characteristic straight chain lipid ratios as a quick method to assess past forest paramo transitions in the ecuadorian andes to assist sustainable reforestation efforts in the andes in northern ecuador the position of the natural upper forest line ufl has to be assessed to this end new vegetation reconstruction techniques to complement traditional reconstruction methods such as pollen analysis are being developed one such technique is the biomarker ratio approach that uses changes in characteristic ratios of extractable lipids such as n alkanes and n alcohols for a quick assessment at high spatial resolution of abrupt past vegetation changes in the present study we assessed the applicability of this approach as indicator of past vegetation shifts from forest to paramo or vice versa in northern ecuador we looked at the n alkane and n alcohol signals of two soil monoliths one taken in a forest patch located 100 m above the current ufl and the other at the same altitude in the surrounding paramo the monoliths were sampled to a depth corresponding to at least 1500 cal yr bc links were sought between biomarker ratio signals in soil organic matter som and those found in plants from the vicinity during previous research based on the outcome an attempt was made at determining whether the forest patch had been part of the continuous forest further down slope during the last 3500 cal years results show that specific ratios of extractable straight chain lipids can be used to get a quick indication of whether an area was forested or not in the past in som the n alkane relationship c29 c31 1 combined with the n alcohol relationship c22 c24 1 was found to be characteristic for dominant forest vegetation while the opposite relationships were indicative for past paramo vegetation the n alkane relationship could be explained very well by the lipid ratios of the current forest and paramo vegetation in the area while agreeing well with the n alkane signal the n alcohol signal was less robust probably partially due to degradation of wax esters in the soil contrary to expectations we conclude that the investigated forest patch was never part of the continuous forest in the vicinity in the past 3500 cal years instead our biomarker ratio approach suggests that the forest patch soil was once covered by paramo vegetation overall this study shows that the biomarker ratio approach in volcanic ash soils can serve as a quick method for an assessment of abrupt vegetation changes such as associated with past ufl shifts at a high spatial resolution and at the location where the shift occurred as such it is highly complementary to traditional proxies such as pollen analysis

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

tectonic and climatic controls on asymmetric half graben sedimentation inferences from 3 d numerical modeling the tectono sedimentary evolution of asymmetric extensional systems driven by the activity of major normal faults or detachments associated with footwall exhumation is often characterized by a sequence of slower faster and ultimately again slower subsidence rates in the center of hanging wall half grabens during their synkinematic and postkinematic evolution we have studied this specific evolution by the means of 3 d stratigraphic numerical modeling that accounts for the variability of the sediment and water flux combined with climatic and sea level variations and sediment compaction the model setup is constrained by observations from the pannonian back arc basin of central europe our modeling predicts the formation of low order tectonic and higher order sea level and climate driven transgressive regressive sedimentary cycles furthermore we model and analyze the autocyclic nature of the depositional systems retrograding prograding cycles are visible on the proximal flank of the half grabens by their different spatial and temporal expressions while depocenters record large water depth variations linked to the specific and episodic activity of normal faults and their migration with time the application to a system of multiple half grabens in the pannonian basin which are activated in different locations at different times and with different kinematics demonstrates a complex interplay between direct sediment sourcing and the sediments ability to bypass trapping subbasins and paleo reliefs created by eroded footwalls

biomarkers as proxies for plant inputs to peats an example from a sub boreal ombrotrophic bog lipid distributions in modern peat forming plants were determined and compared to biomarkers recovered from a ca 2800 year old peat deposited in a dutch ombrotrophic bog the peat section spans a well constrained shift in local hydrology and vegetation and potential molecular proxies for plant inputs were directly tested against the macrofossil record based on the analyses of modern plants low molecular weight lmw n alkanes c23 c25 and high molecular weight n alkanes c33 are proposed as potential biomarkers for sphagnum and non sphagnum inputs respectively consistent with this in the bargerveen peat core lmw n alkane abundances increase and hmw n alkane abundances decrease coincident with the shift to sphagnum dominated peat additional potential proxies include the abundances of c22 alkanedioic acid and phytenyl phytenoate as sphagnum indicators and the abundances of the triterpenoids taraxer 14 ene and taraxast 20 ene as biomarkers for ericaceae rootlets

late holocene changes in vegetation and atmospheric circulation at lake uddelermeer the netherlands reconstructed using lipid biomarkers and compound specific δd analysis we reconstructed middle to late holocene changes in atmospheric circulation patterns and vegetation in northwest europe by applying novel geochemical techniques to the sediment record of lake uddelermeer the netherlands a comparison of higher plant derived leaf wax n alkane distributions archived in the lake sediments with those in living plant material combined with palynological analysis indicates that the vegetation immediately surrounding the lake became more open at 3150 cal a bp while the regional vegetation responded more gradually and 650 years later our record of the hydrogen isotopic composition of plant leaf waxes dwax shows a deuterium enrichment starting from 3500 cal a bp which we interpret as a change in atmospheric circulation a similar dwax record from nearby meerfelder maar germany shows an opposite trend around this time which could be explained by a change in sea level pressure resembling a negative north atlantic oscillation phase this could account for depleted d values of precipitation at meerfelder maar while confounding factors related to the more maritime position of uddelermeer cause the opposite shift there

stratigraphic record of basin development within the san andreas fault system late cenozoic fish creek vallecito basin southern california the fish creek vallecito basin contains a 5 5 km thick section of late miocene to early pleistocene sedimentary rocks exposed in the hanging wall of the west salton detachment fault these deposits preserve a high fidelity record of late cenozoic subsidence and basin filling that resulted from deformation in the san andreas fault system of southern california existing and new paleomagnetic data combined with new u pb zircon ages of two tuffs high in the section show that the section ranges in age from ca 8 0 0 4 ma at the base to ca 0 95 ma at the top geohistory analysis reveals 1 moderate subsidence 0 46 mm yr from ca 8 0 to 4 5 ma 2 rapid subsidence 2 1 mm yr from 4 5 to 3 1 ma 3 moderate subsidence 0 40 mm yr from 3 1 to 0 95 ma and 4 rapid uplift and erosion that has exhumed the section since ca 1 ma onset of sedimentation at ca 8 0 0 4 ma records earliest extension or transtension in the area possibly related to localization of the pacific north america plate boundary in the salton trough and gulf of california alternatively marine incursion at 6 3 ma may be the earliest record of plate boundary deformation in the gulf of california salton trough region a thick interval higher in the section records progradation of the colorado river delta into and across the basin starting ca 4 9 ma progradation continued during an abrupt increase in subsidence rate at 4 5 ma and fluvial deltaic conditions persisted for 1 4 m y during the rapid subsidence phase indicating that delta progradation was driven by a large increase in rate of sediment input from the colorado river uplift and inversion of the basin starting ca 1 0 ma record initiation of strike slip faults that define the modern phase of dextral wrench tectonics in the western salton trough

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

quantification of biotic responses to rapid climatic changes around the younger dryas a synthesis to assess the presence or absence of lags in biotic responses to rapid climatic changes we 1 assume that the 18o in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the younger dryas without any lag at our two study sites of gerzensee and leysin switzerland 2 derive a time scale by correlating the 18o record from these two sites with the 18o record of the grip ice core 3 measure 18o records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes 4 analyse at gerzensee and leysin as well as at two additional sites that lack carbonates and hence a 18o record pollen plant macrofossils chironomids beetles and other insects and cladocera 5 estimate our sampling resolution using the grip time scale for the isotope stratigraphies and the biostratigraphies and 6 summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis we conclude that at the major climatic shifts at the beginning and end of the younger dryas hardly any biotic lags occur within the sampling resolution of 8 30 years and that upland vegetation responded as fast as aquatic invertebrates we suggest that the minor climatic changes associated with the gerzensee and preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site but were more distinct at higher altitudes individualistic responses of plant and animal species to climatic change may reflect processes in individuals e g productivity and phenology in populations e g population dynamics in spatial distributions e g migrations and in ecosystems e g trophic state we suggest that biotic responses may be telescoped together into relatively short periods 50 to 150 years perhaps disrupting functional interactions among species and thus destabilising ecosystems

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

how sediment promotes narrow rifting application to the gulf of california 1 if the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension then sedimentation should also affect that mode we propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning allowing a rift to extend more easily in a narrow rift mode we examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two dimensional regional scale numerical experiments of extending thick continental crust with varied initial thermal conditions geometries rheological parameters and sedimentation properties depending on initial model conditions the onset of rifting in thick continental crust occurs in the narrow rifting wide rifting or core complex mode with continued extension all cases eventually transition to a narrow rifting mode for a system in wide rift mode moderate to fast sedimentation shortens the time needed to transition to a narrow rift in the gulf of california changes in extensional style correlate with sediment thickness with an earlier transition to narrow rifting in the north versus the south we compare our model results to the extensional history of the gulf of california where sediments may have caused the northern and north central domain of the gulf delfin tiburon and guaymas segments to transition to narrow rifting before the south central domains alarcon segment

late quaternary environmental changes in misiones subtropical ne argentina deduced from multi proxy geochemical analyses in a palaeosol sediment sequence abstract a 4 5 m long sediment core from a small basin in the province of misiones ne argentina was analyzed in a multi proxy geochemical approach major and minor elemental composition c org n hi oi 13 c org n alkanes and compound specific 13 c analyses of biomarkers in order to contribute to the reconstruction of the late quaternary environmental and climate history of subtropical south america the results of the elemental analyses and radiocarbon dating indicate different sediment provenances for unit a the holocene unit b the late glacial and the last glacial maximum lgm and unit c the inca huasi wet phase before 40 ka bp a sedimentary hiatus after 40 ka bp is interpreted as a pronounced pre lgm dry phase with landscape erosion deflation multi proxy geochemical characterization of the soil organic matter som shows that especially i the stable carbon isotopic composition 13 c of the grass derived alkanes n c 31 and n c 33 ii the alkane ratio n c 31 n c 27 and iii lacustrine derived short and mid chain alkanes are valuable proxies for the reconstruction of the palaeoenvironment a transition from c3 tree dominance to c4 grass dominance is recorded at the end of the inca huasi wet phase in unit b the ratio n c 31 n c 27 documents forest expansion at the beginning of a late glacial wet phase more positive 13 c values in unit a reflect the increasing contribution of c4 grasses and or cam plants to the som during the holocene and a human impact on the formation of this unit may be possible the results are in good agreement with other tropical subtropical palaeoenvironmental records and highlight the importance and temporal variability of the palaeo south american summer monsoon sasm

control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low viscosity asthenosphere model results reproduce the typical evolution of deformation during continental narrow rifting with early activation of large boundary faults and basin subsidence followed by their abandonment and localization of tectonic activity in internal faults near the centre of the rift the experiments document the strong influence exerted by the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation on the evolution of deformation namely on the amount of bulk extension preceding inward fault migration thin upper and or lower crust and absent or low syn rift sedimentation promote a rapid abandonment of boundary faults and a transition to in rift fault development for low amounts of extension conversely thick upper and or lower crust and high syn rift sediment accumulation favour prolonged slip on boundary faults and delayed development of internal faulting the experimental results suggest that the inward migration of faulting during extension of continental lithosphere results from the interplay between the ductile stresses acting at the base of the upper crust and the total resistance of this brittle layer

asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern south atlantic abstract the southern south atlantic has often been considered a classic example of continental break up in the presence of a starting mantle plume evidence for a mantle plume includes the parana etendeka continental flood basalts which are associated with the rio grande rise and walvis ridge and the wide spread presence of seaward dipping reflectors and high velocity lower crustal bodies along the conjugate margins observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break up in this region and consequent numerical modelling was used to test this we tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated this was done through the use of 37 measurements of initial oceanic crustal thickness from wide angle and multichannel seismic profiles collected along the conjugate margins these measurements show that at 450 km south of the parana etendeka flood basalts the oceanic crust is thicker than the global average at 11 7 km farther south the oceanic crust thins reaching 6 1 km at a distance of 2300 km along strike overall the along strike trend of oceanic crustal thickness is linear with a regression coefficient of 0 7 and little indication of segmentation from numerical models representing extension of the lithosphere we find that observed melt volumes are matched with the presence of a hot layer if we assume this region of hot mantle has a thickness of 100 km its excess temperature relative to the asthenosphere has to decrease from 200 to 50 c north to south this decrease in temperature also seen in published thermobarometry results suggests that temperature was the primary control of volcanism during the opening of the southern south atlantic

a bottom driven mechanism for distributed faulting in the gulf of california rift abstract observations of active faulting in the continent ocean transition of the northern gulf of california show multiple oblique slip faults distributed in a 200 70 km 2 area developed some time after a westward relocation of the plate boundary at 2 ma in contrast north and south of this broad pull apart structure major transform faults accommodate pacific north america plate motion here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the northern gulf where basal shear is distributed between the cerro prieto strike slip fault southernmost fault of the san andreas fault system and the ballenas transform fault we hypothesize that in oblique extensional settings whether deformation is partitioned in a few dip slip and strike slip faults or in numerous oblique slip faults may depend on 1 bottom driven distributed extension and shear deformation of the lower crust or upper mantle and 2 the rift obliquity to test this idea we explore the effects of bottom driven shear on the deformation of a brittle elastic plastic layer with the help of pseudo three dimensional numerical models that include side forces strain localization results when the basal shear abruptly increases in a step function manner while oblique slip on numerous faults dominates when basal shear is distributed we further explore how the style of faulting varies with obliquity and demonstrate that the style of delocalized faulting observed in the northern gulf of california is reproduced in models with an obliquity of 0 7 and distributed basal shear boundary conditions consistent with the interpreted obliquity and boundary conditions of the study area

three dimensional numerical modeling of upper crustal extensional systems 1 we focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models to date few 3 d models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones we use an arbitrary lagrangian eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions plastic materials weaken with accumulating strain to localize deformation a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions controls on the geometry and spacing of three dimensional plastic shear zones are investigated the sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested the model results indicate the primary controls of strain dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction the amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other resulting in a trade off effect between the amount of offset between the initial grabens and the strain weakening ratio the three dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions 1 small offset grabens with a single relay zone 2 intermediate offset grabens with one or more secondary step over graben segments and 3 large offset grabens with limited to no significant segment interaction

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

modern pollen vegetation relationships along a steep temperature gradient in the tropical andes of ecuador the characterization of modern pollen rain assemblages along environmental gradients is an essential prerequisite for reliable interpretations of fossil pollen records in this study we identify pollen vegetation relationships using modern pollen rain assemblages in moss polsters n 13 and lake sediment surface samples n 11 along a steep temperature gradient of 7 c 3100 4200 m above sea level on the western andean cordillera ecuador the pollen rain is correlated to vascular plant abundance data recorded in vegetation relevees n 13 results show that pollen spectra from both moss polsters and sediment surface samples reflect changes in species composition along the temperature gradient despite overrepresentation of upper montane forest taxa in the latter estimated pollen transport distance for a lake laguna llaviucu situated in a steep upper montane forest valley is 1 2 km while a lake laguna pallcacocha in the paramo captures pollen input from a distance of up to 10 40 km weinmannia spp podocarpus spp and hedyosmum sp are indicators of local upper montane forest vegetation while phlegmariurus spp and plantago spp are indicators for local paramo vegetation

seismic structure beneath the gulf of california a contribution from group velocity measurements rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the gulf of california region we compute group velocity maps for rayleigh waves from 10 to 150 s using earthquakes recorded by broad band stations of the network of autonomously recording seismographs in baja california and mexico mainland unm in mexico bor dpp and gor in southern california and tuc in arizona the study area is gridded in 120 longitude cells by 180 latitude cells with an equal spacing of 10 10 km assuming that each gridpoint is laterally homogeneous for each period the tomographic maps are inverted to produce a 3 d lithospheric shear wave velocity model for the region near the gulf of california rift axis we found three prominent low shear wave velocity regions which are associated with mantle upwelling near the cerro prieto volcanic field the ballenas transform fault and the east pacific rise upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the gulf this more detailed finding is new when compared to previous surface wave studies in the region a low velocity zone in northcentral baja at 28on which extends east south eastwards is interpreted as an asthenospheric window in addition we also identify a well defined high velocity zone in the upper mantle beneath central western baja california which correlates with the previously interpreted location of the stalled guadalupe and magdalena microplates we interpret locations of the fossil slab and slab window in light of the distribution of unique post subduction volcanic rocks in the gulf of california and baja california we also observe a high velocity anomaly at 50 km depth extending down to 130 km near the southwestern baja coastline and beneath baja which may represent another remnant of the farallon slab

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

chronology of miocene pliocene deposits at split mountain gorge southern california a record of regional tectonics and colorado river evolution late miocene to early pliocene deposits at split mountain gorge california preserve a record of basinal response to changes in regional tectonics paleogeography and evolution of the colorado river the base of the elephant trees formation magnetostratigraphically dated as 8 1 0 4 ma provides the earliest well dated record of extension in the southwestern salton trough the oldest marine sediments are ca 6 3 ma the nearly synchronous timing of marine incursion in the salton trough and northern gulf of california region supports a model for localization of pacifi c north america plate motion in the gulf ca 6 ma the fi rst appearance of colorado river sand at the miocene pliocene boundary 5 33 ma suggests rapid propagation of the river to the salton trough and supports a lake spillover hypothesis for initiation of the lower colorado river

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

regional hydrogeochemistry of a modern coastal mixing zone in west central florida groundwater samples were collected along flow paths in the unconfined upper floridan aquifer that cross the inland freshwater recharge area and the coastal discharge area a groundwater flow and solute transport model was used to evaluate groundwater flow and mixing of fresh and saline groundwater along a cross section of the unconfined upper floridan aquifer results show that between 8 and 15 of the fresh and 30 31 of the saline groundwater penetrates to the depth in the flow system where contact with and dissolution of gypsum is likely the deeply circulating fresh and saline groundwater returns to the near surface environment discharging caso4 rich water to the coastal area where it mixes with fresh cahco3 groundwater resulting in a prediction of calcite precipitation in the modern mixing zone

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the northern gulf of california analysis of seismic reflection profiles 1 the transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern gulf of california across the obliquely conjugate tiburon upper delfin basin segment structural mapping on a 5 20 km grid of seismic reflection lines of petroleos mexicanos demonstrates that 1000 extension is accommodated on a series of nne striking listric normal faults that merge at depth into a detachment fault the detachment juxtaposes a late neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift two northwest striking dextral oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport a regional unconformity in the upper 0 5 s two way travel time and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the upper delfin and lower delfin basins the maximum length of new crust in both delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge shaped upper sequence that reaches 5 km in thickness a fundamental difference exists between the tiburon delfin segment and the guaymas segment to the south in terms of presence of low angle normal faults and amount of new oceanic lithosphere which we attribute to thermal insulation diffuse upper plate extension and slip on low angle normal faults engendered by a thick sedimentary lid

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

the formation of graben morphology in the dead sea fault and its implications the dead sea fault dsf is a 1000 km long continental transform it forms a narrow and elongated valley with uplifted shoulders showing an east west asymmetry which is not common in other continental transforms this topography may have strongly affected the course of human history several papers addressed the geomorphology of the dsf but there is still no consensus with respect to the dominant mechanism of its formation our thermomechanical modeling demonstrates that existence of a transform prior to the rifting predefined high strain softening on the faults in the strong upper crust and created a precursor weak zone localizing deformations in the subsequent transtensional period together with a slow rate of extension over the arabian plate they controlled a narrow asymmetric morphology of the fault this rift pattern was enhanced by a fast deposition of evaporites from the sedom lagoon which occupied the rift depression for a short time period

modes of extensional faulting controlled by surface processes we investigate the feedbacks between surface processes and tectonics in an extensional setting by coupling a 2 d geodynamical model with a landscape evolution law focusing on the evolution of a single normal fault we show that surface processes significantly enhance the amount of horizontal extension a fault can accommodate before being abandoned in favor of a new fault in simulations with very slow erosion rates a 15 km thick brittle layer extends via a succession of crosscutting short lived faults heave 10 km using simple scaling arguments we quantify the effect of surface mass removal on the force balance acting on a growing normal fault this leads us to propose that the major range bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes

sedimentation and crustal recycling along an active oblique rift margin salton trough and northern gulf of california transtensional basins embedded in the san andreas fault system of southern california united states and northwestern mexico are fi lled with sediment derived from the colorado river which drains a large area of the western u s interior the sediment is rapidly buried heated and mingled with intrusions in the deep basins to form a new generation of recycled crust along the active plate boundary using a range of values for total basin depth relative volume of mantle derived intrusions and composition of early rift deposits the volume of colorado river derived sediment in the basins is bracketed between 2 2 and 3 4 10 5 km 3 similar to the volume of rock that likely was eroded from the colorado river catchment over the past 5 6 m y the volumetric rate of crustal growth by sedimentation is 80 130 km 3 m y km comparable to growth rates in subductionrelated island arcs and slow seafl oor spreading centers sedimentary and basinal processes thus play a major role in crustal evolution and recycling in this setting and may be important at other rifted margins where a large river system is captured following tectonic collapse of a prerift orogenic highland

high frequency floral changes at the paleocene eocene boundary revealed by comparative biomarker and palynological studies abstract the cap d ailly area upper normandy france shows several terrestrial lagoonal sections recording the negative carbon isotope excursion cie associated with the paleocene eocene thermal maximum petm 55 8 ma a study of the biomarkers and spore pollen content of the vasterival section gave complementary information on paleofloral changes that occurred around the paleocene eocene p e boundary aliphatic hydrocarbon fractions revealed a high abundance of vascular plant biomarkers including tricyclic diterpanes derived from conifers fernenes derived from ferns and des a triterpenes derived from angiosperms whereas the vegetation of the depositional environment surroundings seemed alternately dominated by ferns and angiosperms revealed from biomarkers the regional flora seemed dominated strictly by angiosperms such as juglandaceae revealed by pollen spore pollen assemblages revealed no major turnover in the paleovegetation but slight variation in vegetation composition however both biomarkers and spores pollen exhibited two intervals of rapid floral change the first occurred around the p e boundary and the second at the top of the strictly terrestrial unit that coincides with a marine transgression comparison of the data with published paleoclimatic results obtained from the same samples indicated that the first interval of floral change correlates with strong paleohydrological perturbations whereas the second is linked to an increasing marine influence

calcite cementation in the upper floridan aquifer a modern example for confined aquifer cementation models detailed petrographic study of the oligocene suwannee limestone a principal unit of the upper floridan aquifer system indicates that all the calcite cements in the suwannee limestone are precompaction in origin most are clearly associated with either early mineralogical stabilization or oligocene subaerial exposure features none of the cements are in isotopic 18 o and 13 c or trace element fe mn and sr equilibrium with the extant aquifer waters the cements therefore formed prior to the onset of regionally confined freshwater aquifer conditions these rocks have been basically inert with respect to calcite cementation during the past 5 to 10 m y while they have been part of the regional confined aquifer using this modern aquifer as an example it is likely that confined paleoaquifer cementation has been overestimated as an explanation for regional cementation patterns in ancient limestones

slip on normal faults induced by surface processes after the cessation of regional extension insights from three dimensional numerical modelling abstract in regions of active extension normal faulting generates topography that is continuously modified by erosion sediment transport and deposition as shown by previous numerical models with full coupling between tectonics and surface processes the redistribution of mass at the earth s surface accelerates the rate of faulting by affecting the stress state of the crust it remains unknown however how fault slip evolves as a result of ongoing surface processes if regional extension as the main driver of faulting ceases here we use three dimensional finite element modelling to show that surface processes acting on normal fault bounded mountain ranges may sustain fault slip for millions of years even after regional extension has stopped the models consist of two successive phases during the first phase the normal fault accumulates displacement owing to an extensional boundary condition while erosion and sediment deposition are active on the model surface at the beginning of the second phase extension of the model is stopped while the surface processes remain active the results show that in most models normal faulting continues during the second phase at rates of 20 to 70 m ma for more than 1 ma in some experiments normal slip is maintained for 3 ma whereas in other models a short phase of normal faulting is followed by slow reverse slip the maximum amount of normal slip in different experiments reaches up to 90 m during the second model phase if erosion is intensified by increasing the diffusion constant by a factor of 5 the fault accumulates the additional normal slip at a faster rate i e during a shorter time period in contrast a five fold variation of the fluvial erosion constant does not significantly affect the fault slip evolution variations of the fault dip and length have a similar effect on the duration of the phase with additional normal slip as variations of the diffusion constant the fault slip evolution is correlated with the temporal evolution of the erosion and sedimentation rates which decrease more or less gradually after the end of extension ultimately the fault slip behaviour is controlled by the evolution of the differential stress which varies through time due to the redistribution of mass on the model surface that is induced by erosion sediment transport and deposition our results imply that individual normal faults may remain active even if regional extension ceases because surface processes continue to modify the fault generated topography

does slab window opening cause uplift of the overriding plate a case study from the gulf of california abstract complete subduction of an oceanic plate results in slab window opening a key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab window opening generate a detectable topographic signature in the overriding plate we focus on the baja california peninsula which incorporates the western margin of the gulf of california rift the topography and tectonics of the rift flank along the peninsula are strongly bimodal north of the puertecitos accommodation zone the primary drainage divide attains a mean elevation of ca 1600 m above sea level asl above an asthenospheric slab window opened by pacific farallon spreading ridge subduction along this section of the trench at ca 17 15 ma to the south mean topography decreases abruptly to ca 800 m asl excluding the structurally distinct los cabos block at the southern tip of the peninsula above fragments of the oceanic farallon slab which stalled following slab tear off at ca 15 14 ma along the peninsula a low relief surface established atop miocene subduction related volcaniclastic units has been incised by a west draining canyon network in response to uplift these canyons exhibit cut and fill relationships with widespread post subduction lavas here we utilise landsat and digital elevation model dem data integrated with previously published k ar and 40ar 39ar lava crystallisation ages to constrain the onset of rift flank uplift to ca 9 5 ma later than slab window formation in the north and ca 11 10 ma later in the south these greatly exceed response time estimates of ca 2 ma or less for uplift triggered by slab window opening instead uplift timing of the high elevation northern region is consistent with lower lithospheric erosion driven by rift related convective upwelling to the south stalled slab fragments likely inhibited convective return flow preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting

improved reconstruction of palaeo environments through unravelling of preserved vegetation biomarker patterns montane forest composition and specifically the position of the upper forest line ufl is very sensitive to climate change and human interference as a consequence reconstructions of past altitudinal ufl dynamics and forest species composition are crucial instruments to infer past climate change and assess the impact of pre historic human settlement one of the most detailed methods available to date to reconstruct past vegetation dynamics is the analysis of fossil pollen unfortunately fossil pollen analysis does not distinguish beyond family or generic level in most cases while its spatial resolution is limited amongst others by wind blown dispersal of pollen affecting the accuracy of pollen based reconstructions of ufl positions to overcome these limitations we developed a new method based on the analysis of plant specific groups of biomarkers preserved in suitable archives such as peat deposits that are unravelled into the plant species of origin by the newly developed verhib model here we present this new method of biomarker analysis and describe its first application in a peat sequence from a biodiversity hotspot of montane rainforest in the ecuadorian andes we show how a combination of the new biomarker application with conventional pollen analysis from the same peat sequence yields a reconstruction of past forest compositions including ufl dynamics with previously unattainable detail

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

pollen inferred palaeoclimate reconstructions in mountain areas problems and perspectives transfer functions are an efficient tool for the quantitative reconstruction of past climate from low to mid elevation pollen sites however the application of existing methods to high altitude pollen assemblages frequently leads to unrealistic results in the aim of understanding the causes of these biases the standard best modern analogue method has been applied to two high altitude pollen sequences to provide quantitative climate estimates for the lateglacial and holocene periods both pollen sequences laghi dell orgials 2130 m sw aspect and lago delle fate 2240 m e aspect are located in the subalpine belt on opposing sides of the st anna di vinadio valley italian maritime alps different results were obtained from the two sequences the largest differences occurred in palaeotemperature reconstruction with notable differences in both the values and trends at each site these biases may be attributed to 1 a lack of high elevation best modern analogues in the database of modern samples 2 the problem of pollen taxa that have multiple climatic significance 3 problems related to the complexity of mountainous ecosystems such as the phenomenon of uphill transport of tree pollen by wind possible improvements to the reconstruction process are discussed copyright 2006 john wiley sons ltd

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

tephra stratification of volcanic ash soils in northern ecuador we combined proxies traditionally used in stratigraphic research mineral assemblages grain size distribution and element ratios with soil organic carbon contents and radiocarbon dating both at a high vertical resolution to unravel the tephra stratigraphy in volcanic ash soils our results show that soil profiles along an altitudinal transect intersecting the upper forest line in northern ecuador were formed in three distinct tephra deposits although the deposits contained a similar assemblage of minerals we were able to differentiate these deposits because of their characteristic organic carbon distribution grain size distribution and typical sro to na2o cao and crystalline al2o3 ratios unravelling the tephra stratigraphy improved understanding of the vertical distribution of soil organic carbon including paleoecological proxies in the studied soils we demonstrated that bioturbation likely plays an important role in current pedogenesis resulting in overprinting merging mixing of the paleosol surprisingly in spite of bioturbation a linear age depth relationship exists leading to the hypothesis that the active zone of bioturbation shifted upwards during soil formation therefore we conclude that paleoecological proxies are stratified in our soils albeit probably somewhat more crudely than in undisturbed peat bogs or lake sediments

marine incursion synchronous with plate boundary localization in the gulf of california volcanic strata on southwest isla tiburon define the age of interstratified marine rocks and through revision of existing correlations the age of the proto gulf of california marine incursion a 5 7 0 2 ma ash flow was emplaced at the base of the marine section a rhyodacite dike and its related lava flow dated as 11 2 1 3 ma 3 7 0 9 ma and 4 2 1 8 ma intrudes and overlies respectively the marine rocks the 11 2 ma age which was the core datum for a middle miocene proto gulf of california origin for the underlying rocks is discordant with all other isotopic and microfossil ages an alternative interpretation utilizing all available geologic and geochronologic data except this discordant age is that marine strata on isla tiburon are latest miocene to early pliocene age reinterpretation of these strata supports a simplified history of marine incursion into the gulf of california marine rocks as old as 8 2 ma in the southern gulf of california indicate an early marine incursion perhaps flooding a region of more intense proto gulf of california continental extension flooding of the entire basin by 6 5 6 3 ma correlates to the sudden onset of significant pacific north american plate boundary motion within the gulf of california

thermomechanical implications of sediment transport for the architecture and evolution of continental rifts and margins erosion and deposition redistribute mass as a continental rift evolves which modifies crustal loads and influences subsequent deformation surface processes therefore impact both the architecture and the evolution of passive margins here we use coupled numerical models to explore the interactions between the surface crust and lithosphere this interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink if transport is efficient there are two possible outcomes 1 faulting within the zone of extension is longer lived and has larger offsets this implies a reduction of the number of faults and the width of the proximal domain 2 efficient transport of sediment leads to significant deposition and hence thermal blanketing this will induce a switch from brittle to ductile deformation of the upper crust in the distal domains the feedbacks between these two outcomes depend on the extension history the underlying lithospheric rheology and the influence of submarine deposition on sediment transport high erosion sedimentation during early faulting leads to abrupt crustal necking while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains in models where rheological conditions favor the formation of asymmetric conjugate margins only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization these models suggest that passive margin architecture can be strongly shaped by the solid earth structure sea level and climatic conditions during breakup

rhizoliths in loess evidence for post sedimentary incorporation of root derived organic matter in terrestrial sediments as assessed from molecular proxies abstract loess palaeosol sequences are important terrestrial archives for studying quaternary climate change a major assumption for palaeoenvironmental reconstruction based on loess organic matter om is that it represents the signal of syn sedimentary incorporated plant biomass i e predominantly grass vegetation however recent studies on rhizoliths roots encrusted by secondary carbonate formed in loess reveal the possibility of post sedimentary penetration of loess palaeosol sequences by deeply rooting plants this likely led to incorporation of younger root derived om into surrounding loess potentially causing an overprinting of the initial plant derived signal of loess om to obtain information on the source vegetation of rhizoliths and surrounding loess om we examined rhizoliths and loess from nussloch sw germany using alkane and fatty acid fa molecular proxies moreover the lipid composition was compared in transects from rhizoliths via rhizosphere loess towards root free loess for a preliminary assessment towards quantifying the post sedimentary input of root derived om in loess loess om gave a combined signal from shoot and root biomass from grass vegetation confirming the assumption of deposition during glacial periods with scarce grass vegetation cover in contrast the lipid composition of rhizolith om reflected roots of woody vegetation indicating the post sedimentary character of rhizoliths stronger degradation of om together with a large content of microorganism derived fas in the former rhizosphere indicated rhizodeposition associated with high microbial activity in loess adjacent to rhizoliths at least up to a distance of 5 cm rhizosphere loess and reference loess at a distance of 50 70 cm showed a significantly different om composition thereby revealing the incorporation of considerable portions of root derived om into loess in the vicinity of roots further studies are necessary for an exact quantification of this potential overprint of the syn sedimentary loess om which might cause uncertainty in palaeoenvironmental studies

petrogenesis of early cretaceous alkaline dolerite dykes in the dabie orogen china constraints on the timing of lithospheric thinning abstract the dabie orogen in central china has long been proposed to transform from triassic compression related to northward deep subduction to late jurassic to early cretaceous extension involving marked crustal thinning and doming processes zircon u pb ages mineral chemistry and whole rock geochemistry of alkaline dykes intruding early cretaceous intermediate mafic ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension these dykes of dominantly dolerites with minor monzodiorites have zircon u pb ages of 128 3 129 7 ma and are characterized by sr nd isotopic ratios around the enriched mantle i the dolerites can be divided into two groups with the first group having lower incompatible elements but more enriched sr nd isotopes than the second group equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation systematic normative minerals trace elements and isotopic contrasts of early cretaceous gabbros 125 130 ma dolerite dykes of this study and early cretaceous mafic dykes 111 115 ma indicate that the source regions for the dolerites are deeper than those for the host gabbros therefore the lithospheric mantle thickening probably continued until the formation of the dolerites and the lithospheric mantle thinning occurred significantly later than previously proposed the thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125 130 ma but changed to less than that for the source region of tholeiitic basalts before 115 ma suggesting a minimum decrease of 20 km within 10 m y this rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 ma

slip history of the la cruz fault development of a late miocene transform in response to increased rift obliquity in the northern gulf of california abstract the gulf of california rift has accommodated oblique divergence of the pacific and north america plates in northwestern mexico since miocene time due to its infancy its rifted margins preserve a rare onshore record of early continental break up processes and an opportunity to investigate the role of rift obliquity in strain localization we map rift related structures and syn tectonic basins on southern isla tiburon a proximal onshore exposure of the rifted north america margin we integrate analysis and geochronology of syn tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity on southern isla tiburon an early phase of extension initiated between 19 17 ma and 12 2 ma subsequently these normal faults and related basins were cut by the la cruz strike slip fault and buried by deposits of the la cruz basin an elongate fault controlled trough coextensive with the la cruz fault crosscutting relationships show that the nw striking la cruz fault accrued 5 2 km of dextral slip 8 4 ma the la cruz fault and parallel tiburon transform were kinematically linked to detachment faulting that accommodated latest miocene to pliocene oblique opening of the offshore upper tiburon pull apart basin the onset of strike slip faulting on isla tiburon was synchronous with the 8 6 ma onset of transform faulting and basin formation along 1000 km of the reconstructed pacific north america plate boundary this transition coincides with the commencement of a clockwise azimuthal shift in pacific north america relative plate motion that increased the obliquity of the gulf of california rift and formed the gulf of california shear zone the record from the proto gulf of california illustrates how highly oblique rift geometries where transform faults are kinematically linked to pull apart basins enhance the ability of continental lithosphere to rupture and ultimately hasten the formation of new oceanic rift basins

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

the role of long term rifting history on modes of continental lithosphere extension continental lithosphere extension results in complex basin types with differing structural styles subsidence thermal histories and melt production many studies have examined the role of initial rheological layering geothermal gradients and extension rates during a single rifting event this approach neglects the tectonic history of many basins that are marked by multiple rifting events here we address the role of repeated extension on long term lithospheric strain modes and the resulting basins highlighting cases most affected by previous rifting events we use numerical models of a lithosphere undergoing two rifting events of differing extension rates and separated by cooling to show the effect of early events on subsequent evolution the combination of boundary displacement velocity in both events leads to the formation of various rift basin types ranging from narrow to wide to hyperextended and with variation of subsidence patterns degrees of symmetry and melt yield we show that basin type subsidence and melt production might be strongly affected by previous rifting events illustrating cases in which the previous rifting history cannot be neglected our models reproduce the first order features of earth s sedimentary basins and propose a classification to guide the interpretation of extensional basins and their evolution

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

middle miocene extension in the gulf extensional province baja california evidence from the southern sierra juarez new geologic mapping structural studies and geochronology of miocene volcanic and sedimentary rocks in the southern sierra juarez baja california shed light on the extensional history of the gulf extensional province prior to sea floor spreading in the gulf of california the southern sierra juarez is underlain by lower middle miocene rocks including fluvial strata intermediate composition volcanic deposits basalt lava flows and cinder cones and dacite pyroclastic deposits and lavas that nonconformably overlie the cretaceous peninsular ranges batholith the 40 ar 39 ar geochronology indicates that basaltic rocks are 16 90 0 05 ma and dacite pyroclastic deposits are between 16 69 0 11 ma and 15 98 0 13 ma these strata were subsequently cut by two generations of faults first generation faults comprise a dominant set of north south striking west dipping normal faults a secondary set of north south striking east dipping normal faults and a lesser set of variably oriented strike slip faults all three fault sets are temporally and spatially related and were produced by east west extension the dominant west dipping faults which are antithetic to and oblique to the east dipping main gulf escarpment may have been a precursor or an early phase accommodation zone along the escarpment west dipping normal faults are cut by a 10 96 0 05 ma dacite hypabyssal intrusion thus bracketing the age of east west extension between 15 98 0 13 ma and 10 96 0 05 ma hence this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated miocene extension within baja california second generation faults which are comprised of east west striking strike slip faults that cut first generation faults and associated northwest striking northeast dipping normal faults may be related to early development of the transpeninsular strike slip province global plate reconstructions suggest that transtensional motion between the north american and pacific plates along the western margin of baja california began during middle miocene time coeval with east west extension in the southern sierra juarez this observation supports a hypothesis that middle miocene transtensional plate motion was partitioned into two components a strike slip component parallel to active faults along the western offshore margin of baja california and an extensional component normal to the margin but located in what is now the gulf extensional province hence the onset of extension within the circum gulf region was in response to plate boundary processes

effects of a temperature dependent rheology on large scale continental extension the influence of a temperature dependent rheology on large scale continental extension is investigated using a thin viscous sheet model a vertically averaged rheology is used that is consistent with laboratory experiments on power law creep of olivine and that depends exponentially on temperature the behavior of the model depends principally on two parameters the peclet number which describes the relative rates of advection and diffusion of heat and a dimensionless activation energy which controls the temperature dependence of the rheology numerical calculations of such a thin sheet subjected to an extensional boundary condition show the following results at short times following the beginning of extension deformation occurs with negligible change in temperature so that only small changes in lithospheric strength occur due to attenuation of the lithosphere and maximum rates of deformation are located close to the extensional boundary however after a certain time interval thermal diffusion results in lowered temperatures in the lithosphere strongly increasing lithospheric strength and slowing the rate of extension this critical time depends principally on the peclet number and is short compared with the thermal time constant of the lithosphere the changes in strength cause the locus of high extensional strain rates to shift with time from regions of high strain to regions of low strain results of the calculations are compared with observations from the aegean where maximum extensional strains are found in the south near crete but the largest present day extensional strain rates occur about 300 km further north the comparisons support the hypothesis that the observed separation of the loci of maximum strain and maximum present day strain rates in the aegean may be the consequence of changes in lithospheric strength arising from the temperature dependent mechanical properties of lithospheric materials

transtensional rifting in the proto gulf of california near bahia kino sonora mexico continental rifts require focused strain to rupture and form an ocean basin in oblique rifts such as the gulf of california focused transtensional strain associated with strike slip faulting may serve as a catalyst for rupture to test this hypothesis we analyzed structural mapping geochronology paleomagnetism and fault kinematics of pre and synrift rocks exposed in an 200 km 2 coastal mountain belt flanking the eastern rift margin of the northern gulf of california this coastal sonora region hosts the onshore portion of the transform boundary between the upper tiburon and adair tepoca marine basins two early formed oblique rift segments extension commenced here between 11 5 ma and 7 ma resulting in 25 40 of e ne tilting initiation of clockwise vertical axis rotation of fault bounded blocks and minor basin sedimentation rates of deformation prior to 7 ma are unconstrained due to a lack of exposed syntectonic deposits deformation after 7 ma was associated with rapid tilting and the majority of observed clockwise vertical axis rotation and strike slip faulting nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older tilted strata by 5 6 ma deformation in coastal sonora must have largely ceased and migrated westward into the upper tiburon marine basin we document up to 120 total extension and total clockwise block rotations up to 53 in portions of the study area extension and rotation were supplanted by strike slip faulting as deformation proceeded we develop a tectonic model for this coastal sonora fault zone which is bounded by major nw striking transform faults with 10 km of displacement internal to the coastal sonora fault zone the majority of an estimated 6 2 1 1 km of dextral deformation associated with up to 5 7 km of wnw directed extension occurred over the final 1 2 m y of its life span at a strain rate approaching 10 14 s 1 this activity occurred as the plate boundary localized along nascent pull apart basins in the northern gulf of california consistent with the hypothesis that late proto gulf of california dextral shear zones such as the coastal sonora fault zone acted to focus lithospheric scale strain and promoted continental rupture in the wide rift setting of the mexican basin and range

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

reconstruction of late holocene forest dynamics in northern ecuador from biomarkers and pollen in soil cores centuries of human interference have led to large scale reduction of montane forests in the northern ecuadorian andes as a result the natural position of the upper forest line ufl in the area is now subject of scientific debate which is hindering sustainable reforestation efforts uncertainty is fuelled by insufficient precision of fossil pollen spectra to reconstruct the natural ufl position here we tried to resolve this issue by using biomarkers i e plant species specific patterns of n alkanes and n alcohols preserved in soils in the northern ecuadorian andes as additional proxy to reconstruct the natural ufl position to unravel preserved biomarker patterns we used the recently developed verhib model and for the first time assessed its applicability in soil archives changes in holocene biomarker based vegetation composition were directly compared to changes in pollen based vegetation composition from the same soil profiles both proxies proved to be complementary and a combined application allowed for a more accurate reconstruction of past vegetation than with pollen analysis alone we found that the present day ufl in the study area has not been significantly depressed by human interference and was at 3650 m maximally during late holocene times for the moment of post glacial forest development we found a migration lag between pollen earlier and biomarkers later this reflects the difference between the non transported biomarker signal showing spot dating thus in paleoecological studies functionally equalling the information from plant macro remains in peat bogs and the upslope wind blown pollen signal showing an upslope forest expansion up to over a millennium ahead the combined pollen biomarker approach in soil cores shows great potential for vegetation reconstruction however more research of biomarker consistency and preservation is needed before application in other environments

structure of the rift basins in the central gulf of california kinematic implications for oblique rifting abstract we use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central gulf of california our analysis reveals that oblique strain in the central gulf formed two tectono sedimentary domains during distinct deformation stages the eastern domain offshore sonora is bounded by the east and west pedro nolasco faults that may constitute the southernmost segments of the tiburon fault system within this domain the dip slip yaqui fault controlled deposition of 3 9 km of sediments in the half graben yaqui basin the western domain offshore baja california is bounded by the guaymas transform fault which controlled the accumulation of 1 45 km of sediments within a half graben that formed the early guaymas basin the tectono sedimentary activity offshore sonoran likely ranges from late miocene pliocene to late pliocene time while activity in the guaymas basin commenced in late pliocene time extinction of the main faults offshore sonora was nearly coeval to the initiation of the guaymas transform fault our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central gulf the guaymas basin is now a nascent spreading center but prior to this it evolved as a half graben controlled by the guaymas transform fault such drastic transition is not constrained but likely occurred during the pleistocene time and must be localized

initiation evolution and extinction of pull apart basins implications for opening of the gulf of california abstract we present a model for the origin crustal architecture and evolution of pull apart basins the model is based on results of three dimensional upper crustal elastic models of deformation field observations and fault theory and is generally applicable to basin scale features but predicts some intra basin structural features geometric differences between pull apart basins are inherited from the initial geometry of the strike slip fault step over which results from the forming phase of the strike slip fault system as strike slip motion accumulates pull apart basins are stationary with respect to underlying basement and the fault tips propagate beyond the rift basin increasing the distance between the fault tips and pull apart basin center because uplift is concentrated near the fault tips the sediment source areas may rejuvenate and migrate over time rift flank uplift results from compression along the flank of the basin with increasing strike slip movement the basins deepen and lengthen field studies predict that pull apart basins become extinct when an active basin crossing fault forms this is the most likely fate of pull apart basins because basin bounding strike slip systems tend to straighten and connect as they evolve the models show that larger length to width ratios with overlapping faults are least likely to form basin crossing faults and pull apart basins with this geometry are thus most likely to progress to continental rupture in the gulf of california larger length to width ratios are found in the southern gulf which is the region where continental breakup occurred rapidly the initial geometry in the northern gulf of california and salton trough at 6 ma may have been one of widely spaced master strike slip faults lower length to width ratios which our models suggest inhibits continental breakup and favors straightening of the strike slip system by formation of basin crossing faults within the step over as began 1 2 ma when the san jacinto and elsinore cerro prieto fault systems formed

back arc extension in the andaman sea tectonic and magmatic processes imaged by high precision teleseismic double difference earthquake relocation 1 the geometry kinematics and mode of back arc extension along the andaman sea plate boundary are refined using a new set of significantly improved hypocenters global centroid moment tensor cmt solutions and high resolution bathymetry by applying cross correlation and double difference dd algorithms to regional and teleseismic waveforms and arrival times from international seismological centre and national earthquake information center bulletins 1964 2009 we resolve the fine scale structure and spatiotemporal behavior of active faults in the andaman sea the new data reveal that back arc extension is primarily accommodated at the andaman back arc spreading center absc at 10 which hosted three major earthquake swarms in 1984 2006 and 2009 short term spreading rates estimated from extensional moment tensors account for less than 10 of the long term 3 0 3 8 cm yr spreading rate indicating that spreading by intrusion and the formation of new crust make up for the difference a spatiotemporal analysis of the swarms and coulomb stress modeling show that dike intrusions are the primary driver for brittle failure in the absc while spreading direction is close to ridge normal it is oblique to the adjacent transforms the resulting component of e w extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the west andaman fault system after the mw 9 2 sumatra andaman earthquake of 2004 a possible skew in slip vectors of earthquakes in the eastern part of the absc indicates an en echelon arrangement of extensional structures suggesting that the present segment geometry is not in equilibrium with current plate motion demands and thus the ridge experiences ongoing re adjustment

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

petrogenesis of jurassic a type granites associated with cu mo and w sn deposits in the central nanling region south china relation to mantle upwelling and intra continental extension abstract the central nanling region in south china was the site of uplift extensional deformation granite magmatism and hydrothermal activity from 180 to 150 ma information on the age petrological characteristics major and trace element contents whole rock sr and nd and zircon hf isotopic compositions of granitic plutons were synthesized to constrain the sources evolutionary processes and uplift cooling rates of intrusions that host cu mo and w sn deposits in this region the older 180 160 ma cu mo related granitoids are less differentiated a1 type alkaline rocks with faster cooling and uplift rates they have younger nd and hf two stage model ages and higher end t and ehf t values that indicate mantle derived magmas were moderately contaminated by upper crust the younger 160 150 ma w sn related granites are more differentiated a2 type aluminous rocks with slower cooling and uplift rates they have older nd and hf two stage model ages and lower end t and ehf t values that require more upper crustal contamination the time space distribution and characteristics of these granitoids and ore deposits are indicative of centralized mantle upwelling and gradational crustal extension the older rapidly uplifted less contaminated granitoids with cu mo mineralization occur near the center of an intra continental extension bulge produced by mantle upwelling whereas the younger slowly uplifted more contaminated granitoids with w sn mineralization occur on the periphery the results suggest that metals in the cu mo deposits may have come from the mantle whereas those in the world class nanling w sn polymetallic deposits were derived from early mesoproterozoic 1 4 1 6 ga crustal basement of the cathaysia block in south china

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere