 300 cal yr bp this was followed by a second early holocene interval 10 000 8000 cal yr bp during which relatively wet conditions prevailed during the early part of the middle holocene 8000 6500 cal yr bp high amplitude oscillations in tic in owens lake and 18 o in pyramid lake indicate the presence of shallow lakes in both basins during the latter part of the middle holocene 6500 3800 cal yr bp drought conditions dominated owens lake desiccated and lake tahoe ceased spilling to the truckee river causing pyramid lake to decline at the beginning of the late holocene 3000 cal yr bp lake tahoe rose to its sill level and pyramid lake increased in volume

shear zones in clay rich fault gouge a laboratory study of fabric development and evolution abstract clay rich fault rocks have long been recognized to host distinctive fabric elements and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime although the geometries of fabric elements in fault gouges have been described for almost a century the genesis and evolution of these elements during shear and their links to bulk mechanical properties remain poorly understood we characterize the development and evolution of fabric elements with increasing shear in a variety of clay rich experimental gouges over shear strains of 20 and at normal stresses of 2 150 mpa in the double direct shear configuration in addition to sem observations of experiment products at a variety of shear strains we quantified clay fabric intensity and the degree of grain size reduction using x ray texture goniometry xtg and particle size distribution psd measurements we also measured p and s wave velocities during shear to further probe the evolution of shear fabric and gouge properties we find that clay fabric elements develop in a systematic manner regardless of the gouge material riedel shears in the r1 orientation and boundary parallel shears are the dominant fabric elements riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress clay particles rotate into the p orientation shortly after riedels propagate through the layer the riedel shears are through going but are 10 thinner than similar zones observed in coarser granular materials our results suggest that the weakness of clay rich fault gouge may be less a function of anisotropic crystal structure as has been suggested previously and more a consequence of very thin shear surfaces permitting deformation in clay rich materials with minimal dilation or cataclasis the very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal psd s

interactions between low angle normal faults and plutonism in the upper crust insights from the island of elba italy the late miocene to early pliocene zuccale low angle normal fault on the island of elba italy was one of the principal structures active during extensional collapse of the apennine fold and thrust belt we investigate the relationships among the zuccale fault subsidiary footwall fault networks and igneous bodies that were intruded into the immediate footwall of the zuccale fault both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent wnw ese extension direction suggesting that faulting and intrusion overlapped in time structure contour analysis indicates that the zuccale fault has a regional domal morphology the dimensions and spatial location of the dome correlate with the likely subsurface position of the porto azzurro pluton originally intruded at 6 km depth we propose that doming of the zuccale fault may have been related in part to emplacement of the porto azzurro pluton as a tabular intrusion involving some component of vertical inflation and roof uplift the immediate footwall of the zuccale fault is everywhere crosscut by a complex linked network of high and low angle extensional faults with observed displacements of

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

tectonic regime granite emplacement and crustal structure in the inner zone of the northern apennines tuscany italy a new hypothesis abstract geological and geophysical data on the inner part of the northern apennines and the deep 4 km geothermal wells contained therein have been reviewed in order to better characterize the tectonic regime of the region the area is affected by neogene quaternary igneous activity reduced crustal thickness high heat flow and active geothermal systems the available information leads to the following conclusions 1 at present part of the chain including its inner regions are undergoing extensional activity a tectonic situation in southern tuscany dating back to the late tortonian however the simple model of solely extensional tectonics cannot exhaustively account for the phenomena observed in the area as there is ample evidence of compressive events of pliocene and pleistocene ages 2 the geophysical data indicate reduced lithosphere thickness and the uprising of the asthenosphere to shallow crust levels the upper crust has been intruded by many granite bodies and the brittle ductile transition is probably very shallow an integrated geophysical model which accounts for heat flow seismic gravimetric and magnetic data indicates that a regional strong reflector rather than coinciding with the beginning of the ductile crust more likely indicates the presence of deep brines rather than a gneiss core complex and a generalized post collision extensional model cameli et al 1993 we support a more complex geodynamic framework including compressive phases of the internal zone and re activation of the thrust of the apennines as a consequence of crustal thrust emplacements from the late tortonian in the frame of a subducting delaminated lithosphere

new constraints on the evolution of the inner northern apennines by k ar dating of late miocene early pliocene compression on the island of elba italy

structural and lithological control on fluid circulation dilation and ore mineralization rio albano mine island of elba italy abstract we present the results of geological and structural investigation documenting the interaction between hydrothermal fluids and host rock leading to a vein type ore mineralization at shallow crustal depths

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

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

rscm thermometry in the alpi apuane nw tuscany italy new constraints for the metamorphic and tectonic history of the inner northern apennines abstract in this study raman spectroscopy on carbonaceous material rscm is applied for the first time in the northern apennines with particular focus on the alpi apuane nw tuscany italy and surrounding areas in order to constrain peak metamorphic temperatures and their variability in the different continent derived units of the nappe stack peak temperatures in the range of 530 320 c were found in the alpi apuane whereas in the nearby metamorphic core of the monte pisano and punta bianca lower peak temperatures of 305 315 c and 350 c were found respectively the tuscan nappe in la spezia area west of alpi apuane shows temperatures in the range of 295 246 c whereas the same unit in the lima valley east of the alpi apuane shows temperatures lower than 230 c the collected data allowed refining the thermal architecture of the belt and the relationships between deformation early and late folds and low angle normal detachments and the metamorphic architecture of the alpi apuane core these results provide new contraints for the thermo mechanical evolution and exhumation history of the inner northern apennine and its geodynamic setting in particular our data support the interpretation of the alpi apuane as a cold metamorphic core complex in which the preserved paleothermal structure and part of the exhumation are related with crustal thickening while the final exhumation stages depth 15 km and at ambient crustal temperature 350 c are associated with crustal thinning still ongoing in the area

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults

linking thermodynamic modelling lu hf geochronology and trace elements in garnet new p t t paths from the sevier hinterland major element trace element and lu hf geochronological data from amphibolite facies pelitic schist in the raft river and albion mountains of northwest utah and southern idaho indicate that garnet grew during increasing pressure interpreted to be the result of tectonic burial and crustal thickening during sevier orogenesis garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation pressure temperature paths were determined from the pre hiatus portions of the garnet chemical zoning profiles and indicate an increase of 2 kbar and 50 c in the western raft river mountains garnet lu hf dates of 150 1 ma in the western raft river mountains and 138 7 0 7 ma and 132 5 ma in the southern albion mountains indicate the timing of garnet growth lutetium garnet zoning profiles indicate that the lu hf ages are biased towards the post hiatus or outer pre hiatus segments indicating that the determined ages likely post date the recorded p t path history or date the tail end of the paths crustal thickening associated with sevier orogene sis in the western raft river mountains thus began slightly before 150 1 ma in the late jurassic this study shows that integrating p t paths determined from garnet growth zoning with lu hf gar net geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts

mono lake geomagnetic excursion found at summer lake oregon a 25 cm zone of anomalous paleomagnetic directions from lacustrine sediments near summer lake oregon has the same age and the same paleomagnetic signature as the younger part of the proposed mono lake geomagnetic excursion both field observations and rock magnetic studies support the hypothesis that the anomalous directions represent actual geomagnetic field behavior thereby confirming the existence of the mono lake excursion at the oregon site although confirmation of its existence at this site does not make the mono lake excursion any more suitable as a chronostratigraphic horizon this excursion may now be used as an additional constraint on models of geomagnetic field behavior including those that predict the morphology of transitional geomagnetic fields

magnetostratigraphic results from the eastern arctic ocean ams 14c ages and relative palaeointensity data of the mono lake and laschamp geomagnetic reversal excursions summary a new high resolution magnetostratigraphic record from the eastern arctic ocean has yielded further evidence for the existence of the laschamp excursion 37 35 ka the mono lake excursion 27 25 5 ka and possibly another very short excursion 22 ka inferred from steep negative inclinations ages are based on nine ams accelerator mass spectrometry 14c dates oxygen isotope stratigraphy and correlation with odp site 983 estimates of relative palaeointensity variations for the time interval from 80 to 10 ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity the lowest values were obtained for the two excursions and the normal reversed n r and reversed normal r n transitions of the laschamp polarity excursion which itself is characterized by a slight increase of relative palaeointensity during its reversed state the results are in general agreement with palaeointensity studies from other regions indicating that these fluctuations could be global phenomena and that the geomagnetic field of the brunhes chron was very variable in amplitude as well as in geometry the new result is one of the rare records comprising large directional as well as large relative palaeointensity variations

pliocene and pleistocene exhumation and uplift of two key areas of the northern apennines application of different thermochronological methods to two areas of pronounced relief apuane alps and mt falterona of the northern apennines chain documents an average exhumation rate of about 0 7m m yr this result suggests that the general exhumation of the chain is driven mainly by erosion nevertheless the long term exhumation trend seems to be perturbed by shortterm events at higher exhumation rates the time of increased exhumation rates coincides with onset of intramontane sedimentation documenting a contribution of tectonics local normal faulting to the surface uplift the data indicate an eastward shifting of the onset of topographic relief early pliocene in the apuane alps and early pleistocene in the mt falterona area r 2002 elsevier science ltd and inqua all rights reserved

magma emplacement in a thrust ramp anticline the gavorrano granite northern apennines italy 1 magma emplacement in the upper crust is often associated with crustal extension whereby space for intrusions is made by dilation along transtensive and strike slip faults the lenticular shape of several intrusions indicates that a further mechanism for magma intrusion is laccolith emplacement with roof uplift of the overburden additionally melts exploit rheological discontinuities e g sedimentary layering during their ascent we present an emplacement model for intrusion of the shallow level gavorrano granite northern apennine italy which is located at the core of an open anticline the shape of the intrusion and structural features of the host rocks are indicative of a synkinematic emplacement in a growing thrust anticline space was provided by the opening of dilatation zones at the core of anticline as a consequence of different amounts of translation between hanging wall and footwall units which were separated by evaporitic rocks these evaporitic units acted as major decollement layers analog models provide results in good agreement with the structural setting of the anticline and emphasize the possibility that melts filled the dilatation zone with the decollement layers further facilitating intrusion