crossing the several scales of strain accomplishing mechanisms in the hinterland of the central andean fold thrust belt bolivia depictions of structures at outcrop regional and tectonic scales enforce horizontal shortening and vertical thickening as the predominant style of deformation at all scales within the hinterland of the central andean fold thrust belt outcrop scale structures document a progression of strain that created 1 flexural slip folds 2 fold flattening via axial planar cleavage 3 vertical stretching via boudinage and late stage faulting and finally 4 kink folding these examples of intraformational deformation are generally concentrated just beyond the tip lines of thrust faults where fault propagation folds and related structures are well developed fault propagation folding accommodated the accrual of strain indicated by outcrop scale structures while the structures themselves indicate how deformation developed within each individual fold fault propagation fold geometries at a regional scale emerge from the construction of regional balanced cross sections the sections were drawn with careful attention to 1 known map relationships 2 field inspection of key contacts 3 bedding orientation data 4 local seismic control and 5 principles of balance the pervasive ese wnw shortening and vertical elongation seen at the outcrop and regional scales developed during the formation of the central andean backthrust belt in the hinterland of the andes the central andean backthrust belt is a large scale west vergent thrust system along the western side of the eastern cordillera in the generally east vergent andean fold thrust belt of bolivia strain accrual within this west verging zone of deformation is proposed to be a taper building mechanism that allowed the fold thrust belt to continue propagating eastward

fold reactivation involving angular unconformable sequences theoretical analysis and natural examples from the cantabrian zone northwest spain abstract during the reactivation of folds truncated by unconformable layers the unconformity surface adopts various geometrical shapes depending on the fold reactivation mechanisms the paleorelief slope is also a determining factor for location and development of cover structures it can be theoretically shown that the reactivation of flexural flow and flexural slip folds below an unconformity produces folds of the same sense of curvature in layers above and below the unconformity viewed in profile that is to say these mechanisms give rise to a cover synform over a basement synform or antiform over antiform when the axes of both folds are subhorizontal folds reactivated by tangential longitudinal strain also behave in the same way in this case the cover folds may become tight even for initially high angles of unconformity which does not happen with flexural mechanisms the reactivation of chevron folds may give rise to opposite curvatures of basement and cover beds hence paradoxical structures such as a cover synform over a basement antiform or viceversa may be produced even if their axes are horizontal when two adjacent parts of the basement sequence are deformed by different folding mechanisms cover beds and unconformity are folded over the homoclinal sequence of each limb in the basement fold during fold reactivation the unconformity surface usually becomes a surface of decollement owing to the structures that have to be developed in the cover in order to accommodate the changes in length of the unconformity the development of extensional or compressional structures in the cover can be predicted from a geometrical study of the unconformity surface natural examples of reactivated folds under unconformable sequences of carboniferous age in the cantabrian zone show a good agreement with the above mentioned theoretical conclusions conclusions obtained on the distribution of folding mechanisms in the limbs and hinge zones and in layers or multilayers of different nature from the geometrical study of folded unconformable sequences in the cantabrian zone also agree with results from published experimental data

acid volcanic glass and pumice in lower miocene sediments of the northern pacific sse of hawaii abstract acid volcanic glass shards and pumice with sio 2 contents of about 72 2 were found in early miocene pure radiolarian ooze at site va 13 1 sse of hawaii between clarion and clipperton fracture zone an origin from northern central america is suggested

double evaporitic decollements influence of pinch out overlapping in experimental thrust wedges abstract this article focuses on how deformation and displacements are transferred between two decollements located at different stratigraphic levels by means of analogue modeling using brittle viscous sand silicone systems we present results from ten analogue models in which we varied key parameters such as the amount of horizontal offset or overlap between the pinch outs of the upper and lower decollements the total amount of shortening and the planform geometry of the upper decollement results indicate that i structures root onto the basal and upper decollement defining an inner and an outer domain and ii the offset overlap of the decollements controls the geometry of the transition zone located between the two decollements the propagation of deformation into the foreland both in space and time and the deformation style and kinematics in the different domains of the model when the pinch out of the upper decollement is at an angle with the backstop oblique structures form and the geometry and propagation mode of the structures change progressively along strike we compare our experimental results with other silicone sand analogue models and with the natural examples of the southern pyrenees where upper triassic and eocene oligocene syn tectonic evaporites acted as basal and upper decollements respectively

the derwent hunter submarine volcano the product of a long defunct subduction zone abstract the derwent hunter volcano 31 s 156 e is one of a 1400 km long chain of oligocene miocene undersea volcanoes that transect the tasman abyssal plain it developed some 15 16 ma ago and the rocks it contains form an abbreviated differentiation sequence that range in composition from olivine tholeiite to either mugearite or basaltic andesite these rocks generally contain normative hypersthene but do not contain significant quantities of either normative quartz or normative nepheline and their trace element abundances generally lie in the area of overlap between the subalkali and alkali rock series the more primitive rocks contain spinels 36 cr 2 o 3 olivines fo 81 83 plagioclases an 63 70 and less commonly augite set in a glassy mesostasis some specimens contain xenocrysts of anorthitic plagioclase an 85 94 and calcic salite en 36 fs 12 wo 52 the volcano is part of the tasmantid volcanic chain and the latter is one of a number of parallel volcanic chains of oligocene miocene age that arise in eastern australia and the tasman sea both the onshore and offshore volcanoes contain rocks that belong to a transitional petrographic series that comprises tholeiitic transalkalic and alkalic members with one exception the ages of the volcanoes across the various chains are similar for the same latitudes the exception is the easternmost chain i e lord howe where the ages of the individual volcanoes lag behind their counterparts at similar latitudes when considering the origin of these parallel volcanic chains the fundamental problems are 1 a what processes occurred within the asthenosphere to create a volcanic focus that extends for 1000 km in an east west direction and has been active for circa 40 ma 2 b why are the ages of the lord howe seamounts not synchronous with the other volcanic chains 3 c what processes and materials are required to generate the geochemically diverse and complex magmas needed to construct these intraplate volcanoes it is proposed that these magmas evolved from a line of diapirs within the asphenosphere the diapirs began forming at a depth of 650 km as the result of the heating and partial melting of pressure transformed materials that had originally been subducted after traversing the asthenosphere individual diapirs impinged on the base of the lithosphere where they reactivated pre existing fractures and produced extensional rock failure this resulted in the extraction of batches of magma from the diapirs and the composition of each batch was substantially controlled by the degree of partial melting and the ambient pressure at the source of magma extraction intratelluric fractional crystallization limited assimilation of crustal materials high level magmatic differentiation and the mixing of different batches of magma all assisted in the evolution of the more differentiated magmas the diapirs developed along a trend parallel to the long axis of a long defunct subduction zone it is proposed that this subduction zone was linear along most of its length but in the east it terminated in a section that was displaced northwards by a transform fault and the materials subducted by this displaced segment triggered the string of diapirs that eventually produced the lord howe seamount chain

intraplate deformation and basin formation during the tertiary within the northern iberian plate origin and evolution of the almazan basin the tertiary almazan basin was a topographic high during the mesozoic rifting at the northern part of the iberian plate it was bordered by two mesozoic basins 1 the castilian basin with a maximum thickness of more than 2000 m to the south and 2 the cameros basin with a maximum thickness of 9000 m to the north to the east the almazan basin is bordered by paleozoic rocks of the nw se trending aragonian range which underwent little subsidence during the mesozoic the two mesozoic basins show an e w direction during the tertiary nne ssw compression mainly late eocene early miocene both mesozoic basins were inverted and uplifted and the almazan basin with an overall synclinal geometry was filled with syn tectonic terrestrial sediments reaching a maximum thickness of 3500 m deformation during the tertiary was strongly controlled by preexisting structures both mesozoic extensional faults and reactivated variscan folds the depocenter of the tertiary basin migrated southward during the paleogene this migration is consistent with a piggyback transport of the whole basin northward due to the movement on the underlying crustal scale north verging cameros thrust the geometry and sedimentary evolution of the almazan thick skinned piggyback basin was controlled by the uplift associated with the main crustal scale thrust which was the main source area and was separated from the foreland basin since the early stages of filling late eocene this makes the main difference with thin skinned piggyback basins controlled by the foreland thrust belt and separated from the foreland basin only at their late stages

environmental problems and geological implications derived from evaporite dissolution in the barbastro salt anticline ne spain the halite bearing barbastro formation crops out in the core of the barbastro anticline ebro tertiary basin this anticline is traversed perpendicularly by some of the most important pyrenean drainages such as the cinca and noguera ribagorzana rivers the terrace sequences of these fluvial systems have been used as markers to identify and assess dissolution induced subsidence and salt tectonics in the limbs of the anticline terrace deposits underlain by detrital bedrock do not show any evidence of deformation and have a consistent thickness of less than 10 m the deposits of certain terrace levels of the noguera ribagorzana river and its tributary the lo reguer creek are locally thickened filling basins generated by dissolution induced synsedimentary subsidence up to several kilometers long and more than 100 m deep conversely terraces of the cinca river do not show anomalously high thicknesses but local uplifts related to differential upward flow of the halite bearing bedrock locally a minimum uplift rate of 0 3 mm year has been estimated from a 64 ka terrace tilted away from the valley the subsidence hazards occur chiefly in areas where the ground receives artificial water recharge serviceability of some canals has been notoriously affected by evaporite karstification the problem has been mitigated to acceptable levels by grouting numerous buildings of ivars de noguera are severely damaged by dissolution subsidence and possibly by hydrocompaction of gypsiferous silts the pipe network has been replaced to ameliorate the subsidence risk in the cinca river valley cavities with a total volume of about 180 500 m3 have been created by solution mining at depths greater than 500 m no investigation methods are applied in the brine field to monitor the distribution and evolution of artificial voids substantial increase in salinity of the cinca river is another evidence of subjacent evaporite dissolution

multidisciplinary approach to constrain kinematics of fault zones at shallow depths a case study from the cameros demanda thrust north spain thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts e g mylonites sheath folds shear bands in this paper a combination of techniques including structural analysis magnetic methods as anisotropy of magnetic susceptibility and paleomagnetism and paleothermometry is used to define thrusting conditions deformation and transport directions in the cameros demanda thrust north spain three outcrops were analyzed along this intraplate large scale major structure having 150 km of outcropping length 30 km of maximum horizontal displacement and 5 km of vertical throw results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis mixed layer illite smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust kinematic indicators foliation s c structures and slickenside striations indicate altogether a dominant nnw movement of the hanging wall in the western zone and ne in the eastern zone of the thrust thus implying strain partitioning between different branches of the main thrust the study of ams in fault rocks nearly 400 samples of fault gouge breccia and microbreccia indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max k int plane in sites with strong shear deformation paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting in spite of the shallow depth for deformation and a strong deformation or scattering of the magnetic remanence vectors in the fault zone the application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels

the mineralogy and chemistry of the anorogenic tertiary silicic volcanics of s e queensland and n e new south wales australia the late oligocene early miocene volcanism of this region is chemically strongly bimodal the mafic lavas volumetrically dominant comprise basalts hawaiites and tholeiitic andesites while the silicic eruptives are mainly comendites potassic trachytes and potassic high silica rhyolites the comendites and rhyolites have distinctive trace element abundance patterns notably the extreme depletions of sr ba mg mn p cr v and eu and the variable enrichment of such elements as rb zr pb nb zn u and th the trachytes exhibit these characteristics to lesser degrees the comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations e g lree nb y and especially zr and zn the phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases sodic plagioclase albite andesine calcic anorthoclase sanidine quartz ferroaugite ferrohedenbergite ferrohypersthene fayalitic olivine ilmenite titanomagnetite and rarely biotite near annite and fe hastingsitic amphibole accessories include apatite zircon chevkinite ferrohedenbergite bearing rhyolites only and allanite amphibole and botite rhyolites only the comendites generally contain ca poor anorthoclase sanidine quartz fluorarfvedsonite aegirine and aegirine augite zr bearing aenigmatite and ilmenite coexisting fe ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes where present they indicate equilibration temperatures of 885 980 c and fo2 between qfm and wm buffers the magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration which is also consistent with the general paucity of pyroclastics the rarity of hydrous mineral phases and the extreme fe enriched ferromagnesian phenocryst compositions the chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas and the comendites trachytes and certain trachyte rhyolite series are considered to have evolved from a mafic parentage the available oxygen sr and pb isotopic data however point to some modification of the magnas through crustal equilibration processes the remaining high silica rhyolites are considered to most likely represent crustal partial melts but are again modified by extensive fractionation

inverted turtle salt anticlines in the eastern basque cantabrian basin spain abstract halokinetic structures are sometimes masked by subsequent compression in inverted salt related basins making their study difficult the eastern basque cantabrian basin is an outstanding area to study salt tectonics and sedimentation relationships because precursor salt structures have maintained undeformed geometries after the alpine contraction phase based on the interpretation of subsurface data with surface geology a structural model of some of the area s salt structures is proposed according to the model the formation of listric faults and raft tectonics during the first extensional rifting stage resulted in the sinking and collapse of the late jurassic early cretaceous formations giving rise to a turtle back anticline structure with a semicircular geometry the upper triassic keuper salt migrated towards the outer zones of the structure and peripherial troughs were subsequently filled with albian late cretaceous post rift sediments during the eocene alpine contraction the whole area was folded and moved southwards maintaining much of the geometry of the previously formed salt anticlinal structures traditionally the presence of an nne ssw alignment of triassic diapirs in the eastern zone of the basin has been used to place the southern segment of the so called pamplona fault rather than being associated with fault these diapirs are interpreted to rise along the eastern edge of the collapsed extensional turtle structure existing below the eocene urbasa andia synclines no evidence of fault affecting the mesozoic cover series is observed in several composite profiles that cross the pamplona fault alignment it is suggested that the alignement of the triassic diapirs is due to the incidence of the later alpine compression in a previous expulsion rollover rather than the effect of a major alpine strike slip fault

quaternary reactivation of flexural slip folds by diapiric activity example from the western ebro basin spain in the western sector of the ebro basin two types of structures deform the quaternary terraces and pediments developed by the ebro river and its tributaries 1 folds up to 10 km long in the lower levels of fluvial terraces and 2 normal listric faults that produce tilting and rollover anticlines of the quaternary deposits both types of structures are linked to the geometrical and lithological features of the tertiary beds that underlie the quaternary deposits quaternary folds whose axes are parallel to the strike of the tertiary beds are the result of reactivation of tertiary large scale 60 km long folds due to diapirism of their gypsum cores where the gypsum units reach a maximum thickness of 3 000 m this reactivation produced flexural slip in some beds on the limbs of the folds bringing about the folding of the overlying quaternary terraces the mechanism of quaternary folding involves layer parallel shear in alternating tertiary units and folding linked to detachments and reverse layer parallel faults normal listric faults are widespread throughout the studied area they are partly parallel to low dipping tertiary beds and are the result of a nne ssw tectonic extension compatible with minor structures and focal mechanisms of recent earthquakes the relationship between the two kinds of quaternary structures indicates that diapirism of the gypsum cores of the anticlines was activated by extensional tectonics

the role of fluid pressure in the formation of bedding parallel thrust hosted gold deposits sabie pilgrim s rest goldfield eastern transvaal abstract shallowly dipping thrust hosted gold veins occur within the paleoproterozoic transvaal supergroup rocks in the eastern transvaal of south africa internally the veins record episodic events of fluid infiltration and subsequent mineral deposition within discrete local scale thrust faults evidence also exists for subvertical dilation within the individual veins and textures which record open space filling are preserved the probable depth of formation suggests lithostatic pressures in excess of 2 2 kbar and the vein textures indicate numerous reactivation events associated with individual thrust faults occurring in a terrain characterised by low levels of preserved strain fault reactivation may have occurred under conditions of low differential stress in the presence of high pressure fluids with p fluid approaching or exceeding lithostatic levels

karst in gypsum and its environmental impact on the middle ebro basin spain the middle ebro basin in characterised by strong evaporitic profiles from the miocene period and in which gypsum formations are predominant these groups of easy solubility produce in many areas typically karstic landforms although there are certain features specific to them the generated landforms appear on the surface as different morphological types of doline which very often develop on the alluvium deposited by the river ebro and its affluents on the agricultural land of the penaflor villamayor area the functional character of karst causes collapse dolines to appear after heavy storms and these are constantly filled in by farmers motorways and roads that cross these areas as also the industrial estates existing on the outskirts of zaragoza are affected by dissolution processes that bring about continual collapses the infiltration of water from unlined canals causes the generation of numerous dolines along canal banks

evidence for mesoarchean 3 2ga rifting of the pilbara craton the missing link in an early precambrian wilson cycle abstract a sequence of c 3 2 ga low grade sedimentary rocks bimodal volcanic rocks and subvolcanic layered mafic ultramafic sills and dykes occur in greenstone belts that flank the margins of the 3 51 3 24 ga east pilbara terrane of the pilbara craton western australia u pb shrimp zircon data and sm nd model age data suggest that these rocks represent juvenile addition of material to the margins of the craton during a period of major extension accompanied by the local emplacement of granitic rocks this extensional event is also represented in adjacent terranes by thick basalt flows with c 3 2 ga zircon and sm nd model ages and by granitic rocks based on the available age data geochemistry of basaltic rocks and the distribution of this event in space and time within the craton we interpret the c 3 2 ga rocks to reflect widespread rifting of the protocraton and associated thinning of the lithosphere this event resulted in separation of the flanking kurrana and karratha terranes from their parent east pilbara terrane which represents the ancient nucleus of the craton these terranes were then reassembled during subsequent accretion at 3 07 2 905 ga combined the data suggest the preservation of a mesoarchean wilson cycle in the pilbara craton

diapiric deformations in the quaternary deposits of the central ebro basin spain from a study of 24 outcrops in neogene and quaternary deposits of the central ebro basin a number of diapiric deformations have been recognized two principal types being differentiated domal or pillow structures and piercement or intrusive structures the former are incipient diapirs of gypsum piercing structures have reverse faulted contacts not caused by halokinesis here neogene marls are the active plastic material contrasting with the competent behaviour of gypsum these intrusive structures are viscous diapirs which easily pierce the non consolidated low strength quaternary gravel overburden and submit it to a horizontal compressive stress as a consequence reverse faults and flexures develop in it generally normal faults and tension cracks do not appear underlying gypsum beds are frequently pulled up into diapirs and they constitute the structural core density contrast and conditions for plastic flow exist at the marl gravel boundary it seems to have been specially common at the time of quaternary fluvial sedimentation so that much deformation is synsedimentary diapiric phenomena have been very active during early to middle pleistocene time becoming weaker afterwards

magnetobiochronology of lower miocene ramblian continental sediments from the tudela formation western ebro basin spain abstract the calibration of european lower miocene continental stages is poorly constrained due to the lack of robust magnetostratigraphic data from continental sediments of this age in this paper we present the first magnetostratigraphic results from the mammal fossil bearing continental sediments of the lower middle miocene tudela formation western ebro basin spain the study is based on 331 paleomagnetic sites distributed along a 790 m thick composite section the resulting magnetostratigraphy provides an independent correlation to the geomagnetic polarity time scale and allows accurate dating of previously known and new ramblian upper lower miocene fossil localities of the tudela formation the new age constraints derived for the tudela formation fossil localities provide together with a revaluation of other iberian faunas the first magnetochronology for the ramblian continental stage the lower boundary of the ramblian is placed at the lower part of chron c6r ca 20 4 ma and the boundary between the lower and upper ramblian is located at the middle of chron c6n ca 19 6 ma the upper boundary of the ramblian and hence the base of the aragonian is placed at the upper middle part of chron c5cr 16 8 17 ma our results also provide the first magnetostratigraphically dated mn3 faunas in spain which together with a revaluation of other iberian and european faunas provide a revised calibration of european mammal zone mn3 the lower boundary of zone mn3 is placed at the boundary between chrons c6r and c6n ca 20 1 ma and its upper boundary is located at the upper middle part of chron c5cr 16 8 17 ma

hawaiian derived volcanic ash layers in equatorial northeastern pacific sediments abstract volcanic ash layers with thicknesses of up to 30 cm and ages between 1 and 2 m y were found in equatorial northeastern pacific sediments grain size distributions mineralogical composition and alteration of the ashes are described chemical analyses of major trace and rare earth elements and trace element correlations indicate a basaltic composition of the ocean island tholeiitic type comparison with hawaiian tholeiitic rocks indicates the island of maui as the source of material for the ash sequence in the investigated area the composition of an ash layer north of the area indicates it was associated with volcanism of mauna loa or kilauea

thermochronology of the needle falls shear zone a post collisional high strain zone of the trans hudson orogen the needle falls shear zone is a major paleoproterozoic discontinuity separating an archean craton from accreted paleoproterozoic terranes in the western trans hudson orogen it is a 1 km wide subvertical mylonite zone forming the southwestern sector of a northeast trending shear system extending for more than 400 km kinematic data indicate oblique east side up dextral strike slip displacement throughout its evolution from ductile through late brittle deformation for most of its exposed length it marks the northwestern margin of the wathaman batholith the oldest pb evaporation ages of zircons from mylonite average 1857 ma consistent with published zircon ages of the wathaman batholith confirming the batholith is protolith for much of the shear zone the synkinematic metamorphic assemblage of amphibole biotite quartz feldspar signifies deformation under amphibolite facies conditions 500 630 c oxygen isotope fractionations between quartz hornblende and quartz biotite pairs in textural equilibrium

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

a reevaluation of the position of the baie verte brompton line in the quebec appalachians the influence of middle devonian strike slip faulting in gaspe peninsula the baie verte brompton line bbl the surface expression of the taconian suture in the canadian appalachian orogen extends from southern quebec to the northeast end of newfoundland in the que

geomorphic and stratigraphic evidence of incision induced halokinetic uplift and dissolution subsidence in transverse drainages crossing the evaporite cored barbastro balaguer anticline ebro basin ne spain abstract the evaporite cored barbastro balaguer anticline located in the northeastern sector of the ebro tertiary basin ne spain is traversed perpendicularly by several drainages coming from the pyrenees including the cinca noguera ribagorzana and segre rivers the terraces associated with these discordant fluvial systems deposited upon evaporitic and detrital formations across the anticline allow the analysis of geomorphic and stratigraphic anomalies related to the soluble and ductile nature of the halite bearing rocks in the core of the anticline geomorphological mapping reveals that the gravels discontinuously capping the evaporitic core of the anticline together with some alluvial mantles disconnected from the present day river valleys constitute the oldest sediments deposited under exorheic conditions in this sector of the ebro tertiary basin these alluvial deposits inset into the basin fill record an early post capture palaeogeographic stage during which the drainage was dominated by unconfined distributary channels developed on extensive alluvial fans fed by perennial rivers coming from the pyrenees the terraces associated with the present day fluvial valleys correspond to a later palaeogeographic stage characterized by a well integrated drainage confined to fluvial valleys entrenched in the basin fill these terraces show evidence of both dissolution induced subsidence and halokinesis restricted to areas underlain by the evaporites of the anticline core the deposits of the highest terrace levels of the noguera ribagorzana and segre rivers and its tributaries lo reguer creek and farfanya river are locally thickened filling basins generated by dissolution induced synsedimentary subsidence up to several kilometers long and more than 100 m deep subsidence caused by the karstification of the evaporites reached higher magnitude and extent during the early stages of development of the new external drainage network when the more soluble halite bearing units were closer to the new incisional base level most likely the development of a thick caprock rich in less soluble sediments has hampered dissolution and subsidence processes in subsequent stages incision induced halokinetic deformation is locally recorded by differentially uplifted terraces dipping away from the valley the increasing differential loading caused by the entrenchment of the fluvial systems has driven the lateral and upward flow of salt rich evaporites from the valley margins towards the valley bottom a minimum long term uplift rate of 0 3 mm yr has been calculated for a titled osl dated upper pleistocene terrace of the cinca river valley the capture of the ebro basin and its change to exorheic conditions have played an instrumental role in the development of the quaternary gravitational deformations associated with the evaporitic bedrock the new incisional drainage network propitiated the karstification and evacuation of large amounts of evaporites and its entrenchment has created adequate differential loading conditions for the salt to flow towards the valley floors

coarse grained volcanic detritus in deep sea sediments of the northeastern equatorial pacific abstract coarse grained volcanic detritus in near surface sediments of the northeastern equatorial pacific consists mainly of feldspar plagioclase with an35 70or0 4 and magnetite with minor parts of pyroxene augite wo25 32en53 32 and volcanic quartz except for plagioclase 0 005 0 5 of total sediment all concentrations range between 0 and 30 grains m 1 of piston core where differences exist within the stratigraphic sequence and also from mineral to mineral the following relationships have been found plagioclase magnetite volcanic quartz pyroxene contents are low in plio quaternary sediments and in lower miocene pure radiolarian ooze they increase within the late eocene to early oligocene sediments and near the early oligocene hiatus the morphology distribution and alteration of the minerals are described and singlegrain analyses for plagioclase and pyroxene are given the origin of plagioclase pyroxene and magnetite is related to the formation of new oceanic crust at the east pacific ridge transport and distribution by slumping high density currents and reworking of older sediments are assumed the origin of volcanic quartz from potential source areas in northern central america and western mexico and transport and accumulation in deepsea sediments are discussed

gypsum karst evolution in a diapir a case study pinoso alicante spain gypsum deposits are highly soluble and can undergo rapid weathering especially where water intervenes though certain human activities can accelerate the karstification processes an example of karst formed in gypsum at pinoso alicante spain is analysed to show how human activity has impacted on its evolution the karst here is associated with a huge diapir in the triassic terrain that boasts of a large number of highly unusual exokarstic landforms here rock salt has been extracted commercially for over 25 years using solution mining via deep boreholes the brine extracted contains in excess of 300 g l over the whole period more than 12 million tonnes of salt have been extracted using this method after preliminary treatment to remove impurities it travels along a brine pipeline to the coast the enormous volume of the voids generated have probably favoured the acceleration of certain exokarstic and endokarstic processes

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

ash layers and pumice in the central indian basin relationship to the formation of manganese nodules abstract manganese nodules in areas of biosiliceous sedimentation in the central indian basin cib have three main types of nuclei 1 fragments of altered ash layer 2 felsic pumice and 3 indurated residual sediments red clay type morphological variations among nodules are due to the form of the nuclei the type of accretionary process diagenetic or hydrogenetic accretion and thickness of the crust piston cores in the same areas sampled up to four tephra layers interstratified in red clay all of which underlie condensed series of pliocene to recent siliceous oozes the in situ tephra layers and fragments of tephra in the core of the nodules are similar in composition they are completely altered to smectite and smectite zeolite tephra horizons are 10 35 mm thick and have sharp contacts with the red clay rare earth element ree patterns zr hf and nb content and ti al ratios strongly suggest that this altered volcaniclastic material originated as distal fallout from the indonesian volcanic arc iva and was deposited in late neogene time positive ce ce anomalies indicate that alteration of this tephra may have occurred at the sediment water interface prior to burial under hydrogenous conditions rhyolite pumicelapilli are water laid and are variably replaced by fe mn oxides inducing a higher metal content in the bulk nodules pumice constitutes one of the main types of nuclei of diagenetic nodules that are found in the deep sea troughs and plains where biosiliceous deposits are thick petrological characteristics of the pumice suggest that volcanic eruptions from toba in northern sumatra are the most likely source tectonic deformation of the indian plate may have affected the sedimentary sequence causing uplift and outcrop of ash layers on the topographic highs and slopes eroded fragments of these ash layers and waterlaid pumice fragments have contributed significantly to the large extent of mn nodules in the cib

paleomagnetic structural and stratigraphic constraints on transverse fault kinematics during basin inversion the pamplona fault pyrenees north spain 1 the pamplona fault in the pyrenees is a major transverse structure that has been classically interpreted as a strike slip fault however lack of consensus concerning the sense of movement casts doubt on its actual kinematics and as a consequence its role in the cenozoic evolution of the pyrenees remains controversial in order to assess its kinematics we have conducted a paleomagnetic structural and stratigraphic study focused on the mesozoic and tertiary sedimentary rocks that outcrop around the southern segment of the fault restoration of balanced cross sections allows us to examine the present day spatial relationship of the sedimentary sequences on both sides of the fault and to reconstruct the geometry of the extensional basins formed during mesozoic rifting episodes in the bay of biscay and pyrenean domains paleomagnetic results indicate that no significant tectonic rotations occurred around the fault during tertiary inversion of the pyrenees the lack of tectonic rotations and revaluation of previous hypotheses argues against a strike slip movement of the fault we propose a new model in which the pamplona fault is treated as a large scale hanging wall drop fault whose kinematics was determined by variations in the geometry and thickness of mesozoic sequences on both sides of the fault these variations influenced the geometry of the thrust sheet developed during tertiary compression we are unaware of any other transverse fault that has been interpreted in this fashion thus the pamplona fault serves as a case study for the evolution of transverse faults involved in basin inversion processes

geomorphology of the tertiary gypsum formations in the ebro depression spain abstract this paper reviews the current knowledge of the mainly karstic geomorphological features developed in the evaporitic formations of the ebro depression northern spain special emphasis is given to the recently published and unpublished scientific advances the gypsum formations of tertiary age have an extensive outcrop area within the ebro depression here their morphogenesis is controlled mainly by processes of surface and subsurface dissolution acting on the gypsum outstanding landforms in the gypsum terrain include saline lakes developed in flat bottom dolines saladas other characteristic morphologies include karren and gypsum domes which occur on a decimetre scale where the gypsum is covered by quaternary alluvial deposits the karstification processes are especially intense and cause subsidence phenomena karstic subsidence affects stream terraces mantled pediments and infilled valleys which in the region are called vales dissolution induced synsedimentary subsidence has produced interesting geological features which include significant thickening and deformation of the alluvial deposits in contrast to the rapid removal of gypsum by dissolution the amount of gypsum removed by erosion is low water erosion studies carried out on gypsiferous slopes of the ebro depression indicate that the sediment yield ranges from 0 59 to 7 82 t ha year this low yield results from the high infiltration capacity of the soils subsidence caused by gypsum dissolution has important socio economic consequences in the ebro depression the active alluvial karstification of the gypsum causes numerous sinkholes that are harmful to linear structures roads railway lines irrigation channels buildings and agricultural land unforeseen catastrophic subsidence also puts human lives at risk the benefits of such terrains include thickened alluvial deposits which act as valuable water reservoirs and which form excellent sources of aggregates fluvial valleys in this gypsiferous terrain commonly show an asymmetrical geometry with prominent gypsum scarps at one side these gypsum scarps are affected by numerous landslides these slope movements are hazardous may dam rivers and cause flooding of the alluvial plains

mantle dynamics and basalt petrogenesis abstract differentiation at mid ocean ridges generates a layered lithosphere consisting of a basaltic crust immediately underlain by harzburgite and further underlain by pyrolite which has experienced depletion only of highly incompatible elements the body forces driving subduction are concentrated mainly in the upper half of the lithosphere which is relatively cool and brittle during subduction the lower layer of relatively ductile slightly depleted pyrolite is stripped off and resorbed into the upper mantle thereby providing a future source region for morb magmas the slab which sinks to 600 km is comprised mainly of differentiated former basalt and harzburgite which undergo a different series of phase transformations to those experienced by mantle pyrolite in consequence the former basaltic crust remains denser than surrounding mantle whereas former harzburgite becomes relatively buoyant below the 650 km seismic discontinuity the resulting non uniformity in stress distribution causes the slab to buckle at this depth and accrete to form a large relatively cool ovoid megalith of mixed former harzburgite and basaltic crust heating of the megalith occurs over 1 2 b y leading to partial melting of the former basaltic crust the resultant liquids contaminate regions of former harzburgite rendering them fertile in the sense of future capacity to produce basaltic magmas after thermal equilibration the newly fertile former harzburgite becomes buoyant leading to the separation of diapirs which rise into the upper mantle such diapirs rising beneath sub oceanic lithosphere experience small degrees of partial melting to produce ocean island basalts mainly of the alkaline suite diapirs of fertile former harzburgite rising beneath continents become incorporated into the sub continental lithosphere this is a cumulative process and is ultimately responsible for the development of the chemical physical and isotopic characteristics of the sub continental lithosphere including its capacity to produce alkaline magmas when subjected to small degrees of partial melting

from carbonate sulphate interbeds to carbonate breccias the role of tectonic deformation and diagenetic processes cameros basin lower cretaceous n spain interpreting the origin of carbonate breccias requires a detailed analysis because they may be the result of a wide variety of processes that produce similar features this is the case of the lower cretaceous carbonate breccias of the cameros basin previously interpreted as slump or collapse breccias whose origin is interpreted after performing a detailed sedimentary petrographic and tectonic study the studied carbonate breccias consist of angular carbonate mudstone fragments floating in a matrix of calcite and quartz crystals the breccias are interbedded with and laterally associated to alternating layers of carbonate mudstone and calcite and quartz pseudomorphs after gypsum which show strong similirities to the breccia fragments and matrix respectively suggesting that the brecciated beds were originally composed of identical alternating carbonate mudstone and gypsum layers as the unbrecciated layers the breccias are associated with frequently polyharmonic deformation structures which are similarly oriented as the regional tectonic structures indicating that they are related with the alpine contractional deformation of this area of the cameros basin all these features suggest that the carbonate breccias were formed by tectonic deformation of alternating layers of carbonate mudstone and calcium sulphate which have very different rheological behaviours as a result during tectonic deformation sulphate flowed and carbonate layers were broken and displaced producing a breccia of carbonate fragments within a sulphate groundmass afterwards the sulphate groundmass was replaced by quartz and calcite and the breccia acquired its final composition

active flexural slip faulting controls exerted by stratigraphy geometry and fold kinematics flexural slip plays an important role in accommodating fold growth and its topographic expression flexural slip fault fsf scarps may be one of the most commonly occurring secondary structures in areas dominated by active thrusts and folds where fsf scarps are present and what factors control their occurrence however are typically poorly known through an investigation of clearly expressed fsf scarps well preserved fluvial terraces and well exposed bedrock at eight sites in the pamir tian shan convergent zone and kuche fold belt nw china we summarize the most favorable conditions for active flexural slip faulting our study yields six key results first flexural slip operates commonly in well layered beds although uncommonly can occur in massive poorly layered beds as well second in well layered beds the slip surface is commonly located either a close to the contact of competent and incompetent beds or b within thin incompetent beds third fsf scarps are always found overlying steep beds with dips of 30 100 fourth slip surfaces are typically spaced between 10 and 440 m but can reach up to 600 m fifth fsf scarps at most sites can be observed far away from the hinge migrated fold scarps suggesting that compared to hinge migration limb rotation is generally required to accumulate flexural slip and produce associated topographic scarps finally a higher regional convergent rate seems to facilitate the creation of fsf scarps more often than lower rates whereas well preserved old terraces capped by thin deposits are more likely to record fsf scarps than unevenly preserved young terraces with thick sedimentary caps

growth of the central andean plateau by tectonic segmentation is controlled by the gradient in crustal shortening abstract crustal deformation orocline formation and growth of the central andean plateau as well as the mutual relations of these processes with climate are a matter of international debate earthquake focal mechanism fault kinematic paleomagnetic and cross section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model we offer such a model that approximates simple shear in plan view and that predicts the kinematics of first order fault zones in the southern central andes the model indicates that segmentation of the upper crust into rhomb shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the andes an anastomosing network of kinematically coupled reactivated orogen parallel and newly formed ne trending deformation zones encompassing components of sinistral and dextral strike slip res

can flexural slip faults related to evaporite dissolution generate hazardous earthquakes the case of the grand hogback monocline of west central colorado a paleoseismological investigation of flexural slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude the carbondale collapse center in the southern rocky mountains of colorado is a morpho structural depression of 1200 km 2 where miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite bearing evaporites on the western margin of the collapse center the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late laramide grand hogback monocline accompanied by displacement on bedding parallel faults these flexural slip faults rupture unconformable miocene basalts and quaternary mantled pediments generating conspicuous half graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline two trenches dug across flexural slip fault scarps developed in each stratigraphic marker basalt cap mantled pediment revealed unexpected evidence of multiple late quaternary faulting events e g faulted colluvial wedge sharp unconformities with displacement per event values of 1 m three faulting events were inferred from the trench dug in the pediment w around 6

gypsum karst features as evidence of diapiric processes in the betic cordillera southern spain in the triassic evaporite deposits of the betic cordillera halokinesis has played a fundamental role in the morphodynamic evolution of gypsum karst as evidenced by the morphology and distribution of dolines and poljes by data obtained from caves and from the types of water associated with evaporite domes distribution patterns of major exokarstic forms display a subcircular array with collapse sinkholes located in the centre of the domes and solution drawdown dolines on the outer limits of the diapiric forms in addition the least mineralized waters are situated in association with the caprock gypsum hypersaline waters which are also hotter drain deeper flow systems that flow out at the edges of the halokinetic structures the morphodynamic and hydrogeological information constitute a valuable tool for the correct interpretation of the tectonic structure of such complex areas

dike propagation and magma flow in a glassy rhyolite dike a structural and kinematic analysis exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems we use a 5 14 m thick flow banded rhyolite dike in arran scotland to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow based on mesoscale kinematic indicators the dike has concave inward dike margin segments with plumose like structures that record vertical and horizontal propagation of lobes which inflated and linked to form a through going sheet devitrified rhyolite zones at the dike margins show gentle to open folds in contrast glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal curvilinear folds and sheath folds that record sustained progressive deformation the inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions in the dike center fold axes plunge 27 ne along the dike and parallel to elongation lineations combined with shear sense indicators and objects sheared vesicles and asymmetric folds these features indicate that magma flow was obliquely upward to the southwest and locally 60 to the propagation direction of the dike the distribution of structures within the rhyolite indicates local accretion of the now devitrified material to the margins with localization of flow into the center of the dike we find that the initial magma flow direction was controlled by fracture propagation and interaction with the subsequent flow record controlled by accretion and flow localization in the conduit this study demonstrates that analysis of mesoscopic structural and kinematic features several of which have not previously been reported from dikes is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes

late cenozoic stress field and fracturing in the iberian chain and ebro basin spain detailed study of mesoscopic faults and joints affecting neogene and pleistocene deposits in the iberian chain and the ebro basin involving statistical study of fracture directions and palaeostress analysis from striated fault planes together with analysis of macrostructures suggest a complex late cenozoic stress field in which both primary and secondary stress systems are superposed 1 regional n s compression was caused by convergence between the iberian and african plates 2 multidirectional extension was caused by crustal doming in the iberian chain 3 stress trajectories were deflected near major faults 4 small scale swapping of the horizontal tension axes 2 and 3 was caused by development of fractures at right angles to either the primary or the deflected 3 axis

geomorphological and sedimentological features in quaternary fluvial systems affected by solution induced subsidence ebro basin ne spain abstract the quaternary evolution and the morpho sedimentary features of some of the most important rivers in spain ebro and tagus rivers among others have been controlled by subsidence due to alluvial karstification of the evaporitic bedrock the subsidence mechanism may range from catastrophic collapse to slow sagging of the alluvium by passive bending in the ebro basin the mechanisms and processes involved in karstic subsidence were studied through the analysis of present day closed depressions as well as through old subsidence depressions palaeocollapses and solution induced basins and associated deformations recorded in the quaternary alluvial sediments the gallego ebro river system is presented as a case study of channel adjustments and geomorphic and sedimentary evolution of fluvial systems in dissolution induced subsidence areas in this fluvial system evaporite dissolution during particular quaternary time intervals namely early and middle pleistocene have lead to the development of a solution induced basin approximately 30 km long by 8 km wide filled by quaternary deposits with a total thickness in excess of 190 m the main river response to balance the subsidence in the alluvial plain was aggradation in the central reach of the subsiding area and degradation both in the upstream reach and in the valley sides where alluvial fans and covered pediments may prograde over the fluvial sediments the main sinking areas are recognized in the sedimentary record by anomalous thickenings in the alluvial deposits and fine grained sediments deposited in backswamp and ponded areas