geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

iron geochemistry under mussel rafts in the galician ria system galicia nw spain abstract the intensive mussel culture carried out in the past 40 years in the rias of vigo and arousa galicia nw spain has led to substantial changes in the ecology and geochemistry of the seabed in these areas organic c enrichment of the seabed has generated strongly reducing conditions that directly affect the geochemistry of fe and s in the present study a total of six sediment cores were collected from the seabed under mussel rafts and two different layers were distinguished the biodeposit generated by the mussels and the sediment situated immediately below this samples of each were analyzed to determine the ph redox potential sulphate and chloride in the interstitial water as well as total percentage of organic c toc n and s sequential extraction of the samples differentiated six fractions of fe exchangeable carbonate ferrihydrite lepidocrocite goethite and pyrite the contents of total fe fe associated with silicates fe soluble in 1 m hcl and avs fe were also determined in general both the biodeposit and the sediment were anoxic eh 1 were high but significantly lower than in the sediment 1 10 0 20 mol g 1 and there remained large quantities of reactive fe that were susceptible to pyritisation in contrast in the sediment the reactive fe was intensively pyritised and judging from the ratio of toc dop it limited synthesis of pyrite furthermore a plot of the concentration of pyrite s against toc revealed an excess of 15 of pyrite s which is explained by the partial decoupling of pyrite formation from organic matter accumulation caused by the formation of pyrite from the h 2 s generated by the anaerobic oxidation of methane the latter process also appears to favour although to a lesser extent the precipitation of ca carbonate with incorporation of fe

thrusting of the central california margin over the edge of the pacific plate during the transform regime wide angle seismic reflection and refraction data mapped geologic structures and marine magnetic anomalies provide evidence that oceanic crust underlies the coastal margin of california from morro bay northward beyond san francisco we interpret the presence of this oceanic slab to be at least partly the result of thrusting of north america over the pacific plate during pliocene to quaternary time well after the principal subduction regime ended likely causes of this thrusting are the change in plate motions at about 3 5 ma and extension in the basin and range province

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

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

complexity of the deep san andreas fault zone defined by cascading tremor seismic tremors can be used to distinguish plate boundaries analysis of tremors occurring deep beneath the san andreas fault may identify the boundary between the north american plate and the preserved remnant of a subducted slab

northwest margin of california continental borderland marine geology and tectonic evolution the northwest margin of the california continental borderland consists mainly of two northwest trending pre neogene lithologic belts blanketed by miocene and younger strata these belts which are lithologically and structurally correlated with the franciscan complex and great valley sequence of northern california are interpreted to represent facies corresponding to the subduction complex and forearc basin deposits of a late mesozoic and paleogene continental margin arc trench system the outer belt which forms the acoustic basement underlying the continental slope and patton ridge is characterized by 1 moderately high compressional velocities 5 1 to 6 2 km sec 2 discordant and discontinuous seismic reflectors and 3 nonfossiliferous and diverse rock types t at range in metamorphic grade from zeolite bearing arkosic sandstone to blueschist landward the inner great valley sequence belt underlies santa rosa cortes ridge and santa cruz and san nicolas basins in contrast to the outer belt this belt is characterized by 1 intermediate compressional velocities 4 5 to 4 6 km sec 2 concordant and relatively continuous reflectors and 3 a thick turbidite sequence of relatively undeformed and unmetamorphosed fossiliferous cretaceous and paleogene strata the inner belt overlies a 6 6 km sec layer that is probably oceanic crust a relation consistent with the hypothesis that trapped old oceanic crust underlies the forearc basins along parts of some convergent margins well developed wrench fault structures in overlying miocene strata record a transition from paleogene subduction to neogene transform faulting related events include widespread andesitic to dacitic volcanism and concurrent uplift and erosion of miocene and older rocks the proximity of these miocene silicic volcanic rocks to the paleotrench 20 km is best explained as resulting from passage of the southward migrating rivera ridge fault trench triple junction along the margin on the basis of the age of the adjacent sea floor passage of this triple junction occurred about 17 m y ago a date that corresponds closely to the inception of volcanism and uplift in the northwest margin of the borderland a widespread unconformity that separates highly deformed lower and lower middle iocene strata from relatively undeformed younger strata is assumed to mark the subsequent onset of wrench faulting the timing of volcanism and uplift and of the slightly younger wrenching event closely matches the plate tectonic model of atwater and molnar if the northwest margin of the borderland is restored to an earlier tectonic setting off baja california approximately 260 km southeast of its present position

sedimentological characteristics heavy metal distribution and magnetic properties in subtidal sediments ria de pontevedra nw spain the rias of nw spain are coastal ecosystems of high biological productivity and great economic importance they are intensively exploited by man for fish and shellfish there are a number of important centres of population and industrial activity along their margins which serve as sources of contamination in this context it is desirable to achieve the best possible understanding of the physico chemical processes that control spatial and temporal variations in the geochemical mineralogical and sedimentological characteristics of near surface sediments in the ria de pontevedra and in particular the distribution and mobility of heavy metal contaminants thus adequate environmental planning can be achieved for this site and understanding gained for comparable contexts core samples were examined from the inner middle and outer parts of the ria grain size distributions reflect the presence of two main populations one dominated by silt and clay derived mainly from terrestrial sources and the other by fine sand to coarse silt which is derived mainly from continental shelf and ria mouth sources mineralogical analysis shows an abundance of terrestrial intensive weathering products near the ria head a dominance of shelf derived sediment towards the mouth and the presence of several diagenetic minerals whose nature varies with location within the ria in the inner ria the near surface sediments are slightly enriched in pb cu and zn from anthropogenic sources these sediments are fine grained and have a high organic content hence they have a higher potential to sorb contaminants than the coarser grained less organic rich sediments of the mid and outer ria the estimated sedimentation rates for the fine grained organic rich sediments from the inner part of the ria are about 1 mm year 1 the dominant authigenic minerals in the inner ria are iron sulphides whereas in the mid and outer ria iron silicates and oxyhydroxides are more important these differences in authigenic iron mineralogy are clearly reflected by the magnetic properties of the sediments

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

shallow gas accumulation in a small estuary and its implications a case history from in and around xiamen bay 1 various forms of shallow gas have been observed in seafloor strata on the seabed and in the water column during acoustic profiling investigations in 2007 2009 in xiamen bay and adjacent areas acoustically transparent zones acoustic turbidity and gas seepage can be seen in seabed strata pockmarks and accumulation bodies have been found on the seafloor and hummocky features and mushroom shaped gas signatures can be identified in the water column this evidence shows that shallow gas is widely distributed in and around xiamen bay due to degradation of the organic matter transported by the jiulong river the area covered by such features is roughly estimated at 150 km2 and methane flux is estimated to be 150 106 m3 assuming the thickness of gas bearing formations to be 1 m this study shows that even small rivers flowing out onto a continent shelf contribute to recognizable methane flux and are linked to identifiable gas reservoirs in the shallow seabed more detailed studies are required to understand the role of such systems as a component of the global carbon cycle

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

along strike translation of a fossil slab abstract the isabella anomaly is a high seismic velocity anomaly beneath the southern central valley of california that has previously been interpreted to represent a lithospheric drip or delaminated sierra nevada root however recent work suggests that the anomaly is a remnant slab left over from cenozoic subduction attached to the pacific plate underneath the edge of the north american plate this hypothesis requires the slab to translate hundreds of kilometers along strike while remaining intact and attached to the pacific plate despite drag from the surrounding asthenosphere and overriding lithosphere we use 3 d finite element models to simulate this scenario and calculate the viscosity ratio between the slab and the surrounding asthenosphere required for the slab to translate undeformed the required viscosity ratio increases with increasing downdip extent of the slab and decreases with increasing slab dip for geometries approximating that of our proposed slab the required ratio ranges from 10 2 to 10 4 given the thermal and petrologic history of the slab mantle flow laws predict viscosity contrasts greater than or equal to these requirements as such we conclude that along strike translation of a remnant slab is feasible and serves as a possible explanation of the isabella anomaly the significance of this finding extends beyond our general understanding of subduction dynamics in that the presence of such a slab could have implications for the water budget of the san andreas fault and its role in aseismic slip

contributions to atmospheric methane by natural seepages on the uk continental shelf natural gas seepages occur on the united kingdom s continental shelf and although published reports suggest that they are very rare the petroleum industry has identified but not publicly reported many more there is also very little data on the flux of gas from seabed seepages and even less on the contribution of seepages to atmospheric concentrations of gases such as methane potential gas source rocks include quaternary peats as well as petroliferous source rocks such as the carboniferous coal measures and the upper jurassic kimmeridge clays there are also other organic rich sediments which are potential source rocks together these cover a considerable part of the u k continental shelf analogue seismic reflection pinger profiles acquired during the british geological survey s regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles the ability to distinguish targets is critical to an assessment of the distribution of gas seepages both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles the likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth methane the principal constituent of seepage gas is relatively unreactive and sparingly soluble the studies have enabled a new estimate of the distribution of gas seepages on the u k continental shelf and of the contribution to atmospheric methane levels the results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established it is estimated that between 120 000 and 3 5 mtonnes of methane per year come from a continental shelf area of about 600 000 km2 this represents between 2 and 40 of the total united kingdom methane emission it is suggested that similar contributions arise from other continental shelf areas worldwide and that geological sources of atmospheric methane are more significant than is generally acknowledged

crustal structure of bohai sea and adjacent area north china from two onshore offshore wide angle seismic survey lines abstract in 2010 and 2011 onshore offshore surveys along two lines in the bohai area were carried out this paper presents the deep structure from wide angle seismic data along the two lines and try to improve the reliability of deep interfaces by using of the cmp prospecting method the result is as follows 1 the environmental noise of records from obss in shallow water area is stronger than that from deep waters 2 the cmp stacking method is potential for high density obs surveys 3 the thickness of sedimentary layer in bohai sea and surrounding area is 3 5 km the velocity increases with depth from about 2 km s at the seafloor to about 4 5 km s on the sedimentary basement 4 the upper crust thins toward the sea area from the land area the middle and lower crust also vary insignificantly the middle crust is about 4 km and the lower crust is about 15 km the velocities of the middle and lower crust shows gentle variation in vertical but obvious in lateral due to the impact of fault zones 5 the velocity contours nearby the tancheng lujiang tlfz and zhangjiakou penglai zpfz fault zones are u shaped where the velocity changes laterally and the interface undulates significantly velocity disturbances exist in the crust mantle transition zone at the fault zones the tlfz and zpfz could be the upwelling channels of deep material 6 in the eastern part of north china craton ncc the moho gently ranges at the depth of about 30 km with local rises lithosphere thinning occurred predominantly at the top of upper mantle the east boundary of lithosphere thinning is inferred to be on the east of bohai sea

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

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

quaternary evolution of the ria de vigo spain abstract the seismic analysis of the sedimentary infill of the ria de vigo reveals that the infill comprises of a fifth order sequence 18 ky bounded by a major discontinuity surface within the holocene sequence several seismic units have been identified the oldest one originated after the wurm glaciation and the origin of its top discontinuity was eroded during the regression sea level at 120 m of this last glacial episode isotopic stages 2 to 4 18 ky b p in some places below this ria infill another sequence can be identified this older sequence is associated with the pre existing faults and troughs in the deepest parts of the ria and it was deposited during the wurm glaciation miocene pleistocene eroded sediments were moved from the rias area to deeper zones where they accumulated in the lowstand system tract the next sea level rise occured in two steps producing two transgressive units separated by an hiatus surface this cycle was interrupted by the next sea level fall to 55 m marked on the seismic records by another recognizable boundary the next sea level rise took place in several steps back stepping with some stillstands generating another transgressive system tract in the ria from that time on the relative sea level continued rising depositing the most recent unit which represents the highstand system tract separating the highstand system tract and the transgresive unit below there is a consensed section the analysis of 29 cores provides groundtruthing for our hypothesis the identified main lithologic units show a transgressive sequence a granitic rock base an overlying second unit locally wedge shaped and most recently a marine muddy unit covering the present ria seafloor

receiver function and geometric tomography along the monterey microplate to test slab delamination or lithospheric drip models of the isabella anomaly california abstract two hypotheses were tested for the origin of the isabella tomographic anomaly which has been interpreted as either a lithospheric drip jones et al 1994 ducea and saleeby 1998 or a remnant of the farallon plate possibly attached to the monterey microplate wang et al 2013 p wave receiver functions and travel time residuals based on teleseismic events recorded by 41 stations were used to construct simple geometric tomography models of the isabella anomaly and to test whether the monterey microplate or a similar fragment of the farallon slab can be connected with the location of the anomaly the travel time residual pattern was modeled with a slab geometry composed of a high velocity rectangular block and a lithospheric drip was modeled as a dipping cylindrical anomaly both gave statistically similar fits but the cylindrical block appears less well constrained than the rectangular block the top surface of the best fit rectangular block was located at 50 km below the great valley and dips 65 northeast toward the sierra nevada with 100 km thickness west of the isabella anomaly receiver function p to s converted phases from the top and bottom surfaces of the oceanic crust of the monterey microplate were modeled as late arriving negative positive dipole signals such dipoles are observed at times that place their origin at 10 km below the moho at the coast ranges these converted phases from the oceanic crust could also be traced from the coast ranges to the great valley where the top surface of the slab model is located the combined result of the receiver functions and a geometric tomography model is consistent with the fossil slab hypothesis which initiated as a flat subduction system that later went through a delamination process online material figures of receiver functions sorted by back azimuth and ray parameter at 31 stations

characterisation and preliminary quantification of the methane reservoir in a coastal sedimentary source san simon bay ria de vigo nw spain abstract ria de vigo is a river valley flooded by the sea with a bay san simon bay at its innermost part the accumulation of holocene sediment in san simon bay has been studied by the integration of 1 large scale high resolution seismic data and 2 detailed geochemical analysis of a gravity core in san simon bay the majority of the seismic records are obscured by acoustic turbidity which represents gassy sediments but on records from rande strait it is possible to distinguish two quaternary seismic sequences an upper pleistocene sequence sq1 and a holocene sequence sq2 only sq2 is recognized in san simon bay where it is comprised of two seismic units the upper unit represents the hst sediment i e the period of highest sea level a gravity core taken within the gassy zone at 10 m water depth provided 3 55 m of fine grained sediments muds from the youngest seismic unit 4 m thick geochemical analysis show high values 4 to 10 of toc sediment and porewater analyses indicate a distinct sulphate methane transition zone smtz between 60 and 80 cm where sulphate is depleted to 0 4 mm the top of the acoustic turbidity the gas front at 80 cm corresponds to the lower limit of the smtz the methane cannot have been derived from the underlying metamorphic and granitic rocks but was probably derived by microbial degradation of the organic matter in the holocene sediments we estimate that the sediments of the bay contain approximately 1 8 10 6 m 3 of organic carbon and 275 ton of methane

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

geometry and structure associated to gas charged sediments and recent growth faults in the ebro delta spain abstract in the ebro delta ne spain small to large scale synsedimentary deformation has been observed by means of high and low resolution seismics correlated with a network of wells lithoseismic units involved are middle upper pleistocene clays and gravels and holocene deltaic sequence gassy sediments and large river floor pockmarks are identified from seismic profiles along the ebro delta gas charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles most of the gas is believed to be biogenic in origin resulting from the decay of organic matter contained within rapidly developing shelf margin deltas a group of large symmetric river floor pockmarks also occurs along the inner delta plain both on the river floor surface and buried in the sediment column listric fault geometry is a typical scope shaped plane with a steep upper surface 40 60 that passes downward into a horizontal shear plane the shear plane appears seismically as chaotic and convoluted high amplitude reflectors and is correlated with well data showing layers that include gravel sand and clays associated structural features include reverse drag of reflectors antithetic faults and fluid escapes methane water the growth faulting along the ebro delta includes a break away and thin near surface layer created by overstepping of a sandy holocene delta slope the over steeping lies down dip from growth faults along the shelf break the overall growth faults province appears to represent a deeper seated general movement of the delta mass involving differential loading overpressure of fluids diapiric movements and extensional tectonics that cause listric faults and associated structures at different scales the recent growth faults are regularly spaced with the distance between successive faults controlled by the depth to the detachment layer three main fault systems have been identified a first or primary system spaced 5 2 10 5 km above a detachment layer at 55 95 m deep a second system spaced 3 5 6 km and a third system spaced 1 5 2 5 km above a detachment layer at 25 30 m the slope angle calculated by empirical equations ranges between 0 5 and 1 0 for the three systems

oligocene tectonics and sedimentation california abstract during the oligocene epoch california was marked by extensive nonmarine sedimentation in contrast to its pre oligocene and post oligocene depositional history the oligocene continental deposits are especially widespread in southern california and fill a number of small and generally partly restricted basins fluvial facies in many basins prograded over previously deposited lower tertiary turbidites volcanism from widespread centers was associated with the nonmarine sedimentation however some basins remained marine and a few contain oligocene turbidites and pelagic sediments deposited at bathyal depths the oligocene redbeds of california do not form a post orogenic molasse sequence comparable to the old red sandstone or alpine molasse they are synorogenic and record local uplift of basins and surrounding source areas late cretaceous to contemporary orogenesis in california has been generally characterized by the formation of small restricted basins of variable depth adjacent to small upland areas in response to strike slip faulting deposition of oligocene redbeds was associated with climatic change from warm and humid to cold and semiarid and a global lowering of sea level oligocene tectonism occurred during the transition from subduction of the farallon plate to initiation of the modern san andreas transform system however the major influence that caused uplift formation of fault bounded basins and extensive redbed deposition especially in southern california was the approach of the pacific farallon spreading ridge to the western margin of california

gas venting and late quaternary sedimentation in the persian arabian gulf abstract high resolution 3 5 khz echo sounding profiles and piston cores were used to reconstruct the microtopography and late quaternary depositional history of the persian gulf perversive throughout the seafloor of the gulf is an extensive network of pockmarks formed by seepages of thermogenic gas these gas seeps and bottom water exiting the gulf via the strait of hormuz are the most significant processes controlling present day sedimentation in the region erosion by these seeps has been so intense in the baiban shelf in the strait of hormuz as to create a hoodoo like terrain on the outer shelf the surfical geology of the gulf documents a short lived transgression 29 400 to 22 800 years ago during the wisconsin regression which began 125 000 years ago the wisconsin regressive maxima when sea level dropped to 120 130 m about 21 000 20 000 years ago and the climate was dry and eolian and paralic sedimentation characterized the gulf the holocene transgression 18 000 to 12 000 years ago when the climate was more humid than during the climax of the wisconsin regression a dry phase 12 000 to 9000 years ago when the persian gulf was a site of eolian and carbonate deposition and the present sediment cycle during the last 9000 years under a more humid regime it was during the present cycle that southeast trending marl lobes were deposited off iran arabia acquired its hyper arid climate about 3000 years ago and the gulf attained its present configuration about 1000 years ago as a result of the construction of the tigris euphrates delta at its head and tectonism and aggradation along its arabian and iranian flanks

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

gas charged sediments on the inner continental shelf off western india abstract high resolution seismic reflection profiles on the inner continental shelf less than 60 m water depth off india from 10 n to 22 n show 5 35 m of weakly stratified acoustically transparent clays at water depths of 15 50 m the subbottom profiles are characterized by anomalous seismic signatures in the form of acoustic maskings these extend from the underlying late pleistocene unconformity often stretching along the bedding planes and occasionally surfacing on the sea bed these acoustic maskings could be due to shallow hydrocarbon gases mainly methane which might have been derived from biogenic sources or geothermal processes or both

new evidence of perfect overlapping of haploops and pockmarks field is it a coincidence abstract in the shallow waters of the bay of concarneau south brittany france previous studies reported a dense settlement of the species haploops tube dwelling bioengineer amphipod that perfectly overlaps a field of pockmarks a possible mechanistic link may therefore exist between the haploops colonies and the shallow pockmarks to test this hypothesis interferometry sonar and sub bottom profiler chirp data were acquired in two new areas located in the marine estuary of the loire river la lambarde and le croisic the new sites share the same sedimentary and geological characteristics as those found in the bay of concarneau the three sites are similarly located in shallow and muddy environments above a faulted middle eocene formation that is incised by paleo valleys despite different hydrodynamic conditions and estuarine influence in the two new investigated areas the haploops also settle within the pockmarks field limits to better constrain the association pockmarks haploops we compared the sedimentary features seismic signatures and total organic carbon toc content of the sediment active pockmark chimneys displaying highly reworked profiles and slightly higher toc contents suggest a local deeper source of organic matter which might be an important source of nutrient for the haploops in southern brittany the pockmarks haploops spatial overlap is not a simple coincidence and may therefore present a general pattern for identification of settlement preferences of the specie

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

geometry and evolution of the san andreas fault zone in northern california the evolution of the san andreas fault system is controlled by thermal mechanical processes associated with the development and evolution of a narrow slabless window formed beneath the western edge of north america this fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the mendocino triple junction as a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle the plate boundary separating the north american and pacific plates follows a complex three dimensional geometry which varies through time seismic velocity structure heat flow seismicity surface deformation uplift and fault development are controlled by the evolving thermal structure in the region after triple junction passage thermal mechanical models have been used to evaluate the fault system s time varying three dimensional dynamical behavior simulating the principal processes involved in the thermal mechanical evolution of the san andreas fault system results from this modeling indicate that the fault system has essentially a three stage history 1 in the vicinity of the mendocino triple junction the san andreas fault maps the eastern edge of the pacific plate with a broad 100 km zone of asthenospheric mantle separating the pacific and north american plates in the 25 to 90 km depth range 2 between 37 n and 39 n the plate boundary within the mantle separating the pacific and north american plates has developed approximately 40 60 km east of the surface trace of the san andreas fault and lies beneath the hayward calaveras faults and associated faults the surface trace of the plate boundary appears to be connected to the mantle segment via a lower crust subhorizontal detachment surface this fault zone orientation produces the surface deformations observed geodetically in the region 3 south of 37 n the surface fault again overlies the deeper plate boundary apparently as a result of an eastward jump in the surface fault

the diagenetic destruction of porosity in plutoniclastic petrofacies the miocene diligencia and eocene maniobra formations orocopia mountains southern california usa

geophysical evidence for gas geohazards off iskenderun bay se turkey abstract high resolution seafloor and sub surface data were acquired as part of a site survey in iskenderun bay se turkey to characterize the geohazards at the location of the proposed drilling site a 3 km 3 km geophysical study reveals a pockmark field which trends ne and nne similar to the trend of major fault systems in the area the pockmarks with an average diameter of 35 m reach their highest density in the northern part of the detailed survey area with 13 features km2 acoustic anomalies in the seismic records acoustic turbidity blanking enhanced reflectors below the proposed drilling site indicated potential shallow gas beneath it the local seismic anomalies amplitude and frequency parallel to stratigraphy were assigned a low gas risk as a result of the active neotectonics in the area the pockmark field presented a potential hazard for drilling at the original location the geohazard study resulted in moving the proposed drilling site eastward to an area of fewer pockmarks less sub surface seismic anomalies and thus a location interpreted as a lower geohazard environment

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

late cenozoic history and styles of deformation along the southern death valley fault zone california late cenozoic deposits in the southern death valley region have been offset 35 km by right lateral strike slip faulting on the southern death valley fault zone since miocene time virtually all slip took place prior to 1 m y ago along western traces of the fault zone during the past 1 m y the eastern traces of the fault zone have been active and characterized by oblique slip with a lateral component of only a few hundred metres movement along these eastern traces has formed normal faults and gentle to isoclinal folds that have uplifted fan gravel and lacustrine sediments as much as 200 m above the modern alluvial fan surface surveying of the longitudinal profile of the amargosa river which flows within the eastern traces of the fault zone suggests that vertical deformation continues today the 35 km of right lateral offset which is based on matching offset alluvial fan gravel with its source area refines earlier estimates of 8 to 80 km of movement for the southern death valley fault zone and it is consistent with the geometry of a pull apart basin model for central death valley causes for the observed differences in the style and timing of movement of the eastern and western subzones are not well understood the study area however is located a few kilometres north of the intersection of the southern death valley and garlock fault zones the garlock fault zone changes its sense of movement from left lateral strike slip to east vergent thrusting against the southern death valley fault zone the resulting compression may have caused the shift in activity and the change in style of deformation along branches of the southern death valley fault zone

seismicity and state of stress in the la paz los cabos region baja california sur mexico we present the results of an analysis of seismic data collected for seven years in the la paz los cabos region the data set includes earthquakes with magnitudes of up to 3 6 and focal depths mostly between 2 and 14 km the results show that some located epicenters correlated with known faults of the region but others did not suggesting faults that have not yet been recognized in addition even though earthquakes occurred all over the study area zones of stress concentration were identified in such zones the seismicity often developed as earthquake swarms composite fault plane solutions were prepared by using data of those areas the resulting mechanisms for events of the northern part of the study area indicated normal faulting east side down in such case the p axis had a mean vertical angle of 55 in the n12 w direction whereas the t axis was nearly horizontal and with a n68 e average trend this part of the study area is found to be under a predominant tensional stress regime to the southwest of the area the focal mechanisms showed predominant components of strike slip faulting there the t axes were still subhorizontal but the p axes were more horizontal resulting in a mean plunge of about 20 in the n38 w direction on the other hand highly consistent directions of p and t axes from 13 representative events of the gulf of california fault system led to average axes that are close approximations to the principal stresses that drive such a fault system a comparison of these axes with average p and t axes of our region shows that the latter are about 16 to 23 degrees more westerly than the former we propose then either that the earthquakes in the study area occur more in response to local tectonic forces or that our inferred stress axes are only approximations to the principal stresses the complex tectonic situation of the region makes the first possibility feasible but the fact that the earthquakes occurred on pre existing faults favors the second one future seismic data of the region will help to elucidate this uncertainty finally the microearthquakes studied were not generated by the gulf of california fault system that forms the boundary between the pacific and north america plates our results then are evidence of a wide zone of deformation across which subsidiary faulting accommodates part of the relative plates motion

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

expulsion of shallow gas in the skagerrak evidence from sub bottom profiling seismic hydroacoustical and geochemical data a combined high resolution seismic sub bottom profiling and multi beam echo sounding survey in the skagerrak danish sector of the north sea together with gas analyses at a station along the profile exhibit the expulsion of gas mainly methane and the presence of gas charged sediments at shallow depth the echo soundings yield detailed insight into the distribution and shape of typical sea floor features associated with gas seepage such as pockmarks the pockmarks reach dimensions of 800 m in length 300 m in width and 15 m in depth with the long axis running parallel to the slope of the norwegian trench processing of the multi channel high resolution seismic data and the digitally recorded sub bottom profiler signals indicate an internal compressional velocity of about 1050 m s 1 within the gas charged sediments reaching from the sea floor to a sub bottom depth of about 23 m using the lateral distribution and thickness of the gas charged sediments in conjunction with a mean concentration of gas of 3000 ppb the present amount of trapped gas is estimated to be 6 45 1011 g ch4 the flux of methane through the sea bed into the water column appears to be 7 2 1010 g ch4 per year to explain the small difference in size between the methane pool in near surface sediments and the annual flux through the sea bed a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

identification and characterization of fluid escape structures pockmarks in the estremadura spur west iberian margin abstract located on the west iberian margin between cabo carvoeiro and cabo da roca the estremadura spur is a trapezoidal promontory elongated in an east west direction extending until the tore seamount recently a field with more than 70 pockmarks was discovered in the nw region of the estremadura spur outer shelf lourinha monocline pockmarks are the seabed culminations of fluid migration through the sedimentary column and their characteristic seabed morphologies correspond to cone shaped circular or elliptical depressions the characterization of these features and the understanding of the associated fluid escape process are the main objectives of this work here we characterize these structures to understand their structural and stratigraphic control based on 1 seismic processing and interpretation of the high resolution 2d single channel sparker seismic dataset 2 bathymetric and backscatter interpretation and 3 rov direct observation of the seafloor the analysis of the seismic profiles allowed the identification of six seismic units disturbed by the migration and accumulation of fluids the estremadura spur outer shelf has been affected by several episodes of fluid migration and fluid escape during the pliocene quaternary that are expressed by a vast number of seabed and buried pockmarks at present the pockmarks are mainly inactive as the seabed pockmarks are covered by recent sediments it is concluded that the migration of fluids to the seabed occurred over the pliocene quaternary as indicated by the buried pockmarks at different depths below the seabed the vertical stacking of various pockmarks suggests a cyclical fluid flow activity that can possibly be the result of the eustatic sea level variations and the subsequent changes of the hydrostatic pressure

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

the holocene gas system of the ria de vigo nw spain factors controlling the location of gas accumulations seeps and pockmarks abstract high resolution seismic profiles cored sediment gravity cores and vibro cores and radiocarbon data from suitable shell material were used to establish the stratigraphic context of gassy sediments in the ria de vigo mapping of seeps and pockmarks were compared with the stratigraphy of gassy sediment to ascertain the relationships in the ria de vigo three holocene seismic units were identified the first two uh 1 and uh 2 correspond to the transgressive systems tract tst and the third uh 3 corresponds to the highstand systems tract hst the presence of acoustic turbidity within the three seismic units indicates that generation of gas took place during the relative sea level rise that occurred after the younger dryas event i e during the tst and mainly during the hst the relative sea level variations are shown to be a major factor controlling the type fine vs coarse grained and distribution of facies lateral and vertical that generate and or allow gas accumulation or escape acoustic turbidity due to presence of free gas was recognized in the three holocene units with a total of 12 acoustic turbidity fields mapped in the ria de vigo no significant variations in the spatial distribution or extent of the gas fields or in their vertical location below the present sea bed have been observed during the seismic surveys carried out in the ria during the last decade furthermore gas escape from the sediment and active pockmark has been observed therefore we conclude that gas escape seeps pockmark generation and maintenance and methane formation are in equilibrium the combined analysis of high resolution seismic profiles sediment temperature pore water salinity and hydrostatic pressure revealed 1 a direct relationship between depth of free gas and hydrostatic pressure and 2 a direct link between sediment temperature and number of acoustic plumes from seabed to water column more acoustic plumes were detected in summer when the temperature of the sediment was higher than in winter indicating a seasonal variation of seeps within the ria salinity showed no significant differences between sea bottom water and sediment pore water and was therefore not a major factor controlling gas front variations

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

modern analogues to middle ordovician sedimentary mounds and washout depressions abstract mounds and depressions containing dense fossil accumulations occur in a bed of middle ordovician age in the upper ottawa valley canada the appearance of these relict seabed features is similar to modern features containing dense accumulations of live species in the north sea it is suggested that the middle ordovician fossil accumulations relied on a similar local upward flux of minerals and nourishment as north sea communities which live in areas where sediments contain light hydrocarbon gases if further petrological and sedimentological studies on the middle ordovician deposits could confirm such a relationship then it would be the first time relict or fossil pockmarks were found in the sedimentary column

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

geochemical and microbial context of the gassy sediments in the ria de vigo nw of spain abstract multidisciplinary data coming from high resolution seismic profiles gravity core analysis sedimentology and geochemistry radiocarbon dating and metagenomics has been used to establish the stratigraphic framework as well as the geochemical and microbial context of the gassy sediments in the ria de vigo gas appears within the muddy sediments that compound the youngest holocene seismic unit of the infilling of the ria and is mainly methane generated by microbial decomposition in a rich organic matter depositional environment within the ria the biogeochemical zonation is affected by the quality quantity and age of the buried organic matter the 16s ribosomal rna gene amplicon clone libraries sequencing suggests that prokaryotic composition may be linked to the sediment type uncultured archaea common to the marine sub sea floor of the miscellaneous crenarchaeotal group mcg and marine benthic group d mbgd are dominant in these organic rich estuarine and shallow marine sediments the presence of methanomicrobia and anme in the sulphate methane transition zone smtz from the estuarine gassy area indicates anaerobic oxidation of methane aom in these sediments desulfobacteraceae are also homogeneously distributed along the sediment columns even in the methanogenic zone suggesting the existence of a cryptic sulphur cycle driven by iron under the smtz the older sediments retrieved from the non gassy outer area of the ria contain less organic matter and appear to represent mainly a sulphate reducing environment where smtz is not yet reached at the bottom of the sediment core methanogens of various affiliations and traces of methane are detected in this sulphate reducing environment with an unusual increase in concentration towards the sediment surface microbial methanogenesis in the sulphate reducing zone of surface sediments a process recently described in organic rich sediments able to supply methanogenic substrates could reconcile these observations

southward younging apatite u th he ages in the northern california coast ranges due to a northward migrating crustal welt abstract u th he in apatite dating of franciscan formation samples from between 39 47 n and 39 08 n in the northern california coast ranges nccr reveals a southward and downward younging pattern of ages 21 8 0 6 ma to 7 9 0 9 ma these ages are interpreted to represent a northward or northeastward dipping fossil he partial retention zone that was tilted and exhumed by a northward migrating pulse of crustal thickening that begins 150 200 km ahead of the migrating mendocino triple junction mtj k p furlong r govers ephemeral crustal thickening at a triple junction the mendocino crustal conveyor geology 27 1999 127 130 the 8 ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the he partial retention zone when crustal thickening began 5 my before the mtj passed by these rocks have now reached the surface 3 my after passage of the mtj the older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the he partial retention zone prior to crustal thickening ahead of the mtj and which therefore record ages transitional from older subduction related cooling and exhumation to more recent transform related cooling and exhumation in addition to providing support for the model of a migrating crustal welt associated with the mtj the existence of an essentially intact gently tilted fossil partial retention zone in the northern california coast range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region

timing and origin of silicic volcanism in northwestern mexico insights from zircon u pb geochronology hf isotopes and geochemistry of rhyolite ignimbrites from palmarejo and guazapares in southwest chihuahua abstract precise timing and constraints for magma source s are critical to understand the post subduction tectonomagmatic evolution of the world s largest cenozoic silicic large igneous province in western mexico we provide in situ zircon u pb ages and hf isotope compositions of rhyolitic ignimbrites exposed in the palmarejo and guazapares at the northwestern extremity of sierra madre occidental four rhyolite ignimbrite samples from palmarejo yielded zircon weighted mean u pb ages between 23 and 24 ma however five individual analyses from one sample yielded the oldest age of 25 09 0 41 ma mswd 1 7 n 5 a rhyolitic tuff from the guazapares district yielded older individual dates varying from 25 7 to 28 6 ma with a main population yielding a weighted mean age of 27 31 0 36 ma zircon u pb ages suggest that the ignimbrite flare up in southwestern chihuahua remained restricted to the early middle oligocene to early miocene we suggest that the zircon u pb dates 24 ma particularly from guazapares are the antecrystic zircons included from the underlying older ignimbrites the zircon u pb dates of 25 28 ma are related to the culmination of syn extensional first ignimbrite flare up and bimodal volcanism started as early as 35 ma the first ignimbrite flare up is possibly resulted from the detachment of the leading edge of the farallon slab around 35 28 ma the younger age of 24 23 ma from palmarejo can be correlated to the second episode of ignimbrite flare up erupted during the pervasive regional extension normal faulting and onset of formation of metamorphic core complexes this second flare up is resulted by the subsequent events of break off rollback of the farallon plate the positive hf isotope composition ehf t 1 0 to 5 8 and concomitant mafic volcanism suggest partial melting of subcontinental lithospheric mantle and the overlying laramide crust by upwelling of decompressing asthenosphere during post laramide regional extension this interpretation is consistent with the successive detachments fragmental removal and or rollback of the leading edges of the farallon slab beneath the western continental margin of north america we did not find any xenocrystic zircon inheritance neither from the laramide plutonic basement roots of the jurassic guerrero terrane nor from the proterozoic north american crust however zircon inheritance of laramide age has been reported from the guazapares district partial melting of laramide basement was possibly facilitated by the heat advection and ponding of subcontinental lithospheric mantle related magmas in the crust during the cenozoic regional extension

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

subduction of young oceanic lithosphere and extensional orogeny in southwestern north america during mid tertiary time an extensional orogeny deformed the basin and range province probably beginning in the late eocene about 40 3 ma its characteristics include partial melting of the continental lithosphere during the ignimbrite flareup massive ductile extension including detachment faulting and rise of metamorphic core complexes the affected zone became about 1200 km wide possibly double its original width it rose an average of 1 2 km despite crustal thinning locally some of the highest mountains of north america up to 4 3 km high rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons the climax of extension occurred prior to the development of the present basin and range topography modeling of major and trace elements and sr and pb isotopes strongly suggests that mid tertiary volcanic magmas equilibrated and probably originated in the continental lithosphere components attributable to subducted oceanic lithosphere have not yet been identified the rocks seem to belong to two provinces separated by the quartz diorite boundary line of moore 1959 which also marks the western limit of north america at the end of the late paleozoic sonoman orogeny to the west low k rocks rest on a basement of predominantly oceanic accreted terranes to the east high k rocks rest on an autochthonous sialic basement within the high k province potassium variations can be correlated with crustal thickness there is no need to invoke a k h relationship conventional models of plate convergence and back arc extension which involve subduction of old rigid cool and dense oceanic lithosphere may not apply to the mid tertiary basin and range province the overridden farallon plate is more likely to have been young hot ductile buoyant and no denser than continental asthenosphere having been generated in a spreading center close to north america under these conditions motion of the subducting plate slows and slab pull is likely to approach zero even prior to ridge trench collision overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs subducted oceanic lithosphere no longer acted as a heat sink which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting extension and metamorphism had these processes not died down after ridge trench collision the western segment of the cordillera might have separated from north america to form a japanlike archipelago while the basin and range province foundered into an analog to the sea of japan instead of rupturing completely the basin and range province fractured into fault blocks

seafloor gas seeps triggered by a northern california earthquake abstract a series of marine surveys using high resolution seismic reflection and side scan sonar profiling was conducted on the klamath river delta from 1977 to 1985 in november 1980 near the middle of this 8 year time period a major earthquake magnitude 7 occurred in the area the earthquake caused liquefaction of shelf deposits off the klamath river and while mapping the sediment failures we observed an inordinate number of gas vents and acoustic anomalies in the sediment gas charging and water column gas clouds our analyses indicate that a change in gas seepage occurred after the earthquake there was a marked increase in gas abundance immediately after the earthquake a gradual decrease over the next year and stabilization at pre earthquake gas levels within five years or less the acoustic anomalies were observed in pre quaternary rocks as well as in surficial holocene deposits and gas seeps overlay truncated anticlines faults and syncline exposures thus the gas released may have been in part thermogenic in origin a large area 20 km 2 on the delta failed during the earthquake and the abundance of gas seeps and craters in the area suggests that sediment degassing may have been an important process contributing to failure

chemically diverse sporadic volcanism at seamounts offshore southern and baja california compositions of lavas from seven small to medium sized seamounts between lat 34 0n25 n are all highly differentiated trachyte and trachyandesite based on 40 ar 39 ar laser fusion techniques morb like lava from one of the northernedificesisasoldastheunderlying oceanic crust 20 ma indicating that it originatedataspreadingcenter otherseamountlavaagesaremuchyoungerthanthe oceanic crust on which they reside ranging from 16 8 6 0 3 to 7 ma for some of the northern seamounts to 270 616 ka for the trachyte from rocas alijos similar highly evolved lavas cap fossil spreading centers like guadalupe and socorro islands but rocas alijos based on magnetic anomalies is not an abandoned spreading center but may instead have formed on a leaky transform fault some of the seamounts with transitional and alkalic lavas may have formed as part of a short age progressive chain formed by a short lived mantle plume many others aligned along abandoned spreading centers orfaultsandfracturezoneswhichareabundant in the tectonically complex region offshore southern and peninsular california may have resulted from upwelling mantle diapirs in response to localized extension someoftheepisodesofvolcanismappearto havebeencontemporaneouswithvolcanism in the continental borderland and coastal southern california suggesting linkage between extension along the continental margin and the seamount province farther offshore the data available for the abundant volcanic edifices of varying sizes shapes and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism tappingvariablyenriched heterogeneousmantle which occurred sporadically from early miocene to late pleistocene

a seeping sea floor in a ria environment ria de vigo nw spain the occurrence of gas accumulations in the ria de vigo nw spain have been characterized by the authors in previous research pockmarks frequently appear on seismic and sonar records covering ca 45 of the sea floor of the study area which indicates that gas expulsion is not an uncommon phenomena in the coastal ria environment here we report the occurrence of gas seepage for the first time along the coast of nw spain side scan sonar echo sounder and high resolution seismic techniques were used for mapping gas expulsion features some expulsion pockmarks sit over elongated features that represent bottom marks created by anthropogenic activity thus these anthropogenic sites may act as preferential venting zones for gas as well as being potential hazards on a muddy sea bed such as that of the ria where gas accumulates just below the surface of the sea floor

geophysical evidence for miocene extension and mafic magmatic addition in the california continental borderland the california continental borderland is a key region for understanding the effect on crustal structure of tectonic processes associated with the initiation and elongation of the san andreas transform system because the borderland lies near the point where the pacific farallon spreading system first encountered the north american plate margin whereas the central and northern california margins have been extensively investigated by geophysical surveys in recent years the crustal structure of the borderland particularly the outer borderland has received relatively little attention this paper presents three gravity models that stretch from the pacific ocean floor to shore they provide new evidence that the crust of the entire california continental borderland has been extended and infiltrated by mafic melt since early miocene time the gravity models were constructed from interpretations of nearly 750 km of seismic reflection data seismic refraction studies and geologic maps thus minimizing the nonuniqueness of the method the evidence for extension in the models is that the crust of the accretionary complex and forearc basin that make up the borderland is only 22 km thick 35 thinner than comparable crustal sections in central and northern california extension could well have been accommodated by eastward flow of accretionary complex material during middle miocene formation of a metamorphic core complex in the inner borderland the gravity models also show that the accretionary complex is 100 200 kg m 3 more dense than commonly observed to the north this density difference may result from influx of basaltic melts due to mantle upwelling at the rivera triple junction and beneath the highly extended inner borderland the borderland models permit the occurrence of a discrete mafic layer that forms the base of the crust as is seen along the central and northern california coast but do not require such a layer9s presence anywhere except immediately inland of the shelf slope break

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

high resolution shallow seismic identification of gas escape features in the sediments of loch tay scotland tectonic and microbiological associations high resolution seismic reflection profiles 3 5 khz have revealed the presence of extensive interstitial gas accumulation within the sedimentary sequences of loch tay scotland as identified by acoustic turbidity masking the seismic stratigraphy within the central section of the loch in the deepest water area directly above the zone of the seismically active loch tay fault focused flows of gas through the sediment pile to the loch bed via chimneys and pockmarks together with gas seeps within the water column have been identified microbiological observations indicate that the gas is biogenic ch4 produced by both chemoautotrophic which use co2 as a source of carbon and h2 as a source of energy and aceticlastic species which use acetate as a source of carbon and energy of methanogens in the fine grained organic rich deposits that have been focused into the zone of accumulation in the deep central part of the loch the spatial distribution of the gas escape features suggests that earthquake movements along the loch tay fault are responsible for facilitating focused gas escape in this part of the loch by the creation of new pathways and conduits through the sediment pile along which gas can migrate upwards and exit into the water column relict pockmarks and associated chimneys identified in the seismic records indicate that gas escape has been taking place since pleistocene times though the precise timings cannot be ascertained this is the first time that such features have been reported from a lake in the uk

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

uncertainties in reconstructions of the pacific farallon vancouver and kula plates and constraints on the rigidity of the pacific and farallon and vancouver plates between 72 and 35 ma to study the rigidity of the pacific and farallon plates between 72 and 35 ma we analyzed the magnetic anomalies and fracture zones in the pacific ocean between longitudes 125 w 180 and latitudes 43 s 60 n a set of consistent rotations indicates that the pacific and farallon plates between the pioneer and agassiz fracture zones were rigid plates between 72 and 35 ma thus the location of a missing tertiary boundary within the pacific or antarctic plates is restricted to be south of latitude 43 s the farallon plate broke into two plates at the time of the change in the pacific farallon spreading direction north of the pioneer and mendocino fracture zones sometime between 59 and 49 ma the northern fragment termed the vancouver plate was subducted beneath most of north america we present rotations and uncertainties for the histories of motion of the pacific and kula plates from anomaly 32 to anomaly 25 of the pacific and vancouver plates from anomaly 21 to anomaly 13 and of the pacific and farallon plates from anomaly 32 to anomaly 13

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

thermodynamic and vertical velocity structure of two gust fronts observed with a wind profiler rass during mctex high time resolution wind profiler rass observations are used to describe the vertical velocity temperature and reflectivity fields of two gust fronts in detail the first was a freely propagating gust front and the second interacted with a rain cell near the profiler site the first of these shows a large updraft confined to the warm air ahead of the front this updraft coincided with the nonhydrostatic pressure jump the vertical motions within the gust front were an order of magnitude smaller the updraft impinging on the top of the boundary layer excited a clear gravity wave signature in the free troposphere the interaction of the vertical circulation and the weakly precipitating cloud in the second case coincided with explosive growth of the cell with reflectivities increasing from 30 dbz to 50 dbz in 6 min a descending reflectivity core was observed at this time precipitation loading played a significant role in a downdraft behind the gust front head leading to adiabatic warming as no evidence of evaporative cooling in the downdraft was seen a distinct clear air peak was visible in the profiler doppler spectra even during the heavy rain

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

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