constraining the late mesozoic and early tertiary tectonic evolution of southern mexico structure and deformation history of the tierra caliente region abstract we analyze the structure and assess the deformation history of the tierra caliente metamorphic complex tcmc of southern mexico where laramide accretion of exotic terranes is in debate the tcmc consists of a south plunging antiform fault that is bounded on both its eastern and western flanks tierra caliente metamorphic complex rocks show at least two phases of compressional deformation the first and most prominent records a mean tectonic transport direction of 068 this phase is responsible for east verging asymmetrical folding and thrusting of both metamorphic and superjacent sedimentary rocks the second phase has an average transport direction of 232 and is restricted to the western portion of the tcmc a third phase is responsible for normal faulting lack of discernible deformation before late cretaceous time indicates that the main deformation phase is coincident with laramide orogenesis elsewhere in the north american cordillera the stratigraphy structure and deformational histor

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

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

sheath fold morphology in simple shear abstract sheath folds are highly non cylindrical structures often associated with shear zones we investigate the formation of sheath folds around a weak inclusion acting as a slip surface in simple shear by means of an analytical model we present results for different slip surface orientations and shapes cross sections perpendicular to the shear direction through the sheath fold display closed contours so called eye structures the aspect ratio of the outermost closed contour is strongly dependent on the initial slip surface configuration the center of the eye structure is subject to change in height with respect to the upper edge of the outermost closed contour for different cross sections perpendicular to the shear direction this results in a large variability in layer thickness across the sheath fold length questioning the usefulness of eye structures as shear sense indicators the location of the center of the eye structure is largely invariant to the initial configurations of the slip surface as well as to strain the values of the aspect ratios of the closed contours within the eye pattern are dependent on the strain and the cross section location the ratio r of the aspect ratios of the outermost closed contour ryz and the innermost closed contour ry z shows values above and below 1 r shows dependence on the slip surface shape and orientation but not on the number of involved contours using r measurements to deduce the bulk strain type may be erroneous

mid holocene el nino southern oscillation enso attenuation revealed by individual foraminifera in eastern tropical pacific sediments holocene reconstructions of the el nino southern oscillation enso provide valuable perspective on its recent evolution and can be important for assessing its future optimal assessment of past enso variability requires observations from its center of action in the eastern equatorial pacific but these are limited due to paucity of high resolution paleoceanographic archives e g corals here we use a new approach to quantify past enso variance based on the oxygen isotopic composition 18 o of individual foraminifera globigerinoides ruber from deep sea sediments in the enso source region individual g ruber foraminifera behave as monthly recorders of sea surface conditions including enso extremes circumventing the lack of annual resolution in the sediments intrapopulation 18 o distributions derived with this method from a core near the galapagos islands reveal mid holocene reductions in variance of 50 requiring drastic attenuation of the enso amplitude furthermore mg ca thermometry indicates that mid holocene background conditions were accompanied by a stronger zonal temperature gradient that coincided with a northward displaced intertropical convergence zone itcz the results suggest that the position of the itcz is an important factor in the low frequency modulation of enso and could influence its future evolution

concurrence of folding and remagnetization events in the monterrey salient ne mexico abstract carbonate formations from many locations reveal magnetizations that are secondary attaching a numerical age to these remagnetizations would provide information on orogenic processes to contribute to the effort of assigning an age to remagnetization events lower cretaceous formations from the monterrey salient in northeast mexico were sampled individual fold tests reveal eight site pair synfolding remagnetizations set a of eocene age the remanence is a chemical remanent magnetization crm carried by magnetite the magnetization of set b is possibly due to an early carbonate alteration that produced a remanence significantly older than folding a complexity of set b is that the site pairs reveal k versus unfolding percentage diagrams with maximum unfolding peaks at 100 to 125 an explanation of multiple deformation phases with an earlier horizontal folding phase around a non plunging horizontal axis and a later vertical axis rotation is suggested for the past 100 peaks of the b sites the latter deformation rotation about a vertical axes is revealed by a declination strike test in set a sites and site means of the full collection that deviate counterclockwise from the reference declination by combining synfolding results with published 40ar 39ar illite ages of folding fitz diaz et al 2016 remagnetization ages in set a sites of 48 52 ma are obtained the magnetization in set b sites is likely older though not well constrained interestingly prior results from the central sierra madre oriental to the south show two remagnetization events late cretaceous and early eocene in age in a succession from w to e nemkin et al 2015 the age of folding and remagnetization acquisition in set a sites along the monterrey salient are concurrent with the frontal remagnetized folds in the central sierra madre oriental thus the timing of major remagnetization in the monterrey salient occurred in the eocene

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

influence of pre tectonic carbonate facies architecture on deformation patterns of syntectonic turbidites an example from the central mexican fold thrust belt abstract the mexican fold thrust belt in central mexico excellently exposes relatively well preserved syntectonic deposits that overlay rocks with lateral lithostratigraphic changes across the belt we consider the deformational effects of these changes by investigating the geometry kinematics and strain distribution within syntectonic turbidites which are deposited on top of albian cenomanian shallow and deep water carbonate layers field observations and detailed structural analysis at different stratigraphic and structural levels of the late cretaceous syntectonic formation are compared with the deformation as a function of lithological and structural variations in the underlying carbonate units to better understand the effect of these lithostratigraphic variations on deformation kinematics strain distribution and propagation of deformation from our kinematic analyses we conclude that the syntectonic strata are pervasively affected by folding in all areas and that deformation partitioning localized shear zones at the boundaries of this unit particularly along the contact with massive carbonates at the boundaries with massive platformal carbonates the turbidites are strongly deformed by isoclinal folding with a pervasive sub horizontal axial plane cleavage and 70 60 shortening in contrast contacts with thinly bedded carbonate layers basinal facies do not show strain localization and have horizontal shortening of 50 40 that is accommodated by buckle folds with a less pervasive steeply dipping cleavage the mechanical properties variations in the underlying pre tectonic units as a function of changes in lithostratigraphy fundamentally control the deformation in the overlying syntectonic strata which is an effect that could be expected to occur in any deformed sedimentary sequence with such variations

key ring structure gradients and sheath folds in the goantagab domain of nw namibia abstract the concept of deformation phases is one of the corner stones of structural geology but despite its simplicity there are situations where the concept breaks down in the goantagab domain of nw namibia structures in an area of complex deformation can be subdivided into at least four sets attributed to four deformation phases on the basis of overprinting relations three of these sets of structures however formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions these sets of structures show gradational transitions in space that can be understood by a concept of key ring structure gradients where older d a structures are reoriented and overprinted by new structures d a 1 that have similar orientation and seem to grade into d a structures outside the overprinted area this kind of behaviour may be common in inhomogeneous non coaxial flow in the core of the goantagab domain d 2 structures are thus reoriented and overprinted by local d 2b folds and foliations that have the same orientation and style as d 2 structures outside the domain core

observations on late pleistocene cooling and precipitation in the lowland neotropics although accurate reconstructions of amazonian palaeoclimates are central to under standing the distribution and history of neotropical biodiversity current reconstructions based on proxy data are discordant and subject to intense debate we review some current thinking in amazonian climatology and incorporate some new ideas in an attempt to explain the apparently con tradictory records of palaeoprecipitation from across the amazon basin we suggest that palaeoecol ogists need to recognise that organised convective radiation the process that induces most wet season rainfall in amazonia should be treated as a phenomenon related to but separate from the passage of the inter tropical convergence zone itcz decoupling itcz migration from larger con vective activity provides a mechanism to account for observed palaeoclimatic heterogeneity patterns of observed precipitation change are consistent with the long term persistence of closed forest across much of amazonia indicating that the greatest changes in precipitation during the last glacial max imum came during the wet season which would have little negative impact on forest extent neo tropical cooling at the last glacial maximum lgm is widely accepted although the estimates of that cooling range between 1 c and 5 c in answering the basic question what is meant by cooling we observe that interhemispheric ice mass asymmetry may have caused cooling to be manifested differently according to location a terrestrial cooling of ca 5 c as well as radiative cooling and event based cooling combined to induce vegetation change probably both absolute temperature and mean monthly minima were reduced by polar air incursions in the northern neotropics how ever in the southern neotropics mean monthly minima were reduced by more frequent incursions of patagonian air masses but absolute minima may have been largely unchanged copyright 2004 john wiley sons ltd

rapid subtropical north atlantic salinity oscillations across dansgaard oeschger cycles geochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age the dansgaard oeschger cycles were coupled to fluctuations in north atlantic meridional overturning circulation through its regulation of poleward heat flux the balance between cold meltwater from the north and warm salty subtropical gyre waters from the south influenced the strength and location of north atlantic overturning circulation during this period of highly variable climate here we investigate how rapid reorganizations of the ocean atmosphere system across these cycles are linked to salinity changes in the subtropical north atlantic gyre we combine mg ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four dansgaard oeschger cycles spanning 45 9 59 2 kyr ago to generate a seawater salinity proxy record from a subtropical gyre deep sea sediment core we show that north atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials covarying with inferred shifts in the tropical atlantic hydrologic cycle and north atlantic overturning circulation these salinity oscillations suggest a reduction in precipitation into the north atlantic and or reduced export of deep salty thermohaline waters during stadials we hypothesize that increased stadial salinities preconditioned the north atlantic ocean for a rapid return to deep overturning circulation and high latitude warming by contributing to increased north atlantic surface water density on interstadial transitions

holocene shifts of the subtropical shelf front off southeastern south america controlled by high and low latitude atmospheric forcings over the uruguayan shelf and uppermost slope the coalescence of northward flowing subantarctic shelf water and southward flowing subtropical shelf water forms a distinct thermohaline front termed the subtropical shelf front stsf running in a sw direction diagonally across the shelf from the coastal waters at 32 s toward the shelf break at ca 36 s the stsf represents the shelfward extension of the brazil malvinas confluence zone this study reconstructs latitudinal stsf shifts during the holocene based on benthic foraminifera 18o and 13c total organic carbon carbonate contents ti ca and grain size distribution from a high accumulation sedimentary record located at an uppermost continental slope terrace our data provide direct evidence for 1 a southern stsf position to the south of the core site at the beginning of the early holocene 9 4 cal ka bp linked to a more southerly position of the southern westerly winds in combination with restricted shelf circulation intensity due to lower sea level 2 a gradual stsf northward migration bypassing the core site toward the north primarily forced by the northward migration of the southern westerly winds from 9 4 cal ka bp onward 3 a relatively stable position of the front in the interval between 7 2 and 4 0 cal ka bp 4 millennial scale latitudinal oscillations close to 36 s of the stsf after 4 0 cal ka bp probably linked to the intensification in el nino southern oscillation and 5 a southward migration of the stsf during the last 200 years possibly linked to anthropogenic influences on the atmosphere

sedimentation and diagenesis along open and island protected windward carbonate platform margins of the cretaceous el abra formation mexico abstract the windward margin of the mid cretaceous valles san luis potosi carbonate platform in northeastern mexico included open and island protected segments depositional environments and diagenesis vary markedly with margin type sand shoals near the windward open margin are composed of oncoid bioclastic and cross laminated carbonates increasingly restricted and finer lagoonal and tidal flat environments occurred bankwards recording gradually decreasing wave and current energy lithofacies include peloid miliolid cryptalgal laminite lime mudstone and molluscan carbonates islands along the windward margin are composed of rudistid skeletal debris from adjacent reefs lagoonal to tidal flat sediments were deposited bankwards similar lithofacies occur as in these environments along the open margin but they are muddier and contain less diverse fauna the different energy regimes along the margin influenced the distribution and packaging of banktop sediments the bankward transition to low energy restricted environments was gradual across the open margin in contrast muddy sediments with restricted fauna accumulated in close proximity to the island protected margin non cyclic vertical lithofacies successions characterized the open platform margin whereas asymmetric shoaling upward sequences characterized the island protected margin early diagenesis along the open margin was minor burial diagenesis was of major importance thin rinds of marine cement are widespread but meteoric diagenesis was minor burial promoted extensive compaction mg rich connate brines expressed from guaxcama gypsum resulted in dolomitization and lithification thereby precluding further compaction pore fluids resulting from dehydration of guaxcama gypsum to anhydrite yielded pore filling and replacement anhydrite in the el abra formation burial and laramide deformation maastrichtian paleocene resulted in stylolitization and extensive fracturing uplift produced widespread meteoric diagenesis for the first time resulting in dissolution of anhydrite calcitization of dolomite and equant calcite cementation early diagenesis along the windward protected margin was extensive prolonged subaerial exposure resulted in calcrete cementation and secondary porosity subsequent marine cementation and internal sedimentation reduced porosity equant calcite cement precipitated during local upper cretaceous exposure burial diagenesis was limited to traces of dolomite anhydrite and late stage equant calcite cement suggesting similar pore fluids as in the open margin depositional environment determined the type and degree of early diagenesis which strongly influenced subsequent diagenetic events subtidal sediments of the open margin underwent minimal early diagenesis and entered the burial realm much as they were deposited they were subjected to intensive compaction and alteration by brines in contrast sediments exposed to meteoric diagenesis and marine cementation on near backreef islands and tidal flats entered the mesogenetic realm as stabilized lithified rocks much as they are at present

tropical rain forest and climate dynamics of the atlantic lowland southern brazil during the late quaternary palynological analysis of a core from the atlantic rain forest region in brazil provides unprecedented insight into late quaternary vegetational and climate dynamics within this southern tropical lowland the 576 cm long sediment core is from a former beach ridge valley located 3 km inland from the atlantic ocean radio carbon dates suggest that sediment deposition began prior to 35 000 14c yr b p between ca 37 500 and ca 27 500 14c yr b p and during the last glacial maximum lgm ca 27 500 to ca 14 500 14c yr b p the coastal rain forest was replaced by grassland and patches of cold adapted forest tropical trees such as alchornea moraceae urticaceae and arecaceae were almost completely absent during the lgm furthermore their distributions were shifted at least 750 km further north suggesting a cooling between 3 c and 7 c and a strengthening of antarctic cold fronts during full glacial times a depauperate tropical rain forest developed as part of a successional sequence after ca 12 300 14c yr b p there is no evidence that araucaria trees occurred in the atlantic lowland during glacial times the rain forest was disturbed by marine incursions during the early holocene period until ca 6100 14c yr b p as indicated by the presence of microforaminifera a closed atlantic rain forest then developed at the study site

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

newly formed illite preserves fluid sources during folding of shale and limestone rocks an example from the mexican fold thrust belt abstract we combine structural fluid inclusion microthermometry illite crystallinity x ray diffraction xrd and o and h stable isotope analyses of authigenic illite to determine the source of local fluids interacting with rock during folding in anchizonal shales of the mexican fold thrust belt mftb a well exposed train of mesoscopic asymmetrical folds in a sequence of cretaceous limestones interbedded with shale was targeted for this study we test the hypothesis that syn folding vein minerals and clay minerals were formed from the same fluids by comparing the 2 h composition of inclusion fluids in calcite and quartz from veins and from illite concentrates from sheared shale layers and the sources of that fluid five clay size fractions 2 m were separated from eight shale samples in the 40 clay grain size fractions analyzed illite calcite kaolinite smectite chlorite and minor quartz were identified by xrd analysis most samples show different proportions of various clay minerals except for the finer fractions in two of the samples bl3 and bl4 were illite is the only clay phase present the discriminating potential of 18 o values of clay is generally masked by the abundance of calcite in all samples in contrast samples containing chlorite and smectite show very low values in 2 h 75 9 to 53 9 while samples containing illite and kaolinite or pure illite show relatively high 2 h values 33 1 to 50 1 the latter fall within the 2 h range 39 to 49 determined in fluid inclusions of syntectonic veins indicating isotopic equilibrium between water veins fillings and illitic clay during deformation according to fractionation factors at these temperatures 220 250 c the 2 h values and fluid inclusion salinities in the sampled rocks indicate that water active during folding was partly marine and partly meteoric and that the amount of such pore water represented a small fraction of the deformed rock volume our study demonstrates that the h isotopic composition of anchizonal illitic clays commonly present in exhumed fold thrust belts can be used to determine the source s of fluids that were active during deformation

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

comparison of tectonic styles in the mexican and canadian rocky mountain fold thrust belt despite the fact that most fold thrust belts around the world share many features successfully explained by the critical wedge model the details of their geometric evolution and tectonic style development are poorly understood in the classic section of the southern canadian rocky mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices in the mexican fold thrust belt central mexico the age of deformation the overall structural pattern and the total amount of shortening are similar but the individual thrust sheets exhibit much more internal deformation as manifest by metre scale buckle folds one of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly bedded basinal limestone in the central mexico section strain is concentrated toward the margins between platforms and basins in canada thick platform carbonates form continuous resistant units across the front range possible reasons for the differences in tectonic style between the two sections include the dominant lithology distribution of lithologies taper angle of the tectonic wedges amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks fold thrust belts are common features that developed on the edges of orogenic belts worldwide many have been well studied and their characteristic features established dahlstrom 1970 chapple 1978 boyer elliott 1982 price fermor 1985 mcclay 1992 these features include the classic notions of forward propagation of thrusts piggyback style a basal decollement and decreasing intensity of deformation towards the foreland most fold thrust belts are wedge shaped and since the mid 1980s the critical taper model of their development davis et al 1983 dahlen et al 1984 dahlen 1990 has become generally accepted analogue and numerical models have been developed that are able to capture many of the basic features of fold thrust belts and that are broadly consistent with the critical taper theory e g huiqi et al 1992 stockmal et al 2007 from early studies it has been clear that most of the large folds in fold thrust belts are intimately related genetically to the thrusts and extensive studies of thrust related folds in the field and in analogue and numerical models have established the nature of this relationship e g dahlstrom 1969 1970 elliott 1976 suppe 1983 mcclay 1992 wilkerson et al 2002 spratt et al 2004 despite the many similarities among fold thrust belts in general there are significant differences among belts and along strike within individual belts these reflect among other things differences in the strength of the basal detachment and the composition and distribution of the rocks in the fold thrust wedge in this contribution we compare basic elements of the structures developed roughly synchronously in two widely separated parts of a single orogenic belt the north american cordillera the sections we compare are a well known section across the southern canadian rocky mountains and a new section across the sierra madre oriental in central mexico the dimensions of the fold thrust belts in these two locations are similar and both sections are dominated by carbonates the structural style however is quite different we compare structures in the two locations and speculate on what may be responsible for the differences

stratigraphic controls on eformation patterns in fault related folds a detachment fold example from the sierra madre oriental northeast mexico abstract deformation patterns are described by the style intensity and distribution of structures throughout a volume of rock in fault related folds presently observed deformation patterns are controlled by structural factors like kinematic history environmental factors like burial history and stratigraphic factors like layer thickness and competence to demonstrate the role of stratigraphy in controlling fold related deformation patterns we document the stratigraphic variability of mesostructures at one structural position on a map scale detachment fold unique assemblages of faults fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units each exhibiting a characteristic deformation response to the folding process the varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure flexural slip or flexural flow were sequentially or coevally operative in each of the units using the simple theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold related strain by developing a characteristic set of mesostructures we are able to predict general deformation patterns elsewhere in the fold the significance of this approach is that it recognizes that even in a single structure fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section because of this the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances predictions of fold related deformation based solely on fold kinematics are unlikely to resolve these variations

evidence of a late glacial warming event and early holocene cooling in the southern brazilian coastal highlands a high resolution pollen record of the atlantic rain forest arf biome from the coastal serra do tabuleiro mountains of southern brazil documents an 11 960 yr history of vegetation and climate change a marked expansion of weinmannia into the grassland vegetation marks the latter part of the younger dryas reflecting warm and relatively wet conditions between 11 490 and 9110 cal yr bp grasslands became dominant again indicating a long cold and dry phase probably in response to the stronger influence of cold south atlantic seawater and to antarctic cold fronts between 9110 and 2640 cal yr bp four distinct phases with strong or moderate expansions of different arf biome taxa were recorded reflecting warmer and relatively dry conditions with changes in rainfall and length of the annual dry season after 2640 cal yr bp the modern arf biome became established with high amounts of ferns reflecting somewhat cooler and wetter conditions with a reduced annual dry season in particular after 1000 cal yr bp tree ferns increased reflecting wetter conditions with no dry season

reconstructing the shape of large synformal folds based on the solution of the 2d non linear magnetic inverse problem abstract this paper deals with a new approach to reconstructing the shape of a poorly exposed synform that folds strongly magnetic layers the approach is based on the solution of the 2d non linear magnetic inverse problem in areas best suited for 2d modeling the interpretation model consists of two parts i a geometric model imitating a synform cross section and ii a set of elementary layers of a fold characterized by anisotropic magnetic susceptibility the main task is to determine the deepest point of magnetic layers and the direction and amount of their dip it can be solved by a selection of optimal model parameters from a set of random numbers with a priory model constraints the solution is an iterative process which defines intervals of geometrical model parameters that correspond to minimal values of the misfit between observed and modeled magnetic fields the proposed approach is characterized by a stability of the determination of reference fold points coordinates which is sufficient to confidently answer a number of questions important for the geologist these include whether a synformal fold is upright inclined or overturned determining the depth of its trough line which combined with the fold outline on the ground surface allows an evaluation of the hinge line curvature as an example the large paleoproterozoic serpovidny isoclinal synformal anticline in the keivy terrane of the northeastern fennoscandian shield has been chosen according to field observations this synform is a giant sheath fold consisting of the poorly exposed strongly magnetic core and weakly magnetic limbs the 3d morphology of the serpovidny fold reconstructed using the new approach is shown to be very close to the geometry determined from field observations thereby this study favors the conclusion based the sheath geometry of the serpovidny isoclinal synformal anticline that an alpine type tectonics operated in the paleoproterozoic

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

structural development of the peregrina huizachal anticlinorium mexico abstract the sierra madre oriental fold belt is a nw se trending contractional belt that spans nearly the entire length of mexico this area underwent triassic middle jurassic rifting and late cretaceous early cenozoic shortening our structural investigation of the peregrina huizachal anticlinorium in east central mexico shows that its development is characterized by two phases of deformation an early thin skinned phase and a late thick skinned phase a detachment developed at the contact between the rift sedimentary rock and overlying limestone during the thin skinned phase deformation of the upper jurassic cretaceous strata is accommodated by a series of detachment folds and faults the rift boundary faults are sub perpendicular to the late stage compression in east central mexico this makes rift related faults prone to be inverted as reverse faults during the thick skinned phase the basement in the rift is uplifted along two opposing reverse faults and experience internal deformation its uplift contributed to the growth of the peregrina huizachal anticlinorium

the structural architecture of the los humeros volcanic complex and geothermal field abstract the los humeros volcanic complex lhvc is a large silicic caldera complex in the trans mexican volcanic belt tmvb hosting a geothermal field currently in exploitation by the comision federal de electricidad cfe of mexico with an installed capacity of ca 95 mw of electric power understanding the structural architecture of lhvc is important to get insights into the interplay between the volcano tectonic setting and the characteristics of the geothermal resources in the area the analysis of volcanotectonic interplay in lhvc benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3d structural view of the volcano system the lhvc thus represents an important natural laboratory for the development of general models of volcano tectonic interaction in calderas in this study we discuss a structural model of lhvc based on morphostructural and field analysis integrated with well logs focal mechanism solutions and magnetotelluric imaging the structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system caldera collapses and post caldera deformations these inherited weak planes have been reactivated by resurgence faults and post caldera magma driven hydrofractures under a local radial stress field generated by the shallow lhvc magmatic hydrothermal system the local stress field induced caldera resurgence and volcanotectonic faulting the results of this study are important to better constrain the structural architecture of large caldera complexes also our study is useful to understand the structure of the los humeros geothermal field and support the exploration of deeper super hot geothermal systems shgss and engineering of enhanced geothermal systems egss for electric power production in the lhvc and other active resurgent calderas

anatomy of a 10 km scale sheath fold mount hay ridge arunta region central australia the structural record of deep crustal flow 1 mount hay ridge arunta region central australia exposes a 10 km scale antiformal sheath fold formed in the deep crust p 0 6 0 9 gpa in proterozoic time tilting and exhumation during the devonian alice springs orogeny resulted in an oblique cross section such that the once subhorizontal sheath fold now has a moderately dipping gently folded axial plane and overturned southern limb fabric types e g s l l s were mapped in the field and finite strain data were collected from a widely distributed gabbroic granulite unit fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge there is no field evidence for significant strain localization and measured strain magnitudes are consistent across the fold thus we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries the uniformity of foliation lineation and mesoscale fold orientations across mount hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single progressive deformational event based on its geometry the inferred scale of the enclosing shear zone 8 km in thickness and regional tectonic correlations we infer that the mount hay sheath fold formed in a subhorizontal crustal scale flow zone during the paleoproterozoic strangways orogeny 1740 1690 ma the record of deformation is consistent with fold formation either on one side of a channel or through detachment flow

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

southern oscillation signal in south american palaeoclimatic data of the last 7000 years abstract during strong el nino events rainfall anomalies and changes in wind patterns are observed in different regions of south america along the central brazilian coast during the 1983 el nino year the frontal systems were blocked to the south provoking a reversal of the longshore sand transport long duration reversals of longshore transport were also recorded in holocene beach ridge terraces from the rio doce coastal plain this led to the formulation of a model relating these reversals of longshore transport to el nino like conditions el nino like conditions are past average climate situations that generate the same perturbations as the strong el nino events observed during the last decade they are likely to correspond to the long duration low phase of the southern oscillation to confirm this hypothesis we compared the holocene beach ridge record with other palaeoenvironmental records from regions where strong el nino events would have a substancial signal as well 1 water level fluctuations of lake titicaca 2 a pollen and sediment record in an eastern amazonian lake 3 changes of the rio xingu discharge in eastern amazonia and 4 variations of sand supply at the rio piura and rio chira outlets in the sechura desert the occurrences of el nino like conditions were numerous before 3900 3600 yr b p absent between 39003600 and 2800 2500 yr b p and infrequent after 2800 2500 yr b p

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

a stalagmite record of changes in atmospheric circulation and soil processes in the brazilian subtropics during the late pleistocene we present a high resolution 116 000 year carbon stable isotope record from a stalagmite from southern brazil which has been precisely dated using the u series method evaluation of carbon and oxygen isotope ratios together with the speleothem growth history suggest that the carbon isotopic composition of the speleothem is primarily controlled by biogenic co2 supply from the soil which is in turn affected by temperature and secondarily rainfall amount thus the speleothem provides evidence of paleoenvironmental change in southern brazil during the last glacial period predominantly high d 13 c values and low stalagmite growth rates reflect persistent cool conditions during most of the glacial period in subtropical brazil this cooling is probably related to an intensified extratropical circulation with more frequent and intense cold surges reaching a maximum at approximately 19 ky b p this cooling tendency is interrupted during periods of high obliquity values within the full glacial period at 93 85 and 47 40 ky b p and after 19 ky b p when a dramatic decrease in d 13 c marks the deglaciation time in the continent unlike d 18 o the d 13 c record does not exhibit a strong response to precessional forcing instead it shows a strong 40 ky obliquity signal here we propose that local temperature and thus the biological processes in the soil are primarily steered by the gradients of temperature between low and mid high latitudes which influence the meridional heat transport these gradient changes in turn are paced by obliquity r 2006 published by elsevier ltd

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

megafauna from the late pleistocene holocene deposits of the upper ribeira karst area southeast brazil abstract the carbonate caves of the upper ribeira valley southeastern sao paulo state brazil preserve an abundant fossil record of pleistocene holocene south american megafauna nevertheless in comparison with other localities of the country its paleontological knowledge still can be considered sparse and in need of further research this work presents an update taxonomic survey of the megafauna material collected since the beginning of the explorations in the area based on this it discusses paleobiogeographic and paleoecological questions the fossil megafauna of the upper ribeira karst region include eremotherium laurillardi nothrotherium maquinense lestodon armatus catonyx cuvieri ocnopus gracilis ahytherium aff aureum glyptodon clavipes g reticulatus hoplophorus cf toxodon spp stegomastodon waringi equidae hippidion cf tapirus terrestris panthera onca and smilodon populator only p onca and t terrestris still remain in the region despite the occurrence of some typical taxa of the pampean paleobiogeographical province the fossil assemblage recorded shows greater affinity to the mega mammal community of the intertropical region the ecology of species found suggests the previous occurrence of fairly different environmental settings than the existing one in the area there were more temperate climates both dry and wet until the current mesophytic forest was finally established the studied material does not show any kind of chrono correlation given the type of genesis of the cave accumulations these fossils depict different faunistic moments related to climate changes during the quaternary to better understand the information provided by these deposits efforts on dating and detailed taphonomic investigations should be conducted the studies of the quaternary fauna in the state of sao paulo fill an important gap in the intricate biogeographical history of the south american megafauna

south and southeast brazilian grasslands during late quaternary times a synthesis fourteen pollen records from the south s and the southeast se brazilian regions have been synthesised late glacial records from s brazilian highlands document the predominance of grassland campos where today araucaria forests occur records from se brazil show that during pre and full glacial times modern tropical semideciduous forest and cerrado savanna to dry forest were mostly replaced by grassland and some subtropical gallery forest modern montane araucaria forests and cloud forests in se brazil were mostly replaced by grassland during pre and full glacial times there is evidence that the modern tropical atlantic rainforest in s brazil was significantly reduced and replaced by cold adapted forest taxa or grassland during glacial especially during full glacial times the synthesis indicates that grasslands dominated the s and se brazilian landscape during the late pleistocene where today different forest ecosystems exist grassland extended over 750 km from s to se brazil from latitudes of about 28 27 s to at least 20 s these results indicate that climates in the region were markedly drier and 5 7 c cooler during glacial times antarctic cold fronts must have been much stronger and more frequent than today studies from s brazil show that huge areas of late pleistocene campos vegetation were still found on the s brazilian highlands during early and mid holocene times reflecting dry climatic conditions with an annual dry period of probably 3 months modern wet climatic conditions with no or only short dry periods were not established until the late holocene period when araucaria forests replaced large areas of grassland vegetation after about 3000 14c yr b p and especially after 1500 1000 14c yr b p

sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial interglacial cycle recent studies indicate that the tropical freshwater budget of the atlantic is modulated by changes in el nine southern oscillation enso if anomalies of the atlantic freshwater balance persist on the order of decades these might have a large influence on the north atlantic thermohaline circulation thc here we present a sensitivity study in which we use a model scenario for enso behavior during the last 120 kyr to force a simplified model of the thc with corresponding freshwater exchange between the tropical atlantic and pacific if the steady state strength of the north atlantic thc is similar to the present day its response to the forcing is of the order of 1 3 sv 1 sv 10 6 m 3 s 1 no mode changes of the thc are simulated for reasonable values of the coupling constant between freshwater exchange and enso if on the other hand the steady state overturning is significantly weaker a collapse of the thc occurs due to forcing from the tropics the modeled thc variation due to tropical freshwater perturbations are compared to those resulting from middle and high latitude freshwater forcing due to long term growth and decay of northern hemisphere ice sheets it is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates a possible out of phase relationship between deep water formation in the north pacific and north atlantic is also explored

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

insights into fluid flow and water rock interaction during deformation of carbonate sequences in the mexican fold thrust belt analysis of mesoscopic structures and related veins allows the history of deformation and role of fluids to be established for part of the central mexican fold thrust belt mftb the mftb developed in mostly carbonate rocks with prominent lateral facies changes associated with two platforms and two basins fluids played a key role in the deformation both physically and chemically by reducing strength and inducing extensional fracturing through raised pore pressure and by providing the medium for solution transfer on the grain scale veins preserve portions of water related to deformation as fluid inclusions lithology and lithological variations strongly control deformation styles with thrusts dominant in the platforms and folds dominant in the basins and also influence fluid behavior by controlling both porosity and permeability structural observations allow distinguishing veins dominantly calcite of several generations emplaced early during and late after deformation v1 v3 respectively 13c and 18o analyses in calcite from veins and host rock show that the veins confined within thrust slices are isotopically buffered by the host rock and differ in isotopic composition from veins emplaced along major thrusts or crosscutting thrust slices d analyses in fluid inclusions and clay minerals strongly suggest rock interaction with meteoric fluids in the west hinterland and with fluids close to smow in the less deformed eastern foreland side of the cross section by focusing on a single stratigraphic interval exposed across the width of the fold thrust belt we propose a conceptual model that explains the differences in vein rock isotopic composition the differences of in isotopic composition of aqueous fluids active during deformation and the progression of clay dehydration reactions as being related to variations in temperature and intensity of deformation in a growing tectonic wedge

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

cold season synoptic scale waves over subtropical south america the most active winter synoptic scale wave patterns over south america are identified using an extended empirical orthogonal function eeof technique and are physically diagnosed using composite methods results show that the leading modes of short timescale variability propagate along two main paths over the subtropical jet latitudes 308s and over the subpolar jet latitudes 608s this research focuses on the subtropical mode and its evolution over south america the observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves both cyclonic and anticyclonic perturbations display significant modifications in their threedimensional structure as they evolve over extratropical and subtropical south america while the upper level perturbations are mostly unaffected when moving eastward the lower level perturbations advance following the shape of the andes mountains and exhibit an abrupt equatorward migration at the lee side of the mountains as a result of such detachment smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed once the upper level system is on the lee side the perturbations acquire a more typical baroclinic wave structure and low level intensification of the system occurs the latter is largest around 1000 km east of the orography where enhanced moisture transports from tropical latitudes along the eastern portion of the low level cyclone favor precipitation occurrence over southeastern south america those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low level cyclone in addition an intensification of the cyclone once over the ocean was found in 60 of the situations considered which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the brazil current

orientational data on the state of stress in northeastern mexico as inferred from stress induced borehole elongations preferential elongation directions are presented from 46 subvertical wells in northeastern mexico from depth intervals ranging from 161 to 4 912 m the measurements document a remarkably consistent west east to northwest southeast direction of the contemporary least horizontal stress in the upper crust of this intraplate region in the regional neotectonic framework the data suggest that mexico north of the trans mexican volcanic belt is being stretched in west east to northwest southeast direction the measurements from the area of the laramide chihuahua and coahuila fold thrust belts define the southern extend of the basin and range rio grande rift stress province of the southwestern united states into northern mexico the least horizontal in situ stress directions recorded in the gulf coastal plain burgos and tampico misantla basins are oblique to the continental margin of the gulf of mexico but subparallel to the direction measured in the area of the basin and range rio grande rift stress province this suggests that the stress distribution in these basins is not mainly the result of gravitational loading as in the gulf coast stress province of the united states but is controlled by the same lithospheric stress system that characterizes the basin and range rio grande rift stress province this is also supported by the north south trending zone of oligocene quaternary alkaline volcanism that crosses the tampico misantla basin furthermore the data indicate that the active deformation of the mexican ridges fold belt east of the study area in the gulf of mexico cannot be caused by a compressional external tectonic load the direction of the least horizontal in situ stress measured in the area of the laramide sierra madre oriental fold thrust belt in hidalgo and san luis potosi states is consistent with that of the other zones however wellbore elongations were recorded less frequently and show a rather large circular variance which suggests that the minimum and maximum horizontal far field stresses are about equal in this area

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

quantifying frictional variations and erosion in the mexican fold thrust belt abstract the impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin skinned mexican fold and thrust belt mftb is investigated and quantified using numerical finite element simulations these simulations explore the evolution total shortening and structural styles of deforming orogenic wedges the models include spatial lateral variations in the internal friction and the topographic erodibility k using the power law incision rule the mftb exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation the observed kinematics of the mftb requires based on the parameters considered in this study a rheologically heterogeneous crust with at least laterally varying internal friction yet only limited syntectonic and post tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations application of the analytical and homogeneous critical coulomb wedge ccw theory to the mftb closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho tectonic units and structural styles

abrupt changes in high latitude nutrient supply to the atlantic during the last glacial cycle the supply of nutrients to the low latitude thermocline is largely controlled by intermediate depth waters formed at the surface in the high southern latitudes silicic acid is an essential macronutrient for diatoms which are responsible for a significant portion of marine carbon export production changes in ocean circulation such as those observed during the last deglaciation would influence the nutrient composition of the thermocline and therefore the relative abundance of diatoms in the low latitudes here we present the first record of the silicic acid content of the atlantic over the last glacial cycle our results show that at intermediate depths of the south atlantic the silicic acid concentration was the same at the lgm as it is today overprinted by high silicic acid pulses that coincided with abrupt changes in ocean and atmospheric circulation during heinrich stadials and the younger dryas we suggest these pulses were caused by changes in intermediate water formation resulting from shifts in the subpolar hydrological cycle with fundamental implications for the nutrient supply to the atlantic

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

formal subdivision of the holocene series epoch a discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommission on quaternary stratigraphy international commission on stratigraphy this discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommision on quaternary stratigraphy sqs of the international commission on stratigraphy ics considers the prospects for a formal subdivision of the holocene series epoch although previous attempts to subdivide the holocene have proved inconclusive recent developments in quaternary stratigraphy notably the definition of the pleistocene holocene boundary and the emergence of formal subdivisions of the pleistocene series epoch mean that it may be timely to revisit this matter the quaternary literature reveals a widespread but variable informal usage of a tripartite division of the holocene early middle or mid and late and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology we propose an early middle holocene boundary at 8200 a bp and a middle late holocene boundary at 4200 a bp each of which is linked to a global stratotype section and point gssp should the proposal find a broad measure of support from the quaternary community a submission will be made to the international union of geological sciences iugs via the sqs and the ics for formal ratification of this subdivision of the holocene series epoch copyright 2012 john wiley sons ltd

related changes in biodiversity insolation and climate in the atlantic rainforest since the last interglacial abstract the long term colonia record is located in the atlantic rainforest domain in brazil 23 52 s 46 42 20 w 900 m a s l the 780 cm long core co3 provides a coverage of a complete interglacial glacial cycle for the first time in a neotropical rainforest information on the behavior of tropical climates compared to global changes in temperatures indicates specific climate responses in terms of precipitation at these latitudes winter extratropical circulation was very active during the last interglacial and most of the glacial floristic composition of the rainforest changed several times in each phase of expansion twice during the interglacial and three times during glacial episodes araucaria was well developed in the area of sao paulo until the beginning of the first dry phase of the glacial at ca 50 000 yr b p changes in insolation controlled the expansion of the rainforest and the tropical hydrological cycle as evidenced by a strong precession signal however precession had no impact on regional climatic features the two interglacials mis 5e and holocene showed completely different patterns attesting to the continuous evolution of the forest the biodiversity index shannon wiener index remained high during both the interglacial and glacial attesting to the permanence of small patches of rainforest refugia during drier phases the lowest shannon wiener indexes were recorded between 23 000 and 12 000 yr b p and 40 000 and 30 000 yr b p and characterize two marked phases of stress for the rainforest

does the trigger for abrupt climate change reside in the ocean or in the atmosphere two hypotheses have been put forward to explain the large and abrupt climate changes that punctuated glacial time one attributes such changes to reorganizations of the ocean s thermohaline circulation and the other to changes in tropical atmosphere ocean dynamics in an attempt to distinguish between these hypotheses two lines of evidence are examined the first involves the timing of the freshwater injections to the northern atlantic that have been suggested as triggers for the global impacts associated with the younger dryas and heinrich events the second has to do with evidence for precursory events associated with the heinrich ice rafted debris layers in the northern atlantic and with the abrupt dansgaard oeschger warmings recorded in the santa barbara basin

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

upper pleistocene to holocene peatland evolution in southern brazilian highlands as depicted by radar stratigraphy sedimentology and palynology paleoenvironmental interpretation of proxy data derived from peatlands is largely based upon an evolutionary model for ombrotrophic bogs in which peat accumulates in still environments reports on proxies obtained from minerotrophic fens where hydrologic inputs are variable are less common in this study a highland peatland in southern brazil is presented through ground penetrating radar gpr and sedimentological palynological and geochronologic data the radar stratigraphic interpretation suggests a relatively complex history of erosion and deposition at the site since the beginning of marine isotope stage 3 mis 3 interstadial period in spite of this radar stratigraphic and palynologic interpretations converge electromagnetic reflections tend to group in clusters that show lateral coherence and correlate with different sediment types while pollen grains abound and are well preserved as a result the study of minerotrophic fens provides a source of proxies suggesting that ombrotrophic bogs are not the only reliable source of data in wetlands for palynological analysis

moisture transport across central america as a positive feedback on abrupt climatic changes reconstruction of sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years detected strong millennial scale fluctuations that are associated with the migration of the tropical atlantic intertropical convergence zone thus millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic ocean circulation moisture transport from the atlantic to the pacific ocean across central america leads to relatively high salinities in the north atlantic ocean1 and contributes to the formation of north atlantic deep water2 this deep water formation varied strongly between dansgaard oeschger interstadials and heinrich events millennial scale abrupt warm and cold events respectively during the last glacial period3 increases in the moisture transport across central america have been proposed to coincide with northerly shifts of the intertropical convergence zone and with dansgaard oeschger interstadials with opposite changes for heinrich events4 here we reconstruct sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years by comparing palaeotemperature estimates from alkenones and mg ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity we detect millennial scale fluctuations of sea surface salinities in the eastern equatorial pacific ocean of up to two to four practical salinity units high salinities are associated with the southward migration of the tropical atlantic intertropical convergence zone coinciding with heinrich events and with greenland stadials5 the amplitudes of these salinity variations are significantly larger on the pacific side of the panama isthmus as inferred from a comparison of our data with a palaeoclimate record from the caribbean basin6 we conclude that millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic deep water formation

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

the acambay graben active intraarc extension in the trans mexican volcanic belt mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench the central part of the belt is being deformed by the chapala tula fault zone an approximately 450 km long and 50 km wide zone of active extension the volcanic arc and the arc parallel chapala tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the mexican basin and range the acambay graben about 40 km long and 15 km wide is located approximately 100 km northwest of mexico city and is one of the major troughs within the chapala tula fault zone the border faults of the acambay graben acambay tixmadeje in the north and pastores in the south are separated in the west by stepovers from range bounding faults of similar orientation epitacio huerta in the north and venta de bravo in the south the stepovers occur at the intersection of these faults with an older system of basin and range faults an early stage right lateral component of motion along the venta de bravo and pastores faults is inferred on a map scale from a left stepping en echelon array of normal fault segments the divergence of the en echelon segments from the general fault trend decreases gradually from west to east suggesting that the early extension was rotational the present relative displacement along the southern margin of the system on the other hand results in a left lateral strike slip component this is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left oblique slip the striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component this is supported on a map scale by the structure of the right stepover between the acambay tixmadeje and epitacio huerta faults which shows no evidence of local extension or shortening the divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation this could explain why no active extension has been observed to the east of the chapala tula fault zone in the eastern part of the trans mexican volcanic belt during the ms 6 9 acambay earthquake of november 19 1912 surface rupture occurred along both margins of the graben at the base of multiple event scarps along the acambay tixmadeje fault the coseismic rupture is 41 km long the vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum furthermore the change in the vertical surface offset along the fault is approximately proportional to the change in height of the acambay tixmadeje multiple event fault scarp the easternmost part of the ground rupture passes through a plain and not at the base of a multiple event scarp as farther west it may therefore correspond partly to an increase in length of the acambay tixmadeje fault during the 1912 earthquake the slip rate along the southern border of the acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40ar 39ar age of 0 4 0 1 ma this basalt may be displaced up to 15 m by the pastores fault which indicates a middle late quaternary slip rate of 0 04 mm yr furthermore based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake we estimate a recurrence interval of 5000 years for major earthquakes along the faults of the acambay graben

late quaternary araucaria forest grassland campos fire and climate dynamics studied by high resolution pollen charcoal and multivariate analysis of the cambara do sul core in southern brazil abstract late quaternary vegetation fire and climate dynamics have been studied based on high resolution dated pollen and charcoal samples and multivariate data analysis the samples were taken from a 212 cm long sediment core of a bog in the cambara do sul region on the highlands of northeastern rio grande do sul state brazil the records including seven ams radiocarbon dates span 42 840 14c years for the first time extending the reconstruction of past environmental changes on the southern brazilian highlands back to the last glacial maximum lgm and pre lgm times the last 1100 years provide a decadal resolution initially the site was a permanent shallow lake which became seasonally dry after 26 900 14c yr bp seasonal climate with a long annual dry period prevailed until the late holocene the climate was somewhat wetter from 42 840 to 41 470 14c yr bp and from 41 470 to 26 900 14c yr bp than during the lgm and the late glacial period natural fires were rare but became very frequent after 7400 cal bp suggesting human occupation of the southernmost brazilian highlands since that time the records suggest that a species rich campos grassland vegetation existed in the area under a relatively dry and cold climate during glacial times under possibly as low as 10 c the record also suggests that small populations of araucaria were probably only present in refugia of deep and protected valleys and or on wetter coastal slopes campos vegetation existed through the early and mid holocene until 4320 cal yr bp after which araucaria forest expanded into the network of gallery forests along the streams by 1100 cal yr bp the araucaria forest replaced the campos vegetation reflecting the onset of the wettest period with no marked annual dry season the marked expansion of the araucaria forest coincided with the reduction in fire between ad 1520 and 1770 weinmannia became a common taxa in the araucaria forest suggesting a shift to warmer climatic conditions on the highlands this interval was synchronous with a cool phase within the little ice age known from north atlantic land records after about ad 1780 human activities changed the original forest composition first by introducing cattle into the forest and than by selective logging of araucaria trees multivariate analysis of the pollen data shows compositional changes that follow a trajectory alternating undirectional random phases and phases with directional sometimes fast transitions the results also show that compositional changes in the vegetation are slower during cool periods lgm compared to pre lgm and faster in warm periods holocene

paleogeographic evolution of early deep water gulf of mexico and margins jurassic to middle cretaceous comanchean the paleobathymetric configuration of the early gulf of mexico is inferred from 1 cretaceous carbonate shelf margins interpreted from seismic profiles and other stratigraphic data 2 distribution of jurassic and cretaceous platform and basinal facies and 3 hindcasting of subsidence history in the central basin substantial paleogeographic ambiguity results from uncertainty about 1 kinematics and timing of late triassic to jurassic extensional opening of the gulf basin which probably involved major strike slip faulting 2 the magnitude of subsequent compressive deformation on the western and southern basin margins and 3 possible accretion of allochthonous terranes the initial sub sea level topographic depression created by crustal extension is manifested by the distribution of callovian pre marine salt and oxfordian basinal marine facies late jurassic sea floor spreading split the main salt body and created a central deep water 1 km trough from the oxfordian to early neocomian ramp like platform margins generally underwent net retreat except in the northern gulf where major clastic progradation took place carbonate platform margins with sufficient paleorelief for geometric expression on seismic profiles developed early in the cretaceous these cretaceous carbonate margins exhibit a variety of architectural styles including 1 in the northwestern gulf two cycles of progradation coahuilan and comanchean terminated by drowning events 2 in the southwestern gulf a single cycle of progradation 3 continuous aggradation along the florida and campeche escarpments paleobathymetric relief across cretaceous carbonate margins ranges from high and steep to low relief to ramplike the close of comanchean drowning event greatly and permanently reduced the extent of circum gulf carbonate platforms and especially rimmed margins locations of cretaceous carbonate margins were influenced by basement hinge zones and paleohighs developed on attenuated continental crust by the distribution of salt and by clastic progradation eustacy is commonly invoked to explain behavior and architecture of carbonate margins in the gulf but other paleoenvironmental variables such as nutrient abundance are probably important as well

evidence of rainfall variations in southern brazil from trace element ratios mg ca and sr ca in a late pleistocene stalagmite trace element ratios in the bt2 stalagmite from botuveracave southern brazil are explored as a proxy for changes in the local rainfall recharge during the last 116 ky bp mg ca and sr ca ratios measured with an electron microprobe are signif icantly positively correlated with one another throughout the entire record and vary in a way that is very consistent with variations of d 18 o in the same speleothem during the last glacial period we suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of mg and sr into calcite of the stalagmite this interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting therefore we conclude that higher lower mg ca and sr ca values are associated with lower higher levels of recharge into the karstic aquifer as such conditions lead to an increase decrease in the volume of calcite precipitated in the unsaturated zone above the cave during dry wet climate periods trace element variations point to generally dryer wetter conditions during lower high phases of summer insolation in the southern hemisphere these periods coincide with decreased increased activity of the south american summer monsoon as revealed by d 18 o stalagmite records in addition trace element variations show that rather wet conditions persisted through out most of the last glacial period from approximately 70 to 17 ky bp we suggest that during this period the glacial boundary conditions especially ice volume buildup in the northern hemisphere played an important role for monsoon rainfall inten sification in the region 2007 elsevier ltd all rights reserved

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

in situ th pb dating of monazite in fibrous veins direct dating of veins and deformation in the shallow upper crust of the mexican orogen abstract few methods exist for directly dating orogenic events in the upper crust here we report in situ dates from 10 m diameter monazite grown in quartz calcite fibrous veins which precipitated at very low grade metamorphic conditions about 250 c in volcaniclastic and calcareous rocks of the early cretaceous trancas fm in the zimapan basin central mexico our integrated study includes regional and local structural and kinematic analysis of vein and folds detailed textural and mineralogical characterization of vein bearing minerals and texturally controlled th pb geochronologic analysis of monazite by la mc icpms we find that monazite in the veins preferentially crystallized at the interface between vein forming calcite and quartz fibers as well as at the vein host rock interface monazite ages range from 154 to 68 ma with the youngest sub population yielding a weighted mean age of 76 8 0 8 ma coincident with the age of basin wide folding as earlier constrained by ar ar illite dating the oldest monazite ages coincide with the timing of deposition of the volcaniclastic host rock given that veins are ubiquitous in shallow crustal rocks this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid rock interaction during of deformation

a computer model of sheath nappes formed during crustal shear in the western gneiss region central norwegian caledonides abstract the eastern western gneiss region of central norway is part of the deepest exposed norwegian caledonides where basement gneisses and an overlying thrust nappe sequence have been folded into large fold nappes structural analysis of a fold nappe within the central part of the district the grovudal area suggests that it has a strongly sheath like form and that other fold nappes of the western gneiss region may also have sheath like forms the structural history within the grovudal area is dominated by intense east directed subhorizontal shear in an overthrust sense followed by asymmetric refolding with an easterly vergence a computer generated kinematic model was developed to test whether the regional interference patterns could be explained by sheath fold development during this type of deformation the computer model shows that the major regional interference patterns could have been formed by such a kinematic history but does not rule out other possibile histories the proposed kinematic history is however compatible with the regional tectonic history of the main caledonian nappe pile suggesting that the complex nappe interference patterns typical of the region were formed in a kinematically simple but intense ductile deformation associated with caledonian continental imbrication

experimental study of sheath fold development around a weak inclusion in a mechanically layered matrix article i nfo we present quantitative laboratory models investigating the mechanics of sheath fold formation around a weak inclusion in simple shear sheath folds are intriguing highly non cylindrical structures that stimulated extensive field and experimental studies leading to an ongoing debate concerning their formation and evolution through a parametric study we test the influence of a mechanically layered matrix on the devel opment of sheath folds using silicone models our models show how 1 the viscosity ratio between the layers in the matrix and 2 the layer thickness control the shapes of the resulting folds and their visibility all experiments with a weak inclusion resulted in strong deformation of the layers an increase in viscosity ratio however leads to less evolved sheath folds which are shorter and show a larger opening and dip angle in contrast to former studies we show that a mechanically layered matrix does not hinder the forma tion of sheath folds the visibility of the sheath folds in our models strongly depends on the aspect ratio be tween the inclusion height and the layer thickness we observed sheath folds for a ratio larger than 7 5 the experiments reproduced the first order features of natural sheath folds our results challenge previously pub lished studies where sheath folds were considered as purely passive structures we demonstrate that sheath folds readily develop around slip surfaces suggesting that this might be a relevant formation mechanism in nature

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

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

kinematic modeling of cross sectional deformation sequences by computer simulation we derive a kinematic algorithm which permits simulation of postulated cross sectional deformation sequences in sedimentary rocks affected by faulting fault related folding and simple shear the transformations are to a more deformed state and are formulated analytically in terms of the less deformed configuration forward modeling lagrangian description the medium is subdivided into domains of constant dip and homogeneous displacement vector fields which are delimited by the axial planes of the fault inflections the displacement trajectory is of constant length for all the displaced particles throughout the medium and parallel to the underlying active fault segment as a consequence the deformation path is continuous but not smooth and causes an angular style of parallel folding except at the deformation front where the layer thickness is not conserved the inhomogeneity of the displacement vector field across axial planes introduces longitudinal and angular shear strains whereas the area of the medium remains constant these strains which are superposed on the externally applied simple shear make the mapping transformation unconformal we define the stratigraphic layering of the undeformed medium by an approximating body of finite quadrilateral elements evaluate the defined displacement functions for the grid nodes and graph the deformed mesh eventually we create with the algorithm synthetic deformation sequences for simple boundary conditions and try in a specific case study to match the resulting synthetics with the available observational and experimental data

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

general geology and evolutionary history of the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt in the northeastern baltic shield abstract the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt is situated in the northeastern part of the baltic shield and is discontinuously developed over a length of ca 1000 km it comprises five main zones which are separated by longitudinal and a number of transverse syndepositional faults each zone contains a number of lithostratigraphic units at the rank of groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16 000 m the sequence comprises a series of sedimentary volcanic rhythms usually separated by stratigraphic breaks marked by palaeo weathering the groups are separated by major disconformities which may mark record rift inversion or orogenic episodes at least seven distinctive lithostratigraphic markers occur in the belt including two komatiitic volcanic series separated by immature conglomerates two red coloured proto evaporitic formations separated by the alkaline volcanics and two different black shale formations perhaps the main characteristic of the belt is the apparently long history of sedimentary volcanic development at the same site spanning a time period of 2500 to 1800 ma a three stage rift development model is considered 1 an intracontinental rift stage 2500 2100 ma separated by the major inversion 2400 2330 ma and comparable with present day afar triangle and east african rifts 2 a transitional from intracontinental to to intercontinental rift stage 2100 1970 ma with possible short lived spreading about 1990 1970 ma 3 a collision related intercontinental rift stage 1970 1800 ma followed by the svecofenian orogeny 1800 1700 ma the observed distribution of platinum group elements pge mineralization is related to layered gabbro norite complexes fedorova and panskie tundry gora general skaya intrusions dated at 2470 ma which are apparently co magmatic and coeval with the first komatiitic volcanic series seidorechka formation found in the eastern part of the belt only the major chromite occurences are connected with a layered gabbro norite intrusion imandra lopoloth which is co magmatic and coeval between 2440 2330 ma with the second komatiitic volcanic series rusinga polisarka akhmalahti and malbekk formations the ni cu deposits of the belt are connected with a differentiated gabbro werhlite complex which is co magmatic and coeval ca 1970 ma with the specific fe and ti rich ferropicrites with komatiitic affinities

fold dating a new ar ar illite dating application to constrain the age of deformation in shallow crustal rocks we propose a deformation dating method that combines xrd quantification and ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay bearing layers two folds in the frontal segment of the mexican fold thrust belt mftb which was deformed from late cretaceous to eocene are used to illustrate the method and its future potential variations in mineral composition illite polytype crystallite size cs and ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds this analysis examines potential effects of strain variation on illitization and the ar isotopic system along folded layers versus possible regional thermal overprints the ar total gas ages for 9 samples in fold 1 vary between 48 4 and 43 9 ma the of 2m1 detrital illite vs ar total gas ages tightly constrains the age of folding at 43 5 0 3 ma nine ages from three samples in fold 2 range from 76 2 to 62 7 ma which results in a folding age of 63 9 2 2 ma both ages are in excellent agreement with more broadly constrained stratigraphic timing the method offers a novel approach to radiometric dating of clay bearing folds formed at very low grade metamorphic conditions and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts

orbitally driven east west antiphasing of south american precipitation the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales a comparison between a speleothem record of precipitation in northeast brazil and rainfall reconstructions from the rest of tropical south america shows that a similar antiphasing operated in the same hemisphere during the holocene the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales this antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the hadley cells1 2 3 4 here we compare an oxygen isotopic record recovered from a speleothem from northeast brazil for the past 26 000 years with existing reconstructions of precipitation in tropical south america5 6 7 8 during the holocene we identify a similar but zonally oriented antiphasing of precipitation within the same hemisphere northeast brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high whereas the rest of southern tropical south america shows opposite characteristics simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation driven monsoon activity our results suggest that convective heating over tropical south america and associated adjustments in large scale subsidence over northeast brazil lead to a remote forcing of the south american monsoon which determines most of the precipitation changes in the region on orbital timescales

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

interaction of the south american monsoon system and the southern westerly wind belt during the last 14 kyr abstract surface currents and sediment distribution of the se south american upper continental margin are under the influence of the south american monsoon system sams and the southern westerly wind belt swwb both climatic systems determine the meridional position of the subtropical shelf front stsf and probably also of the brazil malvinas confluence bmc we reconstruct the changing impact of the sams and the swwb on sediment composition at the upper rio grande cone off southern brazil during the last 14 cal kyr bp combining sedimentological geochemical micropaleontological and rock magnetic proxies of marine sediment core geob 6211 2 sharp reciprocal changes in ferri and paramagnetic mineral content and prominent grain size shifts give strong clues to systematic source changes and transport modes of these mostly terrigenous sediments our interpretations support the assumption that the sams over se south america was weaker than today during most of the late glacial and entire early holocene while the swwb was contracted to more southern latitudes resembling modern austral summer like conditions in consequence the stsf and the bmc were driven to more southern positions than today s favoring the deposition of fe rich but weakly magnetic la plata river silts at the rio grande cone during the mid holocene the northern boundary of the swwb migrated northward while the stsf reached its northernmost position of the last 14 cal kyr bp and the bmc most likely arrived at its modern position this shift enabled the transport of antarctic diatoms and more strongly magnetic argentinean shelf sands to the rio grande cone while sediment contributions from the la plata river became less important during the late holocene the modern el nino southern oscillation set in and the sams and the austral tradewinds intensified causing a southward shift of the stsf to its modern position this reinforced a significant deposition of la plata river silts at the rio grande cone these higher magnetic silts with intermediate fe contents mirror the modern more humid terrestrial climatic conditions over se south america

thermal modelling of the laramide orogeny testing the flat slab subduction hypothesis abstract the laramide orogeny is the late cretaceous to palaeocene 80 55 ma orogenic event that gave rise to the rocky mountain fold and thrust belt in canada the laramide block uplifts in the usa and the sierra madre oriental fold and thrust belt in mexico the leading model for driving laramide orogenesis in the usa is flat slab subduction whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards producing basement cored block uplifts and arc magmatism in the foreland the thermal models presented here indicate that arc magma generation at significant distances inboard of the trench 600 km during flat slab subduction is problematic this conclusion is consistent with the coincidence of volcanic gaps and flat slab subduction at modern convergent margins lawsonite eclogite xenoliths erupted through the colorado plateau in oligocene time are inferred to originate from the subducted farallon slab and indicate that the laramide flat slab subduction zone was characterised by a cold thermal regime thermal modelling indicates that this regime can be produced by flat slab subduction of old 50 myr oceanic lithosphere at high convergence rates in the canadian and mexican portions of the laramide orogen the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat slab subduction in these regions it is proposed that subduction of an oceanic plateau aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat slab subduction in the usa these findings cast doubt on the flat slab model as a primary means of driving laramide orogenesis along its entire length and instead point to the need for an alternative mechanism for cordilleran wide laramide orogenesis

geochronology and geochemistry of the enderbite series in the lapland granulite belt generation tectonic setting and correlation of the belt1nordsim publication no 322 the lapland granulite belt of the northeastern fennoscandian shield is dominated by blastomylonitic migmatitic metagreywackes and a deformed structurally conformable norite enderbite series the b

timing duration and transitions of the last interglacial asian monsoon thorium 230 ages and oxygen isotope ratios of stalagmites from dongge cave china characterize the asian monsoon and low latitude precipitation over the past 160 000 years numerous abrupt changes in 18o 16o values result from changes in tropical and subtropical precipitation driven by insolation and millennial scale circulation shifts the last interglacial monsoon lasted 9 7 1 1 thousand years beginning with an abrupt less than 200 years drop in 18o 16o values 129 3 0 9 thousand years ago and ending with an abrupt less than 300 years rise in 18o 16o values 119 6 0 6 thousand years ago the start coincides with insolation rise and measures of full interglacial conditions indicating that insolation triggered the final rise to full interglacial conditions