structural traverse across the sierra madre oriental fold thrust belt in east central mexico the exposed structural geometry of the sierra madre oriental foreland fold thrust belt is quantified for an 130 km long strip along the moctezuma river 21 n lat east central mexico the strip extends from the belt s leading edge in the east to its most internal continuous outcrops in the west where the cordilleran structures disappear beneath cenozoic volcanic rocks the style of deformation is controlled mainly by the geometry and lithology of two major cretaceous carbonate banks el doctor and valles san luis potosi the long dimensions of which parallel the cordilleran structural trend the exposed shortening is concentrated on the platform margins 5 km on the eastern edge of the el doctor bank and 10 to 12 km on either edge of the valles san luis potosi platform and is characterized by thrust and fold nappes the thrust faults cut across the platform margin carbonates on 20 inclined tectonic ramps that were partly steepened by imbrication or by subsequent folding but pass in the overlying mechanically weak rocks nearly parallel to bedding the interiors of the platforms appear relatively undeformed whereas the regions between the two platforms zimapan basin and between the eastern edge of the valles san luis potosi platform and the foreland are composed of flexural slip folds resulting from decoupling along several incompetent layers the exposed bulk shortening measures 41 4 km or 25 for the entire traverse retrodeformable sections of the subsurface structure down to basement result in a bulk shortening of 90 1 km or 40 the subsurface deformation model for the western part of the traverse is thin skinned and has geometry similar to that on the surface the model is very speculative in the eastern part where the areal shortening calculated for thin skinned models is much larger than is the exposed linear shortening balance was attained by inferring a blind thrust on an upper detachment in the late cretaceous which has a shortening of 20 to 30 km it could also be attained however by intrabasement decoupling in a depth of 10 to 20 km deformation is brittle in the eastern part of the belt east of the valles san luis potosi platform whereas igneous intrusion controlled higher ductility characterizes the western part of the foreland fold thrust belt

modification of a linear bar trough system by a standing edge wave abstract detailed fluid motions and bathymetric changes in the lacustrine nearshore zone at wymbolwood beach georgian bay canada were monitored during storms to test theoretical postulates about the importance of low frequency wave motions to nearshore circulation of water and sediments eighteen continuous resistance wave staffs and two electromagnetic current meters were deployed along one shore normal and two shore parallel instrument arrays over a period of two months during one intense storm a mode 3 standing edge wave at 0 035 hz was the predominant low frequency motion prior to the storm the bathymetry was characterized by linear bar trough systems and during the storm these features migrated onshore and became rhythmic in response to the length scales and relatively fixed locations of nodal and antinodal lines of the standing edge wave however a well defined crescentic bar as predicted by the model under the condition of zero net sediment flux divergence did not evolve because the hydrodynamic forcing ceased before an equilibrium condition could be attained nevertheless the location of scour and accretion features demonstrated the tendency towards such a crescentic form the offshore and alongshore irregularities of the post storm bathymetry are not readily explained by alternative hydrodynamic interactions

miocene chronostratigraphy paleogeography and play framework of the burgos basin southern gulf of mexico this study characterizes miocene chronostratigraphy and plays in the burgos basin and adjacent south texas within an area of approximately 39 700 km2 15 300 mi2 onshore and offshore to the 500 m 1640 ft isobath using greater than 40 000 linear kilometers 25 000 mi of two dimensional seismic lines 115 onshore wells 9 offshore wells and paleontological data we established a correlation framework of 9 key surfaces upper oligocene to lower pliocene representing major probably third order sequence boundaries and maximum flooding surfaces throughout the basin five of the burgos miocene surfaces coincide with regional chronostratigraphic surfaces from the veracruz and laguna madre tuxpan basins thus establishing a consistent correlation framework throughout much of the mexican gulf coast basin twenty miocene plays are defined by four age divisions lower miocene middle miocene upper miocene 1 and upper miocene 2 and four paleogeographic settings unexpanded and expanded shelf proximal slope and distal slope because of proven high productivity in salt bounded basins in the northern gulf of mexico the onlap of strata onto diapirs in the eastern burgos salt province was evaluated as a fifth setting the paleogeographic provinces and onlap areas exhibit characteristic seismic facies stratal geometries and structural relations a characterization of each one of these being key to the overall play evaluation this play framework provides the means for continuing exploration of miocene strata and evaluation of key play elements reservoir presence and quality seal trap source and migration and timing in this structurally complex underexplored basin the relative importance of these play elements varies systematically for each play especially between the onshore shelf plays and the offshore deep water plays where fault complexity and stratigraphic variability are greater

eastern turkish high plateau as a small turkic type orogen implications for post collisional crust forming processes in turkic type orogens abstract post collisional magmatism may be generated by extensive crustal melting in tibet type collisional environments or by falling out of slabs from under giant subduction accretion complexes in turkic type collisional orogens giving rise to decompression melting of the asthenospheric mantle replacing the removed oceanic lithosphere in turkic type post collisional magmatism the magmatic products are dominantly alkalic to peralkalic and greatly resemble those of extensional regions giving rise to much confusion especially in interpreting old collisional orogenic belts such magmatic regions are also host to a variety of economically valuable ore deposits including gold one place in the world where today active turkic type post collisional magmatism is present is the eastern anatolian high plateau produced after the terminal arabia eurasia collision in the late miocene the plateau is mostly underlain by the late cretaceous to oligocene east anatolian accretionary complex which formed south of the rhodope pontide magmatic arc this subduction accretion complex has been further shortening since the collision but it has also since been domed and became almost entirely covered by at least 15 000 km3 of volcanic rocks the volcanic rocks are calc alkalic in the north transitional in the middle and alkalic in the south of the plateau where the crust is thinnest today less than 38 km the volcanics are derived almost entirely from an enriched mantle the ages of the volcanics also become younger from north to south from about 11 ma to possibly 17th century ad we interpret the origin of the magmatic rocks as the result of decompression melting of the asthenospheric mantle sucked towards the exposed base of the east anatolian accretionary complex as the oceanic lithosphere beneath it fell out the lower density of the hot asthospheric material was the cause of the doming we believe that similar processes dominated the post collisional tectonics of such vast turkic type orogens as the altaids of central asia the late devonian early carboniferous lachlan belt in southeastern australia and the pan african collage of northeast afro arabia it is likely that archaean collisions were dominated by turkic type post collisional events rather than tibetan ones that only became common in the proterozoic

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

geometric and seismic interpretation of the perdido fold belt northwestern deep water gulf of mexico we interpret the perdido fold belt to be composed of detachment folds flanked by megascopic scale kink bands the two previous principal fold models for the perdido fold belt incorporated components of large scale faulting an imbricate fault bend fold model and a high angle reverse faulted detachment fold model we interpret the perdido fold belt to be fold dominated the belt consists of detachment folds overlying a ductile decollement layer the detachment fold crests are bounded by kink bands which are narrow zones of angularly folded strata the low reflectivity low signal to noise zones seen in seismic profiles across the perdido fold belt are poorly imaged sections of the folds that result from steeply dipping bedding in the kink bands the substantial width of these low reflectivity zones their subparallel edges in cross section and their orientation at a high angle to layering as well as their organized spacing and conjugate geometry support a kink band interpretation propagating reverse faults are unlikely in the folds because of the absence of backlimbs overlying the locations of hypothetical faults improved seismic imaging efforts have illuminated steeply dipping bedding that is not cut by faulting in many of the low reflectivity zones in the perdido fold belt

the enigma of the albian gap spatial variability and the competition between salt expulsion and extension the albian gap is a uniquely large up to 65 km wide and 450 km long enigmatic salt related structure in the santos basin offshore brazil it is located near the basin margin and trends ne i e subparallel to the brazilian coastline the gap is characterized by a near complete absence of albian strata above depleted aptian salt its most remarkable feature is an equivalently large equally enigmatic basinward dipping supra salt rollover that contains a post albian sedimentary succession that is up to 9 km thick owing to its unique geometry size and counter regional aspect the origin and evolution of the albian gap has been the centre of debate for 25 years this debate revolves around two competing models that is did it form as a result of thin skinned i e supra salt extension or progradational loading and salt expulsion the extension driven model states that the albian gap and overlying rollover formed as a result of post albian gravity driven extension accommodated by slip on a large counter regional listric normal fault the cabo frio fault conversely the expulsion driven hypothesis states that the albian gap was established earlier during the albian and that post albian deformation was controlled by differential loading vertical subsidence and basinward salt expulsion in the absence of significant lateral extension this study utilizes a large c 76 000 km2 dense 4 8 km line spacing depth migrated 2d seismic dataset that fully covers and thus permits for the first time a detailed quasi 3d structural analysis of the entire albian gap in this study we focus on 1 the evolution of base salt relief and the original salt thickness variations and 2 the geometry of the post albian rollover and its related faults and salt structures to constrain the kinematics of the albian gap and how this relates to the evolution of the base salt relief we also apply novel structural restoration workflows that incorporate flexural isostasy in addition to a detailed sequential reconstruction of the intra gap rollover sequences our results show that the geometry and kinematics of the albian gap vary along strike and that both post albian extension and expulsion play a significant role in its evolution basinward dipping growth wedges salt rollers and listric normal faults record extension whereas sigmoidal wedges halokinetic sequences and upturned near diapir flaps the latter two associated with large diapirs bounding the downdip edge of the gap record basinward salt expulsion and inflation where the albian gap is relatively wide 50 km these processes alternate and operate at approximately equal proportions our results are consistent with the amount of basinward translation inferred from the analysis of ramp syncline basins located downdip on the sao paulo plateau our results seemingly reconcile one of the longest running debates in salt tectonics as well as having more general implications for understanding the regional kinematics and dynamics of salt related structures in other salt basins in particular the controls on the development of large counter regional faults supplementary material uninterpreted versions of the seismic sections are available at https doi org 10 6084 m9 figshare c 5023088

swash oscillations on a natural beach run up swash oscillations were measured on a gently sloping beach face for a variety of incident wave conditions run up energy spectra at wind wave frequencies show an 3 dependence and energy levels that are independent of incident wave height this suggests saturation in contrast run up energy at surf beat periods increase approximately linearly with increasing incident wave energy thus in the inner surf zone where wave breaking limits the energy at wind wave frequencies the principal manifestation of large incident wind waves is energetic surf beat

middle late jurassic tectonostratigraphic evolution of central asia implications for the collision of the karakoram lhasa block with asia mesozoic basins in different regions of central asia provide important records for investigating relationships between intraplate deformation in central asia and tectonic processes at asian boundaries the present study gives a review of the stratigraphic and structural evolution of basins in different regions of central asia during the middle late jurassic it is shown that basins and mountain belts in northwest china experienced compressional deformation and were wholly or partially uplifted during the late middle late jurassic compared to extensively distributed middle jurassic coal bearing strata in northwest china upper jurassic strata characterized by red mudstones and conglomerates have a much smaller distribution in the mean time the tibet pamir plateau also underwent a folding and uplift event and upper jurassic sedimentary rocks are generally missing in the pamir and western tibet the intense compressional deformation and uplift event of the late middle late jurassic from the tibet pamir plateau to northwest china requires a new tectonic model as proposed here we suggest that the karakoram and lhasa blocks were a single giant block which was accreted to asia in the late middle jurassic earliest cretaceous and cross cut by the karakoram fault in the cenozoic during the callovian the western part of the karakoram lhasa block initially collided with the southern asian margin collision and continued convergence during the late middle late jurassic caused sinistral strike slip faulting along the central badakhshan fault and south tian shan suture accommodating crustal shortening in areas to the southeast of the faults the pamir western tibet tarim block qilian qaidam block and bei shan meanwhile the northeastward transpressional motion of the tarim block produced strong compressional stresses to areas north of the tarim block the kyrgyz tian shan central tian shan junggar basin and turfan basin with the northward movement of the karakoram lhasa block the eastern part of the karakoram lhasa block began to collide with the southern asian margin during the latest jurassic earliest cretaceous resulting in strong crustal deformation and thickening in east asia and central asia

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

sediment suspension under waves and currents time scales and vertical structure field measurements of the vertical structure of near bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms data were recorded from both a nonbarred marine shoreface and a barred lacustrine shoreface under both shoaling and breaking waves significant heights of 0 25 1 50m peak periods of 3 and 8 s and in water depths of 0 5 5 0 m sediment concentrations are positively correlated with increasing elevation above the bed but lagged in time the time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near bed oscillatory velocity both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation sediment concentrations appear to respond more slowly to the incident wind wave forcing with distance away from the bed as a result of two factors 1 the sequential increase in concentration induced by a succession of large waves in a group and ii the relative increase in finer sediments with smaller settling velocities bedforms interact with the near bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion and therefore the vertical structure of the suspended sediment concentration at a range of time scales the near bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests concentration gradients in the presence of vortex ripples are large as are the correlation between concentrations measured at different elevations within the fluid

divergent double subduction tectonic and petrologic consequences divergent double subduction involving removal of an oceanic basin from both sides and collision has distinct fingerprints of magmatism and significant implications for the tectono magmatic evolution of orogenic belts the complete evolution of a divergent double subduction system involves four stages 1 initial interaction between the two divergent subduction zones on both sides of a single oceanic plate 2 closure of the oceanic basin when the two overriding plates meet followed by 3 detachment of the oceanic lithosphere from the overlying crust and sedimentary sections 4 sinking and disappearance of the oceanic slab into the mantle detachment of the oceanic slab leads to intense decompressional melting of mantle and triggers melting of accretionary wedge strata and lower crust resulting in large scale magmatic and volcanic activity the main consequences of a divergent double subduction system are that it produces opposed thrust systems extensive long lived granitoid magmatism with mantle isotopic signature and volcanism that evolves toward bimodalism this model can be applied to the western half of the paleozoic lachlan fold belt southeastern australia to explain the wide scale silurian to early carboniferous granitoid magmatism its spatial and temporal relationships and late bimodal magmatism closure of the oceanic basin is thought to have occurred during the early to middle devonian detachment of the oceanic slab led to felsic to intermediate composition magmatism bimodal volcanism involving local basaltic flows is a reflection of subsequent sinking of the slab

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

jurassic evolution of the gulf of mexico salt basin we present a new hypothesis for the jurassic plate tectonic evolution of the gulf of mexico basin and discuss how this evolution influenced jurassic salt tectonics four interpretations some based on new data constrain the hypothesis first the limit of normal oceanic crust coincides with a landward dipping basement ramp near the seaward end of the salt basin which has been mapped on seismic data second the deep salt in the deep water gulf of mexico can be separated into provinces on the basis of position with respect to this ramp third paleodepths in the postsalt sequence indicate that salt filled the gulf of mexico salt basin to near sea level fourth seismic data show that postsalt sediments in the central louann and the yucatan salt basins exhibit large magnitudes of late jurassic salt detached extension not balanced by equivalent salt detached shortening in our hypothesis callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust after salt deposition ended rifting continued for another 7 to 12 m y before sea floor spreading began during this phase of postsalt crustal stretching the salt and its overburden were extended by 100 to 250 km 62 155 mi depending on location sea floor spreading divided the northern gulf of mexico into two segments separated by the northwest trending brazos transform the eastern segment opened from east to west leaving the walker ridge salient in the center of the basin as the final area to break apart in some areas salt flowed seaward onto new oceanic crust first concordantly over the basement as a parautochthonous province then climbing up over stratigraphically younger strata as an allochthonous province

geochronological and geochemical constraints on the origin of the yunzhug ophiolite in the shiquanhe yunzhug namu tso ophiolite belt lhasa terrane tibetan plateau abstract the formation of the shiquanhe yunzhug namu tso ophiolite melange zone snmz within the lhasa terrane tibetan plateau is key to understanding the mesozoic tectonic evolution of this terrane which remains controversial we show that the yunzhug ophiolite in the central segment of the snmz formed at 150 ma based on u pb dating of zircons from a gabbroic sample in a well developed sheeted dike complex geochemically these mafic rocks are dominated by e morb type compositions along with minor amounts of rocks with p morb type compositions the samples also exhibit high end t values and lack negative nb and ta anomalies data for all the samples plot within the morb array on a th yb nb yb diagram therefore these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean and not in a back arc basin as has been previously assumed taking into account the regional geology we propose that the yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the lhasa terrane which provides new insights into the jurassic tectonic evolution of the lhasa terrane

origin and tectonic setting of the giant duolong cu au deposit south qiangtang terrane tibet evidence from geochronology and geochemistry of early cretaceous intrusive rocks abstract we constrain the origin and tectonic setting of the giant duolong porphyry epithermal cu au deposit in the south qiangtang terrane of northern tibet based on new zircon u pb ages and hf isotopic data as well as whole rock major and trace element data from poorly studied ore associated intrusions in the duolong area the la icp ms zircon u pb dating indicates that the ore associated rocks formed between 121 and 126 ma these ore associated rocks are geochemically similar to low k tholeiitic m type granitoids and to mid to high k calc alkaline i type granitoids they have variable and predominantly positive zircon e hf t values 1 4 to 15 6 and variable crustal model ages t c dm hf 176 1122 ma taking into account previous data and the regional geology of the study area we propose that the ore associated rocks originated from fractional crystallization of mantle derived mafic melts and magma mixing of mantle derived mafic and hybrid lower crust derived felsic melts and the hybrid lower crust included a mix of juvenile and older continental material the duolong porphyry epithermal cu au deposit formed within an ensialic forearc of an active continental margin as a result of the northwards subduction of the bangong nujiang ocean crust beneath the south qiangtang terrane

jiangnan orogen in south china developing from divergent double subduction abstract the jiangnan orogen is considered as a continent continent collisional belt resulting from the closure of a meso neoproterozoic ocean separating the southeastern margin of the yangtze block from the northwestern margin of the cathaysia block recent data indicate the existence of early neoproterozoic 1000 825 ma volcanic arc assemblages on both sides of the orogen suggesting that the ocean lithosphere between the yangtze and cathaysia blocks must have undergone divergent double sided subduction during the period of 1000 825 ma the divergent double subduction eventually resulted in the closure of the ocean basin at 825 ma leading to the soft collision of the yangtze and cathaysia blocks to form the jiangnan orogen without involvement of continental deep subduction high grade metamorphism of continental crust and uplift exhumation of high grade metamorphic rocks shortly after the collision the initial detachment of the ocean lithosphere from the overlying crust and sedimentary sections induced underplating of mantle magmas triggering partial melting of accretionary wedge strata to form some peraluminous s type granites in the period 825 815 ma finally the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the banxi group and its equivalent strata including bimodal volcanic rocks were formed in the period 815 750 ma

map view interference of monoclinal folds abstract outcrop observations and laboratory experiments show that many small chevron folds form by interference of monoclinal kink bands in multilayer buckling kink band interference has also been proposed for some map scale folds furthermore fault related folding provides additional mechanisms of monoclinal fold generation other than buckling and thus makes kink band interference all the more conceptually plausible as a significant large scale process in this paper we document several relatively simple examples of map scale monoclinal fold interference including three interfering monoclines of the colorado plateau a seismically imaged example in the perdido foldbelt of the gulf of mexico and a more complex example from seismic mapping in the santa barbara channel california kink band interference has normally been analyzed in cross section here we emphasize the map view phenomena and present a simple balanced three dimensional model of the interference geometry treating the monoclines as two independent kink bands which does not depend of the kink band folding mechanism this model predicts the shape of the jog produced by crossing monoclines and is used to help evaluate the role of interference in the map view geometry of our examples the documentation of these simple examples supports the concept that more complex monoclinal fold interference could be a significant phenomenon in the upper crust

influence of deep louann structure on the evolution of the northern gulf of mexico three aspects of basement structure and rift related salt distribution have especially influenced the evolution of the deep water northern gulf of mexico 1 creation of a basement high toledo bend flexure separating a chain of interior basins from the central louann salt basin 2 segmentation of the central louann salt basin by the brazos transfer fault into eastern and central domains and 3 salt provinces formed during basin opening the toledo bend flexure was reactivated as a hinge during the cenozoic uplift of the north american craton this uplift triggered gravity gliding forming fold belts in the seaward parts of the continental margin the geometry of the toledo bend flexure influenced the position of these fold belts the brazos transfer fault separates the west sector of the study area from the central and east sectors most of the salt in the deep water northern gulf of mexico lay in the central sector which sourced most of the sigsbee salt canopy the western sector was narrower and was subdivided by the east breaks basement high splitting the callovian salt basin in two as the gulf opened created a southward thinning wedge of salt at the seaward end of the northern gulf of mexico we divide this wedge into a series of provinces on the basis of the geometry of the base of the deep salt original salt thickness influenced diapir location the geometry of the sigsbee canopy the geometry and style of later compressional fold belts and petroleum systems

cordilleran deformation along the eastern edge of the valles san luis potosi carbonate platform sierra madre oriental fold thrust belt east central mexico the eastern margin of the cretaceous valles san luis potosi carbonate platform hidalgo queretaro and san luis potosi states became activated during the formation of the sierra madre oriental fold thrust belt a series of thrust sheets with a minimum linear shortening of 3 to 4 km each and 10 to 12 km in total formed where the platform edge was approximately normal to the greatest principal stress trajectories as deduced from fold axes a tear fault developed where the platform edge formed an acute angle with the same stress trajectories the overthrusts cut across the competent layers of the lower cretaceous on tectonic ramps that dip 10 and became partly steepened by subsequent imbrication or folding in the mechanically weak upper cretaceous rocks the thrust faults are nearly parallel to bedding for several kilometres the location of the thrusts is controlled mainly by the lithological and thickness change of the middle cretaceous rocks from an orthotropically layered basin facies into a two to five times thicker homogeneous platform edge assemblage the bank margin was a zone of stress concentration the magnitude of the horizontal tectonic components which caused the deformation may have changed by a factor of two to five across the platform margin due to the change in cross sectional area oblique and layer parallel discrete shears and subordinate tectonic stylolites appear to have been the dominant deformational mechanisms the rocks did not suffer any measurable penetrative ductile deformation ooids present at the base of the xilitla thrust are unflattened but are marked by a closely spaced stylolitic cleavage perpendicular to bedding the deformations are bracketed by the paleontological age of the youngest strata affected by the overthrusts globotruncana contusa planktonic foraminiferal zone and by the isotopic age of a post tectonic pluton 62 2 ma and are thus of late maastrichtian paleocene age

plate detachment asthenosphere upwelling and topography across subduction zones this study analyzes the topography across subduction zones considering the separate contributions of the crust and the mantle lithosphere to the observed surface elevation we have found a transition from a region where the overriding plate is coupled to the descending slab and pulled down along with it to a region where the overriding plate floats freely on the asthenosphere when the subducting slab retreats oceanward rapidly this transition is abrupt and the edge of the overriding plate is uplifted we propose that at some point during rapid slab rollback the overriding plate detaches and rebounds like a boat released from its keel this event is associated with suction of asthenospheric material into the gap that is opened between the plates up to the base of the crust as a result the forearc uplifts and magmatism in the arc increases

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

new instrumentation for the investigation of sediment suspension processes in the shallow marine environment abstract an instrumentation system has been developed that continuously measures flow velocity water depth and suspended sediment concentration profiles in the nearshore zone the system consists of four components 1 an array of electronic sensors including an electromagnetic current meter a surface piercing resistance wave gauge and a five element array of optical back scatterance sensors 2 a signal processing electronics unit 3 a guyed mast support structure and 4 a data acquisition system the small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves 2 m results from an experiment on an exposed ocean beach on the coast of washington u s a indicate that sand suspension events of long duration 15 40 sec relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift work is now in progress to modify the system for self contained operation on the continental shelf field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation

factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models 1 the perdido fold belt pfb is a prominent saltcored deep water structure in the northwestern gulf of mexico it is characterized by symmetric kink banded folds of a 4 5 km thick prekinematic layer and its vicinity to the extensive sigsbee salt canopy we use 2 d finite element numerical models to study the evolution of the pfb as a gravity driven fold belt both in a local context and in the context of the larger scale passive margin influenced by adjacent allochthonous salt structures we show that parameters such as overburden strength salt geometry or salt viscosity determine timing extent and location of the modeled fold belt simplified models of the gulf of mexico show that toe of slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden in this scenario the pfb likely represents the terminal folding of a much larger diachronously formed fold belt system citation gradmann s c beaumont and m albertz 2009 factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models tectonics 28 tc2002 doi 10 1029 2008tc002326

role of subaerial volcanic rocks and mantle plumes in creation of south atlantic margins implications for salt tectonics and source rocks abstract seaward dipping reflectors sdrs represent flood basalts rapidly extruded during either rifting or initially subaerial sea floor spreading evaporites can form on this basaltic proto oceanic crust as in the afar triangle today evidence for sdrs in south atlantic deep water regions comes from proximity to the uniquely large parana etendeka volcanic province onshore the tristan and gough hot spots drilled volcanic rocks and seismic profiles showing sdr provinces more than 100 km wide as much as 7 km thick and thousands of kilometers long sdrs are clearest adjoining the aptian salt basins however we speculate that sdrs are also present but seismically obscured below the salt basins we argue that the conjugate aptian salt basins are post breakup not pre breakup they were separated from the start by a mid oceanic ridge distal salt accumulated on proto oceanic crust not rift basins this hypothesis is supported by seismic stratigraphy and structure magnetic anomalies plate reconstructions and hydrothermal potash evaporites an important implication for exploration is that thick basalts rather than rift age source rocks may underlie distal parts of the salt basins

frequency dependent cross shore suspended sediment transport 1 a non barred shoreface abstract the local time varying and time averaged suspended sediment transports across a marine non barred shoreface were obtained from field measurements of the near bed velocity and sediment concentration vectors using electromagnetic current meters and optical backscatterance suspended solids sensors co spectral analyses of velocity and sediment concentration revealed the contributions of waves of different frequencies to the total transport the transport attributable to quasi steady currents was determined from the product of the time averaged cross shore velocity and sediment concentration estimates of the local time integrated sediment volume flux total and net and the associated erosion or accretion were determined using depth of activity rods suspended sediment transport was associated with 1 local wind forced waves 2 swell 3 low frequency waves group bound long waves and 4 offshore directed mean currents undertow these transport components varied spatially and temporally in response to changes in the wave height to water depth ratio the local net oscillatory transport rate at wind wave and swell wave frequencies was directed onshore predominantly and increased as water depth decreased in association with wave shoaling the local mean sediment transport rate was predominantly offshore and dominated the net transport rate wherever wave induced components were weak relative to the mean currents under breaking waves the suspended sediment transport contribution associated with wind waves and swell waves was reduced as a result of dissipation the net suspended sediment transport rate exhibited a distinct vertical structure reflecting the balance at each elevation between the opposing mean and oscillatory components of transport the local time averaged total and net sediment volume flux and resulting erosion and accretion patterns support the hypothesis of a near balance in sediment flux for the complete storm event

mesozoic diapirism in the pyrenean orogen salt tectonics on a transform plate boundary the mesozoic history of the pyrenean region was dominated by the opening of the north atlantic ocean and the left lateral movement of iberia relative to europe most deformation was tensional or transtensional but this style was interrupted by several short compressional events perhaps related to bends in the transform plate boundary or minor changes in plate movement directions these compressional events were recorded by deformation adjacent to and above salt structures which were the weakest parts of the system and so served as especially sensitive strain barometers the alternation of extension and shortening controlled the location and style of salt structures evaporites were initially deformed during the latest jurassic early cretaceous neocimmerian transpression which formed salt cored anticlines above west northwest east southeast trending basement faults transtension in the aptian albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest southeast and northeast southwest trending basement structures transtension was at least locally interrupted by brief periods of transpression at the aptian albian and albian cenomanian boundaries transpression shortened the diapirs causing rotation uplift and erosion of beds near the salt most diapirs were buried during the late cretaceous in some cases postburial hydrothermal circulation dissolved much of the halite causing the diapirs to sag and the basins to form in roof strata finally the entire region was shortened during the late cretaceous tertiary pyrenean orogeny which greatly distorted many of the preexisting geometries

nd isotopes of siliciclastic rocks from tibet western china constraints on provenance and pre cenozoic tectonic evolution nd isotope data from pre cenozoic siliciclastic rocks from tibet are used to determine variations of source regions with time and hence tectonic history on average the tdm and tchur ages of the tibetan samples are up to 1 83 0 36 ga and 1 25 0 45 ga respectively indicating that tibet is underlain by lower proterozoic basement and has close affinity with the himalayas an nd isotope crisis occurred at the early permian with older samples defining a trend of slight decrease in end values with time while the lower permian rocks sharply increasing 6 5 end units this is related to the coeval widespread basaltic magmatism in tibet which could be responsible for tibet rifting away from northern gondwanaland the central qiangtang meta sedimentary samples have distinct nd isotope characteristics end 0 19 87 1 51 tdm 2 07 0 13 ga tchur 1 78 0 21 ga from the songpan ganzi samples end 0 12 77 2 87 tdm 1 49 0 18 ga tchur 1 08 0 23 ga suggestive that these two groups were derived from differentsourcesandthatthe former werenot theequivalents ofthe latterthat wasunderthrust fromthe jinsasuture andmayrepresent subducted continental crust the mid jurassic flysch rocks in northern lhasa have typical upper crust nd isotopic signatures and could have formed under a passive margin environment their bangong counterparts averagely 6 35 units higher in end values unveil derivations from a likely magmatic arc along the northern bangong margin basaltic pebblewith low nd content and endvalue from the berriasian valanginian rocks in central tibet along with the samples with generally high end values shows that the bangong ophiolites could have been a main source of sediments during the earliest cretaceous time a significant transition of sediment sources occurred at the beginning of the hauterivian stage 120 ma sedimentary basaltic tuff end t 2 27 in the hauterivian lower cenomanian samples in central tibet and common nd signatures for input of mantle like materials in these rocks indicate that mantle derived magmatic rocks could have constituted a significant source the huge hauterivian lower cenomanian sediments in central tibet are thus inferred to have been accumulated under a back arc rifting setting 2007 elsevier b v all rights reserved

1 subduction slab detachment and mineralization the neogene in the apuseni mountains and carpathians abstract the inner carpathians comprise several distinct neogene late stage orogenic pb zn cu ag au ore districts the mineral deposits in these districts are closely related to volcanic and subvolcanic rocks and represent mainly porphyry and epithermal vein deposits which formed within short periods of time in each district here we discuss possible geodynamic and structural controls that suggest why some of the neogene volcanic districts within the carpathians comprise abundant mineralization while others are barren the neogene period has been characterized by an overall geodynamic regime of subduction where primary roll back of the subducted slab and secondary phenomena like slab break off and the development of slab windows could have contributed to the evolution location and type of volcanic activity structural features developing in the overlying lithosphere and visible in the carpathian crust such as transtensional wrench corridors block rotation and relay structures due to extrusion tectonics have probably acted in focusing hydrothermal activity as a result of particular events in the geodynamic evolution and the development of specific structural features mineralization formed during fluid channelling within transtensional wrench settings and during periods of extension related to block rotation in the slovakian ore district of the western carpathians neogene volcanism and associated mineralization were localized by sinistral ne trending wrench corridors which formed part of the extruding alcapa block the baia mare ore district in the eastern carpathians reflects a transtensional wrench setting on distributed oversteps close to the termination of the dragos voda fault there mineralization was spatially controlled by the transtensional dragos voda master fault and associated cross fault systems the golden quadrangle cu au ore district of the southern apuseni mountains reflects an unusual rotated transtensional extensional setting close to the termination of a graben system there fluid flow was probably localized by fault propagation at the inner tip of the graben system the spatial and temporal evolution of the magmatism and its changing geochemical signature from n w to s e strongly suggests a link with the contemporaneous northeastward roll back of the subducted slab and a progressive southeastward detachment during accelerating roll back this geodynamic evolution is further supported by the present day overall and detailed mantle lithospheric density images the present day heat flow patterns the crustal architecture and its interpreted evolution and the spatial and temporal evolution of depocentres around the carpathian arc in contrast to all these features the mineral deposits in the west carpathians east carpathians and apuseni mountains are too synchronous with respect to their individual volcanic history and contrast too much with younger volcanics of similar style but barren in southeastern parts of the carpathians to simply link them directly to the slab evolution in all three districts the presence of magmatic fluids released from shallow plutons and their mixing with meteoric water were critical for mineralization requiring transtensional or extensional local regimes at the time of mineralization possibly following initial compressional regimes these three systems show that mineralization was probably controlled by the superposition of favourable mantle lithospheric conditions and partly independent evolving upper crustal deformation conditions in the 13 to 11 ma period the dominant mineralization formed all across the carpathians and was superimposed on structurally favourable crustal areas with at that time volcanic hydrothermal activity the period may reflect the moment when the upper part of the crust failed under lithospheric extension imposed by the slab evolution this crustal failure would have fragmented the overriding plate possibly breaking up the thermal lid to provoke intensive fluid flow in specific areas and allowed subsequent accelerated tectonic development block rotation and extrusion of a family of sub blocks that are arbitrarily regarded as the tisia dacia or alcapa blocks even though they have lost their internal entity

provenance of mesozoic clastic rocks within the bangong nujiang suture zone central tibet implications for the age of the initial lhasa qiangtang collision abstract the bangong nujiang suture zone separating the lhasa and qiangtang blocks of the tibetan plateau is marked by remnants of the bangong nujiang oceanic basin in the gaize area of central tibet mesozoic sedimentary strata recording the evolution of the basin and subsequent collision between these two blocks include the upper triassic lower jurassic turbidites of the mugagangri group the upper jurassic lower cretaceous sandstone dominated wuga and shamuluo formations and the upper cretaceous molasse deposits of the jingzhushan formation the shamuluo and jingzhushan formations rest unconformably on the underlying mugagangri group and wuga formation respectively in this contribution we analyze petrographic components of sandstones and u pb hf isotopic compositions of detrital zircons from the wuga and jingzhushan formations for the first time based on the youngest detrital zircon ages the maximum depositional ages of the wuga and jingzhushan formations are suggested to be 147 150 ma and 79 91 ma respectively petrographic and isotopic results indicate that sediments in the wuga formation were mainly sourced from the accretionary complex preserved as the mugagangri group in the north while sediments in the jingzhushan formation have mixed sources from the lhasa block the qiangtang block and the intervening suture zone provenance analysis together with regional data suggests that the upper jurassic lower cretaceous wuga and shamuluo formations were deposited in a peripheral foreland basin and a residual sea basin respectively in response to the lhasa qiangtang collision whereas the upper cretaceous jingzhushan formation reflects continental molasse deposition during the post collisional stage the development of the peripheral foreland basin evidenced by deposition of the wuga formation reveals that the age of the initial lhasa qiangtang collision might be the latest jurassic 150 ma

reconstructing in space and time the closure of the middle and western segments of the bangong nujiang tethyan ocean in the tibetan plateau when and how the bangong nujiang tethyan ocean closed is a highly controversial subject in this paper we present a detailed study and review of the cretaceous ophiolites ocean islands and flysch deposits in the middle and western segments of the bangong nujiang suture zone bnsz and the cretaceous volcanic rocks late mesozoic sediments and unconformities within the bnsz and surrounding areas our aim was to reconstruct the spatial temporal patterns of the closing of the middle and western segments of the bangong nujiang tethyan ocean our conclusion is that the closure of the ocean started during the late jurassic and was mainly complete by the end of the early cretaceous the closure of the ocean involved both longitudinal diachronous closure from north to south and transverse diachronous closure from east to west the spatial temporal patterns of the closure process can be summarized as follows the development of the bangong nujiang tethyan oceanic lithosphere and its subduction started before the late jurassic after the late jurassic the ocean began to close because of the compressional regime surrounding the bnsz along the northern margin of the bangong nujiang tethyan ocean collisions involving the arcs back arc basins and marginal basins of a multi arc basin system first took place during the late jurassic early early cretaceous resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the southern qiangtang terrane on the northern side of the ocean however the closure of the bangong nujiang tethyan ocean cannot be attributed to these arc arc and arc continent collisions because subduction and the development of the bangong nujiang tethyan oceanic lithosphere continued until the late early cretaceous the gradual closure of the middle and western segments of bangong nujiang tethyan ocean was diachronous from east to west starting in the east in the middle early cretaceous and being mainly complete by the end of the early cretaceous the bnsz and its surrounding areas underwent orogenic uplift during the late cretaceous

petrogenesis of the basalt andesite dacite association of grenada lesser antilles island arc revisited the basalt andesite dacite association of grenada is produced by fractional crystallization of primary magmas that contain about 15 weight percent mgo and 1 2 weight percent h2o the compositions of primary magmas indicate that they were last in equilibrium with the mantle at depths greater than about 65 km indicating that they must ascend rapidly after they segregate from residual mantle it is inferred that mantle wedge diapirism typically leads to moderately large degrees of batch partial melting in the formation of primary arc magmas 10 30 the trace element geochemical diversity of primary magmas mainly reflects variable degrees of source region enrichment by fluids derived from the subducted slab the degree to which source regions are enriched is likely related to mantle wedge dynamics water pressure suppresses plagioclase crystallization in derivative magmas leading to development of the calc alkaline rather than tholeiitic differentiation trend amphibole crystallization typically occurs only after melt mgo content decreases below about 4 weight percent the island arc crust acts as a density filter as arc crust thickens parental basaltic magmas will have a greater tendency to stagnate at mid crustal levels the observation that basalts predominate in young island arcs while andesites and dacites predominate in more mature arcs is explained by this hydrostatic control the thicker the arc crust the more magmas must differentiate by crystal segregation to become buoyant enough to erupt at the surface

early cretaceous foreland like northeastern qaidam basin tibetan plateau and its tectonic implications insights from sedimentary investigations detrital zircon u pb analyses and seismic profiling abstract the early cretaceous collision between the lhasa and qiangtang terranes is one of the critical tectonic events that shaped the southern margin of the eurasian continent and that impacted the formation of the cenozoic tibetan plateau however how far north this collision affected the structural and sedimentary geology of the early cretaceous remains uncertain to address this issue we present results from an integrated study of lower cretaceous strata in the northeastern qaidam basin neqb northern tibetan plateau based on field observations detrital zircon u pb analyses and seismic profile interpretations the lower cretaceous mainly consists of deltaic fluvial and alluvial sandstone and conglomerate with a pattern of orogeny related molasse deposits and a provenance in the nearby south qilian shan to the north a number of south directed reverse faults developed and formed piggy back basins in the early cretaceous these reverse faults remained active and affected erosion during the late cretaceous leaving an unconformity above the cretaceous strata these observations indicate a foreland like neqb due to the uplift and southward thrusting of the south qilian shan in the early cretaceous we further interpreted the neqb and the south qilian shan to be the northern edge of the compressional regime associated with the lhasa qiangtang collision in the early cretaceous which contrasts with the contemporaneous extensional setting in the north qilian shan and further north we also observed that some early cretaceous faults were rejuvenated as major thrusts in the south qilian shan thrust belt in the cenozoic for the first time this study maps the northern boundary of the late mesozoic qiangtang plateau and highlights a protracted influence on tibetan plateau tectonism since the early cretaceous

subduction of a spreading ridge within the bangong co nujiang tethys ocean evidence from early cretaceous mafic dykes in the duolong porphyry cu au deposit western tibet abstract the intrusion of mafic dykes into a near trench accretionary prism and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism are direct manifestations of the subduction of a spreading ocean ridge and the formation of a slab window in this paper we investigated mafic dykes intruded into the accretionary prism that hosts the duolong porphyry cu au deposit dcad of western tibet la icp ms analysis of u pb in zircon indicates that the dykes formed during the early cretaceous 126 127 ma the dykes are characterized by ehf t values from 2 44 to 11 8 twenty nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions group i has nb 8 31 10 2 ppm nb la 0 71 1 20 and nb u 21 4 37 9 group ii has nb 40 5 52 6 ppm nb la 0 84 1 58 and nb u 18 8 47 8 and group iii has nb 65 7 105 ppm nb la 1 35 2 08 and nb u 36 5 73 8 group i is classified as nb enriched basalts enbs whereas groups ii and iii are classified as high nb basalts hnbs both the enbs and hnbs were derived from an adakite metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene the compositional variations of the studied dykes resulted mainly from mantle source heterogeneity the volume of the slab melts gradually increases from group i enbs to group ii hnbs and group iii hnbs leading to gradually increasing incompatible element concentrations considering their geochemical characteristics and field relationships as well as the unique characteristics of continental margin magmatism in the dcad we propose that the dykes emplaced in an extensional accretionary prism were derived from the northward subduction of a spreading ridge in the bangong co nujiang tethys ocean during the early cretaceous the ridge subduction event was also responsible for the generation of coeval adakites intermediate felsic intrusions the maierze bimodal volcanic rocks as well as the metallogenesis of the dcad

initiation of gravitational collapse of an evaporite basin margin the messinian saline giant levant basin eastern mediterranean regional 2d and 3d seismic data from the levant basin reveal the updip extensional component of thin skinned gravity tectonics on this continental margin because of its youth 5 7 ma the earliest stages of deformation of this salt basin are preserved without the severe structural overprinting common on more mature giant salt tectonic systems extension detaches onto and within messinian evaporites up to 1800 m thick by structural restoration we reconstruct the tectonic evolution of the messinian evaporites using paleobathymetric constraints from wells and seismic profiles this analysis suggests that the mediterranean drawdown during the messinian salinity crisis was 800 m the 10 to 15 km wide extensional domain tracks the landward pinch out of the mobile messinian evaporites against an older late miocene scarp diachronous extension began in the center of the margin in the mid pliocene then spread northward in the late pliocene then finally southward in the early pleistocene extension continues today on many of the most landward faults extensional strain varies greatly along strike from 1 km to as much as 12 15 km comparing observations with four end member conceptual models we infer that both extension and seaward salt flow thinned the evaporite margin and its overburden both processes were triggered by a combination of uplift of the continental shoulder of the dead sea rift and subsidence in the mediterranean basin at least three factors controlled variations in extension 1 degree of tilting of the salt wedge related to the interplay of coastal uplift and basin subsidence 2 presence of pre messinian canyons overlain by landward salients of salt and 3 variations in evaporite facies and the proportion of siliciclastic admixture

paleomagnetic constraints on the mesozoic drift of the lhasa terrane tibet from gondwana to eurasia the mesozoic plate tectonic history of gondwana derived crustal blocks of the tibetan plateau is hotly debated but so far paleomagnetic constraints quantifying their paleolatitude drift history remain sparse here we compile existing data published mainly in chinese literature and provide a new high quality well dated paleomagnetic pole from the ca 180 ma sangri group volcanic rocks of the lhasa terrane that yields a paleolatitude of 3 7 s 3 4 this new pole confirms a trend in the data that suggests that lhasa drifted away from gondwana in late triassic time instead of permian time as widely perceived a total northward drift of 4500 km between ca 220 and ca 130 ma yields an average south north plate motion rate of 5 cm yr our results are consistent with either an indian or an australian provenance of lhasa

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

early cretaceous tectonomagmatic evolution and basin development of western bangong nujiang suture a complete history of soft collision abstract uncertainty surrounding the early cretaceous tectonomagmatic evolution and basin development of the bangong nujiang suture zone bnsz limits understanding of the history of meso tethys ocean and the initial uplift and large scale metallogenesis of the tibetan plateau during the mesozoic here we present new geochronological geochemical and isotopic data for the early cretaceous volcano intrusive rocks including quartz diorites tholeiitic andesites and bajaitic latites around the western segment of the bnsz w bnsz we also report an early cretaceous deep water volcano stratigraphic section newly identified in this region based on these new data and information available for the bnsz we propose that the w bnsz and surrounding regions had already entered a stage of soft collisional land locked basin evolution during the early cretaceous the land locked basin underwent limited uplift related to the initial soft collision during the earliest early cretaceous resulting in the formation of a regional unconformity between shamuluo formation shallow marine sedimentary rocks and underlying ophiolitic melange subsequently the basin experienced continuous subsidence related to sinking and rollback of the bangong oceanic slab leading to the occurrence of deep water flysch deposits e g the zhaga formation the bangong oceanic slab was completely detached from the overlying soft collision zone at ca 113 ma resulting in the strong isostatic uplift of the whole soft collision zone consequently seawater receded from w bnsz and surrounding regions during the late early cretaceous late cretaceous our study also shows that the western segment of the south qiangtang terrane experienced long lived intermediate magmatism 130 104 ma under this soft collision regime intermediate magmas of this age originated from partial melting of juvenile lower crust fractional crystallization of mantle derived mafic magmas and partial melting of metasomatized mantle wedge peridotites and these processes were driven by the break off rollback and complete detachment of the bangong oceanic slab respectively

controls on the evolution of passive margin salt basins structure and evolution of the salina del bravo region northeastern mexico we would like to thank cgg and tgs for permission to use seismic data nancy cottington drafted the diagrams cari breton constructed the geographic information system database eva silverfine ott edited the text the paper was greatly improved by reviews from chris jackson and an anonymous reviewer the project was funded by the applied geodynamics laboratory consortium consisting of the following companies anadarko bhp billiton bp cgg chevron condor ecopetrol emgs eni equinor exxonmobil fieldwood hess ion geophysical midland valley murphy nexen noble petrobras petronas pgs repsol rockfield saudi aramco shell spectrum geo stone talos tgs total west erngeco and woodside additional funding came from the jackson school of geosciences juan i soto also acknowledges the financial support provided by the ministry of education in spain with a salvador de madariaga fellowship publication was authorized by the director of the bureau of economic geology jackson school of geosciences the university of texas at austin

permo carboniferous sequences of gondwana affinity in southwest china and their paleogeographic implications abstract in southwest china permo carboniferous depositional sequences of gondwana affinity were first discovered in the himalayan region by the expedition team of acadimia sinica in 1975 by 1983 gondwana affinity permo carboniferous sequences had been identified also in the lhasa block the south qiangtang block in tibet and the tengchong and baoshan blocks in yunnan these sequences are characterised by the presence of successions which are interpreted as glacio marine deposits and by the presence of gondwana affinity biota the distribution of gondwana affinity permo carboniferous indicates that basins whose remnants are now represented by the yarlung zangbo suture and the bangong nujiang suture did not exist at that time and the vast area south of the lungmu shuanghu jinshajiang zone in tibet and west of the changning menglian belt in yunnan formed a part of the northern margin of gondwana

cretaceous volcanic rocks in south qiangtang terrane products of northward subduction of the bangong nujiang ocean the subduction of the bangong nujiang ocean is a long standing problem in the study of the geological evolution of the tibetan plateau based on recent data acquired from the abushan volcanic rocks in the qiangtang terrane we will discuss the relationship between the cretaceous volcanism and the evolution of the bangong nujiang ocean the abushan volcanic rocks are mainly composed of trachy andesites dacites and rhyolites zircon u pb dating constrains the time of emplacement as the middle cretaceous 102 6 1 6 96 1 2 4 ma major element geochemistry shows that the volcanic rocks belong to the high k calc alkaline and calc alkaline series all of the volcanic rocks are enriched in light rare earth elements lree and some large ion lithophile elements lile rb th and u and are depleted in heavy rare earth elements hree and some high field strength elements hfse nb ta and ti the geochemical characteristics of the abushan volcanic rocks are correlated with those of the volcanic arc rocks in the subduction zone the petrogenesis of the abushan volcanic rocks suggests that they were derived from the partial melting of the relict subducted oceanic crust combined with the input of oceanic sediments oceanic crust will melt upon reaching its hydrous solidus in the amphibolite facies after plate collision this induces the formation of the abushan volcanic rocks our data compared with the previous studies on volcanism in the lhasa terrane leads us to propose that the bangong nujiang ocean crust was subducted beneath the qiangtang terrane and underwent bidirectional subduction during the cretaceous

upper jurassic structure and evolution of the yucatan and campeche subbasins southern gulf of mexico the isthmian salt basin in the southern gulf of mexico can be divided into the yucatan and campeche subbasins separated by a base salt high near the nose of the yucatan platform despite their proximity these two subbasins experienced radically different histories in the period immediately following salt deposition portions of the yucatan subbasin are characterized by large scale locally as much as 60 km 37 mi downdip translation of salt and suprasalt sediments during the late jurassic this translation produced a major detached extensional province at the updip end of the basin which is not compensated by observed shortening downdip we interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening a process mirrored on the conjugate florida margin the campeche subbasin in contrast shows no evidence of significant late jurassic translation detached on salt no large scale extensional or contractional provinces of mesozoic age are evident although some minor translation did occur we suggest that salt in the campeche subbasin was confined at its seaward end which prevented the seaward salt flow experienced in the yucatan subbasin furthermore salt at the seaward end of the campeche subbasin lies 2 3 km 1 2 mi above oceanic crust in contrast to salt lying on crust whose top sits at or below the level of oceanic crust at the seaward ends of the tamaulipas yucatan and florida margins the campeche subbasin thus appears to have been perched relative to other parts of the gulf of mexico

early cretaceous gangdese retroarc foreland basin evolution in the selin co basin central tibet evidence from sedimentology and detrital zircon geochronology the selin co basin in the northern lhasa terrane includes more than 3000 m of upward coarsening lower cretaceous strata and the sedimentary sequence from the flysch to the molasse indicates the evolution of a foreland basin petrographic analysis shows that sandstones are rich in volcanic and sedimentary lithics and most of them fall into recycled orogen and magmatic arc uranium lead u pb ages were determined for 435 detrital zircons from the lower cretaceous strata in the selin co basin relative probability of detrital zircon ages shows the eshaerbu formation was rich in zircon grains with the age of 125 140 and 160 180 ma and the duoni formation was dominated by one main age cluster of 125 150 ma analysis of the potential provenances suggests the early cretaceous zircon grains were primarily derived from the gangdese magmatic arc to the south the youngest zircon ages in the lowermost exposure of the eshaerbru formation are c 130 ma providing a maximum depositional age of sediments in the selin

nearshore water motion and mean flows in a multiple parallel bar system abstract measurements of surface waves and nearshore water motion were made with wave staffs and electromagnetic currentmeters across the straight outer bars of a multiple parallel bar system at wasaga beach georgian bay ontario three sets of experiments were carried out between 1983 and 1985 and this paper examines the characteristics of spatial and temporal variations in incident waves and mean flows incident wave spectra during storms are typical of fetch limited conditions with a prominent peak in the wind wave frequency and a sharp cut off towards lower frequencies there is little evidence of significant energy at infragravity frequencies mean flows perpendicular to the shoreline during storms measured at 0 1 m above the bed are offshore with speeds of 0 04 0 10 m s 1 during moderate storms and 0 25 m s 1 during intense fall storms longshore currents are relatively weak and their direction closely follows changes in the incident wind angle about shore perpendicular there is no evidence of rip cell development and offshore return flow occurs as an undertow uniformly alongshore forming a simple two dimensional net circulation pattern the presence of the undertow very close to the bed is an important control on the direction of suspended sediment transport and thus in maintaining the equilibrium of the nearshore bars

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

long lasting interactions between tectonic loading unroofing post rift thermal subsidence and sedimentary transfers along the western margin of the gulf of mexico some insights from integrated quantitative studies abstract after jurassic rifting numerous carbonate platforms i e the orizaba cordoba and golden lane tuxpan platforms developed during the lower and middle cretaceous episode of thermal subsidence along the western passive margin of the gulf of mexico with intervening basinal domains i e the tampico misantla zongolica veracruz and deep gulf of mexico dgm basins during the late cretaceous paleocene the east verging sierra madre oriental thrust belt developed resulting in tectonic uplift and unroofing of the allochthonous units i e tectonic units made up of former orizaba and cordoba platforms and zongolica basin series this new topography provided also an important source of clastics to feed the adjacent foredeep where coeval tectonic loading accounted for the bending of the foreland lithosphere however shallow water facies or even emersion persisted until the eocene in the forebulge area at the present location of the golden lane preventing locally the clastics to reach the dgm this topographic barrier was ultimately bypassed by the clastics only during the oligocene and neogene once 1 the prograding clastic wedge had exceeded accommodation and 2 the long lasting thermal subsidence of the passive margin could overpass the effect of the bending and force the former bulge to sink numerous paleo thermo meters tmax ro paleo thermo barometers fluid inclusions pvt and coupled forward kinematic and thermal modeling have been used to calibrate and date the progressive unroofing of the thrust belt coupled tectonic and sedimentologic modeling was applied in the foreland to predict the distribution of sand versus shale ratios in the oligocene to plio quaternary clastic sedimentary wedge of the passive margin where gravitational gliding of post eocene series occurred during the neogene along major listric faults mantle dynamics are advocated as the main process accounting for post orogenic uplift and regional tilting of the basement which initiated a massive transfer of sediments from the cordillera towards the gulf of mexico from oligocene onward resulting in a destabilization and gravitational collapse of the western slope of the gulf of mexico in neogene times

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

temporal and spatial variations of late mesozoic granitoids in the sw qiangtang tibet implications for crustal architecture meso tethyan evolution and regional mineralization abstract the western segment of the south qiangtang terrane w sqt is an ideal place to investigate the late mesozoic crustal architecture tectonomagmatic evolution and regional mineralization of this terrane where the emplacement of voluminous late mesozoic granitoids provides favorable conditions to form some world class porphyry cu au mineral deposits as well as a series of fe pb zn and w mineral deposits or occurrences here we synthesize the spatial distribution geological geochronological geochemical and isotopic data of the late mesozoic granitoids and associated mineral deposits or occurrences in w sqt these data suggest that the granitoid magmatism in w sqt lasted from ca 170 ma to ca 110 ma with two magmatic flare ups at ca 170 150 ma and ca 130 110 ma in particular the granitoids formed between ca 170 ma and ca 120 ma are generally younger from north to south indicating a southward migration of magmatism during this period the compositions of the granitoids vary considerably across w sqt becoming more reduced and evolved from the ancient coastal area i e the area close to the bangong nujiang suture zone to the continental interior area moreover the zircon ehf t values of the granitoids also gradually decrease from the ancient coastal area to the interior area indicating the ancient crustal components in the source of the granitoids increase continentward we therefore concluded that an ancient crustal basement with archean to paleoproterozoic ages must exist beneath the interior of the south qiangtang terrane i e the south qiangtang terrane has been an older microcontinent based on this conclusion a comprehensive model involving the flat subduction break off rollback and complete detachment of the bangong nujiang oceanic slab was proposed for the late mesozoic tectonomagmatic and metallogenic evolution of w sqt in particular the thickening of w sqt crust during late jurassic and the occurrence of magmatic gap ca 140 130 ma were interpreted to be associated with the flat subduction of bangong nujiang oceanic slab the development of regional extension recorded by early cretaceous 127 122 ma mafic dikes and bimodal volcanic rocks and the southward migration of granitoid magmatism were attributed to the break off and rollback of bangong nujiang oceanic slab our study also shows that the early cretaceous mantle derived juvenile crust derived highly oxidized and less evolved granitoids are most favourable for the generation of significant porphyry cu au deposits while the middle late jurassic mature crust derived relatively reduced and highly evolved granitoids are highly prospective for pb zn and w mineralisations in addition the granitoids generated by a mixture of mantle derived and mature crust derived components have high potential for skarn fe cu deposits for the w sqt magmatic belt as a whole cu au prospective granitoids fe cu prospective granitoids and pb zn w prospective granitoids exist in the south the middle and the north respectively

tectono stratigraphic evolution of eastern mexico during the break up of pangea a review abstract the lower jurassic cenomanian successions of mexico can be grouped into two different tectono stratigraphic systems that were developed under the influence of two distinct geodynamic processes 1 the mesozoic atlantic system of mexico which occupies the eastern two thirds of the country and was dominantly influenced by the break up of pangea and consequent opening of the atlantic ocean and 2 the mesozoic pacific system of mexico which is exposed along its western margin and was controlled by the subduction of the farallon plate beneath the pacific margin of north america the mesozoic atlantic system of mexico can be subdivided into two tectono stratigraphic subsystems representing the two main tectonic stages of the break up of pangea the early middle jurassic rift subsystem was developed during the initial stage of lithospheric attenuation of western equatorial pangea which is evidenced by the opening of a number of right lateral transtensional troughs that were opened according to the nw se regional extension imposed by north america south america divergence rift related troughs were progressively infilled by continental to marginal marine deposits and locally by volcanic lava flows that display a geochemical signature typical of supra subduction settings based on these characteristics the rift subsystem is interpreted in this work as a hybrid tectono stratigraphic domain that displays characteristics that are typical both of the mesozoic atlantic system of mexico and mesozoic pacific system of mexico reflecting the superposition of the atlantic and pacific tectonic processes during the initial phase of north america south america divergence by late callovian time most of intra continental extension in western equatorial pangea had occurred and was followed by anticlockwise rotation of the yucatan block to its present position and initial seafloor spreading in the gulf of mexico the opening of the gulf of mexico promoted the incursion of marine waters in eastern mexico and the onset of carbonate basins and platforms that make up the late jurassic cenomanian drift and cooling subsystem the lack of arc like igneous rocks in the drift and cooling subsystem suggests that by the late callovian the mesozoic atlantic system of mexico was developed independently of the tectonic processes related to subduction along the pacific margin according to our synthesis the mesozoic atlantic system of mexico represents a unique stratigraphic record that was developed at least in part under the combined influence of two main tectonic processes that are 1 the divergence between north america and south america and 2 the subduction of the farallon plate in light of this the mesozoic atlantic system of mexico can be considered a natural laboratory to explore the evolution of complex geodynamic settings related to the interaction of plate tectonic boundaries

high mg diorites derived from sanukitic hma magmas kyushu island southwest japan arc evidence from clinopyroxene and whole rock compositions abstract high mg diorites that have similar whole rock composition to high mg andesites hmas should not be simply interpreted as rocks solidified from the hma magmas because the high mg diorites may be mafic cumulates derived from a different magma from the hmas the hmas contain unique clinopyroxenes with higher mg and si than those of other sub alkaline series igneous rocks the mg and si are controlled by the source magma composition rather than its crystallized conditions such as pressure and temperature the chemical composition of clinopyroxenes would present important information for the investigation of the source of high mg diorites we considered the source of early cretaceous high mg diorites on kyushu island southwest japan arc based on their clinopyroxene and whole rock compositions the clinopyroxenes have similar chemical characteristics to those in hmas rather than those in other sub alkaline rocks moreover the whole rock compositions are equivalent to the sanukitic hma and do not show features of mafic cumulates this indicates that the high mg diorites solidified from sanukitic hma magmas it is generally believed that the sanukitic hma magmas involve the subduction of a young and or hot oceanic slab was situated in their genesis therefore the occurrence of the high mg diorites suggests that kyushu was situated in the tectonic setting of young and or hot slab subduction in the early cretaceous

tectonic implications of fore arc magmatism and generation of high magnesian andesites alexander island antarctica alexander island situated off the west coast of the antarctic peninsula contains a suite of late cretaceous to early tertiary subduction related magmatic rocks the rocks occupy a fore arc position 100 200 km trenchward of the main arc antarctic peninsula and they become younger northward along the length of the island major and trace element geochemistry for 222 samples shows the suite to be a medium to high k calc alkaline series ranging in composition from picro basalt to rhyolite andesite samples show a large range in mgo and mg with nine samples representing high magnesian andesites sr and nd isotopic data indicate that the andesites range isotopically to more depleted mantle compositions than the associated basalts the dacite rhyolites can be related compositionally to the andesites by assimilation of typical pacific rim accretionary material to produce high magnesian andesite lavas it is necessary to introduce a suitable source of heat into the fore arc thus enabling partial melting of depleted sub fore arc hydrous peridotite a causative link with ridge subduction prior to magmatism is proposed with successive ridge trench collisions producing a temporal migration of the magmatism and high geothermal gradients in an anomalously hot fore arc region

gravitational collapse and neogene sediment transfer across the western margin of the gulf of mexico insights from numerical models the western margin of the gulf of mexico veracruz state mexico displays an extensive neogene gravitational system whereby the neogene siliciclastic sediments are detached from underlying mesozoic carbonates along decollement surface in oligocene underpressured clays rapidly subsiding half grabens develop above the footwall associated with major listric faults whereas mini basins develop in the deepest parts of the slope in conjunction with the growth of west verging compressional features between the high angle normal faults in the west and the thrust anticlines in the east a wide roll over structure has grown progressively resulting in a major topographic break in the morphology of the slope profile coupled forward kinematic modelling thrustpack and stratigraphic modelling dionisos calibrated against seismic profiles and two key nearshore wells have been applied to representative regional transects across the margin in order to quantify the vertical subsidence and horizontal gravitational deformations and to discuss the impact of various key parameters on the distribution of debris flow depositions these simulations are compared with data from distant odp wells and discussed in the scope of current exploration strategies in both the us and mexican portions of the gulf of mexico basin

timing and mechanism of bangong nujiang ophiolite emplacement in the gerze area of central tibet abstract the bangong nujiang suture zone bnsz separates the lhasa terrane from the qiangtang terrane and contains remnants of the bangong nujiang oceanic lithosphere ophiolites despite decades of research when and how the bangong nujiang ophiolites were emplaced remains enigmatic in the gerze area western segment of the bnsz the geochemistry and provenance discrimination of chromian spinels cr spinels from the pre collisional subduction complex mugagangri group and syn collisional peripheral foreland basin succession wuga formation can help us solve this fundamental problem in the bnsz evolution this study compares the geochemistry of cr spinels from the mugagangri group and wuga formation with those from the bangong nujiang ophiolites cr spinels in the bangong nujiang ophiolites have either low tio2 0 01 0 15 and low al2o3 11 74 26 76 indicating an ssz peridotite origin or high al2o3 45 28 49 15 indicating a morb peridotite origin cr spinels from the ultramafic fragments within the mugagangri group have extremely low tio2

cenozoic mineral deposits and subduction related magmatic arcs in mexico spatial distribution of isotopic ages of igneous rocks in northern mexico shows that a magmatic arc commenced at 140 m y b p near a paleotrench progressed 1 000 km eastward by 40 m y b p and then regressed to within 200 km of the continental margin by 18 m y b p during progression a calcic and calc alkaline continental margin volcano plutonic regime developed but farther east high k calc alkaline and alkaline facies formed in northeastern mexico and west texas following regression calc alkaline volcanism was renewed on the pacific margin at 18 m y b p and continued until cessation of subduction by initiation of transform movements between pacific and american plates benioff zone depth estimates from k 57 5 relationships suggest decreasing dip during progression and a variable dip variable depth regime from 140 to 16 m y b p both the dip angle convergence rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m y b p followed by decreasing convergence and sinking rates from 42 to 16 m y b p changes in subduction dynamics are attributed to plate reorganization at 44 m y b p and the approach of the east pacific rise differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time cu mo w ores 106 to 40 m y old in calc alkaline magmas formed during progression whereas mo and caf 2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths and pb zn ag deposits 49 26 m y b p were formed during late arc progression and early regression these geodynamic variations plus sr rb and s isotopic characteristics of the arc terrane suggest that processes associated with subduction and some crustal contamination rather than local compositional anomalies in crust or mantle account for different magma and mineral deposit suites southern mexico exhibits fe and cu zones whereas the overlapping calc alkaline trans mexican chiapanecan arc of late miocene to holocene age contains several base and precious metal deposits

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

differentiated rare element mineralization in an ongonite topazite composite dike at the xianghualing tin district southern china an electron microprobe study on the evolution from niobium tantalum oxides to cassiterite our study characterizes in detail the mineralogical textural and compositional features of a highly evolved composite ongonite topazite dike and its magmatic differentiation history we present compositional data collected by established techniques i e by electron microprobe and wet chemical analysis which provide a detailed framework for future studies that employ state of the art analytical techniques the studied dike referred to as the no 431 dike crops out within the xianghualing area in the nanling range of southern china in close spatial association with jurassic sn nb ta granite plutons the rock samples in the no 431 dike were collected from a structurally lower drill hole and a trench at higher level the ongonite is encountered throughout the dike but the topazite is only revealed along the margin of the upper near surface dike the results of whole rock major and trace element analyses show that the rocks of the no 431 dike are strongly peraluminous with an average acnk value of 1 5 for ongonite and 3 9 for topazite they are enriched in f 1 7 wt and 5 4 wt on average for ongonite and topazite respectively the rocks have low zr hf and nb ta ratios and high levels of ore forming elements including nb ta sn and w silicate and oxide mineral assemblages textures and compositions are also distinct for the two rock types studied in the lower ongonite of the dike there are abundant phenocrysts of k feldspar quartz and albite and microphenocrysts of topaz and zinnwaldite in a matrix dominated by quartz k feldspar and albite characteristic oxides are columbite tantalite tapiolite and microlite but cassiterite is absent the upper ongonite of the dike has a silicate assemblage similar to the lower ongonite columbite mn uranomicrolite and limited amounts of cassiterite are the dominant accessory minerals the topazite is characterized by large amounts of topaz and zinnwaldite intergrown with quartz while k feldspar albite and quartz phenocrysts have rounded shapes and are relatively rare cassiterite is the most abundant ore mineral while nb ta oxide minerals are less abundant we interpret the whole rock compositional trends mineral textures assemblages and compositions to reflect the differentiation of an evolved initially homogeneous magma that separated into aluminosilicate and hydrosaline melts corresponding to crystallization of ongonite and topazite respectively the crystallization of nb ta and sn bearing ore minerals was strongly controlled by the separation of the two melt phases we hypothesize that dike propagation widening subsequent to the initial dike emplacement may have driven the separation of the aluminosilicate and hydrosaline melt phases that crystallized to ongonite in the core and topazite along the margins of the structurally higher part of the dike

geochemical diversity of late miocene volcanism in southern baja california mexico implication of mantle and crustal sources during the opening of an asthenospheric window abstract five main petrologic and geochemical groups can be identified among the middle to late miocene lavas from the western part of southern baja california 1 calc alkaline and k rich andesites emplaced between 15 5 and 11 7 ma 2 adakites and 3 associated niobium rich basalts erupted between 11 7 and 8 5 ma in the santa clara volcanic field vizcaino peninsula 4 10 6 9 2 ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near san ignacio and 5 magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11 7 ma south of san ignacio and between 9 7 and 8 8 ma near la purisima these lavas although spatially and temporally related display very different geochemical signatures their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface these magmas were eventually trap

a cenozoic uplift history of mexico and its surroundings from longitudinal river profiles geodynamic models of mantle convection predict that mexico and western north america share a history of dynamic support we calculate admittance between gravity and topography which indicates that the elastic thickness of the plate in mexico is 11 km and in western north america it is 12 km admittance at wavelengths 500 km in these regions suggests that topography is partly supported by subcrustal processes these results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of north american topography supported by the mantle may exceed 1 km the cenozoic history of magmatism sedimentary flux thermochronometric denudation estimates and uplifted marine terraces imply that north american lithosphere was uplifted and eroded during the last 30 ma we jointly invert 533 mexican and north american longitudinal river profiles to reconstruct a continent scale rock uplift rate history uplift rate is permitted to vary in space and time erosional parameters are calibrated using incision rate data in southwest mexico and the colorado plateau calculated rock uplift rates were 0 15 0 2 mm yr between 25 and10 ma central mexico experienced the highest uplift rates central and southern mexico continued to uplift at 0 1 mm yr until recent times this uplift history is corroborated by independent constraints we predict clastic flux to the gulf of mexico and compare it to independent estimates we tentatively suggest that the loop between uplift erosion and deposition can be closed here mexico s staged uplift history suggests that its dynamic support has changed during the last 30 ma

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

geochronology geochemistry and zircon hf isotopic compositions of mesozoic intermediate felsic intrusions in central tibet petrogenetic and tectonic implications abstract mesozoic intermediate felsic intrusions are widely distributed in the southern qiangtang terrane central tibet zircon u pb dating shows that these intrusions formed in two periods in the jurassic 169 150 ma and cretaceous 127 113 ma they mostly belong to the high k calc alkaline series and show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta and ti and negative ba anomalies on primitive mantle normalized diagrams p 2 o 5 contents decrease with increasing sio 2 content and th contents increase with increasing rb content consistent with the evolution trend of i type magmas these intrusions show a wide range of zircon ehf t values from 19 4 to 11 2 including in rocks with similar sio 2 contents suggesting variable contributions from mantle and qiangtang crustal sources fine grained mafic to intermediate igneous enclaves in jurassic intrusions have similar zircon u pb ages and similar or slightly higher zircon ehf t values to the host rocks suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers magma mixing is also supported by the wide range of zircon hf isotopic compositions ehf t 19 4 to 2 5 from within individual jurassic and cretaceous intrusions and jurassic enclaves the jurassic cretaceous magmas likely formed in a continental arc setting during subduction of the bangong nujiang ocean between 170 and 110 ma and evolved in the upper plate crust by mash processes