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

the structure of the moho in the northern apennines evidence for an incipient slab tear fault abstract this study investigates the geometry of the crust mantle boundary along the northern apennines italy and its relationship to the tectonic evolution of a retreating subduction system using receiver function analysis from teleseismic records the moho depth was computed beneath 26 seismic stations located in key positions along the orogen the results show a relatively shallow 20 km moho in the tyrrhenian side as expected from an extended overriding plate and a deeper moho 40 km in the subducted adriatic plate sw dipping 15 45 beneath the chain stations deployed on the northeastern edge of the plio quaternary extensional basins provide maximum moho depth estimates thus supporting the hypothesis that these basins define the zone where the two plates separate at depth in addition an abrupt change in the moho depth is recognized along strike of the orogen attributed to a 45 km wide trench normal deformation zone that crosses the northern apennines in the proximity of the livorno sillaro lineament this zone is interpreted as an incipient lithospheric tear fault which accommodates varying rates of trench retreat along the length of the subduction zone

variance estimate in frequency domain deconvolution for teleseismic receiver function computation summary in this study the problem of estimating the receiver function variance is discussed the receiver function computation is performed through a frequency domain deconvolution oldenburg 1981 of the vertical seismogram from the radial and tangential seismograms this technique requires a knowledge of the noise contaminating the seismic signals and provides an estimate of the receiver function variance oldenburg 1981 modelled this noise as additive amnion 1992 proposed to estimate the noise level from the power spectral density in a pre signal time window in this paper this approach is used jointly with an additional inversion which yields a sequence of spikes as a model for the true receiver function the resulting residual in a time window preceding the direct p pulse is used in order to evaluate the receiver function variance and the actual noise level involved in the frequency domain deconvolution this variance estimate is compared with the one provided by oldenburg s original method the application of this method to teleseismic recordings of earthquakes 5 1 mb 6 1 with similar backazimuths and distances shows that considering the additive noise only is not sufficient to explain the uncertainty measured from the pre signal in the receiver function moreover the estimated uncertainty seems to be independent of the seismic event magnitude suggesting a signal generated noise affecting the receiver function the proposed approach provides a variance estimate for a single receiver function allowing one to assess the statistical accuracy of its amplitudes in agreement with the uncertainty involved in a stacking procedure this is particularly useful when poor quality waveforms from relatively small earthquakes mb 5 1 are analysed and data from nearly co located events are not available for stacking

seismic evidence for a moho offset and south directed thrust at the easternmost qaidam kunlun boundary in the northeast tibetan plateau abstract during 2006 to 2007 a small array of broadband seismic stations was deployed across the eastern qaidam kunlun border receiver functions derived from this data set show a coherent p to s converted phase from the moho at 65 km to 58 km below sea level b s l from south to north beneath the kunlun mountains the moho is observed at 45 km b s l beneath the qaidam basin a sharp offset of 15 km in the moho depth is observed just beneath the qaidam kunlun border the close correlation between the surface topography and the sharp moho offset substantiates the view that the kunlun crust is weaker than that of the qaidam basin and thickens vertically in response to compression between india through the tibetan plateau and the qaidam basin furthermore a strong steeply north dipping converted phase can be seen from 5 to 15 km b s l north of the north kunlun fault which is likely a major south directed thrust fault partially responsible for the thickening of the kunlun crust

recent tectonic evolution and present stress in the northern apennines italy the present day stress field and its recent tectonic evolution in the northern apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data we have considered 86 wells for breakout analysis with depths down to 6 7 km 125 fault plane solutions of crustal earthquakes with m 5 that occurred between 1988 and 1995 in the northern and central apennines and data of stronger earthquakes m 6 reported in other studies the tyrrhenian coastal region and the apenninic belt are characterized by shmin direction mainly trending ne sw with predominantly normal fault plane solutions along the outer front of the belt and the adriatic offshore shmin is oriented nw se and focal solutions are thrust or strike slip with maximum compression around ne sw conversely south of 43 n breakouts evidence an orthogonal direction of horizontal compression nw se following the southern apennine trend where a widespread ne sw extension was recognized by previous investigations comparing these results to the recent tectonic evolution inferred from structural geology we argue that the extension compression pair characteristic of the post tortonian evolution of the mountain belt has been migrating in time from late miocene to present only in the northern sector of the arc whereas the southern sector underwent a generalized extension at least since middle pleistocene the striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension compression pair has been controlled by progressive retreat of the slab

crustal structure above a retreating trench receiver function study of the northern apennines orogen abstract teleseismic receiver functions recorded along a densely spaced transect of 20 broad band seismic stations highlight systematic spatial variation of crustal thickness across the northern apennines italy revealing the two sides of an active continental orogenic zone the crustal structure has been investigated using a 1d grid search technique based on the coherent arrivals of forward and reverberated p to s converted phases from the moho at the sw end of the transect the tyrrhenian sea and tuscany are characterized by a shallow moho 20 25 km and a sharp impedance contrast at the opposite edge the definition of the crust mantle boundary is problematic suggesting a gradual and or complicated transition from the crust to the mantle in the adriatic domain however where we can resolve it the crustal thickness is larger 35 km the boundary between these two crustal regimes coincides with the high crest of the apennines shear wave splitting measurements an upper mantle texture indicator in the area reveal a correspondence between crustal thickness and mantle fabric transitions we suggest that these spatially coincident changes in the crustal structure and the upper mantle texture beneath high apennines mark the edge of the eurasian plate in 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

syn convergent extension observed using the retreat gps network northern apennines italy 1 we present crustal deformation results from a geodetic experiment retreating trench extension and accretion tectonics retreat focused on the northern apennines orogen in italy the experiment centers on 33 benchmarks measured with gps annually or more frequently between 2003 and 2007 supplemented by data from an additional older set of 6 campaign observations from stations in northern croatia and 187 continuous gps stations within and around northern italy in an attempt to achieve the best possible estimates for rates and their uncertainties we estimate and filter common mode signals and noise components using the continuous stations and apply these corrections to the entire data set including the more temporally limited campaign time series the filtered coordinate time series data are used to estimate site velocity we also estimate spatially variable seasonal site motions for stations with sufficient data the rms scatter of residual time series are generally near 1 mm and 4 mm horizontal and vertical respectively for continuous and most of the new campaign stations but scatter is slightly higher for some of the older campaign data velocity uncertainties are below 1 mm yr for all but one of the stations maximum rates of site motion within the orogen exceed 3 mm yr directed ne relative to stable eurasia this motion is accommodated by extension within the southwestern and central portions of the orogen and shortening across the foreland thrust belt to the northeast of the range the data set is consistent with contemporaneous extension and shortening at nearly equal rates the northern apennines block moves northeast faster than the northern adria microplate convergence between the northern apennines block and the northern adria microplate is accommodated across a narrow zone that coincides with the northeastern apennines range front extension occurs directly above an intact vertically dipping slab inferred by previous authors from seismic tomography the observed crustal deformation is consistent with a buried dislocation model for crustal faulting but associations between crustal motion and seismically imaged mantle structure may also provide new insights on mantle dynamics

upper mantle fabrics beneath the northern apennines revealed by seismic anisotropy 1 we image anisotropic structure of the upper mantle beneath the northern apennines based on body wave data collected during the retreat experiment 2003 2006 joint analysis of anisotropic parameters evaluated from independent data sets teleseismic p wave travel times and shear wave splitting allows us to identify regions of different fabrics both in the mantle lithosphere and in the sublithospheric mantle we recognize three regions the tyrrhenian adriatic and transition with their own anisotropic characteristics the slab parallel flow prevails in the sublithospheric mantle beneath the thin tyrrhenian plate while nearly slab perpendicular orientation of the high velocities dominates on the adriatic side of the region the asthenospheric flow pattern excludes a simple corner flow model to fit the fabric of the upper mantle in the syn convergent extensional tectonics and suggests the end of the subduction roll back we map fossil anisotropy with inclined symmetry axes within two domains of the thick continental adriatic lithosphere we estimate the lithosphere thickness of the tyrrhenian and adriatic plates at 50 km and 80 km respectively the latter being subducted down to no more than 200 km with indications of inherited frozen in anisotropic fabric if a potential detachment at the northern end of the apennine slab exists then it would have to be narrow and in its initial stage synthetic tests of the well known trade off between isotropic heterogeneity and evaluated anisotropic parameters along with combining independent data sets document a sufficient separation of both effects

mantle wedge dynamics versus crustal seismicity in the apennines italy 1 in the apennines subduction italy earthquakes mainly occur within the overriding plate along the chain axis the events concentrate in the upper 15 km of the crust above the mantle wedge and focal solutions indicate normal faulting in the foreland the seismogenic volume affects the upper 35 km of the crust focal solutions indicate prevailing reverse faulting in the northern foreland and strike slip faulting in the southern one the deepening of the seismogenic volume from the chain axis to the foreland follows the deepening of the moho and isotherms the seismicity above the mantle wedge is associated with uplift of the chain axial zone volcanism high co2 flux and extension the upward pushing of the asthenospheric mantle and the mantle derived co2 rich fluids trapped within the crust below the chain axis causes this seismicity all these features indicate that the axial zone of apennines is affected by early rifting processes in northern italy the widespread and deeper seismicity in the foreland reflects active accretion processes in the southern foreland the observed dextral strike slip faulting and the lack of reverse focal solutions suggest that accretion processes are not active at present in our interpretation of the apennines subduction the shallower seismicity of the overriding plate is due to the dynamics uprising and eastward migration of the asthenospheric wedge

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

a major step in the continental moho and its geodynamic consequences the taranaki ruapehu line new zealand summary receiver function analysis reveals a 7 km step like change in crustal thickness across the taranaki ruapehu line tr line of north island new zealand the tr line runs east west between the active andesite volcanoes of mt taranaki and ruapehu and marks the southernmost extent of subduction zone volcanism in new zealand north of the tr line receiver functions show a strong and sharp p to s ps conversion at 25 1 5 km depth which is interpreted as a shallow moho at the tr line the moho ps conversion deepens across a step to 32 km depth and weakens further south the moho deepens to 35 km most of the 7 km step change in crustal thickness occurs over a lateral distance of 8 km yet there is little surface or topographic manifestation of the line given fresnel zone considerations the dip of the moho offset could vary between 90 and 45 gravity seismic attenuation and electrical data all show that the tr line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties such as attenuation qp 1 implied density and electrical resistivity change abruptly across the line an east west oriented cluster of earthquakes with hypocentral depths of 20 40 km is centred on the moho step we propose that the moho step the earthquakes and the rapid change in mantle properties across the tr boundary are causally related processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the tr line

erosion of the continental lithosphere at the cusps of the calabrian arc evidence from s receiver functions analysis 1 mediterranean tectonics has been characterized by an irregular complex temporal evolution with episodic rollback and retreat of the subducted plate followed by period of slow trench migration to provide insight into the geodynamics of the calabrian arc we image the characteristics and lithospheric structure of the convergent apulian and hyblean forelands at the cusps of the arc specifically we investigate the crustal and lithospheric thicknesses using teleseismic s to p converted phases applied to the adria africa plate margin for the first time we find that the moho in the apulian foreland is nearly flat at 30 km depth consistent with previouspreceiver functions results and that the hyblean crustal thickness is more complex which can be understood in terms of the nature of the individual pieces of carbonate platform and pelagic sediments that make up the hyblean platform the lithospheric thicknesses range between 70 120 km beneath apulia and 70 90 km beneath sicily the lithosphere of the forelands at each end of the calabrian arc are continental in nature buoyant compared to the subducting oceanic lithosphere and have previously been interpreted as mostly undeformed carbonate platforms our receiver function images also show evidence of lithospheric erosion and thinning close to mt etna and mt vulture two volcanoes which have been associated with asthenospheric upwelling and mantle flow around of the sides the slab we suggest that as the continental lithosphere resists being subducted it is being thermo mechanically modified by toroidal flow around the edges of the subducting oceanic lithosphere of the calabrian arc

the crustal structure beneath se romania from teleseismic receiver functions summary due to vigorous neogene geodynamic processes including oceanic subduction slab break off and mountain building in the carpathian arc the architecture of the lithosphere in romania is quite complicated to improve the knowledge about the lithosphere asthenosphere system in this region the passive seismological calixto99 experiment was conducted in 1999 in the se part of romania here we present crustal models derived from the analysis of teleseismic recordings with the receiver function rf method of the 120 temporarily installed stations and of the permanent geofon stations mlr and tirr the rf results extend the known crustal models which are based mainly on seismic refraction work and analysis of regional earthquakes we apply a grid search inversion at 30 stations and use two different error estimation methods to determine the moho depth and the average crustal vp vs ratio the complex 3 d intracrustal structure especially the deep sedimentary basins distorts significantly the rf waveforms within the whole station network this leads to ambiguous results at some stations our model of the moho depth has a maximum crustal thickness in the se carpathian mountains at station mlr with a depth of about 45 km and an average crustal vp vs ratio of 1 79 the surrounding crust in se romania has a thickness of mainly 35 40 km the rfs at mlr are characterized by clear azimuthal effects that can be correlated with the variation of the sediment thickness in the foredeep of the carpathian mountains a rf waveform inversion verifies these results and gains improved 1 d s wave velocity models at several stations in se romania

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

sks splitting measurements beneath northern apennines region a case of oblique trench retreat abstract we present here the new observations of seismic anisotropy obtained from sks birefringence analysis we studied 27 teleseismic earthquakes recorded by the temporary seismic network of retreat project in the northern apennines region for each station event couple we calculate the anisotropic parameters delay time and fast polarization direction by minimizing the energy in the transverse component our measurements confirm the existence of two domains the tuscany domain on the south west with respect to the apennines shows mostly nw se fast axes directions with a rotation toward e w direction moving toward the tyrrhenian sea the adria domain north east of the apennines orogen shows more scattered measurements with prevailing n s to nne ssw directions also with back azimuthal dependence the transition between the two domains is abrupt in the northern part of the study region but more gradual in the southern part measured delay times 1 8 s on average suggest that the detected anisotropy is located principally in the asthenosphere beneath the adria domain where the presence of a double layer structure seems consistent a lithospheric contribution is plausible an interpretation in terms of ongoing mantle deformation suggests a differential evolution of the trench retreat process along the northern apennines orogen the orogen parallel anisotropy in the study region is beneath the inner part of the belt instead of beneath its crest and no orogen normal measurements are found in the tuscany side compared to the anisotropy pattern of the typical slab retreat seen in southern part of the northern apennines in the northernmost one the anisotropy suggests that an oblique trench retreat has occurred possibly linked to northern apennines retreat since 5 ma

the database of individual seismogenic sources diss version 3 summarizing 20 years of research on italy s earthquake geology abstract this paper describes the main characteristics the evolution and the structure of the database of individual seismogenic sources diss and particularly of its release of early 2007 the database contains the results of the investigations of the active tectonics in italy during the past 20 years the first two sections of this paper document the recent evolution in mapping and archiving italian active fault data in relation to important achievements in the understanding of italian tectonics some of which were spurred by significant earthquakes the central sections describe the current structure of the database the reasons for its assumptions and data categories its current contents its evolution through several years of improvements the last section describes how the current contents of the database correspond with the existing strain and stress data available from focal mechanism borehole breakout and gps data for the whole of italy the database supplies a fresh and unified view of active and seismogenic processes in italy by building on basic physical constraints concerning rates of crustal deformation on the continuity of deformation belts and on the spatial relationships between adjacent faults both at the surface and at depth

tears or thinning subduction structures in the pacific plate beneath the japanese islands abstract the nature of a subduction zone at depth is affected by the evolution of its tectonic system and the geometry of the trench line can change over time due to slab roll back or the arrival of a distinctive feature with the incoming oceanic lithosphere the configuration of the plate has to accommodate such changes with buckling thinning or the formation of tears depending on the rate of influx to the trench tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones a good example is provided by pacific plate subduction beneath the japanese islands a horizontal tear in the plate below 300 km depth can be recognised at the southern end of the izu bonin arc associated with the change in slab morphology to the much steeper mariana arc beneath southern honshu a break in the fast wavespeeds associated with the pacific plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone in the north close to the hokkaido bend in the subduction zone the reduction in the shear wavespeed anomaly is just as dramatic but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab the thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly the various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone

nature of the crust beneath northwest basin and range province from teleseismic receiver function data received 30 july 2007 revised 12 april 2008 accepted 2 july 2008 published 14 october 2008 1 we utilized teleseismic receiver function techniques complemented by active source refraction seismology to study the crustal structure and continental rift processes responsible for the development of the northwest corner of the basin and range province in northwest nevada our passive seismic array of 28 short period stations spanning 70 km west to east and 5 broadband usarray transportable array stations that extended our aperture to 230 km provided data on crustal properties independent of the results of our active source refraction experiment combining data from the two experiments provides better constraints on moho depth vp vs ratio and the structure and composition of the crust beneath the northwestern basin and range province than possible from either experiment alone our new data indicate moho depths that vary from 29 5 on the east to 36 5 km in the west along the passive source array and vp vs ratios that systematically span a wide range from 1 68 to 1 83 velocity modeling of data collected from the 300 km wide angle refraction reflection survey provides comparable moho depths of 32 37 km under the same region decreasing vp vs is correlated with decreasing crustal thickness with vp vs ratio 1 80 in northeast california and vp vs 1 74 in the thinner crust of the basin and range province we associate the high vp vs of northeast california with mafic additions to the crust during formation of the modoc plateau in a region of accreted paleozoic island arc terrans similar to those exposed in the eastern klamaths and northwestern sierra nevada unusually low vp vs in conjunction with low absolute vp and vs is observed beneath the central part of our array and may be due to the roots of the sierra nevada batholith now dismembered by basin and range faults back azimuthal stacking of our receiver function data suggests a northwest oriented crustal anisotropy fast direction beneath this region parallel to the northwest to southeast oriented cenozoic extension experienced by the basin and range province and suggestive of a similar flow direction in the middle to lower crust

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

lava dome growth and explosive volcanism in the jemez mountains new mexico evidence from the plio pleistocene puye alluvial fan abstract the plio pleistocene puye formation north central new mexico is a 200 km2 volcanogenic alluvial fan shed eastward from the tschicoma volcanic center part of the jemez mountains volcanic field the fan contains 15km3 of volcaniclastic material derived from closely spaced lava domes in the northeastern portion of the tschicoma center interbedded in the fan sediments are at least 25 primary pyroclastic units from explosive eruptions of dacitic and rhyolitic lava domes tephra occur mainly as pumice falls but include several pumiceous ignimbrites and two thick proximal block and ash pyroclastic flows the upper part of the fan also contains rhyolitic plinian deposits erupted from sources in the central portion of the jemez field and basaltic ash derived from the central rio grande rift fanglomeratic pyroclastic and epiclastic facies exhibit considerable lateral variation primary tephra deposits however provide a stratigraphic framework for reconstruction of the growth of individual lava domes while only volcanic domes and lava flows are exposed in the tschicoma center puye tephra layers show that vulcanian subplinian and plinian activity and block and ash pyroclastic flows accompanied many dome forming events degradation of these lava domes produced coarse grained debris flows dominated by lava and dome carapace clasts from these epiclasts it is possible to identify lithologies of lavas that are either currently buried or have been obliterated by erosion or by collapse of the valles calderas the preservation of puye deposits reflects very high rates of aggradation and gradual down faulting in the espanola basin of the central rio grande rift this record of pyroclastic and epiclastic deposition allows detailed interpretation of the evolution of a volcanic center from its pyroclastic and erosion products and is the first example of a volcanogenic alluvial fan in an intracontinental rift setting to be described in detail this study emphasizes that interpretation of any ancient volcanic terrain based solely upon data derived from mapping radiometric dating and chemical analysis of lavas or existing denuded edifices is likely to greatly underestimate both the role of explosive volcanism in the build up of volcanic centers and subsequent estimates of the volumes of material produced