continental growth at convergent margins facing large ocean basins a case study from mesozoic convergent margin basins of baja california mexico abstract mesozoic rocks of the baja california peninsula form one of the most areally extensive best exposed longest lived 160 my least tectonized and least metamorphosed convergent margin basin complexes in the world this convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins a progression from highly extensional phase 1 through mildly extensional phase 2 to compressional phase 3 strain regimes this trend is largely due to the progressively decreasing age of lithosphere that is subducted which causes a gradual decrease in slab dip angle and concomitant increase in coupling between lower and upper plates as well as progressive inboard migration of the arc axis this paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin phase 1 consists of late triassic to late jurassic oceanic intra arc to backarc basins that were isolated from continental sediment sources new progressively widening basins were created by arc rifting and sea floor spreading and these were largely filled with progradational backarc arc apron deposits that record the growth of adjacent volcanoes up to and above sea level inboard migration of the backarc spreading center ultimately results in renewed arc rifting producing an influx of silicic pyroclastics to the backarc basin rifting succeeds in conversion of the active backarc basin into a remnant backarc basin which is blanketed by epiclastic sands phase 1 oceanic arc backarc terranes were amalgamated by late jurassic sinistral strike slip faults they form the forearc substrate for phase 2 indicating inboard migration of the arc axis due to decrease in slab dip phase 2 consists of early cretaceous extensional fringing arc basins adjacent to a continent phase 2 forearc basins consist of grabens that stepped downward toward the trench filled with coarse grained slope apron deposits phase 2 intra arc basins show a cycle of 1 arc extension characterized by intermediate to silicic explosive and effusive volcanism culminating in caldera forming silicic ignimbrite eruptions followed by 2 arc rifting characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites this extensional rifting cycle was followed by mid cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent caused by a decrease in slab dip as well as a possible increase in convergence rate phase 2 fringing arc terranes form the substrate for phase 3 which consists of a late cretaceous high standing compressional continental arc that migrated inboard with time strongly coupled subduction resulted in accretion of blueschist metamorphic rocks with development of a broad residual forearc basin behind the growing accretionary wedge and development of extensional forearc trench slope basins atop the gravitationally collapsing accretionary wedge inboard of this ongoing phase 3 strongly coupled subduction together with oblique convergence resulted in development of forearc strike slip basins upon arc basement the modern earth is strongly biased toward long lived arc trench systems which are compressional therefore evolutionary models for convergent margins must be constructed from well preserved ancient examples like baja california this convergent margin is typical of many others where the early to middle stages of convergence phases 1 and 2 create nonsubductable arc ophiolite terranes and their basin fills in the upper plate these become accreted to the continental margin in the late stage of convergence phase 3 resulting in significant continental growth

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

angular velocities of nubia and somalia from continuous gps data implications on present day relative kinematics abstract this study focuses on the break up of the african tectonic plate into separate nubian and somalian blocks based on recent global positioning system gps data a new unique velocity field has been obtained by processing all available observations of permanent gps stations on africa since 1996 the quantity and distribution of the stations and the length of the time series of observations exceed that of previous studies by a considerable margin allowing one to derive a reliable estimate of the differential motion between the nubia and somalian plates which are considered as a single african block in the prevailing global tectonic plate models the estimated relative pole of rotation of somalia with respect to nubia is located at 54 8 s 37 0 e with magnitude 0 069 ma implying distinct opening in the ethiopian rift of magnitude 7 mm year and azimuth n94 e whereas in southeastern south africa this value is reduced to 2 mm year in almost the same direction this is in accordance with some of the independent geological and geophysical tectonic models of the nubia somalia plate boundary region however the spatial density of the current tracking network is still not optimal to establish the exact location of the entire somalia nubia plate boundary in particular the possible branch east of lake victoria and heading towards the mozambique channel is impossible to confirm or reject at this moment

control of rheological stratification on rifting geometry a symmetric model resolving the upper plate paradox numerical experiments reproduce the fundamental architecture of magma poor rifted margins such as the iberian or alpine margins if the lithosphere has a weak mid crustal channel on top of strong lower crust and a horizontal thermal weakness in the rift center during model extension the upper crust undergoes distributed collapse into the rift center where the thermally weakened portion of the model tears among the features reproduced by the modeling we observe 1 an array of tilted upper crustal blocks resting directly on exhumed mantle at the distal margin 2 consistently oceanward dipping normal faults 3 a mid crustal high strain zone at the base of the crustal blocks s reflector 4 new ocean floor up against a low angle normal fault at the tip of the continent 5 shear zones consistent with continentward dipping reflectors in the mantle lithosphere 6 the mismatch frequently observed between stretching values inferred from surface extension and bulk crustal thinning at distal margins upper plate paradox rifting in the experiment is symmetric at a lithospheric scale and the above features develop on both sides of the rift center we discuss three controversial points in more detail 1 weak versus strong lower crust 2 the deformation pattern in the mantle and 3 the significance of detachment faults during continental breakup we argue that the transition from wide rifting towards narrow rifting with a pronounced polarity towards the rift center is associated with the advective growth of a thermal perturbation in the mantle lithosphere

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

fission track evidence on the initial rifting of the red sea two pulses no propagation fission track analyses indicate that the red sea initially opened simultaneously along its entire length two distinct pulses of uplift and erosion characterized the early stages of rifting in the red sea throughout egypt and in southwestern saudi arabia the first pulse began at 34 million years ago ma the second pulse began in the early miocene 21 to 25 ma and marked the start of the main phase of extension these data support a rigid plate model for continental extension these results also indicate that the initiation of rift flank uplift and therefore rifting and volcanism occurred nearly simultaneously this conflicts with classical models of active and passive extension that predict sequential development of these features

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

large scale potential vorticity anomalies and deep convection the effect on deep convection of large scale potential vorticity pv anomalies and their associated tropospheric stable layers is complex and not well understood this article examines the meteorological events of 9 july 2007 iop7b of the convective and orographically induced precipitation study cops which was dominated by an upper level pv anomaly that stretched from the uk to southern france and as far northeast as denmark three precipitation regions were identified from the case lines of intense storms beneath the pv anomaly less intense more widespread convective precipitation to the east of the pv anomaly and in between a region of no precipitation the latter of these coincided with the high resolution measurements and model analyses from cops the extensive and varied data analyzed in this investigation show that convective available potential energy cape was present in this region the distribution of cape and convective inhibition cin is presented via an innovative pseudo 3d visualization that allows horizontal and vertical interactions to be considered however convection was capped by a complex arrangement of dry layers the base of the key layer was at 750 hpa these dry layers descended separately from the upper troposphere moving around the pv anomaly as it developed from a breaking rossby wave to the west during the seven days before the iop this case adds to other studies that show that descent of complex dry layers is an important mechanism for forming convection inhibiting atmospheric lids in western europe a simple conceptual model is developed that synthesizes the effect of large scale pv anomalies on deep convection from a series of consistent case studies this model has significant implications for storm forecasting and projections of storminess in future climates as it highlights the importance of thin structures that can advect hundreds of km before having an impact copyright 2012 royal meteorological society

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

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

the 40ar 39ar thermochronology of core complexes and other basement rocks in sonora mexico implications for cenozoic tectonic evolution of northwestern mexico 1 northwestern mexico remains an important region for understanding the tectonic evolution of the north american cordillera however many fundamental aspects of the geologic history of the region remain poorly known we present new 40ar 39ar thermochronology of exposed basement rock from throughout sonora mexico including from metamorphic core complexes these results provide new constraints on the late cretaceous to cenozoic tectonic evolution of northwestern mexico following laramide ca 80 50 ma emplacement most sonoran plutons experienced rapid post magmatic cooling to 400 200 c which we attribute to shallow level emplacement following this rapid cooling plutons cooled slowly 300 c until the late oligocene rapid footwall cooling documents that extension at sonoran core complexes began synchronously at ca 25 ma despite the fact that these regions are separated by 200 km extension at most sonoran core complexes continued until ca 15 ma although extension at the magdalena core complex ceased earlier ca 21 20 ma slip at each core complex was 10 30 km and occurred at average rates of 1 9 mm yr this major period of extension primarily occurred during subduction indicating that relative plate motions between the pacific and north american plates cannot be invoked as a driving force for this phase of extension

effect of flood recession on scouring at bed sills the effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined experiments were carried out to study the development of scour holes under time varying hydraulic conditions with no upstream sediment feed triangular shaped hydrographs having recession times up to three times the duration of the rising limb were used traditionally the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood this approach is overly conservative when the flow hydrograph is steep i e during the occurrence of flash floods the actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream the results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady state temporal scale of scour development a method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters results obtained for clear water boundary conditions can be extended to sediment supply tests if specific supply input conditions hold

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

ignimbrite flare up and deformation in the southern sierra madre occidental western mexico implications for the late subduction history of the farallon plate geologic mapping and structural data for the southern part of the smo demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare up in oligocene 31 5 28 ma and early miocene 23 5 20 ma time and that extensional deformation occurred mostly before the transfer of baja california to the pacific plate extensive ignimbrite successions with 40 ar 39 ar ages clustering at 23 and 21 ma cover most of the southern smo thus correlating in age with ignimbrites exposed in southern baja california and central mexico grabens with a 020 to n s orientation developed in the east almost concurrently with this volcanic episode half grabens and nnw striking listric normal fault systems formed at the end of middle miocene as far as 150 km from the present coast a belt of left lateral transpressional structures formed along the southern boundary of the smo during the same period we link these magmatic and tectonic events to the evolution and dynamics of the farallon and north america plates during the miocene particularly we propose that a first detachment of the lower part of the farallon plate in early miocene time produced a transient thermal event and partial melting of the crust via mafic underplating middle miocene extension would be related to a second detachment event resulting from the slowing subduction that preceded the final capture of the magdalena microplate by the pacific plate at 12 5 ma transpression at the southernmost end of the smo occurred along the inland projection of the magdalenacocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the magdalena plate index terms 5480 planetology solid surface planets volcanism 8450 8150 tectonophysics evolution of the earth plate boundary general 3040 8109 tectonophysics continental tectonics extensional 0905 keywords ignimbrite flare up sierra madre occidental western mexico extensional tectonics slab detachment

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

timing of rifting in the southern gulf of california and its conjugate margins insights from the plutonic record the gulf of california is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin previous studies focused along the southwestern rifted margin in baja california indicated rifting was initiated after subduction and related magmatism ceased at ca 14 12 5 ma however the geologic record on the mexico mainland sinaloa and nayarit states indicates crustal stretching in the region began as early as late oligocene the timing of cooling and exhumation of pre and synrift plutonic rocks can provide constraints on the timing and rate of rifting here we present results of a regional study on intrusive rocks in the southern gulf of california sampled along the conjugate baja california and nayarit sinaloa rift margins as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf forty one samples were dated via u pb zircon and 40ar 39ar mineral ages providing emplacement age and thermochronological constraints on timing and rate of cooling we found an extensive suite of early and middle miocene plutons emplaced at shallow depths within the basement cretaceous paleocene peninsular range and sinaloa jalisco batholiths early miocene granitoids occur in an elongated wnw ese belt crossing the entire southern gulf from southern baja california to nayarit and sinaloa most have an intermediate composition 75 sio2 wt was emplaced 20 1 18 3 ma near the end of the early miocene age span and chemical composition of the early miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern sierra madre occidental and in southern baja california suggesting that eruptive sources for the early miocene ignimbrite flare up may also have been located within the southern gulf of california early miocene plutons cooled below the 40ar 39ar biotite closure temperature 350 400 c in less than 2 5 m y which we interpret as evidence of a regional extensional event leading to the opening of the gulf of california a less widely distributed suite of intermediate composition middle miocene granitoids 15 13 ma was sampled from the central western part of the gulf west of the pescadero basin and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore comondu group in baja california our widely spaced sampling of the generally sediment covered igneous crust suggests that middle miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust comondu arc thermobarometry data also indicate a very shallow depth 5 km of emplacement for the middle miocene plutonic rocks some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure early and middle miocene granitoids away from the gulf axis yielded 40ar 39ar cooling ages very close to u pb zircon ages demonstrating rapid cooling to 350 c which we attribute to their shallow emplacement and possibly to exhumation soon after intrusion since comondu age and middle miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction we suggest that intense crustal stretching controlled the pattern and timing of comondu age magmatism rather than the middle miocene magmatism controlling the locus of 12 ma extension

crustal deformation in the baikal rift from gps measurements three years and four campaigns of global positioning system gps measurements 1994 1997 in the baikal rift zone largest active continental rift system in eurasia show crustal extension at a rate of 4 5 1 2 mm yr in a wnw ese direction a comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the baikal rift zone along active faults that currently have the potential for a m 7 5 earthquake the gps derived extension rate in the baikal rift zone is at least two times greater than the prediction of most deformation models of asia this result could reflect the dynamic contribution of the pacific eurasia subduction to intracontinental deformation in asia in addition to the effect of the india eurasia collision

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

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

seismic structure of the southern gulf of california from los cabos block to the east pacific rise 1 multichannel reflection and coincident wide angle seismic data collected during the 2002 premier experiment sea of cortez addressing the development of oblique rifting pescador experiment provide the most detailed seismic structure to date of the southern gulf of california multichannel seismic mcs data were recorded with a 6 km long streamer 480 channel aboard the r v maurice ewing and wide angle data was recorded by 19 instruments spaced every 12 km along the transect the mcs and wide angle data reveal the seismic structure across the continent ocean transition of the rifted margin typical continental and oceanic crust are separated by a 75 km wide zone of extended continental crust dominated by block faulted basement little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust suggesting a constant rate of magmatic productivity since seafloor spreading began oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature 1300 c and passive mantle upwelling at the early stages of seafloor spreading the crustal thickness width of extended continental crust and predicted temperature conditions all indicate a narrow rift mode of extension on the basis of upper and lower crust stretching factors an excess of lower crust was found in the extended continental crust total extension along transect 5w is estimated to be 35 km following crustal extension new oceanic crust 6 4 km thick was formed at a rate of 48 mm a 1 to accommodate plate separation