cretaceous metamorphism and plutonism in the santa cruz mountains salinian block california and correlation with the southernmost sierra nevada new data on the ages of plutonism and metamorphism in the northern santa cruz mountains part of the enigmatic salinian block of california clarify the area9s geologic history uranium lead dates on zircon monazite sphene and apatite and initial pb and sr isotopic ratios provide a fingerprint for comparison to the rest of the salinian block and to other terranes the u pb zircon dates of plutonic rocks in the santa cruz mountains fall into three groups emplacement ages of the two younger undeformed groups are 91 ma and between 99 and 103 ma individual zircon analyses range from concordant to significantly discordant but have simple crystallization histories coupled with either inheritance of precambrian zircon or pb loss the older group of plutons includes recrystallized granite to well foliated orthogneisses that share the regional fabric analyses from the older group are slightly discordant and multistage histories that include inheritance pb loss and homogenization of pb complicate interpretation of their age emplacement ages for the older group are early cretaceous greater than about 103 ma and probably less than 130 ma foliated and unfoliated plutons from the older group appear to bracket the age of salinian deformation and metamorphism between about 103 and 130 ma lower intercept ages for detrital zircon from schist and quartzite further support a cretaceous metamorphic age the detrital zircon population contains a component at least 1 6 ga probably close to 1 7 ga uranium lead cooling ages measured on sphene and apatite are 96 to 93 and 93 to 88 ma respectively this cooling history appears to be a response to intrusion and rapid cooling of the youngest plutons common pb and sr initial isotopic ratios are radiogenic compared with ratios of oceanic rocks and the western parts of the cordilleran batholiths but they are the least radiogenic in the salinian block and fit the local geographic gradient of pb and sr initial ratios linear variation of pb isotope ratios between radiogenic wall rock and the least radiogenic rocks and correlation of inherited zircon with elevated pb isotope ratios suggest that plutons assimilated wall rock isotopic data and geologic histories of basement rocks in the northern santa cruz mountains fit the geographic patterns documented in the rest of the salinian block more intriguingly the santa cruz mountains plutons are strikingly similar to parts of the southern sierra nevada batholith ages of plutonism and metamorphism u pb zircon discordance systematics petrography and initial sr isotopes match closely between the santa cruz mountains crystalline rocks and units in the southernmost sierra nevada batholith these data reinforce the long standing view that the salinian block and sierra nevada batholiths were adjacent during their formation prior to right slip of about 310 km on the san andreas fault the probable westward continuation of the low angle rand fault system into the southern sierra nevada and the salinian block appears to have offset these two crystalline suites about 125 km

block models of crustal motion in southern california constrained by gps measurements 1 we estimate slip rates on major active faults in southern california using a block model constrained by global positioning system measurements of interseismic deformation the block model includes the effects of block rotation and elastic strain accumulation consistent with a simple model of the earthquake cycle our estimates of the right lateral strike slip rate on the san andreas fault vary by at least a factor of 5 from a high of 35 9 0 5 mm yr in the carrizo plain to a low of 5 1 1 5 mm yr through the san bernadino segment shortening across the puente hills thrust and left lateral slip on the raymond hill fault are consistent with both thickening and escape tectonics in the los angeles basin discrepancies between geodetic and geologic slip rate estimates along the san andreas and san jacinto faults as well as in the eastern california shear zone may be explained by a temporal change in fault system behavior we find no substantial evidence for long term postseismic relaxation and infer that the viscosity of the lower crust upper mantle may be relatively high 1019 pa s

evolution and thickness of the lithosphere beneath coastal california many of the tectonic features of the california coast ranges are directly related to the northward migration of the mendocino triple junction along the western edge of north america during the past 20 to 30 m y at the triple junction the western edge of the north american plate slides off the relative northward moving and subducting gorda plate leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying gorda plate two dimensional time dependent thermal modeling of this process constrained by teleseismic delay studies is used to construct a map of lithospheric thickness of coastal california among the implications of this map are that 1 the high heat flow in the coast ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating mendocino triple junction 2 the general elevation and the late cenozoic volcanism in the california coast ranges are responses to a zone of unusually thin 20 to 45 km lithosphere that extends southward behind the mendocino triple junction and 3 the course of some segments of the san andreas fault in both central and southern california appear to deviate from a deeper more fundamental transform boundary separating the pacific and north american plates

cenozoic tectonism in the central basin and range magnitude rate and distribution of upper crustal strain a wealth of pre cenozoic geologic markers across the central basin and range province defined mainly by thrust faults developed within a miogeoclinal wedge makes it one of the best areas on earth to reconstruct large scale continental extension we use this 100 000 km 2 region bounded by the southern sierra nevada on the west and the colorado plateau on the east to illustrate the concepts of correlation and misalignment for regional strain markers and of strain compatibility and kinematic viability in palinspastic reconstructions the results indicate 250 to 300 km of west northwest motion of the sierra away from the plateau accommodated by both crustal thinning and north south shortening tertiary intermontane basin deposits and mineral cooling ages of deeply exhumed rocks constrain the overall kinematics of motion most of the westward motion occurred between 16 and 5 ma at rates near 2 cm yr slowing to 1 to 1 5 cm yr in the last 5 ma we have quantified the partitioning of strain between vertical crustal thinning via normal faults and map view plane strain via conjugate strike slip faults by placing a grid of 10 x 10 km elements on a retrodeformed map of the region and measuring the increase in area of grid elements between the undeformed and present day grids this analysis yields a maximum finite elongation of the basin and range at 36 n to 37 n of 3 4 oriented n73 w map view area balance shows that 20 percent of this elongation is compensated by north south shortening and 80 percent by crustal thinning this yields an average thinning factor for the upper crust of 2 7 between the sierra and plateau inconsistent with the hypothesis that neogene deformation in the central basin and range is predominantly dextral shear plane strain these results in concert with the observation that the southern sierra nevada has similar crustal thickness to the central basin and range support the hypothesis of large scale eastward flow of sierran deep crust during extension

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

holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

development of the eastern california shear zone walker lane belt the effects of microplate motion and pre existing weakness in the basin and range abstract the role that the motion of microplates has played in the development of the eastern california shear zone in western north america is unclear it was previously proposed that shear related to baja california motion has propagated from the gulf of california northward into nevada however there is evidence that the formation of the eastern california shear zone predates the inception of baja california motion thus either baja california began to move northwestward prior to 6 ma or the eastern california shear zone formed by shear strain resulting from different microplate motion than baja california the role of sierra nevada motion on the development of an adjacent shear zone remains untested we present a numerical modeling study that examines the deformational response of western north america to baja california and or sierra nevada microplate motion in particular we study if and under what condition of microplate motion shear strain can localize in the northern and southern part of the eastern california shear zone separated by the garlock fault we find that given the pre existing weakness from miocene basin and range extension a lithospheric strength contrast or normal faults shear strain from either the baja california or sierra nevada microplates can lead to the formation of the eastern california shear zone furthermore the presence of a pre existing weakness explains the earlier initiation of strike slip faulting north of the garlock fault our models suggest that today baja california microplate motion is the major player for present day deformation rates along the eastern california shear zone and was in the past 6 myrs also a major driver of strain localization in southern eastern california shear zone within the mojave block

continuing evolution of the pacific juan de fuca north america slab window system a trench ridge transform example from the pacific rim abstract many subduction margins that rim the pacific ocean contain complex records of cenozoic slab window volcanism combined with tectonic disruption of the continental margin the series of slab windows that opened beneath california and mexico starting about 28 5 ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime the timing and areal extent of the resultant slab window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin the initial pioneer slab window thermally weakened the overlying western transverse ranges and california borderlands region starting about 28 5 ma a second thermal pulse occurred in the same region starting about 19 ma during growth of the monterey slab window this additional heating combined with the capture of a partially subducted monterey plate fragment by the cocos plate initiated the pulling apart and rotation of the adjacent continental margin similarly the capture of guadalupe and magdalena plate fragments by the pacific plate and initiation of the guadalupe magdalena slab window about 12 5 ma are coeval with baja california pulling away from the mexico continental margin with the break along the comondu arc in crust already thermally weakened by about 10 my of volcanism in coastal california distributed crustal extension and subsidence accompanied the new transform plate boundary and continued until the slab windows cooled and plate motion coalesced along a through going system of strike slip faults the transform boundary continues to evolve and forward modeling predicts an instability with the current configuration as a result of convergence between the sierra nevada and peninsular ranges batholiths starting about 2 my in the future the instability may be resolved by a shift in the locus of transform motion from the san andreas fault to the eastern california shear zone or by breaking off another fragment of the mojave or southern sierra nevada crustal blocks and translating it northward

an examination of the nature and dynamics of seismogenesis in south california usa based on non extensive statistical physics abstract we examine the nature of the seismogenetic system in south california usa by searching for evidence of complexity and non extensivity in the earthquake record we attempt to determine whether earthquakes are generated by a self excited poisson process in which case they are independent or by a critical process in which long range interactions in non equilibrium states are expected correlation emphasis is given to background seismicity i e to the rudimentary expression of the seismogenetic system we use the complete and homogeneous earthquake catalogue published by the south california earthquake data centre in which aftershocks are either included or have been removed by a stochastic declustering procedure we examine multivariate cumulative frequency distributions of earthquake magnitudes interevent time and interevent distance in the context of non extensive statistical physics which generalizes the additive boltzmann gibbs thermodynamics to non additive non extensive dynamic systems our results indicate that the seismogenetic systems of south california are generally sub extensive complex and certainly non poissonian background seismicity exhibits long range interaction as evidenced by the overall increase of correlation observed by declustering the earthquake catalogues as well as by the high correlation observed for earthquakes separated by long interevent distances the results compare very well with those obtained in previous work for the seismogenetic systems of north california specifically the walker lane eastern california shear zone on one hand and the san andreas fault system south california continental borderland region on the other appear to comprise fault networks with different dynamics and expression the former exhibits persistent very high correlation and has attributes of stationary self organized criticality soc the latter exhibits high correlation mainly at long ranges and has attributes of evolutionary of self organized criticality all in all our results are compatible with simulations of small world fault networks in which free boundary conditions at the surface allow for self organization and criticality to develop

tracing the subducted oceanic crust beneath the central california continental margin results from ocean bottom seismometers deployed during the 1986 pacific gas and electric edge experiment large aperture seismic data were collected on several ocean bottom seismometers obs deployed along a deep crustal seismic profile that was shot across the central california continental margin the line of shots extends from the oceanic crust seaward of the santa lucia escarpment to the california coast near morro bay and crosses the santa lucia basin santa lucia bank santa maria basin and hosgri fault zone the obs data permit us to trace the subducted oceanic crust from seaward of the santa lucia escarpment to beneath the central part of the santa maria basin a complementary study using onshore recordings of the same shots extends the crustal model onshore just seaward of the santa lucia escarpment the oceanic crust is subhorizontal and covered by a thin layer of low velocity sediment the velocity 4 5 km s and gradient 1 20 1 25 km s km of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern pacific beneath the santa lucia escarpment and santa lucia basin the oceanic crust dips approximately 16 to the east it is overlain by material with a velocity that increases from 4 8 to 6 4 km s at a depth of 1 7 5 5 km below the seafloor beneath the santa lucia basin a low velocity zone may be sandwiched between the subducted crust and this shallow high velocity material which we interpret to represent obducted oceanic crustal material beneath the eastern edge of santa lucia basin the dip of the subducted oceanic crust decreases to less than 2 the configuration of the subducted crust in this region is consistent with imbrication of the subducted crust as suggested by others based on common midpoint reflection data beneath the central santa maria basin the top of the subducted oceanic crust is at a depth of about 14 16 km and the moho is at 19 21 km

geochemical confirmation of the kula farallon slab window beneath the pacific northwest in eocene time plate tectonic models indicate subduction of the kula farallon spreading ridge and thus imply formation of the kula farallon slab window beneath western north america from late cretaceous to middle eocene time seafloor magnetic anomalies however pro vide few constraints on the configuration of the subducted ridge and the location of the slab window we use geochemical data from eocene igneous rocks to provide an estimate of the slab window s position at 50 ma contouring of the trace element ratios k 2o sio2 rb zr and ta ce for lavas of the eocene challis kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character this trend is consistent with a pacific northwest position for the slab window this is supported by the presence of adak ites slab melts near the canada united states border and by ca 51 ma forearc intru sions and volcanic rocks on vancouver island that are regarded as products of ridge subduction a new reconstruction of the slab window s geometry over time indicates that its chemically defined position is kinematically viable

convection tectonics some possible effects upon the earth s surface of flow from the deep mantle until a little more than a century ago the land surface not only was the only part of the earth accessible to humans but also was the only part for which geophysical and geochemical methods could then provide any details since then scientists have developed ways to study the ocean floors and some details of the interior of the earth to ever greater depths these discoveries have followed one another more and more rapidly and now results have been obtained from all depths of the earth new methods have not contradicted or greatly disturbed either old methods or old results hence it has been easy to overlook the great importance of these recent findings within about the last 5 years the new techniques have mapped the pattern of convection currents in the mantle and shown that these rise from great depths to the surface even though the results are still incomplete and are the subject of debate enough is known to show that the convection currents take two quite different modes one of these breaks the st

slip rate of the calico fault implications for geologic versus geodetic rate discrepancy in the eastern california shear zone 1 long term 10 5 years fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the eastern california shear zone ecsz the calico fault is one of a family of dextral faults that traverse the mojave desert portion of the ecsz its slip rate is determined from matching and dating incised pleistocene alluvial fan deposits and surfaces displaced by fault slip a high resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features the oldest geomorphically preserved alluvial fan unit b is displaced 900 200 m from its source at sheep springs wash in the northern rodman mountains this fan deposit contains the first preserved occurrence of basalt clasts derived from the pipkin lava field and overlies quaternary conglomerate deposits lacking these clasts the 40 ar 39 ar dating of two flows from this field yields consistent ages of 770 40 ka and 735 9 ka an age of 650 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 he exposure date of 653 20 ka on a basalt boulder from the surface of unit b this assigned age and offset together yield a mid pleistocene to present average slip rate of 1 4 0 4 mm yr ayounger fan surface unit k records 100 10 m of dextral displacement and preserves original depositional morphology of its surface granitic boulders and pavement samples from this surface yield an average age of 56 4 7 7 ka after taking into account minimal cosmogenic inheritance of granitic clasts the displaced and dated k fans yield a slip rate of 1 8 0 3 mm yr distributed deformation of the region surrounding the fault trace if active could increase the overall displacement rate to 2 1 0 5 mm yr acceleration of slip rate from an average of 1 4 mm yr prior to 50 ka to 1 8 mm yr since 50 ka is possible though a single time averaged slip rate of 1 6 0 2 mm yr satisfies the data these rates are faster than any other paleoseismic or long term slip rate yet determined for other dextral faults in the mojave desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ecsz total displacement across the calico fault diminishes northward as shear is distributed into folding and sinistral faults in the calico mountains this pattern is consistent with an approximately threefold drop in geologic slip rate as the calico fault steps over onto the blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present day strain accumulation in the ecsz

spatial fields of antarctic sea ice concentration anomalies for summer autumn and their relationship to southern hemisphere atmospheric circulation during the period 1979 2009 summer autumn monthly sea ice concentration anomaly sica fields in antarctica obtained from satellite data for the period 1979 2009 were analysed with varimax rotated t mode principal component analysis pca the first three pca scores described the sica spatial behaviour and explained 38 07 of the total variance the related atmospheric circulation characteristics were analysed using 850 hpa height and surface air temperature anomalies for the months clustered by the corresponding sica composites which were based on pca loadings above a 0 3 threshold the principal characteristics of sica can be seen between the ross and weddell seas areas that remained ice covered during the analysis period elsewhere around antarctica small distinct characteristics occur mostly in embayments the leading summer autumn sica pattern shows a structure with two centres of equal sign located one over the weddell and the other over the ross sea southwest pacific ocean sector and a centre of opposite sign over the bellingshausen and amundsen seas the second sica pattern is represented by a dipole over the weddell sea as a result of an increase decrease in sea ice concentration in the northern sector positive phase and a decrease increase in the southern region together with a positive negative centre over the ross and amundsen seas the latter pattern is characterized by equal sign anomalies on both sides of the antarctic peninsula and opposite sign centres all around antarctica with the highest intensity over the ross sea

oligocene and miocene paleogeography of central california and displacement along the san andreas fault recently completed sedimentologic and petrologic studies of oligocene and miocene strata in the temblor range san joaquin basin and santa cruz mountains la honda basin permit detailed reconstructions of paleogeography as well as new estimates of displacement along the san andreas fault during the oligocene and miocene the san joaquin and la honda basins were contiguous the southwestern margin of the san joaquin la honda basin was tectonically unstable and paleogeographically complex with several small deep sea fans fed by sediment derived from nearby subaerial uplifts of franciscan and granitic basement one of these deep sea fans represented by the temblor formation of the southern temblor range and the vaqueros sandstone of the central santa cruz mountains apparently has been displaced about 315 to 320 km by post early miocene post zemorrian about 23 ma right lateral slip along the san andreas fault this new estimate of displacement places an additional constraint on interpretations of san andreas slip history but ambiguities remain the time of initiation of san andreas displacement is poorly known and available data do not permit discrimination between a history of relatively continuous slip versus a history of episodic slip

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

a new model of slab tear of the subducting philippine sea plate associated with kyushu palau ridge subduction we suggest that the kyushu palau ridge kpr plays a key role in the subduction process of the philippine sea plate psp and the origin of the abu volcano in the southwestern japan the 3 d p wave velocity structure was imaged to approximately 300 km beneath the abu volcano using a large number of p wave arrivals from local earthquakes and teleseismic events our results indicate that a high velocity anomaly beneath the abu volcano is associated with the subducting psp however the anomaly is not continuous being interrupted apparently by a low velocity anomaly zone extending northwestwards from 80 km to great depth the psp appears to be tearing and then forms a slab window corresponding to kpr subduction at ca 2 ma the low velocity anomaly may indicate hot upper mantle material rising through the slab window and causing partial melting both of the lower crust of the overriding plate and the oceanic crust of the subducted kpr a new model is presented for slab tearing of the psp associated with the subduction of the buoyant wide and thick kpr and directional change in the motion of the plate contributing to better understanding of the abu volcanism c 2014 elsevier b v all rights reserved

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

space geodetic observation of the deformation cycle across the ballenas transform gulf of california the gulf of california mexico accommodates 90 of north america pacific plate relative motion while most of this motion occurs on marine transform faults and spreading centers several fault segments in the central gulf come close to peninsular baja california here we present global positioning system and interferometric synthetic aperture radar data near the ballenas transform fault separating the peninsula from angel de la guarda island we observe interseismic motion between june 2004 and may 2009 and displacements associated with the 3 august 2009 mw 6 9 earthquake from the interseismic data we estimate a locking depth of 9 12 5 km and a slip rate of 44 9 48 1 mm yr indicating that faults east of angel de la guarda deform at negligible rates and that the ballenas transform accommodates virtually all of the relative motion between the north american plate and the baja california microplate our preferred model for coseismic slip on a finite rectangular fault plane suggests 1 3 m of strike slip displacement along a vertical rupture plane that is 60 km long and extends from the surface to a depth of 13 km in the eastern ballenas channel striking parallel to baja california north america relative plate motion these estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high resolution bathymetric mapping the geodetic moment is 33 higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate coulomb stress changes for adjacent faults in the gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 april 2012 mw 6 9 guaymas earthquake

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

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

three stage evolution of the los angeles basin southern california we propose that an episode of transtension dominated development of the los angeles basin area from 12 to 6 ma following middle miocene transrotation and prior to the modern transpressional regime transtension resulted from the releasing bend of the san gabriel chino hills cristianitos fault which acted as the primary transform boundary in southern california during this episode such an interpretation implies that significant transform motion did not occur on the southern san andreas fault prior to 6 ma and that the gulf of california has opened primarily since 6 ma we propose a three stage model for evolution of the los angeles basin and vicinity within the evolving transform fault system transrotation 18 12 ma transtension 12 6 ma and transpression 6 0 ma timing of these stages correlates with microplate capture events which occurred during conversion from a convergent margin to a transform margin

an apparent shear zone trending north northwest across the mojave desert into owens valley eastern california strain rates measured at four geodetic networks in eastern california situated between northern owens valley and transverse ranges along a small circle drawn about the pacific north america pole of rotation are remarkably consistent each exhibits 0 14 mu rad yr simple right lateral engineering shear strain accumulation across the local vertical plane tangent to the small circle local faults e g owens valley garlock helendale traversing these networks are not as closely aligned with the vertical planes of maximum shear strain accumulation as is the local tangent to the small circle a fifth network slightly east of the small circle shows no significant strain accumulation thus a shear zone trending n35 degree w from near the eastern end of the big bend of the san andreas fault to northern owens valley is indicated by these data this corresponds to the eastern california shear zone proposed on geological evidence by dokka and travis the shear zone carries approximately 8 mm yr of the pacific north america relative plate motion from the san andreas fault north northwest across the mojave desert into owens valley and the northern basin and range province the shear zone observed at the surface may be a manifestation of a through going subcrustal fault

crustal magmatism and lithospheric geothermal state of western north america and their implications for a magnetic mantle abstract the western north american lithosphere experienced extensive magmatism and large scale crustal deformation due to the interactions between the farallon and north american plates to further understand such subduction related dynamic processes we characterize crustal structure magmatism and lithospheric thermal state of western north america based on various data processing and interpretation of gravimetric magnetic and surface heat flow data a fractal exponent of 2 5 for the 3d magnetization model is used in the curie point depth inversion curie depths are mostly small to the north of the yellowstone snake river plain hotspot track including the steens mountain and mcdermitt caldera that are the incipient eruption locations of the columbia river basalts and yellowstone hotspot track to the south of the yellowstone hotspot track larger curie depths are found in the great basin the distinct curie depths across the yellowstone snake river plain hotspot track can be attributed to subduction related magmatism induced by edge flow around fractured slabs curie depths confirm that the great valley ophiolite is underlain by the sierra nevada batholith which can extend further west to the california coast range the curie depths thermal lithospheric thickness and surface heat flow together define the western edge of the north american craton near the roberts mountains thrust rmt to the east of the rmt large curie depths large thermal lithospheric thickness and low thermal gradient are found from the differences between curie point and moho depth we argue that the uppermost mantle in the oceanic region is serpentinized the low temperature gradients beneath the eastern great basin montana and wyoming permit magnetic uppermost mantle either by serpentinization metasomatism or in situ magnetization which can contribute to long wavelength and low amplitude magnetic anomalies and thereby large curie point depths

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

comparison between the chile and mexico triple junction areas substantiates slab window development beneath northwestern mexico during the past 12 10 myr abstract when combined with the miocene recent volcanic record of baja california a parallel drawn between the chile and mexico triple junction areas substantiates slab window development beneath northwestern mexico during the past 12 10 myr the slab free zone manifestations challenge the notion that ridge subduction has not occurred beneath the southern baja california peninsula the geochemically distinctive rocks from the santa clara volcanic field of west central baja california including coeval adakites and niobium enriched basalt are commonly inferred to signal partial melting of the subducting plate at shallow depths and relatively high temperatures before slab dehydration occurs such pt conditions for slab melting have only been observed in association with spreading ridge subduction we propose that slab window development beneath southern baja california and mainland mexico 30 to 18 n resulted from subduction of the east pacific rise

late tertiary tectonic evolution of the seafloor spreading system off the coast of california between the mendocino and murray fracture zones a forward modeling program of two plate spreading on a sphere was utilized to reconstruct the late tertiary tectonic evolutionary history of the seafloor spreading system off the coast of california between the mendocino and murray fracture zones this analytical technique was applied to a unique set of magnetic anomaly data consisting principally of a new digital data set the original raff mason data from one of the most significant marine geophysical surveys the 1955 1956 uscgs pioneer survey all the data critical for the development of seafloor spreading and plate tectonic theory were digitized 32 to 52 n however only the southern half was analyzed in this study investigation of the raff mason data supplemented by additional magnetic anomaly data indicates that the primary mechanism for reorganization of this part of the pacific farallon ridge as the spreading center migrated toward and arrived at the north american trench was rift propagation the model presented here containing a total of 21 propagation episodes from 36 ma to 19 8 ma initiation time of the last propagator shows generally excellent agreement with the isochrons identified from the magnetic anomaly pattern although minor exceptions exist locally in complex areas a step by step evolution of the model is presented in a series of reconstructions documenting the history of spreading between the pacific plate and five smaller plates created from the breakup of the large ancestral farallon plate these plates include the vancouver plate north of the pioneer the farallon plate existing between the murray and pioneer transforms prior to the major reorganization at chron 10 30 33 ma and the arguello monterey and reyes microplates with northwest southeast spreading existing after chron 10

a review on forearc ophiolite obduction adakite like generation and slab window development at the chile triple junction area uniformitarian framework for spreading ridge subduction this paper aggregates the main basic data acquired along the chile triple junction ctj area 45 48 s where an active spreading center is presently subducting beneath the andean continental margin updated sea floor kinematics associated with a comprehensive review of geologic geochemical and geophysical data provide new constraints on the geodynamics of this puzzling area we discuss 1 the emplacement mode for the pleistocene taitao ridge and the pliocene taitao peninsula ophiolite bodies 2 the occurrence of these ophiolitic complexes in association with five adakite like plutonic and volcanic centers of similar ages at the same restricted locations 3 the inferences from the cooccurrence of these sub coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature pressure p t path low grade greenschist facies overprint and amphibolite eclogite transition respectively 4 the evidences that document ridge jump events and associated microplate individualization during subduction of the scr1 and scr 1 segments the chonos and cabo elena microplates respectively the ridge jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation ophiolite emplacement and adakite like production and location in the ctj area kinematic inconsistencies in the development of the patagonia slab window document an 11 km westward jump for the scr 1 spreading segment at 6 5 to 6 8 ma the scr 1 spreading center is relocated beneath the north patagonia icefield npi we argue that the deep seated difference in the dynamically sustained origin of the high reliefs of the north and south patagonia icefield npi and spi is asthenospheric convection and slab melting respectively the chile triple junction area provides the basic constraints to define the basic signatures for spreading ridge subduction beneath an andean type margin

bed topography and surges in ice streams 1 extensive bed topography at the scale of the ice thickness occurs frequently in the form of drumlins by analogy with hard bed sliding ice flow over this type of topography leads to the generation of drag on the ice particularly when ice flow is rapid a crucial difference with classical nye kamb sliding is that the upper free surface of the ice has a significant effect on the sliding process through the formation of a standing wave using a theoretical model we demonstrate that the presence of this wave introduces a non linearity into the sliding motion which can lead to multiple sliding velocities for the same large scale ice stream geometry and that switches between these velocities can cause surging behavior

evidence of upper mantle processes related to continental rifting versus oceanic crust in the gulf of california summary receiver functions from teleseismic events recorded by stations around the gulf of california are used to map the upper mantle seismic discontinuities we observe a mean transition zone thickness comparable to the global average for most of the region a low velocity layer is detected above the 410 discontinuity that varies in thickness along the gulf of california the 660 discontinuity shows complex waveforms south of latitude 30 n as a result of the phase change of garnet to perovskite within the transition zone a complex behaviour of the receiver functions is observed mainly at the southern end of the gulf the north south variations of this zone are likely associated with a slab window at the northern gulf resulting from the cessation of subduction of the farallon plate 12 ma and the subduction of the guadalupe and magdalena microplates at the southern end resulting in a hydrated upper mantle our results suggest that change in rifting styles occurring along the gulf of california mirrors deeper processes in the upper mantle

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

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

slab remnants beneath the baja california peninsula seismic constraints and tectonic implications the formation of the gulf of california has been related to the cessation of subduction of the guadalupe and magdalena microplates various studies have identified features that point to the presence of a slab remnant beneath the baja california peninsula but its depth range and lateral extent remained unclear in this study we used surface wave phase velocity and receiver function data of nars baja stations around the gulf of california to better constrain the location of the slab for stations in central and southern baja california the shear velocity models show an upper mantle high velocity layer with its top in the depth range from 115 to 135km and a thickness varying between roughly 40 and 60km these high velocity anomalies are interpreted as subducted slab remnants in contrast the models for the northern peninsula show no slab signature this change directly correlates with the variation in relative motion between the baja california peninsula and the pacific plate as measured by gps data it is inferred that the stalled slab fragments beneath the peninsula produce strong coupling between baja california and the pacific plate the shear velocity models for stations on the mexican mainland show a layer of higher velocities above a low velocity upper mantle in the north the low velocity mantle starting at a depth of 40km is associated with upwelling as suggested by previous studies further south the transition from higher to lower mantle velocities occurs around 80km depth which is a typical value for the lithosphere asthenosphere boundary furthermore the models show strong crustal thinning towards the gulf both from the peninsula as well as from the mexican mainland

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

subduction control on the curie isotherm around the pacific north america plate boundary in northwestern mexico gulf of california preliminary results abstract spectral analysis of namag magnetic data enabled us to estimate depths to the curie isotherm along the pacific north america plate boundary the estimated depths for the gulf of california and southern baja california peninsula correlate quite well with the depths to the moho available from previous studies based on receiver function and seismic refraction in northernmost baja california and southwestern usa no correlation is observed between depths to the curie isotherm and depths to the mantle obtained from receiver function analysis depths to the curie isotherm are shallower along the coastal plains of sonora and sinaloa differences between receiver function based mantle depths and curie depths are equally large however there is more consistency with depths established by seismic refraction studies four zones with shallow curie isotherm are identified in southern gulf of california gs1 gs2 gs3 and gs4 gs1 is located at the gulf of california mouth the alarcon rise is located between gs1 and gs2 pescaderos basin is located northwest of gs2 gs3 stretches from the carmen basin to the farallon basin gs4 extends from tiburon island up to northern guaymas basin northernmost shallow curie depth zone gn1 comprises upper and lower delfin basins as well as the consag and wagner basins a northern prolongation also covers pinacate volcanic center the southern shallow curie isotherm zones are associated with lithospheric asthenospheric material upwelling zones gn1 also includes a convective component related to the younger magmatism of the pinacate volcanic field deeper curie depths are found at southern baja california peninsula interpreted as associated with thicker continental or stretched crust it is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the magdalena shelf a contrasting pattern features northern baja california deep curie zones to the west contrasting with relative shallower curie isotherm areas to the east the prolongation towards the peninsula of the extinct shirley transform fracture zone roughly separates the two domains observed along the peninsula and suggests that the curie isotherm along the peninsula is controlled by differences subduction of the guadalupe and magdalena microplates major differences pointed out between northern and southern gulf of california might indicate that subduction also played a key role at the beginning of the opening of the gulf of california however currently the curie pattern is controlled by the rifting processes shallow curie zones correlating quite well with the rift basins and the lithosphere asthenosphere upwelling material

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

global eocene tectonic unrest possible causes and effects around the north american plate abstract many of our planet s crises were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere at the paleocene eocene boundary and in the early eocene several mantle plumes continental collision and mid ocean ridge subduction triggered a series of changes in seafloor spreading dynamics we have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 ma interval we revise evidence for changes in seafloor spreading direction in the north atlantic arctic and ne pacific oceans at least two periods of spreading rate highs which are separated by sharp value decrease occurred along the entire eastern north american plate boundary from c25 to c18 time c 57 to 40 ma the collision and incipient subduction of the early eocene siletzia oceanic lip may have caused the sharp decrease in spreading rate at c23 time in the labrador sea and north of charlie gibbs fracture zone the post c23 rapid farallon slab break off and subsequent upper mantle flow upwelling may have led to further variations in north atlantic spreading rates at c22 21 time eastward pacific subduction may have resumed at c 43 ma as indicated by a steady ne pacific seafloor spreading regime which resumed at or shortly after c21 the north atlantic realm shows a delayed response to tectonic events west of north america with an increase in spreading rate south of charlie gibbs fracture zone from c20 to c18 time followed by a steady decrease until the oligocene north american late paleocene early eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle lithosphere interactions may have affected the entire plate and therefore also its eastern boundaries

cretaceous sinistral strike slip along nacimiento fault in coastal california the san andreas and nacimiento faults of coastal california both separate granitic and metamorphic basement rocks of the salinian block from partly coeval but contrasting mesozoic terranes underlain by the franciscan subduction complex by analogy with neogene dextral strike slip along the san andreas fault cretaceous sinistral strike slip can be inferred along the nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount following restoration of known san andreas and inferred proto san andreas dextral displacements reversal of about 560 km 350 mi of postulated sinistral slip on the nacimiento fault brings four major mesozoic lithotectonic belts of california and baja california into simple al gnment as subparallel terranes related to mesozoic subduction along the continental margin neogene deformation within the san andreas transform system involved a elongation of the salinian block by dextral slip along subsidiary faults that branch from the san andreas fault and b dextral rotation of crustal panels within the transverse ranges latest cretaceous and or earliest paleogene dextral slip along a proto san andreas fault followed the san andreas course in central california but diverged westward in southern california nacimiento sinistral displacements occurred in mid cretaceous to early or medial late cretaceous time after cretaceous emplacement of plutons now within the salinian block but prior to deposition of uppermost cretaceous sedimentary sequences in central california available data on mesozoic relative and absolute plate motions in the mesoameric n region support the likelihood of cretaceous sinistral strike slip subparallel to the california continental margin paleotectonic reconstruction of crustal blocks in california and baja california to their inferred mid cretaceous relative positions shows the salinian block inserted on a bias between the flanking mojave and peninsular ranges blocks salinian granitic rocks thus formed an interior part of the mesozoic batholith belt and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks the similar mesozoic terranes that now lie east and west of the salinian block were then adjacent to one another west of the sierra nevada block available paleomagnetic data neither support nor preclude the reconstruction but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents a correct interpretation of the nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal california

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

eocene volcanism above a depleted mantle slab window in southern alaska the caribou creek volcanic fi eld lies along the continent side edge of forearc basin rocks in south central alaska and consists of over 1000 m of shallow dipping basalt and andesite lavas with minor mafi c pyroclastic deposits dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas the basalts are tholeiitic and strongly depleted in the light rare earth elements la yb 0 18 1 5 with concentrations of high fi eld strength elements e g zr hf ti y similar to mid ocean ridge basalt and with variable enrichment in fl uid mobile elements e g cs ba and pb intermediate and felsic rocks show enrichment in the rare earth elements and fl uid mobile elements plus rb and k but retain low la yb ratios 0 48 3 6 a few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites e g sr y up to 52 ten 40 ar 39 ar ages for the caribou creek volcanic rocks range from 49 4 2 2 to 35 6 0 2 ma an adakite like tuff beneath the other volcanic rocks yields an age of 59 0 0 4 ma caribou creek basalts were derived from mid ocean ridge like depleted mantle that was emplaced beneath the southern margin of alaska through a slab window following spreading ridge subduction caribou creek volcanism was coeval with oblique subduction oroclinal bending and right lateral strike slip faulting in south central alaska all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas the basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of jurassic arc basement rocks to form the intermediate and felsic magmas enrichment of the basaltic parent magmas in fl uid mobile elements occurred by contamination from the jurassic arc rocks and or by contamination with metasomatic mantle remnant from preceding subduction high heat fl ow through the slab window induced partial melting of garnet bearing mafi c parts of the jurassic arc basement to form the adakite like rocks the caribou creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime e g crustal extension and that the infl uence of a slab window on continental margin magmatism can be long lived 20 m y

san luis obispo transform fault and middle miocene rotation of the western transverse ranges california a fault disrupted linear belt of lower middle miocene pyroclastic and volcanic rocks was erupted 15 17 m y b p along a trend 140 to 200 km long in the southern coast ranges and along the northern flank of the western transverse ranges of california offset segments of the belt are coincident with the oceanic west huasna santa maria river little pine and lompoc solvang fault zones the volcanic rocks thin eastward and also vary in thickness along strike the latter reflecting the undulation of porpoising highs and lows of basins to account for the linear distribution of over 3000 km2 of bimodal volcanic rock types it is proposed that they were intruded extruded and ejected into a continental submarine margin environment along a feature here named the san luis obispo transform slot the fault was near the mid miocene continental margin and evidently joined the early mid miocene mendocino triple junction with the crest of the east pacific rise subduction of the farallon plate had stopped along the adjacent continental margin before miocene magmatism in this region the inferred position of the east pacific rise crest was too far to the south and the inferred position of the mendocino triple junction was too far to the north between 15 and 20 m y b p to have been directly related to the miocene volcanic event near 35 n latitude at that time a part of the western transverse ranges was transported northwestward immediately following miocene volcanism earlier movement is possible but evidence is inconclusive microplate transport was concurrent with the development of pull apart structures and the 12 to 16 m y old period of volcanism along the southern margin of the microplate within the los angeles basin variable paleomagnetic directions that display clockwise deflections in the oligocene e g morro rock islay hill complex and miocene igneous rocks e g western transverse ranges in the vicinity of slot can be accounted for by extension and rotational tectonics during pull apart development rotation of blocks between faults during the northwestward transport of the western transverse ranges along slot and by rotation and possible northward translation of a part of the western transverse ranges

a revised dislocation model of interseismic deformation of the cascadia subduction zone 1 cas3d 2 a new three dimensional 3 d dislocation model is developed to model interseismic deformation rates at the cascadia subduction zone the model is considered a snapshot description of the deformation field that changes with time the effect of northward secular motion of the central and southern cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc horizontal deformation data including strain rates and surface velocities from global positioning system gps measurements provide primary geodetic constraints but uplift rate data from tide gauges and leveling also provide important validations for the model a locked zone based on the results of previous thermal models constrained by heat flow observations is located entirely offshore beneath the continental slope similar to previous dislocation models an effective zone of downdip transition from locking to full slip is used but the slip deficit rate is assumed to decrease exponentially with downdip distance the exponential function resolves the problem of overpredicting coastal gps velocities and underpredicting inland velocities by previous models that used a linear downdip transition a wide effective transition zone etz partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model the pattern of coseismic deformation is expected to be different from that of interseismic deformation at present 300 years after the last great subduction earthquake the downdip transition from full rupture to no slip should take place over a much narrower zone

heat flow and lithospheric thickness analysis in the patagonian asthenospheric windows southern south america abstract the lithosphere asthenosphere boundary lab is a first order discontinuity essential to understand the earth composition and evolution however its detection has been critical and several regions still lack of coverage southern patagonia in southern most south america is one such area which has been affected by the subduction of a seismic oceanic ridge south chile ridge and formation of an extensive slab window since 12 ma we calculate the lab position by defining the thermal lithospheric thickness of the southernmost patagonia using the thermal conductivity equation to estimate the surface heat flow we used data from seventy five hydrocarbon wells of two from the most productive petroleum basins in argentina the golfo de san jorge and magallanes austral basins our results show the highest heat flow values in the southernmost region over the austral basin and core of the slab window 70 90 mw m2 these values are twice as hot as global average to the north over de golfo de san jorge basin and northern margin of the slab window heat flows are normal 50 60 mw m2 these thermal contrasts from south to north agree with the kinematic reconstructions of the chile ridge and northward widening of the slab window these heat flows values evidence an attenuated lithospheres which thickens eastward from between 25 km to the west trench region to 50 km to the east foreland region

glacial geologic confirmation of an intraplate boundary in the parana basin of brazil the predrift fit of south america and africa remains problematic given the inability to match all parts of the coastlines simultaneously early opening of the south atlantic ocean may have been facilitated by intraplate deformation in either south america or africa but the location of such deformation is controversial and poorly constrained broad scale remote sensing has suggested an intraplate boundary in south america stretching from the cochabamba bend in the andes to the rio grande walvis rise but the inferred boundary crosses the paranaa basin of brazil where any evidence of deformation is obscured by late jurassic flood basalts a subsurface study of the upper paleozoic glaciogenic infill of the paranaa basin the 1300 m thick itarare group ca 300 260 ma identifies repeated episodes of intracratonic rifting on either side of an accommodation zone crossing the basin along the trend of the inferred intraplate boundary renewed influence of this zone during the opening of the south atlantic is suggested by the asymmetric distribution of successive magma units within the serra geral lava pile and by the trend of dike swarms the intraplate boundary identified within the paranaa basin may extend westward along the northern margin of the chaco basin in bolivia

plate deformation at depth under northern california slab gap or stretched slab plate kinematic interpretations for northern california predict a gap in the underlying subducted slab caused by the northward migration of the pacific north america juan de fuca triple junction however large scale decompression melting and asthenospheric upwelling to the base of the overlying plate within the postulated gap are not supported by geophysical and geochemical observations we suggest a model for the interaction between the three plates which is compatible with the observations in this slab stretch model the juan de fuca plate under coastal northern california deforms by stretching and thinning to fill the geometrical gap formed in the wake of the northward migrating mendocino triple junction the stretching is in response to boundary forces acting on the plate the thinning results in an elevated geothermal gradient which may be roughly equivalent to a 4 ma oceanic lithosphere still much cooler than that inferred by the slab gap model we show that reequilibration of this geothermal gradient under 20 30 km thick overlying plate can explain the minor neogene volcanic activity its chemical composition and the heat flow in contrast to northern california geochemical and geophysical consequences of a true slab gap can be observed in the california inner continental borderland offshore los angeles where local asthenospheric upwelling probably took place during the miocene as a result of horizontal extension and rotation of the overlying plate the elevated heat flow in central california can be explained by thermal reequilibration of the stalled monterey microplate under the coast ranges rather than by a slab gap or viscous shear heating in the mantle

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

devonian volcanic rocks of the southern chinese altai nw china petrogenesis and implication for a propagating slab window magmatism induced by ridge subduction during accretionary orogenesis abstract ridge trench interaction is a common tectonic process of the present day pacific rim accretionary orogenic belts and this process may facilitate slab window magmatism that can produce significant thermal anomalies and geochemically unusual magmatic events however ridge trench interaction has rarely been well documented in the ancient geologic record leading to grossly underestimation of this process in tectonic syntheses of plate margins the chinese altai was inferred to have undergone ridge subduction in the devonian and a slab window model is proposed to interpret its high temperature metamorphism and geochemically unique magmatic rocks which can serve as an excellent and unique place to refine the tectonic evolution associated with ridge subduction in an ancient accretionary orogeny for this purpose we carried out geochemical and geochronological studies on devonian basaltic rocks in this region secondary ion mass spectrometry sims zircon u pb dating results yield an age of 376 2 2 4 ma suggesting an eruption at the time of late devonian geochemically the samples in this study have variable sio2 43 3 58 3 wt low k2o 0 02 0 07 wt and total alkaline contents 2 16 5 41 wt as well as fe2o3t mgo ratios showing typical tholeiitic affinity on the other hand the basaltic rocks display morb like ree patterns la yb n 0 90 2 57 and ga yb n 0 97 1 28 and have moderate positive end t values 4 4 to 5 4 which collectively suggest a derivation from a mixing source comprising morb like mantle of a mature back arc basin and subordinate arc mantle wedge these basaltic rocks are characterized by low la yb 1 26 3 69 dy yb 1 51 1 77 and sm yb 0 83 1 32 ratios consistent with magmas derived from low degree 10 partial melting of the spinel lherzolite source at a quite shallow mantle depth considering the distinctive petrogenesis of the basaltic rocks in this region the late devonian basalts in the southern chinese altai is suggested to have witnessed the propagating process of slab window magmatism that was induced by ridge subduction in a nascent rifting stage of a back arc basin

a kinematic model for the formation of the siletz crescent forearc terrane by capture of coherent fragments of the farallon and resurrection plates 1 the volcanic basement of the oregon and washington coast ranges has been proposed to represent a pair of tracks of the yellowstone hotspot formed at a mid ocean ridge during the early cenozoic this interpretation has been questioned on many grounds especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and north america updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55 49 ma and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments this kinematic model indicates that there is no firm reason to reject the near ridge hotspot hypothesis on the basis of plate motions a novel element of the model is the resurrection plate previously proposed to exist between the farallon and kula plates by including the defunct resurrection plate in our reconstruction we are able to model the farallon hotspot track as docking against the oregon subduction margin starting about 53 ma followed by docking of the resurrection track to the north starting about 48 ma accretion of the farallon plate fragment and partial subduction of the resurrection fragment complicates the three dimensional structure of the modern cascadia forearc we interpret the so called e layer beneath vancouver island to be part of the resurrection fragment our new kinematic model of mobile terranes within the paleogene north american plate boundary allows reinterpretation of the three dimensional structure of the cascadia forearc and its relationship to ongoing seismotectonic processes

kinematics of the northern walker lane an incipient transform fault along the pacific north american plate boundary in the western great basin of north america a system of dextral faults accommodates 15 25 of the pacific north american plate motion the northern walker lane in northwest nevada and northeast california occupies the northern terminus of this system this young evolving part of the plate boundary offers insight into how strike slip fault systems develop and may reflect the birth of a transform fault a belt of overlapping left stepping dextral faults dominates the northern walker lane offset segments of a w trending oligocene paleovalley suggest 20 30 km of cumulative dextral slip beginning ca 9 3 ma the inferred long term slip rate of 2 10 mm yr is compatible with global positioning system observations of the current strain field we interpret the left stepping faults as macroscopic riedel shears developing above a nascent lithospheric scale transform fault the strike slip faults end in arrays of n striking normal faults suggesting that dextral shear diffuses into extension in the great basin coeval extension and dextral shear have induced slight counterclockwise fault block rotations which may ultimately rotate riedel shears toward the main shear zone at depth thus facilitating development of a throughgoing strike slip fault

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

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

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

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

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

observed crustal uplift near the southern patagonian icefield constrains improved viscoelastic earth models thirty one gps geodetic measurements of crustal uplift in southernmost south america determined extraordinarily high trend rates 35 mm yr in the north central part of the southern patagonian icefield these trends have a coherent pattern motivating a refined viscoelastic glacial isostatic adjustment model to explain the observations two end member models provide good fits both require a lithospheric thickness of 36 5 5 3 km however one end member has a mantle viscosity near 1 6 1018 pa s and an ice collapse rate from the little ice age lia maximum comparable to a lowest recent estimate of 1995 2012 ice loss at about 11 gt yr in contrast the other end member has much larger viscosity 8 0 1018 pa s half the post lia collapse rate and a steadily rising loss rate in the twentieth century after ad 1943 reaching 25 9 gt yr during 1995 2012

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

a comparison between mid paleozoic new england usa and the modern western usa subduction of an oceanic ridge transform fault system abstract a detailed comparison is made between the mid paleozoic norumbega fault system nfs in maine usa and the san andreas fault system safs of coastal california usa and their tectonic settings the safs formed following subduction of an oceanic ridge transform system and the nfs is interpreted as having formed the same way the parallel evaluation of mid crustal processes associated with the nfs and surface processes associated with the safs gives a unique perspective on modern and inactive crustal scale strike slip fault systems the safs is separated from the cascadia subduction zone by the mendocino triple junction similarly the mid paleozoic nfs is interpreted as having been separated from a convergent zone to the southwest by the interpreted norumbega triple junction the safs cuts a subduction complex the franciscan complex that formed when the margin was still convergent no equivalent subduction complex associated with the nfs has been recognized perhaps because it was eroded away or displaced or because it never formed the safs experienced periods of transpression and transtension whereas the nfs shows transpressive structures only slab window magmatic rocks exist along the safs devonian to earliest carboniferous plutonic rocks along the nfs may in part be slab window magmatic rocks although the effects of the acadian and prior orogenies in the appalachians complicate their identification magmatic rocks along the safs are offset due to the high rate of displacement 20 40 mm yr magmatic rocks along the nfs show little offset consistent with the low rate of displacement 3 mm yr in the model transverse structures are associated with both the safs and nfs other mendocino type triple junctions may have existed in the past but may be difficult to recognize where a past crustal scale intracontinental strike slip fault system terminates on an along strike contemporaneous convergent system such a fault fault trench triple junction may be considered