rift flank uplift at the gulf of california no requirement for asthenospheric upwelling continental rifts are commonly flanked by zones of high elevation but the cause of uplift remains controversial proposed uplift mechanisms include active and induced asthenospheric upwelling and isostatically driven lithospheric flexure discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension here we focus on the well preserved neogene gulf of california rift the western rift margin is characterized by a prominent east facing kilometer scale escarpment which bounds a west tilted topographically asymmetric rift flank we exploit west draining canyons incised into the rift flank to constrain the timing of uplift to between ca 5 6 and 3 2 ma using 40ar 39ar dating of lavas which show cut and fill relationships to the canyons rift flank uplift closely followed the onset of slip on the principal fault of the loreto rift segment at ca 8 6 ma the age of which we obtain from apatite u th he and fission track thermochronologic analysis of rift escarpment exhumation uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca 6 and 3 ma but post dates a proposed asthenospheric upwelling event by 8 10 ma the timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading

la purisima volcanic field baja california sur mexico miocene to quaternary volcanism related to subduction and opening of an asthenospheric window geological mapping and geochemical analyses combined with 40 k 40 ar ages for lavas from the late miocene to quaternary la purisimavolcanicfield bajacaliforniasur provideevidenceforfivevolcanicevents these inturn mayreflectplateinteractionsin the region the oldest event event 1 prior to 11 ma corresponds to the emission of normal to k rich calc alkaline lavas exposed as large mesas in the eastern part of the studied area and as pyroclastic breccias and volcaniclastic sediments to the west it is associated with the end of the comonduarc activity resulting from subduction of the farallon and guadalupe plates between 10 6 and 8 8 ma event 2 magnesian andesites and tholeiites were emplaced at 5 5 ma event 3 and 2 5 ma event 4 small volumes of magnesian andesites erupted in the central and southern parts of the volcanic field finally between 1 2 ma and holocene event 5 numerous basaltic and magnesian andesitic fissural and central emissions resulted in the formation of strombolian cones and associated lava flows mainly distributed within a nnw sse trending graben located se of the town of la purisima magmatic events 2 to 5 occurred well after the supposed end of the subduction event their geochemical characteristics are still typical of subduction modified sources and possibly indicate partial melting of hot slab and formation of an asthenospheric window due to a slab rupture event which followed ridge trench collision prior to the continental breakup of the gulf of california extensional province d 2005 elsevier b v all rights reserved

crustal structure of the peninsular ranges batholith from magnetic data implications for gulf of california rifting 1 a 70 km wide belt of magnetic highs extends 1200 km northwest from the southern tip of the baja california peninsula into southern california the anomalies are caused by the mafic western belt of the peninsular ranges batholith which is exposed extensively along the northern 800 km length of the magnetic belt modeling indicates that the source of the anomalies extends to mid to lower crustal depths the linearity and undisrupted nature of the magnetic belt support the hypothesis that the peninsula has behaved as a rigid block and has resisted significant 50 km strike slip deformation during the tertiary the absence of a corresponding magnetic belt along the coast of mainland mexico to the south suggests that the location of rifting in the gulf of california was influenced by the mafic western peninsular ranges batholithic crust perhaps because it is thermally and mechanically more resistant to deformation than the surrounding crust

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

convective upwelling in the mantle beneath the gulf of california in the past six million years baja california has rifted obliquely apart from north america opening up the gulf of california between transform faults seafloor spreading and rifting is well established in several basins other than hotspot dominated iceland the gulf of california is the only part of the world s seafloor spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments such resolution is needed to address the long standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres here we use rayleigh wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle low shear velocities similar to those beneath the east pacific rise oceanic spreading centre underlie the entire length of the gulf but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart these anomalies are 40 to 90 kilometres beneath the surface at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin we interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle

temporal variations in fumarole gas chemistry at poas volcano costa rica we report the chemical and isotopic composition of fumarole gas discharges collected at poas volcano costa rica from 2001 to 2014 covering a period during which the volcano experienced a series of phreatic eruptions march 2006 to october 2014 the relative abundances of poas c s h o gas species are controlled by reactions involving the so2 h2s and so so2 gas buffers indicating magmatic temperatures of up to 800 c although fumarole outlet temperatures are 120 c for most samples so2 is the dominant sulfur gas and hcl contents are relatively high gas compositional changes within the magma lake hydrothermal system likely result from a combination of several processes including 1 the injection of new and undegassed magma in late 2000 early 2001 2 the heating of the hydrothermal system accompanied by gas pressure build up and 3 hydrofracturing through 2006 these processes culminated in the phreatic eruptions of 2006 and 2008 since 2005 the lake level has declined and is now january 2014 at the lowest level 10 m since the last periods that it dried out completely april 1984 and april 1994 the most recent data of 2014 show high level of degassing from the dome fumaroles and the release of hcl rich and co2 poor gases implies that the magma injected in late 2000 continues to supply volatiles our data show that time series sampling of fumarole gases provides important insights to better understand magmatic and hydrothermal processes at active volcanoes and also to potentially forecast phreatic eruptions

study on sedimentary numerical simulation method of fan delta sand body fan delta was one of the important sediment types for continental clastic rocks the fan delta sand body geometrical shape could be forecasted by using numerical simulation method of dynamic model of sediment transport after mathematic equations of fan delta sand body deposition process were established simulation conditions were designed for fan delta sedimentation process based on deposition characteristics of modern delta the length and width of the calculation field were 20 km and 10 km the flow rate was designed as the most serious flood in 50 years when simulation process carried through the year 2 000 fan delta sedimentation process reached stabilization state with the largest sand thickness of 39 m medium sand was the main sediment by fan delta lithology simulation study a set of technology about reservoir structure simulation and prediction of fan delta was established four sediment units could be identified underwater distributary channel with an average length width ratio of 2 68 and average width thickness ratio of 79 8 debouch bar with average length width ratio of 2 02 and average width thickness ratio of 68 2 distal bar with average lengthwidth ratio of 1 65 and width thickness ratio of 58 3 underwater overflow sedimentation the correlativity among the sand body geometry parameters was preferable the experimental results showed that the dynamic model of sediment transportation could better revealed the fan delta development process by using numerical simulation method and then could forecast the sand body geometrical shape of the fan delta

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

pulling apart the mid to late cenozoic magmatic record of the gulf of california is there a comondu arc the composition of the lithosphere can be fundamentally altered by long lived subduction processes such that subduction modified lithosphere can survive for 100 s myrs incorrect petrotectonic interpretations result when spatial temporal compositional trends of and source contributions to magmatism are not properly considered western mexico has had protracted cenozoic magmatism developed mostly in board of active oceanic plate subduction beneath western north america a broad range of igneous compositions from basalt to high silica rhyolite were erupted with intermediate to silicic compositions in particular showing calc alkaline and other typical subduction related geochemical signatures a major oligocene rhyolitic ignimbrite flare up 300 000 km3 switched to a bimodal volcanic phase in the early miocene 100 000 km3 associated with distributed extension and opening of numerous grabens extension became more focussed 18 ma resulting in localised volcanic activity along the future site of the gulf of california this localised volcanism known as the comondu arc was dominantly effusive and andesite dacite in composition past tectonic interpretations of comondu age volcanism may have been incorrect as these regional temporal compositional changes are alternatively interpreted as a result of increased mixing of mantle derived basaltic and crust derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting guadalupe plate

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

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

westward migration of extension in the northern gulf of california mexico interpretation of industry seismic lines indicates that the eastern margin of the northern gulf of california contains the inactive adair tepoca and upper tiburon basins the western margin is active and includes the wagner consag upper delfin and lower delfin basins these basin systems are separated by a wide basement high across which the upper strata in the inactive basins correspond to the middle and lower strata in the active basins recording the westward migration of strain and subsidence during late pliocene time our results illustrate the formation of an abandoned rift margin along the eastern gulf of california

low temperature thermochronology of northern baja california mexico decoupled slip exhumation gradients and delayed onset of oblique rifting across the gulf of california 1 the northern gulf extensional province displays key structural relationships that characterize the magnitude direction and timing of neogene rift related transtension during the opening of the gulf of california apatite fission track and u th he thermochronology from the sierra san felipe document moderate cooling 4 c myr 7 c myr during the early paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral peninsular ranges beginning at 45 35 ma a period of tectonic quiescence with low cooling rates 1 c myr marks the development of a regional oligocene miocene peneplain rift related exhumation began at 9 7 ma and attains 2 5 km in the hinges of two antiformal megamullions decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension perpendicular folding of the fault surfaces the main faults of the detachment system were active contemporaneously thus forming a mechanically linked array of large displacement normal faults in the hanging wall of the main gulf escarpment the late miocene onset of transtension in the sierra san felipe suggests that widespread deformation may only have localized in the gulf extensional province between 9 and 7 ma some 3 5 ma after a major plate reorganization associated with cessation of subduction in the trench to the west between 12 ma and 9 7 ma plate boundary shearing was likely distributed between the continental borderland west of baja california and the southern basin and range province in mexico

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

uplift of the western margin of the andean plateau revealed from canyon incision history southern peru we explore canyon incision history of the western margin of the andean altiplano puna plateau in the central andes as a proxy for surface uplift u th he apatite data show rapid cooling beginning at ca 9 ma and continuing to ca 5 1 ma in response to incision a minimum of 1 0 km of incision took place during that interval the youngest apatite date and a volcanic fl ow perched 125 m above the present valley fl oor dated at 2 261 0 046 ma 40 ar 39 ar show that an additional 1 4 km of incision occurred between ca 5 1 and 2 3 ma thus we infer that a total of at least 2 4 km or 75 of the present canyon depth was incised after ca 9 ma u th he zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form rather than uplift being accommodated on major surface breaking faults

application of stratigraphic sedimentological forward modeling of sedimentary processes to predict high quality reservoirs within tight sandstone abstract the prediction of high quality reservoirs within tight sandstone is presently a popular and challenging research topic many geophysical characteristics of tight sandstone reservoirs are similar to those of the surrounding rock making it difficult to predict the distribution of these reservoirs in this paper a stratigraphic sedimentological forward modeling is proposed to predict the distribution of high quality tight sandstone reservoirs in the xihu depression east china sea stratigraphic sedimentological forward modeling can evaluate the influences of different geological parameters on sedimentary processes and the results to reproduce the spatial distribution of sedimentary strata in a progradational delta and the overall stratigraphic stacking pattern different parameters were tested by numerical simulation according to the influence of the parameters on the sedimentary process the parameters can be divided into two types high sensitivity parameters transport capacity of the river and sand ratio and low sensitivity parameters maximum initial water depth lake level change and tectonic subsidence the analysis results are suitable for continental sedimentary basins with relatively high deposition rates in the process of optimizing the simulation parameters 236 simulations were carried out by adjusting the high sensitivity parameters and then the low sensitivity parameters an optimal model that can reflect the development characteristics of high quality tight sandstone reservoirs was obtained according to the characteristics of high quality reservoirs the product of the sand ratio and reservoir thickness from the sedimentary simulation results is used as the parameter for evaluating high quality reservoirs the ratio can effectively overcome the difficulties of insufficient amounts of data and seismic prediction and effectively predict the locations of high quality reservoirs in low exploration areas of tight sandstone reservoirs this method also provides a new idea for the prediction of high quality reservoirs in this kind of area in the future

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

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

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

oceanic ridge subduction vs slab break off plate tectonic evolution along the baja california sur continental margin since 15 ma the interaction of the pacific farallon spreading centers with the north american convergent margin off baja california mexico supposedly ceased at 12 ma when plate convergence and seafloor spreading stopped we propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23 n to 27 n famex cruise of the r v l9atalante april 2002 the data unveil a major clockwise rotation of the pacific farallon spreading direction starting ca 14 ma that formed a series of short spreading centers that became extinct ca 8 7 ma we suggest that the transcurrent motion between the pacific and north america along baja california was accommodated by seafloor spreading and oblique convergence along the trench this change in spreading direction was followed by a concomitant progressive demise of both pacific farallon seafloor spreading and farallon north america subduction that are attributed to the break off of the farallon slab this also resulted in the opening of a trench parallel slab window beneath baja california

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

sedimentary evolution of overlapped sand bodies in terrestrial faulted lacustrine basin insights from 3d stratigraphic forward modeling abstract terrestrial faulted lacustrine basins usually have large variabilities in structural subsidence and multiple river sources the facies in such basins are thus characterised by frequently changed facies and complex relationship of overlapped sand bodies e g liaoxi depression of liaodong bay bohai sea china therefore it s a great challenge to quantitatively describe evolution of sand bodies and their overlap relationship in this study a three dimensional stratigraphic forward model is used to solve this problem which is corrected according to well log data after the parameter optimization was verified against borehole strata thickness and lithology this model had a thickness error within 8 and the lithology distribution agreed with the well data as a whole four evolution phases of the shahejie 3 formation were recognised through the modeling in the first phase the lake level rose and the eastern fan delta sand developed in the second phase the fan delta sand scaled up and the braided river delta front sand developed at a small scale in the third phase the lake level shifted from falling to rising the fan delta sand further expanded the scale and the braided river delta developed at a large scale sands on both sides overlapped at a large scale in the fourth phase the lake level rose the lake basin expanded and lake shale was the main lithology in the study area from the sedimentary evolution of es3 formation sand overlapped in the third stage in 39 75 ma two parts of the sand body began to overlap and reached largest scale in 39 6 ma which was approximately 10 km 2 both then retrograded to a separation in 39 ma when the overlap ended coupling stratigraphic forward simulation with uncertainty quantification allowed us to estimate the influence of each stratigraphic parameter on sand overlap fluvial discharge appears to be the most influential parameter while the initial bathymetry and lake level variations exert a minor to moderate impact

multi phased uplift of the southern margin of the central anatolian plateau turkey a record of tectonic and upper mantle processes abstract uplifted neogene marine sediments and quaternary fluvial terraces in the mut basin southern turkey reveal a detailed history of surface uplift along the southern margin of the central anatolian plateau from the late miocene to the present new surface exposure ages 10be 26al and 21ne of gravels capping fluvial strath terraces located between 28 and 135 m above the goksu river in the mut basin yield ages ranging from ca 25 to 130 ka corresponding to an average incision rate of 0 52 to 0 67 mm yr published biostratigraphic data combined with new interpretations of the fossil assemblages from uplifted marine sediments reveal average uplift rates of 0 25 to 0 37 mm yr since late miocene time starting between 8 and 5 45 ma and 0 72 to 0 74 mm yr after 1 66 to 1 62 ma together with the terrace abandonment ages the data imply 0 6 to 0 7 mm yr uplift rates from 1 6 ma to the present the different post late miocene and post 1 6 ma uplift rates can imply increasing uplift rates through time or multi phased uplift with slow uplift or subsidence in between longitudinal profiles of rivers in the upper catchment of the mut and ermenek basins show no apparent lithologic or fault control on some knickpoints that occur at 1 2 to 1 5 km elevation implying a transient response to a change in uplift rates projections of graded upper relict channel segments to the modern outlet together with constraints from uplifted marine sediments show that a slower incision uplift rate of 0 1 to 0 2 mm yr preceded the 0 7 mm yr uplift rate the river morphology and profile projections therefore reflect multi phased uplift of the plateau margin rather than steadily increasing uplift rates multi phased uplift can be explained by lithospheric slab break off and possibly also the arrival of the eratosthenes seamount at the collision zone south of cyprus

uplift of the sierra san pedro martir baja california mexico the sierra san pedro martir of baja california is the highest of the peninsular ranges and has undergone rapid cenozoic uplift recent uplift can be explained as a result of flexural isostasy driven by a previously suppressed crustal root two dimensional modeling of topography and bouguer gravity data for present and preuplift conditions indicates that a flexural isostatic mechanism for uplift is consistent with the available topographic geologic and geophysical data the crustal root underlying the batholithic rocks of the sierra san pedro martir was probably emplaced during cretaceous magmatic arc activity however existence of a vast low relief and low elevation erosion surface mantled with deposits of fluvial systems originating far to the east suggests that by the early tertiary the plutons were unroofed and there was only subtle topographic expression of this crustal thickening we model the remnant crustal root as regionally compensated suppressed beneath a strengthened and continuous elastic plate and topographically expressed as a broad dome the regional distribution of tertiary sediments and preserved drainage patterns are geologic evidence of such doming since the late miocene there has been rapid uplift and apparent drainage reversal near the dome apex the present topography approximates the predicted isostatic response of a doubly broken elastic plate to the previously suppressed root several lines of evidence indicate that a major component of uplift of the sierra san pedro martir has occurred during the pliocene quaternary and that the crustal scale faulting that has allowed the range to rise rapidly toward local isostasy is related to full development of the san andreas gulf of california transform system

dynamics of slab detachment 1 we investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach the study combines results of two dimensional 2d state of the art thermomechanical numerical simulations and a 1d analytical solution for viscous necking under gravity we show that the dominant deformation mechanism during slab detachment is viscous necking independent of the depth of slab detachment when the slab dip is moderate 35 70 slab detachment is partly affected by localized simple shearing in the colder parts of the slab brittle fracturing breaking plays a minor role during slab detachment our 2d thermomechanical models indicate that the duration of slab detachment quantified from the onset of slab thinning until the actual detachment i e vanishing of slab pull force is relatively short 250 km can occur within a short time interval 1 ma which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting exhumation or surface uplift the thinning of the slab during detachment observed in 2d simulations agrees well with predictions from a 1d analytical solution indicating that the 1d solution captures the first order features of the detachment process we also evaluate the impact of shear heating on the duration of slab detachment the predictions of a simple semi analytical solution based on dimensionless parameters agree well with our and previously published results

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

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

tectonic setting of blueschist and island arc terranes of west central baja california mexico diverse oceanic rocks exposed in west central baja california mexico are assigned to three shallowly dipping tectonic units the lower plate is a subduction complex consisting of regionally metamorphosed blueschists and divided into structurally and petrologically distinct subterranes the upper plate consists of triassic lower cretaceous are rocks and overlapping mid cretaceous turbidites a fault bounded serpentinite matrix melange between lower and upper plate rocks contains blueschist eclogite amphibolite and variably serpentinized mafic and ultramafic blocks lower plate blueschists were metamorphosed in the late early cretaceous and uplifted along shallowly dipping normal faults during conditions of steady state subduction exotic blocks of blueschist eclogite and amphibolite are derived from a cryptic terrane or terranes record varied p t t histories and were uplifted via tectonic emplacement or diapiric rise of serpentinite along the normal faults